ETHYLENE POLYMERIZATION WITH HYDRIDOTRIS(PYRAZOLYL)BORATONIOBIUM COMPLEXES AS PRECURSORS

Substituted hydriodotris(pyrazolyl)borato (Tp') niobium complexes of t he type [Tp'NbMe2(PhC=CMe)] polymerize ethylene (under 1 atm, at room temp. and in toluene) when treated with an equimolar amount of B(C6F5) (3). A clear relationship is observed between the stereoelectronic nat ure of the complex and catalytic activity. This is discussed on the ba sis of electrochemical data for the dichloro complexes [Tp'NbCl2(PhC=C Me)], and the rates of alkyl migration in [Tp'Nb(Cl)(R)(PhC=CMe)] (R = CH2Me, CH2SiMe3). For a given electron density at the metal, the bulk y Tp(Me2,4-Cl) yields a catalyst which is more than 6 times as active as the unsubstituted Tp based catalyst.