Undoped Polyacetylene

Pyrolysis of poly(methylacetylene) shows rather similar behaviour 528 , with mesi-tylene as the major product but substantial yields of methyl and proton-enriched products. Thermal decomposition of this polymer sets in at around 150 °C and the mechanism is postulated to involve chain scission followed by cyclization reactions and both electron-proton and electron-methyl exchanges. Pyrolysis of poly(phenyl-acetylene) has been reported to start at 270 °C in nitrogen 529). 

The possible involvement of oxygen in cis-trans isomerisation of polyacetylene and the related question of whether cis- and tram-polymers oxidize at different rates 

Cernia et al.553) examined the effect of exposure on m-trans-isomerisation, using a high intensity mercury arc lamp with no filtering specified. Under these aggressive conditions irradiation at minus 196 °C produced a rapid increase in free-spin concentration and irradiation at temperatures up to 120 °C increased the rate of isomerisation above this temperature the thermal isomerisation is the dominant reaction. 

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. 

---

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

Proton 1/Ti of heavily doped polyacetylene films with different dopants such as FSO3H, FICIO4, iodine, bromine and potassium was measured by Shimizu et al.113 and the behaviour expected for a quasi-ID metal was not found. Some of them showed a time dependence of 1/Ti. They have deduced the temperature dependence of resistivity from the 1/Ti and 1/Ti versus temperature shows T1 5 behaviour above 40 K and deviates from such behaviour below 40 K. Mizoguchi and Kuroda107 have given a comprehensive review of many investigations of NMR relaxation of both 1FI and 13C in undoped polyacetylene. 

Deviating results obtained in older studies are, after all, most likely a consequence of the very limited number of usable reflections in diffraction. Usually this number has been lower than 10, and only in the most recent studies on oriented material has the first layer line been exploited. Discrepancies have usually been ascribed to the presence of undoped polyacetylene, either cis or trans, to general inhomogeneity, a small amount of an unidentified phase, or impurities from handling. Especially in isotropic samples, spurious reflections from such contaminants cause positional shifts and altered intensity ratios that caimot be clarified. 

Optical absorption experiments on the thin semitransparent t/ans-polyacetylene revealed essentially no absorption until the band gap of 1.4 eV was reached. The strong absorption beyond 1.4 eV in the interband region, is consistent with previous studies on undoped polyacetylene(lO). After n-doping with either Ca or Eu" 2 countercations, increased absorption is found in the midgap region (0.7 - 0.9 eV). 

Figure 1.37. Temperature dependence of the a.c. conductivity of undoped polyacetylene. (Reprinted with permission from...

The electrochemical polymerization method was used to prepare layers of conductive polymers on electrodes pre-coated with another conductive polymer or using films of a conductive polymer as electrode. This was the method used to prepare a composite of poly acetylene and polypyrrole [150,151] A sandwichlike composite with layers of polypyrrole adhered to polyacetylene was obtained using undoped polyacetylene electrodes. The use of doped polyacetylene induced pyrrole polymerization in the bulk of the film. A strong improvement in the stability of polyacetylene was observed exposure to ambient temperature of 30 days or to water for 20 minutes did not reduce the conductivity of both materials. 

The simplest part of this problem is evaluating the maximum concentration of carriers. The maximum value might result for a small ion such as Li". Since the radius of interchain voids in polyacetylene is comparable with the radius of Li", relatively little volume expansion will occur upon doping if solvent coinsertion is avoidedHence, to good approximation, the carrier density in (CHLiQ 3Q)x will be 0.3 electrons in the volume of a CH group in undoped polyacetylene (18.6 a ) or n = 1.6 x 10 electrons/cm. ... 

Fig. 23 shows ESR measurements of the decrease of the spin concentration during doping /20/. In undoped polyacetylene there are usually several hundred ppm of neutral solitons. Light doping converts them into charged and spinless solitons, hence the spin concentration decreases on doping. 

---