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. Xray 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 degrees 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 degrees 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.

Automated summary

A novel series of dinuclear nickel complexes containing benzotriazole based 1,3-diamine-bisphenolate (1,3-DiBTP) ligands were synthesized as highly active single component catalysts for the copolymerization of CO2 and epoxides. The best benzoate-bonded dinickel catalyst demonstrated effective activity for CO2-copolymerization of cyclohexene oxide at high pressure and 1 atm, achieving a high turnover frequency and producing narrowly dispersed poly(cyclohexene carbonate) with high selectivity. Additionally, kinetic studies of the copolymerization process revealed first-order dependence on catalyst and epoxide concentrations, with an activation energy of 57.69 kJ mol(-1) for poly(cyclohexene carbonate) generation.