Most of the existing metal carbonyl catalysts for acetylene polymerizations need to be preactivated by chlorine-containing additives or by UV irradiation in halogenated solvents. In this work, we developed a series of "simple" metal carbonyl catalysts of general structure M(CO)(x)L-y (M = Mo, W), none of which require additives or pre-photoirradiation, most of which are air- and moisture-stable, and some of which work well in nonhalogenated solvents. The acetonitrile complexes M(CO)(3)(NCCH3)(3) initiated polymerizations of a variety of mono- and disubstituted acetylenes at room temperature. The arene and diene complexes W(CO)(3)(mes) and Mo(CO)(3)(nbd) (mes = mesitylene, nbd = 2,5-norbornadiene) are tolerant of polar groups and effected polymerizations of functional acetylenes containing ester, ether, and cyano groups. The halogenated complexes MI2(CO)(3)(NCCH3)(2) catalyzed polymerizations of phenylacetylene in toluene. The chlorine-containing acetylene monomers ClC drop CC6H5 and ClC drop CC6H13 were readily polymerized by the Mo complexes in nonhalogenated solvents such as toluene and dioxane, giving polymers with high molecular weights (M-w up to 883 x 10(3)) in high yields (up to 100%).