A new type of polyrotaxane based on the threading of γ-cyclodextrins (γ-CDs) along a highly hydrophobic polymer, polyisobutylene (PIB), is successfully prepared and finely characterized. The used radical coupling associated with tuned reaction time and temperature leads to a fast and controlled necklace synthesis with low reagent consumption. Synthesis exhibits appealing conversion and threading rates with almost 100% and 62–73%, respectively. A combination of well-established SEC and NMR techniques, with a more forefront MALDI-TOF MS approach, provides details on the original PIB and the resulting polyrotaxanes (M w , M n , PDI, and average number of γ-CD threaded). Interestingly, tetramethylguanidinium-2-(4-hydroxyphenylazo)benzoate in DMF for MALDI analysis is revealed as a suitable matrix to overcome solubility troubles widely observed with PIB. Moreover, rotaxanation appears as an alternative to the grafting of polar groups to modify/handle hydrophobic polymers. Such an approach offers new opportunities to achieve the synthesis, with unambiguous evidence, of new supramolecular necklaces based on highly hydrophobic polymers.