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The free radical post reactor modification of polyolefins with polar groups is a very well known methodology to functionalize macromolecular frameworks. The presence of grafted functionalities makes the pristine completely apolar polyolefins more compatible/interactive with polar additives as for example inorganic fillers or pigments etc. Usually, the conventional "grafting from" process involves the use of functionalizing molecules which are attached to polyolefin macroradicals through double bonds substituted by polar groups (maleic anhydride). This process is characterized by a generally very high efficiency, but owing to the not controlled macroradical formation has to be considered not selective. Recently, two TEMPO derivatives, the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (HO-TEMPO) and the 4-benzoyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (BzO-TEMPO), were reported to be a very interesting functionalizing agents because their quick reaction with macroradicals, generated by a peroxide initiator, limits all side reactions due to the presence of an excess of free radicals as for example the cross-linking and/or degradation processes.