DMC-Mediated Copolymerization of CO₂ and PO—Mechanistic Aspects Derived from Feed and Polymer Composition

Round 1

Reviewer 1 Report

This paper entitled “DMC mediated Copolymerization of CO2 and PO - Mechanistic Aspects derived from Feed and Polymer Composition” describes the synthesis of poly(propylene ether carbonates) by the copolymerization of PO and CO2, promoted by a double metal cyanide (DMC) catalyst.

In the text, the authors present the a study of the homopolymerization of PO and the copolymerization of PO with CO2 in batch and semi-batch experiments, using a polypropylene glycol (PPG) as chain transfer agent. The aim was to evaluate the factors that bring to the formation of polyether linkages versus polyether-alt-polycarbonate linkages. In addition, they investigated also the formation of cyclic propylene carbonate as side product, and also the influence of the addition of amines, namely triethyl amine and dodecyl amine. The catalyst employed was a non-well-defined DMC catalyst, and in a specific section of the work, a modified version of the catalyst, namely DMC-NO3, in which the Cl atom of the original complex has been exchanged with a nitrate anion. In a final paragraph, the authors highlighted some mechanistic aspects derived from the results collected.

By carefully reading the manuscript, some questions arose, and also some aspects of the experiments/results should be improved in order to make them more clear and more exhaustive. 

These are the critical issues that I would like to highlight to the authors:

• the catalyst is not described at all. Even if the catalyst was not prepared by the authors, some more details should be given, for example with which metals is it prepared, in which form it is, or other details that can be extracted from the patents cited in the text. This would help the reader to get more into the details of the experiments; this is even more important when the DMC-Cl catalyst is modified to the DMC-NO3 (concerning this point, I’m still wandering how is it possible to modify something that it is almost unknown…).
  For example, I would suggest to move the elemental composition give in Table 8, paragraph 2.6 at the beginning of the Results section, so as to make a brief description of the DMC catalyst. It is important also to give some more examples reported in the literature in the introductory paragraph concerning DMC catalysts.
  In addition, so many metal-based catalysts for the copolymerization of ethers and CO2 are present in the recent literature: a good comparison would be useful.
  Lastly, as a reader, I would prefer read that it is not possible to give further details on the structure because of intellectual property or because the industry that produces it asked not to reveal too many details, rather than “The catalyst was a gift, its identity was not disclosed to us, known is that zinc dichloride was used in its synthesis.”
• A question concerning the catalyst: how can you say that is a new DMC catalyst if its identity was not disclosed to you??? I suggest to remove the wording “new”, it’s inappropriate.
• The results are not presented in a well-ordered manner, and for this they are rather difficult to be understood, and to detect the novelty reported in the article. Even after carefully reading the manuscript several times, it is difficult to understand the strength of this article, and hence to completely approve it. I mean:
  • the tables should report the reaction conditions as footnotes, and “entry x” should be added, both in the tables and in the text, to catch which of the experiments is commented in a specific point.
  • In Table 1 a T is reported, but it is not clear at all to which temperature it is referred to.
  • the results are generally reported in a chaotic manner, and the polymer characterization is not described within the text, sometimes nor in the tables.
  • the results reported in paragraph 2.4 should be introduced by a table summing up the experiments performed and the various conditions, because as they are reported now are unclear at all.
• In the introduction paragraph, I suggest to insert some more review articles that well present the topics discussed, rather than single articles that are not exhaustive enough.
• Considering polymer characterization, it is poor. Thermal characterization would be helpful, especially considering the field of applications for these materials (e.g. food packaging, as reported in the introductory paragraph).
• Paragraph 2: the first section of this paragraph concerns more about the introduction, rather than the specific results. I would suggest to make it shorter and move it to the first paragraph.
• Paragraph 2.5: how much amine do you add to each experiment? To which of the above described experiment did you add the amine? Please, give a better comparison with the tests without the base, and provide a better description of the results before giving the conclusion.
• Paragraph 2.6: a more detailed comparison of the results obtained with DMC-NO3 and those from DMC-Cl is necessary. Moreover, if I were you, I would investigate more in detail this catalyst, it seems promising considering the higher levels of CO2 incorporation.
• Paragraph 3: it would be really really helpful to add numbers to the intermediates reported in Schemes 4-6, and added accordingly in the text. Sometimes it is difficult to understand from which one the reader should start considering them. Moreover, since “No new reaction steps are introduced”, I would suggest to make this section shorter.

MINOR COMMENTS

• Scheme 2: use (a) and (b) instead of (l) and (r) only in the caption, it would make it more clear, and it would be helpful to add the “(a/b)” writings in the text.
• A question concerning this Scheme, part (r): what does happen from the adduct on the top to the one on the right? equilibrium arrows would be more appropriate in this case?
• Some times in the text abbreviations are reported but without being previously defined (e.g. PPG, PPC, etc).
• What does “resp.” mean? Please avoid.
• Figure 3: the axes must start at (0;0). Negative coordinates don’t make sense at all considering that they describe time and volume of CO2 added!!! Moreover, I would suggest to add a dashed line at 60 min, when the dosing of PO ended.
• Figure 5: does it make sense to consider so long time frames, considering that from 300 min on the situation remains almost unchanged, in particular for (a) section?
• Figure 8: use (a) and (b) instead of (top) and (bottom) only in the caption; moreover, the SEC profiles can be moved in the SI file, since they are not fundamental in the text.
• Paragraph 4. Voranol 2000 L is not reported in the text. It can be removed from this section.
• f value should be described better (for example by giving some numerical references), and especially its use should be avoided in the abstract, since it is not descriptive.

Author Response

Dear reviewer, thank you for your time and the valuable comments to our manuscript. We have tried to process your remarks as good as we can. Please find below the changes that have made along your comments. We hope, you find it satisfactory.

 

The catalyst is not described at all. Even if the catalyst was not prepared by the authors, some more details should be given, for example with which metals is it prepared, in which form it is, or other details that can be extracted from the patents cited in the text. This would help the reader to get more into the details of the experiments; this is even more important when the DMC-Cl catalyst is modified to the DMC-NO3 (concerning this point, I’m still wandering how is it possible to modify something that it is almost unknown…). For example, I would suggest to move the elemental composition give in Table 8, paragraph 2.6 at the beginning of the Results section, so as to make a brief description of the DMC catalyst.

The compounds used in the synthesis - Co(CN)₆^3- and ZnCl₂ - were added in 2.1, and reference to Table 9 (former Table 8) is given for the compositional data (moving the Table seemed unattractive, losing the direct comparison to DMC-NO3). An XRD is added to the supplement (Figure S7), this would allow an unambiguous identification of the catalyst, i.e. together with the morphology in Figure 8 and the composition in Table 9.

Modification: DMC solids are extremely insoluble in water, and anion exchange at the surface appears possible by the procedure described. It thus was expected that the core of the solid would not change by dispersing in water, and it was hoped that the chloride anions at the surface could be replaced by exposure to a massive amount of nitrate anions. As it turns out, this seems to have worked.

 

 It is important also to give some more examples reported in the literature in the introductory paragraph concerning DMC catalysts. In addition, so many metal-based catalysts for the copolymerization of ethers and CO₂ are present in the recent literature: a good comparison would be useful.

More references were added:

Ln 44: for other epoxides see [16] ()

Ln 66: reference [49] to the review by Professor Williams is now also cited in the introduction

Ln 77: a further reference was added after an additional sentence: Also blockcopolycarbonates can be obtained by the strategy as was recently summarized in an extensive review, comprising many mechanistic considerations.[58]

Ln 92: an extensive review on DMC was now cited earlier in the introduction [63]

Ln 94: a reference to DMC catalysis in the context of immortal polymerization was added with the text: “The immortal copolymerization of CO2 and epoxides can also be mediated by zinc carboxylate and DMC catalysts, giving a product with the signature of both catalysts.[68]”

A comparison of many catalysts for the copolymerization is available in recent literature (e.g. by professors Williams, Srinivas and Zhang) was not explicitly elaborated except for the general observation of alternating and non-alternating copolymers already given. With respect to the specific comparison no new attempt is made, rather it is pointed to the recent excellent reviews available in the open literature. It was the objective of the current investigation to work out the general mechanistic differences in propoxylation and carboxy propoxylation, and the neutral and anionic intermediates.

    Lastly, as a reader, I would prefer read that it is not possible to give further details on the structure because of intellectual property or because the industry that produces it asked not to reveal too many details, rather than “The catalyst was a gift, its identity was not disclosed to us, known is that zinc dichloride was used in its synthesis.”

Text was modified accordingly: “The catalyst was kindly supplied to us under restrictions of analysis and non-disclosure obligations, known is that zinc chloride and the hexacyano cobaltate trianion Co(CN)₆^3-were used in its synthesis (c.f. Figure S7).

    A question concerning the catalyst: how can you say that is a new DMC catalyst if its identity was not disclosed to you??? I suggest to remove the wording “new”, it’s inappropriate.

“new” in connection to the catalyst was removed as its identity cannot be disclosed (although this is a new DMC catalyst, as we were informed, an XRD is added to the Supplement).

        the tables should report the reaction conditions as footnotes, and “entry x” should be added, both in the tables and in the text, to catch which of the experiments is commented in a specific point.

Running entry numbers were added to all Tables, and reference to the entries is given throughout the text. Reaction conditions (type of CTA, amount, dosing rates and times, amount of catalyst) were made specific and put in Table footers. Polymer product characteristics are added to the Tables.

 

        In Table 1, a T is reported, but it is not clear at all to which temperature it is referred to.

Yes, thank you for the remark, the T column was removed from Table 1 as it is undefined, the explicit reaction conditions are now given in the insert of Figure 1, the Table reports now the polymer properties.

        the results are generally reported in a chaotic manner, and the polymer characterization is not described within the text, sometimes nor in the tables.

The polymer characteristics in terms of Mn, PDI and F were added to the respective Tables, and a new Table was added to the Supplement. The products are all liquids at these masses and the thermal properties follow the Flory-Fox equation. This was added as a comment in lines 134-136, see below

        the results reported in paragraph 2.4 should be introduced by a table summing up the experiments performed and the various conditions, because as they are reported now are unclear at all.

A Table with details was added, reference to experiments and entries are now given.

    In the introduction paragraph, I suggest to insert some more review articles that well present the topics discussed, rather than single articles that are not exhaustive enough.

See above

    Considering polymer characterization, it is poor. Thermal characterization would be helpful, especially considering the field of applications for these materials (e.g. food packaging, as reported in the introductory paragraph).

See above with respect to the matter of mass, its distribution and carbonate content F of all products was now offered in Tables. The matter of thermal properties is not so meaningful as these will dependent on the molecular weight and the carbonate content in most of the products. The dual dependence has been published and can be taken from ChemistryOpen 2019, doi10.1002/open.201900135. It follows with respect to copolymer composition the Fox-Flory equation. Amendment: in line 134, the following statement was added: The products of this study all are liquids, the thermal properties are dependent on the molecular weight and the composition.

    Paragraph 2: the first section of this paragraph concerns more about the introduction, rather than the specific results. I would suggest to make it shorter and move it to the first paragraph.

The matter on induction time was moved to the section 2.5 (line 485-495), where the induction time with added amines is treated. The batch polymerization was incorporated into 2.1 (line 130-141). The length was not changed, the text works appropriate to us this way.

    Paragraph 2.5: how much amine do you add to each experiment? To which of the above described experiment did you add the amine? Please, give a better comparison with the tests without the base, and provide a better description of the results before giving the conclusion.

The amount of amines is in Table 7. The entry 35 is the blank experiment. Text was amended to “Generally, PO consumption in time is the slower, the more TEA is present (Figure 7a, Table 76)” to make a connection to the slower conversion in the presence of TEA. The sentence with “Furthermore…” was removed. It indeed was confusing. The description should be more accessible now.

    Paragraph 2.6: a more detailed comparison of the results obtained with DMC-NO3 and those from DMC-Cl is necessary. Moreover, if I were you, I would investigate more in detail this catalyst, it seems promising considering the higher levels of CO2 incorporation.

Research on DMC-NO₃ is ongoing, and the approach of changing the anions in DMCs to change their catalytic action will be placed in a boarder context when that investigation is finished. It possibly will apply to many DMCs.

A Figure 9 is offered, showing the broadening with increasing yield for conditions giving a the highest carbonate content of F ~ 0.45. A text was added “The distribution broadens to the high molecular weight side, increasingly with the molecular weight. The broadening is like observed for DMC-Cl (Figure 6b), but much more extensively and already at much lower molecular weights.” (line 570)

    Paragraph 3: it would be really really helpful to add numbers to the intermediates reported in Schemes 4-6, and added accordingly in the text. Sometimes it is difficult to understand from which one the reader should start considering them. Moreover, since “No new reaction steps are introduced”, I would suggest to make this section shorter.^

Thank you for the suggestion: The structures in Schemes 5 & 6 have been labeled with N# for non-basic DMC surfaces and B# for basic DMC surfaces. Accordingly, references in the text to structures were added. We also find that this strongly enhances the accessibility. Scheme 4 was not changed, it is more self-explanatory and has not such a detailed description in the main body of text. We could not see that a shortening would improve the understanding, the details of the reaction pathways are central to the observation reported upon, and actually was the purpose of the investigation.

 

MINOR COMMENTS

    Scheme 2: use (a) and (b) instead of (l) and (r) only in the caption, it would make it more clear, and it would be helpful to add the “(a/b)” writings in the text.

Thank you, this was carried out, reference to the difference was made in section 2.5, first paragraph (line 480ff).

    A question concerning this Scheme, part (r): what does happen from the adduct on the top to the one on the right? equilibrium arrows would be more appropriate in this case?

RO^- dissociates from the surface and attacks the coordinated PO. The formulation given is the forward reaction towards the transition state with already dashed bonds in coordinated PO. The caption to the Scheme was extended to point to the equilibrium: ” Scheme 2. Proposed reaction routes in DMC mediated propoxylation of alcohols, external attack of ROH and proton transfer (a) and “insertion” of PO in a zinc alkoxide bond after (reversible) RO- dissociation (b). Formulated in the essence of single zinc site catalysis for simplicity, more interactions will be present; structures would be derived from the 001/100 plane of DMC crystals.[69–71]”

    Some times in the text abbreviations are reported but without being previously defined (e.g. PPG, PPC, etc).

The abbreviations were overhauled and defined on first use. PPG is referring to polypropylene carbonate, PPC to alternating polypropylene carbonate and PPEC to polypropylene ether carbonates.

    What does “resp.” mean? Please avoid.

 It means respectively, the abbreviation was replaced by the full wording.

    Figure 3: the axes must start at (0;0). Negative coordinates don’t make sense at all considering that they describe time and volume of CO2 added!!! Moreover, I would suggest to add a dashed line at 60 min, when the dosing of PO ended.

This has been accordingly altered, a dashed line marks 60 min.

    Figure 5: does it make sense to consider so long time frames, considering that from 300 min on the situation remains almost unchanged, in particular for (a) section?

The Figure pertains to the whole experiment while feeding PO, the fact that no larger changes occur shows that a steady state is reached with respect to PO consumption and dosing are about the same, and the composition of the reaction mixture with respect to PPEC and cPC are not changing anymore. It shows the uniformness of the reaction. It is therefor not changed.

Figure 8: use (a) and (b) instead of (top) and (bottom) only in the caption; moreover, the SEC profiles can be moved in the SI file, since they are not fundamental in the text.

The Figure was replaced into the supplement, and accordingly modified; reference to the Figure S6 was added in the main text.

    Paragraph 4. Voranol 2000 L is not reported in the text. It can be removed from this section.

Thank you, it was removed, details are now added in footers under the Tables.

    F value should be described better (for example by giving some numerical references), and especially its use should be avoided in the abstract, since it is not descriptive

A small text was inserted in the main body at line 182ff, exemplifying F “F thus runs from 0 for a polyether to infinitive for alternating PPC, and reaches the value of 1 for PPEC with equal molar amounts of carbonate and ether linkages”. F is defined in the abstract too. The abstract has limited wording, intermediate defining of F gives the possibility to put more information into it, that is why we prefer not to change it.

Reviewer 2 Report

The authors submitted "DMC mediated Copolymerization of CO2 and PO - Mechanistic Aspects derived from Feed and Polymer Composition" to publish in Catalysts. In this manuscript, the authors investigated of the different (co)polymers and way of comonomer incorporation into macromolecules on the properties copolymers. The topic of this research was important, and the results were also interesting; however, the quality of this manuscript should be further improved before acceptance. I have the following suggestions for improvement of the manuscript.

1. In the page 5, lines 160-162, it is said that “This is typically the result of conditions were PO diffusion is faster than that of the chain ends, a process which is self-enhancing as longer chains have a longer diffusion time and then grow faster once in the vicinity of the catalyst and then become still slower to diffuse away (and thus to grow further).” The diffusion of monomer in reaction feed is important in case of polymerization reactions. However, the molecular weight is also dependent on different kind of active centre as well as on influence of chain termination reaction on the polymerization process. In addition, in Fig. 1 two peaks can be observed which suggest the presence of different active centre. The authors could add further explanation of this phenomenon.
2. The PPG, PPC and ?(=????????ℎ??) should be explained in the text where they first occur.
3. In the page 5, lines 167-169, it is said that “The analysis of the products indicate that only small amounts of CO₂ get incorporated into the polymer chain. Incorporation is more or less independent on the process conditions as polymer composition ?(=????????ℎ??)⁄ adds up to 0.14 at 10 bar and at 36 bar of CO₂ pressure.” The is no result related to copolymerization under 36 bar of CO₂ pressure. The authors did not prove this statement.
4. The shortcuts of CTA, Poly 1a, Poly 1b, Poly 1c in Table 1 should be changed in order to improve clarification.
5. In the page 5, lines 175-177, it is said that “Another experiment in which CO₂ was added just after the exotherm started to arise shows that even then CO₂ can be used to slow down the propoxylation. The increase in temperature could be limited to 130 °C instead reaching the 220 °C (Figure 1d).” In my opinion is not consistent with the data provided.
6. In the page 7, lines 242-243, it is said that “Lower temperatures give a lower PDI which may be related to the lower viscosities of the reaction mixture at lower polymer concentration and in part to the lower importance of backbiting.” please add further explanation on it.
7. In page 9, line 296 in my opinion the average molecular weight of Lupranol 1200 of 450.

Author Response

Dear reviewer, thank you for your time and the valuable comments to our manuscript. We have tried to process your remarks as good as we can. Please find below the changes that have made along your comments. We hope you find the manuscript improved upto standards.

 

    In the page 5, lines 160-162, it is said that “This is typically the result of conditions were PO diffusion is faster than that of the chain ends, a process which is self-enhancing as longer chains have a longer diffusion time and then grow faster once in the vicinity of the catalyst and then become still slower to diffuse away (and thus to grow further).”

The diffusion of monomer in reaction feed is important in case of polymerization reactions. However, the molecular weight is also dependent on different kind of active centre as well as on influence of chain termination reaction on the polymerization process. In addition, in Fig. 1 two peaks can be observed which suggest the presence of different active centre. The authors could add further explanation of this phenomenon.

Thank you for the suggestion: the presence of various catalytic centers is not easy to detect in a living polymerization as chains grow on all the centers, and would thus carry the signature of all catalytic centers. The usual interpretation for this type of bimodal distributions is a diffusion limitation for a fraction of the chains that do not grow proportionally to others and a another part that progressively picks up more and more PO: PO has a faster Brownian motion than the chains (whole coils), the difference becoming larger with increasing molecular weight. The text was amended in the form of an extra sentence: “The presence of various catalytic sites is unlikely an explanation as chains would grow an all centers in an immortal polymerization (that can be achieved with the catalyst).”  

    The PPG, PPC and (=????????ℎ??) should be explained in the text where they first occur.

The abbreviations were overhauled and defined on first use. PPG is referring to polypropylene carbonate, PPC to alternating polypropylene carbonate and PPEC to polypropylene ether carbonates. The factor F was further exemplified in line 182ff: F thus runs from 0 for a polyether to infinitive for alternating PPC, and reaches the value of 1 for PPEC with equal molar amounts of carbonate and ether linkages.

    In the page 5, lines 167-169, it is said that “The analysis of the products indicate that only small amounts of CO2 get incorporated into the polymer chain. Incorporation is more or less independent on the process conditions as polymer composition (=????????ℎ??)⁄ adds up to 0.14 at 10 bar and at 36 bar of CO2 pressure.” The is no result related to copolymerization under 36 bar of CO2 pressure. The authors did not prove this statement.

The number F at 10 or 36 bars of CO2 is equal, the text was amended to include “both”. A Table S1 with more data is offered in the Supplement, reference to Supplement was added. 

    The shortcuts of CTA, Poly 1a, Poly 1b, Poly 1c in Table 1 should be changed in order to improve clarification.

The order of experiments in Table 1 and Figure 1 and 2 was unified. Details on the CTA were added below the Table, and the denomination was changed to PPG_1, _2, and PPEC. The legend (insert) to Figure 2 was accordingly modified.

    In the page 5, lines 175-177, it is said that “Another experiment in which CO2 was added just after the exotherm started to arise shows that even then CO2 can be used to slow down the propoxylation. The increase in temperature could be limited to 130 °C instead reaching the 220 °C (Figure 1d).” In my opinion is not consistent with the data provided.

This was checked, the maximum T in the reactor log of the experiment in Figure 1d was 132°C. The text was amended accordingly in line 181.

  In the page 7, lines 242-243, it is said that “Lower temperatures give a lower PDI which may be related to the lower viscosities of the reaction mixture at lower polymer concentration and in part to the lower importance of backbiting.” please add further explanation on it.

The text was expanded by “at those temperature. The molecular weight distribution may broaden through differences in diffusivity of PO monomer and CTA, which is smaller for low molecular weight CTAs (else gradients in molecular weight near and far from the catalyst crystals may arise on account of differences in propoxylation propensity, PO being more mobile, and only reacting with catalyst near hydroxyls).”   

    In page 9, line 296 in my opinion the average molecular weight of Lupranol1200 of 450

That is true; we decided to report the molecular weights as relative values against Polystyrene standards, this is now uniformly noted. This choice is made as the KMHS constants of the PPECs are not established, and a universal calibration correction to the real mol. mass is not available: in this way, at least a common reference frame is present.

Round 2

Reviewer 1 Report

The manuscript has been greatly improved now.

I would like to suggest minor comments that I found reading the whole manuscript again:

lines 177-178: "It is generally experienced that the presence of CO2
178 substantially decreases the activity of the DMC catalysts for PO ring-opening" if available, references should be added to support this claim

Figure 3a: the dashed line makes sense only if at 60 mins something happens, like in the case of Fig. 3b where at 60 mins the PO dosing ends. I would remove it from Fig. 3a.

And here a few typing errors: 

line 59 and 67: Inoue is written in all-capital letters

line 291: "and" instead of "und"

line 326: remove "of"

line 332: "above" instead of "over"

line 554: "which" instead of "with"

 

Best wishes

Author Response

Dear Reviewer, we are grateful for the renewed intense care of the manuscript. With respect to your comments:

lines 177-178: "It is generally experienced that the presence of CO2 substantially decreases the activity of the DMC catalysts for PO ring-opening" if available, references should be added to support this claim

The following reference was added:

Ln 178: eases the activity of the DMC catalysts for PO ring-opening,[73]

Reference 73: Robertson, N.J.; Qin, Z.; Dallinger, G.C.; Lobkovsky, E.B.; Lee, S.; Coates, G.W. Two-dimensional double metal cyanide complexes : highly active catalysts for the homopolymerization of propylene oxide and copolymerization of propylene oxide and carbon dioxide. Dalt. Trans. 2006, 5390–5395, doi:10.1039/b607963f

Although I have been working with DMCs since the early 2000 years, and the fact was already known at that time, little direct documentation on the CO2 inhibition can be found. The reference given at least discusses this observation.  

 

Figure 3a: the dashed line makes sense only if at 60 mins something happens, like in the case of Fig. 3b where at 60 mins the PO dosing ends. I would remove it from Fig. 3a.

The dashed line was a request from another reviewer, it is hard to make a choice what is better. In the end we decided to keep Fig 3a and 3b more uniformly, and keep the dashed line aso in Fig 3a: it at least indicates where the change of the saturation of the mixture with CO2 and reaction with CO2 are about to expect.

 

And here a few typing errors:

Thank you, these were all taken care of.

line 59 and 67: Inoue is written in all-capital letters

line 291: "and" instead of "und"

line 326: remove "of"

line 332: "above" instead of "over"

line 554: "which" instead of "with"

 

I hope you may find the manuscript publishable in the current form.

Thank you in advance, Gerrit Luinstra