CH2 ═C(R3)--C(O)--O--(R4 O)m --(C6 H4)--R5 where R3 is a hydrogen atom or a methyl group; R4 is an alkylene group having 2 to 6 carbon atoms; R5 is a hydrogen atom or an alkyl group having 1 to 12 carbon atoms; and m is an integer from 0 to 12, preferably from 1 to 8.