Diphenylacetylenes containing both siloxy groups and halogen or methyl substituents (p-tert-BuMe2SiOC6H4C CC6H3R2, R = mp-CI,CI; m,m-CI,CI; m,p-Br,Br; m,m-Br,Br; m,p-Me,Me; m,m-Me,Me, 1a-f, respectively) were polymerized with TaCl5-n-Bu4Sn catalyst to give high molecular weight polymers (2a-f) in good yields. Free-standing membranes were fabricated by casting from toluene solution. Desilylation of these siloxy group-containing polymer membranes with trifluoroacetic acid (TFA) afforded poly(diphenyl acetylene) membranes having hydroxy groups (3a-f). Polymers 2a-f were soluble in low polar solvents such as toluene and CHCl3, while 3a-f were insoluble in these solvents. According to thermogravimetric analysis (TGA), 2a-f and 3a-f exhibited high thermal stability (To similar to 340 and similar to 390 degrees C, respectively). The gas permeability of the halogen-containing polymer membranes was higher than that of methyl group-containing ones. The PCO2/PN2 permselectivity ratios of polymer membranes 3a-f were in the range of 14-49. The points of 3a-f in the PCO2 vs PCO2/PN2 plot were located obviously above Robeson's upper bound. (c) 2006 Elsevier Ltd. All rights reserved.