Modeling the Mechanism of CO₂/Cyclohexene Oxide Copolymerization Catalyzed by Chiral Zinc β-Diiminates: Factors Affecting Reactivity and Isotacticity

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 a 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 the 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-dichloro and dibromo substitutions of the BDI ligand N-aryl group(s) should result in increased rates and enantioselectivities for copolymerization.