espond to formula (I) ##STR2## wherein R1, R2 and R3 are identical or different and stand for C2 -C12 -alkyl or C8 -C20 -aralkyl, and Ar preferably stands for phenyl, diphenyl or naphthyl, and at least one of the groups denoted by R1 or R2 is a C8 -C20 -aralkyl group, and n has a value from 0.5 to 1, preferably from 0.6 to 0.9.