Preparation of double-metal cyanide catalysts incorporating ZnOAc and ZnOtBu components for propylene oxide/CO2 copolymerization

Abstract

Double metal cyanide complex (DMC) catalysts, widely used industrially for the ring-opening polymerization (ROP) of propylene oxide (PO), have garnered attention for PO/CO₂ copolymerization due to their ease of preparation and high catalytic activity, although their ability to incorporate CO₂ remains relatively limited. In this study, we report DMC variants with well-defined compositions, specifically incorporating ZnOAc and ZnOtBu components. These catalysts were synthesized via a distinct route from conventional DMCs by reacting K₃Co(CN)₆ with 2 eq. Zn(OAc)₂, followed by sequential treatment with HCl and LiOAc. Among the variants, the DMCs enriched in ZnOAc and minimized in ZnOtBu content—e.g., [ZnOAc]_1.28[ZnOtBu]_0.10[ZnCl]_0.02[Zn]_0.80[Co(CN)₆][H₂O]₄—exhibited outstanding productivity (up to 5.0 kg g-cat⁻¹ h⁻¹) and enhanced CO₂ incorporation (F_CO2, up to 0.56), yielding poly(ether-co-carbonate) diols with M_n ≈ 3500 Da and narrow dispersity (M_w/M_n = 1.7). SEM images revealed thin plate-like structures with straight lines, resembling laterally fused planks. By correlating the known atomic-level structure of [ZnOAc][Zn][Co(CN)₆][H₂O]₄ with the SEM images, the high performance was reasonably attributed to fractures along the straight lines, which expose the lateral faces of thin planks bearing syn-anti Zn-OC(O-Zn)CH₃ units with Zn–Zn distances of 4.4 Å. These structural features are well-suited to facilitate a bimetallic polymerization mechanism, as proposed for other catalyst systems.