We report a new route for biocompatible polymer end-group modification by means of the Staudinger ligation. This reaction allows the formation of a peptide bond in aqueous media between a phosphine-containing ester functionality and an azide group. Esterification of the two carboxylic acid-containing chain transfer agents (CTAs), 2-(dodecylsulfanylthiocarbonylsulfanyl)-2-methylpropionic acid (1) and 4-cyano-4-(dodecylsulfanylthiocarbonylsulfanyl)pentanoic acid (2), with different appropriate phosphines gave phosphine-containing CTAs. They allowed us to synthesize polystyrene of medium molecular weight via "reversible addition-fragmentation chain transfer" (RAFT) polymerization. 3,6,9-Trioxodecyl azide (TOD-N-3) was then used as model compound to study the Staudinger ligation with the corresponding polymers. Among all CTAs tested, the phosphine-functionalized CTA-4, prepared from 2 and P-borane-(diphenylphosphanyl)methanethiol (6), not only proved to be suitable for RAFT polymerization of styrene but the polymer-bound P-borane-(diphenylphosphanyl)methyl thioester group also showed the best performance in the subsequent polymer analogous Staudinger ligation.