Well-defined fluorescent, α-functional polymethacrylates with PEG side chains were readily designed by nitroxide-mediated polymerization (NMP) from N-hydroxysuccinimidyl (NHS) ester-containing alkoxyamines based on the nitroxide SG1. These polymers were obtained without any purification beyond a simple precipitation. They exhibited tunable reactivities towards nucleophiles depending on the nature of the alkoxyamine used. Copolymers derived from the commercially available SG1-based alkoxyamine led to quantitative coupling with small molecules and a neuroprotective peptide whereas partial conjugation was obtained with lysozyme, used here as a model protein. Complete conjugation to the latter was obtained from similar copolymers exhibiting a less sterically hindered NHS extremity, via appropriate alkoxyamine structure, which represents the very first example of peptide/protein PEGylation with functional polymers derived from NMP.