Novel phenylacetylene (PA) monomers, which have o-silylmethyl groups of different bulkinesses, i.e., o-Me3SiCH2PA, o-Et3SiCH2PA, and o-t-BuMe2Si-CH2PA, polymerized with W and Mo catalysts in high yields. The MoCl5-Ph4Sn catalyst achieved the highest weight-average molecular weights (MBAR(w) 7 x 10(5) - 12 x 10(5)), and the MBAR(w) increased as the ortho-substituent became bulkier (e.g., M(w) of o-t-BuMe2SiCH2PA : 12 x 10(5)). These monomers polymerized in a living fashion by the MoOCl4-n-Bu4Sn-EtOH catalyst. The resulting polymers were soluble in common solvents such as toluene and chloroform. In the UV-visible spectra, a tendency was observed that absorption maxima shifted to longer wavelengths as the substituents became bulkier. Membranes of the polymers were fairly permeable to gases (e.g., oxygen permeability coefficients 30-80 barrers). Though o-(Me3Si)2CHPA also polymerized with W and Mo catalysts, the product polymer was partly insoluble in any solvent.