A new two-step procedure for the preparation of block copolymers by a free radical process is proposed. A trichloromethyl-terminated azo initiator is synthesized and subsequently used to polymerize a first monomer, leading to a trichloromethyl-functionalized polymer. Secondly, the preformed polymer is used as a macroinitiator to initiate the atom transfer radical polymerization (ATRP) of a second monomer, to generate a block copolymer. The principle of this method is illustrated with butyl acrylate as the first monomer, and styrene for the ATRP step.