polymerization of methyl acrylate from uio-66-ddmat for the synthesis of self-assembled metal-organic framework monolayers

Self-Assembly of Polymer-Grafted Metal Organic Framework Monolayers

Metal-organic frameworks (MOFs) are crystalline, porous materials that offer large surface areas while being readily tunable through modifications of the organic ligands or metal nodes1,2. MOFs are constructed from two components: an organic ligand and metal ions (or metal ion clusters referred to as secondary building units, SBUs). MOFs have been investigated for chemical (e.g., gas) storage, separations, catalysis, sensing, and drug delivery. Generally, MOFs are synthesized in the form of crystalline powders; however, for ease of handling in many applications, formulation into other form factors is desirable if not necessary3,4. For example, mixed matrix membranes (MMMs) of MOFs with polymers have been reported as one particularly useful composite of MOFs and polymers5. However, in some cases, MMMs may have limitations due to the incompatibility/immiscibility between MOF and polymer components5,6. Therefore, strategies have been explored to incorporate polymer grafting directly onto MOF particles to form polymer-grafted MOFs.
Inorganic and metallic nanoparticles exhibit unique behavior in terms of optical, magnetic, catalytic, and mechanical properties7,8. However, they tend to aggregate easily after synthesis, which can hinder their processability. To enhance their processability, polymer chains can be grafted onto the particle surface9. Nanoparticles with high grafting density offer excellent dispersion and stability due to favorable enthalpic interactions between surface polymers and the solvent and entropic repulsion interactions between the particles10. Grafting of polymers onto particle surfaces can be achieved through a variety of strategies11. The most straightforward approach is the &#39;grafting to&#39; particle strategy, where functional groups, such as thiols or carboxylic acids, are introduced at the ends of polymer chains to directly bind to the nanoparticle. When complementary chemical groups, such as hydroxyls or epoxides, are present on the particle surface, polymer chains can be grafted onto these groups via covalent chemical approaches12,13. The &#39;grafting from&#39; particle or surface-initiated polymerization method involves anchoring initiators or chain transfer agents (CTAs) to the surface of nanoparticles and then growing polymer chains on the particle surface through surface-initiated polymerization. This method often achieves higher grafting density than the &#39;grafting to&#39; approach. Furthermore, grafting from enables the synthesis of block copolymers, thereby expanding the diversity of polymer structures that can be immobilized on a particle surface.
Examples of polymer grafting onto MOF particles have begun to emerge, largely focused on installing polymerization sites on the organic ligands of the MOF. In a recent study published by Shojaei and coworkers, vinyl groups were covalently attached to the ligands of Zr(IV)-based MOF UiO-66-NH2 (UiO = Universitetet i Oslo, where the terephthalic acid ligand contains an amino substituent), followed by methyl methacrylate (MMA) polymerization to create polymer-grafted MOFs with a high grafting density (Figure 1A)14. Similarly, Matzger and coworkers functionalized the amine groups on a core-shell MOF-5 (a.k.a., IRMOF-3@MOF-5) particles with 2-bromo-iso-butyl groups. Using polymerization initiated by the 2-bromo-iso-butyl groups, they created poly(methyl methacrylate) (PMMA)-grafted PMMA@IRMOF-3@MOF-515.
In addition to functionalizing the ligand of the MOF for grafting from polymerization, new methods that create sites for polymer grafting via coordination to the metal centers (a.k.a., SBUs) of the MOF have also been explored. For example, a ligand that can bind to the MOF metal centers, such as catechol (Figure 1B), can be used to coordinate to exposed metal sites on the MOF surface. Using a catechol-functionalized chain-transfer agent (cat-CTA, Figure 1B) the MOF surface can be functionalized and made suitable for a grafting from polymerization.
Recently, the aforementioned strategy for synthesizing MOFs-polymer composites has also been used for the creation of free-standing MOF monolayers16,17,18. MOFs such as UiO-66 and MIL-88B-NH2 (MIL = Materials of Institute Lavoisier) were surface-functionalized with pMMA using a ligand-CTA strategy (Figure 1B). The polymer-grafted MOF particles were self-assembled at an air-water interface to form self-supporting, self-assembled metal-organic framework monolayers (SAMMs) with a thickness of ~250 nm. The polymer content in these composites was ~20 wt%, indicating that SAMMs contained ~80 wt% MOF loading. Followup studies showed that different vinyl polymers could be grafted onto UiO-66 to produce SAMMs with different characteristics19. Analytical techniques such as thermogravimetric analysis (TGA), dynamic light scattering (DLS), and gel permeation chromatography (GPC) were used to calculate polymer brush height and grafting density of the surface-grafted MOF-polymer composites.
Herein, the preparation of SAMMs from UiO-66-pMA (pMA = poly(methyl acrylate)) is presented. For the polymerization of methyl acrylate (MA), 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT, Figure 1B) is used as the CTA19. The functionalization of the UiO-66 particles with cat-DDMAT is essential for the grafting of pMA. Cat-DDMAT can be synthesized through a two-step acylation procedure from a commercially available CTA&#160;and dopamine hydrochloride19. It is also crucial to use UiO-66 particles of uniform size for the successful formation of SAMMs19; therefore, the UiO-66 used in this study was prepared using the continuous addition method20. The polymerization method employed for forming the polymer-grafted MOF particles is photoinduced reversible addition-fragmentation chain transfer (RAFT) conducted under blue LED light (using an in-house-built photoreactor, Figure 2) with a tris(2-phenylpyridine)iridium (Ir(ppy)3) photocatalyst. RAFT polymerization gives exceptionally narrow polymer dispersity that can be finely controlled. Free CTA is included during the polymerization reaction because the ratio of transfer agent to monomer allows for control over the molecular weight during polymerization. The amount of cat-DDMAT transfer agent on the surface of the MOF particles is small; therefore, excess free CTA is added and the amount of monomer to be used is calculated based off of the amount of free CTA present21. After polymerization, the free polymer produced from the free CTA is removed by washing, leaving only the polymer-grafted UiO-66-pMA. Subsequently, this composite is dispersed in toluene at a high concentration and used to form SAMMs at an air-water interface.

Author Spotlight: Exploring Self-Assembled MOF-Polymer Composites

使用聚合物包覆颗粒的自组装金属有机框架单层的合成和表征

UiO-66-DDMAT丙烯酸甲酯聚合合成自组装金属有机框架单分子膜

首先，在 DMSO 中移出 2 毫升 UiO-66-DDMAT 颗粒分散体，并将其转移到位于搅拌板上的 10 毫升圆底烧瓶中。将搅拌棒放入烧瓶内。开始搅拌分散体。接下来，将 12 微升铱 III 苯基吡啶催化剂原液移液到烧瓶中。现在，在搅拌的同时将 0.45 毫升 DDMAT 储备溶液加入烧瓶中。接下来，将 1.7 毫升丙烯酸甲酯移液到 20 毫升小瓶中。用微量移液管，向小瓶中加入两毫升DMSO。摇晃小瓶以溶解丙烯酸甲酯。缓慢地将丙烯酸甲酯溶液滴入反应瓶中。停止搅拌动作。然后，用隔膜牢固地密封烧瓶。将一根长针连接到氮气供应歧管。将针头插入隔膜，到达烧瓶的内部空气层。将一根短针插入鼻中隔以形成一个出口。打开氮气阀对溶液进行脱气。将长针头下降到烧瓶底部 15 分钟。然后，将针头抬高到内部空气层。脱气后，依次取出短针头，然后取出长针头。然后，关闭氮气阀。现在，将定制的蓝光 LED 光反应器放在搅拌板上。将其连接到电源后，验证蓝光发射。在光照下重新开始搅拌混合物。用铝箔覆盖反应器的上部，以防止过度的光线照射。当反应溶液的粘度增加到无法再搅拌的程度时，请关闭 LED。对于颗粒自组装，首先，在甲苯中产生颗粒的分散体。小心地将大约 10 微升的甲苯分散体滴入装有去离子水的 60 毫米宽的培养皿中。当水面上的甲苯蒸发时，使用铜线制成的环小心地去除单层的一部分。在蒸发剩余的共伴水后，可以观察到独立的单层。从浓缩甲苯分散体中将聚合物-磨砂机金属-有机框架滴在水面上，产生彩虹色单层。使用铜线模具来提升单层，可以形成独立的SAMMS。

Research

JoVE Journal

Chemistry

Polymerization of Methyl Acrylate from UiO-66-DDMAT for the Synthesis of Self-Assembled Metal-Organic Framework Monolayers

Synthesis and Characterization of Self-Assembled Metal-Organic Framework Monolayers Using Polymer-Coated Particles

Metal-organic frameworks (MOFs) are materials with potential applications in fields such as gas adsorption and separation, catalysis, and biomedicine. ...

科研

化学

Surface Modification of UiO-66 with cat-DDMAT for the Synthesis of Self-Assembled Metal-Organic Framework Monolayers