In-situ FTIR spectroscopy of key intermediates in the first stages of ethylene polymerization on the Cr/SiO2 Phillips catalyst: solved the puzzle of the initiation mechanism?

We report here the first experimental observation, by means of in situ FTIR spectroscopy, of key intermediate species in the ethylene polymerization on the Cr(II)/SiO2 Phillips catalyst. We demonstrate that by adopting suitable strategies, it is possible to shed light on one of the remaining questions concerning the system responsible for one-third of the worldwide polyethylene production: the puzzle of the initiation mechanism. "Anomalous" bands in the CH2 stretching region are observed during the first steps of the polymerization reaction and assigned to small cycles on the Cr(II) sites, characterized by a structural strain decreasing with increasing ring dimension. These intermediate species are stable only in presence of a sufficiently high C2H4 pressure and show a peculiar reactivity toward strong ligands, such as NO and O-2. These results allow us to prove that the initiation mechanism follows a metallacycle route, similar to what occurs for several ethylene trimerization and tetramerization catalysts.