Theoretical study on thermal degradation mechanism of polybutylene terephthalate dimer
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摘要: 采用密度泛函理论(DFT)方法M06-2X/6-311G(d)研究了聚对苯二甲酸丁二醇酯(PBT)二聚体的热降解机理，对PBT二聚体热解过程设计了八条可能反应路径，计算了每条反应路径的各基元反应步的热力学及动力学参数。计算结果表明，在PBT初始热解过程中，主链发生协同反应的反应能垒明显低于自由基反应的能垒，因此，通过协同反应生成的对苯二甲酸、对苯二甲酸单丁烯酯、对苯二甲酸二丁烯酯和二对苯二甲酸-1,4-丁二酯是PBT初始热解主要产物。主链通过六元环过渡态进行的协同反应的反应能垒低于通过四元环过渡态的，PBT主链的断裂主要通过六元环过渡态的协同反应而进行。此外，还讨论了PBT主要产物的二次降解反应，研究发现，在二次降解反应过程中主要以协同反应为主，生成1,3-丁二烯、四氢呋喃、苯、CO2、苯甲酸等主要产物。Abstract: Thermal degradation mechanism of polybutylene terephthalate (PBT) dimer was studied by density functional theory (DFT) method M06-2X/6-311G(d). Eight possible reaction paths were designed for the thermal decomposition of PBT dimer, and the thermodynamic and kinetic parameters of elementary reaction steps in each reaction path were calculated. Calculation results show that, in the initial pyrolysis process of PBT, the energy barrier of concerted reaction occurring on the main chain is significantly lower than that of the radical reaction, so terephthalic acid, monobutenyl terephthalate, dibutenyl terephthalate and diterephthalate-1,4-butadiester formed by concerted reaction are main products in PBT initial pyrolysis. The energy barrier of the main chain fracture through the six-membered cyclic transition state is lower than that through the four-membered cyclic transition state, and the main chain fracture of PBT is mainly through the concerted reaction with six-membered cyclic transition state. In addition, the secondary degradation reaction of the main products of PBT pyrolysis was also discussed. It is found that the main products such as 1,3-butadiene, tetrahydrofuran, benzene, CO2 and benzoic acid are mainly generated through concerted reaction in the processes of secondary degradation.
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Key words:
- PBT dimer /
- density functional theory /
- pyrolysis /
- reaction mechanism
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图 1 PBT二聚体的优化结构和键离能值
Figure 1 BDE values and optimal structure of PBT dimer (unit: kJ/mol )
图 11 反应路径（5）的能垒示意图
Figure 11 Schematic diagram of energy barriers in reaction path (5)
图 13 反应路径（6）的能垒示意图
Figure 13 Schematic diagram of energy barriers in reaction path (6)
图 17 反应路径（8）的能垒示意图
Figure 17 Schematic diagram of energy barriers in reaction path (8)
表 1 各热解路径中过渡态唯一的虚频
Table 1 Sole imaginary frequency of various transition states
Freq. i/cm−1 TS1 TS2 TS3 TS4 TS5 TS6 TS7 TS8 −89.58 −1443.48 −60.14 −1406.55 −89.01 −64.48 −1478.36 −76.92 TS9 TS10 TS11 TS12 TS13 TS14 TS15 TS16 −1426.85 −73.06 −1415.01 −86.88 −1017.01 −1135.73 −1015.16 −1908.39 TS17 TS18 TS19 TS20 TS21 TS22 TS23 TS24 −1106.42 −1909.65 −1017.91 −1859.80 −446.01 −1755.78 −1179.75 −58.67 TS25 TS26 TS27 TS28 TS29 TS30 TS31 TS32 −1461.59 −87.20 −1168.96 −65.87 −1535.27 −45.51 −1482.76 −1171.92 TS33 TS34 TS35 TS36 TS37 TS38 TS39 TS40 −516.58 −1897.99 −540.57 −859.71 −152.44 −467.65 −539.92 −885.34 TS41 TS42 TS43 TS44 TS45 TS46 TS47 TS48 −466.44 −154.37 −440.04 −608.36 −464.81 −1904.35 −125.15 −137.97 TS49 TS50 TS51 −234.88 −2034.71 −1800.56 
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