Harada Akira, Osaki Motofumi, Takashima Yoshinori, Yamaguchi Hiroyasu
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
Acc Chem Res. 2008 Sep;41(9):1143-52. doi: 10.1021/ar800079v. Epub 2008 Aug 9.
Synthetic polymers, typically prepared by addition polymerization or stepwise polymerization, are used constantly in our daily lives. In recent years, polymer scientists have focused on more environmentally friendly synthetic methods such as mild reaction conditions and biodegradable condensation polymers, including polyesters and polyamides. However, challenges remain in finding greener methods for the synthesis of polymers. Although reactions carried out in water are more environmentally friendly than those in organic solvents, aqueous media can lead to the hydrolysis of condensation polymers. Furthermore, bulk polymerizations are difficult to control. In biological systems, enzymes synthesize most polymers (proteins, DNAs, RNAs, and polysaccharides) in aqueous environments or in condensed phases (membranes). Most enzymes, such as DNA polymerases, RNA polymerases, and ribosomes, form doughnutlike shapes, which encircle the growing polymer chain. As biopolymers form, the active sites and the substrate-combining sites are located at the end of the growing polymer chain and carefully control the polymerization. Therefore, a synthetic catalyst that could insert the monomers between the active site and binding site would create an ideal biomimetic polymerization system. In this Account, we describe cyclodextrins (CDs) as catalysts that can polymerize cyclic esters (lactones and lactides). CDs can initiate polymerizations of cyclic esters in bulk without solvents (even water) to give products in high yields. During our studies on the polymerization of lactones by CDs in bulk, we found that CDs function not only as initiators (catalysts) but also as supporting architectures similar to chaperone proteins. CDs encircle a linear polymer chain so that the chain assumes the proper conformation and avoids coagulation. The CDs can mimic the strategy that living systems use to prepare polymers. Thus, we can obtain polyesters tethered to CDs without employing additional solvents or cocatalysts. Although CD has many hydroxyl groups, only one secondary hydroxyl group attaches to the polyester chain. In addition, the polymerization is highly specific for monomer substrates. We believe that this is the first system in which the catalyst includes monomers initially and subsequently activates the included monomers. The catalyst then inserts the monomers between the binding site and the growing chain. Therefore, this system should provide a new environmentally friendly route to produce biodegradable functional polymers.
合成聚合物通常通过加成聚合或逐步聚合制备，在我们的日常生活中被不断使用。近年来，聚合物科学家们专注于更环保的合成方法，如温和的反应条件以及可生物降解的缩聚物，包括聚酯和聚酰胺。然而，在寻找更绿色的聚合物合成方法方面仍然存在挑战。尽管在水中进行的反应比在有机溶剂中进行的反应更环保，但水性介质会导致缩聚物水解。此外，本体聚合难以控制。在生物系统中，酶在水性环境或凝聚相（膜）中合成大多数聚合物（蛋白质、DNA、RNA和多糖）。大多数酶，如DNA聚合酶、RNA聚合酶和核糖体，形成甜甜圈状结构，环绕着不断增长的聚合物链。随着生物聚合物的形成，活性位点和底物结合位点位于不断增长的聚合物链的末端，并仔细控制聚合反应。因此，一种能够将单体插入活性位点和结合位点之间的合成催化剂将创建一个理想的仿生聚合体系。在本综述中，我们描述了环糊精（CDs）作为能够使环状酯（内酯和丙交酯）聚合的催化剂。CDs可以在无溶剂（甚至无水）的本体中引发环状酯的聚合反应，以高产率得到产物。在我们对CDs在本体中使内酯聚合的研究过程中，我们发现CDs不仅作为引发剂（催化剂）起作用，还作为类似于伴侣蛋白的支撑结构。CDs环绕线性聚合物链，使链呈现适当的构象并避免凝聚。CDs可以模仿生物系统用于制备聚合物的策略。因此，我们可以在不使用额外溶剂或助催化剂的情况下获得连接到CDs上的聚酯。尽管CD有许多羟基，但只有一个仲羟基连接到聚酯链上。此外，聚合反应对单体底物具有高度特异性。我们认为这是第一个催化剂最初包含单体并随后激活所包含单体的体系。然后催化剂将单体插入结合位点和生长链之间。因此，该体系应该提供一条生产可生物降解功能聚合物的新的环保途径。
