Efficient gas-separation systems comprising gas-permeable membranes are important for energy conservation in various industrial applications. Herein, high-molecular-weight copolymers (2ab and 2ac) were synthesized in good yields by the copolymerization of 1-(p-trimethylsilyl)phenyl-2-(p-trimethylsilyl)phenylacetylene (1a) with 1-phenyl-2-(p-tert-butyl)phenylacetylene (1b) and 1-phenyl-2-(p-trimethylsilyl)phenylacetylene (1c) in various monomer feed ratios using TaCl₅–n-Bu₄Sn. Tough membranes were obtained by solution casting. The copolymers exhibited very high gas permeabilities (P_O2: 1700–3400 barrers). Desilylation of 2ac membranes decreased the gas permeability, but desilylation of 2ab membranes resulted in a significant increase in the gas permeability. The highest oxygen permeability coefficient obtained was 9300 barrers, which was comparable to that of poly(1-trimethylsilyl-1-propyne), a polymer known to have the highest gas permeability.