N-Oxyl radicals formed from

It is well known that HALS compounds (sterically hindered amine light stabilizers) exert excellent photostabilizing effect on polyolefins (1-3). Most of the authors agree that N-oxyl radicals formed from HALS compounds play an important role in the photostabilizing process (4-6). 

These changes are the consequences of the mobility change of the free N-oxyl radicals. The same phenomenon has been observed with the N-oxyl radicals formed from a tertiary amine, e.g. from TINUVIN 622. 

The amount of the N-oxyl radicals in tertiary amine, however, is much less than that in the secondary one. Another difference between the N-oxyl radicals formed from the different kind of amines is that by decreasing the amount of the oxygen above the sample, the signal of the radicals formed from the tertiary amine splits into secondary lines (Figure 3), while the ESR spectrum of the radicals formed from the TINUVIN 770 remains unchanged. 

Figure 3. ESR spectrum of the N-oxyl radicals formed from Tinuvin 622 in benzene solution after decreasing the amount of oxygen above the sample.

This direct reaction taking place between the hindered phenol and N-oxyl radical formed from the HALS compound during processing might be the reason for the decreased N-oxyl radical concentration and the antagonistic effect in the presence of a hindered phenolic ant ioxidant. 

TINUVIN 770 takes place at about 210 Mlxh (megaluxhours), and during this time no carbonyl formation can be detected (7-8). The concentration of the stabilizing transformation product of TINUVIN 770 (the N-oxyl radical) decreases slowly, even at very high dose it can be assumed that the N-oxyl radicals may be regenerated from the product formed in the course of the stabilization (9-10). 

From the standpoint of steric hindrance, an N-oxyl radical should be less stable than the corresponding N-free radical. Murayama (14), however, has shown that an N-free radical has a strong affinity to oxygen, and forms the N-oxyl readily. The N-oxyl is a stable radical owing to the lack of hydrogen on the carbon alpha to the N-atom and the following resonance ... 

Other related to TEMPO additives have been used, e.g., N-hydroxyphthalimide (NHPI) which generates the nitroxyl radical phthahmide-N-oxyl (PINO, Fig. 6). The PINO radical and analogs have been utilized for a range of aerobic oxidation reactions. PINO has been shown to be more reactive than TEMPO, but it is not stable and is usually formed from NHPI in situ by means of an initiator. 

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