Polyacetylene precursor routes

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 crystal size in polyphenylene, as determined from x-ray peak widths, is of the order of 5 nm476) with a disorder parameter g = 0.026 nm. Compression at up to 12kB decreased the (/-spacing perpendicular to the chains, decreased the peak size and increased the disorder slightly. Annealing at temperatures above 250 °C increases the crystal size and perfection 472). The spin concentration increases above 300 °C, but unlike those in polyacetylene, these spins are not mobile477. The crystallinity has variously been estimated as 80% 327) and 20 to 30% 478). It seems to depend on the catalyst used in the Kovacic method. Polyphenylene produced by the precursor route has a crystallinity from 60-80% dependent on the conversion conditions 252). 

Precursor route chemistry, principally used for processing polyacetylene and PPV into thin films ... 

Table 10-2 A summary of precursor routes to unsubstituted polyacetylene...

Figure 10-16 A precursor route to polyacetylene which proceeds via an insertion polymerization followed by a retro-Diels-Alder reaction [50],...

Several variants of this precursor route to polyacetylene and related conjugated polymers have been developed with varying degrees of success [3]. 

In this section we have focused attention on ROMP routes to polyacetylene and closely related structures. Time and space does not allow the discussion of the many other conjugated polymers accessible via direct or precursor routes involving ROMP. 

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. 

F ure 1.5. (a) A generalized two-stage route to polyacetylene, (b) A modified precursor route to polyacetylene. (Reprinted with permission from ref. 40)... 

The precursor route to the formation of conjugated polymer is another milestone in achieving complete solubility for the usually infusible and intractable materials. Two of the best-known precursor methods are the Durham route for polyacetylene [30] and the sulfonium route for poly(phenylene vinylene) [31,32]. The precursor polymers have been dissolved either in organic solvents or in aqueous media. The thermal stability of the precursor polymer, however, places an upper limit on its processing temperature. In order to convert the precursor polymer into its final conjugated form an additional elimination step is always required. 

Swager et al. [214] demonstrated an alternative precursor route to polyacetylene in which the production of volatile byproducts is avoided. The method involves the ring-opening polymerization of benzvalene and the subsequent catalytic isomerization to polyacetylene of low crystallinity (Fig. 5). 

Precursor methods have been developed for polyacetylene and poly(p-phenylene). The precursor approach has been intensively stimulated by developments in the field of poly(p-phenylene viny-lene) and the use of this polymer in light-emitting devices. The Wessling precursor route is one of the most used procedures to prepare this material. The precursor route of Louwet and Van-derzande has overcome many of the limitations inherent in the Wessling precursor route and will most probably allow further interesting developments for the application of poly(p-phenylene vinylene) in polymer electronics. 

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