Catalyst and cocatalysts

With insufficient catalyst these equilibria lie too far to the left, while excess cocatalyst destroys the catalyst and/or terminates the chain. The optimum proportion of catalyst and cocatalyst varies with the system employed and also with the solvent for a specific system. 

Some experiments (entries 3-10, 17, 18, 22-25 Table 1) were performed in liquid propene, which was condensed at -10 °C up to the desired volume in the autoclave. Using liquid propene, the catalyst and cocatalyst solutions were both injected into the autoclave via a pressure burette. 

Natta s bimetallic mechanism stipulates that when the catalyst and cocatalyst components are mixed, the chemisorption of the aluminium alkyl (electropositive in nature) occurs on the titanium chloride solid surface which results in the formation of an electron-deficient bridge complex of the structure shown... 

What is the catalyst and cocatalyst in the most widely used ZNC ... 

Inorganic Lewis acids, specifically, Friedel-Crafts catalysts, such as SnCU, TiCl4, AICI3, AlBr3, and BF3, usually require a cocatalyst (e.g., H20 always considered to be present), and in this case, initiation is also induced by the proton formed [Eq. (13.18)]. Pioneering work on the role of proton acids as catalysts and cocatalysts in acid-catalyzed polymerization was carried out by Polanyi and... 

The cationic polymerization of isobutylene (12) and styrene (13) is initiated readily by Et2AlCl in the presence of an alkyl halide, RC1. The interaction of the catalyst and cocatalyst is presumed to produce the carbonium ion R+, which initiates polymerization, and the corresponding gegenion Et2AlCl2". Alkyl halides with low R-Cl bond dissociation energies—e.g. tertiary, substituted allylic, and benzylic halides—are among the most effective cocatalysts. 

First the influence of the ratio of cocatalyst to catalyst was investigated at a constant pressure of 1500 bar, a temperature of 180°C and 240 s residence time. Using a catalyst concentration of 0.01 mole ppm in the reactor feed, the ratio of the two components was varied between 3600 and 29000 mole Al/mole Zr. An excess of cocatalyst shifts the reaction between catalyst and cocatalyst toward the catalytically active species formed in this reaction. Though catalyst productivity and the rate of polymerization increases with increasing ratio of cocatalyst to catalyst. This is shown in Figure 2 in which the productivity determined from the amount of polymer and the amount of catalyst metal is plotted versus the ratio of aluminium to zirconium. At ratios below 10000 mole Al/mole Zr only low productivities result. At higher... 

Bohm polymerized ethylene [48] on a highly active catalyst, formed by the interaction of Mg(OEt)-, with TiCl4. Et,Al was used as cocatalyst. The activity of the catalytic system depended on the ratio between catalyst and cocatalyst (see Fig. 9)f. After a certain mixing rate (> 500 rev. min "1) has... 

Thus, the terms initiator and coinitiator, as well as catalyst and cocatalyst, must be clearly distinguished. As proposed earlier [68], an initiator is consumed in the initiation process whereas a catalyst remains unchanged during the polymerization. In the polymerization of alkenes initiated directly by Lewis acids (e.g., iodine initiated polymerization of vinyl ethers) [69], the Lewis acid plays both roles. Nevertheless, Lewis acids usually act only as catalysts rather than as initiators, with protonogenic compounds such as adventitious moisture being the initiator. 

The work is divided into three parts. The first (Chapters 3 and 4) covers the catalyst (solid components) with particular regard to preparation, structure and role of the components. The second part (Chapter 5) deals with the reactions which take place between catalyst and cocatalyst and give rise to the formation of polymerization centers. The third part regards the polymerization and, apart from a discussion of number and nature of the active centers, reports the hypotheses about the most probable reaction mechanisms as a function of various kinetic parameters. 

These results may be, at least qualitatively, interpreted on the basis of simple, simultaneous complexation and exchange equilibria between the various catalyst and cocatalyst components. A rough model could, for example, be made up of the following equilibria (where Cat-D indicates a free site on the catalytic surface) ... 

Any interpretation of these polymerisations based on this type of interaction as the basic initiation step (i.e., without the participation of the mtaiomer) is therefore incorrect. On the other hand, our mechanism involving the intervention of a monomer-catalyst complex justifies the necessity of the presence of the mcaiomer when catalyst and cocatalyst are mixed, if initiation rates passing throu a maximum are to be obtained. 

It has been pointed out that the initiating potential of the system MesAl-f-BuQ is hi in solvent methyl chloride, but nil in n entane. On the other hand, AIQ3 and alkylaluminium dichlorides can induce the polymerisation of isobutene in both solvents. These facts do not disprove our interpretation, since it can be readily argued that the chlorine-methyl exchanges between catalyst and cocatalyst are extremely slow in a medium of such alow dielectric constant as n-pentane and that the absence of polymerisation arises precisely from the lack of formation of di- and trichloride. 

Wesslau was the first to demonstrate that, along with certain heterogeneous catalytic systems for polyethylene, there is a certain MWD dependence on the type and number of ligands altogether distributed between catalyst and cocatalyst. With TiCl -aluminium alkyls it was concluded > that, by varying the type of aluminum alkyl, polymers are obtained with a very similar MWD curve, but with the maximum shifted according to the nature of the alkyl group. Instead, different results were found by Russian researchers 59-ieo)... 

Ger. Offen. 2, 742, 585 (1976) Asahi Chem. Ind. AlMgjEtj BUjj.. + SiHClj/TiCU i-BUjAl SL FR68 FR 52 with EtAIClj instead of SiHClj control of MWD by changing catalyst and cocatalyst composition... 

Reaction of dialkylmagnesium compounds with selected chlorinated compounds produces finely divided MgCl that can be used as a support for polyethylene catalysts. Other reagents may be used to produce different inorganic magnesium compounds, also suitable as supports. Examples are shown in Figure 4.1. Treatment of these products with transition metal compounds results in a supported "precatalyst." Typically, the transition metal is subsequently reduced by reaction with an aluminum alkyl and the solid catalyst isolated. The solid catalyst and cocatalyst (usually TEAL) may then be introduced to the polymerization reactor. 

Features - polymerization takes place in "solution" - catalyst residence time short (minutes) - catalyst and cocatalyst must show reasonably good high temperature stability - morphology and psd of catalyst are less important than in other processes - wide range of comonomers may be used... 

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