Diphenylacetylenes having both a trimethylsilyl group and an alkyl group at para positions [Me3SiC6H4C CC6H4R; R = Et (1a), n-Bu (1b), t-Bu (1c), n-C8H17 (1d)] and having only an alkyl group [PhC CC6H4R: R = n-Bu (1B), n-C8H17 (1D)] were synthesized and then polymerized with TaCl5/n-Bu4Sn catalyst to provide the corresponding poly(diphenylacetylene)s (2a, 2b, 2c, 2d, 2B, and 2D). The formed polymers except 2c afforded tough free-standing membranes by casting from toluene solutions. Desilylation reaction of the Si-containing polymer membranes (2a, 2b, 2d) was carried out with trifluoroacetic acid, and the desilylated polymer membranes (3a, 3b, 3d) were obtained. The permeability to O-2, N-2, and CO2 were determined for the obtained polymer membranes. All the desilylated membranes showed lower gas permeability than the Si-containing counterparts. To clarify the effects of the desilylation further, CO2 diffusivity, CO2 solubility, and fractional free volume (FFV) of the polymer membranes were investigated. The FFV and CO2 diffusivity decreased upon desilylation, while CO2 solubility hardly varied. (C) 2009 Elsevier Ltd. All rights reserved.