Diphenylacetylenes having a dimethyloctylsilyl group and an alkyl group at para positions [Me(2)n-C8H17SIC6H4C CC6H4R, R=H (la), t-Pr (1b), t-Bu (1c), n-Bu (1d)] and having only an alkyl group,[PhC CC6H4R, R = t-Pr (1B), t-Bu (1C)] were synthesized and then polymerized with TaCl5/n-Bu4Sn catalyst to provide the corresponding poly(diphenylacetylene)s (2a, 2b, 2c, 2d, 2B, and 2C) The formed polymers afforded tough free-standing membranes by casting from toluene solutions Desilylation reaction of Si-containing membranes (2a-d) was carried out with trifluoroacetic acid to give the desilylated membranes (3a-d). The permeability of these membranes to O-2, N-2, and CO2 were determined All the Si-containing membranes exhibited almost the same gas permeability. The desilylation of Si-containing membranes of 2a-c resulted in large increase of gas permeability No apparent increasing of gas permeability was observed in the desilylation of 2d To clarify the effects of desilylation, CO2 diffusivity (D(CO2)) CO2 solubility (S(CO2)), and fractional free volume (FRO of the polymer membranes were investigated. The S(CO2) values of desilylated membranes were much larger than that of Si-containing counterparts The D(CO2) and FFV of membranes of 2a-c increased through desilylation The desilylated membrane of 3d had small D(CO2) value and almost the same FFV compared with 2d. Further, the comparison of the permeability between three types of membranes with the same chemical structure revealed that the microvoids were not generated by the desilylation of membranes of poly(diphenylacetylene)s containing alkyl groups. (C) 2010 Elsevier Ltd All rights reserved