Syntheses of Polythiophenes

1 Polymerization of Thiophene Monomers with FeCI3 (Chart 2.160) 

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The first controlled chemical syntheses of polythiophene were reported in 1980 by two different groups they used metal-catalyzed coupling of 2,5-dibromothio-phene (Equation 6.3).27 28 In both cases, the dibromothiophene substrate was first reacted with Mg in THF, replacing either the 2- or 5-bromo substituent with MgBr. Self-coupling was then achieved, with either Ni(bipy)Cl227 or M(acac)2 (M = Co, Ni,... 

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. 

Recently, Dai, Su, and coworkers synthesized the block copolymer of polythiophene and polypyridine from the vinyl-terminated polythiophene (Scheme 102) [336]. Meijer and coworkers used an allyl-terminated polythiophene to synthesize a block copolymer of poly(3-hexylthiophene) and polyethylene the ring-opening metathesis polymerization of cyclooctene in the presence of the allyl-terminated polythiophene was followed by hydrogenation (Scheme 103) [337]. 

As in the cases of pyrrole and aniline polymers, both chemical and electrochemical procedures have been employed in the synthesis of polythiophene (PTH) and its derivatives. The thiophene polymers exhibit remarkable stability in air and water [119], The alkyl-substituted derivatives also exhibit a high degree of processability [120,121]. PTH has also been synthesized from bithiophane (the dimer) or terthiophene (the trimer). The resulting poly(2,2 -bithiophene) (PbTH) and polyterthiophene (PtTH) are more ordered than PTH and appear to have the same basic structure as the starting monomer [117,122],... 

Hiraishi, K., Masuhara, A., Nakanishi, H., Oikawa, H., Shinohara, Y, 2009. Evaluation of thermoelectric properties of polythiophene films synthesized by electrolytic polymerization. Jpn. J. Appl. Phys. 48, 071501. 

Polythiophene (PT, Figure 9.4D) is the most easily functionalized of the polymer systems surveyed here. While the bulk of polythiophenes have been used as cathode materials, there are several systems that have been found to n-dope these are used as anode materials. In general, PTs typically exhibit a specific charge of 25-100 Ah/kg and a specific energy of 50-325 Wh/kg. Polythiophene electrochemically synthesized from bithiophene [115-117] or terthiophene [118] exhibits, as predicted, cleaner electrochemistry and more stable battery materials. Poly(3-methylthiophene) (PMT, Figure 9.4E) is well studied [119-122], with specific charge ca. 90 Ah/kg and one report of specific energy of 326 Wh/kg [123]. In nonaqueous... 

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 conditions for polymerization were also foimd to be crucial in relation to polythiophene and polybithiophene films [58,80,84,114-121], The relatively high potential required for the oxidation prevents the use of many metallic substrates. The electrochemical oxidation of substituted thiophenes and thiophene oligomers yields conducting polymers, and these compounds can be electropolymerized at less positive potentials, so it is a good strategy to use these derivatives instead of thiophene (see Sect. 2.2.6). Another approach is the deposition of a thin polypyrrole layer that ensures the deposition of polythiophene on these substrates (eg., Ti, Au) [115]. Interestingly, other polymers as well as copolymers and composites (see Chap. 2) can also be synthesized. 

Shi and co-workers prepared polythiophene films with different roughnesses by electrochemical polymerization of thiophene in boron trifluoride-diethyl etherate (BFEE) [42]. The highest WCA of 116° on polythiophene film was observed. To further increase the WCA, aligned polythiophene nano-tubes were synthesized using anodized aluminium oxide (AAO) as template. The WCA increased to 134° (Fig. 7). As a comparison, the WCA of polythiophene polymerized in acetonitrile solution was measured to be less than 75°. 

A comparison of the bandgaps of polythiophene and thienoacene illustrate this point. Figure 7.7 shows the frontier orbitals of the latter polymer (Tian and Kertesz, 2008). (Note that whereas several thienoacene oligomers have been synthesized and characterized the corresponding polymers have not yet been made.) If all C-C bond distances were equal, which is not the case, as will be emphasized subsequently, the difference between the HOCO orbitals in Figures 7.5 and 7.7 would be minor and the difference would be due only to the different conformations of the underlying polyacetylene-like n-orbital chain. 

K. Akagi, M. Narita, R. Toyoshima, H. Shirakawa, Syntheses and properties of polythiophene derivatives with phenylalkyl and liquid crystalline groups. Mol. Cryst. Liq. Cryst., 318, 157-177 (1998). 

The structure of polythiophene is based on monomer units coupling through their 2,5-positions (a,a ), which preserves the aromaticity in the polymer [192,260]. This is consistent with infrared spectra, in which absorption bands found in the monomer are observed in the polymer. Dopant ions are probably responsible for the formation of new bands in the spectra of doped polythiophene, since the frequencies of the new bands shift with different types of dopants. Furthermore, elec-trochemically synthesized polythiophene exhibits distinctly different absorption bands from chemically prepared poly-2,4-thiophene [261]. 

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