Radical efficiency

This unique formation of a borabenzene ring skeleton is thought to proceed as delineated in Scheme 1. A more detailed explanation together with comments on stoichiometry, reaction conditions, and by-products may be found elsewhere (2,7). However, it should be noted that Scheme 1 is based on the well-known reducing power of cobaltocene and its ability to add radicals efficiently (80), both properties being intimately connected with the uncommon 19-e configuration of CoCp2. 

C olvents have different effects on polymerization processes. In radical polymerizations, their viscosity influences the diffusion-controlled bimolecular reactions of two radicals, such as the recombination of the initiator radicals (efficiency) or the deactivation of the radical chain ends (termination reaction). These phenomena are treated in the first section. In anionic polymerization processes, the different polarities of the solvents cause a more or less strong solvation of the counter ion. Depending on this effect, the carbanion exists in three different forms with very different propagation constants. These effects are treated in the second section. The final section shows that the kinetics of the... 

These values assume that the radical efficiency factor is unity. 

The key feature of Inisurfs is their surfactant behavior. They form micelles and are adsorbed at interfaces, and as such they are characterized by a critical micelle concentration (CMC) and an area/molecule in the adsorbed state. This influences both the decomposition behavior and the radical efficiency, which are much lower than those for conventional, low molecular weight initiators. Tauer and Kosmella [4] have observed that in the emulsion polymerization of styrene, using an Inisurf concentration above the CMC resulted in an increase in the rate constant of the production of free radicals. This was attributed to micellar catalysis effects as described, for example, by Rieger [5]. Conversely, if the Inisurf concentration was below the CMC the rate constant of the production of free radicals decreased with an increase in the Inisurf concentration, which was attributed to enhanced radical recombination. Also note that a similar effect of the dependence of initiator efficiency on concentration was reported by Van Hook and Tobolsky for azobisisobutyronitrile (AIBN) [6]. 

Unfortunately, radicals derived from alkylmercuries are even more limited in what they will react with than radicals made from alkyl halides by the tin hydride method. Styrene, for example, cannot be used to trap alkylmercury-derived radicals efficiently because the radicals react more rapidly with the mercury hydride (which has an even weaker metal-H bond than Bi SnH) than with the styrene. 

Iron should be coordinated by the chelate in such a manner as to prevent direct access of oxygen and hydrogen peroxide. If this is achieved, then hydroxyl-radical production will be reduced to a minimum. Some iron complexes, for instance ethylenediamine tetra-acetic acid (EDTA) (Fig. 3a), generate hydroxyl radicals efficiently while others such as DFO and the hydroxypyridinones (Fig. 3b) do not. By designing chelators that produce extremely stable complexes, the generation of hydroxyl radicals is further minimized. Such stable complex formation would also reduce the tendency for iron redistribution within the body. 

The oxidative dehydrogenation reactions over these catalysts are similar to the gas phase result of shock tube experiments determined by Skinner et al. (ref. 6). This observation supports the fact that the recombination reactions of methyl radicals in the gaseous phase are an important source of ethane and that the ethene is a secondary product derived from ethane. This secondary reaction proceeds in the gaseous phase as well as the catalyst surface. The major role of the MgO surface is to produce the methyl radical efficiently. The active sites for cleaving the H-CH3 bond should be moderated by Li to enforce C2 selectivity. In addition to gas phase oxidation, the direct surface oxidation of the hydrocarbon adsorbate is very significant especially for acidic materials. 

In toluene at 55° the radical efficiency (/) as determined by means of scavengers is about 75%. On silica the only scavenger that proved to be suitable, of several tested, was BDPA. The radical efficiency on silica, as determined from the fading of the BDPA epr signal, is only 10-20%. This was supported by the ratios of dj, dg, and dj coupling products found in experiments with unsymmetrically deuterated azocumene l-dj. 

A molecule adsorbed at an isolated hydrogen bond donor site should diffuse less readily than it might on a surface with many such sites close together, where potential energy minima begin to overlap. Thus in all instances but one, the radical efficiency was lower on the Hq dehydroxy lated silica than on H, silica. The apparent exception was a photolysis, in which the radical efficiency is already low for other reasons. Covering part of the surface with azobenzene also appeared to decrease f, possibly because azocumene blocks some of the hydroxy 1-rich diffusion paths. 

At higher conversions of cumene, cumene hydroperoxide can trap the cumyloxy radical efficiently prior to fragmentation and in those cases where fragmentation does occur, cumyl hydroperoxide also traps the methylperoxy radical... 

Figure 5.13 summarizes the general organic/NO, system from the point of view of the peroxy radicals (RO2), alkoxy radicals (RO), and acyl peroxyacyl radicals (RC(O)OO). At each step in the chain, propagation and termination steps compete with each other. The peroxy radicals efficiently convert NO to NO2 as long as NO levels are sufficiently high. The... 

The peculiarities of the aggregation behaviour of surfactant molecules in non-aqueous media. It is admitted that the aggregates exhibit disc-like structures with small aggregation numbos (<20) [39,40]. Such aggregates would not swell with monomer solutions, but on the other hand, would be able to capture initiator radicals, thus decreasing the overall radical efficiency. 

The high electrocatalytic activity of boron-doped diamond (BDD) is due to the generated highly active hydroxyl radicals that are present in a thin layer adjacent to the anodic surface [1, 2]. Such radicals efficiently oxidize the organic pollutants that achieve this layer. A BDD anode, operating at an overpotential lower than a critical value r]cr, is covered after some time by a polymeric film that inhibits the occurrence of electrochemical oxidation [3]. A critical value of current density ic, corresponds to this critical overpotential. 

Even though it was not possible to separate evidence of conventional chain transfer processes from the definite presence of the uncontrolled polymer radical chains with the data available at that time, deviations from living behavior allowed estimates of radical efficiency to be made. 

Another way to increase the radical efficiency as well as the incorporation yield could be to increase the hydrophilicity of the smaller, highly mobile radicals of the asymmetrical inisurfs. Unfortunately, all synthetic efforts in this direction have not been successful. One must conclude that with respect to inisurfs as reactive surfactants the main drawback of very low radical efficiency still exists. 

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