Synthesis, organogelating and self-sorting properties of non-preorganized oligoamides

Ph.D.This thesis describes the synthesis, characterization, gelation and self-sorting properties of several series of oligoamides. These amide units are anchored on a relatively flexible oligomer backbone and therefore the resulting oligomers are structurally non-preorganized. The aims of this research work are to investigate how the number and the density of amide groups, as well as other functional units (e.g. photo-responsive) can affect gelation, self-sorting and other supramolecular properties of these oligoamides.In the beginning, this thesis provides an outline of the research background of gels and self-sorting in Chapter 1. Chapters 2, 3 and 4 delineate the experimental design concept, synthesis, characterization of the target compounds. We first described the detailed synthesis and structural characterization of the first series of oligoamides [OAT-CO2H-2n and OAT-COPrg-(2n+1)] containing 2-12 CONH units that were prepared via both the (n+1) and the accelerated (n+2) synthetic approaches. Mixed xerogels consisting of a 1:1 weight ratio of two oligomers bearing different amide moieties were found to show different degrees of microphase segregation according to differential scanning calorimetry (DSC), thus proving the existence of narcissistic self-sorting. The relationship between the difference in the number of amide units and the extent of self-sorting was further studied by plotting the phase diagrams. It was also found that the end group of the oligoamides had an important effect on their gelation properties. Those with the carboxylic acid end (OAT-CO2H-2n) would undergo decarboxylation reaction to generate an additional series of oligomers (OAT-H-2n), while those with the propargylamide terminal [OAT-COPrg-(2n+1)] were found to be thermodynamically less stable and pack less orderly.To study how the density of amide units (i.e. number of amide units per weight) can influence the self-sorting behavior, two length-extended series of oligoamides [OATe-CO2H-2m and OATe-COPrg-(2m+1)] containing 2-10 CONH units, in which the spacing unit between the two neighboring amide groups was lengthened, were then prepared. It was found that a decrease in the density of the amide units actually lowered the gel strength. DSC analysis of the mixed 1:1 molar ratio of two oligomers with the same number but different density of CONH units (e.g. OAT-CO2H-2n and OATe-CO2H-2n) also revealed the existence of narcissistic self-sorting.Finally, in order to investigate whether a photo-responsive functional group could produce photo-induced self-sorting behavior, an ABA type photochromic tetraamide gelator (OAD-4) based on two dithienylethene groups were prepared. Compound OAD-4 was able to form translucent gels in various aromatic solvents and exhibited reversible photochromic behavior in solution. The closed-form isomer OAD-4-cc was separated and purified after UV (313 nm) irradiation. Self-sorting property was observed between OAD-4 and OAD-4-cc in the xerogel state.In conclusion, this thesis reported the study of organogelating phenomenon of non-preorganized oligoamides and explored how the number and density of the amide units, and a photochromic structural change could influence the self-sorting property of mixed gel systems. The results indicated that even a slight structural change in the oligomers would affect the gelation strength and attribute to the formation of phase-separated self-sorting network.本文報道了幾種系列低聚酰胺的合成、表徵、成膠以及自分類性質。酰胺單元被鑲嵌在柔軟的骨架上，具有非預先組織的結構特性。本課題的目的是探索酰胺鍵的數量、密度以及光響應基團轉變對低聚物的成膠及自分類特性的影響。第一章論述了關於有機凝膠及自分類現象的研究背景。第二、三、四章则分別闡述了實驗設計理念、低聚物的合成及表徵部分。我們先闡述了低聚酰胺OAT-CO2H-2n和OAT-COPrg-(2n+1) 的合成步驟以及結構表征。其結構包含2‒12個酰胺單元，并通过（n+1）及（n+2）逐步合成法成功製備。我們將具有不同酰胺單元個數的兩種低聚物按照1:1質量比混合并製備成凝膠，再利用差式掃描量熱法（DSC）進行分析，發現其顯示出微相分離現象，進而證明了自分類現象的發生。通過繪製相圖法，我們揭示了酰胺單元個數差異大小對自分類程度的影響。此外，我們發現低聚酰胺的末端基團對其成膠性質具有很重要的影響。在退火過程中，末端為羧基的低聚物會發生脫羧反應生成另一新系列低聚物OAT-H-2n，而末端為炔丙基酰胺的低聚物的分子排列變得更加無序。為了探究分子鏈中酰胺鍵密度會如何影響自分類行為，另兩種低聚物OATe-CO2H-2m和OATe-COPrg-(2m+1)被成功製備，其分子中包含2‒10個酰胺單元，但每兩個臨近酰胺鍵之間鏈段的距離被增長了。當兩種具有相同酰胺鍵數目但不同密度的低聚物按照1:1摩爾比混合形成凝膠時，自分類現象同樣被證明存在。最後，為了探究光響應基團能否引發自分類行為，我們合成了一種基於兩個雙噻吩烯基和四個酰胺基團構成的ABA型凝膠OAD-4。OAD-4可以在多種芳香溶劑中形成半透明凝膠，并在溶液狀態下展示出可逆的光致反應行為，其閉環異構體OAD-4-cc被成功分離。在干凝膠形態下，開環異構體OAD-4和閉環異構體OAD-4-cc之間被證實可發生自分類行為。綜上所述，本論文報道了非預先組織化的低聚酰胺形成有機凝膠現象的研究，并論述了酰胺鍵的數量和密度，以及光響應基團結構的變化對混合凝膠體系中自分類行為的影響。研究證實，一些微小的結構變化都會影響低聚物成膠的強度，并有助於形成自分類網絡結構。Zheng, Kun.Thesis Ph.D. Chinese University of Hong Kong 2018.Includes bibliographical references (leaves 175-187).Abstracts also in Chinese.Title from PDF title page (viewed on 07, January, 2021)