Polymerization of Cyclosiloxanes. I. Kinetic Studies on Living Polymer—Octamethylcyclotetrasiloxane Systems

The kinetics of the anionic polymerization of octamethylcyclotetrasiloxane (D₄) initiated by α-methylstyrene living polymer in tetrahydrofuran was studied. The following kinetic scheme was postulated: Initiation: Propagation: where S^- and M represent the initiator and D₄, respectively. At a living end concentration of 0.0377 mole/l. and a monomer concentration of 1.5 mole/l. in tetrahydrofuran at 25°C. the following kinetic data were obtained: k₁ = 2.3 × 10^?4 l./mole-sec., k₂ < 2.3 × 10^?5 sec.^?1, k₃ = 2.75 × 10^?2l./mole-sec. k₄ ≈ 1.17 × 10^?2 sec.^?1, K₁ > 10 l./mole and K₂ ≈ 2.35 l./mole. The rate constants k₁ and k₃ were found to be dependent on the concentration of anions. This is attributed to the dissociation of ion pairs to free ions at lower concentration. Under the experimental conditions studied the majority of the anions were present in the form of ion pairs. The reactivity of the free ions is about 100 times greater than that of ion pairs. There is no temperature effect on K₂, indicating zero ΔH and positive ΔS in the propagation reaction.     

The reactions of alkoxy radicals with O2. II. n-C3H7O radicals

n-C₃H₇ONO was photolyzed with 366 nm radiation at ?26, ?3, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yields of C₂H₅CHO, C₂H₅ONO, and CH₃CHO were measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?₁ = 0.38 ± 0.04 independent of temperature. The n-C₃H₇O radicals can react with NO by two routes The n-C₃H₇O radical can decompose via or react with O₂ via Values of k₄/k₂ ? k_4b/k₂ were determined to be (2.0 ± 0.2) × 10¹⁴, (3.1 ± 0.6) × 10¹⁴, and (1.4 ± 0.1) × 10¹⁵ molec/cm³ at 55, 88, and 120°C, respectively, at 150-torr total pressure of N₂. Values of k₆/k₂ were determined from ?26 to 88°C. They fit the Arrhenius expression: For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (2.9 ± 1.7) × 10^?13 exp{?(879 ± 117)/T} cm³/s. The reaction scheme also provides k_4b/k₆ = 1.58 × 10¹⁸ molec/cm³ at 120°C and k_8a/k₈ = 0.56 ± 0.24 independent of temperature, where      

Oxidation studies with peroxomonophosphoric acid. II. A kinetic and mechanistic study of dimethyl sulfoxide oxidation in acid and alkaline media

The kinetics of dimethyl sulfoxide (DMSO) oxidation by peroxomonophosphoric acid (PMPA) in aqueous medium at 308 K and I = 0.4 mol/dm³ follow the rate expressions In the pH range from 0 to 2, where k₁ and k₂ are 5.092 × 10^?1 dm³/mol sec and ? 0, respectively; in the pH range from 4 to 7, where k₂ = 8.127 × 10^?3 and k₃ = 2.90 × 10^?3 dm³/mol sec; and in the pH range from 10 to 13.6, where k₄ ? 0, and k₅ = 3.08 × 10^?2 dm³/mol sec. The reaction is interpreted in terms of mechanisms involving an electrophilic and a nucleophilic attack of the peroxomonophosphoric acid species, respectively, in acid and alkaline regions, on the sulfur atom of the sulfoxide molecule giving rise to S-type transition states followed by oxygen-oxygen bond fission to form the products.     

Kinetics of the reversible reaction between arsenious acid and aqueous iodine

The kinetics of the reversible reaction have been studied spectrophotometrically in acid solution under conditions in which both the forward and reverse reactions go to virtual completionand in which the reaction comes to a practical equlibrium. The rates of theforward (R_f) and reverse (R_r) reactions are given by where f, g, h, u, and v have the values (4 ± 1) × 10^?5 mole/1.·s, (4.2 ± 0.2) × 10^?5 mole²/1.²·s, (5.0 · 0.3) × 10^?7 mole³/1.³·s, (1.1 ± 0.1) × 10^?3 1.²/mole²·s, and (3.7 ± 0.2) × 10^?3 1.³/mole³·s at 298.2°K and at an ionic strength of 2.00M maintained by adding sodium chloride. The stoichiometric equilibrium constant under similar conditions is 0.022 ± 0.003. Differentvalues of these parameters were obtained when sodium perchlorate and sodiumnitrate were used to control ionic strength. The results are compared with those from previous reports and a mechanism is proposed based upon an initial rapid equilibrium followed by a rate-determining attack of water upon H₃AsO₃I⁺, H₂AsO₃I, and HAsO₃I^?.     

A pyrolysis mechanism for ammonia

The mechanism of NH₃ pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time-history measurements of the species NH and NH₂ were made using narrow-linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH₂ in this experimental database have also been summarized graphically. Rate coefficients for several reactions which influence the NH and NH₂ profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows: with a rate coefficient of 4.0 × 10¹³ exp(?3650/RT) cm³ mol^?1 s^?1, with a rate coefficient of 1.5 × 10¹⁵T^?0.5 cm³ mol^?1 s^?1 and with a rate coefficient of 5.0 × 10¹³ exp(?10000/RT) cm³ mol^?1 s^?1. The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates. The experimental data from four earlier measurements of the dissociation reaction were reanalyzed in light of recent data for the rate of NH₃ + H → NH₂1 + H₂, and an improved rate coefficient of 2.2 × 10¹⁶ exp(?93470/RT) cm³ mol^?1 s^?1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.     

Temperature and shear dependencies of rheology of poly(acrylonitrile‐co‐itaconic acid) dope in DMF

Poly(acrylonitrile‐co‐itaconic acid) (poly(AN‐co‐IA)) precursor required for carbon fiber production is made into a dope and spun into fibers using a suitable spinning technique. The viscosity of the resin dope is decided by the polymer concentration, polymer molecular weight, temperature, and shear force. The shear rheology of concentrated poly(AN‐co‐IA) polymer solutions in N,N‐dimethylformamide (DMF), in the range of 1 × 10⁵–1 × 10⁶ g mol^?1, has been investigated in the shear rate (γ′) range of 1 × 10¹–5 × 10⁴ min^?1. The zero shear viscosity (η₀) has been evaluated at different temperatures. The temperature dependence of zero shear viscosity conformed to the Arrhenius–Frenkel–Eyring model. The free energy of activation of viscous flow (ΔG_V) values were in the range 5–32 kJ mol^?1 and this value increased with increase in polymer concentration and molecular weight. A master equation for the ΔG_V value of the polymer solution of any and concentration (c) is suggested. The power law fitted well for the shear dependency of viscosity of these polymer solutions. The pseudoplasticity index (n) diminished with increase in polymer concentration and molecular weight. An empirical relation between viscosity (η) and was found to exist at constant shear rate, concentration and temperature. For each , the equation relating n, c, and T was established. Copyright © 2008 John Wiley & Sons, Ltd.     

The oxidation of methyl radicals at room temperature

Using the technique of molecular modulation spectrometry, we have measured directly the rate constants of several reactions involved in the oxidation of methyl radicals at room temperature: k₁ is in the fall-off pressure regime at our experimental pressures (20–760 torr) where the order lies between second and third and we obtain an estimate for the second-orderlimit of (1.2 ± 0.6) × 10^?12 cm³/molec · sec, together with third-order rate constants of (3.1 ± 0.8) × 10^?31 cm⁶/molec² · sec with N₂ as third body and (1.5 ± 0.8) × 10^?30 with neopentane; we cannot differentiate between k_2a and k_2c and we conclude k_2a + (k_2c) = (3.05 ± 0.8) × 10^?13 cm³/molec · sec and k_2b = (1.6 ± 0.4) × 10^?13 cm³/molec · sec; k₃ = (6.0 ± 1.0) × 10^?11 cm³/molec · sec.     

Kinetics of the molybdate catalyzed oxidation of iodide by hydrogen peroxide

The kinetics of the oxidation of iodide by hydrogen peroxide catalyzed by acidic molybdate have been studied by a spectrophotometric stopped-flow method. The results are interpreted in terms of the mechanism and the implied rate law where [mol] is total analytical concentration of molybdate. The values obtained for the rate and equilibrium constants are k₄ = (3.3 ± 1) × 10² 1./mole · s, K₁ = (1.2 ± 0.6) × 10⁴ 1./mole, K₂ = (1.3 ± 0.7) × 10³ 1./mole, and K₃ = (4 ± 3) × 10² 1./mole at 298°K.     

Kinetic investigation of the bromate–ascorbic acid–malonic acid system

According to our experiments the bromide ion concentration exhibits in the bromate–ascorbic acid–malonic acid–perchloric acid system three extrema as a function of time. To describe this peculiar phenomenon, the kinetics of four component reactions have been studied separately. The following rate equations were obtained: Bromate–ascorbic acid reaction: Bromate–bromide ion reaction: Bromide–ascorbic acid reaction: Bromine–malonic acid reaction: k₄ = 6 × 10^?3 s^?1, k_-4 ≥ 1.7 × 10³ s^?1, k₅ ≥ 1 × 10⁷M^?1 · s^?1 Taking into account the stoichiometry of the component reactions and using these rate equations, the concentration versus time curves of the composite system were calculated. Although the agreement is not as good as in the case of the component reactions, it is remarkable that this kinetic structure exhibits the three extrema found.     

Lactone polymers. II. Hydrodynamic properties and unperturbed dimensions of poly-ε-caprolactone

Poly-ε-caprolactone prepared by a dibutylzinc-catalyzed bulk polymerization process was fractionated, and the solution properties of the fractions were studied in benzene and in dimethylformamide. In these solvents at 30°C the Mark-Houwink relations were [η] = 9.94 × 10^?5 M and [η] = 1.91 × 10^?4 M , respectively. The value of K_Θ was found to vary from 1.1 to 1.2 × 10^?3 when determined by three known extrapolation techniques. Poly-ε-caprolactone chains appear to be quite flexible in solution, and the steric hindrance parameter σ had the low value of 1.37. Root-mean-square end-to-end dimensions were approximated from the experimental data and calculated from the Debye-Bueche and the Kirkwood-Riseman theories.     

Rate data for O + OCS → SO + CO and SO + O3 → SO2 + O2 by a new time-resolved technique

Pulsed laser photolysis of O₃ in a large excess of N₂ has been used to generate O(³P) atoms in the presence of OCS. By observing chemiluminescence from the small fraction of electronically excited SO₂ formed in the reaction of SO with O₃, rate constants of (1.7 ± 0.2) × 10^?14 and (8.7 ± 1.6) × 10^?14 cm³/molecule sec have been determined at 296 ± 4 K for the reactions and In addition, it has been shown that any reaction between SO and OCS has a rate constant 10^?14 cm³/molecule sec.     

Chain ion molecular mechanism of ascorbic acid oxidation with hydrogen peroxide. Cu3+ ion as a chain carrier

The kinetics and mechanism of ascorbic acid (DH₂) oxidation have been studied under anaerobic conditions in the presence of Cu²⁺ ions. At 10^?4 ≤ [Cu²⁺]₀ < 10^?3M, 10^?3 ≤ [DH₂]₀ < 10^?2M, 10^?2 ≤ [H₂O₂] ≤ 0.1M, 3 ≤ pH < 4, the following expression for the initial rate of ascorbic acid oxidation was obtained: where χ₂ (25°C) = (6.5 ± 0.6) × 10^?3 sec^?1. The effective activation energy is E₂ = 25 ± 1 kcal/mol. The chain mechanism of the reaction was established by addition of Cu⁺ acceptors (allyl alcohol and acetonitrile). The rate of the catalytic reaction is related to the rate of Cu⁺ initiation in the Cu²⁺ reaction with ascorbic acid by the expression where C is a function of pH and of H₂O₂ concentration. The rate equation where k₁(25°C) = (5.3 ± 1) × 10³M^?1 sec^?1 is true for the steady-state catalytic reaction. The Cu⁺ ion and a species, which undergoes acid–base and unimolecular conversions at the chain propagation step, are involved in quadratic chain termination. Ethanol and terbutanol do not affect the rate of the chain reaction at concentrations up to ≈0.3M. When the Cu²⁺–DH₂–H₂O₂ system is irradiated with UV light (λ = 313 nm), the rate of ascorbic acid oxidation increases by the value of the rate of the photochemical reaction in the absence of the catalyst. Hydroxyl radicals are not formed during the interaction of Cu⁺ with H₂O₂, and the chain mechanism of catalytic oxidation of ascorbic acid is quantitatively described by the following scheme. Initiation: Propagation: Termination:      

Reactions of alkoxy radicals with O2. III. i-C4H9O radicals

i-C₄H₉ONO was photolyzed with 366-nm radiation at ?8, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of i-C₃H₇CHO, Φ{i-C₃H₇CHO}, was measured as a function of reaction of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?₁ = 0.24 ± 0.02 independent of temperature. The i-C₄H₉O radicals can react with NO by two routes The i-C₄H₉O radical can decompose via or react with O₂ via Values of k₄/k₂ ? k_4b/k₂ were determined to be (2.8 ± 0.6) × 10¹⁴, (1.7 ± 0.2) × 10¹⁵, and (3.5 ± 1.3) × 10¹⁵ molec/cm³ at 23 55, and 88°C, respectively, at 150-torr total pressure of N₂. Values of k₆/k₂ were determined from ?8 to 120°C. They fit the Arrhenius expression: For k₂ ? 4.4 × 10¹¹ cm³/s, k₆ becomes (3.2 ± 2.0) × 10^?13 exp{?(836 ± 159)/T} cm³/s. The reaction scheme also provides k_4b/k₆ = 3.59 × 10¹⁸ and 5.17 × 10¹⁸ molec/cm³ at 55 and 88°C, respectively, and k_8b/k₈ = 0.66 ± 0.12 independent of temperature, where      

Polymerization of acrylamide with persulfate–thiomalic acid initiator

The redox system of potassium persulfate–thiomalic acid (I₁–I₂) was used to initiate the polymerization of acrylamide (M) in aqueous medium. For 20–30% conversion the rate equation is where R_p is the rate of polymerization. Activation energy is 8.34 kcal deg^?1 mole^?1 in the investigated range of temperature 25–45°C. M_n is directly proportional to [M] and inversely to [I₁]. The range of concentrations for which these observations hold at 35°C and pH 4.2 are [I₁] = (1.0–3.0) × 10^?3, [I₂] = (3.0–7.5) × 10^?3, and [M] = 5.0 × 10^?2–3.0 × 10^?1 mole/liter.     

Kinetics and mechanism of the reaction of OH with ClO

The kinetics and mechanism of the following reactions have been studied in the temperature range 230–360 K and at total pressure of 1 Torr of helium, using the discharge‐flow mass spectrometric method: 1a : (1a) 1b : (1b) The following Arrhenius expression for the total rate constant was obtained from the kinetics of OH consumption in excess of ClO radical, produced in the Cl + O₃ reaction either in excess of Cl atoms or ozone: k₁ = (6.7 ± 1.8) × 10^?12 exp {(360 ± 90)/T} cm³ molecule^?1 s^?1 (with k₁ = (2.2 ± 0.4) × 10^?11 cm³ molecule^?1 s^?1 at T = 298 K), where uncertainties represent 95% confidence limits and include estimated systematic errors. The value of k₁ is compared with those from previous studies and current recommendations. HCl was detected as a minor product of reaction (1) and the rate constant for the channel forming HCl (reaction (1b)) has been determined from the kinetics of HCl formation at T = 230–320 K: k_1b = (9.7 ± 4.1) × 10^?14 exp{(600 ± 120)/T} cm³ molecule^?1 s^?1 (with k_1b = (7.3 ± 2.2) × 10^?13 cm³ molecule^?1 s^?1 and k_1b/k₁ = 0.035 ± 0.010 at T = 298 K), where uncertainties represent 95% confidence limits. In addition, the measured kinetic data were used to derive the enthalpy of formation of HO₂ radicals: Δ H_f,298(HO₂) = 3.0 ± 0.4 kcal mol^?1. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 587–599, 2001     

Molecular-modulation spectrometry: CH3ONO photolysis and detection of nitroxyl

The method of molecular-modulation spectrometry for studying photochemical reactions has been applied to methyl nitrite photolysis. The infrared absorption of the nitroxyl radical HNO has been observed in the gas phase at 3300 cm^?1. Under the present experimental conditions the steady-state concentration of HNO under steady illumination was 1.1 × 10¹² particles/cc, and the observed modulation amplitude was 4.5 × 10¹⁰ particles/cc. At 25°C and 1 atm of nitrogen, the cross section for infrared absorption by HNO at 3300 cm^?1 is 1.7 × 10^?19 cm². The rate constant ratio b/c was found to be 8.0. From the literature value of the rate constant d , the observed rate constant for the reaction is e = (5 ± 1) × 10^?11 cc/particle sec.     

Magnesium chloride supported high-mileage catalysts for olefin polymerization. X. Effect of hydrogen and catalytic site deactivation

The effect of H₂ on propylene polymerization initiated by a MgCl₂/EB/PC/AlEt₃/TiCl₄–3 AlEt₃/MPT catalyst was studied. Hydrogen increases significantly the initial rate during the early stage of the polymerization to give a higher yield of polymer than reactions without H₂. But H₂ reduces the yield toward the latter stages so that the net effect on the total yield can be quite small. There is no appreciable effect of H₂ on either the isotacticity index or polydispersity of the products. It decreases molecular weight proportional to (p_H2)^1/2. The chain transfer by H₂ resulted in a decrease of total metal polymer bond concentration with time of polymerization. The rate constants of hydrogen chain transfer for the two kinds of isospecific and nonspecific sites are = 5.1 × 10^?3, = 2.7 × 10^?3, = 7.5 × 10^?3, = 4.4 × 10^?3, in units of torr^1/2 sec^?1 at 50°. Hydrogen assists in the deactivation of the catalytic sites as does propylene; rates of the former and the latter vary with (p_H2)^1/2 and [C₃H₆]^1/2, respectively, with k = (12.1 ± 0.9) M^?1 torr^?1/2 sec^?1 and k = (65.3 ± 3.3) M^?3/2 sec^?1 at 50° and A/T = 167. The mechanism for deactivation of catalytic sites are discussed.     

Thermal gas-phase decomposition of chloroethylenes. II. Vinyl chloride

The thermal gas-phase decomposition of vinyl chloride has been studied behind shock waves over the temperature range of 1350-1900°K and the density range of 7 × 10^?7-1.5 × 10^?3 mol/cm³ (at 1600°K) in mixtures of C₂H₃Cl highly diluted with argon. The ultraviolet absorption of C₂H₃C was recorded at 230 nm as a function of time. The decomposition proceeds via molecular elimination of HCl. The unimolecular dissociation rate is pressure dependent at all but the highest pressures applied. Application of modified HKRR theory results in the rate expression for the limiting high pressure rate constant, and in a collision efficiency of derived from the limiting low-pressure rate constant.     

Complex dynamical behavior in the oxidation of hydroxylamine by bromate

Complex dynamical behavior has been observed in the oxidation of hydroxylamine by bromate in acidic sulfate medium. The reaction shows clock type kinetics in closed conditions and an aperiodic oscillations if gaseous products are removed from the system with a constant flow-rate. The reduction kinetics of bromate ions with excess hydroxylamine has been studied in the presence of allyl alcohol. The observed pseudo-first-order rate constant k_obs has been found to follow the expression where [hydroxylamine] is total initial hydroxylamine concentration, K₁ = 0.5 M^?1, K₂ = 10⁶ M^?1, and k = 2.57 × 10³ M^?1 s^?1 at 298.15 K and I = 2.0 M. The rate constant for the bromine oxidation of hydroxylamine in sulfuric aqueous solution has been determined. © 1994 John Wiley & Sons, Inc.     

Bimolecular self-reactions of 2-arylindandione- 1,3-yl radicals studied by flash photolysis

Kinetic and thermodynamic data for reaction (1) of certain C-centered aromatic radicals (referred to in this paper by the numbers I to X) in chlorobenzene: have been obtained. The k₁ values of radicals varied between (1.1 ± 0.2) × 10⁶M^?1·sec^?1 (radical VIII) and (3.6 ± 0.7) × 10⁹M^?1 sec^?1 (radical VI) at 20°C. An investigation of the relationship between the recombination rates of radicals I–VIII and X and the solvent viscosity (mixture of toluene and dibutylphthalate, 0.6 < η < 18.4 cP) has shown that the recombination reactions involving radicals I–IV are limited by diffusion in solvents having a viscosity η> 10 cP and are activation reactions in solvents having a viscosity η < 10 cP. The recombination of radicals VIII and IX is an activation reaction, while that of radicals V–VII is diffusion-controlled in the entire viscosity range. The recombination of radical X is limited, in the viscosity range of 18.4 to 2 cP, by intrusion into the first coordination sphere of the partner, the effect of viscosity on the radical X recombination rate in the specified range being the same as its effect on diffusion-controlled reactions. The possible reasons of the discrepancies between the experimental fast recombination rate constants and the theoretical values calculated by the Debye–Smoluchowski theory are discussed. The equilibrium constant depends strongly on the nature of the substituent in the phenyl fragment: the substituents which increase unpaired electron delocalization in the radical intensify the dissociation of the respective dimer. Long-wave absorption bands have been recorded for radicals I–X and their extinction coefficients obtained. Dimers I–V are thermo- and photochromic compounds.