Role of radicals in UV‐initiated postplasma grafting of poly‐ε‐caprolactone: An electron paramagnetic resonance study

Electron paramagnetic resonance (EPR) measurements were performed on poly‐ε‐caprolactone (PCL) films at different stages of the postplasma‐grafting process. PCL films prepared by solvent casting (SC) or electrospinning (ESP) yield very similar EPR spectra after Ar‐plasma treatment and subsequent exposure to air, but the EPR signal is much stronger in the PCL‐ESP films. The free radicals appear to be mainly, and possibly exclusively, oxygen centered. The radicals generated by UV irradiation in PCL‐ESP films were studied in situ with EPR, using a UV‐LED (λ = (285 ± 5) nm). Their EPR spectrum is distinctly different from the plasma‐induced signal, indicative of carbon‐centered radicals, and appears to be independent of the plasma pretreatment. UV‐induced homolytic splitting of (hydro)peroxide bonds was not observed. Both the plasma‐ and UV‐induced radicals decay at room temperature (RT), even in an inert atmosphere. This study demonstrates the potential of electrospun films and UV‐LEDs for the study of plasma‐ and UV‐generated free radicals with EPR in polyesters, and raises questions with respect to the validity of some generally accepted molecular mechanisms underpinning the postplasma grafting technique for polyesters. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

DFT‐EPR study of radiation‐induced radicals in α‐D‐glucose

The structures of two radiation‐induced radicals in solid‐state α‐D ‐glucose have been identified by means of single‐molecule density function theory (DFT) calculations. Using the original crystalline structure as input, several radical models were created and their geometries optimized. Subsequently, electron paramagnetic resonance (EPR) parameters were calculated. During these calculations, the global orientation of the radical structure was kept fixed with respect to the crystal axes reference frame. This was essential to allow for an easy analysis of the hyperfine tensor principal directions, besides the isotropic and anisotropic coupling constants. By comparing these calculated EPR parameters with their experimentally determined counterparts, a plausible identification of two carbon‐centered glucose radicals was possible. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

ESR studies in γ‐irradiated and PS‐radiation‐grafted poly(vinylidene fluoride)

The fluoropolymer poly(vinylidene fluoride) was irradiated with γ rays to induce a polystyrene (PS) radiation grafting via an indirect method. Electron spin resonance and Fourier transform infrared studies were performed to identify the species that initiated the PS grafting. Specific experiments were performed to prove separately the importance of each kind of radical. A localization of the radicals in this irradiated polymer is proposed, and a kinetic model of the grafting is given. © 2001 John Wiley & Sons, Inc. J Polym Sci B Part B: Polym Phys 39: 1437–1448, 2001     

Identification of radical structures on 1‐pentamethylbenzyl‐3‐ethylimidazoliumsilver(I)bromide and 1,3‐bis(pentamethylbenzyl)‐4,5‐dimethylbenzimidazoliumsilver(I)bromide exposed to gamma rays: an EPR study

1‐Pentamethylbenzyl‐3‐ethylimidazoliumsilver(I)bromide and 1,3‐bis(pentamethylbenzyl)‐4,5dimethylbenzimidazoliumsilver(I)bromide and their Ag⁺ complexes were synthesized and their polycrystal forms were produced by recrystallization in dichloromethane/Et₂O solvent system. Structural determinations were carried out by ¹H NMR and ¹³C NMR with a Varian 400 NMR system using tetramethylsilane as internal standard and CDCl₃ as solvent. The disappearance of acidic N‐heterocyclic carbene proton showed the formation of Ag(I) complexes. Also, elemental analyses were carried out. Electron paramagnetic resonance (EPR) measurements were performed to determine the formed radical structure on the samples irradiated at the room temperature for 72 h by using ⁶⁰Co‐source with dose rate of 0.680 kGy. The EPR measurements were carried out in the temperature range of 200 K–450 K. Identical radicals were determined on the irradiated compounds. It was observed that the shapes of the spectra of the samples were independent of the temperature but, the resonance line intensities changed linearly with the temperature. Also, it was detected that the free radical on the 1‐pentamethylbenzyl‐3‐ethylimidazoliumsilver(I)bromide is not stable compared to that on the 1,3‐bis(pentamethylbenzyl)‐4,5dimethylbenzimidazoliumsilver(I)bromide. Copyright © 2016 John Wiley & Sons, Ltd.     

Electron spin resonance study on chemical‐crosslinking reaction mechanisms of polyethylene using a chemical agent. VI. Effect of α‐methyl styrene dimer

The effect of α‐methyl styrene dimer (AMSD), which is used as a scorch retarder, on the reaction mechanisms of the chemical crosslinking of polyethylene (PE) with dicumyl peroxide (DCP) at high temperatures was investigated using electron spin resonance. When AMSD was added to PE containing DCP, the AMSD radical was observed; however, the PE alkyl radical or allyl radical presence was not detected. At 145 °C, crosslinking was obstructed as a result of the reaction between AMSD and alkyl radicals. As the temperature increased, AMSD fragmented to form 2‐phenyl‐2‐propyl and double bonds in PE. This generation of double bonds, however, accelerated crosslinking at 180 °C and was more effective than when AMSD was not present. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2151–2156, 2001     

γ射线辐射医用级超高分子量聚乙烯自由基的演变

采用γ射线对医用级超高分子量聚乙烯(UHMWPE)进行辐照处理, 利用电子自旋共振波谱仪(ESR)研究了辐照诱导自由基的种类及其在氩气和不同氧分压下的衰减行为. 在氩气中, 辐射诱导UHMWPE主要产生烷基自由基和烯丙基自由基, 总的辐射化学产额约为0.48/100 eV. 室温下烷基自由基的稳定性差, 其寿命仅有 1 d左右. 在含氧气氛中, 自由基主要通过氧化反应而衰减, 其衰减速率随氧分压的增加而增加, 半衰期则由1×10⁵ Pa氩气中的224.0 h降至5×10⁵ Pa O₂气中的1.8 h. 根据此结果推算, 室温下陷落在晶区的自由基迁移至微晶表面的速率非常快, 仅需小时量级.     

HYSCORE and Davies ENDOR study of irradiated ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been studied with different magnetic resonance techniques to elicit information on the nature and the location of radicals generated during high energy irradiation. Field swept electron paramagnetic resonance, pulsed Davies electron nuclear double resonance and hyperfine sublevel correlation spectroscopic measurements allowed extracting for the first time the full ¹H hyperfine coupling tensors of the most abundant radical, i.e. a secondary alkyl radical and to ascertain the formation of allyl radicals in the first stages of the irradiation process. The ¹H hyperfine coupling tensors are analogous to those reported for single crystal irradiated polyethylene, suggesting that radicals generated in UHMWPE are located in the crystalline region of the polymer. Copyright © 2012 John Wiley & Sons, Ltd.     

Abilities of 1‐(9‐anthrylmethyloxy)‐2‐pyridone and related compounds to initiate radical and cationic photopolymerizations

This study explored the abilities of 1‐(9‐anthrylmethyloxy)‐2‐pyridone and related compounds, which absorb long‐wavelength light (>350 nm), to photochemically initiate radical and cationic polymerizations. It was found that the irradiation of the title compounds initiates the radical polymerization of styrene whereas the cationic polymerization of oxetane proceeds in the presence of these photoinitiators to a negligible extent. The behavior of 9‐anthrylmethyloxyl and amidyl radicals in the photopolymerization process of styrene was discussed based on ¹H NMR, UV, and fluorescence spectral data. In addition, the photoinitiation ability of the anthrylmethyloxyl end group was also investigated by using its model compound. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2859–2865, 2004     

Photoinduced controlled/living free‐radical polymerization of 4‐methacryloyl‐1,2,2,6,6‐pentamethyl‐piperidenyl

The photoinduced solution polymerization of 4‐methacryloyl‐1,2,2,6,6‐pentamethyl‐piperidinyl (MPMP), used as a reactive hindered amine piperidinol derivative, was performed. The obtained MPMP homopolymer had a very narrow molecular weight distribution (1.06–1.39) according to gel permeation chromatography. The number‐average and weight‐average molecular weights increased linearly with the monomer conversion, this being characteristic of controlled/living free‐radical polymerizations. Electron spin resonance signals were detected in the MPMP homopolymer and in a polymer mixture solution, and they were assigned to nitroxide radicals, which were bound to the polymer chains and persisted at a level of 10^?9 mol/L during the polymerization. Instead of the addition of mediated nitroxide radicals such as 2,2,6,6‐tetramethyl‐piperidinyl‐1‐oxy (TEMPO), those radicals (>N? O ·) were formed in situ during the photopolymerization of MPMP, and so the reaction mechanism was understood as being similar to that of TEMPO‐mediated controlled/living free‐radical polymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2659–2665, 2004     

Three different tyrosyl radicals identified in L-tyrosine HCl crystals upon gamma-irradiation: magnetic characterization and temporal evolution

High-frequency electron paramagnetic resonance (EPR) and X-band electron-nuclear double resonance (ENDOR) spectroscopies were used to investigate the effect of gamma-irradiation on single crystals of L-tyrosine hydrochloride at room temperature. The oxidation product is the tyrosyl radical formed by hydrogen abstraction from the phenolic group; interestingly, on freshly irradiated crystals, two tyrosyl radicals were identified, characterized by slightly different magnetic parameters. In particular, one of the two radicals, with a gxx value of 2.00621, has its phenoxyl oxygen strongly hydrogen-bonded to one or more donors; to our knowledge, this is the lower gxx value reported for tyrosyl radicals. These two oxidation radicals are found to evolve very slowly to a third, single more stable radical conformation. To interpret the experimental data, a possible molecular scenario is presented, where the process of radical formation can be seen as a hydrogen atom transfer or a proton-coupled electron transfer. These processes seem to be controlled by the specific network of hydrogen-bond interactions present in the crystal. The results are discussed in relation to their relevance for the interpretation of EPR spectra of tyrosyl radicals in biological systems.     

An unusual photo‐oxidation pathway for a poly(2,6‐dimethyl‐1,4‐phenylene oxide) model compound

The long wavelength UV photochemistry was investigated of a model compound for poly(2,6‐dimethyl‐1,4‐phenylene oxide). Irradiation of the phenyl‐capped dimer of 2,6‐dimethyl phenol at wavelengths >290 nm with UVA‐340 fluorescent lamps in the absence of oxygen gave no detectable products after 27 days. Very low conversion to oxidation products was found in the presence of oxygen, but about 20% conversion to products in which solvent had added to the benzylic methyl groups occurred under aerobic conditions when the solvents had readily abstractable hydrogen atoms. A mechanism is proposed involving the facile, reversible abstraction of a benzylic hydrogen by oxygen as a first step in the oxidation. The hydroperoxyl radical that is formed can abstract hydrogen atoms from 2° and 3° carbons of the solvent, and these radicals can combine with the benzylic radicals to give the solvent adducts. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2221–2226     

Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene‐propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene‐propylene copolymer (EPM), and ethylene‐propylene‐diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000     

High performance liquid chromatography/electron spin resonance/mass spectrometry analyses of radicals formed in an anaerobic reaction of 9- (or 13-) hydroperoxide octadecadienoic acids with ferrous ions

The oxidation of linoleic acid yields isomeric acyl hydroperoxides. In order to clarify the relation between the lipid peroxide-derived radicals and the toxicity of the lipid peroxide, identification of the lipid-derived radicals is essential. In this paper, high performance liquid chromatography/electron spin resonance/mass spectrometry (HPLC/EPR/MS) analysis of the radicals was performed for the reaction mixture containing 9-hydroperoxy-(10E,12E)-octadeca-10,12-dienoic acid (9EE-OOH) [or 13-hydroperoxy-(9Z,11E)-octadeca-9,11-dienoic acid (13ZE-OOH)] under an aerobic condition or an anaerobic condition. Following radicals were identified from 9EE-OOH (or 13EZ-OOH) by using high performance liquid chromatography/electron spin resonance spectrometry (HPLC/EPR) and HPLC/EPR/MS: pentyl radical and isomers of epoxylinoleic acid radicals from 13EZ-OOH under an anaerobic condition; 7-carboxyheptyl radical and pentyl radical from 13EZ-OOH under an aerobic condition; 7-carboxyheptyl radical and pentyl radical from 9EE-OOH under an aerobic condition; 7-carboxyheptyl radical from 9EE-OOH under an anaerobic condition. These results showed that the formation of the respective radical species depends on oxygen concentration in the reaction mixtures to a great extent.     

Copper‐Catalyzed Radical/Radical CH/PH Cross‐Coupling: α‐Phosphorylation of Aryl Ketone O‐Acetyloximes

The selective radical/radical cross‐coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper‐catalyzed radical/radical C? H/P? H cross‐coupling has been developed. It provides a radical/radical cross‐coupling in a selective manner. This work offers a simple way toward β‐ketophosphonates by oxidative coupling of aryl ketone o‐acetyloximes with phosphine oxides using CuCl as catalyst and PCy₃ as ligand in dioxane under N₂ atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o‐acetyloximes generates iminium radicals, which could isomerize to α‐sp³‐carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o‐acetyloximes and phosphine oxides were suitable for this transformation.     

Identification of oxidative processes during simulated mastication of uncooked foods using electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is used to measure directly the generation of free radicals during a simulation of the mastication process. This involves the gentle grinding of the food product in the presence of a spin trap, a molecule which reacts selectively with unstable free radicals to generate (more) stable radical adducts, which can then be characterised. With mushrooms of the Agaricus family, adducts consistent with a carbon-centred radical are seen with a wide range of spin traps and this radical has been confirmed as 4-(hydroxymethyl)phenyl. In plant tissues that are rich in ascorbic acid, this molecule competes successfully with spin traps for the free radicals and the (monodehydro)ascorbate radical, formed by the 1-electron oxidation of ascorbic acid, is seen in the EPR spectra. However, with >50% of the plant tissue samples studied in the present experiment, free radicals resulting from oxidation of the spin traps were observed. The formation of such molecules, for which oxygen was found to be necessary, requires the existence of powerful oxidation processes as the plant tissue is broken down. Such pro-oxidant behaviour is contrary to the popular assumption that the beneficial effects of uncooked plant tissues are the result of their high levels of anti-oxidant molecules.     

Reactive intermediates in the initiation step of the photo‐oxidation of MDMO‐PPV

Because oxygen cannot be fully eliminated from organic solar cells, the occurrence of oxidative photo‐degradation of the device in operating conditions has to be considered. Polyphenylene‐vinylene‐based photovoltaic devices have a short lifetime that currently limits their applications. In this article, we focus on various transient species that are potentially involved in the initiation step of the photo‐oxidation of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV). The role of the transient species was investigated by a combination of quenching and sensitization experiments. Complementary information was obtained by transient absorption spectroscopy. Results evidenced the fact that ¹O₂ was not the principal reactive intermediate involved in the photo‐oxidation of MDMO‐PPV. This result was in contradiction with previous reports. It was shown that the MDMO‐PPV^?+ radical cation was generated after excitation. The presence of oxygen and the photo‐aging favored the formation of the radical cation, suggesting that oxygen and photoproducts act as electron acceptors. The charged radicals formed are likely to evolve and give radicals that initiate the oxidation of the polymer by abstraction of the labile hydrogen in α position of the ether function and by addition on the double bonds. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6044–6052, 2009     

Electron paramagnetic resonance spectroscopy of γ-irradiated cellulose and related compounds

EPR studies were carried out on different irradiated cellulosic materials. The shape of the EPR spectrum was found to depend upon the fine structure of cellulose. The peak-to-peak distances of the large peaks on the differential curves were found to be 23.3 and 24.8 gauss, respectively, for the amorphous and recrystallized cellulose. The number of free radicals present per unit weight in the recrystallized cellulose and in the amorphous cellulose were found to be in the ratio of 2.1:1. The EPR spectrum of a sample of irradiated cotton crystallites which was soaked in acetone before being dried was markedly symmetrical, in contrast to the EPR spectra of samples of cotton linters and of cotton crystallites dried from water, and was considered to be a reflection of the lower intermolecular bonding in the former. For corresponding doses, the free radicals on cotton crystallites have the highest g value, followed by cotton linters, followed by regenerated cellulose I. The g value decreased as the irradiation dose was increased.     

Mechanism of action of sensors for reactive oxygen species based on fluorescein-phenol coupling: the case of 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid

We demonstrate the ability of a sensor containing a tethered fluorescein-phenol structure to react with peroxyl radicals and with an oxidizing agent such as potassium ferricyanide. This latter reaction yields the corresponding peroxyl radical as observed by EPR analysis. We propose that the reaction of the sensor with peroxyl and alkoxyl radicals is also initiated by the formation of the phenoxyl radicals, which is followed by radical-radical reactions and product hydrolysis responsible for the release of fluorescein. The proposed mechanism is based on results obtained by laser flash photolysis, HPLC and EPR studies of the reaction of peroxyl and alkoxyl radicals with 4-phenoxylphenol, a molecule used to mimic the behavior of the sensor.     

Spin-polarized peroxyl radical adducts formed from the addition of oxygen to amino acid radicals

[Structure: see text] Peroxyl radicals formed from the addition of oxygen to carbon radicals of N-acetyl glycine, serine, and diglycine are directly observed at room temperature via time-resolved EPR spectroscopy. Isotopic labeling confirms the identity of the N-acetyl glycine and diglycine peroxyl analogues. The peroxyl radicals show unusually strong chemically induced electron spin polarization which is discussed in terms of the radical pair mechanism and spin polarization transfer processes.     

Why is the order reversed? peroxyl‐scavenging activity and fats‐and‐oils protecting activity of vitamin E

The orders of relative tocopherol biopotency, scavenging activity of lipid peroxyl radicals, and quenching activity of singlet oxygen are as follows: jalpha‐tocopherol > β‐tocopherol ? γ‐tocopherol > δ‐tocopherol. However, the reverse is the case for the activity to protect fats and oils from oxidation: jalpha‐tocopherol < β‐tocopherol < γ‐tocopherol δ‐tocopherol. The reason for this reverse has been studied by measuring weight gains of solutions including methyl linoleate and various antioxidants at daily intervals. Antioxidant activity is expressed as an induction period in days. Induction periods are compared with the second‐order rate constants for scavenging of an aroxyl radical by the antioxidants. A plot of the logarithm of the second‐order rate constant vs. the induction period is found to give a good linear fit, and the slope is negative. From these results, it is considered that the larger the peroxyl‐radical‐scavenging activity of a tocopherol, the more favorable the production of the tocopheroxyl radical, and thus the more efficient the oxidation of fats and oils by the tocopheroxyl radical. This is not the case for tocopherols in vivo, because vitamin C and ubiquinol reduce tocopheroxyl radicals formed from tocopherols. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 605–610, 2005