2,5‐Dihydro‐1H‐imidazole‐Based Nitroxides as Prospective Mediators in Living Radical Polymerization

Abstract

Dedicated to the memory of Professor Hanns FischerA number of spectroscopic methods were applied to obtain kinetic parameters of reactions modelling the 2,5-dihydro-1H-imidazole 1-oxide mediated living polymerization of acrylates. The homolysis rate constants of alkoxyamines based on five nitroxides were measured by EPR spectroscopy at different temperatures. The recombination rate constants k c between the corresponding alkyl radicals and the nitroxides were measured by means of laser flash photolysis. The time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP) experiments revealed the negligible contribution of disproportionation in the recombination reaction. In addition, the thermodecomposition of alkoxyamines in the NMR probe showed the absence of intramolecular elimination of hydroxylamines from the corresponding alkoxyamines. Analysis of the kinetic parameters showed that the 2,5-dihydro-1H-imidazole 1-oxide type radicals are promising mediators for the living polymerization of acrylates and methacrylates.Introduction. -Nitroxide-mediated polymerization [1] [2] (NMP) is a fine synthetic technique, which allows for the design of new types of homo-and copolymers with welldefined architectures and narrow molecular-mass distributions [3]. The principal mechanism of NMP (Scheme 1) involves the reversible dissociation of a dormant nitroxide end-capped polymer chain with rate constant k d , the propagation of C-centered radicals with rate constant k p , the cross-coupling reaction between the nitroxide and the growing polymer chain with rate constant k c , and simultaneous irreversible self-termination reactions of the C-centered radicals with rate constant k t .

Can the First Addition of Alkyl Radicals Play a Role in the Fate of NMP?

“…Nitroxides 1 •,30 2 •,30 5 •,31 6 •,32 7 •,20 10 •,33 12 •,19 16 •– 21 •,19, 34, 35 and alkoxyamines20 1a , 2a , 7d , 17e,g , and 18a were prepared as reported. Solvents for synthesis – 1‐bromoethylbenzene, 2‐bromo‐2‐methylpropionic acid methyl and tert ‐butyl ester, methyl and tert ‐butyl, bromo‐2‐methylpropionyl bromide, copper metal, copper bromide, N , N , N ′, N ′, N ″ ‐ pentamethyldiethylenetriamine (PMDETA), benzene, chlorobenzene and tert‐ butylbenzene – were purchased from Aldrich and used as received.…”

“…In recent reports, it was shown that the k c values for the recombination of alkoxycarbonyl‐2‐prop‐2‐yl,19, 20 alkoxycarbonylethyl,19, 20 and 1‐phenylethyl21 radicals – models for the corresponding propagating polymer radicals – with two new families of nitroxides – imidazolidin‐ N‐ oxyl and imidazolin‐ N‐ oxyl radicals (Figure 1) – were in a suitable range ( k c > 10 6 L · mol −1 · s −1 ) for the NMP of the corresponding monomers. Few years ago Aldebbagh et al22, 23 and Chong et al24 showed with some homologues (5‐membered ring nitroxides exhibiting a CO in position 3 of the ring) the possibility of appling alkoxyamines based on such type of nitroxides as initiators/controller for NMP.…”

H‐transfer reaction during decomposition of N‐(2‐methylpropyl)‐ N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐N‐oxyl (SG1)‐based alkoxyamines

“…Up to date, NMP of alkyl methacrylates, in particular methyl methacrylate (MMA), is very demanding, although a number of attempts using different nitroxides or alkoxyamines have been performed 10–19. The main complication for NMP of MMA is the occurrence of side reactions, namely, the intramolecular β‐proton transfer (IPT, Cope‐type elimination20), intermolecular β‐hydrogen atom transfer (IHAT) reactions (reactions (ii) and (iv), respectively, in Scheme ), irreversible decomposition of dormant species (i.e., through NO bond homolysis),21–23 and decomposition of persistent nitroxyl radicals in the course of NMP.…”

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

“…Alkoxyamines were prepared as previously reported for 1a [19], 2a [20], 3a [21]. Deuterated solvents and PhSH were purchased from Aldrich and used as received.…”