Catalyst metallocene, processing

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Attempts to produce coplymers of ethylene and styrene by free radical and by conventional Ziegler-Natta catalysts systems have, over the years, not proved successful. However, in 1998 Dow announced novel polymers using a metallocene process with the intention of commercial polymerisation using a 23 000t.p.a. capacity plant starting in 1999. 

With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

Based on an early process discovered by Natta in the 1950s, soluble transition metal catalysts like metallocenes were developed mainly in the 1950s as initiators for polyolefin syntheses. Others are still now under investigation, like the so-called LTM (for "late transition metal") catalysts. Metallocenes seem... 

Production problems with current metallocene-aluminoxane catalysts reflect the high aluminoxane/metal ratio needed for good catalyst productivity and stereochemical control. Aluminoxane production is slow and relatively inefficient and the large amounts used make post-polymerization catalyst removal processes necessary. These deficiencies are expected to be remedied by different cocatalysts, which are being disclosed in the patent literature. 

To further examine the effects of varying triaryl-borane Lewis acidity on the metallocene catalyst activation process, a series of triarylbones in which two of the aryl groups are CeFs and the third is varied has been synthesized. The triarylborane ArB(C6Fs)2 (76) substituents (at 3.5-positions substituted with... 

All olefin polymerization processes described in Section 2.5 have been tested, and some are being operated commercially with metallocene catalysts. Metallocenes can be used directly in solution processes but need to be supported to be used in slurry and gas-phase processes. In the latter case, the support of choice is Si02. There are several supporting techniques for metallocenes, but this subject is beyond the scope of this chapter [24]. 

For homogeneous catalysts, the process of polymer particle formation generally leads to porous, low-density polymer particles, which can cause significant increase in slurry viscosity and reactor fouling, leading to inadequate reactor temperature control. Additionally, polymer particles with poor powder properties are undesirable for postreactor polymer processing. These problems must be addressed before using soluble metallocene catalysts for industrial production of polyolefins. 

Commercially, Dow appears to have implemented activation by well-defined cocatalyst for its Insite solution-phase metallocene process. Aside from historical priority, one reason for the persistent use of MAO is that the perfluorinated boron compounds are difficult to prepare, thus raising their expense. (One of the intermediates, (CeFslLi, must be handled at very low temperatme to prevent violent decomposition.) Another is that, unlike the MAO systems, catalysts activated by discrete activators have no large excess of alkylaluminum to scavenge poisons from the reaction medium, while common scavengers may hinder activity by consuming the expensive activator. 

Rohrmann, J. Bridged chiral metallocenes, process for their preparation and their use as catalysts. European Patent 0528287B1 (Targor GmbH), November 11, 1998. 

FIG. 36 Metallocene catalysis is just one body of activity, albeit a very large one, that is currently influencing the market. Developers of technology have already begun to separate catalyst and process. 

Key processes, in use today, can be retrofitted with metallocenes to produce designed polymers. The catalyst can be used in solution or slurry processes. It can also be used in gas-phase processes. High catalyst productivity minimizes catalyst removal needs and attendant waste streams. Metallocene catalysts offer process optimization opportunities that benefit both the resin supplier and the convertor. In Table 14a, the demand for metallocene-catalyzed polyolefins, polystyrene, and copolymers has been projected up to the year 2010. 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

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