Propagating center

VI. Some General Features of Propagation Centers in One-Component... 

The propagation center for catalytic polymerization is a chemical compound having an active bond between the catalyst and growing polymer molecule the monomer insertion into this bond occurs as a propagation act. 

In catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain mainly on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. 

The propagation centers of the catalysts of olefin polymerization contain the active transition metal-carbon olefin polymerization may be divided into two vast classes according to the method of formation of the propagation center two-component and one-component.1... 

Two-component systems are obtained by the interaction of transition metal compounds of groups IV-VIII of the periodic system with or-ganometallic compounds of groups I-III elements (Ziegler-Natta catalysts). An essential feature of the formation of the propagation centers in these catalysts is the alkylation of the transition metal ions by an organo-metallic cocatalyst. 

One-component catalysts cause polymerization without organometallic activators in this case the formation of the propagation centers takes place at the interaction of the transition metal compound with the monomer. 

It is necessary to note the limitation of the approach to the study of the polymerization mechanism, based on a formal comparison of the catalytic activity with the average oxidation degree of transition metal ions in the catalyst. The change of the activity induced by some factor (the catalyst composition, the method of catalyst treatment, etc.) was often assumed to be determined only by the change of the number of active centers. Meanwhile, the activity (A) of the heterogeneous polymerization catalyst depends not only on the surface concentration of the propagation centers (N), but also on the specific activity of one center (propagation rate constant, Kp) and on the effective catalyst surface (Sen) as well ... 

The results of the investigation of chromium oxide catalysts accumulated up to now permit the following main stages in the process of the propagation center formation to be singled out ... 

Formation of the Active Component (the Precursor of the Propagation Centers)... 

In the propagation centers of chromium oxide catalysts as well as in other catalysts of olefin polymerization the growth of a polymer chain proceeds as olefin insertion into the transition metal-carbon tr-bond. Krauss (70) stated that he succeeded in isolating, in methanol solution from the... 

The following stage of the propagation center formation occurs through the reduction of Cr(VI) to the lower oxidation state. The compounds of Cr(II) seem to be active in polymerization in the solution of bis-triphenyl-silyl-chromate (109). For the formation of these compounds the following scheme taking into account the results (110) concerning the study of the reaction of bis-triphenylsilyl-chromate with olefins was considered (109) ... 

Here KP is the propagation rate constant, catalyst effective surface, C the monomer concentration near the catalyst surface, and Na the surface concentration of propagation centers. 

As for the dependence of the polymerization rate V on the monomer concentration some authors have also found first-order kinetics (84, 90, 96, 99), but sometimes deviations from the first order were observed (38, 51, 88) that may be connected with a change in the number of propagation centers with monomer concentration. 

Here Ceq is the ethylene concentration equilibrium to the concentration in a gaseous phase, Kp the propagation rate constant, N the concentration of the propagation centers on the catalyst surface, Dpe the diffusion coefficient of ethylene through the polymer film, G the yield of polymer weight unit per unit of the catalyst and y0at, ype are the specific gravity of the catalyst and polyethylene. 

If the dissolving of a portion of the polymer takes place, diffusional restriction may occur as a result. Such a case was observed in (98) where a decrease of the polymerization rate (slurry process in cyclohexane) with temperature rise from 75° to 90°C was found despite the increase in the number of propagation centers. At a further increase of the polymerization temperature (>115°C) polymerization becomes a solution process that may also proceed with no diffusional restrictions (94). 

The most obvious reason for the polymerization rate variation with time is the change in the number of propagation centers (88, 89). According to Zakharov et al. (115) this change may be determined by the concurrence of the following reactions in the polymerization medium ... 

In Ballard et al. (9a, 118, 123) the formation of the propagation centers from benzyl and allyl compounds of transition metals were supposed to be a result of the insertion of the monomer into the bond between the organic ligand and metal in the initial compound according to the following schemes ... 

According to the data on the number of propagation centers (N) the propagation rate constant (Kp) is calculated ... 

A. Methods Used for the Determination of the Number of Propagation Centers... 

To determine the number of propagation centers in one-component catalysts, in principle the same methods used to study two-component catalysts of olefin polymerization may be applied Qsee (18, 160, 160a) ]. The most widely used methods for the determination of the number of propagation centers in polymerization catalysts are ... 

Inhibition methods where the number of propagation centers is calculated by considering the quantity of the inhibitor added and the resulting decrease of the polymerization rate. 

It is evident [see Eq. (5), Section II[] that for catalysts of the same or similar composition the number of active centers determined must be consistent with the catalytic activity it can be expected that only in the case of highly active supported catalysts a considerable part of the surface transition metal ions will act as propagation centers. However, the results published by different authors for chromium oxide catalysts are hardly comparable, as the polymerization parameters as a rule were very different, and the absolute polymerization rate was not reported. 

The data on the determination of the number of the propagation centers on chromium oxide catalysts by the inhibition method were given in several papers water (61), carbon tetrachloride (167), and diethylamine (69) were used as inhibitors. It was found that the number of propagation centers is about 10% (61), 1% (167), and 20% (69) of the total content of chromium in the catalyst. 

In Hogan (69) it was supposed that in a highly active catalyst containing 0.01% of chromium all the chromium ions act as active centers. According to this it was calculated that in the catalyst containing 1% of chromium on silica the number of propagation centers reached 10% of the supported chromium. 

Several determinations of the number of propagation centers by the quenching technique have been carried out (98, 111). As a quenching agent methanol, labeled C14 in the alkoxyl group, proved to be suitable in this case. The number of active centers determined by this technique at relatively low polymerization rates (up to 5 X 102 g C2H4/mmole Cr hr at 75° and about 16 kg/cm2) (98, 111, 168) in catalysts on silica was about... 

The reactivity of the propagation centers in oxide polymerization catalysts depended on the nature of the transition metal, support, activation temperature of the catalyst, and type of reducing agent (168a). 

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