Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Simple Potassium Amidate Complexes Containing Polycyclic Aryl Group†
Round 1
Reviewer 1 Report
This is a paper that describes a series of amide associated K complexes and their application in ring-opening polymerization of L-LA and rac-LA. If the author can provide the data I think is needed and answer my questions, I suggest that this paper can be accepted.
1. In the beginning the authors should mention that rac-LA ROP to produce isotactic PLA with high melt point is the necessary for PLA application. However, the authors should briefly introduce the method of ROP by using metal catalysts. The following suggested review to justify the use of metal catalysts as the main method for synthesizing PCL.
Coordination Chemistry Reviews 475 2023 214847
Polym. Chem., 2022, 13, 1618–1647
Dalton Trans., 2022, 51, 1241–1256
Coordination Chemistry Reviews 392 (2019) 83–145
Catalysts 2020, 10, 800
Chin. J. Chem. 2020, 38, 282－286
Org. Process Res. Dev. 2020, 24, 1435−1442
Dalton Trans., 2020, 49, 16533–16550
2. In Figures 1 and 2, what are a, b, and c? For the packing diagram, the authors should remodify it. I cannot understand how the repeat unit bridge together.
3. The data of LA ROP in hexane should be removed because LA and PLA cannot dissolve in hexane. This data is useless.
4. The authors mentioned that The conversion by K1-K4 are following in the order: K3(NO2) > K2(Br) > K1(H) > K4(CHPh2), but K6 (OCH3, 95%) > K5 (CH3, 85%) > K1(H,78%), K7(F, 78%). Why? The authors should explain the opposite catalytic tendency between R1 and R2 of these K complexes.
5. In Page 6, line 176, the authors mentioned that “the bigger π-conjugated complexes not showing higher activity”. However, in Page 11, line 265-267, “Generally, all these complexes K1-K10 showed better activity than K11 bearing phenyl ring, to some extent indicating the bigger conjugated aryl ring may help to increase activity.” I'm confused about whether the bigger conjugated aryl ring help or didn’t help to increase activity.
6. I can’t entirely agree with the path 1 in possible mechanism (Scheme 2). If path 1 is possible, the authors should obtain the PLA (run 9 of Table 2) without MeOH quenching, and producing PLA was analyzed by MALDI-TOF. The PLA with amide end chain should be observed. In addition, when amide initiated LA and was far away from K atom, amide cannot influence the catalytic activity of K catalyst. After that, the polymerization rate of all K catalysts should be the same. Herein, the authors should investigate the kinetic polymerization rate of these K complexes and prove it. Moreover, if amide was far from K atom, amide cannot influence K catalysts to reveal the selectivity of producing rac-PLA, but Table 4 revealed the opposite result.
7. The authors indicate that selectivity decreases with increasing polymerization time because of the increasing side reaction within longer time. Why? The authors should clearly explain the connection between the two.
The grammar and typesetting of the manuscript need to be confirmed.
Author Response
please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript describes on the iso-selective and mechanistic studies of ring-opening polymerization of rac-lactide catalyzed by some potassium-based complexes. The authors intend to demonstrate the π interactions between the potassium and aryl ring will enhance the activity and stereoselectivity for ROP of rac-lactide. This research article is quite interesting for those doing ring-opening polymerization of cyclic esters. According to the contents described, this reviewer recommends this article to be published in this journal after some revisions (listed below).
(a) The elemental analysis data (or references for preparation) should be reported for L1-L10 and K1-K10.
(b) Based on the data reported in this manuscript, the mechanisms reported in Scheme 2 should be re-considered.
For the case of presence of BnOH, the plausible mechanism should be ligand assisted activated monomer mechanism (reference: Inorg. Chem. Front., 2017, 4, 261–269).
For the case of absence of BnOH, the monomers could be activated by potassium complex followed by the insertion of the monomers. There is no evidence for the ligand end-caped polymers. The products are quite weird after hydrolysis.
(c) The R values for K10 are too high for the data collection under 170K.
(d) Some wrong typing should be corrected. (ex. line 42, 81, ‘steroselectivity’ should be ‘stereoselectivity’; line 87, ‘compound’ should be ‘compounds’…..etc.).
Author Response
To Referee 2:
Qa: The elemental analysis data (or references for preparation) should be reported for L1-L10 and K1-K10.
Answer: Thank you very much for your kind suggestions. 1H NMR and 13C NMR are fine.
Qb:Based on the data reported in this manuscript, the mechanisms reported in Scheme 2 should be re-considered. For the case of presence of BnOH, the plausible mechanism should be ligand assisted activated monomer mechanism (reference: Inorg. Chem. Front., 2017, 4, 261–269). For the case of absence of BnOH, the monomers could be activated by potassium complex followed by the insertion of the monomers. There is no evidence for the ligand end-caped polymers. The products are quite weird after hydrolysis.
Answer: Thank you very much for your kind suggestions. The proposed mechanism were revised. In the presence of BnOH, it was ligand assisted activated monomer mechanism. For the case without BnOH, the mechanism is proposed as coordination insertion ring expansion mechanism, the back bitting will lead to cyclic product or quenching with MeOH afforded the linear polymer capped with MeO. The related references were added and revised.
Qc: The R values for K10 are too high for the data collection under 170K.
Answer: Thank you very much for your kind suggestions. Although the R-value is high, CIF-check reports showed that there are no type A or type B errors in the crystal. We think that the structure of K10 was fine.
Qd: Some wrong typing should be corrected. (ex. line 42, 81, ‘steroselectivity’ should be ‘stereoselectivity’; line 87, ‘compound’ should be ‘compounds’…..etc.).
Answer: Thank you very much for your kind suggestions. We corrected the mistakes.
Reviewer 3 Report
1. Source and purity of all chemicals used should be specified in the experimental section.
2. In all the figures, the authors have not marked a,b,c in each map.
3. In fig.2c, the authors have mentioned the packing interactions, but not discuss it in detail for the interactions.
4. as to the ring opening polymerization, some work could be cited, such as Polym. Chem., 2022, 13, 2351–2361; J. Org. Chem. 2019, 84, 14627−14635 and Org. Chem. Front., 2020,7, 3515-3520
5 Some of the figures should be updated due to its poor resolution.
Author Response
Q1: Source and purity of all chemicals used should be specified in the experimental section.
Answer: Thank you very much for your kind suggestions. We added a description of the source of the chemicals.
Q2: In all the figures, the authors have not marked a,b,c in each map.
Answer: Thank you very much for your kind suggestions. We have modified the figures.
Q3: In fig.2c, the authors have mentioned the packing interactions, but not discuss it in detail for the interactions.
Answer: Thank you very much for your kind suggestions. In this part, we only show the specific crystal morphology of the complex and some discussions were added.
Q4: as to the ring opening polymerization, some work could be cited, such as Polym. Chem., 2022, 13, 2351–2361; J. Org. Chem. 2019, 84, 14627−14635 and Org. Chem. Front., 2020,7, 3515-3520
Answer: Thank you very much for your kind suggestions. These references are not related with this manuscript; therefore, we cannot cite them in the manuscript.
Q5: Some of the figures should be updated due to its poor resolution.
Answer: Thank you very much for your kind suggestions. We have checked the all the figures and some were updated.
Reviewer 4 Report
The manuscript describes a series of new potassium complexes for the ring-opening polymerization of lactide. The synthesized compounds were well characterized by different techniques such as NMR, MALDI-TOF, DSC, XRD, and GPC. Synthesized complexes showed high catalytic activity even at low temperatures with good stereoselectivity. I think this paper will be of certain interest to scientists (especially to polymer chemists) and can be accepted for publication after major revision according to the comments given below:
1) It is difficult to understand the chemical structures in Chart 1. They are so close to each other. The same for Scheme 1. The upper left structure in Scheme 1 is incorrect due to the absence of H at the nitrogen atom.
2) From the experimental part, 2 equivalent of potassium tert-butoxide was used to treat L1-L10.
3) How was the instability of complexes in CDCl3 and d6-benzene established? Does this mean that the complexes are also unstable in THF and MeOH? What about toluene? Could this instability be due to the presence of water in the solvents?
4) Thermal stability measurements are required for a series of new compounds K1-K10 due to their specific application (ROP of LA above room temperature).
5) Does the benzyl alcohol react with K1-K10? Probably, 1H NMR in DMSO could help to understand this.
6) Add more details about the polymerization procedure, because the solubility of LA in toluene is very low at room temperature. In the article, polymerization was provided at a 5.0 M solution of LA in toluene. To the best of my knowledge and experience, the appropriate concentration for the ROP polymerization of LA in toluene at 80 deg C is about 1.0 M, which is limited by the monomer solubility. Thus, the catalyst (initiator) should be injected into the reactor after the complete dissolution of LA. Just an example from literature: crude-lactide solubility in toluene at 23ºC is 70 mg per mL (Kazuomi K. US Patent: Process for producing lactides and process for purifying crude lactides, US5463086A).
7) Typo Mn calcd in the head of Table 1. What do table captions (Conditions:.. 2 mL toluene.) mean? Is it the total amount of toluene?
8) Add GPC curves to show bimodal distribution (lines 145-148) in Supplementary Information.
9) Line 152-153: the concentration of monomer is the same. The difference in the ratio between components.
10) I don’t agree with a conclusion about broad distribution (line 161).
11) The text does not contain references to Figure 4, Figure 5, Figure 6, or Scheme 2. Refer to Path I in Scheme 2 in the text (lines 207-215).
12) The obtained results in part 2.3 about two possible mechanisms can explain the results in part 2.2, such as the conclusion in line 147.
13) The next question: is it possible to calculate the percentage of chains obtained by Path I and Path 2 based on the 1H NMR? Maybe the calculation of functionality (doi: 10.1002/pola.27110) can help.
14) I think the suggestion of why the PDI for Run 7, Table 3 is so low and does not fit the trend should be provided. Is it just a typo or error?
15) Conclusion in lines 249-250: is it on side transesterification reaction? Or the K2-K10 is incorporated into the polymer chain due to polymerization time (like chain transfer to catalyst – Path II).
16) Is it possible to perform integral 1H NMR analysis for 4-dibenzhydrylnaphthalen (line 382) in CDCl3 due to overlapping signals at the 7.26 ppm region?
17) Pm=0.64 is not “much” higher than Pm=0.61.
18) How does the polarity of the solvent affect stereoselectivity? Line 262. Typo at steroselectivity.
19) Table 3: caption f is not used.
20) The DSC results are not entirely clear. Do polymers have a glass transition temperature?
Generally, the more isotactic polymer has a more clear melting signal. The polymers obtained with K2, K8, and K10 are almost identical but with different PDI and Pm. Thus, the observation of DSC results does not follow the Pm trend and should be explained.
21) I’m not sure if the grouping of references (26a, 26b, 26c) is allowed for this journal.
22) A lot of problems with the experimental part:
*) not all chemicals were listed(rac-LA, dichloromethane, naphthylamine, and etc.);
*) ε-CL was indicated (line 344) but not used in the work;
*) GPC – what is the vendor's name, what are the columns (precolumns), and what vendor of polystyrene standards;
*) somewhere missing space (400MHz, 0.63g, 0.79g, 5mmol), somewhere extra space (N-( dibenzhydrylnaphthalen-1-yl));
*) incorrect number of significant figures(sf) was used throughout the chapter: for example, 0.99 g, 5.3 mmol (2 sf), not 5 mmol (1 sf) at line 370;
*) wrong numbers: the catalytic amount of zinc chloride (5.2 g, 4 mmol) at line 378 (probably 0.52 g of ZnCl2); 4-methylbenzoyl chloride (3.98 g, 20 mmol) at line 395 must be 26 mmol; and more.
*) petroleum tetrahydrofuran acetate (v : v = 30 : 1) is wrong (line 381), probably petroleum ether-ethyl acetate;
*) wrong yield for L7 (from 1.49 g should be 38%, not 80%);
*) How has DSC analysis been performed?
Thus, the whole experimental part should be revised accordingly. I have only provided examples.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I am satisfied with the author's response and modification. I suggest accepting this manuscript.
Author Response
Thank you very much for your kind comments.
Reviewer 2 Report
The citation format is not consistent for references 6, 7, 8, 9 and 18. Please check it.
Author Response
Q1: The citation format is not consistent for references 6, 7, 8, 9 and 18. Please check it.
Answer: Thank you very much for your kind suggestions. We have corrected the format of references 6, 7, 8, 9 and 18.
Reviewer 4 Report
Thank you very much for your kind answers to my questions. I am sure that this is a terrific manuscript that should be published.
I only have two points to add:
1) Disagree with the conclusion about "a very broad distribution" (lines 164-167). It's not "very", it is just broad or broader.
2) Indicate how the Mncalcd was obtained (maybe reference, or formula, or figure in SI).
Author Response
Q1: Disagree with the conclusion about "a very broad distribution" (lines 164-167). It's not "very", it is just broad or broader.
Answer: Thank you very much for your kind suggestions. We changed the description about that.
Q2: Indicate how the Mncalcd was obtained (maybe reference, or formula, or figure in SI).
Answer: Thank you very much for your kind suggestions. Actually the formula of Mncalcd was listed in the table footnotes, Mn(calcd) = M(LA) × conversion × ([LA]0/[BnOH]0)+MBnOH. There were no contents about Mncalcld in supporting information, therefore, the how the Mncalcd was obtained were not added in SI.
