The Durham Route to Polyacetylene

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 aromatic residue may be any of a large number of such units but the favourite for academic study has been the perfluoromethylxylene derivative shown, which smoothly eliminates at around room temperature to give a polyacetylene containing 25 % of trans- and 75 % of m-units. After transformation and isomerization at 80 °C, the polyacetylene produced is a continuous dense film. The physical chemistry of the transformation and isomerization reactions has been studied in detail229,230) and the properties of the polyacetylene are reviewed 231). The great advantage of this route is that the precursor is a soluble polymer so that it can be characterized and the physical form of the polyacetylene can be controlled. 

The perfluoromethylxylene group eliminated in the transformation of the precursor is very convenient because it is volatile enough to be easily removed from the poly- 

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The Durham route to polyacetylene 103, 104) involves the metathesis polymerization of 1 to give a soluble but thermally unstable high polymer (Scheme 5.2). Slowly at room temperature, or more rapidly at 80 °C, the polymer undergoes a retro-Diels-Alder reaction. This reaction results in elimination of a substituted benzene and formation of amorphous polyacetylene. An enormous weight loss accompanies the conversion, but high-density films were produced with no apparent voids. The kinetics of the transformation reaction were extensively studied (JOS). 

More recently, a Pd(II) salt was shown to catalyze the 1,2-insertion polymerization of a 7-oxanorbornadiene derivative (Fig. 10-16) [50]. The resulting saturated polymer, when heated, gives polyacetylene via a retro-Diels-Alder reaction. (This reaction is reminiscent of the Durham route to polyacetylene discussed below). One advantage of this technique over other routes is that it employs a late transition metal polymerization catalyst. Catalysts using later transition metals tend to be less oxophilic than the d° early transition metal complexes typically used for alkene and alkyne polymerizations [109,110]. Whereas tungsten alkylidene catalysts must be handled under dry anaerobic conditions, the Pd(II)-catalyzed reaction of water-insoluble monomers may be run as an aqueous emulson polymerization. 

The Durham-route to polyacetylene An intelligent combination of organic and polymer chemistry leading to an interesting material. In the Editors view a most creative piece of modem... 

Hguie 1 The Durham route to polyacetylene. Monomer (A) is polymerised to the precursor polymer (B) which is processed from solution and is converted to polyacetylene by thermal elimination of hexafluoro-orthoxylene, initially to cu-rich and that to all trans iscnner. 

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. 

Polyacetylene may also be produced from a soluble precursor polymer by the Durham route, described earlier. In- this case the soluble precursor can be studied by conventional solution methods, provided that it is kept cold enough to prevent transformation. The molecular weight of the precursor has been determined by light scattering and low-temperature GPC 326) and corresponds to a polyacetylene chain with a molecular weight of about 200,000, with Mw/Mn of about 2. 

In our own studies of the isomerisation of the 75% e/s-polyacetylene produced by the Durham route 347> we have been unable to detect any effect on the isomerisation process of illumination with modest levels of light. We do find that the isomerisation is markedly affected by even trace amounts of oxygen, which lead to a change in the apparent order of reaction and a marked lowering of activation energy, somewhat similar to the observations of Chien and Yang 447) for conventional polymer. However, polymers prepared by the Durham route are very different from Shirakawa polymer and it would be unwise to extrapolate our results to Shirakawa materials. 

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...

In principle, conjugated materials may either be directly synthesized via metathesis polymerization of acetylene or 1-alkynes, via ROMP of various cyclooctatetraenes (COTs) or via ROMP of polyene precursors as realized in the Durham route [107-111]. The first direct polymerization of acetylene to yield black untreatable unsubstituted polyacetylene was achieved with W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2 [112]. In order to obtain soluble polymers, polyenes were prepared via the ROMP of a polyene-precursor, 7,8-bis(trifluoromethyl)tricyclo[4.2.2.02 5]deca-3,7,9-triene (TCDTF6), using initiators such as W(N-2,6-i-Pr2-C6H3)(CH-t-Bu)(OC-t-Bu)2) (Scheme 5.10) [113, 114]. 

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. 

The Durham route allows the preparation of polyacetylene in the form of continuous uniform and featureless films, as opposed to the entangled fibrillar morphology obtained by the direct polymerization of acetylene. 

As schematically shown in Figure 6.18, an unpaired 7i-electron is associated with the soliton in trans-polyacetylene. In this case, ENDOR spectroscopy can directly measure the spin density distribution of the soliton by the study of hyperfine coupling [98], according to the discussion in the preceding section. In fact ENDOR observations of the spin density distribution close to those predicted theoretically in the case of finite electron correlation have been reported independently for stretch-oriented cA-rich samples prepared by the conventional Shirakawa method [102-105] and for stretch-oriented trans samples prepared by the Durham route [99,106,107]. 

ENDOR frequency determination with the aid of ENDOR-induced ESR in cis-rich samples gives the half-width of the spin distribution of 18 carbon sites and the ratio of the peak value of the negative density to that of the positive density of p(l)/p(0)%0.44 [104,105], Similar results to those in cis-rich samples, that is similar spectra with resolved structures for the stretch direction and similar spectral frequencies, have been reported for stretch-oriented trans-polyacetylene prepared by the Durham route using pulsed ENDOR techniques [99,106,107], The unpaired electrons observed in Durham samples were conjectured to be trapped solitons from nearly temperature-independent ENDOR spectra. In the studies of this system the first successful application of TRIPLE resonance in polyacetylene has been reported [99,107], Readers can find reviews on the results of Durham samples [2,99] as... 

NBE and 7.8-bis(trifluoromethyl)tricyclo[4.2.2.0 ]-deca-3.7.9-triene may be used to generate ABA-triblock copolymers of NBE (A-block) and polyacetylene (B-block) via the Durham route ° ° (Scheme 39).354,355 -ppig poly-NBE blocks sufficiently solu-... 

The advantages of the Durham route are (1) contaminating catalyst residues can be removed because the precursor polymers are soluble and can be purified by dissolution and reprecipitation and (2) the precursors can be cast as films or drawn and oriented prior to conversion to the all-trans form of polyacetylene. This allows a degree of control over the morphology of the final product which in the pristine state appears to be fibrous and disordered. Because conductivity increases by alignment of the polymer chains, stretching the film or fiber assists this process and this can be performed using the prepolymer. 

Many of these problems have been solved by Feast (1985), who developed a very elegant synthetic method, now commonly known as the Durham route. This is a two-stage process in which soluble precursor polymers are prepared by a metathesis ringopening polymerization reaction, and these are subsequently heated to produce polyacetylene by a thermal elimination reaction. An example of the method is given below ... 

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 Durham route leads to dense continuous films of polyacetylene, and the morphology details of films or fibres produced from the precursor polymer is dependent upon the type of conversion utilized. Consequently, the morphological considerations can vary from amorphous materials to highly oriented stretched films. 

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