Catalysts, anionic coordinative

In studying two-component polymerization catalysts, beginning with Feldman and Perry (161), a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of the anionic coordination mechanism of olefin polymerization occurring... 

According to detailed XRD analyses, the two catalyst preparation procedures under study formed solid solutions. The application of sol-gel method led to improved selectivity to olefins in the reaction of propane ODH, compared to the simple procedure of evaporation and decomposition. However, the propane conversion on the sol-gel catalysts was lower at the same experimental conditions, while the catalysts surface area was higher. Moreover, the sol-gel samples presented higher basicity as shown by C02 TPD. It could be explained by a better incorporation of Nd into the AEO lattice, creating cationic vacancies for attaining electroneutrality and thus rendering the nearby oxide anions coordinatively unsaturated and more basic. 

CuBr/QUINAP System The CuBr/QUlNAP system was initially used in the enan-tioselective synthesis of proparyl amines via the reaction of alkynes and enamines (Scheme 5.5). It was rationalized that the enamines reacted with protons in terminal alkynes in the presence of copper catalyst to form zwitterionic intermediates in which both the generated iminiums and alkyne anions coordinate to the copper metal center. After an intermolecular transfer of the alkyne moiety to the iminium ion, the desired products were released and the catalyst was regenerated. The combination of CuBr as catalyst and the chiral ligand QUEMAP is crucial for the good reactivities and enantioselectivities seen in the reaction. Another potential... 

A kinetic chain reaction usually consists of at least three steps (1) initiation, (2) propagation, and (3) termination. The initiator may be an anion, a cation, a free radical, or a coordination catalyst. Although coordination catalysts are the most important commercially, the ionic initiators will be discussed first in an attempt to simplify the discussion of chain-reaction polymerization. 

The discussion of Mnetic work will be here preceded by a summarized description of the chemical nature of the polymerization, to which we have attributed a mechanism of anionic coordinated type. Such a definition of the reaction mechanism depends upon the fact that the catalyst is a complex in which, generally, a transition metal acts as a coordinating agent and that a carbon atom, which belongs to the extremity of a growing polymeric chain, is coordinated to such a complex and, in the activated state, it possesses a negative charge. 

The data here related on the kinetics of the propylene polymerization and of the transfer processes and the studies of the catalysts carried out with C-labelled alkylaluminums, derive from a series of researches mostly carried out some time ago, when the knowledge of the mechanism of the considered catalytic processes was still rather limited. Nevertheless, it helped remarkably to know these new processes of anionic coordinated polymerization their true catalytic nature (which regard to a-TiCU) differentiates them from the more usual polymerization processes (radicalic) which, actually, are not catalytic. They substantially contributed to demonstrate that the anionic coordinated polymerization is a step-wise addition process in which each monomeric unit inserts itself into a metal carbon bond of the catalytic complex. 

A phenoxy anion coordinates to a Cu(II) catalyst and an electron is transferred from the coordinated phenoxy anion to the Cu(II) ion. This electron-transfer step [Eq. (22)] is an intracomplex process, and it is expected that the electronic and steric factors of the amine ligands(L) in the Cu complex as well as the electronic... 

POLYALLOMER RESINS. These are block copolymers prepared by polymerizing monomers in the presence of anionic coordination catalysts. The polymer chains in polyallomers are composed of homopolymerized segments of each of the monomers employed. The structure of a typical polyallomer can be represented as ... 

One catalyst system which allows for the copolymerization of BD and St is based on NdV and is activated by MAO. It is important to note that this catalyst system does not contain a halide donor. By the addition of cy-clopentadienyl derivatives (e.g. cyclopentadiene, indene, anthracene etc.) St incorporation is increased. It may be speculated that by the influence of MAO a proton is abstracted from the cyclopentadienyl derivatives added and that the resulting cyclopentadienyl-type anions coordinate to the active Nd sites [498,499]. In this way the activity pattern of Nd is changed and copolymerization of BD and St is made possible. As shown in the attached Table 28 increases of St incorporation occur simultaneously with decreases of cis-1,4-contents. 

Mossbauer spectroscopy has been quite successful in identifying catalyst components. Mossbauer spectroscopy provides quantitative site populations, easily discriminating between various metal oxidation states and anion coordinations, and it can lead to phase compositions or distributions between phases of the isotope under investigation. It also gives quantitative population distributions of local distortion environments and local chemical environments, via extracted quadrupolar splitting distributions. 

As in the case of the zinc catalysts, active catalysts are formed by reaction of alkyl aluminium compounds with water. It is generally felt that since aluminium compounds are usually fairly strong Lewis acids, the catalysts also are somewhat more acidic in nature. Thus a coordinate cationic mechanism is generally favoured for these polymerizations. In contrast, a more anionic coordinate mechanism is usually suggested for the zinc catalysts. In fact, as will be seen in the discussion of the higher cyclic ethers, some of these catalysts are distinctly able to initiate true cationic polymerizations. However, the catalysts under discussion here as applied to epoxides are clearly considered to be coordinate. 

Table 8. Catalysts of anionic-coordinative polymerization based on a-ally) complexes of transition metals [49, 68]...

The chain propagation via-the anionic coordination mechanism, as shown above, involves insertion of an oriented monomer molecule. In more detail, since the catalyst fragment G (containing transition metal) is coordinated with both the... 

The difference between this principle and the four preceeding ones is that the range of its application is limited to anionic and anionic-coordination polymerization. Only in the anionic propagation mechanism, is the formation of triplet intermediates possible (Scheme 5). This is probably one of the main reasons for the high structure- and stereoregulating ability of anionic and anionic-coordination catalytic systems including Zie r-Natta catalysts and is one of their distinguishing features. 

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