Further suitable compounds for component B are chlorine-containing aluminum-organic compounds, for example dialkyl aluminum monochlorides of the formula R5.sub. 2 AlCl or alkyl aluminum sesqui-chlorides of the formula R5 3 Al2 Cl3, wherein R5 is a hydrocarbon radical having from 1 to 16 carbon atoms, preferably an alkyl radical having from 1 to 16, especially from 2 to 12 carbon atoms.