Distributed polyacetylene polymers

The degeneracy of the ground state of polyacetylene influences its charge distribution. In fact, upon doping the charges, which in other polymers, such as the heterocyclics, would pair to form bipolarons, are here readily separated to form two positively charged solitons ... 

In general, doping tends to lead to a loss of x-ray order in polyacetylene and polyphenylene, suggesting that dopant ions may be distributed more or less at random. The structural models shown in Fig. 16 are clearly idealised as only limited order is seen even in cation-doped polymer. The anion dopants are much larger and apparently disrupt the structure too much for any sign of regularity to be seen, except in the case of iodine. 

Pekker, S. Janossy, A. Chemistry of doping and distribution of dopants in polyacetylene. In Handbook of Conducting Polymers Skotheim,... 

When PVAPAC films, optimized with respect to conjugation length distribution and PAC concentration, are stretched in a controlled manner they turn into highly dichroic transparent neutral grey POLPAC filters (Fig. 1.10). The absence of coloration is indicative of an exceptionally highly ordered PAC state in these novel all polymer broadband polarizers based on polyacetylene (POLPAC ). 

The interaction of long-chain molecules such as polymers is a problem area where the nature of polarization response can be a significant concern on its own. An example is from a study of parallel hexatriene molecules carried out to represent a truncated form of solid-state polyacetylene [192]. This smdy included both ab initio calculations and an electrostatic model using polarizability, a, and second hyperpolarizability, y, tensors distributed to the carbon centers. The ab initio calculations on a single hexatriene molecule were used to find the distributed tensors for the electrical analysis. The objective in this smdy was not the interaction energy, but the effect on each molecule s polarizability and hyperpolarizability due to intermolecular interaction. The ab initio evaluations benchmarked the electrostatic model calculations both for... 

This chapter has provided some examples of the ways in which conjugated polymers can be prepared. While the account is not of course exhaustive, and indeed many extremely important synthetic routes have not been included, such as the formation of polyacetylene by the Durham route,it does serve to illustrate that the range of synthetic techniques vary from the simple to the extremely sophisticated. Electrochemical synthesis is largely in the former classihcation, however, it does have considerable potential in the design of materials for molecular electronics since it will allow patterns to be formed on the electrode surface. With the continuing demand for new materials both for electronic and power distribution needs, it is to be expected that this area will continue to develop in the foreseeable future. 

Blending of polyacetylene with polybutadiene provides an avenue for property enhancement as well as new approaches to structural studies. As the composition of the polyacetylene component is increased, an interpenetrating network of the polymer in the polybutadiene matrix evolves from a particulate distribution. The mechanical and electrical properties of these blends are very sensitive to the composition and the nature of the microstructure. The microstructure and the resulting electrical properties can be further influenced by stress induced ordering subsequent to doping. This effect is most dramatic for blends of intermediate composition. The properties of the blend both prior and subsequent to stretching are explained in terms of a proposed structural model. Direct evidence for this model has been provided in this paper based upon scanning and transmission electron microscopy. 

The unpaired electron spin in trans type polyacetylene (PA) plays an important role as a soliton for the conduction. The PA developed by Shirakawa [40] is a semiconductor, but changes to a conducting polymer by adding dopants. The distribution of the spin along the chain is symmetrical like a wave centering around the midpoint of the soliton. The neutral soliton of the excited state is not a conduction carrier. However, when dopants like As P5,12 and similar agents are introduced and abstract electrons from the solitons, the formed carbanium ion solitons are converted to conduction carriers. When dopants like Li, Na and similar substances add electrons to the solitons, carboanion solitons also change to conduction carriers. 

---