Poly-α-ester degradation studies. VI. Thermal degradation of poly(3-pentylidene carboxylate)

The thermal degradation of poly(3-pentylidene carboxylate) has been studied kinetically over the temperature range 200–300°C using thermogravimetry, gas evolution analysis, and rheogoniometry together with isolation and analysis of the reaction products. The observed behavior is completely different from that previously reported for poly(isopropylidene carboxylate) and poly(methylene carboxylate). Whereas in the latter cases the decomposition occurs by a first-order intramolecular ester interchange process characterized by an activation energy in the region of 27 kcal mole^?1, poly(3-pentylidene carboxylate) decomposition occurs by random chain scission superimposed on a first-order hydrogen abstraction process. The activation energy associated with this decomposition reaction is in the region of 47 kcal mole^?1, and the major degradation products are cis- and trans-2-ethyl crotonic acid.     

Thermochemical studies of sym-dichlorobis (2,4,6-trichlorophenyl) urea

Thermal decomposition of sym-dichlorobis (2,4,6-trichlorophenyl) urea occurs by two steps: the first at 150–184°C accompanied by a 26% weight loss and +(16.6±0.7) kcal mole^?1 and the second by a 40% weight loss and ?(17.4±1.0) kcal mole^?1. The decomposition pressure follows the equation ln p=A + B/T + C/T² where A = 149.89, B=9.45·10 [4] and C=1.48·10 [7]. The decomposition products are 2,4,6-trichlorophenyl isocyanate 2,4,6-trichloroaniline, chlorine, 1,2,3,5-tetrachlorobenzene, 2,2′,4,4′,6,6′-hexachlorobiphenyl, 2,2′,4,4′-tetrachlorobiphenyl, 2,2′,4,4′,6-pentachlorobiphenyl and ammonium chloride.     

The Thermal Decomposition of Polydichlorophosphazene

Abstract

The thermal decomposition of polydichlorophosphazene has been examined. The decomposition was found to be first-order with an activation energy of 22.5 ± 2 kcal mole^?1. The products comprised a wide range of cyclic and linear dichlorophosphazenes. It is suggested that the decomposition reaction is initiated at the ends of the macromolecules.     

Thermal decomposition of sodium azide

The thermal decomposition of sodium azide has been investigated in the temperature range 240–365°C. Three values for the activation energy, 37.0, 59.0 and 14 kcal mol^?1 have been obtained depending on the temperature range of study. The mechanism of decomposition seems to involve excited azide ions (through internal conversion) and excitations. The activation energy of 14 kcal mol^?1 appears to be associated with the promotion of electron in the presence of sodium metal.     

Kinetic study of the dehydration of modified Y zeolites

The dehydration of ferric exchanged Y zeolites is studied by thermal analysis. Their DTA shows three endotherms in the temperature range 80–450°C. The order of reaction and apparent energies of activation are calculated using various equations. The order of dehydration is nearly one and the apparent energy of activation is 4–8 kcal mole^?1. The effect of heating rate is studied. The energy of activation as determined by the Kissinger and Ozawa method is about 12 kcal mole^?1, which is comparable with the heat of adsorption of water determined by the gravimetric method, and is more acceptable.     

Flash photolysis studies of the reaction of NH2 radicals with NO

The kinetics of the reaction NH₂ + NO → N₂ + H₂O were studied, using a conventional flash photolysis system. A value of k₁ = (1.1 ± 0.2) × 10¹⁰ & mole^?1 s^?1 was obtained at room temperature and in the pressure range 2–700 torr in the presence of nitrogen. A slight negative temperature coefficient was observed between 300 and 500 K, equivalent to a negative activation energy of 1.05 ± 0.2 kcal mole^?1.     

Studies of Thermal Transesterification of Phenylurethane by High Pressure Liquid Chromatograph

Thermal transesterification of phenylurethane with n-octanol was carried out in DMSO. It was found that the reaction followed first-order kinetics with an average rate constant of 8.630 × 10^?5 sec ^?1at 140°C. High pressure liquid chromatograph technique was employed to analyze chemical species in the course of the reaction. The reaction obeyed Arrhenius equation closely between 133°C and 155°C with activation energy of 29.6 kcal/mole and entropy of activation of ?8.2 cal/mole deg at 140°C. An intramolecular cyclic intermediate mechanism was proposed for this reaction.     

The Catalytic Oxidation of Hydrogen on Nickel Oxides

The catalytic oxidation of hydrogen on highly-dispersed and sintered nickel oxides has been studied by a static method and the existence of two different kinetic rcgions established. Between 0 and 100°C the initial catalytic activity was not ionary and a strong poisoning effect of the reaction product was observed at all temperatnres up to 250°C. The activation energy of the reaction based on the initial reaction rates on freshly- outgassed oxide surfaces had a low value of 1–2 kcal. mole^?1 with both preparations. Between 250 and 350°C stationary catalytic activity was observed and the activation energy of the reaction was significantly higher, 12–14 kcal . mole^?1. The change of the activation energy is discussed in terms of the participation in the reaction of oxygen species in the catalyst surface layer which have different reactivities in the two temperature regions. A close analogy is noted between the carbon monoxide and hydrogen oxidation reactions on nickel oxide and a compensation effect is illustrated for a series of oxidation reactions on the oxide.     

Thermodynamics of the lanthanum-hydrogen system at 917°K

The binary system lanthanum-hydrogen has been studied at pressures up to 1 atm at 917°K by a calorimetric-equilibrium method. From the calorimetric measurements we found the enthalpy of formation of LaH₂ at 917°K to be ?45.7 kcal mole^?1 with an estimated uncertainty at ±0.3 kcal mole^?1. This result is about 4 kcal mole^?1 less negative than the values derived indirectly from plateau pressure equilibrium measurements by Mulford and Halley and by Korst and Warf. A comparison between the calorimetric and equilibrium measurements at 917°K provides information on the partial entropy of hydrogen in lanthanum and in the dihydride LaH_2±δ. The excess entropy of hydrogen in lanthanum is about 6 cal K^?1 mole^?1 at 917°K: this value is essentially fully accounted for by the estimated vibrational entropy contribution of the hydrogen atoms. In LaH_2±δ the partial entropy of hydrogen changes from small negative values at X ≈ 1.95 to positive values for X > 2. This entropy change is explained by an assumed intrinsic disorder of hydrogen in LaH₂ of about 0.02.     

Application of DNMR,group theory and molecular mechanics in the elucidation of the inversion process in the flexible molecule perhydrotetra-azapyrene

The inversion process in cis-10b,10c-perhydro-1H,6H-3a,5a,8a,10a-tetraazapyrene (1) has been investigated by ¹H and ¹³C NMR. The free energy of activation is found to be 14.95 ± 0.2 kcal mole^?1 at 45°. Application of group theoretical techniques led to a graph representing the essential symmetry properties of the potential energy surface for conformational change. The energies of intermediates on this graph were then estimated using molecular mechanics calculations. This combined approach suggests that the total inversion proceeds via a conformation of C_2v symmetry with two non-chair piperazine rings, calculated to be 6.8 kcal mole^?1 less stable than the ground state conformation (C₂ symmetry).     

Radiation-Induced Solid-State Polymerization of Derivatives of Methacrylic Acid. VII. A Differential Scanning Calorimetric Investigation of the Dehydration and Polymerization of Barium Methacrylate

Abstract

Barium methacrylate monohydrate showed two dehydration steps (at 80 and 110°C), in small crystals, but only one step (at 80° C) in fine powder. The equilibrium water vapor pres-sure was independent of the degree of hydration and its temperature dependence was found to be log₁₀ p_H2O (Torr)=10.70 - 2858/T, giving ΔH (dehyd)=54 ±4 kJ mole^?1. Small doses of γ-irradiation at 78° C suppressed the second de-hydration endotherm and shifted the first to higher temperatures. Larger radiation doses caused the endotherm to diminish and be re-placed by a polymerization exotherm. The variations in position with scan rate gave E_a=163 kJ mole^?1, similar to the value from isothermal polymerization rate measurements. There was no evidence of polymerization in the anhydrate except at the decompo-sition temperature.     

On the thermal decomposition of K2PdCl4 in a hydrogen atmosphere

The decomposition of single crystals and powders of K₂PdCl₄ in a hydrogen atmosphere was investigated by means of thermogravimetry (TG) at temperatures between 85 and 170°C, and by optical microscopy. The rate of decomposition is controlled by a combined process of nucleation and growth. The activation energy was calculated to be 15.2 ± 0.5 kcal mol^?1 for single crystals and 13.5 ± 0.4 kcal mol^?1 for powders. The results are compared with those obtained for K₂PtCl₄.An attempt was made to explain the differences in the orientation relationships, previously determined by X-ray diffraction, between K₂PtCl₄ and K₂PdCl₄, Rb₂PdCl₄ and K₂PdBr₄ and their decomposition products with a different kinetic behaviour.     

The addition of methyl radicals to tetrafluoroethylene

The addition of methyl radicals to tetrafluoroethylene in the gas phase has been studied over the temperature range 80–180°C, using a material balance method. Arrhenius parameters of 10^11.95±0.23 (mole^?1 cm³ sec^?1) and 5.7 ± 0.4 (kcal/mole) have been measured for the addition reaction. Electrophilic reagents such as O or CF₃ appear to react almost equally readily with ethylene and tetrafluoroethylene but methyl radicals add much more rapidly to tetrafluoroethylene than to ethylene, the difference in reactivity being principally due to an activation energy difference of ～2 kcal/mole.     

Kinetics of the II-III phase transformation of polytetrafluoroethylene at high pressure

The rate of transformation from helical to all-trans-planar polytetrafluoroethylene (PTFE) at high pressures has been determined by monitoring the Raman spectra of PTFE following pressure jumps from the stability field of PTFE II to pressures between 9 and 14 kbar at temperatures between 0 and ?30°C. The transformation kinetics can be described by Avrami's equation for nucleation and growth kinetics with an exponent of 0.5, although observations at lower temperatures suggest that even smaller values of the exponent may be appropriate. At 10 kbar and 0°C, the specific rate constant is 0.51 min^?1/2. The energy and volume of activation are 11 kcal mole^?1 and ?7 cm³ mole^?1, respectively. The values of these parameters suggest that the transformation mechanism involves propagation of helix reversal planes along the several adjacent chains leaving all-trans material in their wakes.     

A study of molecular motion in solid cyclopropane and cyclopropane clathrate deuterate by nuclear magnetic resonance absorption and relaxation methods

Proton magnetic resonance absorption and spin-lattice relaxation measurements have been carried out for cyclopropane clathrate deuterate from 77 to 290 K together with spin—lattice relaxation measurements on solid cyclopropane from 90 to 146 K. The absorption measurement for the type I structure deuterate indicates the presence of an isotropic rotation of the cyclopropane molecule from about 230 K, while in the type II structure deuterate isotropic rotation of the enclathrated cyclopropane is present over all of the range of stability of the clathrate (~250 to 278 K). The spin-lattice relaxation measurements give an activation energy of 0.83 ± 0.03 kcal mole^?1 for the barrier to reorientation (not assigned) of the cyclopropane molecules inside the clathrate deuterate cavities. In solid cyclopropane the barrier associated with the threefold axis rotation is found to be 4.8 ± 0.2 kcal mole^?1.     

The nitric oxide catalyzed positional isomerization of 3-methylene-1,5,5-trimethylcyclohexene and the stabilization energy in the 3-methylenecyclohexenyl radical

The gas phase, nitric oxide catalyzed positional isomerization of 3-methylene-1,5,5-trimethylcyclohexene (MTC) into 1,3,5,5-tetramethyl-1,3-cyclohexadiene (TECD) has been studied for temperatures ranging between 296° and 425°C. The major reaction was first order with respect to nitric oxide and to MTC. The major side product, mesitylene, usually amounted to less than 10% of the TECD isomer formed. Only at high temperatures and large conversions has up to 20% been observed. Conditioned pyrex or quartz vessels coated with KCl have been used. The nitric oxide catalyzed isomerization is apparently a homogeneous process, as demonstrated by the insensitivity of the observed rate constants towards a 15-fold increase in the surface to volume ratio of the reaction vessels. However, a residual, presumably heterogeneous, thermal isomerization of the starting material could not be eliminated. Good mass balances were obtained for both NO and hydrocarbons. After correcting for the thermally induced conversion the observed rate constants for the nitric oxide catalyzed isomerization yield log k₁ (1 mole^?1 sec^?1) = (10.7 ± 0.2) – (37.3 ± 0.9)/θ where θ is 2.303 × 10^?3 RT (kcal mole^?1). Plotting log k₁ versus the ratio of the starting materials (MTC/NO)₀ it was found that for temperatures ≥ 365°C the rate constants were systematically too high. Using extrapolated values for the higher temperature range yields the more reliable corrected Arrhenius equation log k = 8.6 – 31.7/θ. The reaction mechanism is outlined and the implications with respect to the stabilization energy generated in the MTC? radical intermediate and the activation energy of the backreaction MTC? + HNO are discussed. Using for the activation energy E_?1 of the backreaction (R? + HNO) a literature value of 9.2 ± 0.9 kcal mole^?1 reported for the cyclohexadiene? 1,3? system, this yields 23.4 ± 2 kcal mole^?1 for the stabilization energy in the methylenecyclohexenyl radical, which is to be compared with the corresponding values for the allyl (10.2 ± 1.4), methallyl (12.6 ± 1) pentadienyl (15.4 ± 1) and cyclohexadienyl (24.6 ± 0.7) radicals. The pre-exponential factor agrees well with the value of (8.4 ± 0.2) reported by Shaw and co-workers for the similar reaction of NO with 1,3-cyclohexadiene. It is noteworthy that HNO, acting as sole hydrogen donor in the system, is surprisingly stable under the reaction conditions used. Nitrous oxide, HCN, H₂O and N₂ are observed in the product mixture of experiments carried out to high conversions at higher temperatures.     

Kinetics of the reactions of isopropoxy radicals with NO,NO2, and O2

A fluorescence excitation spectrum of (CH₃)₂CHO (isopropoxy radical) is reported following photolysis of isopropyl nitrite at 355 nm. Rate constants for the reaction of isopropoxy with NO, NO₂, and O₂ have been measured as a function of pressure (1–50 Torr) and temperature (25–110°C) by monitoring isopropoxy radical concentrations using laser-induced fluorescence. We have obtained the following Arrhenius expressions for the reaction of isopropoxy with NO and O₂ respectively: (1.22±0.28)×10^?11 exp[(+0.62±0.14 kcal)/RT]cm²/s and (1.51±0.70)×10^?14 exp[(?0.39±0.28)kcal/RT]cm³/s where the uncertainties represent 2σ. The results with NO₂ are more complex, but indicate that reaction with NO₂ proceeds more rapidly than with NO contrary to previous reports. The pressure dependence of the thermal decomposition of the isopropoxy radical was studied at 104 and 133°C over a 300 Torr range using nitrogen as a buffer gas. The reaction is in the fall-off region over the entire range. Upper limits for the reaction of isopropoxy with acetaldehyde, isobutane, ethylene, and trimethyl ethylene are reported.We have performed the first LIF study of the isopropoxy radical. Arrhenius parameters were measured for the reaction of i-PrO with O₂, NO, NO₂, using direct radical measurement techniques. All reactions are in their high-pressure limits at a few Torr of pressure. The rate constant for the reactions of i-PrO with NO and NO₂ reactions exhibit a small negative activation energy. Studies of the i-PrO + NO₂ reaction produce data which indicate that O(³P) reacts rapidly with i-PrO. Unimolecular decomposition studies of i-PrO indicate that the reaction is in the fall-off region between 1 and 300 Torr of N₂ and the high-pressure limit is above 1 atmosphere of N₂.     

Preparation and degradation of copolymers of methyl methacrylate with alkali metal methacrylates—IV. The relationship of product yields to sequence distribution

Experimental yields of methanol and methyl methacrylate (MMA), produced in degradation to 500° of copolymers of MMA with lithium, sodium and potassium methacrylates (KMA) respectively, have been compared with the amounts expected on the basis of composition and sequence distribution. Rates of formation of these products under isothermal conditions have also been measured and activation energies for MMA formation at various compositions in the KMA-MMA copolymer series have been evaluated. The activation energy changes from 35 ± 5 kcal mole^?1 in PMMA to 52 ± 5 kcal mole^?1 in a 75 mole% KMA copolymer, indicating increasing difficulty in depolymerization to MMA as the MMA sequences become shorter. The primary route to methanol is by a cyclization involving adjacent ester and salt units in the chain, giving anhydride rings and metal methoxide as the initial products. Methanol yields and the positions of the maxima in the yield vs copolymer composition curves, however, are found to be inconsistent with those predicted from sequence distribution calculations. It is argued that water retained by the copolymers plays a key part in the reaction scheme by converting the metal methoxide to methanol and hydroxide; the latter then causes conversion of ester groups to salt units, so permitting further cyclization in copolymers initially rich in MMA. Mechanisms are discussed in detail.     

Kinetics of the dissociation and cis-trans isomerization of o,o'-azodioxytoluene (dimeric o-nitrosotoluene)

The dimer-monomer reactions were investigated for the system cis and transo,o'-azodioxytoluene-o-nitrosotoluene in acetonitrile solvent. For the reaction cis dimer-monomer the following thermodynamic and activation parameters have been derived: ΔH°=58.5±2.5 kJ mole^?1, ΔS°=206.2±3.8 J mole^?1 K^?1, ΔH^≠=63.6±3.3 kJ mole^?1, ΔS^≠=6.3±0.3 J mole^?1 K^?1. The corresponding values for the reaction trans dimer-monomer are: ΔH°=45.6±2.1 kJ mole^?1, ΔS°=162.7±7.1 J mole^?1 K^?1, ΔH^≠=80.8±2.9 kj mole^?1, ΔS^≠=-13.4±0.8 mole^?1 K^?1. There is no evidence of a direct cis-trans isomerization (i.e. a reaction not proceeding via the monomer). NMR and various perturbation techniques monitoring the visible absorption of the monomer were employed.     

The fluorobasicities of ReF7 and IF7 as measured by the enthalpy change ΔH°(EF7(g) → EF6+(g) + F−(g))

Iridium hexafluoride oxidizes ReF₆ (via an ReF₆⁺ salt) and at room temperatures IrF₆, ReF₆, ReF₇ and (IrF₅)₄ are each present in the equilibrium mixture. From these and related findings: ΔH°(ReF₆ → ReF₆⁺ + e^?) 1092 ± 27 kj mole^?1(261 ± 6 kcal mole^?1), and thermodynamic data are selected to yield ΔH°(ReF_7(g) → ReF₆⁺_(g) + F^?_(g))=893 ± 33 kj mole^?1(213 ± 8 kcal mole^?1). From observations on the stability of IF₆⁺BF₄^? and the lattice enthalpy evaluation for the salt, ΔH°(IF_7(g) → IF₆⁺_(g) + F^?_(g))= 870 ± 24 kj mole^?1(208 ± 6 kcal mole^?1).