Mechanism for alternating copolymerization of CO2 and propylene oxide in diethylzinc–water catalytic system: A DFT study

Abstract

The mechanism of alternating copolymerization of CO2 with propylene oxide (PO) catalyzed by the diethylzinc–water system has been studied by means of density functional theory (DFT) calculations. Six most plausible molecular models for the active species of the diethylzinc–water systems (C2H5ZnOH, C2H5(ZnO)2H, C2H5(ZnO)3H, C2H5(ZnO)4H, HOZnOH, C2H5ZnOC2H5) have been constructed. Possible reaction pathways and their corresponding reaction energy barriers have been investigated. It is found that the reaction follows monometallic mechanism and is initiated by PO insertion into the ZnOR bond of the catalyst. The ring-opening of PO is inclined to occur via the methineoxygen bond (CCHO) cleavage. The rate-determining step is the second molecular PO insertion into Zn–carbonate group. CO2 insertion proceeds easily and the reverse reaction of CO2 insertion is also easy to occur. It is likely that the most possible active species is the condensed species with repeated Zn–O group, which shows high Gibbs free energy barrier to the formation of cyclic propylene carbonate. The consecutive PO insertion to give polyether and consecutive CO2 insertion to give dicarbonate linkages have been found to be disfavored. The non-condensed species could be excluded due to its easy formation of cyclic propylene carbonate.