Thangaraj Ravikumar, Fiser Béla, Qiu Xuanbing, Li Chuanliang, Viskolcz Béla, Szőri Milán
Institute of Chemistry, University of Miskolc, Miskolc-Egyetemváros A/2, H-3515 Miskolc, Hungary.
Higher Education and Industrial Cooperation Centre, University of Miskolc, H-3515 Miskolc, Hungary.
Polymers (Basel). 2022 Oct 5;14(19):4183. doi: 10.3390/polym14194183.
2,4- and 2,6-isomers of toluene diisocyanates (2,4-TDI and 2,6-TDI) are important raw materials in the polyurethane industry. These reactive compounds associate even under ambient conditions to form oligomers, changing the physicochemical properties of the raw material. Kinetically and thermodynamically relevant dimerization reactions were selected based on G3MP2B3 calculations from all possible dimers of phenyl isocyanate using these isocyanates as proxies. As it turned out, only the formation of the diazetidine-2,4-dione ring (11-dimer, uretdione) resulted in a species having an exothermic enthalpy of formation (-30.4 kJ/mol at 298.15 K). The oxazetidin-2-one ring product (1-2-dimer) had a slightly endothermic standard enthalpy of formation (37.2 kJ/mol at 298.15 K). The mechanism of the relevant cyclodimerization reactions was investigated further for 2,4-TDI and 2,6-TDI species using G3MP2B3 and SMD solvent model for diazetidine as well as oxazetidin-2-one ring formation. The formation of the uretdione ring structures, from the 2,4-TDI dimer with both NCO groups in the meta position for each phenyl ring and one methyl group in the para and one in the meta position, had the lowest-lying transition state (Δ= 94.4 kJ/mol) in the gas phase. The one- and two-step mechanisms of the TDI cyclotrimerization were also studied based on the quasi-G3MP2B3 (qG3MP2B3) computational protocol. The one-step mechanism had an activation barrier as high as 149.0 kJ/mol, while the relative energies in the two-step mechanism were significantly lower for both transition states in the gas phase (94.7 and 60.5 kJ/mol) and in ODCB (87.0 and 54.0 kJ/mol).
甲苯二异氰酸酯的2,4 - 异构体和2,6 - 异构体（2,4 - TDI和2,6 - TDI）是聚氨酯工业中的重要原料。这些活性化合物即使在环境条件下也会缔合形成低聚物，从而改变原料的物理化学性质。基于G3MP2B3计算，以这些异氰酸酯为替代物，从苯基异氰酸酯的所有可能二聚体中选择了动力学和热力学相关的二聚反应。结果表明，只有二氮杂环丁烷 - 2,4 - 二酮环（11 - 二聚体，脲二酮）的形成产生了一种具有放热生成焓的物种（在298.15 K时为 - 30.4 kJ/mol）。恶唑环丁烷 - 2 - 酮环产物（1 - 2 - 二聚体）具有略微吸热的标准生成焓（在298.15 K时为37.2 kJ/mol）。使用G3MP2B3和用于二氮杂环丁烷以及恶唑环丁烷 - 2 - 酮环形成的SMD溶剂模型，进一步研究了2,4 - TDI和2,6 - TDI物种相关环二聚反应的机理。脲二酮环结构的形成，来自每个苯环中两个NCO基团处于间位且一个甲基处于对位和一个处于间位的2,4 - TDI二聚体，在气相中具有最低的过渡态（Δ = 94.4 kJ/mol）。还基于准G3MP2B3（qG3MP2B3）计算协议研究了TDI环三聚反应的一步和两步机理。一步机理具有高达149.0 kJ/mol的活化能垒，而两步机理中气相中两个过渡态的相对能量（94.7和60.5 kJ/mol）以及在邻二氯苯中（87.0和54.0 kJ/mol）明显更低。
