An application of cation switching in small molecule activation was reported by Kita and Miller in 2014 (FIG. 2cii). In this work, an iridium PCN-pincer complex was prepared containing an aza-crown ether macrocycle, which serves as a hemilabile ligand and cation receptor. When sodium or lithium tetrararylborate salts were added to a CD2Cl2/Et2O solution of the compound, the free energy of azacrown ether dissociation from iridium is lowered due to the favorable interaction of the alkali metal ion and the macrocycle. This cation-activated iridium species catalyzed H/D exchange with D2 significantly faster than the unactivated complex. This concept was later extended to catalytic positional olefin isomerization, where a three-state catalyst system was discovered. An Ir chloride complex was inactive for isomerization of allylbenzene; removal of the chloride produced a cationic species with hemilabile Ir-O interactions that was a slow catalyst; addition of Li + salts to this cationic catalyst resulted in over 1,000 increase in the rate of isomerization. The rate enhancement is attributed to cation-crown interactions making olefin binding more favorable, increasing the amount of iridium that is actively engaged in catalysis. The same approach was then used to achieve positional isomerization with cation-switchable regioselectivity: without salts added, alkenes were isomerized from the 1-to the 2-position; under the same conditions but with added Na + salts, 3-alkenes were observed instead. In 2015, Do and coworkers applied cation-tuning to polymerization catalysis. A new family of nickel phenoxyimine complexes bearing polyethylene glycol (PEG) chains that can coordinate secondary metals (FIG. ) was developed. On the basis of solution and solid-state studies, it was demonstrated that the addition of M + (where M + = Li + , Na + , or K + ) to the nickel complexes can produce 1:1 and 2:1 nickel: alkali species. The association constants between Ni and M + correlated with size matching between the ionic radius of M + and the chain length of the PEG chelator (i.e., larger cations require longer PEG chains and vice versa). It was reported that combining Na + or K + with the nickel catalysts featuring tri-or tetraethylene glycol chains increased the ethylene polymerization activity and gave polymers with higher molecular weight and branching density relative to the nickel catalysts alone. In subsequent work, the researchers showed that cation-tuning could be applied to other olefin polymerization platforms and catalyst nuclearity could be controlled through suitable ligand design. A unique catalyst system for chemoselective ring opening polymerization (ROP) and copolymerization (ROCOP) was described by Romain and Williams in 2014 (FIG. 2civ). 71 Dizinc-acetate and dizinc-carbonate species could catalyze ROP of cyclohexene oxide (CHO) and CO2 to poly(cyclohexene carbonate) (PCHC). However, these species were inactive toward -caprolactone (CL), even when the reactions were carried out in neat CL at 80 °C. When the dizinc-acetate complex was treated with CHO, a dizinc-alkoxide intermediate capable of initiating CL ROP was produced. Sequential ROP and ROCOP were accomplished by using epoxide as a chemical switch. First, the dizinc-acetate complex was combined with excess CHO to furnish the corresponding dizinc-alkoxide initiator, which catalyzed the conversion of CL to poly(caprolactone) (PCL). Next, CO2 was added to generate a dizinc-carbonate species that promoted ROCOP of CHO and CO2. This step led to further polymer chain growth, resulting in formation of a PCL-PCHC block copolymer. The reverse order of monomer addition to furnish PCHC-PCL block copolymers was also achieved. In this case, CO2 was purged from the reaction mixture once the PCHC block had formed to produce a second PCL block.