Union Carbide Polymerization Catalysts: from Uncovering Active Site Structures to Designing Molecularly-Defined Analogs

21 July 2022, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


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 its active sites. Based on a combination of IR, EPR spectroscopies, labelling experiments, and DFT modelling, we identified monomeric surface-supported Cr(III) hydrides, (≡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 involves 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 analogue, CpCr(CH(SiMe3)2)2 grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure as the UC catalyst, further supporting the proposed active site structure.


C-H activation
CO adsorption
hyperfine spectroscopy
Union Carbide catalyst
Ethylene Polymerization
Cr hydrides

Supplementary materials

Supporting Information
Methods and additional data.


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