It was found that alkylboron halide, such as ethylboron dichloride and diethylboron chloride, was a highly active catalyst for the alternating copolymerization of isobutylene and acrylic ester. The alternating sequential structure of the resulting copolymers was proved by analysis of n.m.r. spectra. The Lewis acidity of the catalyst, the molar ratio of catalyst to acrylic ester and the molar ratio of isobutylene to acrylic ester played important roles in determining the regularity of the alternating sequence. Copolymer molecular weight had an intimate correlation with the Lewis acidity of the catalyst. The polymerization rate was maximum at about -50 degrees C and decreased rapidly above 0 degrees C. The present copolymerization proceeded via a complexed radical mechanism. The cocatalytic effect of oxygen was remarkable and the molecular weight controlling effect of mercaptan was notable. Alkylboron halide has various advantages compared with alkylaluminium halide.