Abstract The ring-opening metathesis of cyclopentene is catalyzed by WCl 6 in the presence of 1-alkynes. In an investigation of the reaction mechanism, several mono- and disubstituted alkynes were tested for their activating effect and their influence on the stereochemistry of the resulting poly-1-pentenylene. With 1-alkynes yields of poly-1-pentenylene are as high as 90%. The trans content of the polymer approaches 80%. Increasing chain length and steric bulk of the alkyne substituent reduce the yield and the trans content significantly. Disubstituted alkynes inhibit the metathesis reaction. The oligomeric reaction side products which are formed in low yield were analyzed by GC/MS and NMR. The initial reaction of the system WCl 6/phenylacetylene/cyclopentene is the formation of polyphenylacetylene and some polycyclopentene by WCl 6. As the active metathesis catalyst is produced, metathesis ensues, producing a high trans content of poly-1-pentenylene which remains constant over a period of days. Based on these results a reaction mechanism is proposed involving a carbyne complex as the starting metathesis catalyst.