An IR study of the polymerization mechanism of the Phillips catalyst

Abstract

The IR investigation of the catalytic center of the chromium(II) surface compound on silica gel (the active center of the Phillips catalyst) showed a broad band at 2750 cm −1 (previously observed by other scientists), assigned to the stretching vibrations, and a new IR band at 1448 cm −1, assigned to the deformation vibration of a methylene group at the chromium(II) surface ion ((-O-) 2Cr-CH 2-R, carbane polymerization mechanism). The known alkylchromium dichloride complexes (Cl 2Cr-R(THF) 3), which are the best models at the moment for the Phillips catalyst, show C-H vibrations from the alkyl group between 2790 and 2800 cm −1. On adsorption of CO on the chromium(II) surface compound, the CO stretching vibration is shifted to 2176 cm −1 and the above two IR bands are replaced by normal methylene stretching vibrations (2852 and 2919 cm −1) and by a methylene deformation vibration at 1466 cm −1. At higher polymerization temperatures (above 150 °C) methyl groups (bands at 2879, 2962, 1456 and 1379 cm −1) are observed in the produced polymer and also after reaction with CO. Adsorbed CO has now a stretching vibration of 2171 cm −1. The latter results are explained by a carbene polymerization mechanism (CRCH-R-CH 3). The increase of the polymerization rate with time in the industrial polymerization is explained by a slow conversion of the polymerization sites from a carbane (slow polymerization) to a carbene polymerization mechanism (faster polymerization).