A common way to carry out redox-switchable catalysis is to design redox-active ancillary ligands that are attached to a redox-inactive metal, which serves as the site for catalysis. This strategy was employed in the first example of redox-switchable catalysis (1995). A rhodium complex supported by a cobaltacene bis(phosphine) as the redox-active moiety was used for the hydrogenation and isomerization of alkenes. Despite this first example being applied to catalysis involving small molecules, the utility of redox-switchable catalysis has been exploited with more success in polymerization catalysis. In this area, a titanium complex containing two redox-active ferrocene moieties appended to a salen ancillary ligand (FIG. ) demonstrated redox modulation when it was used for the polymerization of lactide, with the doubly reduced version being more active than the doubly oxidized form of the catalyst. Since this report, several groups have utilized the ferrocene moiety for redox-switchable polymerization. Notably, Diaconescu and coworkers have developed a number of polymerization catalysts whose ancillary ligands feature ferrocene as the redox-switchable entity 54 . Compared to the titanium complex with two peripheral ferrocene units, the chelating ligands used by the Diaconescu position the ferrocene moiety in close proximity to the redox-inactive site for catalysis. This organization has proven to be advantageous because greater differences in the reaction rate were observed between the oxidized and reduced states of a catalyst. For example, while both forms of the above titanium complex demonstrated some activity for lactide polymerization, a yttrium complex showed complete on/off activity for lactide polymerization. An alternative method for redox-switchable catalysis is to use redox-active metals that double as the redox-switching moiety and the site for catalysis (FIG 2bi). Catalysts based on several different redoxactive metals have been explored using this strategy, with the most notable examples being ring-opening polymerization catalysts using cerium 56 and bis(imino)pyridine iron complexes. Notably, the iron complexes were the first to use an electrochemical potential instead of chemical redox reagents for redoxswitching (FIG. ). These catalysts showed a similar behavior as the polymerization catalysts that utilize redox-active ancillary ligands, demonstrating that it is not necessary to separate the redoxswitching entity from the catalytically active entity. Fig. Different types of switchable catalysis as temporal control. a| Switchable catalysis using different external stimuli. b| Redox-switchable catalysis. Design of a redox-switchable metal catalyst (i). Redox-switchable polymerization using electrochemical setup (ii). c| Chemoswitchable catalysis. Anion coordination leads to allosteric change which unblocks the catalytic active center for the ring opening polymerization of -caprolactone (i). Metal cation coordination onto the hemilabile crown ether moiety promotes the hydrogen activation reaction (ii). Metal cation coordination to the oligomeric ethylene glycol chain increases ethylene polymerization activity (iii). Presence of CO2 stagnates the polymerization of -caprolactone and initiates the ring opening copolymerization of CO2 and cyclohexene oxide (iv). d| Photoswitchable catalysis. Photoinduced E/Z isomerization and its application in catalyzing amidation (i). Photoinduced ring closing and its application in catalyzing ring-opening polymerization (ii).