Highly CO2-permeable and -permselective poly(diphenylacetylene)s having imidazolium salts: Synthesis, characterization, gas permeation properties, and effects of counter anion

Diphenylacetylenes having a 2-bromoethoxy group [p-(BrCH2CH2O)C6H4CCC6H4-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)2NLi, and the membranes 3a (TFAc−) and 3a (Tf2N−) were obtained. The carbon dioxide permeability coefficients (PCO2) of membranes of 2a and 2c were 250 and 21 barrers, respectively. The PCO2/PN2 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/N2 selectivity (PCO2/PN2 = 31–44), although the CO2 permeability decreased. The increase in the CO2/N2 selectivity was attributed to the significant increase of CO2/N2 solubility selectivity. The PCO2 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.