Catalytic Copolymerization of Carbon Dioxide and Cyclohexene Oxide by a Trinuclear Cyclohexane-bridged Tetradentate Schiff Base Chromium Complex

The development of catalytic systems is a central area of research in carbon dioxide (CO2) and epoxy copolymerization. A novel trinuclear cyclohexane-bridged tetradentate Schiff base chromium complex 1 was synthesized as a catalyst for the ring-opening copolymerization (ROCOP) of CO2 and cyclohexene oxide (CHO), resulting in the formation of poly (cyclohexene carbonate) (PCHC). The impact of polymerization temperature, CO2 pressure, reaction time, and catalyst loading on complex 1's polymerization activity was systematically investigated. It was observed that, with the addition of the co-catalyst PPNN3 (PPN= bis(triphenylphosphine)iminium), complex 1 exhibited enhanced catalytic activity for the ROCOP of CO2 and CHO under mild conditions. In contrast, the mononuclear tetradentate Schiff base chromium complex 2 system showed low activity under the same conditions. Compared to complex 2, complex 1 achieved a higher CHO conversion rate (70%) and 85% PCHC selectivity, with a turnover frequency (TOF) of 419 h-1, which is 5.3 times greater than that of complex 2. Additionally, the polymer produced by complex 1 had a molecular weight of 13790 g/mol, which is higher than that produced by complex 2 (8800 g/mol) and the commercial Salen CrCl catalyst (9610 g/mol). By varying the amounts of complex 1 and CHO, PCHC with different molecular weights (6000 g/mol to 14000 g/mol) and low dispersity can be easily obtained. Notably, the activation energy barrier for polycarbonate formation in the complex 1 system was 21.63 kJ/mol, compared to 32.88 kJ/mol in the complex 2 system.