a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group and R12 and R13 may be the same or different and are a hydrogen atom, a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group, Z is an organic group which has no OH group in a side chain and has one or more carbon atoms and which may contain an oxygen atom, each of a and b is 0 or an integer of 1