Porous dipeptide crystals and organic frameworks as polymerization vessels

Our work was aimed at performing polymerizations in porous materials and controlling solid state reactions of the new adducts with polymers. The project exploits the unprecedented potentials of porous materials presently in use, which range from fully-organic covalent frameworks and metal-organic frameworks to porous molecular crystals of biological origin, while the properties induced to the polymers include stereochemistry, chain alignment and morphology. The extraordinary surface areas (> 5000 m2/g) and pore capacity exhibited by porous aromatic frameworks (PAFs) interconnected by covalent bonds, allow sorption of a large amount of monomers to form high-molecular-mass polymers tightly interwoven with the porous matrix [1]. Polyacrylonitrile (PAN) obtained by this way could undergo in-situ thermal transformation to conductive, semi-conductive polymers and carbon nano-fibers. PAN was also synthesized in the form of isotactic polymer within the nanochannels of dipeptide crystals which were used as sacrificial polymerization vessels [2]. The crystalline matrix sublimed away over 250 °C after a polymer intramolecular reaction to yield a rigid 'ladder' polymer, which retained the morphology of the crystal scaffold. In a third example, the metal-organic host framework can participate with two reactive vinyl pendant groups in the polymerization, resulting in a cross-linked network [3]. The crystal scaffold of the host was removed except at cross-linking points, which act as clipping points for the aligned polymer chains. Although the polymer chains are atactic, they are kept in register by the molecular clips and chain-periodicity was ascertained by XRD and TEM. An intriguing aspect of porous crystalline frameworks is the dynamics of molecular elements in the pore-walls. Since these mobile elements behave as rotors and are exposed collectively to the structural voids in the porous material, their motion could be switched on and off by intervening molecular species migrating into the crystal channels. The fast rotor dynamics was hampered by guests occupying the crystal voids and released after absorbate removal. This effect was realized in mesoporous hybrid materials and porous molecular crystals.