Cationic tetranuclear macrocyclic CaCo 3 complexes as highly active catalysts for alternating copolymerization of propylene oxide and carbon dioxide.

We found that a cationic hetero tetranuclear complex including a calcium and three cobalts exhibited high catalytic activity toward alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO 2 ). The tertiary anilinium salt [PhNMe 2 H][B(C 6 F 5 ) 4 ] was the best additive to generate the cationic species while maintaining polymer selectivity and carbonate linkage, even under 1.0 MPa CO 2 . Density functional theory calculations clarified that the reaction pathway mediated by the cationic complex is more favorable than that mediated by the neutral complex by 1.0 kcal mol -1 . We further found that the flexible ligand exchange between Ca and Co ions is important for the alternating copolymerization to proceed smoothly.