Electron donating complexing agents (CAs) are one of the key components to achieve active Zn(II)-Co(III) double metal cyanide (DMC) catalysts for the ring-opening polymerization (ROP) of propylene oxide (PO). The effect of the type of CAs on the ROP of PO is mechanistically studied by exploiting a series of dicarbonyl compounds, alpha-, beta-, and gamma-diketones and B-ketoesters, as CAs for DMC catalysts. The keto and/or enol modes of complexation of CAs on the catalyst surface is defined by combining structure-sensitive characterization tools such as infrared, absorption and X-ray photoelectron spectroscopies, finding keto complexation exhibits higher activity than enol complexation. Polymerizations of 1,2-epoxyhexane, glycidyl isopropyl ether, epichlorohydrin, and 1,2-epoxytetradecane are also conducted to get further insights on the mechanistic pathways of DMC-catalyzed ROP of epoxides. The interaction of oxygen atoms from both CAs and PO with the dormant sites during the activation of catalyst and the consequent formation of active sites by the removal of the coordinated CAs has been investigated using H-1 NMR analysis combined with DFT calculations. Once the active sites are initiated in cationic pathway and started to propagate, the DMC catalysts are fragmented into small pieces, allowing the unexpected high activity of the DMC catalysts. The propagation proceeds in either cationic or coordinative route depending on the presence of polyol initiator. The further understanding of the mechanistic pathways using a series of new dicarbonyl CAs expands the variety and scope of DMC-catalyzed ROP of epoxides. (C) 2019 Elsevier Inc. All rights reserved.