s ##STR4## CH.sub.2.dbd.CHSi(OC.sub.3 H.sub.7).sub.3, CH.sub.2 .dbd.CHSi(OC.sub.4 H.sub.9).sub.3,CH.sub.2 .dbd.CHSi(OC.sub.6 H.sub.13).sub.3,CH.sub.2 .dbd.CHSi(OC.sub.8 H.sub.17).sub.3 orCH.sub.2 .dbd.CHSi(OC.sub.10 H.sub.21).sub.3 ;a compound represented by the formula (IV): ##STR5## wherein R.sup.1, R.sup.2, R.sup.5 and a are as defined above, and n is an integer of 1 to