Highly Active, Bifunctional Co(III)-Salen Catalyst for Alternating Copolymerization of CO₂ with Cyclohexene Oxide and Terpolymerization with Aliphatic Epoxides

Abstract

The cobalt(III) complex of a salen ligand bearing one quaternary ammonium salt on the three-position of one aromatic ring is a highly active catalyst for the alternating copolymerization of cyclohexene oxide (CHO) and CO2 to afford the corresponding poly(cyclohexene carbonate) (PCHC) at various temperatures. The cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation at high temperatures up to 120 °C and even under low CO2 pressures of 0.1 MPa. Also, the cobalt(III)-salen complex could operate very efficiently for the terpolymerization of CHO and aliphatic epoxides with CO2 to provide selectively polycarbonates with a narrow polydispersity at high temperatures. The polycarbonates resulting from the terpolymerization of equimolar CHO and propylene oxide (PO) with CO2 have a close content for both cyclohexene carbonate and propylene carbonate units. This is ascribed to the presence of CHO significantly inhibiting the reactivity of PO and thereby causing a matched reactivity for both epoxides during the terpolymerization. The competition coordination of CHO and PO to the electrophilic cobalt(III) ion has no effect on regioselective ring-opening of PO in the terpolymerization.