Polymer networks can have a range of desirable properties such as mechanical strength, wide compositional diversity between different materials, permanent porosity, convenient processability and broad solvent compatibility1,2. Designing polymer networks from the bottom up with new structural motifs and chemical compositions can be used to impart dynamic features such as malleability or self-healing, or to allow the material to respond to environmental stimuli3,4,5,6,7,8. However, many existing systems exhibit only one operational state that is defined by the material’s composition and topology3,4,5,6; or their responsiveness may be irreversible7,9,10 and limited to a single network property11,12 (such as stiffness). Here we use cooperative self-assembly as a design principle to prepare a material that can be switched between two topological states. By using networks of polymer-linked metal–organic cages in which the cages change shape and size on irradiation, we can reversibly switch the network topology with ultraviolet or green light. This photoswitching produces coherent changes in several network properties at once, including branch functionality, junction fluctuations, defect tolerance, shear modulus, stress-relaxation behaviour and self-healing. Topology-switching materials could prove useful in fields such as soft robotics and photo-actuators and also provide model systems for fundamental polymer physics studies.
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We thank the National Science Foundation (CHE-1629358 for J.A.J. and CHE-1506722 for X.L.) for support of this work. This work was supported in part by the MRSEC Program of the National Science Foundation under award number DMR-1419807. Y.G. thanks ExxonMobil for an ExxonMobil–MIT Energy Fellowship. A.P.W. and E.A.A. acknowledge funding from the Research Corporation for Scientific Advancement through a Cottrell Scholars Award. X-ray scattering experiments were performed at the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory (DE-AC0206CH11357). We thank G. Clever for discussions.
Nature thanks S. Sheiko and T. Sirk for their contribution to the peer review of this work.