Kinetics of Styrene Polymerization to Syndiotactic Polystyrene over Metallocene Catalyst on Flat Surface, Silica Nanotube Reactors and Porous Silica Particles

The geometry of a catalyst support has a strong influence on the catalytic activity of heterogeneous metallocene catalysts for polymerization of o.-olefins and styrene. In this study, the catalytic activities of Cp*Ti(OCH3)(3)/MAO catalyst for styrene polymerization to syndiotactic polystyrene (sPS) have been investigated using three different types of support materials with different geometries: flat surface derived from a silicon wafer, cylindrical pores in silica nanotube reactor arrays, and random tortuous and interconnected pores in porous silica particles. With the same catalyst immobilization technique applied to these support materials, the highest titanium loading per unit support surface area has been obtained with a flat surface catalyst whereas porous silica particles of 15 am average pore size has the lowest titanium loading per pore surface area. The metallocene catalyst supported on a flat silica surface also exhibited the highest catalyst activity per mole of titanium among different types of supported catalysts investigated in this study. The flat surface catalyst renders minimal mass transfer resistance for the catalytic components (metallocenes and MAO) as well as monomer and all the active sites are fully exposed to monomer and available for polymerization. The relatively low catalyst activity of the silica particle supported catalyst is attributed to the limited exposure of active sites to cocatalyst as well as monomer because of geometric obstructions. The catalyst deactivation kinetics for the different types of supported catalysts was very similar in the early stage of polymerization and fitted well by the first order deactivation kinetics with about 20 min of catalyst half-life. The sPS synthesized with all these supported catalysts show that sPS molecules assemble to a bundle of nanofibrils of about 30 nm in diameter.