R13 stands for an alkyl group of 1 to 20 carbon atoms or an aromatic group of 6 to 20 carbon atoms, or R14 O--CO--R15 --, wherein R14 stands for an alkyl group of 1 to 20 carbon atoms and R.sup. 15 stands for an alkylene group of 1 to 4 carbon atoms, and n is an integer in the range of 1 to 6).