Themosensitive/N-isopropylacrylamide/Reaction kinetics/Radiolysis. The pulse radiolysis technique has been employed to determine the initiation and propagation rates of different transient species involved in the polymerization of N-isopropylacrylamide (NIPA) in aqueous solutions. Polymerization by anionic mechanism has been observed to be faster than by the free-radical mechanism. The kinetic, spectroscopic and redox properties of the transient species formed upon reaction of primary

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... species of water radiolysis with NIPA have been evaluated. The one-electron oxidation potential for the formation of a radical cation is quite high (>2 V), but the one-electron reduction potential is low (in the range of -0.3 to -0.7 V). The radical anion of NIPA is able to undergo an electron-transfer reaction with MV 2+ , and has a pK a value of 3.2. The tert-butyl alcohol radical was also able to initiate polymerization. Gamma radiation-induced polymerization studies showed that the reaction of H • / • OH/e aq -/tert-butyl alcohol radicals with NIPA results in a nearly equal yield of the gel fraction. The hydrogel is observed to have very little swelling below pH 3 and above pH 10. The linear polymer of NIPA formed by irradiating dilute aqueous solution is found to be a thermosensitive polymer with lower a critical solution temperature (LCST) of~33°C. The diameters of polymer molecules were 290 and 20 nm at temperature below and above LCST, respectively.