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Visualization of on-surface ethylene polymerization through ethylene insertion

Polyethylene production through catalytic ethylene polymerization is one of the most common processes in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be directly inserted into the metal–carbon bond during chain growth, which has been awaiting microscopic and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored ethylidene (CHCH3), serves as the chain initiator (self-initiation), which subsequently grows by ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization pathway at the molecular level. Description Imaging polyethylene synthesis The long-accepted mechanism for olefin polymerization growth is that the polymer elongates at one end through olefin insertion into the metal–carbon bond that anchors the chain to the active catalyst. Using in situ scanning tunneling microscopy, Guo et al. visualized this process for polyethylene on an iron carbide surface formed from Fe(110) (see the Perspective by Wintterlin). Triangular iron sites at the boundary between two neighboring carbide domains could oligomerize ethylene molecules at room temperature and allowed for real-time imaging of the growing chain. —PDS In situ scanning tunneling microscopy images direct ethylene insertion during polymerization on an iron carbide surface.