(764h) 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.