Alkoxyamine dissociation rate constant

We can now plot the temperature dependence of k using the Arrhenius equation given in Equation 10.10, see Fig. 10.5 a (from the slope) and Aa (from the extrapolated intercept 

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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. 

It was however quickly realized that the main problem with TEMPO was the low value of the dissociation rate constant, fed, of the corresponding alkoxyamine. Moad and Rizzardo showed that alkoxyamine homolysis rate is governed by a combination of polar, steric, and electronic factors the steric one being predominant (see Section 3.10.3.1 for details). Many variations in the nature of the alkyl groups linked to the carbon in the a-position to the aminoxyl function were then investigated. For example, Mannan et and Miura et developed many six-membered cyclic nitroxides with spiro stmctures (18-24), which could be either mono (19) or disubstituted (21). The increase of the steric hindrance due to the spiro stmcture increased the dissociation rate constant of the (macro) alkoxyamines and led to successful NMP of S at 70 °C and -butyl acrylate (nBA) at 120 °C using nitroxide... 

A drastic change of nitroxide stmcture was witnessed with the use of the commercially available DBNO (27). In particular, Moad and Rizzardo showed that the dissociation rate constant of a DBNO-based alkoxyamine was higher than any similar alkoxyamines based on cyclic nitroxides bearing tetra-methyl alkyl groups on the vicinity of the aminoxyl function. The first experimental studies were performed by the group of Catala where it was shown that the polymerization of styrene and substituted styrene monomers could be carried out at 90 ° C with all the criteria of control/livingness. However, the polymerization rate was independent of the alkoxyamine concentration and remained governed by the production of thermal radicals in the medium. The tert-butyl-tert-amyl nitroxide 28 was tested by Moad et to control the polymerization of MMA and appeared to be inefficient. 

All these results highlighted the cmcial role of the alkoxya-mine structure for NMP. The best alkoxyamine should then present a high dissociation rate constant value and a rate constant of first monomer addition at least equal to the propagation rate constant. This makes the determination of the dissociation and recombination rate coefSdents of model alkoxyamines of high importance for further improvement of this process. 

To avoid tedious synthetic and polymerization studies, the estimation of the dissociation rate constant, predicted on the basis of alkoxyamine and nitroxide stmctures, is highly desirable. 

Variations of the initial alkoxyamine concentration [I]o and independently measured rate constants k,, k,, and ktR nicely confirmed that the system fulfills the conditions (20) and reproduced the value of the parameter combination taken from the experimental data. The same was found for other alkoxyamines.16-23 However, it must be mentioned that the observation of such kinetics requires precautions with respect to the purity of chemicals, solvents, and containers. This is necessary because even very minor side reactions may strongly interfere with the many cycles of dissociation and recoupling that are necessary to provide a clear kinetic manifestation of the effect. 

Importantly in NMP, the decomposition of alkoxyamines can occur through the reaction in Scheme 5, in which the P-proton abstraction by a nitroxide is assumed to take place in the solvent cage in both dissociation and combination processes. In this scheme, the rate constant of decomposition fe ec (the rate=fedec[P-X]) wiU be proportional to fed imda the DC equilibrium and will take the relation... 

Table 1. Rate and Equilibrium Constants for the Reversible Dissociation of Polymeric Alkoxyamines and Low Molecular Model Compounds, Frequency Factors, and Activation Energies of Dissociations...

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