Cellulose bond scission

As reported in literature (13) the irradiation of a cellulose sample at 254 nm results in glucosidic bond scission and C - H bond scission. Recent results in photosensitized experiments (15) show that the same behaviour can be obtained ... 

Resins may influence phototendering of rayons. Wood (26) has reported that viscose rayon fabrics treated with urea formaldehyde or thiourea formaldehyde resin are protected from the degradative effects of mercury vapor lamp radiation. The mechanism of the protective effect is not fully understood as yet. Possibly resins can quench free radicals formed during irradiation. Work with resin-treated cotton indicates that simultaneous scission of cellulose chain molecules and resin-cellulose bonds occurs on exposure to light (63). 

Other studies have shown that in the thermal treatment of cellulose at temperatures below 300 °C, the rate of weight loss can be accelerated by oxidation reactions such as the degradation of cellulose by atmospheric oxygen. When cotton cellulose was heated at 190 °C for 50 h, carboxyl and carbonyl groups formed at a linear rate. When rates of glycosidic bond scission at 170 °C in nitrogen and in air were compared, the rate in nitrogen was close to one-half of the rate in air (30). 

Extensive kinetic investigations have also indicated that the activation energy of the overall thermal-decomposition process is substantially lowered by the addition of sodium chloride and sodium carbonate. Madorsky and coworkers have, therefore, proposed that these salts catalyze the dehydration of cellulose by scission of the C —O bonds (bonds a, b, and c in 1 see p. 438), and that this results in destruction of the hexose units and increases the yield of water and char at the expense of levoglucosan. This theory has found substantial support in subsequent experiments and publications however, it may be noted here that Golova and associates" consider that inorganic salts promote the cleavage of C—C, rather than C—O, bonds in the macromolecule. 

Subsequent reactions are strongly dependent on the chemical nature of the polymer. Recombination of radicals to form a new chemical bond is often observed and is the key process in radiation-induced cross-linking. Examples of polymers in which cross-linking is favored include polyolefins such as polyethylene (PE), natural rubber, or poly-dimethylsiloxane (PDMS). In other polymers, including most fluori-nated polymers, poly(methyl methacrylate) (PMMA), and natural polymers such as DNA and cellulose, chain scission is favored, leading to degradation of the polymer (for a more comprehensive list, see Drobny [2, p. 21]). 

Depolymerization of some natural polymers is another typical example. Milling of chitin or chitosan, at ambient temperature, leads to cleavage of the cellulose polymeric chain. Scission of 1,4-glucosidic bonds takes place, and the radicals formed recombine. Based on electron spin resonance, Sasai et al. (2004) monitored both the homolysis and the radical recombination. The recombination led to the formation of midsize polymeric chains only. Some balance was established between the homolytic depolymerization and the size-limited recombination of the radicals primarily formed. 

When oxycelluloses are placed in alkaline solution, large reductions in molecular weight take place because of the scission of glycosidic bonds. As a consequence, the oxidation of cellulose does not, in general, lead directly to rupture of the molecules, but renders those linkages near the point of attack extremely susceptible to alkaline cleavage. < Several mechanisms have been proposed for explaining these effects. 

NaOH, a considerably different crystal structure results (cf. Fig. 4). The chain conformation departs from 2 symmetry and forms, Instead, a threefold helix. The helices pack antiparallel In a hexagonal fashion, with a relatively large separation distance. The unit cell contains more than 60% of non-cellulose constituents — NaOH and water -- surrounding each helix with a llquld-llke structure. The presence of a large number of Na" Ions quite likely results In the formation of many secondary bonds between the cellulose hydroxyls and the Ions, forcing a scission of the remaining Intramolecular hydrogen bonds that are present In the Na-cellulose I structure. 

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