Organically Modified Polyoxometalate Hybrid Supramolecular Polymers: Self-Assembly And Assembling Structure

Supramolecular polymer materials have attracted enormous attention in recentyears due to their unique structure and potential applications. Much progress have beenmade on both fabrication of supramolecular polymer units and their self-assemblyarchitecture. One of the hot pots focuses on the building of hybrid supramolecularpolymer through rational designing, tailoring and realizing its functionalization.Polyoxometalate is one of the best choices to realize these aims. Furthermore, thedrawbacks of POM cluster’s negative effect due to the rigidity of their crystal structurecan be overcome through preparing POM-hybrid supramolecular polymer system.Therefore, it is novel research which refers to optimizing supramolecular polymersystems through rational design and tailoring, and its functionalization, regulatingpolyoxometalate cluster features through dynamic assembly of supramolecular polymer.To realize rational tailoring and optimization of POM-hybrid supramolecularpolymer structure, it may adopt the strategy of covalent and non-covalent grafting aorganic functional group on the surface of POM cluster. Thereby regulating equilibriumof non-covalent interaction to reversible control assembly architecture of POM-hybridsupramolecular polymer, and realizing functionalization of supramolecular polymer and the opening or closing of polyoxometalate features. According to this strategy, wesuccessfully designed and processed POM-hybrid supramolecular polymer and itsstructure was rational tailored and optimized, and realized reversible controlling ofPOM-hybrid supramolecular polymer assembly architecture through stimulus-responseof thermal, pH and the polarity of solvent. It provides a certain reference for furthermulti-functionalizing supramolecular polymer and reversible controllingpolyoxometalate features.Firstly, to exploit a new kind of POM-hybrid supramolecular polymer withthermal-responsive properties. We synthesized two POM-based hydrogen-bondingsynthon through systematically grafting two adeninyl (A) groups onto MnMo6cluster(POM) via “tris” linker, and which were encapsulated by two different surfactants:tetrabutyl ammonium (TBA) and dimethyldioctadecyl ammonium (DODA). We calledthem TBA-A-POM-A and DODA-A-POM-A. The obtained supramolecular polymerunits were characterized through1H NMR, FT-IR, ESI-MS, elemental analysis and TGAto confirm its composition and structure. TBA counterions are replaced completely byDODA. Furthermore, we systemically studied the thermal-induced self-assemblybehavior of the as-prepared POM-hybrid supramolecular polymer. Two types ofhierarchical self-assembled architectures were obtained. The resultant hybridsupramolecular polymer exhibited thermal-induced dynamic self-assembly behaviorswhich greatly depended on the ambient temperature and the chain length of cationicsurfactants. With the encapsulation of a short surfactant tetrabutyl ammonium, thesupramolecular polymer assembled into fibrous, rod-like, and tubular architecturesrespectively upon heating; while for the case of using a long surfactantdimethyldioctadecyl ammonium as counter ions, the assemblies of the complextransformed from fibers to spheres with the increased temperature. Moreover, the twotypes of transformations were both reversible during a cooling process. The relatedmechanism was investigated by combining multiple characterization methods including X-ray crystallography, XPS, FT-IR and temperature-dependent1H NMR, whichindicated that such a thermal-induced morphological transformation resulted from asynergy effect of the variation of the multiple hydrogen bonds among the hybridsupramolecular polymer units and the rearrangement of the surfactants surrounding theMnMo6clusters.Secondly，to build the hybrid supramolecular polymer through rational design andtailoring, two kinds of organic-inorganic hybrid supramolecular polymer based onpolyoxometalate were synthesized through symmetrically grafting two thyminyl (T) andtwo adeninyl (A) groups onto Anderson-type MnMo6clusters (POM), respectively. Thecounter ions of this pair of polymer units (TBA-A-POM-A and TBA-T-POM-T) wereexchanged by sodium cation, and then the clusters was encapsulated by TDA. bytetrakis (decyl) ammonium (TDA). The obtained supramolecular polymer units werecharacterized through1H NMR, FT-IR, ESI-MS, elemental analysis and TGA toconfirm its composition and structure. Every POM cluster was encapsulated completelyby three TDA. Furthermore, we systemically studied the hydrogen bonding-inducedsupramolecular polymerization of the POM-hybrid supramolecular polymer units.Through comparing the1H NMR spectra of TDA-A-POM-A,TDA-T-POM-T+TDA-A-POM-A, and TDA-T-POM-T in the same concentration inCDCl3, we confirm that hydrogen bonding were formed between A and B. Throughcalculating the association constant of hydrogen bonding (Ka) by equilibrium formula,we found the Ka is101.8L mol-1. And then we characterize the growing ofsupramolecular polymer chains by variable concentration NMR, With increasing theconcentration of the peak of hydrogen binding (-NH2in the cycle of adeninyl) shifted tolow field and changed to broaden, this result imply that the base-pair interaction becomestronger and the supramolecular polymer chains were growing during the increasing ofthe concentration. And we also confirmed that the polarity of solvent influenced theself-assembly architecture of the hybrid supramolecular polymer by NMR and SEM. Furthermore, we used the bola surfactant to cross-linked supramolecular polymer chainsby electrostatic interaction between cation group of bola surfactant and POM cluster,and induced the hybrid supramolecular polymer based on POM to form gel. All theseexperimental results show that1) this method is feasible to build the hybridsupramolecular polymer through rational design and tailoring, which is realized bycovalent and non-covalent grafting on the surface of POM cluster;2) the temperatureand the polarity of solvent are all influenced the self-assembly architecture of the hybridsupramolecular polymer, and the viscosity of the supramolecular polymer was improvedgreatly by cross-linking between supramolecular polymer chains.Thirdly, to realize instantaneous and reversible gelation of hybrid supramolecularpolymer, two types of hybrid supramolecular polymer gel system were obtained throughmixing appropriate dicarboxylic acid additives with TBA-P-POM-P（P was representedpyridyl） in acetonitrile, and DODA-A-POM-A by itself in DMSO or DMF. In thegelation process, two systems were confirmed to supramolecular polymerize intopolymer chains through unidirectional hydrogen bonding between supramolecularpolymer units, which drove the polymer units into one-dimensional fibers. The fiberswere demonstrated to further intertwine together through the lateral hydrogen bondingor van der Waals interaction of side alkyl chains and forming three-dimensionalnetworks. The distance between POM cluster units was proved to be a criterion for theformation of gels, which is related to the counterion density along the supramolecularpolymer chains and the solvent/fiber interfacial energy. The gelation behavior of hybridsupramolecular polymer can be simply controlled through adjusting the length ofdicarboxylic acids. More interestingly, the hybrid supramolecular polymer gelsexhibited quick response to pH and environmental temperature reversibly. Scanningelectron microscopy, transmission electron microscopy and X-ray diffraction were usedto characterize the gel morphology and structure. All these results show that gelator canstabilize solvent，and thus forming supramolecular polymer gel by the synergetic effect of hydrogen bonding and van der waals force of alkyl chains. The transformation ofsol-gel can also be realized by adjusting pH and temperature.In conclusion, in this dissertation, we focus on the study of building the hybridsupramolecular polymer based on polyoxometalate by inducing hydrogen bindingsupramolecular polymerization, reversible controlling of assembly architecture,assembly of polyoxometalate-based hybrid supramolecular polymer gel and realizingdifferent modulation strategy, such as enviromental temperature, polarity of solvent andpH. All these results can provide references for preparing a novel hybrid supramolecularpolymer. We also believe that the method of building hybrid supramolecular polymerthrough rational design and tailoring, and the strategies of modulate reversible assemblyarchitecture and the law of assembly can provide reference basis for development ofhybrid supramolecular polymer, providing a new research idea in hybrid supramolecularpolymer area.