Direct polycondensation of carboxylic acids and amines catalyzed by 3,4,5-trifluorophenylboronic acid
Direct polycondensation of carboxylic acids and amines catalyzed by 3,4,5-trifluorophenylboronic acid
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DOI：
10.1021/ma000085o
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发表时间：
2000-05-16
期刊：
影响因子：
5.5
通讯作者：
Yamamoto, H
中科院分区：
文献类型：
--
作者：
Ishihara, K;Ohara, S;Yamamoto, H
导论.聚酰胺用于生产合成纤维和工程树脂。芳香族聚酰胺由于其优异的热、机械和化学性能而被称为高性能聚合物。1合成聚酰胺有三种常规方法。一种是热聚合，这是用于具有高分子量的脂肪族聚酰胺的众所周知的工业方法。然而，通过熔融缩聚难以获得具有高分子量的芳族聚酰胺。1b，2，3这主要是通过由于苯基的共振效应，芳香胺的反应性低于脂肪胺来解释的。第二种方法是酸和胺在化学计量的缩合剂的存在下聚合。第三种方法涉及低温溶液聚合，其中活化的酰基衍生物如酰氯与胺反应形成聚酰胺。该方法仍然可用于制备芳族聚酰胺，即使使用酰基卤在环境上是不期望的。直接缩聚只产生化学计量量的水作为副产物，是最理想的路线，无论是环境还是工业。据我们所知，没有已知的可重复使用的催化剂用于在热条件下生产聚酰胺的该路线。我们最近发现3，4，5-三氟苯基硼酸（1）是胺和羧酸（方程式1）酰胺缩合反应的高效催化剂。4在芳基上带有吸电子取代基的芳基硼酸表现为耐水、耐酸和耐碱的热稳定的刘易斯酸，并且可以容易地在空气中处理。活性（酰氧基）硼配合物2是通过1与羧酸缩合原位生成的。4.本文报道了1催化的酰胺直接热缩聚反应，不仅合成了脂肪族聚酰胺，而且合成了芳香族聚酰胺和聚酰亚胺。结果和讨论。首先，在各种条件下研究了六亚甲基二胺和己二酸直接缩聚形成尼龙6，6。结果总结见表1。六亚甲基二胺和己二酸的1：1盐在10摩尔%的1存在下在回流下在邻二甲苯中进行缩聚，同时除去水（在索氏套管中的4 μ m分子筛）20小时，以89%的产率获得尼龙6，6（条目2）。由用于通过六氟-2-丙醇（HFIP）洗脱的GPC估计得到的尼龙的数均分子量（Mn）和重均分子量（Mw）分别为2680和8330。尼龙6，6在邻二甲苯中的不溶性似乎使高聚物的形成变得困难。由于尼龙6，6溶解在间甲酚中，因此在间甲酚和邻二甲苯的1：3（v/v）混合溶剂中在10摩尔%的1存在下回流进行缩聚（条目1）。间甲酚在溶剂中的比例确定为20 v/v %，因为过量的间甲酚抑制1的催化活性。Mn和Mw值分别增加到4690和22400。将反应混合物加热到150 ℃以上对于获得较高分子量的尼龙6，6是无效的，这可能是由于副反应。有趣的是，在没有催化剂1的对照实验中，在热条件下根本没有得到尼龙6，6（条目3对条目4）。这种简单的方法可能是合成低聚尼龙6，6的最有效的方法之一，尽管聚合物的分子量永远不够高。接下来，我们探讨了直接缩聚是否...
Introduction. Polyamides are used in the production of synthetic fibers and engineering resins. Aromatic polyamides are particularly well-known as high-performance polymers due to their excellent thermal, mechanical, and chemical properties. 1 There are three conventional methods for the synthesis of polyamides. One is thermal polymerization, which is a well-known industrial process for aliphatic polyamides with high molecular weight. However, it is difficult to obtain aromatic polyamides with a high molecular weight by molten polycondensation. 1b, 2, 3 This has been explained primarily by the low reactivity of aromatic amines compared with that of aliphatic amines because of the resonance effect of phenyl groups. The second method is polymerization of acids and amines in the presence of a stoichiometric amount of condensing agents. The third method involves low-temperature solution polymerization, where activated acyl derivatives such as acid chlorides are reacted with amines to form polyamides. This approach is still useful for preparing aromatic polyamides even though the use of acid halides is environmentally undesirable. Direct polycondensation that produces only a stoichiometric amount of water as a byproduct is the most ideal route, both environmentally and industrially. As far as we know, there are no known reusable catalysts for this route to produce polyamides under thermal conditions. We recently found that 3, 4, 5-trifluorophenylboronic acid (1) is a highly effective catalyst for the amide condensation of amines and carboxylic acids (eq 1). 4 Arylboronic acids bearing electron-withdrawing substituents at the aryl group behave as water-, acid-, and base-tolerant thermally stable Lewis acids and can be easily handled in air. An active (acyloxy) boron complex 2 is generated in situ by the condensation of 1 with carboxylic acids. 4 In this paper, we describe a successful direct thermal amide polycondensation catalyzed by 1 to form not only aliphatic polyamides but also aromatic polyamides and polyimides. Results and Discussion. Initially, the direct polycondensation of hexamethylenediamine and adipic acid to form nylon 6, 6 was examined under various conditions. The results are summarized in Table 1. The polycondensation of a 1: 1 salt of hexamethylenediamine and adipic acid in the presence of 10 mol% of 1 was carried out at reflux in o-xylene with removal of water (4 Å molecular sieves in a Soxhlet thimble) for 20 h to obtain nylon 6, 6 in 89% yield (entry 2). The numberaverage molecular weight (Mn) and the weight-average molecular weight (Mw) of the nylon obtained were estimated to be 2680 and 8330, respectively, from GPC for elution by hexafluoro-2-propanol (HFIP). It seems that the insolubility of nylon 6, 6 in o-xylene makes high polymer formation difficult. Since nylon 6, 6 dissolves in m-cresol, polycondensation was carried out in a 1: 3 (v/v) mixed solvent of m-cresol and o-xylene at reflux in the presence of 10 mol% of 1 (entry 1). The ratio of m-cresol in the solvents was determined to be 20 v/v%, since excess m-cresol inhibits the catalytic activity of 1. The Mn and Mw values were increased to 4690 and 22 400, respectively. Heating the reaction mixture to over 150 C was not effective for obtaining a higher molecular weight of nylon 6, 6, presumably due to side reactions. Interestingly, nylon 6, 6 was not obtained at all in a control experiment without catalyst 1 under thermal conditions (entry 3 versus entry 4). This simple procedure may be one of the most effective methods of synthesizing oligomeric nylon 6, 6, although the polymer molecular weights were never high enough. Next, we explored whether direct polycondensation …