The Synthesis and Evaluation of New α-Hydrogen Nitroxides for ‘Living’ Free Radical Polymerization

Abstract

Three N-alkoxyamines were synthesized for use in nitroxide-mediated radical polymerization. Upon thermolysis, they generate new acyclic a-hydrogen nitroxides: one adamantyl substituted and two diol-containing nitroxides. The initiators were tested in polymerization reactions in direct comparison with the initiator derived from the nitroxide TIPNO.Nitroxide-mediated 'living' free radical polymerization (NMRP) has become a very attractive method for the controlled polymerization of olefins, as monomers bearing a wide variety of functionality can be tolerated under the polymerization conditions. The resulting polymers generally display good control over both molecular weight and polydispersity, and the 'living' nitroxide endcap allows for the preparation of nanoscopic materials with highly designed architecture. 1 Other 'living' radical techniques such as Atom Transfer Radical Polymerization (ATRP) mediated by a metal complex, 2 and Reversible Addition Fragmentation Transfer (RAFT) mediated by a thiocarbonyl intermediate 3 have also been developed. ATRP has the advantage that it can be run at lower temperatures, and it functions well with methacrylates, but amine containing monomers sometimes coordinate to the metal catalyst, interfering with the polymerization process. Polymers made by ATRP contain traces of metal, derived from the metal catalyst. RAFT polymerization is effective with a wider range of monomers, including electron rich vinyl acetates, which are not good substrates for NMRP and ATRP. However sulfur-containing impurities may lead to undesirable colored polymers prepared by the RAFT process.Thus all three methods constitute valuable options in the methodologies available for producing designed polymers using free radical intermediates, yet there is room for improvement. With NMRP, an important achievement would be developing a system that would allow polymerizations to be carried out at temperatures lower than the 105-125 °C that are typically employed. The ability to polymerize electron rich olefins in a controlled manner would further extend the versatility of NMRP. For all of these goals, the characteristics of the nitroxide end-cap are key to improving the polymerization profile. Previously, we 4 and the group of Tordo 5 have introduced a-hydrogen nitroxides TIPNO 1 and SG-1 2, which are effective in the polymerization of styrenes and a variety of electron poor olefin monomers. Very recently a number of cyclic 6 and acyclic 7 new nitroxides have been introduced for improved efficacy in NMRP. Herein we present work on initiators based on several new acyclic a-hydrogen nitroxides for NMRP, in which the N-tert-butyl has been modified.

Nitroxide: Anwendungen in der Synthese und in der Polymerchemie

“…[226] Von TEMPO abgeleitete Regulatoren 113 unterliegen allerdings der gravierenden Einschränkung, dass sie zwar in der Lage sind, Styrolderivate kontrolliert zu polymerisieren, sich jedoch nicht für die Polymerisation von Acrylaten eignen. Für diese wurde eine Reihe neuer Nitroxide entwickelt und in Form von Alkoxyaminen als Initiatoren/Regulatoren in der NMP getestet (114, [227] 115, [228] 116, [229] 117, [230] 118, [231] 119, [232] 120, [233] 121, [234] 122, [235] 123, [236] 124, [237] 125, [238] 126, [239] 127, [240] 128, [241] 129 [242] und 130 [243] in Abbildung 3). Darunter finden sich auch ausgefeilte Varianten (114,118,126,127), die eine kontrollierte NMP von Acrylaten und darüber hinaus von Acrylsäure, Dienen und Acrylamiden ermöglichen.…”

Application of Stable Radicals as Mediators in Living‐Radical Polymerization

“…86,87 In spite of the low reaction temperatures these reactions were quite fast, not due as suggested by the authors to the higher k d of the N-O bond but instead due to the lower rate of recombination (k c ) of the nitroxide with the transient macroradical, 82 The synthesis and application of a nitroxide containing one adamantyl substituent to living radical polymerization has been reported. 89, 90 Braslau concluded that steric bulk and an inherent instability of the nitroxide were both factors in whether a nitroxide could mediate the polymerization of acrylate monomers. This prompted the synthesis of 1-adamantyl-3-methyl-2-phenylazabutane-1-nitroxide, 13, an acyclic nitroxide containing both an α hydrogen and an adamantyl group.…”

Synthesis of versatile TIPNO-based alkoxyamines

“…Since its discovery [1,2], nitroxide mediated radical polymerization (NMRP) has been improved especially with the emergence of the nitroxides of second generation [3][4][5][6][7][8][9][10][11] whose s are versatile initiators/mediators of NMRP as they provide the control of molecular weight, polydispersity and end-group functionality during polymerization of almost all vinylic monomers polymerizable by a radical process. The C-O bond of an alkoxyamine is a relative robust bond except under strong oxidative [12], reductive conditions [13] or when it is heated during polymerization [1].…”