Model studies on the mechanism of hals stabilization

Hindered Amine Light Stabilizers (HALS) are know to inhibit the photo-oxidation of polymers. A key reaction in their stabilization mechanism is believed to be the conversion of a hindered aminoether into a nitroxyl radical. Several different possible mechanisms for this conversion were explored. One, the elimination of the aminoether to form an olefin and hydroxylamine (an intermediate in the formation of a nitroxyl), while possible at high temperatures, cannot account for the inhibitory activity we observed for secondary and primary aminoethers. Direct radical displacement by peroxy radicals was also considered. However, the products predicted by this reaction pathway were not observed. Finally, oxidation of the nitrogen by a peroxy radical, by either electron transfer or a radical attack on the nitrogen, was investigated. While electron transfer was shown to be unlikely, direct oxidation of the aminoether nitrogen was supported by our results. A detailed mechanism for the reaction of both alkyl- and acyl-peroxy radicals with aminoethers is proposed.     

Effect of hindered piperidine light stabilizer molecular structure and UV absorber addition on the oxidation of HDPE. Part 2: Mechanistic aspects—molecular modeling and electron spin resonance spectroscopy study

The first in this series of papers explored the effect of the structural characteristics of 2,2,6,6‐tetramethylpiperidine‐based hindered amine light stabilizers (HALS) on the thermal and photostabilization of high‐density polyethylene. In the second part, the energies (stabilities) of the nitroxyl radicals and various intermediate species have been predicted using AccuModel® and related to stabilization performance and electron spin resonance (ESR) spectral data. Nitroxyl radicals with low predicted stability generally afforded improved thermal and photostabilization. ESR spectra were used to obtain values of nitroxyl radical concentration ([>N? O·]) and g‐factor as a function of pre‐aging time for combinations of pre‐aged HDPE and >N? H HALS. Demethylation reactions of > N‐methyl HALS resulted in uselessly weak ESR spectra. The HALS that afforded poor thermal oxidative stabilization gave rise to pronounced minima in [>N? O·] that coincided with a maximum in hydroperoxide concentration. The g‐factor values indicated that a predominant nitroxyl canonical form generally promoted superior thermal oxidative stabilization, whereas a predominance of the dipolar N·⁺?O^? form promoted superior photo‐oxidative stabilization. These trends may be related to greater radical‐scavenging and peroxide‐trapping effects, respectively. Molecular modeling and ESR spectra can therefore provide valuable insight into the effectiveness of HALS and stabilization mechanisms. J. Vinyl Addit. Technol. 10:159–167, 2004. © 2004 Society of Plastics Engineers.     

Catalytic oxidation of alcohols using molecular oxygen mediated by poly(ethylene glycol)-supported nitroxyl radicals

The selective catalytic oxidation of alcohols over a mixture of copper(I) chloride and a number of linear ‘linker-less’ or ‘branched’ poly(ethylene glycol)-supported nitroxyl radicals of the 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) family as a catalyst system has been investigated in the presence of molecular oxygen in a batch reactor. It is found that the activity profile of the polymer-supported nitroxyl radicals is in good agreement with that of low-molecular weight nitroxyl catalysts, for example, allylic and benzylic alcohols are oxidised faster than aliphatic alcohols. The oxidations can be tuned to be highly selective such that aldehydes are the only oxidation products observed in the oxidation of primary alcohols and the oxidations of secondary alcohols yield the corresponding ketones. A strong structural effect of the polymeric nitroxyl species on catalytic activity that is dependent upon their spatial orientation of the nitroxyl radicals is particularly noted. The new soluble macromolecular catalysts can be recovered readily from the reaction mixture by solvent precipitation and filtration. In addition, the recycled catalysts demonstrate a similar selectivity with only a small decrease in activity compared to the fresh catalyst even after five repetitive cycles.     

In situ detection of hindered amine stabilizer consumption in polymer through oxidation by indirect electron spin resonance

The stabilization mechanisms of hindered amine stabilizers (HAS) involve various oxidation products of the piperidine structure. Thus, we propose a novel method, based on electron spin resonance (ESR) spectroscopy, to check in situ the consumption of the total piperidyl species (intact HAS and all of its byproducts) in polypropylene (PP) films through photooxidation. First, the concentration of nitroxyl radicals produced upon irradiation in stabilized PP has been measured by direct ESR analysis. Then, the changes of concentration of the overall stabilizing species have been detected by indirect ESR, after conversion of the overall HAS derivatives into nitroxyl-free radicals by exposure of photooxidized PP to peracetic acid vapor at room temperature. Results were compared with those obtained by the conventional Fourier transform infrared method in the particular case of [2,2,6,6-tetramethyl piperidinyl]sebacate, and the reliability of both techniques was discussed. Thus, we assume that indirect ESR experiments consist of a easy, accurate, and very sensitive method to monitor the time evolution of a low-molecular weight HAS–NH concentration in PP upon photooxidation. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1107–1114, 1998     

Catalytic oxidation of alcohols using molecular oxygen mediated by poly(ethylene glycol)-supported nitroxyl radicals

The selective catalytic oxidation of alcohols over a mixture of copper(I) chloride and a number of linear ‘linker-less’ or ‘branched’ poly(ethylene glycol)-supported nitroxyl radicals of the 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) family as a catalyst system has been investigated in the presence of molecular oxygen in a batch reactor. It is found that the activity profile of the polymer-supported nitroxyl radicals is in good agreement with that of low-molecular weight nitroxyl catalysts, for example, allylic and benzylic alcohols are oxidised faster than aliphatic alcohols. The oxidations can be tuned to be highly selective such that aldehydes are the only oxidation products observed in the oxidation of primary alcohols and the oxidations of secondary alcohols yield the corresponding ketones. A strong structural effect of the polymeric nitroxyl species on catalytic activity that is dependent upon their spatial orientation of the nitroxyl radicals is particularly noted. The new soluble macromolecular catalysts can be recovered readily from the reaction mixture by solvent precipitation and filtration. In addition, the recycled catalysts demonstrate a similar selectivity with only a small decrease in activity compared to the fresh catalyst even after five repetitive cycles.     

On increasing the inhibiting property of amine antioxidant in contact with copper and its compounds

The mechanism of increasing efficiency of amine‐type antioxidant (AO)‐N‐plenyl‐2‐naphthylamine (Neozon D)—used to inhibit oxidation of polyethylene (PE) under conditions of contact between the polymer and metallic copper has been investigated. It has been learned experimentally, that the hypothesis of formation of complex compounds by reaction of AO and copper‐containing products—resulted from contact oxidation of PE melt on copper—is invalid. A different mechanism of raising the inhibiting efficiency of amine AO is suggested, according to which copper catalyzes transformation of Neozon D to nitroxyl radicals. The latter are excellent inhibitors for oxidative transformations and appreciably enhance polymer resistance to oxidation. An amine‐type AO can be converted to nitroxyl radicals in two ways: (i) before AO is added to the polymer; (ii) during direct contact between the AO and metallic copper (i.e., copper as metal) which is a component of the metalopolymeric material (e.g., polymer‐coated substrate; filler particles introduced into a polymer) that is subjected to oxidation. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Chemiluminescence studies of the degradation and stabilization of polymers

In order to predict the safe service life of a polymer from its rate of oxidation, a detailed understanding of degradation and stabilization reactions in the induction period is required. The mechanism by which an oxidizing polymer emits weak visible light (chemiluminescence) is reviewed and the use of the technique to study the early stages of the oxidation of nylon 66 and polypropylene is considered. Steady-state chemiluminescence experiments are described to measure the activation energy of initiation of nylon 66 oxidation and to study the mechanism of the dark reaction of a hindered amine light stabilizer (and its nitroxyl radical) with polypropylene hydroperoxide. Non-stationary experiments, in which the steady state is perturbed by UV irradiation and gas switching, demonstrate the reactions of long lived alkyl radicals in the induction period. The observation of stress-induced chemiluminescence from nylon 66 shows the effect of even a low fiber load is to greatly increase the rate of oxidation.     

Readily Accessible Oxazolidine Nitroxyl Radicals: Bifunctional Cocatalysts for Simplified Copper Based Aerobic Oxidation

Using a two‐step procedure a range of bifunctional oxazolidine nitroxyl radicals have been prepared. The application of these co‐catalysts to the copper‐based aerobic oxidation of alcohols was then investigated. From these studies it was found that the parent tetramethyloxazolidine nitroxyl radical L1 was competent for oxidation in the presence of 2,2′‐bipyridine, and the bifunctional pyridyl‐containing nitroxyl radicals L2 and L3 could be used in the absence of additional coordinating ligands. Following optimization, the scope of this simplified transformation was explored, demonstrating that a range of primary and secondary benzylic alcohols are readily oxidized. In addition, the oxidation of allylic alcohols and hydroquinone can be achieved.     

Mechanisms of antioxidant action: Photoantioxidant activity of polymer-bound hindered amines. II. Bis acrylates

1-Acryloyl-4-acryloyloxy-2,2,6,6-tetramethylpiperidine (AATP) has been covalently attached to polypropylene under conditions of reactive processing to give substantial concentrations of essentially 100% grafted antioxidant (AATP-B). Solution of the concentrates showed that at high concentration of grafted antioxidant some homopolymerized AATP was present and the polymer undergoes transient cross-linking during processing. Concentrates of bound AATP (AATB-B) when used as conventional additives for polypropylene are very effective photoantioxidants due to conversion to the related polymer-bound nitroxyl radicals by photosensitized oxidation. A typical commercial hindered phenol, Irganox 1076, when used at low concentration in combination with -B also acts as a photosensitizer for nitroxyl formation. Unlike low molar mass-hindered piperidinoxyl, polymer-bound nitroxyls are effective thermal antioxidants that synergize very effectively with low concentrations of Irganox 1076.     

The role of hindered piperidine (HALS) compounds for the stabilization of polypropylene against oxidation reactions caused by ozone and oxidative products formed during photolysis of ozone

The role of hindered piperidine (HALS) photostabilizers for the photostabilization of polypropylene films against oxidative reactions caused by ozone and oxidative species formed during photolysis of ozone under UV irradiation (i.e., atomic oxygen and singlet oxygen) has been investigated and discussed. It has been found that ozonization and/or photoozonization of 2,2,6,6-tetramethyl-piperidine (I) and piperidinoxy radical (2,2,6,6-tetramethylpiperidino-1-oxy) (II) gives a high yield (91–98%) 2,6-dimethyl-2-hydroxy-6-nitro-heptane. Ozonization and/or photoozonization of HALS probably occur by a similar mechanism as reported for (I) and (II). HALS photostabilizer reacts with ozone and/or photolysis products of ozone (atomic oxygen and/or singlet oxygen) and loses its function in the photostabilization process. For that reason protection of polypropylene against photooxidative degradation and photoozonization is of great commercial importance.     

微反应器内基于氮氧自由基催化剂连续氧气/空气氧化反应的研究进展

连续液相氧气氧化技术代替传统氧化技术已经成为氧化反应发展的一大趋势。但是,分子氧通常需要被合适的催化剂进行活化后才能进行高选择性氧化。近年来,氮氧自由基催化剂因其能够在温和条件下高效地催化氧气氧化反应而取得了快速发展。此外,可持续的绿色氧化工艺不仅依赖于高效环保的催化体系,还需要依托能够强化传质和反应性能的反应器技术。本文介绍了连续微反应氧化技术中常用的微反应器,归纳总结了以氮氧自由基及其衍生物为催化剂的空气/氧气氧化反应在连续有机合成中的研究进展。最后,针对现阶段氮氧自由基催化的连续液相氧化技术的潜在挑战,对该技术在精细化工领域中的应用进行了展望。     

Characterization of new hindered amine light stabilizers

New polymerizable urethane functional monomers containing hindered amine (III, III′) and their copolymers with styrene (S) and methyl methacrylate (MMA) are characterized and tested as hindered amine light stabilizers (HALS). Experimental results indicated that the monomers possess high photostabilizing effectiveness comparable to that of Tinuvin 770 and Tinuvin 765. Their copolymers with S and MMA showed lower photostabilizing effectiveness mainly due to the low hindered amine content in the copolymers. The copolymers, however, showed very high resistance to liquid extraction. UV and ESR analyses indicate that the photostabilizing mechanism of these HALS is through the scavenging action of nitroxyl radicals formed from degradation of the substrate polymer. © 1993 John Wiley & Sons, Inc.     

Photo-oxidation mechanisms of poly(2,6-dimethyl-1,4-phenylene oxide).

A re-examination of the photo-oxidation mechanism of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) at both long and short irradiation wavelengths under accelerated conditions is reported. The formation rate of poly-2,6-dimethylphenoxy (PDMP) radicals, have been observed by esr spectroscopy, while benzylic-type radicals have been detected by spin trapping experiments using nitrosodurene under UV irradiation conditions. The changes of FTIR spectra obtained under accelerated photo-oxidative conditions show the formation of primary oxidation products such as aromatic aldehydes and quinones. The direct formation of radical cations and superoxide anions by a photo-induced electron-transfer reaction is suggested under our conditions. In addition, some results on the of photo-oxidation PPO/PS blends as a function of polystyrene tacticity are also reported. In the case of syndiotactic polystyrene the blend photostability shows some differences in comparison with the blend containing atactic polystyrene. This effect has been ascribed to the absence of excimers in syndiotactic polystyrene which are not revealed by fluorescence measurements. Finally, the chemical modification of PPO with bromine gave a bromomethylated PPO with different amounts of CH₂ Br groups, which has been used for further substitution reactions with alkoxy sodium salts of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 4-hydroxy-1,2,2,6,6-pentamethylpiperidine (PMPP). The accelerated photo-oxidation of a modified copolymer PPO-PMPP containing a 60% PMPP moieties as side groups and its blends with pure PPO has been followed by FTIR and esr spectroscopies. Some comparisons with a commercially available hindered amine light stabilizers (HALS) and a modified copolymer PPO-PMPP are also reported under the same photo-oxidative conditions.     

某些高分子受阻胺对聚丙烯稳定化作用的研究

众所周知,受阻胺光稳定剂对聚烯烃有良好的光稳定效率,已引起世界的注意。然而,小分子量的受阻胺易于挥发,从而影响它的效率发挥,为了克服这个缺点,使小分子受阻胺的高分子化是近年来受阻胺光稳定剂发展的方向,本文主要是研究三种商品化高分子受阻胺光稳定剂对聚丙烯的稳定化作用及其效率和机理。     

对二嗪N,N`—二氧化物光还原的电子自旋共振研究

报道了利用原位电子自旋共振(ESR)技术研究对二嗪N,N′-二氧化物光解的结果。研究结果表明,化合物5,6,7和8通过光还原过程从溶剂分子夺取氢原子分别生成稳定的自由基,记录出它们分辨良好的ESR谱。文中还讨论了对二嗪N,N′-二氧化物的光化学反应的自由基机理。     

Mechanisms of antioxidant action: the antioxidant activity of nitrones in polypropylene

Aldonitrones are effective melt stabilizers for polypropylene, the most effective being benzaldonitrones containing a partially hindered 4-hydroxy group in the aromatic ring. They form nitroxyl radicals in the polymer during processing, and these and the related hydroxylamines are believed to be involved in a catalytic CB? A/CB? D antioxidant cycle during mechano- and photooxidation. During thermal oxidation (oven aging) they behave as stoichiometric CB-D antioxidants.     

国产光稳定剂BW—10LD在聚丙烯生产中的应用研究

对国产受阻胺类光稳定剂BW－10LD与同类进口产品Tinuvin 622LD的理化性能进行了对比分析,研究了BW－10LD对聚丙烯老化性能的影响及其在PP生产中的应用,并对受阻胺类光稳定剂的作用机理进行了探讨。     

Wirksamkeit sterisch gehinderter amine bei anspruchsvollen applikationen von polypropylen

The influence of some types of antioxidants on the efficiency of low molecular hindered amine light stabilizers (HALS) has been investigated in a comparison with the behaving of high molecular HALS. Most of the antioxidants used exhibit favourable influence. Convenience of both low and high molecular HALS in compounds depends on the character of the stabilised product and the technological conditions for its production.     

E.s.r. study of u.v.-irradiated polypropylene: effect of the stereoregularity on the behaviour of free radicals produced by u.v. irradiation

The effect of stereoregularity on the behaviour of free radicals produced in polypropylene by ultra-violet light at low temperature was investigated by an electron spin resonance method. Ultra-violet (u.v.) light is absorbed mainly by carbonyl groups produced in polypropylene by oxidation. The radicals produced at low temperature are identified as ---CH₂---CH(ĊH₂)---CH₂--- and methyl. A large yield of radicals is obtained in samples of poor tacticity. Some of the ---CH₂---CH(ĊH₂)---CH₂--- radicals convert to acyl and some to ---CH₂---Ċ(CH₃)---CH₂--- on warming the sample. The conversion to acyl is remarkable in atactic and stereoblock polypropylene, and that to ---CH₂---C(CH₃)---CH₂--- in isotactic polypropylene. These acyl and ---CH₂---Ċ(CH₃)---CH₂--- radicals return to ---CH₂---CH(ĊH₂)---CH₂--- by illumination, respectively, by visible and u.v. light at 77K. The decay of radicals is attributed to the local motion or microbrownian motion of molecules.     

Antioxidant action of a lipophilic nitroxyl radical,cyclohexane-1-spiro-2′-(4′-oxyimidazolidine-1′-oxyl)-5′-spiro-1″-cyclohexane,against lipid peroxidation under hypoxic conditions

Nitroxyl radicals are known to act as radical scavenging antioxidants. In the present study, a lipophilic nitroxyl radical, cyclohexane-1-spiro-2′-(4′-oxyimidazolidine-1′-oxyl)-5′-spiro-1″-cyclohexane (nitroxyl radical I) was synthesized and its antioxidant capacity was assessed in comparison with a hydrophilic nitroxyl radical, 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempol). Both nitroxyl radical I and Tempol inhibited methyl linoleate oxidation induced by free radicals, and the efficacy increased with decreasing partial pressure of oxygen, the effect being more pronounced for nitroxyl radical I than Tempol. Their hydroxylamines inhibited lipid peroxidation more effectively than their corresponding parent nitroxyl radicals. In liposomal membranes, a synergistic effect was observed in the combination of nitroxyl radical I with ascorbic acid, whereas only an additive effect was observed between Tempol and ascorbic acid. The present study suggests that nitroxyl radical I and its hydroxylamine may act as potent antioxidants, especially in combination with ascorbic acid under hypoxic conditions.