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標題: Synthesis, Characterization and Applications of Poly(urea/malonamide) Dendrimers
作者: 陳志平
Chen, Chih-Ping
關鍵字: Dendrimer
出版社: 化學工程學系
摘要: 欲製備複雜之分子結構,常使用去保護/保護基的方法,然而去保護/保護基的方法,會導致合成步驟增加及產率下降等問題,因此具高反應選擇性及高產率化學之合成步驟,即成為論文之重心。研究中將利用發展出具反應選擇性的中間體,進行複雜分子之合成,即規則樹枝狀聚合物(dendrimer)之研究。論文將分別以收斂及發散方式,進行dendrimers的合成,雖然文獻提到使用所謂hypercore, hypermonomer及double exponential growth等方式,可加速dendrimer之合成,然這些方式,亦使用了保護及活化等步驟。研究中將利用具有選擇性之雙官能基單體(4-Isocyanato-4’(3,3-dimethyl-2,4-dioxo-azetidino)diphenylmethane (MIA))為核心化合物,進行dendrimers材料之製備及應用,內容主分二部份,首先為dendrimer系統之選擇,由於異氰酸塩官能基(isocyanate)可分別與氫氧基及胺基產成urethane及urea鍵結,再配合由azetidine-2,4-dione官能基與一級胺所形的malonamide鍵結,進行dendrimer材料之研究。論文的第二部份,即進行dendrimers的應用, 研究主要進行自我凝聚(supramolecular)高性能材料的製備研究。材料形成原理,為利用第二作用力,如氫鍵、凡得瓦力及配位共價鍵等作用力,因此其會因外在環境而產生可逆行為。若高分子中含大量的氫鍵,會使材料產生熱可逆的物理性交聯,且能達到共價鍵的化學交聯般的強度。研究即以此為主軸,選擇聚胺酯(polyurethane)系統,希望藉由改變dendrimer的代數及含量,研究其相分離程度對系統之影響,並對材料的熱性質、機械及拉力等性質進行評估。另外亦將dendrimer改質為末端具雙醇之dendritic diols,利用此預先設計之確定大小和代數的樹枝單體,製備出side chain dendritic polyurethanes材料。由於dendrimers具有多量尿素官能基,可產生大量氫鍵,預期產生熱可逆的物理性交聯來補強聚胺酯材料的強度,並能使PU材料具備優良的微觀相分離狀態,繼而使材料具備優良的物理性質。
Protection/deprotection chemistry has often been utilized in synthesis of complex molecules. However, the protection/deprotection steps often bring about unacceptable synthetic costs, lengthy procedures, and decrease in overall yield. Therefore, a highly selective chemistry, capable of a specific reaction by one functional group at a time without resorting to protection stretegies can provide a tremendous synthetic utility and advantage. The preparation of dendrimers via convergent or divergent routes is the case in point where syntheses have been relied heavily upon deprotection or activation steps. Several modified approaches such as syntheses of hypercore, hypermonomer, and double exponential growth have been advanced recently to reduce overall synthetic steps. However, these approaches still required partial deprotection or activation methodology in the total scheme at some points. Therefore, an intermediate capable of undergoing selective fuctionalization especially via a simple addition reaction would be a much preferred precusor. With that aim in mind, an effective candidate has been identified. As a result, a new and facile way of preparing dendrons has been developed via a sequentially selective addition approach. The key discovery of this methodology was realized from our lab's earlier investigation on ring-opening reactions of azetidine-2,4-dione with various substrates. On the other hand, the segmented polyurethanes have been focused upon incorporating the various precisely structure-controlled macromolecules as the association units(dendrimer). Since the early work of Cooper and Tobolsky, it has been established that multi-block copolymers composed of hard and soft segments are generally microphase-separated into high-Tg (hard; sometimes crystalline) domains and relatively low-Tg (soft) domains. The resulting microstructure of soft-rich domains reinforced by rigid hard-rich domains exhibits excellent elastomeric properties, making these polymers useful as elastomers, textiles, coatings, biomaterials, etc. Herein a unique type of the polyurethane elastomers have been achieved, by incorporating the various of precise structure-controlled macromolecules as the end-capped hard segments, and the poly(tetramethylene oxide) glycol (PTMO) as soft segments. By merging both the dendrimer and the polymer concepts, a series of side chain dendritic polyurethanes was synthesized, and dendritic diols of corresponding generation, with hard segment contents ranging from 21 to 35 wt % . It has been found that polyurethane elastomers possess improved mechanical properties compared to traditional polyurethane elastomers without bearing dendritic wedges due, in part, to the strong hydrogen bonding in the hard domain. The degree to which these polymers microphase separate significantly affects many of the physical properties of these materials. The dendritic polymers with hydrogen bond-rich structure would be an excellent building block for synthesize the high performance materials with reversible physical bonding. This would lead wide variety of potential applications.
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