Research

We currently have 3 research initiatives within the Moore group:

Self-Healing Polymers and Mechanochemical Transduction

Foldamers

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Modular construction is a powerful way to achieve diversity and complexity from a simple set of position-interchangeable building blocks. Chain molecules built from a regular repeating unit provide a molecular-level example of the concept, which takes on added meaning if the sequence also folds into a conformationally ordered state. There is no finer example that illustrates the power of modularity than the functional diversity that comes from combinations of the 20 amino acids to produce polypeptide heterosequences. The concept is so simple and widely appreciated that it requires little explanation. By analogy to the polypeptide chain, it is easy to imagine, at least in principle, artificial sequences capable of performing molecular functions prescribed by chemists, rather than dictated by the boundaries of biology. This idea traces its roots back to the earliest days of peptide structure determination; nonetheless, it still remains an elusive goal.

Chain molecules that fold into compact conformations can produce concave surfaces that define the walls of nanoscale cavities and crevices. However, given the enormous number of conformations available even to short oligomers, it remains a major challenge to find novel backbones that favor collapsed conformations and generate only a small number of cavity shapes and sizes. Recent advances in the field of foldamers have provided promising oligomers that exhibit a high degree of conformational order. In particular, we have previously shown that m-phenylene ethynylene (m-PE) oligomers adopt a unique helical conformation stabilized by solvophobic interactions. The helical conformation produces an internal cavity lined with non-polar surfaces, and it forms complexes with hydrophobic molecules of appropriate size. Given their form and function, helical m-PE oligomers can now be included among the cavitands – open-ended molecular containers that surround their guests to a large extent.

Synthetic molecular containers are of interest for reasons that generally have to do either with recognition or with catalysis. Two major problems that limit the potential of molecular containers are (i) the moderate dimensions and restricted shape variations of the cavities, and (ii), the inability to position desired functional groups at prescribed locations on the cavity’s concave surface. The potential of PE foldamers to address these limitations is a consequence of helically-segmented verses laterally-paneled walls. Each segment of a foldamer contributes to the cavity’s geometry (size and shape) according to connectivity and bond angle constraints. Moreover, each segment can be appended with internally-directed side chains that shape the cavity’s space and polarity in a piecewise, modular fashion.

To capitalize on the customizable space that is potentially available with foldamer-based cavitands, two objectives that are just beyond current capabilities must be satisfied. First, heterosequences rather than homosequences must be prepared. Consequently, a rapid and reliable (ultimately automated) synthetic method must be developed for PE heterosequences (including heterosequence libraries). Second, theory and computational protocols must be developed to guide the rational and systematic selection of focused heterosequence libraries. In particular, these algorithms must identify sequences that fold to create a cavity that is complementary in volume, shape, and polarity to a guest, intermediate, or transition state of interest.

A main objective of our current research is to show that the space inside a molecular container can be customized to bring about specific recognition. Success here will set the stage for our long-term objective of using this capability to control reactivity. Along these lines, we have plans to develop the concept of “catalytic reactive sieving”. The idea is to create a catalyst that selectively activates substrates of a certain size. This will be achieved by placing a catalytic activating group in the middle of the oligomer sequence.

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Structural polymers are susceptible to damage in the form of cracks, which often develop deep within a material where detection is difficult and repair is nearly impossible. To extend the lifetime of such materials, we have undertaken a project aimed at inventing polymers that repair themselves automatically. A key aspect to self-healing capability is the development of repair mechanisms that are activated by damage. In addition to these damage-induced triggering mechanisms, the chemistry involved in the healing process is critical and the requirements are challenging. For example, the repair agent must be highly reactive when damage is encountered, yet possess long-term stability while dormant. There must be an efficient means to deliver the healing agents to the site of damage, the repair reactions must be fast kinetics under ambient conditions, and the repair agent must possess exceptional structural and adhesive properties. Development of novel chemistries and tailoring existing ones is the focus of this program. Because the problem is multidisciplinary involving mechanics, chemistry, processing, and engineering, we work closely with our collaborators at the Beckman Institute.

Conclusive demonstration of self-healing was obtained with a system based on ring-opening metathesis polymerization (ROMP) chemistry. Dicyclopentadiene (DCPD), a highly stable monomer with excellent shelf life, was encapsulated in microcapsules with 0.2-micron thick shell made of poly(urea-formaldehyde). A small volume fraction of microcapsules was dispersed in an epoxy resin along with Grubbs’ olefin metathesis catalyst. The embedded microcapsules were shown to rupture in the presence of a crack and release the DCPD monomer into the crack plane. Contact with the embedded Grubbs’ catalyst initiated polymerization of the DCPD and rebonded the crack plane. We plan to continue this approach, further optimizing the catalyst system, monomeric healing agent, and matrix material.

We also plan to develop new mechanisms for triggering repair reactions. Stress-induced reactions in polymers have long been studied but they have never been used for the purpose of repairing damage. The stress-induced reactions studied to date involve radical mechanisms resulting from homolytic bond rupture. Our immediate goal is to develop a triggering mechanism that can be used productively to reinforce the polymer. By designing appropriate monomers, we believe that it will be possible for a strain field (as might be encountered in the region of damage) to impart sufficient energy to a polymer backbone to trigger a reaction, and subsequently initiate a radical polymerization. Our other interests lie in the creation of self-assessing polymers. We are also investigating mechanically responsive triggers that undergo color changes or fluoresce upon stress activation.

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Self-Assembly of 2D Finite Organic Grids

            An important challenge for organic chemists is to build non-natural macromolecules of multi-nanometer dimensions with precise control over structure and functionality.¹ In other words, to build large structures while avoiding several limitations of current finite organic nanostructures: flexibility and/or uniform and symmetrical functionalization. We wish to address these issues by synthesizing finite, shape persistent molecular grids. Such materials have a clear application as nanofiltration membranes. Also, with precise control over the placement of functionality, these structures could be used as a scaffold for more complicated molecular devices. A long- term target is the organization of multiple chromophores, donors, and acceptors for solar energy conversion.²

Figure 1.

Our goal is to build such structures by crosslinking oligomeric strands. Specifically, our goal is to use dynamic covalent chemistry (e.g., imine formation/metathesis) to self-assemble meta-phenylene ethynylene oligomers into two-dimensional nanostructures (Figure 1). We have recently explored the limits of intermolecular oligomer crosslinking by assembling [n]-rung molecular ladders (n = 3–6, Figure 2). We have found that this approach quickly becomes inefficient with the longer oligomers. New approaches, based on the intramolecular crosslinking of comb-shaped macromolecules or the programming of self-assembly instructions into the oligomer sequence are currently being developed.

Figure 2.

(1)        Stupp, S.I. Chem. Rev. 2005, 105, 1023.

(2)        Lewis, N.S. Basic Research Needs for Solar Energy Utilization; U.S. Dept. of Energy: http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf, 2005.

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