ESR study of primary radical formation during mechanical degradation of poly(2,6-dimethyl-p-phenylene oxide) solid

Radical formation during mechanical degradation of solid poly(2,6-dimethyl-p-phenylene oxide) (PPO) was investigated by electron spin resonance (ESR). The ESR spectrum of PPO fractured at room temperature in air consisted of eight lines with a separation of about 5.5 gauss with g = 2.0043, indicating a small asymmetry. For PPO fractured in liquid nitrogen, a similar spectrum was observed at ?196°C in air or in vacuo. These spectra have been identified as belonging to a 2,6-dimethyl-substituted phenoxy radical and thus indicate the occurrence of main-chain rupture. The phenyl radical which was expected to be formed together with a 2,6-dimethyl-substituted phenoxy radical could not be detected, but at temperatures below ?46°C a small hump was observed at g = 2.034. By subtracting the spectrum observed after decay of this hump from the original one, the resulting curve was the characteristic asymmetric spectrum of a peroxy radical, which was presumably formed by the reaction between a phenyl radical and oxygen. The radical decay curve showed two stepwise-decaying regions; one located in the temperature region between about ?120°C and ?80°C where only a small number of radicals decayed, another located in the temperature region from about ?30°C to 100°C where almost all mechanically formed radicals decayed. The latter radical decay, which occurred considerably below the glass-transition temperature of PPO, was attributed to the molecular motions associated with the mechanical β^* relaxation on the basis of the activation energy and the temperature region.     

Radiation-induced polymerization of tetraoxane in the solid state

The radiation-induced polymerization of tetraoxane in the solid state has been investigated in air and in vacuo. The polymerization rate was higher in air than in vacuo, whereas the molecular weight of the polymer obtained at high conversion in air was considerably lower than in vacuo. A large decrease in the molecular weight with increasing polymer yield observed in air may be explained mainly by degradation during polymerization.     

ESR study of the generation and decay of peroxy radicals in mechanically destructed polyamide 6

Polyamide 6 has been mechanically destructed in vacuo. At -70°, the ESR spectrum corresponds to the sum of the component spectra of three radicals NH?HCH₂, ·CH₂NHCO, and ·CH₂CONH. After introducing air into the ampoule, the spectrum changes even at -70°; the changes have been studied up to 0°. The spectrum of the peroxy radical ROO· (with line width 1.57 mT, g₁ = 2.0089 and g_| = 2.0301) predominates.     

In vacuo photocurrent measurements of plasma polymerized styrene films

Photoconductivity measurements were carried out on plasma polymerized styrene (PPS) in vacuo, in air, and in subsequently re-evacuated conditions. Samples with two gold electrodes (Au-PPS-Au) and gold-aluminum electrodes (Au-PPS-Al) were used. Positive short circuit photocurrents in the direction of illumination were observed in both types of specimens. In the case of Au-PPS-Au, photocurrent threshold was about 1.6 eV in vacuo for the virgin film, which is attributed to the energy of hole generation in bulk. Residual free radicals were assumed to be the acceptors in this mechanism. In the case of Au-PPS-Al, electron injection from aluminum was thought to be a more likely mechanism, with an estimated barrier height of approximately 2.6 eV.     

Radical reactions in the coordination field of transition metals. VII—H-transfer between phenolic and aryl-aminic H-donors and their radicals

An ESR method for studying the mechanism of H-transfer reactions between H-donors of different reactivity (A₁H, A₂H…) and their free radicals (A₁; A₂^.…) in non-polar solvents at ambient temperature is presented. The new technique is based on a pulsed initiation of various secondary phenoxy or nitroxy radicals in binary mixtures of hindered phenols, unhindered phenols, partially hindered thiobisphenols and diphenylamine, employing a high concentration of free RO₂^. and coordinated (Co^III)RO₂^. tert-butyl peroxy radicals generated in the redox-reaction of Co(acac)₂ with tert-butyl hydroperoxide. The consecutive H-transfer reactions proceed to equilibrium until the most stable radicals are formed. In this way criteria are obtained for ranking the compared free and coordinated phenoxy radicals according to their relative stabilities. The secondarily generated phenoxy radicals from unhindered phenols after coordination to Co^III are stabilized and cannot take part in further H-transfer reactions.     

Photodegradation of cellulose acetate fibers

The photodegradation of cellulose acetate fibers by ultraviolet light in vacuo at 77°K and at ambient temperature was studied. Three kinds of light sources with different wavelengths between 2353 and 6000 Å were employed. ESR studies at 77°K show that several kinds of free radicals are produced from cellulose diacetate (CDA) and cellulose triacetate (CTA) fibers when irradiated with light of wavelength shorter than 2800 Å. Among these methyl radicals formed decayed within 210 min at 77°K. When the temperature was raised above 77°K, radical transformation occurred at 87°K and most of the free radicals decayed at 193°K, whereas the cellulosic radicals were stable at this and even at higher temperatures. Ultraviolet spectroscopy studies revealed that the main chromophores are the carbonyl function of the acetyl group and acetal groups in the polymer. The photodegradation of the polymers at ambient temperature resulted in the formation of gaseous products (mainly CO, CO₂, and CH₄), together with the loss of bound acetic acid content and sample weight. Decreases in viscosity and reduction of tensile strength and elongation were also observed in the irradiated samples, revealing that the overt effects of ultraviolet light on cellulose acetate fibers are interpreted in terms of free-radical reactions ultimately leading to main-chain and side-group scissions, unsaturation, and the formation of small molecule fragments. Among these, main-chain scission took place predominantly in CDA fiber and side-group scission in CTA fiber. The mechanism of the fundamental photochemical degradation processes of cellulose acetate fibers is elucidated.     

High-resolution NMR studies on radiolytic reactions in polyethylene

High-resolution NMR spectra was obtained for linear polyethylene powder irradiated by ⁶⁰Co γ-rays in vacuo or in air. For the sample irradiated in vacuo, the spectra consist of two components having different line width. The narrow component was assigned to soluble molecules and the broad one to gel. When samples are irradiated in air, their spectra show no broad component. The GPC curves obtained for the samples irradiated in air shift to high elution counts (low molecular weight) upon increase of the irradiation dose. It is concluded that oxidative degradation is the predominant reaction in the sample irradiated in air.     

Photodegradation of aramids. II. Irradiation in air

A series of isomeric fully aromatic polyamides (aramids) were photodegraded in the presence of oxygen. Films and fibers of these aramids gave carboxylic acids as the major products when measured by infrared spectroscopy and potentiometric titration. These acids probably resulted from the oxygen interception of the radicals generated by photocleavage of the amide bonds. In contrast to results found upon irradiation in the absence of oxygen, carboxylic acid formation was accompanied by a rapid loss in molecular weight, and a decrease in useful mechanical properties. Quantum yields for carboxylic acid formation were ≤5.5 × 10^?5 mole/einstein and decreased along the aramid series roughly in agreement with increases in T_g. The photo-Fries rearrangement product was observed in aramid fibers irradiated in air, whereas no rearrangement product was seen in films irradiated in air.     

Electron spin resonance study of the radiation-induced polymerization of N-vinylcarbazole

When crystals of N-vinylcarbazole are γ-irradiated at 77°K., the ESR spectrum observed before warming consists of three peaks attributed to a radical–cation with the unpaired spin associated mainly with the nitrogen atom. Above 90°K. polymerization occurs, initiated by the cation, and the spectrum changes to that of an alkyl type of radical, ?N? ?H? CH₂, trapped in the polymer. Single crystals were used for a detailed analysis of the nuclear hyperfine parameters of the observed radicals.     

Photolysis of poly(ethylene terephthalate)

The photolysis of poly(ethylene terephthalate) films was studied in vacuo with light of wavelengths 2537 and 3130 A. A very stable filter system which cuts out the 3025 A. line was developed to isolate 3130 A. from a mercury spectrum. Despite the fact that the penetration of 2537 A. light was limited to a depth of a ca. 10³ A. whereas 3130 A. light was more uniformly absorbed it was possible to demonstrate that the quantum yields for CO and CO₂ formation were in agreement for the two wavelengths. Quantum yields for fractures and crosslinks were estimated by sol-gel analysis. An absorption maximum which develops near 13 μ after exposure of poly(ethylene terephthalate) to light or γ-rays was attributed to the formation of groups formed by elimination of CO and CO₂. ESR spectra for trapped radicals were tentatively assigned to the components p-C₆H₃· and ·O? CH₂? CH₂? . It is suggested that the former radicals combine to form crosslinks. Quantum yields (× 10⁴) with 3130 A. light are: CO, 6; CO₂, 2; crosslinks, 5.5; trapped radicals, 1.5; With 2537 A. light, quantum yields are: CO, 6–9; CO₂, 2–3; the network formed was not characterized as to crosslinks and fractures; trapped radicals were observed to exist but not determined.     

Approximate valence-shell electron unrestricted Hartree-Fock calculations with orthogonalized atomic orbitals: Proton hyperfine splittings in benzyl and related radicals

A modified INDO procedure has been used to calculate the proton hyperfine splittings in benzyl and the isoelectronic anilino, phenoxy and 2-azabenzyl as well as 2- and 3-thenyl radicals. The present procedure differentiates between s-, p- and d-orbitals on an atom in estimating various integrals involving them, satisfies the rotational invariance requirements and employs an orthogonalized basis set of atomic orbitals for obtaining core-Hamiltonian matrix elements. The calculations based on using the exponents which depend only on the type of orbital and the nature of atom fail to provide correct relative order of ortho and para proton splittings in benzyl as well as anilino, phenoxy and 2-azabenzyl radicals. On the other hand, use of the exponents which are modified according to the charge densities in various orbitals leads to a high absolute value for para proton splitting compared to that for ortho proton splitting which in case of all these radicals is in agreement with experiment. A spin density calculation on benzyl, anilino and phenoxy radicals considering the variation of one-center one-electron and one-center two-electron integrals for different protons with their charges is found to yield further improvement in the relative order of ortho and para proton splittings in all these radicals. In 2- and 3-thenyl radicals the role of 3d-orbitals on sulfur has also been examined. To our knowledge, no unrestricted INDO calculations including 3d-orbitals on sulfur have been reported in the literature so far.     

Electron spin resonance study of ethylene–acrolein copolymer irradiated with ultraviolet light

When ethylene–acrolein copolymer was irradiated at ?196°C with ultraviolet light, a sharp singlet spectrum with a g value of about 2.001 was predominant. This spectrum is attributed to acyl radicals, which are produced by dissociation of a hydrogen atom from an aldehyde group. At the same time it is supposed that dissociation of formyl groups also took place to give alkyl radicals, CO, and H₂. The alkyl radicals reacted with CO molecules to give acyl radicals at ?78°C under vacuum. Peroxy radicals were produced when the sample irradiated at ?196°C in the presence of air was treated at ?78°C. The sample irradiated at ?196°C was warmed to near 0°C and an apparent singlet spectrum with a g value of about 2.004 was observed. This spectrum was tentatively assigned as due to free radicals of the type      

Theoretical study on the reaction of the phenoxy radical with O2, OH,and NO2

Unlike the chemistry underlying the self‐coupling of phenoxy (C₆H₅O) radicals, there are very limited kinetics data at elevated temperatures for the reaction of the phenoxy radical with other species. In this study, we investigate the addition reactions of O_2, OH, and NO₂ to the phenoxy radical. The formation of a phenoxy‐peroxy is found to be very slow with a rate constant fitted to k = 1.31 × 10^?20T^2.49 exp (?9300/T) cm³/mol/s in the temperature range of (298–2,000 K) where the addition occurs predominantly at the ortho site. Our rate constant is in line with the consensus of opinions in the literature pointing to the observation of no discernible reaction between the oxygen molecule and the resonance‐stabilized phenoxy radical. Addition of OH at the ortho and para sites of the phenoxy radical is found to afford adducts with sizable well depths of 59.8 and 56.0 kcal/mol, respectively. The phenoxy‐NO₂ bonds are found to be among the weakest known phenoxy‐radical bonds (1.7–8.7 kcal/mol). OH‐ and O₂‐initiated mechanisms for the degradation of atmospheric phenoxy appear to be negligible and the fate of atmospheric phenoxy is found to be controlled by its reaction with NO₂. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Production of radicals in polyoxymethylene by ultraviolet photolysis: An EPR study

Polyoxymethylene (POM) was photolyzed at 2537 and at 3130 Å at ?196°C. The EPR spectra of the radical intermediates were recorded. Photolysis in vacuo produces a small number of radicals, apparently due to the presence of traces of chromophores. Photolysis in oxygen, however, is a type of photo-oxidation. The radicals HCO, , CH₃·, and HOO· were detected and identified as intermediate products of photolysis. Hydrogen atoms and hydroxyl radicals were too reactive (i.e., mobile) at ?196°C to be observed. Alkoxy and alkyl radicals and the POM peroxy radical were probably formed as well but could not be characterized with certainty.     

Thermal degradation of a polyphenylquinoxaline in air and vacuum

A series of poly[2,2′-(1,4-phenylene)-6,6′-bis(3-phenylquinoxalines)] were prepared. These polymers had all the same repeating unit but differed in molecular weight and polymer chain endings. The thermal degradation characteristics in air and vacuum were determined by isothermal weight loss measurements. The temperature coefficients of thermal degradation (apparent activation energies) were also determined. Whereas the apparent activation energies for degradation in air showed a considerable dependency on the type of polymer chain endings, no such effect was observed upon pyrolysis in vacuo. A possible chain-end unzipping mechanism of degradation in air is postulated to explain these results.     

The very low-pressure pyrolysis of phenyl ethyl ether,phenyl allyl ether,and benzyl methyl ether and the enthalpy of formation of the phenoxy radical

A value of the enthalpy of formation of the phenoxy radical in the gas phase, ΔH°_,298K (?O·, g) = 11.4 ± 2.0 kcal/mol, has been obtained from the kinetic study of the unimolecular decompositions of phenyl ethyl ether, phenyl allyl ether, and benzyl methyl ether

1 Trivial names for ethoxy benzene, 2-propenoxy (allyloxy) benzene, and α-methoxytoluene, respectively

at very low pressures. Bond fission, producing phenoxy or benzyl radicals, respectively, is the only mode of decomposition in each case. The present value leads to a bond dissociation energy BDE(?O—H) = 86.5 ± 2 kcal/mol,

2 1 kcal = 4.18674 kJ (absolute)

in good agreement with recent estimates made on the basis of competitive oxidation steps in the liquid phase. A comparison with bond dissociation energies of aliphatic alcohols, BDE(RO—H) = 104 kcal/mol, reveals that the stabilization energy of the phenoxy radical (17.5 kcal/mol) is considerably greater than the one observed for the isoelectronic benzyl radical (13.2 kcal/mol). Decomposition of phenoxy radicals into cyclopentadienyl radicals and CO has been observed at temperatures above 1000°K, and a mechanism for this reaction is proposed.     

Electron spin resonance study of poly-3,3-bis(chloromethyl) oxetane irradiated with electron beams and ultraviolet light

Poly-3,3-bis(chloromethyl)oxetane (poly-BCMO) was irradiated at ?196°C with electron beams and ultraviolet light, and observed ESR spectra were compared. A three-line spectrum (coupling constant of about 21 gauss) and a two-line spectrum (coupling constant of about 18 gauss) were observed after irradiation with electron beams in vacuo. They were attributed to free radicals and respectively. On the other hand, a three-line spectrum (coupling constant of about 20 gauss) and an asymmetric singlet spectrum were observed after ultraviolet irradiation in vacuum. They were assigned to free radicals and ? CH₂? O·, respectively. Mechanisms of radical formation were discussed in each case. When poly-BCMO was irradiated with electron beams at ?196°C in the presence of air, peroxy radicals were produced after subsequent treatment at ?78°C. The reaction between alkyl radicals and oxygen molecules was found to be diffusion-controlled.     

Solid-state polymerization of 1,2,3,4-diepoxybutane initiated by cobalt-60 γ-radiation

The solid-state polymerization of 1,2,3,4-diepoxybutane appears to proceed “insource” by an ionic mechanism and has an overall activation energy of 0.4 kcal./mole with an intensity dependency of 0.99. There is a rapid increase in the rate of polymerization just prior to the melting point and a very low rate for the liquid-phase reaction. Limiting conversions of 5% polymer are observed at ?196°C. for irradiation in vacuo. No limiting conversion was observed when the monomer was polymerized in the presence of air or in vacuo at ?78°C. Under all polymerization conditions the reactions were characterized by the absence of an induction period.     

Characteristics of Chemiluminescence during Oxidation of Organic Substances Inhibited by Vegetable Phenols

An increased intensity in the induction period was observed when the inhibition of the oxidation of hydrocarbons by vegetable phenols was studied by the method of chemiluminescence. It was shown that the additional emitters of chemiluminescence are o- or p-quinones formed in the reaction between phenoxy and peroxy radicals.     

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.