New sulfonated poly(arylene ether sulfone) copolymers with high molecular weights were successfully synthesized with controlled degrees of disulfonation of up to 70 mol % via the direct copolymerization of sulfonated aromatic dihalides, aromatic dihalides, and one of four structurally distinct bisphenols. The disodium salts of the 3,3'-disulfonated-4,4'-dichlorodiphenyl sulfone and 3,3'-disulfonated-4,4'-difluorodiphenyl sulfone comonomers were synthesized via the sulfonation of 4,4'-dichlorodiphenyl sulfone or 4,4'-difluorodiphenyl sulfone with 30% fuming sulfuric acid at 110 degreesC. Four bisphenols (4,4'-bisphenol A, 4,4'-bisphenol AF, 4,4'-biphenol, and hydroquinone) were investigated for the syntheses of novel copolymers with controlled degrees of sulfonation. The composition and incorporation of the sulfonated repeat unit into the copolymers were confirmed by H-1 NMR and Fourier transform infrared spectroscopy. Solubility tests on the sulfonated copolymers confirmed that no crosslinking and probably no branching occurred during the copolymerizations. Tough, ductile films were solvent-cast that exhibited increased water absorption with increasing degrees of sulfonation. These copolymers are promising candidates for high temperature proton-exchange membranes in fuel cells, which will be reported separately in part II of this series. (C) 2003 Wiley Periodicals, Inc.