tert-Butylphenyl-terminated disulfonated poly(arylene ether sulfone) random copolymers with a sulfonation degree of 35mol% (BPS35) and controlled molecular weights (M-n), 20-50 kg mol(-1), were successfully prepared by direct copolymerization of the two activated halides, 4,4'-dichlorodiphenyl sulfone (DCDPS) and 3,3'-disulfonate-4,4'-dichlorodiphenyl sulfone (SDCDPS) with 4,4'-biphenol and the endcapper, 4-tert-butylphenol. Dilute viscosity measurements of the BPS35 random copolymers were successfully conducted in NMP containing various concentrations of LiBr from 0.01 to 0.2 M and mostly at 0.05 M according to the measured theory. The effects of salt concentration and molecular weights of the copolymers on the viscometric behavior were studied and compared with published data for sulfonated polystyrene. The charge density parameter (xi) for the BPS35 copolymers was determined to be smaller than 1, suggesting that no counterion condensation occurs. Studies of the effect of ionic strength (I) on the intrinsic viscosities ([eta]) under theta condition were obtained by plotting [eta] vs. I-1/2 and extrapolating to infinite ionic strength. For salt-free BPS35 solutions, the viscometric behavior was shown to fit well with the Liberti-Stivala equation, providing a way to determining intrinsic viscosity when the copolymer charge is fully screened. Intrinsic viscosity and molecular weight characterization of BPS35 copolymers by SEC and static light scattering are also presented. The results are very useful for characterizing polymeric electrolyte membrane (PEM) for fuel cells, reverse osmosis and ionic transducer membranes. (c) 2008 Elsevier Ltd. All rights reserved.