Cationic copolymerizations of vinyl ether monomers having a triethylene glycol segment [2-(2-(2methoxyethoxy)ethoxy)ethyl vinyl ether (TEGMVE)] with vinyl ether monomer having a methacrylate unit [2-(2-vinyloxyethoxy)ethyl methacrylate: VEEM] were performed to obtain crosslinkable copolymers [poly(TEGMVE-VEEM)] with different composition rates. Methacrylate groups in poly(TEGMVE-VEEM)s partially reacted to give crosslinked polymer membranes. The free-standing membranes of poly(TEGMVE-VEEM)s obtained at monomer feed ratios of 6:1, 10:1, and 14:1 were too brittle, while their composite membranes with PTFE sheet were tough enough to measure gas permeability. All the membranes of poly(TEGMVE-VEEM) showed high CO2/N-2 selectivity (P-CO2/PN2, = 35-54) because of their high CO2 solubility selectivity (S-CO2/S-N2, = 38-61). The gas permeability increased as the composition ratio of VEEM decreased, and the composite membrane of poly(TEGMVE-VEEM)(14:1) showed the highest CO2 permeability (P-CO2 = 410 barrers) among the present polymer membranes. (c) 2014 Elsevier Ltd. All rights reserved.