Oxidative depolymerization, cellulose

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. 

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. 

Although white-rot fungi also secrete H2O2 (104), they have not been found to depolymerize cellulose oxidatively. One reason might be that they possess oxalate decarboxylase, which decomposes oxalate, whereas brown-rot fungi apparently do not (98). This problem deserves further investigation. 

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... 

The process operated by ACI 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 cellulose 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 allow oxidative depolymerization and hence a lowering of the dope viscosity. 

Free-radical initiation is the main method of forming cellulose copolymers. In some cases, macrocellulosic radicals are formed directly and initiate polarization of vinyl monomers on to cellulose. In other cases, free radicals are formed which may abstract hydrogen atoms from cellulose to form macrocellulosic radical sites. Complexations of metallic ions with cellulose and subsequent release of the metallic ions in a lowered state of oxidation also result in the formation of macrocellulosic radical sites. In all cases, oxidative depolymerization of the cellulose molecule is also initiated. 

A second degradation process is oxidation, often photo-induced especially by exposure to light not filtered for uv. The radicals resulting from this reaction promote depolymerization of the cellulose, as well as yellowing and fa ding of paper and media. Aging causes paper to become more crystalline and fragile, and this can be exacerbated particularly if the paper is subjected to poor conditions. 

Depolymerization, e.g., polyethylene terephthalate and cellulose hydrolysis Hydrothermal oxidation of organic wastes in water Crystallization, particle formation, and coatings Antisolvent crystallization, rapid expansion from supercritical fluid solution (RESS)... 

Specifically, D-glucose < maltose < maltotriose < amylose < starch < amylo-pectin < cellulose (Greenwood, 1967). Trends indicated are that thermochemical stability increases with the DP, branching, and 1,4-fi bonding. Chemical bonds other than 1,4-a and 1,4-(3 introduce heat and acid instability. Either of these two bonds is less easily depolymerized when the sixth pyranose carbon is oxidized to the carboxyl group rather than esterified for this reason, low-methoxyl pectin is more stable than high-methoxyl pectin. 

Especially important are the radical-redox reactions by which transition metals promote free radical generation by catalyzing the decomposition of the peroxides formed during bleaching. This faster free radical generation results in higher oxidation rates and a more extensive depolymerization of the cellulose. 

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