A New Curing Kinetic Model and Its Application to BPSER/DDM Epoxy System

A new model has been deduced by assumed autocatalytic reactions. It includes two rate constants, k ₁ and k ₂, two reaction orders, m and n, and the initial concentration of [OH]. The model proposed has been applied to the curing reaction of a system of bisphenol-S epoxy resin (BPSER), with4,4'-diaminodiphenylmethane (DDM) as a curing agent. The curing reactions were studied by means of differential scanning calorimetry (DSC). Analysis of DSC data indicated that an autocatalytic behavior showed in the curing reaction. The new model was found to fit to the experimental data exactly. Rate constants, k ₁ and k ₂ were observed to be greater when curing temperature increased. The activation energies for k ₁ and k ₂ were 95.28 and 39.69 kJ mol^–1, respectively. Diffusion control was incorporated to describe the cure in the latter stages. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gas phase ionization of 1,3-propanedial tautomeric forms: A theoretical study

Two stable 1,3-propanedial tautomers and three their anions have been studied theoretically at MP2 and DFT levels of theory. The energies, structural parameters, ionization potentials, and vibration frequencies have been calculated at the two theoretical levels in order to compare the accuracy of the methods used. The ionization potential of the end form of 1,3-propanedial enol form was estimated to be 752 kJ mol^?1; the first and second potentials of the diketo form of 1,3-propanedial are 661 and 1239 kJ mol^?1, respectively.     

Isothermal cure kinetics of uncatalyzed and catalyzed diglycidyl ether of bisphenol-A/carboxylated polyester hybrid powder coating

The thermal cure behavior of diglycidyl ether bisphenol-A/carboxylated polyester hybrid powder coating system in the absence and presence of catalyst was monitored using differential scanning calorimetry. Curing temperatures were between 160 and 200?°C. The experimental results showed an autocatalytic behavior of the reaction, which could be described by the model proposed by Kamal. This model includes two rate constants k ₁ and k ₂ and two reaction orders m and n. The activation energies E _a1 and E _a2 of these rate constants were 51.7 and 42.3?kJ/mol for uncatalyzed cure reaction and 40.6 and 35.0?kJ/mol for externally catalyzed reaction. The average order of the overall reaction was found to be 2.45 and 2.72 for uncatalyzed and catalyzed system, respectively. Except for the late stage of cure reaction, the model agreed well with the experimental data, especially at high temperatures and in externally catalyzed cure reaction. A diffusion factor was introduced into the model to account for the effect of diffusion on the cure rate. The modified model greatly improved the predicated data at the late stage of cure reaction.     

Effects of methyl groups on the geometry and conformational equilibrium of 1,3-dioxanes

The geometries and energies of polymethyl-1,3-dioxanes were studied by molecular mechanics calculations. Buttressing effects of the Me groups are discussed. The chair/twist conformational equilibrium of 1,3-dioxanes having two syn-axial Me groups in the chair were calculated, and a twist form (the 1,4-twist) is found to be more stable than the chair only for 9 and 14, chair and 2,5-twist form are of comparable energies for 10 and 13, and the chair is considerably favored in 11 and 12. The chair/1,4-twist energy difference of 1 was calculated to be only 16.4 kJ mol^?1. Ring inversion of 1 goes through a transition state with C-C-C-O coplanar with a calculated activation enthalpy of 28.3 kJ mol^?1.     

Temperature dependence of the self-decay rate constants of some phenoxyl radicals in aqueous solution

The Arrhenius parameters of the bimolecular rate constants for the decay of several phenoxyl radicals in aqueous solution were measured. The p-halophenoxyl radicals (F, Cl, and Br) decay in a diffusion controlled reaction as the activation energies are the same as that of diffusion of water (16 ± 1.5 kJ · mol^?1). The A factors are 10^{12.2 ± 0.2}. For alkyl and alkoxy substituted phenoxyl, slightly higher activation energies were found (19.5 ? 21.9 kJ · mol^?1). © 1993 John Wiley & Sons, Inc.     

Enthalpies of sublimation and fusion of monophenylurea and diphenyl-1,3 urea

The sublimation enthalpies of monophenylurea (MPhU) and diphenyl-1,3 urea (1,3-DPhU) have been derived from the dependence of their vapour pressures on temperature, as measured by the torsion-effusion method. Values obtained are: 136 kj mol⁻¹ for MPhU and 152 kJ mol⁻¹ for 1,3-DPhU, where the estimated errors are comprised within 6 kJ mol⁻¹Enthalpies and temperatures of fusion have been measured by differential scanning calorimetry, leading to 23.7 kJ mol⁻¹ and 420.6 K for MPhU, and 34.6 kJ mol⁻¹ and 512 K for 1,3-DPhU. Poor reproducibility of results for 1,3-DPhU seems be due to the beginning of decomposition. No solid-to-solid transitions have been revealed from r.t. to fusion for both compounds.     

Reaction rates for nucleophiles with cyanogen chloride: Comparison with two other digonal carbon compounds

The hydrolysis kinetics of CICN have been reinvestigated from pH 0.0–10.5 and from 18–40°C. In the pH range from 1–5, the hydrolysis rate is invariant and the activation parameters (ΔH? = 84 kJ mol^?1 and ΔS? = ?84 J mol^?1 K^?1) are consistent with water attack. In basic solution the rate is first order each in CICN and OH^? concentrations with parameters ΔH? and ΔS? equal to 82 kJ mol^?1 and + 54 J mol^?1 K^?1, respectively. The rate constants with 20 other donors have been measured. Nitrogen nucleophiles are more reactive than oxygen donors, and an alpha-effect is seen. The constants follow a pattern indicative of attack at carbon. Cyanate in its acid form reacts with nucleophiles. Further points on the cyanate rate–pH profile have been obtained. A chromate-catalyzed hydrolysis can contribute between pH 5–10. Some studies were made of the reaction of cyanate with hydrogen peroxide. Free energy correlations are presented.     

Synthesis and cure kinetics of diphenyl(diphenylethynyl)silane monomer

Diphenyl(diphenylethynyl)silane ((ph–C≡C)₂–Si–ph₂) (DPDPES) was synthesized by the Grignard reaction. The corresponding isothermal and non-isothermal cure kinetics of DPDPES were analyzed by using differential scanning calorimetry (DSC), and the molecular structure was characterized by H-NMR. The results showed that all the cure curves were typically sigmoid shape and cure reactions could be described by an autocatalytic kinetic model by isothermal DSC. The kinetic data, for example, activation energy (E) and frequency factor (A), were 119.22 kJ/mol and 4.67 × 10⁷ (s^?1), respectively. The non-isothermal DSC analyses showed that E and A were 162.12 kJ/mol and 1.32 × 10⁹ (s^?1), respectively, and the reaction order was 0.94. Based on the research work of this paper, it can be said that the cure reaction of DPDPES monomer was of autocatalytic and diffusion-controlled characteristics, and the effect of the diffusion was more evident at low temperature. The cure reaction of DPDPES was a first-order kinetic reaction.     

Acetylation of Glycerol over Highly Stable and Active Sulfated Alumina Catalyst: Reaction Mechanism,Kinetic Modeling and Estimation of Kinetic Parameters

The Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic model was developed for acetylation of glycerol over highly stable and active 2 M SO₄²⁻/γ‐Al₂O₃ catalyst. The apparent reaction rate constants were determined by numerically solving the differential rate equations using ode23 tool in MATLAB coupled with the genetic algorithm optimization technique. The estimated rate constants were used to obtain the activation energy and pre‐exponential factor by using the Arrhenius equation. The estimated activation energy for direct acetylation of glycerol to monoacetylglycerol and diacetylglycerol was 7.2 kJ mol⁻¹, for acetylation of monoacetylglycerol to diacetylglycerol was 37.1 kJ mol⁻¹, and for acetylation of diacetylglycerol to triacetylglycerol was 26.6 kJ mol⁻¹, respectively.     

Conformational Analysis. XIX properties and reactions of 1,3-oxathianes VIII A 1H NMR conformational study of methyl-substituted derivatives

The addition of thioacetic acid to unsaturated alcohols or acids was utilized to obtain mercaptoalkanols which were condensed with suitable carybonyl compounds to prepare 24 methyl-substituted 1,3-oxathianes. The ¹H NMR spectra of the 1,3-oxathiane products were recorded at 60, 100 and/or 300 MHz and fully analysed. The results are best explained by a chair form which is completely staggered in the C-4? C-5? C-6 moiety ψ₄₅ or (ψ₅₆=60±1°). 1,3-Oxathianes having syn-axial 2,4- (and/or 2,6-) methyl-methyl interactions exist appreciably, if not exclusively, in twist forms. The vicinal coupling constants lead to the conformational free energies of axial methyl groups at C-4, ΔG°=7.4±0.4 kJ mol^?1, and at C-5, ΔG°=3.7±0.3 kJ mol^?1, in good agreement with previous estimates. They also show that both r-4,cis-5,trans-6- and r-4,trans-5,trans-6- trimethyl-1,3-oxathianes greatly favour the chiar form where the methyl group at C-4 is axial. The chair-twist energy parameters are reestimated at ΔH°_CT 27.0 kJ mol^?1, ΔS°_CT 11.6J mol^?1K^?1, and ΔG°_CT(298) 23.5 kJ mol^?1 for a 2,5-twist form.     

Investigation of the kinetics of formation of 1 : 1 complex between chromium(III) with 1,3-propanediamine-N,N′-diacetate-N,N′-di-3-propionate ion in aqueous acidic media

The kinetics of formation of the 1?:?1 complex of chromium(III) with 1,3-propanediamine-N,N′-diacetate-N,N′-di-3-propionate (1,3-pddadp) were followed spectrophotometrically at λ _max?=?557?nm. The reaction was first-order in chromium(III). Increasing the 1,3-pddadp concentration from 2.2?×?10^?2 to 0.11?mol?dm^?3 accelerated the reaction rate. Increasing the hydrogen ion concentration from 1.995?×?10^?5 to 6.31?×?10^?4 mol?dm^?3 retarded the reaction rate. The reaction rate was also retarded by increasing ionic strength and dielectric constant of the reaction medium. A mechanism was suggested to account for the results obtained which involves ion-pair formation between the various reactants. Values of 22?kJ?mol^?1 and ?115?J?K^?1 mol^?1 were obtained for the energy and the entropy of activation, respectively, which indicate an associative mechanism. The logarithm of the formation constant of the 1?:?1 complex formed was 11.3.     

Cure kinetics of thermosetting bisphenol E cyanate ester

Resin injection repair is a common method to repair delamination damage in polymer matrix composites (PMCs). To repair high-temperature PMCs, the resin should have a very low viscosity, yet cure into a compatible adhesive with high temperature stability. Normally, thermosetting polymers with high glass transition temperatures (T _g) are made from monomers with high room temperature viscosities. Among the high temperature resins, bisphenol E cyanate ester (BECy, 1,1’-bis(4-cyanatophenyl)ethane), is unique because it has an extremely low viscosity of 0.09–0.12 Pa s at room temperature yet polymerizes as a cross-linked thermoset with a high T _g of 274°C. BECy monomer is cured via a trimerization reaction, without volatile products, to form the high T _g amorphous network. In this study, the cure kinetics of BECy is investigated by differential scanning calorimetry (DSC). Both dynamic and isothermal experiments were carried out to obtain the kinetic parameters. An autocatalytic model was successfully used to model isothermal curing. The activation energy from the autocatalytic model is 60.3 kJ mol⁻¹ and the total reaction order is about 2.4. The empirical DiBenedetto equation was used to evaluate the relationship between T _g and conversion. The activation energy of BECy from the dynamic experiments is 66.7 kJ mol⁻¹ based on Kissinger’s method, while isoconversional analysis shows the activation energy changes as the reaction progresses.     

Study of the kinetics and activation parameters of reduction of Mn(III) to Mn(II) by SO3 2− ion in (MnSiW11O40H2)5- heteropoly ion

In order to gain an insight into the mechanism of reduction of Mn(III) heteropoly ions, and also to establish the conditions for use of some of these ions as oxidizing agent, following measurements have been made. The pseudo-first order rate constants, k_obs, have been determined and specific rate constants, k, were calculated from the plots of k_obs against SO₃ ^2? concentrations. A plot of ln(k₂/T) against inverse temperature gives enthalpy of activation as 10.67 kJ mol^?1 and entropy of activation as ?237.90 J K^?1 mol^?1. Effects of ionic strength and pH have also been studied over a limited range.     

Synthesis and nonisothermal reaction of a novel acrylonitrile-capped poly(propyleneimine) dendrimer with epoxy resin

A novel acrylonitrile-capped poly(propylene imine) dendrimer (PAN4) was synthesized and characterized with FTIR and ¹H-NMR. PAN4 and its precursor (poly(propylene imine) dendrimer (1.0GPPI) were employed to cure bisphenol A epoxy resin (DGEBA), and the nonisothermal reaction kinetics of DGEBA/PAN4 and DGEBA/1.0GPPI was systematically investigated using a differential scanning calorimeter (DSC) in a comparative way. The apparent activation energies determined with the Kissinger method were 59.7 kJ/mol for DGEBA/1.0GPPI and 53.9 kJ/mol for DGEBA/PAN4. Applied the Málek method, it was found that a two-parameter autocatalytic model (SB(m, n)) could well simulate the reaction rates, and further analysis of the reaction rate constants showed PAN4 could cure DGEBA at a greatly decreased rate by a factor a more than ten compared with 1.0GPPI control.     

Kinetische untersuchung der reversiblen dimerisierung von o-phenylendioxidimethylsilan

The reversible dimerisation of o-phenylenedioxydimethylsilane (2,2-dimethyl-1,3,2-benzodioxasilole) has been studied by ¹H NMR spectroscopy. The kinetics of this reaction can be described quantitatively by a bimolecular 10-ring formulation reaction and a monomolecular backreaction. The thermodynamic and kinetic parameters are: ΔH⁰ = ?43 kJ mol^?1; ΔS⁰ = ?112 J mol^?1 K^?1; ΔG⁰₂₉₈ = ?9.6 kJ mol^?1; ΔH³₂₉₈ = 57 kJ mol^?1; ΔS³₂₉₈ = ?129 J mol^?1 K^?1; ΔG³₂₉₈ = 96 kJ mol^?1; E_a = 60 kJ mol^?1; A = 3.17 × 10⁶ l mol^?1 s^?1. Remarkable is the low activation energy of formation of the ten-membered ring, considering that two SiO bonds have to be cleaved during the reaction. Transition states and possible structures of the ten-membered heterocycle are discussed.     

PREPARATION OF 1,3-DIPHOSHAALLENE FROM 1,2-DIPHOSPHACYCLOPROPANES: A THEORETICAL INVESTIGATION

Abstract

The ring opening of diphosphacyclopropane (1a), mono- (1b) and di-fluorodiphosphacyclopropane (1c) with methyllithium to give diphosphaallene is examined at the 3-21G(?) and 6-31G? Hartree-Fock level. In the first step, the diphosphacyclopropane opens to give the stable Li⁺/diphosphaallyl anion pair. The next step, formation of the phosphaallene, is endothermic unless an ionic salt (LiF) is produced, which can be further stabilized by solvent. The overall reaction energetics are 148.7 kJ Mol^?1 for 1a, -169.7 kJ mol^?1 for 1b, and - 137.8 kJ mol^?1 for 1c. The calculated ring strain energy for 1a is 61.8 kJ mol^?1.     

A density functional theory investigation on the mechanism and kinetics of dimethyl carbonate formation on Cu2O catalyst

A theoretical analysis about the mechanism and kinetics of dimethyl carbonate (DMC) formation via oxidative carbonylation of methanol on Cu₂O catalyst is explored using periodic density functional calculations, both in gas phase and in solvent. The effect of solvent is taken into account using the conductor‐like screening model. The calculated results show that CO insertion to methoxide species to produce monomethyl carbonate species is the rate‐determining step, the corresponding activation barrier is 161.9 kJ mol^?1. Then, monomethyl carbonate species reacts with additional methoxide to form DMC with an activation barrier of 98.8 kJ mol^?1, above reaction pathway mainly contributes to the formation of DMC. CO insertion to dimethoxide species to form DMC is also considered and analyzed, the corresponding activation barrier is 308.5 kJ mol^?1, suggesting that CO insertion to dimethoxide species is not competitive in dynamics in comparison with CO insertion to methoxide species. The solvent effects on CO insertion to methoxide species involving the activation barriers suggest that the rate‐determining step can be significantly affected by the solvent, 70.2 kJ mol^?1 in methanol and 63.9 kJ mol^?1 in water, which means that solvent effect can reduce the activation barrier of CO insertion to methoxide species and make the reaction of CO insertion to methoxide in solvents much easier than that in gas phase. Above calculated results can provide good theoretical guidance for the mechanism and kinetics of DMC formation and suggest that solvent effect can well improve the performance of DMC formation on Cu₂O catalyst in a liquid‐phase slurry. © 2012 Wiley Periodicals, Inc.     

Chemical equilibria 6. Measurement of equilibrium constants for the dehydrogenation of 2-methylpropan-1-ol by a vapour-flow technique

Equilibrium constants for 2-methylpropan-1-ol + 2-methylpropanal + hydrogen have been calculated from measurements of the composition of mixtures formed by passing the vapour over a catalyst at several temperatures in the range 473 to 563 K. Equations relating the changes in enthalpy and entropy of the dehydrogenation reaction to temperature were derived from the equilibrium constants with the aid of heat capacities. By coupling these changes with other thermodynamic data, the standard enthalpy of formation and the standard entropy of 2-methylpropanal at 298.15 K were calculated to be ?(215.7 ± 1.3) kJ mol^?1 and (331.2 ± 1.7) J K^?1 mol^?1 respectively, in the gas state, and ?(247.3 ± 1.8) kJ mol^?1 and (238.3 ± 4.4) J K^?1 mol^?1 respectively, in the liquid state.     

Rotation about the C?N bond in 2-aza-1,3-butadiene and the N?N bond in 2,3-diaza-1,3 butadiene: A molecular orbital study

The geometry and energy of 2-aza-1,3-butadiene and 2,3-diaza-1,3-butadiene have been calculated using the 6-31G* basis set as a function of the CNCC and CNNC dihedral angles, respectively. With the 2-aza derivative potential minima are located at 0° (trans) and at about 130° for a gauche structure approximately 9.5 kJ mol^?1 less stable than the trans. Potential maxima are at about 75° giving a gauche barrier height of approximately 19 kJ mol^?1 relative to the trans structure, and at 180° (cis) giving a barrier height of approximately 14.5 kJ mol^?1 relative to the 130° gauche structure. With the 2,3-diaza derivative the gauche barrier has disappeared and there are a series of gauche structures in the region 70°–100° of almost equal energy 12.5-15 kJ mol^?1 less stable than the trans. In addition the cis barrier is much greater, nearly 70 kJ mol^?1 relative to the trans structure. Inclusion of electron correlation, accounting for about 50% of the correlation energy, produces no significant changes in the shape of the potential energy curves. There are systematic and progressive changes in almost all the geometrical parameters as the ?CH? groups in butadiene are replaced by ?N? . The outward tilt and compression within the methylene groups show adverse steric interactions to be operative in the cis structures. The values of V_nn indicate that gauche structures of both the 2-aza and the 2,3-diaza derivatives near the cis structure are more compact (as with butadiene), and gauche structures of the 2-aza derivative near the trans structure are less compact (as with butadiene). Originating in the changes in bond lengths and bond angles, rotation-independent nuclear–nuclear interactions again play an important role.     

Gold Nanoparticles Formation via Au(III) Complex Ions Reduction with l‐Ascorbic Acid

In this paper, the kinetics and mechanism of gold nanoparticles formation during the redox reaction between [AuCl₄]− complex and l ‐ascorbic acid under different conditions were described. It was also shown that reagent concentration, chloride ions, and pH influence kinetics of nucleation and growth. To establish rate constants of these stages, the model of Finke and Watzky was applied. From Arrhenius and Eyring dependencies, the values of activation energy (22.5 kJ mol⁻¹ for the nucleation step and 30.3 kJ mol⁻¹ for the growth step), entropy (about −228 J K⁻¹ mol⁻¹ for the nucleation step and −128 J K⁻¹ mol⁻¹ for the growth step), and enthalpy (19.8 kJ mol⁻¹ for nucleation and 27.8 kJ mol⁻¹ for particles growth) were determined. It was also shown that the disproporationation reaction had influence on the rate of nanoparticles formation and may have impact on final particles morphology.