up comprising from 2 to 12 carbon atoms, X is a chlorine or bromine atom and y is zero, one or a fraction between zero and one, and organoaluminium compounds of the formula Al(R8)3-z Xz, in which R8 is an alkyl group comprising from 2 to 12 carbon atoms, X is a chlorine or bromine atom and z is zero, one or two, or a fraction between zero and two.