Insights into the thermal degradation mechanisms of polyethylene terephthalate dimer using DFT method

The thermal degradation mechanisms of polyethylene terephthalate (PET) dimer were studied by the B3P86 density functional theory (DFT) approach at 6-31++G (d, p) base set in this paper. Seven possible reaction paths were designed and analyzed, and the thermodynamic parameters for all reactions were computed. The calculated results indicate that the bond dissociation energy values (BDEs) of C-C bonds on the main-chain are the smallest, followed by those of C-O bonds. The kinetics analysis indicates that the concerted reactions are obviously liable to occur rather than radical reactions in the initial thermal decomposition process. In the processes of initial reactions, all concerted reactions occurred by six-membered cyclic transition states (TSs) are more prone to carry out than those happened by four-membered cyclic transition states. The research results show that the primary products of PET dimer pyrolysis are terephthalic acid, vinyl terephthalate, CH₃CHO and divinyl terephthalate. CH₃CHO is mainly formed by a concerted reaction in the initial degradation process, and CO₂ is mainly produced by the decarboxylation via a concerted reaction and CO is mainly produced by the decarbonylation of a radical in secondary degradation.