Recombination, alkoxyamines

The exchange in the alkoxyamine-based polymer occurs in a radical process that is tolerant of many functional groups. The exchange process is therefore applicable to polymers with various functional groups. TEMPO-based polyester 43 and polyurethane 44 were synthesized for studies of the scrambling of disparate polymers imder thermodynamic control (Fig. 8.11) [37], Two kinds of TEMPO-based polymers were mixed and heated in a closed system. After 24 hours when the crossover reaction achieved equilibrium, GPC and NMR analyses revealed that they were totally scrambled through bond recombination on the TEMPO units. 

In the photoinduced cleavage of N-alkoxyamines, where the transient and the persistent radical are an alkyl and an aminoxyl radical, Fischer et al. used time-resolved CIDNP to clarify the mechanism of cross-termination and to determine the rate constant of the recombination. They concluded that disproportionation plays no role in this system, and that no unknovm side reactions occur. Later, Ananchenko and Fischer compared the recombinations of the transient ferf-butyl and 2-hydroxy-... 

H) Scavenging of an alkyl radical by an aminoxyl radical Among the several methods already developed to prepare alkoxyamines, the easiest approach involves the in situ generation of carbon-centered radicals followed by its recombination with the nitroxide. Many different methods already exist to prepare carbon-centered radicals. The ones used to prepare alkoxyamines are described in Figure 8 and the most used are detailed in the following paragraphs. 

The use of a very labile SGl-based alkoxyamine drastically improved the controlled character of a bulk styrene potymer-ization. In that case, a crowded tertiary alkyl radical moiety instead of a secondary alkyl one increased the value from 5.0 X 10 to 1.7 X 10 s at 90 °C. Additionally, Benin and Charleux research groups showed that a very labile SGl-based alkoxyamine (BlocBuilder, Table 4, 71) allowed the polymerization of nBA to be successfully performed without any initial addition of free nitroxide. In this case, high leads to the in situ production of free nitroxide in the early stage of the polymerization and this forces the recombination of alkyl radicals with nitroxides even at low monomer conversion. Chauvin et propagation rate constant of the monomer ... 

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. 

Actually, the alkoxyamines formed by recombination between a nitroxide and a transient alkyl radical are able to dissociate upon thermal activation and generate again the initial radicals as schematized in Figure 8.2. Depending upon the structure of the two free radicals involved, this dissociation, which occurs by homolytic rupture of the C-ON bond, will be observed in a varying range of temperatures. 

---