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 M(w)/M(n) less than or equal to 1.10. The p olymerizations exhibited an increase in molecular weight in direct pro portion to the ratio of the monomer consumed to the initial initiator concentration and also exhibited internal first-order kinetics with re spect to monomer concentration. The optimum ratio of ligand-to-copper( I) halide for these polymerizations was found to be 2:1, which tentati vely indicates that the coordination sphere of the active copper(I) ce nter contains two bipyridine ligands. The exclusive role for this copp er(I) complex in ATRP is atom transfer, since at typical concentration s that occur for these polymerizations (approximate to 10(-7)-10(-8) M ), polymeric radicals were found not to react with the copper(I) cente r in any manner that enhanced or detracted from the observed control. ATRP also exhibited first-order kinetics with respect to both initiato r and copper(I) halide concentration; however, the polymerization kine tics were not simple inverse first-order with respect to the initial c opper(II) halide concentration. The latter observation was found to be due to the persistent radical effect, which resulted in an increase i n copper(n) concentration during the initial stages of the polymerizat ion. This phenomenon also has the effect of regulating the polymerizat ion by ensuring that the rate of radical combination and/or disproport ionation is sufficiently less than the rate of propagation.