Dinuclear chromium complexes with [OSSO]-type ligands in the copolymerization of epoxides with CO2 and phthalic anhydride

Abstract

In this study, a new family of dinuclear chromium complexes (1–3) containing bis-thioether-diphenolate ligands has been introduced for the binary copolymerization of carbon dioxide and epoxides and ternary copolymerization of the latter with phthalic anhydride. Complex 1 (0.1 mol%) in combination with (bis(triphenylphosphine)iminium chloride) (PPNCl, 0.5 mol) at 45 °C and 20 bar of CO₂ showed high regioselective copolymerization to polypropylene carbonate (PPC) with conversion (up to 91%) and selectivity (up to 95%). The data presented in this work have consistently shown that complex 1 displayed higher catalytic activity in copolymerizing epoxides with CO₂ than complexes 2, 3, and the analogous mononuclear 4. At the same time, complex 1 showed good catalytic properties in the terpolymerization of epoxides/CO₂/phthalic anhydride among the tested complexes 2 and 4. In the case of cyclohexene oxide and vinylcyclohexene oxide, a selectivity of more than 99% towards polycyclohexene carbonate (PCHC) and polyvinylcyclohexene carbonate (PVCHC) with a TOF as high as 41 h⁻¹ in the poly(ester-block-carbonate) was observed. Notably, a conversion higher than 99% towards the polyester block was also observed for all the studied epoxides. A bimetallic intramolecular cooperative mechanism was proposed for the copolymerization of propylene oxide and CO₂ based on the first-order dependence with respect to complex 1 by the kinetic investigations.