A tandem polymerization methodology, chain walking polymerization (CWP) followed by atom transfer radical polymerization, was developed for efficient synthesis of nanoparticles for bioconjugation. Using the chain walking palladium-alpha-diimine catalyst (catalyst 1), dendritic polymers bearing multiple initiation sites were synthesized and used as macroinitiators for subsequent Cu(I)-mediated ATRP. Control of molecular weight and size of the water-soluble core-shell polymeric nanoparticles was achieved by tuning reaction conditions. Addition of an N-acryloyloxysuccinamide (NAS) monomer at the end of the ATRP afforded NHS-activated polymer nanoparticles. Conjugation with both small dye molecules and protein (ovalbumin) yielded nanoparticle conjugates with relatively high dye or protein per particle ratio. With the efficient synthesis and good biocompatibility, these nanoparticles may find many potential applications in bioconjugation.