Electricity-Conducting Polymers

Most polymeric materials, as we usually know them, are insulators, yet conducting polymers were first prepared in 1862 by Letherby who formed polyaniline [60] by electro polymerization. It was only during the past 25–30 years, however, that organic polymers, capable of conducting electric current, created considerable interest [61]. Many polymeric materials were synthesized since. At present, it is not completely understood by what mechanism the electric current passes through them. We do know, however, that all conductive polymers, or intrinsically conducting polymers according to Inzelt [62], are similar in one respect. They all consist of extended p-conjugated systems, namely alternating single and double bonds along the chain. One of the early known conductive polymers is polysulfurnitride (SN)x, an inorganic material that tends to be explosive, but becomes superconducting at 0.3 K [63]. Since then many other conductive polymers evolved. The most investigated ones appear to be polyacetylene, polyaniline, polypyrrole, polythiophene, poly (phenylene sulfide), and poly (phenylene vinylene) and their derivatives. Many derivatives of these materials and other similar ones also been reported. The mechanism of electro polymerization that is used to form many conductive polymers is also still not fully understood. According to Diaz et al. [64], the process involves a sequence of coupling steps, with each step being activated by two species. The polymer- forming process requires two electron per step It also includes partial oxidation of the polymer. Thus, the polymer formation and the polymer oxidation occur simultaneously. Diaz proposed a chain propagation process for the polymer formation. Although that is still accepted by many, his mechanism is now being questioned. Heinze et al. [65] suggest that the process of polymer formation consists of oligomer formation and oxidation followed by s-coupling of chains. All known conducting polymers have backbones of contiguous sp²-hybridized carbon centers. In each of these centers, one valence electron resides in a pz orbital. It is orthogonal to the other three sigma bonds. When the material is oxidized or reduced, that removes some of the delocalized electrons. The electrons then obtain high mobility. As a result, the conjugated p-orbitals form a one-dimensional electronic band. The electrons within that band become mobile when it is partially emptied. Depending upon the chemical structure, some polymer can also be self-oxidizing or reducing. Structural disorder in these polymer molecules interferes with electron mobility. Thus, for instance, polyacetylene exhibits conductivity of 0.1–10 kS/cm. Stretch-orienting this polyacetylene, a process that aligns the chains and removes much of the disorder, increases conductivity to 80 kS/cm [66]. Polyacetylene can be shaped into a silvery looking film. The polymer is more thermodynamically stable in the trans form and converts from cis to trans when heated above 150C. Partial oxidation of the film, with iodine or other materials, transforms it and increases its conductivity 109-fold. The process of transforming a polymer to its conductive form through chemical oxidation or reduction is called doping.

Two types of polyacetylene doping are possible:

The doping process can be reversed and conductive polymers can be undoped again by applying an electrical potential. It causes the dopant ions to diffuse in and out of the structure. Improvements in preparations of polyacetylene came from several developments. One is the use of metathesis polymerization of cyclooctatetraene, catalyzed by a titanium alkylidine complex. The product has improved conductivity, though it is still intractable and unstable. By attaching substituents, it is possible to form soluble and more stable materials that can be deposited from solution on various substrates. Substitution, however, lowers the conductivity. This is attributed to steric factors introduced by the substituents that force the double bonds in the polymeric chains to twist out of coplanarity [67]. A family of substituted polyacetylenes were prepared [68] that were actually formed from ethynyl pyridines by a polymerization reaction that takes place spontaneously by a quaternization process:

where X is a bromine or an iodine. Like other substituted polyacetylenes, these materials are fairly stable in air and are soluble in polar solvents, also in water. The conductivity of these polymers is improved over previously reported substituted polyacetylenes to within the range of semiconductors. Preparation of a highly conductive polyacetylene was achieved when Ziegler–Natta catalyst was used by Shirakawa in aged silicone oil at 150C. It is believed that this reaction results in formation of polymers with less defects in the structures. The conductivity of these materials, when doped, actually approaches that of copper [62]. Considerable progress has been achieved in development of catalyst systems for living polymerization of various substituted acetylenes during the last 10 or 15 years [69]. Nowadays, there are available single-component catalysts based on stable carbene complexes and multicomponent catalysts based on MoOCl₄ and WOCl₄, both operating in metathesis mode, as well as Rh(diene) complexes operating in the Ziegler–Natta mode. For instance, a living polymerization of phenyl acetylene and the synthesis of an end functionalized poly (phenyl acetylene) by using Rh-based catalyst systems, [RhCI(nbd)]₂/Ar₂C=C (Ph)Li/PPh₃ (Ar = Ph, 4-Me2-NC₆H₄), was reported [70]. Also, use of (triphenyl vinyl) lithium that has functional groups, such as the dimethylamino groups, results in formation of end-functionalized poly (phenyl acetylene) s, which quantitatively contained functional groups at the initiating chain end. The polymerization of phenyl acetylene by a catalyst prepared from [RhCl(norbornadiene)]2, Ph₂C=C(Ph)Li, and PPh₃ proceeds smoothly in benzene to give quantitatively a yellow polymer with the number average molecular weight of 5,400 and the polydispersity ratio of 1.14. The reaction can be illustrated as follows:

Masuda and colleagues reported [71] that they synthesized poly(anthryacetylenes)-bearing oligo oxyethylene units by using a transition metal catalyst, WC16, in 30 and 34% yields. The polymers were black solids. These polymers are soluble in chloroform, tetrahydrofuran, acetone, etc., but insoluble in alcohols, aliphatic hydrocarbons, etc. The UV–VS spectra of the polymers showed absorption maxima and band edges at around 570 and 750 nm, respectively, indicating that the polymer chains possess highly extended conjugation. These polymers exhibited blue emission (emission maxima 470 nm) upon photo-excitation at 380 nm. One of the polymers (n = 4) showed a fairly large ionic conductivity (4.1 x 10 S/cm) at 80C upon doping with Li (CF₃SO₂)₂N.

In 1979, it was demonstrated that polypyrrole can be formed as a film by electrochemical oxidative polymerization of the pyrrole monomer in acetonitrile. The polymers that form on the surface of the electrode can be peeled off as flexible, shiny blue-black films. Subsequently, in 1982 it was shown that thiophene can also be electro polymerized oxidatively at the anode. The method allows control over the oxidative potential during the polymerization, yielding doped films with optimized polymer properties. Both polypyrrole and polythiophene differ from polyacetylene in that both form during the polymerization in the doped form and that both are stable in air. They are, however, less conductive than the doped polyacetylene. The exact structures of polypyrrole and polythiophene prepared that way are still not fully established. The process of oxidative polymerization involves very reactive cation radical intermediates. Much of the coupling of the heterocyclic rings together is at the 2 and 5 positions. X-ray photoelectron spectroscopy shows that the polypyrrole formed in this manner has about 30% of the linkages at other than 2 and 5 positions. They might be in the 2 and 3 positions. This introduces “defects” into the hypothetically ideal chain and reduces the conjugation length and with it the conductivity. The flexible films of polypyrrole that form upon electrochemical oxidation are not only stable in air and water, but may also be heated to 200C without much change in electrical properties. The oxidative polymerization of pyrrole can be illustrated as follows:

A regioselective synthesis of a highly conductive poly(3-alkylthiophene) s was reported [72]. Following synthetic procedure was used:

The iodine doped, unoriented poly (3-dodecyl thiophene) s exhibits average conductivity of 600 S/ cm and a maximum conductivity of 1,000 S/cm [72].

More recently, McCullough and coworkers [73] reported using the Grignard reaction to synthesize poly (3-dodecyl thiophene)

The molecular weights of the products were found by them to range from 10,000 to 50,000. Earlier, McCullough et al. [74] reported that they have developed an end group functionalization method that enables the synthesis of many well-defined block copolymers that form nano wires with high electrical conductivity. They claim to have discovered that nickel-initiated regioregular poly merization of alkyl-thiophenes proceeds by a chain growth mechanism. They also observed that the degree of polymerization of poly(alkyl-thiophenes) increases with conversion and can be predicted by the molar ratio of monomer to nickel initiator. On the basis of their experimental results, they concluded that nickel-initiated cross-coupling polymerization is essentially a living system, with low polydispersities. Irvin et al. [75] reported forming a poly (3,4-difluoro thiophene) by anodic polymerization:

Because of the very electron-poor nature of the monomer, electro deposition could only be accomplished with a strong Lewis acid, BF3EtO. Mendez and Weder improved considerably the conductivity of poly (3,4-diethylene, dioxy thiophene) by cross-linking [76]. The polymers were oxidatively synthesized in the presence of four different oxidative cross-linkers,

The cross-linking compounds were incorporated into the polymer in amounts of 0.5–2%. As a result, in a maximum case, the conductivity increased by as much 36%.

Polyaniline was first prepared at the beginning of the last century. Several oxidation states are known. The conductivity and the color of the material vary progressively with oxidation. Only one form, however, known as the emeraldine salt, is truly conducting. The material can be prepared readily by electrochemical or chemical oxidation of aniline in aqueous acid media. Common oxidants, such as ammonium peroxydisulfate, can be used. Flexible emeraldine films can be cast from solutions of N-methyl pyrrolidone and made conductive by protonic doping. This is done by dipping the films in acid or by exposing them to acid vapors. The process results in protonation of the imine nitrogen atoms:

The conductivity of the emeraldine salt increases with decrease in pH of the acid used to dope it. In this respect, polyaniline, in its emeraldine form, differs from other conductive polymers because it does not require partial oxidation or reduction for doping. Protonation of the imine nitrogens is sufficient to make it a very conductive material. Adifferent method of synthesizing polyaniline was reported [77]. It uses an enzyme, horseradish peroxidase, in the presence of hydrogen peroxide to polymerize aniline. To prevent reactions at the ortho positions of the phenyl rings that yield insoluble branched materials, a polyelectrolyte template, like sulfonated polystyrene, was used. The polyelectrolyte aligns the monomers, dopes the polyaniline to the conducting form, and forms an irreversible complex with the polyaniline to keep it water-soluble [77]. The conductivity of the complex increases with increasing polyaniline to sulfonated polystyrene molar ratios. Conductivities of 0.005 S/cm are obtained with the pure complex and increase to 0.15 S/cm after additional doping by exposure to HCl vapor [77]. Lee reported [78] a new synthesis of polyaniline, where aniline hydrochloride monomer acts as a surfactant. The organic phase serves to diffuse water away from water-insoluble oligomers. As a result, the chains grow, because the monomer radicals can meet the active polymer chain ends at the organic aqueous interfaces. This results in formation of a polymer with high structural integrity. This enables the electrons or holes to travel in a perpendicular direction.

Hanetal. [79] reported preparation of highly conductive and thermally stable, self-doping propylthiosulfonated polyanilines. The polymer was illustrated as follows:

Thematerialwasfoundtobeabetterconductorthansulfonatedpolyanilineandmorethermallystable. Poly (phenyl vinylene) can be synthesized by several routes. One of them is step growth polymerization [80] by the Witting reaction:

There are several other routes to poly (phenyl vinylene). One of them is through a metathesis ring opening polymerization:

The polymer can be doped with iodine, acids, and ferric chloride. Alkoxy-substituted poly (phenyl vinylene) is easier to oxidize and exhibits higher conductivity.

Natori et al. reported [81] the synthesis of a copolymer that consists of p-phenylene, p-phenylene vynelene, and styryl amine:

Orientation of the films yields large increases in conductivity. Thus, films of doped, oriented poly (phenylene vinylene) [81] not only have the strength of high-performance polymers, but also their conductivities measure as high as 104 S/cm. This is approximately 1,000 times greater than that of the unoriented films. Another interesting material consists of the doped forms of covalently linked siloxane phthalocyanin (Pc) complexes, [Si(Pc)O]n. In these polymers, the planar phthalocyanin units are apparently stacked face-to-face and form columns, due to the silicon-oxygen-silicon bonds. The polymers appear to be intrinsically metallic systems. The principal pathways of conductivity are perpendicular to the phthalocyanin planes. The extended p–p systems that form result from face to face are a pathway for the electron [67]. There are numerous applications for conducting polymeric materials. These applications include use in electronics, in organic solar cells that convert light to electricity and others.

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مواضيع ذات صلة

The Electron Transfer Process

Energy Transfer Process

Interaction of Light with Organic Molecules

The Nature of Light

Immobilized Reagents

Immobilized Nonenzymatic Catalysts

Immobilized Catalysts

Special Gels for Drug Release

Support Materials Other Than Polystyrene

Materials Based on Polystyrene

Polymer Supports for Reagents, Catalysts, and Drug Release

Degradation of Polymeric Materials by Ionizing Radiation