Penultimate Effect in Ethylene−Styrene Copolymerization and the Discovery of Highly Active Ethylene−Styrene Catalysts with Increased Styrene Reactivity

For the first time commercially relevant catalysts for the copolymerization of ethylene and styrene
have been identified. The catalysts maintain very high copolymer efficiencies at relatively high reactor
temperatures without sacrificing styrene comonomer reactivity. The observations which led to this discovery
are based upon the kinetic analysis of ethylene−styrene copolymerization using constrained geometry
catalyst (η5-C5Me4)(SiMe2-N-t-Bu)TiMe2 (1). This analysis revealed a substantial styrene penultimate
monomer effect. Inherent reactivity of 1 toward styrene is greatly improved when the penultimate monomer
on the growing polymer chain is styrene rather than ethylene. The presence of a penultimate styrene effect
led to the hypothesis that catalysts bearing aromatic moieties in close proximity to the active site could
lead to enhancement of styrene reactivity for this catalyst family. This hypothesis was born out by two new
constrained geometry catalysts, one having two phenyl substituents placed in the 3 and 3‘ positions of the
Cp ring (2) and the other with a 2,2‘-biphenyl fragment attached to the Cp ring (3). Both catalysts exhibit
higher activity than that of 1 and, more importantly, much higher styrene reactivity leading to copolymers
with substantially increased styrene content (21.5% for 2, 30.6% for 3) as compared to 1 (11%) under the
same polymerization conditions. Analysis of the X-ray crystal structures of 2 and 3 shows no overriding
structural arguments for the increased performance. Outstanding polymerization characteristics achieved
with 3 make this catalyst a candidate for commercial production of ethylene−styrene resins in a solution
process.