In this perspective, recent developments on palladium and nickel mediated chain walking olefin polymerization and copolymerization with polar functionalized comonomers are described. First, the chain walking polymerization mechanism is discussed followed by its implications in olefin polymerization and copolymerization. Then, recent advances in catalyst design are provided. Special attention is paid to the influence of ligand structures on the catalytic properties. Subsequently, the applications of these chain walking polymerization catalysts in the synthesis of functionalized hyperbranched polymers and copolymers are summarized. Finally, some recent developments and perspectives on very fast and very slow chain walking polymerization catalysts are discussed.

A series of sterically demanding α-diimine ligands bearing electron-donating and electron-withdrawing substituents were synthesized by an improved synthetic procedure in high yield. Subsequently, the corresponding Pd complexes were prepared and isolated by column chromatography. These Pd complexes demonstrated unique properties in ethylene polymerization, including high thermal stability and high activity, thus generating polyethylene with a high molecular weight and very low branching density. Similar properties were observed for ethylene/methyl acrylate copolymerization. Because of the high molecular weight and low branching density, the generated polyethylene and ethylene/methyl acrylate copolymer were semicrystalline solids. The (co)polymers had unique microstructures originating from the unique slow-chain-walking activity of these Pd complexes.

The synthesis, characterization, and olefin (co)­polymerization studies of a series of palladium complexes bearing phosphine phosphonic amide ligands were investigated. In this ligand framework, substituents on three positions could be modulated independently, which distinguishes this class of ligand and provides a great deal of flexibilities and opportunities to tune the catalytic properties. The palladium complex with an o-MeO-Ph substituent on phosphine is one of the most active palladium catalysts in ethylene polymerization, with 1 order of magnitude higher activity than the corresponding classic phosphine-sulfonate palladium complex. Meanwhile, the polyethylene generated by this new palladium complex showed ca. 6 times higher molecular weight in comparison to that by the classic phosphine-sulfonate palladium complex. In ethylene/methyl acrylate copolymerization, the new palladium complex showed lower activity, generating copolymer with similar methyl acrylate incorporation and much higher molecular weight. The new palladium complex was also able to copolymerize ethylene with other polar monomers, including butyl vinyl ether and allyl acetate, making it one of the very few catalyst systems that can copolymerize ethylene with multiple industrially relevant polar monomers.