COMONOMER EFFECTS WITH HIGH-ACTIVITY TITANIUM-BASED AND VANADIUM-BASED CATALYSTS FOR ETHYLENE POLYMERIZATION

High-activity titanium- and vanadium-based catalysts for ethylene poly merization frequently show an increase in reaction rate in the presenc e of an alpha-olefin. The magnitude of this increase depends on the sp ecific alpha-olefin. The results show propylene > 1-butene > 1-hexene in increasing initial reaction rates. Addition of certain electron-don or compounds to these catalysts can lower the magnitude of the comonom er 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 in fluence the kinetic order of the reaction. These results can be interp reted using a mechanistic model involving two vacant coordination posi tions at the active sites. In this model electron donors and comonomer s are viewed as Lewis-base ligands which influence features of chain p ropagation and chain termination. As Lewis-base ligands, the comonomer s can also increase the number of active sites available for polymeriz ation. Catalyst deactivation following the initial co-monomer rate inc rease is believed to be caused by reaction with the Lewis bases (alpha -olefin included) in the system and by possible reduction in the oxida tion state of the metal centers. The most acidic metal centers activat ed by the comonomer are most reactive to Lewis bases and deactivate mo st rapidly. Veratrole (1,2-dimethoxybenzene) can be employed as a prob e for estimating the number of bis-vacant coordination sites in vanadi um-based catalysts. Addition of low levels of veratrole led to signifi cant deactivation of the vanadium-based catalyst. (C) 1993 John Wiley & Sons, Inc.