Mechanisms of antioxidant action: Metal complexes as u.v. stabilisers in polyethylene

In order to evaluate the importance of u.v. screening, peroxide decomposition and excited state quenching in the mechanism of action of a variety of metal complex stabilisers, screening and additive studies were carried out in high density polyethylene and in model hydrocarbons. All the metal complexes studied were found to be u.v. screening agents when contained in a separate phase between the u.v. source and the polymer and all were rather more effective than a typical commercial u.v. absorber (2-hydroxy-4-octyl-oxybenzophenone, HOBP). However, when used as additives, all the stabilisers had an additional function. In the case of the transition metal dibutyl dithiocarbamates (NiDBC and FeDBC), the additional stabilisation mechanism is consistent with their known peroxide decomposing behaviour. In the case of the 2-hydroxy-acetophenone oxime complexes (NiOx and CuOx), the initial function is a stoichiometric reaction with hydroperoxides to give non-radical products. An amine complex of nickel bis-octylphenol sulphide (NiBOPS) appeared to behave primarily as a screen. Only one of the metal complexes (FeOx) appeared to be capable of functioning as an excited state quencher and this complex is effectively sensitised by triplet carbonyl to give a photo-pro-oxidant effect in cumene. It is concluded that the concept of excited state quenching, although it may occur with some metal complexes, is not a sufficient criterion of u.v. stabilising activity.     

Mechanism of antioxidant action. Transformation involved in the antioxidant function of nickel dithiolates—2

The initial transformation products from nickel dialkyl dithiophosphates and nickel alkyl xanthates have been identified by spectrophotometric and kinetic methods. A common mechanism of action is involved for both nickel complexes and the distribution of products from complexes during their reaction with hydroperoxides suggests a greater contribution from an ionic mechanism in the case of dithiophosphate. Nickel dialkyl dithiophosphate is more stable to u.v. light than nickel alkyl xanthate and its reaction with tert. butyl hydroperoxide in the presence and absence of light is 7–8 times faster than that of the xanthate.     

Mechanisms of antioxidant action. Effect of the polymer medium on the antioxidant efficiency of the dithiolates

Peroxide decomposing antioxidants (e.g. nickel dithiophosphates and thiophosphoryl disulphides) control hydroperoxide formation during processing and on exposure to light. However, these additives are more efficient u.v. stabilizers in polypropylene (PP) than in low density polyethylene (LDPE). It is suggested that this difference results from the more rapid formation of hydroperoxides in the more oxidisable substrate under normal processing conditions. In contrast, nickel xanthates are completely destroyed in PP under the same processing conditions and the transformation products obtained in this case are less effective u.v. stabilizers than the original xanthates. Nickel dialkyl dithiophosphates stabilise both LDPE and PP very effectively, while nickel alkyl xanthates are much less effective u.v. stabilisers in both matrices. However, the difference between the efficiencies of the two dithiolates is much less in the case of LDPE. The nickel dithiophosphates and xanthates effectively synergise with the commercial u.v. absorber Cyasorb u.v. 531 (HOBP) but they show antagonism towards a typical chain breaking antioxidant, Irganox 1076, during u.v. exposure. They are however synergists under thermal oxidative conditions.     

Photostabilising action of Zn and Ni salts of 3,5-DI-t.-butyl-4-hydroxybenzoic acid in polypropylene

The photostabilising action of two metal chelates of the commercial antioxidant Shell acid (3,5-di-t.-butyl-4-hydroxybenzoic acid) viz. its zinc and nickel salts, is examined, using i.r. and derivative u.v.-visible spectroscopy and hydroperoxide analysis. The polymeric matrix was commercial polypropylene. The results show that both salts operate by inhibiting hydroperoxide formation during melt processing and thermo-oxidative conditions. The latter effect is attributed to a chain breaking donor mechanism which terminates, not alkylperoxyl, but macroalkyl radicals. Both salts are relatively efficient photostabilisers when compared with the parent antioxidant, probably because of their good photostability to both polychromatic and monochromatic radiation. The latter also shows that the nickel salt may also operate by scavenging alkoxyl and hydroxyl radicals during photo-oxidation. The addition of calcium stearate in conjunction with the salts produces some interesting effects which are explained in terms of metal exchange.     

Photo-stabilising action of metal chelates in polypropylene—Part II: Photolysis versus photo-sensitised oxidation under monochromatic irradiation

The photo-stabilising action of three metal chelates in unprocessed and processed polypropylene is examined using normal and second order derivative ultraviolet and infra-red spectroscopic techniques and hydroperoxide analysis. The effects of photolysis with 254 nm light versus photo-sensitised oxidation with 365 nm light are compared. For each exposure condition the rate of carbonyl formation in the polymer is compared with the rate of decomposition of the metal complex. On photolysis, carbonyl growth commences well before the complete destruction of the complexes and none offers protection to the polymer. In fact, all three chelates behave as photo-sensitisers, indicating that stabiliser photolysis products are photo-active. On photo-sensitised oxidation, while the initial hydroperoxide concentration appears to control the onset of carbonyl growth in the polymer, the rate of decomposition of the complexes shows no dependence on hydroperoxide concentration. Solution experiments indicate that there are no dark reactions with hydroperoxides apart from one of the nickel chelates (Cyasorb UV 1084) at high concentrations (～ 10^?2m) only. Essentially, the metal chelates operate by scavenging macroalkyl radical species (P·) and not alkoxy (PO·) and hydroxy radicals (·OH) during photo-oxidation. They also inhibit hydroperoxide formation during processing and one of the nickel chelates (UV 1084) gives products during the early stages of photo-oxidation which appear to operate as effective stabilisers.     

PHOTOCHEMISTRY OF cis-POLYISOPRENE AND ITS SINGLET OXYGEN ADDUCT

Abstract— Reaction of singlet oxygen (¹Δ_g, ¹O₂) with cis -polyisoprene yields an allylic hydroperoxide with an olefinic double bond shifted in the polymer chain. The photochemical decomposition of the resultant hydro-peroxide and the subsequent polymer chain scission kinetics have been studied in the absence of oxygen. Quantum yields of hydroperoxide decomposition range from 3.1 to 8.4 in cyclohexane, depending on the initial amount of hydroperoxide in the polymer. On the other hand, the quantum yields for polymer chain scission are low, and vary with the frequency of the incident light. The ratio for number of polymer scissions per number of hydroperoxy groups decomposed is of the order of 10^-2. The polymer chain degradation is sensitized by the addition of ketones. Based on these data, a reaction mechanism for the overall photodegradation of the cis -polyisoprene initiated by singlet oxygen is proposed.     

The role of hydroperoxides in photo-oxidative degradation of cis-1,4-polybutadiene

Hydroperoxides undergo various types of homolytic reactions on exposure to u.v. radiation. Free radicals formed from the photodecomposition of the hydroperoxide group (OOH) are oxy (HO.) and peroxy (HOO.) radicals which participate in further reactions. In cis-1,4-polybutadiene, they may initiate free radical oxidations. Cleavage of alkoxy (RO.) radicals and crosslinking of polymer radicals through polymer peroxides in the presence of air in solid film nearly balance. Most polymer radicals produced in the absence of oxygen undergo cross-linking but form peroxy radicals (POO.) in its presence. This paper presents results on the photodecomposition of tert-butyl hydroperoxide, cumyl hydroperoxide and 2,5-dimethyl-2,5-dihydroperoxyhexane in cis-1,4-polybutadiene in film and in solution.     

Protection of poly(2,6-dimethyl-1,4-phenylene oxide) against u.v. irradiation

The photo-oxidative degradation and stabilization of poly(2,6-dimethyl-1,4-phenylene oxide) films have been examined in the temperature range 263–313 K in air with u.v. light of 253.7 nm. The changes in weight-average molecular weight, quantum efficiencies, carbonyl index, hydroperoxide concentration and i.r. spectra have been followed in the absence and presence of the stabilizer. The heats of activation of the systems have been calculated and the mechanism of stabilization has been postulated. A saturation limit in photostabilization of the polymer was achieved beyond 0.6 wt% of zinc di-n-hexyl dithiophosphate.     

Photostabilization of polypropylene. II. Stabilizers and hydroperoxides

The effects of various polyolefin photostabilizers on the thermal and photodecomposition of model and polymeric hydroperoxides have been investigated, both in the liquid and solid phases. Several extremely effective ultraviolet stabilizers belonging to the metal chelate class can cause the rapid thermal decomposition of model and polymeric hydroperoxides. Although stabilizers did not reduce the quantum yield for polypropylene hydroperoxide photolysis, several additives can scavenge hydroxyl and macroalkoxy radical products which result from hydroperoxide photolytic cleavage. Ultraviolet stabilizers which were found to trap radicals were able to prevent the photodegradation of polypropylene which already contained a significant concentration of hydroperoxide groups. Highly effective polypropylene ultraviolet stabilizers probably operate by a range of mechanisms including hydroperoxide decomposition, radical scavenging and singlet oxygen quenching.     

The effects of UV radiation on anodic wave of human serum

Polarographic anodic oxidation wave that can be correlated to total antioxidant capacity (TAC) in Human serum corresponds to 2H+ and 2e process. The limiting current of the wave, which is proportional to TAC, is strongly influenced by UV light intensity and irradiation time. A mechanism has been proposed to explain the effects of UV on anodic wave related to antioxidant behavior. Results show that decomposition processes follow to the excitation a transitory product at three different light intensities at 340 nm. The number of decomposition products increase with increasing light intensity. The rate of oxygen radical capture ability of the serum antioxidants might be estimated by comparing the changes of anodic wave in the absence and presence of naturally dissolved oxygen.     

Inhibitive effects of transition metal thiopicoline anilides on the photo-oxidative degradation of poly(2,6-dimethyl-1, 4-phenylene oxide) films

The changes of physical, chemical and mechanical properties of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) films under u.v. light were investigated. The inhibitive effects of transition metal (Zn, Cr) thiopicoline anilides on the photo-oxidative degradation, both in the presence and absence of copper stearate as oxidation accelerator, were examined. In both cases Zn(II) and Cr(III) thiopicoline anilides showed a pronounced inhibitive effect on the degradation of the polymer, especially the Zn compound. The inhibitive action of these additives is discussed from the viewpoint of molecular weight, elongation, gel, carbonyl and hydroperoxide content of the polymer.     

Photo-oxidation of polymers—VII: A re-investigation of the photo-oxidation of polystyrene based on a model compound study

Fluorescence quenching experiments indicate that energy transfer occurs from cumene excited at 254 nm to cumene hydroperoxide. Quantum yields show that the sensitized decomposition of the hydroperoxide occurs quantitatively and that 2-phenylpropanol-2 is the main photoproduct. In the presence of oxygen, this process plays a dominant role in the initiation of the photo-oxidation. When benzophenone is excited to the first triplet state by irradiation at 365 nm in the presence of cumene hydroperoxide, phosphorescence quenching experiments and laser flash photolysis suggest that an exciplex is formed. This exciplex dissociates into cumylperoxy and ketyl radicals in such a way that 80% of the excited ketone molecules are transformed into the corresponding pinacol. In the presence of oxygen, benzophenone primarily initiates the photo-oxidation of cumene by hydrogen abstraction but, as cumene hydroperoxide is formed, formation and reaction of the exciplex become progressively more and more important. The photochemical behaviour a fluorenone is quite different from that of benzophenone. The sensitized decomposition of cumene hydroperoxide occurs in the presence of that ketone. Surprisingly, fluorenone also initiates the photo-oxidation of cumene; the mechanism of that reaction is discussed. The whole set of results provides a sound basis for the interpretation of the photo-oxidation of polystyrene in various conditions.     

Synthesis of poly(arylene ethylene) oligomeric hydroperoxides and their use as initiating agents of radical polymerization

The oxidation to hydroperoxide of poly(arylene ethylenes) (PAE) by oxygen carried out in solutions at 80–110°C. The effect of initiating additions and the nature of solvent relative to the content of hydroperoxide groups in oxidized PAE were investigated. The oxidation to hydroperoxides in PAE occurs at the methylene groups, and the synthesized hydroperoxides are secondary peroxides. The decomposition of PAE hydroperoxides in toluene and chlorobenzene at concentrations of 0.006–0.03 mole/l. for hydroperoxide in the presence and absence of N-phenyl-α-naphthylamine (PNA) was studied. The decomposition of one hydroperoxide has been studied in the presence of cobaltous and manganese resinates and of PNA in chlorobenzene at 30–50°C. The addition of PNA to a chlorobenzene solution of PAE hydroperoxide containing cobaltous or manganese resinate accelerates the hydroperoxide decomposition, reduces the activation energy, and changes the reaction order from the second-order to first-order. The synthesized hydroperoxides initiate the radical polymerization of styrene and methyl methacrylate. The initiating activity of one of the synthesized hydroperoxides of PAE for polymerization of styrene (60°C) in the presence and absence of activating addition of manganese resinate was also evaluated.     

Diethyl-2,2′-bipyridyl-palladium(II), a case for the study of combination vs. disproportionation products.

Diethyl-2,2′-bipyridyl-palladium(II) is synthesized and its decomposition reactions in absence and presence of methyl acrylate are compared with those of the corresponding nickel and platinum complexes. Depending on the reaction conditions either CC-coupling or disproportionation can be induced.     

Decomposition of polypropylene hydroperoxide by hindered amines

The hindered amine-induced decomposition of polypropylene hydroperoxide was studied in the solid state and in the presence of a liquid solvent and the polymer was compared with model hydroperoxides. The high reactivity of the macrohydroperoxides appears to be related to the adjacent, hydrogen-bonded hydroperoxide groups that occur in the polymer. The hindered amines are converted to nitroxides in the reaction via hydroxylamine intermediates. Amine-induced decomposition of polypropylene hydroperoxide is faster in the absence of a liquid solvent for the amine than in the presence of the solvent, probably because of the strong amine-hydroperoxide association that occurs in the solid state. The decomposition process in the solid state is sufficiently rapid for the reaction to contribute to the effectiveness of hindered amines in the light stabilization of polymers.     

The effects of thermal processing on PVC—IX. A polymer reactive u.v. stabilizer as a thermal stabilizer for PVC

It is shown that a thiol u.v. stabilizer (4-ethoxymercaptoaceto-2-hydroxybenzophenone, EBHPT) becomes efficiently bound to PVC during normal processing at 170° in the presence of dibutyltin maleate (DBTM). This combination is an effective melt stabilizer and shows synergism as a heat stabilizer at 140°. This is shown to be due to the protection of DBTM by the PD antioxidant function in EBHPT and a synergistic optimum is observed. In the absence of DBTM or in the presence of a thiotin stabilizer binding of the u.v. stabilizer is incomplete.     

Diffusion mechanism of the synergism of u.v.-absorbers and antioxidants

The mechanism of synergism in mixtures of u.v.-absorbers with antioxidants is considered. The synergism is attributed to diffusion of the antioxidant from the deeper-lying layers of polymer film, protected from the action of light by a u.v.-absorber, towards surface layers where the photoreaction takes place. The effect of the antioxidant diffusion process on the duration of the induction period of the unbranched chain oxidation reaction of the polymer is analysed. The experimental investigation of the polybutadiene photo-oxidation process has verified the diffusion mechanism so allowing finding of the optimum relationship between the concentrations of u.v.-absorber and antioxidant and explanation of the observed enhancement of the effect of synergism in thicker polymeric films, for higher antioxidant diffusion coefficients and lower light intensities.     

The Dual Function of PhOH Included in the Coordination Sphere of the Nickel Complexes in the Processes of Oxidation with Dioxygen

The role of ligands in the regulation of the catalytic activity of Ni-complexes (Ni(acac)₂) in green process-selective ethylbenzene oxidation with O₂ into α-phenyl ethyl hydroperoxide is considered in this article. The dual function of phenol (PhOH) included in the coordination sphere of the nickel complex as an antioxidant or catalyst depends on the ligand environment of the metal. The role of intermolecular H-bonds and supramolecular structures (AFM method) in the mechanisms of selective catalysis by nickel complexes in chemical and biological oxidation reactions is analyzed.     

Nickel nitrosyl complexes with the diphenylphosphinoethane ligand

Summary Four-coordinate nickel nitrosyl complexes of the general formula Ni(NO)X(Dppe) (Dppe=Ph₂PCH₂CH₂PPh₂) have been prepared byin situ formation of Ni(NO₂)X(Dppe), (X= Cl, Br, I or SCN) followed by reduction with triphenylphosphine, or carbon monoxide, and/or DMF. Oxygenation of the nitrosyl complexes gives the corresponding nitro products and as indicated by u.v.-vis spectroscopy involves formation of an intermediate. The oxygenation rate increases markedly in the presence of light or of a catalytic amount of benzoyl peroxide and a tentative explanation is offered for these observations. Ionic adducts are formed in reactions between the nitrosyl complexes and donor molecules.Paper presented in part at the XXth ICCC Conference.     

Singlet oxygen and polymer photooxidations. II. Photodegradation of an olefinically unsaturated polymer

A method based on infrared spectral changes in thin films was devised and used to evaluate the relative effectiveness of various types of additives on photodegradative processes in an olefinically unsaturated polymer. Nickel chelates of the thiobisphenol-amine complex type are shown to be quantitatively more effective in retarding the photodegradation of these polymers than other additives having greater singlet oxygen quenching efficiencies, ultraviolet absorption, or radical antioxidant properties. Photosensitization with a singlet oxygen sensitizer (fluoranthene) rapidly produces degradative effects in the polymer which are identical with those of its direct degradation by light. The sensitized effects are also retarded by the nickel chelates. Photosensitization by a hydrogen atom-abstracting sensitizer (a benzophenone) proved considerably less effective. The effectiveness of the nickel complexes appears to be due to a combination of their ability to quench singlet oxygen and precusor excited states and their relative stability under photoexposure conditions.