Nitroxide-Mediated Living Radical Polymerization:  Determination of the Rate Coefficient for Alkoxyamine C−O Bond Homolysis by Quantitative ESR

Abstract

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 = 1.10 × 1014 s-1 and E act = 133.2 kJ mol-1 for alkoxyamine 1 and A = 1.02 × 1016 s-1 and E act = 140.9 kJ mol-1 for its polymeric analogue.

Homolysis and Decomposition of Alkoxyamines Containing PROXYL and TEMPO Residues: A Comparison

“…This may be due to restricted rotation around the alkoxyamine O-C-C-O bond in the transition state; the molecule will have to adopt a conformation where the alkoxy fragment C-O bond and the breaking alkoxyamine C-O bond are antiperiplanar to enable the anomeric stabilization. The A value for 1 is also much lower than those reported for the same species by Bon et al [12] At present we have no explanation for this difference, but have confidence in our own data based on the agreement between the values of A and E a for 4 with those reported in the literature. [13] E a for 5 is also in line with reported values for other 1-phenethyl derived alkoxyamines, although A is around one order of magnitude lower.…”

Effect of the Penultimate Unit on the CON Bond Homolysis in SG1‐Based Alkoxyamines

“…7) observed for SG1-based alkoxyamines has also been recently reported for TEMPO-based alkoxyamines 13 by Georges. [31] It is worth mentioning that the difference observed between the two isomers of 13 accounts for the difference observed between the k d measured by Bon et al [7] and Fukuda et al [9] Indeed, Bon measured k d using the initial slope method and, therefore, measured the decay of the faster homolizing isomer. This underlines the importance of the choice of techniques for measurement and the data processing procedure.…”

“…This approach links the expected control and polydispersity index to the values of k c,ds and k d,ds for the macro-alkoxyamine (the so-called dormant species) for a given monomer. [4] According to the scarce studies on chain length effect [5][6][7][8][9][10] and the effect of the penultimate unit, [11][12][13][14] macro-species are generally expected to be close to those for the corresponding molecular alkoxyamine. [4] Our objective was to successfully apply NMP to the methyl methacrylate monomer (MMA), so we developed new alkoxyamines that we expected to be efficient.…”

Diasteromeric Effect on the Homolysis of the C–ON Bond in Alkoxyamines: A DFT Investigation of 1,3-Diphenylbutyl-TEMPO

“…However, no ambiguous discussions were provided on the origin of this effect. At a first glance, although this led to apparently contradictory measurements [10,15,16], such small differences in k d may look unimportant, but nevertheless, we have recently shown that the fate of NMP [11,12,17] and the occurrence of side-reactions [18] depends on such small differences.…”