A study of the polymerization of styrene, methyl methacrylate, acrylonitrile, vinyl acetate, and vinyl chloride initiated by various metal acetylacetonates [Me(acac)x] has been made. It was found that Mn(acac)3was the most effective initiator, and Co(acac)3, Mn(acac)2, Cu(acac)2, and Cr(acac)3showed moderate activity for the polymerization of methyl methacrylate at 60°C. However, the other, Me(acac)x, had no effect or served as inhibitors. The addition of some additives such as halogen compounds did not accelerate polymerization of methyl methacrylate by Mn(acac)3, From the results of polymerization and copolymerization of methyl methacrylate by Mn(acac)3, it was concluded that the polymerization proceeded via an ordinary radical mechanism and the activation energy for initiation was 25.2 kcal/mole. The initiation mechanism of vinyl polymerization by Me(acac)xwas studied on the basis of the complex formation with the monomer.