Polyethers dissolving metals

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

A significant advance in overcoming this problem is the use of macrocyclic polyethers in catalytic quantities in such media. Macrocyclic polyethers, such as 18-crown-6, the structure of which is shown in Figi 5.3, have a pronounced ability to coordinate metal ions. This ability allows for more salt to be dissolved in the organic solvent equally important, coordination of the metal ion leaves a relatively poorly solvated anion behind. The anions behave under these conditions as highly reactive species, sometimes termed naked anions. A study of the relative rates of... 

The importance of crown ethers derives from their ability to sequester specific metal cations in the center of the polyether cavity. 18-Crown-6, for example, binds strongly with potassium ion. As a result, a solution of 18-crown-6 in a nonpolar organic solvent will dissolve many potassium salts. Potassiirm permanganate, KMn04, dissolves in toluene in the presence of 18-crown-6, for instance, and the resulting solution is a valuable reagent for oxidizing alkenes. 

Immersions of polymer microtensile specimens in solutions of metal ions or lipid emulsions at elevated temperatures for 16 weeks have been reported [32]. Temperatures of 37, 70, and 90°C are recommended with sampling monthly to establish trends. For pacemaker leads, the solutions should include all the metals found within the device, a base, and an acid. For example, aqueous solutions of 1 M AgNOs or 0.1 M C0CI2 (acetylacetanoate) can assess oxidation. Immersion in 1 N acetic acid. Ringer s solution, and 1.0 N HCl can assess hydrolytic resistance. Immersion in 20% intralipid (soybean) emulsion can assess the propensity to absorb lipids. However, in our experience, none of the above in vitro tests appear to be reliably predictive of performance in pacemaker lead insulation. Why The in vivo environment cannot be duplicated in vitro. For example, the oxidation state of an ion varies as a function of what it is dissolved in. Distilled water containing a metal ion does not represent the environment within a lead. This accelerated test predicts that Ag+ will oxidize and degrade polyether polyurethanes while Co will not. Multiple in vivo studies clearly demonstrate exactly the opposite [14, 33]. Traces of cobalt will degrade the polymer in vivo whereas silver will not. 

The polymer component in these batteries fulfills the function of a medium for ionic transport and a separator. The polymers are polyethers, PEO, or PPO. However, the lithium salts, dissolved in these polymers, have 100-fold lower conductivity than that of a lithium salt dissolved in water. The low conductivity below 70 °C, the reactivity of the interface with the lithium metal electrode, and the issues related to mechanical properties and electrochemical stability need to be resolved before the lithium polymer battery has acceptable performance. The use of inorganic composite membranes, described in a subsequent section, has been shown to result in improved ionic conductivity. 

---