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Polyacetylene (IUPAC name: polyethyne) is an organic polymer with the repeating unit (C2H2)n. It is an important polymer, as the discovery of polyacetylene and its high conductivity upon doping helped to launch the field of organic conductive polymers. Many different methods have been developed for the synthesis of polyacetylene, with synthesis from precursors leading to more controlled reactions. Doping of the polymer can be achieved using both p-type and n-type dopants. Different conformations of the polymer as well as different dopants can have an effect on the properties of the material, leading to a range of conductivities. Conductivity in this polymer is believed to arise from the creation of charge transfer complexes. Furthermore, the linear arrangement of pi electrons along the polymer chain yields many interesting properties in the material, including anisotropic optical, magnetic, electrical, and mechanical properties.

The interest in polyacetylene research by chemists and physicists alike arose from a collaborative project between Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid. Their interest in both the synthesis and doping of polyacetylene led to many significant discoveries to the field of 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." Despite the promise of this polymer in the field of conductive polymers, many of its properties such as instability to air and difficulty with processing have lead to difficulty in its processing for large scale use in many consumer goods.

Structure of polyacetylene
Polyacetylene 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. The different isomers of the polymer can be achieved by changing the temperature at which the reaction is conducted. This cis form of the polymer is thermodynamically less stable than the trans isomer. Despite the conjugated system exhibited by the polyacetylene backbone, not all of the carbon-carbon bonds in the material are equal: a distinct alternation exists, where every second bond has some double-bond character.

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 can be replaced by a functional group. Substituted polyacetylenes tend to be more rigid than saturated polymers. Furthermore, placing different functional groups as substituents on the polymer backbone leads to bending of the polymer chain out of conjugation.

Synthesis from pure acetylene
A variety of methods have been developed to synthesized polyacetylene, from pure acetylene as well as other monomers. One of the most common methods uses titanium catalyst and aluminum catalysts, known as Ziegler–Natta catalysts, with gaseous acetylene. Mechanistic studies suggest that this polymerization involves metal-insertion into the triple bond of the monomer.

By varying the apparatus and catalyst loading, Hideki Shirakawa and coworkers were able to synthesize 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 CoNO3/NaBH4 system, which was stable to both oxygen and water.

Polyacetylene can also be synthesized by radiation polymerization of pure acetylene, in gaseous, liquid and solid phases. Glow discharge radiation, γ-radiation, and ultraviolet irradiation have been used. The phase affects the final structure of the polymer; gas-phase polymerization typically produces irregular cuprene, while liquid-phase polymerization, conducted at -78 °C produces linear cis-polyacetylene and solid phase polymerization, conducted at -296 °C produces trans-polyacetylene.

I. Ring-opening metathesis polymerization
Polyacetylene can be synthesized by ring-opening metathesis polymerization (ROMP) from cyclooctatetraene, avoiding the use of the explosive acetylene monomer. This synthetic route also provides a facile method for adding solubilizing groups to the polymer while maintaining the conjugation.

II. Synthesis from precursor polymers
Various synthetic pathways have been developed to avoid using pure acetylene as a monomer. Short segments of polyacetylene can be obtained by dehalogenation of poly(vinyl chloride).

Several methods have also been developed to synthesis polyacetylene via thermal conversion of precursor polymers. In the Durham-precursor route, polymers are prepared via ring-opening metathesis polymerization, and a subsequent heat-induced reverse Diels-Alder reaction yields the final polymer, as well as a volatile side product.

Properties of polyacetylene
Shirakawa and coworkers have extensively studied the structure of polyacetylene films using both infrared and Raman  spectroscopy, and found that 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 ration of 60:40 cis:trans. Films containing the cis form appear coppery, while the trans form is silvery. The synthesis and processing of polyacetylene films affects the properties. Increasing the catalyst ratio creates thicker films with a greater draw ratio, allowing them to be stretched further. Lower catalyst loadings leads to the formation of dark red gels, which can be converted to films by cutting and pressing between glass plates. A foam-like material can be obtained from the gel by displacing the solvent with benzene, then freezing and subliming the benzene. Polyacetylene has a bulk density of 0.4 g/cm3, while density of the foam is significantly lower, at 0.02-0.04 g/cm3. The morphology consists of fibrils, with an average width of 200 Ǻ. These fibrils form an irregular, web-like network, with some cross-linking between chains.

In spite of its interesting properties, polyacetylene has few applications due to its instability in air. While both cis and trans-polyacetylene show high thermal stability, exposure to air causes a large decrease in the flexibility and conductivity. Infrared spectroscopy shows formation of carbonyl groups, epoxides, and peroxides. Coating with polyethylene or wax can slow the oxidation temporarily, while coating with glass increases stability indefinitely.

Doping of Polyacetylene
When polyacetylene films are exposed to vapors of electron-accepting compounds (p-type dopants), the electrical conductivity of the material increases by orders of magnitude over the undoped material. 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. Charge-transfer occurs from the polymer to the acceptor compound; the polyacetylene chain acts as a cation and the acceptor as an anion. The “hole” on the polymer backbone is weakly associated with the anionic acceptor by Coulomb potential. Polyacetylene doped with p-type dopants retain their high conductivity even after exposure to air for several days.

Electron-donating (n-type) dopants can also be used to create conductive polyacetylene. n-Type dopants for polyacetylene include lithium, sodium, and potassium. As with p-type dopants, charge-transfer complexes are created, where the polymer backbone is anionic and the donor is cationic. The increase in conductivity upon treatment with an n-type dopant is not as significant as those achieved upon treatment with a p-type dopant. Polyacetylene chains doped with n-type dopants are extremely sensitive to air and moisture.

The conductivity of polyacetylene varies depending on structure and doping. Undoped trans-polyacetylene films have a conductivity of 4.4 x 10-5 Ω-1cm-1, while cis-polyacetylene has a lower conductivity of 1.7 x 10-9Ω-1cm-1 Doping with bromine causes an increase in conductivity to 0.5 Ω-1cm-1, while a higher conductivity of 38 Ω-1cm-1 is obtained through doping with iodine.

History of Polyacetylene
Cuprene was one of the earliest reported acetylene polymers. Its highly cross-linked nature lead to no further studies in the field for quite some time. 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. After this first reported synthesis, few chemists were interested in polyacetylene because the product of Natta’s preparation was an insoluble, air sensitive, and infusible black powder. The next major development of polyacetylene polymerization was made by Hideki Shirakawa’s group who were able to prepare silvery films of polyacetylene. Shirakawa discovered that the polymerization of polyacetylene could be achieved at the surface of a concentrated solution of the catalyst system of Et3Al and Ti(OBu)4 in an inert solvent such as toluene. While Shirakawa was developing methods to synthesize high quality films of polyacetylene, Alan Heeger and Alan MacDiarmid were studying the metallic properties of polythiazyl [(SN)x], a covalent inorganic polymer. Alan Heeger’s research interests were centered on the metal-insulator transition of one-dimensional systems. Polythiazyl caught his 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. Upon meeting Shirakawa, Heeger and MacDiarmid’s interest shifted to looking into the potential of polyacetylene as a conductive material. From his experience with (SN)x, MacDiarmid wanted to modify the polyacetylene films synthesized by Shirakawa by iodide treatment. Upon doping with I2, the researchers discovered that the conductivity of polyacetylene increased seven orders of magnitude over the undoped polymer. Shirakawa, MacDiarmid, and Heeger obtained similar results upon treating polyacetylene with Cl2 and Br2. These dopant materials had the largest room temperature conductivity observed for a covalent organic polymer, and this seminal report was key in furthering the development of organic conductive polymers. Further studies by Shirakawa, MacDiarmid, and Heeger were aimed at optimizing these results. Shirakawa developed methods to further control the cis/trans isomer ratio and demonstrated that cis-polyacetylene doping led to higher conductivity than doping of trans-polyacetylene. They also discovered that doping cis-polyacetylene with AsF5 produced higher conducting materials than doping with I2. In 2000, Shirakawa, Heeger, and MacDiarmid were recognized for their achievements by receiving the Nobel Prize in Chemistry “for the discovery and development of electrically conductive polymers.” The discovery of polyacetylene as a conductive organic polymer has led to many more developments in this field. The principle interest in using polymers as conductive materials is in inexpensive manufacturing using solution-processing for film-forming polymers. However, despite the promise of polyacetylene, this polymer suffers from many drawbacks including instability in air and insolubility in solvents making it essentially impossible to process the material. Therefore, much attention in recent years has shifted to other conductive polymers for application purposes.