ELECTRONIC EFFECTS IN ZIEGLER-NATTA POLYMERIZATION OF PROPYLENE AND ETHYLENE USING SOLUBLE METALLOCENE CATALYSTS

The polymerization of ethylene was studied by using a series of (eta-6-5,6-X2C9H5)2ZrCl2 catalysts (4a, X = H; 4b, X = CH3; 4d, X = OCH3; 4e, X = Cl), and the polymerization of propylene and ethylene was studied by using a number of the corresponding racemic, ethylene-bridged analogues (5a, X = H; 5b, X = CH3; 5d, X = OCH3). The presence of electron-withdrawing substituents on the indenyl ring in catalysts 4a-e led to a decrease in catalytic activity for ethylene polymerization and a decrease in polymer molecular weights. Ethylene-bridged catalysts produced lower molecular weight polyethylene than their unbridged counterparts but possessed similar activities toward ethylene. Similar trends in catalytic activity toward propylene were observed by using the ethylene-bridged catalysts, but the molecular weight of polypropylene was not as sensitive to changes in electron density at the metal center. Surprisingly, the stereoselectivity of propylene insertion was sensitive to electronic effects; an increase in electron density at the metal center led to a decrease in the stereoselectivity of propylene insertion.