A coordination insertion mechanism is proposed for the polymerization of alkynes initiated by transition metal acetylides on the basis of some experimental results. A transition metal acetylide coordinates with an alkyne resulting in the formation of a penta-coordinated complex, which is a preliminary activation and rate-determining step. The polymeric chain begins by insertion of the pi -coordinated monomer molecule into the metal-carbon sigma -bond. For large-bulk monomers such as p-diethynylbenzene and its derivatives, the insertion is almost trans-type to give rich trans polymers. On the other hand, for small-bulk monomers such as phenylacetylene, cis insertion easily occurs to yield cyclotrimers. The propagation of the polymer chain can be interrupted when the sigma -bonded monomer transfers its acidic hydrogen to the growing chain.