Polyacetylene

Polyacetylene (IUPAC name: polyethyne) usually refers to an organic polymer with the repeating unit . The name refers to its conceptual construction from polymerization of acetylene to give a chain with repeating olefin groups (a conjugated polyene). This compound is conceptually important, as the discovery of polyacetylene and its high conductivity upon doping helped to launch the field of organic conductive polymers. The high electrical conductivity discovered by Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid for this polymer led to intense interest in the use of organic compounds in microelectronics (organic semiconductors). This discovery was recognized by the Nobel Prize in Chemistry in 2000. Early work in the field of polyacetylene research was aimed at using doped polymers as easily processable and lightweight "plastic metals".

Structure

Polyacetylene consists of a long chain of carbon atoms with alternating single and double bonds between them, each with one hydrogen atom. The double bonds can have either cis or trans geometry. The controlled synthesis of each isomer of the polymer, cis-polyacetylene or trans-polyacetylene, can be achieved by changing the temperature at which the reaction is conducted. The cis form of the polymer is thermodynamically less stable than the trans isomer. Despite the conjugated nature of the polyacetylene backbone, not all of the carbon–carbon bonds in the material are equal: a distinct single/double alternation exists. Linear polyacetylene was first prepared by Giulio Natta in 1958. The resulting polyacetylene was linear, of high molecular weight, displayed high crystallinity, and had a regular structure. X-ray diffraction studies demonstrated that the resulting polyacetylene was trans-polyacetylene. In parallel with Shirakawa's studies, Alan Heeger and Alan MacDiarmid were studying the metallic properties of polythiazyl [(SN)_x], a related but inorganic polymer. Polythiazyl caught Heeger's interest as a chain-like metallic material, and he collaborated with Alan MacDiarmid who had previous experience with this material. By the early 1970s, this polymer was known to be superconductive at low temperatures. Further studies led to improved control of the cis/trans isomer ratio and demonstrated that cis-polyacetylene doping led to higher conductivity than doping of trans-polyacetylene.

To account for such an increase in conductivity in polyacetylene, J. R. Schrieffer and Heeger considered the existence of topologically protected solitonic defects, their model is now known as the Su–Schrieffer–Heeger model, which has served as model in other contexts to understand topological insulators.

Synthesis

From acetylene

thumb|left|Ziegler–Natta scheme

A variety of methods have been developed to synthesize polyacetylene. One of the most common methods is via passing acetylene gas over a Ziegler–Natta catalyst, such as Ti(OiPr)₄/Al(C₂H₅)₃. This method allows control over the structure and properties of the final polymer by varying temperature and catalyst loading. Mechanistic studies suggest that this polymerization involves metal insertion into the triple bond of acetylene.

thumb|Mechanism of polyacetylene synthesis from acetylene and a metal catalyst

By varying the apparatus and catalyst loading, Shirakawa and coworkers synthesized polyacetylene as thin films, rather than insoluble black powders. They obtained these films by coating the walls of a reaction flask under inert conditions with a solution of the Ziegler–Natta catalyst and adding gaseous acetylene resulting in immediate formation of a film. Enkelmann and coworkers further improved polyacetylene synthesis by changing the catalyst to a Co(NO₃)₂/NaBH₄ system, which was stable to both oxygen and water. This synthetic route also provides a means for introducing solubilizing groups to the polymer while maintaining the conjugation. Polymers with linear groups such as n-octyl had high conductivity but low solubility, while highly branched tert-butyl groups increased solubility but decreased conjugation due to polymer twisting to avoid steric crowding. They obtained soluble and conductive polymers with sec-butyl and neopentyl groups, because the methylene (CH₂) unit directly connected to the polymer reduces steric crowding and prevents twisting.

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More efficient methods for synthesizing long polyacetylene chains exist and include the Durham precursor route in which precursor polymers are prepared by ring-opening metathesis polymerization, and a subsequent heat-induced retro-Diels–Alder reaction yields the final polymer, as well as volatile side products. p-Type dopants include Br₂, I₂, Cl₂, and AsF₅. These dopants act by abstracting an electron from the polymer chain. The conductivity of these polymers is believed to be a result of the creation of charge-transfer complexes between the polymer and halogen. and Raman spectroscopy, and found that the structure depends on synthetic conditions. When the synthesis is performed below −78 °C, the cis form predominates, while above 150 °C the trans form is favored. At room temperature, the polymerization yields a ratio of 60:40 cis:trans. A foam-like material can be obtained from the gel by displacing the solvent with benzene, then freezing and subliming the benzene. Coating with polyethylene or wax can slow the oxidation temporarily, while coating with glass increases stability indefinitely.