Trimethylsilylated derivatives of hexafluoropolysulfone (6FPSf) and tetramethylhexafluoropolysulfone (TM6FPSf) were prepared by reaction of lithiated polymer intermediates with halotrimethylsilane electrophiles for a membrane gas separation study to compare improvements in gas permeabilities with those of unmodified polymers. Ortho sulfone substituted 6FPSf and TM6FPSf having pendant trimethylsilyl (TMS) groups were obtained by a two-step process involving first direct lithiation of 6FPSf and TM6FPSf solutions with n-butyllithium to afford ortho sulfone dilithiated intermediates, followed by reaction with a TMS electrophile. The corresponding ortho ether 6FPSf derivative was obtained by lithiation of dibrominated 6FPSf at the bromine sites, followed by reaction with a TMS electrophile. The degree of substitution (DS) of the TMS groups was greater than or equal to2.0 and was dependent on the molar ratio of n-butyllithium and electrophile quantity, electrophile reactivity, and reaction conditions. Detailed structural characterization and DS of the modified polymers were obtained by nuclear magnetic resonance spectroscopy. The glass transition temperatures and thermal stabilities were determined by differential scanning calorimetry and thermogravimetric analysis, respectively. Polymer chain d-spacing was investigated using wide-angle X-ray diffraction. Polymer free volume was calculated from the polymer density and specific van der Waals volume. The polymer gas permeability coefficients (P) were measured for He, CO2, O-2, and N-2. Both CO2 and O-2 permeabilities of modified 6FPSf increased by 8-9-fold (P(CO2) = 110 barrers, P(O-2) = 28 barrers) compared with those of the starting polymer 6FPSf (P(CO2) = 12 barrers, P(02) = 3.4 barrers), which gives P and a properties very close to the upper bound line.(1) Almost no reduction in permselectivity (alpha) was observed for the CO2/N-2 gas pair, while the O-2/N-2 gas pair showed moderate reduction.