Issue 37, 2022

Union carbide polymerization catalysts: from uncovering active site structures to designing molecularly-defined analogs

Abstract

The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of their active sites. Based on a combination of IR, EPR spectroscopies, labeling experiments, and DFT modeling, we identified monomeric surface-supported Cr(III) hydrides, ([triple bond, length as m-dash]SiO)Cr(Cp)–H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involve a C–H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(III) alkyl species that insert ethylene through a Cossee–Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analog, CpCr(CH(SiMe3)2)2 grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure to the UC catalyst, further supporting the proposed active site structure.

Graphical abstract: Union carbide polymerization catalysts: from uncovering active site structures to designing molecularly-defined analogs

Supplementary files

Article information

Article type
Edge Article
Submitted
30 Jul 2022
Accepted
01 Sep 2022
First published
15 Sep 2022
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2022,13, 11091-11098

Union carbide polymerization catalysts: from uncovering active site structures to designing molecularly-defined analogs

D. Trummer, A. G. Nobile, P. Payard, A. Ashuiev, Y. Kakiuchi, D. Klose, G. Jeschke and C. Copéret, Chem. Sci., 2022, 13, 11091 DOI: 10.1039/D2SC04235E

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