and with at least one aluminum compound selected from hydrocarbylaluminum compounds with general formula AlR???mY3-m, where R??? is a hydrocarbyl radical containing 1 to 6 carbon atoms, Y is a chlorine or bromine atom and m is a number from 1 to 3 (i.e., tris(hydrocarbyl)-aluminum compounds, chlorinated or brominated hydrocarbylaluminum compounds) and aluminoxanes.