The polymerization of acrylonitrile initiated with the imidazole-copper (II) complexes was studied in dimethylsulfoxide solution. The ability of the complexes to initiate polymerization seems to depend on their anion. 2-Substituted imidazole-copper (II) complexes of the type, CuL4X2(L = imidazole as ligand and X = anion), were also found to initiate vinyl polymerization. From the data of electronic spectra in dimethylsulfoxide solution, the initiation mechanism is discussed in terms of the formation of the active species by the interaction of the complex with monomer molecules.