Under carbon monoxide pressure, cupric alkoxides such as cupric methoxide, Cu(OCH3)2, and cupric acetylacetonate methoxide, Cu(acac)(OCH3), initiate the polymerization of vinyl monomers. The microstructure of polybutadiene and the composition of styrene-methacrylate copolymer by these catalyst systems have indicated a free radical mechanism. The mechanism of the initiation was examined by the end group of product polymer and the analysis of the reaction between the catalyst components. Reduction of Cu(OCH3)2and Cu(acac)(OCH3) with carbon monoxide to Cu(OCH3) and Cu(acac), respectively, was responsible for the initiating activity. The decomposition of these unstable cuprous species produces methoxyl and acetylacetonyl radicals which initiate the polymerization.