Copper(0)-mediated radical polymerization (single electron transfer-living radical polymerization) is an efficient polymerization technique that allows control over the polymerization of acrylates, vinyl chloride and other monomers, yielding bromide terminated polymer. In this contribution, we investigate the evolution of the end-group fidelity at very high conversion both in the presence and in the absence of initially added copper (II) bromide (CuBr2). High resolution electrospray-ionization mass spectroscopy (ESI-MS) allows determination of the precise chemical structure of the dead polymers formed during the polymerization to very high monomer conversion, including post polymerization conditions. Two different regimes can be identified via ESI-MS analysis. During the polymerization, dead polymer results mainly from termination via disproportionation, whereas at very high conversion (or in the absence of monomer, that is, post-polymerization), dead polymers are predominantly generated by chain transfer reactions (presumably to ligand). The addition of CuBr2 significantly reduces the extent of termination by both chain transfer and disproportionation, at very high monomer conversion and under post-polymerization conditions, offering a convenient approach to maintaining high end-group fidelity in Cu(0)-mediated radical polymerization. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5313-5321, 2011