Polyoxometalate-Polymer Hybrids Constructed Via Covalent Bonds:Synthesis, Catalysis And Vesicle Structures

Polyoxometalates (abbreviated as POMs), are a class of anionic metal-oxygen clusters, which consist of early transition metals in their highest oxidation states. Different from other metal oxides, POMs have molecularly defined structures. Due to these discrete structures, POMs always show unique properties. With the advancement of technology, investigations on POMs-based materials applied in catalysis, pharmacy, energy, and photoelectricity have captured considerable attention. However, there are still problems on their practical application:(1) Most POMs are normally crystalline powders with poor processability;(2) POMs are heavy metal contained clusters;(3) POMs have poor compatibility with other materials. Therefore, development of hybrid POM-based materials with easy processability and sustainability becomes a very important topic.Polymers, with good mechanical performance and easy processability, have been widely used in people’s daily life and also high-tech fields. However, limited with their compositions and structures, there are only a few functional polymers. According to the advancement of technology, design and preparation of post-functional polymer materials are required.Consequently, development of functional materials based on POM-polymer hybrids can fit the requirement of novel functional materials. Such hybrids can both have the functions of POM clusters and easy processability of polymers. To create synergistic effect, the hybridization between POMs and polymers needs to be at molecular level. In this way, they can make the best of both. In most cases, such hybridization is based on electrostatic interactions since it is a cost-effective and convenient method. However, these materials often suffer from the leakage of POMs as a consequence of the relatively weak connection between POMs and polymers. Thereupon, another hybridization approach arises, in which strong covalent links are established. It is a very promising approach, but so far only a few examples have been reported. That is because such approach needs prior covalent modification of the POM clusters. Considering all the above, herein we propose our work proposal. We focus on materialization of POMs by developing POM-polymer hybrids via covalent links. POMs with Wells-Dawson structure are selected because they can be modified easily.In the first part, a novel POM-polymer based catalyst was designed and prepared. The work was inspired by the great research needs of applicable POM catalysts and the great social needs of air pollution control. Previous studies have clearly demonstrated that sustainable catalysts are generally prepared to be heterogeneous. Therefore, considering practically industrial utilization, novel heterogenization strategies of POM-based catalysts need to be developed. Thus, we focus on covalent immobilization of POM clusters on solid support. Such covalent immobilization strategies, especially covalent immobilization of POM clusters on polymer support, are rarely reported. Here, macroporous resin was employed as the solid support due to its stability and wide use in industrial application. First, K10[α2-P2W17O61]·H2O was selected as the catalytic component. A two-azido-containing POM derivative (A-POM) was synthesized and characterized through1H and31P NMR and FT-IR methods. Second, the resin was post-functionalized in order to introduce alkynyl groups into the channel surface. According to acid-base titration, the reaction conversion was calculated to be76%. Ninhydrin method was also used to test the reaction. Third, via click chemistry, a chemical synthesis consistent with the goals of green chemistry, POM clusters were covalently immobilized onto the channel surface of the macroporous resin. According to TGA data, the loading amount was calculated to be0.15mmol/g. Besides, XPS, FT-IR, BET methods were also used for support.In the second part, the catalytic activity of the heterogeneous catalyst was appraised. First, the oxidization of tetrahydrothiophene (THT), a well-known gasoline contaminant, was used as benchmark reaction to appraise catalytic performance. The progress of the reaction could be monitored by HPLC. Second, hydrogen peroxide was selected as the environment-friendly oxidant and the catalytic condition was optimized. The catalytic reaction was confirmed with100%selectivity. The oxidation reaction exhibited pseudo-first-kinetics and agreed well with other reports. Third, the catalyst was separated simply by filtration and there was no POM residual in the filtrate. That is because of the strong covalent link between POM clusters and resin. Fourth, the recovered catalyst was reused for a number of cycles and no activity loss was observed. Besides, the heterogeneous nature of catalysis was proved through catalyst filtration test.In the third part, the macromolecule-to-amphiphile conversion process of a POM-PS hybrid and structures of the assembled hybrid vesicles were investigated. Before, our group had reported the synthesis of linear POM-PS hybrid polymers. The impacts of solution concentration, temperature, and time on the self-assembly morphology had also been discussed. Here, we focused on the macromolecule-to-amphiphile conversion process and how POM head impacted the vesicle structure. The POM-PS hybrid used here was composed of a POM head and a long PS tail. The number average molecular weight of PS was86000g/mol. First, a solution of the hybrid (2.5mg/mL in DMF) was placed in a vial and a certain amount of H+-resin was added in order to covert the hybrid to an amphiphile. The ion-exchange method is widely used but how it actually works has rarely been reported. Second, a molecule (Bu4N+-POM-Tris) was designed and prepared in order to further research the ion-exchange method. As above, a same ion-exchange process was adopted. By using1H NMR method, all the Bu4N+counterions were demonstrated to be replaced by H+through such a method. Third, the hybrid vesicle structure with a large membrane thickness was further investigated. Different from diblock copolymer, PS chains were required to adopt a highly stretched conformation during self-assembly process due to the impact of the giant POM head, leading to an increase of the membrane thickness. The significance of this work is to gain an understanding and control of the macromolecule-to-amphiphile conversion process. Such a fundamental understanding provides us with an opportunity to prepare POM-containing vesicles with some "value-adding" functionalities.In summary, we achieve the materialization of POMs by developing heterogeneous catalyst via covalent immobilization and fabricating nanomaterial via self-assembly. Final conclusions are given as follows:1. An effective immobilization strategy was proposed by a covalent link between POM clusters and resin. The usage of ’click chemistry’ made the preparation green.2. The heterogeneous catalyst exhibited high efficiency and selectivity in oxidation of THT. No POM species or its fragments were detected in the filtrate, stemming from the strong covalent bonding.3. Because of the sub-millimeter size, the heterogeneous catalyst could be recovered via simple filtration. And it could be reused at least five times without detectable catalytic activity loss.4. Through a designed molecule Bu4N+-POM-Tris, all the Bu4N+counterions were demonstrated to be replaced via ion-exchange method. Due to the impact of the giant POM head, PS chains were required to adopt a highly stretched conformation during self-assembly process, leading to an increase of the membrane thickness.