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Novel sulfonated poly(arylene ether)s derived from ω,ω′-dihydroxypoly(1,4-phenylene ether)s (DHPE) and mixed dihalide monomers of 3,3′-disulfo-4,4′-dichlorodiphenylsulfone disodium salt (S-DCDPS) and 2,6-dichlorobenzonitrile were synthesized. As DHPE has molecular weight distribution with phenylene rings lower than ca. 10 units, experimental determination of its average molecular weight made it possible to be used as a monomer to prepare sulfonated poly(arylene ether)s. Membranes derived from resultant polymers showed a similar level of proton conductivity to those of other common sulfonated poly-(arylene ether)s; however, its softening temperature was much lower. Therefore, good MEA was able to be obtained by hotpressing at relatively lower temperature without shrinking around electrode. The membrane with an ion exchange capacity of 0.9 showed lower methanol permeability and dimensional change in methanol solution assuming a condition of DMFC than standard perfluorosulfonic acid polymer membrane (PFSA). ■ INTRODUCTIONSolid polymer electrolyte membrane fuel cell (PEMFC) is attracting much attention as a new power source for transportation, stationary, portable, and consumer electronics power applications from the viewpoint of environmental and new energy issues. Polymer electrolyte membranes (PEM) made of sulfonic acid-containing aromatic condensation polymers are considered as possible candidates for an alternative of perfluorosulfonic acid polymers (PFSA) membrane, such as Nafion, because their backbone structures are generally heat-and solvent-resistant with good mechanical properties. 1−3 Although PFSA showed much higher stability against radical oxidation and has been used as a standard membrane material for PEMFC, accumulation of studies for sulfonated aromatic condensation polymers may create new characteristic materials.Sulfonic acid-containing aromatic condensation polymers are considered as essentially good structure for especially direct methanol fuel cell (DMFC) because they are stable against hydrolysis and show relatively low methanol permeability. Sulfonated polymers derived from poly(arylene ether ketone)-s, 4−6 poly(arylene ether sulfone)s, 7−10 poly(arylene thioether)-s, 11 other poly(arylene ether)s, 12−14 polyimides, 15−18 poly-(phenylene)s, 19 and other types of polymers have been extensively studied so far. As methanol crossover from anode to cathode induces decrease in fuel efficiency and reduction of cell voltage, these aromatic condensation polymer shows a great advantage against PFSA membrane. Although their performance of polymer electrolyte membrane is highly expected in this point of view, drawback in the processability to prepare membrane−electrode assembly (MEA) exists. As MEA is usually prepared by hot-pressing of membrane with electrode containing PFSA ionomer as a binder, high glass transition temperatures of aromatic condensation polymers sometimes result in poor adhesion between them. The poor adhesion does not only induce difficulty to prepare good MEA but also increases in ME...