In 2010, Mirkin and coworkers reported an elegant strategy to regulate catalysis using anions. In this work, a supramolecular triple layer catalyst comprising an aluminum salen complex flanked by two rhoudium nodes featuring biaryl blocking groups was developed (FIG. ). In the closed form, the rhodium centers are ligated by the amino donor of the supporting ligand, which positions the biaryl units above and below the aluminum active site. Because aluminum is inaccessible due to the steric bulk of the amino arms, the catalyst cannot react with substrates. In the open form, chloride anions are bound to rhodium so that the amino groups are freely dangling in the structure, opening up access to incoming monomers. Toggling the catalyst between the open and closed forms by either adding or removing Cl -, respectively, allowed controlling the molecular weight of the growing polymer in the ring opening polymerization of caprolactone. Remarkably, the catalyst could be activated and deactivated and then reactivated without a substantial loss in activity.