Influence of Monomer Sequence on the Cyclization Behavior of Poly(acrylonitrile-co-acrylamide)

Round 1

Reviewer 1 Report

Please see attached marked-up copy, supplementary information, and the reviewer comments below.

Comment 1: The sequence of monomers changes the polymer composition, structure and properties of the bulk polymer especially with variable feed ratio. Therefore the range of structural properties need to be assessed if the monomer sequence is changed. The authors should indicate why they have selectively chosen to study the influence only on thermal properties when the monomer sequence and the polymerization protocol has been optimized. 

 I see later in the paper that it is the cyclization reaction that has been investigated with thermal treatment using DSC so the clarity around that needs to be provided throughout the text. The mention of thermal behaviour of copolymers in the title and in the text is confusing. Please see comments in the marked-up copy.

 Comment 2: Page 2, Lines 46-52

The introduction of the various polymerization mechanisms for AN with various monomers and their impact on thermoxidative stability comes a little out of context. It is a little unclear what is predicted with ATRP versus free radical polymerization and then becomes possible for RAFT. Is it the monomer sequence control to relate to thermoxidative stability? A brief clarification or rephrasing will help clarify this.  Please see comments in the marked-up copy.

 Comment 3: Page 2

The mention of batch polymeriszation also comes out of context. Batch polymerization versus block or random needs to be briefly introduced for the importance to the urgent work to be understood in context.

 Comment 4: Page Line 55

What is the narrow temperature range? Is it a mild temperature range? An why is this important?

 

Comment 5: Page 4, lines 127-130.

The first sentence appears to convey that this work has been done in references 30 and 31 already and is not novel. It is important to highlight what this work contributes as novel or as optimization that makes it relevant for PAN copolymer synthesis. However, it is the cyclization reaction investigation of a complex mechanism for acrylamide cyclization with batch and random monomer sequencing. So that could be more explicitly clarified.

Comments for author File: Comments.pdf

Author Response

We are very thankful to the referee for useful comments that gave us an opportunity to look at the paper from a different perspective. The particular thanks for the marked-up copy, which we have studied carefully. We have taken into account all the comments and questions given both in the attached file and separately on the site. Below the response for the comments is given. All the changes in the text are marked in red.

Comment 1: The sequence of monomers changes the polymer composition, structure and properties of the bulk polymer especially with variable feed ratio. Therefore the range of structural properties need to be assessed if the monomer sequence is changed. The authors should indicate why they have selectively chosen to study the influence only on thermal properties when the monomer sequence and the polymerization protocol has been optimized. I see later in the paper that it is the cyclization reaction that has been investigated with thermal treatment using DSC so the clarity around that needs to be provided throughout the text. The mention of thermal behaviour of copolymers in the title and in the text is confusing. Please see comments in the marked-up copy.

Response: Acrylonitrile copolymers are attractive as carbon fibers precursors. That is why their chemical modification during thermal treatment in inert and air atmosphere are important. So, in the present study we have concentrated our efforts on the investigation of cyclization reaction depending from the copolymer microstructure. We have extended the introduction part, abstract and conclusions, as well as results and discussion. Besides, we have analyzed triad composition of four copolymers and have made some conclusions about correlation between triad composition and cyclization behavior of copolymers. Solution properties and spinnability of the copolymers are also important. However, we have decided to limit ourselves in this paper to cyclization behavior. Also thermal behavior was replaced either by cyclization behavior or thermal treatment depending on context.

Comment 2: Page 2, Lines 46-52: The introduction of the various polymerization mechanisms for AN with various monomers and their impact on thermoxidative stability comes a little out of context. It is a little unclear what is predicted with ATRP versus free radical polymerization and then becomes possible for RAFT. Is it the monomer sequence control to relate to thermoxidative stability? A brief clarification or rephrasing will help clarify this. Please see comments in the marked-up copy.

Response: We have thoroughly revised introduction in accordance with comments.

Comment 3: Page 2: The mention of batch polymeriszation also comes out of context. Batch polymerization versus block or random needs to be briefly introduced for the importance to the urgent work to be understood in context.

Response: We have specified in the paper and in the table three methods to perform copolymerization. Batch (all reagents are introduced in copolymerization simultaneously), continuous (acrylamide is introduced to the acrylonitrile polymerization with a given rate), semi-continuous (acrylamide is introduced in the reaction mixture with a given rate after acrylonitrile polymerization is started). The necessary comments are given in the experimental part and in the discussion section.

Comment 4: Page Line 55: What is the narrow temperature range? Is it a mild temperature range? An why is this important?

Response: For cyclization reaction the wide temperature range and low intensity of exo-effect are required. Otherwise the copolymer will become brittle and unsuitable for further transformation into carbon fiber. All the required comments are given in the text.

Comment 5: Page 4, lines 127-130: The first sentence appears to convey that this work has been done in references 30 and 31 already and is not novel. It is important to highlight what this work contributes as novel or as optimization that makes it relevant for PAN copolymer synthesis. However, it is the cyclization reaction investigation of a complex mechanism for acrylamide cyclization with batch and random monomer sequencing. So that could be more explicitly clarified.

Response: In fact, the results obtained in the present work are new. We agree that the novelty should be emphasized. So, we have extended this part and make its more clear.

Finally, we would like again to thank referee for very useful comments and questions.

Reviewer 2 Report

The authors prepared random, gradient and block gradient poly(acrylonitrile-co-acrylamide) copolymers copolymers by using dibenzyl trithiocarbonate and batch/semibatch polymerization. And differential scanning calorimetry and Fourier transform infrared spectroscopy were used to investigate the difference of thermal behavior of copolymers in argon and air. However, some issues exist in this work. Please address the following issues:

1.      The author needs to provide more characterization means to analyze the structure and performance of the copolymer, for example, the quantitative analysis of the copolymer should provide more evidence.

2.      It is suggested to increase the comparison (including cost, productivity, environmental protection, etc.) with similar studies.

3.      The preparation method and steps of copolymer should be thoroughly detailed so that readers can repeat.

4.      The author should emphasize the innovation and the significance of the research in the abstract and conclusion.

Author Response

We are very thankful to the referee for very useful comments and questions. All of them were taken into account during revision of the text. All the changes in the text are marked in red. Below the response for the comments is given.

Comment 1: The author needs to provide more characterization means to analyze the structure and performance of the copolymer, for example, the quantitative analysis of the copolymer should provide more evidence.

Response: We completely agree with the comment. The copolymers were analyzed by NMR spectroscopy (Fig.3, Table 3, Fig. S2). The triad composition has been estimated. The discussion was supplemented with the information about copolymer microstructure and its influence on the cyclization of the copolymers.

Comment 2: It is suggested to increase the comparison (including cost, productivity, environmental protection, etc.) with similar studies.

Response: We are thankful for this proposal. Carbon fibers production consists from multiple stages including polymer synthesis, fiber molding, thermal stabilization and carbonization. In the present paper, we consider the synthesis and thermal stabilization only. From the viewpoint of the cost, productivity, etc. the suggested method is comparable with conventional free radical polymerization. However, the advantage of RAFT copolymerization is the better control of molecular characteristics of the polymer. We have thoroughly revised introduction and have given some comparison.

Comment 3: The preparation method and steps of copolymer should be thoroughly detailed so that readers can repeat.

Response: The experimental part presents the general procedure, while Table S1 in ESI file (supporting information) contains detailed quantities of all the reagent used in the experiments 1 – 7.

Comment 4: The author should emphasize the innovation and the significance of the research in the abstract and conclusion.

Response: We completely agree and have done the corresponding changes.

Reviewer 3 Report

In this article researchers aimed to find answer to a very crucial question via synthesizing acrylamide copolymers with varied monomer distributions. They have utilized RAFT based copolymers and controlled addition of copolymers to tune the polymer microstructures. The work is well sought and important addition to the polymer literature. I would recommend publication of this article with some minor correction/ suggestion

1.       Introduction is missing some major and seminal citation of the current field such as Liu Y, Kumar S. Recent Progress in Fabrication, Structure, and Properties of Carbon Fibers. Polymer Reviews. 2012;52:234-58. Or Jian Hao, Yaodong Liu, Chunxiang Lu, Effect of acrylonitrile sequence distribution on the thermal stabilization reactions and carbon yields of poly(acrylonitrile-co-methyl acrylate),Polymer Degradation and Stability etc.

2.       Formulation copolymer distribution is not presented clearly, authors are recommended to add NMR spectra to show tell the initial copolymer distribution

3.       Fig 1 legends are not properly labeled creates confusion to readers, authors are recommended to add some text to the legends, same comment for Fig 2 and all other figures

 

4.       For two controlled atmosphere what is the rational between choosing them. We see one reaction dominates in inert atmosphere whereas in air atmosphere it choses different route. Is there any previous literature evidence that that air (20% O2/N2) helps to create any radical pathway?

 

Author Response

We are very thankful to the referee for very useful comments and questions. All of them were taken into account during revision of the text. All the changes in the text are marked in red. Below the response for the comments is given.

Comment 1: Introduction is missing some major and seminal citation of the current field such as Liu Y, Kumar S. Recent Progress in Fabrication, Structure, and Properties of Carbon Fibers. Polymer Reviews. 2012;52:234-58. Or Jian Hao, Yaodong Liu, Chunxiang Lu, Effect of acrylonitrile sequence distribution on the thermal stabilization reactions and carbon yields of poly(acrylonitrile-co-methyl acrylate),Polymer Degradation and Stability etc.

Response: We have thoroughly revised introduction and the mentioned references were added (ref. 5,6)

Comment 2: Formulation copolymer distribution is not presented clearly, authors are recommended to add NMR spectra to show tell the initial copolymer distribution

Response: We completely agree with the comment. The copolymers were analyzed by NMR spectroscopy (Fig.3, Table 3, Fig. S2). The triad composition has been estimated. The discussion was supplemented with the information about copolymer microstructure and its influence on the cyclization of the copolymers.

Comment 3: Fig 1 legends are not properly labeled creates confusion to readers, authors are recommended to add some text to the legends, same comment for Fig 2 and all other figures

Response: In most of the cases we have used the numbering of the samples from Table 1 as the numbering of the curves. It was done to shorten the capture of the figures. Otherwise, the captures become overloaded with the information. We have checked carefully the legend and the captures and revised them.

Comment 4: For two controlled atmosphere what is the rational between choosing them. We see one reaction dominates in inert atmosphere whereas in air atmosphere it choses different route. Is there any previous literature evidence that that air (20% O2/N2) helps to create any radical pathway?

Response: Thank you for the question. We have not seen such publications for the copolymers of acrylonitrile and acrylamide. Typically, the authors use vacuum, inert atmosphere or air. Meanwhile, there were some publications where the mixed atmosphere was used for copolymers of acrylonitrile with comonomers inert to cyclization. It should be interesting to study its influence on the cyclization of the copolymers containing comonomer,  which is able to accelerate cyclization.

Round 2

Reviewer 2 Report

The authors have improved the manuscript completely, so I suggest to published as it is.