Visualization of on-surface ethylene polymerization through ethylene insertion

Area of Science:

• Catalysis
• Polymer Chemistry
• Surface Science

Background:

• Catalytic ethylene polymerization is a cornerstone of the chemical industry, producing polyethylene.
• The widely accepted Cossee-Arlman mechanism describes chain growth via ethylene insertion into metal-carbon bonds.
• Experimental, molecular-level confirmation of this mechanism has been lacking.

Purpose of the Study:

• To provide direct, microscopic, and spatiotemporal experimental evidence for the ethylene polymerization mechanism.
• To visualize the process of ethylene polymerization at the molecular level.
• To elucidate the initiation and propagation steps in ethylene polymerization on a specific catalytic surface.

Main Methods:

• In situ visualization of ethylene polymerization using scanning tunneling microscopy (STM).
• Utilizing a carburized iron single-crystal surface as the catalyst.
• Observation of polymerization intermediates and chain growth dynamics.

Main Results:

• Ethylene polymerization was observed to occur on specific triangular iron sites at the boundary of carbide domains.
• A novel self-initiation pathway was identified, involving a surface-anchored ethylidene intermediate.
• Subsequent ethylene insertion into this intermediate was visualized, confirming chain growth.

Conclusions:

• Direct experimental evidence confirms the ethylene polymerization pathway at the molecular level.
• The findings validate and refine the understanding of the Cossee-Arlman mechanism.
• The study highlights the role of specific surface sites and intermediates in catalytic polymerization.