AbstractMgCl2‐supported CoBr2catalysts were prepared from mixture of MgCl2and CoBr2L2(L = triphenylphosphine or pyridine) in toluene. Polymerization of 1,3‐butadiene was conducted over the catalysts combined with ordinary alkylaluminums as cocatalyst. The CoBr2(PPh3)2/MgCl2catalyst gave polybutadiene with approximately 80% of 1,2 units, whereascis‐1,4‐poly‐butadiene was obtained with the CoBr2(C5H5N)2/MgCl2catalyst.Addition of triphenylphospine to the latter catalyst caused a marked increase in the content of 1,2 units. The content of 1,2 units could be thus controlled in the range from 0 to 80% by changing the amount of triphenylphosphine.On the other hand, the CoBr2(PPh3)2/MgCl2catalyst with very low content of CoBr2(PPh3)2hardly displayed any activity. Addition of dimethoxydiphenylsilane to the catalyst gave polybutadiene containing 90 % of 1,2 units with a fairly high