Ionic conducting polymers are a fascinating research field with wide-spread applications, ranging from ultrafiltration or separation membranes, ion exchange resins, to membranes for advanced energy applications. The synthesis of ionomers is realized by post-polymerization modification or by direct copolymerization of ionic and non-ionic monomers.
The LIME group has extensively worked on synthesis of ionomers applying different strategies. Starting from existing polymers, we have introduced Van der Waals bonds (organic-inorganic Class I hybrids) or covalent bonds (“cross-links” or Class II hybrids) reporting different methods for the functionalization of polyarylene systems, among them, classical organic reactions (electrophilic aromatic substitution, chloromethylation reactions, etc.), metalation routes, sol-gel synthesis, electrochemical reactions, solvothermal synthesis. Direct copolymerization such as polycondensation and catalytic olefin polymerization are currently applied for new polymers.
The continuous optimization of synthesis parameters, the choice of different polymers and/or functionalization groups and the improvement of casting procedures and treatments of polymers, led to good results in terms of ionic conductivity, selectivity and stability.
Some representative examples:
- formation of cross-linked sulfonated aromatic polymers (European Project LoLiPEM) by solvothermal macromolecular synthesis, without any addition of cross-linker, to exploit the memory effect on the conductivity;
- synthesis of ionomers based on PPO grafted with the sterically hindered strong base 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) to explore the concept of “intrinsic” ionic conductivity. The TBD nucleophilic properties were enhanced to drive the reaction towards the formation of the tertiary amine;
- formation of organic/inorganic Class II nano-hybrids introducing silanol units into the macromolecules. Different methods for the functionalization of polyarylene systems were applied depending on the nature of the backbone. In the case of silylated and sulfonated PEEK (SOSiPEEK) the synthesis proceeds via sulfochlorination followed by lithiation and subsequent reaction with a silicon compound.
- synthesis of amphoteric ionomers (European Project CREATE). The ionic groups were introduced by a) sol-gel reactions with different ormosil materials; b) carboxylation by CO2 followed by chloromethylation and reaction with trimethylamine; c) chloromethylation with direct reaction with a amino acid.