Nitroxide structures

Nitroxide Structure Nitroxide Structure Nitroxide Structure... 

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. 

Nitroxide Structure Nitroxide Structure Nitroxidc Stmeture... 

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

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]

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