Complex [^tBuOCO]WC(^tBu)(THF)₂ (1) {where ^tBuOCO = [2,6-(^tBuC₆H₃O)₂C₆H₃]³⁻, THF = tetrahydrofuran} polymerizes acetylenes (R-phenylacetylene (R = H, p-OMe, p-F, 3,5-diCF₃), 1-decyne, 3,3-dimethyl-1-butyne, and trimethylsilylacetylene) to form π-conjugating polymers. Upon treating 1 with 2 equiv. of phenylacetylene in toluene-d₈ at −35 °C, two isolable products form. These two products are [O₂C(^tBuC)W(η²-HCCPh)] (2-^tttBu) and [O₂C(PhC)W(η²-HCC^tBu)] (2-Ph) {where OC(^tBuC)O = [2,6-(^tBuC₆H₃O)₂C₆H₃(^tBuC)]⁴⁻, OC(PhC)O = [2,6-(^tBuC₆H₃O)₂C₆H₃(PhC)]⁴⁻} and derived from an apparent reductive alkylidyne migratory insertion into a metal–arene bond. Complexes 2-^tttBu and 2-Ph polymerize acetylene and a wide variety of monosubstituted acetylenes including phenylacetylene derivatives, 1-decyne, 3,3-dimethyl-1-butyne and trimethylsilylacetylene. With a substrate to catalyst loading ratio of 25 000 : 1, complex 2-^tttBu polymerizes phenylacetylene with a turnover number (TON) of 17 233. Additionally, 2-^tttBu polymerizes phenylacetylene and 1-decyne with catalytic activities up to 5.64 × 10⁶ g_PPA mol⁻¹ h⁻¹ and 7.98 × 10⁶ g_PA mol⁻¹ h⁻¹, respectively. 2-^tttBu also polymerizes the disubstituted acetylene, 1-phenyl-1-propyne. NMR spectroscopic and single crystal X-ray structural studies provide compelling evidence for polymer chain growth via an insertion ring-expansion mechanism.