Mechanistic studies of palladium(II)- and nickel(II)-catalyzed olefin polymerization

The development of cationic diimine-based palladium and nickel complexes has allowed an unprecedented opportunity to investigate transition metal-catalyzed insertion polymerization. In contrast to these late transition metal catalysts, the study of Ziegler-Natta and early metallocene polymerization systems is limited by the nature of the active species. Although the basic steps of initiation, propagation, and chain transfer are nearly identical, palladium and nickel diimine catalysts give polymers of ethylene and ;Palladium analogs, although less active than nickel for ethylene polymerization, are more robust and undergo reactions which exhibit rates ideally suited to observation and measurement using variable temperature NMR spectroscopy. For these reasons, cationic palladium alkyl complexes, including ;The structure of an agostic species lies somewhere in between an olefin hydride complex and a discrete metal alkyl complex. Stable ;Diimine complexes of palladium and nickel polymerize...