The organoaluminum oxy-compounds used as the catalyst components may be shown as benzene-soluble aluminoxanes represented by the following general formulas (1) and (II). ##STR7## wherein R may be the same or different and is a hydrocarbon group such as methyl, ethyl, propyl or butyl, preferably methyl or ethyl and especially methyl, and m is an integer of at least 2, preferably at least 5.