Li Taohong, Cao Ming, Liang Jiankun, Xie Xiaoguang, Du Guanben
The Yunnan Province Key Lab of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China.
Key Lab for Forest Resources Conservation and Utilisation in the Southwest Mountains of China, Southwest Forestry University, Ministry of Education, Kunming 650224, China.
Polymers (Basel). 2017 Jun 2;9(6):203. doi: 10.3390/polym9060203.
Base-catalyzed urea⁻formaldehyde condensation reactions were investigated by using a quantum chemistry method. It was found that monomethylolurea or ,'-dimethylolurea can produce the methyleneurea intermediate (⁻HN⁻CO⁻N=CH₂) with the catalysis of base. The E1cb (unimolecular elimination of conjugate base) mechanism was identified for the formation of such an intermediate. The potential energy barrier was theoretically predicted to be 59.6 kJ/mol for the E1cb step, which is about half of that of previously proposed S2 (bimolecular nucleophilic substitution) mechanism. In the subsequentcondensation reactions, Michael addition reactions that lead to different condensed structures can occur between the methyleneurea intermediate and the anions produced from methylolureas under alkaline conditions. Based on the theoretical calculations on the kinetics and thermodynamics of the selected reactions, the competitive formations of methylene linkages, ether linkages and uron were discussed in combination with our previous experimental observations.
采用量子化学方法研究了碱催化尿素-甲醛缩合反应。研究发现，在碱的催化下，一羟甲基脲或N,N'-二羟甲基脲可生成亚甲基脲中间体（-HN-CO-N=CH₂）。确定了该中间体形成的E1cb（共轭碱单分子消除）机理。理论预测E1cb步骤的势能垒为59.6 kJ/mol，约为先前提出的S2（双分子亲核取代）机理的一半。在随后的缩合反应中，在碱性条件下，亚甲基脲中间体与羟甲基脲产生的阴离子之间会发生导致不同缩合结构的迈克尔加成反应。基于对所选反应动力学和热力学的理论计算，结合我们之前的实验观察结果，讨论了亚甲基键、醚键和环亚胺基的竞争形成。
