In Situ Polymerization

Reaction Scheme

Materials that are polymerizable in situ are currently used in a wide range of commercial and medical applications. For example, dimethacrylate-based composite resins are commonly photopolymerized for dental restorations; cyanoacrylate-based formulations, which polymerize upon exposure to trace amounts of moisture, are used as adhesives and suture-less wound closures; and fibrinogen-based surgical sealants, which crosslink upon the addition of thrombin, are used for hemostasis. Unfortunately, these in situ polymerizable materials have drawbacks that limit their usability and efficacy; radical (meth)acrylate polymerizations are inhibited by the presence of oxygen, cyanoacrylates are brittle and fibrinogen gels are incapable of resisting high stresses and burst pressures. To address the shortcomings of current approaches to in situ polymerization, we are developing novel polymerizable resins that react upon exposure to environmentally-borne initiation stimuli to yield materials with readily tailorable chemical and mechanical properties.

Polymer Mechanochemistry

We are exploring the intersection of mechanochemistry and polymer science by employing mechanisms such as degenerative addition–fragmentation chain transfer, and mechanical activation of colored, fluorescent, and reactive mechanophores to create polymeric materials that are able to respond to their environment both diagnostically and remedially. Significant research effort has been devoted to attaining low shrinkage and shrinkage stress, and improving the fatigue resistance of polymer composites; however, as stress and fatigue development in these materials is not readily observable, the integrity of a polymer composite is difficult to accurately evaluate in the field. Composite resins that self-report their stress and fatigue state would afford a convenient approach to non-destructive assessment and ensure that material damage is rectified prior to catastrophic failure. We are developing polymerizable mechanophores that, upon incorporation in composite resins, manifest visible cues at raised stresses and enable rearrangement of the polymer connectivity for stress relaxation and flaw elimination.

Dynamic Covalent Self-assembly

Self-assembly processes are often based upon weak intermolecular interactions such as hydrogen bonding, π-stacking, or van der Waals interactions. One consequence of the relative weakness of these transient interactions is that the assembled structures are often fragile and susceptible to mechanical degradation. Covalent bonds typically exhibit bond energies that are over an order of magnitude higher than those for hydrogen bonds and could conceivably provide a route for the fabrication of far more mechanically robust assemblies. However, the creation of exquisite nanostructures by self-assembly and the toughness imparted by covalent bonds are generally perceived as mutually exclusive owing to the prevalent irreversibility of covalent bondgenerating reactions. Fortunately, several 'dynamic' covalent interactions, covalent bond-forming reactions whose addition products can be reverted back to the constituent reactants under particular reaction conditions, are known, enabling the error correction mechanism that is essential for the selective fabrication of supramolecular structures. We are utilizing dynamic covalent chemistries to mediate nanostructure assembly that, as a result of the enormously greater strength and directionality offered by covalent bonds in comparison to the weaker interactions observed in biology, combines complexity AND toughness.