AbstractComplexes of aluminum alkyl (AlR3), formed with certain kinds of Lewis bases, induce anionic polymerization of polar vinyl compounds such as acrylonitrile (AN) or methyl methacrylate (MMA), whereas AlR3 by itself cannot polymerize AN and MMA anionically. When the ligand Lewis base is bidentate, the resultant complex with a formal coordination number of five shows high catalytic activity for anionic polymerization even if the basicity of the ligand is small. On the other hand, when the ligand Lewis base is monodentate, the complex is four‐coordinated and shows catalytic activity for the polymerization of AN and MMA only when the basicity of the ligand is sufficiently large. The initiation reaction for the polymerization of 1‐substituted vinyl compounds such as MMA should be the conjugate addition of one alkyl group of the activated AlR3 to the monomer. With 1‐unsubstituted vinyl compounds such as AN or methyl acrylate (MA), a part of the initiation and the chain transfer reactions seem to take place through the abstraction of the 1‐hydrogen of the vinyl monomer molecules.