Disclosing multiple factors influencing enantioselective copolymerization of CO2 with meso-epoxides using β-diiminate Zn catalysts†

Abstract

The enantioselective ring-opening copolymerization (ROCOP) of cyclohexene oxide (CHO) and carbon dioxide (CO₂) to produce isotactic poly(cyclohexene carbonate) (iPCHC) was systematically investigated using chiral C₁-symmetric zinc β-diiminate (BDI) catalysts. A combination of density functional theory (DFT), molecular steric descriptors (%V_Bur), and the activation strain model (ASM) was employed to elucidate the mechanistic pathways and factors governing enantioselectivity. We found that chiral monomeric BDI catalysts exhibit intrinsic enantioselective properties in meso-desymmetrization polymerization catalysis, which are significantly enhanced upon formation of dimeric complexes with anti and syn conformations. The predicted enantioselectivity, arising during the CHO ring-opening step, explains the experimental combination of selected stereocenters on the ligand and preferred stereochemistry of the polymer chain. This study identifies key factors influencing ROCOP enantioselectivity, including monomer deformation, ligand steric effects dictated by the number of chiral centers, and noncovalent interactions, all contributing additively to the observed selectivity. These insights provide a better understanding of the mechanistic origins of enantioselectivity in CHO/CO₂ ROCOP and offer guidance for the design of more efficient catalysts.