Ethylene/propylene copolymer rubbers

A process comprising contacting a mixture comprising ethylene, propylene, and, optionally, a diene, under polymerization conditions, with a catalyst system comprising: (a) a catalyst precursor, in independent or prepolymer form, consisting of vanadium (acetylacetonate)3; (b) optionally, a support for said precursor; (c) a cocatalyst consisting of a trialkylaluminum; and (d) a promoter, which is a saturated or an unsaturated aliphatic halocarbon having at least 3 carbon atoms and at least 6 halogen atoms or a haloalkyl substituted aromatic hydrocarbon wherein the haloalkyl substituent has at least 3 halogen atoms.