Polystyrene-Supported Salen-Cobalt Catalyst for the Copolymerization of CO2 and Propylene Oxide

Abstract

A novel polystyrene-supported Salen-cobalt(III) complex (PS-Salen-Co(III)-OAc) is prepared via covalent immobilization onto functionalized polystyrene microspheres. The catalyst is comprehensively characterized by FT-IR, XPS, SEM-EDS, and ICP-OES, confirming successful immobilization and preservation of the catalyst sites. This heterogeneous system is applied in the alternating copolymerization of carbon dioxide (CO₂) and propylene oxide (PO), achieving high polycarbonate yields (up to 76%), excellent chemoselectivity (up to 99%), and narrow molecular weight distributions in the presence of a chain transfer agent (Ð as low as 1.04) under mild conditions (30°C, 500 psi). The introduction of 1,2-propanediol as a chain transfer agent further enhances the control over molecular weights. However, catalyst reuse is hindered by substantial mass gain due to covalent grafting of the polycarbonate product onto the supporting polystyrene matrix via the surface hydroxyl groups. Attempts to mitigate this grafting through hydroxyl capping and reduction are partially effective. A degradation-recycling strategy, based on controlled depolymerization of surface-grafted polymer chains via a back-biting mechanism, significantly reduces mass gain and restores catalytic activity in subsequent cycles. This study provides a practical and efficient platform for sustainable CO₂ utilization through regenerable heterogeneous catalysis.