AbstractPolymerization oft‐butylacetylene initiated by a tungsten carbene complex (I) was investigated under various conditions.t‐Butylacetylene was polymerized in high yield in chlorinated hydrocarbons such as CHCI3and CCI4and in the presence of AICI3. The molar mass of the polymers formed was as high as 8.2 ×104∼ 3.2 × 105. Moreover, when the solution of tungsten carbene complex in CCI4was irradiated with UV light, the molar mass was remarkably enhanced. This phenomenon may occur because tungsten dichlorocarbene immediately forms. Apparent rate constants were determined by gas chromatography. NMR spectra show a predominant characteristic of highciscontent in the structure of poly(t‐butylacetylene) in contrast to poly(phenylacetylene). Theciscontent increasedca. 11% when poly(t‐butylacetylene) was heated in decalin at 110°C. X‐Ray diffraction diagrams show that poly(t‐butylacetylene) has an amo