Radicals nitroxy

Controlled radical polymerization stable nitroxy radicals, R-X/ Cu(I)/ligand, thiocar-bamates (transferters) Heat... 

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end or monomeric radical and the nitroxy radical. Monomer is inserted into this bond when it opens thermally. The free radical necessary to start the reaction can be created by adding a conventional radical initiator in combination with, e.g., TEMPO or by starting the reaction with a preformed adduct of the monomer with the nitroxy radical using so-called unimolecular initiators (Hawker adducts). 

This equation seems to be a key reaction point in the antioxidant action of these amines >N radicals in the presence of oxygen are transformed via peroxy radical intermediates into nitroxy radicals. The nitroxy radicals are very persistent and react efficiently with radicals produced on polyolefine degradation. Such radical interception blocks the chain in radical oxidation and therefore causes the actual antioxidant phenomenon of sterically hindered amines (Brede et al. 1998). 

E. G. Rozantsev, Free Nitroxy Radicals (Plenum Press, New York, 1970). 

Nitroxy radicals may also reac unsaturated bonds in polymers according to the reactions ... 

The first radicals we looked at, the alkylaminotropone nitroxy radicals (1), were unstable (2 ). However, since Neiman et al. ( 3) had reported that 2,2,6,6-tetramethyl-4-oxopiperidine-l-oxyl (2) was extremely stable, we turned to nitroxyl radicals derived from fully hindered amines. In this section we will describe the synthesis of new stable nitroxyl radicals and their evaluation in polypropylene. 

Tetramethyl-l-piperidinyloxy (TEMPO), a nitroxy radical (Figure 10.3), is an effective catalyst for the oxidation of primary alcohols in the presence of secondary alcohols [9]. Iwabuchi and co-workers reported excellent catalytic activity of 2-azaadamantane-N-oxyl (1-Me-AZADO) towards a variety of alcohols. Oxidation of secondary alcohols was effected by means of 1 mol% of 1-Me-AZADO (Equation 10.4) [10]. 

In the course of thermo-oxidative degradation of PAs stabilized with antioxidants, such as phenols and secondary aromatic amines and hindered nitroxy radicals chemiluminescence is observed. This effect is attributed to redox inhibition reactions. 

Yoshida and Sugita [72, 73] have described the synthesis of polytetrahydrofuran (PTH F) possessing a nitroxy radical by terminating the Hving cationic ring-opening polymerization (CROP) of THF with sodium 4-oxy TEMPO. The polymer obtained in this way acted as a counter-radical in the polymerization of styrene, in the presence of a free radical initiator, to yield PSt-f)-PTHF (Scheme 11.16). 

The in situ approach leads to the formation of multiple nitroxide initiating species and there is not perfect stoicheometry between the initiating and mediating species. With the presynthesis approach, a pure compound can be used to initiate polymerization which should lead to cleaner chemistry. However, it has been believed that excess nitroxy radicals is important to achieve narrow polydis-persity and good end-group purity (249). 

The formation of nitroxy] radicals testifies to the fact that main-chain decomposition by reaction (24) and side-chain ester group cleavage by reaction (26) take place in the polymer. Thus, the interaction of NO-, with double bonds is able to initiate free-radical reactions of polymer degradation when hydrogen atom abstraction reactions from C-H bonds are inefficient. 

Hydroxyl amine ethers (> NO-R) react with peroxy radicals and generate nitroxy radicals, resulting in an slowdovm of chain oxidation degradation as shown in Formula 4.26. 

However, it is well established that >N-H are converted into nitroxy radicals >N-0°, these latter being extremely efficient alkyl traps. 

To obtain a planar-conjugated main chain, poly(phenylenevinylene) (PPV) was suggested as a potential candidate for an effective backbone ft)r p-conjugated polyradicals because of its developed conjugation, coplanarity, and solubility after substitution on the phenylene ring. Nishide et al. [23-25] synthesized poly(1,4- and 1,2-phenylenevinylene) (13-16) bearing phenoxy and nitroxy radicals, the unpaired electrons of which are delocalized to the p-conjugated main chain, as one of the polyradicals with a PPV... 

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