Nitroxide-mediated living radical polymerization: Determination of the rate coefficient for alkoxyamine C-O bond homolysis by quantitative ESR

The rate coefficient for alkoxyamine C-O bond homolysis has been determined over a range of temperatures for both 2-tert-butoxy-1-phenyl-1-(1-oxy-2,2,6,6-tetramethylpiperidinyl)ethane (1) and a polystyrene-TEMPO (approximately 75 units) adduct using quantitative ESR. In a thermostated solution of the alkoxyamine species in toluene the nitroxide concentration was monitored via ESR. Measurements were performed under atmospheric conditions to create pseudo-first-order kinetics with oxygen to serve as a scavenger for the carbon-centered radicals. The acquired kinetic data for alkoxyamine 1 were compared with data obtained via nitroxide-exchange experiments using HPLC to monitor the alkoxyamine concentrations. The data for the polystyrene alkoxyamine were compared with literature data obtained by Fukuda via his GPC method. The quantitative ESR measurements proved to be an easy and more important an accurate method to determine values for the rate coefficient for alkoxyamine C-O bond homolysis. The Arrhenius parameters obtained via this method were A = 1.1(0) x 10(14) s(-1) and E-act = 133.(2) kJ mol(-1) for alkoxyamine 1 and A = 1.0(2) x 10(16) s(-1) and E-act = 140.(9) kJ mol(-1) for its polymeric analogue.