Deactivation of active centers

Temperature effects on the polymerization activity and MWD of polypropylene have been examined in the range of —78 °C to 3 °C 82 The MWD of polypropylene obtained at temperatures below —65 °C was close to a Poisson distribution, while the MWD at higher temperatures above—48 °C became broader (Slw/IWIii = 1.5-2.3). At higher temperatures the polymerization rate gradually decreased during the polymerization, indicating the existence of a termination reaction with deactivation of active centers. It has been concluded that a living polymerization of propylene takes place only at temperatures below —65 °C. 

Rate decay is mainly ascribed to a chemical deactivation of active centers. Nevertheless, in the case of ethylene, it appears that diffusive phenomena play also a certain role in the drop of the polymerization rate88 94. Moreover, diffusivity of monomer in the reaction medium may restrict polymerization rate, as can be concluded from the dependence of catalytic activity on catalyst concentration 95... 

Actually, studies on the propylene polymerization at atmospheric pressure carried out in our laboratories 101 > have demonstrated that R0 and the deactivation rate depend, in a complex manner, on both the organoaluminum and external donor concentrations (see Sect. 6.1.2 and 6.1.3). The kinetic curves obtained cannot be reduced to a single model for the deactivation of active centers according to a simple 1 st and 2nd order law, but rather they seem to follow a more complicated behavior. This is not surprising if one considers that the decay of polymerization rate is probably the effect of an evolution, in time, of a plurality of different catalytic species having different stability, reactivity and stereospecificity (see Sect. 6.3). 

The availability of such data would be very useful for a direct experimental verification of those reaction models which provide for competitive adsorption of the monomer, the aluminum alkyl and the hydrogen on the catalyst surface86 87. The studies carried out to-date on the effect of temperature do not even permit to establish clearly, whether the decline in catalytic activity, observed in propylene polymerization above 60-70 °C with TiCl4. EB/MgCl2—AlEt3 type catalysts 45,69,98), is due to an irreversible deactivation of active centers or to some other phenomena. 

Any of Eqs. [8], [9] and [10] allow the determination of C and also of relevant rate constants in the simplest case defined above. Complications arise, when these rates are time-or polymer yield-dependent. C is seldom time-invariable formation and deactivation of active centers are quite often difficult to express by simple kinetic laws. Nevertheless, the fundamental Eq. [7] should be valid, and Ivanov et al. 35) and Ermakov and Zakharov 6> suggested its modified version applicable to the non-stationary kinetics of polymerization, if only transfer reactions occur. Supposing that kp, ktr, [M], and [X] are time-independent and substituting C by Rp/kp[M], equation ... 

Fig. 2. Number of active centers vs. temperature at different cocatalyst concentrations. Concentration of AlEtj in mol/1. (I) 0 (2) 6x10 (3) 1.5 xlO (4) 3x10 (5) 3x10 (6) 1.5. Data calculated considering the reversible deactivation of active centers due to adsorption of AlEt,. For the temperature region, given in the frame, average activation energies are indicated...

In115 116) the cationic polymerization of cyclic ethers was examined theoretically and experimentally with regard to the nature of MWD variation. A theoretical analysis was made of how MWD is affected by the depolymerization reactions, monomole-cular deactivation of active centers, recombination of active centers, chain transfer by hydroxyl-containing compounds, chain transfer to the monomer, and ether oxygen of the polymer chain, as well as via the end hydroxyl group. 

Lower catalytic activity than homogeneous catalysts because of poor accessibility of the active sites for the substrate, steric effects of the matrix, incompatibility of solvent and polymer, deactivation of active centers. 

Therefore immobilization of active centers on the supports is perhaps one possibility of diminishing the prevailing role of side reactions 4 and 5, and thereby of enhancing the efficiency of metal complex catalysts for polymerization of olefins. It was expected that spatial isolation of MX (as immobilization of enzymes prevented their deactivation) would lead to a decrease in bimolecular deactivation of active centers and in turn, to a cooperative stabilization preventing monomolecular termination. Instead, as earlier studies have shown (Fig. 12-6) [69] polymer-immobilized complexes are stable over time. Macromolecular metal complexes for polymerization processes can be used as powders, films, fiber... 

Living polymerization is defined as chain polymerization in which chain termination and irreversible chain transfer are absent. The rate of chain initiation is usually larger than the rate of chain propagation with the result that the number of kinetic-chain carriers is essentially constant throughout the reaction. Reversible (temporary) deactivation of active centers can take place in a living polymerization, and all the macromolecules formed possess the potential for further growth. The term controlled polymerization, on the other hand, indicates control of a certain kinetic feature of a polymerization or structural aspect of the polymer. ... 

To avoid the deactivation of active centers, which led to formation of allyl carban-ions (and hence afforded homopolymers), a rapidly reacting 1,1-diphenylethylene... 

The alkoxy groups should not be larger than ethoxy. If secondary or tertiary carbons are present, especially if these are very dose to the oxygen atom, the deactivation of active centers becomes sterically hindered. 

Note 2 In a living polymerization, the reversible (temporary) deactivation of active centers can take place (see reversible chain deactivation). 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

Conversely, controlled immobilization of enzymes at surfaces to enable high-rate direct electron transfer would eliminate the need for the mediator component and possibly lead to enhanced stability. Novel surface chemistries are required that allow protein immobilization with controlled orientation, such that a majority of active centers are within electrontunneling distance of the surface. Additionally, spreading of enzymes on the surfaces must be minimized to prevent deactivation due to irreversible changes in secondary structure. Finally, structures of controlled nanoporosity must be developed to achieve such surface immobilization at high volumetric enzyme loadings. 

The only uncertainty about the nature of active centers that remains, concerns their aggregation state. In order to measure the aggregation number, we have performed viscometric measurements on the polymer solutions in benzene, before and after deactivation of the active species. 

The maximum value in the number of active centers was observed at a mole ratio of anisole/V(acac)3 of 0.5. The presence of anisole did not influence the syndiotactic-specificity of the active center, suggesting that anisole is not involved in the active center. Ueki et al.103> have concluded that anisole functions in the initiation reaction of V(acac)3 with A1(C2H5)2C1 inhibiting the deactivation step (28) in Section 4.2, resulting in an increase of the number of active vanadium species. 

Chien s kinetic model [48,49], unlike Ewen s model described above, is for the systems in which more than one active species is present. The model assumes the presence of multiple active center types, chain transfer to MAO, chain transfer by /3-H elimination (see p. 801), and first-order deactivation reactions of active centers. Chien applied the model in the study of ethylene polymerization with Cp2ZrCl2/MAO catalyst and propylene polymerization with Et(Ind)2ZrCl2/MAO and Et(H4lnd)2ZrCl2/MAO catalysts. 

Surface titanium hydrides (CCTi-H) are highly reactive compounds and can be partially deactivated in side reactions by interaction with components of the catalytic system. As a result of these side reactions, the number of active centers for polymerization in the presence of hydrogen is as a rule lower than the total number of the centers containing titanium-polymer bonds (Cp + Cd) during polymerization without hydrogen (Table 7). In the case of ethylene polymerization, dormant centers (Cd) are not formed. But, for polymerization in the presence of hydrogen,... 

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