Cocatalysts contacting

Rudler and coworkers reported that in the case of moderately acid-sensitive epoxides the use of biphasic reaction conditions (H2O/CH2CI2) proved to be sufficient in order to obtain the epoxides with good selectivity because under biphasic conditions the contact of epoxides with water is minimized. Because of the lability of pyridines under the reaction conditions employed, alternative and more stable cocatalysts such as pyrazole (12 mol%, biphasic conditions), bipyridine (6 mol%, biphasic conditions H20/CH2Cl2) and bipyridine-A,Af -dioxide (1.2mol%) were employed together with MTO (Scheme Pyrazole is stable against oxidation and with this additive... 

Modified methylaluminoxanes exhibit much improved storage stability and several are highly soluble in aliphatic hydrocarbons. (Manufacturers of polyethylene prefer to avoid toluene because of toxicity concerns, especially if resins are destined for food contact.) Most importantly, because yields are higher, modified methylaluminoxane formulations are less costly than MAO. However, since modified methylaluminoxanes contain other types of alkylaluminoxanes, they do not match the performance of conventional methylaluminoxane in some single site catalyst systems. Consequently, modified methylaluminoxanes should be considered niche cocatalysts for single site catalysts. 

The active center in this reaction is presumably a carbonium ion ion pair, as shown above, which can vary in structure and reactivity from a free carbonium ion at one extreme to a contact ion pair (or even a readily dissociated covalent compound) at the other. The initiator, which consists of the catalyst shown in the equation and generally a cocatalyst, has a controlling effect on the structure of the ion pair because it provides the counterion, Y, for the active center. Hence, small changes in the composition of the initiator as well as in monomer structure, reaction solvent, and temperature can cause profound changes in both the rates of the propagation and termination reactions and in the structure of the polymer formed. For this reason, polymerization reactions have been referred to as "chemical amplifiers" in that the polymer molecule is formed by hundreds or thousands of propagation reactions followed by one termination reaction. 

The catalyst and metal alkyl cocatalyst can be brought into contact in a number of ways, depending on the commercial process. In a slurry or solution polymerization process, it is most convenient to simply feed a solution of the cocatalyst directly into the reactor, where it comes in contact with the catalyst in dilute solution and in the presence of ethylene and any comonomer. This procedure allows for continuous adjustment of the cocatalyst concentration for control of polymer properties. 

In a variation of this approach, the catalyst and cocatalyst can also be fed first into a stirred contacting vessel where they are allowed to react with each other at higher concentrations for typically half an hour, followed by injection into the polymerization reactor. The whole process is continuous, thus permitting real-time adjustments to be made. This approach also allows cocatalyst and catalyst to be mixed in the absence of monomer, and at a temperature different from the polymerization... 

In another method of contacting catalyst and cocatalyst, the catalyst is treated with concentrated cocatalyst in a batch process. The catalyst is then dried and fed to the reactor as a single powder. This approach is most often used in the fluidized-bed process, in which a solvent is not available and in which the cocatalyst does not have high volatility. Both Cr(VI) oxide and bis(triphenylsilyl) chromate catalysts can be used as the catalyst component (see Section 3.8). Typically the latter is used, and treated with diethylaluminum ethoxide. 

In solution and slurry polymerizations cocatalysts are typically added in an amount equivalent to 0.5 to 10 ppm by weight of the solvent or diluent, or about 1 molecule per Cr atom in the reactor. Larger amounts can result in loss of activity, probably caused by an attack on the Cr-O-Si attachment bonds. To avoid such damage, it is usually better to have olefin present during the contacting, because it seems to protect the site [238,681],... 

Although the cocatalyst can be added to the reactor directly, a more efficient method of modifying the catalyst (which requires the use of less cocatalyst to achieve a more effective response) is to contact the catalyst and cocatalyst in the absence of monomer in a pre-contacting vessel. This procedure allows the catalyst and cocatalyst to react with each other when the concentrations are higher. Although the reduced catalyst, Cr(II) /silica, is more reactive with the cocatalyst, the hexavalent form, Cr(VI) /silica is also capable of reaction. 

This phenomenon is rather specific. That is, a Cr/silica catalyst is preferred, not a Cr/silica-titania or a Cr/silica-alumina. It must be activated at a high temperature or treated with fluoride, perhaps to reduce potential ligands. Then it must be reduced in CO. When contacted with some cocatalysts, especially aluminum alkyls, the catalyst then becomes highly sensitive to H2. As illustrated in Table 60, in this series of experiments there was a huge jump in MI only when all of these treatments were combined. Activation at 600 °C does not work unless the catalyst also contains fluoride. Activation at 800 °C is effective without fluoride, but the effect is more pronounced with fluoride. The data shown in Figure 214 illustrate the huge shift in the MW distribution resulting from this combination of catalyst and reaction variables. 

In another experiment, the order of contacting was reversed. Another sample of the catalyst was charged to the empty reactor followed by the same amount of cocatalyst. Isobutane was then added, and finally, after about 5 min, ethylene was also added. One hour later, polymerization was stopped and the reactor was opened to allow recovery of the polymer. This time it contained 2.26 mol% branching and had a density of... 

The formal contact ion pair is the catalyst. This speculation is supported by the fact that other organoalumoxanes, like ethylalumoxane, are not suited as cocatalysts. Even if they can form cages, the corresponding monomeric aluminum triorganyls are too bulky to fit into these cages. The less the ion interactions, the better the catalyst s activity. Bulky ligands at the transition metal can indeed keep the MAO anion at a certain distance and produce a more or less naked metallocene monomethyl cation. As a consequence, the activity can be increased by factors of 5 or 6 (see section III.3.6, Figure 15). 

Toxicology Toxic by inh., ing., skin contact target organs liver, kidneys, bladder Precaution Pyrophoric spontaneously flamm. in air reacts violently with water Storage Handle and store under nitrogen Uses Cocatalyst for single-site catalyst systems for polymer prod. 

The requirement for an external donor when using catalysts containing an ester as internal donor is due to the fact that, when the catalyst is brought into contact with the cocatalyst (most commonly AlEts), a large proportion of the internal donor is lost as a result of alkylation and/or complexation reactions. In the absence of an external donor, this leads to poor stereoselectivity due to increased mobility of the titanium species on the catalyst surface [360]. 

In contrast to ester internal donors, the diethers, having greater affinity towards MgCU than towards AIR3, are not displaced from the catalyst surface on contact with the cocatalyst [361]. Consequently, highly isotactic poly(propene) can be obtained even in the absence of an external donor. 

Several alcohol-, ether-, acid-, ester- and ketone-functional alkenes (Fig. 12) were tested as comonomers in polymerization experiments [19,21] A bridged zirconocene complex rac-Et(Ind)2ZrCl2 was selected as catalyst for the studies because it is a relatively good copolymerization catalyst and capable of both ethylene and propylene polymerizaticais. MAO was used as cocatalyst. MAO and the comonomers were pre-contacted for 15 min in the reactor just before the start of the polymerization. 

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