A copolymer including a repeating unit represented by (I) wherein: Y is selected from: a carboxylic acid, sulfonic, phosphorous acid and phosphoric acid and their corresponding salt or ester; imino, amide, nitrile, hydrogen, hydroxyl and alkyl comprising from 1 to 6 carbon atoms; and R1, R2, R3, and R4 are independently selected from: hydrogen, alkyl groups comprising from 1 to 6 carbon atoms, and R1 and R2 may collectively form a ketone group or a 9, 9′-fluorene group, and R3 and R4 may collectively form a ketone group or a 9, 9′-fluorene group; R5 and R6 are independently selected from: a bond and an alkylene group comprising from 1 to 6 carbon atoms; R7 is selected from: hydrogen, alkyl, aryl, aralkyl and heteroaryl groups comprising from 1 to 8 carbon atoms which may be unsubstituted or substituted with carboxylic acid, sulfonic acid and phosphoric acid and their corresponding salt or ester, imino and amide; and X and X′ are independently selected from: a carboxylic acid, sulfonic acid and phosphoric acid and their corresponding salt or ester, imino and amide; nitrile, hydrogen, alkyl having from 1 to 6 carbon atoms and alkoxy having from 1 to 6 carbon atoms.