Nitroxyl radicals were used as functionalizing agents during the free radical postreactor modification process of polyolefins carried out in the melt. 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) free radicals were successfully grafted onto a polyethylene-based material (ethylene-co-1-octene copolymer) by coupling reaction with polymer macroradicals; these last were formed by H-abstraction through peroxide addition. The macromolecular structure of the functionalized polyolefins was assessed by H-1-NMR, FTIR spectroscopy, and SEC measurements which were used to evidence the grafting site, to evaluate the grafting level and to highlight the occurrence of chain extension through cross-linking side reactions. Indeed the use of proper model compounds allowed the preparation of accurate FTIR calibration curves for the quantitative determination of the functionalization degree. Besides the high temperature SEC analysis highlighted that this fast and simple coupling reaction between macroradicals and nitroxyl free radicals grants the grafting of functionalities onto the polyolefin backbone by contemporarily preventing the side reactions liable of the structure and MW modification of the pristine polymer. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 781-795, 2011