TECHNOLOGY FOCUS - POLYMERS - Preparation (claimed): 5-100 mol% of phenoxy group-containing aromatic diamine of formula (6) and 0-95 mol% of aromatic diamine, and aromatic tetracarboxylic acid are reacted to obtain phenoxy sulfonated aromatic polyimide.X=same as defined above.Preferred Polymer: The polyimide contains 5-100 mol% of structural unit of formula (1) and 0-95 mol% of structural unit of formula (3).Ar3=Ar1;andAr4=bivalent group having at least one aromatic ring and which does not contain -SO3X group.Preferred Property: The fracture stress after immersing polymeric electrolyte film having thickness of 30 microns in pressurized hot water at 130degreesC for 190 hours and bending to 180degrees, is 50 MPa or more. The film has proton conductivity of 0.10 S/cm or more at 60degreesC and 100% relative humidity, 0.004 S/cm or more in 50% relative humidity, 0.03 S/cm or more at 120degreesC and 50% relative humidity, after immersing in 130degreesC or more pressurized hot water for 190 hours. Preferred Definitions:Ar1,Ar3=aromatic group of formula (4 or 5).    Z,Y'=direct bond, CO, O, CH2 or SO.    EXAMPLE - 2,2'-diphenoxy-4,4'-diamino biphenyl (in g) (6.001) was cooled and dissolved in concentrated sulfuric acid (991). Fuming sulfuric acid (3 ml) was added slowly and the contents were stirred at 0degreesC for 0.5 hour. The contents were heated to 40degreesC and cooled to room temperature after 2 hours. The reaction liquid was poured into ice and white solid was precipitated. The precipitate was filtered and dried to obtain 2,3'-bis (3-sulfo phenoxy) benzidine (2,2'-BSPOB) as white solid product (6.04) having melting point of 300degreesC or more. 2,2'-BSPOB (1.053) and triethyl amine (0.67 ml) were added and dissolved in m-cresol (9 ml). 1,4,5,8-naphthalene tetracarboxylic acid dianhydride (0.532) and benzoic acid (0.337) were added and stirred at 180degreesC for 4 hours. The reaction mixture was cooled to 80degreesC and m-cresol (25 ml) was added. Excess acetone was added and the solid precipitated was filtered and dried. The obtained product had solution viscosity of 4.1 dl/g. The obtained product was dissolved in m-cresol, flow-spread on a glass plate and dried to form triethyl amine salt type sulfonated polyimide film with water absorptivity of 190 wt.%, proton conductivity of 0.023 S/cm at 50% relative humidity and excellent hot water resistance.