Electron spin resonance studies of plasma-induced polystyrene radicals

Plasma-induced polystyrene radicals were first studied by electron spin resonance (ESR). The room temperature ESR spectrum was compared with those obtained by γ-irradiation, UV-irradiation, and mechanical fracture. It was found that even less than a few seconds of plasma-irradiation gave rise to a large amount of polystyrene radicals and the ESR spectrum consisted of two types of spectra, a triplet and a single broad line. The spectral feature of the triplet was nearly identical with that of γ-irradiated polystyrene. Thus, it was assigned to the structure of a cyclohexadienyl-type radical formed by a nearly random addition of a hydrogen atom to the aromatic ring. The single broad line, thought to be an outline of multicomponent spectrum, was assigned to an immobilized dangling-bond sites at the plasma-induced crosslinked portion of the polystyrene surface.     

Electron spin resonance studies of propagating radicals in polymerization. I. Methacrylate propagating radicals

Ferric chloride-photosensitized free-radical initiation was used to generate propagating radicals in polymerization of methacrylic acid (MAA), allyl methacrylate (AMA), methyl methacrylate (MMA), 1,3-butylene dimethacrylate (1,3-BDMA), hydroxypropyl methacrylate (HPMA), lauryl methacrylate (LMA), hexyl methacrylate (HMA), and methacrylamide (MA) in rigid glasses of methanol or acetone at near liquid nitrogen temperatures. The formation and conformational changes of these propagating radicals at different temperatures were studied by electron spin resonance (ESR) spectroscopy. When methanol was the rigid glass, ·CH₂OH radicals were formed initially and were stable below ?160°C. As the temperature of the rigid glass was increased, the ·CH₂OH radicals reacted with monomer to yield propagating radicals. With the exception of the propagating radical for methacrylamide, the propagating radicals of the methacrylates examined initially generated five-line ESR spectra which gradually changed to nine-line spectra, as temperature of the rigid glass was increased. It was concluded that one type of propagating radical was formed in all cases. However, when the temperature of the rigid glass was increased, the single structural conformation that initially allowed one of the methylene hydrogens and methyl group to interact with the unpaired electron to generate only a five-line spectrum was changed to yield a second conformation that allowed interaction to generate an additional four-line spectrum. Finally, a mixture of the propagating radical for methacrylate monomer in two structural conformations was obtained, and an ESR spectrum consisting of nine lines (5 + 4 lines) was generated. In the case of the propagating radical for methacrylamide this change to yield two structural conformations evidently was hindered, so that only an ESR spectrum consisting of five lines was generated.     

Electron spin resonance studies of propagating radicals in polymerization. II. Acrylate propagating radicals

Ferric chloride-photosensitized free-radical initiation was used to generate propagating radicals in polymerization of acrylic acid (AA), methyl acrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), acrylamide (A), and diacetone acrylamide (DAA) in rigid glasses of methanol, ethanol, n-propanol, isopropanol, or acetone at near liquid nitrogen temperatures. When the temperatures of the glasses were increased, primary radicals derived from the solvents reacted with the monomers to yield propagating radicals. Formation and conformational changes of the propagating radicals at different temperatures were studied by electron spin resonance (ESR) spectroscopy. It was concluded that one type of propagating radical was formed in all cases. However, when the temperature of the rigid glass was increased, the structural conformation of the radical that initially allowed the near-equivalent interaction of the α-hydrogen and one of the β-hydrogens with the unpaired electron generated a three-line spectrum.     

Electron spin resonance studies of several anion radicals of nitro-substituted thioacylpiperidines and morpholines

The radical anions of m- and p- nitro-substituted derivatives of several classes of N-(thioacyl)-piperidines and -morpholines have been studied by ESR spectroscopy. The anion radicals were found to be centred on the nitro group, and the distribution of the unpaired electron was found to be dependent on the extent of conjugation between the thiocarbonyl group and the aromatic moiety of the molecule. The hfs constants were discussed in terms of the spin densities calculated by the McLachlan procedure.     

Electron spin resonance spectra of transient tin-centered radicals in solution

ESR data for short-living stannyl radicals in solution are presented. The radicals are generated in the cavity of the ESR spectrometer by UV irradiation of di-t-butyl peroxide with the appropriate tin hydride. The radicals R₃Sn^. (R = Me, n-Pr, n-Bu) show multiplets caused by interaction with the β-protons (a_H = 0.30–0.31 mT, g = 2.0160–2.0163) and line broadenings depending on [R₃SnH]. The radicals Ph_nMe_3−nSn^. (n = 1, 2) exhibit only splittings due to the methyl protons (a_H = 0.30 mT). The observed linewidths show that a_o,p 0.05 mT, a_m 0.03 mT. The radicals Ph_nEt_3−nSn^. (n = 0–2) show no splittings caused by the methylene protons because of exchange narrowing effects. The g values decrease with increasing n from 2.0163 and 2.015 (n = 0) to 2.0023 (n = 3) because of increasing deviations from the planar conformation at the radical center. The line broadening and exchange narrowing effects are caused by rapid hydrogen exchange between the radicals and hydride molecules (k 10⁶ M⁻¹ s⁻¹); in Zavitsas' model, the relatively high k values are the consequence of the large Sn---H bond lengths which diminish the repulsive forces between the terminal Sn atoms in the transition state [R₃Sn--H--SnR₃]^.. The observation of line broadenings in the NMR spectrum of Me₃SnH during irradiation with di-t-butyl peroxide confirms the ESR results.     

Electron spin resonance of radicals in γ-irradiated ethylene-propylene-polyene terpolymers

The ESR spectra resulting from γ-irradiation of the following terpolymers: ethylene-propylene-5-ethylidene-2-norbornene (EPENB), ethylene-propylene-l-isopropylidendicyclopentadiene (EPIPDCP) and ethylene-propylene-tricyclo-5.2.1.0^2.6-deca-2.5.8-triene (EPDDCP) have been investigated. Measurements with ethylene-propylene copolymer and isotactic polypropylene have been made for comparison. The results suggest that, during the irradiation at 77 K, most of the radicals can be accounted for in terms of loss of hydrogen atoms from the main EP chains. On warming above the glass transition temperature, some of the primary radicals react with termonomer units yielding new allyl type radicals. The dominant mechanism of reaction seems to be double bond addition in the case of EPIPDCP and EPDDCP and hydrogen abstraction in the case of EPENB.     

Electron spin resonance spectra of α-sulfonyl iminoxy radicals

Syn- andanti-α-sulfonyl iminoxy radicals have been generatedin situ by lead tetra-acetate oxidation of α-oximino sulfones. The ESR spectra of the geometrical isomers have been assigned and the¹⁴N and¹H hyperfine splitting constants are discussed in terms of the proposed structures.     

Electron spin resonance studies on radiation graft copolymerization. I. Grafting initiated by alkyl radicals trapped in irradiated polyethylene

The behavior of trapped radicals in polyethylene which is irradiated in air at room temperature, under grafting of methylmechacrylate or butadiene has been studied by electron spin resonance. Part of the alkyl radicals are converted to allyl radicals by reaction with double bonds and the others disappear by recombination under vacuum. The active species of grafting are alkyl radicals when the vapor pressure of monomers is relatively high, while at low pressure allyl radicals also play a role as well as alkyl radicals. In the grafting at 20°C, the grafting yields depend mainly on the decay rate of alkyl radicals which come out of the crystalline regions of polyethylene. The decay rate of alkyl radicals and the rate of grafting at the initial stage increase with decreasing crystallite size of polyethylene.     

Electron spin resonance spin trapping of thiyl,radicals from the decomposition of thionitrites

Alkyl thiyl radicals produced by the homolytic decomposition of thionitrites are detected by ESR spin trapping using 5,5-dimethyl-1-Δ-pyrroline-N-oxide (DMPO).     

Electron spin resonance studies on radiation graft copolymerization in polyethylene. II. Grafting initiated by allyl radicals trapped in irradiated polyethylene

The regions trapping allyl radicals in irradiated polyethylene and the reactivity of the radicals with various monomers have been studied by electron spin resonance. Most of the allyl radicals are trapped in defects or surfaces of the crystallite, and only 10% of the radicals are trapped inside the crystallites. Since the reactivity of the allyl radicals depends on whether they come in contact with reactive monomers, the rate of reaction is related to the concentration of the monomers. When grafting reactions are initiated by allyl radicals, the rate of initiation is rapid and the rate of termination at the initial stage is also very fast. Consequently the yield of grafting increases at the initial stage but soon levels off, which is in contrast to the grafting behavior with alkyl radicals.     

Electron spin resonance studies of anisotropy in semiconducting polymeric films

X‐band electron spin resonance was employed to study the structural anisotropy in several polythiophene derivatives. Because of the dominating homogeneous width, the obtained absorption spectra were Lorentzian‐shaped. Information about the structural anisotropy was obtained from the position and width of the absorption peak. Qualitatively, the anisotropy was in full agreement with earlier results from X‐ray diffraction, including a flip in molecular orientation with respect to the film substrate between solution‐cast and spin‐cast films. With the Monte Carlo technique, the spectra were fitted with a biaxial g tensor, an anisotropy parameter S, and an intrinsic width σ. The simulations showed that g could be treated as pseudo‐uniaxial, with the unique axis along the side chains rather than along the ring normal. Closed‐form analytical expressions relating g to the anisotropy were obtained and used for a quantitative assessment of the molecular anisotropy. Because the molecular g tensor for these materials was not known, a known value of S for one of the samples obtained by X‐ray diffraction was used for normalization. Fairly consistent values were obtained for both g and S. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3011–3025, 2003     

Dimethylsilyl- and methylene-bridged aromatic groups : III. Electron spin resonance spectra of anion radicals

From the ESR spectra of anion radicals of bis(4-biphenylyl)dimethylsilane and 4,4′-bis(4-biphenylyldimethylsilyl)biphenyl, the delocalization of spin density is inferred. The bridging SiMe₂-groups separate practically independent π-systems. The rate of electron transfer between biphenyl sub-units is less than ～ 10⁶ sec^?1 at 223 K.     

Electron spin resonance studies on deuterated nitroxyl spin probes used in Overhauser‐enhanced magnetic resonance imaging

The electron spin resonance studies were carried out for 2 mm concentration of ¹⁴N‐labeled and ¹⁵N‐labeled 3‐carbamoyl‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl, 3‐carboxy‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl, 3‐methoxycarbonyl‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl and their deuterated nitroxyl radicals using X‐band electron spin resonance spectrometer. The electron spin resonance line shape analysis was carried out. The electron spin resonance parameters such as linewidth, Lorentzian component, signal intensity ratio, rotational correlation time, hyperfine coupling constant and g‐factor were estimated. The deuterated nitroxyl radicals have narrow linewidth and an increase in Lorentzian component, compared with undeuterated nitroxyl radicals. The dynamic nuclear polarization factor was observed for all nitroxyl radicals. Upon ²H labeling, about 70% and 40% increase in dynamic nuclear polarization factor were observed for ¹⁴N‐labeled and ¹⁵N‐labeled nitroxyl radicals, respectively. The signal intensity ratio and g‐value indicate the isotropic nature of the nitroxyl radicals in pure water. Therefore, the deuterated nitroxyl radicals are suitable spin probes for in vivo/in vitro electron spin resonance and Overhauser‐enhanced magnetic resonance imaging modalities. Copyright © 2017 John Wiley & Sons, Ltd.     

Electron spin resonance study on plasma-induced surface radicals of poly(ethylene naphthalate)

We have undertaken Ar plasma irradiation on poly(ethylene naphthalate) (PEN) powder, and the radicals formed were studied by electron spin resonance (ESR). The room temperature ESR spectrum of plasma-irradiated PEN shows a five-line spectrum separated with nonbinomial intensity distribution, indicating that the spectrum is an outline of multicomponent spectra. The systematic computer simulation of the observed ESR spectra disclosed that the spectra consist of two types of radicals in structural term: the major radicals formed were assigned to dihydronaphthalenyl-type radicals generated by a nearly random addition of a hydrogen atom to the naphthalene ring, and immobilized dangling bond sites at the surface crosslinked moiety.     

Electron spin resonance studies of phosphatidylcholine interacted with cholesterol and with a hopanoid in liposomal membrane.

The effects of bacteriohopane-32-ol (Monol) on liposomal membrane composed of dipalmitoylphosphatidylcholine (DPPC) or egg yolk phosphatidylcholine (egg PC) were compared with those of cholesterol (Chol) in the change of fluidity using a spin label. The fluidity change close to the polar head groups caused by temperature increase in the DPPC membrane containing Monol was different from that of Chol. Chol had a condensing effect on DPPC membrane, whereas Monol had only a slight effect except when used at 20 mol%. Near the hydrophobic end, Chol incorporation into DPPC led to fluidization below transition temperature (Tm) and condensation above Tm. Monol incorporation into DPPC had only a fluidizing effect below Tm. On the other hand, in egg PC membrane Chol had the condensing effect at any temperature, whereas Monol had only slight effect. These results suggest that Monol may have a role in supporting constant membrane fluidity under drastic conditions.