METALLOSUPRAMOLECULAR POLYMERS
Contact: Dr. Gina Fiore
Metallopolymers are organic-inorganic hybrid materials that feature metal complexes within a polymer matrix. These materials are attractive because they combine the advantages of polymers – low cost, ease of processing, good mechanical properties – with the unique properties of metal complexes (e.g., optical, magnetic, electronic, catalytic). One important subset of these materials is the class of metallosupramolecular materials, where the metal-ligand interaction is dynamic in nature and thus acts as the supramolecular motif. In collaboration with Prof. Stuart Rowan (Case Western Reserve University), a number of projects focus on the exploration and exploitation of metallosupramolecular polymers. Various new classes of organic-inorganic hybrid materials have been explored with the objective to merge the structure of known polymer systems with the advantages of a dynamic (reversible) polymerization process. These materials are synthesized by the self-assembly polymerization of ditopic macromolecules via metal-ligand binding, allowing for access to materials that are otherwise difficult to process through conventional means.
Chemical structures of macromonomers based on Mebip ligands and their associated films produced by metallo-polymerization with Zn2+ and Fe2+.
Initial studies have made use of the following building blocks: (i) ditopic macromonomers based on low molecular weight poly(2,5-dialkoxy-p-phenylene ethynylene) (PPE), poly(2,5-dioctyloxy-p-xylylene) (PPX), and hydrogenated poly(ethylene-co-butylene) (Kraton) cores, (ii) the 2,6-bis(1’-methylbenzimidazolyl)- pyridine (Mebip) ligand as the binding unit; and (iii) metal salts, which cause linear chain extension (Zn2+ and Fe2+) or branching/cross-linking (La3+). The new metallopolymers prepared from these building blocks can be readily solution-processed and represent illustrative examples for metallosupramolecular polymers with good mechanical properties. Exploiting the change in fluorescence of the macroligands upon metal binding and the strong, and sometimes rather selective binding affinity of certain metals to analytes of interest, this framework has been utilized for the development of fluorescent sensory systems that exploit competitive binding. While it was demonstrated that the supramolecular metallopolymerization of endcapped macromolecules (PPE, PPX, and Kraton) can be employed to create metallopolymers that combine ease of processing with good mechanical characteristics, an understanding of what exactly determines these properties is still lacking. Research in our group seeks to develop a predictive understanding between molecular structure, morphology, and mechanical properties of supramolecular metallopolymers.
TOWARDS SELF-HEALING POLYMERS…
One particularly intriguing feature of certain supramolecular metallopolymers is their ability to (autonomously) heal after being damaged. Research on “self-healing” polymers is a growing area of research and has received significant commercial interest. One early concept to attain this functionality is the incorporation of monomer-filled capsules, which release the healing agent when damaged. Another approach is the use of reversible chemical motifs, such as moieties introduced by Diels-Alder reactions. In collaboration with Prof. Stuart Rowan (Case Western Reserve University), a novel class of optically healable supramolecular polymers based on Mebip end-capped Kraton and appropriate metal salts has been developed. Upon exposure to appropriate stimuli (e.g., light, heat), certain supramolecular motifs can easily disengage, causing a controlled local decrease of the polymer’s molecular weight concomitant with an increase of the crack-healing rate. The use of light as a stimulus for dissociation of the supramolecular motif brings several distinct advantages, including the possibility to heal locally and to specifically target defects sites. Current research efforts on this project seek to develop a better understanding for the parameters that govern the mechanical properties, light responsiveness, and healing ability of metallosupramoleculear polymers.
Schematic representation of the concept of healable materials based on light-responsive metallosupramolecular motifs.
Controlled photo-healing of a film of [Mebip2(Kraton)•Zn(NTf2)2]n with multiple scratches.
