Active centre inactive

Butyllithium initiation of methylmethacrylate has been studied by Korotkov (55) and by Wiles and Bywater (118). Korotkov s scheme involves four reactions 1) attack of butyllithium on the vinyl double bond to produce an active centre, 2) attack of butyllithium at the ester group of the monomer to give inactive products, 3) chain propagation, and 4) chain termination by attack of the polymer anion on the monomer ester function. On the basis of this reaction scheme an expression could be derived for the rate of monomer consumption which is unfortunately too complex for use directly and requires drastic simplification. The final expression derived is therefore only valid for low conversions and slow termination, and if propagation is rapid compared to initiation. The mechanism does not explain the initial rapid uptake of monomer observed, nor the period of anomalous propagation often observed with this initiator. The assumption that kv > kt is hardly likely to be true even after allowance is made for the fact that the concentration of active species is much smaller than that of the added initiator. Butyllithium disappears almost instantaneously but propagation proceeds over periods from tens to hundreds of minutes. The rate constants finally derived therefore cannot be taken seriously (the estimated A is 2 x 105 that of k ) nor can the mechanism be regarded as confirmed. 

From this, it can be deduced that open network structures of A1F3 exhibit catalytic activity. Since the catalytic process occurs at the surface of these solids and taking into account the catalytic inactivity of the compact a-AlF3 phase, it can be concluded that the particular surface structure arising from the bulk MF6 octahedra arrangement and the availability of the active centres, but not the size of the surface area, are of crucial importance for the catalytic process. 

The 1,4 and 3,4 additions are the proper initiation reactions leading to the same active centre, though perhaps at different rates. Addition to the carbonyl group yields products whose participation in polymerization is still subject to controversy. Some authors assume that reaction (36) yields inactive compounds [165], A considerable part of the initiator is also consumed in the formation of very short chains [168, 169], Trimers are probably formed... 

Valuable data on the properties of active centres are obtained from kinetic measurements. They reveal the simultaneous existence of several centre types. The stable centres are active during the whole course of polymerization in addition, some fraction of decaying centres is also present. Isotactic centres exhibit stereoregulating ability and are, moreover, extremely active. Centres may oscillate between active and inactive (dormant) forms and some centres selectively polymerize enantiomers from a racemate. External effects, caused by specific properties of centres, will be discussed in subsequent chapters. In addition, the centres on which dienes are polymerized will be treated in Chap. 5, Sect. 4. The structure of these centres is a function of the coordinated diene, and it is therefore better presented together with propagation. 

Mutual transition of a dormant form into a living form and vice versa must be spontaneous, without external intervention. The concentrations of living and dormant forms are usually connected by an equilibrium. During macromolecule propagation, the centre can oscillate between the active and inactive states generally, the two forms have different mean life times. 

Dormant centres should not be confused with the inactive form of chain ends to which active centres are sometimes transformed by termination and which can be reversed to living forms only by slow re-initiation. Centres whose reactivity has been lost by termination are dead. The existence of living, dormant and dead centres is manifested in the mechanism and kinetics of the whole polymerization. The consequences of their occurrence will be discussed in the appropriate parts of the subsequent text (see Chap. 5, Sect. 8.1). 

Equilibria between various forms of living centres were treated in Chap. 5, Sect. 8.1. Equilibria of similar character control the arrangement and reactivity of all ionic centres. When polymerization-inactive structures participate in the equilibria, the number of active centres is reduced by the equilibrium amount of inactive forms. This phenomenon is usually not considered as termination the unreactive particles are treated as dormant. In the course of polymerization, however, the physico-chemical parameters of the system change as a function of the monomer-polymer transformation. Changes in permittivity, viscosity and the amount of polymer can cause shifts in ionization and dissociation equilibria. The kinetic manifestations of such changes are identical with the occurrence of termination. 

Some reactions can cause active centres to lose part of their reactivity. When such a transformation results in an inactive species, we speak of termination. When less active centres are formed which are still able to propagate, we speak of retardation. Naturally there is no sharp boundary between these two phenomena. Sometimes it is even hard to decide if termination of a part of the centres or a reduction in the reactivity of all centres is taking place. 

Unlike THE this system has an inherent termination reaction since the polymer yields are always less than 100%. When triethyl sulphonium tetrafluoroborate, Et3S" BF4, was found to be inactive as an initiator it was concluded that termination involved reaction of a backbone sulphur atom with an active centre, to form a stable tertiary sulphonium ion, viz. 

Organic reactions have been performed through the combination of incompatible basic and acid catalysts by two general methodologies, namely by using a mixture of two supported catalysts with different active centres on separate supports or multifunctional single catalysts where incompatible reagents are rendered mutually inactive by confinement effect. 

As seen (Fig. 2) the metal (Co, Zn)-phase is much better spread over the support surface in the inactive catalyst 5. The (Co+Zn) Si ratio characterizing the covering of the support is 1.25 in the sample 3 and 1.64 in the sample 5. This effect proves that the deposit of the active component covers and eliminates the catalytic active centres at the bare smface of the support. Thus, the support calcination at temperature higher than 950 °C leads to the decrease of the surface area and content of Si02 on the surface due to the interphase difhision and consequently to the decrease of SiOj accessibihty to a reactant. 

Accepting the existence of active sites implies also that there are also inactive centres or perhaps overactive centres that are quickly inactivated by the destructive chemisorption that occurs so easily with hydrocarbons. The accidental or deliberate removal of such sites by autogenic toxins (i.e. carbonaceous deposits , ethylidyne etc.), or by other poisonous species such as sulfur compounds, allows the remaining active centres to exhibit reactions of a structure-insensitive type that could not take place while the overactive centres were in existence. This situation may arise not only through surface heterogeneity, but also on a plane surface by operation of the Principle of Maximum Occupancy (Section 4.2) by which the preferred first reaction is that which utilises the largest size of active... 

Where Cat, M, A , P are the catalyst, monomer, growing chain (active centre), and inactive macromolecule respectively k , and k, are rate constants of the initiation reactions, chain transfer to monomer, and chain termination respectively. 

If is assumed thaf fhe active centres of the catalyst surface are distributed in a regular geometrical pattern determined by the lattice structure. It can be assumed that the area of cafalysf surface has uniform active cenfres and all these centres behave similarly. If active surfaces are nof uniform and if, during adsorption and reaction, some active centres become inactive, the result is an increase in the energy of activation and a decrease in the heat of adsorption (Suresh and Keshav, 2012). 

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