prov:value
| - ogen atom or a C3 -C10 alkenyl group, at least one of the radicals R14 to R17 being an alkenyl group, or ##STR4## wherein each of R18, R19, R20, R21, R22 and R23, 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
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