Polymers of conjugated diene hydrocarbons and a method for their preparation which comprises (A) polymerizing the conjugated diene to form a polymer having substantially a trans-1,4 structure in the presence of a catalyst prepared by mixing a vanadium compound selected from vanadium trichloride and the product of vanadium tetrachloride and titanium tetrachloride with at least one organo aluminum compound and with up to 3 moles of a titanium compound per mole of vanadium compound where the titanium compound is selected from the group consisting of titanium triacetylacetonate and the compounds having the formula Ti(OR'')4 and (B) mixing oxygen with the polymerized diene hydrocarbon and catalyst to reduce the molecular weight of the polymer.