ISSN:
1022-1352
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
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 · 109 dm3 · mol-1 · s-1 and 1,5 · 108 dm3 · mol-1 · s-1, respectively. Upon OH attack two kinds of radicals are generated: (i) tertiary α-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 α-hydroxyalkyl radicals in both systems. In the presence of oxygen, the carbon-centered radicals are converted into the corresponding peroxyl radicals. The α-hydroxyalkylperoxyl radicals eliminate HO2·/O2· in spontaneous and base-catalyzed reactions (rate constants ≈700 s-1 and ≈2,3 · 109 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 · 108 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 1010 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.
Additional Material:
11 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/macp.1994.021950427
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