Review Reports - Mechanistic Pathways and Product Selectivity in Pyrolysis of PE, PP and PVC: A Foundation for Applied Chemistry in Europe

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper proposes a correlation framework of polymerization and depolymerization，which emphasizes the decisive role of polymer microstructure in pyrolysis pathways and product selectivity. However, it lacks an in-depth quantitative analysis of the correlation between the specific mechanisms by which different polymerization processes (e.g., differences between Ziegler-Natta catalysts and metallocene catalysts in coordination polymerization, and different initiator systems in radical polymerization) regulate polymer microstructure and subsequent pyrolysis behavior.

For instance, although it has listed that the linear structure of HDPE and the branched structure of LDPE lead to different pyrolysis product distributions, it does not clearly specify how the regulatory range of key parameters such as branching degree and stereoregularity during polymerization quantitatively affects the yield ratio of olefins, BTX, and paraffinic oils/waxes in pyrolysis products. It is recommended to supplement relevant quantitative research data or kinetic models to enhance the scientific validity and practicality of this framework.

Author Response

Thank you very much for your constructive comment and detailed suggestions.

In direct response to your concerns, we have substantially revised the manuscript and added a new dedicated subsection (line 454),

(2.3) “Quantitative Correlation of Polymerization Conditions, Microstructure and Resulting Product Selectivity”,

which systematically links polymerization method, catalyst system, initiator type, reaction conditions, and feed composition to key microstructural parameters (branching degree, tacticity, crystallinity, defect content). For each polymer family (HDPE, LDPE, PP, PVC), we now provide explicit quantitative ranges compiled from the recent literature and correlate these microstructural descriptors with pyrolysis product yields (olefins, paraffinic oils/waxes, BTX aromatics, gases, and HCl evolution), supported by new tables and mechanistic explanations.

Representative values and trends are summarized in Tables 1–3, which clarify how variations in branching, tacticity and defect levels quantitatively affect the relative amounts of oil/wax, light olefins and BTX under practical pyrolysis conditions. The new subsection also consolidates the influence of catalyst type, polymerization control variables (e.g., hydrogen addition, initiator concentration), and operating parameters such as heating rate and vapor residence time on product distribution, with brief mechanistic discussion and references to kinetic models where available.

If you see any remaining issues or further opportunities for improvement, we would be grateful for your additional guidance.

Reviewer 2 Report

Comments and Suggestions for Authors

This is a very interesting review work, systematizing the depolymerization process of HDPE, LDPE, PP and PVC. However, before it is accepted for publication, I recommend that you consider the following points:

Provide an explanation of each abbreviation before its first use in the manuscript, e.g., SP-PLP-EPR, Et₃Al, MAO, BTX. There are many abbreviations in the paper, including the abstract; consider adding the nomenclature at the end of the manuscript.
According to the abstract, the manuscript analyzes, among others, "...(temperature, heating rate, vapor residence time, pressure)." It seems that "heating rate" and "residence time" are only mentioned in general terms. Please add some specific values how i) vapor residence time, e.g., ACS Sustainable Chem. Eng. 2021, 9, 43, 14443–14450, and ii) and heating rate affect polymer degradation, e.g., Thermochimica Acta 749 (2025) 180023, or "vapor residence time"
Lines 326-327: "This pathway has a relatively low activation energy ⁻¹, making it kinetically favorable under high-pressure industrial conditions [96]." The energy value is probably missing here - there is only -1.
Some content and values are repeated,

lines 404-406 "Activation energies for termination processes vary depending on the mechanism and radical chain length. Disproportionation requires 25–39kJmol⁻¹, while recombination occurs at 18–24kJmol⁻¹ [106]." and lines 343-345 "Activation energy for termination is also chain-length dependent: short chains exhibit Ea of 25–39kJmol⁻¹, while longer radicals show values of 18–24kJmol⁻¹ [106]"
lines 395–403; 540–577; 779–818 describe the PVC, HDPE, and PP sections. The texts are nearly identical in structure, describing the same three mechanisms (recombination, disproportionation, crosslinking) and similar relationships (temperature, viscosity, radical concentration). I recommend shortening the descriptions for each polymer, avoiding repeating the full definitions of the mechanisms.
lines 984–992; 1074–1084. Both fragments report that silica–alumina and ZnO lower the activation energy, and HZSM-5 drives the reaction towards BTX.
In these sections (HDPE, LDPE, PP, PVC), the pattern repeats: initiation → propagation → termination, with very similar descriptions of the mechanisms. Perhaps it would be worth adding a summary section with a comparison table (indicating differences) instead of repeating the descriptions in the text?

5. Please complete or clarify some of the references in the bibliography so that nothing is missing (e.g. names of scientific journals, etc.). "Natta, G. Olefin Polymerization with Ziegler-Natta Catalyst. 1963, 1–6."; "doi:https://doi.org/10.1002/0471238961.0618050519011403.a01.pub2"; "Alghamdi, M.M.; Russell, G.T. On the Activation Energy of Termination in Radical Polymerization , as Studied at Low 1527 Conversion. 2024."; "Barner-kowollik, C.; Buback, M.; Egorov, M.; Fukuda, T.; Goto, A.; Friedrich, O.; Russell, G.T.; Vana, P.; Yamada, B.; 1529 Zetterlund, P.B. Critically Evaluated Termination Rate Coefficients for Free-Radical Polymerization : Experimental Methods. 1530 2005, 30, 605–643, doi:10.1016/j.progpolymsci.2005.02.001. "

Author Response

Comment 1:
Provide an explanation of each abbreviation before its first use in the manuscript, e.g., SP-PLP-EPR, Et₃Al, MAO, BTX. There are many abbreviations in the paper, including the abstract; consider adding the nomenclature at the end of the manuscript.
Response 1:
Thank you for this suggestion. We agree with this comment. Therefore, every abbreviation is now explained at its first occurrence in the manuscript text, including SP-PLP-EPR, Et₃Al, MAO, BTX, and all others. Additionally, a comprehensive list of abbreviations and nomenclature has been added at the end of the manuscript for clarity.

Comment 2:
According to the abstract, the manuscript analyzes, among others, "(temperature, heating rate, vapor residence time, pressure)." It seems that "heating rate" and "residence time" are only mentioned in general terms. Please add some specific values how i) vapor residence time, e.g., ACS Sustainable Chem. Eng. 2021, 9, 43, 14443–14450, and ii) heating rate affect polymer degradation, e.g., Thermochimica Acta 749 (2025) 180023, or "vapor residence time".
Response 2:
We agree with this comment. Specific values for heating rate and vapor residence time have been added and discussed in the revised manuscript. These include quantitative examples from referenced literature on their effects on polymer degradation (see section 2.3, page 17, lines 635-657).

Comment 3:
Lines 326-327: "This pathway has a relatively low activation energy ⁻¹, making it kinetically favorable under high-pressure industrial conditions ." The energy value is probably missing here - there is only -1.
Response 3:
Thank you for pointing out this issue. The missing activation energy value has been inserted in the revised manuscript (lines 339-341).

Comment 4a:
Some content and values are repeated,
lines 404-406 and lines 343-345: Descriptions of activation energies for termination processes are repeated.

Response 4a:
We agree and have removed duplicate sentences describing activation energies for termination. These are now concisely presented once with all quantitative details included (see lines: 356-358).

Comment 4b:
Lines 395–403; 540–577; 779–818: The texts are nearly identical in structure, describing the same three mechanisms (recombination, disproportionation, crosslinking) and similar relationships (temperature, viscosity, radical concentration).

Response 4b:
We agree and have shortened and clarified these polymer-specific mechanism descriptions. Now, the general definitions are given once and referenced in subsequent polymer sections, with each polymer’s unique features emphasized. (see lines: 409-418; 749-784;985-993.

Comment 5c:
Lines 984–992; 1074–1084. Both fragments report that silica–alumina and ZnO lower the activation energy, and HZSM-5 drives the reaction towards BTX.

Response 5c:
Thank you for your comment. In the revised manuscript, catalyst effects for each polymer have been consolidated into unified sections to remove duplication and clarify details. Activation energy values and product selectivity for major catalysts have been specifically added, and catalyst and initiator roles are now discussed in separate dedicated subsections (see lines:1123-1184;1270-1285).

Comment 5d:
Please complete or clarify some of the references in the bibliography so that nothing is missing (e.g. names of scientific journals, etc.).

Response 5d:
We have thoroughly reviewed the bibliography and completed all incomplete references, including adding missing journal names, publication years, volumes, issues, pages, and publisher details for books and chapters.