Nitroxide compounds kinetics

Jousset, S., Hammouch, S. O., and Catala, J. M. (1997). Kinetic studies of the polymerization of p-tert-butylstyrene and its block copolymerization with styrene through living radical polymerization mediated by a nitroxide compound. Macromolecules, 50(21) ... 

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 kinetic parameter for the radical dissociation of a carbon—halogen terminal was obtained with the use of an isolated polystyrene with a terminal C—Br bond in the presence of a copper catalyst and a conventional radical initiator with a long half-life.282-283 The result was compared with that of low molecular weight compounds of similar carbon-halogen bonds.163 The second-order rate constant of the model compound 1-13 (X = Br), an effective initiator for styrene, is comparable to that of the polymer terminal. Alternatively, rate constants can be obtained by using a combination of nitroxide-exchange reactions and HPLC analysis.242... 

Nitroxides and Nitroso- and Nitro-compounds. Further examples of the formation of 2 1 adducts from bistrifluoromethyl nitroxide and halogenated olefins [those used were CF CHF, CHjtCFj, CFg CFCI, CFjtCFBr, CF2 C(CFa)2, and COjrCCls] have been quoted, and perfiuorobutadiene has been shown to give a 2 1 and, under forcing conditions (250°C), a 4 1 adduct. Kinetic studies have established the following order of susceptibility towards attack by the nitroxide ... 

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