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 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 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 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.     

Nature of plasma-induced radicals on crosslinked methacrylic polymers studied by electron spin resonance

Plasma-induced radicals of several crosslinked methacrylic polymers such as poly(ethyleneglycol dimethacrylate) (PEDMA), poly(2-hydroxyethylmethacrylate) (PHEMA) and polymethacrylamide (PMAAm) were studied by electron spin resonance (ESR). The observed ESR spectra did not exhibit a drastic difference in the spectral feature caused by the effect of crosslinking. All the spectral features can be represented by “nine-line spectra” as a major spectral component similar to those of linear methacrylic polymers such as polymethacrylic acid (PMAA). A pronounced effect of crosslinking, however, has emerged on the specific formation in the radical structure and the stability of radicals formed, especially in PEDMA. The formation of fewer kinds of radical in PEDMA is apparently caused by the high degree of crosslinking which leads to a suppression of the occurrence of depolymerization on plasma irradiation.     

Electron spin resonance studies of spin-labelled polymers—I segmental relaxation of polystyrene in solution

From an analysis of the widths of the hyperfine lines in the ESR spectrum of a nitroxidelabelled polystyrene in toluene solution, the correlation times for rotational diffusion at three temperatures have been measured. The values agree well with published data from NMR studies of polystyrene in solution. The value of the activation energy for the relaxation process, 4·3 kcal. mole⁻¹, is close to published values for dielectric relaxation of para-substituted polystyrenes. These comparisons provide strong evidence for associating the ESR correlation times with some form of segmental motion of the polymer backbone.     

Electron spin resonance studies of spin-labeled polymers. IV. Slow-motional effects in solid polystyrene

Shape changes in the electron spin resonance spectrum of spin-labeled polystyrene have been studied as a function of temperature in the range 77–340°K. Rotational correlation times have been calculated by using recent theories of slow-motional effects on ESR spectra. Three models were used, namely, Brownian, moderate jump diffusion, and large jump diffusion. The moderate jump model gave correlation times in good accord with those found by other techniques for the δ relaxation in polystyrene. The relaxation was found to be independent of molecular weight.     

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 study of new N-alkoxyphosphoramidyl and N-alkylphosphoramidyl radicals

The title radicals have been generated and studied by esr spectroscopy. The N-alkoxyphosphoramidyl radicals are formed by hydrogen abstraction from the corresponding protic parent compounds while the N-alkylphosphoramidyls can be generated by photolysis of cyclopropane solution of the corresponding N-bromo derivatives. Both types of radical exhibit esr data characteristic of a π electronic configuration.     

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 studies of spin labeled alternating functional copolymers

The spin-labeled and spin-probed alternating copolymers of poly(phenylvinyl alkyl ethers) with various alkyl groups (Me, Et, sec-Bu) in the ether moiety were investigated by electron spin resonance (ESR). Measurements were taken in different solvents over a wide range of concentration. Studies reveal that the molecular motion of spin-labels and spin-probes depends on the bulkiness of the alkyl group at the solvent concentration in which the effect of segmental rotation is dominant. The implication of the type of chemical bonding on the nitroxide motion is discussed.     

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 spectra of some phosphonyl radicals and related species

The isotropic ESR spectra of a number of phosphonyl radicals (X₂$\text{P}$O), the dimethylphosphinyl radical, and the phosphoranyl radical (MeO)₃$\text{P}$OBu-t, are described, and accurate values of the phosphorus hyperfine splittings and g-factors are reported. For X₂$\text{P}$O, the value of a(P) increases and the g-factor decreases as the electronegativity of X increases. There is a linear relationship between a(P) for X₂$\text{P}$O and ¹J(PH) for X₂P(O)H, but the same relationship does not hold for Me₂P- and Me₂PH. The spectrum of the di-n-hexylphosphonyl radical shows coupling to two pairs of α-methylene protons, and this non-equivalence is attributed to the pyramidal structure of the phosphonyl radical.