Nanoporous materials for confined polymerization and molecular rotor dynamics. (Invited Oral Presentation)

The extraordinary surface areas (BET> 5000 m2/g) and pore capacity exhibited by porous aromatic frameworks (PAFs), which are themselves organic polymeric architectures[1] interconnected by covalent bonds, allow sorption of a large amount of monomers to form high-molecular-mass polymers tightly interwoven with the porous matrix [2]. Polyacrylonitrile (PAN) polymerized by this methodology could undergo in-situ thermal transformation to semi-conductive or conductive polymers and carbon nano-fibers. PAN was also synthesized in the form of isotactic polymer within the nanochannels of dipeptide porous crystals which were used as sacrificial polymerization vessels [3]. The crystalline matrix sublimed away at 250 °C after the polymer intramolecular reaction to yield a rigid 'ladder polymer', which retained the morphology of the crystal scaffold. Morphological control has also been obtained starting from mesoporous silica to fabricate polymeric micro-objects. In the case of metal-organic host framework the innovative idea was to make the host participate in the polymerization with two reactive vinyl pendant groups, that resulted in a cross-linked network [4]. The crystal scaffold of the host was removed except where it participates in the cross-linking reaction and acts as clipping point for the aligned polymer chains. Although the polymer chains grow in the atactic configuration, they exhibit periodic order since they are kept in register by the molecular clips. Chain-periodicity was ascertained by XRD and TEM. Moreover, molecular rotors were built in porous organic frameworks, combining the features of porosity and dynamics. The intervention of guests entering from the liquid or vapor phases, such as melted alkanes or iodine, could modulate the motional behavior of the rotors, as shown by 2H NMR. References 1. Angew. Chem. Int Ed. 2014, 53, 1043. 2. Angew. Chem. Int Ed. 2012, 51, 10136. 3 Angew. Chem. Int Ed. 2012, 51, 9258. 4. Nature Chem. 2013, 5, 335.