wherein R1 and R2 each is a bivalent hydrocarbon group having a linear or branched chain having 1 to 36 carbon atoms, n is 1 to 15, X and Y each is at least one substituent selected from the group consisting of a carboxyl group, a hydroxyl group and a hydrogen atom, and at least one of X and Y is a carboxyl group or a hydroxyl group; in the case where n is 2 to 15, the moiety of (???R2???COO???)n