independently of the others, is a hydrogen atom, C1 -C4 alkyl or C3 -C10 alkenyl, at least one of the radicals R18 to R23 being an alkenyl group, and a is a number from 0 to 10, and C) a compound of formula V ##STR19## wherein R24 and R25 are identical or different and each is hydrogen or C1 -C10 alkyl, or of formula VI, ##STR20## wherein R26 and R27 are identical or different and each is hydrogen