Role of zinc chloride and complexing agents in highly active double metal cyanide catalysts for ring-opening polymerization of propylene oxide

Abstract

Several double metal cyanide (DMC) catalysts based on Zn3[Co(CN)6]2 were prepared by an impregnation method. Infrared spectroscopy (IR), wide-angle X-ray diffraction (WAXD), and elemental analysis were used to characterize these DMC catalysts. The WAXD patterns showed that pure Zn3[Co(CN)6]2 had very high crystallinity (98.8%) with a typical cubic lattice structure, while DMC catalysts prepared with excess ZnCl2 and/or complexing agents had low crystallinity. Furthermore, no free ZnCl2 was found in DMC catalysts prepared with excess ZnCl2 (from WAXD characterization), although it seemed that free ZnCl2 existed in the catalyst (from elemental analysis). The characteristic absorptions (νCN and νCo–C) in the IR spectra of DMC catalysts prepared with excess ZnCl2 were shifted to higher wave numbers than those in pure Zn3[Co(CN)6]2. These results showed that a new chemical linkage was produced between Co(CN)63− and ZnCl2 (or zinc center complexes) in the precipitation reaction. Excess ZnCl2 in DMC catalyst preparation was necessary to ensure its high catalytic activity for ring-opening polymerization (ROP) of propylene oxide (PO), while complexing agents were unnecessary in the composition. The role of the complexing agents was mainly to promote the formation of the amorphous DMC catalyst and thus increase the catalytic activity. A possible heterogeneous coordinative cationic polymerization mechanism is proposed for a reasonable interpretation of the ROP of PO.