NOVELTY - A block copolymer (I) comprises hydrophilic segment and benzonitrile-containing hydrophobic segment.    USE - Block copolymer used for anion exchange film fuel cell (claimed).    ADVANTAGE - The film is developed with ion transfer channel, and has high water content, low swelling property, high ion conductivity and superior fuel cell performance.    DETAILED DESCRIPTION - A block copolymer of formula (I) comprises hydrophilic segment and benzonitrile-containing hydrophobic segment.    m=5-50;    n=5-50;    R=H or group of formula (i)-(iv); and    Ar1 and Ar2=group of formula (a)-(d).    An INDEPENDENT CLAIM is included for preparation of block copolymer (I) comprising:    (A) taking 3,3-bis(4-hydroxy-3,5-dimethylphenyl)phthalide and difluoro monomer under nitrogen protection and anhydrous potassium carbonate and toluene, dissolving into nonproton solvent, reacting at 140-150 degrees C for 4-6 hours, heating at 160-180 degrees C for 10-12 hours, adding 5% difluoro monomer, reacting at 160-180 degrees C for 1-2 hours, continuously cooling, precipitating using alcohol aqueous solution, filtering, washing, and drying to obtain hydrophilic segment;    (B) taking 2,6-difluorobenzonitrile and bishydroxy aromatic monomer, feeding under nitrogen protection anhydrous potassium carbonate and toluene, dissolving into nonproton solvent, reacting at 140-150 degrees C for 4-6 hours, heating at 160-180 degrees C for 10-12 hours, adding 5% hydroxy-containing aromatic ring monomer, continuously reacting at 160-180 degrees C for 1-2 hours, cooling, precipitating using alcohol aqueous solution, filtering, washing, and drying to obtain hydrophobic segment;    (C) taking under nitrogen protection hydrophilic segment, hydrophobic segment, anhydrous potassium carbonate and toluene, dissolving into nonproton solvent, reacting at 140-150 degrees C for 3-4 hours, heating at 160-180 degrees C for 18-20 hours, cooling, precipitating using alcohol aqueous solution, filtering, washing, and drying to obtain monomer;    (D) dissolving into 1,1,2,2-tetrachloroethane, adding N-bromosuccinimide and initiator, reacting, cooling, precipitating using methanol, filtering, washing, and drying; and    (E) adding functional agent, dissolving into nonproton solvent, coating into glass plate, heating, to evaporate solvent to obtain solid film, dipping into alkali liquid, and washing.