Polystyrenes with different content of peroxide groups – from polystyrene with the peroxide group content of [OO]=1.7×10−3mol/g to alternating copolymer of styrene with oxygen (polystyrene peroxide (PSP)) [OO]=6.6×10−3mol/g – are synthesized by radical polymerization in the presence of oxygen. The composition and structure of the polymers are investigated by size exclusion chromatography (SEC), elemental analysis, TGA-DSC, IR and NMR spectroscopy. The synthesized polymers are tested as a macroinitiators for radical polymerization of methyl methacrylate (MMA). The MMA polymerization kinetics is studied by isothermal calorimetry. It is found that the polymerization rate increases with the decrease of peroxide group content in the macroinitiator. It is shown that the macroinitiators may be used to obtain block copolymers.