Incidentally, the copolymers synthesized by the above-mentioned processes are inferred to have the structures represented by the general formula (9): ##STR3## wherein each of A1 to A4 is a group selected from the group consisting of sulfonic group, carboxyl group and their alkali metal salts, ammonium salts and substituted ammonium salts; each of B1 to B4 is a group selected from the group consist