Cationic copolymerizations of vinyl ether monomers [2-methoxyethyl vinyl ether: MOVE, 2-(2-methoxyethoxy)ethyl vinyl ether: MEEVE, 2-adamantyl vinyl ether: AdVE, 2-(2-vinyloxyethoxy)ethyl methacrylate: VEEM] were performed to obtain three types of vinyl ether copolymers [poly(MOVE-AdVE)s, poly(MEEVE-AdVE)s, and poly(MEEVE-VEEM)s] with different composition rates. Poly(MOVE-AdVE) and poly(MEEVE-AdVE) obtained at monomer feed ratio of 1:1 exhibited the glass transition temperatures (T-g) of 55 and 28 degrees C, respectively, but the T-g's of copolymers were near or lower than room temperature when the feed ratio of AdVE decreased. Poly(MOVE-AdVE)s and poly(MEEVE-AdVE) with T-g's above room temperature afforded free-standing membranes by casting them from toluene solutions. They exhibited relatively high CO2 permeability and high CO2/N-2 separation factors (P(CO2) = 22-36 barrers, P(CO2)/P(N-2) = 19-40). The T-g's of poly(MEEVE-VEEM)s were very low and around -70 degrees C irrespective of the difference of monomer feed ratio. Methacrylate groups in poly(MEEVE-VEEM)s partially reacted under heating to give crosslinked polymer membranes. The crosslinked membranes showed high CO2/N-2 selectivity, especially the poly(MEEVE-VEEM) membrane possessing the highest ratio of MEEVE exhibited high CO2 permeability and high selectivity (P(CO2) = 120 barrers, P(CO2)/P(N-2) = 55). (C) 2012 Elsevier Ltd. All rights reserved.