A process is described for polymerizing acetylene to selectively produce partially crystalline polyacetylene in its cis and trans configurations, which when doped with halogen additives, are useful as electrical conductors or semi-conductors. The polymerization is conducted in a solution containing a lithium dicyclopentadienyl titanium hydride catalyst in a liquid organic solvent therefor, at a temperature in the range of about -120.degree. to +250.degree. C. Lower process temperatures, such as below -60.degree. C., favor the formation of the cis form, and higher temperatures, above 0.degree. C., favor the formation of the trans form.