Polyacetylene formed polymers

The examples of polyacetylenes whose main chain is directly bonded to heteroaromatic rings (e.g., silole, carbazole, imidazole, tetrathiafulvalene, ferrocene) are increasing in number. Such polymers are usually obtained by one of catalysts (W, Mo, and Rh). The formed polymers are expected to display interesting (opto)electronic properties such as electrochromism, cyclic voltammetry, electroluminescence, and so on. 

In 1982, Soga et al. 256> showed that exposure of acetylene to AsFs at low temperatures leads to rapid polymerization (in our experience this reaction can be explosively violent). The product is a solid polymer which is heavily arsenic-doped and has a conductivity several orders of magnitude lower than a conventional sample of polyacetylene saturation-doped from the gas phase. Aldissi and Liepins 2S7) have adapted this reaction to the preparation of soluble polyacetylene by adopting AsF3 as the reaction solvent. They claim that polymerization of acetylene with AsF5 is very rapid, giving a polymer which is soluble in common solvents. However, elemental analysis shows that the polyacetylene formed contains about one As atom per 10 CH units and this is not removed on repeated reprecipitations. It seems likely that the As atoms form part of the chain backbone, conferring sufficient flexibility to allow dissolution. It is claimed that films of soluble polyacetylene can be doped but very little information has been published. 

Acetylene selectivity polymerizes in the presence of Ziegler catalysts whose components have low Lewis acidity [e.g., Ti(0-n-Bu)4—Et3Al(l 4)J. Cis-polyacetylene forms at low temperature, and trans-polyacetylene at high temperature (Eq. (1)). When doped, a polyacetylene film shows metallic conductivity, and hence the application of polyacetylene to polymer batteries and solar cells is now under intensive study 1-3). 

Poly(2c), poly(3), and poly(4a) are interesting and representative Si-containing polyacetylenes in the sense that they are high-molecular-weight, totally soluble, film-forming polymers. 

Polyacetylene synthesis has long been a goal of polymer chemists and materials scientists because its rigid conjugated system could be an organic electrical conductor. Two approaches are outlined below. Propose mechanisms for how polyacetylene forms in both approaches. What are the structures of byproducts E and F 120... 

How much do the results we have obtained here tell us about the fundamental limits to the mobilities of carriers in devices fabricated with polymer that is very much better ordered than the polyacetylene that we have used here There are recent reports of very much improved mobilities for devices based on sexithiophene (the six repeat unit oligomer of polythiophene), with a value of 0.4 cm /Vsec now reported [73], and there is now considerable interest in the development of polymer FETs as large area thin film transistors, with interest in polythiophene derivatives [74] and in poly(arylenevinylenes) such as poly(2,5-thienylene vinylene) [75]. We can see from the optical characterisations of the MIS devices that the surface layer of polyacetylene formed on SiC>2 is very much more disordered than the bulk material, but we have not made FET devices with the polymer insulator layers which give better ordered structures as characterised optically. 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

Fig. 1. (a) Comparison of normalised electrical conductivity of individual MWCNTs (Langer 96 [17], Ebbesen [18]) and bundles of MWCNTs (Langer 94 [19], Song [20]). (b) Temperature dependence of resistivity of different forms (ropes and mats) of SWCNTs [21], and chemically doped conducting polymers, PAc (FeClj-doped polyacetylene [22]) and PAni (camphor sulfonic acid-doped polyaniline [2. ]) [24]. 

Formation of living polymers is not restricted to norbornene. For example, Grubbs successfully polymerized cyclooctatetraene to polyacetylene, and demonstrated the living nature of this polymer by forming block polymers with cyclooctadiene 19). 

It was also observed that, with the exception of polyacetylene, all important conducting polymers can be electrochemically produced by anodic oxidation moreover, in contrast to chemical methoconducting films are formed directly on the electrode. This stimulated research teams in the field of electrochemistry to study the electrosynthesis of these materials. Most recently, new fields of application, ranging from anti-corrosives through modified electrodes to microelectronic devices, have aroused electrochemists interest in this class of compounds... 

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