Process of preparing butadiene-styrene copolymer with nickel compound and alkylaluminoxane compound catalysts

A process for preparing a butadiene-styrene copolymer which comprises polymerizing a butadiene monomer and styrene monomer in the presence of a catalyst, wherein said catalyst comprises:

• (A) a nickel compound, wherein said nickel compound is selected from the group consisting of a nickel salt of an organic carboxylic acid and an organic complex compound of nickel, wherein said nickel salt of an organic carboxylic acid is represented by the following formula:
• (R.sup.6 --CO--O).sub.p Ni
• wherein R.sup.6 is selected from the group consisting of an aliphatic-hydrocarbon, an alicyclic-hydrocarbon and an aromatic-hydrocarbon and p is the valence of nickel, and
• (B) an alkylaluminoxane compound, wherein said alkylaluminoxane compound is a compound represented by formulas (I) or (II), or a mixture thereof:
• ##STR1## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are each an alkyl having from 1 to about 10 carbon atoms, n is an integer from 1 to about 100, and m is an integer from 2 to about 50, and wherein formula (I) represents a linear structure and formula (II) represents a cyclic structure, and
• wherein said nickel compound is present in an amount of 0.001 to 10 mmole per 100 g of the combined amount of said butadiene monomer and said styrene monomer, and a molar ratio of aluminum in said alkylaluminoxane compound to nickel in said nickel compound is 1 to about 1.times.10.sup.5.