Controlled/“Living” Radical Polymerization. Kinetics of the Homogeneous Atom Transfer Radical Polymerization of Styrene

The homogeneous atom transfer radical polymerization (ATRP) of styrene using solubilizing 4,4'-dialkyl substituted 2,2'-bipyridines yielded well-defined polymers with Mw/Mn ≤ 1.10. The polymerizations exhibited an increase in molecular weight in direct proportion to the ratio of the monomer consumed to the initial initiator concentration and also exhibited internal first-order kinetics with respect to monomer concentration. The optimum ratio of ligand-to-copper(I) halide for these polymerizations was found to be 2:1, which tentatively indicates that the coordination sphere of the active copper(I) center contains two bipyridine ligands. The exclusive role for this copper(I) complex in ATRP is atom transfer, since at typical concentrations that occur for these polymerizations (≈10-7−10-8 M), polymeric radicals were found not to react with the copper(I) center in any manner that enhanced or detracted from the observed control. ATRP also exhibited first-order kinetics with respect to both initiator and copper(I) halide concentration; however, the polymerization kinetics were not simple inverse first-order with respect to the initial copper(II) halide concentration. The latter observation was found to be due to the persistent radical effect, which resulted in an increase in copper(II) concentration during the initial stages of the polymerization. This phenomenon also has the effect of regulating the polymerization by ensuring that the rate of radical combination and/or disproportionation is sufficiently less than the rate of propagation.