Functionalized thiophene monomers

The electropolymerization of thiophene monomers and the quality of the resulting polymer can be strongly influenced by the nature of the substituent grafted in the [ -position. The substitution of thiophene in the P-position may be of interest for several reasons (i) the electropolymerization can be more regioselective, (ii) introduction of electron-rich substituents decreases the oxidation potential of the monomer and (iii) the presence of substituents such as alkyl chains might improve the mechanical properties of the resulting polythiophenes. 

P -Substitution of thiophene by electron-releasing and electron-withdrawing groups 

Among the electron-donating substituents, diaikylamino groups such as pyrrolidino exhibit the most pronounced resonance effects and the strongest donor character, which is reflected in very negative Hammett constants, e.g. Op = —0.83 for N(CH3)2. Accordingly, owing to the enhanced stability of its related radical 

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P. Vallat, J. P. Lamps, F. Schosseler, M. Rawiso and J. M. Catala. Quasi-controlled polymerization through a nickel catalyst process of a functionalized thiophene monomer kinetic studies and apphcation to the synthesis of regioregular poly(thiophene-3-acetic acid). Macromolecules 40(7), 2600-2602 (2007). 

There have been a number of different synthetic approaches to substituted PTV derivatives proposed in the last decade. Almost all focus on the aromatic ring as the site for substitution. Some effort has been made to apply the traditional base-catalyzed dehydrohalogenation route to PTV and its substituted analogs. The methodology, however, is not as successful for PTV as it is for PPV and its derivatives because of the great tendency for the poly(u-chloro thiophene) precursor spontaneously to eliminate at room temperature. Swager and co-workers attempted this route to synthesize a PTV derivative substituted with a crown ether with potential applications as a sensory material (Scheme 1-26) [123]. The synthesis employs a Fager condensation [124] in its initial step to yield diol 78. Treatment with a ditosylate yields a crown ether-functionalized thiophene diester 79. This may be elaborated to dichloride 81, but pure material could not be isolated and the dichloride monomer had to be polymerized in situ. The polymer isolated... 

Other workers have shown that polymerization of functional thiophenes from aqueous media is possible if surfactants are used to help solubilize the monomer.18 19 20 21 In the case of SDS being used as a solubilizing agent for bithiophene,18 it was also found to lower the oxidation potential and inhibit the dissolution of the... 

Because poly thiophene itself is prone to overoxidation during polymerization, most practical work has been carried out using alkylated thiophenes, which have higher overoxidation potentials. Synthesis of functionalized thiophenes (such as alkylated monomers) is much easier to achieve than that of its pyrrole counterpart. The decreased activity of the sulfur group compared to that of the -NH group means that the laborious steps involved in protecting the heteroatom during synthesis are not required for thiophene. 

The pol5mier can be irreversibly oxidized and decomposed at a rate comparable to the pol3mierization of the corresponding monomer, which is one of the disadvantages of electrochemical polymerization and limits its application for many thiophene monomers with functional side groups. 

Functionalized conjugated polymers obtained by polymerization of thiophene monomers derivatized by covalently attached functional groups have been developed for almost two decades. However, while the field remains very active, it has been subjected to considerable changes in its objectives and methods during the last 10 years. Recent years have witnessed the use of new functionalized PT derivatives for advanced applications such as chemical, electrochemical, or bioelectrochemical sensors, and optoelectronic devices [12]. 

The use of bithiophenic precursors instead of thiophenic monomers leads to a decrease of the electropolymerization potential while maintaining a sufficient reactivity of the radical cation to produce an efficient polymerization [8,10]. Thus, functionalized PTs with groups sensitive to oxidation can be electrogenerated from tailored bithiophenic precursors. Additionally, the insertion of one EDOT unit in the precursor structure leads to a further decrease of the polymerization potential and enhances the efficiency of the electropolymerization process. 

Berlin et al. [166] reported the synthesis of n-doped polycationic polythiophenes using thiophene monomer functionalized with ammonium groups (see Figure 1.20 (7)). The polymer exhibits reversible p-and n-doping characteristics. The n-doping process and the associated expulsion of anions, is reportedly fast and independent of cation size, and the in situ conductivity is reportedly 2 x 10 S/cm. Kumar et al. synthesized a fully sulfonated n-doped polyaniline electrochemically using an acetonitrile-water (4 1) mixture [180]. The n-doping was confirmed... 

Different strategies have been pursued to prepare electrogenerated functional PTs [13, 63, 71]. One of the most efficient approaches consisted in the introduction of a functional group at one internal position of a 2,2 -bithiophene structure 13 (Scheme 9.14). Compared with -substituted thiophene monomers 12, the decrease of the oxidation potentials of bithiophene derivatives 13 allows for the introduction of functional... 

Because of the strong coloration of PT, especially in the partly and fully oxidized state, resonance Raman spectroscopy has been employed suceessfully to elucidate the molecular structure and changes thereof as a function of applied electrode potential and other experimental parameters [930]. Resonance Raman and infrared spectra of polythiophene obtained ex situ under various, frequently unsatisfactorily defined experimental conditions have been reported [931-935]. These early results were, in part, inconsistent. In particular, the assignment of bands not found in the thiophene monomer to modes of the newly created polymer was repeatedly contradictory. 

In an analogous way, soluble substituted poly(p-phenylene)s were prepared from substituted benzene rings functionalized with MgCl and Br in para position (see Formula 16.10). It proved to be necessary to add LiCl in addition to the Ni-catalyst to achieve low DPs (1.17-1.27) and Mns up to 36 kDa [130]. Furthermore, it was feasible to synthesize diblock copolymers, when the poly(p-phenylene) was prepared first and used as a macroinitiator for thiophene monomers. The same approach also allowed to perform CCPs of 2,5-dihalo N-hexylpyrrol, but again... 

While the incorporation of these solublizing groups at the 3-position has yielded PTs that have surmounted the intractability issue and also contain fewer (3-defects, such substitution of the thiophene core necessarily results in the loss of ring symmetry. With these functionalized monomers, two different types of a-linkages now exist since the 2- and 5-positions are no longer equivalent [231, 232]. 

Electroadsorption—adsorption carried out with an applied electrode potential. The quantity deposited is a function of deposition time, multilayer formation being possible, as is the case with thionine. On the other hand, application of a potential, in the correct conditions, in the presence of a molecule susceptible to polymerization, can produce radicals, initiating polymerization and subsequent electrode modification. Examples of these conducting polymer monomers are pyrrole, N-phenylpyrrole and W-methylpyrrole, aniline, and thiophene. 

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