J represents an unsubstituted or substituted alkylene group, preferably an alkylene group which has from 1 to 10 carbon atoms (including substituted alkylene groups, such as methylene, ethylene, propylene, trimethylene, butylene, hexylene), an unsubstituted or substituted arylene group (including substituted arylene groups such as phenylene) or a --(CH2 CH2 O)m (CH2)n group, a ##STR11## group (whe