Pristine polyacetylene

Give reasons why pristine polyacetylenes formed with coordination catalysts of various types at low temperature are of the cis configuration. 

The utility of classical antioxidants such as hindered amines, phenols, and nitrones for the stabilization of pristine polyacetylene (29), poly(methyl acetylene) (30), and poly(l,6-heptadiyne) (31) has been examined. Poly(methyl acetylene), although dopable to only low conductivities (10" S/cm), has similar oxidative behavior to polyacetylene and serves as a good model for other polyenes. In general, the improvement in stability of poly(methyl acetylene) was limited, but combinations of hindered phenols and hydroperoxide scavengers resulted in a factor of 5 decrease in the oxidation rate (30) as monitored by the appearance of IR absorption bands attributable to carbonyl groups. These degradation rates are still too high for the use of these polyenes in an unprotected environment. The compatibility of such stabilizers with the dopants commonly used for polyacetylene was not studied. 

The addition of N-bromosuccinimide (NBS) to pristine polyacetylene stabilized the undoped polymer (32). The enhanced stability was attributed to the reaction of NBS with the numerous free radicals found in polyacetylene, as evidenced by the decreased rate of oxygen uptake in treated samples and increased final conductivities for iodine-doped polyacetylene. The conductivity of the undoped polymer rose from 10 S/cm for untreated samples to greater than 10 S/cm for NBS-treated samples. A slight amount of Br was detected in treated samples this finding indicates that some doping accompanies the treatment. The stability of doped polymer samples was not significantly improved by this treatment. 

Pristine polyacetylene is a typical organic semiconductor irrespective of its cis-trans content. The room-temperature conductivity measured by a conventional two-probe method and the energy gap determined by equation 19 are 1 x 10 S cm and 0.56 eV for 92.5% trans-polymQv, and 4 x 10" Scm and 0.93 eV for 80.0% cw-polymer, respectively In equation 19 (T is the electrical conductivity at T and ACg is the energy gap... 

Ahlgren and Krische [92] reported the stability of electrochemically deposited polypyrrole on pristine polyacetylene (A-type) or predoped (B-type) with... 

In order to understand the physical properties of polyacetylene doped with divalent ions, it is important to consider the theory of conductivity of polyacetylene doped with monovalent ions. One of the most unusual characteristics of polyacetylene is that small amounts of dopant ions give rise to enormous increases in electrical conductivity without causing any increase in the number of unpaired electrons. In fact, the small level of paramagnetism observed in pristine polyacetylene actually decreases on doping (8). This is in contrast to what occurs in traditional semiconductors, such as silicon, where dopants increase both conductivity and paramagnetism. An explanation has been offered by the soliton theory of conductivity (9,10). 

Diffusive Motion of the Soliton in Pristine Polyacetylene Detected by the ESR Line Width... 

On this basis, the plot of v vs. R (Fig. 6) keeps its validity also for the IR spectra of doped PA, and the doping-induced bands can be accounted for, in frequencies and intensities, as the modes which contain, to various extents, the. Si mode, just as in the case of the Raman spectra of pristine polyacetylene. 

Figure 4.1 7c-electronic band structure possibilities for pristine polyacetylene. 

The lower the doping concentration, the stronger the temperaturedependence. Not only do iodine-doped samples behave this way but all samples investigated so far. The conductivity of pristine polyacetylene drops... 

It remained for a chemist and a physicist, MacDiarmid and Heeger, respectively, to appreciate the potential of doping for improving the cond uctivity of pristine polyacetylene. In 1977, Shirakawa, MacDiarmid, and Heeger reported that exposure of polyacetylene to Ch, Br2, or I2 produced a material with conductivities as high as 10 S cm, and the field of conducting polymers was born. Suddenly, materials that always had been viewed as insulators had the potential to transform the electronics industry. For this work, Shirakawa, MacDiarmid, and Heeger shared the 2000 Nobel Prize in Chemistry. 

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