Cao Ming, Li Taohong, Liang Jiankun, Wu Zhigang, Zhou Xiaojian, Du Guanben
The Yunnan Province Key Lab of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China.
College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China.
Polymers (Basel). 2016 Nov 8;8(11):391. doi: 10.3390/polym8110391.
The reactions of di-hydroxymethylurea with phenol under alkaline (pH = 10), weak (pH = 6) and strong acidic (pH = 2) conditions were investigated via the C-NMR method. Based on the proposed reaction mechanisms, the variations of the structures of different condensed products were analyzed and the competitive relationship between self- and co-condensation reactions was elucidated. The required experimental conditions for co-condensations were clearly pointed out. The main conclusions include: (1) the self-condensation between urea formaldehyde (UF) or phenol formaldehyde (PF) monomers were dominant while the co-condensations were very limited under alkaline conditions. This is because the intermediates produced from urea, methylolurea and phenol are less reactive in co-condensations with respect to self-condensations; (2) under weak acidic conditions, the self-condensations occurred exclusively among the UF monomers. The co-condensation structures were not observed; and (3) the co-condensations became much more competitive under strong acidic conditions as the relative content of the co-condensed methylenic carbon accounts for 53.3%. This result can be rationalized by the high reactivity of the methylolphenol carbocation intermediate toward urea and methylolurea. The revealed reaction selectivity and mechanisms may also be applied to the synthesis of those more complex co-condensed adhesives based on natural phenolic and amino compounds.
通过碳核磁共振（C-NMR）方法研究了二羟甲基脲与苯酚在碱性（pH = 10）、弱酸性（pH = 6）和强酸性（pH = 2）条件下的反应。基于所提出的反应机理，分析了不同缩合产物结构的变化，并阐明了自缩合反应和共缩合反应之间的竞争关系。明确指出了共缩合所需的实验条件。主要结论包括：（1）在碱性条件下，脲醛（UF）或酚醛（PF）单体之间的自缩合占主导，而共缩合非常有限。这是因为由尿素、羟甲基脲和苯酚产生的中间体在共缩合中相对于自缩合的反应活性较低；（2）在弱酸性条件下，自缩合仅发生在UF单体之间。未观察到共缩合结构；（3）在强酸性条件下，共缩合变得更具竞争力，因为共缩合亚甲基碳的相对含量占53.3%。羟甲基苯酚碳正离子中间体对尿素和羟甲基脲的高反应活性可以解释这一结果。所揭示的反应选择性和机理也可应用于基于天然酚类和氨基化合物的更复杂共缩合胶粘剂的合成。
