A method of preparing a block copolymer of Formula (B) wherein P1 represents a substantially aqueously soluble polymeric component and P2 represents a substantially aqueously insoluble polymeric component, comprises admixing an aqueously soluble polymer macro-chain transfer agent comprising P1 with a monomer (M2) and initiating an aqueous dispersion-type radical addition fragmentation chain transfer (RAFT) polymerization. (P1) is derived from a monomer (M1) selected from monomers of the Formulae (M1A), (M1B) and/or (M1C) where R1, R10 and R11 represent a substituent of (M1A) or (M1C) which allows P1 to be at least partially aqueously soluble, R2 represents H, CH3 or CN, RS represents one or more substituents of the aromatic ring effective to allow P1 to be at least partially aqueously soluble, and monomer M2 is selected from: where R3 is a substituent of (M2) which allows P2 to be substantially aqueously insoluble, and R4 and R6 independently represent H or methyl.