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 as a complexing agents. The DMC catalyst of the molecular formula, Zn2.3Cl1.0[Co(CN)(6)](1.0)center dot 2.0 'BuOH center dot 1.0H(2)O, is characterized by gas sorption measurements, infrared spectroscopy and X-ray powder diffraction. The structure of DMC catalyst with negligible surface area and broadened X-ray diffraction peaks is different from that of Prussian blue analogue, Zn-3[Co(CN)(6)](2)center dot 12H(2)O of microporous crystalline materials. The PO polymerization behavior is tunable by combining it with various imidazolium based ionic liquids (lLs) as external additives. Thus, (1) they make the zinc-monomer bond faster activated during the initial stage of polymerization, (2) they make the zinc-monomer bond more active, (3) they stabilize the polymerization centers and prevent their decomposition, and (4) they improve important polymer properties such as molecular weight, viscosity and unsaturation level. The maximum rate of polymerization (R-p,R-max) of DMC catalyst increases from 2587 to 27,222 g-polymer/g-cat h by combining with 1-ethyl-3-methylimidazolium chloride (emimCl, [emimCl]/[Zn] = 1.25) at] 15 degrees C. The induction period as the time to reach Rp,max becomes short from 321 min for DMC catalyst to 29 min for DMC/emimCl binary catalyst. The unsaturation value of polyol (0.017 mequiv./g) produced by DMC decreases to 0.005 mequiv./g by simply combining with IL. The molecular weight polyol produced by DMC catalyst increases from M-n=3700 to more than 6000, and the viscosity of polyol decreases by combining with lLs. (c) 2007 Elsevier B.V. All rights reserved.