. . . . "(a) a group ???[C(O)]vw (CR11b 2)ww (SiR11c 2) (OSiR11c 2)vv R11c in which each group R11b is the same or different and is hydrogen or alkyl of 1 to 4 carbon atoms, each group R11c is the same or different and is alkyl of 1 to 4 carbon atoms or aralkyl, for example benzyl or phenethyl, vw is 0 or 1, ww is from 0 to 6 with the proviso that vw and ww are not both 0, and vv is from 0 to 49;" . . .