Constrained Geometry Single Site Catalysts for Olefin Polymerization

Constrained Geometry Single Site Catalysts for Olefin Polymerization

ABSTRACT

Polyolefins are commercially important plastics. These are available in compounds of widely different structure as copolymers, in polymer mixtures as filled compositions and as cross-linkable products etc. Single site polymerization catalysts, with the generic name of metallocenes, are currently used industrially to produce various polyolefins with more precise molecular architecture than those can be produced by any conventional polymerization methods like Zeigler-Natta type catalysts. Metallocenes are known for the stereoselective polymerization of a range of polymers. It is now a well-accepted fact that metallocene offers several advantages over the traditional Zeigler-Natta catalysts. Many metallocenes are up to hundred-fold more active because the initiators are homogeneous and nearly each atom of transition metal is active. Stereoreselectivity can be controlled and varied to produce different stereoregular products by appropriate choice of the metallocene ligands and reaction conditions. In single site polymerization, there are different kinetic processes, including catalyst activation and deactivation, chain propagation, chain transfer by beta-hydrogen elimination etc. These reactions are mediated by the metal/ligand/counter-ion complex, where the key challenge is to first understand, and then rationally design, the ligands and the counterion to produce a polymer with a desired molecular architecture. The major impact of Metallocene catalysts on the polyolefin industry is related to the fact that these catalysts can produce polymers with controlled microstructure. Presently polymer manufacturing companies the world over are examining the possibility of using the metallocene catalyst to produce polyolefins. In this paper, the development of the metallocene catalysts and co-catalysts from their discovery to their present state of the art is presented. This paper also focuses on various mechanistic aspects of applications of single site catalyst systems in polymer industry.