Persistent-radical effect

In the nitrosoacetanilide system, Binsch and Riichardt recognized that a relatively high concentration of a persistent radical forms, which efficiently oxidizes the phenylcyclohexadienyl intermediate to biphenyl (equation 39) and produces a second species, from which the radical is regenerated in a catalytic cycle. Two possibilities, 63 and 64, exist for the identity of the pesistent radical, and both of these can reach ESR-detectable 

Some early examples of a preference for unsymmetrical radical coupling were drawn together in a short paper by Perkins in 1964. ° Intriguingly, the first correct analysis of an example of this kind of behavior dates from Bachmann and Wiselogle clearly understood the role of triphenyl-methyl in accounting for their observations that, in solution at 100°C, pentaphenylethane dissociates rapidly and reversibly into triphenylmethyl and diphenylmethyl, but dimerization of the diphenylmethyl to form detectable quantities of tetraphenylethane (which is stable under the reaction conditions) occurs only very slowly. 

The decomposition of 31 in benzene, like that of nitrosoacetanilide, does not give phenylcyclohexadienyl dimers, since the outcome is again governed by the presence of a persistent radical, in this case triphenylmethyl. A limited 

Biological antioxidants such as a-tocopherol (65, vitamin E) serve to inhibit free radical chain oxidation, and the mechanisms of their reactions have attracted close attention. The chain-breaking reaction of such phenols with peroxyl radicals is by hydrogen transfer (equation 101). 

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The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

NMP of S in heterogeneous media is discussed in reviews by Qiu et at.,205 Cunningham,206 207 and Schork et a/.208 There have been several theoretical studies dealing with NMP and other living radical procedures in emulsion and miniemulsion."09 213 Butte et nr/.210 214 concluded that NMP (and ATRP) should be subject to marked retardation as a consequence of the persistent radical effect. Charlcux209 predicted enhanced polymerization rates for minicmulsion with small... 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

In combination ATRP, the catalyst is again present in its more stable oxidized form. A slow decomposing conventional initiator e.g. AIBN) is used together with a normal ATRP initiator. Initiator concentrations and rate of radical generation arc chosen such that most chains arc initiated by the ATRP initiator so dispersities can be very narrow.290 The conventional initiator is responsible for generating the activator in situ and prevents build up of deactivator due to the persistent radical effect. Reverse or combination ATRP are the preferred modes of initiation for ATRP in emulsion or miniemulsion (Section 9.4.3.2).290 291... 

Emulsion polymerization has proved more difficult. N " Many of the issues discussed under NMP (Section 9.3.6.6) also apply to ATRP in emulsion. The system is made more complex by both activation and deactivation steps being bimolecular. There is both an activator (Mtn) and a deactivator (ML 1) that may partition into the aqueous phase, although the deactivator is generally more water-soluble than the activator because of its higher oxidation state. Like NMP, successful emulsion ATRP requires conditions where there is no discrete monomer droplet phase and a mechanism to remove excess deactivator built up in the particle phase as a consequence of the persistent radical effect.210 214 Reverse ATRP (Section 9.4,1,2) with water soluble dialky 1 diazcncs is the preferred initiation method/87,28 ... 

Although more studies need to be performed to study the scope and generality of this system, the use of amine hydrochloride salts as initiators for controlled NCA polymerizations shows tremendous promise. Fast, reversible deactivation of a reactive species to obtain controlled polymerization is a proven concept in polymer chemistry, and this system can be compared to the persistent radical effect employed in all controlled radical polymerization strategies [37]. Like those systems, success of this method requires a carefully controlled matching of the... 

Fischer H (2001) The persistent radical effect a principle for selective radical reactions and living radical polymerizations. Chem Rev 101 3581-3610... 

Financial support by The Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged, as is the assistance and encouragement of many individuals. These include Professor John Perkins, who provided much of the text dealing with spin traps and the persistent radical effects, and Professors John Lorand, John Warkentin, Keith Ingold, and Paul Engel. 

The synthesis of mixed peroxides formed from /-butyl hydroperoxide and carbon-centred radicals has been studied. The reactions were strongly effected by solvents as well as catalytic amounts of Cun/Fem. The kinetic data suggest that the conditions for the Ingold-Fischer persistent radical effect are fulfilled in these cases.191 The use of Cu /Cu" redox couples in mediating living radical polymerization continues to be of interest. The kinetics of atom-transfer radical polymerization (ATRP) of styrene with CuBr and bipyridine have been investigated. The polymer reactions were found to be first order with respect to monomer, initiator and CuBr concentration, with the optimum CuBr Bipy ratio found to be 2 1.192 In related work using CuBr-A-pentyl-2-... 

Scheme 10.10 The persistent radical effect (PRE) as a function of reversible and irreversible termination, i.e. cross-coupling and self-termination, respectively.

The mechanism of this unusual transformation has been studied very carefully and compelling evidence for the intermediacy of radicals has been presented. One of the arguments is the low diastereoselectivity of the dimerizations that is typical for a radical process. Moreover, it has been demonstrated that the persistent radical effect is overcome by reduction of the Co(H) metalloradicals to the catalytically... 

Paramagnetic Metal Complexes Valence Tautomerism and Persistent Radical Effect... 

Wetter, C. and Studer, A. 2004. Microwave-assisted free radical chemistry using the persistent radical effect. Chemical Communications, 174-75. 

Synthetic applications 39 Enantioselective free radical reactions 40 Living free radical polymerization 40 Persistent-radical effect 41... 

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