Number Of Free Radicals Research Articles

Steady‐state analysis of emulsion polymerisation in a seeded, continuous stirred‐ tank reactor

AbstractA steady‐state model is presented for emulsion polymerisation in a continuous stirredtank reactor (CSTR) which is fed with a narrow particle‐size seed latex. The rather general kinetic model of Ugelstad et al 1,5 for monodisperse particles is applied to the polymerisation among latexes of broad particle size distribution in a CSTR.The model accounts for free radical absorption and desorption from the latex particles, for termination in the particles and the water phase, and for polymerisation within the particles. Results presented include the latex particle size distributions in the CSTR effluent stream and the number of free radicals per particle, averaged over these size distributions.

Influence de l'intensité et de la longueur d'onde de la lumière sur le rendement des radicaux libres dans les semences de tomate et de carotte

Influence of light intensity and wavelength on the yield of free radicals in seeds of tomato and carrot. Seeds of tomato (Lycopersicum esculentum L. cv. Gribovsky‐1180) and carrot (Daucus carota L. cv. Santanet) were irradiated by continuous light of different wavelengths (white, blue, green, red, far‐red and He‐Ne laser) and different power density. Free radical output was measured using Electron Spin Resonance spectra (ESR). The dependence of the content of free radicals (FR) on the light power density was established. The yield of FR in the seeds irradiated with monochromatic light of the same power density was inversely proportional to the wavelength. The ESR spectra showed maximum absolute number of FR for white light. Successive irradiations gave cumulative effects on the output of free radicals in both directions: increase by the action of white light and decrease by the action of red and far‐red light. The contact of irradiated seeds with steam decreased the amount of FR.

Free radicals in coals and coal conversion. 4. Investigation of the free radicals in selected macerals upon liquefaction

This study of the nature and role of free radicals in coals and coal conversion considers free radicals from selected macerals in the liquefaction process. High-purity macerals were liquefied to different extents at high temperature (425–480 °C) in the presence of naphthalene or tetralin and hydrogen or nitrogen. The initial concentrations of free radicals as well as the residual concentrations have been measured. The dependence of free radical concentrations on solvents, gases, and temperature is reported. Resinites show the least number of free radicals. They also liquefy almost completely under all conditions used. Fusinites have the highest concentration of residual free radicals and they are unaffected by solvent, gas or temperature changes: The vitrinites show intermediate free radical concentrations that depend very strongly on the solvent used, and on the temperature, but they are unaffected by the gas used. A correlation has been established between free radical concentration and degree of conversion.

Mechanism of cellulose smoldering retardance by sulfur

The addition of elemental sulfur is effective in preventing cotton smoldering, a process which leads to a major fraction of fire loss. The objective of this work is to determine the mechanism of the observed retardance. Double thickness samples of cotton terrycloth approximately 18 mm by 100 mm, were smoldered in a pyrex cylinder. The samples contained 0, 5, 8 and 10 percent sulfur by weight. The air velocity over the face of the sample was about 10 cm/s. Ten percent treated samples self-extinguished or smoldered with difficulty. The smoldering velocities and peak temperatures of the other samples were determined using embedded thermocouples; both decreased with increasing sulfur concentration. Calculations and experiments indicate that the sulfur does not simply act as a heat sink as has been suggested for many flame retardants. Electron paramagnetic resonance data showed that a reduction in the number of free radicals in the char by a factor of two resulted from the addition of five percent sulfur. X-ray fluoresence measurements showed that the concentration of bound sulfur in the char was comparable to that of eliminated free radicals. Mass spectrometric studies indicated that the major vapor phase products of sulfur oxidation Mass spectrometric studies indicated that the major vapor phase products of sulfur oxidation were SO 2 and COS, but these were present in much lower concentration than O 2 and CO 2 . This result eliminates the possibility of oxygen depletion by reaction with sulfur as an important contribution to the retardancy. Rather, the sulfur vaporizes and reacts with the radical sites in the char. In the absence of sulfur, these sites would oxidize exothermically and regenerate to continue the smoldering. As a result of some radical sites being poisoned, less heat is released, and the resulting pyrolysis of further fuel is retarded. Thus the critical properties of sulfur are vaporization at the correct temperature, reactivity with the char, and stability of the resulting adduct.

Kinetics of emulsion polymerisation in the unsteady state

Equations are developed which show how the number of free radicals in polymer particles changes with time in emulsion polymerisation. The treatment is confined to cases where the total number of particles is constant. Unlike the procedures which have been published previously, the present treatment, which is relatively simple, allows for both radical desorption from the particles and radical combination within the particles. Inclusion of intraparticle combination leads to predictions of more realistic values of the numbers of radicals within the particles. The new procedure is shown to apply over a wider range of operating variables than the previous treatments. If the rate at which radicals enter the particles is time-independent, the efficiency of radical capture by the particles is expected to be low.

Energy for formation of a number of free radicals that take part in acid-base equilibrium

1. The energies and heats of formation of nine pairs of free radicals, which take part in acid-base equilibrium, in the gas phase were calculated. 2. When the difference in the energies of forming the acid and basic forms of the radical is ΔE 10.5.

Free radical reduction of hemin c

With the electron pulse radiolysis technique, the rates of reduction of hemin(III) c by a number of free radicals have been determined in aqueous solution at neutral pH. Second-order rate constants for hemin(III) c reduction typical approach the diffusion controlled limit (approx. 109 M−1· s−1). Corresponding reduction rates for ferricytochrome c are discussed in terms of possible electron transfer mechanisms.

Theoretical analysis of rates of emulsion polymerization

The importance of emulsion and seeded polymerizations that proceed with a small average number of free radicals per particle (text-decoration:overlinen) has recently been recognized. We now show how to generate analytical solutions to the set of differential equations that describe polymerizations for which text-decoration:overlinen is small due to the rapid transfer of reactivity from the particles. The steady state approximation is not invoked. The analysis exploits methods devised originally to describe gaseous, unimolecular decomposition reactions to which the emulsion polymerizations are analogous. As an example of their usefulness the analytical solutions are applied to the seeded polymerization of vinyl acetate. It is inferred that despite the absence of new nucleation, the efficiency of free radical capture by the particles is remarkably small (< 0.1 %); possible reasons for this are discussed.

Calculations of the isotropic hyperfine coupling constants in free radicals

For a number of free radicals the results of non-empirical (ab initio) and semi-empirical (INDO, DEPAC, CNDO/SP) calculations of the isotropic hyperfine coupling constants are compared.

Electron Spin Resonance Studies of γ-Irradiated Dinucleotides and Related Compounds at 77°K

Polycrystalline samples of 19 dinucleotides, barium diethyl phosphate, and 3′-cytidylic acid have been γ-irradiated and observed at 77°K by electron spin resonance. In 18 of the dinucleotides and barium diethyl phosphate a doublet having a hyperfine splitting of 117-124 G and g value (spectroscopic splitting factor) of 1.998-2.000 is observed. It is concluded that the doublet hyperfine interaction is due to31 P. The free radical postulated to account for the observed doublet is $({\rm RO})({\rm R}^{\prime}{\rm O}){\rm PO}_{2}$ , with the unpaired electron localized on one or both of the nonester oxygens. A similar radical, $({\rm HO})({\rm RO}){\rm PO}_{2}$ , is the postulated assignment for the observed 145-G hyperfine doublet observed in 3′-cytidylic acid. Depending on the individual dinucleotide, the free radical giving rise to the 120-G doublet after a dose of ∼10 MR accounts for from ∼0.1% to ∼5% of the total number of free radicals produced.

On the calculation of the hyperfine coupling constants in free radicals

Abstract For a number of free radicals the INDO and CNDO/SP calculations were carried out with the parameter values recommended by Pople et al. for INDO. The more simple and sufficiently effective CNDO/SP approach proves to be a useful alternative to the INDO method.

Formation of free radicals during mechanical degradation of elastomers

Solithane 113 (an amorphous polyurethane elastomer) was prepared by curing equal proportions of castor oil and trifunctional isocyanate for 6 hr 45 min at 170 F. The sample material was mechanically degraded by grinding below and above its glass transition point at liquid nitrogen and room temperatures. The EPR spectra of ground samples were recorded and the number of free radicals were determined by a computer double-integration of the recorded spectra and by a comparison of the values with those of a standard material. Curves of EPR spectra suggest that different molecular mechanisms may be active in degradation of this material below and above its glass transition temperature.

Sensitivity of Artemia eggs to -irradiation. IX. A correlation between radiation-induced free radicals and hatchability in eggs exposed to -mercaptoethylamine.

SummaryBoth free-radical and hatchability measurements were made on the same samples of Artemia eggs treated with MEA. MEA present at the time of irradiation protected eggs under both dry and wet conditions, but was not effective when administered after irradiation. Radiation protection was accompanied by a reduction in the number of free radicals. The effect of MEA was to increase the rate of decay of radiation-induced radicals. However, transfer of unpaired electrons to the sulphur nucleus in MEA was not found.The protective mechanism of MEA is discussed in the light of current hypotheses.

Polymerization of vinyl acetate in aqueous media. Part IV. Influence of preformed (“seed”) latex upon polymerization kinetics and particle size

AbstractThe polymerization of aqueous solutions of vinyl acetate initiated by K2S2O8, has been studied in the presence of different concentrations of a fine “seed” latex, both in the absence and in the presence of a surfactant (sodium hexadecyl sulfate). Both kinetics and final latex size were measured, the latter showing whether fresh nucleation had occurred or not. The results suggest that surfactant adsorbed on the latex surface greatly influences the capture, by the latex surface, of oligomers formed in the aqueous phase and this, in turn, markedly affects both nucleation and kinetics. The number of free radicals per particle does not appear to be constant even with the finest particles used (ca. 50 mμ).

ESR-resonance in doped GaAs and GaP

The feasibility of lithium lactate (Li-lactate: CH3CH(OH)COOLi) as an electron spin resonance (ESR) dosimeter material has been studied and compared with lactose and a conventional alanine dosimeter. The ESR spectra of Li-lactate and lactose with the spectroscopic splitting g-factors of 2.0029 ± 0.0003 and 2.0043 ± 0.0003 respectively show hyperfine splitting with an intensity ratio of 1:3:3:1 and hyperfine coupling constants (A/gβ) of 1.82 ± 0.06 and 1.88 ± 0.06 mT, respectively. The radical formation efficiency (G-value), i.e. the number of free radicals per 100 eV for alanine, Li-lactate and lactose were 0.95, 1.0 and 3.0, respectively. The sensitivity of Li-lactate to γ-rays is 10 times higher than that of alanine and 0.3 of lactose. The signal stability of Li-lactate was greater than that of lactose.

Electron spin resonance spectroscopy of irradiated DNA

Much work has been done in the last few years to study the effect of ionizing radiation on DNA . In a typical experiment a DNA sample is sealed off under vacuum in a quartz tube and irradiated at one end. The sample is then moved to the other (unirradiated) end and placed in the e. s. r. spectrometer. Results take the form, first, of spectra, which can obviously be obtained under a variety of irradiation and measurement conditions (variation of temperature has been a common procedure) and, secondly, of yield curves, where the area under the absorption curve is taken as a measure of the total free radicals and the radical concentration is plotted against dose. The curves usually show an exponential increase—the free radical concentration levelling off between 10 17 and 10 18 spins/g at a dose of about 1 Mrad. From the initial steep part of the curve the yield or G -value may be estimated as the number of free radicals produced by 100 eV of absorbed energy. Typical values for DNA lie between ½ and 2.

Pyrolysis of n-octane in the presence of ammonia

ONE of the methods of increasing olefin content in pyrolytic gases (gas-phase, high-temperature cracking) is by carrying out the process with different diluents and initiating additives [1-5]. Most studies in this field have sought to study the mechanism of cracking [1-3, 6-10]. It was found that, below 700 °, the process takes place mainly by the radical-chain mechanism. At higher temperatures it was shown that the importance of radical reactions is less and direct molecular conversions increase [8]. Above 700 ° paraffins are very unstable, they decompose with the formation of a large number of free radicals. Consequently, the introduction of initiating additives, the function of which is to produce and increase radical concentration, cannot have any significant effect. Under similar conditions an increased olefln yield may be expected as a result of introducing inhibitors, which cause a loss of radicals promoting the formation of chains in the secondary reactions during which olefins are used. In papers [4, 5] it was proposed to introduce regulating additives during paraffin cracking; these additives increased the yield of lower olefins. Unfortunately, these regulating compounds have no industrial value as they increase the material cost due to the use of additives or result in products which cause a deterioration in processes of pyrolytic gas separation. This paper deals with the pyrolysis of n-octane in the presence of ammonia and steam. EXPERIMENTAL t All experiments were carried out in a quartz reactor, the dimensions and shape of which have been previously described [11]. The reactor was placed in a tubular furnace, in which the required temperature was maintained by means of art ]gPD-12 automatic potentiometer. The raw material was fed from an automatic burette, ammonia -- from a cylinder through a needle valve. The amount of NH a to be fed was determined by a rheometer, excess was removed through a pressure gauge containing glycerine. Water was added to the raw material line of the reactor from a burette. After pyrolysis the gases passed through traps cooled with ice, a flask

Energy Dependence of Free Radical Production in Alanine

It has been shown by Bradshaw et al. (1) that the amino acid dl-alanine in dry powdered form is a sensitive and accurate y-ray dosimeter. When it is exposed to directly and indirectly ionizing radiations, free radicals are formed and their relative concentration can be measured by electron spin resonance (ESR) spectrometry. The free radicals are stable, decaying only about 5% in 90 days (1). Evidence indicates that the free radical CH3-C--COOH is formed in alanine after exposure to ionizing radiation at room temperature (2, 3). In the work of Bradshaw et al., it was found that the ESR amplitude, which is proportional to the number of free radicals formed in the alanine sample, was proportional to the measured exposure dose and presumably to the absorbed dose. Rotblat and Simmons (4) have shown in electron experiments that in alanine the ESR amplitude saturates in the megarad dose region, indicating that this substance can be used over a wide range of doses. In an attempt to measure the characteristics of alanine as a neutron dosimeter and thereby add to the data presented in reference 1, results of irradiations with neutrons and low-energy X-rays that were not in agreement with those of Bradshaw et al. (1) indicated the necessity of more knowledge of the dependence of free radical production in amino acids on particle energy and mass (5, 6). This paper is a report of these measurements and of further experiments to measure the properties of alanine as an Xand y-ray dosimeter.

Kinetics of Disappearance of Radicals Trapped in Irradiated Polyethylene

Development of a technique for determining the concentration of alkyl radicals trapped in irradiated polyethylene using radio-iodine is described. It is applied to study the kinetics of alkyl radical disappearance in irradiated polyethylene and polypropylene, and the kinetics are interpreted in terms of a simple model. The radicals are assumed to be formed in isolated spherical spurs and rapidly attain an approximately uniform concentration within a spur. This concentration decreases with time due to second-order recombination accompanied by a diffusion-like expansion of the spur. Recombination is increased by hydrogen and the diffusive expansion is increased by hydrogen and by preirradiation. Both processes are faster in polypropylene than in polyethylene. The number of free radicals per spur after the irradiation decreases with increase in radiation dose due probably to the effect of the hydrogen produced by irradiation.

Emulsion copolymerization of butadiene and styrene in the presence of carbon black

AbstractCarbon blacks inhibit and retard free radical polymerization in bulk and solution. The mechanism of this is discussed in terms of the chemical groups present on the carbon surface. It is shown that the number of free radicals which react during the induction period (polymerization of styrene with AIBN) is much less than the number of quinone groups but of the same order of magnitude as the number of easily oxidizable groups. Inhibition by carbon black was also found in an emulsion polymerization system with cumene hydroperoxide as initiator (no reducing components). By use of highly active redox recipes, reasonable rates of polymerization were obtained at both 50°C. (sugar‐iron‐hydroperoxide recipe) and 5°C. (peroxide‐polyamine recipe), with as much as 40 parts of black per 100 parts of monomers. In these systems, the carbon black was charged as an aqueous dispersion stabilized with Daxad 11; the latices were stable and showed evidence of association between some of the carbon black and some of the latex particles. The coagulated rubber stocks, when compounded and cured, gave properties closely similar to those of dry mixed stocks.