Alkane C–H Bond Activation by Cr(II)/Silica Polymerization Catalyst: Part 1. Tagging Initiating Species by Oxidation

Abstract In this report, the initiation of the Phillips commercial polymerization catalyst has again been probed, this time using its simpler cousin, Cr(II)/silica. This divalent catalyst is already reduced and therefore needs only an alkylation reaction to initiate the first polymer chain. Alkane and aromatic solvents, usually considered inert, were found in this study to chemisorb onto the Cr(II) catalyst and yield reactive species, possibly live alkyls. Quenching the catalyst by exposing it to dry air for 30 s generated many oxidized products, suggesting the existence of live alkyls on the Cr. Several potential explanations were considered and tested, by varying Cr‐alkane reaction order and time. Since the commercial Cr(II) catalyst is always stored in alkane prior to ethylene contact, this reaction provides a new insight into how initiation might occur industrially. The Cr(II) catalyst was also reacted with alkenes, which were then similarly air‐quenched. Once again, a preference for reaction with the allylic H was observed. Among the resulting oxidation products were found oxidized oligomers, representing oligomeric chains which were alive on the Cr at the time of air introduction. This provides a simple and effective way of counting active sites, which seem to constitute a relatively large portion of the total Cr content.