It includes, for example, the following polymers. ##STR4## (R1 and R2 are an alkyl group and may be the same or different.) ##STR5## (R1 and R2 are an alkyl group and may be the same or different.) ##STR6## (R1 and R2 are an alkyl group and may be the same or different.) ##STR7## (R1 and R2 are an alkyl group and may be the same or different.)