roup, and X represents ##SPC3##--COOM, --SO3 M or --COO(CH2)m SO3 M, where M is a cation and m is 3 or 4; and c. the balance of repeating units of the formula (III) ##EQU11## where R3 represents a hydrogen atom or a methyl group, R4 and R5 each represents a hydrogen atom, a lower alkyl group or a substituted alkyl group thereof wherein the substituents are selected from hydroxy, halogen, alkoxy, a