Nitroxides radical decomposition

Electron spin resonance (ESR) spectroscopy can be advantageously used to measure the radical concentrations of the nitroxide radicals (XV and XVI) produced, since these are much more stable then the R- radicals. Of greater importance, ESR can be used to determine the structure of R% since the ESR of the nitroxide radical is quite sensitive to the structure of R. (For this purpose, nitroso spin traps are more useful, since the R group in the nitroxide radical is nearer to the lone electron.) This can allow a determination of the structures of radicals first formed in initiator decomposition, the radicals that actually initiate polymerization (if they are not identical with the former) as well as the propagating radicals [Rizzardo and Solomon, 1979 Sato et al 1975],... 

Various stable radicals such as nitroxide, triazolinyl, trityl, and dithiocarbamate have been used as the mediating or persistent radical (deactivator) for SFRP. Nitroxides are generally more efficient than the others. Cyclic nitroxide radicals such as 2,2,6,6-tetramethyl-l-piper-idinoxyl (TEMPO) have been extensively studied. SFRP with nitroxides is called nitroxide-mediated polymerization (NMP). Polymerization is carried out by two methods that parallel those used in ATRP [Bertin et al., 1998 Georges, 1993 Flawker, 1997 Flawker et al., 2001], One method involves the thermal decomposition of an alkoxyamine such as... 

An alternative, but considered (192) a less likely possibility, was that Ru NO induced radical decomposition the case of an organosilver compound was cited as precedent (190). However, we note that in that work a nitroxide and not a spin trap was used. 

Polymerization can be started using an alkoxyamine as initiator such that, ideally, no reactions other than the reversible activation of dormant species and the addition of monomer to carbon-centered radicals take place. The alkoxyamine consists of a small radical species, capable of reacting with monomer, trapped by a nitroxide. Upon decomposition of the alkoxyamine in the presence of monomer, polymeric dormant species will form and grow in chain length over time. Otherwise, polymerization can be started using a conventional free-radical initiator and a nitroxide. The alkoxyamine will then be formed in situ when an initiator molecule decomposes, and, after adding a monomer unit or two, is trapped by a nitroxide. 

K. Murayama and T. Yoshioka, Stable free radicals. IV. Decomposition of stable nitroxide radicals, Bull. Chem. Soc. Jpn. 1969, 42, 2307-2309. 

If the decomposition of the transient nitroxide radical yields HNO and subsequently NaO, the latter cannot be accurately determined in solutions presaturated with N2O. Therefore, in experiments requiring determination of N2O as an end-product, H2O2 was used as an e q scavenger (reaction 4) under anoxic conditions where fes[H202] < [Pg.318]

The kinetics of the decomposition of the transient nitroxide radicals derived from aceto-HX and SAHA are similar RC(0)N0 decays via a second-order reaction, which is followed by two consecutive first-order reactions. Typical kinetic traces are shown in Figure 2. 

In the reaction of p-nitrophenyl isopropyl ether with radicals produced by the photo-induced decomposition of AIBN, the ESR spectrum obtained is very similar to that observed for p-nitrosophenyl isopropyl ether under similar reaction conditions. Accordingly, the ESR spectra observed for these compounds correspond to that for the nitroxide radicals(II) in the reaction scheme shown in Figure 3. 

The rate of t-butoxy radical addition to the double bond of 4 relative to MMA was determined by the radical trapping technique devised in our laboratories some years ago (5). In this, t-butoxy radicals are generated (by thermal decomposition of di-t-butyl peroxyoxalate) in the presence of the olefin and a stable nitroxide radical. The t-butoxy radical adds to the olefin and the carbon-centered radical so formed is trapped by the nitroxide to give a stable alkoxyamine (Scheme 1). The method functions effectively because nitroxides do not react with olefins nor... 

Some limitations of the method arise due to side reactions involving the nitroxide. However, such problems can usually be avoided by the correct choice of nitroxide and reaction conditions. Nitroxides, while stable in the presence of most monomers, may act as oxidants or rcductants under suitable reaction conditions.516 The induced decomposition of certain initiators (e.g. diacyl peroxides) can be a problem (Scheme 3.94).166 177 There is some evidence that nitroxides may disproportionate with alkoxy radicals bearing a-hydrogens,123 Side reactions with thiols have also been identified.4 18... 

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 efficiency of these inhibitors may depend on reaction conditions. For example the reaction of radicals with stable radicals (e.g. nitroxides) may be reversible at elevated temperatures (Section 7.5.3) triphenylmethyl may initiate polymerizations (Section 7.5.2). A further complication is that the products may be capable of undergoing further radical chemistry. In the case of DPPH (22) this is attributed to the fact that the product is an aromatic nitro-compound (Section 5.3.7). Certain adducts may undergo induced decomposition to form a stable radical which can then scavenge further. 

Various methods have been used to form low molecular weight alkoxyamine initiators for NMP. Most involve forming an appropriate carbon-centered radical in the presence of a nitroxide. Initiators that generate carbon-ccntcrcd radicals may be thermally decomposed in the presence of a nitroxide. For example, alkoxyamine 100 is formed by decomposition of AIBN in the presence of TEMPO (Scheme 9.19). 1,1 Carbon-centered radicals may also be generated photochemically. 70... 

Separate experiments in which tert.-butoxy radicals were produced thermally in benzene from di-tert.-butyl peroxyoxalate failed to reveal any direct reaction of these radicals with amine II. Even at higher temperatures (A/ 150°C, dichlorobenzene, +00+ decomposition), the +0 radicals attacked neither amine II nor nitroxide I. The earlier described experiments of ketone photooxidation showed additionally that amine II displays no specially marked reactivity towards peroxy radicals. 

Matyjaszewki et al. [229,236] pointed out the importance of the bimolecular exchange reaction (Eq. 19) to control the molecular weight and its distribution. Simulation revealed a decrease in the Mw/Mn values during polymerization, but the contribution in the actual polymerization is still ambiguous [237-240]. Reports have also addressed the importance of the decomposition of the alkoxyamine such as the disproportionation of the propagating radical and the nitroxide for the control of the polymerization [229,236,241 ]. 

Decomposition of azo compounds and peroxides provides the alkoxyamine by the nitroxide-trapping of the primary radicals [29]. The radicals produced by hydrogen abstraction with oxy radicals are also trapped by the nitroxide [242, 243]. In the photoreaction, alkoxyamines were isolated with high yields [244]. The reactions of Grignard reagents with nitroxides [215] and the coupling reaction of sodium nitroxides with bromo compounds [234,235] are also used. The hydrolysis of 56 followed by the reaction with acyl or alkyl halides afforded alkoxyamines with various functional groups,63 (Eq. 68) [245-251] ... 

Laser flash photolysis at wavelengths within the charge-transfer absorption bands of 2,2,6,6-tetramethylpiperidine-./V-oxyl (TEMPO) and carbon tetrachloride yields theoxoam-monium chloride of TEMPO 291 (Xmax = 460 nm) and the trichloromethyl radical in an essentially instantaneous 18 ps) process152. The primary photochemical reaction is an electron transfer from TEMPO to carbon tetrachloride followed by immediate decomposition of the carbon tetrachloride anion radical to chloride and trichloromethyl radical (equation 140). The laser flash photolysis of TEMPO and of other nitroxides in a variety of halogenated solvents have confirmed the generality of these photoreactions152. 

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