Pentafluorophenyl sulfone was prepared by oxidation of pentafluorophenyl sulfide. Ethynyl terminated fluorinated poly(arylene ether sulfone) (EFPAESO) was synthesized via nucleophilic aromatic substitution from 4,4'-(hexafluoroisopropylidene) diphenol or 4,4'-(trifluoromethylphenylisopropylidene) diphenol with an excess of pentafluorophenyl sulfone, followed by reaction with 3-ethylnylphenol. The molecular weights (M(n)s) of the polymers determined by GPC with polystyrene standard were in the range of 6,400-17,200 and polydispersities (M-w/M(n)s) were in the range of 2.25-3.19. This EFPAESO showed very high thermal stability up to 479 degreesC for 5% weight loss in TGA in air. T-g of the polymer was changed from 148 to 196 degreesC after curing. The cured films showed good chemical resistance and high thermal-stability. At 1550 nm wavelength, the refractive indices of the copolymer films were in the range of 1.5037-1.5504 and birefringences were in the range of 0.0021-0.0025. The optical loss for EFPAESO was less than 0.37 dB/cm at 1550 nm wavelength. (C) 2003 Elsevier Science Ltd. All rights reserved.