Nitroxide species

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

Aromatic bromides were electrochemically reduced in the presence of a spin marker, such as r-butylphenylnitrone, and studied by ESR. In weakly hydrogen donor solvent (DMSO), the radicals are cleanly trapped by nitrone 234 forming the nitroxide species 235243(equation 128). 

In NMP, living macrospecies can be temporarily trapped by a nitroxide species X resulting in the formation of dormant macrospecies (RjX), which are the targeted polymer molecules for CRP, in contrast with the typical dead polymer product P in other chain-growth polymerizations. For a sufficiently fast deactivation (k eact, Scheme 10.2), this dormant state is favored and the contribution of dead polymer molecules is minimized. This favoritism is enhanced as X does not undergo self-termination. 

The first approach for cellulose modification followed in the Bamer-Kowollik team employs the photoinduced functionalization of photoinitiator-modified substrates with preformed raYrax/cfe-functionalized macromolecules, which are derived from commercially available photoinitiators and nitroxides [DEL 12a]. The approach is based on the generation of radicals at the cellulose surface by mild UV irradiation (5 -max 311 nm) of an immobilized photoinitiator, followed by radical trapping with a nitroxide species. To evaluate the new photochemical grafting method, preliminary studies were carried out based on the nitroxide end functionalization of PEG in solution. Moreover, the light-induced radical technique was used to eouple two synthetie maeromolecular strands. 

Nitroxide species are also formed in polymeric materials containing hindered amine stabilizers (HAS an example is TINUVIN 770 figure 23.9b) these are oxidized during the stabilization processes to form stable HAS-NO radicals (Figure 23.10) that have been used successfully as EPR spin probes of the host material [39,68]. 

It is known that, whenever excitation bands of the detection and pump pulses are close, a reduction of echo amplitude due to a Bloch-Siegert mechanism takes place. This effect is particularly important for Gd(iii)-nitroxide pairs as the pump pulse is set up to flip low-spin nitroxide species, it has a higher turning angle ( 3-4ti, instead of ti) for the Gd(iii) species. Therefore, for the same frequency offset the effect of echo reduction is stronger for Gd(iii)-nitroxide pairs as compared to pairs of identical spin-centres, like nitroxide-nitroxide or Gd(iii)-Gd(iii). 

Fig. 6 Multiple pathway RE in nitroxide-Dy(iii)-02 three spin system. The RE induced by Dy(iii) on nitroxide species increases in the presence of O2 (upper curve in the plot and upper scheme) due to the indirect RE mechanism via dioxygen. In the absence of O2 (lower curve in the plot and lower scheme) real RE value is measured. Data from ref. 60.

Figure Bl.16.16 shows an example of RTPM in which the radical species is TEMPO (10), a stable nitroxide radical, while the triplet state is produced by photoexcitation of benzophenone (11) [45].

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). 

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... 

The very small number of growing polymer chains, when compared to the monomer concentration results in a very low overall concentration of free control agent and leads to inefficient capping of chain ends. One solution to this problem is the addition of a free or unbound control agent to the polymerization medium. This can take the form of a low molecular weight alkoxyamine, ATRP initiator, RAFT agent or, alternatively, free deactivator such as nitroxide or Cu(II). This species is often called a sacrificial agent. This solution also leads to the formation of free polymer that must ultimately be removed from the brush. 

One other aspect of the photolysis of coordinate spin labeled derivatives is of interest. Nitroxides are good free radical scavengers (123). As a result, when methyl-cobalamin is photolyzed in the presence of a nitroxide, the methyl radical generated will react with the free nitroxide and cause disappearance of the ESR spectrum (123). However, once the nitroxide is coordinated it is no longer susceptible to attack by free radicals. Thus the nitroxyl function is quite well protected from approach by other species. 

Nitroxides have been shown to associate quite strongly with model hydroperoxides in the liquid phase (17.). We have now found evidence for NO /hydroperoxide group association in solid PPH films based both on the e.s.r. spectra of N0> species and... 

PPH in the presence of III (or its bis-nitroxide) we have found that the doubly grafted species V is generated in much larger quantities (23.) ... 

The reactions of radicals with oxygen are diffusion-controlled it-S-S. Moreover, as has been previously shown, the isobutyryl radical a could readily be captured by a nitroxide. It is therefore not easy to see why reaction between oxygen and the species a does not also occur. 

Various hybrid compounds comprised of two types of nitroxide radicals and either a pentamethine (Cy5) or trimethine cyanine (Cy3) were synthesized by Sato and co-workers [32]. These compounds seem to be promising fluorescent chemo-sensors for the measurement of reducing species such as Fe2+, ascorbic acid, and hydroxyl radicals. 

Sato S, Tsunoda M, Suzuki M, Kutsuna M, Takido-uchi K, Shindo M, Mizuguchi H, Obara H, Ohya H (2009) Synthesis and spectral properties of polymethine-cyanine dye-nitroxide radical hybrid compounds for use as fluorescence probes to monitor reducing species and radicals. Spectrochim Acta A 71 2030-2039... 

This equation is similar to that for the ordinary polymerization, indicating that Rp is independent of the concentration of P-N. In fact, the polymerization rate experimentally determined in the presence of P-N agreed with the rate of thermally initiated polymerization without any initiators. The production of the polymer induced a decrease in the Rvalue because of the gel effects, resulting in an increase in the rate. The suppressed gel effects in the presence of TEMPO have also been reported [233]. Catala et al. interpreted the independence of the polymerization rate from the nitroxide concentration with the terms of the association of domant species. However, there is no experimental evidence for the association [229,234,235]. 

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