Monte Carlo simulation of density dependence of molecular motion and radical decay in solid polymers

It was found experimentally that after increasing pressure, the decay of free-radicals in solid polymers is slowed down (Szöcs F, Rostašová O, Tiňo J, Plac̆ek J, European Polym J, 1974;10:725). Since the mechanism for decay is associated with molecular mobility, a Monte Carlo method has been used for studying the effect of the polymer density on molecular mobility and free-radical decay in a model system with the parameters close to those of polyethylene. Increased pressure is correlated with higher density of the polymer system. Rotational motions were found to be considerably limited at increased density (ρ=0.85 g cm⁻³ versus 0.81 g cm⁻³). Consequently, free-radical decay is slowed down at the higher density in accord with the experimental results.     

ESR study of pressure and temperature dependence of free-radical decay in poly(vinyl chloride)

Gamma-irradiated poly(vinyl chloride) was annealed at various temperatures (80 to 140°C) and pressures (0.1 to 800 MN m^?2). Rate constants and activation volumes of free-radical decay were determined from the changes of the electron spin resonance (ESR) spectra before and after annealing. The pressure and temperature dependence of the rate constant is compared with the pressure and temperature dependence of the mechanical relaxation parameters. The correlation between the kinetics of radical reactions and the kinetics of the molecular motions in the α-relaxation region is discussed.     

Monte-Carlo simulation of free-radical decay as a probe of local and collective dynamics in polymer molecules

The effect of molecular mobility on the stability of free radicals in amorphous polymers was investigated by a Monte-Carlo (MC) method. Crank, crankshaft-like, kink and double kink were used as the various types of movements of submolecular structures. This work introduces the librational motions of these structures and formulates the methodology for their incorporation into the MC method. The results show that the presence of librational motion significantly influences both transfer and the decay of free radicals.     

ESR study of pressure dependence of free-radical decay in irradiated polystyrene

Radicals giving the usual triplet ESR spectrum have been generated in polystyrene by γ-irradiation at room temperature. The decay of the radicals has been investigated in the temperature interval between 90° and 200° and pressures ranging from 1 to 8000 atm. The effect of pressure on the mechanism of the free-radical decay is discussed. There are two regions of free-radical decay showing different activation volumes: V_I = 11.5 cm³/mole and V_II = 66 cm³/mole. The correlation between molecular motion in the α-relaxation region and radical decay is pointed out.     

Radiation chemistry of polybutadiene and butadiene-styrene block copolymers. II. Kinetics and mechanisms of free-radical decay reactions

The decay of free radicals produced in polybutadiene, polystyrene, and block copolymers of butadiene and styrene by γ irradiation at 77 K has been studied at ?110°C in the case of polybutadiene and at ?95°C for the other samples. The free-radical decay rate is best interpreted in terms of an equation based on a second-order decay mechanism of a fraction of the free radicals decaying in the presence of other nondecaying free radicals. Hydrogen gas accelerates the free-radical decay. Increase of radiation dose increases the fraction of the radicals that decay, while increase of the fraction of styrene segments decreases the decaying fraction. In pure polybutadiene the higher the cis content, the greater fraction of decaying free radicals, but the second-order decay constant is less in the high-cis-content polybutadiene and is also less at the higher dose, probably owing to the hindrance of the radiation-produced crosslinks on the free-radical decay. The decrease of the second-order constant with increase of dose is also true for all the block copolymers studied.     

Simulation of initiated nonbranched chain oxidation of hydrogen: Oxygen as an autoinhibitor

A simple kinetic equation describing nonmonotonic (with a maximum) dependence of the hydrogen peroxide and water formation rate on the oxygen concentration was derived by the quasi-steady-state treatment in terms of the nonbranched-chain free-radical mechanism of noncatalytic hydrogen oxidation involving the relatively low-reactive radical HO ₄ ^? , which inhibits the chain process. Possible nonchain pathways of the free-radical oxidation of hydrogen, the energetics of key radical-molecule reactions, and the routes of ozone decay from the energetics standpoint via the reaction with the HO^? radical in the upper atmosphere (including the addition yielding the HO ₄ ^? radical, which can be an intermediate in the sequence of conversions of biologically hazardous UV radiation energy) were considered.     

Chlorine‐Enhanced Surface Mobility of Au(100)

Motivated by experimental studies of two‐dimensional Ostwald ripening on Au(100) electrodes in chlorine‐containing electrolytes, we have studied diffusion processes using density functional theory. We find that chlorine has a propensity to temporary form AuCl complexes, which diffuse significantly faster than gold adatoms. With and without chlorine, the lowest activation energy is found for the exchange mechanism. Chlorine furthermore reduces the activation energy for the detachment from kink sites. Kinetic Monte Carlo simulations were performed on the basis of extensive density functional theory calculations. The island‐decay rate obtained from these Monte Carlo simulations, as well as the decay rate obtained from the theoretical activation energies and frequency factors when inserted into analytical solutions for Ostwald ripening, are in agreement with experimental island‐decay rates in chlorine‐containing electrolytes.     

Effect of water density on methanol oxidation kinetics in supercritical water

We oxidized methanol in supercritical water at 500 degrees C to explore the influence of the water concentration (or density) on the kinetics. The rate increased as the water concentration increased from 1.8 to 5.7 mol/L. This effect of water density on the kinetics observed experimentally was quantitatively reproduced by a previously validated mechanism-based, detailed chemical kinetics model. In this model, reactions of OH radicals with methanol were the fastest methanol removal steps. The rates of these removal steps increased with water density at 500 degrees C because the OH radical concentration increased. The OH radical concentration increased with density because the rates of the steps H + H2O = OH + H2 and CH3 + H2O = OH + CH4, which produce OH radicals, increased. Thus, the main role of water in accelerating methanol oxidation kinetics at 500 degrees C is as a hydrogen donor to a radical (R) in steps such as R + H2O = OH + RH. This system provides a striking example of SCW being involved on the molecular level in the free-radical oxidation as a reactant in elementary steps.     

Influence of pressure of free radical decay in irradiated polyethylene

Information is given on the effect of high pressure on the decay of free radicals in irradiated polyethylene. The dependence of the rate constant for decay on pressure (1–13,000 atm) was determined at 80, 90, 100 and 110°. The mechanism of the effect of pressure on the rate constant and activation energy for free radical decay is discussed.     

Radiolysis of poly(vinyl chloride)

Allyl free-radical intermediates are detected by ultraviolet absorption at 255 mu in poly(vinyl chloride) irradiated at ?196°C and stored at 25°C. In vacuum at 25°C, allyl radicals are converted into polyenyl free radicals and polyenes. From the nature of allyl radical decay in vacuum, radical chain transfer between polyenyl radicals and poly(vinyl chloride) is inferred. Allyl and polyenyl free radicals are scavenged by oxygen on post-irradiation storage in air.     

Structural effects on the OH-promoted fragmentation of methoxy-substituted 1-arylalkanol radical cations in aqueous solution: the role of oxygen acidity

A kinetic and product study of the OH- -induced decay in H2O of the radical cations generated from some di-and tri-methoxy-substituted 1-arylalkanols (ArCH(OH)R*+) and 2- and 3-(3,4-dimethoxyphenyl)alkanols has been carried out by using pulse- and gamma-radiolysis techniques. In the 1-arylalkanol system, the radical cation 3,4-(MeO)2C6H3CH2-OH*+ decay at a rate more than two orders of magnitude higher than that of its methyl ether; this indicates the key role of the side-chain OH group in the decay process (oxygen acidity). However, quite a large deuterium kinetic isotope effect (3.7) is present for this radical cation compared with its a-dideuterated counterpart. A mechanism is suggested in which a fast OH deprotonation leads to a radical zwitterion which then undergoes a rate-determining 1,2-H shift, coupled to a side-chain-to-ring intramolecular electron transfer (ET) step. This concept also attributes an important role to the energy barrier for this ET, which should depend on the stability of the positive charge in the ring and, hence, on the number and position of methoxy groups. On a similar experimental basis, the same mechanism is suggested for 2,5-(MeO)2C6H3CH2OH*+ as for 3,4-(MeO)2C6H3CH2OH*+, in which some contribution from direct C-H deprotonation (carbon acidity) is possible. In fact, the latter process dominates the decay of the trimethoxylated system 2,4,5-(MeO)3C6H2CH2-OH*+, which, accordingly, reacts with OH- at the same rate as that of its methyl ether. Thus, a shift from oxygen to carbon acidity is observed as the positive charge is increasingly stabilized in the ring; this is attributed to a corresponding increase in the energy barrier for the intramolecular ET. When R=tBu, the OH- -promoted decay of the radical cation ArCH(OH)R*+ leads to products of C-C bond cleavage. With both Ar = 3,4- and 2,5-dimethoxyphenyl the reactivity is three orders of magnitude higher than that of the corresponding cumyl alcohol radical cations; this suggests a mechanism in which a key role is played by the oxygen acidity as well as by the strength of the scissile C-C bond: a radical zwitterion is formed which undergoes a rate-determining C-C bond cleavage, coupled with the intramolecular ET. Finally, oxygen acidity also determines the reactivity of the radical cations of 2-(3,4-dimethoxyphenyl)ethanol and 3-(3,4-dimethoxyphenyl)propanol. In the former the decay involves C-C bond cleavage, in the latter it leads to 3-(3,4-dimethoxyphenyl)propanal. In both cases no products of C-H deprotonation were observed. Possible mechanisms, again involving the initial formation of a radical zwitterion, are discussed.     

Kinetics of polymeric network synthesis via free-radical mechanisms - polymerization and polymer modification

The kinetics of polymeric network formation via free radical mechanisms is an attractive research area because there are many phenomena which are not well understood and in addition, the commercial potential for crosslinked systems is great. Recently, a large research/development program was initiated at the McMaster Institute for Polymer Production Technology (MIPPT) to investigate the fundamentals and applications of polymeric network, in particular, the kinetics of synthesis via free-radical mechanisms and network characterization. The research on crosslinking involved both theoretical developments and experimentation. Herein is provided a comprehensive summary of this work. In the experimental polymerization, two comonomers, methyl methacrylate (MMA) / ethylene glycol dimethacrylate (EGDMA) and acrylamide (AAm) / N,N-methylene bisacrylamide (Bis), as model systems were studied in considerable detail. Measurements included: monomer conversions, radical concentrations, sol/gel fractions, crosslink densities (equilibrium swelling and swollen-state ¹³C-NMR) over the entire range of divinyl monomer levels as a function of polymerization time. In the polymer modification, high density polyethylenes were crosslinked using peroxides and γ-radiation. For this system, crosslinking and chain scission occur simultaneously. In the theoretical studies, it was shown that in general, network formation by free-radical mechanisms is highly irreversible requiring that the classical equilibrium gelation theories after Flory/Stockmayer be generalized. The general model which was developed using the pseudo-kinetic rate constant method predicts the existence of a crosslink density distribution (crosslink density of a primary polymer chain depends on its birth time) with a variance which can vary widely depending on network synthesis conditions.     

Modeling stable free-radical polymerization

A kinetic model has been developed for stable free-radical polymerization (SFRP) processes by using the method of moments. This model predicts monomer conversion, number-average molecular weight, and polydispersity of molecular weight distribution. The effects of the concentrations of initiator, stable radical, and monomer, as well as the rate constants of initiation, propagation, termination, transfer, and the equilibrium constant between active and dormant species, are systematically investigated by using this model. It is shown that the ideal living-radical polymerization having a linear relationship between number-average molecular weight and conversion and a polydispersity close to unity is the result of fast initiation, slow propagation, absence of radical termination, and a high level of dormant species. Increasing stable radical concentration helps to reduce polydispersity but also decreases polymerization rate. Thermal initiation significantly broadens molecular weight distribution. Without the formation of dormant species, the model predicts a conventional free-radical polymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2692–2704, 1999     

Kinetic approach to radiation–induced grafting in the polyethylene–styrene system

To investigate the mechanism of radiation-induced grafting in this system, the increase of monomer concentration in the polyethylene film in styrene vapor was evaluated by measuring the weight increase and formulated to be V([M_∞] ? [M]). The decay of radical concentration was also measured by ESR and the rate constant of the decay was determined. The alkyl type radical was affected only a little by styrene, while the allyl type radical was much affected by styrene. A new computer investigation method was proposed to clarify the reaction mechanism. The data obtained were substituted into differential equations and used to calculate the pattern of increase of the degree of grafting for the preirradiation method with reaction in the vapor phase. Results of these calculations suggest that only allyl type radicals induce grafting reactions and that the grafting reaction seldom occurs in the region of grafted polystyrene.     

Multimode Jahn-Teller and pseudo-Jahn-Teller interactions in the cyclopropane radical cation: complex vibronic spectra and nonradiative decay dynamics

The complex vibronic spectra and the nonradiative decay dynamics of the cyclopropane radical cation (CP+) are simulated theoretically with the aid of a time-dependent wave packet propagation approach using the multireference time-dependent Hartree scheme. The theoretical results are compared with the experimental photoelectron spectrum of cyclopropane. The ground and first excited electronic states of CP+ are of X2E' and A2E' type, respectively. Each of these degenerate electronic states undergoes Jahn-Teller (JT) splitting when the radical cation is distorted along the degenerate vibrational modes of e' symmetry. The JT split components of these two electronic states can also undergo pseudo-Jahn-Teller (PJT)-type crossings via the vibrational modes of e', a1' and a2' symmetries. These lead to the possibility of multiple multidimensional conical intersections and highly nonadiabatic nuclear motions in these coupled manifolds of electronic states. In a previous publication [J. Phys. Chem. A 2004, 108, 2256], we investigated the JT interactions alone in the X2E' ground electronic manifold of CP+. In the present work, the JT interactions in the A2E' electronic manifold are treated, and our previous work is extended by considering the coupling between the X2E' and A2E' electronic states of CP+. The nuclear dynamics in this coupled manifold of two JT split doubly degenerate electronic states is simulated by considering fourteen active and most relevant vibrational degrees of freedom. The vibronic level spectra and the ultrafast nonradiative decay of the excited cationic states are examined and are related to the highly complex entanglement of electronic and nuclear degrees of freedom in this prototypical molecular system.     

THE PHOTOREDUCTION OF METHYLENE BLUE BY AMINES—II. AN INVESTIGATION OF THE DECAY OF SEMIREDUCED METHYLENE BLUE

Abstract— The decay of semireduced methylene blue (MB') formed in the photoreduction of the dye by aryl- and alkylamines was examined by flash photolysis. The second order decay of MB in methanol produced using arylamines and 1,4-diazabicyclooctane as photoreducing agents led to complete regeneration of the dye and amine. The rate constants for a series of N.N-dimethylanilines showed a small substituent effect ( p = 0.69 ± 0.16) compatible with recombination of MB' with arylamine derived radical cations. A study of the solvent effect on the recombination process revealed the importance of the stability of the radical cation toward the solvent and also indicated that the reaction approaches the rate of diffusion. The photoreduction of the dye by most alkylamines examined resulted in permanent bleaching of methylene blue. The second order decay of MB' produced using tertiary aliphatic amines led exclusively to formation of the leuco-dye with no apparent regeneration of methylene blue; this process was tentatively assigned to a reaction of MB' with α-amino radicals formed from the amine radical cation. It was concluded that the methylene blue-sensitized photooxidation of amines involves a Type I process and possible mechanisms are discussed.     

Pressure effects on the electron spin relaxation of several radicals in solution

The pressure effect on the decay rate of chemically induced dynamic electron spin polarization (CIDEP) was investigated on several free-radical intermediates in photolysis, and the spin-lattice relaxation times for these radicals were estimated from the decay rates of CIDEP signals at various pressures. The spin-lattice relaxation rates were retarded by increasing external pressure. From the pressure dependence of the spin-lattice relaxation rates the activation volume was estimated. The activation volumes of these radicals divide into two groups; ≈30 cm³ mol⁻¹ for negative ions and ≈10 cm³ mol⁻¹ for neutral radicals.     

Double-layer effects and distance dependence of electron transfer in reduction of nitro aromatic radical anions

The first example of the effect of an electric double layer on the reduction of electrochemically generated radical species is reported. The anion radical of methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (pesticide bifenox) is electrochemically reduced in acetonitrile to a phenylhydroxylamine derivative in a process involving three electrons. This heterogeneous reaction is strongly influenced by the concentration and nature of the cation of the indifferent electrolyte. Depending on the type of tetraalkylammonium cation, the redox potential changes by 0.45 V. The kinetic parameters were obtained for five tetraalkylammonium hexafluorophosphate salts. The Frumkin correction, which assumes that the outer Helmholtz plane coincides with the reaction site, was applied to kinetic data of the radical anion reduction. The correction of the apparent rate accounted for the observed effect only in the case of tetramethylammonium salt. The presence of higher tetraalkylammonium homologues causes deviations from the predicted dependence of the electron-transfer rate on the phi2 potential of the outer Helmholtz plane. Hence, the nature of the cation of the electrolyte exerts a further effect extending beyond the electrostatic repulsion only. The corrected rate of electron transfer decreases exponentially with increasing size of the alkyl chain of the indifferent electrolyte cation in the order methyl > ethyl > propyl > butyl > hexyl. The rate decay is characterized by an exponent beta = 0.83. This confirms that the reaction plane for the reduction of the bifenox radical anion is different for each electrolyte. Due to this fact the Frumkin correction cannot fully account for the observed dependence of the heterogeneous rate on the solution composition. The observed effect is not specific to the bifenox radical. A similar influence of the concentration and nature of the cation of the indifferent electrolyte was observed for other nitro compounds, namely, nitrobenzene, nitrobenzoate, and nitrofen.     

ESR studies of chlorophyll one-electron photochemistry in solution: kinetics of reversible quinone reduction and general redox mechanisms

Abstract— ESR studies have been made of the kinetics of semiquinone radical formation and disappearance resulting from the reversible photosensitization of reduction or oxidation, by chlorophyll, pheophytin or hematoporphyrin, of several quinone-hydroquinone pairs in various solvents. The rate of radical decay was found to be second order with respect to the radical concentration in all systems. Radical formation rates were determined by the initial production rate minus the decay rate. The kinetic constants for single electron transfer between triplet porphyrins and quinones or hydroquinones were determined usingβ-carotene as a quencher in aqueous pyridine, and by measuring the initial rate of radical formation at various concentrations of quinones and hydroquinones in methanol and ethanol. These constants were found to be approximately the same in a given solvent for benzoquinone and hydroquinone with all porphyrins, though the rates differed in different solvents: pyridine-water ～ 10⁶I./mole sec, and methanol and ethanol ～ 5X 10⁴l./mole sec. Trimethylquinone and its hydroquinone also give similar rate constants for radical formation in pyridine-water, ～ 10⁶ l./mole sec. The second order radical decay constants for both benzoquinone and hydroquinone in pyridine-water were the same, ～ 10⁵ I./mole sec, with either chlorophyll, pheophytin or hematoporphyrin as sensitizer. The same activation energy, 6900 cal/mole, was found for chlorophyll-benzoquinone and hydroquinone in aqueous pyridine; 5500 cal/mole was obtained for these systems in ethanol. In methanol and ethanol solutions of chlorophyll, the same radical decay rate constants, ～10⁶ I./mole sec, were observed for both benzoquinone and hydroquinone. Also, the same decay constants, ～ 10⁶ I./mole sec, were found for trimethylquinone and its hydroquinine in pyridine-water. These latter two compounds gave extremely small steady-state ESR signals in ethanol compared with aqueous pyridine. We have also observed that the steady-state signal obtained with chlorophyll-menadione in ethanol-water was much enhanced by the presence of NADH. In contrast, NAD⁺ was found to decrease radical production, by increasing the decay rate, in the chlorophyll-hydroquinone system in aqueous pyridine. These results are discussed in terms of possible mechanisms for radical formation and disappearance. The most likely possibility is considered to be a one-electron oxidation or reduction of the porphyrin triplet, followed by radical disproportionation and redox reactions between the disproportionation products.