NOVELTY - Branched polyaryl ether copolymers comprising building blocks of substituted polyaryl compounds (A1) and (A2), and building blocks (B), which are derived from compounds having at least three hydroxy functions and present at 0.1-10 wt.% based on the total weight of the copolymers, are new.    USE - The polyaryl ether copolymer are useful: in polymer blends; and for producing membranes including fuel cell membranes, ultra filtration membranes and gas separation membranes (all claimed).    ADVANTAGE - The branched polyaryl ether copolymer: exhibits high water absorption; has higher elongation at break in the moist state than the linear products; and has high mechanical strength including good tensile strength.    DETAILED DESCRIPTION - Branched polyaryl ether copolymer comprising building blocks of substituted polyaryl compounds of formulae (A1) and (A2); and building blocks (B), which are derived from compounds having at least three hydroxy functions and present at 0.1-10 wt.% based on the total weight of the copolymer, are new.    Q, T1, Y1 = bond, -O-, -S-, -SO2-, S=O, C=O, -N=N-, -R1aC=CR1b or -CR1CR1d-;    R1a, R1b = H or 1-12C alkyl; either    R1c, R1d = 1-12C alkyl, 1-12C alkoxy or 6-18C aryl (all optionally substituted by F and/or Cl) or H; or    CR1cR1d = 3-12C cycloalkyl (optionally substituted 1-6C alkyl), where at least one of Q, T1 and Y1 is different from -O-, and at least one of Q, T1 and Y1 is -SO2-;    A1, A2 = 6-18C aryl (optionally substituted by 1-12C alkyl, 6-18C aryl, 1-12C alkoxy or halo);    t, q = 0-3;    m = 0-4; and    n = 1-4.    INDEPENDENT CLAIMS are also included for:    (1) preparation of the branched polyaryl ether copolymers;    (2) a polymer blend comprising at least one polyaryl ether copolymer and at least one polymer comprising polyether sulfones, polysulfones, polyether ketones, polyetherimides, polyimides, polybenzimidazoles, polyamidimides or polyamides; and    (3) preparing polymer blends comprising mixing branched building blocks (A1), (A2) and (B) in solution with the further polymer(s).