Atom transfer radical polymerization (ATRP) can be successfully carried out at low catalyst concentration in the presence of both non-radical or radical-based reducing agents. To be able to conduct controlled polymerizations, several important requirements should be met. The catalytically active complexes in both oxidation states should be very stable, even in the presence of excess destabilizing compounds, such as monomers and reducing agents or the products of their oxidation. The ratio of the stability constants of the higher to that of the lower oxidation state complexes should be large. The association constant of halide ions to the higher oxidation state metal complex (halidophilicity) should also be large, which guarantees both high catalytic activity and efficient radical deactivation. Reducing agents should be chosen that can only regenerate the lower oxidation state activating complex, but that cannot participate in side reactions with the monomer, polymer, or other reaction components. Only highly active ATRP initiators afford well-defined polymers with molecular weights close to the expected ones based on the monomer to initiator ratio.
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