Thiophene oxidative polymerization

The synthesis involves the nickel-catalyzed coupling of the mono-Grignard reagent derived from 3-alkyl-2,5-diiodothiophene (82,83). Also in that year, transition-metal hahdes, ie, FeCl, MoCl, and RuCl, were used for the chemical oxidative polymerization of 3-substituted thiophenes (84). Substantial decreases in conductivity were noted when branched side chains were present in the polymer stmcture (85). 

Another example of the effect of the polymerization method on the optical properties of the resulting polymer is the synthesis of polymer 402 (Scheme 2.64) [486]. Polymers obtained by oxidative polymerization of the corresponding 3-(methoxyphenyl)thiophene with FeCls (CHCls-soluble fraction), and with Mg/Ni(dppp)Cl2 or Ni(cod)2 polymerizations of the corresponding dibromothiophene derivatives showed somewhat different maxima in absorption (and PL emission) spectra 405 (520), 433 (555), and 435 (560) nm, respectively. 

The observed catalytic effect of the crown ether appears to be dependent on the nucleophile employed in both polymerization and corresponding model reactions. Not surprisingly, it appears that the stronger the nucleophile employed, the smaller the catalytic influence of the crown ether. For example, with potassium thiophen-oxide yields of polymer or model products were almost quantitative with or without catalyst. By contrast, the reaction of PFB with potassium phthalimide, a considerably weaker nucleophile, affords 6 in 50% with catalyst and in 2-3% without catalyst under identical conditions. However, it may be that this qualitative difference in rates is, in fact, an artifact of different solubilities of the crown complexed nucleophiles in the organic liquid phase. A careful kinetic study of nucleophilicity in catalyzed versus non-catalyzed reactions study is presently underway. 

Oxidative polymerizations of thiophene and pyrrole and their derivatives are also under study to obtain polymers with conducting and optoelectronic properties [Jin et al., 2000 ... 

Vanadiaxerogels are especially interesting for their redox properties, which promote the catalytical activity of enzymes such as glucose oxydase [644] or induce the oxidative polymerization of organic monomers, such as aniline, pyrrol, and thiophene [873],... 

The oxidative polymerization has been proposed to proceed via a radical coupling that involves the coupling of neutral radicals or cation radicals. The former case corresponds to the oxidative polymerization of phenols and dithiols in which the neutral radical is formed by one-electron transfer after dissociation of a hydron from the monomer, or by the elimination of a hydron after the oxidation. The latter case takes place when the cation radical formed by one-electron oxidation exists as a stable species. The cation radicals then couple with each other, and the dimer is formed through solvent-catalyzed hydron elimination from the intermediate dication. Oxidative polymerization of pyrrole and thiophene uses this mechanism [57-62]. 

When considering the synthesis of phospholes, one has to forget most of the classical and powerful methods employed for the preparation of thiophenes and pyrroles. For example, Paal-Knorr condensation, direct ortho-lithiation, halogenation with NBS or I2/Hg2+ and Vilsmeier-Haack formylation are not operative in phosphole chemistry. Likewise, no chemical or electrochemical oxidative polymerization... 

Polymerization of thiophenes by oxidative coupling has been discussed earlier <1996CHEC-II(2)491>. The generally accepted mechanism for the electropolymerization of thiophene may also be valid in the case of chemical oxidative polymerization. The steps involved are formation of a radical cation, spin-pairing of two such radical cations to form a dihydrodimer dication, loss of protons with concomitant rearomatization, and repetition of this cycle with the dimer. Couplings take place at the position of highest unpaired-electron spin density. 

Summaries on the synthesis, properties, and uses of polythiophenes are included in two general reviews on poly thiophenes [259,260]. A synopsis of important aspects of polythiophenes are also included in several reviews on various aspects of conducting polymers [221-226], Cation radicals are the propagating species in both electrochemical and chemical oxidative polymerizations of thiophene and its derivatives. The polymer obtained by this method is linked primarily by a,a-linkages. However, other types of linkages (a,f3 and /3,/3) are present in varying amounts (Fig. 59). Substituted thiophene derivatives can couple in a head-to-tail or head-to-head manner. 

The direct oxidative polymerization of a thiophene monomer has also been successfully employed to prepare polythiophene, for example, using FeCl3 in chloroform solvent.32 MoC15 may be similarly employed as both the oxidant and dopant.33 Subsequent reduction with ammonia of the doped polythiophene products from these FeCl3 and MoC15 oxidations provides the neutral polymer. 

The oxidative polymerization of diphenyl disulfides or of thiophenal with quinones at room temperature has been reported to produce pure poly(p-phenylene sulfides) [37cJ. This will be described in more detail in Section 4. 

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. 

Conducting poly thiophene (PTh)-SWCNTs composites were synthesized by the in situ chemical oxidative polymerization method.Using an excitation wavelength of 514 nm, the authors studied the pristine and the derived materials. The Raman spectrum for the SWCNT PTh composites is clearly an addition of the corresponding spectra of PTh and SWCNTs, demonstrating that SWCNTs served as templates in the formation of a co-axial nanostructure for the composites. ... 

We have demonstrated that oxidative polymerization reactions leading to conducting polymers can be carried out within the channel systems of zeolites. Acidic zeolite forms are required to synthesize intrazeolite polyaniline by analogy to the oxidative coupling of aniline in acidic solutions. The presence of intrazeolite oxidants such as Cu(II) and Fe(III) ions is fundamental for the polymerization of pyrrole, thiophene and 3-methylthiophene. The degree of polymer chain oxidation and probably the chain lengths are influenced by the dimension ity of the zeolite channels. 

Figure 3.20 Proposed mechanisms for ferric chloride oxidative polymerizations of thiophenes.

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