0 or less carbon atoms, ???(Y???O)??? is an arrangement of one kind of structure defined by the formula (12) or a random arrangement of two or more kinds of structures defined by the formula (12) in which R10 and R11 may be the same or different and are 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 ato