Single‐Site Catalysts

Abstract

The recent rapid development of organometallic single‐site catalysts (SSCs) has revolutionized polymer synthesis and polyolefin production technologies. These well‐defined catalyst systems generally consist of a transition metal atom, such as titanium, zirconium, iron, nickel, or palladium, complexed with an organic ligand set and an initiating group(s). Activation with methylaluminoxane or other weakly coordinating anions generates the cationic active species responsible for olefin coordination and polymer chain growth. Representative examples of SSCs include the bis(cyclopentadienyl)‐type Group 4 metallocenes 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, catalyst generation, polymerization of ethylene, propylene, cycloolefins, dienes, styrene, as well as as functional olefins, and the application of SSCs in enantioselective polymerization are covered. Finally, heterogenization of SSCs for use in industrial processes is briefly discussed.

C–H Activation and Olefin Insertion as Sources of Multiple Sites in Olefin Polymerization Catalyzed by CpAlkylHf(IV) Complexes

“…The catalytic performance of metallocenes is primarily determined by the well-defined Cp-ligand structure, and modulation of the steric and electronic properties of the active site allows the production of tailored polymers. , Compared to heterogeneous Ziegler–Natta and Chrome catalysts, which typically generate broad and/or multimodal polymer compositions, metallocenes and other molecular catalysts can produce polyolefins with a narrower molecular weight distribution (MWD) and chemical composition distribution (CCD) . Such catalysts are usually classified as “single site catalysts” . However, despite the moniker, molecular catalysts often produce polymers or copolymers with broad/multimodal MWD and CCD.…”