Two-component Catalysts

Among one-component polymerization catalysts subhalides of the transition metals are most similar in composition to the traditional Ziegler-Natta catalysts. In this connection, the study of the simplest one-component catalyst of this type (especially TiCl2) is of great importance for the clarification of still disputable problems of the mechanism of polymerization by two-component catalysts. 

Fig. 3. Comparison of the rates of polymerization by one-component and two-component catalysts (75°C, ethylene pressure 5.5 kg/cm2, polymerization in hydrocarbon solvent). Curve 1—TiCh (specific surface 24 m /g). Curve 2—TiCfi + AlEtjCl (Al Ti = 4, specific surface of TiClj 20 m2/g the same sample of TiClj was used for the preparation of TiCls). Curve 3—TiCfi + AlEtjCl (Al Ti = 4).

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 ... 

However, in olefin polymerization by two-component catalysts during polymerization not only active transition metal-polymer bonds are formed, but also inactive aluminum-polymer ones, as a result of the transfer process with the participation of a co-catalyst (11, 162-164). The aluminum-polymer bonds are quenched by tritiated alcohol according to the scheme (25), so an additional tagging of the polymer occurs. The use of iodine (165, 166) as a quenching agent also results in decomposing inactive metal-polymer bonds. 

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

The processes of reversible adsorption of the coordination" inhibitors (including the adsorption of organometallic compounds) result in an increase in the lifetime of the transition metal-carbon bond. It is possible that due to this, in the case of propylene polymerization by two-component catalysts based on TiCU, at low temperatures a long-term increase of molecular weight with time was observed (192,193). 

The rearrangement reaction of 1 requires a two component catalyst system consisting of a Lewis acid and a soluble source of iodide anion. The process concept had a number of requirements for the catalysts in order to achieve commercially useful operation. These were ... 

Norbomen-2-yl 4-(l-(3-(2,2,5-trimethyl-4-phenyl-3-azahexoxy))ethyl)ben-zyl ether, (V), was previously prepared by the authors (4) and used in the controlled free radical copolymerization of ethylene and 5-norbomen-2-ol. Polymerization were carried out in the presence of the two component catalyst mixture of the current application. 

Isoalkanes can also be synthesized by using two-component catalyst systems composed of a Fischer-Tropsch catalyst and an acidic catalyst. Ruthenium-exchanged alkali zeolites288 289 and a hybrid catalyst290 (a mixture of RuNaY zeolite and sulfated zirconia) allow enhanced isoalkane production. On the latter catalyst 91% isobutane in the C4 fraction and 83% isopentane in the C5 fraction were produced. The shift of selectivity toward the formation of isoalkanes is attributed to the secondary, acid-catalyzed transformations on the acidic catalyst component of primary olefinic (Fischer-Tropsch) products. 

Trost, B. M. McEachem, E. J. Toste, F. D. A two-component catalyst system for asymmetric allylic alkylations with alcohol pronucleophiles./. Am. Chem. Soc. 1998, 120, 12702-12703. 

The two component catalyst consists of a 1 3 molar ratio of tris(dibenzylidene-acetone)dipalladium and bis(2-diphenylphosphino)phenyl)ether, respectively. 

The application of this technique is based on a well documented concept according to which active centers in two component catalysts are formed via alkylation of transition metal compounds by organometals (cocatalysts), e.g. ... 

The two-component catalytic systems used for olefin polymerization (Ziegler-Natta catalysts) are combinations of a compound of a IV-VIII group transition metal (catalyst) and an organometallic compound of a I-III group non-transition element (cocatalyst) An active center (AC) of polymerization in these systems is a compound (at the surface in the case of solid catalysts) which contains a transition metal-alkyl bond into which monomer insertion occurs during the propagation reaction. In the case of two-component catalysts an AC is formed by alkylation of a transition metal compound with the cocatalyst, for example ... 

When using, however, two-component catalysts alcohols also react with inactive metal-polymer (aluminium-polymer) bonds which are formed in the chain transfer reactions with a cocatalyst. It is expedient to use the alcohol method only for catalytic systems and polymerization conditions for which the number of inactive metal-polymer bonds is low. Such a case is the polymerization of 4-methyl-1-pentene on vanadium trichloride activated with various organoaluminium compounds. For this system the influence of catalyst composition and polymerization conditions on Cp and kp was determined by quenching the polymerization with tritiated alcohol. 

Such a mechanism of carbon monoxide interaction with active centers is compatible with the data on the slow copolymerization of CO with ethylene found for the ethylene polymerization by some one-component catalysts This copolymerization may proceed also in the case of two-component catalysts resulting in an increase of the number of radioactive tags in the polymer with time (see Fig. 1). Arguments have been given that the rapid increase of polymer radioactivity in the initial period (5-10 min) is due to the insertion of the first CO molecule into the active metal-carbon bond. 

However, the comparative data on (Table 1) and the stereoregularity of polymer fractions (Table 6) for one- and two-component catalysts based on titanium chlorides indicate that the cocatalyst does not influence the reactivity and stereospecificity of the propagation centers. Its effect on the overall polymerization rate is apparently due to the change in the total number of active centers and the ratio of isospecific and non-stereospecific centers. 

A two-component catalyst system consisting of two different transition metal complexes (Rh-Pd) has been applied to the enantioselective allylation of diethyl 1-cyanoethylphosphonate. The reaction proceeds with high enantioselectivity to give optically active phosphonic acid derivatives (Scheme 6.11). ° ... 

Sawamura, M., Sudoh, M., and Ito, Y., An enantioselective two-component catalyst system. Rh-Pd-catalyzed allylic alkylation of activated nitriles, 7. Am. Chem. Soc., 118, 3309, 1996. 

Oxides of a variety of metals on finely divided inert support materials initiate polymerization of ethylene and other vinyl monomers by a mechanism that is assumed to be similar to that of heterogeneous Ziegler-Natta polymerization that is, initiation probably occurs at active sites on the catalyst surface [2j. Unlike the traditional Ziegler-Natta two-component catalyst systems, the supported metal-oxide catalysts are essentially one-component systems. Among the metals that have been investigated for these catalyst... 

A two component catalyst system has provided promising results in the alkylation of a 7t-allyl moiety with a prochiral nucleophile (Scheme 44). 

The presence of zinc oxide in thorium catalysts increased the amount of liquid products. The search for a possible substitute for thorium oxide led to the discovery that a two-component catalyst, composed of aluminum and zinc oxides, was capable of catalyzing the formation of higher hydrocarbons as well as that of isobutane. However, the space-time yields obtained with the catalysts of this type are lower than those obtained with the corresponding Th02 catalysts. 

Aluminum and chromium oxides used as two-component catalysts showed little activity. However, it is interesting to note that catalysts of this type, operated at 500°C., produced liquid hydrocarbons composed almost exclusively of aromatics. 

Two component catalyst composition for trimerizing isocyanate to iso-cyanurates [36]. 

Care must be taken with this reaction, and an alternative procedure is to use a two-component catalyst system by reacting the lactam with a base to produce an activated monomer. This then reacts with a promoter such as the acyl lactam, which initiates the ring-opening growth of the linear polymCT. For a series of cychc lactams, the reaction rates are a function of ring size and are in the order of 8- > 7- > 11- 5- or 6-membered rings. These are important commercial processes, and nylon-4 is also prepared vising this type of reactioiL... 

Processes 4 and 5 in Figure 2.13 are more recent developments entailing dimerization of acrylonitrile. Although formally analogous to the Monsanto process they are quite different because they proceed via catalysis rather than electrolysis. One of these processes developed by Halcon [176] involves liquid-phase reaction of acrylonitrile at 30°C under a nitrogen pressure of 0.7 MPa in the presence of a two-component catalyst system. 

Another unsolved problem concerns the regeneration of two component catalysts because each catalytic function surely requires its own regeneration conditions. 

On the other hand, two-component catalyst systems, mainly WOCI4 and WCle in combination with a suitable cocatalyst, have been widely employed on a laboratory scale. These were the first catalysts developed for the metathesis of unsaturated fatty acid esters. Although they are more sensitive to moisture and air, they are cheaper and easier to handle than the alkylidene complexes. In this sense, the use of W(OAr)2Cl4 has... 

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