The photolysis in polymer matrices of dyes containing a benzothioxanthene chromophore linked with a hindered amine

Novel dyes based on a benzothioxantheneimide chromophore covalently linked with a sterically hindered amine (HAS) were prepared and their light stability was tested in polymer matrices. The following dyes: 2-(2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINH) and N-alkoxy derivative 2-(1-(1′-phenylethoxy)-2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej] isoquinoline-1,3-dione (BTXINOR) were prepared. For comparison the parent dye without HAS structural unit benzothioxanthene-3,4-dicarboxylic anhydride and the N-alkyl derivative 2-(1-dodecyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXID) and the stable nitroxyl radical 2-(1-oxo-2,2,6,6-tetramethyl-4-piperidyl)-thioxantheno[2,1,9-dej]isoquinoline-1,3-dione (BTXINO) were also tested. Their spectral properties, absorption and fluorescence have been examined in polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC) and in isotactic polypropylene (PP). The light stability of these dyes and model compounds were examined in thin polymer films. The photolysis rate was monitored by UV spectroscopy and for all additives under study it was in the range 10⁻⁴–10⁻³ h⁻¹. The rate of decomposition was the lowest for the parent amine BTXINH in PMMA, PS and PVC. The rate constants of photolysis are about 10 times higher for all adducts and the lowest rate of decomposition in PP matrix was observed for BTXINOR. A distinct stabilization effect of HAS structural unit on the dye decomposition was not observed. The light stability of the dyes was more influenced by the selection of the polymer. The photolysis proceeds rather fast in PP, and moderately in PS and PVC compared to PMMA.     

Synthesis and polymerization of 4-o-vinylbenzyl-2,2,6,6-tetramethylpiperidine

Some hindered amine polymers containing 2,2,6,6-tetramethylpiperidine structure were synthesized. A new monomer, 4-O-vinylbenzyl-2,2,6,6-tetramethylpiperidine, was homopolymerized and copolymerized with some vinyl monomers in the presence of AIBN as an initiator. The obtained amine polymers were oxidized with hydrogen peroxide to obtain the polymers having nitroxyl radical moiety.     

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.     

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.     

Photostabilization of commercial polypropylene by hindered piperidine compounds: An ESR and luminescence study

The photostabilization of commercial polypropylene by a hindered piperidine stabilizer, bis [2,2,6,6-tetramethyl-4-piperidinyl] sebacate (I) and by a model N-oxy radical compound, 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy (II) is examined using ESR and luminescence spectroscopy. ESR spectroscopy shows that I operates through the formation of a stable nitroxyl radical in the polymer. On the other hand, II disappears rapidly during the early stages of photo-oxidation but continues to act as an effective stabilizer. A low steady-state equilibrium concentration of nitroxyl radicals is believed to be responsible for the high photostabilizing efficiency of I. Both compounds also inhibit the photolysis of the luminescent α,β-unsaturated impurity groups present in the polymer; possible mechanisms are discussed.     

Mechanism of autoreduction of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium cation in alkaline medium

Autoreduction of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium ion to nitroxyl radical in alkaline medium involves a number of parallel and consecutive reactions. The primary products of the reaction of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium with hydroxide ion are three nitroso compounds and N-hydroxy-2,2,6,6-tetramethylpiperidine N-oxide. Isomerization of the nitroso compounds and elimination of acetone from the N-oxide give cyclic hydroxylamines which reduce the initial cation to nitroxyl radical, being oxidized to nitrones.     

Thermal and photo-oxidative behaviour of hindered piperidine compounds in polypropylene: Importance of hydroxylamine in stabilisation

The thermal oxidation behaviour of three hindered piperidine compounds has been examined in polypropylene film by oven ageing in air at 140°C together with its subsequent effect on rate of photo-oxidation. The amine, bis[2,2,6,6-tetramethyl-4-piperidinyl] sebacate, shows a rapid growth in nitroxyl radical concentration due to reaction with hydroperoxides whereas the bis-nitroxyl derivative is relatively stable and decreases in concentration very slowly, giving the hydroxylamine. The ageing process results in an improvement in photo-stability and this is associated with the reaction of the nitroxyl radical with a tertiary radical to give the hydroxylamine and ethylenic unsaturation. The mono nitroxyl compound behaves differently during the early stages of ageing in that there is a rapid decrease in concentration and this is followed by a rapid fall in photo-stability. Only during the latter stages of ageing does hydroxylamine production appear to control photo-stability.     

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.     

The Synthesis of New Type Diazo Compounds by 4-hydroxy-2,2,6,6-Tertramethyl-1-Piperidinyloxy 1 as The Phases Transfer Dehydrogenation Catalyst

It is well known that 4-hydroxy-2,2,6,6-tertramethyl-1-piperidinyloxyl is a stable nitroxyl radical. It can be used as antioxidants¹ and spin labeled compounds to mark protein, biomembrance and nucleic acid etc. Furthermore, as an efficient inhibitor, it can prevent olefin polymerizing by free radical. However, using 4-hydroxy-2,2,6,6-tertramethyl-1-piperidinyloxyl as the phase transfer catalyst has not been reported so far.     

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.     

The formation of nitroxyl radicals in reactions of dimethyldioxirane with 2,2,6,6-tetramethylpiperidine and 2,2,5,5-tetramethyl-3-imidazoline-3-oxide derivatives

The oxidation of secondary and tertiary amines, derivatives of 2,2,6,6-tetramethylpiperidine and 2,2,5,5-tetramethyl-3-imidazoline-3-oxide, by dimethyldioxirane results in the formation of nitroxyl radicals, which induce the decomposition of dimethyldioxirane. Chemiluminescence in the IR and visible regions during the reaction of dimethyldioxirane with 2,2,6,6-tetramethyl-4-hydroxypieeridine-1-oxyl was observed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2501–2503, December, 1998.     

Mechanism of electrochemical oxidation of spatially hindered amines of 2,2,6,6-tetramethylpiperidine series

Electrooxidation of spatially hindered amines of the series of 2,2,6,6-tetramethylpiperidine in the medium of acetonitrile results in formation of radical cations, aminyl radicals, nitroxyl radicals, and oxoammonium salts identified using the methods of cyclic voltammetry and ESR spectroscopy.     

Formation of polymer-bonded nitroxyl radicals in the UV stabilization of polypropylene by a bifunctional hindered amine light stabilizer

The formation of polymer-bonded stabilizer derivatives during photooxidation of isotactic polypropylene, which contains the hindered amine light stabilizer bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate and the corresponding bisnitroxyl radical, was investigated. The photooxidized polypropylene films that contain these additives were studied by ESR, IR, and nitrogen analysis before and after exhaustive solvent extraction of the photooxidized films. ESR showed that under the conditions in use a maximum of 20% of the nitroxyl radicals formed from the hindered amine was bonded to the polymer chain. Regeneration of nitroxyl radicals from the polymer-bonded stabilizer derivatives under photooxidative conditions indicated that the stabilizer was bonded to the polymer chain by O-alkyl-substituted hydroxylamine linkages >N? O? PP.     

Electrochemical oxidation of 1-chloro(bromo)-2,2,6,6-tetramethylpiperidines

Electrochemical oxidation of 1-haloamines of the 2,2,6,6-tetramethylpiperidine series results in the formation of relatively stable radical-cations, detected by the methods of cyclic voltammetry and ESR spectroscopy. The final products of electrochemical oxidation of these haloamines are stable nitroxyl radicals.     

The rotational mobility of fulleropyrrolineoxyl in toluene solution

The rotational mobility of a stable fullerene-based nitroxyl radical (fulleropyrrolineoxyl) in toluene was studied. The rotational correlation time of fulleropyrrolineoxyl was found to be about eight times longer than that of 4-oxy-2,2,6,6-tetramethylpiperidine-1-oxyl. The activation energy of the rotational motion of both probes was determined and their hydrodynamic radii estimated.     

Mechanisms of antioxidant action: Behaviour of a hindered piperidine and related oxidation products during processing and photo-oxidation of polypropylene

Evidence is presented to show that a commercial hindered piperidine (I(A)) is not an effective melt stabiliser for polypropylene whereas related nitroxyl radicals (II(A)) and (II(B)) and hydroxylamine (III(B)) are highly effective. These results are explained on the basis of an oxidative transformation of the piperidine to the nitroxyl during processing and the involvement of the latter in a cyclical regenerative process in which the nitroxyl acts as a chain-breaking acceptor (CB-A) antioxidant and the derived hydroxylamine as a chain-breaking donor (CB-D) antioxidant.The same CB-A/CB-D cycle operates during photo-oxidation of polymer films containing each of the additives. The nitroxyl radical concentration reaches a stationary concentration in the polymer, irrespective of whether it is added as nitroxyl or as parent amine. The derived hydroxylamines are substantially more effective than the nitroxyl radicals as ultraviolet stabilisers.     

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.     

Photo-stabilisation of polypropylene by a hindered piperidine compound: Influence of hydroperoxide and carbonyl groups

The influence of prior thermally generated hydroperoxides and added benzophenone on the photo-stabilising action of a hindered piperidine compound, bis[2,2,6,6-tetramethyl-4-piperidinyl]-sebacate, in polypropylene film has been examined. The hindered piperidine compound was found to effectively inhibit both the hydroperoxide and carbonyl group sensitised photo-oxidation of the polymer. Using ESR spectroscopy, evidence is presented to show that the hindered piperidine compound reacts stoichiometrically with the hydroperoxide groups generated by thermal oxidation to give a stable nitroxyl radical. The photo-stabilising effects observed are discussed in relation to our understanding of the ultraviolet anti-oxidant action of the hindered piperidine systems.     

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.     

Reactions of stabilizer compounds. IV. ESR studies of the formation of stable nitroxyl radicals in the photooxidation of polypropylene containing hindered amine light stabilizers

ESR studies of the formation of stable nitroxyl radicals in the photooxidation of polypropylene containing 2,2,6,6-tetramethyl-4-hydroxypiperidine (II) and the commercial light stabilizer bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate (III) are described. Spectra of the stable nitroxyl radicals in solid polymer and in solution after extraction from the polymer were obtained. The extract from photooxidized polypropylene containing the bifunctional stabilizer (III) showed only the mononitroxyl radical. The possible formation of polymer-bonded nitroxyl radicals from (III) is discussed.