Polarized olefins as enabling (co)catalysts for the polymerization of γ-butyrolactone
Polarized olefins as enabling (co)catalysts for the polymerization of γ-butyrolactone
复制标题
DOI：
10.1039/c8py00784e
复制
发表时间：
2018-07-14
影响因子：
4.6
通讯作者：
Naumann, Stefan
中科院分区：
文献类型：
--
作者：
Walther, Patrick;Frey, Wolfgang;Naumann, Stefan
合适的方法来解决。通过开环聚合（ROP）制备丁内酯（GBL）是最近才出现的。鉴于相应的脂肪族聚酯的令人兴奋的性质，需要有机催化或简单的金属基催化剂体系。为了扩展目前有限的一组合适的催化剂并探索基于磷腈的“超碱性”阴离子途径之外的机理，已经将所谓的N-杂环烯烃（NHO）用于GBL均聚。发现它们可以在有机催化装置中应用，但也可以在本体条件（-36 ℃）下与锂盐组合作为协同刘易斯对。无金属设置仅在引发剂（BnOH）存在下成功，其中通过使用六元NHO 1，3-二甲基-2-（1-甲基亚乙基）四氢嘧啶（3，70%转化率，0.5mol%NHO负载）实现了140-230的TON。MALDI-ToF MS分析显示，生成了环状和线性物质的混合物，由此环状材料的比例在聚合的后期阶段增加。相反，当几种NHO与锂盐一起作为刘易斯对（NHO、LiX（X = Cl、I、OTf）和BnOH）使用时，发现大环的形成被延迟。在不存在质子引发剂（BnOH）但存在锂盐的情况下，聚合机理可以从阴离子型转变为两性离子型ROP。正如MALDI-ToF研究所强调的那样，根据NHO的化学结构，可能发生三种不同的情况。首先，高度亲核的NHO（7）将形成稳定的两性离子物种：NHO直接连接到链上并使其阳离子化。作为一个坏的离去基团，NHO部分不能被取代，并且仅生成线性聚（GBL）。在这些条件下，亲核性较低的空间位阻NHO（6）仍将形成两性离子，但可以发生取代，有利于大环聚（GBL）拓扑结构。最后，空间位阻但强碱性的NHO（4）优选内酯的烯醇化而不是亲核开环，因此在阴离子聚合途径上操作，与磷腈催化相当。总的来说，发现所制备的聚（GBL）在低聚合物/高低聚物分子量（1000-9000 g mol（-1），GPC）的范围内，其中有机催化方法提供比在锂盐存在下的反应显著更高的转化率。
Suitable methods to polymerize.-butyrolactone (GBL) by ring-opening polymerization (ROP) have emerged only recently. In view of the exciting properties of the corresponding aliphatic polyester, organocatalytic or simple metal-based catalyst systems are called for. To extend the currently limited set of suitable catalysts and to explore mechanisms beyond the "superbasic", anionic pathway based on phosphazenes, so-called N-heterocyclic olefins (NHOs) have been employed for GBL homopolymerization. It was found that they can be applied in an organocatalytic setup but also in combination with lithium salts as cooperative Lewis pairs under bulk conditions (-36 degrees C). Metal-free setups succeed only in the presence of initiator (BnOH), whereby TONs of 140-230 have been realized by using the six-membered NHO 1,3dimethyl- 2-(1-methylethylidene) tetrahydropyrimidine (3, 70% conversion, 0.5 mol% NHO loading). MALDI-ToF MS analysis revealed that a mixture of cyclic and linear species is generated, whereby the proportion of cyclic material increases in the later stages of the polymerization. Conversely, when several NHOs were employed with lithium salts as Lewis pairs (NHO, LiX (X = Cl, I, OTf) and BnOH), the formation of macrocycles was found to be retarded. In the absence of protic initiator (BnOH) but presence of lithium salt, the polymerization mechanism can change from an anionic one to a zwitterionic ROP. As underlined by MALDI-ToF investigations, three different situations can occur, depending on the chemical structure of the NHO. Firstly, highly nucleophilic NHOs (7) will form stable zwitterionic species: the NHO is directly attached to the chain and cationizing it. As a bad leaving group, the NHO moiety cannot be substituted and exclusively linear poly(GBL) is generated. Less nucleophilic, sterically hindered NHOs (6) will still form zwitterions under these conditions, but substitution can occur, favouring a macrocyclic poly (GBL) topology. Finally, sterically encumbered but strongly basic NHOs (4) prefer enolization of the lactone over nucleophilic ring-opening, thus operating on an anionic polymerization pathway, comparable to phosphazene catalysis. Overall, the prepared poly(GBL) was found to be in the range of low polymeric/high oligomeric molecular weights (1000-9000 g mol(-1), GPC) with the organocatalytic approaches delivering significantly higher conversion than reactions in the presence of lithium salts.