# Synthesis and Mechanism of Conduction in Conducting Polymers Conducting polymers are a special class of organic polymers that possess electrical conductivity while retaining many of the useful characteristics of conventional polymers, such as low density, flexibility, ease of processing, and chemical versatility. Unlike ordinary polymers, which generally act as electrical insulators because their valence electrons are localized in covalent bonds, conducting polymers contain an extended conjugated π-electron system along their polymer backbones. Important examples include polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(phenylene). Their electrical conductivity arises mainly from the presence of delocalized electrons and, in most cases, from oxidation or reduction processes known as doping. The synthesis of conducting polymers can be carried out by several methods, but chemical oxidative polymerization and electrochemical polymerization are particularly important. The nature of the monomer, oxidizing agent, solvent, temperature, and doping conditions strongly influences the structure and conductivity of the resulting polymer. In chemical oxidative polymerization, a suitable monomer containing a conjugated structure is polymerized in the presence of an oxidizing agent. For example, aniline can be oxidatively polymerized to form polyaniline. Common oxidants include ammonium persulfate, potassium persulfate, ferric chloride, and related oxidizing agents. The reaction is generally performed in an acidic aqueous medium. Initially, the aniline monomer undergoes oxidation to form a radical cation. These reactive species couple with other oxidized monomer species, followed by deprotonation and further oxidation. Repetition of these elementary steps produces longer conjugated chains. Polypyrrole is similarly prepared by oxidative polymerization of pyrrole. Oxidants such as ferric chloride or ammonium persulfate can be used. During polymerization, pyrrole molecules are oxidized to radical cations, which couple predominantly at their α-positions. Subsequent deprotonation and oxidation produce an extended conjugated polypyrrole chain. The counter-ions originating from the oxidizing agent or electrolyte can remain associated with the positively charged polymer and act as dopant ions. Polythiophene and its substituted derivatives can also be synthesized through oxidative polymerization. The choice of substituents on the thiophene ring permits modification of solubility, optical properties, morphology, and conductivity. Conjugated polymers can also be synthesized through coupling reactions, such as palladium-catalyzed cross-coupling reactions, which provide better control over molecular structure and are especially useful for preparing well-defined functional materials. Electrochemical polymerization is another important method for producing conducting polymers. In this process, the monomer is dissolved in an electrolyte solution and an electrical potential is applied between electrodes. Oxidation of the monomer occurs at the anode, generating radical cations. These species combine to form dimers and higher oligomers, which undergo further oxidation and coupling. The growing polymer deposits directly on the electrode surface as a thin conducting film. Electrochemical synthesis has the