Rheological study on the cure kinetics of two-component addition cured silicone rubber

The cure kinetics for two-component silicone rubber formed by addition reaction was studied by the rheological method. The influence of reaction temperature (T) on the cure kinetics was explored in detail. It was observed that the data of gel time (t _gel, i.e. the time when the reaction reaches the gel point) or a specific reaction time (t _nc) (defined as the reaction time before which time the influence of confinement of network on the diffusion of reaction components can be neglected) versus T obey certain functional relationship, which was well explained by the cure kinetics model of thermoset network. The cure kinetics for the two-component silicone rubber can be well fitted by the Kamal-Sourour(autocatalyst) reaction model rather than Kissinger model. When the reaction time was before or equal to t _nc, the reaction order obtained by the Kamal-Sourour reaction model was 2, which was consistent with the reaction order inferred from the two components chemical reaction when the diffusion of reaction components was not influenced by the formed cross-linked polymer network. When the reaction time was larger than t _nc, such as to the end of reaction (t _e), the influence of confinement of network on the diffusion of reaction components cannot be neglected, and the reaction order obtained by the Kamal-Sourour reaction model was larger than 2. It was concluded that the confinement effect of network had a greater influence on the cure kinetics of the silicone rubber. The reaction rate constants (k _r) under different temperatures were also determined by Kamal-Sourour reaction model. The activation energy (E) for the two-component silicone rubber was also calculated from the results of lnt _gel, lnt _nc, and lnk _r versus 1/T, respectively. The three values of E were close, which indicated that above analyses were self-consistent.     

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.     

A model study for coatings containing hexamethoxymethylmelamine

Using a model reaction we have studied the crosslinking chemistry of hydroxy-functional polymers and hexamethoxymethylmelamine. The transetherification of optically active monofunctional alcohols and hexamethoxymethylmelamine was monitored with polarimetry and ¹H-NMR. The reaction rate constants for both the forward (k₁) and the backward (k_?1) reaction of the sulphonic-acid-catalyzed alcoholysis were determined. Primary and secondary alcohols showed the same reaction rate and activation energy (E_a = 96 kJ/mol) for the forward reaction. However, the backward reaction in the equilibrium is considerably slower for primary alcohols than for secondary alcohols, with activation energies of E_a = 96 and 79 kJ/mol, respectively. When amine salts of sulphonic acids are used as catalysts, the E_a is increased from 97 to 116 kJ/mol in the case of primary alcohols. In concentrated aprotic solutions the reaction order in acid is 2.5. The same order in acid is found for the alcoholysis of acetaldehyde diethyl acetal. All the results strongly support the statement that the crosslinking reaction proceeds by an Sn-1 mechanism. The results of this model study are compared with results obtained in network-forming reactions. The important role of the evaporation of the condensation product methanol is discussed.     

HTPB／TDI,HDI聚合反应动力学研究

对端羟基聚丁二烯／甲苯二异氰酸酯，端羟基聚丁二烯／己二异氰酸酯甲苯溶液体系进行了反应动力学研究，用基团分析方法计算了相应体系的活化能，并对无催化剂和有催化剂的体系作了比较。结果表明，二丁基二月桂酸象对上述体系有强的催化作用，使体系的活化能降低，反应速度加快。对于对端差基聚丁二烯／甲苯二异氰酸酯体系，无催化剂时前后期反应活化能分别为２９．１ｋＪ／ｍｏｌ、３７．４ｋＪ／ｍｏｌ，有催化剂时前后期反应活化     

Kinetics of the reaction CH + N2 rarr; [M]⇒[M] products in the range 10–620 torr and 298–1059 K

The gas-phase reaction of CH(X² Π) radicals with molecular nitrogen was studied in the temperature range 298–1059 K at total pressures between 10 and 620 torr. CH radicals were generated by excimer laser photolysis of CHCIBr₂ at 248 nm and were detected by laser-induced fluorescence. The investigated reaction shows a strong temperature and pressure dependence. At pressures of 20, 100, and 620 torr the Arrhenius plots exhibit a strong decrease of the rate constant with increasing temperature. The rate constant is well described by, with E₀ in kJ/mol. The pressure dependence was studied at temperatures of 298, 410, 561, and 750 K. The rate constants for each temperature were fitted by the Troe formalism. From the calculated values of k₀ and k_infinity, the Arrhenius expressions, were obtained with E₀ (k₀) and E_A (k_infinity) in units of kJ/mol. Within the range of 298–750 K the temperature dependence of the broadening factor is well described by F_c = 0.029 + (173.3/T). © 1996 John Wiley & Sons, Inc.     

Polyesterification reactions of adipic acid-based polyesters

A series of studies of uncatalyzed and catalyzed polyesterification experiments have been investigated using different glycols at different temperatures. The experimental results for uncatalyzed reactions agreed quite well with the kinetic equation as proposed by P. J. Flory. However, Flory proposed a second-order reaction for an externally added strong acid as a catalyst. In contrast to Flory's theory, a mixed mechanism, based on a combination of carboxylic acid groups from the raw material and an external organotin catalyst, was proposed for the catalyzed reaction. In this article, the reaction rate of catalyzed and uncatalyzed polyesterifications using different glycols was compared, and the effect of different temperatures and catalyst levels was also discussed. The reaction rate constant decreases in the following order for both catalyzed and uncatalyzed polyesterification: 1,6-HD > 1,4-BD > NPG > DEG > EG ? PG. © 1993 John Wiley & Sons, Inc.     

Kinetics and mechanism of the oxidation of acrylic acid and methyl methacrylate

The kinetics of the initiated oxidation of acrylic acid and methyl methacrylate in the liquid phase were studied volumetrically by measuring oxygen uptake during the reaction. Both processes proceed via the chain mechanism with quadratic-law chain termination. The oxidation rate is described by the equation w = k ₂/(2k ₆)^1/2[monomer]w _i ^1/2 , where w _i is the initiation rate and k ₂ and k ₆ are the rate constants of chain propagation and termination. The parameter k ₂/(2k ₆)^1/2 is 7.58 × 10^?4 (l mol^?1 s^?1)^1/2 for acrylic acid oxidation and 2.09 × 10^?3 (l mol^?1 s^?1)^1/2 for the oxidation of methyl methacrylate (T = 333 K). For the oxidation of acrylic acid, k ₂ = 2.84 l mol^?1 s^?1 (T = 333 K) and the activation energy is E ₂ = 54.5 kJ/mol; for methyl methacrylate oxidation, k ₂ = 2.96 l mol^?1 s^?1 (T = 333 K) and E ₂ = 54.4 kJ/mol. The enthalpies of the reactions of RO ₂ ^? with acrylic acid and methyl methacrylate were calculated, and their activation energies were determined by the intersecting parabolas method. The contribution from the polar interaction to the activation energy was determined by comparing experimental and calculated E ₂ values: ΔE _μ = 5.7 kJ/mol for the reaction of RO ₂ ^? with acrylic acid and ΔE _μ = 0.9 kJ/mol for the reaction of RO ₂ ^? with methyl methacrylate. Experiments on the spontaneous oxidation of acrylic acid provided an estimate of the rate of chain initiation via the reaction of oxygen with the monomer: w _i,0 = (3.51 ± 0.85) × 10^?11 mol l^?1 s^?1 (T = 333 K).     

Enzymatic hydrolysis of protein: Mechanism and kinetic model

The bioreaction mechanism and kinetic behavior of protein enzymatic hydrolysis for preparing active peptides were investigated to model and characterize the enzymatic hydrolysis curves. Taking into account single-substrate hydrolysis, enzyme inactivation and substrate or product inhibition, the reaction mechanism could be deduced from a series of experimental results carried out in a stirred tank reactor at different substrate concentrations, enzyme concentrations and temperatures based on M-M equation. An exponential equation dh/dt = aexp(-bh) was also established, where parameters a and b have different expressions according to different reaction mechanisms, and different values for different reaction systems. For BSA-trypsin model system, the regressive results agree with the experimental data, i.e. the average relative error was only 4.73%, and the reaction constants were determined as K _m = 0.0748 g/L, K _s = 7.961 g/L, k _d = 9.358/min, k ₂ = 38.439/min, E _a = 64.826 kJ/mol, E _d = 80.031 kJ/mol in accordance with the proposed kinetic mode. The whole set of exponential kinetic equations can be used to model the bioreaction process of protein enzymatic hydrolysis, to calculate the thermodynamic and kinetic constants, and to optimize the operating parameters for bioreactor design. __________ Translated from Journal of Tianjin University, 2005, 38(9) (in Chinese)     

Proton catalyzed hydrolytic deamination of cytosine: a computational study

Two pathways involving proton catalyzed hydrolytic deamination of cytosine (to uracil) are investigated at the PCM-corrected B3LYP/6-311G(d,p) level of theory, in the presence of an additional catalyzing water molecule. It is concluded that the pathway involving initial protonation at nitrogen in position 3 of the ring, followed by water addition at C4 and proton transfer to the amino group, is a likely route to hydrolytic deamination. The rate determining step is the addition of water to the cytosine, with a calculated free energy barrier in aqueous solution of ΔG ^≠=140 kJ/mol. The current mechanism provides a lower barrier to deamination than previous work based on OH ^? catalyzed reactions, and lies closer to the experimental barrier derived from rate constants (E _a = 117  ±  4 kJ/mol).     

Absolute rate constants for the addition of hydroxymethyl radicals to alkenes in methanol solution

Absolute rate constants and their temperature dependencies were determined for the addition of hydroxymethyl radicals (CH₂OH) to 20 mono- or 1,1-disubstituted alkenes (CH₂ = CXY) in methanol by time-resolved electron spin resonance spectroscopy. With the alkene substituents the rate constants at 298 K (k₂₉₈) vary from 180 M^?1s^?1 (ethyl vinylether) to 2.1 middot; 10⁶ M^?1s^?1 (acrolein). The frequency factors obey log A/M^?1s^?1 = 8.1 ± 0.1, whereas the activation energies (E_a) range from 11.6 kJ/mol (methacrylonitrile) to 35.7 kJ/mol (ethyl vinylether). As shown by good correlations with the alkene electron affinities (EA), log k₂₉₈/M^?1s^?1 = 5.57 + 1.53 · EA/eV (R² = 0.820) and E_a = 15.86 ? 7.38 · EA/eV (R² = 0.773), hydroxymethyl is a nucleophilic radical, and its addition rates are strongly influenced by polar effects. No apparent correlation was found between E_a or log k₂₉₈ with the overall reaction enthalpy. © 1995 John Wiley & Sons, Inc.     

Calorimetric investigation of polymerization reactions. IV. Curing reaction of polyester fumarate with styrene

The curing reaction of polyester fumarate with styrene was investigated with a differential scanning calorimeter (DSC) operated isothermally. The change in rate of cure was followed over the whole range of conversion. The rate of cure is accelerated by the gel effect to about ten to fifty times the rate of model copolymerization of diethyl fumarate with styrene. This autoacceleration is much enhanced for systems with higher crosslinking densities and at lower temperatures. The results confirm that both termination and propagation steps of the curing reaction are controlled by diffusion of polymeric segments and monomer molecules over almost the whole range of conversion. The final extent of conversion is short of completion for isothermal cure and even for postcure of polyester fumarate with styrene because of crosslink formation. The final conversion of isothermal cure decreases with increasing crosslinking density and shows a maximum with increasing reaction temperature. This temperature dependency of the final conversion is caused by the difference in the activation energies for two propagation rate constants k_pf and k_ps, which were evaluated to be 7–10 and 5–8 kcal/mole, respectively, for the intermediate stage of the curing reaction.     

Exploring antibiotic resistant mechanism by microcalorimetry

In an effort to probe the reaction of antibiotic hydrolysis catalyzed by B3 metallo-??-lactamase (M??L), the thermodynamic parameters of penicillin G hydrolysis catalyzed by M??L L1 from Stenotrophomonas maltophilia were determined by microcalorimetric method. The values of activation free energy ??G _?? ^?? are 88.26, 89.44, 90.49, and 91.57?kJ?mol^?1 at 293.15, 298.15, 303.15, and 308.15?K, respectively, activation enthalpy ??H _?? ^?? is 24.02?kJ?mol^?1, activation entropy ??S _?? ^?? is ?219.2511?J?mol^?1?K^?1, apparent activation energy E is 26.5183?kJ?mol^?1, and the reaction order is 1.0. The thermodynamic parameters reveal that the penicillin G hydrolysis catalyzed by M??L L1 is an exothermic and spontaneous reaction.     

Kinetics and mechanism of atmospheric reactions of partially fluorinated alcohols

Gas-phase reactions typical of the Earth’s atmosphere have been studied for a number of partially fluorinated alcohols (PFAs). The rate constants of the reactions of CF₃CH₂OH, CH₂FCH₂OH, and CHF₂CH₂OH with fluorine atoms have been determined by the relative measurement method. The rate constant for CF₃CH₂OH has been measured in the temperature range 258–358 K (k = (3.4 ± 2.0) × 10¹³exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(1.5 ± 1.3) kJ/mol). The rate constants for CH₂FCH₂OH and CHF₂CH₂OH have been determined at room temperature to be (8.3 ± 2.9) × 10¹³ (T = 295 K) and (6.4 ± 0.6) × 10¹³ (T = 296 K) cm³ mol^?1 s^?1, respectively. The rate constants of the reactions between dioxygen and primary radicals resulting from PFA + F reactions have been determined by the relative measurement method. The reaction between O₂ and the radicals of the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH) have been investigated in the temperature range 258–358 K to obtain k = (3.8 ± 2.0) × 10⁸exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(10.2 ± 1.5) kJ/mol. For the reaction between O₂ and the radicals of the general formula C₂H₄FO (? HFCH₂O, CH₂F?HOH, and CH₂FCH₂?) at T = 258–358 K, k = (1.3 ± 0.6) × 10¹¹exp(?E/RT) cm³ mol^?1 s^?1, where E = ?(5.3 ± 1.4) kJ/mol. The rate constant of the reaction between O₂ and the radicals with the general formula C₂H₃F₂O (?F₂CH₂O, CHF₂?HOH, and CHF₂CH₂?) at T = 300 K is k = 1.32 × 10¹¹ cm³ mol^?1 s^?1. For the reaction between NO and the primary radicals with the general formula C₂H₂F₃O (CF₃CH₂? and CF₃?HOH), which result from the reaction CF₃CH₂OH + F, the rate constant at 298 K is k = 9.7 × 10⁹ cm³ mol^?1 s^?1. The experiments were carried out in a flow reactor, and the reaction mixture was analyzed mass-spectrometrically. A mechanism based on the results of our studies and on the literature data has been suggested for the atmospheric degradation of PFAs.     

Determination of Activation Energies and the Optimum Temperatures of Hydrolysis of Starch by α-Amylase from Porcine Pancreas

The present paper reports the determination of the activation energies and the optimum temperatures of starch hydrolysis by porcine pancreas α-amylase. The parameters were estimated based on the literature data on the activity curves versus temperature for starch hydrolysis by α-amylase from porcine pancreas. It was assumed that both the hydrolysis reaction process and the deactivation process of α-amylase were first-order reactions by the enzyme concentration. A mathematical model describing the effect of temperature on porcine pancreas α-amylase activity was used. The determine deactivation energies E_a were from 19.82 ± 7.22 kJ/mol to 128.80 ± 9.27 kJ/mol, the obtained optimum temperatures T_opt were in the range from 311.06 ± 1.10 K to 326.52 ± 1.75 K. In turn, the values of deactivation energies E_d has been noted in the range from 123.57 ± 14.17 kJ/mol to 209.37 ± 5.17 kJ/mol. The present study is related to the starch hydrolysis by α-amylase. In the industry, the obtained results the values E_a, E_d, T_opt can be used to design and optimize starch hydrolysis by α-amylase porcine pancreas. The obtained results might also find application in research on the pharmaceutical preparations used to treat pancreatic insufficiency or prognosis of pancreatic cancer.     

Micelle Catalyzed Oxidation of D-Mannose by Cerium (IV) in Sulfuric Acid

Kinetics of D-mannose oxidation by cerium (IV) was studied in a sulfuric acid medium at 40°C both in absence and presence of ionic micelles. In both cases, the rate of the reaction was first-order in D-mannose and cerium (IV), which decreased with increasing [H₂SO₄]. This suggested that the redox reaction followed the same mechanism. The reaction proceeded through formation of an intermediate complex, which was proved by kinetic method. The complex underwent slow unimolecular decomposition to a free radical that reacted with cerium (IV) to afford the product. The catalytic role of cationic cetyltrimethylammonium bromide (CTAB) micelles was best explained by the Menger-Portnoy model. The study of the effect of CTAB also indicated that a negatively charged species was the reactive form of cerium (IV). From the kinetic data, micelle-cerium (IV) binding and rate constants in micellar medium were evaluated. The anionic micelle of sodium dodecyl sulfate plays no catalytic role. The oxidation has the rate expression: -d[Ce(IV)]=k1Kc1[D-mannose][Ce(IV)]dt Different activation parameters for micelle catalyzed and uncatalyzed paths were also calculated and discussed.     

A kinetic study of diffusion in the electrothermal atomizer with a graphite filter by laser excited atomic fluorescence

Laser excited atomic fluorescence-electrothermal atomizer (LEAFS-ETA) was used to study atomization and diffusion mechanisms in a novel diffusive graphite tube atomizer. The atomizer design included a hollow graphite cylinder mounted between two graphite rods which served as electrodes. One of the rods had a small graphite insert with a sampling hollow and could move backwards and forwards. After the sample was introduced into the hollow, the electrodes tightly sealed the graphite cylinder ensuring that the insert was directly in the center of the furnace. The furnace assembly was then heated and the vaporized sample diffused through the hot graphite wall. The atomic fraction of the sample vapor was excited by a laser beam which was directed along the graphite tube surface so that no gap remained between the beam and the tube surface. Fluorescence vs. time profiles for three elements — Cu, Ag and Ni — were obtained within the temperature range of 1400–2600 K. The rate constants of specific processes were measured from the decay portions of the fluorescence signals under the assumption of first-order kinetics. The Arrhenius plots were constructed and the activation energies, E_a were evaluated from their slopes. The plots obtained for Cu and Ag consisted of two linear parts, the corresponding values of E_a were: 195 kJ/mol and 77 kJ/mol for Cu (1550 K < T < 2600 K) and 238 kJ/mol and 97 kJ/mol for Ag (1430 K < T < 2280 K). The Arrhenius plot for Ni was linear within the temperature range of 1770–2530 K resulting in an E_a equal to 161 kJ/mol. The diffusion coefficients were evaluated on the basis of a steady-state diffusion model out of a hollow cylinder. The values for the diffusion coefficients were: 3.7·10⁻⁴−2.0·10⁻³ cm²/s (1750–2600 K) for Cu, 6.5·10⁻³−1.4·10⁻³ cm²/s (1750–2280 K) for Ag and 5.6·10⁻⁵−1.5·10⁻³ cm²/s (1770–2530 K) for Ni.     

Ag(I)‐Catalyzed Chlorination of Linezolid during Water Treatment: Kinetics and Mechanism

The kinetic and mechanistic study of Ag(I)‐catalyzed chlorination of linezolid (LNZ) by free available chlorine (FAC) was investigated at environmentally relevant pH 4.0–9.0. Apparent second‐order rate constants decreased with an increase in pH of the reaction mixture. The apparent second‐order rate constant for uncatalyzed reaction, e.g., k″_app = 8.15 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 0.076 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C and for Ag(I) catalyzed reaction total apparent second‐order rate constant, e.g., k″_app = 51.50 dm³ mol⁻¹ s⁻¹ at pH 4.0 and k″_app. = 1.03 dm³ mol⁻¹ s⁻¹ at pH 9.0 and 25 ± 0.2°C. The Ag(I) catalyst accelerates the reaction of LNZ with FAC by 10‐fold. A mechanism involving electrophilic halogenation has been proposed based on the kinetic data and LC/ESI/MS spectra. The influence of temperature on the rate of reaction was studied; the rate constants were found to increase with an increase in temperature. The thermodynamic activation parameters E_a, ΔH^#, ΔS^#, and ΔG^# were evaluated for the reaction and discussed. The influence of catalyst, initially added product, dielectric constant, and ionic strength on the rate of reaction was also investigated. The monochlorinated substituted product along with degraded one was formed by the reaction of LNZ with FAC.     

Kalorimetrische Untersuchungen der Umwandlungskinetik von Ketten in amorphem Schwefel

The transformation of the chains in the amorphous sulphur was investigated by calorimetric method at the temperature range from 288 to 303 K. The results satisfy the equationX=1?exp [?(kt)²] (X-transformation degree,t—time,k andz—constants). It was found that thez values increasevs. the temperature The activation energy is equal toE=21 kJ/mol with standard deviation 5 kJ/mol. The results were explained on the basis of the theory of the nucleation and the growth of the nuclei.     

Origins of Large Rate Enhancements in the Nazarov Cyclization Catalyzed by Supramolecular Encapsulation

The self‐assembled supramolecular host [Ga₄L₆]^12? ( 1 ; L=N,N‐bis(2,3‐dihydroxybenzoyl)‐1,5‐diaminonaphthalene) catalyzes the Nazarov cyclization of 1,3‐pentadienols with extremely high levels of efficiency. The catalyzed reaction proceeds at a rate over a million times faster than that of the background reaction, an increase comparable to those observed in some enzymatic systems. A detailed study was conducted to elucidate the reaction mechanism of both the catalyzed and uncatalyzed Nazarov cyclization of pentadienols. Kinetic analysis and ¹⁸O‐exchange experiments implicate a mechanism, in which encapsulation, protonation, and water loss from substrate are reversible, followed by irreversible electrocyclization. Although electrocyclization is rate determining in the uncatalyzed reaction, the barrier for water loss and for electrocyclization are nearly equal in the assembly‐catalyzed reaction. Analysis of the energetics of the catalyzed and uncatalyzed reaction revealed that transition‐state stabilization contributes significantly to the dramatically enhanced rate of the catalyzed reaction.