ABSTRACT
The advent of homogeneous metallocene catalysis has allowed chemists to make significant progress in understanding the steric, electronic, and dynamic structures of the active organometallic species in olefin polymerizations, as described in previous chapters. The polymer structures obtained using these well-defined homogeneous systems can be qualitatively related to the structures of the metallocene catalysts, and vice versa. This significant achievement has stimulated many researchers to develop nonmetallocene olefin polymerization catalysts, that is, catalysts that do not have any η5-cyclopentadienyl (Cp) type ligands. Drawing on the knowledge obtained from metallocene catalysts, many electrophilic (cationic) nonmetallocene complexes based on both early and late transition metals have been investigated as potential olefin polymerization catalysts.1,2 Propylene polymerization has been examined in those cases where the steric properties of the catalyst were expected to provide stereoselective polymerization. These nonmetallocene catalysts generally have ligands containing oxygen and/or nitrogen (sometimes sulfur and phosphorus), and these heteroatoms are usually incorporated into a chelate ligand structure. Even though many well-defined nonmetallocene catalysts have been developed since the late 1980s, their use to achieve propylene polymerization with comparable stereoselectivity to heterogeneous Ziegler-Natta and chiral ansa-metallocene catalysts has remained a challenge, until quite recently.
