Polymerizations of propylene oxide (PO) have been carried out by using double metal cyanide (DMC) catalyst prepared by reacting ZnCl2 and K-3[Co(CN)(6)] in the presence of tert-butyl alcohol ((BuOH)-Bu-t) as a complexing agent. The catalytic activity and the induction period for PO polymerizations catalyzed by DMC are tunable by using various quaternary ammonium salts (QAS) as external additives. The DMC/QAS binary catalyst improves polymer properties as well such as molecular weight, molecular weight distribution, viscosity, and unsaturation level. (c) 2007 Elsevier Ltd. All rights reserved.