least one aluminum compound selected from the group consisting of ethylaluminum sesquichloride, tris(hydrocarbyl)aluminum compounds, chlorinated and brominated 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, and aluminoxanes. 2.