52 | December 2021 www.facs.website control of various polymer network structures becomes possible. For example, if the synthesis of a polysaccharide is carried out within the 3D channels of [Cu3(btc)2] (btc = benzene-1,3,5-tricarboxylate), polymeric particles possessing mesopores are obtained24. As this type of material efficiently takes up guests such as drugs and peptide molecules, it is hoped to see use in biological applications such as drug delivery systems. Research is also being carried out into the precise control over bridged structures by carrying out host–guest cross-polymerization between monomer guests and reaction sites embedded into MOF ligands. For example, a ‘click’ coupling reaction proceeds efficiently between a tetraalkynyl guest and a MOF host possessing two azide groups on each of its ligands25. Upon removal of the MOF’s metal atoms, a polymer particle with a shape reflecting that of the original MOF particle is obtained, and exposing this particle to solvent then expands it isotropically into a gel. More recently, success has also been found in designing anisotropically-expanding gels by using pillared-layer 2D MOFs as templates26. If all different types of polymeric material were mutually miscible, we could freely tune the functional properties of these polymers by simply mixing them, bringing about possibilities that would be unobtainable with mere homopolymers. However, the vast majority are instead mutually immiscible, resulting in the spontaneous phase separation of polymer mixtures. In our work, we have shown that mutually immiscible polymer species (such as polystyrene and methyl methacrylate) can be mixed on the molecular level by using the non-equilibrium approach of encapsulating different species of polymer inside a MOF’s pores, then removing the MOF27. Integrating MOF-polymer functions By introducing polymer chains into MOF nanospaces, one can precisely control the Figure 5: Fabrication of a perfectly alternating donor–acceptor architecture at the molecular level. number of interacting polymer chains, their alignment, and their surrounding environment. Accordingly, it becomes possible to characterize the physical properties of either one or several polymer chains in isolation, which are distinct from those in the bulk state. Accordingly, we have revealed that the nanoconfinement in MOF channels has marked effects on the dynamics as well as the thermal behaviors of the encapsulated polymer chains28,29. By enclosing conductive polymers like polythiophene in MOF pores, changes in hole mobility could be induced depending on the number of polymer chains aggregated together30. Preparing such a host–guest complex gives a material possessing a combination of both porosity and conductivity, and this was further developed into sensor materials capable of detecting guest NO2 gas at the ppb level by measuring their impact on host conductivity31. It has also been shown that donor-acceptor structures can be rationally assembled at the molecular level by forcing them to reflect anisotropic MOF framework structures. By first preparing a MOF with titanium oxide nanowires (acceptors) present in its pores, then synthesizing polythiophene chains (donors) in the 1D channels, a perfectly alternating array structure was achieved (Figure 5)32. Investigations showed that this array creates long-lived charge separation states, with the half-life of the charged species exceeding 1 millisecond – about 1,000 times longer than that of any other reported titanium oxide system. Results like this provide a useful guide to building new materials to raise the efficiency of photoelectric devices, and draw attention to their possible applications in solar cells. The above methods al l focus on the post-synthetic introduction of polymers to form host–guest complexes. In contrast, if we instead use polymers which incorporate ligands that can make up a MOF, we may create hybrids where these materials are connected directly by covalent bonds. Cohen and co-workers are researching what they call a ‘polyMOF’ – a complex where polymers with repeating units containing terephthalate (a common MOF ligand) are incorporated directly into the MOF structure by mixing them in during the MOF synthesis process33. Building complexes like this enhances properties such as the stability and hydrophobicity of the MOF, and there are even cases where the MOF crystals form structures resembling thin films. Recently, the Johnson group has been carrying out the synthesis of polyMOF nanoparticles using block copolymers34. In this system, MOF crystals are formed around blocks of ligand moieties in the polymer while the remaining blocks encircle the outside, resulting in nanoparticles with decorated surfaces. In short, the use of block copolymers as precursors enables simultaneous control over both the MOF nanoparticles and their surfaces. Practicality issues arise in many cases when MOFs in particle form are considered for use in real-world implementations. As such, there are times when MOFs are simply mixed with By enclosing conductive polymers like polythiophene in MOF pores, changes in hole mobility could be induced depending on the number of polymer chains aggregated together
RkJQdWJsaXNoZXIy NDU2MA==