Gas-phase thermolysis of sulfur compounds. Part VIII. Chloromethyl allyl,cyanomethyl allyl, 1-cyanoethyl allyl and neopentyl allyl sulfides

The pyrolyses of four alkyl allyl sulfides with substituents on the α? C atom of the alkyl moiety have been studied in a stirred-flow system over the temperature range 340-400°C and pressures between 2 and 12 torr. The only products formed are propene and thioaldehydes. The reactions showed first-order kinetics with the rate coefficients following the Arrhenius equations: Chloromethyl allyl sulfide: Cyanomethyl allyl sulfide: 1-cyanoethyl allyl sulfide: Neopentyl allyl sulfide: The effects of these and other substituents on the reactivity is discussed in relation with the stabilization of a polar six-centered transition state. The results support a non-concerted mechanism where the 1–5 α? H atom shift is assisted by its acidic character.     

Dielectric behaviour of copolymers based on 2,2,2-trifluoroethyl methacrylate and cyano co-monomers

In the frame of a research aiming at developing new dielectric polymers containing CCN and CF substituents with strong dipole moment, statistical copolymers based on cyano monomers such as acrylonitrile (AN), methacrylonitrile (MAN), methylvinylidene cyanide (MVCN) and 2,2,2-trifluoroethylmethacrylate (MATRIF), were synthesized and characterized. Elemental analysis has shown that the molar percentages of AN and MAN in the copolymers were 45 mol.%, while only 5 mol.% of MVCN was incorporated in the poly(MVCN-co-MATRIF) copolymer. These copolymers exhibit glass transition temperatures, T_g, in the range of 70-90 °C. The dynamic dielectric analyses and their complex permittivities of these copolymers were studied versus the temperature and the frequency. Evidence of an α-relaxation phenomenon in the glass transition region, which is confirmed by the Vogel-Fulcher-Tammann (VFT) temperature dependence of the relaxation times, was assigned to the cooperative reorientation motions of the cyano groups. The values of dielectric strength (Δε) for the copolymers based on MATRIF were determined by Havriliak-Negami (HN) fitting from the dispersion curves, and can be related to the polarity of the monomer unit and to the packing of the macromolecular chains. These relaxations are sometimes overlapped by conduction phenomena due to ionic impurities at low frequencies and high temperatures dipolar losses. In the glassy state, the permittivity values of AN and MAN copolymers show an increase of polarity which makes them candidates for some applications amongst advanced electrical materials such as dielectric layer for capacitors.     

Kinetics of the Diels–Alder addition of acrolein to cyclohexa-1,3-diene and its reverse reaction in the gas phase

The Diels–Alder addition of acrolein to cyclohexa-1,3-diene has been studied between 486 and 571°K at pressures ranging from 55 to 240 torr. The products are endo- and exo-5-formylbicyclo[2.2.2]oct-2-ene (endo- and exo-FBO), and their formations are second order. The rate constants (in l./mole · sec) are given by The retro-Diels–Alder pyrolysis of endo-FBO has also been studied. In the ranges of 565–638°K and 6–38 torr, the reaction is first order, and its rate constant (in sec^?1) is given by The reaction mechanism is discussed. The heat of formation and the entropy of endo-FBO are estimated.     

Characterization of polymers under large deformation. I. Repeated chemical stress relaxation of vulcanized rubbers

In this investigation, repeated chemical stress relaxation measurements were carried out to observe the relaxation behavior at large deformation. It was found that the repeated chemical stress relaxation curves were affected by both measurement temperature and the extension ratio of rubber. It was suggested on the basis of the results that temperature and mechanical stimulus have a similar effect on the stress relaxation curves. Thus we propose the following Arrhenius type equation for high extension ratios: where α is the extension ratio and A, B are constants determined experimentally. On the other hand, from this equation and the usual Arrhenius equation, a universal equation for the extension ratio and the temperature was derived as follows: where, T₀ is voluntary temperature. The curves obtained by this equation were quite consistent with those obtained experimentally at different temperatures and extensions at large deformation.     

Effects of diluent on molecular motion and glass transitions in polymers. II. The system polyvinylchloride–tetrahydrofuran

Dielectric measurements, differential thermal analyses, and density measurements are reported on concentrated solutions of polyvinylchloride in tetrahydrofuran. The relaxation processes observed between 80 and 400°K have been classified into four types. From the analysis of experimental data, the primary process at the highest temperature and the process at the lowest temperature are assigned, respectively, to segmental motion of the polymer and motion of the solvent. Activation plots for the primary process conform to the Vogel–Tamman equation. The dielectric glass-transition temperature T'_g (defined as the temperature at which the dielectric relaxation time is 100 sec) determined with this equation agrees well with the glass-transition temperature T_g from thermal analysis. Therefore, T_g can be represented by an expression of the form The parameters of the Vogel–Tamman equation A and B are nearly independent of concentration, whereas T_o depends strongly on concentration. The dipole moment per monomeric unit calculated from the experimental data changes with concentration and exhibits steep increments around 30% and 90% by weight. The width of the distribution of the relaxation time also increases with the concentration. The results were compared with those for the system polystyrene–toluene studies in the same way.     

Temperature dependence of the rate constant for the reaction OH + H2S

Absolute rate constants for the reaction of OH with H₂S have been measured over the temperature range of 239–425 K using the flash photolysis–resonance fluorescence technique. The results showed that the rate constants deviate slightly from Arrhenius behavior but can still be represented adequately by the following Arrhenius equation: Comparisons with recent literature values are presented.     

The reaction of cyclohexyl radicals with carbon tetrachloride

The photolysis of azocyclohexane, carbon tetrachloride, and cyclohexane at 360 nm has been investigated over a wide temperature range. At moderate temperatures a chain reaction ensues from which the following approximate rate constants could be determined assuming 2CCl₃. → C₂Cl₆, k₅ = 10^9.7 (303–673K): The really striking feature of the results is that they show that termination in bicyclohexyl [reaction (7)] is extremely slow: The root-mean-square rule for estimating the cross-combination rate is also followed. The photolysis of carbon tetrachloride and cyclohexane at 250 nm has also been investigated. The reaction is complicated by the occurrence of two concurrent photolytic processes, the main one yielding trichloromethyl radicals and chlorine atoms, and the subsidiary one yielding dichlorocarbene and molecular chlorine. Nonetheless the results from this reaction can be interpreted in the medium temperature range 360–430K, where long chains are present, in terms of the rate constants derived from the azocyclohexane system.     

Electron spin resonance study of free radicals in the radiation grafting of butadiene to poly(vinyl chloride)

The kinetics of the disappearance of propagating butadiene radicals produced in the grafting of butadiene to poly(vinyl chloride) was followed by electron spin resonance measurements. The radical species observed in the temperature range 213–298°K is the conjugated allylic radical: The reaction conditions were adjusted to provide a highly viscous medium in which the propagation reaction took place. Under these conditions the radicals were stabilized and a precise ESR spectrum was obtained. Arrhenius plots of the second-order decay rate constants yield an activation energy of 5 kcal/mole. This suggests that the bimolecular termination reaction is controlled by the segmental motion of the grafted side chains.     

Kinetics of the reactions of cyclopropane derivatives. III. Gas-phase unimolecular isomerization of cyclopropyl cyanide to the cyanopropenes

Cyclopropyl cyanide isomerizes in the gas phase at 660°–760°K and 2–89 torr to give mainly cis- and trans-crotonitrile and allyl cyanide, with traces of methacrylonitrile. The reactions are first order, homogeneous, and unaffected by the presence of radical-chain inhibitors. The rate constants are given by Overall: cis-Crotonitrile: trans-Crotonitrile: Allyl cyanide: where the error limits are standard deviations. On the basis of a biradical mechanism, it is deduced that the ? CH? CN radical center is resonance stabilized by ca. 30 kJ mole^?1. Approximate equilibrium data are given for interconversion of the 1- and 3-cyanopropenes.     

The unimolecular elimination kinetics of benzaldoxime in the gas phase

The kinetics of the gas‐phase elimination of benzaldoxime was determined in a static reaction system over the temperature and pressure range 350°C–400°C and 56–140 Torr, respectively. The products obtained were benzonitrile and water. The reaction was found to be homogeneous, unimolecular, and tend to obey a first‐order rate law. The observed rate coefficient is represented by the following Arrhenius equation: According to kinetic and thermodynamic parameters, the reaction proceeds through a concerted, semi‐polar, four‐membered cyclic transition state type of mechanism. © 2007 Wiley Periodicals, Inc. 39: 145–147, 2007     

TI(III) oxidation of alkenes and alkynes and Ru(III) catalysis in the oxidation of alkenes: A kinetic and mechanistic study

The oxidation of trans-stilbene, phenylacetylene, and diphenylacetylene by Tl(OAc)₃ in aqueous acetic acid medium in the presence of HClO₄ follows the rate law in [H⁺] of 0.1–1.0M, the [H⁺] dependence below 0.1M being marginal. The reactions are strongly dielectric dependent. The order of reactivity among the substrates is styrene > phenylacetylene and trans-stilbene > diphenylacetylene. A mechanism involving the oxythallation adduct by the Tl⁺(OAc)₂ species has been discussed. The use of Ru(III) as a homogeneous catalyst brings a change in the kinetic orders for trans-stilbene, the rate law being The formation constants K for the Ru(III)–alkene π complex at 40, 50, and 60°C are 90.14M^?1, 105.2M^?1, and 127.7M^?1, respectively. Interestingly the oxidation of phenylacetylene and diphenylacetylene does not undergo catalysis by Ru(III). The mechanism involving the metal–arene π complex is discussed.     

The mechanism and rate parameters for the pyrolysis of n-hexane in the range 723–823 K

The pyrolysis of n-hexane has been investigated in the ranges 723–823 K and 10–100 Torr at up to 3% decomposition. The reaction is homogeneous and free from the self-inhibition by olefin products observed for several other alkanes. The products of the reaction are hydrogen, methane, ethane, ethene, propene, but-1-ene, and pent-1-ene, with smaller amounts of propane. It is shown that the results are in quantitative agreement with a conventional Rice-Herzfeld chain mechanism terminated by the combination and disproportionation of ethyl radicals, but with the mechanism extended so as to include the unimolecular isomerizations via a six-membered cyclic transition state between 1-hexyl and 2-hexyl (1-methylpentyl) radicals. The overall rate constant of initiation is estimated to be given by The rate constant for the reaction is given by which when combined with published data gives an Arrhenius plot curved upwards at low values of 1/T as has been observed for several other hydrogen abstraction reactions of methyl and of ethyl. Estimates are made of rate constants and ratios of rate constants for several reactions of the free radicals involved in the reaction. It is suggested that the minor product propane arises mainly from a hydrogen abstraction by 1-propyl from hexane with a contribution from a minor termination process involving ethyl and methyl.     

Gas and liquid phase kinetics of the unimolecular isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane: The radical stabilization energy of an α-bromine atom

The kinetics of the gas phase isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane to 2-chloro-5-bromohexa-1,5-diene have been measured in a static system over the temperature range of 135–215°C, with a variation in the total pressure from 0.6 to 400 torr. For these conditions the rate constants are well represented by the Arrhenius equation: where θ = 2.303RT kcal/mole. Transition state estimates for the biradical mechanism for the isomerization of bicyclo[2.2.0]hexanes are shown to be in good agreement with these Arrhenius parameters. By comparison of the activation energy with that for the isomerization of bicyclo[2.2.0]hexane and 1,4-dichlorobicyclo[2.2.0]hexane, the radical stabilization energy of an α-bromine atom is shown to be 1.0 ± 1.8 kcal/mole. Rates are also reported in the liquid phase at temperatures of 155°C and 175°C with diphenyl ether, nitrobenzene, and dimethylsulfoxide as solvents. The observed rate constants are all faster (by a factor of 1.1–1.7) than those measured in the gas phase and display no correlation between rate and solvent polarity.     

Time-resolved vibrational chemiluminescence: Rate constants for the reactions of F atoms with H2O and HCN,and for the relaxation of HF (v = 1) by H2O and HCN

Rate constants have been determined at (298 ± 4) K for the reactions: and the relaxation processes: Time-resolved HF(1,0) emission was observed following the photolysis of F₂ with pulses from an excimer laser operating on XeCl (λ = 308 nm). Analysis of the emission traces gave first-order constants for reaction and relaxation, and their dependence on [H₂O] and [HCN] yielded:      

Alternating copolymerization of benzofuran with crotononitrile and α-chloroacrylonitrile

The copolymerizations of benzofuran with α,α- or α,β-disubstituted acrylic monomers were studied. The alternating copolymer of benzofuran and crotononitrile was prepared in the presence of an excess amount of crotononitrile with respect to benzofuran, ethylaluminum dichloride, and azobisisobutyronitrile. The intrinsic viscosity of copolymers was 0.1–0.2 dl/g. Crotononitrile is known to possess a polar carbon–carbon double bond from ¹³C-NMR spectroscopy but the alternating copolymerizability with benzofuran is low. It was found that the order of alternating copolymerizability of acrylic monomers is as follows: This fact may be attributed to the steric hindrance of the β-methyl of crotononitrile. The induced shifts by complexation with ethylaluminum dichloride on ¹³C-NMR spectra of the two isomers of crotononitrile are almost same but the copolymerizability of cis isomer is higher than that of trans isomer. α-Chloroacrylonitrile shows the highest alternating copolymerizability with benzofuran in the presence of weak Lewis acid such as ethoxyaluminum chloride. Alternating copolymerizability of acrylic monomers seems to be in proportion to their e value. The reactivity of cis- and trans-crotononitrile may depend on the nature of a ternary complex composed of aluminum compound, crotononitrile, and benzofuran.     

Thermal reaction of hydrogen–butene-2-cis mixtures at 500°C: Hydrogenation,hydrogenolysis, and thermal reaction of the olefin

The thermal reaction of hydrogen–butene-2-cis mixtures has been studied in a static system at low extent of reaction around 500°C. Hydrogen does not affect the thermal reaction itself of the olefin, but gives rise to new stoichiometries of hydrogenolysis and hydrogenation, which are specified: The reaction is described in terms of a molecular and free-radical mechanism. It is shown that the key process for the hydrogenolysis–hydrogenation reaction is (1) and that the rate constant of this process can be determined from either propylene, or methane, or butene-1 formations: with θ = 4.57 × 10^?3 T kcal/mol. Other rate constants are estimated and agree with literature data.     

Effects of alkenes on the thermal decomposition of propane part II. Influence of ethylene

Experiments with propane-ethylene mixtures in the temperature range 760–830 K resulted in refinement of the role of ethylene inhibition in the decomposition of propane. The source of the rate-reducing effect of ethylene is the reaction This replaces the decomposition chains more slowly by means of the reactions than H-atoms do by direct H-abstraction from propane. Analysis of the ratios of the product formation rates showed that the selectivity of the ethyl radical for the abstraction of hydrogen of different bond strengths from propane was practically the same as that of the H-atom. The ratio of the rate constants of hydrogen addition to ethylene and methyl-hydrogen abstraction from propane by the H-atom (3) was determined as was that of the decomposition and the similar H-abstraction of the ethyl radical Interpretation of the influence of ethylene required the completion of the mechanism with further initiation of the reaction besides termination via ethyl radicals.     

Dynamic relaxation behavior of copolymers of n-butyl methacrylate with n-butyl acrylate and of 2-hydroxyethyl methacrylate with ethyl acrylate,n-butyl acrylate,and dodecyl methacrylate

The effect of the α-methyl group on the mobility of the main and side chains of methacrylateacrylate copolymers has been investigated. Poly(ethyl acrylate) shows a small secondary loss maximum (attributed to the rotation of ? COOR side chains) at 145 K, while in the case of poly(n-butyl acrylate) this relaxation process is smeared out or possibly absent. On the contrary, poly(n-butyl methacrylate) and poly(2-hydroxyethyl methacrylate) exhibit secondary relaxations at about 278 and 301 K, respectively. From the dynamic mechanical response spectra of methacrylate-acrylate copolymers one can see that the removal of the α-methyl group causes a qualitative change in the molecular mechanism of the secondary relaxation, presumably as a consequence of the different participation of the main chains. The existing data, however, are insufficient to quantify these differences. The low-temperature relaxation attributed to internal motion within the side groups is not distinctly affected by the presence of α-methyl groups. If both components of the copolymer display the low-temperature relaxation (above 77 K), the loss maxima preserve their identity to a large extent. The effect of copolymer composition on the main (glass) transition temperature has been described by means of a one-parameter equation.     

Diluent effects on molecular motions and glass transition in polymers. I. Polystyrene–toluene

The effects of diluent on molecular motions and glass transition in the polystyrene–toluene system was studied by means of dielectric, thermal, and NMR measurements. Three dielectric relaxations were observed between 80 and 400°K. On the basis of NMR measurements on solutions in toluene and in deuterated toluene, relaxation processes were assigned to segmental motions of polystyrene, rotations of toluene, and the local motions of polystyrene and toluene in order of appearance from the high-temperature side. The concentration dependence of the relaxation strength and of the activation energy for the primary relaxation (that at the highest temperature) show a step increment at about 50% by weight. The activation plots for the primary process were expressed by the Vogel–Tamman equation. With this equation, the temperatures at which the mean dielectric relaxation time becomes 100 sec is determined. This agrees well with the glass-transition temperature T_g and hence T_g in concentrated solution is expressed by in terms of the parameters A, B, and T₀ of the Vogel–Tamman equation. The values of A and B are, respectively, about 12 and 0.65 and independent of the concentration. The physical meaning of these parameters is discussed.