Confined polymerization in porous materials

Introduction: Porous materials with permanent porosity have recently emerged as an exciting research field with potential applications in gas storage, separation and catalysis. Porous aromatic frameworks and periodic mesoporous organosilicas showed high robustness and extraordinary chemical stability that made them ideal for supporting chemical reactions without framework destruction. Confined polymerization in nanopores allowed the synthesis of innovative nanostructured materials and nanocomposites with extended interfaces that couldn’t be obtained otherwise. Materials and methods: Porous aromatic framework (PAF-1) was synthetized through a Yamamoto-type Ullmann cross-coupling. P-phenylene silica (PSS) was prepared by a template synthesis. These porous frameworks were characterized with nitrogen adsorption measurement, IR spectroscopy, DSC, TGA and ss-NMR. The polymerizations were performed in the liquid phase: a solution of AIBN (azobisisobutyronitrile) in distilled acrylonitrile was diffused inside the pores of the matrixes and the samples were heated to activate the polymerizations. The nanocomposites were prepared either with stoichiometric amount of monomers or excess monomers. The nanocomposites were characterized with nitrogen adsorption measurement, thermogravimetric analysis and electron microscope techniques (SEM, HR-TEM). The interactions between the porous matrixes and the engendered polymer were investigated with fast magic-angle-spinning 2D 1H-13C HETCOR NMR. The nanocomposites were heated at different temperatures (300-1000°C) under an inert atmosphere. Results: The confined polymerization of acrylonitrile inside the pores of PAF-1 and PSS with stoichiometric amount of monomers led to the fabrication of nanocomposites with intimate relationship between host and guest. The matrixes and the polymer form extensively interdigitated nanophases through multiple interactions. By treating the PAF-1/PAN nanocomposite at 300°C the formation of a ladder polymer was observed, thanks to the cyclization of PAN. This process enhanced the electronic properties and permitted the fabrication of a 3D network of two rigid and nonmeltable materials that were not expected to be blended effectively otherwise. The thermal treatment at 300°C of the PSS/PAN nanocomposite similarly led to the formation of a ladder polymer inside the channel of the mesoporous silica. By treating the PSS/PAN at 1000°C structural changes were observed both in the polymer and in the matrix: the first formed a graphitic structure while the second showed the cleavage of C-Si bonds and the formation of siliceous species. This material contained a carbonaceous structure and a silica nanophase that were not easily blended otherwise. Discussion: The present results show that it is possible to obtain linear polymers in situ within a 3D polymer architecture. In conclusion, confined polymerization is an interesting and unusual methodology that leads to the fabrication of novel nanomaterials with extended interfaces