Para-substituted 1-chloro-2-phenyl acetylenes [ClC&3bond; CC6H4-p-R; R = F (1), Cl (2), Me (3), Br (4), 1 (5), CF3 (6), SiMe3 (7), SiEt3 (8), Si(n-Pr)(3) (9), and Si(i-Pr)(3) (10)] were polymerized with a catalyst generated by UV irradiation Of MO(CO)(6) in CCl4 to provide the corresponding polymers. The formed polymers except poly(5) were soluble in common organic solvents and afforded free-standing membranes by the casting method except for poly(10) whose molecular weight was rather low. Compared to poly(1-chloro-2-phenyl acetylene) (PCPA), all the polymers in this study exhibited high gas permeability. As the size of para-substituents increased, both the fractional free volume (FFV) and gas permeability increased and then decreased. Consequently, poly(6), which has a relatively bulky CF3 group, showed the largest FFV (0.258) and the highest gas permeability (P-O2 = 280 barrers). Poly(9) and poly(10) were more gas-permeable than PCPA despite their small FFV values and showed large diffusion coefficients, which might be due to large local mobility of the substituents.