In this case, an oxygen atom, a nitrogen atom and a silicon atom each can be present in the hydrocarbon group or the end thereof as --O-- or ???C???O bonds; --NR'-- bond wherein R' is a hydrogen atom, an aryl group or an alkyl group having 1 to 8 carbon atoms, particularly 1 to 6 carbon atoms; or a straight chain or cyclic organosiloxy group and organosilylene group as follows: ##STR6## wherein R