AbstractThe effect of the catalytic amount of H2O was investigated with the EtnAlCl3‐n‐VOCl3catalyst system on the alternating copolymerization of acrylic monomers with diolefins and styrene. The presence of the catalytic amount of H2O produced an improvement in the yield and in the molecular weight as well as the structure of copolymer with the EtnAlCl3‐n‐VOCl3catalyst system. The efficiency of the aluminum components in the EtnAlCl3‐n‐VOCl3system appears with AlEt3and especially with Et1.5AlCl1.5. The catalytic activity was found to depend upon the H2O EtnAlCl3‐nmolar ratio and was also affected by the order of mixing of the catalyst components and the monomers. Effective catalyst could be prepared when the catalyst components (except VOCl3) were premixed without presence of monomers. The possible catalytic behavior of H2O