The invention provides a method of preparing an aqueous dispersion of polymer encapsulated particulate material, the method comprising: providing a dispersion of the particulate material in a continuous aqueous phase, the dispersion comprising RAFT agent as a stabiliser for the particulate material; and polymerising ethylenically unsaturated monomer under the control of the RAFT agent to form polymer at the surface of the dispersed particulate material, thereby providing the aqueous dispersion of polymer encapsulated particulate material; wherein polymerisation of the ethylenically unsaturated monomer comprises: (a) polymerising a monomer composition that includes ionisable ethylenically unsaturated monomer so as to form a base responsive water swellable RAFT polymer layer that encapsulates the particulate material; and (b) polymerising a monomer composition that includes non-ionisable ethylenically unsaturated monomer so as to form an extensible, water and base permeable RAFT polymer layer that encapsulates the base responsive water swellable RAFT polymer layer.