A novel and robust route for the synthesis of a new amphiphilic brush copolymer, poly(glycidyl methacrylate)-graft-polyethylene glycol (PGMA-g-PEG), with high grafting densities of 97%-98% through a "grafting onto" method via carbon dioxide chemistry is reported. PGMA-g-PEG can self-assemble and form stable spherical core-shell micelles in aqueous solution. Besides, the obtained PGMA-g-PEG polymer contains hydroxyurethane structures as the junction sites between the PGMA backbone and PEG side chain, which can be used for further modification.