Revising the mechanism of polymer autooxidation

The basic scheme for autooxidation of polymers, originally developed by Bolland, Gee and co-workers for rubbers and lipids, is now widely applied to all types of polymeric materials. According to their scheme, the reaction that makes this process autocatalytic, referred to as the propagation step, is a hydrogen abstraction from the next substrate by the peroxyl radical (ROO˙ + RH → ROOH + R˙). In this study, using advanced quantum-chemical methods, we have shown that this step is actually characterised by largely positive Gibbs free energy (10-65 kJ mol(-1)) for most regular polymers with saturated chains (polypropylene, polyethylene, polyvinyl chloride, polyvinyl acetate, polyurethane, poly(methyl methacrylate) etc.) and even some polymers with unsaturated fragments (polystyrene, polyethylene terephthalate). Neither elevated temperature, nor solvation makes this process thermodynamically favourable. Only when the formed radical centre is conjugated with adjacent double bonds (as in polybutadiene) or captodatively stabilised by two suitable functional groups (such as a carbonyl and a lone pair donor such as oxygen or nitrogen), is the propagation step exoergic. Instead, we show that it is the presence of structural defects, such as terminal or internal double bonds, formed either during polymerisation or in the degradation process itself, that is responsible for the autooxidation of most polyesters and most polyalkenes. Recognition of the real mechanism of autooxidation in polymers is a key to developing strategies for the prevention of their degradation.     

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.     

微波辐照实现PVC-PS之间的Friedel-Crafts接枝反应

在四氢呋喃、环己酮等溶液中实现了聚氯乙烯(PVC)与聚苯乙烯(PS)的Friedel-Crafts接枝反应,考察了加热方式、反应温度、催化剂种类、催化剂用量对接枝反应的影响,通过FTIR表征了PVC-g-PS接枝物的结构,探讨了反应的机理和特点.研究发现,油浴加热和微波辐照都能使PVC和PS发生接枝反应,但微波辐照效果更好,油浴加热,接枝率仅0.27%,而微波辐照接枝率高达23%;油浴加热时产物少且多为间位,微波辐照时产物多且多为对位和邻位.无水AlCl3、无水FeCl3、无水SnCl4皆对PVC与PS之间的Friedel-Crafts反应具有催化活性,其中无水AlCl3的催化活性最高,其最佳用量为质量分数3%,此时,接枝率达到29%.当无水AlCl3过高时,PVC降解,产生双键,接枝率下降.表明微波辐照是实现PVC与PS之间Friedel-Crafts接枝反应的有效途径.     

龙脑烯醛同单重态氧反应的立体选择性

用竹红菌甲素匹配高压钠灯光敏氧化龙脑烯醛,高产率并有立体选择性地获得反应主产物,α-(2,2-二甲基-3-亚甲基-4-羟基-1-环戊烷基)乙醛。反应具有协同的“ene”反应特性。反应的立体选择性被龙脑烯醛的分子构象,取代基的空间位阻效应和烯丙基氢的轴向定位所控制。羰基同环戊烯基的相互隐蔽的分子构象对反应的立体选择性起关键作用。     

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.     

Separation into Phases of Polymer Mixtures in Solution

When three different polymers are dissolved in a common solvent and the total polymer concentration is above a certain value, three liquid layers are obtained because of the limited compatibility of the polymer components with one another. Tests have been carried out at room temperature on solutions of polystyrene t polyvinyl acetate + polyvinyl chloride in tetrahydrofuran, and polystyrene t polyvinyl acetate t atactic polypropylene in benzene. Analysis of the individual layers yielded the following results: each polymer is always contained in each layer; the mixing ratios of the polymers in the three layers are generally different from one another and differ also from the mixing ratio of the starting mixture; each of the three layers is distinguished from the others by the fact that a specific polymer has been accumulated therein in preference to the other polymers. Concerning the quantitative distribution of the polymers among the layers, some laws could be found which can be explained qualitatively on the grounds of molecular-physical considerations. A solution of polystyrene t polyvinyl acetate t polyvinyl chloride t atactic polypropylene in tetrahydrofuran (equal amounts of polymers, total concentration about 10 g/100 ml) forms four layers, the composition of which was also determined by analysis.

When solutions of polymer mixtures are evaporated, separation into phases takes place because the polumers become incompatible with one another when the concentration exceeds a certain value. As a result, strongly inhomogeneous films are obtained after evaporation of the solvent. If the inhomogeneity of the films is not taken into consideration, misleading results may be obtained in the analysis of the films.

It is shown that in the system polystyrene + polyvinyl acetate, with tetrahydrofuran as solvent, surprising phenomena appear on the addition in stages of cyclohexane. The phenomena are due to the overlapping of separating and dissolving processes which have, in part, opposite actions.     

Degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers. Effect of internal unsaturation on the stability of Saran copolymers

The thermal degradation of vinylidene chloride/methyl acrylate/phenylacetylene (VDC/MA/PA) terpolymers containing a constant 9 wt % methyl acrylate and small but varying amounts of phenylacetylene has been examined in the solid phase and in bibenzyl solution. Thermally promoted degradative dehydrochlorination, largely uncomplicated by methyl chloride formation, readily occurs at temperatures approaching 200°C. Incorporation of phenylacetylene into the polymer structure greatly facilitates degradative dehydrochlorination. Indeed, the presence of phenylacetylene induces the formation of polyene segments during the polymerization so that all the terpolymers, even at very low phenylacetylene loading, are tan in color. The decreased stability of polymers containing internal unsaturation arises from an increased rate of initiation for the degradation reaction. The propagation rate is largely unaffected by the level of unsaturation initially present in the polymer. Thus random double bonds have been identified as the principal defect sites responsible for the facile degradation of Saran copolymers. Species which promote the degradation of Saran polymers probably do so by facilitating the introduction of double bonds into the structure. The ratio of hydrogen chloride to stilbene formed for degradation of the terpolymers in bibenzyl solution is ca. 35:1. This is strongly reminiscent of PVDC degradation and suggests that for degradation of either the homopolymer or Saran copolymers the chain-carrying allylic radical pair does not dissociate to any appreciable extent as dehydrochlorination occurs.     

C_(60)敏化的顺-1,4-聚丁二烯的光氧化

Ｃ６０敏化的顺 １，４ 聚丁二烯的光氧化周涛陈双基（信阳师范学院化学系河南４６４０００）（北京大学分校化学系北京１０００８３）李子臣周锡煌李福绵（北京大学化学系北京１００８７１）关键词Ｃ６０，顺 １，４ 聚丁二烯，光氧化自从能以宏观量制备Ｃ６０...     

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.     

SENSITIZED PHOTOOXYGENATION REACTIONS AND THE ROLE OF SINGLET OXYGEN†,‡

Abstract— In this paper we discuss various theoretical and experimental aspects of the role of singlet oxygen in sensitized photooxygenation reactions. New spectroscopic observations on the photosensitized production of singlet oxygen molecules are presented. The various factors which control the generation and reactions of singlet oxygen molecules are considered in detail. A relatively simple theoretical procedure is developed to predict the relative reactivities of ¹σ, ¹δ and ³σ oxygen toward various organic acceptors, and is used to discuss the chemical and photochemical properties of some of the oxygenation products. Finally, the properties of dioxetanes are examined in connection with the role which they may play in chemi- and bioluminescence. While we have said rather little about photodynamic reactions per se , the results presented in this paper strongly support the suggestion that many of the observed photodynamic effects could be due to reactions of singlet oxygen. Clearly a careful reexamination of various photodynamic effects at the molecular level to establish whether or not reactions of singlet oxygen are involved is now in order.     

Degradation of vinylidene chloride/phenylacetylene (VDC/PA) copolymers. Effect of internal unsaturation on poly(vinylidene chloride) stability

The thermal degradation of vinylidene chloride/phenylacetylene copolymers containing small but varying amounts of phenylacetylene has been examined in both the solid phase and in bibenzyl solution. Incorporation of phenylacetylene into the poly(vinylidene chloride) structure greatly facilitates degradative dehydrochlorination. Indeed, the presence of phenylacetylene promotes the formation of polyene segments during the polymerization process so that all the copolymers, even at very low phenylacetylene loading, are tan in color. The decreased stability of polymers containing interal unsaturation arises from an increased rate of initiation for degradative dehydrochlorination. The propagation rate is largely unaffected by the level of unsaturation initially present in the polymer. The ratio of hydrogen chloride to stilbene formed for degradation of these copolymers in bibenzyl solution is approximately 35:1. This suggests that the chlorine atom of the initially-formed radical pair preferentially abstracts an adjacent hydrogen atom rather than interacting with solvent, i.e., the chain-carrying radical pair does not dissociate appreciably as the unzipping dehydrochlorination occurs. Thus random double bonds introduced in a variety of ways may be identified as principal defect sites responsible for the initiation of the degradative dehydrochlorination of poly(vinylidene chloride). Species which promote the degradation of poly(vinylidene chloride) probably do so by facilitating the introduction of random double bonds into the structure.     

PROBING PHOTODYNAMIC DAMAGE IN NUCLEIC ACIDS WITH A DAMAGE-SPECIFIC DNA BINDING PROTEIN: A COMPARISON OF THE B and Z DNA CONFORMATIONS

—We have employed a damage-specific DNA binding protein from human cells as a probe for base damage in polymers irradiated with white light in the presence of methylene blue. Protein-recognizable damage is introduced only into guanine-containing polymers and quenching of damage introduction by H₂O and sodium azide suggest the involvement of a singlet oxygen mechanism. Using poly d(G-m⁵C), we have demonstrated that the left-handed double helical Z conformation is much less susceptible to guanine photooxidation than is the usual B conformation. We speculate that this difference in reactivity may reflect steric hindrance at the purine C-4 position and could provide some insight into the initial steps of the reaction between singlet oxygen and guanine in nucleic acid polymers.     

HALOMETHYLATION OF POLYSTYRENE AND SUBSEQUENT PYRIDINATION VIA A HOMOLYTIC PATHWAY

Chloromethylation of polystyrene （PS） with two different chloromethylating systems methylal/thionyl chloride and paraformaldehyde/trimethylchlorosilane was studied. Soluble chloromethylated polystyrene with a degree of substitution of 89% was obtained. The Conant-Finkelstein reaction on the chloromethylated PS afforded soluble iodomethylated polystyrene with a degree of substitution as high as 96%. The reaction conditions of Minisci were employed to radically pyridinate PS via its iodomethylated derivative. Polyelectrolytes were formed which could be converted to normal polymers by treatment with a 20% aqueous solution of NaOH.     

Head-to-Head Polymers

Head-to-head polymers of α-olef ins, vinyl and acrylic monomers synthesized by indirect methods have shown similarities and differences in some of their behavior when compared to their counterparts of the common head to tail polymers. Head-to-head polyole-fins, such as polystyrene or polypropylene, were prepared by 1,4-polymerization of the corresponding 2,3-disubstituted butadienes-1,3 followed by hydrogenation of the remaining double bonds, polyacrylates by alternating copolymerization of ethylene and maleic anhydride, followed by esterification of the polymer. Head-to-head polyacrylates were reduced to poly(allyl alcohols) which were then acrylated to the acetates and benzoates. Head-to-head polyvinyl halides) were obtained from 1,4-polybutadiene by chlorination or bromination. The head-to-head polymers were characterized by spectroscopic analysis, some molecular weight characterization was done, and their thermal behavior was studied. The blending behavior with the corresponding head-to-tail polymers was studied in some cases; in others (head-to-head polystyrene), the blending with other polymers was investigated and the codegradation of the polymers was evaluated.     

THE ROLE OF SUPEROXIDE AND SINGLET OXYGEN IN LIPID PEROXIDATION

Abstract— An investigation into the mechanism of lipid peroxidation catalyzed by xanthine oxidase showed a dependence upon superoxide, singlet oxygen and adenosine 5'-diphosphate chelated iron (ADP-Fe³⁺). In the absence of ADP-Fe³⁺ or in the presence of superoxide dismutase there is complete inhibition of enzymatic peroxidation. Initiation of peroxidation likely occurs through an ADP-perferryl ion complex formed by ADP-Fe³⁺ and superoxide. Use of the singlet oxygen trapping agent 2,5-diphenylfuran showed that singlet oxygen does not participate in the initiation of peroxidation but rather in the propagation of peroxidation. The mechanisms of NADPH-cytochrome P450 reductase-catalyzed and ADP-Fe²⁺ catalyzed lipid peroxidation parallel that of xanthine oxidase in that initiation occurs through a superoxide dismutase-sensitive reaction and that singlet oxygen is present during propagation of lipid peroxidation.     

Theory of chemical bonds in metalloenzymes XII: Electronic and spin structures of metallo–oxo and isoelectronic species and spin crossover phenomena in oxygenation reactions

The broken-symmetry (BS) and multideterminant approaches to atomic oxygen (O), molecular oxygen (O₂) and iron–oxo (Fe(IV)O) core in P450 have elucidated electronic structures of the ground triplet and excited singlet states, which indicate isoelectronic characteristics of the species. The dissociation processes of the O–O and Fe–O double bonds are also examined to clarify the radical character, namely O-atom property responsible for radical mechanism of hydroxylations of alkanes and epoxidation of alkenes. This isolobal analogy has indeed enabled us to propose possible reaction mechanisms of oxygenation reactions by the Fe(IV)O species on the basis of available theoretical and experimental results for O and O₂. Similarly, an isolobal analogy of the σ^* bond among Fe(IV)O, dioxirane, peracids, etc. indicates the common electrophilic property for the oxygenation reactions. The small energy gaps between the high- and low-spin states of the transition structures and intermediates generated in the oxygenation reactions are found to be origins for spin crossover phenomena along the reaction pathways of these reactions.     

The effect of 1,3-diphenylisobenzofuran on the photo-oxidative degradation of cis-1,4-polybutadiene

1,3-Diphenylisobenzofuran (DPBF) is easily photo-oxidized by two mechanisms viz free radical oxidation and singlet oxygen oxidation. The final products of DPBF oxidation by these two mechanisms are the same. Using light in the range 280–480 nm, DPBF is an effective sensitizer of photooxidative degradation of polybutadiene in the solid and in solution. In a system with methylene blue (MB) in methanol-benzene solution (0.5:9.5) where free radicals from MB and ¹O₂ are formed during irradiation with visible light, DPBF is oxidized by both ¹O₂ and free radical mechanisms. DPBF cannot stop free radical degradation of PB initiated by MB radicals in MB-methanol-benzene solution. These results show that the DPBF is an ineffective stabilizer for polydienes against ¹O₂ and free radical oxidation. It rather acts as a sensitizer for photo-oxidation of polydienes.     

Contribution of size exclusion chromatography and thermal analysis methods in the study of the radiochemical behavior of plastic materials

Sterilization by irradiation techniques is considered to be an improved method to treat polluted products of human consumption. Secondary effects, that may be generated in the components of packaging materials, could induce organoleptic damage and some physico-chemical modifications, observed especially in plastic materials. In order to demonstrate any such secondary changes, we have investigated the effects of beta-irradiation (a treatment which could induce either a reticulation or a cleavage in a polymeric sequence) on three polymers; namely, low density polyethylene, polystyrene and polyvinyl chloride. The techniques used to identify irradiation-induced modifications of these polymers were size exclusion chromatography and thermal analysis. Thermal analysis showed a decrease in the percentage of crystallinity of polyethylene after its radiosterilization. Exclusion chromatography highlighted a reticulation for irradiated polyvinyl chloride and polystyrene.     

食品塑料包装材料中化学物质的残留和分析

综述了近几年食品塑料材料中残留的化学物质和分析进展，内容包括聚乙烯、聚苯乙烯，聚氯乙烯、聚碳酸酯、三聚氰胺树脂及成型品。     

Immiscible polydiene blocks in linear copolymer and terpolymer sequences

Synthesis, molecular, and morphological characterization of two linear diblock copolymers consisting of two polydienes with specific geometric isomerisms and two triblock terpolymers with a combination of the same polydienes with polystyrene are investigated for both lower and very high molecular weights. This work is inspired from a previous research study which demonstrated that linear ABC terpolymers consisting of polystyrene, poly(butadiene), and poly(isoprene), with specific geometric isomerisms for the polydienes, lead to 3‐phase microphase separated systems. We report also the coexistence of the core‐shell double gyroid and the 3‐phase 4‐layer alternating lamellae morphologies with the majority fraction being the lamellar structure. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1238–1246