example of synthesis of literature review

Literature Syntheis 101

How To Synthesise The Existing Research (With Examples)

By: Derek Jansen (MBA) | Expert Reviewer: Eunice Rautenbach (DTech) | August 2023

One of the most common mistakes that students make when writing a literature review is that they err on the side of describing the existing literature rather than providing a critical synthesis of it. In this post, we’ll unpack what exactly synthesis means and show you how to craft a strong literature synthesis using practical examples.

This post is based on our popular online course, Literature Review Bootcamp . In the course, we walk you through the full process of developing a literature review, step by step. If it’s your first time writing a literature review, you definitely want to use this link to get 50% off the course (limited-time offer).

Overview: Literature Synthesis

• What exactly does “synthesis” mean?
• Aspect 1: Agreement
• Aspect 2: Disagreement
• Aspect 3: Key theories
• Aspect 4: Contexts
• Aspect 5: Methodologies
• Bringing it all together

What does “synthesis” actually mean?

As a starting point, let’s quickly define what exactly we mean when we use the term “synthesis” within the context of a literature review.

Simply put, literature synthesis means going beyond just describing what everyone has said and found. Instead, synthesis is about bringing together all the information from various sources to present a cohesive assessment of the current state of knowledge in relation to your study’s research aims and questions .

Put another way, a good synthesis tells the reader exactly where the current research is “at” in terms of the topic you’re interested in – specifically, what’s known , what’s not , and where there’s a need for more research .

So, how do you go about doing this?

Well, there’s no “one right way” when it comes to literature synthesis, but we’ve found that it’s particularly useful to ask yourself five key questions when you’re working on your literature review. Having done so,  you can then address them more articulately within your actual write up. So, let’s take a look at each of these questions.

1. Points Of Agreement

The first question that you need to ask yourself is: “Overall, what things seem to be agreed upon by the vast majority of the literature?”

For example, if your research aim is to identify which factors contribute toward job satisfaction, you’ll need to identify which factors are broadly agreed upon and “settled” within the literature. Naturally, there may at times be some lone contrarian that has a radical viewpoint , but, provided that the vast majority of researchers are in agreement, you can put these random outliers to the side. That is, of course, unless your research aims to explore a contrarian viewpoint and there’s a clear justification for doing so. 

Identifying what’s broadly agreed upon is an essential starting point for synthesising the literature, because you generally don’t want (or need) to reinvent the wheel or run down a road investigating something that is already well established . So, addressing this question first lays a foundation of “settled” knowledge.

Need a helping hand?

2. Points Of Disagreement

Related to the previous point, but on the other end of the spectrum, is the equally important question: “Where do the disagreements lie?” .

In other words, which things are not well agreed upon by current researchers? It’s important to clarify here that by disagreement, we don’t mean that researchers are (necessarily) fighting over it – just that there are relatively mixed findings within the empirical research , with no firm consensus amongst researchers.

This is a really important question to address as these “disagreements” will often set the stage for the research gap(s). In other words, they provide clues regarding potential opportunities for further research, which your study can then (hopefully) contribute toward filling. If you’re not familiar with the concept of a research gap, be sure to check out our explainer video covering exactly that .

3. Key Theories

The next question you need to ask yourself is: “Which key theories seem to be coming up repeatedly?” .

Within most research spaces, you’ll find that you keep running into a handful of key theories that are referred to over and over again. Apart from identifying these theories, you’ll also need to think about how they’re connected to each other. Specifically, you need to ask yourself:

• Are they all covering the same ground or do they have different focal points  or underlying assumptions ?
• Do some of them feed into each other and if so, is there an opportunity to integrate them into a more cohesive theory?
• Do some of them pull in different directions ? If so, why might this be?
• Do all of the theories define the key concepts and variables in the same way, or is there some disconnect? If so, what’s the impact of this ?

Simply put, you’ll need to pay careful attention to the key theories in your research area, as they will need to feature within your theoretical framework , which will form a critical component within your final literature review. This will set the foundation for your entire study, so it’s essential that you be critical in this area of your literature synthesis.

If this sounds a bit fluffy, don’t worry. We deep dive into the theoretical framework (as well as the conceptual framework) and look at practical examples in Literature Review Bootcamp . If you’d like to learn more, take advantage of our limited-time offer to get 60% off the standard price.

4. Contexts

The next question that you need to address in your literature synthesis is an important one, and that is: “Which contexts have (and have not) been covered by the existing research?” .

For example, sticking with our earlier hypothetical topic (factors that impact job satisfaction), you may find that most of the research has focused on white-collar , management-level staff within a primarily Western context, but little has been done on blue-collar workers in an Eastern context. Given the significant socio-cultural differences between these two groups, this is an important observation, as it could present a contextual research gap .

In practical terms, this means that you’ll need to carefully assess the context of each piece of literature that you’re engaging with, especially the empirical research (i.e., studies that have collected and analysed real-world data). Ideally, you should keep notes regarding the context of each study in some sort of catalogue or sheet, so that you can easily make sense of this before you start the writing phase. If you’d like, our free literature catalogue worksheet is a great tool for this task.

5. Methodological Approaches

Last but certainly not least, you need to ask yourself the question: “What types of research methodologies have (and haven’t) been used?”

For example, you might find that most studies have approached the topic using qualitative methods such as interviews and thematic analysis. Alternatively, you might find that most studies have used quantitative methods such as online surveys and statistical analysis.

But why does this matter?

Well, it can run in one of two potential directions . If you find that the vast majority of studies use a specific methodological approach, this could provide you with a firm foundation on which to base your own study’s methodology . In other words, you can use the methodologies of similar studies to inform (and justify) your own study’s research design .

On the other hand, you might argue that the lack of diverse methodological approaches presents a research gap , and therefore your study could contribute toward filling that gap by taking a different approach. For example, taking a qualitative approach to a research area that is typically approached quantitatively. Of course, if you’re going to go against the methodological grain, you’ll need to provide a strong justification for why your proposed approach makes sense. Nevertheless, it is something worth at least considering.

Regardless of which route you opt for, you need to pay careful attention to the methodologies used in the relevant studies and provide at least some discussion about this in your write-up. Again, it’s useful to keep track of this on some sort of spreadsheet or catalogue as you digest each article, so consider grabbing a copy of our free literature catalogue if you don’t have anything in place.

Bringing It All Together

Alright, so we’ve looked at five important questions that you need to ask (and answer) to help you develop a strong synthesis within your literature review.  To recap, these are:

• Which things are broadly agreed upon within the current research?
• Which things are the subject of disagreement (or at least, present mixed findings)?
• Which theories seem to be central to your research topic and how do they relate or compare to each other?
• Which contexts have (and haven’t) been covered?
• Which methodological approaches are most common?

Importantly, you’re not just asking yourself these questions for the sake of asking them – they’re not just a reflection exercise. You need to weave your answers to them into your actual literature review when you write it up. How exactly you do this will vary from project to project depending on the structure you opt for, but you’ll still need to address them within your literature review, whichever route you go.

The best approach is to spend some time actually writing out your answers to these questions, as opposed to just thinking about them in your head. Putting your thoughts onto paper really helps you flesh out your thinking . As you do this, don’t just write down the answers – instead, think about what they mean in terms of the research gap you’ll present , as well as the methodological approach you’ll take . Your literature synthesis needs to lay the groundwork for these two things, so it’s essential that you link all of it together in your mind, and of course, on paper.

Psst… there’s more!

This post is an extract from our bestselling short course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

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• University of Oregon Libraries
• Research Guides

How to Write a Literature Review

• 6. Synthesize
• Literature Reviews: A Recap
• Reading Journal Articles
• Does it Describe a Literature Review?
• 1. Identify the Question
• 2. Review Discipline Styles
• Searching Article Databases
• Finding Full-Text of an Article
• Citation Chaining
• When to Stop Searching
• 4. Manage Your References
• 5. Critically Analyze and Evaluate

Synthesis Visualization

Synthesis matrix example.

• 7. Write a Literature Review

• Synthesis Worksheet

About Synthesis

Approaches to synthesis.

You can sort the literature in various ways, for example:

How to Begin?

Read your sources carefully and find the main idea(s) of each source

Look for similarities in your sources – which sources are talking about the same main ideas? (for example, sources that discuss the historical background on your topic)

Use the worksheet (above) or synthesis matrix (below) to get organized

This work can be messy. Don't worry if you have to go through a few iterations of the worksheet or matrix as you work on your lit review!

Four Examples of Student Writing

In the four examples below, only ONE shows a good example of synthesis: the fourth column, or  Student D . For a web accessible version, click the link below the image.

Long description of "Four Examples of Student Writing" for web accessibility

• Download a copy of the "Four Examples of Student Writing" chart

Click on the example to view the pdf.

From Jennifer Lim

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• Next: 7. Write a Literature Review >>
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Get Organized

• Lit Review Prep Use this template to help you evaluate your sources, create article summaries for an annotated bibliography, and a synthesis matrix for your lit review outline.

Synthesize your Information

Synthesize: combine separate elements to form a whole.

Synthesis Matrix

A synthesis matrix helps you record the main points of each source and document how sources relate to each other.

After summarizing and evaluating your sources, arrange them in a matrix or use a citation manager to help you see how they relate to each other and apply to each of your themes or variables.  

By arranging your sources by theme or variable, you can see how your sources relate to each other, and can start thinking about how you weave them together to create a narrative.

• Step-by-Step Approach
• Example Matrix from NSCU
• Matrix Template
• << Previous: Summarize
• Next: Integrate >>
• Last Updated: Sep 26, 2023 10:25 AM
• URL: https://guides.library.jhu.edu/lit-review

How to Write a Literature Review - A Self-Guided Tutorial

• Literature Reviews: A Recap
• Reading Journal Articles
• Does it describe a Literature Review?
• 1. Identify the question
• 2. Review discipline styles
• Searching article databases - video
• Finding the article full-text
• Citation trails
• When to stop searching
• Citation Managers
• 5. Critically analyze and evaluate
• 6. Synthesize
• 7. Write literature review
• Additional Resources

You can meet with a librarian to talk about your literature review, or other library-related topics.

You can sort the literature in various ways, for example:

Synthesis Vizualization

Four examples of student writing.

In the four examples below, only ONE shows a good example of synthesis: the fourth column, or  Student D . For a web accessible version, click the link below the image.

Long description of "Four Examples of Student Writing" for web accessibility

• Download a copy of the "Four Examples of Student Writing" chart

Synthesis Matrix Example

From Jennifer Lim

Synthesis Templates

Synthesis grids are organizational tools used to record the main concepts of your sources and can help you make connections about how your sources relate to one another.

• Source Template Basic Literature Review Source Template from Walden University Writing Center to help record the main findings and concepts from different articles.
• Sample Literature Review Grids This spreadsheet contains multiple tabs with different grid templates. Download or create your own copy to begin recording notes.
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• Next: 7. Write literature review >>
• Last Updated: May 23, 2024 11:00 AM
• URL: https://libguides.ucmerced.edu/literature-review

How to Synthesize Written Information from Multiple Sources

Shona McCombes

Content Manager

B.A., English Literature, University of Glasgow

Shona McCombes is the content manager at Scribbr, Netherlands.

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Saul Mcleod, PhD

Editor-in-Chief for Simply Psychology

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, PhD., is a qualified psychology teacher with over 18 years of experience in further and higher education. He has been published in peer-reviewed journals, including the Journal of Clinical Psychology.

On This Page:

When you write a literature review or essay, you have to go beyond just summarizing the articles you’ve read – you need to synthesize the literature to show how it all fits together (and how your own research fits in).

Synthesizing simply means combining. Instead of summarizing the main points of each source in turn, you put together the ideas and findings of multiple sources in order to make an overall point.

At the most basic level, this involves looking for similarities and differences between your sources. Your synthesis should show the reader where the sources overlap and where they diverge.

Unsynthesized Example

Franz (2008) studied undergraduate online students. He looked at 17 females and 18 males and found that none of them liked APA. According to Franz, the evidence suggested that all students are reluctant to learn citations style. Perez (2010) also studies undergraduate students. She looked at 42 females and 50 males and found that males were significantly more inclined to use citation software ( p < .05). Findings suggest that females might graduate sooner. Goldstein (2012) looked at British undergraduates. Among a sample of 50, all females, all confident in their abilities to cite and were eager to write their dissertations.

Synthesized Example

Studies of undergraduate students reveal conflicting conclusions regarding relationships between advanced scholarly study and citation efficacy. Although Franz (2008) found that no participants enjoyed learning citation style, Goldstein (2012) determined in a larger study that all participants watched felt comfortable citing sources, suggesting that variables among participant and control group populations must be examined more closely. Although Perez (2010) expanded on Franz’s original study with a larger, more diverse sample…

Step 1: Organize your sources

After collecting the relevant literature, you’ve got a lot of information to work through, and no clear idea of how it all fits together.

Before you can start writing, you need to organize your notes in a way that allows you to see the relationships between sources.

One way to begin synthesizing the literature is to put your notes into a table. Depending on your topic and the type of literature you’re dealing with, there are a couple of different ways you can organize this.

Summary table

A summary table collates the key points of each source under consistent headings. This is a good approach if your sources tend to have a similar structure – for instance, if they’re all empirical papers.

Each row in the table lists one source, and each column identifies a specific part of the source. You can decide which headings to include based on what’s most relevant to the literature you’re dealing with.

For example, you might include columns for things like aims, methods, variables, population, sample size, and conclusion.

For each study, you briefly summarize each of these aspects. You can also include columns for your own evaluation and analysis.

The summary table gives you a quick overview of the key points of each source. This allows you to group sources by relevant similarities, as well as noticing important differences or contradictions in their findings.

Synthesis matrix

A synthesis matrix is useful when your sources are more varied in their purpose and structure – for example, when you’re dealing with books and essays making various different arguments about a topic.

Each column in the table lists one source. Each row is labeled with a specific concept, topic or theme that recurs across all or most of the sources.

Then, for each source, you summarize the main points or arguments related to the theme.

The purposes of the table is to identify the common points that connect the sources, as well as identifying points where they diverge or disagree.

Step 2: Outline your structure

Now you should have a clear overview of the main connections and differences between the sources you’ve read. Next, you need to decide how you’ll group them together and the order in which you’ll discuss them.

For shorter papers, your outline can just identify the focus of each paragraph; for longer papers, you might want to divide it into sections with headings.

There are a few different approaches you can take to help you structure your synthesis.

If your sources cover a broad time period, and you found patterns in how researchers approached the topic over time, you can organize your discussion chronologically .

That doesn’t mean you just summarize each paper in chronological order; instead, you should group articles into time periods and identify what they have in common, as well as signalling important turning points or developments in the literature.

If the literature covers various different topics, you can organize it thematically .

That means that each paragraph or section focuses on a specific theme and explains how that theme is approached in the literature.

Source Used with Permission: The Chicago School

If you’re drawing on literature from various different fields or they use a wide variety of research methods, you can organize your sources methodologically .

That means grouping together studies based on the type of research they did and discussing the findings that emerged from each method.

If your topic involves a debate between different schools of thought, you can organize it theoretically .

That means comparing the different theories that have been developed and grouping together papers based on the position or perspective they take on the topic, as well as evaluating which arguments are most convincing.

Step 3: Write paragraphs with topic sentences

What sets a synthesis apart from a summary is that it combines various sources. The easiest way to think about this is that each paragraph should discuss a few different sources, and you should be able to condense the overall point of the paragraph into one sentence.

This is called a topic sentence , and it usually appears at the start of the paragraph. The topic sentence signals what the whole paragraph is about; every sentence in the paragraph should be clearly related to it.

A topic sentence can be a simple summary of the paragraph’s content:

“Early research on [x] focused heavily on [y].”

For an effective synthesis, you can use topic sentences to link back to the previous paragraph, highlighting a point of debate or critique:

“Several scholars have pointed out the flaws in this approach.” “While recent research has attempted to address the problem, many of these studies have methodological flaws that limit their validity.”

By using topic sentences, you can ensure that your paragraphs are coherent and clearly show the connections between the articles you are discussing.

As you write your paragraphs, avoid quoting directly from sources: use your own words to explain the commonalities and differences that you found in the literature.

Don’t try to cover every single point from every single source – the key to synthesizing is to extract the most important and relevant information and combine it to give your reader an overall picture of the state of knowledge on your topic.

Step 4: Revise, edit and proofread

Like any other piece of academic writing, synthesizing literature doesn’t happen all in one go – it involves redrafting, revising, editing and proofreading your work.

Checklist for Synthesis

•   Do I introduce the paragraph with a clear, focused topic sentence?
•   Do I discuss more than one source in the paragraph?
•   Do I mention only the most relevant findings, rather than describing every part of the studies?
•   Do I discuss the similarities or differences between the sources, rather than summarizing each source in turn?
•   Do I put the findings or arguments of the sources in my own words?
•   Is the paragraph organized around a single idea?
•   Is the paragraph directly relevant to my research question or topic?
•   Is there a logical transition from this paragraph to the next one?

Further Information

How to Synthesise: a Step-by-Step Approach

Help…I”ve Been Asked to Synthesize!

Learn how to Synthesise (combine information from sources)

How to write a Psychology Essay

Related Articles

Student Resources

How To Cite A YouTube Video In APA Style – With Examples

How to Write an Abstract APA Format

APA References Page Formatting and Example

APA Title Page (Cover Page) Format, Example, & Templates

How do I Cite a Source with Multiple Authors in APA Style?

How to Write a Psychology Essay

Library Guides

Literature reviews: synthesis.

• Criticality

Synthesise Information

So, how can you create paragraphs within your literature review that demonstrates your knowledge of the scholarship that has been done in your field of study?  

You will need to present a synthesis of the texts you read.  

Doug Specht, Senior Lecturer at the Westminster School of Media and Communication, explains synthesis for us in the following video:  

Synthesising Texts  

What is synthesis? 

Synthesis is an important element of academic writing, demonstrating comprehension, analysis, evaluation and original creation.  

With synthesis you extract content from different sources to create an original text. While paraphrase and summary maintain the structure of the given source(s), with synthesis you create a new structure.  

The sources will provide different perspectives and evidence on a topic. They will be put together when agreeing, contrasted when disagreeing. The sources must be referenced.  

Perfect your synthesis by showing the flow of your reasoning, expressing critical evaluation of the sources and drawing conclusions.  

When you synthesise think of "using strategic thinking to resolve a problem requiring the integration of diverse pieces of information around a structuring theme" (Mateos and Sole 2009, p448). 

Synthesis is a complex activity, which requires a high degree of comprehension and active engagement with the subject. As you progress in higher education, so increase the expectations on your abilities to synthesise. 

How to synthesise in a literature review: 

Identify themes/issues you'd like to discuss in the literature review. Think of an outline.  

Read the literature and identify these themes/issues.  

Critically analyse the texts asking: how does the text I'm reading relate to the other texts I've read on the same topic? Is it in agreement? Does it differ in its perspective? Is it stronger or weaker? How does it differ (could be scope, methods, year of publication etc.). Draw your conclusions on the state of the literature on the topic.  

Start writing your literature review, structuring it according to the outline you planned.  

Put together sources stating the same point; contrast sources presenting counter-arguments or different points.  

Present your critical analysis.  

Always provide the references. 

The best synthesis requires a "recursive process" whereby you read the source texts, identify relevant parts, take notes, produce drafts, re-read the source texts, revise your text, re-write... (Mateos and Sole, 2009). 

What is good synthesis?  

The quality of your synthesis can be assessed considering the following (Mateos and Sole, 2009, p439):  

Integration and connection of the information from the source texts around a structuring theme. 

Selection of ideas necessary for producing the synthesis. 

Appropriateness of the interpretation.  

Elaboration of the content.  

Example of Synthesis

Original texts (fictitious): 

Synthesis: 

Animal experimentation is a subject of heated debate. Some argue that painful experiments should be banned. Indeed it has been demonstrated that such experiments make animals suffer physically and psychologically (Chowdhury 2012; Panatta and Hudson 2016). On the other hand, it has been argued that animal experimentation can save human lives and reduce harm on humans (Smith 2008). This argument is only valid for toxicological testing, not for tests that, for example, merely improve the efficacy of a cosmetic (Turner 2015). It can be suggested that animal experimentation should be regulated to only allow toxicological risk assessment, and the suffering to the animals should be minimised.   

Bibliography

Mateos, M. and Sole, I. (2009). Synthesising Information from various texts: A Study of Procedures and Products at Different Educational Levels. European Journal of Psychology of Education,  24 (4), 435-451. Available from https://doi.org/10.1007/BF03178760 [Accessed 29 June 2021].

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• Next: Criticality >>
• Last Updated: Nov 18, 2023 10:56 PM
• URL: https://libguides.westminster.ac.uk/literature-reviews

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Chapter 7: Synthesizing Sources

Learning objectives.

At the conclusion of this chapter, you will be able to:

• synthesize key sources connecting them with the research question and topic area.

7.1 Overview of synthesizing

7.1.1 putting the pieces together.

Combining separate elements into a whole is the dictionary definition of synthesis.  It is a way to make connections among and between numerous and varied source materials.  A literature review is not an annotated bibliography, organized by title, author, or date of publication.  Rather, it is grouped by topic to create a whole view of the literature relevant to your research question.

Your synthesis must demonstrate a critical analysis of the papers you collected as well as your ability to integrate the results of your analysis into your own literature review.  Each paper collected should be critically evaluated and weighed for “adequacy, appropriateness, and thoroughness” ( Garrard, 2017 ) before inclusion in your own review.  Papers that do not meet this criteria likely should not be included in your literature review.

Begin the synthesis process by creating a grid, table, or an outline where you will summarize, using common themes you have identified and the sources you have found. The summary grid or outline will help you compare and contrast the themes so you can see the relationships among them as well as areas where you may need to do more searching. Whichever method you choose, this type of organization will help you to both understand the information you find and structure the writing of your review.  Remember, although “the means of summarizing can vary, the key at this point is to make sure you understand what you’ve found and how it relates to your topic and research question” ( Bennard et al., 2014 ).

As you read through the material you gather, look for common themes as they may provide the structure for your literature review.  And, remember, research is an iterative process: it is not unusual to go back and search information sources for more material.

At one extreme, if you are claiming, ‘There are no prior publications on this topic,’ it is more likely that you have not found them yet and may need to broaden your search.  At another extreme, writing a complete literature review can be difficult with a well-trod topic.  Do not cite it all; instead cite what is most relevant.  If that still leaves too much to include, be sure to reference influential sources…as well as high-quality work that clearly connects to the points you make. ( Klingner, Scanlon, & Pressley, 2005 ).

7.2 Creating a summary table

Literature reviews can be organized sequentially or by topic, theme, method, results, theory, or argument.  It’s important to develop categories that are meaningful and relevant to your research question.  Take detailed notes on each article and use a consistent format for capturing all the information each article provides.  These notes and the summary table can be done manually, using note cards.  However, given the amount of information you will be recording, an electronic file created in a word processing or spreadsheet is more manageable. Examples of fields you may want to capture in your notes include:

• Authors’ names
• Article title
• Publication year
• Main purpose of the article
• Methodology or research design
• Participants
• Measurement
• Conclusions

  Other fields that will be useful when you begin to synthesize the sum total of your research:

• Specific details of the article or research that are especially relevant to your study
• Key terms and definitions
• Strengths or weaknesses in research design
• Relationships to other studies
• Possible gaps in the research or literature (for example, many research articles conclude with the statement “more research is needed in this area”)
• Finally, note how closely each article relates to your topic.  You may want to rank these as high, medium, or low relevance.  For papers that you decide not to include, you may want to note your reasoning for exclusion, such as ‘small sample size’, ‘local case study,’ or ‘lacks evidence to support assertion.’

This short video demonstrates how a nursing researcher might create a summary table.

7.2.1 Creating a Summary Table

  Summary tables can be organized by author or by theme, for example:

For a summary table template, see http://blogs.monm.edu/writingatmc/files/2013/04/Synthesis-Matrix-Template.pdf

7.3 Creating a summary outline

An alternate way to organize your articles for synthesis it to create an outline. After you have collected the articles you intend to use (and have put aside the ones you won’t be using), it’s time to identify the conclusions that can be drawn from the articles as a group.

  Based on your review of the collected articles, group them by categories.  You may wish to further organize them by topic and then chronologically or alphabetically by author.  For each topic or subtopic you identified during your critical analysis of the paper, determine what those papers have in common.  Likewise, determine which ones in the group differ.  If there are contradictory findings, you may be able to identify methodological or theoretical differences that could account for the contradiction (for example, differences in population demographics).  Determine what general conclusions you can report about the topic or subtopic as the entire group of studies relate to it.  For example, you may have several studies that agree on outcome, such as ‘hands on learning is best for science in elementary school’ or that ‘continuing education is the best method for updating nursing certification.’ In that case, you may want to organize by methodology used in the studies rather than by outcome.

Organize your outline in a logical order and prepare to write the first draft of your literature review.  That order might be from broad to more specific, or it may be sequential or chronological, going from foundational literature to more current.  Remember, “an effective literature review need not denote the entire historical record, but rather establish the raison d’etre for the current study and in doing so cite that literature distinctly pertinent for theoretical, methodological, or empirical reasons.” ( Milardo, 2015, p. 22 ).

As you organize the summarized documents into a logical structure, you are also appraising and synthesizing complex information from multiple sources.  Your literature review is the result of your research that synthesizes new and old information and creates new knowledge.

7.4 Additional resources:

Literature Reviews: Using a Matrix to Organize Research / Saint Mary’s University of Minnesota

Literature Review: Synthesizing Multiple Sources / Indiana University

Writing a Literature Review and Using a Synthesis Matrix / Florida International University

 Sample Literature Reviews Grid / Complied by Lindsay Roberts

Select three or four articles on a single topic of interest to you. Then enter them into an outline or table in the categories you feel are important to a research question. Try both the grid and the outline if you can to see which suits you better. The attached grid contains the fields suggested in the video .

Literature Review Table  

Test yourself.

• Select two articles from your own summary table or outline and write a paragraph explaining how and why the sources relate to each other and your review of the literature.
• In your literature review, under what topic or subtopic will you place the paragraph you just wrote?

Image attribution

Literature Reviews for Education and Nursing Graduate Students Copyright © by Linda Frederiksen is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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What is a Literature Review? How to Write It (with Examples)

A literature review is a critical analysis and synthesis of existing research on a particular topic. It provides an overview of the current state of knowledge, identifies gaps, and highlights key findings in the literature. 1 The purpose of a literature review is to situate your own research within the context of existing scholarship, demonstrating your understanding of the topic and showing how your work contributes to the ongoing conversation in the field. Learning how to write a literature review is a critical tool for successful research. Your ability to summarize and synthesize prior research pertaining to a certain topic demonstrates your grasp on the topic of study, and assists in the learning process. 

Table of Contents

• What is the purpose of literature review? 
• a. Habitat Loss and Species Extinction: 
• b. Range Shifts and Phenological Changes: 
• c. Ocean Acidification and Coral Reefs: 
• d. Adaptive Strategies and Conservation Efforts: 

How to write a good literature review 

• Choose a Topic and Define the Research Question: 
• Decide on the Scope of Your Review: 
• Select Databases for Searches: 
• Conduct Searches and Keep Track: 
• Review the Literature: 
• Organize and Write Your Literature Review: 
• How to write a literature review faster with Paperpal? 
• Frequently asked questions 

What is a literature review?

A well-conducted literature review demonstrates the researcher’s familiarity with the existing literature, establishes the context for their own research, and contributes to scholarly conversations on the topic. One of the purposes of a literature review is also to help researchers avoid duplicating previous work and ensure that their research is informed by and builds upon the existing body of knowledge.

What is the purpose of literature review?

A literature review serves several important purposes within academic and research contexts. Here are some key objectives and functions of a literature review: 2  

1. Contextualizing the Research Problem: The literature review provides a background and context for the research problem under investigation. It helps to situate the study within the existing body of knowledge. 

2. Identifying Gaps in Knowledge: By identifying gaps, contradictions, or areas requiring further research, the researcher can shape the research question and justify the significance of the study. This is crucial for ensuring that the new research contributes something novel to the field. 

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3. Understanding Theoretical and Conceptual Frameworks: Literature reviews help researchers gain an understanding of the theoretical and conceptual frameworks used in previous studies. This aids in the development of a theoretical framework for the current research. 

4. Providing Methodological Insights: Another purpose of literature reviews is that it allows researchers to learn about the methodologies employed in previous studies. This can help in choosing appropriate research methods for the current study and avoiding pitfalls that others may have encountered. 

5. Establishing Credibility: A well-conducted literature review demonstrates the researcher’s familiarity with existing scholarship, establishing their credibility and expertise in the field. It also helps in building a solid foundation for the new research. 

6. Informing Hypotheses or Research Questions: The literature review guides the formulation of hypotheses or research questions by highlighting relevant findings and areas of uncertainty in existing literature. 

Literature review example

Let’s delve deeper with a literature review example: Let’s say your literature review is about the impact of climate change on biodiversity. You might format your literature review into sections such as the effects of climate change on habitat loss and species extinction, phenological changes, and marine biodiversity. Each section would then summarize and analyze relevant studies in those areas, highlighting key findings and identifying gaps in the research. The review would conclude by emphasizing the need for further research on specific aspects of the relationship between climate change and biodiversity. The following literature review template provides a glimpse into the recommended literature review structure and content, demonstrating how research findings are organized around specific themes within a broader topic. 

Literature Review on Climate Change Impacts on Biodiversity:

Climate change is a global phenomenon with far-reaching consequences, including significant impacts on biodiversity. This literature review synthesizes key findings from various studies: 

a. Habitat Loss and Species Extinction:

Climate change-induced alterations in temperature and precipitation patterns contribute to habitat loss, affecting numerous species (Thomas et al., 2004). The review discusses how these changes increase the risk of extinction, particularly for species with specific habitat requirements. 

b. Range Shifts and Phenological Changes:

Observations of range shifts and changes in the timing of biological events (phenology) are documented in response to changing climatic conditions (Parmesan & Yohe, 2003). These shifts affect ecosystems and may lead to mismatches between species and their resources. 

c. Ocean Acidification and Coral Reefs:

The review explores the impact of climate change on marine biodiversity, emphasizing ocean acidification’s threat to coral reefs (Hoegh-Guldberg et al., 2007). Changes in pH levels negatively affect coral calcification, disrupting the delicate balance of marine ecosystems. 

d. Adaptive Strategies and Conservation Efforts:

Recognizing the urgency of the situation, the literature review discusses various adaptive strategies adopted by species and conservation efforts aimed at mitigating the impacts of climate change on biodiversity (Hannah et al., 2007). It emphasizes the importance of interdisciplinary approaches for effective conservation planning. 

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Writing a literature review involves summarizing and synthesizing existing research on a particular topic. A good literature review format should include the following elements. 

Introduction: The introduction sets the stage for your literature review, providing context and introducing the main focus of your review. 

• Opening Statement: Begin with a general statement about the broader topic and its significance in the field. 
• Scope and Purpose: Clearly define the scope of your literature review. Explain the specific research question or objective you aim to address. 
• Organizational Framework: Briefly outline the structure of your literature review, indicating how you will categorize and discuss the existing research. 
• Significance of the Study: Highlight why your literature review is important and how it contributes to the understanding of the chosen topic. 
• Thesis Statement: Conclude the introduction with a concise thesis statement that outlines the main argument or perspective you will develop in the body of the literature review. 

Body: The body of the literature review is where you provide a comprehensive analysis of existing literature, grouping studies based on themes, methodologies, or other relevant criteria. 

• Organize by Theme or Concept: Group studies that share common themes, concepts, or methodologies. Discuss each theme or concept in detail, summarizing key findings and identifying gaps or areas of disagreement. 
• Critical Analysis: Evaluate the strengths and weaknesses of each study. Discuss the methodologies used, the quality of evidence, and the overall contribution of each work to the understanding of the topic. 
• Synthesis of Findings: Synthesize the information from different studies to highlight trends, patterns, or areas of consensus in the literature. 
• Identification of Gaps: Discuss any gaps or limitations in the existing research and explain how your review contributes to filling these gaps. 
• Transition between Sections: Provide smooth transitions between different themes or concepts to maintain the flow of your literature review. 

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Conclusion: The conclusion of your literature review should summarize the main findings, highlight the contributions of the review, and suggest avenues for future research. 

• Summary of Key Findings: Recap the main findings from the literature and restate how they contribute to your research question or objective. 
• Contributions to the Field: Discuss the overall contribution of your literature review to the existing knowledge in the field. 
• Implications and Applications: Explore the practical implications of the findings and suggest how they might impact future research or practice. 
• Recommendations for Future Research: Identify areas that require further investigation and propose potential directions for future research in the field. 
• Final Thoughts: Conclude with a final reflection on the importance of your literature review and its relevance to the broader academic community. 

Conducting a literature review

Conducting a literature review is an essential step in research that involves reviewing and analyzing existing literature on a specific topic. It’s important to know how to do a literature review effectively, so here are the steps to follow: 1  

Choose a Topic and Define the Research Question:

• Select a topic that is relevant to your field of study. 
• Clearly define your research question or objective. Determine what specific aspect of the topic do you want to explore? 

Decide on the Scope of Your Review:

• Determine the timeframe for your literature review. Are you focusing on recent developments, or do you want a historical overview? 
• Consider the geographical scope. Is your review global, or are you focusing on a specific region? 
• Define the inclusion and exclusion criteria. What types of sources will you include? Are there specific types of studies or publications you will exclude? 

Select Databases for Searches:

• Identify relevant databases for your field. Examples include PubMed, IEEE Xplore, Scopus, Web of Science, and Google Scholar. 
• Consider searching in library catalogs, institutional repositories, and specialized databases related to your topic. 

Conduct Searches and Keep Track:

• Develop a systematic search strategy using keywords, Boolean operators (AND, OR, NOT), and other search techniques. 
• Record and document your search strategy for transparency and replicability. 
• Keep track of the articles, including publication details, abstracts, and links. Use citation management tools like EndNote, Zotero, or Mendeley to organize your references. 

Review the Literature:

• Evaluate the relevance and quality of each source. Consider the methodology, sample size, and results of studies. 
• Organize the literature by themes or key concepts. Identify patterns, trends, and gaps in the existing research. 
• Summarize key findings and arguments from each source. Compare and contrast different perspectives. 
• Identify areas where there is a consensus in the literature and where there are conflicting opinions. 
• Provide critical analysis and synthesis of the literature. What are the strengths and weaknesses of existing research? 

Organize and Write Your Literature Review:

• Literature review outline should be based on themes, chronological order, or methodological approaches. 
• Write a clear and coherent narrative that synthesizes the information gathered. 
• Use proper citations for each source and ensure consistency in your citation style (APA, MLA, Chicago, etc.). 
• Conclude your literature review by summarizing key findings, identifying gaps, and suggesting areas for future research. 

Whether you’re exploring a new research field or finding new angles to develop an existing topic, sifting through hundreds of papers can take more time than you have to spare. But what if you could find science-backed insights with verified citations in seconds? That’s the power of Paperpal’s new Research feature!  

How to write a literature review faster with Paperpal?

Paperpal, an AI writing assistant, integrates powerful academic search capabilities within its writing platform. With the Research feature, you get 100% factual insights, with citations backed by 250M+ verified research articles, directly within your writing interface with the option to save relevant references in your Citation Library. By eliminating the need to switch tabs to find answers to all your research questions, Paperpal saves time and helps you stay focused on your writing.   

Here’s how to use the Research feature:  

• Ask a question: Get started with a new document on paperpal.com. Click on the “Research” feature and type your question in plain English. Paperpal will scour over 250 million research articles, including conference papers and preprints, to provide you with accurate insights and citations. 
• Review and Save: Paperpal summarizes the information, while citing sources and listing relevant reads. You can quickly scan the results to identify relevant references and save these directly to your built-in citations library for later access. 
• Cite with Confidence: Paperpal makes it easy to incorporate relevant citations and references into your writing, ensuring your arguments are well-supported by credible sources. This translates to a polished, well-researched literature review. 

The literature review sample and detailed advice on writing and conducting a review will help you produce a well-structured report. But remember that a good literature review is an ongoing process, and it may be necessary to revisit and update it as your research progresses. By combining effortless research with an easy citation process, Paperpal Research streamlines the literature review process and empowers you to write faster and with more confidence. Try Paperpal Research now and see for yourself.  

Frequently asked questions

A literature review is a critical and comprehensive analysis of existing literature (published and unpublished works) on a specific topic or research question and provides a synthesis of the current state of knowledge in a particular field. A well-conducted literature review is crucial for researchers to build upon existing knowledge, avoid duplication of efforts, and contribute to the advancement of their field. It also helps researchers situate their work within a broader context and facilitates the development of a sound theoretical and conceptual framework for their studies.

Literature review is a crucial component of research writing, providing a solid background for a research paper’s investigation. The aim is to keep professionals up to date by providing an understanding of ongoing developments within a specific field, including research methods, and experimental techniques used in that field, and present that knowledge in the form of a written report. Also, the depth and breadth of the literature review emphasizes the credibility of the scholar in his or her field.  

Before writing a literature review, it’s essential to undertake several preparatory steps to ensure that your review is well-researched, organized, and focused. This includes choosing a topic of general interest to you and doing exploratory research on that topic, writing an annotated bibliography, and noting major points, especially those that relate to the position you have taken on the topic. 

Literature reviews and academic research papers are essential components of scholarly work but serve different purposes within the academic realm. 3 A literature review aims to provide a foundation for understanding the current state of research on a particular topic, identify gaps or controversies, and lay the groundwork for future research. Therefore, it draws heavily from existing academic sources, including books, journal articles, and other scholarly publications. In contrast, an academic research paper aims to present new knowledge, contribute to the academic discourse, and advance the understanding of a specific research question. Therefore, it involves a mix of existing literature (in the introduction and literature review sections) and original data or findings obtained through research methods. 

Literature reviews are essential components of academic and research papers, and various strategies can be employed to conduct them effectively. If you want to know how to write a literature review for a research paper, here are four common approaches that are often used by researchers.  Chronological Review: This strategy involves organizing the literature based on the chronological order of publication. It helps to trace the development of a topic over time, showing how ideas, theories, and research have evolved.  Thematic Review: Thematic reviews focus on identifying and analyzing themes or topics that cut across different studies. Instead of organizing the literature chronologically, it is grouped by key themes or concepts, allowing for a comprehensive exploration of various aspects of the topic.  Methodological Review: This strategy involves organizing the literature based on the research methods employed in different studies. It helps to highlight the strengths and weaknesses of various methodologies and allows the reader to evaluate the reliability and validity of the research findings.  Theoretical Review: A theoretical review examines the literature based on the theoretical frameworks used in different studies. This approach helps to identify the key theories that have been applied to the topic and assess their contributions to the understanding of the subject.  It’s important to note that these strategies are not mutually exclusive, and a literature review may combine elements of more than one approach. The choice of strategy depends on the research question, the nature of the literature available, and the goals of the review. Additionally, other strategies, such as integrative reviews or systematic reviews, may be employed depending on the specific requirements of the research.

The literature review format can vary depending on the specific publication guidelines. However, there are some common elements and structures that are often followed. Here is a general guideline for the format of a literature review:  Introduction:   Provide an overview of the topic.  Define the scope and purpose of the literature review.  State the research question or objective.  Body:   Organize the literature by themes, concepts, or chronology.  Critically analyze and evaluate each source.  Discuss the strengths and weaknesses of the studies.  Highlight any methodological limitations or biases.  Identify patterns, connections, or contradictions in the existing research.  Conclusion:   Summarize the key points discussed in the literature review.  Highlight the research gap.  Address the research question or objective stated in the introduction.  Highlight the contributions of the review and suggest directions for future research.

Both annotated bibliographies and literature reviews involve the examination of scholarly sources. While annotated bibliographies focus on individual sources with brief annotations, literature reviews provide a more in-depth, integrated, and comprehensive analysis of existing literature on a specific topic. The key differences are as follows: 

References 

• Denney, A. S., & Tewksbury, R. (2013). How to write a literature review.  Journal of criminal justice education ,  24 (2), 218-234. 
• Pan, M. L. (2016).  Preparing literature reviews: Qualitative and quantitative approaches . Taylor & Francis. 
• Cantero, C. (2019). How to write a literature review.  San José State University Writing Center . 

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Writing in the Health and Social Sciences: Literature Reviews and Synthesis Tools

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• Resources for Dissertation Authors
• Citation Management and Formatting Tools
• What are Literature Reviews?
• Conducting & Reporting Systematic Reviews
• Finding Systematic Reviews
• Tutorials & Tools for Literature Reviews

Systematic Literature Reviews: Steps & Resources

These steps for conducting a systematic literature review are listed below . 

Also see subpages for more information about:

• The different types of literature reviews, including systematic reviews and other evidence synthesis methods
• Tools & Tutorials

Literature Review & Systematic Review Steps

• Develop a Focused Question
• Scope the Literature  (Initial Search)
• Refine & Expand the Search
• Limit the Results
• Download Citations
• Abstract & Analyze
• Create Flow Diagram
• Synthesize & Report Results

1. Develop a Focused   Question 

Consider the PICO Format: Population/Problem, Intervention, Comparison, Outcome

Focus on defining the Population or Problem and Intervention (don't narrow by Comparison or Outcome just yet!)

"What are the effects of the Pilates method for patients with low back pain?"

Tools & Additional Resources:

• PICO Question Help
• Stillwell, Susan B., DNP, RN, CNE; Fineout-Overholt, Ellen, PhD, RN, FNAP, FAAN; Melnyk, Bernadette Mazurek, PhD, RN, CPNP/PMHNP, FNAP, FAAN; Williamson, Kathleen M., PhD, RN Evidence-Based Practice, Step by Step: Asking the Clinical Question, AJN The American Journal of Nursing : March 2010 - Volume 110 - Issue 3 - p 58-61 doi: 10.1097/01.NAJ.0000368959.11129.79

2. Scope the Literature

A "scoping search" investigates the breadth and/or depth of the initial question or may identify a gap in the literature. 

Eligible studies may be located by searching in:

• Background sources (books, point-of-care tools)
• Article databases
• Trial registries
• Grey literature
• Cited references
• Reference lists

When searching, if possible, translate terms to controlled vocabulary of the database. Use text word searching when necessary.

Use Boolean operators to connect search terms:

• Combine separate concepts with AND  (resulting in a narrower search)
• Connecting synonyms with OR  (resulting in an expanded search)

Search:  pilates AND ("low back pain"  OR  backache )

Video Tutorials - Translating PICO Questions into Search Queries

• Translate Your PICO Into a Search in PubMed (YouTube, Carrie Price, 5:11) 
• Translate Your PICO Into a Search in CINAHL (YouTube, Carrie Price, 4:56)

3. Refine & Expand Your Search

Expand your search strategy with synonymous search terms harvested from:

• database thesauri
• reference lists
• relevant studies

Example: 

(pilates OR exercise movement techniques) AND ("low back pain" OR backache* OR sciatica OR lumbago OR spondylosis)

As you develop a final, reproducible strategy for each database, save your strategies in a:

• a personal database account (e.g., MyNCBI for PubMed)
• Log in with your NYU credentials
• Open and "Make a Copy" to create your own tracker for your literature search strategies

4. Limit Your Results

Use database filters to limit your results based on your defined inclusion/exclusion criteria.  In addition to relying on the databases' categorical filters, you may also need to manually screen results.  

• Limit to Article type, e.g.,:  "randomized controlled trial" OR multicenter study
• Limit by publication years, age groups, language, etc.

NOTE: Many databases allow you to filter to "Full Text Only".  This filter is  not recommended . It excludes articles if their full text is not available in that particular database (CINAHL, PubMed, etc), but if the article is relevant, it is important that you are able to read its title and abstract, regardless of 'full text' status. The full text is likely to be accessible through another source (a different database, or Interlibrary Loan).  

• Filters in PubMed
• CINAHL Advanced Searching Tutorial

5. Download Citations

Selected citations and/or entire sets of search results can be downloaded from the database into a citation management tool. If you are conducting a systematic review that will require reporting according to PRISMA standards, a citation manager can help you keep track of the number of articles that came from each database, as well as the number of duplicate records.

In Zotero, you can create a Collection for the combined results set, and sub-collections for the results from each database you search.  You can then use Zotero's 'Duplicate Items" function to find and merge duplicate records.

• Citation Managers - General Guide

6. Abstract and Analyze

• Migrate citations to data collection/extraction tool
• Screen Title/Abstracts for inclusion/exclusion
• Screen and appraise full text for relevance, methods, 
• Resolve disagreements by consensus

Covidence is a web-based tool that enables you to work with a team to screen titles/abstracts and full text for inclusion in your review, as well as extract data from the included studies.

• Covidence Support
• Critical Appraisal Tools
• Data Extraction Tools

7. Create Flow Diagram

The PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) flow diagram is a visual representation of the flow of records through different phases of a systematic review.  It depicts the number of records identified, included and excluded.  It is best used in conjunction with the PRISMA checklist .

Example from: Stotz, S. A., McNealy, K., Begay, R. L., DeSanto, K., Manson, S. M., & Moore, K. R. (2021). Multi-level diabetes prevention and treatment interventions for Native people in the USA and Canada: A scoping review. Current Diabetes Reports, 2 (11), 46. https://doi.org/10.1007/s11892-021-01414-3

• PRISMA Flow Diagram Generator (ShinyApp.io, Haddaway et al. )
• PRISMA Diagram Templates  (Word and PDF)
• Make a copy of the file to fill out the template
• Image can be downloaded as PDF, PNG, JPG, or SVG
• Covidence generates a PRISMA diagram that is automatically updated as records move through the review phases

8. Synthesize & Report Results

There are a number of reporting guideline available to guide the synthesis and reporting of results in systematic literature reviews.

It is common to organize findings in a matrix, also known as a Table of Evidence (ToE).

• Reporting Guidelines for Systematic Reviews
• Download a sample template of a health sciences review matrix  (GoogleSheets)

Steps modified from: 

Cook, D. A., & West, C. P. (2012). Conducting systematic reviews in medical education: a stepwise approach.   Medical Education , 46 (10), 943–952.

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• Research Guides

Literature Review: A Self-Guided Tutorial

Using a synthesis matrix.

• Literature Reviews: A Recap
• Peer Review
• Reading the Literature
• Using Concept Maps
• Developing Research Questions
• Considering Strong Opinions
• 2. Review discipline styles
• Super Searching
• Finding the Full Text
• Citation Searching This link opens in a new window
• When to stop searching
• Citation Management
• Annotating Articles Tip
• 5. Critically analyze and evaluate
• How to Review the Literature
• 7. Write literature review

A synthesis matrix visually represents your research by organizing your sources by themes:

• Sample Synthesis Matrix Example provided by Ashford University Writing Center .
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• URL: https://libguides.williams.edu/literature-review

Literature Review Basics

• What is a Literature Review?
• Synthesizing Research
• Using Research & Synthesis Tables
• Additional Resources

Synthesis: What is it?

First, let's be perfectly clear about what synthesizing your research isn't :

• - It isn't  just summarizing the material you read
• - It isn't  generating a collection of annotations or comments (like an annotated bibliography)
• - It isn't  compiling a report on every single thing ever written in relation to your topic

When you  synthesize  your research, your job is to help your reader understand the current state of the conversation on your topic, relative to your research question.  That may include doing the following:

• - Selecting and using representative work on the topic
• - Identifying and discussing trends in published data or results
• - Identifying and explaining the impact of common features (study populations, interventions, etc.) that appear frequently in the literature
• - Explaining controversies, disputes, or central issues in the literature that are relevant to your research question
• - Identifying gaps in the literature, where more research is needed
• - Establishing the discussion to which your own research contributes and demonstrating the value of your contribution

Essentially, you're telling your reader where they are (and where you are) in the scholarly conversation about your project.

Synthesis: How do I do it?

Synthesis, step by step.

This is what you need to do  before  you write your review.

• Identify and clearly describe your research question (you may find the Formulating PICOT Questions table at  the Additional Resources tab helpful).
• Collect sources relevant to your research question.
• Organize and describe the sources you've found -- your job is to identify what  types  of sources you've collected (reviews, clinical trials, etc.), identify their  purpose  (what are they measuring, testing, or trying to discover?), determine the  level of evidence  they represent (see the Levels of Evidence table at the Additional Resources tab ), and briefly explain their  major findings . Use a Research Table to document this step.
• Study the information you've put in your Research Table and examine your collected sources, looking for  similarities  and  differences . Pay particular attention to  populations ,   methods  (especially relative to levels of evidence), and  findings .
• Analyze what you learn in (4) using a tool like a Synthesis Table. Your goal is to identify relevant themes, trends, gaps, and issues in the research.  Your literature review will collect the results of this analysis and explain them in relation to your research question.

Analysis tips

• - Sometimes, what you  don't  find in the literature is as important as what you do find -- look for questions that the existing research hasn't answered yet.
• - If any of the sources you've collected refer to or respond to each other, keep an eye on how they're related -- it may provide a clue as to whether or not study results have been successfully replicated.
• - Sorting your collected sources by level of evidence can provide valuable insight into how a particular topic has been covered, and it may help you to identify gaps worth addressing in your own work.
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What Synthesis Methodology Should I Use? A Review and Analysis of Approaches to Research Synthesis

Kara schick-makaroff.

1 Faculty of Nursing, University of Alberta, Edmonton, AB, Canada

Marjorie MacDonald

2 School of Nursing, University of Victoria, Victoria, BC, Canada

Marilyn Plummer

3 College of Nursing, Camosun College, Victoria, BC, Canada

Judy Burgess

4 Student Services, University Health Services, Victoria, BC, Canada

Wendy Neander

Associated data, additional file 1.

When we began this process, we were doctoral students and a faculty member in a research methods course. As students, we were facing a review of the literature for our dissertations. We encountered several different ways of conducting a review but were unable to locate any resources that synthesized all of the various synthesis methodologies. Our purpose is to present a comprehensive overview and assessment of the main approaches to research synthesis. We use ‘research synthesis’ as a broad overarching term to describe various approaches to combining, integrating, and synthesizing research findings.

We conducted an integrative review of the literature to explore the historical, contextual, and evolving nature of research synthesis. We searched five databases, reviewed websites of key organizations, hand-searched several journals, and examined relevant texts from the reference lists of the documents we had already obtained.

We identified four broad categories of research synthesis methodology including conventional, quantitative, qualitative, and emerging syntheses. Each of the broad categories was compared to the others on the following: key characteristics, purpose, method, product, context, underlying assumptions, unit of analysis, strengths and limitations, and when to use each approach.

Conclusions

The current state of research synthesis reflects significant advancements in emerging synthesis studies that integrate diverse data types and sources. New approaches to research synthesis provide a much broader range of review alternatives available to health and social science students and researchers.

1. Introduction

Since the turn of the century, public health emergencies have been identified worldwide, particularly related to infectious diseases. For example, the Severe Acute Respiratory Syndrome (SARS) epidemic in Canada in 2002-2003, the recent Ebola epidemic in Africa, and the ongoing HIV/AIDs pandemic are global health concerns. There have also been dramatic increases in the prevalence of chronic diseases around the world [1] – [3] . These epidemiological challenges have raised concerns about the ability of health systems worldwide to address these crises. As a result, public health systems reform has been initiated in a number of countries. In Canada, as in other countries, the role of evidence to support public health reform and improve population health has been given high priority. Yet, there continues to be a significant gap between the production of evidence through research and its application in practice [4] – [5] . One strategy to address this gap has been the development of new research synthesis methodologies to deal with the time-sensitive and wide ranging evidence needs of policy makers and practitioners in all areas of health care, including public health.

As doctoral nursing students facing a review of the literature for our dissertations, and as a faculty member teaching a research methods course, we encountered several ways of conducting a research synthesis but found no comprehensive resources that discussed, compared, and contrasted various synthesis methodologies on their purposes, processes, strengths and limitations. To complicate matters, writers use terms interchangeably or use different terms to mean the same thing, and the literature is often contradictory about various approaches. Some texts [6] , [7] – [9] did provide a preliminary understanding about how research synthesis had been taken up in nursing, but these did not meet our requirements. Thus, in this article we address the need for a comprehensive overview of research synthesis methodologies to guide public health, health care, and social science researchers and practitioners.

Research synthesis is relatively new in public health but has a long history in other fields dating back to the late 1800s. Research synthesis, a research process in its own right [10] , has become more prominent in the wake of the evidence-based movement of the 1990s. Research syntheses have found their advocates and detractors in all disciplines, with challenges to the processes of systematic review and meta-analysis, in particular, being raised by critics of evidence-based healthcare [11] – [13] .

Our purpose was to conduct an integrative review of the literature to explore the historical, contextual, and evolving nature of research synthesis [14] – [15] . We synthesize and critique the main approaches to research synthesis that are relevant for public health, health care, and social scientists. Research synthesis is the overarching term we use to describe approaches to combining, aggregating, integrating, and synthesizing primary research findings. Each synthesis methodology draws on different types of findings depending on the purpose and product of the chosen synthesis (see Additional File 1 ).

3. Method of Review

Based on our current knowledge of the literature, we identified these approaches to include in our review: systematic review, meta-analysis, qualitative meta-synthesis, meta-narrative synthesis, scoping review, rapid review, realist synthesis, concept analysis, literature review, and integrative review. Our first step was to divide the synthesis types among the research team. Each member did a preliminary search to identify key texts. The team then met to develop search terms and a framework to guide the review.

Over the period of 2008 to 2012 we extensively searched the literature, updating our search at several time points, not restricting our search by date. The dates of texts reviewed range from 1967 to 2015. We used the terms above combined with the term “method* (e.g., “realist synthesis” and “method*) in the database Health Source: Academic Edition (includes Medline and CINAHL). This search yielded very few texts on some methodologies and many on others. We realized that many documents on research synthesis had not been picked up in the search. Therefore, we also searched Google Scholar, PubMed, ERIC, and Social Science Index, as well as the websites of key organizations such as the Joanna Briggs Institute, the University of York Centre for Evidence-Based Nursing, and the Cochrane Collaboration database. We hand searched several nursing, social science, public health and health policy journals. Finally, we traced relevant documents from the references in obtained texts.

We included works that met the following inclusion criteria: (1) published in English; (2) discussed the history of research synthesis; (3) explicitly described the approach and specific methods; or (4) identified issues, challenges, strengths and limitations of the particular methodology. We excluded research reports that resulted from the use of particular synthesis methodologies unless they also included criteria 2, 3, or 4 above.

Based on our search, we identified additional types of research synthesis (e.g., meta-interpretation, best evidence synthesis, critical interpretive synthesis, meta-summary, grounded formal theory). Still, we missed some important developments in meta-analysis, for example, identified by the journal's reviewers that have now been discussed briefly in the paper. The final set of 197 texts included in our review comprised theoretical, empirical, and conceptual papers, books, editorials and commentaries, and policy documents.

In our preliminary review of key texts, the team inductively developed a framework of the important elements of each method for comparison. In the next phase, each text was read carefully, and data for these elements were extracted into a table for comparison on the points of: key characteristics, purpose, methods, and product; see Additional File 1 ). Once the data were grouped and extracted, we synthesized across categories based on the following additional points of comparison: complexity of the process, degree of systematization, consideration of context, underlying assumptions, unit of analysis, and when to use each approach. In our results, we discuss our comparison of the various synthesis approaches on the elements above. Drawing only on documents for the review, ethics approval was not required.

We identified four broad categories of research synthesis methodology: Conventional, quantitative, qualitative, and emerging syntheses. From our dataset of 197 texts, we had 14 texts on conventional synthesis, 64 on quantitative synthesis, 78 on qualitative synthesis, and 41 on emerging syntheses. Table 1 provides an overview of the four types of research synthesis, definitions, types of data used, products, and examples of the methodology.

Although we group these types of synthesis into four broad categories on the basis of similarities, each type within a category has unique characteristics, which may differ from the overall group similarities. Each could be explored in greater depth to tease out their unique characteristics, but detailed comparison is beyond the scope of this article.

Additional File 1 presents one or more selected types of synthesis that represent the broad category but is not an exhaustive presentation of all types within each category. It provides more depth for specific examples from each category of synthesis on the characteristics, purpose, methods, and products than is found in Table 1 .

4.1. Key Characteristics

4.1.1. what is it.

Here we draw on two types of categorization. First, we utilize Dixon Woods et al.'s [49] classification of research syntheses as being either integrative or interpretive . (Please note that integrative syntheses are not the same as an integrative review as defined in Additional File 1 .) Second, we use Popay's [80] enhancement and epistemological models .

The defining characteristics of integrative syntheses are that they involve summarizing the data achieved by pooling data [49] . Integrative syntheses include systematic reviews, meta-analyses, as well as scoping and rapid reviews because each of these focus on summarizing data. They also define concepts from the outset (although this may not always be true in scoping or rapid reviews) and deal with a well-specified phenomenon of interest.

Interpretive syntheses are primarily concerned with the development of concepts and theories that integrate concepts [49] . The analysis in interpretive synthesis is conceptual both in process and outcome, and “the product is not aggregations of data, but theory” [49] , [p.12]. Interpretive syntheses involve induction and interpretation, and are primarily conceptual in process and outcome. Examples include integrative reviews, some systematic reviews, all of the qualitative syntheses, meta-narrative, realist and critical interpretive syntheses. Of note, both quantitative and qualitative studies can be either integrative or interpretive

The second categorization, enhancement versus epistemological , applies to those approaches that use multiple data types and sources [80] . Popay's [80] classification reflects the ways that qualitative data are valued in relation to quantitative data.

In the enhancement model , qualitative data adds something to quantitative analysis. The enhancement model is reflected in systematic reviews and meta-analyses that use some qualitative data to enhance interpretation and explanation. It may also be reflected in some rapid reviews that draw on quantitative data but use some qualitative data.

The epistemological model assumes that quantitative and qualitative data are equal and each has something unique to contribute. All of the other review approaches, except pure quantitative or qualitative syntheses, reflect the epistemological model because they value all data types equally but see them as contributing different understandings.

4.1.2. Data type

By and large, the quantitative approaches (quantitative systematic review and meta-analysis) have typically used purely quantitative data (i.e., expressed in numeric form). More recently, both Cochrane [81] and Campbell [82] collaborations are grappling with the need to, and the process of, integrating qualitative research into a systematic review. The qualitative approaches use qualitative data (i.e., expressed in words). All of the emerging synthesis types, as well as the conventional integrative review, incorporate qualitative and quantitative study designs and data.

4.1.3. Research question

Four types of research questions direct inquiry across the different types of syntheses. The first is a well-developed research question that gives direction to the synthesis (e.g., meta-analysis, systematic review, meta-study, concept analysis, rapid review, realist synthesis). The second begins as a broad general question that evolves and becomes more refined over the course of the synthesis (e.g., meta-ethnography, scoping review, meta-narrative, critical interpretive synthesis). In the third type, the synthesis begins with a phenomenon of interest and the question emerges in the analytic process (e.g., grounded formal theory). Lastly, there is no clear question, but rather a general review purpose (e.g., integrative review). Thus, the requirement for a well-defined question cuts across at least three of the synthesis types (e.g., quantitative, qualitative, and emerging).

4.1.4. Quality appraisal

This is a contested issue within and between the four synthesis categories. There are strong proponents of quality appraisal in the quantitative traditions of systematic review and meta-analysis based on the need for strong studies that will not jeopardize validity of the overall findings. Nonetheless, there is no consensus on pre-defined criteria; many scales exist that vary dramatically in composition. This has methodological implications for the credibility of findings [83] .

Specific methodologies from the conventional, qualitative, and emerging categories support quality appraisal but do so with caveats. In conventional integrative reviews appraisal is recommended, but depends on the sampling frame used in the study [18] . In meta-study, appraisal criteria are explicit but quality criteria are used in different ways depending on the specific requirements of the inquiry [54] . Among the emerging syntheses, meta-narrative review developers support appraisal of a study based on criteria from the research tradition of the primary study [67] , [84] – [85] . Realist synthesis similarly supports the use of high quality evidence, but appraisal checklists are viewed with scepticism and evidence is judged based on relevance to the research question and whether a credible inference may be drawn [69] . Like realist, critical interpretive syntheses do not judge quality using standardized appraisal instruments. They will exclude fatally flawed studies, but there is no consensus on what ‘fatally flawed’ means [49] , [71] . Appraisal is based on relevance to the inquiry, not rigor of the study.

There is no agreement on quality appraisal among qualitative meta-ethnographers with some supporting and others refuting the need for appraisal. [60] , [62] . Opponents of quality appraisal are found among authors of qualitative (grounded formal theory and concept analysis) and emerging syntheses (scoping and rapid reviews) because quality is not deemed relevant to the intention of the synthesis; the studies being reviewed are not effectiveness studies where quality is extremely important. These qualitative synthesis are often reviews of theoretical developments where the concept itself is what is important, or reviews that provide quotations from the raw data so readers can make their own judgements about the relevance and utility of the data. For example, in formal grounded theory, the purpose of theory generation and authenticity of data used to generate the theory is not as important as the conceptual category. Inaccuracies may be corrected in other ways, such as using the constant comparative method, which facilitates development of theoretical concepts that are repeatedly found in the data [86] – [87] . For pragmatic reasons, evidence is not assessed in rapid and scoping reviews, in part to produce a timely product. The issue of quality appraisal is unresolved across the terrain of research synthesis and we consider this further in our discussion.

4.2. Purpose

All research syntheses share a common purpose -- to summarize, synthesize, or integrate research findings from diverse studies. This helps readers stay abreast of the burgeoning literature in a field. Our discussion here is at the level of the four categories of synthesis. Beginning with conventional literature syntheses, the overall purpose is to attend to mature topics for the purpose of re-conceptualization or to new topics requiring preliminary conceptualization [14] . Such syntheses may be helpful to consider contradictory evidence, map shifting trends in the study of a phenomenon, and describe the emergence of research in diverse fields [14] . The purpose here is to set the stage for a study by identifying what has been done, gaps in the literature, important research questions, or to develop a conceptual framework to guide data collection and analysis.

The purpose of quantitative systematic reviews is to combine, aggregate, or integrate empirical research to be able to generalize from a group of studies and determine the limits of generalization [27] . The focus of quantitative systematic reviews has been primarily on aggregating the results of studies evaluating the effectiveness of interventions using experimental, quasi-experimental, and more recently, observational designs. Systematic reviews can be done with or without quantitative meta-analysis but a meta-analysis always takes place within the context of a systematic review. Researchers must consider the review's purpose and the nature of their data in undertaking a quantitative synthesis; this will assist in determining the approach.

The purpose of qualitative syntheses is broadly to synthesize complex health experiences, practices, or concepts arising in healthcare environments. There may be various purposes depending on the qualitative methodology. For example, in hermeneutic studies the aim may be holistic explanation or understanding of a phenomenon [42] , which is deepened by integrating the findings from multiple studies. In grounded formal theory, the aim is to produce a conceptual framework or theory expected to be applicable beyond the original study. Although not able to generalize from qualitative research in the statistical sense [88] , qualitative researchers usually do want to say something about the applicability of their synthesis to other settings or phenomena. This notion of ‘theoretical generalization’ has been referred to as ‘transferability’ [89] – [90] and is an important criterion of rigour in qualitative research. It applies equally to the products of a qualitative synthesis in which the synthesis of multiple studies on the same phenomenon strengthens the ability to draw transferable conclusions.

The overarching purpose of emerging syntheses is challenging the more traditional types of syntheses, in part by using data from both quantitative and qualitative studies with diverse designs for analysis. Beyond this, however, each emerging synthesis methodology has a unique purpose. In meta-narrative review, the purpose is to identify different research traditions in the area, synthesize a complex and diverse body of research. Critical interpretive synthesis shares this characteristic. Although a distinctive approach, critical interpretive synthesis utilizes a modification of the analytic strategies of meta-ethnography [61] (e.g., reciprocal translational analysis, refutational synthesis, and lines of argument synthesis) but goes beyond the use of these to bring a critical perspective to bear in challenging the normative or epistemological assumptions in the primary literature [72] – [73] . The unique purpose of a realist synthesis is to amalgamate complex empirical evidence and theoretical understandings within a diverse body of literature to uncover the operative mechanisms and contexts that affect the outcomes of social interventions. In a scoping review, the intention is to find key concepts, examine the range of research in an area, and identify gaps in the literature. The purpose of a rapid review is comparable to that of a scoping review, but done quickly to meet the time-sensitive information needs of policy makers.

4.3. Method

4.3.1. degree of systematization.

There are varying degrees of systematization across the categories of research synthesis. The most systematized are quantitative systematic reviews and meta-analyses. There are clear processes in each with judgments to be made at each step, although there are no agreed upon guidelines for this. The process is inherently subjective despite attempts to develop objective and systematic processes [91] – [92] . Mullen and Ramirez [27] suggest that there is often a false sense of rigour implied by the terms ‘systematic review’ and ‘meta-analysis’ because of their clearly defined procedures.

In comparison with some types of qualitative synthesis, concept analysis is quite procedural. Qualitative meta-synthesis also has defined procedures and is systematic, yet perhaps less so than concept analysis. Qualitative meta-synthesis starts in an unsystematic way but becomes more systematic as it unfolds. Procedures and frameworks exist for some of the emerging types of synthesis [e.g., [50] , [63] , [71] , [93] ] but are not linear, have considerable flexibility, and are often messy with emergent processes [85] . Conventional literature reviews tend not to be as systematic as the other three types. In fact, the lack of systematization in conventional literature synthesis was the reason for the development of more systematic quantitative [17] , [20] and qualitative [45] – [46] , [61] approaches. Some authors in the field [18] have clarified processes for integrative reviews making them more systematic and rigorous, but most conventional syntheses remain relatively unsystematic in comparison with other types.

4.3.2. Complexity of the process

Some synthesis processes are considerably more complex than others. Methodologies with clearly defined steps are arguably less complex than the more flexible and emergent ones. We know that any study encounters challenges and it is rare that a pre-determined research protocol can be followed exactly as intended. Not even the rigorous methods associated with Cochrane [81] systematic reviews and meta-analyses are always implemented exactly as intended. Even when dealing with numbers rather than words, interpretation is always part of the process. Our collective experience suggests that new methodologies (e.g., meta-narrative synthesis and realist synthesis) that integrate different data types and methods are more complex than conventional reviews or the rapid and scoping reviews.

4.4. Product

The products of research syntheses usually take three distinct formats (see Table 1 and Additional File 1 for further details). The first representation is in tables, charts, graphical displays, diagrams and maps as seen in integrative, scoping and rapid reviews, meta-analyses, and critical interpretive syntheses. The second type of synthesis product is the use of mathematical scores. Summary statements of effectiveness are mathematically displayed in meta-analyses (as an effect size), systematic reviews, and rapid reviews (statistical significance).

The third synthesis product may be a theory or theoretical framework. A mid-range theory can be produced from formal grounded theory, meta-study, meta-ethnography, and realist synthesis. Theoretical/conceptual frameworks or conceptual maps may be created in meta-narrative and critical interpretive syntheses, and integrative reviews. Concepts for use within theories are produced in concept analysis. While these three product types span the categories of research synthesis, narrative description and summary is used to present the products resulting from all methodologies.

4.5. Consideration of context

There are diverse ways that context is considered in the four broad categories of synthesis. Context may be considered to the extent that it features within primary studies for the purpose of the review. Context may also be understood as an integral aspect of both the phenomenon under study and the synthesis methodology (e.g., realist synthesis). Quantitative systematic reviews and meta-analyses have typically been conducted on studies using experimental and quasi-experimental designs and more recently observational studies, which control for contextual features to allow for understanding of the ‘true’ effect of the intervention [94] .

More recently, systematic reviews have included covariates or mediating variables (i.e., contextual factors) to help explain variability in the results across studies [27] . Context, however, is usually handled in the narrative discussion of findings rather than in the synthesis itself. This lack of attention to context has been one criticism leveled against systematic reviews and meta-analyses, which restrict the types of research designs that are considered [e.g., [95] ].

When conventional literature reviews incorporate studies that deal with context, there is a place for considering contextual influences on the intervention or phenomenon. Reviews of quantitative experimental studies tend to be devoid of contextual considerations since the original studies are similarly devoid, but context might figure prominently in a literature review that incorporates both quantitative and qualitative studies.

Qualitative syntheses have been conducted on the contextual features of a particular phenomenon [33] . Paterson et al. [54] advise researchers to attend to how context may have influenced the findings of particular primary studies. In qualitative analysis, contextual features may form categories by which the data can be compared and contrasted to facilitate interpretation. Because qualitative research is often conducted to understand a phenomenon as a whole, context may be a focus, although this varies with the qualitative methodology. At the same time, the findings in a qualitative synthesis are abstracted from the original reports and taken to a higher level of conceptualization, thus removing them from the original context.

Meta-narrative synthesis [67] , [84] , because it draws on diverse research traditions and methodologies, may incorporate context into the analysis and findings. There is not, however, an explicit step in the process that directs the analyst to consider context. Generally, the research question guiding the synthesis is an important factor in whether context will be a focus.

More recent iterations of concept analysis [47] , [96] – [97] explicitly consider context reflecting the assumption that a concept's meaning is determined by its context. Morse [47] points out, however, that Wilson's [98] approach to concept analysis, and those based on Wilson [e.g., [45] ], identify attributes that are devoid of context, while Rodgers' [96] , [99] evolutionary method considers context (e.g., antecedents, consequences, and relationships to other concepts) in concept development.

Realist synthesis [69] considers context as integral to the study. It draws on a critical realist logic of inquiry grounded in the work of Bhaskar [100] , who argues that empirical co-occurrence of events is insufficient for inferring causation. One must identify generative mechanisms whose properties are causal and, depending on the situation, may nor may not be activated [94] . Context interacts with program/intervention elements and thus cannot be differentiated from the phenomenon [69] . This approach synthesizes evidence on generative mechanisms and analyzes contextual features that activate them; the result feeds back into the context. The focus is on what works, for whom, under what conditions, why and how [68] .

4.6. Underlying Philosophical and Theoretical Assumptions

When we began our review, we ‘assumed’ that the assumptions underlying synthesis methodologies would be a distinguishing characteristic of synthesis types, and that we could compare the various types on their assumptions, explicit or implicit. We found, however, that many authors did not explicate the underlying assumptions of their methodologies, and it was difficult to infer them. Kirkevold [101] has argued that integrative reviews need to be carried out from an explicit philosophical or theoretical perspective. We argue this should be true for all types of synthesis.

Authors of some emerging synthesis approaches have been very explicit about their assumptions and philosophical underpinnings. An implicit assumption of most emerging synthesis methodologies is that quantitative systematic reviews and meta-analyses have limited utility in some fields [e.g., in public health – [13] , [102] ] and for some kinds of review questions like those about feasibility and appropriateness versus effectiveness [103] – [104] . They also assume that ontologically and epistemologically, both kinds of data can be combined. This is a significant debate in the literature because it is about the commensurability of overarching paradigms [105] but this is beyond the scope of this review.

Realist synthesis is philosophically grounded in critical realism or, as noted above, a realist logic of inquiry [93] , [99] , [106] – [107] . Key assumptions regarding the nature of interventions that inform critical realism have been described above in the section on context. See Pawson et al. [106] for more information on critical realism, the philosophical basis of realist synthesis.

Meta-narrative synthesis is explicitly rooted in a constructivist philosophy of science [108] in which knowledge is socially constructed rather than discovered, and what we take to be ‘truth’ is a matter of perspective. Reality has a pluralistic and plastic character, and there is no pre-existing ‘real world’ independent of human construction and language [109] . See Greenhalgh et al. [67] , [85] and Greenhalgh & Wong [97] for more discussion of the constructivist basis of meta-narrative synthesis.

In the case of purely quantitative or qualitative syntheses, it may be an easier matter to uncover unstated assumptions because they are likely to be shared with those of the primary studies in the genre. For example, grounded formal theory shares the philosophical and theoretical underpinnings of grounded theory, rooted in the theoretical perspective of symbolic interactionism [110] – [111] and the philosophy of pragmatism [87] , [112] – [114] .

As with meta-narrative synthesis, meta-study developers identify constructivism as their interpretive philosophical foundation [54] , [88] . Epistemologically, constructivism focuses on how people construct and re-construct knowledge about a specific phenomenon, and has three main assumptions: (1) reality is seen as multiple, at times even incompatible with the phenomenon under consideration; (2) just as primary researchers construct interpretations from participants' data, meta-study researchers also construct understandings about the primary researchers' original findings. Thus, meta-synthesis is a construction of a construction, or a meta-construction; and (3) all constructions are shaped by the historical, social and ideological context in which they originated [54] . The key message here is that reports of any synthesis would benefit from an explicit identification of the underlying philosophical perspectives to facilitate a better understanding of the results, how they were derived, and how they are being interpreted.

4.7. Unit of Analysis

The unit of analysis for each category of review is generally distinct. For the emerging synthesis approaches, the unit of analysis is specific to the intention. In meta-narrative synthesis it is the storyline in diverse research traditions; in rapid review or scoping review, it depends on the focus but could be a concept; and in realist synthesis, it is the theories rather than programs that are the units of analysis. The elements of theory that are important in the analysis are mechanisms of action, the context, and the outcome [107] .

For qualitative synthesis, the units of analysis are generally themes, concepts or theories, although in meta-study, the units of analysis can be research findings (“meta-data-analysis”), research methods (“meta-method”) or philosophical/theoretical perspectives (“meta-theory”) [54] . In quantitative synthesis, the units of analysis range from specific statistics for systematic reviews to effect size of the intervention for meta-analysis. More recently, some systematic reviews focus on theories [115] – [116] , therefore it depends on the research question. Similarly, within conventional literature synthesis the units of analysis also depend on the research purpose, focus and question as well as on the type of research methods incorporated into the review. What is important in all research syntheses, however, is that the unit of analysis needs to be made explicit. Unfortunately, this is not always the case.

4.8. Strengths and Limitations

In this section, we discuss the overarching strengths and limitations of synthesis methodologies as a whole and then highlight strengths and weaknesses across each of our four categories of synthesis.

4.8.1. Strengths of Research Syntheses in General

With the vast proliferation of research reports and the increased ease of retrieval, research synthesis has become more accessible providing a way of looking broadly at the current state of research. The availability of syntheses helps researchers, practitioners, and policy makers keep up with the burgeoning literature in their fields without which evidence-informed policy or practice would be difficult. Syntheses explain variation and difference in the data helping us identify the relevance for our own situations; they identify gaps in the literature leading to new research questions and study designs. They help us to know when to replicate a study and when to avoid excessively duplicating research. Syntheses can inform policy and practice in a way that well-designed single studies cannot; they provide building blocks for theory that helps us to understand and explain our phenomena of interest.

4.8.2. Limitations of Research Syntheses in General

The process of selecting, combining, integrating, and synthesizing across diverse study designs and data types can be complex and potentially rife with bias, even with those methodologies that have clearly defined steps. Just because a rigorous and standardized approach has been used does not mean that implicit judgements will not influence the interpretations and choices made at different stages.

In all types of synthesis, the quantity of data can be considerable, requiring difficult decisions about scope, which may affect relevance. The quantity of available data also has implications for the size of the research team. Few reviews these days can be done independently, in particular because decisions about inclusion and exclusion may require the involvement of more than one person to ensure reliability.

For all types of synthesis, it is likely that in areas with large, amorphous, and diverse bodies of literature, even the most sophisticated search strategies will not turn up all the relevant and important texts. This may be more important in some synthesis methodologies than in others, but the omission of key documents can influence the results of all syntheses. This issue can be addressed, at least in part, by including a library scientist on the research team as required by some funding agencies. Even then, it is possible to miss key texts. In this review, for example, because none of us are trained in or conduct meta-analyses, we were not even aware that we had missed some new developments in this field such as meta-regression [117] – [118] , network meta-analysis [119] – [121] , and the use of individual patient data in meta-analyses [122] – [123] .

One limitation of systematic reviews and meta-analyses is that they rapidly go out of date. We thought this might be true for all types of synthesis, although we wondered if those that produce theory might not be somewhat more enduring. We have not answered this question but it is open for debate. For all types of synthesis, the analytic skills and the time required are considerable so it is clear that training is important before embarking on a review, and some types of review may not be appropriate for students or busy practitioners.

Finally, the quality of reporting in primary studies of all genres is variable so it is sometimes difficult to identify aspects of the study essential for the synthesis, or to determine whether the study meets quality criteria. There may be flaws in the original study, or journal page limitations may necessitate omitting important details. Reporting standards have been developed for some types of reviews (e.g., systematic review, meta-analysis, meta-narrative synthesis, realist synthesis); but there are no agreed upon standards for qualitative reviews. This is an important area for development in advancing the science of research synthesis.

4.8.3. Strengths and Limitations of the Four Synthesis Types

The conventional literature review and now the increasingly common integrative review remain important and accessible approaches for students, practitioners, and experienced researchers who want to summarize literature in an area but do not have the expertise to use one of the more complex methodologies. Carefully executed, such reviews are very useful for synthesizing literature in preparation for research grants and practice projects. They can determine the state of knowledge in an area and identify important gaps in the literature to provide a clear rationale or theoretical framework for a study [14] , [18] . There is a demand, however, for more rigour, with more attention to developing comprehensive search strategies and more systematic approaches to combining, integrating, and synthesizing the findings.

Generally, conventional reviews include diverse study designs and data types that facilitate comprehensiveness, which may be a strength on the one hand, but can also present challenges on the other. The complexity inherent in combining results from studies with diverse methodologies can result in bias and inaccuracies. The absence of clear guidelines about how to synthesize across diverse study types and data [18] has been a challenge for novice reviewers.

Quantitative systematic reviews and meta-analyses have been important in launching the field of evidence-based healthcare. They provide a systematic, orderly and auditable process for conducting a review and drawing conclusions [25] . They are arguably the most powerful approaches to understanding the effectiveness of healthcare interventions, especially when intervention studies on the same topic show very different results. When areas of research are dogged by controversy [25] or when study results go against strongly held beliefs, such approaches can reduce the uncertainty and bring strong evidence to bear on the controversy.

Despite their strengths, they also have limitations. Systematic reviews and meta-analyses do not provide a way of including complex literature comprising various types of evidence including qualitative studies, theoretical work, and epidemiological studies. Only certain types of design are considered and qualitative data are used in a limited way. This exclusion limits what can be learned in a topic area.

Meta-analyses are often not possible because of wide variability in study design, population, and interventions so they may have a narrow range of utility. New developments in meta-analysis, however, can be used to address some of these limitations. Network meta-analysis is used to explore relative efficacy of multiple interventions, even those that have never been compared in more conventional pairwise meta-analyses [121] , allowing for improved clinical decision making [120] . The limitation is that network meta-analysis has only been used in medical/clinical applications [119] and not in public health. It has not yet been widely accepted and many methodological challenges remain [120] – [121] . Meta-regression is another development that combines meta-analytic and linear regression principles to address the fact that heterogeneity of results may compromise a meta-analysis [117] – [118] . The disadvantage is that many clinicians are unfamiliar with it and may incorrectly interpret results [117] .

Some have accused meta-analysis of combining apples and oranges [124] raising questions in the field about their meaningfulness [25] , [28] . More recently, the use of individual rather than aggregate data has been useful in facilitating greater comparability among studies [122] . In fact, Tomas et al. [123] argue that meta-analysis using individual data is now the gold standard although access to the raw data from other studies may be a challenge to obtain.

The usefulness of systematic reviews in synthesizing complex health and social interventions has also been challenged [102] . It is often difficult to synthesize their findings because such studies are “epistemologically diverse and methodologically complex” [ [69] , p.21]. Rigid inclusion/exclusion criteria may allow only experimental or quasi-experimental designs into consideration resulting in lost information that may well be useful to policy makers for tailoring an intervention to the context or understanding its acceptance by recipients.

Qualitative syntheses may be the type of review most fraught with controversy and challenge, while also bringing distinct strengths to the enterprise. Although these methodologies provide a comprehensive and systematic review approach, they do not generally provide definitive statements about intervention effectiveness. They do, however, address important questions about the development of theoretical concepts, patient experiences, acceptability of interventions, and an understanding about why interventions might work.

Most qualitative syntheses aim to produce a theoretically generalizable mid-range theory that explains variation across studies. This makes them more useful than single primary studies, which may not be applicable beyond the immediate setting or population. All provide a contextual richness that enhances relevance and understanding. Another benefit of some types of qualitative synthesis (e.g., grounded formal theory) is that the concept of saturation provides a sound rationale for limiting the number of texts to be included thus making reviews potentially more manageable. This contrasts with the requirements of systematic reviews and meta-analyses that require an exhaustive search.

Qualitative researchers debate about whether the findings of ontologically and epistemological diverse qualitative studies can actually be combined or synthesized [125] because methodological diversity raises many challenges for synthesizing findings. The products of different types of qualitative syntheses range from theory and conceptual frameworks, to themes and rich descriptive narratives. Can one combine the findings from a phenomenological study with the theory produced in a grounded theory study? Many argue yes, but many also argue no.

Emerging synthesis methodologies were developed to address some limitations inherent in other types of synthesis but also have their own issues. Because each type is so unique, it is difficult to identify overarching strengths of the entire category. An important strength, however, is that these newer forms of synthesis provide a systematic and rigorous approach to synthesizing a diverse literature base in a topic area that includes a range of data types such as: both quantitative and qualitative studies, theoretical work, case studies, evaluations, epidemiological studies, trials, and policy documents. More than conventional literature reviews and systematic reviews, these approaches provide explicit guidance on analytic methods for integrating different types of data. The assumption is that all forms of data have something to contribute to knowledge and theory in a topic area. All have a defined but flexible process in recognition that the methods may need to shift as knowledge develops through the process.

Many emerging synthesis types are helpful to policy makers and practitioners because they are usually involved as team members in the process to define the research questions, and interpret and disseminate the findings. In fact, engagement of stakeholders is built into the procedures of the methods. This is true for rapid reviews, meta-narrative syntheses, and realist syntheses. It is less likely to be the case for critical interpretive syntheses.

Another strength of some approaches (realist and meta-narrative syntheses) is that quality and publication standards have been developed to guide researchers, reviewers, and funders in judging the quality of the products [108] , [126] – [127] . Training materials and online communities of practice have also been developed to guide users of realist and meta-narrative review methods [107] , [128] . A unique strength of critical interpretive synthesis is that it takes a critical perspective on the process that may help reconceptualize the data in a way not considered by the primary researchers [72] .

There are also challenges of these new approaches. The methods are new and there may be few published applications by researchers other than the developers of the methods, so new users often struggle with the application. The newness of the approaches means that there may not be mentors available to guide those unfamiliar with the methods. This is changing, however, and the number of applications in the literature is growing with publications by new users helping to develop the science of synthesis [e.g., [129] ]. However, the evolving nature of the approaches and their developmental stage present challenges for novice researchers.

4.9. When to Use Each Approach

Choosing an appropriate approach to synthesis will depend on the question you are asking, the purpose of the review, and the outcome or product you want to achieve. In Additional File 1 , we discuss each of these to provide guidance to readers on making a choice about review type. If researchers want to know whether a particular type of intervention is effective in achieving its intended outcomes, then they might choose a quantitative systemic review with or without meta-analysis, possibly buttressed with qualitative studies to provide depth and explanation of the results. Alternately, if the concern is about whether an intervention is effective with different populations under diverse conditions in varying contexts, then a realist synthesis might be the most appropriate.

If researchers' concern is to develop theory, they might consider qualitative syntheses or some of the emerging syntheses that produce theory (e.g., critical interpretive synthesis, realist review, grounded formal theory, qualitative meta-synthesis). If the aim is to track the development and evolution of concepts, theories or ideas, or to determine how an issue or question is addressed across diverse research traditions, then meta-narrative synthesis would be most appropriate.

When the purpose is to review the literature in advance of undertaking a new project, particularly by graduate students, then perhaps an integrative review would be appropriate. Such efforts contribute towards the expansion of theory, identify gaps in the research, establish the rationale for studying particular phenomena, and provide a framework for interpreting results in ways that might be useful for influencing policy and practice.

For researchers keen to bring new insights, interpretations, and critical re-conceptualizations to a body of research, then qualitative or critical interpretive syntheses will provide an inductive product that may offer new understandings or challenges to the status quo. These can inform future theory development, or provide guidance for policy and practice.

5. Discussion

What is the current state of science regarding research synthesis? Public health, health care, and social science researchers or clinicians have previously used all four categories of research synthesis, and all offer a suitable array of approaches for inquiries. New developments in systematic reviews and meta-analysis are providing ways of addressing methodological challenges [117] – [123] . There has also been significant advancement in emerging synthesis methodologies and they are quickly gaining popularity. Qualitative meta-synthesis is still evolving, particularly given how new it is within the terrain of research synthesis. In the midst of this evolution, outstanding issues persist such as grappling with: the quantity of data, quality appraisal, and integration with knowledge translation. These topics have not been thoroughly addressed and need further debate.

5.1. Quantity of Data

We raise the question of whether it is possible or desirable to find all available studies for a synthesis that has this requirement (e.g., meta-analysis, systematic review, scoping, meta-narrative synthesis [25] , [27] , [63] , [67] , [84] – [85] ). Is the synthesis of all available studies a realistic goal in light of the burgeoning literature? And how can this be sustained in the future, particularly as the emerging methodologies continue to develop and as the internet facilitates endless access? There has been surprisingly little discussion on this topic and the answers will have far-reaching implications for searching, sampling, and team formation.

Researchers and graduate students can no longer rely on their own independent literature search. They will likely need to ask librarians for assistance as they navigate multiple sources of literature and learn new search strategies. Although teams now collaborate with library scientists, syntheses are limited in that researchers must make decisions on the boundaries of the review, in turn influencing the study's significance. The size of a team may also be pragmatically determined to manage the search, extraction, and synthesis of the burgeoning data. There is no single answer to our question about the possibility or necessity of finding all available articles for a review. Multiple strategies that are situation specific are likely to be needed.

5.2. Quality Appraisal

While the issue of quality appraisal has received much attention in the synthesis literature, scholars are far from resolution. There may be no agreement about appraisal criteria in a given tradition. For example, the debate rages over the appropriateness of quality appraisal in qualitative synthesis where there are over 100 different sets of criteria and many do not overlap [49] . These differences may reflect disciplinary and methodological orientations, but diverse quality appraisal criteria may privilege particular types of research [49] . The decision to appraise is often grounded in ontological and epistemological assumptions. Nonetheless, diversity within and between categories of synthesis is likely to continue unless debate on the topic of quality appraisal continues and evolves toward consensus.

5.3. Integration with Knowledge Translation

If research syntheses are to make a difference to practice and ultimately to improve health outcomes, then we need to do a better job of knowledge translation. In the Canadian Institutes of Health Research (CIHR) definition of knowledge translation (KT), research or knowledge synthesis is an integral component [130] . Yet, with few exceptions [131] – [132] , very little of the research synthesis literature even mentions the relationship of synthesis to KT nor does it discuss strategies to facilitate the integration of synthesis findings into policy and practice. The exception is in the emerging synthesis methodologies, some of which (e.g., realist and meta-narrative syntheses, scoping reviews) explicitly involve stakeholders or knowledge users. The argument is that engaging them in this way increases the likelihood that the knowledge generated will be translated into policy and practice. We suggest that a more explicit engagement with knowledge users in all types of synthesis would benefit the uptake of the research findings.

Research synthesis neither makes research more applicable to practice nor ensures implementation. Focus must now turn seriously towards translation of synthesis findings into knowledge products that are useful for health care practitioners in multiple areas of practice and develop appropriate strategies to facilitate their use. The burgeoning field of knowledge translation has, to some extent, taken up this challenge; however, the research-practice gap continues to plague us [133] – [134] . It is a particular problem for qualitative syntheses [131] . Although such syntheses have an important place in evidence-informed practice, little effort has gone into the challenge of translating the findings into useful products to guide practice [131] .

5.4. Limitations

Our study took longer than would normally be expected for an integrative review. Each of us were primarily involved in our own dissertations or teaching/research positions, and so this study was conducted ‘off the sides of our desks.’ A limitation was that we searched the literature over the course of 4 years (from 2008–2012), necessitating multiple search updates. Further, we did not do a comprehensive search of the literature after 2012, thus the more recent synthesis literature was not systematically explored. We did, however, perform limited database searches from 2012–2015 to keep abreast of the latest methodological developments. Although we missed some new approaches to meta-analysis in our search, we did not find any new features of the synthesis methodologies covered in our review that would change the analysis or findings of this article. Lastly, we struggled with the labels used for the broad categories of research synthesis methodology because of our hesitancy to reinforce the divide between quantitative and qualitative approaches. However, it was very difficult to find alternative language that represented the types of data used in these methodologies. Despite our hesitancy in creating such an obvious divide, we were left with the challenge of trying to find a way of characterizing these broad types of syntheses.

6. Conclusion

Our findings offer methodological clarity for those wishing to learn about the broad terrain of research synthesis. We believe that our review makes transparent the issues and considerations in choosing from among the four broad categories of research synthesis. In summary, research synthesis has taken its place as a form of research in its own right. The methodological terrain has deep historical roots reaching back over the past 200 years, yet research synthesis remains relatively new to public health, health care, and social sciences in general. This is rapidly changing. New developments in systematic reviews and meta-analysis, and the emergence of new synthesis methodologies provide a vast array of options to review the literature for diverse purposes. New approaches to research synthesis and new analytic methods within existing approaches provide a much broader range of review alternatives for public health, health care, and social science students and researchers.

Acknowledgments

KSM is an assistant professor in the Faculty of Nursing at the University of Alberta. Her work on this article was largely conducted as a Postdoctoral Fellow, funded by KRESCENT (Kidney Research Scientist Core Education and National Training Program, reference #KRES110011R1) and the Faculty of Nursing at the University of Alberta.

MM's work on this study over the period of 2008-2014 was supported by a Canadian Institutes of Health Research Applied Public Health Research Chair Award (grant #92365).

We thank Rachel Spanier who provided support with reference formatting.

List of Abbreviations (in Additional File 1 )

Conflict of interest: The authors declare that they have no conflicts of interest in this article.

Authors' contributions: KSM co-designed the study, collected data, analyzed the data, drafted/revised the manuscript, and managed the project.

MP contributed to searching the literature, developing the analytic framework, and extracting data for the Additional File.

JB contributed to searching the literature, developing the analytic framework, and extracting data for the Additional File.

WN contributed to searching the literature, developing the analytic framework, and extracting data for the Additional File.

All authors read and approved the final manuscript.

Additional Files: Additional File 1 – Selected Types of Research Synthesis

This Additional File is our dataset created to organize, analyze and critique the literature that we synthesized in our integrative review. Our results were created based on analysis of this Additional File.

Chapter 5: Writing a Summary and Synthesizing

5.5 Synthesis and Literature Reviews

Literature reviews : synthesis and research.

Why do we seek to understand the ways that authors or sources “converse” with one another? So that we can synthesize various perspectives on a topic to more deeply understand it.

In academic writing, this understanding of the “conversation” may become the content of an explanatory  synthesis  paper – a paper in which you, the writer, point out various various themes or key points from a conversation on a particular topic. Notice that the example of synthesis in “What Synthesis Is” acknowledges that guns and gun control inspire passionate responses in Americans, that more than one kind of weapon is involved in gun violence, that guns in America are both legally and illegally owned, and that there are many constituencies whose experience with guns needs to considered if sound gun-control policy is to be achieved. The writer of this synthesis isn’t “pretending” to be objective (“Although gun violence is a problem in American today, people who want to increase gun control clearly don’t understand the Second Amendment”); nor is the writer arguing a point or attempting to persuade the audience to accept one perspective. The writer is making a claim about gun control that demonstrates his or her deepest understanding of the issue.

Another assignment that you may complete that also applies your synthesis skills is a l iterature review .  Literature reviews are often found in the beginning of scholarly journal articles to contextualize the author’s own research. Sometimes, literature reviews are done for their own sake; some scholarly articles are  just  Literature reviews.

Literature reviews (sometimes shortened to “lit reviews”) synthesize previous research that has been done on a particular topic, summarizing important works in the history of research on that topic. The literature review provides context for the author’s own new research. It is the basis and background out of which the author’s research grows. Context = credibility in academic writing. When writers are able to produce a literature review, they demonstrate the breadth of their knowledge about how others have already studied and discussed their topic.

• Literature reviews are most often  arranged by topic or theme , much like a traditional explanatory synthesis paper.
• If one is looking at a topic that has a long history of research and scholarship, one may conduct a chronological  literature review, one that looks at how the research topic has been studied and discussed in various time periods (i.e., what was published ten years ago, five years ago, and within the last year, for example).
• Finally, in some instances, one might seek to craft a literature review that is organized  by discipline or field. This type of literature review could offer information about how different academic fields have examined a particular topic (i.e., what is the current research being done by biologists on this topic? What is the current research being done by psychologists on this topic? What is the current research being done by [ insert academic discipline] on this topic?).

A Literature Review offers  only  a report on what others have already written about. The Literature Review does not reflect the author’s own argument or contributions to the field of research. Instead, it indicates that the author has read others’ important contributions and understands what has come before him or her. Sometimes, literature reviews are stand alone assignments or publications. Sometimes, they fit into a larger essay or article (especially in many of the scholarly articles that you will read throughout college. For more information on how literature reviews are a part of scholarly articles, see chapter 10.5 )

A Guide to Rhetoric, Genre, and Success in First-Year Writing by Melanie Gagich & Emilie Zickel is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Literature Reviews

• Introduction: What, Who & Why
• Define a topic
• Identify keywords
• More search tips
• Understand & Analyse

How to synthesise

Synthesis, a written example, synthesising tools.

• More resources
• Accessing help
• Systematic Style Reviews Guide

Synthesis & theme

Synthesising the content of your analysis means you need to explain and provide an original interpretation of what you've read by highlighting relationships (or lack thereof), between your sources.

Organise and categorise your content into themes or patterns. Examples of themes include:

• Chronological
• Geographical
• Theory, issue or question
• Importance (most to least); or
• Topical (general to specific).
• Synthesis Matrix
• 5 ways to tame the literature dragon
• Using a matrix to organise your notes

How not to write.

Smith (1970) reported that bilbies come out at night and eat chocolates. Jones (1972) described the variety of beetles eaten by bilbies on their daytime trips. Wheeler (1974) reported that bilbies eat only apples.

How to write.

The elusive bilby has provoked considerable disagreement over such essential facts as whether it is diurnal or nocturnal, and what constitutes its staple diet. Smith (1970) considered them to be nocturnal whereas Jones (1972) reported that they are daytime foragers. A similar disagreement about food preference can be observed in Smith (1970) who  reported bilbies had a fondness for chocolate, and in Jones (1974) who believed bilbies eat beetles and Wheeler (1974) who maintained that apples were the staple food. However, neither chocolate nor apples are indigenous to the bilby habitat, and it seems improbable that they are the main foodstuffs for bilbies.

Grouping papers by theme

Use this matrix to group papers according to themes you have identified in your topic.

• Literature review matrix by theme

Answering a specific question

Use this matrix to group papers according to the questions you asked when analysing your sources.

• Literature review matrix by question

 Remember, it is common to use more than one method to record your notes.

Evaluating or scoring resources as you go can be helpful, you may like to add a column to your matrix for recording some type of coding system such as a + or -  or numerical value.

Spreadsheets: Creating Matrixes using spreadsheets can be useful if you have a lot of resources and you need to sort the information you have collected.

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• Next: Write >>
• Last Updated: May 13, 2024 5:20 PM
• URL: https://libguides.jcu.edu.au/litreview

Conduct a literature review

• Introduction
• Identify the right articles
• Uncover related articles
• Read, summarize & synthesize
• Put it all together

Questions? Ask Me!

Read and summarize

Now it's time to read and evaluate each of the articles you found in your research. 

If you find an article isn't relevant or helpful, you can exclude it from your review. However, make sure that you don't exclude an article just because its findings don't match with other articles. Remember that part of a literature review is to point out inconsistencies! 

Take notes as you read! 

Make note of common themes you encounter in your readings. You might even want to create an annotated bibliography. You won't turn this in, but it will serve as a useful summary for you when you go to write the actual review. 

A useful tool for writing literature reviews is to create a  synthesis matrix . Each row represents a common theme or point; each column represents an article you read. 

The table below is a sample matrix created by Becky Menendez .

Things to note:

• The authors' names are listed across the top row
• The subtopics are listed across the left-most column
• Include some quotations, but for the most part, paraphrase or summarize
• Include page numbers for direct quotations in case you need to refer back to them
• << Previous: Uncover related articles
• Next: Put it all together >>
• Last Updated: Dec 15, 2023 9:48 AM
• URL: https://libraryguides.oswego.edu/how-to/conduct-lit-review

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Using Evidence: Synthesis

Synthesis video playlist.

Note that these videos were created while APA 6 was the style guide edition in use. There may be some examples of writing that have not been updated to APA 7 guidelines.

Basics of Synthesis

As you incorporate published writing into your own writing, you should aim for synthesis of the material.

Synthesizing requires critical reading and thinking in order to compare different material, highlighting similarities, differences, and connections. When writers synthesize successfully, they present new ideas based on interpretations of other evidence or arguments. You can also think of synthesis as an extension of—or a more complicated form of—analysis. One main difference is that synthesis involves multiple sources, while analysis often focuses on one source.

Conceptually, it can be helpful to think about synthesis existing at both the local (or paragraph) level and the global (or paper) level.

Local Synthesis

Local synthesis occurs at the paragraph level when writers connect individual pieces of evidence from multiple sources to support a paragraph’s main idea and advance a paper’s thesis statement. A common example in academic writing is a scholarly paragraph that includes a main idea, evidence from multiple sources, and analysis of those multiple sources together.

Global Synthesis

Global synthesis occurs at the paper (or, sometimes, section) level when writers connect ideas across paragraphs or sections to create a new narrative whole. A literature review , which can either stand alone or be a section/chapter within a capstone, is a common example of a place where global synthesis is necessary. However, in almost all academic writing, global synthesis is created by and sometimes referred to as good cohesion and flow.

Synthesis in Literature Reviews

While any types of scholarly writing can include synthesis, it is most often discussed in the context of literature reviews. Visit our literature review pages for more information about synthesis in literature reviews.

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Didn't find what you need? Email us at [email protected] .

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• Published: 29 May 2024

Recent advances in structurally elaborate triptycenes, triptycene-containing polymers and assemblies: structures, functions and applications

• Fumitaka Ishiwari   ORCID: orcid.org/0000-0002-0200-4510 1 , 2   nAff4 ,
• Yoshiaki Shoji 1 , 2 , 3 ,
• Colin J. Martin 1 &
• Takanori Fukushima   ORCID: orcid.org/0000-0001-5586-9238 1 , 2 , 3  

Polymer Journal ( 2024 ) Cite this article

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• Polymer synthesis
• Self-assembly

Triptycene, a rigid propeller-shaped molecule, was first synthesized in the early 1940s. More recently, many triptycene-containing polymers and molecular assemblies have been developed for a wide range of applications, including guest recognition, material transport, separation, catalysis, and as device components. The advantages of triptycenes lie in their ability to introduce a variety of functional groups on their three-dimensional backbone, with changes in substitution patterns as well as the type of substituents present having a significant impact on the material properties. In this review, we describe the synthesis of triptycene derivatives and polymers, detailing selected examples of triptycene-containing functional polymers. We also focus on the construction of triptycene-based two-dimensional assemblies and polymers, where space-filling designs based on rigid propeller-shaped skeletons are essential. Through a thorough literature survey, future directions and possibilities for the development of triptycene-containing functional materials are discussed.

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Introduction.

Triptycene is a rigid molecule with a three-dimensional (3D) skeleton consisting of three phenylene rings arranged with interblade angles of 120° (Fig.  1a ) [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. Triptycene derivatives have been utilized in many fields, including supramolecular chemistry and polymer and materials science, and researchers have taken advantage of the structural features of triptycenes in the development of functional molecules, polymers, and assemblies. For example, a number of triptycene-containing polymers (trip-polymers) have been reported [ 2 , 10 ], many of which exhibit microporosity, can serve as low dielectric materials, and show specific assembly properties arising from the large free volumes present around the rigid triptycene skeleton. Importantly, the properties of trip-polymers largely depend on their substitution patterns, the symmetry of the constituent triptycene units, and the type of substituents present. To date, synthetic methods for accessing triptycene derivatives have been intensely developed, allowing the introduction of various substituents at different positions (Fig.  1b ). These structurally elaborate triptycene derivatives are useful for the design of various functional polymers (Fig.  1c–f ). Our group has also been engaged in the development of trip-polymers that exhibit unique assembly properties, for which triptycene units with particular substitution patterns [ 11 ], different from the majority of previously reported trip-polymers, are essential ( vide infra ).

a Molecular structure of triptycene with carbon numbering. b Synthesis and ( c – f ) schematic drawings of trip-polymers with ( c ) network, ( d ) linear, ( e ) linear ladder, and ( f ) hyperbranched structures

In this review, based on a thorough literature survey, we describe the synthesis and structures of triptycene derivatives and trip-polymers with various substitution patterns while summarizing recent research trends. To the best of our ability, we attempted to avoid including examples of trip-polymers that have already been described in the book and review by C-F. Chen et al. [ 1 , 8 ]. We also describe the formation of two-dimensional (2D) assemblies and related polymers using particular types of triptycenes. Triptycene derivatives, when appropriately functionalized, self-assemble into either porous or dense (nonporous) 2D structures through intermolecular π-stacking and/or nested packing of their propeller-blade moieties. These triptycene-based 2D assemblies are useful for the construction of polymeric materials featuring long-range structural order. We also describe various applications of triptycene-based 2D assemblies, including the development of polymer materials with enhanced properties for high-performance organic electronic devices.

Literature survey

Our manual survey of academic papers (other than patents) using Google Scholar ® in March 2024 with the keywords “triptycene + polymer” and “iptycene + polymer” revealed that 364 research publications reported a total of 923 triptycene- and iptycene-containing polymers from 1968 to the present. Note that (i) when polymers with the same structure were reported in different publications, we included them multiple times in the count, (ii) copolymers were counted separately, and (iii) metal-organic frameworks were excluded. Figure  2 shows histograms of the numbers of published papers (Fig.  2a ) and trip-polymers (Fig.  2b ) reported in these papers, where the colors in Fig.  2b corresponds to the substitution patterns of the constituent triptycene units. Figure  2c shows the percentage of polymers with each substitution pattern of triptycene relative to all trip-polymers; linear polymers linked at the 1,4-positions (338 examples) are the most common, followed by network polymers linked at the 2,7,14(15)-positions (189 examples), linear ladder polymers linked at the 2,3,6,7-positions (118 examples), linear polymers linked at the 2,6(7)-positions (92 examples) and network polymers linked at the 2,3,6,7,14,15-positions, including 2D/3D covalent organic frameworks (COFs) (107 examples) [ 12 ]. These polymers account for ca . 90% of the total number of trip-polymers. Both the reported number of papers and the number of trip-polymers rapidly increased around 2010. In the following, we describe the research trends related to triptycene-based polymers and assemblies in chronological order.

a Histogram of the number of annual publications on trip-polymers. b Histogram and distribution of the number and substitution patterns of trip-polymers reported per year. c Summary of the distribution of substitution patterns in reported trip-polymers

Early-stage examples including engineering polymers, fluorescent sensors, and low-dielectric materials

The first trip-polymer was reported in 1968 [ 13 ]. In that period, colorless, heat-resistant engineering plastics were the focus of attention, and structurally rigid triptycene derivatives with a small π-conjugated system were predicted to be potent building blocks in this area [ 14 ]. In 1998, Swager et al. reported a seminal paper describing the detection of explosives (TNT; trinitrotoluene) using π-conjugated polymers containing pentiptycene in the main chain [ 15 ]. Since then, a series of related papers have been published. Swager et al. also developed low- k polymers that serve as insulating interlayer materials for high-density electronic devices [ 16 ]. Trip-polymers with large free volumes around their polymer backbones tend to have low relative permittivity.

PIMs, COFs and 2D polymers (2000s~)

In 2004, McKeown et al. reported ladder polymers with rigid and contorted main chains that yielded microporous freestanding membranes featuring very high specific surface areas [ 17 ] and named them “polymers of intrinsic microporosity (PIMs)” [ 9 , 18 , 19 ]. Since then, PIMs have been studied extensively as membranes for gas separation and storage. Structurally rigid triptycenes, to which a variety of substituents can be introduced, are excellent building blocks for PIMs. Yaghi et al. and others introduced the concept of covalent organic frameworks (COFs) using similar porous solid materials in 2005 [ 12 , 20 ]. Propeller-shaped triptycenes also serve as useful building blocks for 2D and 3D COFs, and an increasing number of triptycene-based COFs have been reported since 2013. The synthesis of exfoliable triptycene-based 2D polymers with single-crystal-like structural order has been achieved using triptycene derivatives consisting of extended propeller blades [ 21 , 22 ].

Ion-conducting membranes (2010s~)

In 2011, a trip-polymer that serves as an excellent proton exchange membrane for fuel cells was first reported [ 23 ]. Then, triptycene-based anion exchange membranes for use as alkaline fuel cells were reported in 2017 [ 24 ]. While the initial focus in the development of these ion-exchange membranes was on introducing dense ionic functional groups on the triptycene moieties, later works shifted toward designs that take advantage of the microporous nature of trip-polymers.

Self-assembled materials (2010s~)

The abovementioned examples of trip-polymers all exhibit porosity and utilize micro- and nanopores, e.g., material transport and separation. In 2015, our research group reported that triptycene derivatives with alkoxy groups at the 1,8,13-positions self-assemble to fill the free volume between neighboring triptycene molecules, forming a 2D nested hexagonal packing arrangement, which has been applied in the development of polymers with new functionalities [ 11 ].

Chiral polymers (2010s~)

Triptycenes substituted at two or more positions, such as 2,6- or 1,5-substituted derivatives, are inherently chiral [ 7 ]. However, until recently, such chiral triptycenes have not been actively explored. Recently, chiral polymers utilizing this triptycene chirality have been reported, and their functions have attracted increased amounts of attention.

In the following sections, we will discuss general synthetic methods for trip-polymers, and their structures and applications. This review builds upon the examples found in the 2013 book of C-F. Chen et al., in which they describe a chapter on trip-polymers [ 1 ]. As such, we will focus on results published after this date.

Synthetic routes for accessing triptycene derivatives and triptycene-containing polymers

As shown in Fig.  1b , there are two major ways to synthesize trip-polymers: direct polymerization of unsubstituted triptycene or polymerization using a triptycene monomer (trip-monomer) substituted with a reactive functional group. Unsubstituted triptycene ( 1 ) easily reacts at its 2,7,14(,15)-positions under Friedel-Crafts-type conditions. Although monomer ( 1 ) can be easily prepared, this polymerization method does not yield network polymers containing triptycenes linked at the 2,7,14(,15)-positions. Trip-polymers linked at other substitution positions require the synthesis of trip-monomers, in which reactive substituents are introduced at the desired positions in advance. One method is to introduce substituents at the 2(,3)(,6),7,14(,15)-positions by Friedel–Crafts-type reactions with 1 . The main trip-monomers synthesized starting with this method are shown in Fig.  3 . The Friedel–Crafts conditions used include nitration with nitric acid [ 25 , 26 ], acylation using Lewis acid catalysts such as aluminum chloride or tin chloride, and formylation [ 27 ]. For nitration, 2-monosubstituted ( 2 ), 2,6- ( 3 ) or 2,7-disubstituted ( 4 ) and 2,6,14- ( 5 ) or 2,7,14-trisubstituted ( 6 ) nitro triptycene derivatives have been reported. The ratios of formation can be controlled somewhat selectively by varying the amounts of reagents used, and importantly, all isomers can be separated using silica gel column chromatography (except for optical isomers). This has allowed for the synthesis of trip-monomers with the desired number and patterns of nitro substituents. Amino-substituted triptycenes ( 7 – 11 ), obtained by reducing nitro moieties, are frequently used as monomers in the synthesis of polyamides, polyimides, and Tröger’s base-containing PIMs [ 9 , 19 ]. Sandmeyer-type reactions via diazotization of the amino forms have led to the insertion of halo substituents, yielding bromo ( 12 – 16 ) and iodo ( 17 – 21 ) triptycenes [ 26 , 28 ] along with an azide-substituted trip-monomer ( 22 ) [ 29 , 30 ], which have been used in various transition-metal-catalyzed couplings and click polymerizations. In addition, 2,6-diaminotriptycene ( 8 ) [ 31 ] and its derivatives [ 32 ] are relatively easy to separate optically using chiral HPLC, and chiral polymers with interesting optically active functions have been obtained, which will be described in detail later. Although most of the trip-monomers shown here are chiral, many optically active monomers and polymers are expected to be developed in the future.

Synthesis of trip-monomers from unsubstituted triptycene 1

Baeyer–Villiger oxidation of acylated trip-monomers ( 23 – 32 ) with m CPBA and subsequent hydrolysis have led to hydroxy trip-monomers for use in polyester and polyether synthesis ( 33 – 37 ) [ 27 , 33 ]. These compounds have also been synthesized by other methods [ 34 ] and will be described later. Nitration and reduction of 2,6(,7)-dihydroxytriptycene ( 34,35 ) yields a trip-monomer with two hydroxy and two amino groups ( 38 ) [ 33 ]. This type of monomer is important in the preparation of polybenzimidazoles [ 35 ] through thermal rearrangement to the corresponding hydroxy group-containing polyimides, which have been shown to act as excellent gas-separation membranes [ 36 ]. Another monomer related to gas separation membranes, the di- ortho -brominated 2,6(,7)-diaminomonomer ( 39 ), which is useful in the synthesis of polyimides with reduced free rotation, has recently been reported [ 37 , 38 ]. A route to synthesize triaminotriptycenes via Beckmann-rearrangement ( 40 ) from acetylated triptycenes ( 23 – 32 ) is also known [ 27 ].

Amino- and/or bromo-substituted monomers can be synthesized by reacting with unsubstituted triptycene 1 to yield, for example, 2,3,6,7,14,15-hexasubstituted trip-monomers. For the di-amination ( 42 ), tetra-amination ( 43 ) and hexa-amination ( 44 ) of triptycenes, a multistep synthesis via protection, nitration ( 41 ), deprotection, and reduction of triaminotriptycene ( 10, 11 ) was originally reported [ 39 ]. Later, Mastalerz et al. reported that amino-substituted derivatives can be synthesized as the corresponding hydrochloride salts in two steps from 1 using fuming nitric acid [ 40 ]; however, the yield from 1 is only 18%. On the other hand, for the synthesis of hexabromotriptycene ( 45 ), bromination of 1 using Br 2 in the presence of iron proceeded in relatively high yield [ 41 ]. Very recently, the addition of I 2 was shown to increase the yield to 90% [ 42 ]. It has been reported that hexaaminotriptycene ( 44 ) can be synthesized efficiently by palladium-catalyzed Buchwald–Hartwig amination of hexabromide with benzophenone imine, followed by deprotection with hydrochloric acid [ 43 ]. This hexa-aminated trip-monomer is widely used as a building block for COFs and network polymers. Sonogashira couplings with di-, tri-, and hexa-brominated monomers have led to ethynyl-substituted trip-monomers ( 46 – 48 ) that can be used in further Sonogashira couplings and click polymerizations. 2,3,6,7,14,15-Hexaethynyltriptycene ( 48 ) can be used for Masamune–Bergman polymerizations, resulting in interesting porous network polymers [ 44 , 45 ], which will be discussed in detail later. Hexa-hydroxylated trip-monomers ( 50 ) can also be synthesized by treating the hexa-brominated trip-monomers with sodium methoxide to introduce methoxy groups ( 49 ) and then removing the methyl group using BBr 3 [ 46 ]. This approach has been used in the preparation of COFs and PIMs. In addition to the synthetic methods described thus far, it is also possible to prepare amino substituents from acyl groups by Beckmann rearrangement and hydrolysis [ 27 ] and hydroxyl substituents from amino groups by Sandmeyer-type reactions [ 25 ], but as of yet, no polymers have been obtained using monomers in these ways. Moreover, direct borylation of 1 using an iridium catalyst and bis(pinacolato)diboron has been reported to yield 2,6,14(15)-tris[(pinacolato)boryl]triptycene as a mixture of regioisomers [ 47 ]. The propeller blades of these triboryltriptycenes can be extended using Suzuki coupling and dehydrogenative cyclization. Thus, triboryltriptycenes could be useful as building blocks for network polymers.

Another method to synthesize trip-monomers is to introduce functional groups into anthracenes, benzynes, or quinones, which are common precursors in the synthesis of triptycenes. In addition to simple modification reactions, anthracene derivatives with various functional groups can be derived from commercially available anthraquinone. Anthracenes can also be synthesized using aromatic electrophilic substitution reactions between relatively electron-rich benzenes and reagents such as aldehydes or dichloromethane. A number of commercially available substituted benzyne precursors, including anthranilic acids and derivatives with triflate and trimethylsilyl groups, have been used.

The synthesis of the most common 1,4-substituted triptycene derivatives is shown in Fig.  4 . para -Benzoquinone ( 51 ) has been used as a dienophile with anthracenes ( 52, 53 ) in Diels-Alder reactions, followed by aromatization to the 1,4-dihydroxy trip-monomer ( 54, 55 ) [ 16 , 17 ]. From this, a perfluorosulfonated intermediate ( 56 ) can be formed and converted into a 1,4-diethynyl trip-monomer ( 57 ) by Sonogashira coupling [ 48 ]. From the 1,4-dihydroxy trip-monomer ( 54 ), a para -benzoquinone derivative ( 59 ) can be obtained by oxidation, and from the imine derivative ( 60 ), it can be prepared through condensation with hydroxylamine, which can be subsequently reduced to yield the 1,4-diamino trip-monomer ( 61 ) [ 49 , 50 ]. Compound 61 can undergo a Sandmeyer-type reaction to yield the 1,4-diiodo trip-monomer ( 62 ) [ 48 ], which has been used in a further Sonogashira reaction to synthesize the 1,4-diethynyl trip-monomer ( 57 ) [ 51 , 52 ]. The para -benzoquinone form of pentiptycene ( 64 ) can be synthesized using an excessive amount of anthracene in a Diels–Alder reaction with para -benzoquinone or by allowing the para -benzoquinone form of triptycene to undergo a second Diels–Alder reaction with anthracene in acetic acid [ 16 , 48 ]. A reduction of 64 yields the 1,4-dihydroxypentiptycene monomer ( 63 ) [ 52 ], whereas a nucleophilic attack of 64 with a TMS-acetylide ( 65 ) and subsequent oxidation ( 66 ) and deprotection yields the 1,4-diethynylpentiptycene monomer ( 58 ) [ 16 , 48 ]. However, it has been noted that the equivalent 1,4-diaminopentiptycene cannot be synthesized by a similar route [ 4 , 48 , 53 ].

Synthesis of 1,4-substituted trip-monomers

It has been reported that functionalized triptycenes and pentiptycenes can be obtained by reacting substituted anthracenes with benzoquinone 51 . The reaction of 1,8-dimethoxyanthracene ( 53 ) with 51 yields 1,4-dihydroxytriptycene ( 55 ), which has two hydroxy substituents protected by methyl groups [ 54 ]. In 2021, a highly functionalized pentiptycene monomer was synthesized by Guo et al. [ 55 ]. They synthesized 2,3-dimethoxyanthracene ( 71 ) from phthalic anhydride ( 67 ) and dimethoxybenzene ( 68 ) by stepwise Friedel–Crafts reactions ( 69 ) and subsequent reduction/aromatization ( 70, 71 ). The reaction of 71 with para -benzoquinone 51 gave a pentiptycene ( 72 ) bearing upper and lower methoxy groups as a mixture of syn- and anti -isomers. These isomers can be separated by means of silica gel column chromatography. The central para -benzoquinone moiety of 72 can be reduced ( 73 ) and etherified ( 74 ). The use of ceric ammonium nitrate (CAN) for the reaction with 74 resulted in the selective oxidation of the dimethoxybenzene moieties to yield the ortho -quinone pentiptycene ( 75 ). This was reduced via a hydrogenation reaction to yield a tetrahydroxy derivative ( 76) [ 55 ], which has been used in the preparation of PIMs [ 9 ].

Figure  5 shows the synthetic pathway for accessing triptycenes with hydroxyl, carboxyl, carboxylic acid chloride, isocyanate, and amino groups as polymerizable substituents at the 9,10-positions. The 9,10-positions of unsubstituted anthracene ( 77 ) were chloromethylated ( 78 ) and converted to acetoxy groups via nucleophilic substitution ( 79 ), followed by a Diels–Alder reaction with benzyne generated from anthranilic acid ( 80 ) and amyl nitrite to yield 9,10-diacetoxymethyltriptycene ( 81 ). The acetyl group can then be deprotected to the 9,10-dihydroxymethyl trip-monomer ( 82 ) [ 14 , 15 ]. It is also possible to synthesize a 9,10-dihydroxyethyl trip-monomer ( 86 ) through the introduction of a cyano group ( 83 ) via nucleophilic substitution of the chloromethyl group of 78 and subsequent functional group transformation followed by a Diels–Alder reaction ( 84, 85 ) [ 15 ]. Oxidation of the 9,10-dihydroxymethyl trip-monomer ( 82 ) with chromium trioxide yields the 9,10-dicarboxyl trip-monomer ( 87 ), and further treatment with thionyl chloride yields the 9,10-dichlorocarbonyl trip-monomer ( 88 ). This synthetic method, reported in 1968 and 1969 [ 13 , 14 ], is still used today. For example, compound 87 has been utilized for the synthesis of MOFs [ 56 ]. 9,10-Diamino Trip-monomer ( 91 ) [ 14 , 15 ] can be synthesized from 88 through a reaction with sodium azide, a thermal Curtius rearrangement of the resulting benzoyl azide ( 89 ), and the subsequent hydrolysis of a diisocyanate derivative ( 90 ). 9,10-Functionalized triptycene derivatives such as 87 and 91 have been used as building blocks for polyesters and polyamides. Moreover, an interesting synthetic method has been developed for 9,10-dihydroxylated derivatives ( 94, 95 ). For example, anthracene substituted with hydroxyl and ethynyl groups at the 9- and 10-positions ( 92 ) reacts with norbornadienes or terminal alkynes in the presence of a rhodium catalyst in a [2+2+2] cycloaddition to form 9,10-dihydroxytriptycene 94 [ 57 , 58 ]. Cycloaddition with terminal alkynes or norbornadienes affords derivatives with or without a substituent at the 14-position, respectively. This synthetic protocol can be applied to the synthesis of a highly substituted, 1,4-dibrominated derivative ( 95 ) from a corresponding precursor ( 93 ) [ 57 ]. Sonogashira coupling polymerization using 95 yields polyphenylene ethynylenes, which can be transformed to polyphenylene vinylenes through intramolecular hydrooxygenation of alkyne moieties.

Synthesis of 9,10-functionalized trip-monomers

Figure  6 summarizes the trip-monomers that can be synthesized from functionalized anthraquinones. The chloro groups of 1,5-dichloroanthraquinone ( 96 ) can be converted to cyano groups ( 97 ) using copper cyanide and then to carboxylic acids ( 98 ) by hydrolysis [ 59 ]. The anthraquinone skeleton is reduced to anthracene by treatment with zinc under basic conditions ( 99 ), which is followed by methylation of the carboxylic acid moieties to yield the methyl ester derivative ( 100 ). Subsequently, 100 reacts with benzyne generated from anthranilic acid ( 80 ) and isoamyl nitrite to yield a triptycene with ester groups at the 1,5-positions ( 101 ). Hydrolysis of 101 yields the 1,5-dicarboxytriptycene monomer ( 102 ). Here, the 1,5-diaminotriptycene monomer ( 106 ) can be synthesized through acid chloride ( 103 ), benzoyl azide ( 104 ), and isocyanate ( 105 ) intermediates, similar to the case of 91 in Fig.  5 [ 59 ]. 1,5-Dicarboxytriptycene ( 102 ) is chiral, and its enantiomeric separation has been achieved by the formation of diastereomeric salts with the naturally occurring optically active alkaloids synconidine or synconine. Thus, ( R,R )- and ( S,S )-1,5-dicarboxytriptycene have been obtained using synconidine and synconine, respectively [ 59 ]. The use of these 1,5-dicarboxytriptycenes results in the formation of the corresponding optically active 1,5-diamino trip-monomers ( 106 ), which can be further transformed into optically active 1,5-dihydroxy trip-monomers ( 107 ) by reaction with water via a diazonium salt [ 59 ].

Synthesis of trip-monomers from functionalized anthraquinones

The synthesis of 1,5-dihydroxy trip-monomer ( 107 ) has also been reported using the scheme shown in the lower part of Fig.  6 [ 60 ]. A sodium salt of anthracene-1,5-disulfonic acid ( 109 ), synthesized from the corresponding anthraquinone ( 108 ), is subjected to alkali fusion, affording 1,5-dihydoxytriptycene ( 110 ). After reacting with acetic anhydride, the resulting 1,5-diacetoxyanthracene ( 111 ) reacts with benzyne, followed by hydrolysis to yield the 1,5-dihydroxy trip-monomer ( 107 ) [ 60 ]. For 2,6-dihydroxytriptycene ( 34 ) [ 34 ], 2,6-dihydroxyanthraquinone ( 113 ) has been used as a starting material. Compound 113 is converted to 2,6-dimethoxyanthracene ( 116 ) via methylation and reduction (in no particular order) [ 61 , 62 ], and then 116 reacts with benzyne followed by demethylation using boron tribromide to yield 2,6-dihydroxytriptycene 34 . Therefore, 1,5-dihydroxyanthraquinone ( 117 ) may provide an alternative synthetic route to 1,5-dihydroxytriptycene 107 , which does not involve alkali fusion. It has been reported that 1,5-dimethoxyanthracene ( 119 ) derived from 117 , upon reaction with para -benzoquinone ( 51 ), yields 1,5-dimethoxytriptycene monobenzoquinone ( 120 ) [ 63 ]. para -Benzoquinone is useful in the synthesis of benzoquione derivatives of triptycene [ 64 ]. 2,6-Diaminoanthracene ( 122 ), obtained by the reduction of 2,6-diaminoanthraquinone ( 121 ) [ 65 ], has been reacted with 1,4-epoxy-1,4-dihydronaphthalene ( 123 ) under Diels–Alder conditions to form 124 , followed by dehydration using perchloric acid to yield 2,6-diaminobenzotriptycene ( 125 ) [ 66 ]. The Diels–Alder reaction using 123 , which can be carried out even in the presence of amino functionalities, is useful for the synthesis of benzotriptycene derivatives. Compound 125 has been used for the synthesis of microporous polyimides and PIMs consisting of Tröger’s base moieties [ 66 ].

Figure  7 summarizes the synthesis of highly functionalized trip-monomers using multisubstituted anthracene derivatives. These triptycene derivatives have attracted much attention in recent years as monomers for use in the preparation of ladder polymer-based PIMs and microporous polyimides. The Friedel–Crafts reaction of 1,2-dimethoxybenzene ( 68 ) with various aldehydes ( 126 ) in the presence of sulfuric acid yields the corresponding hexa-substituted 9,10-dialkyl-2,3,6,7-tetramethoxyanthracenes ( 127 ). By subjecting these anthracenes to a Diels–Alder reaction with benzyne generated from unsubstituted or dimethoxy anthranilic acid ( 80 or 128 ), triptycene derivatives methoxy-substituted at the 2,3,6,7- ( 129 ) or 2,3,6,7,14,15-positions ( 130 ) are obtained [ 46 , 67 , 68 , 69 ]. Deprotection of these methoxy groups using boron tribromide yields the corresponding triptycene 2,3,6,7-tetrahydroxy ( 131 ) or 2,3,6,7,14,15-hexahydroxy ( 132 ) derivatives [ 46 , 67 , 68 , 69 , 70 ]. Related hexahydroxytriptycene ( 50 ) devoid of alkyl substituents at the 9- and 10-positions (i.e., R 1 = H for 132 ) was prepared from 2,3,6,7,14,15-hexabromotriptycene 45 (Fig.  3 ) [ 46 ]. The reaction of furan ( 136 ) with benzyne derived from the lithiation of substituted benzenes ( 133 – 135 ) yields 1,4-epoxy-1,4-dihydronaphthalene derivatives ( 137 ). These compounds undergo Diels–Alder reactions with hexa-substituted anthracenes ( 127 ) to yield epoxy products ( 138 ), which, through dehydration and demethylation by acid treatment, are transformed into highly substituted 2,3,6,7-tetrahydroxybenzotriptycene ( 139 ) [ 69 , 70 ]. 2,3,6,7-Tetracyanotriptycene derivatives ( 141 ) were obtained by palladium-catalyzed cyanation of the corresponding triflated compound ( 140 ) derived from 131 [ 71 ]. Hydrolysis of the cyano groups of 141 yields 2,3,6,7-tetracarboxyltriptycene ( 142 ). The corresponding acid anhydride derivatives ( 143 ) were obtained by reacting with acetic anhydride [ 71 ]. An acid anhydride derivative devoid of alkyl substituents at the 9- and 10-positions (R 1 = H for 142 ) can be synthesized by oxidation of 2,3,6,7-tetramethyltriptycene ( 147 ) using potassium permanganate [ 72 ]. 2,3,6,7-Tetramethylanthracene ( 145 ), the precursor of 147 , is obtained by a Friedel–Crafts reaction using ortho -xylene ( 144 ), dichloromethane, and aluminum chloride. The Diels–Alder reaction between 145 and benzyne yields 147 . Similarly, 2,3,6,7,14,15-hexamethyltriptycene ( 148 ) can be obtained using anthranilic acid with two methyl groups ( 146 ) instead of anthranilic acid ( 80 ) as a benzyne precursor. Compound 148 can be transformed to tri-acid anhydride 150 through successive oxidation ( 149 ) and intramolecular condensation [ 73 ]. 1,3,6,8-Tetramethylanthracene ( 152 ) can be prepared through a Friedel–Crafts reaction of meta -xylene ( 151 ) in dichloromethane [ 74 ]. 1,3,6,8-Tetramethyltriptycene ( 153 ) [ 74 ] and its monobenzoquinone derivative ( 154 ) [ 75 ] are obtained by the reaction of 152 with benzyne generated from anthranilic acid ( 80 ) and para -benzoquinone ( 51 ), respectively. Both 153 and 154 can be selectively nitrated at the 2,7-positions flanked by methyl groups ( 155 [ 74 ] and 156 [ 75 ]) using a mixture of potassium nitrate and trifluoroacetic acid. Reduction of these nitrated products using hydrazine and metal catalysts yields 1,3,6,8-tetramethyl-2,7-diaminotriptycene ( 157 ) and 13,16-dihydroxy-1,3,6,8-tetramethyl-2,7-diaminotriptycene ( 158 ). The latter has been used as a building block for redox-active PIMs [ 75 ].

Synthesis of trip-monomers from multifunctionalized anthracenes

Figure  8 summarizes the synthesis of the 1,8,13-substituted triptycene derivatives. Our group reported that, through the Diels–Alder reaction of 1,8-dimethoxyanthracene ( 53 ) and benzyne generated from a methoxy-substituted precursor ( 162 ), 1,8,13-trimethoxytriptycene ( 165 ) can be prepared along with its 1,8,16-isomer as a minor product (1,8,13/1,8,16 = 2/1) [ 11 ]. Compounds 53 and 162 are synthesized in two steps from 1,8-dihydroxyanthraquinone ( 163 ) and in four steps from guaiacol ( 159 ), respectively. Similarly, 1,8-dimethoxytriptycene ( 164 ) can be obtained using 53 and benzyne [ 76 ]. By reacting these methoxy-substituted triptycenes with boron tribromide, we have shown that 1,8-dihydroxy and 1,8,13-trihydroxytriptycenes ( 166, 167 ) can be synthesized [ 11 , 77 ]. A wide variety of substituents can be introduced to these hydroxylated derivatives via ether linkages, and the resulting alkoxytriptycenes have been found to exhibit particular 2D assembly properties [ 11 , 77 , 78 , 79 , 80 , 81 ], which will be described in detail later. The various 1,8(,13)-substituted derivatives developed thus far include those with terminal olefin moieties ( 168, 171, 172 ) [ 77 , 80 , 81 ] and acryloyl groups ( 169 ) [ 78 ]. Compounds 171 and 172 are used as end modifiers for polydimethylsiloxane (PDMS) [ 80 , 81 ]. We also prepared a 1,8,13-triptycene initiator with hydroxy termini ( 170 ) [ 79 ]. Mastalerz et al. also reported a synthetic method for 1,8,13-trihydroxytriptycene 167 at approximately the same time and showed that the ortho -positions of its hydroxy groups can be formylated ( 173 ) by the reaction using hexamethylene tetraamine in the presence of trifluoroacetic acid [ 82 ]. From hexa-substituted monomer 173 , a triptycene-containing 3D COF was synthesized through the formation of a salphen complex [ 83 ].

Synthesis of 1,8,13-substituted trip-monomers

The synthesis of bridgehead-substituted 1,8,13-triptycenes, which involves selective functionalization of the 10-position of 1,8-substituted anthracenes, requires somewhat laborious, multiple protection/deprotection steps [ 84 ]. Converting the methoxy groups of 53 to acetoxy groups ( 174 ) allows selective bromination at the 10-position using N -bromosuccinimide (NBS). The acetoxy groups of the resulting 175 were then converted back to methoxy groups. The synthesized 10-bromo-1,8-dimethoxyanthracene ( 176 ) is lithiated at the 10-position using n -butyllithium and subsequently treated with N , N -dimethylformamide (DMF), yielding 10-formyl-1,8-dimethoxyanthracene ( 177 ). A bridgehead-substituted 1,8,13-triptycene skeleton ( 179 ) can be synthesized by protecting the formyl group of 177 with ethylene glycol ( 178 ) and then treating it with in situ generated methoxybenzyne. Successive deprotection of 179 using hydrochloric acid and boron tribromide yields 10-formyl-1,8,13-trihydroxytriptycene ( 181 ) [ 84 ]. The formyl group of 181 can be converted to an ethynyl group ( 182 ) at this stage using the Ohira–Bestmann reagent [ 84 ]. Moreover, terminal ethynyl groups at this position are available for click reactions using copper(I) iodide and triethylamine [ 85 ]. The hydroxyl groups of 182 can be functionalized via etherification, and the remaining ethynyl group can be functionalized by nucleophilic substitution through acetylide or Sonogashira coupling. This protocol allows for the synthesis of bifunctional derivatives ( 183 ) carrying azide and terminal ethynyl groups [ 86 ]. This AB 3 -type monomer was found to undergo polymerization in the assembled state, which will be discussed later. 1,8,13-Substituted triptycenes are relatively recently developed derivatives, and their synthesis has been thoroughly reviewed by Shindo et al. [ 87 ].

Recent progress in triptycene-containing polymers

Linear polymers.

Here, we present some recent examples of triptycene-containing linear polymers with unique structures and interesting properties. Swager et al. reported the synthesis of polyethersulfone ( poly-1 ) by polycondensation using 13,16-dihydroxy-1,8-dimethoxytriptycene ( 55 ), bisphenol A ( 184 ) and difluorodiphenylsulfone ( 185 ) (Fig.  9 ) [ 54 ]. The methoxy groups of poly-1 were deprotected with boron tribromide to form poly-2 with hydroxyl substituents, and bulky pyrazolium chloride ( 186 and 187 ) was attached to the hydroxyl groups to form a cross-linked network polymer ( poly-3 ) with a structure in which the pyrazolium ions are densely aggregated (referred to as an ionic highway). Films of poly-3 are reported to show both anion-conducting properties and high stability resulting from suppressed swelling due to its cross-linked nature.

Highly conductive and stable triptycene-containing anion-exchange polymer with an “ionic highway”

Swager et al. synthesized a simple triptycene-containing polyethersulfone ( poly-4 ) from 1,4-dihydroxytriptycene ( 54 ) and 185 , chloromethylated its triptycene moieties, and then treated the resulting poly-5 with N -methylimidazole ( 188 ) to obtain poly-6 with N -methylimidazolium chloride pendants (Fig.  10 ) [ 24 ]. This polymer not only forms anion exchange membranes for alkaline fuel cells but also provides a scaffold for metal nanoparticles [ 24 , 88 , 89 ]. For example, poly-6 was composited with poly(4-vinylpyridine) and single-walled carbon nanotubes (SWCNTs), and the counter anion (Cl – ) was exchanged with tetrachloroaurate (AuCl 4 – ). Upon treatment with sodium borohydride, small (<5 nm) gold nanoparticles (Au NPs) were formed in the composite film (Fig.  10 ). A field-effect transistor (FET) incorporating a Au NP composite layer was reported to serve as a chemo FET sensor for the detection of carbon monoxide and other gaseous molecules [ 88 ]. Moreover, bimetallic Pd/Pt nanoparticles (PdPt NPs) with a size of 1 nm can be synthesized within the poly-6 film (Fig.  10 ). PdPt NPs react with hydrogen gas with high sensitivity in an oxygen atmosphere, and it has been reported that these materials can be applied as very sensitive hydrogen gas sensors [ 89 ]. In these systems, the use of triptycene units allows for the dense accumulation of functional groups as well as the formation of microporous structures.

a Synthesis of triptycene-containing anion exchange polymers and their application in ( b ) chemo-FET sensors (e.g., CO gas) and ( c ) H 2 gas sensors. b Adapted with permission [ 88 ] Copyright 2019, American Chemical Society. c Adapted under terms of the CC-BY 4.0 license [ 89 ] Copyright 2020, Elsevier

Conjugated polymers derived from the 9,10-diethynylpentiptycene monomer 58 have been used as components of highly responsive chemosensors due to their porous nature [ 15 ]. Pentiptycene-containing poly-7 with a benzothiadiazole unit composited with SWCNTs has been reported to serve as a sensor for the chemiresistive detection of solvent vapors such as benzene, toluene and xylene (i.e., BTX) (Fig.  11 ) [ 90 ]. Pentiptycene-containing poly-8 with semiperfluoroalkyl side chains has been reported to function as a fluorescent polymer sensor for the detection of poly(fluoroalkyl) substances (PFAS) (Fig.  11 ) [ 91 ].

Pentiptycene-containing π-conjugated polymers applied for chemosensing

Swager et al. reported that a palladium-catalyzed reaction between phenolic hydroxyl groups and aryl halides can be used in condensation polymerization to obtain a variety of polyaryl ethers that cannot be synthesized through standard aromatic nucleophilic substitution methods (Fig.  12 ) [ 92 ]. Typically, 1,4-dihydroxy-6,14-di- tert -butyl-triptycene ( 189 ), 2,7-dibromospirobifluorene ( 190 ) and aromatic dibromides with various photocatalytically active sites ( 191 – 196 ) were reacted in the presence of a palladium catalyst ( 197 ) and a monophosphine ligand ( 198 ) to afford the corresponding polyethers ( poly-9 – poly-16 ). The obtained polymers are solution-processable and form microporous films that exhibit photocatalytic abilities, as shown in Fig.  12 [ 93 ]. A ruthenium catalyst can also be used to introduce perfluoroalkyl groups into the spirobifluorene moiety. The resulting polymer carrying fluorophilic side chains ( poly-9-C 17 F 35 ) can be used for the modification of the surface inside perfluoroalkoxy alkane (PFA) tubes. Using these modified PFA tubes, photoreactions can be conducted allowing the reagent to flow while irradiating with light [ 93 ].

Triptycene-containing solution-processable porous photocatalysts obtained by palladium-catalyzed carbon–oxygen bond formation

Triptycene is widely used as a constituent in PIMs due to its rigidity and high intramolecular free volume (Fig.  13 ) [ 9 , 18 ]. McKeown et al. reported dioxane-forming ladder polymerization using tetrafluoroterephthalonitrile ( 199 ) and 2,3,6,7-tetrahydroxybenzotriptycene with very bulky substituents ( 139 ) [ 69 , 70 ]. The resulting poly-17 was reported to exhibit highly selective gas permeability, significantly outperforming the 2008 Robeson upper bounds for O 2 /N 2 , H 2 /N 2 , CO 2 /N 2 , H 2 /CH 4 and CO 2 /CH 4 selectivities. Using a similar polymerization method, Guo et al. reported the synthesis of a ladder-type polymer ( poly-18 ) with pentiptycene in the main chain from a tetrahydroxylated pentiptycene ( 76 ), a tetramethyl spirobindan tetraol ( 200 ) and 201 [ 55 ]. High CO 2 /CH 4 permselectivity was reported for poly-18 . McKeown et al. also reported ladder polymerizations by the reaction of dimethoxymethane with 2,6-diaminotriptycene ( 8 ) and 2,6-diaminobenzotriptycene ( 125 ) to yield Tröger’s base-containing PIMs poly-19 [ 94 ] and poly-20 [ 66 ], respectively. Condensation of a trip-monomer with two hydroxy groups and two amino groups ( 38 ) and 4,4’-(hexafluoroisopropylidene)diphthalic anhydride ( 201 ) in the presence of isoquinoline yielded polyimide poly-21 , which, after thermal treatment, was transformed into thermally rearranged polybenzoxazole poly-22 [ 36 ]. Li, Yi, Ma et al. reported that a polyimide membrane ( poly-23 ) obtained from 2,6-diaminomonomer ( 39 ) with ortho -bromo groups and pyromellitic anhydride ( 202 ) showed ca . 8 times greater gas permeability than the corresponding polyimide devoid of bromo groups [ 38 ]. Moreover, when the poly-23 membrane was heated to 550 °C, debromination occurred yielding a carbon molecular-sieve membrane, which displayed an almost 9-fold increase in gas permeability while maintaining permselectivity, with CO 2 /N 2 and CO 2 /CH 4 selectivities of 29.2 and 30.9, respectively, and an unprecedentedly high CO 2 permeability coefficient of 20639 barrer [ 38 ].

Triptycene-containing PIMs useful in gas-separation membranes

Gong, McKeown et al. developed a triptycene-containing polyimide poly-24 with a hydroquinone moiety by reacting 13,16-dihydroxy-1,3,6,8-tetramethyl-2,7-diaminotriptycene ( 158 ) with pyromellitic dianhydride ( 202 ) in the presence of isoquinoline (Fig.  14 ) [ 75 ]. This polymer has a rigid and randomly contorted main-chain structure in which the rotation of the diimide unit is inhibited by the methyl groups on the triptycene moiety. Accordingly, poly-24 shows excellent solubility in aprotic polar solvents and can be easily cast into freestanding films. Upon treatment with CAN, poly-24 is oxidized to form the para -benzoquinone derivative poly-25 , which undergoes a four-step redox process with excellent reversibility (Fig.  14 ). Lithium-ion battery cells using easily solution-processable poly-24 as a component of the cathode material have been reported to exhibit stable cycling performance [ 75 ].

Triptycene-containing redox-active PIMs useful as electrodes for lithium-ion batteries

Despite the fact that many trip-monomers have chirality, there are only a few examples of optically active triptycene polymers [ 7 , 31 , 32 ]. Examples of the synthesis of such systems are shown in Fig.  15 . The polycondensation of optically active 2,6-diaminotriptycene ( 8 ), which can be obtained by means of chiral HPLC, with acid anhydride 4,4’-(hexafluoroisopropylidene)diphthalic anhydride ( 201 ) in the presence of isoquinoline yields triptycene-containing polyimide poly-27 [ 31 ]. This polymer serves as an optically active PIM, and its freestanding film can be used for enantioselective membrane separation. Chen et al. synthesized a chiral triptycene with dihydroacridine blades ( 203 ) from optically active 8 [ 95 ]. Compound 203 was then reacted with bis(4-bromophenyl)sulfone ( 204 ) in the presence of a palladium catalyst to produce optically active poly-28 . This polymer shows thermally activated delayed fluorescence (TADF) properties, where the triptycene and sulfone moieties serve as electron donors and acceptors, respectively, and emits circularly polarized luminescence (CPL) with a dissymmetry factor ( g lum ) on the order of 10 –3 [ 95 ]. Using poly-28 , circularly polarized organic light-emitting diodes (OLEDs) were successfully fabricated by means of solution processing. Ikai et al. synthesized optically pure 2,6-diiodotriptycene ( 18 ) from the corresponding optical isomer of 8 [ 96 ]. This was then used in a Suzuki coupling polymerization with ethynyl-substituted diboronic ester ( 205 ) to yield optically active poly-29 . Upon treatment with trifluoroacetic acid, poly-29 undergoes a geometrically selective Friedel–Crafts reaction, yielding the ladder polymer poly-30 , whose conformation is fixed in a one-handed helical structure [ 96 ]. It has been reported that poly-30 exhibits CPL and can be used as a stationary phase for chiral column chromatography.

Optically active triptycene-containing polymers

Network polymers

Triptycene can be easily tri- and hexa-functionalized by Friedel–Crafts-type reactions (Fig.  3 ). This approach is also useful for the synthesis of triptycene-containing network polymers; however, the resulting polymers are usually insoluble and devoid of regular structures. Many examples of this type of polymer have been described in a recent review by C-F. Chen et al. [ 8 ], so only selected examples are presented here (Fig.  16 ). McKeown et al. reported that porous 3D network polymers ( poly-31 ) with a Brunauer–Emmett–Teller (BET) specific surface area (S BET ) of 1750 m 2 /g can be obtained by the reaction of unsubstituted triptycene ( 1 ) with dichloromethane in the presence of aluminum trichloride (Fig.  16a , top) [ 97 ]. Furthermore, poly-31 can be easily functionalized with nitro ( poly-32 ), amino ( poly-33 ), or sulfo ( poly-34 ) groups [ 98 ]. McKeown et al. synthesized a Tröger’s base-containing network polymer ( poly-35 ) by the reaction of 2,6,14- or 2,7,14-triaminotriptycene ( 10, 11 ) with dimethoxymethane in the presence of trifluoroacetic acid (Fig.  16a , middle). They found that poly-35 serves as a base catalyst for the Knoevenagel condensation of benzaldehyde and malonitrile [ 99 ]. Moreover, the use of various diamines for copolymerization with triaminotoriptycene allows for tuning of the pore size and catalytic activity of the resulting network polymers [ 100 ]. Baek et al. reported a very unique solid-state polymerization of triptycene derivatives [ 45 ]. They found that 2,3,6,7,14,15-hexaethynyltriptycene ( 48 ) forms stable crystalline materials that incorporate acetone and water molecules, but rapid heating causes desorption of the solvent. This triggers Masamune–Bergman cyclization of the ortho -diethynyl moiety to generate highly reactive radicals, leading to explosive reactions and the formation of poly-36 with a porous structure (Fig.  16b, c ).

a Selected examples of triptycene-containing nonregular network polymers. b , c Explosive reactions of 48 forming poly-36 triggered by solvent desorption. b , c Adapted under terms of the CC-BY 4.0 license [ 45 ] Copyright 2017, Springer Nature

Porous 2D polymers

Triptycene derivatives that undergo porous 2D hexagonal packing typically have structures with blades laterally extended through the 2,3-, 6,7- and 14,15-positions (Fig.  1 ). In 2014, King et al. reported a triptycene derivative ( 206 ) with photoreactive tetrafluoroanthraceno blades (Fig.  17a , left and center) [ 101 ]. This extended triptycene derivative forms porous hexagonal packing in the crystalline state, with a configuration in which the tetrafluoroanthraceno blades are intermolecularly π-stacked (Fig.  17b , left). Then, light irradiation was used to induce [4+4] cycloadditions between the blades, with photoirradiation (460 nm) of a single-crystal sample at 223 K leading to dimerization ( 207 ) (Fig.  17b , center) and further photoirradiation at 400 nm forming a 2D polymer ( poly-37 ) (Fig.  17b , right). This two-step photochemical reaction proceeds in a single-crystal-to-single-crystal manner. It has been reported that crystalline samples of this 2D polymer can be exfoliated to monolayer sheets by heating to 50 °C in N -methyl-2-pyrrolidone (NMP). Before this study, the same group reported in 2013 that light irradiation of single crystals of a derivative with nonfluorinated anthraceno blades ( 208 ) produced a 2D polymer (Fig.  17a , right) [ 102 ]. However, the photopolymerization of 208 was reported to be “not single-crystal to single-crystal transformation”. In 2021, Lackinger et al. reported that vacuum deposition of 206 on a hexacosane-passivated graphite substrate followed by thermal annealing produced a crystalline monolayer film (with a domain size of up to 500 nm) consisting of a porous hexagonal lattice similar to that of a single crystal [ 103 ]. When this monolayer sample is irradiated with light using a high-power LED, on-surface photocycloadditions proceed, yielding a 2D polymer.

a Molecular structures of 206 and 208 , which feature photoreactive tetrafluoroanthraceno and anthraceno blades, respectively. b Schematic illustrations of the two-step single-crystal-to-single-crystal transformations of 206 into its 2D polymer poly-37

Based on a similar molecular design, King et al. reported the formation of Langmuir–Blodgett (LB) films at the air/water interface using triptycene derivative 209 , which contains a hydrophilic diethylene glycol moiety at the bridgehead position that serves as an anchor for the water layer (Fig.  18a ) [ 104 ]. A chloroform/hexane solution of 209 was spread on the air/water interface and compressed at 1 °C using an LB trough, and the mean molecular area (MMA) was calculated from the compression isotherm. Phase changes were observed at approximately 155–135 and 80 Å 2 , and the former MMA was assigned to porous hexagonal p 6 packing. STM measurements of a LB film of 209 transferred onto a HOPG substrate show a porous structure that is consistent with the simulated pattern (Fig.  18b ). Upon irradiation with 365 nm light, the LB film of 209 underwent photopolymerization, and the obtained thin-film polymer was found to be mechanically hard enough to deform a paper Wilhelmy plate during the Langmuir experiment.

a Formation of a LB film composed of amphiphilic propeller-shaped 209 , which carries a diethylene glycol chain that anchors the molecule to the water surface. b STM image (left) and simulated p 6 lattice of 209 (right). c Structures of a series of amphiphilic propeller-shaped molecules ( 210, 211, and 212 ) designed for the formation of structurally well-defined LB films and their crosslinked 2D polymers. d Schematic illustration of plasmon-induced [4+4]-cycloaddition/2D polymerization of LB films 210 and 211 . b Adapted with permission [ 104 ] Copyright 2015, American Chemical Society. Adapted under terms of the CC-BY 4.0 license [ 107 ] Copyright 2021, Springer Nature

Similar to 209 , several amphiphilic extended triptycene derivatives have been reported to form highly ordered LB films with porous hexagonal structures ( 210 [ 105 ], 211 [ 106 ] and 212 [ 106 ], Fig.  18c ). Schlüter et al. reported the synthesis of 2D polymers using 211, 212 and a 1:1 mixture of 211 and 212 using the LB technique and light irradiation [ 106 ]. After being transferred onto a Au(111) substrate, the obtained 2D polymers were characterized by tip-enhanced Raman spectroscopy (TERS), which allowed for the evaluation of the conversion number ( X ) of the crosslinked structures (i.e., anthracene dimer moieties). The results showed high conversion numbers, with averages of X  = 87.7 ± 1.8 ( 211 ), 92.0 ± 1.7 ( 212 ), and 94.1 ± 2.1% (copolymer). Based on these values, along with the random-growth model, the calculated crystallinities of the 2D polymers were 95.8 ± 1.2, 98.2 ± 1.1 and 99.0 ± 1.4% for 211, 212 and 211/212 (1/1), respectively. In 2021, Shao, Lan, Zenobi, and coworkers performed TERS measurements on LB films of 210 and 211 transferred onto Au(111) substrates using silver chips and found that visible laser irradiation (633 nm) caused intermolecular [4+4] cycloadditions between the blades in the 2D assembly as plasmon-induced chemical reactions (PICRs) (Fig.  18d ) [ 107 ].

Nonporous 2D assemblies and polymers

Triptycene derivatives that form dense and nonporous 2D hexagonal packing are characterized by structures in which substituents are introduced at either the 1,8,13- or 4,5,16-positions (Fig.  1 ). In derivatives with these substitution patterns, the free volume around the triptycene backbone can be retained, which is favorable for the formation of 2D nested packing. In fact, most triptycene derivatives reported to form nonporous 2D hexagonal structures in crystals or in the liquid-crystalline state have these substitution patterns [ 87 , 108 ]. It has been found that 1,8,13-substituted triptycene derivatives can exhibit 2D assembly ability even when another substituent is introduced at the bridgehead position ( vide infra ). Note that unsubstituted triptycene ( 1 ) does not form nested packing in the crystal or on the substrate surface [ 109 , 110 ]. Therefore, it is essential to choose appropriate substituents and substitution patterns to realize dense 2D hexagonal assemblies.

With the aim of constructing large-area, highly ordered organic thin films through molecular self-assembly, our group developed a tripodal triptycene ( 213 ) with long-chain alkoxy groups introduced at the 1,8,13-positions (Fig.  19a ) [ 11 ]. In the bulk state, this compound forms a “2D nested hexagonal packing + 1D layer stacking” structure (Fig.  19b ). The assembly of 213 on a solid substrate yields oriented thin films with 2D hexagonal sheets stacked parallel to the substrate (Fig.  19b ) on a variety of inorganic and polymer substrates simply by, e.g., spin-coating or vacuum deposition [ 11 , 111 , 112 ]. Importantly, in thin films of 213 , the ordered structure formed on nanometer-length scales can propagate to macroscopic length scales. For example, in through-view XRD measurements of a vacuum-deposited film of 213 (50 nm in thickness) formed on a sapphire substrate (2.0 cm in diameter), the in-plane azimuthal angle dependence of the peak intensity originating from the 2D hexagonal lattice does not change when the measurement position on the film is changed [ 11 ]. This observation indicates that the orientation of the 2D hexagonal lattice is aligned across the entire film.

a Molecular structures of 1,8(,13)-substituted triptycene derivatives 213 – 215 having alkoxy side chains. b Schematic illustrations of a “2D nested hexagonal packing + 1D layer stacking” structure formed by the self-assembly of 213 – 215

Derivatives with different alkoxy chain lengths and those with only two or one long-chain alkoxy group also form 2D+1D structures similar to that of 213 , whereas their thermal stability largely depends on the triptycene substitution pattern (Fig.  19a ) [ 11 , 113 ]. For example, the 2D+1D assembly of 1,8-bis(dodecyloxy)tripycene 214 (m.p. 134 °C) shows significantly lower thermal stability than that of 213 (m.p. 211 °C). However, the assembly of 1,8-bis(dodecyloxy)-13-methoxytriptycene 215 , in which one of the long-chain alkoxy groups in 213 is replaced by a methoxy group, exhibits greater relative thermal stability (m.p. 231 °C). These differences are directly reflected in the properties of the soft materials incorporating the triptycene units ( vide infra ).

Tripodal triptycenes are also useful as components in organic electronic devices [ 113 , 114 , 115 , 116 , 117 ]. We have found that the performance of flexible organic thin-film transistor devices can be significantly improved by forming several layers of 215 on the surface of a parylene-based polymer dielectric layer [ 114 , 115 , 116 , 117 ]. This triptycene-based surface modifier covers structural defects on the surface of the polymer substrate and lowers and homogenizes the surface energy. This leads to improved crystallinity of the organic semiconductor on the dielectric layer and overall performance of the transistors. Although SAMs can be used for the surface coating of inorganic substrates, they cannot be applied to polymer substrate surfaces. Tripodal triptycenes, which can form highly ordered and perfectly oriented films regardless of substrate type, provide a powerful tool for improving the performance of organic electronic devices.

Importantly, tripodal triptycenes exhibit excellent 2D assembly ability even when various functional groups are incorporated [ 84 , 86 ]. For example, compound 216 , in which a sterically bulky spherical C 60 is introduced via an ethynyl group at the bridgehead position, as well as its precursor 217 with a terminal ethynyl group (Fig.  20a ), also forms 2D+1D structures in the bulk state [ 84 ]. The spin-coated thin films of 216 are oriented similarly to those of 215 , with a 2D+1D structure consisting of 2D layers of densely arranged C 60 units (Fig.  20b ). Time-resolved microwave conductivity (TRMC) measurements of these thin films revealed the anisotropic conduction of photocarriers in the in-plane direction. Thus, tripodal triptycenes are useful supramolecular scaffolds [ 118 ] that promote the 2D assembly of various functional groups and molecular units, making it possible to improve their anisotropic properties. Face-selective introduction of an azide group and three terminal ethynyl groups into a tripodal triptycene ( 183 ) also leads to assembly into a 2D+1D structure both in the bulk state and on solid substrates (Fig.  20c ) [ 86 ]. When this assembly is heated, intermolecular uncatalyzed Huisgen cycloadditions maintain the assembly structure. The resulting polymer is insoluble and possibly composed of a highly entangled polycatenane-like cyclic structure (Fig.  20c ). The solid-phase polymerization of the oriented film of 183 proceeded while retaining the surface nanoterraced structure.

a Molecular structure of C 60 -appended tripodal triptycene 216 and its precursor 217 . b Schematic illustrations of the 2D+1D structure formed by the self-assembly of 216 . c Molecular structure (left) and schematic assembly structures of 183 before and after catalyst-free thermal Huisgen cycloaddition

The incorporation of 1,8,(13)-substituted triptycenes into polymers induces long-range ordered structures through 2D assembly of the triptycene units, greatly enhancing the mechanical properties of the polymers [ 77 , 78 , 79 , 80 ]. Such specific 2D assembly can be achieved by incorporating the triptycene units at various sites of polymers, including at both ends [ 80 , 81 ], in the main chain [ 77 ], side chains [ 78 ] and at various branch points [ 79 ]. Figure  21a shows the chemical structures of poly(dimethylsiloxane) (PDMS) derivatives ( poly-38 – poly-40 ) with 1,8(,13)-triptycene units at both ends ( poly-38 and poly-39 ) along with a 1,4-substituted derivative ( poly-40 ). For example, hydrosilylation of olefin-appended 1,8-substituted triptycene 171 (Fig.  8 ) with hydride-terminated PDMS (PDMS-H, M n = 18 or 24 kDa) affords poly-38 [ 80 ]. In sharp contrast to liquid PDMS-H, the telechelic polymer poly-38 forms a highly viscous solid that exhibits birefringence (Fig.  21b ). Rheological measurements revealed that the complex viscosity of poly-38 (approximately 10 5  Pa·s) is 10 4 times greater than that of PDMS-H (approximately 10 1  Pa·s). Small- and wide-angle XRD measurements revealed that poly-38 forms a 2D+1D structure with long layer spacings of 18–20 nm, in which 2D sheets of terminal triptycene units are stacked one-dimensionally via PDMS domains (Fig.  21c ). Telechelic poly - 39 synthesized from 1,8,13-substituted triptycene 172 (Fig.  8 ) also forms a 2D+1D structure but has an even greater mechanical strength than poly-38 and behaves as a thermoplastic, resulting in a freestanding film without any covalent cross-linking (Fig.  21b ) [ 81 ]. It has also been shown that poly-39 films exhibit self-healing properties. The only structural difference between poly-39 and poly-38 is the presence or absence of a methoxy group at the 13-position of the terminal triptycene units, and it is thus surprising that such a tiny change in substituent relative to the entire polymer has a significant impact on the mechanical and thermal properties. Moreover, the structural and physical properties of PDMS are largely unchanged in poly-40 (Fig.  21a, b , right), where 1,4-substituted triptycene units without 2D assembly ability are introduced at both ends of the PDMS.

a 1,8(,13)-Substituted triptycene-containing telechelic polymers poly-38 and poly-39 and 1,4-substituted triptycene-containing poly-40 . b Photographs of bulk samples of PDMS-H and the telechelic polymers. c Schematic illustrations of the assembly structures of the telechelic polymers

Ring-closing metathesis of 1,8-olefin-appended 168 (Fig.  8 ) yields macrocyclic olefin monomer 218 , which has been used for the synthesis of main chain-type triptycene-containing polymers (Fig.  22 ) [ 77 ]. We synthesized the homopolymer poly-41 by ring-opening metathesis polymerization of 218 and the copolymer poly-42 by copolymerization of 218 with cyclooctene [ 77 ]. Both poly-41 and poly-42 form 2D hexagonal sheet structures with triptycene units and lamellar structures filled with polymer chain domains, which leads to a marked improvement in the mechanical properties of the polymers. In poly-43 without 2D assembly ability, no improvement in physical properties was observed [ 77 ]. While the triptycene polymers poly-44 and poly-45 , which have oligosiloxane- and ester-based main chains, respectively, also form 2D+1D structures, the urethane-containing poly-46 does not (Fig.  22 ) [ 77 ]. Presumably, the hydrogen bonding ability of the urethane group may be superior to the assembling ability of the triptycene, hampering structural ordering of the polymer.

Polymers incorporating 1,8-substituted triptycene units ( poly-41, poly-42, poly-44 – poly-46 ) in the main chains with a 1,4-substituted unit ( poly-43 )

We synthesized diblock ( poly-47 ), ABA triblock ( poly-48 ) and random ( poly-49 ) copolymers by reversible addition-fragmentation chain-transfer (RAFT) polymerization of 1-acryloyl-8-methoxytriptycene 169 (Fig.  8 ) and n-butyl acrylate (Fig.  23a ) [ 78 ]. The thermal stability of these triptycene-containing copolymers is comparable (2% weight loss temperature: ca . 300 °C) to that of the precursor polymer ( poly-50 ) without triptycene units. However, while poly( n -butyl acrylate) derivatives, including poly-50 , are liquids, the copolymers are all viscous solids. For diblock poly-47 , both the melting ( T m ) and crystallization ( T c ) temperatures shift toward higher values with increasing triptycene content. While their triptycene contents are comparable, diblock poly-47 has slightly greater T m and T c values than triblock poly-48 , whereas the enthalpy changes associated with the phase transitions are in the same range. XRD measurements revealed that both diblock and triblock copolymers form 2D+1D structures with long-range ordering. In contrast, the random copolymer poly-49 does not form a 2D+1D structure and shows no clear phase transition. It is worth noting that rheological measurements show that diblock poly-47 and triblock poly-48 exhibit complex viscosities 10 4 –10 5 times greater than that of random poly-49 over a wide temperature range. Therefore, in the design of side-chain-type triptycene polymers, the presence of block segments is essential for achieving long-range ordering. Accordingly, we designed poly-51 with triptycenes at both ends of a poly( N -isopropylacrylamide) (PNIPAM) segment (Fig.  23b ) [ 78 ]. This triblock copolymer yields hydrogels (60 wt% water content) without chemical cross-linking due to the formation of a 2D+1D structure with a 2D triptycene sheet and a hydrated PNIPAM domain. The hydrogel became turbid at 60 °C, and the PNIPAM domain exhibited a lower critical solution temperature (LCST) upon increasing the temperature. VT-XRD measurements of the hydrogel revealed that the interlayer distances decreased with increasing temperature while maintaining the 2D structure. These findings are expected to be applied to the future development of soft actuators that exhibit anisotropic motion in response to stimuli.

a Molecular and schematic structures of diblock poly-47 , ABA triblock poly-48, and random poly-49 carrying 1,8-substituted triptycene-based side chains as well as poly-50 , a precursor of poly-48 . b Molecular structure of ABA triblock polymer poly-51 with a PINIPAM segment and ( c ) photographs of its hydrogel. c Adapted with permission [ 78 ] Copyright 2023, American Chemical Society

Conclusions

As outlined in this review, triptycene is a versatile scaffold that can be exploited by introducing a number of substituents in various patterns at its blades and bridgehead positions. It is also possible to impart characteristic redox properties to the blade sites. One of the main topics in the development of triptycene-based functional materials, diverse polymers, and molecular assemblies is the design of nanospaces (microporosity) in the assembly state, which is being explored for use in guest recognition, material transport, separation, and catalysis. Rigid three-bladed propeller molecules are suitable for forming porous or nonporous 2D lattices, and they provide excellent building blocks for forming 2D sheet structures that are not easily accessible. Their ability to form dense 2D sheets stems from their characteristic space-filling structure, which enables precise control over the structures and properties of surfaces and interfaces. It is expected that in the future, the development of materials for the realization of unique electronic states that reflect the two-dimensional nature and topology of assemblies will accelerate, leading to new functionalities. From various perspectives, triptycenes will continue to drive the development of functional materials that cannot be realized using planar molecules. Organic synthesis technologies will be the driving force to enable the precise design of such triptycene derivatives.

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Acknowledgements

This work was supported by JSPS KAKENHI (JP21H05024 and JP21H04690 for TF) and a Grant-in-Aid for Transformative Research Areas (A) “Condensed Conjugation” (JP20H05868 for TF). This work was also supported in part by the Research Program of “Five-Star Alliance” in “NJRC Mater. & Dev.”.

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Fumitaka Ishiwari

Present address: Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka, 565-0871, Japan

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Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501, Japan

Fumitaka Ishiwari, Yoshiaki Shoji, Colin J. Martin & Takanori Fukushima

Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501, Japan

Fumitaka Ishiwari, Yoshiaki Shoji & Takanori Fukushima

Research Center for Autonomous Systems Materialogy (ASMat), Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8501, Japan

Yoshiaki Shoji & Takanori Fukushima

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Ishiwari, F., Shoji, Y., Martin, C.J. et al. Recent advances in structurally elaborate triptycenes, triptycene-containing polymers and assemblies: structures, functions and applications. Polym J (2024). https://doi.org/10.1038/s41428-024-00920-x

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Received : 29 March 2024

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Published : 29 May 2024

DOI : https://doi.org/10.1038/s41428-024-00920-x

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