ABSTRACT
24The investigation was undertaken to determine the antioxidative activity of a range of fUllerenes C60 and C70 generally manufactured in practice in order to rank them according to their comparative efficiency. The model reaction of cumene-initiated (2,2’-azo-bisisobutyronitrile [AIBN]) oxidation was employed herein to determine rate constants for addition of radicals to fullerenes. Kinetic measurements of oxidation rate in the presence of different fullerenes showed that the antioxidative activity as well as the mechanism and mode of inhibition were different for fullerenes C60 and C70 and fullerene soot. All fullerenes—C60 of gold grade, C60/C70 (93/7, mix 1), C60/C70 (80 ± 5/20 ± 5, mix 2), and C70 operated in the mode of an alkyl radical acceptor, whereas fullerene soot surprisingly retarded the model reaction by a dual mode similar to that for the fullerenes and with an induction period like many of the sterically hindered phenolic and amine antioxidants. For the C60 and C70, the oxidation rates were found to depend linearly on the reciprocal square root of the concentration over a sufficiently wide range, thereby fitting the mechanism for the addition of cumyl alkyl R· radicals to the fullerene core. This is consistent with the gathered literature data of the more readily and rapid addition of alkyl and alkoxy radicals to the fullerenes compared with peroxy radicals. Rate constants for the addition of cumyl R· radicals to the fullerenes were determined to be k (333K) = (1.9 ± 0.2) χ 108 (C60); (2.3 ± 0.2) χ 108 (C60/ C70, mix 1); (2.7 ± 0.2) χ 108 (C60/C70, mix 2); and (3.0 ± 0.3) χ 108 (C70) M−1 s−1. The incremental C70 constituent in the fullerenes leads to a respective increase in the rate constant. The fullerene soot inhibits the model reaction according to the mechanism of trapping of peroxy radicals: The oxidation proceeds with a pronounced induction period and kinetic curves are linear at the semilogarithmic coordinates. For the first time, the effective concentration of inhibiting centers and inhibition rate constants for the fullerene soot have been determined to be fn[C60-soot] = (2.0 ± 0.1) χ 10−4 mol g−1 and k inh = (6.5 ± 1.5) χ 103 M−1 s−1, respectively. The kinetic data obtained specify the level of antioxidative activity for the commercial fullerenes and scope of their rational use in different composites. The results may be helpful for designing an optimal profile of composites containing fullerenes.
