Active centre, concept

Recently, Barelko et al. [150-155] put forward a new version of this theory. They suggested a branching-chain process mechanism based on the concepts implying the existence of a two-dimensional gas of adsorbed atoms (adatoms) on solid surfaces which are in equilibrium with their crystal lattice. According to the suggested hypothesis, the active centre is the adatom. The energy evolved in the course of a reaction on the adatom can be applied to break out another atom from the lattice, i.e. to form a new active centre. This process is a step of branching. But the decay of the active centre takes place due to the return of the adatom back into the lattice. 

Monomer reactivity is a broad concept, and it can not always be limited only to reactions of the double or triple bonds of a vinyl or acetylene group. Weakly polar monomers, such as styrene or butadiene, react almost exclusively by their double bonds. The anionic polymerization of polar monomers, such as a, /S-unsaturated esters and nitriles, is accompanied by many side reactions. A fairly large amount of oligomers and side products are formed, and these may affect the active centres, thus indirectly modifying propagation. 

Data on the reduction of halides of other transition metals by Et3Al and Et2AlCl are very numerous [a frequently used organometal is (iso-Bu)3Al, and there are many others], nevertheless a closed concept concerning the fine structure and function of the generated active centres cannot be formulated at present [188]. 

The indicated procedure for analyzing the kinetics of non-stationary polymerizations is evidently unacceptably simplified, and may serve only as a very rough model for the analysis of actual situations. An exact general procedure should include thermodynamic principles (floor and ceiling polymerization temperatures), detailed concepts on the generation and decay of active centres and on transfer (especially degradative). 

This example takes us at once to the heart of a problem which it ought to be the objective of this Discussion finally to resolve. In 1925, a concept of the catalytic surface was formulated which emphasized heterogeneity or, as it came to be expressed, the concept of active centres . A variety of evidence on the properties of technical catalysts, which were the only catalysts then extensively studied, contributed to this concept of active centres. This evidence included observations on adsorption by catalysts both active and inactivated by heat treatment. It attempted to account for the great influence of poisons and promoters, present in... 

This concept was first mathematically developed by Clark and Bailey according to an exponential distribution of active centres with respect to adsorption energy. The Langmuir-Hinshelwood mechanism for adsorption and reaction was found to fit experimental results (ethylene polymerization over chromium oxide-silica-alumina catalyst) more closely than the Rideal mechanism. 

This concept led Cossee and Arlman to propose for TiClj a believable model for the active centre. Recently this hypothesis has been extended, with convincing results, to the examination of a particular type of TiClj while Corradini et al. 

The intercept in Fig. 14 gives, therefore, the ion-pair rate coefficient and the slopes of the lines yield kpKj. Conductance measurements can be used to determine and hence a value for kp is obtained. The validity of these concepts can be checked by carrying out the polymerization in the presence of sodium tetraphenylboride. This salt dissociates to a much greater extent than polystyrylsodium and its presence suppresses the ionization of the latter by a common ion effect. Under appropriate conditions a simple first order reaction in active centres can be observed in its presence with a rate equal to that measured by extrapolation to infinite concentration of active centres. 

Present views concerning the operation mechanism of ZN catalysts are not conclusive. Cossee [288, 289] assumes that, in the first step, donor-acceptor interaction occurs between the transition metal and the monomer. A a bond is formed by the overlap of the monomer n orbital with the orbital of the transition metal. A second n bond is formed by reverse (retrodative) donation of electrons from the orbital of the transition metal into the antibonding 7T orbital of the monomer. In the following phase, a four-centre transition complex is formed with subsequent monomer insertion into the metal-carbon bond. This, in principle, monometallic concept is criticized by the advocates of the necessary presence of a further metal in the active centre. According to them, the centre is bimetallic. Monometallic centres undoubtedly exist on the other hand, technically important ZN catalysts are multicomponent systems in which each component has its specific and non-negligible function in active centre formation. The non-transition metal in these centres is their inherent component, and most probably the centre is bimetallic. Even present ideas concerning the structural difference in centres producing isotactic and atactic polymers are not united. 

Frennet s/ree potential site is a closely related concept to that of the active centre, as is Campbell s true ensemble requirement ... 

It is assumed in all models that the mineral s surface has amphoteric properties and active centres neutral as well as positively or negatively charged. The standard state is viewed as interaction of infinitely diluted solution with a neutral surface. All models are based on the concept of coordination interaction between functional groups of minerals and cations, ligands, and OH in the water solution. At that, all processes are... 

A key problem of the catalytic polymerisation theory is known to be that of the AC structure. Experimental studies and theoretical developments in the ionic and ionic-coordination polymerisation of unsaturated compounds revealed a considerable amount of new information about the nature of AC, for example [50-53]. The most important concept is the polycentrism of cataljnic systems as this problem is currently becoming much more important [49]. Researches have advanced from single examples of active centre types, in some catalytic systems, to an awareness of the fact that in some rare cases, the catalytic system can be considered to include only one type of active centre. 

The filler localization experimentally determined by the use of our wetting concept is correlated to the wetting behaviour of rubber as follows the more the filler surface is wetted by a rubber phase, the more the filler is included and distributed in this phase. When filler is mixed with a single rubber, for instance SBR, a part of the rubber chains will be bonded to the active centres, which are available on the filler surface. If the rubber is extracted from the filled uncured rubber compound with a suitable solvent, the rubber molecules bound on the filler surface remain in the rubber-filler gel. In our concept, we are interested in the fraction of rubber in this rubber-filler gel. The wetting... 

The new concept tested was to crosslink the monomer in presence of a radical photoinitiator which, upon exposure to UV, produces active centres, permitting the self-healing of the damaged composites.[8], A schematic of our system is briefly described in Figure 2. 

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