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. The density functional theory (DFT) method of B3P86/6–31++G (d, p) has been used to investigate the pyrolysis mechanisms of polyethylene terephthalate (PET) dimer in this study. In the initial degradation of PET dimer, the concerted reaction is the main pyrolysis reaction, and terephthalic acid, vinyl terephthalate, divinyl terephthalate formed by concerted reactions may be main pyrolysis products. The radical reaction is the main competitive pyrolysis reaction. In the secondary degradation processes, the degradation reaction of terephthalic acid, vinyl terephthalate, and divinyl terephthalate are investigated. In all reaction processes of PET thermal degradation, acetaldehyde is mainly formed by a concerted reaction in the initial degradation process, CO2 is mainly produced by the decarboxylation via a concerted reaction and CO is mainly produced by the decarbonylation of radicals in secondary degradation. [Display omitted] •The decomposition mechanisms of PET dimer were investigated by using DFT method.•The synergistic reaction occurred via six-membered cyclic transition state is the most favored initial reaction.•The primary pyrolysis products are terephthalic acid, vinyl terephthalate, divinyl terephthalate and acetaldehyde.•Ace