Synthesis, photochemical stability and photo-stabilizing efficiency of probes based on benzothioxanthene chromophore and Hindered Amine Stabilizer

New types of combined chromophore-HAS (Hindered Amine Stabilizer) fluorescence probes were prepared and their photo-stability as well as their photo-stabilizing efficiency was tested in polypropylene film. The chromophore was benzothioxanthene and HAS was 2,2,6,6-tetramethylpiperidine. They were prepared by a one-step synthesis from benzothioxanthene-3,4-dicarboxylic anhydride (BTXA) by condensation with 4-amino-2,2,6,6-tetramethylpiperidine to obtain the probe in the form of parent amine (BTXI-NH) and 4-amino-1-oxy-2,2,6,6-tetramethylpiperidine in the case of stable nitroxyl radical form (BTXI-NO). Kinetics of BTXA photo-decomposition followed by UV spectroscopy were similar to kinetics of photo-decomposition for derivatives with covalently linked HAS i.e. BTXI-NH and BTXI-NO. It consists of two parts, slower in the beginning and very fast at the end of the process. Better stability of BTXI-NO in comparison with BTXI-NH can be explained by the presence of stable nitroxyl radical in its structure. The rate of decomposition of the fast part is similar for both additives. As soon as the decomposition starts there is no influence of NO or NH on this process. Times represented the start of the fast decomposition of probes correspond to the induction period of photo-oxidation of PP films. Possible effect of mutual interaction of chromophore and HAS coupled in one molecule (BTXI-NH or BTXI-NO) during photo-oxidation was followed by comparison with the course of photo-oxidation of PP film containing physical mixture of chromophore in the form of BTXA and HAS in the form of parent amine (4-hydroxy-2,2,6,6-tetramethylpiperidine) and stable nitroxyl radical (4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl).     

Photostabilizing activity of N-acylated hindered amine

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 N? H 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.     

Photostabilizing activity of tertiary hindered amines

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.     

Electrochemical modeling of the reaction of 1-Chloro- and 1-Bromo-2,2,6,6-tetramethylpiperidine photolysis

Electrolysis of 1-Chloro- and 1-Bromo-2,2,6,6-tetramethylpiperidines yields free nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl. The reaction mechanism is suggested, which is based on the intermediate formation of aminyl radical. Concurrently with the nitroxyl radical formation, electrochemical chlorination of 2,2,6,6-tetramethylpiperidine occurs. It is shown that the 2,2,6,6-tetramethylpiperidine can be used as a mediator in the electrochemical oxidation of alcohols.     

Polypropylene photo-stabilization by hindered secondary amines. A spectroscopic investigation

The conversion products from a commercially important photo-stabilizer based on 2,2,6,6-tetramethylpiperidine have been identified and quantified during the photo-oxidation of polypropylene. From a comparison with model compounds and model systems, direct evidence for the dominant involvement of grafted substituted hydroxylamine species was obtained by infra-red spectroscopy. Non-stabilizing conversion products, including N-acyloxy compounds, were also identified and the overall mechanism of photo-protection by hindered amine stabilizers is discussed.     

Electrochemical conversions of triacetoneamine

The studies of electrosyntheses of 2,2,6,6-tetramethylpiperidine derivatives, including nitroxyl radicals, are generalized. The reactions of reduction, reductive amination, and halogenation are discussed. Special attention is paid to electrosyntheses with nitroxyl radicals of 2,2,6,6-tetramethylpiperidine and 2,2,5,5-tetramethylpyrroline as the initial reagents.     

Synthesis and characterization of star-shaped polytetrahydrofuran

Star polymers of tetrahydrofuran (polyTHF) were prepared by reaction of living polyTHF with diethylene triamine (DETA) in the presence of 2,2,6,6-tetramethylpiperidine (TMP) as proton trap. The influence of several reaction parameters on the end-capping reaction was studied in order to produce stars having varying number of arms. With a starting living polyTHF with a molecular weight of 1600, star-shaped polymers containing up to seven polymer chains could be prepared. When the molecular weight of the living polyTHF was 8000, at most five polymer chains could be grafted on the DETA. Some physical properties of the star polymers were compared with those of their linear counterparts of the same molecular weight.     

Mechanism of electrochemical oxidation of 1-chloro-2,2,6,6-tetramethylpiperidine

In contrast to 2,2,6,6-tetramethylpiperidine and other aliphatic amines, at the electrochemical oxidation of 1-chloro-2,2,6,6-tetramethylpiperidine a sufficiently stable cation-radical is formed. Its formation is confirmed by the data of cyclic voltammetry and electron paramagnetic resonance. Further transformation of the cation-radical leads to the formation of 2,2,6,6-tetramethylpiperidin-1-oxyl.

     

Hypervalent iodine oxidation. Synthesis of spin labeled, 1-oxyl-2,2,6,6-tetramethylpiperidine derivatives

Oxidation of 1-oxyl-2,2,6,6-tetramethyl-4-piperidone ( 1 ) with [I,I-bis(acetoxy)iodo]benzene in methanolic potassium hydroxide yields 1-oxyl-4,4-dimethoxy-3-hydroxy-2,2,6,6-tetramethylpiperidine ( 2 ). Treatment of 2,2,6,6-tetramethyl-4-piperidone ( 3 ) with [I,I-bis(acetoxy)iodo]benzene in methanolic potassium hydroxide gave 3-methoxy-2,2,6,6-tetramethyl-4-piperidone ( 4 ) which on oxidation with 30% hydrogen peroxide and catalytic amount of sodium tungstate gave 1-oxyl-3-methoxy-2,2,6,6-tetramethyl-4-piperidone ( 5 ). The esr spectra of 1-oxyl-4,4-dimethoxy-3-hydroxy-2,2,6,6-tetramethylpiperidine ( 2 ) as well as 1-oxyl-3-methoxy-2,2,6,6-tetramethyl-4-piperidone ( 5 ) show three lines.     

Synthesis and properties of 3,5-dibromo-4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl radical

Bromination of 1-hydroxy-4-oxo-2,2,6,6-tetramethylpiperidine gives 3,5-dibromo-1-hydroxy-4-oxo-2,2,6,6-tetramethylpiperidine hydrobromide. Oxidation of the latter generates 3,5-dibromo-4-oxo-2,2,6,6-tetramethylpiperdine-1-oxyl radical, which represents a convenient acylating spin trap.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 73–74, January, 1992.     

Electrochemical synthesis of 2,2,6,6-tetramethylpiperidine

A preparative method for the production of 2,2,6,6-tetramethylpiperidine based on the electrochemical reduction of 4-oxo-2,2,6,6-tetramethylpiperidine in 30% sulfuric acid on cadmium or lead electrodes was developed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 358–359, March, 1984.     

Oxidation of hydroxylamines to nitroxyl radicals with FREMY's salt : Nitroxyl aldehydes

Derivatives of 1-hydroxy-2,2,6,6-tetramethylpiperidine were oxidized by potassium nitrosodisulfonate (FREMY's salt) to 2,2,6,6-tetramethylpiperidine-1-oxyls. A triphenylphosphine-alkylene reaction, a Grignard reaction, and esterification resulted in derivatives with aldehyde groups.     

Synthesis of 2,2,6,6-Tetramethylpiperidine Derivatives

Diazotization of 4-amino-2,2,6,6-tetramethylpiperidine in acetic or sulfuric acid affords 2,2,6,6- tetramethyl-1,2,3,6-tetrahydropyridine in high yield. Under the same conditions, the corresponding nitroxyl radical transforms into 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl.     

The inhibited photo-oxidation of polystyrene and styrene/HALS copolymers

The influence of molecular weight and the mode of binding of HALS stabilisers on their efficiency in the inhibited photo-oxidation of polystyrene and styrene/HALS copolymers was investigated. Photo-oxidation was performed with radiation of λ > 310 nm and 366 nm. The following functionalised HALS stabilisers were prepared: 4-methacryloylamino-2,2,6,6-tetramethylpiperidine, 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine, 4-methacryloyloxy-1,2,2,6,6-pentamethylpiperidine and 15-methacryloyl-7,15-diazadispiro [5.1,5.3] hexadecane-14,16-dione. Binding by copolymerisation is substantially less efficient than addition of low molecular weight stabiliser.     

Electron spin resonance studies on the oxidation mechanism of sterically hindered cyclic amines in TiO2 photocatalytic systems

A sterically hindered cyclic amine, 4-hydroxy-2,2,6,6-tetramethylpiperidine (HTMP), is converted to the corresponding aminoxyl radical (nitroxide radical), 4-hydroxy-2,2,6,6-tetramethyl piperidine 1-oxyl (TEMPOL radical) as a result of a photocatalytic reaction in TiO2 aqueous suspension. The time profile of the radical formation and the effect of additives, such as SCN-, I-, methanol, and H2O2, on the initial formation rate were measured in order to elucidate the reaction mechanism. The experimental observations indicated that the direct photocatalytic oxidation of HTMP followed by reaction with O2 is the dominant process in the formation of TEMPOL radicals. Electrochemical measurements showed that HTMP is oxidized at 0.7 V (vs NHE), which is consistent with the proposed mechanism. The possibility of other processes, involving reactions with singlet molecular oxygen, superoxide radical, and hydroxyl radical, were excluded from the reaction mechanism.     

Polyolefin photo-stabilisation mechanisms. Reactions of tetramethylpiperidine derivatives in model systems

The relative rate of cyclohexyl radical scavenging by oxygen or 4-oxo-2,2,6,6-tetramethylpiperidino-N-oxyl has been measured at 25°C, together with the relative rates of cyclohexyl peroxy radical attack on cyclohexane or substituted hydroxylamines derived from 2,2,6,6-tetramethylpiperidine. These competitive processes are important in the stabilisation of polymers against photo-oxidative destruction by piperidine derivatives. Peroxy radical scavenging by the substituted hydroxylamine appears to be considerably more important than alkyl radical scavenging by the nitroxide, although both processes are essential for prolonged ultra violet (uv) stabilisation of a polymer. However, a comparison of experimental photo-protection with that predicted by the measured rate constant ratios shows that other processes are needed to account entirely for the observed stabilisation. Other factors which may be involved in piperidine photo-protection of polymers include thermal decomposition of the substituted hydroxylamine to reform nitroxide in polar(oxidised) zones and especially the association of nitroxides with hydroperoxide groups (the dominant photo-initiator in degrading polyolefins).     

Functionalization of sterically hindered catechol and <Emphasis Type="Italic">o</Emphasis>-benzoquinone with 2,2,6,6-tetramethylpiperidine 1-oxyl

Sterically hindered 4,6-di-tert-butyl-3-formylcatechol and 3,6-di-tert-butyl-o-benzoquinone react with 4-amino-2,2,6,6-tetramethylpiperidine 1-oxyl to give new chelate ligands of the o-quinone type bearing a 2,2,6,6-tetramethylpiperidine 1-oxyl neutral radical moiety. Structures of the synthesized compounds were established by ESR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray diffraction analysis.     

Nucleophilic Addition of Highly Hindered Amines to Electron-Deficient Acetylenes

Even the most highly hindered amines such as diisopropylamine, isopropyl-t-butylamine, 2,2,6,6-tetramethylpiperidine, 4-oxo-2,2,6,6-tetramethyl piperidine and hexamethyldisilazane react with electron-deficient alkynes to give the Michael addition products.     

Photo-stability of yellow-green emitting 1,8-naphthalimides containing built-in s-triazine UV absorber and HALS fragments and their acrylonitrile copolymers

Two novel 1,8-naphthalimide dyes, containing active fragments of both 2-(2-hydroxyphenyl)-1,3,5-triazine UV absorber and 2,2,6,6-tetramethylpiperidine radical scavenger as well as a polymerisable allyl group, were designed as multifunctional yellow-green emitting fluorophores capable simultaneously of chemically fluorescent dyeing and photo-stabilisation of polymers. Their basic photo-physical characteristics have been determined and are discussed. It was shown that combination of different structural units in the 1,8-naphthalimide molecule does not result in their interaction through intramolecular fluorescence quenching due to an electronic energy transfer. The ability of the combined dyes to copolymerise with acrylonitrile was demonstrated as polyacrylonitriles stable to solvents and with an intense colour and fluorescence were obtained. Photo-degradation of the new fluorophores and their influence on the photo-stability of the coloured copolymers have been studied and compared to other similar fluorescent dyes, not containing either UV absorber or hindered amine fragment in their molecules as well as not containing both of them. Novel fluorophores showed the best photo-stability in both solution and polymer. A significant photo-stabilising effect towards photo-destruction of polyacrylonitrile was found, which might be caused by a possible “synergism” of two stabiliser fragments differing in their action.     

Synthesis, structure, and biological activity of the cis-[4-amino-2,2,6,6-tetramethylpiperidine-N,N′]dichloropalladium(ii) complex

A previously unknown palladium complex was synthesized by the reaction of 4-amino-2,2,6,6-tetramethylpiperidine with PdCl₂. The X-ray diffraction study showed that the complex formed by 4-amino-2,2,6,6-tetramethylpiperidine with PdCl₂ is a chelate, in which the bidentate ligand adopts a boat conformation with the nitrogen atoms occupying cis positions at the palladium atom. The resulting cis-[4-amino-2,2,6,6-tetramethylpiperidine-N,N′]dichloro-palladium(ii) complex exhibits strong antimetastatic activity against experimental B16 melanoma at moderate toxicity.