Simple mixing of H3Co(CN)(6) and ZnCl2 in methanol resulted in precipitates of [ZnCl](+)(2)[HCo(CN)(6)](2-), constituting a new type of double metal cyanide (DMC) catalyst exhibiting a high performance in carbon dioxide (CO2)/propylene oxide (PO) copolymerization. High-molecular-weight poly(propylene carbonate-co-propylene oxide)s [poly(PC-co-PO)s] (M-n approximate to 40,000) were consistently obtained with high carbonate fractions (approximate to 60 mol %) and a high selectivity (>95%) with the new type of DMC catalyst. Conventional preparation of the DMC catalyst using K3Co(CN)(6) and ZnCl2 required removing KCl through thorough washing and resulted in lower carbonate fractions (10-40 mol %), which depended on the washing conditions. Feeding hydrophobic diols such as 1,10-decanediol as chain transfer agent preserved the high carbonate fraction (approximate to 60%) and enabled precise control of the molecular weight, including preparation of a low-molecular-weight poly(PC-co-PO)-diol (M-n approximate to 2000), which was a flowing viscous liquid with a low T-g (-30 degrees C) suitable for polyurethane applications. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4811-4818