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Incredible electrical conductivity: How is polyacetylene promoted to "plastic metal" by doping?
Polyacetylene (IUPAC name: polyacetylene) has always been a representative of organic polymers, with a repeating unit with a structure of [C2H2]n.The concept of this polymer comes from the polymerization of acetylene, forming long chains with alternating double bonds.In this field, polyacetylene is considered to be of high importance, because its discovery not only unveils the door to research on organic conductive polymers, but also attracts great attention for its high conductivity after doping.This discovery has attracted aroused interest in the application of organic compounds in microelectronics, especially organic semiconductors, and was awarded the Nobel Prize in Chemistry in 2000.
The improved conductivity of polyacetylene has enabled this material to develop towards lightweight and processability, and is expected to become an ideal material for "plastic metal".
The structure of polyacetylene is formed from carbon atoms, with single and double bonds alternately between each other; each carbon atom is attached with a hydrogen atom.This polymer can control the synthesis of its cis or trans isomers by changing the reaction temperature.Although the main chain of polyacetylene has conjugated properties, its carbon-carbon bonds are not completely equal, but there is obvious alternation of single and double bonds.For the application of polyacetylene, its instability in the air and difficulty in processing, the possibility of commercialization is limited.
History of polyacetylene
In early research on polyacetylene, the earliest reported acetylene polymer was "Cuprene", which affected later research in this field.In 1958, Giulio Natta first synthesized linear polyacetylene, a polymer with high molecular weight and high crystallinity, but it attracted little attention due to its deadly air sensitivity.
It was not until Hideki Shirakawa's research team discovered that linear polyacetylene could be converted into silver films, and the conductivity value of polyacetylene was re-recognized until this time.
Shirakawa et al.'s experiments showed that when polyacetylene is doped with I2, its conductivity is increased by seven orders of magnitude.This discovery makes polyacetylene an important milestone in organic conductive materials.With further improvements and research, scientists found that cis-polyacetylene has better conductivity than trans-polyacetylene, and the use of other dopants such as AsF5 can further improve the conductivity, even reaching a level close to that of copper.
Synthesis and doping
There are many methods for synthesis of polyacetylene, the most common is the polymerization of acetylene gas through Ziegler-Natta catalyst.Different catalyst configurations and conditions allow scientists to accurately control the structure and properties of polymers.In addition, polyacetylene can also be synthesized by cyclic open chain polymerization (ROMP), which provides the possibility for subsequent introduction of functional substances.
During the doping process of polyacetylene, by exposing it to the vapor of electron-accepting compounds, the conductivity will increase dramatically, which means that the polymer will follow the direction of emerging electronic technologies.
For example, p-type dopants such as Br2, I2, etc. can effectively improve the conductivity of polyacetylene, resulting in the formation of a charge transfer complex.With the introduction of n-type dopants such as lithium, sodium and potassium, although their conductivity increases are not as obvious as p-type doping, corresponding studies are also underway.
Properties and applications of polyacetylene
The structure and properties of polyacetylene depend heavily on the synthesis conditions, which can obtain different cis to trans ratios at different temperatures.The conductivity of polyacetylene films has changed considerably without doping, and it is even more amazing after doping.
Although polyacetylene has good conductivity at room temperature, its flexibility and conductivity will be greatly reduced after contacting air, and even oxidation will occur.
Therefore, although polyacetylene is expected to play a role in electronics and other materials science applications, current commercial applications are not clear due to its own instability and processing difficulties.Researchers may turn their attention to other conductive polymers, such as polythiophene, polyaniline, etc.
Does these difficulties and challenges mean that in the future, polyacetylene can still break through its limitations and bring us new application possibilities?
