Cis-1,4-polybutadiene can be synthesized by polymerizing 1,3-butadiene monomer with a three component nickel catalyst system containing (a) an organonickel compound, (b) an organoaluminum compound, and (c) 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 (a) an organonickel compound, (b) an organoaluminum compound, (c) a fluorine containing compound, and (d) 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.