Initiation homogeneous reactions

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

A number of possibilities exist for the direct one-step oxidation of methane. These include homogeneous gas-phase oxidation, heterogeneously initiated homogeneous reaction, heterogeneous catalytic oxidation, and photochemical and electrophilic oxidations. Several review articles give detailed treatment of the homogeneous and catalytic oxidations.18-27 Some important conclusions are as follows. 

The formation of propene oxide as a side product of the acrolein formation or dimerization reactions is reported by many authors. Daniel et al. [95,96] demonstrated that propene oxide is formed by surface-initiated homogeneous reactions which may involve peroxy radical intermediates. The epoxidation is increased by a large void fraction in the catalyst bed or a large postcatalytic volume. In view of these results, the findings of Centola et al. [84] are understandable, as the wall of the empty reactor may have been sufficiently active to initiate the reaction. 

Dispersion polymerization is defined as a type of precipitation polymerization by which polymeric microspheres are formed in the presence of a suitable steric stabilizer from an initially homogeneous reaction mixture. Under favorable circumstances, this polymerization can yield, in a batch process, monodisperse, or nearly monodisperse, latex particles with a relatively large diameter (up to 15 pm) [103]. The solvent selected as the reaction medium is a good solvent for both the monomer and the steric stabilizer, but a non-solvent for the polymer being formed and therefore a selective solvent for the graft copolymer. This restriction on the choice of solvent means that these reactions can be carried out... 

A typical experimental procedure is illustrated in Fig. 2. Mixing the 2-naphthaldehyde-derived A-Boc imine with isovaleraldehyde in the presence of (S)-proline (20 mol%) in acetonitrile at 0 °C resulted in an initially homogenous reaction mixture (Fig. 2a). After complete consumption of the starting material (10 h), a large amount of the de-... 

During dispersion polymerization polymer particles are formed from an initially homogeneous reaction mixture by polymerization in the presence of a polymeric steric stabilizer. The process is applicable to monomers which yield polymers that are insoluble in a solvent for the monomer. Styrene has been polymerized in alcohols, with steric stabilizers such as poly(A -vinylpyrrolidone) (see Fig. 1-4 for monomer structure) or hydroxypropyl cellulose. Hydrocarbon... 

The authors provided only a qualitative interpretation of their results. They proposed that the better performance of the monolith relied on the beneficial effects of concentration and temperature boundary layers. The expected effect of interphase mass transfer limitations was, in fact, a decrease of O2 concentration at the gas—catalyst interface, with better control of the vanadium valence state and consequently a partial suppression of consecutive oxidation reactions (due to a more important kinetic dependence of deep oxidations on O2 concentration than the desired selective oxidation). The presence of heat transfer limitations was then believed to establish a catalyst temperature higher than the gas-phase temperature, thus promoting heterogeneously-initiated homogeneous reactions at the gas-catalyst interface, with production of peroxo radical species and eventually propylene. ... 

During the dispersion polymerization, the polymer precipitates from an initially homogeneous reaction mixture containing monomer, initiator, steric stabilizer, and solvents. Under favorable conditions, monodisperse polymer particles stabilized by a steric barrier of dissolved polymer are formed. The early work, mainly done in nonaqueous media such as aliphatic hydrocarbons, was thoroughly reviewed by Barrett [90]. Most of the studies dealt with polymer particles in the 0.1- to 2- jun size range. 

Experiments that have taken advantage of the electrochemically initiated homogeneous reactions in the ES ion source can be distinguished into two basic groups. In the first group. 

The rate of formation of the products of steam reforming is extremely slow in the homogeneous reaction, and the reaction only proceeds at a useful rate in the presence of the catalyst. This indicates that adsorption of the reactant gases on the catalyst surface is a necessary step in the initiation of a useful reaction. The nature of the adsorption processes can be described by the equations ... 

Equilibrium Compositions for Single Reactions. We turn now to the problem of calculating the equilibrium composition for a single, homogeneous reaction. The most direct way of estimating equilibrium compositions is by simulating the reaction. Set the desired initial conditions and simulate an isothermal, constant-pressure, batch reaction. If the simulation is accurate, a real reaction could follow the same trajectory of composition versus time to approach equilibrium, but an accurate simulation is unnecessary. The solution can use the method of false transients. The rate equation must have a functional form consistent with the functional form of K,i,ermo> e.g., Equation (7.38). The time scale is unimportant and even the functional forms for the forward and reverse reactions have some latitude, as will be illustrated in the following example. 

The theory of detonation has also been extended to study the process of initiation of reaction by the commonest means used in practice, namely, by the shock wave arising from another high explosive. Campbell, Davis and Travis have studied the initiation by plane shock waves of homogeneous explosives, particularly nitromethane. Initiation occurs at the boundary of the explosive after an induction period which is of the order of a microsecond and which depends markedly on initial temperature. During the induction period the shock wave has proceeded through the explosive and compressed it. The detonation initially in compressed explosive has a velocity some 10% above normal, but the detonation soon overtakes the... 

Similar reactions occur with all aliphatic halides and the rates of substitution are related to the degree of ionic character of the carbon-halogen bond. For preparation purposes, trityl bromide or propargyl bromide are more convenient than allyl bromide. The compounds obtained are listed in Table XI. They were obtained pure and characterized fully. Zr (allyl) 3Br and Zr(allyl)2Br2 are sufficiently soluble in toluene for polymerizations to be initially homogeneous. Their relative reactivities are listed in Table XI. In all cases hydrogen was used to reduce the molecular weight of the polymer formed. In this respect the polymer derived from Zr (allyl )3Br was more readily modified than that from Zr (allyl) 4, but in order to avoid... 

CH ligands, (2) to initiate homogeneous catalytic reactions such as hydrogenation, hydroformylation, and the water gas shift reaction, and (3) to study the mechanism of thermal reactions by the photochemical preparation of possible intermediates. 

The initial polymerizations were conducted with both homogeneous and emulsion conditions. In the homogeneous reactions, a mixture of metha-nolrdichloroethane was used to dissolve the sulfated monomer, and the initiator was introduced as a solution in dichloroethane. These reactions did not proceed to completion, even upon heating. During the reaction, the formation of a precipitate was observed, which could be attributed to the growing polymer chain. Although the monomer is soluble in methanol as a triethylammonium salt, the... 

The homogeneous reaction was found to be catalysed by small percentages of nitric oxide and acetylene dichloride without any detectable change in the overall stoichiometry. This observation suggests the occurrence of additional initiation processes of the type... 

In the CE mechanism (Scheme 2.2), a first-order (or pseudo-first-order) homogeneous reaction precedes the electron transfer step. In the case where the initial electron transfer is fast enough not to interfere kinetically, the electrochemical response is a function of two parameters the first-order (or pseudo-first-order) equilibrium constant, K, and a dimensionless kinetic... 

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