New Sterically Hindered Nitroxides for the Living Free Radical Polymerization:  X-ray Structure of an α-H-Bearing Nitroxide

Abstract

The synthesis of three new sterically hindered Hawker−Braslau type alkoxyamines for the nitroxide-mediated living radical polymerization is described. Efficient polymerizations can be performed at 105 °C. With alkoxyamine 11 controlled polymerization of n-butyl acrylate and styrene is possible even at 90 °C. AA and AB diblock copolymers can be prepared in a controlled manner using alkoxyamine 11. Kinetic EPR experiments for the determination of the C−O bond activation energies of the new alkoxyamines are described. Furthermore, 1H NMR alkoxyamine decomposition studies are presented. These kinetic experiments are used for the discussion of the polymerization results. Furthermore, the X-ray structure of a Hawker-Braslau type nitroxide is presented. X-ray data and ESR data are used to explain the high stability of these nitroxides.

Nitroxide decomposition: Implications toward nitroxide design for applications in living free‐radical polymerization

“…41 These polymerizations demonstrated good control with styrene monomers, but 45,56,[82][83][84][85][86][87] has allowed the extension of NMP to a much wider range of nonstyrenic olefin monomers. These a-hydrogen-bearing nitroxides are stable and easily handled at room temperature but do undergo bimolecular disproportionation under the thermal conditions of polymerization.…”

Design concept for α-hydrogen-substituted nitroxides

“…Angle compression below the optimal 90°angle for hydrogen atom abstraction would increase the energy barrier for hydrogen atom abstraction and nitrone formation. This is due to poor overlap between the C-H s-bond being broken and the singly-occupied molecular orbital localized on the N-O bond that will be part of the nitrone double bond being formed 33 ; similar considerations have been put forth to explain the stability of the acyclic radicals of the TIPNO (7)/ SG1 (8) family based on conformational stabilities 27,34,35 . (iii) During nitrone formation the R 4 substituent would move into the plane of the R 5 substituent.…”

Recent advances in controlled/living radical polymerization in emulsion and dispersion

“…A class of persistent radical precursors is alkoxyamines, alkylated derivatives of nitroxides that generate nitroxides as mediators (deactivators) and alkyl radicals as initiating species upon their decomposition. [115][116][117][118][119][120] Although TEMPO is most efficient in polymerization of St at high temperature, other nitroxides and alkoxyamine derivatives allow for low temperature polymerization of St as well as (meth)acrylates.…”