Oxidation potentials monomers

Compound 1216 was then reacted with allylamine in the presence of Et3N to give A, A -diallyl-9//-carbazole-3,6-dicarboxamide 1217, subsequent treatment of compound 1217 with phosgene gave 3,6-dicarboximidoyl dichloride derivative 1218. Ring closure of dichloride 1218 was carried out under basic conditions to give 9-ethyl-3,6-di(l//-pyrrol-2-yl)-9//-carbazole 1219, a relatively low oxidation potential monomer with electron-rich pyrrole rings as terminal electropolymerizable moieties, in an overall synthetic yield of 5.8%. 

Sotzing, G.A., J.R. Reynolds, A.R. Katritzky, J. Soloducho, S. Belyakov, and R. Musgrave. 1996. Poly[bis(pyrrol-2-yl)arylenes] Conducting polymers from low oxidation potential monomers based on pyrrole via electropolymerization. Macromolecules 29 1679-1684. 

Shi et al. have developed another method for the electrochemical polymerization of high oxidation potential monomers in boron fluoride ethyl ether (BFEE) which could yield highly conducting PT films (Scheme 9.4) [32]. As observed in the case of the electropolymerization of 3-methylthiophene, bithiophene 2T and terthiophene 3T, such improvement stems from the lower oxidation potentials at which the electropolymerization occurs in BFEE compared with those required in common electrolytes. Recent development of this strategy by the Reynolds group has shown that thiophene, 3-methylthiophene, 3-bromothiophene and 3,4-dibromothiophene can be polymerized in BFEE to yield homogeneous, electroactive polymer films, where their electrochemical polymerization in common electrochemical solvents has proved much more difficult [33],... 

Naphthalenedicarboxylic Acid. This dicarboxyhc acid, a potential monomer in the production of polyester fibers and plastics with superior properties (105), and of thermotropic Hquid crystal polymers (106), is manufactured by the oxidation of 2,6-dialkylnaphthalenes (107,108). 

The above strategy was tested [27] with a 3-layer LED consisting of a poly(2,5-thienylene vinylene) (PTV) layer, known to have particularly low oxidation potential [28], followed by a layer of l,4-fcrs-(4 -diphenylaminostyryl)-2,5-di-methoxy-benzene (DASMB) [29] and a layer of 2-(4-biphenyl)-5-(4-tcrt-butyl-pheenyl)-1,3,4-oxadiazol (PBD) dispersed in polystyrene (PS) in a 20 80 ratio. Films of poly-(2,5-thienylene-a-bromoethylcne) were obtained by vapor phase pyrolysis of 2,5-W.v-(bromomethyl)lhiophcne and subsequent vapor deposition of the quinoid monomers onto a cold substrate following a previously published procedure [30]. They were converted to PTV by temperature-induced elimination of HBr. 

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... 

The ideal electropolymerization scheme (Eq. (5.5.39)) is further complicated by the fact that lower oligomers can react with nucleophilic substances (impurities, electrolyte anions, and solvent) and are thus deactivated for subsequent polymerization. The rate of these undesired side reactions apparently increases with increasing oxidation potential of the monomer, for example, in the series ... 

The nucleophilic reaction with the solvent is of crucial importance. Monomers with lower oxidation potentials (aniline and pyrrole) can easily be polymerized even in aqueous electrolytes. For monomers with higher oxidation potentials, aprotic solvents must be used, such as acetonitrile... 

Tetra(o-aminophenyl)porphyrin, H-Co-Nl TPP, can for the purpose of electrochemical polymerization be simplistically viewed as four aniline molecules with a common porphyrin substituent, and one expects that their oxidation should form a "poly(aniline)" matrix with embedded porphyrin sites. The pattern of cyclic voltammetric oxidative ECP (1) of this functionalized metal complex is shown in Fig. 2A. The growing current-potential envelope represents accumulation of a polymer film that is electroactive and conducts electrons at the potentials needed to continuously oxidize fresh monomer that diffuses in from the bulk solution. If the film were not fully electroactive at this potential, since the film is a dense membrane barrier that prevents monomer from reaching the electrode, film growth would soon cease and the electrode would become passified. This was the case for the phenolically substituted porphyrin in Fig. 1. 

Incidentally, oxidation data of the pyrrole monomers show an interesting increase in oxidation potentials when containing heavier substituents (Table 25). However, the ionization potential of N -methylpyrrole (7.95 V) is smaller than that of pyrrole (8.21 V). The accepted linear relationship between ionization potential and oxidation potential210 would have it the other way round. Considering, however, that trimethylsilyl and trimethylgermyl groups are weak electron donors211, it is plausible that a nonelectronic effect is responsible for the observed trend and the potential shifts are associated with steric effects. 

The oxidation potential of the polymer is not very different from that of the vinylferrocene monomer, which in turn is not very different from that of ferrocene. 

The mechanism of electropolymerization is still not fully understood. The one certainty is that in the very first step the neutral monomer is oxidized to a radical cation. It must have an oxidation potential that is accessible via a suitable solvent-electrolyte system and should react more quickly with identical species than with nucleophiles in the electrolyte solution. Therefore, as a general rule, polymerization without defects becomes less successful with increasing oxidation potential of the starting monomer, for example, in... 

The polmerisation reaction proceeds between the radical cations of the monomer and those of the continuing forming oligomers since the latter, following the growth of the chain length, acquire an oxidation potential... 

Fig. 9.5 Oxidation potentials of some selected examples of monomers and of their corresponding polymers. (SCE is a saturated calomel electrode.)...

Investigations into the effect of ultrasound upon these polymerisation processes began in the mid 1980 s when Akbulut and Toppare [81] examined the potentiostatic control of a number of copolymerisations. In such copolymerisations initiation takes place once a potential in excess of the oxidation potential of either monomer has been applied. However, often potentials even higher than these are required due to the formation at the electrode of a polymer film. These films create a resistance to the passage of current in the bulk medium with consequent reductions in the possible electrochemical reactions and therefore reductions in the rate and the yield. The use of ultrasound has been rationalised in terms of its removal of this layer in a... 

These polymerizations depend upon the ability to oxidize the monomer to a radical cation, whose further reactions lead to polymer. Since the oxidation potentials of the polymers are lower than those of the corresponding monomer, the polymer is simultaneously oxidized into a conducting state so that it is non-passivating. Some of the more important electrochemically-synthesised structures are discussed in more detail below and Chandler and Pletcher U4) have reviewed the electrochemical synthesis of conducting polymers. Detailed discussion in terms of thermodynamic parameters is impossible because the polymerizations are irreversible, so that E0 is undefined for the monomer-polymer equilibrium. 

Because the oxidation potential of the polymer is lower than that of the monomer, the polymer is electrochemically oxidized into a conducting state, kept electrically neutral by incorporation of the electrolyte anion as a counter-ion. This is an essential since precipitation of the unoxidized, insulating polymer would stop the reaction. Both coulometric measurements and elemental analysis show approximately one counter-ion per four repeat units. An important feature is the fact that the polymerization is not reversible whereas the oxidation of the polymer is. If the polymer film is driven cathodic then it is reduced towards the undoped state. At the same time neutrality is maintained by diffusion of the counter-ions out of the film and into the electrolyte. This process is reversible over many cycles provided that the film is not undoped to the point where it becomes too insulating. It is possible to use it to put new counter-ions into the film, allowing the introduction of ions which are too nucleophilic to be used in the synthesis. The conductivity of the film for a given degree of oxidation depends markedly on the counter-ion, varying by a factor of up to 105. 

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