Homolysis of alkoxyamines

Marque, S., Fischer, H., Baier, E., and Studer, A., Factors influencing the C-O bond homolysis of alkoxyamines effect of H-bonds and polar substituents, J. Org. Chem., 66,1146, 2001. 

PRE explains the low level of termination observed in NMP and other SRMP s as well as in ATRP. Reversible homolysis of alkoxyamines, the key step of NMP, is a typical example of a reaction governed by the PRE. The kinetic equations describing this process were experimentally verified in 1998 by Fischer and coworkers with the model alkoxyamine 8 (Scheme 4.6). 

The reversible homolysis of 8 (Scheme 4.6) and the reversible reactions between the dormant and active chains in the NMP (Scheme 4.5) are similar reactions obeying the same kinetic laws. The only difference is that the starting alkoxyamine R R NOR is rapidly transformed into the macroalkoxyamines R R N0M R (dormant chains) and at the end of the process into the final polymer R R NOMpR. If the conversion of monomer occurs within the intermediate time range (Figure 4.1), the result will be the living and controlled polymer R R NOMpR contaminated with only small amounts of free nitroxide R R NO and dead chains resulting from significantly diminished termination reactions. 

Bon SAP, Chambard G, German AL. Nitroxide-mediated Uving radical polymerization determination of the rate coefficient for alkoxyamine C-O bond homolysis by quantitative ESR. Macromolecules 1999 32 8269-8276. 

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... 

However, alkoxyamines R R N0R with high kj can only be used for initiation if the H-transfer reaction between R R N0 and R (Scheme 4.5, reaction (8)) is insignificant (<0.5%) and cannot compete with the addition of R to the monomer. Bagryanskaya et showed that NMP of styrene at 120 °C was controlled with 28A1 but failed with 28A5, which undergoes faster homolysis. Around 3% of H-transfer occurs between 28 and A5 compared to 0% with Al. On the other hand, AS adds to styrene more slowly than A1 but couples faster with 28. Consequently, the H-transfer reaction is preferred over the addition of AS to monomer and disrupts NMP. 

The chromophore must be attached close to the nitroxide moiety to ensure an efficient intramolecular energy transfer from the chromophore to the NO-C bond which should also undergo homolysis in the macroalkoxyamine. However, direct attachment of the aromatic chromophore to the alkoxyamine nitrogen, as in 44, leads to competitive cleavage of the N-OC bond because the resulting aminyl radical is resonance stabilized (see Section 6.3). On the other hand, alkoxyamine 45 undergoes a selective NO-C photolysis. Linear increase of Mn with conversion was observed during NMPP of -butyl acrylate with 45, as expected for controlled polymerization (Scheme 4.18). ... 

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