A PROCESS FOR PRODUCING BUTADIENE POLYMER WHICH COMPRISES POLYMERIZING 1,3-BUTADIENE IN THE PRESENCE OF A CATALYST SYSTEM CONSISTING ESSENTIALLY OF (A) AT LEAST ONE OF NICKEL AND COBAL COMPOUNDS, (B) A TRIALKYL ALUMINUM COMPOUND, (C) A BENZOTRIFLUORIDE COMPOUND REPRESENTED BY THE FORMULA:

1-R1,2-R2,3-R3,4-R4,5-R5,6-(CF3-)BENZENE

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