Terminus-dependent insertion of molten poly(ethylene glycol) into a flexible metal-organic framework

Insertion of polymers into microporous metal–organic frameworks (MOFs) appeared recently as a powerful strategy to accomplish very difficult separations. However, the dynamics of their insertion remains to be fully unravelled. In this article, we describe the spontaneous insertion of poly(ethylene glycol) (PEG) into a flexible MOF. Thanks to the structural changes upon PEG insertion, in situ powder X-ray diffraction (PXRD) analysis could be performed. For PEGs with identical main chains, the insertion speed was strongly dependent on the nature of the terminus. Furthermore, a concentration dependence for terminal groups was observed, providing insight into the pore-opening mechanism, and suggesting a mildly cooperative terminus-first insertion. This establishes polymers as very valuable tools to study MOF dynamics, as their extended size allows domains with clearly different properties within the same macromolecule.