The development of efficient carbon dioxide capture and separation technologies is at the forefront of the priorities in climate change policies. Poly(ionic liquid)s (PILs) have been emerging as extremely promising materials for the fabrication of membranes for CO 2 separation. This work is a step forward to evaluate the effect of the PIL-based copolymers chemical structures in the preparation and performance of membranes for CO 2 /N 2 separation. In particular, imidazolium-based homo- and copolymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) copolymerization of different imidazolium salts and characterized by nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) analysis. The membrane forming ability of the synthesized PILs, as well as the influence of different side chain groups (ethyl, pentyl, benzyl, and naphthyl) at the imidazolium ring, were evaluated using the solvent casting technique. In order to improve membrane forming ability and CO 2 separation performance, different amounts of free ionic liquid (IL), [C 2 mim][NTf 2 ], were added into the synthesized homo- and copolymers, and PIL-IL composite membranes were prepared. The CO 2 and N 2 permeation properties of the prepared free-standing PIL-IL membranes were measured at 20 °C and 100 kPa, and the results obtained were compared through the Robeson plot.