Formation of free radicals in photoirradiated cellulose. VIII. Mechanisms

Abstract

SynopsisPhotoinduced free radicals in cellulose were studies by means of ESR spectroscopy. Plausible mechanisms have been suggested for each type of intermediate radical to account for the ESR spectra observed and for the product balances. Based on the principles of photochemistry of cellulose, coupled with the findings of the ESR spectra, it becomes evident that localization of the energy absorbed in the cellulose molecule leads to formation of free radicals in terms of chain scission, dehydrogenation, dehydroxylation, and dehydroxymethylation. Emphasis has been placed on the correlation of the mechanisms of photodegradation and the mechanisms of photoinduced graft copolymerization.

Electron spin resonance study of radicals generated in cellulose/N‐methylmorpholine solutions after flash photolysis at 77 K

“…The established radical II has increased hyperfine interaction withp rotons (A H˛³ 2.2 mT) and less withˇprotons. 22 It was suggested that the radicals V and VI have similar hyperfine interactions with˛protons with an accuracy of 0.2-0.3 mT. The interaction withˇprotons gives a HFS with the hyperfine constant near 0.4-0.5 mT (doublet for V and triplet for VI radicals), which cannot be resolved because of the large (up to 1 mT) electron-˛proton dipole-dipole interaction causing an additional inhomogeneous line broadening.…”

“…22,23 Nevertheless, ESR experiments of pure cellulose were separately carried out because of the expected different results connected with using short UV wavelength (248 nm) and larger power of illumination, in contrast to results obtained before (e.g. Ref.…”

“…Their presence is supported not only by ESR, but by other physical methods as well. 22 The formation of radicals III, IV and VI is connected with the occurrence of other radicals such as ž H and ž OH that may take part in further reactions with NMMO. Moreover, radical III was detected in experiments with visible light excitation described before.…”

“…8(a) and (b)). This photoinduced process is observed in many cases and described using the photoreaction: 22 …”

In Textiles

“…This degrading effect on cotton was noticed over a century ago. Electron spin resonance study of light -irradiated cotton has established the reason of degradation as free -radical generation on cellulose [2,3] . An understanding of this phenomenon has also opened up possibilities for improvement of the properties of cotton fabric by grafting a variety of monomers onto it [3 -5] .…”