Biocompatible polymer coatings for magnetic nanoparticles have shown to drastically increase their usability towards biomedical applications. The coatings imprint characteristics such as stability, resistance to non-specific adsorption and tolerance in complex media for biomedicine. Herein, a thorough investigation towards the anionic ring-opening polymerization of glycidol on the surface of carbon-coated cobalt nanoparticle was performed. Reaction parameters that influence polymer growth have been investigated. Thereafter, a maximal achievable hyperbranched polyglycidol Mw of up to 1148 g mol−1 under optimal reaction conditions was obtained. With this coating, the dispersion stability of the particles could be substantially increased, the non-specific adsorption of proteins could be decreased to 10% while retaining an efficient magnetic separation.