Why is fluoroethylene carbonate (FEC) essential for initiating methyl methacrylate (MMA) polymerization with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)?

LiTFSI alone is a poor initiator for MMA polymerization due to the strong C–F and C–S bonds in the TFSI⁻ anion, which hinder the formation of a reactive anionic species. A very high LiTFSI concentration (>6 M) is required to drive the reaction, making the system impractical. FEC, a carbonate solvent with a high dielectric constant, is introduced to enhance LiTFSI dissociation and lower the initiation energy barrier.

FEC facilitates MMA polymerization by increasing the dielectric constant of the medium, promoting LiTFSI dissociation into free TFSI⁻ anions. This reduces the bond dissociation energy of LiTFSI from 4.047 eV to 0.381 eV, enabling TFSI⁻ to act as an anionic initiator at a much lower LiTFSI concentration (0.4 M). Additionally, FEC’s electron-withdrawing –CF₃ group weakens the electron-donating ability of the nitrogen in TFSI⁻, further enhancing its reactivity toward the MMA double bond.