Molecular olefin polymerization catalysts: From metallocenes to half-sandwich complexes with functionalized cyclopentadienyl ligands

Abstract

This personal account summarizes the development of molecularly defined organometallic polymerization catalyst precursors based on cyclopentadienyl metal complexes. Departure from the bent metallocene structure provided opportunities to considerably expand the coordination sphere of early transition metal centers relevant to olefin polymerization. Thus, half-sandwich metal complexes such as those featuring linked amido-cyclopentadienyl ligands (“constrained geometry catalysts“) allowed copolymerization of ethylene with larger comonomers, while mono(cyclopentadienyl) complexes of titanium and rare earth metals offered new opportunities to develop catalysts for the stereoselective polymerization of monomers such as styrene. The quest for molecular understanding of heterogeneous Ziegler-type catalysts has inspired the organometallic chemistry of the early transition metals, initially limited to that of zirconocenes. The importance of modifying cyclopentadienyl ligands for the rational design of homogeneous polymerization catalysts is highlighted.