Chemical polymerization polythiophene synthesis

Early progress in polythiophene chemistry was achieved by the synthesis of mono- and dialkoxy-substituted thiophene derivatives developed by Leclerc [6] and industrial scientists at Hoechst AG [7-9]. However, most polymers of mono- and dialkoxythiophenes exhibited low conductivity in the oxidized, doped state. A breakthrough in this area was the synthesis of polymers of the bicyclic 3,4-ethylenedioxythiophene (EDT or EDOT) and its derivatives—electrochemically polymerized by Heinze et al. and chemically polymerized by Jonas et al. of the Bayer Corporate Research Laboratories [10,11]. In contrast to the nonbicyclic polymers of mono- and dialkoxythiophenes, PEDT has a very stable and highly conductive cationic doped state. The low HOMO-LUMO bandgap of conductive PEDT allowed the formation of a tremendously stable, highly conductive ICP [12]. Technical use and commercialization quickly followed today ICPs based on PEDT are commercially available in multiton quantities. 

Chemical synthesis involves either condensation polymerization, where the growth of polymer chains proceeds by condensation reaction, or addition polymerization where the growth is dependent on radical, anion, cation formation at the end of polymer chain. Figure 13.4 is a schematic representation of the oxidative chemical polymerization of polythiophene [24]. In general, oxidative chemical polymerization is carried out in the... 

Synthesis of polythiophene is possible by electrochemical and chemical polymerization techniques. The technique used in electrochemical synthesis is similar to that used for polypyrrole. As the starting materials, dimer [60-62], trimer [61, 63] and tetramer [61] can be used. 

SCHEME 2.60 Synthesis of polythiophene via chemical oxidation polymerization. 

Thiophene, pyrrole and their derivatives, in contrast to benzene, are easily oxidized electrochemically in common solvents and this has been a favourite route for their polymerization, because it allows in situ formation of thin films on electrode surfaces. Structure control in electrochemical polymerization is limited and the method is not well suited for preparing substantial amounts of polymer, so that there has been interest in chemical routes as an alternative. Most of the methods described above for synthesis of poly(p-phenylene) have been applied to synthesise polypyrrole and polythiophene, with varying success. 

Summaries on the synthesis, properties, and uses of polythiophenes are included in two general reviews on poly thiophenes [259,260]. A synopsis of important aspects of polythiophenes are also included in several reviews on various aspects of conducting polymers [221-226], Cation radicals are the propagating species in both electrochemical and chemical oxidative polymerizations of thiophene and its derivatives. The polymer obtained by this method is linked primarily by a,a-linkages. However, other types of linkages (a,f3 and /3,/3) are present in varying amounts (Fig. 59). Substituted thiophene derivatives can couple in a head-to-tail or head-to-head manner. 

Although most metal-containing polythiophenes have been synthesized by electropolymerization on an electrode surface, there are many reasons to chemically synthesize these polymers. Chemical synthesis may allow isolation of soluble polymers, enabling complete solution characterization (GPC, light scattering, NMR, etc.) and facilitating conductivity studies. Moreover, it can enable improved thin-film preparation and film deposition onto nonconducting substrates. Finally, monomers that are unsuitable for electropolymerization may be polymerized by chemical methods. 

Chen et al. reported the synthesis of sulfur/polythiophene composites with core/shell structure through an in-situ chemical oxidative polymerization method for LIB cathode. Using a low viscosity electrolyte of 1,3-dioxolane (DOL)/dimethoxy ethane (DME), the composite with 72 wt% of sulfur was cycled at a current density of 100 mA/g and retained 74% of its initial capacity (1120mAh/g) after 80 cycles [47]. Polythiophene (PTh) coated with ultrathin MnO nanosheets was synthesized through one-step aqueous/ organic interfacial method for LIB anode application. The as-synthesized MnOj-polythiophene nanocomposite delivered a reversible capacity of 720 rnAh/g and retained 500 mAh/g after 100 cycles at a high current density of 500 mA/g [48]. 

The chemical synthesis of self-doped polythiophene was first reported by Ikenoue et al. [21]. They synthesized self-doped poly(3-(3 -thienyl)pro-panesulfonate) in aqueous media using ferric chloride as an oxidizing agent. The electrochemical polymerization of the monomer, 3-(3 -... 

F ure 8.13. Various processing routes of the soluble polythiophenes in combination with the doping. The materials are assumed to be prepared oxidatively either by electrochemical polymerization or via chemical synthesis. Reprinted with permission from Reference 51. Copyright 1989 Materials Research Society. 

Finally, we will concentrate on the chemical reactivity of silyl derivatives of thiophene. The oxidative polymerization of various silyl monomers lead to polythiophene. The evaluation of this new polymerization reaction implies a precise characterization of the produced conjugated materials. Knowledge and the control of the pertinent parameters which direct the properties of the conjugated systems are essential. Also required is the development of methods which allow a precise characterization of the samples. The role of vibrational infrared and Raman spectroscopy is of fundamental importance in this field. Optical spectroscopy is one of the few tools for unravelling the structure of these materials and understanding their properties. First, new criteria based on infrared, Raman and photoluminescence spectroscopy which allow precise estimates of the conjugation properties will be reported. Then the synthesis and characterization of polythiophene samples arising from the oxidative polymerization of silyl thiophene will be presented. 

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