Polymerization catalysis

Organometallic catalysts play an important role in the polymerization of ol efins, dienes and styrene. A great part of the chemical industry is using t hese catalysts to produce increasing amounts of plastic materials. Catalyst s such as metallocenes, half-sandwich, nickel and iron complexes, are able to tailor the microstructure of the polymer chain. New properties are obtai ned. Metallocenes in combination with methylalumoxane or perfluorinated ber ates as cocatalyst form extremely active catalysts for the polymerization o f olefins and are now used in industrial processes. Meanwhile a great numbe r of symmetric and chiral zirconocenes have been synthesized that give tota lly different structures of isotactic, syndiotactic, stereoblock, or isoblo ck polymers. The isotactic sequence length of polypropylene is influenced b y the nature of the ligands of the metallocenes. It has become possible to polymerize cyclic olefins, like cyclopentene or norbornene, with different zirconocenes or nickel and palladium catalysts without any ring opening rea ction. The crystalline polycycloalkenes show extremely high melting points which are between 400 and 600 degreesC. Copolymers of cycloalkenes with eth ene are amorphous and transparent with a glass transition temperature betwe en 120 and 160 degreesC and they can be used as materials for optical discs and fibers. Half-sandwich titanocenes are able to catalyze the formation o f syndiotactic polystyrene. (C) 2000 Elsevier Science B.V. All rights reser ved.