Nitroxide structures Structure

Nitroxide Structure Nitroxide Structure Nitroxide Structure... 

Spin-adduct spectra often reveal splittings to substituent atoms other than hydrogen. Indeed, since the spin-trapping technique provides a convenient route to many nitroxides containing structural features likely to be of spectroscopic interest, it has frequently been used to this end. Chlorine and bromine splittings, as well as those from fluorine, have been encountered and many nitroxide spectra have been reported in which there is splitting from a second nitrogen, from phosphorus, or even from a metal atom. 

New Fluorinated Nitroxides. Generation, Structure, Reaction and Application ... 

Table 1 displays rate data for alkoxyamine-termi-nated polymers and low molecular model compounds and shows some important trends. At about the same temperature, the dissociation rate constants Ad of alkoxyamines (Schemes 12 and 30) with the same leaving radical (polystyryl, 1-phenylethyl) increase in the order 3 (TEMPO) < 6 < 8 (DEPN) < 1 (DBNO) by a factor of about 30. Acrylate radicals dissociate markedly slower than styryl radicals from 1 (DBNO), but there is no appreciable difference for 8 (DEPN). The dependence of Ad on the nitroxide structure has been addressed by Moad et al.104 They found the order five membered ring < six membered ring < open chain nitroxides and pointed out additional steric (compare 3 and 6) and polar effects. 

The irreversible decay of the dormant alkoxamine chains stops the monomer conversion rather abruptly at the time t = H( nk. For methyl methacrylate polymerizations this stop has been observed, and it has been demonstrated that it is caused by a considerable fraction of cross-disproportionation between the nitroxide and the propagating radicals.51,97112 Unfortunately, the factors governing disproportion-ation-to-combination ratios in radical—radical reactions are not well understood up to now, but stereo-electronic effects are certainly very important.112 Hence, one cannot yet predict a nitroxide structure that will allow living methacrylate polymerizations up to large conversions. 

This point of view is supported by the unusually high values of the partial rate factors of the a- and y-positions in the homolytic phenylation of pyridine-N-oxide ) (a 139, 1.5, y 31.2) compared with those of pyridine (a 1.8, /S 1.0, y 1.2). This behavior can in fact be ascribed to the higher stability of the cr-complex which has a nitroxide type structure (4)... 

Nitroxide Structure Nitroxide Structure Nitroxidc Stmeture... 

Tabic 9.4 Seven- and Eight-Membered Ring Nitroxides for NMP Nitroxide Structure Nitroxide Structure Nitroxide Structure... 

Unsubstantiated assignment several related radicals derived from other substituted piperidone oximes reported all assigned nitroxide anion structures. ... 

Hence, they were able to selectively compare the influence of these three intertwined effects and they concluded that each effect depends on the nitroxide structure. It was shown that the polar effect mainly depends on the nitroxide moiety. For instance, the influence of the polarity of the leaving alkyl group on the homolysis is weak for TEMPO ([Pg.296]

There are only a few fimctional groups that contain an unpaired electron and yet are stable in a wide variety of structural environments. The best example is the nitroxide group, and numerous specific nitroxide radicals have been prepared and characterized. The unpaired electron is delocalized between nitrogen and oxygen in a structure with an N—O bond order of 1.5. 

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... 

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 ... 

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