Persistent radical

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

An intriguing class of persistent radicals are those formed by the one-electron oxidation of the hexagonal prismatic clusters Li2[E(N Bu)3] 2 (3.21, E = S, Se). The air oxidation of 3.21 produces deep blue (E = S) or green (E = Se) solutions in toluene. The EPR spectra of these solutions consist of a septet (1 3 6 7 6 3 1) of decets (Eig. 3.5). This pattern results from interaction of the unpaired electron with three equivalent 7=1 nuclei, i.e., and three equivalent I = 3/2 nuclei, i.e., Ei. It has been proposed that the one-electron oxidation of 3.21 is accompanied by the removal of an Ei" cation from the cluster to give the neutral radical 3.22 in which the dianion [S(N Bu)3] and the radical monoanion [S(N Bu)3] are bridged by three Ei" cations. 

The triisopropylmethyl radical (33) is another example of a persistent radical. In this case, both disproportionation and combination are substantially retarded by steric factors.168 169 In the preferred conformation shown, the (3-hydrogens lie in a plane orthogonal to the orbital bearing the free spin. 

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

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 is most commonly used for S polymerization. For S polymerizations carried out at temperatures greater than 100 °C, thermal initiation provides some rate enhancement and a mechanism for controlling the excess of nitroxide that is formed as a consequence of radical-radical termination and the persistent radical... 

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,... 

A number of other chemistries which involve C-O bond cleavage have been reported.226 22 Druliner226 has reported on systems where NCO, 112, 113 or related species is the persistent radical. Homolysis rates for these systems were stated to he suitable for MMA polymerization at ambient temperature. The use of NCO has also been studied by Grande et al. z most recently for AA polymerization.2 0 Although control during AA homopolymerization was poor the process yielded NCO- terminated PAA that could be used to make PAA-block-PMMA.230... 

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 ... 

Very much more persistent radicals, with a half-life of up to a year, and with the structures [(Me3Si)2CH]3Sn- and [(MejSiljCHljRSn-(R = Pr , Bu , Me, Et, Bu, or cyclopentadienyl) have been prepared by the photolysis of [(Me3Si)2CH]2Sn(II), or by photolyzing a mixture of the appropriate halide [ Me3Si)2CH]2RSnX with an "electron rich olefin (299, 300). 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

The persistent radical anion II was obtained by chemical or electrochemical reduction of the parent neutral compound. The EPR spectrum of II is composed of a triplet of triplet (ap(2P)=3.50 mT and ap(2P)=0.89 mT) characteristic of a planar conjugated structure (Fig. 9) [87]. Amazingly, the dianion III was found to be paramagnetic exhibiting an EPR spectrum composed of a distorted dou-... 

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... 

The term persistent is typically applied to long-lived radicals7" which in a majority of cases can be characterized by ESR spectroscopy. " However, the lifetime of those radicals can vary rather broadly from several minutes to months, depending on the environment around the radical center. In this contribution, we will use the terminology of persistent radicals for those radical species with a relatively long lifetime however, those persistent radicals that can be isolated as individual room temperature stable compounds and in many cases characterized by X-ray crystallography will be specifically named as stable radicals (for the chemistry of stable radicals, see Section 2.2.4). 

Many persistent radicals centered on the heavier group 14 elements are currently known. " In this section we will emphasize only several of the most remarkable of them the first representatives synthesized by Lappert and co-workers, recent examples of the persilyl-substituted silyl radicals prepared by Matsumoto s group, and the latest representatives reported by Apeloig s group. 

The first landmark syntheses of persistent radicals of the heavier group 14 elements were reported nearly 30 years ago by Lappert s group. ° The first radical of this series, [(Me3Si)2CH]3Sn , was prepared by photolysis of the stable stannylene [(Me3Si)2CH]2Sn with visible light in benzene (Scheme 2.26). ° =... 

Carbon-centered organic radicals are highly reactive trivalent species with only one nonbonding electron. While most known radicals have their unpaired electron in a pure p- or a delocalized Ji-orbital, there are also examples of radicals centered in s/t" hybrid o-orbitals, such as the well known phenyl and cyclopropyl radicals. The first radical reported in the literature is credited to Gomberg s landmark paper in 1900 when he postulated the formation of triphenylmethyl radical 36, also known as tri-fyj 99,100 jj-jjyj j-adical is an example of a persistent radical that exists in equilibrium... 

Spin traps which scavenge a reactive radical to give a more persistent radical may be used to detect the intermediacy of such transient radicals in sorting out a reaction mechanism.39... 

Except for very electron-rich donors that yield stable, persistent radical cations, the ox values are not generally available.64 Thus the cation radicals for most organic donors are too reactive to allow the measurement of their reversible oxidation potentials in either aqueous (or most organic) solvents by the standard techniques.65 This problem is partially alleviated by the measurement of the irreversible anodic peak potentials E that are readily obtained from the linear sweep or cyclic voltammograms (CV). Since the values of E contain contributions from kinetic terms, comparison with the values of the thermodynamic E is necessarily restricted to a series of structurally related donors,66 i.e.,... 

The ladder polysilanes are highly a-conjugated systems, and they are easily oxidized and reduced to give unique oxidation products and persistent radical anions. 

Fundamental knowledge on the structures and properties of the ladder polysilanes has accumulated in our research for the past 15 years. Some results were unpredictable, including the silicon double helix structure, the domino oxidation, the formation of persistent radical anions, the Diels-Alder reactions at the 1,4-positions of anthracene, etc. These results let us recognize that the construction of novel structures will open the new chemistry. 

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