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.

DFT studies on pyrolysis mechanisms of tetrabromobisphenol A (TBBPA)

Tetrabromobisphenol A (TBBPA) is the most widely used brominated flame retardant. In order to better understand the decomposition process of TBBPA and clarify the evolution process of the main pyrolysis products, the density functional theory (DFT) method PBE0/6-311G(d) has been used to investigate the pyrolysis mechanisms of TBBPA in this study. Seven possible pyrolysis reaction paths were proposed, and the kinetic parameters in all pyrolysis paths were calculated. The calculation results indicate that in initial degradation of TBBPA without the involvement of hydrogen radical, the demethylation reaction is the main pyrolysis reaction channel, and the keto-enol tautomerization reaction is the main competitive pyrolysis reaction channel. The brominated cyclohexadienone formed through the keto-enol tautomerization is prone to further debromination to generate Br radical. The involvement of hydrogen radical significantly lowers the energy barrier of TBBPA decomposition. When a hydrogen radical is involved in the pyrolysis process, the debromination reaction becomes the major pyrolysis reaction channel, and the homolytic cleavage of C_aromatic-C bond becomes the major competitive pyrolysis reaction channel.