TECHNOLOGY FOCUS - POLYMERS - Preparation (Claimed): Preparation of (I) comprises: reacting substituted phenol compounds of formulae (III) and (IV) to give substituted polyphenylether compounds of formula (V); reacting (V) with a halogen to give substituted halogenated polyphenylether compounds of formula (VI); and sulfonating (VI) to give substituted polyphenylether compounds of formula (VII) (representative of (A)). Preferred Components: The ratio of m to n is 1:9 to 9:1. The polyphenylether-based copolymer has a weight average molecular weight of 10000-200000. The polymer electrolyte membrane has a proton conductivity of 1x 10-3/cm or higher, and a methanol permeability of 5x 10-7 cm2/S or less. Preferred Definitions:    R1, R2 = CH3;    R3, R4 = H; and    Ar1, Ar2 = 1,4-phenylene-diyl.    EXAMPLE - A condenser was mounted on a 500 ml, 3-necked flask, and 2,6-dimethylphenol (12.25 g), 2,6-diphenylphenol (2.745 g), copper(I) chloride (0.15 g) and pyridine (50 ml) were dissolved in 4-chlorotoluene (400 ml) in the flask. The mixture solution was reacted for 18 hours in an oxygen bubbling condition to give a polymer. The color of the reaction solution changed from bright yellow to dark brown during the reaction. After the reaction was complete, the polymer solution was precipitated in methanol (2l) to obtain a solid of polyphenylether. A condenser was mounted on a 100 ml, 3-necked flask, and polyphenylether (5 g) was dissolved in chloroform (5 g) in the flask. A mixed solution of chloroform (1 ml) and bromine (1 ml) was placed into a dropping funnel and slowly drop wise added into the solution. Then, the mixed solution was reacted for 1 hour. After the reaction was complete, the reaction solution was precipitated in methanol (800 ml) to obtain a brominated polyphenylether as solid. A condenser was mounted on a 500 ml, 3-necked flask, and brominated polyphenylether (5 g) was dissolved in chloroform (250 g) in the flask. A mixed solution of chloroform (60 ml) and chlorosulfonic acid (9 ml) was placed into a dropping funnel and slowly drop wise added into the solution. Then, the mixed solution was reacted 1 hour while stirring. After the reaction was complete, the reaction solution was precipitated in methanol solution (2l) diluted with water in a ratio of 5:5 to obtain a solid. Next, the solid was washed several times with methanol and distilled water until the pH reached 7 to give polyphenylether-based copolymer, where M1-M3 are sodium, m is 0.9, n is 0.1 and X is bromine and the weight average molecular weight is 50000.