SALT SKELETONS AND POROUS POLYMERS: THE SOLID-STATE REACTIVITY OF HALOGENOACETATES. Matthias Epple, Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
By simply heating salts of halogenoacetic acids, a solid-state polymerization takes place, e.g., with sodium chloroacetate:
Na+ -OOC-CH2-Cl (s) NaCl (s) + 1/n [-OOC-CH2-]n (s)
This pure solid-state reaction leads quantitatively to an intimate mixture of the simplest polyester, called polyglycolide, and sodium chloride. NaCl is deposited in a polymer matrix as cubes with edge lengths of 1-2 um. Upon burning the polymer, a skeleton of Nacl cubes remains. Extracting the sodium chloride with water leaves the water-insoluble polyglycolide matrix as highly porous crystal dotted with "inverse NaCl crystals", i.e. cubic holes with edge lengths of 1-2 um. This porous polyglycolide may be of high interest in pharmaceutical technology, because it is easily biodegradable in the body.
This unusual reaction takes place in a couple of halogenoacetates with different combinations of metal cation and halogen. Unfortunately, it was impossible so far to determine the crystal structure of any halogenoacetate (and none is reported in the literature!). Therefore, the course of the reaction was studied by X-ray diffractometry and X-ray absorption spectroscopy (EXAFS). Both techniques were applied ex situ and in situ, i.e., at elevated temperature. Beside these structural techniques, thermal analysis and solid-state NMR were applied.
M.Epple, L. Troger, J. Chem. Soc., Dalton Trans., 1996, 11-16.