Pure hexa-adducts (PS)(6)C-60 can be prepared by addition of PS-Li onto fullerene C-60. These model polymer stars offer an opportunity to study the thermal stability of the bond between the fullerene and a grafted chain simply by SEC. At temperatures around 100 degreesC, the hexa-adduct is slowly converted into adducts of lower functionality and ungrafted chains. The experimental results are coherent with a step by step mechanism where the 6-arm stars are converted to 5-arm stars, then these latter to 4-arm stars and so on; the kinetics constant decreasing if the functionality of the stars decreases. The thermal stability decreases if the length of the grafts increases.