Mechanism of the Comonomer Effect in LLDPE from Ethylene/1-Hexene Using a Quinoline-Amine Hafnium Catalyst

• Published in: Macromolecules Vol. 57; no. 14; pp. 6741 - 6749
• Main Authors: Speer, Joshua M., Medvedev, Grigori A., Caruthers, James M., Lamb, Jessica V., Delferro, Massimiliano, Abu-Omar, Mahdi M.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 23.07.2024
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/acs.macromol.4c00977

Polymers of ethylene and copolymers of ethylene and 1-hexene are synthesized by utilizing a single-site hafnium catalyst activated with tris­(pentafluorophenyl)­borane. Polymer production increases sharply as compared to the homopolymerization of ethylene with the addition of just a small amount of 1-hexene–a phenomenon known as the “comonomer effect.” A mechanism behind the comonomer effect is proposed where the activation-initiation step plays the key part. When only a small fraction of the precatalyst is initially active, high molecular weight (170 000 g mol–1, Đ 1.6) polymer chains grow, forming a physical gel structure that traps catalyst, monomer, and solvent molecules, thereby shutting down the reaction–this scenario plays out in the case of homopolymerization of ethylene. 1-Hexene, when present, slows down the chain growth, thereby lowering the molecular weight and preventing the formation of a gel, which in turn allows for more catalytic sites to be initiated; the reaction continues, resulting in a larger amount of polymer of lower molecular weight (25 000 g mol–1). The proposed mechanism is validated in a series of experiments where targeted variation of the activation/initiation steps is shown to produce, depending on the condition, either a small amount of high molecular weight physical gel or a large amount of low molecular weight polymer.