cis - 1 , 4 - polybutadiene can be synthesized by polymerizing 1 , 3 - butadiene monomer with a three component nickel catalyst system containing an organonickel compound , an organoaluminum compound , and a fluorine containing compound . however , the molecular weight of the cis - 1 , 4 - polybutadiene prepared is typically too high to be utilized as a non - oil extended rubber . this invention is based upon the discovery that para - styrenated diphenylamine acts to reduce the molecular weight and to improve the processability of cis - 1 , 4 - polybutadiene prepared with such nickel based catalyst systems . the use of para - styrenated diphenylamine as a modifier in such polymerizations does not change the microstructure of the cis - 1 , 4 - polybutadiene produced . accordingly , the present invention specifically discloses a process for producing cis - 1 , 4 - polybutadiene having a reduced molecular weight and improved processability which comprises polymerizing 1 , 3 - butadiene in the presence of an organonickel compound , an organoaluminum compound , a fluorine containing compound , and para - styrenated diphenylamine ; wherein the organoaluminum compound and the fluorine containing compound are brought together in the presence of the para - styrenated diphenylamine . since para - styrenated diphenylamine is an excellent antidegradant , it also provides the rubber synthesized in its presence with antioxidant protection .