Asymmetric Copolymerization of Cyclopentene Oxide and CO2 Using a Dinuclear Zinc-AzePhenol Catalyst: Enlightened by DFT Calculations

摘要

Optically active polycarbonates (PCs) are considered as candidates for new and valuable materials because of their well-defined chemical structures and special physical properties. Previous studies on asymmetric alternating copolymerization of cyclopentene oxide (CPO) and CO2 regarding chiral zinc catalysts provided poly(cyclopentene carbonate) (PCPC) with moderate enantioselectivity, and thus, the development of highly efficient catalysts for this enantioselective polymerization is highly desirable. This research work is enlightened by the DFT calculations. In this paper, we clearly describe the use of intramolecular dinuclear zincAzePhenol complex as a high performance catalyst for the asymmetric copolymerization of CPO and CO2, affording completely alternating PCPC under very mild conditions (1 atm CO2, 30 degrees C) in 98% yield with >99% enantioselectivity for (S,S)-configuration. The dinuclear catalyst is prepared in situ from the reaction of multidentate semiazecrown ether ligand and ZnEt2, followed by treatment with an alcohol additive. In addition, our previous studies indicated that this catalyst also showed excellent enantioselectivity in the asymmetric copolymerization of cyclohexene oxide (CHO) and CO2. In order to obtain more information on the mechanism of the catalytic copolymerization, the chemical structures of PCPC are characterized by H-1 NMR and C-13 NMR spectroscopy, and the nonlinear effect is also investigated in this copolymerization. A plausible catalytic cycle for the present reaction system is outlined. The reaction of chiral ligand with ZnEt2, followed by the ethyl group exchange with EtOH, affords the ethoxy-bridged dinuclear zinc complex. The copolymerization reaction is initiated by the insertion of CO2 into the ZnOEt bond to give a carbonateester-bridged complex. The two zinc centers are situated sufficiently close to each other to allow a synergistic effect in the copolymerization, meaning that one zinc atom acts as Lewis acid to activate the epoxide, the other is responsible for carbonate propagation through the nucleophilic attack of carbonate ester at the back side of the cis-epoxide by a six-membered transition state. Furthermore, the dinuclear zinc structure of the catalyst remains intact throughout the catalytic copolymerization. The proposed mechanism implies that the intramolecular dinuclear zinc catalyst is very important for future research into the copolymerization of other epoxides with CO2.