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 Sep 7;9(9):426. doi: 10.3390/polym9090426.
The base-catalyzed resorcinol-formaldehyde condensation reactions were theoretically investigated in this study by employing a quantum chemistry method. The condensation reaction includes two steps: (1) formation of the quinonemethide (QM) intermediate from hydroxymethylresorcinol; (2) Michael addition between the quinonemethide and resorcinol anion. The first step is the rate-determining step. Two mechanisms, unimolecular elimination of the conjugate base (E1cb) and water-aided elimination (WAE), were identified for the formation of QM. The hydroxymethylresorcinol anion produces neutral QM while the dianion produces a quinonemethide anion (QMA). The calculated potential energy barriers suggested that the QMA formation is much more favorable. Although resorcinol-formaldehyde and phenol-formaldehyde condensations share a common mechanism, the former would be faster if the QMA participates in condensations. The potential energy barriers for formation of 2-QM, 4-QM, 6-QM, 2-QMA, and 4-QMA were calculated. The results show that the formations of 6-QM and 4-QMA have relatively lower energy barriers. This rationalized previous experimental observations that the 2,4-(2,6-) and 6,6'-(4,4'-) methylene linkages were dominant, whereas the 2,2'-linkage was almost absent. The resorcinol-phenol-formaldehyde co-condensations were also calculated. The cold-setting characteristic of phenol-resorcinol-formaldehyde co-condensed resin can be attributed to participation of resorcinol quinonemethides in condensations.
本研究采用量子化学方法对碱催化间苯二酚 - 甲醛缩合反应进行了理论研究。缩合反应包括两个步骤：（1）由羟甲基间苯二酚形成醌甲基化物（QM）中间体；（2）醌甲基化物与间苯二酚阴离子之间的迈克尔加成反应。第一步是速率决定步骤。确定了形成QM的两种机制，即共轭碱的单分子消除（E1cb）和水辅助消除（WAE）。羟甲基间苯二酚阴离子产生中性QM，而二价阴离子产生醌甲基化物阴离子（QMA）。计算得到的势能垒表明QMA的形成更有利。尽管间苯二酚 - 甲醛和苯酚 - 甲醛缩合反应具有共同的机制，但如果QMA参与缩合反应，前者会更快。计算了形成2 - QM、4 - QM、6 - QM、2 - QMA和4 - QMA的势能垒。结果表明，6 - QM和4 - QMA的形成具有相对较低的能垒。这合理化了先前的实验观察结果，即2,4 -（2,6 -）和6,6'-（4,4'-）亚甲基键占主导，而2,2'-键几乎不存在。还计算了间苯二酚 - 苯酚 - 甲醛共缩合反应。苯酚 - 间苯二酚 - 甲醛共缩合树脂的冷固化特性可归因于间苯二酚醌甲基化物参与缩合反应。
