Metal-mediated reactions irreversibility

The simple coordination chemistry characteristic of the majority of protein-metal interactions is replaced in certain cases by irreversible covalent modifications of the protein mediated by the metal ion. These modifications are essential for the function and are templated by the structure of the protein, as no other proteins are required for the reaction to occur. These self-processing reactions result in the biogenesis of redox cofactors in some enzymes (amine oxidases, galactose oxidase, cytochrome c oxidase) and activation of hydrolytic sites in others (nitrile hydratase). The active sites of all of these enzymes are bifunctional, directing not only the catalytic turnover reaction of the mature enzyme but the modification steps required for maturation. 

Shaver and coworkers [319] investigated the mechanism of bis(imino)pyridine ligand framework for transition metal systems-mediated polymerization of vinyl acetate. Initiation using azobisisobu-tyronitrile at 120°C results in excellent control over poly(vinyl acetate) molecular weights and polymer dispersities. The reaction yields vanadium-terminated polymer chains which can be readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible halogen transfer from the parent complex to a radical derived from azobisisobutyronitrile generates the active species. 

ATRP is a catalytic process and can be mediated by many redox-active transition metal complexes. The most frequently used metal is Cu however, ATRP has also been successfully carried out using Ru, Fe, Mo, Os, etc. [7]. The key limitation of normal ATRP (as it was initially defined) is the large amount of catalyst loading (up to ca. 1 mol%) compared with monomer. This residual metal creates difficulties in purification of the final product [8]. Also, in ATRP, as in any radical process, radical termination occurs but involves only about 1-10% of all chains. Radical termination leads to irreversible transformation of a fraction of the activator to deactivator, leading to a decrease in the reaction rate. 

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