"(R1)2 AlO(Al(R1)-O)m Al(R1)2 in which R1 denotes an alkyl radical, e.g. of 1 to 6 carbon atoms, such as methyl or ethyl, and m is a number ranging from 2 to 40, preferably from 10 to 20, or a cyclic aluminoxane corresponding to the general formula: ##STR1## in which R1 and m are as defined above." . . . .