Abstract: There is disclosed a process for the regulation of the molecular weight of a high-cis 1,4-polybutadiene prepared by polymerizing 1,3-butadiene in a solution of an aliphatic and/or cycloaliphatic or aromatic solvent system employing as a catalyst system a mixture of (1) an organonickel compound, (2) an organoaluminum compound, and (3) a fluorine containing compound selected from the group consisting of hydrogen fluoride and hydrogen fluoride complexes which are prepared by complexing hydrogen fluoride with a ketone, an aldehyde, a nitrile, a mineral acid containing oxygen, an ester, an ether, an alcohol, a phenol or water; said polymerization being conducted in the presence of small amounts of olefins selected from the group consisting of 1-butene, isobutylene, cis and trans-2-butene and allene.