Observation and analysis of a slow termination process in the atom transfer radical polymerization of styrene

Under conditions in which the rate of polymerization is slow, we have observed the slow elimination of HBr from the polymer endgroups in the ATRP of styrene. Experimental evidence indicates that this process is likely due to the solvent effect on the stability of 1-PEBr at 110 degrees C. A second elimination reaction was observed in a hydrocarbon solvent. The major contribution to the second elimination process comes from the reaction of the Cu(II) species, formed after atom transfer, with the growing polymeric radical which presumably occurs via a one electron oxidation process. The bimolecular rate constant for the reaction of the growing polymeric radical with the Cu(II) species in an atom transfer process is approximately 10(3) to 10(4) times greater than for the same in a termination process. Thus, the chemoselectivity for atom transfer is very high, and the effect of this termination reaction is minimal under conditions in which the concentration of monomer is high and the concentration of Cu(II) species is at the minimum necessary to ensure good molecular weight control. These data also suggest that effect of this reaction is negligible for styrene polymerizations yielding low molecular weight polymer and that it should result in an upper molecular weight limit to styrene ATRP. (C) 1997 Elsevier Science Ltd.