www.asiachem.news December 2021 | 53 various polymers to form mixed-matrix membranes. Compared to the conventional inorganic fillers, MOFs’ hybrid organic-inorganic structures display high compatibility with polymers. Complex membranes constructed using this method have already shown excellent functionality as gas separation membranes, nanofiltration membranes, and solid electrolytes35. Separating desired polymer chains Polymers are made up of chains containing countless monomer units, and can thus have any number of diverse and distinct structures. While sequence disorder is of course present in the case where a mixture of several monomers is used, a statistical distribution of molecular weights is observed even if only one monomer type is used. Stereo- and regiochemical control are often difficult to achieve, and thus the polymeric materials we use in our dai ly lives consist of mixtures of various types of polymer chain. It goes without saying that the polymers’ structures determine their physical properties, and so the isolation of specific desired polymers from mixtures is extremely important both academically and industrially. This is no simple task, however, due to the complete ineffectiveness of standard chemical separations (distillation, recrystallization, solvent extraction, etc.) with respect to polymer-polymer mixtures. Essentially, even if there were a dif ferent moiety present at some point in a polymer chain, we could not usually single that moiety out and isolate only the polymer chains that possess it in a designated location on the chain. On top of that, polymers undergo self-entanglement unique to the chain-like nature of their structures, so even in solution they exist in states which constantly bend and knot themselves together, leaving us no option but to process them as ‘clumps’. For this reason, one could discriminate between polymers based on ‘clump’ size (which is proportional to chain length) using size-exclusion chromatography, but recognition and separation according to some minute change of structure within a long chain (large ‘clump’) was impossible. Recently, our group has sought a resolution to this problem by methods utilizing the adsorption of polymers into the nanospaces of MOFs29,36,37. While such a description makes it sound simple, at the time it was considered common sense that the very idea of polymers adsorbing into channels on the sub-nanometer scale would be impossible, due to the great loss of entropy it would entail. In fact, there were no reported examples of their adsorption into conventional microporous materials – zeolites – either. Nevertheless, by shunning ‘common sense’ we were able to find that many polymers do, in fact, spontaneously penetrate into MOF Figure 7: Well-defined nanopores of MOFs serve as a universal workspace for highprecision polymer recognition and separation, which is not possible with conventional methods. Figure 6: Time-evolved snapshots from MD simulations of PEG (Mw = 600 g/mol) intercalated in [Zn2(terephthalate)2(triethylenediamine)]n.
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