An organoaluminum reaction product of A.) a ligand of the formula I, wherein R.sub.1 represents an alkyl, aryl, arylalkyl, or alkylaryl group, a C.sub.3-24 silyl group, or hydrogen, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.12, and R.sub.13 are the same or different and represent an alkyl, aryl, arylalkyl, or alkylaryl group, a C.sub.3-24 silyl group, halide, or hydrogen, with the proviso that at least one of the groups R.sub.4 and R.sub.13 represents hydrogen, R.sub.8 represents an alkoxy, alkyl, aryl, arylalkyl, or alkylaryl group, a C.sub.3-24 silyl group, halide, hydroxyl radical, or hydrogen, E represents O, S, Se, or Te, and n is an integer from 1 to 4, and B.) an aluminum compound of formula AlR.sub.9R.sub.10R.sub.11, wherein R.sub.9 and R.sub.10 are the same or different and represent C.sub.1-20 alkyl, aryl, arylalkyl, or alkylaryl group, or hydrogen, and R.sub.11 is a C.sub.1-20 alkyl, aryl, arylalkyl, alkylaryl or alkoxy group, hydrogen, or halogen is useful as a polymerization catalyst, particularly for the homopolymerization or copolymerization of an alkylene oxide.