Active centre rapid

In my opinion, an active centre of alkene polymerization in the liquid phase is not a single chemical entity to be visualized by a single (and simple) chemical formula. Probably a set of compounds, of complexes with variable composition, a dynamic system where the effects of individual components are mutually complementary or overlapping is really in play. The same macroscopic effect (centres of equal activity and iso-specific regulating ability) can be obtained with various starting organometals and donors. In such a system, subsystems may exist each of which is externally manifested as an individual active centre (rapid or slow, isotactic, with a tendency to transfer or termination, or living, etc.) [225],... 

In all cases an enzymic process is composed of several consecutive reaction steps. Even the simplest Michaelis-Menten type rapid equihbrium mechanism involves two steps, the binding of the substrate, S, to a specific site in the active centre, and the chemical transformation of the bound S to product P, during which the enzyme becomes free again. The Michaelis constant characterizes the affinity of the enzyme to its... 

As described in section 3.3, correlation studies between CS plane defects observed by ETEM and and parallel reaction chemistry (under conditions similar to those used in ETEM) indicate that the CS planes which eliminate supersaturation of anion vacancies are a consequence of catalytic activity and not active oxygen exchange sites for catalysis as was originally believed. They are secondary or detrimental to catalysis. The correlation results strongly suggest that anion point defects are active centres in the rapid diffusion of oxygen in... 

One problem encountered in the field is the apparent irre-producibility of the results of different workers, even those in the same laboratory. This is particularly the case with molar mass distribution and steric triad composition. The explanation of these apparent inconsistencies comes with the realization that the mechanisms are eneidic and the polymer properties are primarily determined by independent active centres of different reactivities and stereospecificities whose relative proportions are set at the initiation step, which is completed in the first seconds of the polymerization. The irreproducibilities arise from irreproducibilities in the initiation step which had not been thought relevant. Ando, Chfljd and Nishioka (12) noted that these rapid exothermic reactions tend to rise very significantly above bath temperature (we have confirmed this effect) and allowed for this in considering the stereochemistry of the propagation reaction. However our results show that the influence on the initiation reactions can have a more far-reaching effect. 

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. 

COLEMAN and Fox (18) have pointed out that the non Bernoullian sequence distribution observed in some of these systems can be formed without the hypothesis of penultimate effects. All that is required is that two or more types of active species be present which do not rapidly interconvert. Each can add monomer at its own rate and with its own characteristic regulating effect. No penultimate effect is necessary but the sequence distribution will be non-Bernoullian. This type of mechanism is particularly attractive in the explanation of stereoblock polymer formation in the lithium alkyl systems in toluene with small amounts of ether present. The presence of at least two species of active centres has been inferred from an examination of polymer fractions obtained from butyllithium initiated polymerizations (19) in toluene. The change in molecular weight distribution with time suggests the presence of two... 

A criterion was obtained [133] under the fulfilment of which the diffusion can be treated as rapid and not taken into consideration for the surface reaction kinetics kj(ap) [In (L/r0) -1.39] -4 1, where k is the interaction constant of adsorbed substances with active centres. It is evident that at L k r0 this relationship is met. It is this relationship that is the condition for the applicability of the ideal adsorbed layer kinetics but all the limitations imposed for its derivation (the reaction is monomolecular and active centres are taken for a square lattice) should be remembered. 

In terms of the branched-chain model this fact can be interpreted as follows. On rapidly cooling the catalyst, we preserve the high concentration of active centres achieved during "ignition . Hence, next time, "ignition will take place without an induction period. However, we believe the "memory effect can be interpreted on the basis of the ideal adsorbed layer model. 

Other vinyl monomers, such as acrylonitrile, methacrylonitrile, tert.-butyl vinyl ketone and methyl isopropenyl ketone, polymerize at 203 K, i. e. most probably by non-radical mechanisms. Even here, conversion of monomer to polymer is not complete, and utilization of the initiator is low. Only the polymerization of acrylate momomers proceeds to full monomer consumption at low temperatures. Additional monomer, even when introduced after some delay, is also polymerized. This indicates that a part of the active centres remains living for some time. However, the number of high-molecular-weight chains is lower than the number of added initiator molecules. At the same time, initiation is very rapid [163]. 

The structure of the hydrocarbon group also affects reactions leading to the start of polymerization. Initiation by butyllithium leads to the rapid formation of a relatively large amount of lithium methoxide, whereas by the reaction of diphenylhexyllithium with methyl methacrylate only a small amount of methoxide is formed slowly in both cases, the reaction proceeds according to scheme (36) or by cyclization or termination of the active centres [170] by 1,2 addition [i. e. again in analogy to reaction (36)]... 

The activation described is due to an increase in the concentration of active centres. The polymerization rate can, of course, also be enhanced by increased reactivity of the centres. To solvent-separated ion pairs and to free ions, monomers in general are added more rapidly than to contact ion pairs. When we succeed in separating the active centre ions by the addition of a strongly solvating compound, the initiation and propagation rates will be enhanced, and changes affecting the mode of addition may appear [212, 213 ]. 

Very rapid initiations are known, manifested by an instantaneous start to the polymerization after which the number of active centres is not further increased. Polymerizations with slow initiation are also quite frequent, starting only after some inhibition and/or induction period. In the course of these polymerizations, the concentration of active centres is not usually constant. A stationary state is not excluded, of course but it occurs much less frequently than with radical polymerizations. 

An active centre is formed by the addition of a primary radical (for example from an initiator) to the monomer. A polar primary radical may considerably affect the electronic configuration of the centre, and this effect will decay with the number of added monomers. The first additions may proceed by a different mechanism and may even be more rapid or slower than additions at later stages, when the inductive effect has been suppressed. 

The energetic state of an active centre is determined by an equilibrium between the chemical and physical factors within the centre itself and in its immediate vicinity. Usually the equilibrium is rapidly established so that, for the subsequent chemical reaction, the centres are ready in the same energy state, i.e. they are equally reactive. Situations cannot, however, be excluded where the rate of energy equilibration is comparable with the rate of the... 

No support can be regarded as inert with respect to the active centres. By its universally positive effect on the activity of centres, MgCl2 is superior to any other support. In spite of the great technical importance of Mg in active centres, generally not much is known of their structure in third-generation catalysts (or perhaps because of its positive effects all the important producers have published hundreds of patents, but the crucial factors may still be kept secret). It is suspected that the separation (dilution) of transition metal atoms by a barrier of Mg atoms enables the majority of transition metals to become part of the active centres on these centres, the polymer grows more rapidly than on centres without Mg. Mutual contact of the centres is hindered, bimolecular termination of centres (transition metal reduction to a less active oxidation state) is limited, and the centres live longer. 

Very rapid reactions are studied by means of the stopped-flow technique. In some cases this technique makes direct observation of active centres possible, as well as the determination of their immediate concentrations even in non-stationary polymerizations [261-264]. 

Thus the addition of the monomer to the active centre must be repeated about one thousand times without interruption by another reaction. In other words, propagation must be about one thousand times more rapid than termination or any of the many possible transfer processes. 

When the active centre concentrations change during propagation, the whole polymerization is non-stationary. Kinetically the process becomes more complicated and often even experimental control of the process becomes more difficult. On the other hand, a non-stationary condition can be utilized in studies of the elementary polymerization steps. To this end, the non-stationary phases of radical polymerizations are suitable, where outside these phases the process is essentially stationary [23-25]. Hayes and Pepper [26] called attention to the existence and solution of a simple non-stationary case caused by slower decay of rapidly generated cationic centres. In more complicated cases, exact analysis of the causes of a non-stationary condition is often beyond present possibilities. Information from the process kinetics is often not conclusive. It should be mentioned that, even when the condition d[Ac]/dt = 0 is strictly valid, polymerizations may be non-stationary, particularly in those cases when during propagation the more active form of the centres is slowly transformed to the less active form or vice versa. 

Fig. 6. Types of conversion curves. Conversion curve 1, 2,4 polymerization with rapid initiation rate decreases (1) only inconsequence of monomer consumption (living polymerization) (2) due to the consumption of monomer and of active centres (3), (5) polymerization with slow initiation Atind is the time interval of the concentration growth of active centres (4), (5) polymerization with an inhibition period tinh (A and A are points of inflection).

In the previous sections, methods of qualitatively controlling the course of propagation were described. Indirect control as well as the quantitative effects caused by intentional control of the other partial processes in polymerization have still to be mentioned. The separation of initiation from propagation alters the kinetic character of the whole reaction. With ionic polymerizations, initiation can be separated from propagation by the selection of conditions suitable for rapid initiation. With radical polymerizations, this is not possible. Therefore both partial processes must be separated in space. Fortunately, radical active centres operate both in polar and in non polar media. Thus it is not difficult to confine initiation and propagation to mutually immiscible components of the medium. Emulsion polymerization remains the most important representative of quantitative control of propagation. 

Rakova and Korotkov compared the rates of homopolymerization and copolymerization of styrene and butadiene [226], Styrene polymerizes very rapidly and butadiene slowly. Their copolymerization is slow at first, with preferential consumption of butadiene. When most of the butadiene is consumed, the reaction gradually accelerates yielding a product with a high styrene content. In the authors opinion, this is caused by selective solvation of the active centres by butadiene only after butadiene has polymerized, does styrene gain access to the centres [227], A similar behaviour was observed by Medvedev and his co-workes [228] and by many others. In our laboratory we observed this kind of behaviour in the cationic polymerization of trioxane with dioxolane. Although trioxane is polymerized much more rapidly than dioxolane, their copolymerization starts slowly, and is accelerated with progressing depletion of dioxolane from the monomer mixture [229],... 

In the simplest case, with rapid initiation and participation of a single type of active centre, the rate of propagation is equal to the polymerization rate, and kp is the overall polymerization rate constant. Rapid initiation can be established in ionic processes the presence of several kinds of centres means unequal numbers of monomer molecule additions to different centres. Long macromolecules will be formed on "rapid centres, shorter ones on "slow centres. A practical example of this situation is anionic living polymerization with the participation of contact and solvent-separated ion pairs, and of free ions. 

The equilibrium is shifted completely to the right. Thanks to the presence of the phenolate group at the chain ends, the macromolecules could be counted by UV spectrophotometry (polytetramethylene oxide is transparent in the UV range). With the counter-ion B = BF termination was slow with B- = AlCl it was rapid. The active centres can be counted using the Saegusa and Matsumoto method and, together with a determination of the degree of polymerization and the concentration of macromolecules, the elementary constants can be determined (the dead-stop method). 

Kucera et al. combined anionic and cationic polydimethylsiloxane. With the ratio of active centres 1 1, a perfectly stable polymer was produced which did not depolymerize even under conditions where a trace of acid or base would lead to a rapid decomposition of all polymer chains [105]. This was the first combination of macroions described in the literature. 

Active centres of ionic polymerizations do not usually decay by mutual collisions as the radical centres. The stationary state, when it exists at all, results from quite different causes, mostly specific to the given system. Therefore the kinetics of ionic polymerizations is more complicated and its analysis more difficult. The concentration of centres cannot usually be calculated. On the other hand, ionic systems with rapid initiation give rise to the kinetically very simple living polymerizations (see Chap. 5, Sect. 8.1). 

Radical polymerizations are almost always considered as kinetically stationary. However, the stationarity conditions are not always fulfilled. Living polymerizations with rapid initiation are stationary, but the character of the medium should not significantly change during polymerization in order to prevent shifts in the equilibria between ion pairs and free ions. All other polymerizations are non-stationary even, to some extent, living polymerizations with slow initiation. It is usually very difficult to define initiation and termination rates so as to permit exact kinetic analysis. When the concentration of active centres cannot be directly determined, indirect methods must be applied, and sometimes even just a trial search for best agreement with experiment. 

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