Studies on the photooxidation mechanism of polymers. I. Photolysis and photooxidation of polystyrene

A new mechanism has been proposed for the photooxidation of polystyrene as film and in benzene. The initial stage of the photooxidative degradation may involve reactions of singlet oxygen with polystyrene molecules. Singlet oxygen may be formed in the reaction between excited benzene ring in polystyrene molecule and molecular oxygen. The addition of singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene reduces the rate of polymer degradation in benzene solution. The mechanisms of the photolysis of polystyrene as film and in benzene solution, in vacuo and in the presence of oxygen, are discussed and interpretations proposed. The pronounced yellowing of polystyrene during the photooxidation process is interpreted as a reaction involving benzene ring-opening photooxidation in polystyrene molecule. These results were obtained by comparing ultraviolet and infrared spectra in experiments of photooxidation of pure liquid benzene and polystyrene film.     

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.     

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.     

Dehydrochlorination reactions in polymers. Part II. Chlorinated polystyrene

The dehydrochlorination of chlorinated polystyrene was studied in the temperature range 120–220°C., when HCl was the sole volatile product. The dehydrochlorination was accompanied by the slow development of color and chain scission. The elimination rate fell too rapidly with reaction extent to be accounted for by reduction of reagent. Solution studies in o-dichlorobenzene indicated that the reaction was initially first-order in polymer concentration but was retarded by the polyene reaction products. The overall elimination was interpreted as a radical process in which the product was an active retarder. These observations are also valid for the elimination of HBr from brominated polystyrene.     

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.     

A study of the mechanism of photocrosslinking polymers by quinones

Photocrosslinking of poly(methyl methacrylate) (PMMA) was studied in the solid state in the presence of various quinones. For the study of photocrosslinking mechanism, a PMMA film containing p-benzoquinone (Q) was irradiated with UV light (λ > 370nm) and then purified by reprecipitation. It was found that the reprecipitated polymer has quinone-type moieties, besides the hydroquinone-type moieties, chemically bound to the polymer chain. The reprecipitated polymer film also crosslinking efficiency was higher than that of PMMA film containing Q. It was concluded that the formation of quinone-type structure during irradiation played an important role in the photocrossing of PMMA containing Q.     

Thermal degradation of polystyrene—4. Decomposition of oxygen-containing polymers

The thermal decomposition of two polystyrene samples prepared by radical polymerisation in the presence of oxygen was studied in the temperature range 280–300°C where volatile formation is negligible. The energy of activation for chain scission of the oxygen-containing polymers was lower than that of a reference sample prepared under vacuum. All three polymers contained the same proportion of weak bonds and it was therefore concluded that these structures are not copolymerised peroxide groups.     

聚苯乙烯热降解机理的理论研究

采用密度泛函理论B3LYP/6-311G（d）方法,对聚苯乙烯（PS）热降解反应机理进行了研究。PS热降解的主要产物是苯乙烯,其次是甲苯、α-甲基苯乙烯、乙苯和二聚体等芳烃化合物。PS热降解反应主要包括主链C-C键均裂、β-断裂、氢转移和自由基终止等反应。针对以上各类反应进行了路径设计和理论计算分析,对参与反应的分子的几何结构进行了优化和频率计算,获得了各热降解路径的标准动力学和热力学参数。计算结果表明,苯乙烯主要由自由基的链端β-断裂反应形成;二聚体主要由分子内1,3氢转移的反应形成;α-甲基苯乙烯由分子内的1,2氢转移后进行β-断裂形成;甲苯由苯甲基自由基夺取主链上的氢原子形成;乙苯由苯乙基自由基夺取氢原子形成。动力学分析表明,苯乙烯形成所需要的能垒低于其他产物形成所需要的能垒,故苯乙烯为主要的热降解产物;这与相关实验结果基本一致。     

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.     

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.     

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.     

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 coordination polymers : Part II. Coordination polymers of 1,6-dihydroxyphenazine

Coordination polymers of 1, 6-dihydroxyphenazine (I) with Cu(II), Zn(II), Ni (II), Co(II) and Hg(II) were prepared and studied. All the polymers formed were dark blue to black powders, very insoluble in the common solvents, apparently of low molecular weight, and decomposed at temperatures ranging from 200 to 280°. The zinc polymer was the most stable, mercury the least. Infrared studies confirmed the proposed chelation reaction between the metal ion and (I). The polymers of Co and Hg were amorphous, whereas those of Cu, Zn, and Ni were crystalline. When used as a spot test reagent, (I) gave instant color reactions with 15 common ions but neither the sensitivity nor selectivity of the reagent is impressive.     

Study on the intermediate of photooxidative degradation of phenylenevinylene oligomers

The results of the investigation on photooxidative degradation of five phenylenevinylene oligomers are as follows. The sequence of the photodegradation rate in self-sensitized reaction is the same as that in the biacetyl-sensitized reaction and the rate of photooxidative degradation increases upon the incorporation of electron-donating groups and decreases upon the incorporation of electron-accepting groups on the oligomers. The self-sensitized reaction rate of oligomers increases as the solvent changing from benzene to deuterated benzene and decreases with the addition of the singlet oxygen (¹O₂) scavenger. ¹O₂ is shown to be the main reactive intermediate in self-sensitized photolysis of phenylenevinylene oligomers, which was directly confirmed by ESR spin trapping experiments.     

Mechanistical studies on the electron-induced degradation of polymers: polyethylene, polytetrafluoroethylene, and polystyrene

Mechanisms of the electron-induced degradation of three polymers utilized in aerospace applications (polyethylene (PE), polytetrafluoroethylene (PTFE), and polystyrene (PS)) were examined over a temperature range of 10 K to 300 K at ultra high vacuum conditions (～10(-11) Torr). These processes simulate the interaction of secondary electrons generated in the track of galactic cosmic ray particles in the near-Earth space environment with polymer material. The chemical alterations at the macromolecular level were monitored on-line and in situ by Fourier-transform infrared spectroscopy and mass spectrometry. These data yielded important information on the temperature dependent kinetics on the formation of, for instance, trans-vinylene groups (-CH=CH-) in PE, benzene (C(6)H(6)) production in PS, fluorinated trans-vinylene (-CF=CF-) and terminal vinyl (-CF=CF(2)) groups in PTFE together with molecular hydrogen release in PE and PS. Additional data on the radiation-induced development of unsaturated, conjugated bonds were collected via UV-vis spectroscopy. Temperature dependent G-values for trans-vinylene formation (G(-CH=CH-) ≈ 25-2.5 × 10(-4) units (100 eV)(-1) from 10-300 K) and molecular hydrogen evolution (G(H(2)) ≈ 8-80 × 10(-5) molecules (100 eV)(-1) from 10-300 K) for irradiated PE were calculated to quantify the degree of polymer degradation following electron irradiation. These values are typically two to three orders of magnitude less than G-values previously published for the irradiation of polymers with energetic particles of higher mass.     

The role of surface in the atomic mechanism of fracture of polymers

Highly oriented polymer films were stretched along the axis of orientation. Using i.r. transmission and reflection spectroscopy, the scission of polymer molecules under stress in the volume and in the surface layer ~ lμm thick was studied. The accumulation of scissions of chemical bonds in the surface layer was found to be at least an order more intense than that in the volume. It is shown that this phenomenon is due to considerable heterogeneity of external load distribution over chemical bonds near the surface. It is inferred that the surface plays a leading role in the fracture mechanism of polymers.The intensity of the intermolecular interactions in the volume and on the surface of polymers was studied and was found to be lower in the surface layer. It is proposed that the role of surface in the fracture process results from weakening of the intermolecular interactions.     

Thermal degradation of polymers with phenylene units in the chain. II. Sulfur-containing polyarylenes

Poly(p-phenylene sulfide), a poly(arylene sulfone), and a poly(arylene sulfonate) were subjected to thermal degradation in vacuo, at temperatures between 250 and 620°C. The volatile and solid degradation products were analyzed by mass spectroscopy, infrared spectroscopy, and elemental analysis. The major decomposition product of poly-(phenylene sulfide) is a condensate, which consists of di- and trimeric chain fragments, dibenzothiophene, and possibly thianthrene. The residual polymer loses two thirds of its sulfur as hydrogen sulfide, however, one third is retained even at 620°C. The most characteristic decomposition reaction of the polysulfone and of the polysulfonate is the almost complete removal of the sulfur as sulfur dioxide. The elimination of sulfur dioxide is practically complete at 450°C for the polysulfone and at 350°C for the polysulfonate.     

光氧化反应的研究 II: 9-苯甲叉芴的电子转移光氧化反应

芳香族烯烃, 芳烃以及稠环芳烃是光氧化反应中的重要研究对象, 本文以9-苯甲叉芴(BF)作为底物, 发现它在9,11-二氰蒽(DCA)敏化作用下易发生电子转移反应, 反应生成芴酮和BF的相应环氧化物, 用紫外光谱测出BF的电荷转移配合物的络合物稳定常数,用循环伏安法测定BF的氧化电位. 由Rehm-Weller方程计算出DCA敏化的单电子转移过程的自由能变化, 并观察到BF对DCA有明显的荧光猝灭作用.     

Metal chelates of porphyrin derivatives as sensitizers in photooxidation processes of sulfur compounds and in photodynamic therapy of cancer

A number of porphyrin derivatives based on hematoporphyrin, 5,10,15,20-tetrasubstituted porphyrins, phthalocyanines, and naphthalocyanines were prepared either as low-molecular compounds or bonded with methoxypoly(ethylene glycol) or attached to silica of low surface area. The low-molecular weight and the polymer-bonded porphyrins exhibit comparable triplet lifetimes and activities in the photosensitized formation of singlet oxygen. For photoninduced processes, the monomeric state of sensitizers is fundamentally important. The porphyrins have been investigated as sensitizers for photooxidation of thiolates and sulfides, which occursvia singlet oxygen, and, therefore, is much more efficient than the corresponding catalytic dark oxidation. Polymer-bonded porphyrins and long-wavelength absorbing naphthalocyanines incorporated in liposomes exhibitin vivo high accumulation in tumor tissues. Under irradiation, singlet oxygen is produced, and efficient phototherapeutic effects are observed, which may be used for photodynamic cancer therapy.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2071–2082, December, 1994.D. Wöhrle is very thankful to G. Schulz-Ekloff (Bremen), M. Shopova (Sofia), I. Okura (Tokyo), B. Roeder (Berlin), G. Graschew (Heidelberg), and their coworkers for fruitful cooperation. The research was supported by the German Research Society (Deutsche Forschungsgemeinschaft, 436 BUL 113/51/0, Wo 237/ 12-1), the German Cancer Service (Deutsche Krebshilfe, W 15/89/Sch12), Bremen University (FSP 2/90), and the Federal Department of Scientific Research and Tech nology (Bundesministerium far Forschung und Technotogie, Bonn, 13N6290).