Kinetics initial stages

Lu et al. [86] also studied the effect of initiator concentration on the dispersion polymerization of styrene in ethanol medium by using ACPA as the initiator. They observed that there was a period at the extended monomer conversion in which the polymerization rate was independent of the initiator concentration, although it was dependent on the initiator concentration at the initial stage of polymerization. We also had a similar observation, which was obtained by changing the AIBN concentration in the dispersion polymerization of styrene conducted in isopropanol-water medium. Lu et al. [86] proposed that the polymerization rate beyond 50% conversion could be explained by the usual heterogenous polymer kinetics described by the following equation ... 

Detailed investigations indicated an interesting mechanism for azide openings catalyzed by 2 [6]. Chloride-epoxide addition products were observed in the initial stages of the ARO reaction with 2 in amounts commensurate with the catalyst loading. Azide complex 3, characterized as the TH F adduct, was isolated from the reaction mixture and proved to be an active and recyclable catalyst for the ARO, pointing to the role of 2 as that of a precatalyst. Kinetic experiments revealed a second-order dependence on the concentration of 3, a zero-order dependence on azide source, and inverse-order dependence on epoxide concentration. The sue-... 

Obtaining Kinetic Samples for Reactive Extrusion. To develop and test kinetic models, homogeneous samples with a well defined temperature-time history are required. Temperature history does not necessarily need to be isothermal. In fact, well defined nonisothermal histories can provide very good test data for models. However, isothermal data is very desirable at the initial stages of model building to simplify both model selection and parameter estimation problems. 

Both schemes accommodate the kinetics, the primary isotope effect and the induction factor, which indicates that Cr(IV) is the initial stage of reduction of the oxidant. RoCek s mechanism does not accord with the solvent isotope effect of 2.5 per proton, which has just the value to be expected for acid-catalysis, for the O-H bond cleavage should be subject to a primary isotope effect of about 7. The ester mechanism is not open to this criticim. 

The kinetics of the initial stages of the oxidation of some a-hydroxy-carboxylic such as lactic, malic and mandelic acids by chromic acid have been studied by Bakore and Narain . The initial reaction resembles the oxidation of a secondary alcohol to ketone. The authors concluded that the rate determining step involves C-H bond rupture at the a-carbon atom. The rate of oxidation of these acids is reduced to one-half by the addition of manganous ions, when the concentration of the latter is commensurable with that of the acids. 

Initial Stage Mercury electrodes can be used to study the kinetics of the initial step of cathodic metal ion discharge without comphcations due to subsequent steps. Here, the primary reaction product, metal atoms, do not form nuclei or crystallites but continue to exist as an amalgam or solution in mercury. We must remember, however, that even the kinetics of the initial step depends on the electrode material hence, the laws found for mercury cannot be used for other metals. 

Therefore, the kinetics of generation of defects in surface-adjacent layers is similar to kinetics of emission of O-atoms. (The estimates indicate that the maximum concentration of vacancies in this case may attain the value of 10 for a sample with area 1 cm ). If one assumes that the emission of oxygen atoms is caused by processes of annihilation of vacancies in the sample, then the coincidence in time dependence of stationary concentration of defects can be indicative that these processes are limited by generation of defects, which, in its turn, is controlled by processes of formation of oxide phase in surface-adjacent silver layers. Oxidation, especially at initial stage, is characterized by intensive formation of defects [54]. 

At the initial stage of bulk copolymerization the reaction system represents the diluted solution of macromolecules in monomers. Every radical here is an individual microreactor with boundaries permeable to monomer molecules, whose concentrations in this microreactor are governed by the thermodynamic equilibrium whereas the polymer chain propagation is kinetically controlled. The evolution of the composition of a macroradical X under the increase of its length Z is described by the set of equations ... 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

KINETICS OF AUTOINITIATED HYDROCARBON OXIDATION 4.7.1 Initial Stage of Autoxidation... 

Although in determining the onset temperature and the global kinetics in the initial stage of the reaction the, corrections are of lesser importance, these corrections have a dramatic effect on the maximum self-heat rate and the maximum temperature. Care must be taken in the interpretation of such data from an experiment with a phi-factor greater than 1. For direct simulations of plant situations, a phi-factor of 1.0 to 1.05 is used [89],... 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

Krumpolc, M. and Malek, J., Esterification of benzenecarboxylic acids with ethylene glycol, IV. Kinetics of the initial stage of polyesterification of terephthalic acid with ethylene glycol catalyzed by zinc oxide, Makromol. Chem., 171, 69-81 (1973). 

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