Comonomer effects with high‐activity titanium‐ and vanadium‐based catalysts for ethylene polymerization

Publisher Website
Google Scholar
Abstract
High‐activity titanium‐ and vanadium‐based catalysts for ethylene polymerization frequently show an increase in reaction rate in the presence of an α‐olefin. The magnitude of this increase depends on the specific α‐olefin. The results show propylene > 1‐butene > 1‐hexene in increasing initial reaction rates. Addition of certain electron‐donor compounds to these catalysts can lower the magnitude of the comonomer effect and, in some cases, totally eliminate such an effect. Among the classes of electron‐donor compounds examined were ether‐alcohols, ether‐esters, amino‐alcohols, alkoxysilanes, siloxanes, and phosphine oxides. Reaction kinetics show that the presence of a comonomer can influence the kinetic order of the reaction. These results can be interpreted using a mechanistic model involving two vacant coordination positions at the active sites. In this model electron donors and comonomers are viewed as Lewis‐base ligands which influence features of chain propagation and chain termination. As Lewis‐base ligands, the comonomers can also increase the number of active sites available for polymerization. Catalyst deactivation following the initial comonomer rate increase is believed to be caused by reaction with the Lewis bases (α‐olefin included) in the system and by possible reduction in the oxidation state of the metal centers. The most acidic metal centers activated by the comonomer are most reactive to Lewis bases and deactivate most rapidly. Veratrole (1,2‐dimethoxybenzene) can be employed as a probe for estimating the number of bis‐vacant coordination sites in vanadium‐based catalysts. Addition of low levels of veratrole led to significant deactivation of the vanadium‐based catalyst. © 1993 John Wiley & Sons, Inc.