Steady-state model for olefin polymerization with a two-site vanadium catalyst in a continuous stirred-tank reactor

In this study, a process for continuous EP(D)M production is examined and a mechanistic kinetic model is developed to explain the behavior exhibited by this vanadium-catalyzed solution polymerization process. The catalyst system without promoter and without hydrogen, produces polymer with bimodal molecular weight distributions (MWDs), while the addition of catalyst promoter causes an order of magnitude increase in catalyst productivity and eliminates the higher-MW component in the MWD. The addition of hydrogen also precludes bimodal MWDs, regardless of the presence of promoter. In all cases, the polymerization rate has a zero-order rather than a first order response to monomer concentrations. The zero-order response of polymerization rate to monomer concentration is described using a mechanism of monomer coordination to form a stable complex prior to insertion. The bimodal MWDs at high monomer feeds (corresponding to low monomer conversion), in the absence of catalyst promoter and hydrogen, are explained by a two-site type catalyst model in which both monomer insertion and the formation of the second-site type occur after the monomer forms a stable coordinated complex with the first catalyst site type. The model reconciles the molecular weight development with these seldom-discussed features of vanadium catalysis.