Especially suitable compounds for the component B are aluminum trialkyls of the formula AlR5.sub. 3 or aluminum dialkyl hydrides of the formula AlR5 2 H, 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 4 carbon atoms, for example Al(C2 H5)3, Al(C2 H5)2 H Al(C3 H7)2 H, Al(i-C4 H9)3, Al(i-C4 H9)2 H.