TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The polyimide is substituted by bivalent or trivalent group, which is not coupled with sulfonic acid group, when 90 mol% or less of group (Ar2-C(O)-X) of polyimide has aromatic ring. The polyimide is substituted by bivalent group, which is not coupled with sulfonic acid group, when 2-90 mol% of group (Ar2-C(O)-X) of polyimide has aromatic ring. The polyimide is substituted by trivalent group, which is not coupled with sulfonic acid group, when 2-30 mol% of group (Ar2-C(O)-X) of polyimide has aromatic ring. The polyimide is substituted by bivalent and/or trivalent group, which is not coupled with sulfonic acid group, when 90 mol% or less of group (Ar2-C(O)-X) dispersed statistically or unevenly dispersed in block shape in polyimide chain has aromatic ring. The polyimide has cross-linked structure substituted by trivalent group, which is not coupled with sulfonic acid group, when 90 mol% or less of group (Ar2-C(O)-X) of polyimide has aromatic ring. Preferred Definitions:    X=group of formula (17);    Z=aromatic ring, -O-, -S-, -SO2-, -CF2- or -C(CF3)2-;    m=1-30;and    n=1-2.    EXAMPLE - 4-(4-(3,5-dinitro benzoyl)-2-sulfo phenoxy)benzene-1,3-disulfonic acid (in g) (3.35), stannous chloride dihydride (11.63), ethanol (50 ml) and water (10 ml) were added to a flask under nitrogen atmosphere. Concentrated hydrochloric acid (8.75 ml) was added, and reaction mixture was stirred at room temperature for 4 hours. The mixture was filtered, and filtrate was vacuum-dried at 60degreesC for 10 hours to obtain a solid product (2.1). The obtained solid product (2.176) and triethyl amine (2.1 ml) were dissolved in cresol (16 ml). 1,4,5,8-naphthalene tetracarboxylic acid dianhydride (1.072) and benzoic acid (0.68) were added, and stirred at 180degreesC for 4 hours and 80degreesC for 10 hours. N-methyl pyrrolidone (10 ml) was added, and stirred at 180degreesC for 10 hours. Pyrrolidone was further added after cooling, and reaction liquid was precipitated, filtered, washed and dried. The obtained product was dissolved in dimethyl sulfoxide, and flow-spread on glass plate. The coating was dried at 80degreesC for 10 hours, and sulfonated polyimide film was obtained. The film was immersed in methanol for 2 days, and sulfuric acid solution for 2 days. Proton exchange was carried out, and film was water-washed and dried to form proton-type sulfonated polyimide film. The film had high ion exchange capacity.