(2-Methyl-5-tert-butylphenyl)acetylene (2Me5tBuPA) polymerized in the presence of MoCl5-n-Bu4Sn catalyst in 1,4-dioxane to give a high molecular weight polymer (MBAR(w) 8x10(5). Not only MoCl5- but also WCl6-based catalysts were effective. The polymer obtained was a dark brown solid, soluble in organic solvents such as toluene and CHCl3, and formed a free-standing film by solution casting. The onset temperature of weight loss of the polymer in air was 260-degrees-C. Its oxygen permeability coefficient was 43 barrers, which is twice that of natural rubber. Differences from the results (usually lower molecular weight, not film-forming, and thermally less stable) of both poly(phenylacetylene) and poly[(o-methylphenyl)acetylene] are attributable to the synergistic steric effect of the o-methyl and m-tert-butyl groups.