Dual-site supported metallocene catalyst design for bimodal polyolefin synthesis



The processability of various polyolefins may be enhanced by producing one having a bimodal molecular weight distribution. Such polymers can be produced by loading the reactor with a catalyst on which two different catalytic sites are impregnated. The catalysts must be loaded with a proper ratio between the two catalytic sites to produce the desired polymer. The total loading should enable a maximum polymer production in the specified residence time while keeping the maximum transient temperature below a specified maximum, in order to avoid local melting and sheet formation. A model is presented that enables this catalyst design based on kinetic information about the initiation, propagation, and deactivation reaction rates of the two individual catalytic sites. The model simulations provide insight about the factors affecting the maximum safe loading of the two catalytic sites. We show here how to utilize kinetic information about each of the two catalytic sites to predict the optimum loading of dual-site metallocene catalysts. As the maximum temporal temperature occurs during the initial stages of the reaction, the site that leads to a faster generation of a temperature peak, exerts a stronger bound on the total polymer production than the second site. The larger the propagation and/or the initiation rate constants or their activation energies are, the lower should be the catalyst loading. The deactivation rate constants affect both the relative site loading of the two sites as well as the total production. The total polymer production can be increased by decreasing the deactivation rate of the more active site. © 2007 American Institute of Chemical Engineers AIChE J, 2007