Modeling the mechanism of CO2/cyclohexene oxide copolymerization catalyzed by chiral zinc β-diiminates: factors affecting reactivity and isotacticity

Shao, H.; Reddi, Y.; Cramer C. J.
ACS Catal. 2020, 10, 8870 (doi:10.1021/acscatal.0c02299).

Copolymerization of CO₂ and cyclohexene oxide (CHO) upcycles CO₂ into the value-added, chemically recyclable, thermoplastic poly(cyclohexene carbonate) (PCHC). Using density functional theory, the Zn-catalyzed copolymerization mechanism has been characterized with particular focus on the effects of chiral β-diiminate (BDI) ligands as they influence the reactivity and enantioselectivity in the epoxide ring-opening step, where the latter is required for isotacticity. Theory indicates that both mono- and binuclear forms of the catalyst are involved along the reaction path, with the turnover-limiting step being ring-opening of the epoxide mediated by a binuclear catalyst. Subsequent CO₂ insertion is predicted to be kinetically facile and preferentially mediated by a mononuclear catalyst. The predicted preference for epoxide opening to give R,R-stereocenters in the copolymer when N-(4-(((1S,2S)-2-(benzyloxy)cyclohexyl)amino)-5,5,5-trifluoropent-3-en-2-ylidene)-2,6-dimethylaniline is used as the BDI ligand agrees with experiment and is attributed to differential ligand distortions associated with key non-bonded interactions in the competing transition-state structures. Further analysis predicts that 2,6-dicholoro and dibromo substitutions of the BDI ligand N-aryl group(s) should result in increased rates and enantioselectivities for copolymerization.