OH-RADICAL-INDUCED CROSS-LINKING AND STRAND BREAKAGE OF POLY(VINYL ALCOHOL) IN AQUEOUS-SOLUTION IN THE ABSENCE AND PRESENCE OF OXYGEN - A PULSE-RADIOLYSIS AND PRODUCT STUDY

Hydroxyl radicals were generated radiolytically in N2O- or N2O/O2(4:1)-saturated aqueous solutions of poly(vinyl alcohol) (PVAL) or its low-molecular-weight model compound pentane-2,4-diol (PD). Using the pulse radiolysis technique, the rate constant of OH with PD and PVAL has been determined to be 2,3.10(9) dm3.mol-1.s-1 and 1,5.10(8) dM3.Mol-1.S-1, respectively. Upon OH attack two kinds of radicals are generated: (i) tertiary alpha-hydroxyalkyl radicals and (ii) alkyl radicals. The former rapidly reduce tetranitromethane yielding the stable nitroform anion, and from its yield it is calculated that 70-75% of the radicals are alpha-hydroxyalkyl radicals in both systems. In the presence of oxygen, the carbon-centered radicals are converted into the corresponding peroxyl radicals. The alpha-hydroxyalkylperoxyl radicals eliminate HO2./O2.- in spontaneous and base-catalyzed reactions (rate constants almost-equal-to 700 s-1 and almost-equal-to 2,3.10(9) dm3.mol-1.s-1, respectively). There are marked differences in the rates of the bimolecular radical decay reactions. The carbon-centered radicals of PD decay with a rate constant of 5.10(8) dm3.mol-1.s-1, whereas those of PVAL decay, under pulse-radiolytic conditions, mainly intramolecularly (loop formation), with rate constants which reach, at high radical numbers per macromolecule, values as high as 10(10) dM3.Mol-1.s-1. With PD it was shown by product studies that the radicals mainly (90%) disproportionate and dimerize only to 10%, thus limiting the yield of crosslinks in PVAL to the latter value. The bimolecular decay rates of the organic peroxyl radicals in O2-containing solutions are more difficult to investigate because of the effective unimolecular HO2.-elimination reaction. However, it is estimated that the peroxyl radicals of both compounds decay (under otherwise equal conditions) more than one order of magnitude slower than the carbon-centered radicals. The measured yields of strand breaks in oxygenated solutions as well as the yields of intermolecular crosslinks under anoxia, determined with PVAL by low-angle laser light-scattering at different dose-rates and pH values, support these conclusions.