Researchers have shown that the network structure of framework materials called polymer-linked metal-organic cages (polyMOCs) can be adjusted to tune their elasticity. PolyMOCs are related to metal-organic frameworks (MOFs). In both, metal-organic units are connected together into three-dimensional extended networks. But in MOFs the connectors are generally rigid organic molecules and the materials are crystalline solids, whereas in polyMOCs the connectors are elastic polymers and the materials are noncrystalline and gel-like. Jeremiah A. Johnson of MIT and coworkers have now introduced large cages with variable numbers of connectors to rationally adjust polyMOC elasticity (Nat. Chem. 2015, DOI: 10.1038/nchem.2390). In the materials some polymer chains connect MOC junctions to other junctions, and other chains form loops in which both ends bond to the same junction. The researchers found that as the number of connector chains increases, the materials get stiffer. They also discovered that they could replace some of the loops with functional groups without modifying polyMOC elasticity. PolyMOCs “open a new playground for materials design,” Johnson says, noting that potential applications include water purification.