Some kinetic parameters of the degradation of natural rubber induced by chloranil and iron (III) chloride, in solution

The degradation of natural rubber (NR) induced by chloranil and FeCl3, in solution, was investigated by efflux time measurements and by FTIR and H-1 NMR spectroscopies. GC-MS was used to investigate the influence of chloranil on residues generated during thermal degradation of NR. 2-Methyl-2-butene (MB) was used as a model compound for the reactions. The results show that the reaction occurs by the scission of the double bond. The reactions were characterised by second-order kinetics, independent of the catalyst. The rate constants determined from efflux time data show a strong contribution of the physical entanglements: if the reactants are diluted 10-fold the value of the rate constant decreases significantly. Thus, the rate constants were more realistically calculated from the changes in the C=C concentration measured by H-1 NMR. The rate constants for the reaction of MB and chloranil (or FeCl3) are lower than for the corresponding NR degradation. The presence of chloranil does not alter the chromatogram profile obtained by GC-MS for the thermal degradation of NR. It is concluded that chloranil decreases the activation parameters for the degradation because the degradation products are the same as those obtained thermally in absence of chloranil. Activation parameters such as energy, enthalpy, entropy and Gibbs energy were determined. Activation energies (in 0 mol(-1)) for the degradation of NR are 60.1 for NR/chloranil and 41.9 for NR/FeCl3. From the values of activation parameters, it was suggested that the activation step of the reaction induced by chloranil is more thermally dependent than the reaction induced by FeCl3 that, other-wise, is more entropically dependent. (C) 2002 Elsevier Science Ltd. All rights reserved.