. "(a) a group ???[C(O)]rs(CR13b 2)ss (SiR13c 2) (OSiR13c 2)rr R13c in which each group R13b is the same or different and is hydrogen or alkyl of 1 to 4 carbon atoms, each group R13c is the same or different and is alkyl of 1 to 4 carbon atoms or aralkyl, for example benzyl or phenethyl, rs is 0 or 1, ss is from 0 to 6, with the proviso that rs and ss are not both 0, and rr is from 0 to 49; or" . . . . . .