Studies on the photooxidation mechanism of polymers. II. The role of quinones as sensitizers in the photooxidative degradation of polystyrene

During the quinone-sensitized photooxidative degradation of polystyrene film and its solution in benzene, an initial rapid decrease of average molecular weight has been observed by GPC and viscosity measurements. The reaction rates are strongly increased by quinones such as p-quinone, duroquinone, anthraquinone, and chloranil. It has been suggested that this photosensitized degradation of polystyrene occurs by a singlet oxygen reaction which might be related to an energy transfer mechanism from excited triplet states of quinones to molecular oxygen. The photooxidative degradation of polystyrene in solution can be diminished by addition of typical singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene.     

Studies on the photooxidation mechanism of polymers. V. Oxidation of polybutadienes by singlet oxygen from microwave discharge and in dye-photosensitized reactions

The singlet oxygen oxidation of cis- and trans- 1,4-polybutadienes was studied by using singlet oxygen generated in a microwave generator and in dye-photosensitized reactions of these polymers in the solid state and in solution. It was shown that the reactions of singlet oxygen result in formation of hydroperoxide groups, whereas ultraviolet oxidation by molecular oxygen in addition leads to formation of carbonyl groups. During dye-photosensitized oxidation of polydienes in benzene solution, a very rapid decrease in the molecular weight was observed.     

Studies of the photooxidation mechanism of polymers. III. Role of tetrahydrofuran in the photooxidative degradation of poly(vinyl chloride)

The influence of tetrahydrofuran (THF) on photooxidative degradation of poly(vinyl chloride) in films cast from THF solution was studied. THF is partially retained in the polymer matrix in amounts of 6–8% after casting and drying the film. The last 2–3% is very difficult to remove. By use of thermogravimetric analysis, density measurements, and gas permeability measurements, it was shown that THF residues can be removed by preheating the PVC samples to 80°C. THF forms a charge-transfer complex with oxygen which is easily photolyzed. During this reaction hydroperoxide radicals are formed. Molecular weight distribution curves by gel-permeation chromatography (GPC) show that THF in the presence of air promotes the photodegradation of PVC. Attention has been given to the correct interpretation of the infrared absorption spectra of PVC films containing THF residues and ultraviolet-irradiated in air.     

光氧化反应的研究VII. 赤霉酸甲酯的单线态氧反应

本文初次探讨赤霉酸甲酯的敏化光氧化反应规律及其在合成应用上的可能性。     

Photodecomposition of polystyrene on long-wave ultraviolet irradiation: A possible mechanism of initiation of photooxidation

The long-wave (λ < 3000 Å) photo-oxidation of polystyrene in solution at 25°C has been studied osmometrically. Two types of chain scission have been observed: a purely photo process which occurs completely independently of oxygen and which is attributed to fission of photolabile groups in the polymer, and another process associated with random photolyses of the products of oxidation Scavenger experiments with ¹³¹I₂ have shown that approximately two iodine atoms are incorporated per chain scission when photolysis is carried out in solution (benzene, hexafluorobenzene, methylene chloride) under high vacuum conditions in the presence of ¹³¹I₂. No iodine incorporation or chain scission occurs when ionically prepared polystyrenes are treated similarly. The nature of the photolabile bond has been discussed, and there is some evidence for a peroxidic linkage arising from oxygen copolymerization in the chains. It is suggested that fission of the photolabile groups contributes to the initiation of the long-wave photooxidation of the polymer.     

Studies on the photooxidation mechanism of polymers. IV. Effect of ultraviolet light (2537 Å) on solid PVC particles suspended in different liquids

The photolysis of virgin PVC powder suspended in water, methanol, n-hexane, aqueous NH₄OH (30 wt-%), and 0.1 wt-% iodine in methanol and also as dry powder was studied. The mechanism of photolysis of PVC powder has been investigated by using ESR spectroscopy, conductivity titration, gel-permeation chromatography (GPC), and absorption spectroscopy. Photolysis of PVC has been found to occur by a free-radical mechanism. ESR spectroscopy permits a partial identification of several different types of free radicals in PVC such as alkyl, polyenyl, and peroxy radicals. An interpretation is proposed of the mechanism of formation of conjugated polyene structures, and also a new explanation of the crosslinking mechanism, in which transfer of unpaired electrons to double bonds occurs, is suggested. It has also been found that conjugated double bonds can photosensitize free-radical formation as a result of increased ultraviolet absorption due to polyene structures.     

Singlet oxygen and photooxidation of polymers

The rates of accumulation of oxygen-containing species in photooxidation have been determined for a series of polymers such as polystyrene, polyisoprene, and polybutadiene in a wide range of singlet oxygen stationary concentrations in the polymers. An increase in singlet oxygen (¹O₂) concentration was achieved by introducing a dye which is a ¹O₂ donor into a polymer and by irradiating a sample with an additional source of light absorbed by the dye only. To decrease ¹O₂ concentration, 1,2,5-trimethyl-4-hydroxyphenylpiperidine, which is a quencher of ¹O₂, was introduced into a sample. The ¹O₂ stationary concentration in an oxidized polymer was measured via singlet oxygen oxidation of 2,2,6,6-tetramethyl-4-oxypiperidine which leads to the formation of the corresponding nitroxyl radical. The photooxidation rate has been found to vary only slightly with ¹O₂ concentration, even when the concentration changes 10–25-fold. Thus, ¹O₂ does not participate appreciably in the process of polymer photooxidation. It may be due to the lowering of ¹O₂ reactivity toward unsaturated groups in the polymer matrix as compared with that in solution.     

Effect of photooxidation of polystyrene on its dielectric properties. II. Use of dielectric loss measurements to study photooxidation

Dielectric loss measurements have been applied to study the short-wave (λ = 254 nm) and long-wave (λ > 300 nm) photooxidation of polystyrene which had been prepared using both free radical and anionic initiator. The high-frequency loss peak, attributable to carbonyl products of oxidation is a sensitive monitor of the rate of oxidation, the sensitivity being greater than that of manometric measurements. The data are specific to carbonyl compounds and the technique is particularly suitable for studying the surfaces, on which most of the above photooxidation occurs with polystyrene. Effects of radiation intensity and oxygen pressure were also studied. The measurements have permitted a more detailed study of the long-wave photooxidation process than has previously been undertaken, and the results emphasize differential reactivities of radically and anionically prepared polymers. Results are interpreted in terms of initiation by both impurities and charge-transfer complexes.     

Dielectric studies of photodegradation and photooxidation of polystyrene

The effects of vacuum photolysis at 254 nm and the short-(λ = 254 nm) and long-wave (λ > 300 nm) photooxidations on the dielectric constants (?′) and dielectric losses (?″) of polystyrene have been investigated at 25 ± 1°C. Dielectric constants generally increase on photodegradation, but more pronounced increases occur in the low-frequency region on vacuum photolysis. It is suggested that such increases are associated with Maxwell-Wagner-Sillars polarization. Increases in dielectric losses are observed in three main frequency regions: around 10² Hz, 10⁴ Hz, and at 3 × 10⁶ Hz. The lowest frequency loss which occurs in both the vacuum-irradiated and in the photooxidized samples is attributed to a combination of the effects of interfacial polarization and of the increased direct-current conductivity of the polymer which occurs as a result of these reactions. The 10⁴ Hz dispersion associated only with photooxidation is related to the presence of small, volatile, polar oxidation products, like ketones. The loss peak overlaps the intrinsic γ-relaxation of polystyrene, and it appears that the motions of small molecules, or of relaxing dipoles in these, are coupled to the phenyl group rotational vibrations. The high-frequency losses (3 × 10⁶ Hz) are ascribed to orientation polarization of carbonyl dipoles attached to chain ends, these compounds being produced as a result of hydroperoxide decompositions. A good correlation between the carbonyl dipole relaxation strength and carbonyl concentration is observed. It would appear that relaxation strength measurements could provide quantitative kinetic information for polystyrene oxidation.     

Energy-consuming micromechanisms in the fracture of glassy polymers. I. Chain scission in polystyrene

The number of chain scissions per unit area that occur during the fracture of partially annealed latex films from M_n ? 180,000 g/mol polystyrene particles of about 275 Å radius were measured and correlated to annealing times. A curve with four regimes was found. At short annealing times the curve is nearly flat, in what is called the chain pull-out regime. In the second regime, the number of chains broken per unit area increases with a 0.8 power of annealing time as entanglement of the diffusing polymer chains increases in neighboring host particles. This is in good agreement with Wool's theory which predicts a 0.75 power dependence. Then, after reaching a peak, the number of scissions decreases in the third regime, indicating a change in fracture mechanism. The number of chain scissions increases again in the fourth regime, as final healing of the film interface takes place. Fracture surface analysis reveals a rough surface for short annealing times and a smooth surface for longer annealing times. The number of polymer chain scissions per unit area of fracture surface showed no dependence on initial molecular weights for t ? τ_r where t and τ_r are annealing and relaxation times, respectively. The number of chain bridges crossing a unit area of interface was suggested as the basic molecular property. © 1992 John Wiley & Sons, Inc.     

竹红菌甲素光敏氧化吲哚类化合物的研究 I: 关于色氨酸光敏氧化机制的初步探讨

本文利用吸收光谱、荧光光谱和闪光光解研究了竹红菌甲素和色氨酸的相互作用.通过猝灭实验、氘代效应和溶剂效应探讨了竹红菌甲素敏化的色氨酸光氧化机制, 证明该反应是以单重态氧过程为主, 并辅以电子转移的机制. 另外, 考察了反应条件对反应的影响, 发现底物浓度、溶解氧浓度、溶剂的性质和溶液的PH值均对反应有很大的影响.     

Chain and photochain mechanisms of photooxidation of polymers

The mechanisms of photooxidation of the popular commercial polymers polystyrene (PS), polyethylene (PE), and an ethylene-carbon monoxide copolymer (polyketone, PK) differing in the polymer chain structure and the nature and concentration of chromophore groups are considered. In the case of the formation of photosensitive intermediates in polystyrene, taken as an example, the photochain oxidation mechanism was revealed and thoroughly studied, according to which the polymer “burns” into complete oxidation products (CO₂, H₂O) with a degree of conversion of ≥50% and a kinetic-chain length of l = 10³–10⁴ units. The hydroperoxide mechanism plays a minor role in the photooxidation of PS, it is a short-chain process (as in the case of thermal oxidation, l ∼ 10) and does not exceed 1.5% of the total amount of absorbed oxygen. Carbonyl groups, as weak photoinitiators, induce in PE and PK the conventional radical chain mechanism of photooxidation with degenerate branching of kinetic chains on hydroperoxide groups and other oxidations products.     

Studies of orientation and structure of crazed matter in polystyrene. I. Optical measurements

The optical birefringence and refractive index have been measured for crazes grown in specimens of varying thickness. The birefringence was found to consist of the sum of a small negative orientation term and a large positive form term. The latter term could be altered by filling the craze with liquids of various different refractive indexes. Two crazes, which showed a relatively constant width as the specimen thickness changed, could be described by a model with craze having a constant refractive index and birefringence but with impervious dense skins on either side. The numerical value of the form birefringence was approximately 0.6 of that predicted from a model of parallel rods which is not surprising as crazes have a networklike structure. The values of craze refractive index were in good agreement with other measurements. The existence of skins, of thickness approximately 300 nm, places some doubt on the relevance of thin-film electron microscope observations to the situation in the bulk. Other crazes which tapered in width were found to show both varying skin thickness and refractive index along their length.     

Antioxidants and stabilizers—LXXV. Effect of 2,6-ditert.-butyl-4-methylphenol on the photooxidation of high-impact polystyrene

High impact polystyrene (HIPS) was used in an investigation of the stabilizing effect of 2,6-ditert.-butyl-4-methylphenol (I, BHT) in photooxidation. Changes in the polymer due to photooxidation were examined by reflexion spectroscopy in the u.v. and visible ranges, and by i.r. spectroscopy. It was shown that BHT, 3,5,3′,5′-tetratert.-butylstilbenequinone (II) and 2,6-ditert.-butyl-1,4-benzo-quinone (III), which are important products of the transformation of BHT, possess specific stabilizing properties. Using DSC, it was demonstrated that I-III interfere with the photooxidative process mainly because of their antioxidative properties. In the overall mechanism of action of I in the stabilization of HIPS, the predominant role is played by its major transformation product i.e. quinone II.     

The mechanism of photooxidation of the menaquinones

Irradiation of menaquinone-1 under nitrogen gives rise to metastable isomeric o-quinone methides, one of which reacts with oxygen in the dark to form the same hydroperoxide obtained previously from direct or dye sensitized photolysis of the quinone in the presence of oxygen.     

Studies on photooxidation (VIII)——Electron transfer photooxygenation mechanism of acenaphthenone as electron donor

The photooxidation and its electron transfer (ET) mechanism of acenaphthenone (ANO) sensitized by 9,10-dicyanoanthracence (DCA) are investigated. It has been found that the reaction with a stepwise manner led to the formation of 1,8- (3'-hydroxy)-β-naphthalene lactone and 1,8-naphthalenedicarboxylic anhydride. By cyclic voltammetry, fluorescence quenching, exciplex emission, co-sensitbation of biphenyl/DCA as well as CIDNP studies, it is verified that ANO can behave as an electron donor to undergo ET reaction with singlet DCA which is a thermodynamically-favored process.     

Electrochemical Studies on the Photolysis of Iron-Arene Complex Photoinitiator

Iron-arene complex was first introduced by researchers in Ciba-Geigy as an effective photoinitiator for the photocuring of epoxy resin. The photolysis of these complexes has been studied by some authors. The electrochemical study reported in this note provides further evidence for the photolysis process which is crucial for the understanding of the photoinitiation mechanism.     

Blends of bisphenol a polycarbonate and acrylic polymers. I. A chemical reaction mechanism

The chemical reaction at high temperature (above 200°C) between PC and PMMA has been very recently highlighted. However, no clear reaction mechanism has been proposed to explain this phenomenon. We suggest a reaction mechanism following two steps. The first step consists of hydrolysis of the ester bonds of the PMMA leading to acid pendant groups. These acids can then either ring close into glutaric anhydride, or acidolyze the carbonate bonds of PC during the second step. At the same time, benzoic acid produced by PC degradation could further react with PMMA or acidolyze the carbonate groups leading to the crosslinking of the system. A satisfactory contact between the reacting units is a key point in the proces. Significant amounts of PC-PMMA copolymer are obtained when the reaction is performed from a miscible system. On the contrary, no reaction occurs during melt mixing. Understanding the process enables us to specify the conditions, in which no chemical reaction takes place. In nonreactive conditions, PC/PMMA blends remain immiscible for several hours at 300°C. The thermodynamic UCST proposed in the literature is just an artifact due to the occurrence of the chemical reaction. © 1995 John Wiley & Sons, Inc.