Abstract: The present invention is a multistage process for the preparation of a co)polymer composition. Said process comprises the steps of: i) polymerizing in a first stage a reaction mixture comprising one or more monomer with optionally a rubber, a comonomer, a polymerization initiator, a chain transfer agent, a decomposition catalyst and/or a solvent, in the presence of one or more stable free radical precursor which provides a stable free radical during the polymerization process and/or one or more stable free radical such that the polymerization proceeds via a mediated free radical polymerization mechanism until a partial conversion of monomer(s) to (co)polymer is obtained and ii) polymerizing in a second stage the partially converted reaction mixture under reaction conditions that proceed via a free radical polymerization mechanism to obtain a further degree of conversion of monomer(s) to (co)polymer.