Crystal-lattice controlled photopolymerization of di(benzylammonium) (Z,Z)-muconates

A new type of photoreaction of the ammonium salts of butadiene-1,4-dicarboxylic acid in the solid state is reported. When the crystals of a series of di(benzylammonium) (Z,Z)-muconate (2,3-hexadienedioate) and its related derivatives were prepared and photoirradiated, either a tritactic polymer or an (E,E)-isomer was obtained as the photoproduct according to the structure of the ammonium part. The polymers produced were confirmed to be of high molecular weight and stereoregular by viscometry and NMR spectroscopy after solid-state polymer transformation. X-ray crystal structure analysis revealed that the crystal structures of benzylammonium muconates were classified into columnar-type and sheet-type on the basis of the molecular arrangement. Both types involve two-dimensional hydrogen bond networks between the primary ammonium cations and the carboxylate anions. In the former structure, the stacking of the diene moieties was suitable for topochemical polymerization to yield a tritactic polymer, while the latter has molecular packing favoring the isomerization or no reactivity. The stereocontrol of the tritactic polymer is in good agreement with the results expected from the crystal structure. This indicates that the molecular packing determines a reaction pathway in the crystalline state.