The invention provides a method of polymerizing monomer to form polymer at the surface of solid particulate material, said method comprising: providing a dispersion of said solid particulate material in a continuous hydrophilic liquid phase, said dispersion comprising a hydrophilic RAFT agent as a stabilizer for said solid particulate material, and said continuous hydrophilic liquid phase comprising one or more ethylenically unsaturated monomers; and polymerizing said one or more ethylenically unsaturated monomers under the control of said hydrophilic RAFT agent to thereby form polymer at the surface of said solid particulate material.