In the general formula (II) which represents the repeating unit II, R3 and R4, independently of one another, that is, they may be the same or different in each repeating unit as is case with the aforestated R1 and R2, are each a hydrogen atom, an alkyl group having 1 to 4 carbon atoms such as a methyl group or an ethyl group, an alkenyl group having 2 to 4 carbon atoms such as vinyl group or allyl