Durham polyacetylene synthesis

This, the second stage of the Durham polyacetylene synthesis (and presumably identical to the homopolymer [96570-67-1]) is capable of explosion with one sixth the energy of TNT. 

The Durham precursor route to polyacetylene is an excellent example of the application of organic synthesis to produce a precursor polymer whose structure is designed for facile conversion to polyacetylene. Durham polyacetylene was first disclosed by Edwards and Feast, working at the University of Durham, in 1980 227). The polymer (Fig. 6 (I)) is effectively the Diels-Alder adduct of an aromatic residue across alternate double bonds of polyacetylene. The Diels-Alder reaction is not feasible, partly for thermodynamic reasons and partly because it would require the polymer to be in the all m-conformation to give the required geometry for the addition to take placed 228). However, the polymer can be synthesised by metathesis polymerization of the appropriate monomer. 

The synthesis of Durham polyacetylene [98] and Wes-sling-Lenz polyphenylenevinylene [99,100] in the 1980s raised a lot of interest, and not only because of the importance of these polymers for material science. They were successful examples of the so-called indirect or precursor approach in which an insoluble target polymer is liberated from a well-characterizable precursor by some well-understood, cleanly proceeding chemistry. The polymer chemist s community, which, for good reasons, tends to be skeptical toward this kind of approach, realized that the precursor approach can be a powerful method in spite of all the problems inherently associated with it. In view of these developments, which date back... 

A drawback to the Durham method for the synthesis of polyacetylene is the necessity of elimination of a relatively large molecule during conversion. This can be overcome by the inclusion of strained rings into the precursor polymer stmcture. This technique was developed in the investigation of the ring-opening metathesis polymerization (ROMP) of benzvalene as shown in equation 3 (31). 

The monomer is the highest energy stage of the Durham synthesis of polyacetylene since the next stage, metathesized polymer, is known to be potentially explosive, the monomer is not likely to be absolutely safe. 

Scheme 5.2. Synthesis of polyacetylene by the Durham route. The precursor polymer is soluble and can be processed prior to conversion to poly acetylene.

Scheme 5.3. A revised synthesis of polyacetylene by the Durham route. When 2 is included in the polymer, the conversion to polyacetylene first requires that the diene be generated at 80 °C) before the retro-Diels-Alder reaction...

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. 

Each of the above techniques has resulted in high quality polyacetylene films. The major drawback for these techniques has been the intractable nature of polyacetylene, resulting in difficulties in purification, characterization, and processing. To eliminate these processability problems, it is necessary to employ a precursor route such as the widely studied Durham method to polyacetylene (65,66). In this process, an acetone-soluble precursor polymer is formed processing occurs in this phase of the synthesis before conversion to the intractable polyacetylene. This approach is shown in equation 2. 

The synthesis of polyacetylene by the Shirakawa [1,2] and Durham [3,4] methods in the 1970s ignited an explosion of research into the chemistry, physics and applications of conducting polymers that has continued... 

Figure 11.1 Synthesis of frans-polyacetylene 4 by the Durham precursor route.

The synthesis of highly-oriented films of polyacetylene via the Durham precursor route has allowed an investigation of the intrinsic anisotropic properties of the material. In particular, we have been able to study the role of inter-chain transitions in determining the photoexcitation properties of the material and find that long-lived photocarriers are preferentially excited by such transitions. 

---