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

21 July 2022, Version 2
This content is an early or alternative research output and has not been peer-reviewed by Cambridge University Press at the time of posting.

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 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.

Keywords

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

Supplementary materials

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Supporting Information
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Methods and additional data.
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