The investigation on the molecular weigth and its distribution of 1

2-polybuta-diene prepared with MoCl4OR-i-Bu2AlORn catalyst system has been performed by viscosity and GPC methods and the way for regulation of the molecular weight and distribution has been searched. 1

2-polybutadiene with Mw/Mn = 1.2-2.0 can be prepared with MoCL4OR-i-Bu2AlOR' catalyst system at 30-70℃. There is a linear relation between the Mw/Mn and polymerization temperature. It was found by extrapolating that this catalyst system can possibly initiate living polymerization at about -18℃. The molecular weigth and its distribution can be regulated by polymerization temperature or presence of polar additive compounds. Allyl halides are very effective for this purpose and allyl iodide is the best.