Synthesis of Short-Side-Chain Sulfonyl Fluoride Monomers for Perfluorinated Polymer Electrolyte Membranes

Perfluorinated polymer electrolyte membranes (PEMs), prepared from copolymers of tetrafluoroethy-lene and perfluorinated sulfonyl fluoride monomers (SO2F monomers), CF2＝CF[OCF2CF(CF3)]mO-(CF2)nSO2F, have been employed in chlor-alkali process, polymer electrolyte fuel cell, and polymer electrolyte water electrolysis. Compared to long-side-chain (LSC) PEMs, short-side-chain (SSC) PEMs have shorter side chains, resulting in higher glass transition temperature, lower equivalent weight, and higher ionic conductivity. However, SSC-SO2F monomers (m＝0, n＝2 or 4) cannot be synthesized in the same way as LSC-SO2F monomer (m＝1, n＝2). Therefore, they are synthesized using special reaction reagents and equipment. We have developed novel synthetic methods for SSC-SO2F monomers with different side-chain lengths using commercially available reagents. Two synthetic methods for SSC-SO2F monomer (m＝0, n＝2) were developed using CF3CF(COF)O-(CF2)2SO2F, a synthetic intermediate of LSC-SO2F monomer (m＝1, n＝2), as the starting material. One method used imidazole as the protecting group and the other used cyclic compound and vinyl sulfonic anhydride. Additionally, we investigated two synthetic routes for SSC-SO2F monomer (m＝0, n＝4) using the same commercially available reagents, but in different order of use, starting with I(CF2)4I. As a result, we chose the route of first synthesizing the sulfonyl fluoride group, followed by the vinyl ether group.