Linear or cyclic aluminoxanes, which are preferred, such as those described in U.S. Pat. Nos. 4,897,455 to Wellborn, Jr. and 4,912,075 to Chang and dimeric compounds of the formula (Y)2 --Al--O--Al--(Y)2 where each Y is the same or different and is an alkyl containing from about 1 to about 6, preferably from about 2 to about 4, carbon atoms can also be used as the cocatalyst.