Ethylene and propylene were polymerized over the siloxy-substituted bis(indenyl) ansa-metallocenes, rac-[ethylenebis(2-(tert-butyldimethylsiloxy)indenyl)]zirconium dichloride (1), rac-[ethylenebis(2-(tert-butyldimethylsiloxy)-4,5,6,7-tetrahydroindenyl)]zirconium dichloride (2), and rac[ethylenebis(2-(tert-butyldimethylsiloxy)indenyl)]hafnium dichloride (3), using methylaluminoxane (MAO) and trityl tetrakis(pentafluorophenyl)borate as cocatalysts. Especially complex 1 forms a highly active catalyst precursor for the polymerization of alpha-olefins. Synthesis and characterization of 2 and 3 are described. The 1/MAO and 3/MAO catalyst systems show high polymerization activities at exceptionally low [Al] : [Zr] ratios [(100-250):1]. For both catalysts, extensive chain transfer to aluminum is observed as the predominant chain-transfer mechanism. Preliminary results from copolymerization of ethylene with 1-hexene and 1-hexadecene over 1/MAO indicate a good comonomer response.