The polymerization of 1-[(p-trimethylsilyl)phenyl]-2-(2-fluorenyl)acetylene (1c) with TaCl5-n-Bu4Sn produced a high molecular weight polymer (2c), whose M-w reached 3.4 x 10(6). 1-[(p-Trimethylsilyl)-phenyl]-2-(2-phenanthryl)acetylene (1d) and 1-[(p-trimethylsilyl)phenyl]-2-(4-biphenyl)acetylene (le) were also polymerized to give high molecular weight polymers (2d and 2e) in good yields. In contrast, the polymerization of 1-[(p-trimethylsilyl)phenyl]-2-(2-anthryl)acetylene (1f) provided no polymer probably owing to the steric reason. The formed polymers 2c and 2d were soluble in common solvents such as toluene and chloroform and afforded free-standing membranes by casting from toluene solution. The onset temperatures of weight loss of polymers 2c and 2d in air were about 430 degreesC, indicating very high thermal stability. The membranes of 2c and 2d obtained by the casting method showed high gas permeability; e.g., their oxygen permeability coefficients (Po-2) were 1650 and 460 barrers, respectively. The membranes of 2c and 2d were desilylated with trifluoroacetic acid to provide the membranes of poly [1-phenyl-2-(2-fluorenyl)acetylene] (3c) and poly[1-phenyl-2-(2-phenanthryl)acetylene] (3d). The desilylated poly(diarylacetylene)s exhibited even higher Po-2 values of 2150 and 1300 barrers, respectively. These desilylated polymers were insoluble in any solvents and showed higher thermal stability than those of 2c and 2d.