independently of the other hydrogen, C.sub.1 -C.sub.4 -alkyl or halogen; and (Alk*) is C.sub.1 -C.sub.6 -alkylene, and (Alk**) is C.sub.2 -C.sub.12 -alkylene.The following preferences apply to the variables contained in formulae (2a)-(2e): R.sub.1 is preferably hydrogen or C.sub.1 -C.sub.4 -alkyl, in particular hydrogen or methyl.