Kinetics and mechanism of acid-catalyzed decomposition of triazenes in aqueous ethanolic solutions

The kinetics and nature of the end products of the acid decomposition of triazenes (R-N=N-NHR, where R is aryl, and R is aryl or alkyl) in aqueous ethanolic solutions of varying composition have been studied. A hydrolysis mechanism which includes protonation of the triazene molecule (R-N+H=NH-R), formation of an _N-transition stafte (R-N=N-N+H₂-R), and dissociation of this state has been proposed to explain the literature data and the opposite ionic effects observed in the present study. This mechanism explains the roles of the acid anion and the water molecule in the hydrolysis of triazenes. An analytical expression has been obtained from the hydrolysis constant which agrees with the experimental results.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 1, pp. 23–30, January–February, 1993.     

Kinetics and mechanism of diethanolamine degradation in aqueous solutions containing carbon dioxide

The problem of diethanolamine (DEA) degradation in gas-treating processes was quantified through a detailed kinetic study. This reaction was found to be catalyzed by CO₂, and degradation occurs in a successive manner to 3-(2-hydroxyethyl)oxazolidone-2, to N,N,N′-tris(2-hydroxyethyl)ethylenediamine and then to N,N′-bis(2-hydroxyethyl)piperazine. A reaction mechanism consistent with these observations was proposed and tested through kinetic analyses. A satisfactory kinetic model which can be of practical use was derived.     

Reaction mechanism studies on isoquinoline with hydroxyl radical in aqueous solutions

The reaction mechanism between isoquinoline and ·OH radical in aqueous dilute solutions under different conditions was studied by pulse radiolysis. The main characteristic peaks in these transient absorption spectra were attributed and the growth-decay trends of several transient species were investigated. Under neutral or alkaline conditions, the reaction of ·OH radical and isoquinoline produces OH-adducts with respective rate constants of 3.4 × 10⁹ and 6.6 × 10⁹ mol⁻¹ · dm³ · s⁻¹ while under acidic conditions, the isoquinoline was firstly protonated and then ·OH added to the benzene ring and produced protonated isoquinoline OH-adducts with a rate constant of 3.9 × 10⁹ mol⁻¹ · dm³ · s⁻¹. With a better understanding on radiolysis of isoquinoline, this study is of help for its degradation and for environmental protection. __________ Translated from Journal of Fudan University (Natural Science), 2006, 45(6): 774–778 [8BD1;81EA: 590D;65E6;5B66;62A5;(自然科学版)]     

Effect of pH on radical polymerization of butadiene 1-carboxylic acid in aqueous solutions

Radical polymerization of butadiene 1-carboxylic acid (Bu-1-Acid) was carried out in aqueous solutions at 50°C with ammonium persulfate (APS) as an initiator. Kinetic studies led to the rate equation, R_p = k[APS]^1/2 [Bu-1-Acid]¹ at pH 6.8. The overall activation energy for the polymerization was 16.0 kcal/mole. The polymerization rate R_p of Bu-1-Acid decreased with an increase of pH in the range 2.4–6.8 and increased with an increase of pH in the range 6.8–8.4. Moreover, in the pH range 8.4–13, the rate of polymerization was not affected by the pH of the system. In copolymerization with acrylonitrile, the trends of changes in the monomer reactivity ratios r₁, r₂ and Q-e values caused by changes in pH were similar to trends found in homopolymerization described above. In addition, it was observed that the resultant polymer was extended in alkaline solution and contracted in acidic solution.     

Kinetics and mechanism of diethyl sulfide oxidation by sodium peroxoborate in aqueous solutions

The kinetics of diethyl sulfide (Et₂S) oxidation by aqueous sodium peroxoborate (Na₂[В₂(О₂)₂(ОН)₄]) solutions in a wide acidity range (from [HClO₄] = 1 mol/L to рН 12) has been studied using a kinetic distribution method. The kinetic data together with the results of ¹¹B NMR spectroscopy demonstrate that the monoperoxoborate B(O₂H) ₃ ^- (OH) and diperoxoborate B(O₂H)₂(OH) ₂ ^- anions are the active species in Et₂S oxidation by sodium peroxoborate at рН 8–12. It is assumed that, at a high acidity of the medium ([HClO₄] = 0.05–1.0 mol/L), peroxoboric acid (ОН)₂ВООН or its protonated form (OH)₂BOOH ₂ ⁺ are direct reactants along with Н₂О₂ and HOOH ₂ ⁺ .     

Kinetics and mechanism of acetylene polymerization

The kinetics of acetylene polymerization initiated by Ti(OBu)₄/4AlEt₃ catalyst was studied by radioquenching with C^*O to count the number of active sites [C] and by CH₃OT^* to determine the total metal polymer bonds [MPB] and M?_n of the polymer. The amount of quenching agent and time of reaction required and the kinetic isotope effect for CH₃OT^* were determined. The effects of Al/Ti ratio, catalyst aging, catalyst concentration, temperature, and monomer pressure on the polymerization were investigated. Detailed kinetic data on the variation of rate of polymerization, R_p, [C] [MPB], and M?_n with time were obtained at 298 and 195°K. The results required the assumption that the catalytic species C, is initially active and within less than 30 min all are converted by bimolecular kinetics to a far less active species. Analysis of the data yielded rate constants of propagation and termination and their energies of activation. Estimates of chain transfer efficiency were obtained. The mechanisms for the propagation, termination, and transfer processes were discussed. By drawing on our earlier EPR results we propose probable structures for the catalytically active species.     

Kinetics and mechanism of the reactions of uracil with hydrogen peroxide in aqueous solutions

The kinetics of the reactions of 5-hydroxy-6-methyluracil and 6-methyluracil with hydrogen peroxide in aqueous solutions has been investigated. The reactions proceed via two competitive mechanisms, namely, free-radical and nonradical ones. The iron ions present in water as impurities make possible the free-radical mechanism.     

Kinetics and mechanism of triphenylphosphine quarternization with unsaturated carboxylic acids in the medium of acetic acid

Kinetics of reaction of triphenylphosphine with a series of unsaturated carboxylic acids and related esters in the medium of acetic acid as well as in mixtures of acetic acid and acetonitrile was studied by spectrophotometry. The data obtained show that the process of quarternization should be described by kinetic equation of third order, the proton transfer occurs from the solvent. A stepwise mechanism of interaction including formation of prereaction complex on the reaction pathway is proposed.     

Amino-substitutedgem-diphosphonic acids: Dissociation mechanism and the structure of species in aqueous solutions

The mechanism of dissociation of amino-substitutedgem-diphosphonic acids R₂N(CH₂) _n CR'(PO₃H₂)₂ with different lengths of the alkylidene chain and different substituents at the N atom was studied by vibrational (IR, Raman) and NMR (¹H,¹⁴N,³¹P) spectroscopy using data of conformational analysis (molecular mechanical) data. The important role of intramolecular H-bonds and cyclic solvates for the stabilization of various conformations and tautomeric forms of ions was demonstrated. The spectral data that allow one to consider thegem-diphosphonate group as a single acidic center were found. Translated fromIzvestiya Akademii Nauk Seriya Khimicheskaya, No. 6, pp. 1051–1064, June, 2000.     

Kinetics of radical polymerization of dimethylphenyl methacrylates

The kinetics of free radical polymerization of all six existing isomeric dimethylphenyl methacrylates were studied in bulk at 60°C, initiated by dilauroyl peroxide, using dilatometry. The different polymerization rates observed, resulting from the different monomer structures, are discussed in terms of steric and other effects, considering also the previous results for tolyl methacrylates. © 1993 John Wiley & Sons, Inc.     

Kinetics of ring-opening radical polymerization of vinyloxiranes

Ring-opening radical polymerization has been followed by the flash photolysis method. As an initiator radical, the arylthiyl radical (ArS^˙) was used; the addition reaction rate constants of ArS^˙ to vinyl monomer containing epoxy-ring such as vinyloxiranes have been determined at first. The rate constant of ring opening reaction of the adduct radical changing to ArSCH₂CH ? CROCH₂^˙ were determined in the form of the relative values, which were converted to the absolute values. The ring-opening reaction rate constants were ca. 10⁶ s^?1, which indicates that the ring-opening rates are faster than usual radical propagation rates. The rate constants for vinyl monomer with 5-member ring (1,4-epoxy-1,4-dihydronaphthalene) were similarly evaluated.     

Kinetics of nanoconfined benzyl methacrylate radical polymerization

The effect of nanoconfinement on the kinetics of benzyl methacrylate radical polymerization is investigated using differential scanning calorimetry. Controlled pore glass (CPG), ordered mesoporous carbons, and mesoporous silica are used as confinement media with pore sizes from 2 to 8 nm. The initial polymerization rate in CPG and mesoporous silica increases relative to the bulk and increases linearly with reciprocal pore size; whereas, the rate in the carbon mesopores decreases linearly with reciprocal pore size; the changes are consistent with the rate being related to the ratio of the pore surface area to pore volume. Induction times are longer for nanoconfined polymerizations, and in the case of CPG and carbon mesopores, autoacceleration occurs earlier, presumably due to the limited diffusivity and lower termination rates for the confined polymer chains. The molecular weight of the polymer synthesized in the nanopores is generally higher than that obtained in the bulk except at the lowest temperatures investigated. The equilibrium conversion under nanoconfinement decreases with decreasing temperature and with confinement size, exhibiting what appears to be a floor temperature at low temperatures.     

Stereocontrol using Lewis acids in radical polymerization

The radical polymerization of various (meth)acrylamides in the presence of Lewis acids such as Yb(OTf)₃ and Y(OTf)₃ was carried out. The polymerization with Lewis acids led to highly isotactic polymers, while the polymers synthesized without Lewis acids were atactic or syndiotactic. The dependence of the polymer properties on the tacticity was also demonstrated.     

Kinetics of radical polymerization—XXXVII. Investigation of molecular-inhibitors in the radical polymerization of acrylonitrile

The effect of molecular inhibitors with different reactivities was studied for the homogeneous (solution) and heterogeneous bulk-polymerization of AN. p-Nitro-acetophenone, which acts as a weak retarder in solution, strongly decreases the accelerating character of the bulk polymerization. Aromatic nitroso-compounds are strong inhibitors in both homogeneous and heterogeneous polymerizations. The length of the inhibition period depends linearly on the inhibitor concentration. The character and kinetics of the polymerization after the inhibition period are not affected by the nitroso compounds. A novel method has been introduced to determine the length of inhibition period for accelerating heterogeneous polymerization. In every studied system, a considerable stoichiometric anomaly was observed, and attributed to the hot radical effect.     

Atom transfer radical polymerization in aqueous dispersed media

During the last decade, atom transfer radical polymerization (ATRP) received significant attention due to its exceptional capability of synthesizing polymers with pre-determined molecular weight, well-defined molecular architectures and various functionalities. It is economically and environmentally attractive to adopt ATRP to aqueous dispersed media, although the process is challenging. This review summarizes recent developments of conducting ATRP in aqueous dispersed media. The issues related to retaining “controlled/living” character as well as colloidal stability during the polymerization have to be considered. Better understanding the ATRP mechanism and development of new initiation techniques, such as activators generated by electron transfer (AGET) significantly facilitated ATRP in aqueous systems. This review covers the most important progress of ATRP in dispersed media from 1998 to 2009, including miniemulsion, microemulsion, emulsion, suspension and dispersed polymerization.      

Kinetics and mechanism of hydrogen-atom abstraction from rhodium hydrides by alkyl radicals in aqueous solutions

The kinetics of the reaction of benzyl radicals with [L(1)(H(2)O)RhH{D}](2+) (L(1)=1,4,8,11-tetraazacyclotetradecane) were studied directly by laser-flash photolysis. The rate constants for the two isotopologues, k=(9.3±0.6) × 10(7) M(-1) s(-1) (H) and (6.2±0.3) × 10(7) M(-1) s(-1) (D), lead to a kinetic isotope effect k(H)/k(D)=1.5±0.1. The same value was obtained from the relative yields of PhCH(3) and PhCH(2)D in a reaction of benzyl radicals with a mixture of rhodium hydride and deuteride. Similarly, the reaction of methyl radicals with {[L(1)(H(2)O)RhH](2+) + [L(1)(H(2)O)RhD](2+)} produced a mixture of CH(4) and CH(3)D that yielded k(H)/k(D)=1.42±0.07. The observed small normal isotope effects in both reactions are consistent with reduced sensitivity to isotopic substitution in very fast hydrogen-atom abstraction reactions. These data disprove a literature report claiming much slower kinetics and an inverse kinetic isotope effect for the reaction of methyl radicals with hydrides of L(1)Rh.