Core-shell nanoparticles (NPs) with a boron-rich core were synthesized by RAFT-mediated polymerization-induced self-assembly using a new methacrylic boronate ester monomer. Under specific conditions, sub-100 nm spherical NPs could be obtained at high conversions by either emulsion or dispersion RAFT polymerization using poly(oligo(ethylene glycol) methacrylate) (POEGMA) dithiobenozate-based chain transfer agents. Phenylboronic acid surface-functionalized NPs were obtained using a telechelic POEGMA. Primary data on biocompatibility is provided and suggests suitability as boron delivery agent for boron neutron capture therapy.