a catalyst composition comprising : a complex comprised of v 2 x 3 m and alcl 2 r 2 wherein x = cl , br , or i , or mixtures thereof ed = an electron donor , which is an ether , a phosphine , a ketone , an isocyanide , or an ester , said electron donor having 2 to 20 carbon atoms r = an alkyl having up to 14 carbon atoms m = an integer from 3 to 6 , said complex impregnated into an inorganic oxide support ; sufficient trialkylaluminum compound adsorbed on the inorganic oxide support to provide a molar ratio of adsorbed trialkylaluminum compound to vanadium of about 2 . 5 : 1 to about 10 : 1 ; a halocarbon promoter ; and a hydrocarbyl aluminum cocatalyst .