A novel regio-controlled poly(2,5-diphenethoxy-p-phenylene) partially functionalized with sulfonic acids has been developed for a proton exchange membrane. Poly (2,5 -diphenethoxy-p-phenylene) was prepared via the oxidative-coupling polycondensation using iron(111) trichloride as an oxidant. A high molecular weight polymer over 270,000 in the weight-average molecular weight was quantitatively obtained in mild conditions. This polymer was then reacted with two and four equimolar trimethylsilylchlorosulfonate in dichloromethane to give the corresponding sulfonic acid-functionalized polymers, whose functionalities were 0.69 and 1.19 per a polymer unit, which were translated to be 1.73 and 2.49 mequiv/g in ion exchange capacity (IEC), respectively. These polymers showed excellent proton conductivity up to 2 x 10(-1) S/cm at 80 degrees C and 95% relative humidity. (c) 2007 Elsevier Ltd. All rights reserved.