Thermal stability of the fullerene-chain link in 6-arm PS stars with a C60 core

The thermal stability of well-defined hexa-adducts (PS)(6)C-60 in solution at temperatures around 100degreesC has been studied by multi-detector Size Exclusion Chromatography. The degradation reaction corresponds to a quantitative release of the polystyrene arms from the fullerene core through thermal cleavage of the PS-C-60 link. From the kinetics of formation of cut arms and the progressive decrease of the stars' functionality, we could establish that the reaction follows a stepwise "breaking" mechanism where a 6-arm star is first converted to a 5-arm star, then to a 4-arm star, and so on down to the ungrafted arm. Furthermore, not only does the thermal stability of the PS-C-60 bond increase if the functionality of the star decreases, but the difference is large enough to allow determination of the kinetics constants for the first three steps. The activation energy for the breaking of an arm-C-60 link is about 65 kJ/mol. The stability of (PS),C-60 slightly decreases with an increase of the arm length. MALDI-TOF mass spectroscopy has shown that both C-C bonds in alpha and beta positions to C-60 can be cut, but the breaking of the direct fullerene-arm bond is favored. We have also found that a polyisoprene-C-60 bond is about seven times less stable than a PS-fullerene link upon heating. (C) 2004 Wiley Periodicals, Inc.