Abstract: A catalyst system comprising:(a) a catalyst precursor consisting essentially of the reaction product of vanadium (acetylacetonate).sub.3 and a compound having the formula AlR.sub.(3-a) X.sub.a wherein each R is independently alkyl having 1 to 14 carbon atoms; each X is independently chlorine, bromine, or iodine; and a is 1 or 2; the atomic ratio of aluminum to vanadium in the precursor is 1:1 to 3:1; and the oxidation state of the vanadium in the reaction product is plus 2;(b) optionally, a support for said precursor; and(c) a halide substituted hydrocarbyl aluminum cocatalyst; and(d) optionally, a promoter, which is a chlorinated ester having at least 2 chlorine atoms; a saturated or 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.