Silica-immobilized zinc β-diiminate catalysts for the copolymerization of epoxides and carbon dioxide

A synthetic protocol has been developed to prepare silica-supported Zn-beta-diiminate catalysts for the copolymerization of cyclohexene oxide (CHO) and CO2. Multiple strategies have been developed for the immobilization of these beta-diiminate zinc complexes onto the surface of model silica materials such as mesoporous SBA-15 and controlled-pore glass (CPG). The beta-diiminate ligand has been modified to incorporate a C=C double bond or an alkane spacer with a trimethoxysilyl end group, allowing immobilization via direct reaction of the alkoxysilanes with silanols on the surface or via AIBN-promoted C=C bond coupling with thiol-functionalized silica. The immobilization process has been followed using FT-Raman spectroscopy and thermogravimetric analysis, whereas polymers have been characterized by GPC and NMR. The resulting silica-supported catalysts exhibit good activity in the alternating copolymerization of CHO and CO2, leading to polymers with varying degrees of carbonate linkages (copolymerization) relative to ether linkages (homopolymerization of epoxide). Immobilizing the complexes on the silica support leads to catalysts that give more polymeric ether linkages than their corresponding homogeneous analogues. Control studies indicate that this is due, at least in part, to starvation of the active site of CO2, especially at later stages of the polymerization.