R3 O(O)C--R1 --C(O)--OCH2 CH2 --m R2 wherein R1 denotes the residue of an aliphatically unsaturated discarboxylic acid, R2 denotes a halogen atom or an organic residue other than a hydroxyl group, R3 denotes an alkyl, aryl, alkylaryl, aralkyl, or alkoxyalkylene glycol residue, and m an integer not less than 5.