Charles E. Diesendruck

Biomimetic Self‐Healing

Self-healing is a natural process common to all living organisms which provides increased longevity and the ability to adapt to changes in the environment. Inspired by this fitness-enhancing functionality, which was tuned by billions of years of evolution, scientists and engineers have been incorporating self-healing capabilities into synthetic materials. By mimicking mechanically triggered chemistry as well as the storage and delivery of liquid reagents, new materials have been developed with extended longevity that are capable of restoring mechanical integrity and additional functions after being damaged. This Review describes the fundamental steps in this new field of science, which combines chemistry, physics, materials science, and mechanical engineering.

Continuous Self-Healing Life Cycle in Vascularized Structural Composites

J. F. Patrick Civil and Environmental Engineering Department Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana , IL 61801 , USA K. R. Hart, Prof. S. R. White Aerospace Engineering Department Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana , IL 61801 , USA E-mail: swhite@illinois.edu Dr. C. E. Diesendruck, Prof. J. S. Moore Chemistry Department Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana , IL 61801 , USA B. P. Krull, Prof. N. R. Sottos Materials Science and Engineering Department Beckman Institute for Advanced Science and Technology University of Illinois at Urbana-Champaign Urbana , IL 61801 , USA E-mail: n-sottos@illinois.edu

Proton-Coupled Mechanochemical Transduction: A Mechanogenerated Acid

A novel mechanophore with acid-releasing capability is designed to produce a simple catalyst for chemical change in materials under mechanical stress. The mechanophore, based on a gem-dichlorocyclopropanated indene, is synthesized and used as a cross-linker in poly(methyl acrylate). Force-dependent rearrangement is demonstrated for cross-linked mechanophore samples loaded in compression, while the control shows no significant response. The availability of the released acid is confirmed by exposing a piece of insoluble compressed polymer to a pH indicator solution. The development of this new mechanophore is the first step toward force-induced remodeling of stressed polymeric materials utilizing acid-catalyzed cross-linking reactions.

Mechanically triggered heterolytic unzipping of a low-ceiling-temperature polymer

Biological systems rely on recyclable materials resources such as amino acids, carbohydrates and nucleic acids. When biomaterials are damaged as a result of aging or stress, tissues undergo repair by a depolymerization–repolymerization sequence of remodelling. Integration of this concept into synthetic materials systems may lead to devices with extended lifetimes. Here, we show that a metastable polymer, end-capped poly(o-phthalaldehyde), undergoes mechanically initiated depolymerization to revert the material to monomers. Trapping experiments and steered molecular dynamics simulations are consistent with a heterolytic scission mechanism. The obtained monomer was repolymerized by a chemical initiator, effectively completing a depolymerization–repolymerization cycle. By emulating remodelling of biomaterials, this model system suggests the possibility of smart materials where aging or mechanical damage triggers depolymerization, and orthogonal conditions regenerate the polymer when and where necessary. Strong acoustic fields applied to solutions of linear polymers typically result in mid-chain scission, yielding products half the molecular weight of the original. Now it has been shown that poly(o-phthalaldehyde), a polymer with a ceiling temperature below room temperature, undergoes chain scission and subsequent depolymerization to monomers. Introduction of an appropriate initiator to the monomer regenerates poly(o-phthaladehyde) macromolecules.

End Group Characterization of Poly(phthalaldehyde): Surprising Discovery of a Reversible, Cationic Macrocyclization Mechanism

End-capped poly(phthalaldehyde) (PPA) synthesized by anionic polymerization has garnered significant interest due to its ease of synthesis and rapid depolymerization. However, alternative ionic polymerizations to produce PPA have been largely unexplored. In this report, we demonstrate that a cationic polymerization of o-phthalaldehyde initiated by boron trifluoride results in cyclic PPA in high yield, with high molecular weight, and with extremely high cyclic purity. The cyclic structure is confirmed by NMR spectroscopy, MALDI-TOF mass spectrometry, and triple-detection GPC. The cyclic polymers are reversibly opened and closed under the polymerization conditions. Owing to PPAs low ceiling temperature, cyclic PPA is capable of chain extension to larger molecular weights, controlled depolymerization to smaller molecular weights, or dynamic intermixing with other polymer chains, both cyclics and end-capped linears. These unusual properties endow the system with great flexibility in the synthesis and isolation of pure cyclic polymers of high molecular weight. Further, we speculate that the absence of end groups enhances the stability of cyclic PPA and makes it an attractive candidate for lithographic applications.

Depolymerizable, adaptive supramolecular polymer nanoparticles and networks

Incorporation of supramolecular cross-linking motifs into low-ceiling temperature (Tc) polymers allows for the possibility of remendable polymeric networks and nanoparticles whose structure and chemical backbones can be dynamically modified or depolymerized as desired. Herein, we demonstrate the synthesis of phthalaldehyde–benzaldehyde copolymers bearing a pendant dimerizing 2-ureido-pyrimidinone (UPy) motif. The UPy moiety promotes single-chain polymeric nanoparticle formation through non-covalent cross-linking at intermediate concentrations and results in reversible polymer network formation at high concentrations. Furthermore, due to the low Tc polymer backbone within such macromolecules, the materials depolymerize to monomer under appropriate conditions. We envision that the synthesis of such depolymerizable, adaptive supramolecular polymeric materials may find use in materials capable of self-healing and remodeling as well as in triggered release applications or the development of nanoporous structures.

N-Arylation of Tertiary Amines under Mild Conditions

A transition-metal-free procedure for the N-arylation of tertiary amines to sp(3) quaternary ammonium salts is described. The presented conditions allow for the isolation of trialkylaryl, dialkyldiaryl, and novel triarylalkyl ammonium salts, including N-chiral quaternary ammonium salts. The reaction works at room temperature, open to air with electron-rich or -poor benzyne precursors and different tertiary amines, allowing the synthesis of a broad range of N-aryl ammonium salts that have applications in a variety of fields.

BF3-promoted electrochemical properties of quinoxaline in propylene carbonate

Electrochemical and density functional studies demonstrate that coordination of electrolyte constituents to quinoxalines modulates their electrochemical properties. Quinoxalines are shown to be electrochemically inactive in most electrolytes in propylene carbonate, yet the predicted reduction potential is shown to match computational estimates in acetonitrile. We find that in the presence of LiBF4 and trace water, an adduct is formed between quinoxaline and the Lewis acid BF3, which then displays electrochemical activity at 1–1.5 V higher than prior observations of quinoxaline electrochemistry in non-aqueous media. Direct synthesis and testing of a bis-BF3 quinoxaline complex further validates the assignment of the electrochemically active species, presenting up to a ∼26-fold improvement in charging capacity, demonstrating the advantages of this adduct over unmodified quinoxaline in LiBF4-based electrolyte. The use of Lewis acids to effectively “turn on” the electrochemical activity of organic molecules may lead to the development of new active material classes for energy storage applications.

The mechanochemical production of phenyl cations through heterolytic bond scission

High mechanical forces applied to polymeric materials typically induce unselective chain scission. For the last decade, mechanoresponsive molecules, mechanophores, have been designed to harness the mechanical energy applied to polymers and provide a productive chemical response. The selective homolysis of chemical bonds was achieved by incorporating peroxide and azo mechanophores into polymer backbones. However, selective heterolysis in polymer mechanochemistry is still mostly unachieved. We hypothesized that highly polarized bonds in ionic species are likely to undergo heterolytic bond scission. To test this, we examined a triarylsulfonium salt (TAS) as a mechanophore. Poly(methyl acrylate) possessing TAS at the center of the chain (PMA-TAS) is synthesized by a single electron transfer living radical polymerization (SET-LRP) method. Computational and experimental studies in solution reveal the mechanochemical production of phenyl cations from PMA-TAS. Interestingly, the generated phenyl cation reacts with its counter-anion (trifluoromethanesulfonate) to produce a terminal trifluoromethyl benzene structure that, to the best of our knowledge, is not observed in the photolysis of TAS. Moreover, the phenyl cation can be trapped by the addition of a nucleophile. These findings emphasize the interesting reaction pathways that become available by mechanical activation.

Intramolecular Cross-Linking: Addressing Mechanochemistry with a Bioinspired Approach

Many of the attractive properties in polymers are a consequence of their high molecular weight and therefore, scission of chains due to mechanochemistry leads to deterioration in properties and performance. Intramolecular cross-links are systematically added to linear chains, slowing down mechanochemical degradation to the point where the chains become virtually invincible to shear in solution. Our approach mimics the immunoglobulin-like domains of Titin, whose structure directs mechanical force towards the scission of sacrificial intramolecular hydrogen bonds, absorbing mechanical energy while unfolding. The kinetics of the mechanochemical reactions supports this hypothesis, as the polymer properties are maintained while high rates of mechanochemistry are observed. Our results demonstrate that polymers with intramolecular cross-links can be used to make solutions which, even under severe shear, maintain key properties such as viscosity.