Radical stability nitroxides

The reaction of radicals with nitroxides is reversible. 09 This means that the highest temperature that the technique can reasonably be employed at is ca 80 °C for tertiary propagating species and ca 120 °C for secondary propagating species.22 These maximum temperatures are only guidelines. The stability of alkoxyamines is also dependent on solvent (polar solvents favor decomposition) and the structure of the trapped species. This chemistry has led to certain alkoxyamines being useful as initiators of living polymerization (Section 9.3.6). At elevated temperatures nitroxides are observed to add to monomer albeit slowly. 3IS 5" 523... 

If a stabilizer is present then this reaction sequence is modified (Figure 2) by the competing reactions for the removal of polymer alkyl (P-) and alkyl peroxy (POO) radicals. The nitroxide, R2NO, whether pre-formed, as in the case of TMDBIO above, or formed in situ through the oxidation of a hindered amine stabilizer, may scavenge only carbon centred radicals, P-. The hindered phenol antioxidant, AH, in contrast, will scavenge only oxygen centred radicals, and in... 

Nitrogen-based radicals like nitroxides and verdazyl derivatives fulfill the basic requirements, since they share high stability and persistency and can be functionalized to coordinate metal ions. Charged radicals such as tetracyanoeth-ylene [68, 69] and tetracyanoquinone radical anions have also been reported [70], and a special case is represented by the o-quinone ligands that can be found in different oxidation states in valence tautomeric compounds [71]. Recently, PTM radicals substituted with carboxylate groups have been used to obtain metal-radical coordination polymers, which, in some cases, exhibit porous structures and relevant magnetic properties. Example of such porous magnets will be reported in detail in Section 4.3.3 [72]. 

The monomers used in anaerobic adhesives and sealants generally contain at least one free-radical stabilizer, such as hydroquinone or />-methoxyphenol. It was found that ben-zoquinone, naphthoquinone, and similar compounds provided improved shelf stability without retarding the anaerobic cure [56]. It was also found that anaerobic formulations could be stabilized with a stable nitroxide free radical such as di-/-butyl nitroxide (LIV) [57]. The use of a soluble metal chelating agent such as tetrasodium EDTA (V) was found to be an effective method of stabilizing an anaerobic formulation against small amounts of metal contamination [58]. 

The second category is that of free radicals traps. For example, hindered amines are excellent light-stabilizing compounds their oxidation generates stable free radicals called nitroxide that are able to efficiently trap reactive free radicals. Very recently, such nitroxide radicals were used to prevent a... 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Prior to the development of NMP, nitroxides were well known as inhibitors of polymerization (Section 5.3.1). They and various derivatives were (and still are) widely used in polymer stabilization. Both applications are based on the property of nitroxides to efficiently scavenge carbon-centered radicals by combining with them at near diffusion-controlled rates to form alkoxyamines. This property also saw nitroxides exploited as trapping agents to define initiation mechanisms (Section 3.5.2.4). 

A wide range of nitroxidcs and derived alkoxyamincs has now been explored for application in NMP. Experimental work and theoretical studies have been carried out to establish structure-property correlations and provide further understanding of the kinetics and mechanism. Important parameters are the value of the activation-deactivation equilibrium constant K and the values of kaa and (Scheme 9.17), the combination disproportionation ratio for the reaction of the nilroxide with Ihe propagating radical (Section 9.3.6.3) and the intrinsic stability of the nitroxide and the alkoxyamine under the polymerization conditions (Section 9.3.6.4). The values of K, k3Cl and ktieact are influenced by several factors.11-1 "7-"9 ... 

Another important factor is the stability of the nitroxide. Some degree of instability appears beneficial. This can compensate for the buildup of nitroxide that would occur as a consequence of radical-radical termination and which might otherwise inhibit polymerization. 

The presence of /3-hydrogen in the nitroxide radical may lead to disproportionation reactions. In spin-trapping experiments, N-t-butyl-a-phenyl nitrone yields rather unstable spin adducts. This type of radical can be stabilized by coordination to Nin. The Ni11 complex with N-oxy-A-r-butyl-(2-pyridyl)phenylmethanamine (923) reveals a distorted octahedral geometry with antiferromagnetic interactions between the unpaired electrons of the metal ion and the radical spins.00... 

M. x 108M 1s 1 at 25°C, but may be appreciably lower in the solid state. In comparison k2 for oxygen competition for the alkyl radical is 2 x 109M-1s 1. Thus for air-saturated PPH ([02] 8 x 10-1,M)reaction7 will be protection against xenon irradiation was improved as compared to the parent piperidine by about 25, but the nitroxide itself was reduced to the 1 x 10 M level within the first lOOh and persisted at this level until brittle failure (7,) In contrast the parent amine is completely destroyed in the first lOOh of xenon exposure. 

When the lifetime of the radicals is very short and direct ESR detection is not an option, spin trapping is used to detect radicals at ambient temperatures. The method is based on the scavenging of radicals, P, by a spin trap, leading to the formation of a spin adduct with higher stability in most cases, this adduct is a nitroxide radical. 

O—C(/2) coupling seems particularly rapid in vinyl nitroxides (Camaggi et al., 1972) and there are few authenticated examples of esr detection of such radicals except in circumstances where steric effects block the coupling reaction (Ahrens et al., 1975) or substituents stabilize the nitroxide by conjugation (Aurich et al., 1975). 

In addition to the stabilization by suitable substituents and the absence of other termination reactions than recombination, it is the strength of the bond formed in the dimerization which is a necessary cofactor for the observation of free radicals by esr spectroscopy. The stability of nitroxides [4] or hydrazyls [5] (Forrester et al., 1968) derives not only from their merostabilized or captodative character but also from a weak N-N bond in the dimer. The same should be the case for captodative-substituted aminyls... 

Nitroxide radicals are widely used as spin labels in biology, biochemistry and biophysics to gain information about the structure and the dynamics of biomolecules, membranes, and different nanostructures. Their widespread use is related to an unusual stability, which allows researchers to label specific sites and to detect the most informative EPR parameters (g and hyperfme tensors) that are very sensitive to interactions with the chemical surroundings. Figure 2.1 collects all the radicals used in the following to illustrate the different aspects mentioned in the preceding section. 

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