Copolymers of n-butyl acrylate and phosphonium ionic liquid monomers possessing various alkyl substituents and counterions were synthesized through a combination of conventional free radical copolymerization and anion exchange. Differential scanning calorimetry and dynamic mechanical analysis provided the thermal and mechanical properties of these phosphonium cation-containing random copolymers. Factors including alkyl chain length of phosphonium substituents, counterion type, as well as ionic concentration significantly influenced the association of phosphonium cations. Phosphonium ionomers with trialkyl substituents on phosphonium cations did not display the characteristic small-angle X-ray scattering peak, suggesting the absence of ionic clusters. However, low q peaks in wide-angle X-ray diffraction was indicative of significant concentration fluctuations wherein the ionic monomeric units associated. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 166-173, 2012