When polyolefins are functionalised to modify its chemical structure and polarity, some other secondary reactions take place as well. The grafting of a different monomer onto the main chain of the polymer involves the use of a free radical initiator, such as a peroxide, or irradiation of the sample (gamma rays or ultrasound), that not only provide the free radicals for the grafting but also for main chain modifications. Different reaction processes such as functionalisation in solution, reactive extrusion or irradiation produce different structures. This book deals with modifications of the main chain as a consequence of polyolefin grafting through different techniques, such as functionalisation in solution and reactive extrusion and through polymer irradiation with gamma rays and ultrasound.