Kinetic study of H‐atom transfer in imidazoline‐, imidazolidine‐, and pyrrolidine‐based alkoxyamines: Consequences for nitroxide‐mediated polymerization

Abstract

The H‐atom transfer reaction was studied for a series of imidazoline, imidazolidine, and pyrrolidine‐based alkoxyamines containing either isobutyrate‐2‐yl or 1‐phenylethyl alkyl fragments. The CO bond homolysis rate constants and activation energies were determined by 1H NMR product analysis as a function of temperature. Inter‐ and intramolecular H‐atom transfer reactions were distinguished by examination of alkoxyamine thermolysis products in the absence and in the presence of a radical scavenger (thiophenol or deuterated styrene). A correlation between the structure of the nitroxyl fragment of alkoxyamine and the H‐transfer reaction was found. The high steric demands of the substituents on the nitroxyl part of the isobutyrate‐2‐yl alkoxyamine decrease both types of reaction. For alkoxyamines containing the 1‐phenylethoxyamines, neither inter‐ nor intramolecular H‐atom transfer was observed. Controlled polymerization of methylmethacrylate initiated with imidazoline‐based alkoxyamine was observed, although the polymer obtained was not living. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6579–6595, 2009

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.…”

Chemically Induced Dynamic Nuclear Polarization during the Thermolysis of Alkoxyamines: A New Approach to Detect the Occurrence of H-Transfer Reactions

“…0.5%, sensitivity of CIDNP is a valuable tool for screening the occurrence of H-transfer reaction in alkoxyamine. Contrary to 2a, alkoxyamine 3a exhibits high fraction of H-atom transfer (at 95 °C, 30% and 15% for intramolecular proton transfer and intermolecular H-transfer reactions, respectively) [10]. Thus, as mentioned above, one would expect positive polarization on a(-H).…”

“…In previous articles [9,10] we have reported the kinetics of thermolysis with long time delays between scans or at low temperatures where all NMR lines were nonpolarized. This article concerns the cases of high thermolysis temperature and short time delays when radical concentration is high enough to produce intensive Chemically Induced Dynamic Nuclear Polarization (CIDNP).…”

“…Such experiments provide information on the impact of intra/intermolecular H-transfer reaction on the decay of alkoxyamine. This approach was successfully applied to investigate the H-atom transfer reactions for a series of imidazoline, imidazolidine, and pyrrolidine-based alkoxyamines containing either isobutyrate-2-yl or 1-phenylethyl alkyl fragments [10].…”

“…The measurement of NMR spectra during the decomposition of alkoxyamines 1a, 2a, [9] and 3a [10], in the absence and in the presence of scavenger was previously performed. In this article we report the analysis of CIDNP signals observed during the decomposition of alkoxyamines 1a, 2a, and 3a (Chart 1) in the presence of scavenger.…”