Aminosilane-modified poly(ethylene glycol) (PEG), PEG-block-poly(propylene glycol) (PPG)-block-PEG (PEPEG), and PPG-block-PEG-block-PPG (PPEPG) diacrylate prepolymers are synthesized as new membrane materials for gas separation. By combining the in situ sot gel process and cross-linking process together, the silane-modified prepolymers form flexible nanocomposite membranes with a maximum CO2 permeability of 274 barrer and CO2/N-2 selectivity of about 30. The CO2 permeabilities of silane cross-linked PEPEG and PPEPG membranes are higher than those of silane cross-linked PEG membranes, whereas the CO2/N-2 and CO2/H-2 selectivities increase as the PEG content increases in the membrane. A proper aminosilane/prepolymer reaction ratio benefits the gas separation performance of the membrane due to the formation of different cross-linking structures and composites.