A new methodology is developed to predict CO2 sorption isotherms and CO2 induced plasticization behaviors of glassy polyimides using only fully atomistic simulations, which combines quantum-level charge calculations with integrated molecular dynamics (MD) and Monte-Carlo (MC) simulations. The methodology is validated by reproducing the sorption isotherms of Matrimid and 6FDA-DAM up to 35 bar. Structural changes occurring in a polymer due to CO2 sorption is characterized over a pressure range in terms of change in available CO2-accessible free volume in the polymer structure that is in equilibrium with a given CO2 bulk pressure (CAFV+) with respect to its value before sorption is used to characterize plasticization. It is found that the sorption pressure yielding the minimum value of this change can be correlated to the plasticization pressure of polyimides. Next, this methodology was applied to copolyimides containing pBAPS (bis [4-(4-aminophenoxy) phenyl] sulfone) group for which no high pressure experimental data were reported: 6FDA/BTDA-pBAPS, 6FDApBAPS/DABA and 6FDA-pBAPS/mPDA. Radial Distribution Function analyses were performed in order to understand effect of sorption sites on plasticization. Sorption simulations up to 35 bar and CAFV+ analyses suggested that among these three copolyimides, 6FDA-pBAPS/DABA and 6FDA-pBAPS/mPDA are more plasticization resistant compared to Matrimid (R) and 6FDA-DAM, with potential to be used in CO2/CH4 separation.