First steps in radiation-induced hydrogel synthesis: radical formation and oligomerization in dilute aqueous N-isopropylacrylamide solutions |
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Authors: | gnes Sfrny Lszl Wojnrovits |
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Institution: | Institute of Isotope and Surface Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525, Budapest, P.O. Box 77, Hungary |
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Abstract: | The reactions of hydroxyl radical, hydrogen atom and hydrated electron intermediates of water radiolysis with N-isopropylacrylamide (NIPAAm) were studied by pulse radiolysis in dilute aqueous solutions. OH, H and eaq? react with NIPAAm with rate coefficient of (6.9±1.2)×109, (6.6±1)×109, and (1.0±0.2)×1010 mol?1 dm3 s?1. In OH and H radical addition to the double bond mainly -carboxyalkyl type radicals form, (OHCH2CH√C(N-i-C3H7)O and CH3CH√C(N-i-C3H7)O). In reaction of eaq? oxygen atom centered radical anion is produced (CH2CHC√(N-i-C3H7)O?), the anion undergoes reversible protonation with pKa=8.7. There is also an irreversible protonation on the β-carbon atom that produces the same radical as forms in H atom reaction (CH3CH√C(N-i-C3H7)O). The -carboxyalkyl type radicals at low NIPAAm concentration (0.1–1 mmol dm?3) mainly disappear in self-termination reactions, 2kt,m=8.4×108 mol?1 dm3 s?1. At higher concentrations the decay curves reflect the competition of the self-termination and radical addition to monomer (propagation). The termination rate coefficient of oligomer radicals containing a few monomer units is 2kt≈2×108 mol?1 dm3 s1. |
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