Thiophene monomer

By using oligothiophenes (thienyls) instead of the monomer thiophene in the indophenin reaction, for example (Figures 3.17), the electron donor can be inserted into the indophenin molecule in a controlled manner, changing the physical properties, especially the... 

Several monomers (thiophene and substituted thiophenes, dihalobenzenes, 4,4 -dibromobiphenyl and 2,7-dibromo-9,10-dihydrophenanthrene) were polymerized and the corresponding conjugated aromatic polymers obtained in high yield. As the dehalogenating Ni(0) complex, a mixture of bis(l,5-cyclooctadiene) nickel and a neutral ligand L was employed. Table 6.1 shows the principal results reported. 

Detaiis - eleotrolysis of monomer (thiophene) In aoetio aold leads to ... 

Similar to furan and benzofuran monomers,thiophene and 1-benzothiophene " are also known to copolymerize in a 1 1 fashion with MA. Thiophene will not undergo free-radical homopolymerization or copolymerization with monomers such as styrene, methyl methacrylate, or acrylonitrile. However, when an equimolar mixture of thiophene and MA are combined with chloroform containing AIBN and heated at 60°C, a white copolymer starts to precipitate after only a few minutes reaction time. Varying the molar feed ratios showed that both yields and reduced viscosities were maximum for equimolar feeds. Copolymerizations in solution from 1 bar to 3 kbars show that rates and yields are increased with pressure.Through composition studies and comparing the NMR spectrum of the copolymer with the NMR spectra of 2,3- and 2,5-dihydrothiophene, structure 88 was assigned to the copolymer. 

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. 

Polyheterocycles. Heterocychc monomers such as pyrrole and thiophene form hiUy conjugated polymers (4) with the potential for doped conductivity when polymerization occurs in the 2, 5 positions as shown in equation 6. The heterocycle monomers can be polymerized by an oxidative coupling mechanism, which can be initiated by either chemical or electrochemical means. Similar methods have been used to synthesize poly(p-phenylenes). 

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... 

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. 

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

A number of approaches are available to improve the morphology and homogeneity of electrochemically deposited conducting polymer films. Priming of the electrode surface with a monolayer of adsorbed or covalently bonded monomer leads to more compact deposits of polyaniline,87,88 poly thiophene,80 and polypyrrole.89,90 Electrode rotation has been shown to inhibit the deposition of powdery overlayers during poly(3-methylthiophene) deposition.81... 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

There are large numbers of naturally occurring representatives, especially of pyrrole that include the important polypyrroles (porphyrins and corrins), and the nitropyrrole antibiotics such as pyr-rolomycins and pyrroxamycin. Derivatives of furan have been used as fungicides and A-vinylpyr-rolidone is an important monomer for the production of blood plasma extenders and for cosmetic applications. On account of the similarity in the pathways for the aerobic degradation of monocyclic furan, thiophene, and pyrrole, all of them are considered here. Anaerobic degradation of furans is discussed in Part 2 of this chapter. 

Many substituted thiophenes have also been electrochemically polymerised [19,54,399-405] (Table 4) as have thiophene dimers [21,37,55,251,400,406], trimers [21, 83,407], and tetramers [256,406], with the thiophene dimer giving rise to higher quality films than does the monomer [37, 395,408]. Several polycyclic monomers including a thiophene ring have also been polymerised [408-416], as have a series of compounds consisting of two thiophene rings linked by a polyene chain (Fig. 23c). The polymerisation of dithieno-thiophene (Fig. 23d) results in a polymer which shows remarkable similarity to polythiophene in its properties [409,410,414],... 

In general the mechanism of polymerisation for thiophene appears to be similar to that of pyrrole (Section 4.11.2), occurring via a radical coupling mechanism [423] giving mainly a-a linkages [293,400,405], and involves oligomer as well as monomer radicals, with evidence to suggest that the polymerisation reaction occurs at a lower... 

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