ESR studies of vinyl polymerization. II. Initial reactions of vinyl esters with redox systems in aqueous media

ESR measurements of transient radicals during redox polymerization of various vinyl esters in aqueous solutions have been made by using the rapid-mixing flow method. The initiation was by means of hydroxyl and amino radicals from the systems titanous chloride-hydrogen peroxide and titanous chloride-hydroxylamine, respectively. The well resolved hyperfine structures obtained at monomer concentrations of about 0.05 mole/1. are unambiguously assigned to the monomer radicals formed by addition of initiator radicals to monomers. At higher monomer concentrations, additional weak signals attributed to the growing polymer radicals were observed. The effect of reaction conditions on the signal intensity has been studied in particular for vinyl acetate. The coupling constants of monomer radicals from various vinyl esters (acetate, propionate, butyrate, crotonate, and isopropenyl acetate) were obtained and the spin densities calculated. From the ESR spectra, the monomer radicals have a conformation with the substituent R (R = HO or NH₂) of R? CH₂? CH(OCOR′) locked in a position above or below the radical plane. This is tentatively interpreted as due to formation of intramolecular hydrogen bonds to ring structures or complexes with titanium ions. In addition, hydrogen abstraction reactions of some model compounds for poly(vinyl acetate) have been briefly studied in relation to chain transfer and grafting reactions.     

High static pressure alters spin trapping rates in solution. Dependence on the structure of nitrone spin traps

Using a competitive spin trapping method, relative spin trapping rates were quantified for various short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the competitive spin-trapping system by employing high-pressure electron spin resonance (ESR) equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was used as a spin trap. Activation volumes (DeltaDeltaV(double dagger)) for the two spin trapping reactions were calculated to be -17-(-9) cm(3) mol(-1) for the 4-POBN system.     

Reactions of oxygen free radicals with copper complexes in pyridine: differentiation between superoxide and hydroxyl radicals

Superoxide radicals are generated in tetraalkylammonium hydroxide-pyridine-H₂O₂ and detected by spin trapping and by reduction of nitroblue tetrazolium. The addition of low-molecular-weight copper complexes (e.g., of anti-inflammatory drugs) decreases the concentration of superoxide. Depending on the solvent, different oxygen radicals are generated by the addition of hydrogen peroxide to solutions of the copper complexes; hydroxyl radicals are observed in aqueous solutions whereas superoxide is found in pyridine.     

光诱导氧自由基与嘧啶碱基及其核苷反应的ESR研究

活性氧在破坏核酸结构,攻击其嘌呤碱基和嘧啶碱基,导致变异的出现和累积方面起重要作用。在生物体内,超氧自由基(O_2)是一个非常重要的活性氧,是氧分子单电子还原时首先生成的产物。由O_2衍生成过氧化氢(H_2O_2),羟基自由基(OH),单线态分子氧(~1O_2),     

DETECTION OF ACTIVE RADICALS FORMED DURING THE SONOLYSIS OF POLYMERS

Active radicals formed during the sonolysis of PVK (polyvinyl carbazole) and PMMA (polymethylmethacrylate) have been studied with the combination of spin thepping technique and ESR spectroscopy-The results show that: 1. For PVK, radicals CA and--CH_2--CH--are produced during the ultrasonic irradiationand only the later can be trapped by MNP. 2. For PMMA, radicals --C and --CH_2 are grnerated by sonolysis and both can be trappedby ND.     

CHARACTERIZATION OF RADICAL ADDUCTS FORMED DURING PHOTOCHEMICAL SPIN TRAPPING IN LIPOSOMES

There exists an urgent need to monitor radical reactions in biological membranes. With the exception of a few studies in model and natural membranes, most biological spin trapping reactions have been confined to homogeneous media. In the present work we have devised a methodology by which spin trapping reactions can be investigated in liposomes composed of a fully-saturated phospholipid, dimyristoylphosphatidylcholine. Using photochemical spin trapping, we have detected and characterized the formation and partitioning of spin adducts in liposomes. Benzophenone (a lipid-soluble photosensitizer) and benzoyl peroxide (a lipid-soluble radical initiator) have been used to generate free radicals from hydrogen donors during photolysis in liposomes. Both water-soluble and lipid-soluble spin traps have been used to trap the radicals.     

ZnS胶体溶液中某些无机盐光化学过程中产生的阴离子自由基的ESR研究

本文用自旋捕捉与ＥＳＲ技术相结合的方法研究了ＺｎＳ胶体溶液中某些无机盐光化学过程中产生的无机阴离子自由基，并对自由基产生的机理进行了讨论     

Enhancement in Chemiluminescence by Carbonate for Cobalt(II)‐catalyzed Oxidation of Luminol with Hydrogen Peroxide

Chemiluminescence (CL) from the cobalt(II)‐catalyzed oxidation of luminol with hydrogen peroxide was dramatically enhanced by the presence of carbonate. The CL signal increases by several orders of magnitude over a wide range of concentrations of Co(II), luminol, or hydrogen peroxide. A limit of detection of 10^?12 M for Co(II) and luminol and 10^?8 M for hydrogen peroxide can be achieved. The CL emission spectrum exhibits a maximum at 425 nm, indicating that the excited 3‐aminophthalate is the emitting species. ESR spin‐trapping experiments revealed a large increase in the production of hydroxyl and carbonate radicals by the presence of carbonate, which is responsible for the enormous CL enhancement. Uric acid, ascorbic acid, acetaminophen, and p‐hydroxyphenyl acetic acid are capable of scavenging the radicals, thereby inhibiting the CL emission. The inhibition of CL intensity can be used to determine these substances at the sub‐micromolar level.     

利用ESR技术研究α-蒎烯,β-蒎烯光敏氧化反应机理

本文主要利用电子顺磁共振(ESR)自旋捕获技术研究9,10 二氰基蒽(DCA)敏化α-蒎烯(αP),β-蒎烯(βP)光氧化反应.提供了在乙腈中α-蒎烯和β-蒎烯的光氧化反应过程中存在超氧负离子基(O₂^-)和单重态氧(¹O₂)的直接证据;在四氯化碳溶剂中只捕获到¹O₂;在正己烷中没有捕获到O₂^-或¹O₂.ESR实验结果进一步证明在乙腈中光敏氧化反应的¹O₂可能来自O₂^-和反应底物α、β-蒎烯正离子自由基之间的电荷复合(CR).     

酯-二苯酮光化夺氢反应活泼自由基的ESR研究

自旋捕捉技术与ESR相结合,为检测反应过程中产生的自由基活泼中间体提供一个方便且有效的方法。近年来,国内外研究工作者利用这一方法,对固相,液相和气相中产生的活泼自由基已进行了广泛的研究^[1-12]。     

PHOTOINDUCED GENERATION OF HYDROGEN PEROXIDE AND HYDROXYL RADICALS IN MELANINS

The hydrogen peroxide produced during photolysis of melanin pigments has been measured using an oxidase electrode. The photooxidation has been shown to occur via the superoxide intermediate. In the presence of superoxide dismutase the rate of photo-induced production of hydrogen peroxide is increased, reflecting the ability of melanin to scavenge superoxide radicals. Evidence for metal-ion dependent formation of hydroxyl radicals during photooxidation of melanin pigments was obtained using electron spin resonance-spin trapping procedures. Superoxide dismutase increased the rate of formation of hydroxyl radicals in the system. Mechanisms of metal ion-induced production of hydroxyl radicals during photolysis of melanin pigments are discussed.     

Applications of esr techniques to the study of free radical processes and kinetics in acrylic polymerizations

Electron spin resonance (ESR) techniques have been applied to a detailed study of batch and semicontinuous emulsion polymerization of methyl methacrylate; butyl acrylate and styrene have been briefly studied. Quenching of samples from the polymerization reactor followed by ESR analysis are useful, but we have developed continuous flow techniques which are preferable in many cases. ESR techniques can provide valuable information relating to the nature of free radical reactions, the concentration of propagating free radicals, and the kinetic processes in these reactions. Direct ESR analysis is most valuable but is not applicable to all systems. Spin trapping techniques can be useful for systems not accessible by direct analysis.     

ESR spin trap investigations on aqueous glucose solutions irradiated by ultrasound and gamma-rays

Applying 5,5-dimethylpyrroline-N-oxide (DMPO) as the spin trap, the radical adducts produced by ultrasound and γ-irradiation of water and aqueous solutions of d-glucose are compared. In the ESR-spectra obtained for sonolysis and radiolysis of water, H- and OH-adducts are present. In glucose solutions for both types of irradiation the H-adduct is also detected. However, OH-adduct is completely lacking and there is no evidence for the formation of adducts originating from glucosyl radicals. To explain these experimental observations the following mechanism is proposed: the OH- and H-radicals produced by sonolysis or radiolysis in water abstract carbon bond hydrogens from glucose generating glucosyl radicals. This explains the lack of OH-adducts. The glucosyl radicals stabilise by H-transfer to the spin trap. This explains the presence of H-adducts and the lack of glucosyl-adducts. The splitting constants of three additional radical adducts, which are present in minor concentrations as well in water as in glucose solutions, were evaluated. They are supposed to be spin adducts resulting from H-abstraction from DMPO.     

Photochemistry of naphthalene diimides: EPR study of free radical formation via photoredox process

Earlier studies have shown that on exposure to UVA, hydroperoxynaphthalene diimide (IA) generates hydroxyl radicals, induces DNA strand scission, and kills cells.Here we employed electron paramagnetic resonance (EPR) and spin trapping to investigate the free radical photochemistry of IA and that of related naphthalene diimides, which are devoid of the hydroperoxyl moiety (N,N'-bis[2-methyl]-1,4,5,8-naphthaldiimide [IB], N,N'-bis[2-thiomethyl-2-methoxyethyl]-1,4,5,8-naphthaldiimide [IC]) and therefore are unable to generate hydroxyl radicals. It is shown that on UV irradiation (>300 nm) in air-free methanol or ethanol solutions all these naphthalene diimides undergo one-electron reduction to corresponding anion radicals, positively identified by EPR. With EPR and a spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO), we found that the photogeneration of the naphthalene diimide radicals is concomitant with the formation of radicals from the solvents, presumably through electron/hydrogen atom abstraction by photoactivated diimides. Irradiation of IA, IB or IC in the presence of oxygen generates superoxide, which was detected as a DMPO adduct. The high photoreactivity of IB and IC supports the notion that hydroperoxide IA can induce oxidative damage via photoprocesses that are independent of *OH generation. These observations could be pertinent to the application of naphthalene diimides as selective photonucleases, PDT anticancer agents or both.     

溴化银微晶和羧酸盐水溶液体系中的光生自由基的形成

利用ESR电子自旋捕捉技术,研究了甲酸钠,乙酸钠,草酸钾等羧酸盐的水溶液及其含有AgBr的分散体系在不同光照条件下自由基的产生.结果表明:某些羧酸盐(特别是甲酸盐)自身具有光解性,而且AgBr的存在对其光解过程有促进作用.这对研究羧酸盐掺杂乳剂尤其是甲酸盐掺杂乳剂的增感机理有重要意义.     

Effects of pulsed ultrasound on the adsorption of n-alkyl anionic surfactants at the gas/solution interface of cavitation bubbles

Sonolysis of argon-saturated aqueous solutions of the nonvolatile surfactants sodium dodecyl sulfate (SDS) and sodium 1-pentanesulfonate (SPSo) was investigated at three ultrasonic frequencies under both continuous wave (CW) and pulsed ultrasound. Secondary carbon-centered radicals were detected by spin trapping using 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS) and electron paramagnetic resonance (EPR) spectroscopy. Following sonolysis, -*CH- radicals were observed for both surfactants under both sonication modes. Under CW at 354 kHz, the maximum plateau -*CH- radical yield was higher for SPSo than for SDS, indicating that SDS, which is more surface active under equilibrium conditions, accumulates at the gas/solution interface of cavitation bubbles to a lesser degree, compared with the less surface active surfactant, SPSo. However, after sonolysis (354 kHz) under pulsed ultrasound with a pulse length of 100 ms and an interval of 500 ms, the -*CH- radical yield at the plateau concentrations was higher for SDS than for SPSo due to increased amounts of SDS accumulation on the bubble surfaces. In contrast to the findings following sonolysis at 354 kHz, sonolysis of aqueous surfactant solutions at 620 kHz and 803 kHz showed a higher -*CH- radical yield for SDS compared with SPSo under CW but lower -*CH- radical yield with increasing pulsing interval, indicating a frequency dependence on accumulation. Results indicate that pulsing the ultrasonic wave has a significant effect on the relative adsorption ability of n-alkyl surfactants at the gas/solution surface of cavitation bubbles.     

An application of spin trapping to radical polymerizations

Radical polymerizations and copolymerizations were carried out in the presence of phenyl tert-butyl nitrone(PBN) and tert-nitrosobutane (t-BuNO), and the structure of the spin adduct formed was investigated by ESR spectroscopy. PBN adducts gave the same ESR pattern, and the variation of the splitting constant was not large enough to warrant its use for the structure assignment. Therefore, the subsequent trapping experiments were performed with t-BuNO. The polymerization mixture of representative monomers showed esr patterns that are indicative of the propagating radical being trapped. These trapped radicals were not necessarily very stable and, in most cases, disappeared after long reaction periods. In the case of α-methyl substituted monomers, additional nine-line spectra were observed which were attributed to trapping of the radical species formed by hydrogen abstraction from the α-methyl group. The tert-butyl radical which was formed by decomposition of t-BuNo was probably responsible for the hydrogen abstraction. In the case of styrene, methyl acrylate, and methyl methacrylate, characteristic ESR patterns of the propagating radicals were observed with polymers which were prepared in the presence of t-BuNO and purified by reprecipitation. Simultaneous trapping of different propagating radicals was attempted in several copolymerization systems. However, this was generally unsuccessful, because of the large difference in reactivities of the propagating radical with t-BuNo.     

Influence of Electronic Effects on the Reactivity of Triazolylidene‐Boryl Radicals: Consequences for the use of N‐Heterocyclic Carbene Boranes in Organic and Polymer Synthesis

A small library of triazolylidene‐boranes that differ only in the nature of the aryl group on the external nitrogen atom was prepared. Their reactivity as hydrogen‐atom donors, as well as that of the corresponding N‐heterocyclic carbene (NHC)‐boryl radicals toward methyl acrylate and oxygen, was investigated by laser flash photolysis, molecular orbital calculations, and ESR spin‐trapping experiments, and benchmarked relative to the already known dimethyltriazolylidene‐borane. The new NHC‐boranes were also used as co‐initiators for the Type I photopolymerization of acrylates. This allowed a structure–reactivity relationship with regard to the substitution pattern of the NHC to be established and the role of electronic effects in the reactivity of NHC‐boryl radicals to be probed. Although their rate of addition to methyl acrylate depends on their electronegativity, the radicals are all nucleophilic and good initiators for photopolymerization reactions.     

ESR study of spin-trapping with two glycosylated analogues of PBN able to target cell membrane lectins

The spin trapping behaviour of the two galactosylated nitrones LAMPBN and TA1PBN, both of which are able to target cells through recognition by cell membrane lectins, were widely investigated on a variety of free radicals in aqueous media. The ESR spectra of the more amphiphilic nitrone, TA1PBN, were interpreted in the light of the LAMPBN trapping results. The spin adducts of nitrone TA1PBN, which may be better distributed inside the cell, were surrounded by two distinct environments due to the tensioactive organisation of the trap and gave two different ESR signals for each radical trapped.     

过硫酸盐和脂肪环胺体系引发机理的ESR研究

用ESR方法研究了过硫酸盐和脂肪环胺吗啉,哌啶及其N-烷基取代衍生物体系引发反应的初级自由基,结合端基分析,证实了这类脂肪环胺与过硫酸盐反应产生的初级自由基都能引发烯类单体聚合。当所用的是脂环仲胺时,其初级自由基为氮中心自由基;用脂环叔胺时,初级自由基是N-烷基的α-碳中心自由基。