Polymerization of 1,3-butadiene with the CoBr2(PPh(3))(2)/MgCl2/SiPh(2)(OCH3)(2)-Al(CH3)(3) catalyst system was conducted in the presence of ethylene. It was found that addition of ethylene causes a marked decrease in the molecular weight of polybutadiene although ethylene is not incorporated at the chain-end. The structure of chain-ends was analyzed using very low molecular-weight polybutadiene produced in the presence of ethylene, which led to the conclusion that 1,2 and 2,1 insertion reactions predominate at the initiation and propagation steps, respectively.