CO2/propylene oxide copolymerization catalyzed by monoimidazole-modified cobaltoporphyrins: structure-driven dual selectivity and mechanistic insights

Three monoimidazole-modified cobaltoporphyrins with different substitution positions were designed and synthesized as bifunctional homogeneous catalysts for the alternating copolymerization of propylene oxide (PO) and CO 2 to produce biodegradable poly(propylene carbonate) (PPC) under mild conditions without a cocatalyst. Their catalytic performance strongly depends on the ligand skeleton, which determines product selectivity. Under optimal conditions, catalyst X- o 4 showed high PO conversion, polymerization selectivity, high molecular weight polycarbonate, and narrow molecular weight distribution. Kinetic studies using X- o 4 and bisimidazole-modified J- o 4 gave apparent activation energies for polycarbonate and cyclic carbonate. Based on structural and experimental data, a competition mechanism between polymerization and cycloaddition is proposed. Density functional theory (DFT) calculations support this mechanism, explaining the temperature-dependent selectivity and the different behaviors of mono- and bis-imidazole catalysts.