(693d) Surface-Initiated Polymerization of Ionic Liquids

Ionic liquids (ILs) are organic salts that are liquid at or near room temperature (i.e. below 100°C). Ionic liquids possess unique materials and solvent properties that have led to their use in multiple applications, including as solvents, electrolytes, catalysts and polymer plasticizers. The ability to grow polymer films with IL functionality as side chains can enable coatings with versatile and tailorable properties. We report the surface-initiated ring-opening metathesis polymerization (SI-ROMP) of ionic liquids to grow poly(ionic liquid) films on gold and silicon substrates. The kinetics of film growth are rapid, with thicknesses approaching 200nm within 15 min of polymerization in a 0.1 M monomer solution and substantial film growth observed at monomer concentrations as low as 0.05 M.  The ionic liquid consists of the cation 1-norbornylmethylene-3-dimethylimidazolium (N1-dMIm⁺) with various anions that can be easily interchanged to tune the film properties. The dimethylimidazolium monomer was much more reactive toward SI-ROMP than the methylimidazolium analogue due to the acidic proton at C­­₂ of the methylimidazolium ring that can be removed to form a carbene complex with Grubb’s catalyst rendering the catalyst inactive for SI-ROMP. Reflectance absorption infrared spectroscopy was used to quantify the extent of anion exchange in the films. Ellipsometry and profilometry measurements were utilized to quantify the thicknesses of the films. The dependence of ion transfer through the film on the ionic liquid anion was characterized in different solvents by electrochemical impedance spectroscopy. Atomic force microscopy and tribometry were used to quantify the influence of the anion and solvent film swelling on the frictional forces. The film stability, which can be a problem when growing polymers via SI-ROMP, was greatly enhanced by modifying the olefin moieties along the polymer backbone.