Intramolecularly Dinuclear Magnesium Complex Catalyzed Copolymerization of Cyclohexene Oxide with CO2 under Ambient CO2 Pressure: Kinetics and Mechanism

The intramolecularly dinuclear magnesium complexes generated in situ from the reaction of multidentate semi-azacrown ether ligands with n- Bu 2 Mg followed by treatment with alcohol additive were found to be efficient catalysts for the copolymerization of CO 2 and cyclohexene oxide (CHO), affording the completely alternating poly(cyclohexene carbonate) (PCHC) under extremely mild conditions (1 atm pressure of CO 2, temperatures ranging from 20 to 60 °C). Ligand substituent effects were observed to dramatically influence both the activity and the chemoselectivity of the catalysis, exemplifying the fine-tunability of this type of ligands. With regard to the mechanistic studies, end-group analysis of the copolymer by MALDI-TOF mass spectroscopy revealed the predominance of the butoxy-initiated CO 2 /CHO copolymerization process. The butoxy-containing catalytically active species in the present system was proposed to possess a dinuclear structure similar to an isolated model Mg complex which was characterized by X-ray crystal structural analysis. Rate studies performed on the copolymerization using the in situ prepared catalyst (molar ratio of 5a: n -Bu 2 Mg: n -BuOH = 1:2:0.4) at 60 °C demonstrate a zero-order dependence on CO 2 pressure and approximately first-order dependence on CHO and the catalyst concentration. Furthermore, the relative propensity of polycarbonate vs cyclic carbonate formation in the present system was evaluated by a comparative kinetic study of the temperature effect using the in situ IR technique. On the basis of these findings, a plausible bimetallic mechanism was tentatively proposed for the present reaction system.