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

Synthese en karakterisering van zelf-geassembleerde metaal-organische raamwerk monolagen met behulp van polymeer-gecoate deeltjes

Polymerisatie van methylacrylaat uit UiO-66-DDMAT voor de synthese van zelf-geassembleerde metaal-organische raamwerkmonolagen

Pipeteer om te beginnen twee milliliter van een UiO-66-DDMAT-deeltjesdispersie in DMSO en breng deze over in een kolf van 10 milliliter met ronde bodem op de roerplaat. Plaats een roerstaaf in de kolf. Begin met roeren van de dispersie.Pipetteer vervolgens 12 microliter Iridium III-fenylpyridinekatalysator in de kolf. Voeg nu al roerend 0,45 milliliter van de DDMAT-bouillonoplossingen toe aan de kolf. Pipetteer vervolgens 1,7 milliliter methylacrylaat in een injectieflacon van 20 milliliter.Voeg met een micropipet twee milliliter DMSO toe aan de injectieflacon. Schud de injectieflacon om het methylacrylaat op te lossen. Breng de methylacrylaatoplossing langzaam druppelsgewijs in de reactiekolf.Stop met roeren. Sluit de kolf vervolgens goed af met een septum. Sluit een lange naald aan op een stikstoftoevoerspruitstuk.Steek de naald door het septum om de interne luchtlaag van de kolf te bereiken. Steek een korte naald door het septum om een uitlaat te creëren. Open de stikstofklep om de oplossing te ontgassen.Laat de lange naald 15 minuten naar de bodem van de kolf zakken. Til vervolgens de naald op naar de interne luchtlaag. Verwijder na het ontgassen achtereenvolgens de korte naald, gevolgd door de lange naald.Sluit vervolgens de stikstofklep. Plaats nu een op maat gemaakte LED-fotoreactor met blauw licht op de roerplaat. Controleer na het aansluiten op de voeding de emissie van blauw licht.Begin het roeren van het mengsel opnieuw onder blootstelling aan licht. Bedek het bovenste deel van de reactor met aluminiumfolie om overmatige blootstelling aan licht te voorkomen. Schakel de LED uit wanneer de viscositeit van de reactieoplossing toeneemt tot het punt waarop deze niet meer kan worden geroerd.Voor zelfassemblage van deeltjes moet eerst een dispersie van de deeltjes in tolueen worden gemaakt. Laat voorzichtig ongeveer 10 microliter van de tolueendispersie vallen op een 60 millimeter brede petrischaal gevuld met gedeïoniseerd water. Wanneer het tolueen op het wateroppervlak is verdampt, gebruik dan een lus van koperdraad om voorzichtig een deel van de monolaag te verwijderen.Na verdamping van het resterende co-associated water kon een vrijstaande monolaag worden waargenomen. Het druppelen van polymeer-grafter metaal-organische raamwerken op water uit een geconcentreerde tolueendispersie resulteerde in een iriserende monolaag. Het gebruik van een mal van koperdraad om de monolaag op te tillen, maakte de vorming van vrijstaande SAMMS mogelijk.

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. ...

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