First steps in radiation-induced hydrogel synthesis: radical formation and oligomerization in dilute aqueous N-isopropylacrylamide solutions

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 e aq − react with NIPAAm with rate coefficient of (6.9±1.2)×10 9, (6.6±1)×10 9, and (1.0±0.2)×10 10 mol −1 dm 3 s −1. In OH and H radical addition to the double bond mainly α-carboxyalkyl type radicals form, (OHCH 2CH C(N- i-C 3H 7)O and CH 3CH C(N- i-C 3H 7)O). In reaction of e aq − oxygen atom centered radical anion is produced (CH 2CHC (N- i-C 3H 7)O − ), the anion undergoes reversible protonation with p K a=8.7. There is also an irreversible protonation on the β-carbon atom that produces the same radical as forms in H atom reaction (CH 3CH C(N- i-C 3H 7)O). The α-carboxyalkyl type radicals at low NIPAAm concentration (0.1–1 mmol dm −3) mainly disappear in self-termination reactions, 2 k t,m=8.4×10 8 mol −1 dm 3 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 2 k t≈2×10 8 mol −1 dm 3 s 1.