Polystyrene metallocene catalysts

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

Figure 34 A polymer-supported metallocene catalyst (51) with a weakly coordinating anion, [B(C6F5)4] , produced from lightly cross-linked, chloromethylated polystyrene beads for olefin polymerization. (Adapted from ref. 75.)...

For example, it is possible to synthesize isotactic as well as syndiotactic polypropylene in high configurational purity and high yields. The same holds for syndiotactic polystyrene. Furthermore, metallocene catalysts open the possibility to absolutely new homopolymers and copolymers like, e.g., cycloolefin copolymers (COG) and even (co)polymers of polar monomers.The simplest metallocene catalyst consists of two components. The first one is a n-complex (the actual metallocene) that can be bridged via a group X and therefore can become chiral ... 

Metallocenes are very versatile catalysts for the production of polyolefins, polystyrene and copolymers. Some polymers such as syndiotaetic polypropene, syndiotactic polystyrene, cycloolefin copolymers, optically active oligomers, and polymethylenecycloalkenes can be produced only by metallocene catalysts. It is possible to tailor the microstructure of polymers by changing the ligand structure of the metallocene. The effect and influence of the ligands can more and more be predicted and understood by molecular modeling and other calculations. 

Group 4 metallocene catalysts are, in addition to polyethylene and polypropylene, able to generate syndiotactic polystyrene, to polymerize cycloolefins (cyclopentene, nor-bomene, and their substituted compounds), and to give access to various copolymers. During the polymerization of cycloolefins, only the double bond is opened and not the ring. 

Polystyrene and olefin-styrene co-polymerization with post-metallocene catalysts 1145... 

With the fast developments in the plastic industry, some of the lesser known plastics will either find future usage or already be used for devices, general medical instruments and apparatus or as implant aids. Certain plastics now involve alloys, i.e. mixtures of thermoplastics, and thermoplastic and thermoset resins. Improvements in what were the economic five plastics, i.e. polyethylenes, polypropylenes, polyvinylchlorides, polystyrenes and polyesters, are constantly occurring. Use of metallocene catalysts is likely to produce plastics of a controlled chain length. 

Metallocene catalysts were commercialized in 1991 by Exxon Chemical for the industrial production of ethylene-propylene (EP) elastomers in solution polymerization using zirconocene catalysts [37], As a result of extensive research of different metallocenes applied to the stereoregular control of polymeric materials, these systems were able to produce novel polymers such as syndiotactic polystyrene and ethylene-styrene copolymers, which were not possible to produce with traditional Ziegler-Natta catalysts. 

Metallocene catalysts (Fig. 4.3) were first discovered in the early 1950s by Natta and Breslow, and were first used to make polyethylene in 1957. These catalysts were used to produce syndiotactic polystyrene in 1984 and syndiotactic polypropylene (FINA) in 1986. However, commercialization for polyethylene did not come until the mid-1990s, since until that time the advantages the new catalyst systems offered were not fully appreciated. Metallocene catalysts employed today commonly contain a co-catalyst to increase the catalyst activity. 

Syndiotactic polystyrene is available commercially under the trade name of Questra. This material is produced with the aid of a metallocene catalyst and is sold in several grades [181]. 

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