A catalyst composition is provided with an activator in combination with a primary catalyst system. The primary catalyst system is a vanadium compound and an organometallic compound of the formula RMgX, LiAlr.sub.4, or R.sub.A Al.sub.2 X.sub.B, wherein R is a hydrocarbon radical having 1 to 12 carbon atoms, X is a halogen atom; A is a number from 2 to 6, and B is a number from 0 to 4, with the proviso that the sum of A and B is 6 and that at least one of the groups attached either to the vanadium compound or to the organometallic compound is a halogen atom. The activator is a chlorinated tetrahydrofurfuryl tetrahydrofuroate, an alkyl ester of chlorinated tetrahydrofuroic acid wherein the alkyl group has 1 to 8 carbon atoms, a chlorinated alkylene bistetrahydrofuroate wherein the alkylene group has 2 or 3 carbon atoms, a chlorinated oxy-bis [alkylene (2-furoate)], wherein the alkylene group is ethanediyl, 1,3 propanediyl, alkyl substituted ethanediyl or alkyl substituted 1,3 propanediyl wherein the alkyl group on the ethanediyl or 1,3 propanediyl has 1 to 8 carbon atoms, or bis (trichloromethyl)sulfone.