A new approach in the modification of polyethylene involving the thermolysis of peroxyketals in the molten polymer is presented. Three cyclic and one acyclic peroxyketals were decomposed in polyethylene. The grafting of an ester function by coupling of an alkyl radical bearing such a function, arising from the peroxyketals, and a polymer radical, generated from the polyethylene, was identified. The efficiency of this reaction was measured by infrared spectroscopic determination of the carbonyl of the ester and by saponification of this function. The influence of the concentration of peroxyketal in the polyethylene, of the temperature of reaction, and of the nature of the radical coupling with the one arising from the polymer was investigated. Besides the yields of the functionalization, the extent of "crosslinking" of the polyolefin was determined. (C) 2001 Elsevier Science Ltd. All rights reserved.