(a) a group ???[C(O)]tu (CR12b 2)uu (SiR12c 2) (OSiR12c 2)tt R12c in which each group R12b is the same or different and is hydrogen or alkyl of 1 to 4 carbon atoms, each group R12c is the same or different and is alkyl of 1 to 4 carbon atoms or aralkyl, for example benzyl or phenethyl, tu is 0 or 1, uu is from 0 to 6, with the proviso that tu and uu are not both 0, and tt is from 0 to 49; or