page_title: Chain walking
section_title: Mechanism
hierarchical_section_title: Chain walking / Mechanism
caption_attribution_description: English: Structure of polymers obtained in CW polymerization of ethene vs. higher olefins
context_page_description: In polymer chemistry, chain walking 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. This reaction gives rise to branched and hyperbranched/dendritic hydrocarbon polymers. This process is also characterized by accurate control of polymer architecture and topology. 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.
context_section_description: 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. β-Hydride elimination then occurs to give a hydride-alkene complex. Subsequent reinsertion of the M-H into the C=C bond, but in the opposite sense gives a metal-alkyl complex.  
  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 α-olefin polymerization. The variation of CW by changing T, monomer concentration, or catalyst switch can be used to produce block copolymer with amorphous and semi-crystalline blocks or with blocks of different topology.
caption: Structure of polymers obtained in CW polymerization of ethene vs. higher olefins
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