Polyacetylenes organic conductors

Charged solitons may travel when subject to an electric field, and therefore the doped polyacetylene turns out to be a good conductor (organic metal). 

A second type of soHd ionic conductors based around polyether compounds such as poly(ethylene oxide) [25322-68-3] (PEO) has been discovered (24) and characterized. These materials foUow equations 23—31 as opposed to the electronically conducting polyacetylene [26571-64-2] and polyaniline type materials. The polyethers can complex and stabilize lithium ions in organic media. They also dissolve salts such as LiClO to produce conducting soHd solutions. The use of these materials in rechargeable lithium batteries has been proposed (25). 

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

NMR experiments on representative systems like CPs and organic conductors are presented in the next section. Majority of the work reported in CP and organic conductors are concentrated on doped polyacetylene (PANI), polypyrrole (PPY) and poly thiophene (PT). But, before discussing the experimental results, it is better to review the NMR theory as applicable to CPs/organic conductors. A brief presentation of the same has been given here for the ready reference. 

Polyacetylene synthesis has long been a goal of polymer chemists and materials scientists because its rigid conjugated system could be an organic electrical conductor. Two approaches are outlined below. Propose mechanisms for how polyacetylene forms in both approaches. What are the structures of byproducts E and F 120... 

Twenty years ago, a new class of organic polymer materials was discovered with the new property of electronic conductivity comparable to metallic conductors [55]. The first representative was the polyacetylene followed in the subsequent years by several other polymers, such as polyaniline, polypyrrole, and polythiophene (see Fig. 7). The neutral structure is shown in Fig. 7. This structure has properties comparable to a semiconductor. The metal like conductance is obtained by chemical or electrochemical oxidation (shown for polypyrrole in Fig. 8). In this example, up to 30% of the pyrrole rings can be oxidized. The positive charge of the heterocyclic ring... 

Organic compounds are generally good insulators, and metals conduct electricity. However researchers have been successful in making organic compounds that are conductors. Acetylene can be polymerized in the presence of a catalyst to polyacetylene, a typical plastic that does not conduct electricity. 

One of the interesting aspect of conjugated polymers is their ability to behave as electrical conductors (44) (see Electrically-Conducting Polymers). The metallic appearance of many polydiacetylene crystals suggests they would be good candidates as electrical conductors. However, these organic materials are insulators with conductivities less than 10 Polyacetylene also shows low conductivity... 

Polyacetylene is the prototype for new organic materials, be it as conducting polymers, polymeric light-emitting devices, materials for non-linear optics or for molecular electronics. Furthermore, polyacetylene is the prototype of a one-dimensional conductor. 

Our polyacetylene band structure has an Eg of zero. Figure 17.6 shows different ways of achieving this. We can have a partially filled band, or we can have two bands that overlap or touch (as in Figure 17.4 C) at the Fermi level. Either way we have zero band gap, and such a system is a metal and is expected to be a conductor. Based on this, we conclude that at the level of HMO theory, polyacetylene is an organic metall We hasten to add that the real situation, beyond HMO, is more complicated, such that real polyacetylene is a semiconductor. We will return to this below. Still, given the success of HMO theory in making interesting predictions about molecular properties, not always quantitatively correct but usually qualitatively cor-... 

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