TECHNOLOGY FOCUS - POLYMERS - Preparation: No general preparation given. Preferred Units: The fused-aromatic-ring structural unit is preferably of formula (3), more preferably of formula (4). The polyarylene copolymer further consists of an aromatic-sulfonic-acid-containing structural unit of formula (5).AA1=group of formula (aa1);D'=-O-, or -CR'2-;P"=group of formula (a, b or c);P'=group of formula (d);p=0.01-1;Y'=-CO-, -SO2-, -SO-, single bond, -(CF2)u-, or -C(CF3)2-;u=1-10;Z=direct bond, -(CH2)l-, -C(CH3)2-, -O-, or -S-;l=1-10;Ar=aromatic group having substituent chosen from -SO3H, -O(CH2)pSO3H, and -O(CF2)pSO3H;p=1-12;c,d=0-3;andk=1-4. Preferred Definitions:    P'=bivalent group consisting of aromatic ring chosen from naphthalene, anthracene, tetracene and pentacene;and    group of formula (2)=nitrogen-containing heterocyclic ring group chosen from pyrrole, thiazole, isothiazole, oxazole, isoxazole, pyridine, imidazole, imidazoline, imidazolidine, pyrazole, 1,3,5-triazine, pyrimidine, pyridazine, pyrazine, indole, quinoline, isoquinoline, benzimidazole, benzoxazole, benzthiazole, tetrazole, tetrazine, triazole, carbazole, acridine, quinoxaline, quinazoline, or their derivatives.    EXAMPLE - (In g) 2,7-dihydroxynaphthalene (30.8), 2-bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane (96.8), 2,6-dichlorobenzonitrile (92.9), and potassium carbonate (86.2) were taken in a flask. After vacuum-drying, sulfolane (675 mL) and toluene (270 mL) were added and heat-refluxed at 130 degrees C in nitrogen atmosphere. Water and toluene were removed, and 2,6-dichlorobenzonitrile (31) was added and stirred for 3 hours. The insoluble substance was removed, and a precipitate obtained by filtration was washed with methanol and vacuum-dried. The dry product was dissolved in tetrahydrofuran (0.4 kg), poured into methanol (3 L), and reprecipitated. The solidified substance was filtered and vacuum-dried, to obtain an oligomer (144) with a yield of 80%. The oligomer had a number average molecular weight of 5600 and a weight average molecular weight of 7400. 3-(2,5-Dichlorobenzoyl)neopentylbenzenesulfonate (37.67), 2,5-dichloro-4'-(1-imidazolyl)benzophenone (1.49), bis(triphenylphosphine)nickel dichloride (2.62), sodium iodide (0.45), triphenylphosphine (10.49), and zinc (15.69) were polymerized with N,N-dimethylacetamide (152 mL) and lithium bromide (32.6) at 115 degrees C for 7 hours. The solution was stirred for 7 hours and poured into acetone (2 L), and a product was precipitated. The precipitate was washed and dried, to obtain a polyarylene copolymer having a weight average molecular weight of 163000 and a sulfonic-acid equivalent of 2.38 meq/g. A proton-conducting membrane obtained using the copolymer had an proton conductivity of 0.35 S/cm.