50 | December 2021 www.facs.website to impart not only a spatial structure but also electronic properties and chemical reactivity. They therefore have value as vessels for the construction of custom polymers with tailor-made structures and functionality. MOFs, first developed in the 1990s, have mostly seen applications using gases and solvents, aiming for the adsorption and separation of these micromolecules. In contrast, this article describes MOF nanospaces’ ability to produce functional macromolecular (polymeric) materials. 500 publications per year now involve some combination of MOFs and polymers together, and it’s not hard to see why as it becomes increasingly clear that the skillful use of MOFs allows nano-level chemical manipulation of not only organic polymers, but biological and inorganic polymers as well. As the possibilities for combinations of MOFs and polymers are endless, this area will continue to expand into a wide-reaching field with strong impact on various academic disciplines and industries4-8. Controlling polymer primary structures DNA and proteins show us that biological systems can exert such strict control over their polymerizations’ regiochemistry, stereoregularity, molecular weight, and sequence that they can’t help but seem like the ultimate system when measured by the standards we currently apply to synthetic polymerization. Life’s key to producing this kind of elaborate polymer structure is the highly accurate transcription of the molecular information held by nucleic acids, and it is within the organized nanospaces of enzymes that it does this. Similarly, when the micropores of MOFs are used as a vessel for polymerization, precise control over the structures of the polymers that come out becomes possible (Figure 2)4-6. The history of this effort starts in 2005, with our development of the first polymerization method using a MOF nanospace9. Using the 1D channels of [M 2(L)2(ted)] n (M = Cu2+ or Zn2+, L = terephthalate or its derivatives, ted = triethylenediamine), it became clear that radical polymerization of vinyl monomers in their pores proceeds in a manner resembling living radical polymerization9,10. Recently, Schmidt and Antonietti have also repor ted that they can exer t greater control over the molecular weight by carrying out reversible addition-fragmentation chain transfer (RAFT) polymerization inside MOF spaces11. Atom transfer radical polymerization (ATRP) utilizing the metals inside MOFs has also become possible, with developments progressing into precision polymerization systems using recoverable catalysts12. When copolymerization is carried out in a MOF, monomer reactivities differ greatly to their behavior in solution. Recently, it was observed that immobilizing monomers on a MOF framework through coordinate or covalent bonding caused wideranging changes to the makeup of the resulting copolymer Polymerization of vinyl monomers in MOF pores brings about polymers with stereoregularity that sensitively responds to the size and shape of the MOF space, as well as the presence or absence of unsaturated coordination sites10,13,14. This makes the formation of highly isotactic polymers, which are otherwise difficult to achieve by radical polymerization, possible. By also using unsaturated coordination sites as polymerization catalysts, Dinca and co-workers achieved highly stereospecific polymerization of butadiene monomers using a MOF possessing cobalt-substituted metal sites15. Combining the aforementioned RAFT Figure 1: Schematic representation of MOFs. Figure 2: Polymerizations in MOFs allow multi-level controls over the structures of polymers, depending on the nanoporous structures of MOF templates.
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