A series of diphenylacetylene derivatives [(CH3)(3)SiC6H4C CC6H4R; R=H (1a), CH3 (1b), n-C4H9 (1c), n-C8H17 (1d), PhC CC6H4R; R = CH3 (2b), n-C4H9 (2c), n-C8H17 (2d), t-C4H9 (2e)] were synthesized and then polymerized with TaCl5/n-Bu4Sn to afford the corresponding poly(diphenylacetylene)s (3a-d and 4b-e). These poly(diphenylacetylene)s were sulfonated using acetyl sulfate, which gave the sulfonated poly(diphenylacetylene)s (5a-d and 6b-e). Except 6b, the degrees of sulfonation were found to be around 1.0 from the titration of methanol-solutions of sulfonated polymers. Sulfonated polymer 5a exhibited the lowest gas permeability and the highest CO2 permselectivity among the present polymers. The gas permeability coefficients of 5b, 5c, and 5d became lager in this order. The presence of alkyl groups would dilute the effect of sulfonic acid and weak the intermolecular interaction by the steric hindrance. Sulfonated poly(diphenylacetylene)s having n-butyl and t-butyl groups (6c and 6e) showed almost the same CO2 permselectivity, while they showed quite different CO2 permeability. The spherical bulky t-butyl groups more effectively prevent the intermolecular interaction than the linear n-butyl groups in sulfonic acid-containing poly(diphenylacetylene)s. (c) 2012 Elsevier Ltd. All rights reserved.