Polyacetylene doping sites

PA-polymethyl methacrylate graft copolymers were the products of the polymerization of methyl methacrylate on polyacetylene doped by Na [107]. Polystyrene, polyisoprene, and cis-1,4-polybutadiene were used as polymer carriers [108,109]. Acetylene was polymerized with Ti(OBu)4-AlEt3 in a toluene solution of the polymer carrier. The authors considered the formation of graft copolymers to be the result of the nucleophilic effect of a growing PA chain on the electrophilic sites in the polymer carrier. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Furthermore, the fact that the scan exhibits two peaks suggests the presence in the basic polyacetylene structure of at least two (if not more) different structural sites for the doping process, as effectively confirmed by independent structural studies (Shacklette et al, 1985). Finally, the... 

A soliton is a giant solitary wave produced in canals by a cancellation of nonlinear and dispersive effects. The connection between aqueous solitons and tsunamis ("harbor waves") is not definitively established. In "doped" conducting polyacetylene, a neutral soliton is a collective excitation of a polyacetylene oligomer that has amplitude for several adjacent sites [57],... 

About 15 years ago, it was discovered that /ra/w-polyacetylene, or (CH), could be doped to high electrical conductivity [1]. Since then, it has become important to understand the mechanism of charge storage in polyene chains. It is now well established that excess electronic charges, introduced by either electron acceptors or donors, or through optical absorption, are accommodated as charged solitons in (CH) [2,3,4]. TTie wavefuncdons of these self-localized states typically extend over 15-20 carbon sites in long polyene chains [5]. 

Doping may impose further structural effects that depend on the size and nature of the dopant species. Although there may still not be complete accord on the crystal structure of lithium-doped polyacetylene, it appears that at low doping levels entropic factors are important in inserting a small nonaggregating ion, such as Li", into polyacetylene (PAc), with the dopant occupying sites with minimal strain or disruption of the host lattice. Iodine, on the other hand, in the form of IJ and IJ (and possibly higher polyiodides), produces structures in which anions cluster in columns or sheets to form intercalated layers between polymer chains. ... 

Figure 14 also explains why polypyrrole and polyparaphenylene are not as environmentally stable as polythiophene. Undoped poly pyrrole is susceptible to oxidation by oxygen in air, since the potential for undoped polypyrrole is below the threshold for oxygen reduction. On the other hand, /7-doped polyparaphenylene has an oxidation potential greater than that for water and is unstable toward moisture. F-doped polyparaphenylene is reduced to a less doped, less conducting state. Although oxidized and neutral polyacetylene appear to be within the stability window, experimental results show that polyacetylene is not stable [293]. The chemical reactivity of oxygen toward the carbon-carbon double bonds or defect sites in polyacetylene may make polyacetylene unstable in air. 

Jons. In the present case we need to fill the entire n band with electrons (one extra filectron per site). The result is the structure of fibrous ilfur, selenium, and tellurium. These contain chains of atoms in which all the bond lengths are the same. (The in, however, has distorted so that it is not planar.) Polyacetylene itself may be made conducting by doping either with electron donors or acceptors. The removal of some electron density from the filled band (1330) or the addition of density to die empty band (13.31) leads to a conducting (metallic) situation (cf. 13.4). 

Fig. 6.4 Relative x vs. w curves, Eq. (14), for three Ug modes of rra/i5-polyacetylene coupled to 7r-electron fluctuations. The X = 0 0 and 1.0 values are fixed by the reference force field and the coupling constants gi. The IR frequencies of doped and photoexcited samples fall on the curves at larger , while the Raman frequencies of A-site troni-polyenes fit with smaller . (From Ref. 56.)...

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