Three dense polymer membranes composed of main-chain, poly(imidazolium)s (imidazolium ionenes) with decyl ("C10") spacer groups were fabricated and shown to be selective for CO2-based separations. An imidazolium ionene with the bromide counterion (Polymer 1) exhibited good H-2/CO2 ideal (pure component) selectivity (6.0), while H-2 permeability was slightly lower than 1 x 10(-10) cm(3)(STP) cm cm((-2)) s((-1)) cm Hg(-1) (1 barrer). A "Robeson Plot" was used to compare the separation performance of Polymer 1 to other dense polymer membranes. Polymer 2 was structurally similar to Polymer 1 with the exception of the bulky Tf2N anion in place of the bromide. This polymer was found to have good CO2/CH4 and CO2/N-2 ideal selectivities (20 and 24, respectively), but no H-2/CO2 separation performance with a selectivity of 1.0. A stable composite film made from an imidazolium ionene (Polymer 2) and a room-temperature ionic liquid (RTIL) was also fabricated and showed large CO2 permeability enhancements compared to the analogous neat polymer membrane, with little or no sacrifice in CO2/N-2 and CO2/CH4 ideal selectivity. The separation performance of Polymer 2 and the Polymer 2-RTIL composite films were compared to previously studied "side-chain" poly(imidazolium)s (poly(RTIL)s) and other dense polymer membranes on "Robeson Plots." Polymer 2 and the Polymer 2-RTIL composite films exhibited CO2 permeabilities comparable to poly(RTIL)s. This is the first known study and application of imidazolium ionenes for CO2 light gas separation membranes. (C) 2009 Elsevier B.V. All rights reserved.