Brian J. Adzima

Spatial and temporal control of the alkyne–azide cycloaddition by photoinitiated Cu(II) reduction

The click reaction paradigm is focused on the development and implementation of reactions that are simple to perform while being robust and providing exquisite control of the reaction and its products. Arguably the most prolific and powerful of these reactions, the copper-catalysed alkyne-azide reaction (CuAAC) is highly efficient and ubiquitous in an ever increasing number of synthetic methodologies and applications, including bioconjugation, labelling, surface functionalization, dendrimer synthesis, polymer synthesis and polymer modification. Unfortunately, as the Cu(I) catalyst is typically generated by the chemical reduction of Cu(II) to Cu(I), or added as a Cu(I) salt, temporal and spatial control of the CuAAC reaction is not readily achieved. Here, we demonstrate catalysis of the CuAAC reaction via the photochemical reduction of Cu(II) to Cu(I), affording comprehensive spatial and temporal control of the CuAAC reaction using standard photolithographic techniques. Results reveal the diverse capability of this technique in small molecule synthesis, patterned material fabrication and patterned chemical modification.

Photopolymerization Reactions Using the Photoinitiated Copper (I)‐Catalyzed Azide‐Alkyne Cycloaddition (CuAAC) Reaction

The first bulk photopolymerization of multifunctional alkyne and azide monomers using the CuAAC reaction is successfully carried out from low molecular weight, nonviscous monomer resins. Compared to other traditional step-growth bulk photopolymerization, this approach readily provides crosslinked, high glass transition temperature polymers that incorporate triazole linkages throughout the polymer structure with great temporal control.

Externally Triggered Healing of a Thermoreversible Covalent Network via Self-Limited Hysteresis Heating

Thermosets are assumed to be crosslinked by irreversible covalent bonds that break only during material fracture or destructive decomposition. [ 1 ] Once a crack is formed, either by a fabrication defect, mechanical deformation, or material fatigue, the material’s strength rapidly deteriorates, generally necessitating its replacement. However, thermoreversible covalent adaptable networks are novel materials capable of undergoing a reversible gel-to-sol transition because they are crosslinked by thermoreversible bonds. [ 2 ] Herein, we use the terms sol and gel as per the polymer defi nitions, where the gel represents the infi nite macromolecular network and sol represents the portion of the material that is not attached to the macromolecular network. Covalent crosslinks are an order of magnitude stronger than hydrogen bonds, yet they permit the material to be reversibly transitioned from a crosslinked solid to a nongelled oligomeric state. [ 3 ] As a result, the material is both mechanically strong and readily able to heal fractures and other defects. [ 4 , 5 ] Unfortunately, thermoreversible healing mechanisms are often limited by irreversible side reactions that occur at elevated temperatures (well beyond the sol-to-gel transition temperature). [ 2 ] Additionally, strategies for selectively heating a material that is either spatially confi ned or surrounded by other thermally sensitive materials have their own set of challenges. In this communication, we demonstrate the fracture healing of a novel Diels–Alder (DA) crosslinked network embedded with magnetically susceptible particles that heat in the presence of an electromagnetic fi eld. This responsiveness allows the material to be heated in situ while also exploiting the self-limiting heating behavior of ferromagnetic particles, which minimizes irreversible reactions. As a consequence, the material is unchanged even after ten cycles of fracture and repair, achieving its native properties after each and every fracture and repair cycle. Furthermore, while photocuring in many optically opaque materials is intractable, the electromagnetic fi elds used in this study are not readily absorbed. Thus, our approach allows for an externally triggered depolymerization and subsequent polymerization in materials such as composite laminates. Two strategies dominate the fi eld of self-healing materials: systems that release reactive monomers at the site of fracture [ 6 ]

3D Photofixation Lithography in Diels–Alder Networks

3D structures are written and developed in a crosslinked polymer initially formed by a Diels-Alder reaction. Unlike conventional liquid resists, small features cannot sediment, as the reversible crosslinks function as a support, and the modulus of the material is in the MPa range at room temperature. The support structure, however, can be easily removed by heating the material, and depolymerizing the polymer into a mixture of low-viscosity monomers. Complex shapes are written into the polymer network using two-photon techniques to spatially control the photoinitiation and subsequent thiol-ene reaction to selectively convert the Diels-Alder adducts into irreversible crosslinks.

Temperature Dependent Stress Relaxation in a Model Diels–Alder Network

The effect of temperature on the complex shear modulus (G*(ω)) of a model reversible covalent network formed by the Diels–Alder reaction was studied. The gel temperature of 119°C and the functional group conversion at this temperature were determined by the Winter–Chambon criterion. The complex modulus of the cross-linked network was measured from 110°C to 121°C, near the gel temperature, to determine the frequency ranges over which stress relaxation could occur. The crossover time was found to have a strong dependence on temperature (Ea ∼ 260 kJ mol–1); greater than would be expected from a typical thermally-activated retro-Diels–Alder process. Low frequency scaling of G*(ω) over the experimental frequency and temperature range was interpreted to be a result of the existence of a distribution of transient clusters in these thermoreversible covalent gels.

Towards understanding the kinetic behaviour and limitations in photo-induced copper(I) catalyzed azide–alkyne cycloaddition (CuAAC) reactions

The kinetic behaviour of the photo-induced copper(i) catalyzed azide-alkyne cycloaddition (CuAAC) reaction was studied in detail using real-time Fourier transform infrared (FTIR) spectroscopy on both a solvent-based monofunctional and a neat polymer network forming system. The results in the solvent-based system showed near first-order kinetics on copper and photoinitiator concentrations up to a threshold value in which the kinetics switch to zeroth-order. This kinetic shift shows that the photo-CuAAC reaction is not susceptible from side reactions such as copper disproportionation, copper(i) reduction, and radical termination at the early stages of the reaction. The overall reaction rate and conversion is highly dependent on the initial concentrations of photoinitiator and copper(ii) as well as their relative ratios. The conversion was decreased when an excess of photoinitiator was utilized compared to its threshold value. Interestingly, the reaction showed an induction period at relatively low intensities. The induction period is decreased by increasing light intensity and photoinitiator concentration. The reaction trends and limitations were further observed in a solventless polymer network forming system, exhibiting a similar copper and photoinitiator threshold behaviour.

Covalently Crosslinked Diels-Alder Polymer Networks

This project examines the utility of cycloaddition reactions for the synthesis of polymer networks. Cycloaddition reactions are desirable because they produce no unwanted side reactions or small molecules, allowing for the formation of high molecular weight species and glassy crosslinked networks. Both the Diels-Alder reaction and the copper-catalyzed azide-alkyne cycloaddition (CuAAC) were studied. Accomplishments include externally triggered healing of a thermoreversible covalent network via self-limited hysteresis heating, the creation of Diels-Alder based photoresists, and the successful photochemical catalysis of CuAAC as an alternative to the use of ascorbic acid for the generation of Cu(I) in click reactions. An analysis of the results reveals that these new methods offer the promise of efficiently creating robust, high molecular weight species and delicate three dimensional structures that incorporate chemical functionality in the patterned material. This work was performed under a Strategic Partnerships LDRD during FY10 and FY11 as part of a Sandia National Laboratories/University of Colorado-Boulder Excellence in Science and Engineering Fellowship awarded to Brian J. Adzima, a graduate student at UC-Boulder. Benjamin J. Anderson (Org. 1833) was the Sandia National Laboratories point-of-contact for this fellowship.