Chain walking or chain running is a mechanism that operates during some alkene polymerization reactions. This reaction gives rise to branched and hyperbranched hydrocarbon polymers. This process is also characterized by accurate control of polymer architecture and topology. The positions of branches on the polymers are controlled by the choice of a catalyst. The potential applications of polymers formed by this reaction are diverse, from drug delivery to phase transfer agents, nanomaterials, and catalysis.

Catalysts

The discovery of the catalysts used for typical chain walking reactions can be traced to the 1980-1990s. Nickel and palladium complexes of α-diimine ligands were known to efficiently catalyze polymerization of alkenes. The images on the side illustrate two catalysts that promote chain walking reactions:

Currently nickel and palladium complexes bearing α-diimine ligands, such as the two examples shown, are the most thoroughly described chain walking catalysts in scientific literature.

Mechanism

Chain walking occurs after the polymer chain has grown somewhat on the metal catalyst. This is the first resting state of the catalyst and assumes the form [Pd(diimine)(alkene)(chain)]+. The initial step in the chain walking process is then a β-hydride elimination. Before this reaction can occur, the alkene ligand (the monomer) dissociates to produce a highly unsaturated cation. This cation is stabilized by an agostic interaction of the coordinately-saturated transition metal with the C-H bond in the next carbon further down the chain. β-Hydride elimination then occurs to give a hydride-alkene complex, followed by reinsertion of the M-H in the opposite sense. This process moves the metal from the end of a chain to a secondary carbon center. At this stage a molecule of ethylene associates with the palladium complex to give [Pd(diimine)(alkene)(ethylene)]⁺. As a result this traps the catalyst at this next position on the carbon chain back-bone. At this second resting state, the ethylene molecule dissociates from the metal center and joins the carbon chain. Eventually many branches can form, thereby giving a hyperbranched topology.