Comprehensive insights into polycarbonate (PC) pyrolysis mechanisms: Reaction pathways and formation processes of phenolics, gases, dibenzofuran, xanthone

Abstract

In order to deeply explore the pyrolysis mechanism of polycarbonate (PC) and the evolution of products, in this paper, carbonate dimer was used as a model compound of PC and its degradation processes were comprehensively studied through quantum chemical calculations. The calculations results show that in the initial stage of degradation, the end-carboxyl group of PC is most susceptible to decarboxylation via concerted reaction with an energy barrier of 155.5 kJ/mol, followed by the scission of the C-CH₃ bond of the alkane chain and the C-O bond of the carbonate group, with bond dissociation energies of 301.5 and 344.5 kJ/mol. The involvement of H radical significantly promotes the reactivity in bimolecular reaction system, and the energy required to scission the C-CH₃ bond and the C-O bond was reduced to 121.7 and 153.3 kJ/mol, respectively. The addition of H radicals on the benzene ring observably reduces the activation energy for bridge bond breaking, with energy barriers of 36.1 and 39.0 kJ/mol. The final degradation products mainly include a variety of phenolic compounds (e.g., phenol, bisphenol A, etc.), and gaseous products such as CO₂ and CO. CO₂ is mainly generated through concerted decarboxylation reaction, while CO is formed by the decarbonylation reaction of the radical IM14. The formation of dibenzofuran is mainly achieved through the reaction of H radicals or CH₃ radicals with diphenyl ether, and the evolutionary mechanism of xanthone is proposed.