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 stimuli3–8. However, many existing systems exhibit only one operational state that is defined by the material’s composition and topology3–6; 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.Using topology-switching metal–ligand cages to crosslink polymer networks produces gels whose chemical and mechanical properties can be radically and reversibly switched on irradiation.

中文翻译：

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以金属-有机笼作为交联的聚合物网络中的光开关拓扑

聚合物网络可以具有一系列理想的特性，例如机械强度、不同材料之间广泛的成分多样性、永久孔隙率、方便的加工性和广泛的溶剂兼容性 1,2。使用新的结构基序和化学成分自下而上设计聚合物网络可用于赋予动态特征，如延展性或自愈性，或使材料对环境刺激做出反应 3-8。然而，许多现有系统仅表现出一种由材料的成分和拓扑结构定义的操作状态3-6；或者它们的响应性可能是不可逆的 7,9,10 并且仅限于单个网络属性 11,12（例如刚度）。在这里，我们使用协同自组装作为设计原则来制备可以在两种拓扑状态之间切换的材料。通过使用聚合物连接的金属-有机笼网络，笼子在辐射时会改变形状和大小，我们可以用紫外线或绿光可逆地切换网络拓扑。这种光开关同时在几个网络属性中产生连贯的变化，包括分支功能、结波动、缺陷容限、剪切模量、应力松弛行为和自愈。拓扑切换材料可以证明在软机器人和光致动器等领域很有用，还可以为基础聚合物物理研究提供模型系统。使用拓扑切换金属配体笼交联聚合物网络产生凝胶，其化学和机械性能可以从根本上改变并可逆地开启辐照。