Purifying Fluoropolymer Building Blocks

Host-guest chemistry and crystallization may be practical for separating and purifying industrially useful fluorinated monomers. α,ω-Diiodoperfluoroalkanes (DIPFAs), for example, are key intermediates for products such as fluorinated elastomers, but there is no high-yield method for purifying them. Now, Pierangelo Metrangolo of the Politecnico di Milano, in Italy, and colleagues demonstrate that organic salts known as bis(trimethylammonium) alkane diiodides can act as hosts, reversibly encapsulating DIPFAs via a selective reaction between the former's iodide anions and the latter's terminal iodine substituents (Science 2009, 323, 1461). The researchers added dissolved, size-matched ionic salts to mixtures of DIPFAs and found that specific salts could crystallize particular DIPFAs from solution or vapor. Heating releases the purified DIPFAs. The team confirmed that matching the size of the host cavity to the guest DIPFA is critical to selectivity by conducting competitive experiments. Four commercially available DIPFAs were dissolved in solution with one host salt, and only the DIPFA that matched the size of the cavity in the salt was encapsulated. The researchers suggest that this method may also be useful for purifying other intermediates such as α,ω-diiodoperfluoropolyethers.