Single‐Site Catalysts

AbstractThe recent rapid development of organometallic single‐site catalysts (SSCs) has revolutionized polyolefin research and permitted the production of new specialty polymers and commodity polyolefins with improved properties. These well‐defined catalyst systems generally consist of a transition metal atom, such as titanium, zirconium, hafnium, iron, nickel, or palladium, complexed with an organic ligand set and an initiating group. In most cases, activation with methylaluminoxane or compounds bearing a weakly coordinating anions generates cationic‐active species responsible for olefin coordination and polymer chain growth. Representative examples of SSCs include the bis(cyclopentadienyl) or bis(phenoxyimine)‐type group 4 catalysts as well as the diimine complexes of nickel and palladium. By variation of the organic ligand and thus the steric and electronic environment of the metal center, these catalysts can be tailored to control the olefin polymerization reaction in an unprecedented fashion. Almost any vinyl monomer, irrespective of molecular weight or steric hindrance, can be polymerized by choosing the proper catalyst. Virtually all feasible poly(α‐olefin) microstructures ranging from atactic to isotactic, hemiisotactic, syndiotactic, and stereoblock polymers can be produced by rational modification of the catalyst structure. Functional monomers are readily copolymerized with the less oxophilic late transition metal catalysts. Entirely new materials, not accessible with traditional Ziegler–Natta catalysts, have emerged, including high melting syndiotactic polystyrene and cycloaliphatic polymers. This article provides a broad overview of SSCs and their application in olefin polymerization. Evolution and classification of SSCs, polymerization of ethylene, propylene, as well as functional olefins, macromolecular architecture bearing polyolefin blocks are covered. Finally, heterogenization of SSCs for use in industrial processes is described.