Siloxy-substituted group IV metallocene catalysts

The synthesis, characterization and olefin polymerization behavior of siloxy functionalized group IV bis(indenyl)-type metallocene catalysts is reviewed. Ethylene bridged 2- and 1-siloxy-substituted bis(indenyl) ansa-metallocenes are commonly obtained in moderate yields and high excess of the desired racemic diastereomer. In combination with methylaluminoxane (MAO) or other activators, the 2-substituted complexes form highly active catalyst systems for isospecific polymerization of propylene, homopolymerization of ethylene and copolymerization of ethylene with higher alpha-olefins. The 1-siloxy substituted catalysts are aspecific and show low activities in polymerization of propylene but show high activities in homo- and copolymerization of ethylene. The active catalysts can be generated with unusually low [Al]:[M] ratios (M = Zr, Hf), commonly ranging from (100-250):1 as compared with the conventional metallocene/MAO catalyst systems. The observations are attributed to the stabilizing and electron-donating effect from the sterically bulky Lewis-basic siloxy-substituent.