Oxidative depolymerization

The process operated by ACl is outlined in Figure 7. Bales of cotton linter are opened, cooked in dilute caustic soda, and bleached with sodium hypochlorite. The resulting highly purified ceUulose is mixed with pre-precipitated basic copper sulfate in the dissolver, and 24—28% ammonium hydroxide cooled to below 20°C is added. The mixture is agitated until dissolution is complete. If necessary, air is introduced to aUow oxidative depolymerization and hence a lowering of the dope viscosity. 

The mineral talc is extremely soft (Mohs hardness = 1), has good sHp, a density of 2.7 to 2.8 g/cm, and a refractive index of 1.58. It is relatively inert and nonreactive with conventional acids and bases. It is soluble in hydroduoric acid. Although it has a pH in water of 9.0 to 9.5, talc has Lewis acid sites on its surface and at elevated temperatures is a mild catalyst for oxidation, depolymerization, and cross-linking of polymers. 

By using this technique acrylamide, acrylonitrile, and methyl acrylate were grafted onto cellulose [20]. In this case, oxidative depolymerization of cellulose also occurs and could yield short-lived intermediates [21]. They [21] reported an electron spin resonance spectroscopy study of the affects of different parameters on the rates of formation and decay of free radicals in microcrystalline cellulose and in purified fibrous cotton cellulose. From the results they obtained, they suggested that ceric ions form a chelate with the cellulose molecule, possibly, through the C2 and C3 hydroxyls of the anhy-droglucose unit. Transfer of electrons from the cellulose molecule to Ce(IV) would follow, leading to its reduction... 

Macrocellulosic radicals initiated by ionizing radiation can be divided into two classes one class in which the radiant energy is selectively absorbed by the cellulose molecule and another class in which the radiant energy is randomly absorbed by the cellulose molecule. In both cases, after localization of the absorbed radiant energy, oxidative depolymerization of cellulose is initiated, and macrocellulosic radicals are formed (3). 

Conditions selected so that the copolymer contained about the same amount of cellulose, 77-82% AN = acrylonitrile DMF = N,N-dimethylformamide. b Initial molecular weight of cellulose 7.1 X 105 for ionizing radiation molecular weight of cellulose was determined at dosage indicated no determination was made on possible oxidative depolymerization of cellulose by chemical redox systems. c Based on poly (acrylonitrile) recovered from acid hydrolysis of copolymer and on intrinsic viscosity method. 

The mechanism proposed in Figure 3 can easily be applied to the oxidative depolymerization of the lignin macromolecule by nitrobenzene or copper(II) in aqueous alkali. A recent model of softwood lignin (24) contains 27% free benzylic hydroxyl groups and 12% aromatic ring-conjugated double bonds (stilbene... 

This initial oxidative depolymerization of cellulose evidently opens up the wood cell wall structure so that cellulolytic and hemi-cellulolytic enzymes can reach their substrates despite the presence of lignin. Solubility of wood in 1% NaOH increases markedly on brown-rot attack IS) and reflects cellulose depolymerization and the opening up of the wood structure. 

Phenols are oxidized by NaBiO3 to polyphenylene oxides, quinones, or cyclohexa-2,4-dienone derivatives, depending on the substituents and the reaction conditions [263]. For example, 2,6-xylenol is oxidized in AcOH to afford a mixture of cyclohexa-dienone and diphenoquinone derivatives (Scheme 14.123) [264] and is oxidatively polymerized in benzene under reflux to give poly(2,6-dimethyl-l,4-phenylene) ether (Scheme 14.124) [265]. Substituted anilines and a poly(phenylene oxide) are oxidatively depolymerized by NaBiO, to afford the corresponding anils [266]. Nal iO, oxidizes olefins to vicinal hydroxy acetates or diacetates in low to moderate yield [267]. Polycyclic aromatic hydrocarbons bearing a benzylic methylene group are converted to aromatic ketones in AcOH under reflux (Scheme 14.125) [268]. 

Scheme 5.5. Oxidative depolymerization of poly-l,4-phenylene oxides with sodium bismuthate [73CC562].

The effect of density of WPC on its durability in terms of oxidative depolymerization of its plastic matrix is described in detail in Chapter 15. Briefly, porosity in WPC, which is directly related to the decrease of density (specific gravity) of the material, provides a chemically reactive area for oxygen. Oxygen flows into pores and attacks WPC from inside, particularly at elevated temperatures. An increase in temperature by every 10°C accelerates the oxidative destruction of WPC by about... 

Solar radiation, effect of, 503 Stress, effect of, 502 Temperature, effect of, 503 Oxidative depolymerization, 205 Oxidative induction time (OIT), 39, 114,... 

T rimethy Isiloxy-terminated Thermal oxidation depolymerization at 350-420°C 126 (115)... 

Stark K, Taccardi N, Bosmann A, Wasserscheid P (2010) Oxidative depolymerization of lignin in ionic liquids. ChemSusChem 3 719... 

Oxidative depolymerization of cellulose occurs during the bleaching of wood pulp under alkaline conditions (alkali-oxygen and alkali-peroxide bleaching processes) leading to losses in pulp strength and involve the reactions outlined above. The oxidation at the C-2, -3 or -6 hydrot ls usually is initiated by hydroxyl radicals... 

Acid-butanol assay uses an acid-catalyzed oxidative depolymerization of coti-densed tannins to yield red anthocyanidins. Proanthocyanidins depolymerize when treated in hot mineral acid to produce colored anthocyanidins with an absorbance maximum around 550 nm [88,89]. The anthocyanidins result from the autoxidatimi of the carbocations initially formed by cleavage of the interflavonoid bond. Irmi sulfate can be added to the reagent to accelerate the autoxidation reaction that increases color intensity and consistency of anthocyanidin yield [90]. 

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