The influence of the N‐alkyl group of tertiary hindered amines on the photostabilization of polymers was studied. The photostabilizing effects of the tertiary amine derivatives of 4‐benzoyloxy‐2,2,6,6‐tetramethylpiperidine (1a) in polypropylene were compared. All tertiary amine derivatives having α‐H to hindered N showed higher effectiveness than 1a. Model liquid phase photoxidations were carried out by irradiating (UV‐lamp) the solutions of tertiary hindered amines containing tert‐butyl hydroperoxide as a photoinitiator. The tertiary hindered amines were oxidized more easily than corresponding parent hindered amine and converted to the parent amine, which was identified as its salt, resulting from the carboxylic acid produced from the N‐alkyl group by oxidation. The thermal reaction of the tertiary hindered amines with tert‐butyl hydroperoxide was also studied in the liquid phase. The tertiary hindered amines decomposed tert‐butyl hydroperoxide more rapidly than the parent secondary hindered amine, and generated the parent amine. It was also found that the photostabilizing effects of tertiary hindered amines for polyolefins were higher than that of the parent secondary hindered amine.
The hindered secondary amines, 2,2,6,6-tetramethyl-4-oxopiperidine (II) and 1,9-diaza-2,2,8,8,10,10-hexamethyl-4-oxo-spiro[5.5]undecane (VI), were oxidized with m-chloroperbenzoic acid, and the corresponding hydroxylamine (III) and N-oxyl-hydroxylamine (VIII), respectively, were isolated. The further oxidation of III and VIII with the peracid afforded the corresponding nitroxide radical IV and IX, in good yields respectively. The stoichiometry of the oxidation reaction of the hydroxylamine III to the nitroxide radical IV was elucidated on the basis of the ESR signal intensities of oxidation products at various molar ratios with the peracid and III. An ESR study of the 1-15N-labelled N-oxyl-hydroxylamine (XII) indicated that there was an intramolecular hydrogen transfer between >N–OH and >N–O · groups in the N-oxyl-hydroxylamines VIII and XII.
Photolysis of the hindered N-Chloroamines, 1-chloro-2,2,6,6-tetramethyl-4-oxopiperidine (IIa), 1-chloro-2,2,6,6-tetramethylpiperidine (IIb), and 1-chloro-2,2,5,5-tetramethyl-4-oxoimidazolidine (IIc), in benzene solution were carried out in an ESR spectrometer cavity. The ESR spectra of the corresponding amino radicals IIIa, IIIb, and IIIc were observed in evacuated solutions. In solutions containing oxygen, it was found that amino radicals IIIb and IIIc, readily reacted with oxygen to give the corresponding stable nitroxide radicals IVb and IVc, from the shapes of spectra and g-values. Amino radical IIIa did not react with oxygen. Although the amino radicals could not be isolated, their formation was confirmed by the isolation of a coupling product with a benzyl radical generated from dibenzylmercury.
The influence of the acyl groups of N‐acylated hindered amine on the photostabilization of polymers was discussed. The effects of the N‐acyl derivatives of 4‐benzoyloxy‐2,2,6,6‐tetramethylpiperidine (6) in the photostabilization of polypropylene were compared. Although the stabilizing effects of all the N‐acyl derivatives were lower than that of the N‐oxyl derivative (7), N‐acryloyl (4) and N‐benzoyl (5) derivatives showed higher effects than that of parent NH 6. To discuss the stabilizing mechanism of N‐acylated hindered amine, the reaction of N‐acylated hindered amine with model hydroperoxide (tert‐butyl hydroperoxide) was carried out at an elevated temperature (132°C). N‐Acylated hindered amine decomposed tert‐butyl hydroperoxide more rapidly than corresponding secondary hindered amine, and changed into the parent secondary amine.
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