3.10 - Nitroxide-Mediated Polymerization

Since its introduction in the early 1990s, nitroxide-mediated radical polymerization (NMP) has been widely adopted for the preparation of a panoply of new polymer architectures. While NMP provides a number of advantages for the preparation of specific types of polymers, several inherent limitations with NMP have led to the more widespread use of other reversible-deactivation radical polymerization (RDRP) methods, chiefly atom transfer radical polymerization (ATRP) and reversible addition-fragmentation-chain transfer (RAFT) polymerization, in polymer synthesis. This short review discusses strategies that have been adopted for dealing with the difficulties of NMP and also surveys the use of NMP to prepare new polymers and copolymers over the past several years.

Nitroxide-Mediated Radical Polymerization: Limitations and Versatility

The reaction of readily available TEMPONa with aryl diazonium salts allows for clean generation of the corresponding aryl radicals along with TEMPO. Aryl radical addition to alkenes with subsequent TEMPO trapping provides the corresponding oxyarylation products in good to excellent yields. These experimentally easy to conduct transformations occur in the absence of any transition metal under mild conditions, and the process shows broad functional group compatibility.

Transition-Metal-Free Oxyarylation of Alkenes with Aryl Diazonium Salts and TEMPONa

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes2P—C6H10—B(C6F5)2] 1b reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH 3b and FLP-NO—CH2Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes2P—CHR1—CHR2—B(C6F5)2] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NO—CHMePh 6b derived from 1-bromoethylbenzene undergoes NO—C bond cleavage at 120 °C with an activa...

N,N-addition of frustrated Lewis pairs to nitric oxide: an easy entry to a unique family of aminoxyl radicals.

High-level ab initio molecular orbital theory calculations are used to identify the origin of the remarkably high inhibition stoichiometric factors exhibited by dialkylamine-based radical-trapping antioxidants. We have calculated the free energy barriers and reaction energies at 25, 80, and 260 °C in the gas phase and in aqueous solution for a broad range of reactions that might, potentially, be involved in amine/nitroxide cycling, as well as several novel pathways proposed as part of the present work, including that of N-alkyl hindered amine light stabilizer activation. We find that most of the literature nitroxide regeneration cycles should be discarded on either kinetic or thermodynamic grounds; some are even inconsistent with existing experimental observations. We therefore propose a new mechanistic cycle that relies on abstraction of a β-hydrogen atom from an alkoxyamine (R1R2NOCHR3R4). Our results suggest that this cycle is energetically feasible for a range of substrates and provides an explanation...

New insights into the mechanism of amine/nitroxide cycling during the hindered amine light stabilizer inhibited oxidative degradation of polymers.

In a recent article, we have showed that the nitroxide mediated polymerization of methyl methacrylate was possible up to 80% conversion for reasonable masses M n = 60,000 g mol -1 when 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide (DPAIO) was used as control agent. We have claimed that the success of this experiment relied on the absence of H-transfer reaction both in the alkoxyamine and between alkyl and nitroxyl radical. In this article, the decomposition of 4-nitrophenyl 2-(2,2,6,6-tetramethylpiperidine-1-yloxy)-2-methylpropionate (la) and 4-nitrophenyl 2-(2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxy)-2-methylpropanoate (2a) has been studied by 1 H NMR in the presence and in the absence (persistent radical effect condition) of scavenger (thiophenol PhSH). At temperature lower than the one used for polymerization, fast and quantitative H-transfer reaction was observed for la whereas no H-transfer reaction was observed for 2a. The scavenging technique proved for the first time that the H-transfer was an intermolecular process for la. However, the slow side-reaction of N-OC bond homolysis, which did not impede the control of the polymerization but may exert a detrimental effect on the livingness, was observed and quantified for 2a.

Hydrogen‐transfer reaction in nitroxide mediated polymerization of methyl methacrylate: 2,2‐Diphenyl‐3‐phenylimino‐2,3‐dihydroindol‐1‐yloxyl nitroxide (DPAIO) vs. TEMPO

Imidazoline and Imidazolidine Nitroxides as Controlling Agents in Nitroxide Mediated Polymerization

Thermal decomposition of alkoxyamines in the presence of scavengers was found to proceed with the formation of chemically induced nuclear polarization detected by 1H NMR. The distinctive Chemically Induced Dynamic Nuclear Polarization (CIDNP) features were studied using the example of three alkoxyamines: 4-nitrophenyl 2-(2,2,6,6-tetramethylpiperidin-1-yloxy)-2-methylpropanoate (1a), 4-nitrophenyl 2-(2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxy)-2-methylpropanoate (2a) and 4-nitrophenyl 2-(2,2,5,5-tetramethyl-4-phenyl-2H-imidazol-1-oxy)-2-methylpropanoate (3a) in the presence of PhSH. The analysis of CIDNP signs of methacrylate protons allows us to conclude on the occurrence of hydrogen atom transfer reaction in geminate radical pair formed in alkoxyamine thermolysis. Thus, CIDNP is a fast and sensitive method to detect the occurrence of intra/intermolecular hydrogen transfer in alkoxyamine thermolysis.

https://www.mdpi.com/2073-4360/2/4/364/pdf

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

Maria Edeleva

Papers

Hydrogen‐transfer reaction in nitroxide mediated polymerization of methyl methacrylate: 2,2‐Diphenyl‐3‐phenylimino‐2,3‐dihydroindol‐1‐yloxyl nitroxide (DPAIO) vs. TEMPO

Imidazoline and Imidazolidine Nitroxides as Controlling Agents in Nitroxide Mediated Polymerization

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