With the aim of synthesis block copolymers by traditional free radical polymerization，a diblock copolymer，polystyrene-b-poly(methyl methacrylate)(PS-b-PMMA)，was prepared by free radical polymerization with a tetra-functional peroxide initiator JWEB50.The unique feature of this initiator is the fact that one initiator contains four peroxide groups which can be sequential decomposited.The polymerization of the formation of the block copolymer was carried out by two stages.First，the polymerization of styrene(St)using JWEB50 was investigated and the polystyrene with peroxide(PSI)was obtained，which was used in the second stage to initiate the second monomer，methyl methacrylate(MMA).Selective solvent extraction was used for separation of the block polymer from the homopolymers.The separation technique was found to be efficient by gel permeation chromatography(GPC).The block copolymer was characterized by gel permeation chromatography(GPC)，1H-nuclear magnetic resonance(1H NMR)，Fourier transform infrared spectrometer(FTIR).The results showed that the polystyrene-b-poly(methyl methacrylate)could be synthesised with JWEB50.The transmission electron microscope(TEM)analysis showed that the polymer could be used as the compatilizer to improve the compatibility of PS/PMMA blends.