The aluminoxane may be of the formula R'2 AlO(R'AlO)m AlR'2 where each R' is independently selected from the group consisting of alkyl, cycloalkyl, aryl or alkyl-substituted aryl and has 1-20 carbon atoms, especially alkyl or cycloalkyl of 1-6 carbon atoms, and m is 0 or an integer, preferably 0 to 4.