Abstract1‐(p‐t‐Butylphenyl)‐2‐phenylacetylene and 1‐(p‐n‐butylphenyl)‐2‐phenylacetylene were polymerized in catalytic systems based on TaCl5to give new polymers in high yields. These monomers were more reactive than diphenylacetylene (DPA) in copolymerization. Unlike poly (DPA), the present polymers were soluble in toluene, CHCl3, etc. owing to the high configurational entropy induced by thepara‐substituents. Their relative weight‐average molecular weights determined by GPC were in the range of 6 × 105–36 × 105, and films could be obtained by solution casting. These polymers were fairly thermally stable, as seen from their high onset temperatures (320–380°C) of weight loss in TGA in air. The oxygen permeability coefficient of the polymer witht‐Bu group was 1100 barrers, the highest among those of all the hydrocarbon polym