Active centers deactivation

Here Nx is the active center deactivated by X and P is the polymer molecule. 02, C02, C2H2 appear to be X-type inhibitors. 

Here Ny is the active center deactivated by Y. H20 is likely to be a Y-type inhibitor. To explain the steady-state period of polymerization it may be assumed that some quantities of Y are adsorbed on the support surface. 

The above comments should not be taken as claims that anisole and diphenyl ether cannot be metallated by organolithium species. For example, alkyllithiums are known (38,39,40) to react with anisole, usually in the ortho position. However, these reactions are generally slow, particularly at ambient temperature and when the ether is diluted with a hydrocarbon solvent. Our results merely indicate that active center deactivation via metallation of these aromatic ethers is not a serious problem during the time span of our measurements with species that are, at least, partially delocalized (33J ... 

Higginson and Wooding 277) also reported a transfer reaction to solvent for the case of the polymerization styrene in ammonia initiated by potassium amide. There was no termination event in their kinetic scheme, i.e., active center deactivation via a spontaneous termination event was not considered to be a significant event. 

Termination. By some reaction, generally involving two polymers containing active centers, the growth center is deactivated, resulting in dead polymer ... 

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

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]. 

GP 3] ]R 3h] [R 4a] Safe operation in the explosive regime was demonstrated [103]. Catalytic runs with 1-butene concentrations up to 10 times higher than the explosion limit were performed (5-15% 1-butene in air 0.1 MPa 400 °C). A slight catalyst deactivation, possibly due to catalyst active center blockage by adsorption, was observed under these conditions and not found for lower 1-butene concentrations. Regeneration of the catalyst is possible by oxidation. 

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. 

Pore-confinement of the catalytically active center may have favorable effects as demonstrated for (i) protection of the reactive sites from deactivation processes... 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

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. 

Interesting situations develop when multiple (>2) substituents are placed in fairly close proximity. Depending on the donor or acceptor nature of the substituents and that of the carbon, center in question, and the number of bonds separating them, there will be an activating or deactivating effect. Typical scenarios are summarized in the following diagrams. 

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. 

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. 

The elimination of the sodium hydride was explained by the process given by Margeri-son and Nyss 289). Following Schmitt 296), Comyn and Glasse also proposed 309) that reaction of the anions formed in the a-methylstyrene system would yield deactivated species via reaction with the solvent, THF. Their kinetic study showed 310) that the process given in Eq. (68) was second order in monomer and first order in active centers, which are not consumed in the reaction. The sequence shown as Eq. (69) was found to be first order in active center concentration and in the dimer which is the product of Eq. (68). 

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

Due to the strong deactivation, it is close to impossible to determine kinetic parameters because all data measured are only snapshots on the deactivation trajectory. Moreover, since the active center of the Li-doped MgO catalyst is unknown [12], it is not reasonable to calculate values such as TON or TOF. That the apparent activation energy also depends on the state of deactivation is also shown in [11],... 

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