Chemically polymerized polythiophene

Figure 13.4 Representation of oxidative chemical polymerization polythiophene.

In all cases of electrochemicaHy or chemically polymerized unsubstituted polypyrrole, the final polymer is intractable in both the conducting and insulating forms. In contrast, a broad number of substituted polythiophenes have been found to be processible both from solution and in the melt. The most studied of these systems ate the poly(3-alkylthiophenes) (P3AT). 

Composites of polypyrrole and poly(vinyl chloride) have been prepared by several groups (64-67). Polythiophene-poly(vinyl chloride) composites have also been prepared (68). The electropolymerization of pyrrole on poly(vinyl chloride)-coated electrodes yielded composites with mechanical properties (tensile strength, percent elongation at break, percent elongation at yield) similar to poly(vinyl chloride) (65) but with a conductivity of 5-50 S/cm, which is only slightly inferior to polypyrrole (30-60 S/cm) prepared under similar conditions. In addition, the environmental stability was enhanced. Morphological studies (69) showed that the polypyrrole was not uniformly distributed in the film and had polypyrrole-rich layers next to the electrode. Similarly, poly(vinyl alcohol) (70) poly[(vinylidine chloride)-co-(trifluoroethylene)] (69) and brominated poly(vinyl carbazole) (71) have been used as the matrix polymers. The chemical polymerization of pyrrole in a poly(vinyl alcohol) matrix by ferric chloride and potassium ferricyanide also yielded conducting composites with conductivities of 10 S/cm (72-74). 

A large number of optically active polythiophenes have been prepared through the chemical polymerization of monomers bearing chiral substituents covalently bound... 

From X-ray diffraction, it is known that semicrystalline polythiophene powder consists of completely co-planar molecules [64], in contrast to the oligomers with chain length of and above three. The crystallinity of powders of chemically coupled polythiophene prepared by monomer oxidation with iodine, increases from 35% as synthesized up to 56% after annealing at 753 K for 30 minutes [65]. At the same time the residual iodine content decreased from 3.17% as synthesized to 0.13% after the heat treatment. Whereas annealing at 753 K leads to a first degradation of the polymer, heat treatment at 673 K results in polythiophene with chains of approximately 1200 thiophene units. Electrochemically polymerized polythiophene gives a completely different X-ray diffraction pattern [66],... 

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. 

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. 

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. 

Since the discovery of a highly conductive polyacetylene film in 1977 [33], various conductive materials have been developed based on the polymerization of five-membered heteroaromatics represented by polythiophene (2). Polyselenophene (3) was also obtained by chemical polymerization [34-36] or electrochemical polymerization of selenophene (Scheme 6.2) [37-39]. The bandgap energy of polyselenophene... 

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. 

Polythiophenes can be synthesized by electrochemical polymerization or by chemical methods. Although, because of several distinct advantages, electropolymerization has been for a long period the preferred method at the laboratory scale, the recent development of solution-processible PTs has triggered a strong renewal of interest in chemical polymerization, which remains the most suitable method as far as industrial production is envisioned. 

Electronically conducting polymers such as polyaniline, polypyrrole, polythiophene, and their derivatives can be electro-chemically polymerized from their respective monomers. They... 

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. 

SCHEME 2.60 Synthesis of polythiophene via chemical oxidation polymerization. 

ETEROAROMATics FURAN AND THIOPHENE. The chemical transformation of thiophene at high pressure has not been studied in detail. However, an infrared [441,445] study has placed the onset of the reaction at 16 GPa when the sample becomes yellow-orange and the C—H stretching modes involving sp carbon atoms are observed. This reaction threshold is lower than in benzene, as expected for the lower stability of thiophene. The infrared spectrum of the recovered sample differs from that of polythiophene, and the spectral characteristics indicate that it is probably amorphous. Also, the thiophene reaction is extremely sensitive to photochemical effects as reported by Shimizu and Matsunami [446]. Thiophene was observed to transform into a dark red material above 8 GPa when irradiated with 50 mW of the 514.5-nm Ar+ laser line. The reaction was not observed without irradiation. This material was hypothesized to be polythiophene because the same coloration is reported for polymeric films prepared by electrochemical methods, but no further characterization was carried out. 

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