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

Abstract

The intramolecularly dinuclear magnesium complexes generated in situ from the reaction of multidentate semi-azacrown ether ligands with n-Bu2Mg followed by treatment with alcohol additive were found to be efficient catalysts for the copolymerization of CO2 and cyclohexene oxide (CHO), affording the completely alternating poly(cyclohexene carbonate) (PCHC) under extremely mild conditions (1 atm pressure of CO2, 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 CO2/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-Bu2Mg:n-BuOH = 1:2:0.4) at 60 °C demonstrate a zero-order dependence on CO2 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.