End-group modification of polymers prepared by reversible additionfragmentation chain transfer (RAFT) polymerization was accomplished by the conversion of trithiocarbonate or dithioester end-groups into a pyridyl disulfide (PDS) functionality. Several different polymers, such as poly(methyl methacrylate), polystyrene, poly(oligoethylene glycol-acrylate), poly(hydroxypropylacrylamide), and poly(N-isopropylacrylamide) were prepared by RAFT polymerization, and subjected to aminolysis in the presence of 2,2 -dithiodipyridine to yield thiol-terminated polymers with yields in the range 6590 dependent on the polymer structure. Furthermore, this PDS end-group was utilized to generate higher-order architectures, such as diblock copolymers with high yields and selectively. In addition, the PDS end-groups were used for the bioconjugation of different biomolecules, such as oligonucleotides, carbohydrates, and peptides. The successful modification of well-defined polymers was confirmed by a combination of UV-vis, NMR spectroscopy, and gel permeation chromatography.