DFT investigations on the ring-opening polymerization of substituted cyclic carbonates catalyzed by zinc-{β-diketiminate} complexes

The ring-opening polymerizations of γ-methyl substituted six- and seven-membered ring cyclic carbonates, namely trimethylene carbonate (TMC-γMe) and the one-carbon larger homologue tetramethylene carbonate (7CC-γMe), using [(BDIiPr)Zn(N(SiMe3)2)] (BDIiPr = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)-imino)-2-pentene) as a catalyst precursor in the presence of benzyl alcohol (BnOH) as an initiator have been investigated by means of density functional theory (DFT) calculations. This computational study highlights the very active nature of the zinc-alkoxide catalyst obtained via alcoholysis of the Zn–N bond of [(BDIiPr)Zn(N(SiMe3)2)] by HOBn, leading to the replacement of N(SiMe3)2 by an –OBn alkoxide with concomitant release of HN(SiMe3)2. For both TMC-γMe and 7CC-γMe, the initiation and the propagation steps occur according to a three-step process: first, a nucleophilic attack of the alkoxide group followed by the ring-opening of the tetrahedral intermediate and finally the decoordination of the carbonate arm. In the methyl-substituted monomers, the presence of a stereogenic center and, more importantly, the dissymmetry of the monomers raise issues of the stereo- and regio-selectivity during the ROP process. In agreement with experimental results, for both carbonates, the relative Gibbs-free energies of the intermediates and the activation barriers involved in the ROP of both enantiomers (R and S) are very close, which is consistent with the formation of atactic polymers. In the same way, due to the presence of the γ-methyl substituent, two different products can be obtained upon cleavage of the acyl–oxygen bonds during the initiation step, and four different products during the propagation step. For both 7CC-γMe and TMC-γMe, computations indicate a preferential ring-opening at the most hindered oxygen–acyl O1–C(O)O bond, i.e. the one closest to the Me substituent, in agreement with the regioselectivities experimentally observed. The relaxation of the steric constraints inside the growing polymer chain appears to be an important feature. From a thermodynamic point of view, the overall polymerization process is exergonic (7CC-γMe) or almost athermic (TMC-γMe), in agreement with the easier polymerizability of seven- vs. six-membered carbonate rings, as also observed with unsubstituted carbonates.