(b) a siloxane group ???(CR16a 2)qq (SiR16b 2) (OSiR16b 2)ppR16b in which each group R16a is the same or different and is hydrogen or alkyl of 1 to 4 carbon atoms or aralkyl, for example benzyl or phenethyl, each group R16 is alkyl of 1 to 4 carbon atoms, qq is from 1 to 6 and pp is from 0 to 49.