Alternating Copolymerization of Carbon Dioxide with Epoxides Using Highly Active Dinuclear Nickel Complexes: Catalysis and Kinetics.

A novel series of well-defined dicarboxylate dinuclear nickel complexes containing benzotriazole based 1,3-diamine-bisphenolate (1,3-DiBTP) ligands were readily synthesized through a one-pot procedure, which were highly active single-component catalysts for copolymerization of CO2 and epoxides. X-ray structural determination of dinickel complexes 1-11 indicates that the DiBTP ligand acted as a N,O,N,N,O,N-hexadentate framework to chelate two nickel atoms, and two carboxylates are nonequivalently coordinated. The best benzoate-bonded dinickel catalyst 6 displayed the effective activity for both high-pressure and 1 atm CO2-copolymerization of cyclohexene oxide (CHO) in a controllable manner. Noteworthily, a high turnover frequency up to 9600 h-1 could be reached at 140 °C and a CO2 pressure of 20.7 bar utilizing a low catalyst loading of 0.01 mol %, and the same copolymerization conditions were capable of producing narrowly dispersed poly(cyclohexene carbonate) (PCHC) having >99% polycarbonate selectivity. In addition to CO2/CHO copolymerization, 4-vinyl-1,2-cyclohexene oxide or cyclopentene oxide was also applied to efficiently copolymerize CO2 under conditions of 80 °C and 20.7 bar initial CO2 pressure. Kinetic studies of CO2/CHO copolymerization catalyzed by 6 were investigated. Such polymerization revealed first-order dependence for both catalyst 6 and CHO concentrations, and the activation energy for PCHC generation by 6 is 57.69 kJ mol-1. A possible polymerization mechanism for CO2-copolymerization of CHO was proposed based on kinetics and structural studies of the obtained polycarbonates.