Allyl halide or benzyl chloride can greatly regulate the molecular weight of polymer during the polymerization of butadiene in hydrogenated gasoline with PeCl3-(i-C4H9)3Al-phen catalyst system. It was found that allyl chloride is the most effective regulator of molecular .weight among them. The [17] of polybutadiene can be lowered from over 1.0 M3/kg to less than 0.4 M3/kg by the addition of allyl chloride

while the polymerization keeps high yield. Polybutadiene rubber with mid-vinyl structure was obtained and it has good properties.