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In advanced polymer synthesis, metallocene polymerization is a coordination polymerization in which an olefin is enchained at a well-defined sandwich complex of an early transition metal, typically a Group 4 metallocene or ansa-metallocene, after activation to a cationic metal–alkyl species. The concept is bounded by the combination of a bent-metallocene or constrained-geometry precursor, a weakly coordinating counteranion generated by a cocatalyst, and a migratory-insertion mechanism that places each monomer into a metal–carbon bond. Essential features include a single-site active center whose ligand geometry dictates stereochemical control, and an ion-pair environment in which the anion remains associated with the cationic metal without occupying a coordination site required for monomer binding ^[1][2]. The process is distinguished from classical multi-site Ziegler–Natta polymerization by the molecularly defined ligand sphere and the resulting uniformity of active sites, and from late-metal olefin polymerization by the metallocene architecture and the typical use of methylaluminoxane or related borate activators ^[3]. Conceptual limits exclude polymerizations in which the metal is not bound to cyclopentadienyl-type ligands and in which chain growth does not proceed by coordination–insertion.