Monocyclopentadienyl trialkoxy compounds of zirconium and hafnium [CpZr(OR)(3) and CpHf(OR)(3)], which cannot be prepared by earlier methods, are synthesized in situ by reacting bis(cyclopentadienyl) magnesium with Zirconium tetraalkoxides [Zr(OR)(4)] and with hafnium tetraalkoxides [Hf(OR)(4)], respectively. The generation of CpZr(OR)(3) and CpHf(OR)(3) compounds involves a highly effective cyclopentadienyl ligand (Cp) transfer reaction from the Cp ligand of bis(cyclopentadienyl) magnesium to Group 4 metal alkoxides. The utilization of CpZr(OR)(3) and CpHf(OR)(3) compounds as catalyst precursors for mediating alpha-olefin polymerizations was carefully investigated. Results show that highly active catalysts can be generated by reacting these mono-Cp Zr and Hf alkoxides with methylaluminoxane. The resulting active CpZr(OR)(3)/MAO and CpHf (OR)(3)/MAO catalysts induce ethylene polymerization and ethylene/propylene copolymerization with high activity (e.g., 1820 kg PE/mole of catalyst-h). (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2040-2049, 2010