When we talk of polyacetylene, we usually denote an organic polymer with the chemical formula (C2H2)n. The name polyacetylene talks about its theoretical construction from polymerization of acetylene to produce a chain by way of repeating olefin groups. Theoretically, this compound is significant especially as the finding of polyacetylene as well as its elevated conductivity upon doping that facilitated the open up the field of organic conductive polymers. Three scientists - Hideki Shirakawa, Alan MacDiarmid and Alan J Heeger are credited with discovering the high conductivity of this polymer and their discovery resulted in deep interest in using organic compounds, which are known organic semiconductors, in microelectronics. This discovery by these scientists was recognized by awarding them the Nobel Prize in Chemistry in the year 2000. Nearly all initial researches in polyacetylene were undertaken keeping in view that doped polymers were not only processed easily, but they were considered to be lightweight "plastic metal". Initially, this polymer showed lots of promise of being used as conductive polymers, but several properties of polyacetylene like its unsteadiness to air as well as the difficulty in processing it have eventually resulted in avoiding the use of this compound in commercial use.
Polyacetylenes are basically organic compounds that occur naturally in nature. However, when we say this we actually refer to polynes, which are compounds that contain various acetylene groups ("poly" denoting many) and not chains of olefin groups (in this case "pol" means polymerization of).
Aside from occurring in the nature, it is also possible to produce polyacetylene by means of radiation polymerization of acetylene. Various radiation methods, including ultraviolet radiation glow-discharge radiation and γ-radiation have been used to produce polyacetylene. However, all these methods keep away from using any solvent or catalyst, but they need low temperature in order to produce standard polymers.
Polyacetylene comprises lengthy carbon atom chains having alternating single and double bonds in between. Each bond has one hydrogen atom. The double bonds can comprise cis geometry or trans geometry. It is possible to achieve controlled synthesis of all isomers of the polymer - cis-polyacetylene or trans-acetylene by adjusting the temperature required to undertake the reaction.
Compared to the tans isomer, the polymer's cis variety is less stable thermodynamically. In spite of the polyacetylene backbone's conjugated nature, all the carbon-carbon bonds in this compound are not equal. In fact, there is a distinctive single/ double alternation. Moreover, it is possible to replace all the hydrogen atoms by functional groups. The substituted polyacetylenes have a propensity to be additionally unyielding compared to the saturated polymers. In addition, substituting the different functional groups on the backbone of the polymer results in polymer chain's bending out of conjugation.
Among the earliest reported acetylene polymers cuprene is one. The nature of curpene is highly cross-linked and this is the reason why there was no further research in the field of polyacetylene for some time. Italian chemist Giulio Natta was the first to prepare linear polyacetylene in 1958. The resultant polyacetylene was linear, exhibited elevated level of crystallinity, was of elevated molecular weight and its structure was regular. X-ray diffraction studies showed that the resultant polyacetylene was trans-polyacetylene. Following the synthesis that was reported for the first time, a number of chemists showed interest in polyacetylene since the product prepared by Nattta was air sensitive, insoluble and an infusible black powder.
Japanese chemist and Nobel laureate Hideki Shirakawa's group made the subsequent major development in the field of polyacetylene polymerization. In fact, this group was able to make silvery films of polyacetylene. This group also found for the first time that it was possible to achieve polyacetylene polymerization on the surface of catalyst system of of Et3Al and Ti(OBu)'s concentrated solution in an inert solvent like toluene. Parallel to the studies undertaken by Shirakawa's group, scientists Alan MacDiarmid and Alan J Heeger were examining the metallic attributes of a related, but inorganic polymer called polythiazyl [(SN)x]. Heeger took interest in polythiazyl, a chain-like material. Soon he collaborated with American chemist Alan MacDiarmid, who was already experienced with this material. By the beginning of the 1970s, it was known that this polymer was superconductive even at low temperatures.
When polyacetylene was doped with I2, the conductivity of this polymer augmented seven times. The results were similar when CI2 and Br2 were used. In fact, these substances demonstrated the conductivity at the maximum room temperature that was observed for any covalent organic polymer. This decisive report was vital in advancing organic conductive polymer development. Studies undertaken later resulted in enhanced control of the ratio of cis-isomer and trans-isomer. It also showed that doping of cis-polyacetylene resulted in improved conductivity compared to doing trans-polyacetylene. In addition, doping cis-acetylene using AsF5 enhanced the conductivities of the materials further and brought their conductivity almost near copper. In addition, it was also discovered that treating the catalyst used for polymerization with heat resulted the films to have higher conductivity.
While the discovery of polyacetylene in the form of an organic conductive polymer had given rise to lots of developments in the field of material science, today it does not have any commercial use. However, conducting polymers have some use in low-cost solution-processing for polymers that are used in film. Hence, in recent years, a lot of interest has been on other conductive polymers owing to their various applications, such as polyaniline and polythiophene.
Polyacetylenes have several disadvantages that include their unsteadiness in the air and insolubility in solvents, which made it very difficult to process this material. While cis-polyacetylene as well as trans-polyacetylene demonstrates very high thermal constancy, when they are exposed to air, their flexibility as well as conductivity decreases considerably. When you expose polyacetylene to air, their backbone is oxidized by O2. On the other hand, infrared spectroscopy of polyacetylenes demonstrates carbonyl group, peroxide and epoxide formation. However, the oxidation process can be slowed down considerably by coating substances with either polyacetylene or wax. On the other hand, coating using glass improves the stability for an indefinite period.