Preparation of double-metal cyanide catalysts incorporating ZnOAc and ZnO<i>t</i>Bu components for propylene oxide/CO₂ copolymerization

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 ZnO<i>t</i>Bu components. These catalysts were synthesized <i>via</i> 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 ZnO<i>t</i>Bu content-<i>e.g.</i>, [ZnOAc]_1.28[ZnO<i>t</i>Bu]_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 (<i>F</i>_CO₂, up to 0.56), yielding poly(ether-<i>co</i>-carbonate) diols with <i>M</i>_n ≈ 3500 Da and narrow dispersity (<i>M</i>_w/<i>M</i>_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 <i>syn-anti</i> 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.