Investigations of Chain Shuttling Olefin Polymerization Using Deuterium Labeling

Abstract

Chain shuttling among olefin polymerization catalysts with differing monomer responses has been developed for the preparation of block copolymers with enhanced performance. The chain shuttling agent (CSA) is usually a main group alkyl complex that is able to exchange polymeryl chains with the catalysts. We have performed a diverse yet complementary set of polymerization experiments using a hafnium pyridyl−amide catalyst (1) and Oct3Al as CSA. Ethylene/1-hexene copolymerization experiments using different amounts of CSA were quenched with D2O after varied reaction times. In ethylene homopolymerizations with fairly constant polymer yields, Mw/Mn is less than 2 and decreases monotonically with the addition of CSA, indicative of chain shuttling. However, less than half of the available Al sites are occupied by polymer chains. This indicates that chain shuttling (polymeryl−polymeryl exchange) is kinetically competitive with chain transfer (polymer−alkyl exchange) in this system. This approach also allows a more complete accounting of the fates of each polymeryl and alkyl chain by using multiple characterization techniques (GPC, GC, and 2H NMR). The mass balance agreement among these techniques is excellent, which validates our techniques and our basic understanding of chain shuttling processes. The results indicate that chain transfer to aluminum is the dominant termination mechanism under these conditions. Furthermore, 2H and 13C NMR experiments indicate that chain transfer from 1 to Oct3Al occurs significantly faster after an ethylene insertion than after a hexene insertion.