Polymerized ionic liquids: Effects of counter‐anions on ion conduction and polymerization kinetics

Abstract

A novel imidazolium‐containing monomer, 1‐[ω‐methacryloyloxydecyl]‐3‐(n‐butyl)‐imidazolium (1BDIMA), was synthesized and polymerized using free radical and controlled free radical polymerization followed by post‐polymerization ion exchange with bromide (Br), tetrafluoroborate (BF4), hexafluorophosphate (PF6), or bis(trifluoromethylsulfonyl)imide (Tf2N). The thermal properties and ionic conductivity of the polymers showed a strong dependence on the counter‐ions and had glass transition temperatures (Tg) and ion conductivities at room temperature ranging from 10 °C to −42 °C and 2.09 × 10−7 S cm−1 to 2.45 × 10−5 S cm−1. In particular, PILs with Tf2N counter‐ions showed excellent ion conductivity of 2.45 × 10−5 S cm−1 at room temperature without additional ionic liquids (ILs) being added to the system, making them suitable for further study as electro‐responsive materials. In addition to the counter‐ions, solvent was found to have a significant effect on the reversible addition‐fragmentation chain‐transfer polymerization (RAFT) for 1BDIMA with different counter‐ions. For example, 1BDIMATf2N would not polymerize in acetonitrile (MeCN) at 65 °C and only achieved low monomer conversion (< 5%) at 75 °C. However, 1BDIMA‐Tf2N proceeded to high conversion in dimethylformamide (DMF) at 65 °C and 1BDIMABr polymerized significantly faster in DMF compared to MeCN. NMR diffusometry was used to investigate the kinetic differences by probing the diffusion coefficients for each monomer and counter‐ion in MeCN and DMF. These results indicate that the reaction rates are not diffusion limited, and point to a need for deeper understanding of the role electrostatics plays in the kinetics of free radical polymerizations.