Metallocene-Catalyzed Alkene Polymerization and the Observation of Zr-Allyls

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 103; no. 42; pp. 15349 - 15354
• Main Authors: Landis, Clark R., Christianson, Matthew D.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 17.10.2006; National Acad Sciences
• Series: Polymerization Special Feature
• Subjects: Active sites; Alkenes; Anions; Binding sites; Catalysts; Kinetics; Materials; Metallography; Metathesis; Physical Sciences; Polymerization; Polymerization Special Feature; Polymers; Rapid quenching; Reaction kinetics; Studies; Temperature effects
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0602704103

Single-site polymerization catalysts enable exquisite control over alkene polymerization reactions to produce new materials with unique properties. Knowledge of catalyst speciation and fundamental kinetics are essential for full mechanistic understanding of zirconocene-catalyzed alkene polymerization. Currently the effect of activators on fundamental polymerization steps is not understood. Progress in understanding activator effects requires determination of fundamental kinetics for zirconocene catalysts with noncoordinating anions such as [B(C₆F₅)₄⁻. Kinetic NMR studies at low temperature demonstrate a very fast propagation rate for 1-hexene polymerization catalyzed by [(SBI)Zr(CH₂SiMe₃)][B(C₆F₅)₄] [where SBI is rac-Me₂Si(indenyl)₂] with complete consumption of 1-hexene before the first NMR spectrum. Surprisingly, the first NMR spectrum reveals, aside from uninitiated catalyst, Zr-allyls as the sole catalyst-containing species. These Zr-allyls, which exist in two diastereomeric forms, have been characterized by physical and chemical methods. The mechanism of Zr-allyl formation was probed with a trapping experiment, leading us to favor a mechanism in which Zr-polymeryl undergoes α-H transfer to metal without dissociation of coordinated alkene followed by σ-bond metathesis to form H₂ and Zr-allyl. Zr-allyl species undergo slow reactions with alkene but react rapidly with H₂ to form hydrogenation products.