Novel diphenylacetylene monomers 1c-h possessing tert-butyl or trialkylsilyl and siloxy groups (p-R-C6H4C equivalent to CC6H4-P-OSiMe(2)t-Bu: 1c R = t-Bu, 1eR = SiMe3, 1g R = SiMe(2)t-Bu; p-R-C6H4C=-=CC6H4-M-OSiMe(2)t-Bu: 1d R = t-Bu, 1f R = SiMe3, 1h R = SiMe(2)t-Bu) were polymerized with TaCl5-n-Bu4Sn catalyst. Monomers 1c and 1e gave high molecular weight polymers in good yields. Free-standing membranes of polymers 2c-f were fabricated by casting from toluene solution. Desilylation of membranes of polymers 2c and 2d with trifluoroacetic acid (TFA) yielded poly(diphenylacetylenes) having tert-butyl and hydroxy groups (3c and 3d). Polymers 2c-f and 2h were soluble in nonpolar solvents such as hexane, cyclohexane and toluene, while 3c and 3d were insoluble in these solvents. According to thermogravimetric analysis (TGA), 2c-f, 3c and 3d exhibited high thermal stability. The gas permeability of all the membranes in the present study was much higher than that of the corresponding polymers without bulky groups, indicating that incorporation of bulky groups into the polymers improves the gas permeability. The PCO2/PN2 permselectivity ratios of polymers 3c and 3d were 23 and 14, respectively. The CO2 permeability and CO2/N-2 permselectivity of 3c and 3d were positioned above Robeson's upper bound line. (c) 2006 Elsevier B.V. All rights reserved.