On the phillips catalyst site with high catalytic activity

Abstract

FT-IR spectroscopy has been employed to study two catalyst samples with O.5% Cr and O.074% Cr on silica gel in ethylene polymerization at room temperature and in the adsorption of CO. In one of the two series of experiments studied, samples were reacted with ethylene at 200 °C prior to further reaction with ethylene at room temperature. It was observed that the previously reported broad band at 2750 cm −1 arising from the hydrocarbon group connecting the living polymer to the chromium(II) site was split into two bands situated at 2768 and 2725 cm −1, respectively. These bands have been assigned to the asymmetric and symmetric stretching vibrations of the methylene group. Previous experiments had shown that CO adsorption onto the active chromium(II)-A polymerization site led to bands in the IR spectrum at 2176 and 2170 cm −1 (the latter after 200 °C/ethylene treatment) and which exhibited a shift from 2179 cm −1 for CO adsorbed on a chromium(II)-A surface species before reaction with ethylene. Similar shifts have now been observed with bands arising from CO adsorption on to chromium(II)-C surface species situated at 2187, 2181 and 2191 cm −1, respectively. A model for polymerization initiation and for the mechanism of the polymerization reaction is proposed, which involves dinuclear surface species and explains several results obtained with the (reduced) Phillips catalyst.