Diphenylacetylenes having a 2-bromoethoxy group [p-(BrCH2CH2O)C6H4C CC6H4-p-R; R = SiMe3 (1a), H (1c)] were synthesized and then polymerized with TaCl5/n-Bu4Sn catalyst to provide the corresponding poly(diphenylacetylene)s (2a and 2c). These polymers afforded tough free-standing membranes by casting from toluene solutions. Substitution of 1-methylimidazole to the membrane (2a) gave an imidazolium salt-containing poly(diphenylacetylene) [3a (Br-)]. Membrane of 3b (Br-) was obtained by desilylation of 3a (Br-) using HBr. Counter anions of 3a (Br-) could be exchanged by the treatment of the membrane with CF3COOK and (CF3SO2)(2)NLi, and the membranes 3a (TFAc-) and 3a (Tf2N-) were obtained. The carbon dioxide permeability coefficients (Pan) of membranes of 2a and 2c were 250 and 21 barrers, respectively. The P-CO2/P-N2 values of 2a and 2c were 16 and 11, respectively, which are ordinary values among all the polymers. In contrast, introduction of 1-methylimidazole into the membranes remarkably increased the CO2/N-2 selectivity (P-CO2/PN2 = 31-44), although the CO2 permeability decreased. The increase in the CO2/N-2 selectivity was attributed to the significant increase of CO2/N-2 solubility selectivity. The P-CO2 values of 3a (Br-), 3a (TFAc-), and 3a (Tf2N-) were 11, 23, 53, respectively, indicating that the CO2 permeability increased as the counter anion became bulkier. (C) 2013 Elsevier Ltd. All rights reserved.