Thiophene, chemical polymerization

Around April 1998, Monsanto sold the product line to Ormecon Chemie, a subsidiary of Zipperling Kessler, based in Ammersbek (near Hamburg), Germany. As of this writing, this firm continues to produce bulk Versicon. The PVC dispersion, called "Incoblend", is claimed to be injection-moldable. Neste Oy, Helsinki, Finland, and Hexcel Corp., also test marketed P(ANi) and P(3AT) powders for example one of the product lines of Neste Oy included poly(3-octyl thiophene) chemically polymerized using FeCla and with a weight-average MWt of ca. 89,000 [157]. 

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

As outlined earlier, three methods of polymerization have been established for the preparation of thiophenes, viz. electrochemical polymerization [189, 190], oxidative chemical polymerization using Lewis acid catalysts such as FeCl3 [191,192], and step-growth condensation polymerization using transition metal-catalyzed coupling reactions [lj]. 

Figure 2.6. The x-ray diffraction patterns of (a) chemically [46] and (b) electrochemically [40] polymerized thiophene. The curves for the chemically polymerized sample show observed, fitted and difference patterns (top to bottom). (Reproduced by permission of Hiithig Wepf Publishing, Zug, Switzerland.)...

Fell et al. [72] studied the correlation between the composition and the strucmre of co-polymers of hexyl and octyl thiophenes, obtained by chemical polymerization with FeCl3, and performed also a comparative study of the corresponding polymer mixtures [71 ]. The co-polymers, P[(HT) x(OT) ], of molar fractions x = 0, 0.3, 0.5 and 1.0, were cast from chloroform solutions into films that could be stretch oriented, and were also obtained in the form of stretched threads. Diffraction studies showed patterns very similar to those from the pure polymers, with well developed direction dependent Bragg diffraction peaks, and direction dependent diffuse features. Whereas the b-axis parameter stayed almost constant throughout the series, the a-axis parameter was found to vary linearly with the molar fraction x. 

Polymer materials possess varying physical and structural properties dependent upon the route of synthesis and polymerization, as was discussed in Section 4.1. Properties also depend upon their actual physical form or state. The as prepared samples obtained by chemical polymerization are in the state of powder. Polymer films are prepared by dissolving the powder in an appropriate organic solvent and then letting the solution dry by slow evaporation. Common solvents with poly(alkylthiophene)s are chloroform, tetrachlormethane, toluene, tetrahydrofuran, decaline, anisole, thiophene and some others, whereas acetone. 

Several tricyclic monomers required for oxidative (electrochemical or chemical) polymerization have been prepared by coupling reactions. Thus, l,4-diiodo-2,5-dimethylbenzene was coupled with 2-thienylmagnesium bromide in the presence of nickel(II) bis(diphenylphosphino)propane dibromide (NidpppBr2). However when methoxy groups were present in the benzene ring ortho to the site of coupling, the decreased electrophilicity necessitated the use of a more nucleophilic thiophene derivative, 2-thienylcopper (Equation (60)). 

The synthesis of conjugated polymers is highly dependent on the effective carbon-carbon single bond generation between unsaturated carbons in aromatic molecules. Aromatic units in conjugated polymers can be benzene, aniline, pyrrole, thiophene, carbazole, naphthalenediimide, perylenediimide (PDI), or their derivatives, etc. Although monomers are various, their synthetic methods can be mainly classified into chemical and electrochemical polymerizations. Chemical polymerization includes chemical oxidative polymerization and metal-catalyzed coupling condensation. 

SnOj hollow spheres/poly-thiophene nanohybrid (PTh-SnOj) In-situ chemical polymerization Pellet NO [24]... 

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. 

Vanderzande et al. reported the facile synthesis to 5,6-disubstituted-l,3-dithienylbenzo[c]thiophenes 3.10 via Pd°-catalyzed coupling reaction of 5,6-dichloroterthiophenes 3.9 with an alkyl Grignard reagent (Scheme 1.30) [309, 321]. Chemical polymerization of the 5,6-modified monomers with FcCIb yielded polymers with bandgaps of 1.4-1.8 eV, which are similar to that of poly(dithienylbenzo[c]thiophene) P3.3 [309]. Application of these polymers as donors and fullerene PCBM as acceptor in bulk heterojunction solar cells (BHJSC) was also investigated and reported. An overall power conversion efficiency of 0.3 % and an internal power conversion efficiency of 24% were obtained for PMMA-poly-P3.9c-PCBM (1 2 6) blended devices [321]. 

Wudl et al. synthesized a thienopyrazine-based monomer 3.21f (Chart 1.47) by attaching octyl groups at the 3-position of the thiophene moiety [334]. Monomer 3.21f was chemically polymerized with FeC to generate polymer P3.21f. The polymer covered a very broad absorption range from 300 to 980 nm. 

Only solid iron(III) chloride was active as a polymerization oxidant for 3-alkylthiophene. The soluble part of iron(lll) chloride was inert. The solubility of iron(III) chloride in chloroform and the consuming effect of evolved hydrogen chloride gas explained the extra amount of iron(III) chloride that was necessary initially to obtain high conversion in polymerization. A feasible polymerization mechanism for 3-alkylthiophene was developed on the basis of the crystal structure of iron(III) chloride and quantum chemical computations of thiophene derivatives. Polymerization was proposed to proceed through a radical mechanism rather than a radical cation one. 

As for thiophene, this polymerization can be performed either electrochemically on a Pt electrode dipped into an electrolytic solution or chemically using FeCla in chloroform. While polythiophene is insoluble and intractable, poly(3-alkylthiophene) is soluble in common solvents such as THF or chloroform and allow easy characterization. The molecular weight can be estimated using GPC chromatography. In Table 14.15 are reported the experimental values obtained after calibration of the GPC column using polystyrene standards. 

With the aid of NMR spectroscopy ( H- H NOESY) for PDDT and its copolymer with 3-methylthiophene in deuterated chloroform, the configurational structures were determined [90,91]. The 3-dodecylthiophene monomer is proved to be attacked predominantly at the p position of the thiophene ring (head) with a probability of 82% during the electrochemical polymerization [90]. PHT prepared by using a zero-valent nickel complex contains a larger proportion of HH units than HT units [92]. Chemically polymerized poly-(3-cyclohexylthiophene) contains HT and HH-TT coupled components in the ratio of 7 3 [93]. 

Electrically conductive PTs are prepared by polymerization in an electrolyte in the presence of polymeric compounds with sulfonic acid groups [692]. The electrochemical polymerization of thiophene and Nafion gives a self-supporting film showing a uniform distribution of sulfur across the film and a reversible voltammogram [693]. Alkoxy substituted thiophene derivatives are electro-chemically polymerized in a solvent in the presence of a proton source in the form of a Bronsted acid [694]. The electrochemical preparation of poly-(3-alkyloxythiophene) without a Brdnsted acid has also been studied [695]. 

The synthetic methods used to polymerize the 3-alkyl thiophene do not differ substantially from these employed for thiophene. The good choice of the solvent is important to ensure a complete dissolution of the monomer and the electrolyte in the electrosynthesis case. Chemically polymerization is based on the Grignard coupling method used by Yamamoto et al. [71] and later revised by Kobayashi et al. [72]. Some polymerizations carried out with chemical oxidants are also known with poly(3-alkyl-thiophene) [73]. 

Thiophene-Pyridme. In an effort to combine the electron-donating properties of thiophene with the electron-accepting nature of j dine, Mitsuhara et al. [784] electropolymerized 2,5-, 2,6-, and 3,5-[di(2-thienyl)]pyridine derivatives. Others have carried out chemical polymerization of 2-(2-thienyl)pyridine and 2,5-di(2-pyridyl)thiophene by dehalogenative polycondensation of the corresponding dihaloaromatic compounds using a zero-valence nickel complex [785,786] (Fig. 49d). 

Ihiophene-Pyrrole-Furan. There are only a few examples in which three different heterocycles have been incorporated into a polymer. One of those is obtained by the chemical polymerization of an a-linked three-ring system containing thiophene, N-methylpyrrole, and furan [778,782] (Fig. 59). 

In contrast to thieno[3,4-fe]thiophene (2), electropolymerization of which was not reported until 2001 [42], electrochemical and chemical polymerizations of thieno [3,2-( ]thiophene (1) were conducted in the 1980s [18], thus electrochemical polymerization of thieno[3,2-b]thiophene (1), results in poly(thieneno[3,2-b]thiophene) (PTT, 85) with the units linked through the 2- and 5-positions. 

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