Polyacetylene (IUPAC name: polyethyne) usually refers to an organic polymer with the repeating unit [C2H2]n. 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". 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 led to avoidance in commercial applications. Compounds called polyacetylenes also occur in nature, although in this context the term refers to polyynes, compounds containing multiple acetylene groups ("poly" meaning many), rather than to chains of olefin groups ("poly" meaning polymerization of).
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. 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 a twisted conformation of the polymer chain to interrupt the conjugation.
History One of the earliest reported acetylene polymers was named cuprene. Its highly cross-linked nature led 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, infusible black powder. The next major development of polyacetylene polymerization was made by Hideki Shirakawa's group who prepared silvery films of polyacetylene. They 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. 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. Shirakawa, Heeger, and MacDiarmid collaborated on further development of polyacetylene. Upon doping polyacetylene with I2, the conductivity increased seven orders of magnitude. Similar results were achieved using Cl2 and Br2. These materials exhibited 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 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. Doping cis-polyacetylene with AsF5 further increased the conductivities, bringing them close to that of copper. Furthermore, it was found that heat treatment of the catalyst used for polymerization led to films with higher conductivities. 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
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.
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. Polyacetylene can also be produced via photopolymerization of acetylene, avoiding the use of catalysts and solvents. Glow-discharge, gamma, and ultraviolet irradiation have all been used. This method however requires cryogenics to produce usable polymers. Gas-phase polymerization typically produces irregular cuprene, whereas liquid-phase polymerization, conducted at −78 °C produces linear cis-polyacetylene, and solid-phase polymerization, conducted at still lower temperature, produces trans-polyacetylene.
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