new catalyst system prepared by UV irradiation of a toluene solution of W(CO)6 and a Lewis acid polymerized phenylacetylene (e.g., polymer yield 50%, M(n)BAR 5.0x10(4) with SnCl4 (0.5 equivalent to W(CO)6) as Lewis acid). This catalyst afforded high molecular weight polymers (M(n)BAR 4x10(5)-5x10(5)) from phenylacetylenes with ortho-substituents (e.g., o-Me3Si and o-CF3). The Mo(CO)6-based counterpart was rather effective to acetylenes having electron-withdrawing groups (e.g., ClC=CPh). Polymerization of phenylacetylene occurred as well when SnCl4 was added after UV irradiation of a toluene solution of W(CO)6 and the monomer; this suggests that the polymerization proceeds via metal carbenes having coordinated SnCl4.