www.asiachem.news December 2021 | 51 and ATRP methods, simultaneous control over molecular weight, stereoregularity, and terminal structure has also become possible16. When several kinds of monomers are simply mixed together and polymerized, the monomers usually connect randomly to one another, resulting in copolymers with no particular predetermined sequence. In contrast, 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 wide-ranging changes to the makeup of the resulting copolymer17,18. For example, mixing 5-vinylisophthalate (S) and copper ions gives a MOF with a perfectly repeating space of 0.68nm between the S units which are to act as monomers (Figure 3a)18. After introducing acrylonitrile (A) into the 1D channels of this MOF, carrying out copolymerization between the host and guest, and finally digesting the MOF, a polymer is obtained which has a structure consisting of perfectly repeating AAAS units (Figure 3b)18. This implies that precisely three A monomers fit and polymerize between each S embedded regularly along the 1D pores, and is a fascinating system for imprinting the periodicity of a MOF into a polymer. Recently, Sada and co-workers found that the step-growth polymerization of immobilized monomers in a MOF is also capable of controlling the molecular weight of polymers, in contrast to solution processes19. Between all these studies, it has become clear that polymerization within MOF pores can play a strong part in the precision-controlled synthesis of polymers, regardless of the polymerization mechanism. Aligning polymer chains Polymers synthesized within MOF pores form in a state where their orientation is perfectly restricted by the crystalline space. If, perhaps, the template MOF could be removed without disturbing that state, it follows that it would be possible to control the aggregation structure of the polymers inside according to the dimensionality of the MOF used. Following this principle, it was discovered that polythiophene synthesized in a MOF’s 1D pores give rod-shaped particles when said MOF is removed20. Polymer chains in these particles are highly aligned along their long axis, and possess electrical conductivity on the order of 1000 times higher than that of polythiophene synthesized in solution. Further, when polythiophene chains are inserted into a chiral MOF, it has been shown that even upon removal of the MOF mold (that is, in the complete absence of a source of chirality), the isolated polythiophene continues to exhibit chirality21. While this method of control over orientation and conformation is effective with respect to rigid, conjugated polymers, it remains unsuitable for the control of soft, vinyl polymers. To that end, a polystyrene network wherein polymer chains are perfectly aligned along one axis was synthesized by partially introducing divinylated ligands – species capable of bridging the styrene chains – into the walls of a 1D channel MOF, then polymerizing styrene within22. In this system, polymer networks were crosslinked while completely aligned in a MOF space, so the alignment of the polymer chains was conserved after MOF destruction, and even withstood heat and solvent treatment. Recently, the synthesis of ultrathin-film 2D polymers of monomolecular thickness has been demonstrated by crosslinking within the 2D space of a ‘pillared-layer MOF’ (Figure 4)23. These polymers display unique viscoelastic properties due to their unique topological structure, which completely excludes any interweaving of polymer chains. While the control over polymer network structures such as functional gels and adsorbent materials holds great importance to advanced materials science and technology, the standard polymerization reactions carried out in solution inevitably form randomly arranged crosslinks, making such control dif ficult. By using a MOF possessing 3D connected pores as a template, however, Figure 3: a) Formation of a MOF with styryl groups (S) periodically aligned along the 1D channels. b) Copolymerization of acrylonitrile with the MOF provides sequenceregulated copolymers reflecting on the periodicity of the MOF channels. Figure 4: a) Crystal structure of a pillared-layer MOF and an MD simulation snapshot of the MOF containing styrene monomers. b) AFM image and height profile of ultrathin polystyrene film obtained from the MOF.
RkJQdWJsaXNoZXIy NDU2MA==