Remolding and Deconstruction of Industrial Thermosets via Carboxylic Acid-Catalyzed Bifunctional Silyl Ether Exchange.

Convenient strategies for the deconstruction and reprocessing of thermosets could improve the circularity of these materials, but most approaches developed to date do not involve established, high-performance engineering materials. Here, we show that bifunctional silyl ether, i.e., R'O-SiR2-OR'', (BSE)-based comonomers generate covalent adaptable network analogues of the industrial thermoset polydicyclopentadiene (pDCPD) through a novel BSE exchange process facilitated by the low-cost food-safe catalyst octanoic acid. Experimental studies and density functional theory calculations suggest an exchange mechanism involving silyl ester intermediates with formation rates that strongly depend on the Si-R2 substituents. As a result, pDCPD thermosets manufactured with BSE comonomers display temperature- and time-dependent stress relaxation as a function of their substituents. Moreover, bulk remolding of pDCPD thermosets is enabled for the first time. Altogether, this work presents a new approach toward the installation of exchangeable bonds into commercial thermosets and establishes acid-catalyzed BSE exchange as a versatile addition to the toolbox of dynamic covalent chemistry.

Luminescence and Stability of 1,4,5-Triaryl-1,2,3-Triazoles.

The fluorescence, phosphorescence, and photochemical properties of di- and triaryl-substituted-1,2,3-triazoles are reported in this work. The ease of synthesis of regioisomers of substituted triazoles enables a systematic study on the correlation between regiochemistry and excited state properties, which include the solvent dependence of fluorescence, energy gap between singlet and triplet emitters, and propensity to photon-triggered transformations. The triazoles that carry electron (e)-donor and e-acceptor aryl substituents show high fluorescence quantum yields in weakly polar solvents and exhibit solvent-dependent fluorescence. The luminescence properties of these compounds in glass matrices at 77 K are characterized. The thermal and photo-stability, two parameters that are crucial to their potential utilities in optical devices, of these compounds are determined. The position of the e-donor substituent has a significant impact on the fluorescence emission energy and solvent sensitivity, singlet-triplet energy gap, and photochemical reactivity and stability. The experimental observations on the structural correlation with the photophysical and photochemical properties are explained by quantum chemical calculations. This study provides a rationale on the placement of substituent on a donor-acceptor type fluorophore to maneuver a range of photo-related properties.

Structure-Reactivity-Property Relationships in Covalent Adaptable Networks.

Polymer networks built out of dynamic covalent bonds offer the potential to translate the control and tunability of chemical reactions to macroscopic physical properties. Under conditions at which these reactions occur, the topology of covalent adaptable networks (CANs) can rearrange, meaning that they can flow, self-heal, be remolded, and respond to stimuli. Materials with these properties are necessary to fields ranging from sustainability to tissue engineering; thus the conditions and time scale of network rearrangement must be compatible with the intended use. The mechanical properties of CANs are based on the thermodynamics and kinetics of their constituent bonds. Therefore, strategies are needed that connect the molecular and macroscopic worlds. In this Perspective, we analyze structure-reactivity-property relationships for several classes of CANs, illustrating both general design principles and the predictive potential of linear free energy relationships (LFERs) applied to CANs. We discuss opportunities in the field to develop quantitative structure-reactivity-property relationships and open challenges.

Using Visible Light to Tune Boronic Acid-Ester Equilibria.

We report a series of azobenzene boronic acids that reversibly control the extent of diol binding via photochemical isomerization. When the boronic acid is ortho to the azo group, the thermodynamically favored E isomer binds weakly with diols to form boronic esters. The isomerization of the (E)-azobenzene to its Z isomer enhances diol binding, and the magnitude of this enhancement is affected by the azobenzene structure. 2,6-Dimethoxy azobenzene boronic acids show an over 20-fold enhancement in binding upon E → Z isomerization, which can be triggered with red light. Competition experiments and computational studies suggest that the changes in the binding affinity originate from the stabilization of the (E)-boronic acids and the destabilization of the (E)-boronic esters. We demonstrate a correlation between diol binding and the photostationary state, such that different wavelengths of irradiation yield different quantities of the bound diol. Higher binding constants for the Z isomer relative to the E isomer were observed with all diols investigated, including cyclic diols, nitrocatechol, biologically relevant compounds, and polyols. This photoswitch was employed to "catch and release" a fluorophore-tagged diol in buffered water. By tethering this photoswitch to a poly(ethylene glycol) star polymer, we can tune the stiffness of covalent adaptable hydrogels using different wavelengths of visible light. This paper describes how structural modifications of azobenzenes can influence the isomerism-dependent thermodynamics of their dynamic covalent bonds with small molecules and macromolecules.

Fluorescence of Hydroxyphenyl-Substituted "Click" Triazoles.

The structural and optical properties of hydroxyphenyl-substituted-1,2,3-triazole molecules ("click" triazoles) are described. "Click" triazoles are prepared from the copper(I)-catalyzed azide-alkyne cycloaddition reactions. The alkyne-derived C4 substituent of a "click" triazole engages in electronic conjugation more effectively with the triazolyl core than the azide-derived N1 substituent. Furthermore, triazolyl group exerts a stronger electron-withdrawing effect on the N1 than the C4 substituent. Therefore, the placement of an electron-donating group at either C4 or N1 position and the presence or the absence of an intramolecular hydrogen bond (HB) have profound influences on the optical properties of these compounds. The reported "click" triazoles have fluorescence quantum yields in the range of 0.1-0.3 and large apparent Stokes shifts (8000-13 000 cm-1) in all tested solvents. Deprotonation of "click" triazoles with a C4 hydroxyphenyl group increases their Stokes shifts; while the opposite (or quenching) occurs to the triazoles with an N1 hydroxyphenyl substituent. For the triazoles that contain intramolecular HBs, neither experimental nor computational results support a model of excited state intramolecular proton transfer (ESIPT). Rather, the excited state internal (or intramolecular) charge transfer (ICT) mechanism is more suitable to explain the fluorescence properties of the hydroxyphenyl-substituted "click" triazoles; specifically, the large Stokes shifts of these compounds.

Internal Catalysis in Dynamic Hydrogels with Associative Thioester Cross-Links.

Thioesters are an essential functional group in biosynthetic pathways, which has motivated their development as reactive handles in probes and peptide assembly. Thioester exchange is typically accelerated by catalysts or elevated pH. Here, we report the use of bifunctional aromatic thioesters as dynamic covalent cross-links in hydrogels, demonstrating that at physiologic pH in aqueous conditions, transthioesterification facilitates stress relaxation on the time scale of hundreds of seconds. We show that intramolecular hydrogen bonding is responsible for accelerated exchange, evident in both molecular kinetics and macromolecular stress relaxation. Drawing from concepts in the vitrimer literature, this system exemplifies how dynamic cross-links that exchange through an associative mechanism enable tunable stress relaxation without altering stiffness.

Tailoring Dynamic Hydrogels by Controlling Associative Exchange Rates.

Dithioalkylidenes are a newly-developed class of conjugate acceptors that undergo thiol exchange via an associative mechanism, enabling decoupling of key material properties for sustainability, biomedical, and sensing applications. Here, we show that the exchange rate is highly sensitive to the structure of the acceptor and tunable over four orders of magnitude in aqueous environments. Cyclic acceptors exchange rapidly, from 0.95 to 15.6 M-1s-1, while acyclic acceptors exchange between 3.77x10-3 and 2.17x10-2 M-1s-1. Computational, spectroscopic, and structural data suggest that cyclic acceptors are more reactive than their acyclic counterparts because of resonance stabilization of the tetrahedral exchange intermediate. We parametrize molecular reactivity with respect to computed descriptors of the electrophilic site and leverage this insight to design a compound with intermediate characteristics. Lastly, we incorporate this dynamic bond into hydrogels and demonstrate that the characteristic stress relaxation time (τ) is directly proportional to molecular kex.

Reversibly tuning hydrogel stiffness through photocontrolled dynamic covalent crosslinks.

Controlling the physical properties of soft materials with external stimuli enables researchers to mimic and study dynamic systems. Of particular interest are hydrogels, polymer networks swollen by water with broad applicability to biomedicine. To control hydrogel mechanics with light, researchers have relied on a limited number of photochemical reactions. Here we introduce an approach to reversibly tune hydrogel mechanics with light by manipulating the stability of dynamic covalent crosslinks at the molecular level. The equilibrium between a boronic acid and diol to form a boronic ester can be altered by the configuration of an adjacent azobenzene photoswitch. By irradiating branched polymers bearing azobenzene-boronic acid and diol end groups with two different wavelengths of light, we can stiffen or soften the resulting hydrogel. Alternating irradiation induces reversible mechanical changes. Rheological characterization reveals that the hydrogels are viscoelastic, exhibiting stress relaxation on the order of seconds, and the stiffness is tuned independently of the crossover frequency. We have also demonstrated that this approach can be extended to use visible light for both softening and stiffening. These photocontrolled dynamic covalent crosslinks provide a versatile platform for tunable dynamic materials.