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A novel six-membered ring hyperbranched polyimide (HBPI) has been synthesized by condensation polymerization of a difunctional monomer, 9,9-fluorenylidenebis(4,1-phenylene)bis(oxy)-4,4′-bis(1,8-naphthalic anhydride) (FBPNA), and a trifunctional monomer, tris(4-aminophenyl)amine (TAPA), at the molar ratio of FBPNA/TAPA = 1:1 in m-cresol at 180°C for 20 h. The resultant HBPI is further modified via end-capping reaction with 4-phenoxy-1,8-anhydride naphthalene (PNA). Post-sulfonation is performed in concentrated sulfuric acid at different temperatures (50, 60, and 70°C) for the pristine HBPI and 50°C for the PNA-modified polyimides to give various sulfonated HBPIs. Freestanding and tough membranes have been successfully fabricated by casting the polymer solutions containing a cross-linker, bisphenol A epoxy resin, at 80°C. The ion exchange capacities of the resultant membranes are in the range of 1.35–2.21 meq g−1 depending on the degree of chemical modification and the sulfonation conditions. Membrane properties such as water uptake, swelling ratio, proton conductivity, and radical oxidative stability are investigated.