be used a compound of lanthanum neodymium or u201c didymium u201d which is a mixture of rare earth elements containing approximately 72 neodymium 20 lanthanum and 8 praseodymium is preferred preferably component a is soluble in hydrocarbon polymerization medium examples being the carboxylates alkoxides and diketones examples of compounds for use as component a are u201c didymium u201d versatate derived from versatic acid a synthetic acid composed of a mixture of highly branched isomers of c10 monocarboxylic acids sold by shell chemicals praseodymium 2 2 6 6 tetramethyl 3 5 heptane dione lanthanum u201c didymium u201d and especially neodymium u201c versatate u201d are preferred on the grounds of ready solubility ease of preparation and stability although the neodymium species of the disclosed rare earth catalysts are instantly preferred the invention broadly relates to the polymerization of butadiene conducted in the presence of a catalyst system containing a compound of a rare earth element i e an element having an atomic number of 57 to 71 inclusive the rare earth catalysts of the present invention are any complexes of a metal belonging to the series of the lanthanides having an atomic number of 57 to 71 in which a ligand is directly bound to the metal and is a monovalent and monodentate organic radical including but not limited to u2014 r 1 co 2 u2014 or 1 u2014 nr 1 r 2 u2014 sr 1 u2014 pr 1 r 2 and u2014 a r 1 n.