Over the past several years, polymerizable ionic liquids (poly(ILs)) have been studied as potential materials for use in gas separation membranes, typically with a focus on CO2 removal. ILs present a versatile platform for membrane design as monomer synthetic strategies provide multiple dimensions of control over cation/anion chemical structures. The molecular composition of IL monomers can be directly correlated to measurable structure-property-performance relationships, including gas transport, in the resultant poly(IL) materials. A unique feature of the "bottom-up" design approach for poly(ILs) is that it readily allows for the introduction of unique combinations of functional groups not typically found in, or easily introduced to, conventional polymer materials. The flexibility afforded in monomer design also allows for a straightforward means of comparing performances of structural isomers (e.g. n-propyl vs. i-propyl). However, such comparisons are rarely reported for ILs, let alone poly(IL) materials. Here, we have synthesized several styrene-based IL monomers containing branched- and cycloalkyl groups appended to the imidazolium ring. These monomers were used to form poly(IL) membranes which were analyzed in terms of their permeabilities toward CO2, N-2 and CH4 as well as their selectivities for CO2 relative to the other species. Relative to their n-alkyl counterparts, poly(ILs) with branched and cyclic functionalities exhibited similar to 20% larger CO2/N-2 and CO2/CH4 selectivities, yet gas permeabilities dropped by more than 50%. It is proposed that the improvements in selectivity associated with branched and cyclic functionalities is due to smaller fractional free volumes (FFV) compared to analogous n-alkyl chains, which is supported by computational studies using COSMOTherm. (C) 2015 Elsevier B.V. All rights reserved.