R4 in the repeating unit represented by the above-described general formula (III) is a divalent hydrocarbon group of carbon number 1-20 (some or all of the hydrogen atoms in these hydrocarbon groups can be substituted with halogen atoms independently) or a group represented by --R5 --X--R6 -- (each of R1 and R6 is a divalent aromatic hydrocarbon group, and a hydrogen atom on its aromatic ring may