Polymerizations of propylene oxide were carried out with double metal cyanide (DMC) catalysts based on Zn-3[Co(CN)6](2). Through the control of the type and amount of ZnX2 (X = F, Cl, Br, or I) during the preparation of the catalyst, the catalytic activity, induction period, and unsaturation level in the polyether polyols could be tuned. The DMC catalysts were characterized by X-ray photoelectron spectroscopy, infrared spectroscopy, and X-ray powder diffraction. In general, ZnBr2 was the most effective zinc halide with respect to the properties of the resulting polymers as well as the activity and induction period. The average rates of polymerizations of DMC catalysts prepared with ZnCl2, ZnBr2, and ZnI2 were 889, 1667, and 784 g of polyoxypropylene/g of catalyst h, respectively, with induction periods of about 53, 5, and 60 min, respectively, at 115 degrees C. The DMC catalysts produced polyoxypropylenes with an ultralow unsaturation level (0.0025-0.0057 mequiv/g) and a narrow molecular weight distribution (1.07-1.42) without high-molecular-weight tails; this resulted in a low viscosity (962-3950 cP). According to the results collected from catalyst characterizations and polymerizations, the active sites of DMC-catalyzed polymerization had mainly coordinative characters. The presence of free anions accelerated the ring-opening procedure and thus enhanced the propagation rate and shortened the induction period. (c) 2005 Wiley Periodicals, Inc.