The invention relates to a method for preparing a copolymer having a backbone based on methacrylate units, comprising a step involving the polymerization of one or more precursor methacrylate monomers of said units in the presence of: a polymerization initiator; at least RAFT-type transfer agent which can generate a primary radical; and at least one comonomer selected from among styrene monomers and acrylate monomers. The invention also relates to the resulting copolymers capable of being obtained according to this process, in particular block copolymers, in particular block copolymers, and to the use thereof as additives for plastic materials.