Silica Nanofoam (NF) Supported Single- and Dual-Site Catalysts for Ethylene Polymerization with Morphology Control and Tailored Bimodal Molar Mass Distributions

Abstract

Highly active single- and dual-site catalysts supported on silica nanofoams (NF) enable the control of both polyethylene (PE) morphology and tailoring of bimodal PE molar mass distribution in catalytic ethylene polymerization. In a templating process, aqueous polystyrene (PS) nano suspensions are mineralized and calcinated at 600 °C, thus producing NF with specific surface area of 1200 m2/g and average pore diameter varying between 20 and 80 nm. Mineralization of the aqueous PS nano suspensions in a water-in-oil emulsion affords spherical NF with average diameter of 40 μm and control of pore size. The methylalumoxane- (MAO-) tethered NF immobilizes single-site catalysts such as metallocene (nBuZr), bisiminopyridine iron(II) (Fe) and constrained geometry chinolyl cyclopentadienyl chromium(III) complexes (Cr-1). Immobilization of binary blends of Fe/Cr-1 and nBuZr/Cr-1 affords NF-supported dual-site catalysts. The catalyst activity, PE particle size and molecular weight distribution varies as a function of the NF pore size. Typically, macroporous NF (pore size of 75 nm) are effective supports for Cr-1, producing ultrahigh molecular weight polyethylene (UHMWPE, Mw > 106 g/mol). Dual -site catalysts such as nBuZr/Cr-1 on mesoporous NF (pore size of 20 nm) enable tailoring of bimodal PE molar mass distribution with UHMWPE content increasing with increasing Cr-1/nBuZr molar ratio.