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摘要: 采用密度泛函理论B3LYP/6-311G（d）方法，对聚苯乙烯（PS）热降解反应机理进行了研究。PS热降解的主要产物是苯乙烯，其次是甲苯、α-甲基苯乙烯、乙苯和二聚体等芳烃化合物。PS热降解反应主要包括主链C-C键均裂、β-断裂、氢转移和自由基终止等反应。针对以上各类反应进行了路径设计和理论计算分析，对参与反应的分子的几何结构进行了优化和频率计算，获得了各热降解路径的标准动力学和热力学参数。计算结果表明，苯乙烯主要由自由基的链端β-断裂反应形成；二聚体主要由分子内1，3氢转移的反应形成；α-甲基苯乙烯由分子内的1，2氢转移后进行β-断裂形成；甲苯由苯甲基自由基夺取主链上的氢原子形成；乙苯由苯乙基自由基夺取氢原子形成。动力学分析表明，苯乙烯形成所需要的能垒低于其他产物形成所需要的能垒，故苯乙烯为主要的热降解产物；这与相关实验结果基本一致。Abstract: The reaction mechanism of thermal degradation of polystyrene has been studied theoretically using density functional theory B3LYP/6-311G(d) method. The main products of PS thermal degradation are styrene, followed by aromatic compounds such as toluene, α-methylstyrene, ethylbenzene and dimer. It is reported that the thermal degradation of PS mainly includes the homolytic reaction of carbon-carbon bond, β-cleavage reactions, hydrogen transfer reaction and free radical termination reaction. Base on the above reaction, the pathways was designed, and the theoretical calculation and analysis were carried out. Furthermore, the geometrical structure of all the reaction molecules was optimized for the reaction process and the reaction frequency was calculated, which were obtained the standard kinetic parameters and thermodynamic parameters of each thermal degradation path. The results of calculation show that the major formation mechanism of styrene is the chain-end β-cleavage reactions of free radicals, the producing of dimer mainly depends on the intermolecular 1, 3 hydrogen transfer reactions, α-methylstyrene is generated through intermolecular 1, 2 hydrogen transfer and β-cleavage reactions, the radicals of benzyl and phenethyl captured hydrogen atoms to form toluene and ethylbenzene, respectively. Kinetic analysis exhibit that the energy barrier for the formation of styrene was lower than that needed for the generating of other products, so styrene was the main thermal degradation product. This is consistent with the relevant experimental results.
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Key words:
- polystyrene /
- peaction mechanism /
- density functional theory /
- thermal degradation
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图 1 间规、无规、同规聚苯乙烯(PS)模型化合物的结构式和主要键的键离能
Figure 1 Structural formula of the syndiotactic, random, and isotactic polystyrene (PS) model compounds and their bond energy of the primary bond (unit: kJ/mol)
图 2 苯乙烯、乙苯形成的热降解路径示意图
Figure 2 Thermal degradation pathways to form styrene and ethylbenzene
图 3 苯乙烯、乙苯形成过程中的反应能垒示意图
Figure 3 Schematic diagram of the reaction energy barrier during the formation of styrene and ethylbenzene
图 4 苯乙烯、乙苯形成过程中反应分子的优化几何结构
Figure 4 Optimized geometry of reactive molecules in the formation of styrene and ethylbenzene (unit: nm)
图 6 二聚体形成过程中的反应动力学示意图
Figure 6 Schematic diagram of reaction kinetics for during the formation of dimer
图 7 二聚体形成过程中反应分子的优化几何结构
Figure 7 Optimized geometry of reactive molecules during the dimer formation (unit: nm)
图 8 α-甲基苯乙烯、甲苯形成的热降解路径示意图
Figure 8 Thermal degradation pathway to form α-methylstyrene and toluene
图 9 α-甲基苯乙烯、甲苯形成过程中的反应能垒示意图
Figure 9 Schematic diagram of the reaction energy barrier during the formation of α-methylstyrene and toluene
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