Polyacetylene conductive hybrids

Conductive Hybrids Based on Polyacetylene Copolymers and Blends... 

All conducting polymers have a common feature a long chain of sp2 hybridized carbon atoms, often with nitrogen or sulfur atoms included in the chains. Polyacetylene, the first conducting polymer, is also the simplest, consisting of thousands of —CH=CH -units ... 

Unlike (SN), most polymers correspond to closed-shell systems where all the electrons are paired. Such a configuration leads to insulating or semiconducting properties as noted previously. Polyacetylenes and related conjugated polymers, for example, have conductivities that classify them as semiconductors. The carbon atom in polyacetylene is sp hybridized, which leaves one p electron out of the bond-forming hybrid orbitals. In principle, such a structure might be expected to give rise to extended electronic states formed by overlap of the p (tt) electrons and thus provide a basis for metallic behavior in polymers. 

A spectacular example of a clear relationship between chemical and stnictural homogeneity and improved electrical properties can be found in polyacetylene. C CP MAS NMR studies of initially synthesized polyacetylene [22,23] indicated the presence of a small fraction of sp hybridized carbons in addition to sp carbons expected for a perfect polyene chain. Drastic reduction in the number of these sp defects observed in polyacetylene prepared by the method developed by Naarman and Theophilou, resulted in an important improvement of the conductivity of this polymer in the doped state [24],... 

The first conducting polymer to be synthesised was polyacetylene. When polymerised, acetylene (ethyne) forms a silvery flexible film of polyacetylene. Acetylene (ethyne) has a formula C2H2. The carbon atoms are linked by a triple bond, consisting of 1 sp-hybrid a bond and two n bonds (Scheme 13.2). Generally, polymerisation leads to the aU-cii polymer. At room temperature this changes to the thermodynamically stable ail-trans form. These two forms are geometrical isomers (see Section S2.1). Both are poor insulators, with the trans form having a conductivity similar to that of silicon (approximately 10 S m ), and the cis form with a conductivity similar to that of water (approximately 0.1 S cm ). 

Fig. 2.5. Ah initio calculated quasi-particle band structure, (a) truns-polyacetylene, where the valence and conduction bands are denoted as tt and tt, respectively, and the four bands below the valence band are formed from the three sp hybrids and the hydrogen Is orbital, (b) poly(paro-phenylene vinylene), where the valence, 7Ti, conduction, ttJ, and nonbonding bands, 7T2 and > are shown. Reprinted with permission from M. Rohlling and S. G. Louie, Phys. Rev. Lett. 82, 1959, 1999. Copyright 1999 by the American Physical Society.

The structure of polyacetylene contains alternating carbon-carbon single and carbon-carbon double bonds. Recall from Chapter 9 that the carbon atoms in a C=C double bond are sp hybridized and that it is the leftover p orbital on each carbon atom that overlaps to form the pi bond. Each carbon atom in polyacetylene is identical (structurally and electronically), so there is extended overlap of the p orbitals throughout the polymer chain. The p electrorrs are delocalized— that is, they can move throughout the network of overlapping orbitals that exterrd the length of the polymer chain. By being able to move electrorrs from one errd of the polymer chain to the other, polyacetylene can conduct electricity much like a wire. That is, polyacetylene is a plastic that conducts electricity ... 

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