Abstract
The decomposition temperatures, Td, and the activation energies, ΔE, in the initial reaction of depolymerization are obtained by thermogravimetry for poly (n-alkyl methacrylate) s synthesized under the same conditions.
The observed ΔE values in the range of 40 to 80 kJ mol-1, are much smaller than the carbon-carbon bond energy in the literature. This is interpreted in terms of an orbital theory : The difference in energy between the highest occupied molecular orbital of the starting polymer and the singly occupied molecular orbital of the resulting radical, calculated by the extended Hückel method for a model compound of poly (methyl methacrylate), is close to the observed ΔE value. The value of activation energy of unzipping reaction has been estimated to be about 90 kJ mol-1. This suggests that the rate-determing step is not necessarily the initiation (carbon-carbon bond cleavage) reaction.
In the course of the calculation, it was found to depend on the conformation of the polymer molecule. This explains an experimental result that the larger the number of carbons of n-alkyl group, the lower the decomposition temperature.
