NOVELTY - Polyarylene block copolymer comprising a polymer segment (A) having a sulfonic acid group and a polymer segment (B) having substantially no sulfonic acid group is new, where (B) is substituted poly(phenylene) unit (I).    USE - The polyarylene block copolymer is useful: in a polymer electrolyte (used for an electrode electrolyte and in an electrode paste), proton conductive membrane and membrane-electrode assembly for a solid polymer fuel cell (all claimed); and in electrolytes for primary and secondary batteries, solid polymer electrolytes for fuel cells, display elements, various sensors, signaling media, solid condensers and ion exchange membranes etc.    ADVANTAGE - The polyarylene block copolymer: has high proton conductivity, reduced swelling in hot water and reduced shrinkage in drying and high power generation performance and high water resistance; and provides fuel cell with durability against humidity change.    DETAILED DESCRIPTION - Polyarylene block copolymer comprising a polymer segment (A) having a sulfonic acid group and a polymer segment (B) having substantially no sulfonic acid group is new, where (B) is substituted poly(phenylene) unit of formula (I).    R1-R4 = halo, 1-20C hydrocarbon, 1-20C halogenated hydrocarbon, NO2 or nitrile;    E = direct bond, O, S, CO-, SO2, SO, CONH, COO, (CF2)i, (CH2)j or CR1a2, cyclohexylidene or fluorenylidene;    R1a = aliphatic hydrocarbon, aromatic hydrocarbon or halogenated hydrocarbon; either    L = substituted diphenyl moiety of formula (a1), (b1) or (c1); or    at least one of L = (a1) or (b1);    A = O or S;    R5-R10 = halo, 1-20C hydrocarbon or 1-20C halogenated hydrocarbon;    R2a = 1-20C hydrocarbon or 1-20C halogenated hydrocarbon;    D = direct bond, O, S, CO, SO2, SO, CONH, COO, (CF2)k, (CH2)-l, CR1a2, cyclohexylidene or fluorenylidene;    i, j, n, k, l = 1-10;    a-d, q, e, f, g, h, u, s, r = 0-4;    p = 1-200, preferably 2-150; and    m = 0-14.    INDEPENDENT CLAIMS are also included for:    (1) a membrane-electrode assembly for a solid polymer fuel cell, where an anode electrode and a cathode electrode are provided on one side and the other side of proton conductive membrane comprising the polyarylene block copolymer;    (2) a membrane-electrode assembly for a solid polymer fuel cell, where at least one of an anode electrode and a cathode electrode comprises polymer electrolyte comprising the polyarylene block copolymer;    (3) an electrode paste comprising the polymer electrolyte, an electrode catalyst and a solvent; and    (4) an electrode for a fuel cell, comprising the polymer electrolyte and an electrode catalyst; and    (5) substituted poly(phenylene) compounds of formula (II).    Z1 = an atom or a group comprising halo, SO2CH3, SO2CF3 or NO2.