Polythiophene oxidative chemical polymerizations

Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most important conducting polymers industrially due to its superior electrical properties and high thermal stability [3,186-196]. PEDOT is also one of the few polythiophenes that can be synthesized through simple oxidative chemical polymerization without using any catalyst. 

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... 

Figure 13.4 Representation of oxidative chemical polymerization polythiophene.

SCHEME 2.60 Synthesis of polythiophene via chemical oxidation polymerization. 

The chemical analysis of the oxidized polymer is given in Table I. The analysis indicates there are approximately 0.2 CIO 4 anions/ thiophene ring. lso, there is excess H, ca. 1H/2 thiophene rings thus some hydrogenation may have occurred during the simultaneous oxidation and polymerization. Other researchers have found similar occurrences in polypyrrole and polythiophene. (1,10,11)... 

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. 

Nicolas et al. also synthesized semi-fluorinated polythiophenes (Scheme 4) [52, 53]. The monomers were chemically polymerized by oxidation with FeCls, or electrochemically polymerized in acetonitrile containing BU4NPF6 as the supporting electrolyte. The electrochemically synthesized films showed rough surfaces. The poly(fluorinated thiophene) films electropolymerized from the monomer with n = 8 and m = 2 showed a WCA of 153°, while the corresponding spin-coated films exhibited a much smaller WCA, due to their smooth surfaces. Their results indicated that the length of the fluorinated chain had weak influence on the surface property of the resulting film. 

Polyfurans. Relative to polythiophenes and polypyrroles, little has been reported on polyfurans (PFu) this is likely a result of the high oxidation potential of furan (>1.7 V vs Ag/Ag+), which results in side reactions during polymerization (145). While chemical polymerization of furan has been reported (146), the majority of PFu publications focus on electrochemically prepared PFu. 

In the quest for a soluble and processable conducting polythiophene, alkylthiophenes were polymerized. Poly(3-methylthiophene) (PMT) was chemically synthesized and was found to be insoluble [9, 20-22]. The first chemical synthesis of environmentally stable and soluble poly(3-alkylthiophenes) (PATs) [23-25] was reported by Elsenbaumer in 1985 (Scheme 4). Very shortly after this report, other groups [26, 27, 28] also reported both the chemical and electrochemical preparation of PATs. Poly(3-alkylthiophene), with alkyl groups longer than butyl, ean readily be melt- or solution-processed into films which after oxidation ean exhibit reasonably high electrical conductivities of 1-5 S cm [23-28]. 

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

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. 

---