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 13 790 g mol⁻¹, which is higher than that produced by complex 2 (8800 g mol⁻¹) and the commercial Salen CrCl catalyst (9610 g mol⁻¹). By varying the amounts of complex 1 and CHO, PCHC with different molecular weights (6000 g mol⁻¹ to 14 000 g mol⁻¹) 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⁻¹, compared to 32.88 kJ mol⁻¹ in the complex 2 system.