eacted in sequence with (4) at least one first halogenated aluminum compound having the formula Rn 2 AlY3-n, wherein R2 represents a hydrocarbon having 1 to 20 carbon atoms, Y is a halogen atom, and 1???n<3; then (5) at least one silicon compound selected from the group consisting of polysiloxanes and silanes, and mixtures thereof; then (6) at least one second halogenated aluminum compound selecte