Process for medium vinyl polybutadiene

A process for the preparation of medium vinyl polybutadiene which comprises copolymerizing butadiene with small amounts of divinylbenzene in a hydrocarbon solvent system employing as a catalyst an alkyllithium compound, which process has been modified by mixtures of polar compounds, at least one of which is a strong polar compound and is selected from the group consisting of hexamethylphosphoric acid triamide (HMPA), N,N,N'N'-tetramethylethylene diamine (TMEDA), ethylene glycol dimethyl ether (glyme), diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme) and tetraethyleneglycol dimethyl ether (tetraglyme), in which the mole ratio of the strong polar compound to the alkyllithium ranges between about 0.15/1 and 0.50/1, and at least one of which is a weak polar compound and is selected from the group consisting of tetrahydrofuran (THF), 1,4-diazobicyclo[2.2.2]octane (DABCO), diethylether, triethylamine, tri-n-butylamine, tri-n-butylphosphine and p-dioxane, in which the mole ratio of the weak polar compound to the alkyllithium ranges between about 0.90/1 to about 1.50/1, in which the temperature is maintained constant throughout the polymerization and in which the mole ratio of the divinylbenzene to active alkyllithium catalyst ranges between about 0.10 and about 0.90, and in which the amount of alkyllithium catalyst is between about 0.10 and 1.0 millimoles of active alkyllithium per hundred grams of butadiene.