Quinonoid polymers

It was subsequently shown that the polymers contain semi-quinonoid structures 47 proposed to arise from a-p coupling of radicals 46 as shown in Scheme 9.15., (M It was also suggested that 47 could be subject to radical-... 

Pezzella, A. Panzella, L. Crescenzi, O. Napolitano, A. Navaratman, S. Edge, R. Land, E. J. Barone, V. d lschia, M. Short-lived quinonoid species from 5,6-dihydroxyindole dimers en route to eumelanin polymers integrated chemical, pulse radiolytic, and quantum mechanical investigation. J. Am. Chem. Soc. 2006, 128, 15490-15498. 

A reaction mechanism in which the precursor polymer undergoes a redox reaction followed by loss of the bridge hydrogens is proposed. The resulting conjugated aromatic/quinonoid polymers generally have very small semiconductor band gaps in accord with predictions of recent theoretical calculations. 

Preliminary measurements of electrical conductivity of the conjugated derivatives of PBTAB, PBTB and PTTB obtained by the above treatment with bromine vapor are poor semiconductors with a conductivity of the order 10 °S/cm which apparently is not due to doping. Subsequent electrochemical or chemical doping of these polymers lead to 4-6 orders of magnitude increase in conductivity. Ongoing studies of the electrical properties of these conjugated polymers with alternating aromatic/quinonoid units will be reported elsewhere. 

Phenyl and triphenylmethyl radicals generated from 6 contribute to the initiation and the termination, respectively, resulting in polymer 18 because of the remarkably different reactivities of these radicals (Eq. 21). The co-chain end terminated with 1 thermally redissociates to induce further polymerization. Therefore, the polymerization proceeded via a mechanism close to the model in Eq. (18). The recombination product of methyl isobutyryl radical and 1 was reported to have a quinonoide structure [82], suggesting a similar structure of the chain end, 18b. 

First of all p-xylene is dehydrogenated to obtain its dimer (i.e., di-/ -xylene). This is done by using superheated steam at 950°C. The dimer formed is a crystalline solid at room temperature and it is heated to 600°C at 1 mm pressure when it sublimes and forms and equilibrium mixture of diradical and a quinonoid. This equilibrium mixture when quenched to 50°C over metal surface results in the formation of a linear polymer known as... 

In principle, it should be possible to design and synthesise a polymer with a sufficiently small band gap that it would be conducting without doping. Bredas 4I1) has discussed theoretical calculations of the band-gap of polymers based on polyisothianaphthene. A number of polymers have band-gaps predicted to be less than 1 eV, with polyisonaphthothiophene having a predicted band-gap of almost zero. Jenekhe412 has made polymers with a band-gap of 0.73 eV, based on polythiophene with alternating aromatic and quinonoid units (Fig. 12). 

Based on the evidence obtained from the amount and nature of transformation products formed, a mechanism of melt stabilising action of tocopherol in PP and PE has been proposed, see Scheme 6 [34]. It is well known that, like other hindered phenols, a-tocopherol is rapidly oxidised by alkylperoxyl radicals to the corresponding tocopheroxyl radical (a-Toe, Scheme 6a). Further oxidation of the tocopheroxyl radical in the polymers leads to the formation of coupled and quinonoid-type products, e.g. SPD, TRI, DHD (see Figs. 8 and 9). Dimerisation of the intermediate o-quinone methide (QM) leads to the formation of the quinonoid-type dimeric coupled product, SPD (Scheme 6 reaction d). 

The 4,7-dinitro-4,5,6,7-tetrahydro[l,2,5]oxadiazolo[3,4-6]pyrazine (67) has been tested as an explosive <85USP4539405>. The o-quinonoid nature of the [l,2,5]thiadiazolo[3,4-d]pyridazines results in their 4,7-diaryl derivatives being used as fluorescent yellow-red dyes for organic polymers... 

Figure 9 Aromatic and quinonoid forms of fused bithiophene polymers.

It is predicted that the ground-state structures of the fused bithiophene polymers are of the aromatic forms, which are more stable than the quinonoid ones by 3.4-7.1 kcal mol. Optimized structural parameters for both aromatic and quinonoid forms are listed in Table 22. In the aromatic forms, the short bonds are longer than those of PT and the long bonds shorter. Especially, C(2)-C(3) (C(6)-C(7)), C(3)-C(4) (C(5)-C(6)), and C(4)-C(5) bonds of the aromatic forms become similar in length, showing quinonoid character. 

Table 22 Optimized geometrical parameters for the fused bithiophene polymers A1-A5 (bond lengths in A and bond angles in deg) aromatic A) and quinonoid (Q) forms...

Besides these enzyme substrates, a number of biological molecules are likely to give rise to fairly stable and hence observable free radicals. The more important of these are the quinonoid molecules, particularly vitamin Q quinone (ubiquinone), vitamin E quinone, vitamins K, Ks and vitamin E quinone, the flavins and flavoproteins and the important neurochemicals dopa, dopamine, and closely related phenolic and quinonoid molecules. In many of these cases, the generation of free radicals from these molecules should occur in vivo, but as yet only a few radicals such as the ascorbyl radical and the bacteriochlorophyll radical have been directly identified in intact systems. Free radicals from melanins (polymers from dopaquinone) have been demonstrated both in vivo and in vitro, but these radicals are so stable that it has not yet been possible to identify a biological role for the radicals per se. 

The quinonoid form of p-xylylene reacts as a diradical would. It can be produced in quantitative yield by vacuum vapor phase pyrolysis of di-p-xylylene at 550 to 600° C (30). Condensation of the monomer to crystalline polymer does not occur in the gas phase under the vacuum con-... 

Bamberger was able to isolate a low molecular weight polymer having a composition corresponding to a quinonoid monomeric structure 62 from -tolyl azide. 

Fluorescent naphthol-based polymers were prepared by HRP-catalyzed polymerization of 2-naphthol in AOT/isooctane reverse micelles to give the polymer microspheres.31 The precipitated polymer was soluble in a range of polar and nonpolar organic solvents and possessed quinonoid structure. The reverse micellar system induced the peroxidase-catalyzed copolymerization of p-hydroxythiophenol and />ethylphenol, yielding the thiol-containing polyphenol particles.32 The attachment of CdS to the particles gave the CdS—polymer nanocomposite showing fluorescence characteristics. 

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