ng to the formula ##STR7## in which R3 is a linear or branched alkyl group having 1 to 5 C atoms or a phenyl group and the free valencies of the oxygen atoms bonded to the silicon and aluminium atoms in the case of the units (I'), (II') and/or (III') and also (IV') as in the case of heterosiloxane skeletons are saturated by a silicon atom of a similar or a different unit or by an aluminium atom, w