A study of the catalyst formed when reacting lithium-alkyls with Cr(II)/SiO2:: ethylene polymerisation, diffuse reflectance infrared Fourier transform spectroscopy and gas chromatography results

The Cr(II)/SiO2 catalyst modified by tert-butyl lithium and trimethylsilylmethyl-lithium have been used in homopolymerisations of ethylene. Both catalysts produces branched polyethylene by incorporating oligomers formed in situ, but the number of branches are different for the two modifiers. The surface species formed in the reaction between the catalysts and the lithium-alkyl have been studied by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and by analyses of reaction products by gas chromatography. The lithium-alkyls react primarily with silanol groups and with siloxane bridges, but some also react with active chromium sites. For the tert-butyl lithium modified catalyst, isobutene pi-bonded to chromium sites are observed, which is believed to be the result of a reaction between tert-butyl lithium and a chromium-oxygen bond followed by a beta-hydrogen elimination. The reaction between lithium-alkyl and a chromium-oxygen bond is suggested to be responsible for the formation of oligomer producing sites for these catalysts, and the different behaviour of the two modified catalysts is a consequence of the different reactivities of the two Lithium-alkyls. (C) 1998 Elsevier Science B.V. All rights reserved.