Copolymerization of Ethylene and Methyl Acrylate by Dibenzocycloheptyl-Substituted Aryliminopyridyl Ni(II) Catalysts

Round 1

Reviewer 1 Report

The manuscript entitled “Copolymerization of Ethylene and Methyl Acrylate by Dibenzocycloheptyl-substituted Aryliminopyridyl Ni(II) catalysts” includes interesting research results using various Ni catalysts. However, there are several points to be improved in this current manuscript as follows.

1. The polymerization mechanism must be explained. How to produce linear and branched polymers in the polymerization?

2. Polymerization results using Ni catalyst without substitution group is interesting to compare the other polymerization results.

3. The molecular weight of polymers should be evaluated using gel permeation chromatography.

4. Control experiment without AlEt2Cl activation is interesting.

Author Response

REVIEWER 1

The manuscript entitled “Copolymerization of Ethylene and Methyl Acrylate by Dibenzocycloheptyl-substituted Aryliminopyridyl Ni(II) catalysts” includes interesting research results using various Ni catalysts. However, there are several points to be improved in this current manuscript as follows. 

1. The polymerization mechanism must be explained. How to produce linear and branched polymers in the polymerization?

Response 1: The polymerization mechanism leading to linear and branched polymers both in ethylene homopolymerization and copolymerization with methyl acrylate by Ni and Pd catalysts is well known in the literature and it is discussed in detail e. g. in refs. 5-8, 18-20. The mechanism leading to both in-chain and at the end of chain MA incorporation is well established in the literature (see e.g. refs 18-20). Statements concerning this matter have been added in the introduction.

2. Polymerization results using Ni catalyst without substitution group is interesting to compare the other polymerization results.

Response 2: This work addresses the performance in ethylene-MA copolymerization of iminopyridyl Ni complexes bearing bulky substituents at the imino moiety and no substituents at the pyridino moiety. A statement concerning a comparison with the previously reported Ni catalysts bearing substituents at both moieties (ref 28) has been added.

3. The molecular weight of polymers should be evaluated using gel permeation chromatography.

Response 3: GPC measurements have been done.

4. Control experiment without AlEt2Cl activation is interesting.

Response 4: A control experiment using complex 2 without AlEt2Cl activation resulted in no polymer formation. A statement has been introduced in the text.

Reviewer 2 Report

In this contribution, five dibenzocycloheptyl-substituentedaryliminopyridyl Ni (II) complexes with different ortho-substituents are tested in the copolymerization of ethylene with methyl acrylate. The effects of ortho-substituents on the catalyst activity, selectivity of methyl acrylate incorporation, and the effects of ethylene pressure are systematically studied. Given that Macromol aims to encourage scientists to publish the theoretical and experimental results in as much detail as possible, this manuscript is encouraged to enrich discussions according to the following questions and comments before making further decisions.

1. Iminopyridyl Ni(II) complexes produce only oily products soluble in methanol, but dibenzocycloheptyl-substituted aryliminopyridyl Ni(II) complexes produce solid polymers precipitated in acidified methanol (Reference 27 and Line 107). Although, both catalysts produce polymers with comparable molecular weights (i.e., Mn of 3.6 kDa vs. 3.4 kDa for Run 5 in Reference 27 vs. Run 1 in this manuscript), why do those resulting polymers show different solubility?

2.  In Figure 1 and S4, what do the vinylic protons correspond to? Unreacted monomers or unsaturated chain ends/moieties? In addition, in Figure 3 and 4, the proton peaks between 3.6 and 3.9 ppm are assigned to different types of methine in methyl acrylate repeating units. In Figure 3B, why are the minor peaks concentrated in the pentane-soluble fractions?

3. Why does the DSC analysis show two endothermic peaks? Are those two peaks ascribed to the melting of crystalline polyethylene or the evaporation of volatile components? Can the TGA-FTIR detect any volatile species when heating copolymers?

4. Why do the dibenzocycloheptyl-substituted aryliminopyridayl ligands enable higher molecular weights and lower branching of ethylene-methyl acrylate copolymers? What is the contribution of their steric hindrance? Why do the electronegative substituents at the arylimino moiety increase the methyl acrylate content and decrease the molecular weight?  

5. Increasing the pressure of ethylene from 6 atm to 30 atm, Complex 2 produces a copolymer with a lower methyl acrylate content, a lower molecular weight, and fewer branches (Run 2 vs. Run 11). However, Run 2 shows a major peak at 3.69 ppm associated with the methoxy protons of in-chain methyl acrylate at 6 atm (Figure 1). whereas, Run 11 shows three peaks of comparable intensities at 30 atm (Figure 6A). Why does high pressure promote more end-of-chain methyl acrylate?

6. Other amendments: 

Line 197, Chart 1 should be scheme 1.

Table 1, MAb should be MA^b, 1000 C'sc should be 1000 C's^c.

Line 120, define TCDE, 1,1,2,2-tetrachloroethane-d₂ or C₂D₂Cl₄.

Line 81, does DSC detect crystallization temperatures of the ethylene-methyl acrylate copolymers?

Author Response

In this contribution, five dibenzocycloheptyl-substituentedaryliminopyridyl Ni (II) complexes with different ortho-substituents are tested in the copolymerization of ethylene with methyl acrylate. The effects of ortho-substituents on the catalyst activity, selectivity of methyl acrylate incorporation, and the effects of ethylene pressure are systematically studied. Given that Macromol aims to encourage scientists to publish the theoretical and experimental results in as much detail as possible, this manuscript is encouraged to enrich discussions according to the following questions and comments before making further decisions.

1. 1. Iminopyridyl Ni(II) complexes produce only oily products soluble in methanol, but dibenzocycloheptyl-substituted aryliminopyridyl Ni(II) complexes produce solid polymers precipitated in acidified methanol (Reference 27 and Line 107). Although, both catalysts produce polymers with comparable molecular weights (i.e., Mn of 3.6 kDa vs. 3.4 kDa for Run 5 in Reference 27 vs. Run 1 in this manuscript), why do those resulting polymers show different solubility?

Response 1: The polymer or run 5 in ref 27 mentioned by the reviewer was an ethylene homopolymer with more branches than the sample of run 1 of this paper (73 branches per 1000 C's vs 47).

2.  In Figure 1 and S4, what do the vinylic protons correspond to? Unreacted monomers or unsaturated chain ends/moieties? In addition, in Figure 3 and 4, the proton peaks between 3.6 and 3.9 ppm are assigned to different types of methine in methyl acrylate repeating units. In Figure 3B, why are the minor peaks concentrated in the pentane-soluble fractions?

Response 2: The resonances of the unsaturated end groups of such branched low-molecular weight polyethylenes have been attributed in detail on the basis of literature assignments: a specific reference has been added, ref 32. The resonances of the different types of methine of MA units have been assigned by a comprehensive study involving 1-D and 2-D NMR analysis in ref 28. The soluble fraction of figure 3B (as well as the one of figure 6B) has a lower MW and reasonably a higher content of MA inserted at the end of chain. Also, the formation of traces of MA homo-oligomers cannot be ruled out, although the detailed experiments done in ref 28 showed that the NMR resonances are different from those of the copolymer fragments.

3. Why does the DSC analysis show two endothermic peaks? Are those two peaks ascribed to the melting of crystalline polyethylene or the evaporation of volatile components? Can the TGA-FTIR detect any volatile species when heating copolymers?

Response 3: The two peaks are due to the presence of fractions having different MW, degree of MA incorporation and branching content, as shown by the fractionation experiments. Accordingly, the two peaks are present also in the crystallization process during the cooling.

4. Why do the dibenzocycloheptyl-substituted aryliminopyridayl ligands enable higher molecular weights and lower branching of ethylene-methyl acrylate copolymers? What is the contribution of their steric hindrance? Why do the electronegative substituents at the arylimino moiety increase the methyl acrylate content and decrease the molecular weight?

Response 4: Dibenzocycloheptyl substituents reasonably function as many other bulky substituents in Ni-based catalysts which lead to higher molecular weight polymers disfavoring chain termination by associative displacement, as extensively discussed, e. g. in ref 6 and, for this class of complexes, in ref 31. In ref 31 the decrease of molecular weight of ethylene homopolymers for complexes having electronegative substituents at the arylimino moiety was also observed. A possible explanation implying a weak interaction between the coordinated olefin and the ortho-F in the transition state favoring beta-H elimination was proposed. In the case of ethylene-MA copolymerization, we have not discussed in detail the performance of complexes 4 and 5 because they are almost inactive.

5. Increasing the pressure of ethylene from 6 atm to 30 atm, Complex 2 produces a copolymer with a lower methyl acrylate content, a lower molecular weight, and fewer branches (Run 2 vs. Run 11). However, Run 2 shows a major peak at 3.69 ppm associated with the methoxy protons of in-chain methyl acrylate at 6 atm (Figure 1). whereas, Run 11 shows three peaks of comparable intensities at 30 atm (Figure 6A). Why does high pressure promote more end-of-chain methyl acrylate?

Response 5: It is not surprising that the content of end-of-chain MA is higher in short-chain copolymers. The lower selectivity of MA incorporation at higher ethylene pressure (and thus at lower [MA]), paralleling a lower molecular weight was also observed for the pyridylimino Ni complexes reported in ref 28 (cf run 2 vs run 15 of ref 28). In ref 28, we suggested that the concentration of MA affects the molecular weight of the copolymers through k-O coordination of MA to the Ni catalyst site, disfavoring the b-agostic alkyl Ni intermediates which are precursors of chain termination.

6. Other amendments: 

Line 197, Chart 1 should be scheme 1.

Table 1, MAb should be MA^b, 1000 C'sc should be 1000 C's^c.

Line 120, define TCDE, 1,1,2,2-tetrachloroethane-d₂ or C₂D₂Cl₄.

Line 81, does DSC detect crystallization temperatures of the ethylene-methyl acrylate copolymers

Response 6: These corrections have been done. Crystallization temperatures of the ethylene-methyl acrylate copolymers are detected in the DSC (see the SI).

Round 2

Reviewer 2 Report

My questions and comments have been addressed.