Abstract: A new binary rare earth chloride catalyst composed of NdCl₃·2Phen(Phen=1, 10-phenanthroline)and HAl (i-Bu)₂ for stereospecific polymerization of butadiene has beenpresented. In contrast to other type of rare earth chloride catalysts the present systemis characterized by its ability to control the molecular weight of the product and itsspecific mode of formation. The general rule of butadiene polymerization with the cata-lyst was studied. It was found that HAl(i-Bu)₂ is mainly the chain transfer agent inthe reaction and the HAl(i-Bu)₂/monomer ratio is a key parameter controlling themolecular weight of polybutadiene obtained.The possible role of the phen molecule inmolecular weight controlling has been discussed.