Dendronized Polymer Organogels from Click Chemistry: A Remarkable Gelation Property Owing to Synergistic Functional‐Group Binding and Dendritic Size Effects

Abstract

The use of 1,3-dipolar cycloaddition reactions between azides and alkynes (click chemistry) has been extremely successful as a versatile synthetic tool to construct novel polymeric systems.[1] Whereas the main thrust has been focused on building up highly elaborate polymeric architectures, such as block copolymers, star polymers, dendrimers, and hyperbranched polymers, it is also noted that the physical properties of such poly(triazole)-based materials are little studied.[1c]Additionally, although there are many examples of the synthesis of dendrimers using click chemistry, only a few concern the preparation of dendronized polymers.[2] These are polymers incorporating multiple dendron segments stemming from a linear polymer backbone and are commonly prepared by graft-to, graft-from, or macromonomer polymerization approaches.[3] The major challenges for these approaches are the difficulty in ensuring complete dendron coverage in the graft-to and graft-from strategies, and the sometimes poor polymerization efficiency in the macromonomer strategy. To improve the synthetic efficacy, it is necessary to make use of reactions that offer perfect conversion efficiency (such as click chemistry). Herein we wish to report a) the successful click synthesis of two different series of dendronized polymers (DPs), AmDP1-AmDP3 and EsDP1-EsDP3, from heterobifunctional amide-linked macromonomers (AmM1-AmM3) and ester-linked macromonomers (EsM1--EsM3), respectively, b) the novel and unique organogelation property of one such poly(triazole)-based dendronized polymer AmDP2, c) the remarkable functionalgroup synergistic effect on polymer interchain H-bonding, owing to the placing of many amide functionalities in close proximity along the polymer chain, and most importantly d) that the macromolecular interactions among the dendronized polymer chains are strongly influenced by the size of dendritic appendages and the nature of the linker functionality. To our knowledge, synthesis of dendronized polymers by AB-type macromonomer polymerization has not been reported before. Moreover, although physical organogels based on dendrimers [4] and linear polymers [5] are known, those based on click poly(triazole) polymers [6] and dendronized polymers [7] are extremely rare.The click macromonomer polymerization is basically a step-growth polymerization. Therefore, to achieve a higher degree of polymerization (DP), it must be ensured that the AB-hetero-bifunctional monomers AmM1-AmM3 are of perfect purity and structural homogeneity. For this purpose, we made use of the symmetrical aliphatic hydrocarbon-based Meldrums acids 1-3[8] as our starting materials. Simple functional-group transformations then led to the target amide-linked macromonomers AmM1-AmM3 in good yields and high purities (see Supporting Information). The macromonomers AmM1-AmM3 were then polymerized in the presence of sodium ascorbate and CuSO 4 in a 1:1:1 solvent mixture of THF, DMF, and water at 25 8C for 4 days. To counteract the poor solubility of the products, DMF was added to m...

Click‐Dendronized Poly(amide–triazole)s—Effect of Dendron Size and Polymer Backbone Symmetry on Self‐Assembling and Gelation Properties

“…Similar to 1-G1, the poor solubility properties of 2-G1 G1 in THF precluded its SEC examination. In addition to the expected polymer peak, the SEC chromatograms of all examined dendronized polymers also showed the presence of many low-molecular-weight (LMW) peaks, suggesting the formation of cyclic oligomers during the copolymerization process [9] (see Supporting Information for details). The calculated M w , PDI, degree of polymerization (DP) values, and weight percentage of LMW oligomers (up to [3+3]mer) are given in Table 1.…”

“…Hence, sequential introduction of the azide or acetylene functionalities was used. The known monoacid monoesters 7-Gm [9] were first coupled to propargyl amine in the presence of 1-hydroxybenzotriazole (HOBt) and a coupling reagent to produce the propargyl amides 8-Gm in 80-92 % yield. It was found that EDCI was useful for preparation of the first-generation (G1) dendrimer.…”

“…Finally, coupling of the acids with propargyl amine again under the same coupling conditions afforded the target diacetylenes 5-Gm in 80-94 % yield. The target diazides 4-Gm were similarly prepared in 67-79 % by reacting the known acids 10-Gm [9] with 4-(azidomethyl)benzyl amine under the standard coupling conditions. The strucScheme 1.…”

“…[9] To ensure good polymerization efficiency, a 1:1:1 mixture of THF, DMF, and water was required to maintain homogeneous conditions, at least in the initial stage of the polymerization. After stirring at 25 8C for four days, the polymer was isolated either by precipitation in aqueous HCl, or by extraction with CH 2 Cl 2 followed by solvent evaporation.…”

“…[8] We earlier reported the interesting self-assembling properties of a series of clickdendronized poly(amide-triazole)s 1-Gm (m = 1-3) prepared from the polymerization of AB-type monomers 3-Gm, in which a synergistic cooperative hydrogen-bonding effect and dendrimer-generation-specific organogelating properties were found. [9,10] Despite these findings, the mechanism by which triazole and the dendritic units, as well as the polymer backbone, exert their control on the supramolecular properties still deserve further investigations. Herein, we disclose additional research findings based on the study of a new series of click-dendronized poly(amidetriazole)s 2-Gm Gn (m = 1-3; n = 1-3) bearing alternating dendritic side chains of different size, and provided additional insights of the structural factors on their organogelating and self-assembling properties.…”