Polymers containing CO2-philic groups are of great interest for CO2/light gas separation membranes because the affinity toward CO2 can effectively increase CO2 solubility and thus permeability. In this study, polysulfones (PSUs) modified with different degrees of benzyldimethylamine (DMA), benzyltrimethylammonium fluoride (TMAF), and benzyltrimethylammonium iodide (TMAI) were synthesized using sequential post-functionalization reactions and investigated for CO2/N-2 and CO2/CH4 separation. The physical properties of these polymers were studied, including density, fractional free volume, and glass transition temperature. In contrast to the conventional wisdom that tertiary amines exhibit an affinity toward CO2, this study convincingly shows that the DMA substituent has a minimal impact on CO2 solubility and CO2/light gas solubility selectivity in PSUs under dry condition. On the other hand, incorporating TMAF and TMAI in PSU significantly increases CO2 solubility. Particularly, introducing TMAI with a molar ratio of 1.07 relative to PSU repeating units increases CO2/CH4 solubility from 4.4 to 5.2, CO2/CH4 permeability selectivity from 21 to 45, and CO2/N-2 permeability selectivity from 24 to 33 at 35 degrees C, while the CO2 permeability decreases from 5.6 to 1.7 Barrers. The effect of these functional groups in PSUs on gas diffusivity and diffusivity selectivity can be satisfactorily described by the free volume model. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1239-1250