Chain walking
In polymer chemistry, chain walking (CW) or chain running or chain migration is a mechanism that operates during some alkene polymerization reactions. CW can be also considered as a specific case of intermolecular chain transfer (analogous to radical ethene polymerization). This reaction gives rise to branched and hyperbranched/dendritic hydrocarbon polymers. This process is also characterized by accurate control of polymer architecture and topology.[1] The extent of CW, displayed in the number of branches formed and 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.[2]
Catalysts[edit]
Catalysts that promote chain walking were discovered in the 1980-1990s. Nickel(II) and palladium(II) complexes of
Mechanism[edit]
CW occurs after the polymer chain has grown somewhat on the metal catalyst. The precursor is a 16 e− complex with the general formula [ML2(C2H4)(chain)]+. The ethylene ligand (the monomer) dissociates to produce a highly unsaturated 14 e− cation. This cation is stabilized by an agostic interaction.
This process, a step in the chain walk, moves the metal from the end of a chain to a secondary carbon center. At this stage, two options are available: (1) chain walking can continue or (2) a molecule of ethylene can bind to reform the 16e complex. At this second resting state, the ethylene molecule can insert to grow the polymer or dissociate inducing further chain walking. If many branches can form, a hyperbranched topology results. Therefore, ethene only homopolymerization can provide branched polymer whereas the same mechanism leads to chain straightening in
References[edit]
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