Polyacetylene

Polyacetylene (IUPAC name: polyethyne) is an organic polymer with the repeat unit (C2H2)n. The high electrical conductivity discovered for these polymers in the 1970’s accelerated interest in the use of organic compounds in microelectronics. Polyacetylenes are also known where the H atoms are replaced with alkyl groups.

Structure of polyacetylene
The polymer consists of a long chain of carbon atoms with alternating single and double bonds between them, each with one hydrogen atom. Schematically the structure of polyacetylene is shown below. One distinguishes trans-polyacetylene, with all double bonds in the trans configuration, from cis-polyactylene, with all double bonds in the cis configuration. Each hydrogen atom could be replaced by a functional group.

Preparation
Acetylene polymerizes in a similar fashion to ethylene: the polymerization can be effected with anionic, cationic, and radical initiators. Polyacetylene is generally not prepared by polymerizing acetylene, which is a highly flammable gas that uncontrollably oligomerizes at high concentrations. The most common synthesis utilize ring-opening polymerization ("ROMP") of molecules like cyclooctatetraene and substituted derivatives thereof. Depending on the method of preparation, some polyacetylenes are also called acetylene black or polyacetylene black.

Conductivity and the Nobel Prize
As prepared with a Ziegler-Natta catalyst, with high levels of catalyst, the polyacetylene is a silver, non-conductive film. It was shown in 1976 that oxidation of this material with iodine results in a 108-fold increase in conductivity. The conductivity of this doped material approaches the conductivity of the best available conductor, silver. This was one of the first known examples of a conductive organic polymer. The Nobel Prize in Chemistry in 2000 was awarded to Alan J. Heeger, Alan G MacDiarmid, and Hideki Shirakawa for this work.