Poly acetylene, doping

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. 

Most conducting polymers, such as doped poly(acetylene), poly(p-pheny-lene), and poly(/ -phenylene sulfide), are not stable in air. Their electrical conductivity degrades rapidly, apparently due to reaction with oxygen and/or water. Poly(pyrrole) by contrast appears to be stable in the doped conductive state. 

Figure 13 shows the irreversible conversion of a nonconjugated poly (p-phenylene pentadienylene) to a lithiun-doped conjugated derivative which has a semiconducting level of conductivity (0.1 to 1.0 S/cm) (29). Obviously, the neutral conjugated derivative of poly (p-phenylene pentadienylene) can then be reversibly generated from the n-type doped material by electrochemical undoping or by p-type compensation. A very similar synthetic method for the conversion of poly(acetylene-co-1,3-butadiene) to polyacetylene has been reported (30), Figure 14. This synthesis of polyacetylene from a nonconjugated precursor polymer containing isolated CH2 units in an otherwise conjugated chain is to be contrasted with the early approach of Marvel et al (6) in which an all-sp3 carbon chain was employed.

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Table 5.3 Examples of electronically conducting polymers, y is the level of electrochemical doping and k is the maximum electrical conductivity. Except for poly acetylene and polyparaphenylene, only p-doping is considered... 

In 1958, Natta and co-workers polymerized acetylene for the first time by using a Ti-based catalyst. This polymerization proceeds by the insertion mechanism like the polymerization of olefins. Because of the lack of processability and stability, early studies on polyacetylenes were motivated by only theoretical and spectroscopic interests. Thereafter, the discovery of the metallic conductivity of doped polyacetylene in 1977 stimulated research into the chemistry of polyacetylene, and now poly acetylene is recognized as one of the most important conjugated polymers. Many publications are now available about the chemistry and physics of polyacetylene itself. 

FIGURE 6.3 Conductivities of doped poly acetylenes conductivities of insulators, semiconductors and metals are given for comparison. 

Would doping of poly acetylene with (a) Rb, (b) H2SO4 give an rt-type or a p-type conductor ... 

When poly acetylene is doped with chloric (VII) acid, HCIO4, part of the acid is used to oxidise the polyacetylene and part to provide a counteranion. The oxidation reaction for the acid is given in Equation (6.6). Write a balanced equation for the overall reaction. 

Recently, intense interest has been paid on doped poly(acetylene) because its film 68) showed markedly high conductivity on doping69 70 71 and the n- and p-type conductivities are depending on the dopants. The confirmation of p-n junction formation with the p- and n-type -fCH T, has roused great expectations to produce a polymer film solar cell.72a)... 

The doped poly(acetylene) forms various junctions such as a) a p-n junction from p- and n- -f CH, b) a hetero-Schottky junction from the inorganic semiconductor and metalic -f CH-, and c) a heterojunction from the inorganic semiconductor and semiconducting The bandgaps of -(-CH (trans- 0.6 eV, cis- 0.9 eV)... 

A typical battery is composed of poly(acetylene) electrodes which are dipped in propylene carbonate containing tetraalkyl ammonium salt as doping material (Fig 33). By charging poly(acetylene) is doped as represented by Eqs (27) and (28). 

Others Semiconductors or insulators Doped poly acetylene, TTF-TCNQ... 

If the principles, so far outlined, are valid then it is to be expected that n-type doping of polyacetylene would lead to a decrease in stability towards oxidation, and this is indeed so 578). However, the introduction of electrons into the chain can also give a new instability in that the oxidation potential can fall to the point where the polymer is able to reduce water and it becomes hydrolytically unstable. Thus n-type doped polyacetylene reacts rapidly with water and with alcohols, with partial hydrogenation of the chain and a rapid decrease in conductivity 579,580,581). Whitney and Wnek 582) have used the reaction of n-doped polyacetylene with alkyl halides and other reagents to prepare functionalized poly acetylene films. 

Electron microscopy revealed that the morphology of 6% Cl doped PITN depends on the substrate on which it is deposited and that it is a relatively "open" structure, although not as open as poly(acetylene). Selected area electron diffraction on the same sample showed the material to be partially crystalline (three diffraction rings could be seen). 

Poly acetylene can be doped with large anions of H3PM012O40. The doping increases not only the conductivity of the polymer but also the catalytic activity. The HPA is distributed nearly uniformly over the cross-section of the polymer film. For the conversion of ethanol, the catalyst exhibits acid-base activity as well as redox activity. Through the pulse reaction, it has been shown that the rate of formation of ethylene and diethyl ether increased 10 times and the rate of formation of acetaldehyde increased 40 times [98]. 

The presence of a Pauli-like term is associated with a nonzero density of states at the Fermi level it has been presented by different authors as evidence for metallic behavior. Before mentioning other possible interpretations, we would point out that the decomposition x = Xc + XP s not unambiguous. Indeed, in the case of poly acetylene in the heavily doped regime (y > 6%), the existence of Pauli susceptibility is well established [83], since in all the temperature range x is almost constant (Xp Xc)-But this is generally not the case in other compounds in which a significant Curie term remains present. 

A major reason for the failure of poly(acetylene)s in the above-mentioned applications is related to their inherent instability versus moisture and oxygen, and their high susceptibility to decomposition/rearrangement in the partially oxidized/doped state. Nevertheless, poly(ene)s stabilized by appropriate ligand systems and/or incorporated into cyclic structures are believed to exhibit similar stabilities to poly(thiophene)s, poly(pyrrole)s, poly(p-phenylene)s, PPV, and so on. In the following, we will outline the basic concepts of poly(ene)s as well as reviewing the structures that have been realized so far. 

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