Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli‐responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid‐like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re‐equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

The effect of thiol substitution in radical thiol–ene reactions has been studied by using model, monofunctional thiols as well as multifunctional thiol monomers along with the assessment of their subsequent polymerization reactions and polymer mechanical behavior. FT-IR was used to monitor the polymerization rate and quantify the overall conversion. While the total conversion was observed to range from 70% to 100%, the polymerization rate was found to decrease by as much as 10-fold as the thiol substitution was changed from primary to tertiary. Analogous multi-thiol monomers of similar structure but varying substitution were synthesized to observe the effect of substitution type on polymerization kinetics and polymer behavior. Methylation at the α-carbon was varied from primary to tertiary to observe these differences. Mechanical properties were assessed by using dynamic mechanical analysis and water sorption experiments, where the glass transition temperatures were found to be within 1–2 °C as thiol substitution varied. Furthermore, primary thiol films absorbed 1–3% more water than secondary thiol films. Resin shelf stability experiments were performed by using rheometry to measure storage time-dependent viscosity changes, and it was found that secondary thiol films remained relatively stable for up to 100 times longer than their primary counterparts. It was concluded that while there are differences under relatively slow initiation conditions, at typical initiation rates all three thiol substitutions may be made to react at similar rates for both monofunctional and polymeric systems.

Of importance for adhesive materials, particularly pressure-sensitive adhesive (PSA) systems, is the ability to increase bulk toughness without reduction of adhesion. Previous approaches for increasing PSA durability sacrifice permanent cross-linking or adhesive potential, limiting performance. In this work, covalent adaptable networks (CANs) derived from thiol-thioester exchange (TTE) are utilized as a basis for adhesive films. Tensile and single-lap shear tests were conducted for adhesive materials containing no filler, 15 wt % nanoparticles functionalized with thioester-containing acrylate, or 15 wt % nanoparticles functionalized with nonthioester-containing acrylate. Additionally, fatigue experiments were conducted on unfilled adhesives. Results indicate that TTE improves toughness, adhesion, and fatigue in unfilled materials. Filled adhesives with activated TTE showed a nearly fourfold increase in adhesion with slightly reduced toughness compared to uncatalyzed filled specimens. This work has implications in many industries, from biomedical to automotive, as toughness and fatigue resistance are important considerations for adhesive applications.

A new strategy is reported for the design and synthesis of high sulfur-containing materials for potential use in covalent adaptable networks and optical materials by combining photomediated thiol–ene- and disulfide–ene-based polymerization reactions. Taking advantage of the relative reaction rates to differentiate sequentially between the thiol–ene and disulfide–ene conjugations, these reactions were performed semiorthogonally to produce polymer networks of controlled architecture. Kinetic analysis demonstrates that the thiol–ene reaction is approximately 30 times faster than the disulfide–ene reaction, enabling spatial and temporal manipulation of material properties via dual-cure networks and photopatterning. A two-stage polymerization approach was implemented with increases in modulus in the second stage of 2–3 orders of magnitude accompanied by increases in the glass-transition temperature of more than 15 °C. Additionally, the thiol–ene reaction in the presence of a disulfide yields materials capable of simultaneous network development and stress relaxation through dynamic bond exchange during in situ polymerization.

The thiol–thioester reaction has emerged as a promising method for developing covalent adaptable networks (CANs) due to its ability to exchange rapidly under low temperature conditions in a number of solvents, orthogonality among other functional groups, and tunability. Here, the effects of thiol substitution (i.e., primary vs secondary) were assessed with respect to their reactivity in two dynamic thioester reactions: the thiol–thioester exchange and the reversible thiol–anhydride addition. Model NMR experiments were conducted using small-molecule compounds to observe how polymers of similar components would behave in thiol–thioester exchange. It was determined that the K eq was near unity for mixtures of primary thiols and secondary thioesters, and vice versa, in both a polar solvent, DMSO-d 6, and at most slightly favors primary thioesters in a relatively nonpolar solvent, CDCl3. Dielectric spectroscopy and stress relaxation experiments were used to determine the relaxation times and activation energies of the two thioester-containing networks: Thiol-ene networks, which undergo thioester exchange, displayed activation energies of 73 and 71 kJ/mol from dielectric measurements and 36 and 53 kJ/mol from stress relaxation for the primary and secondary thiols, respectively. Thiol–anhydride-ene networks, which undergo both thioester exchange and reversible thiol–anhydride addition, displayed activation energies of 94 and 114 kJ/mol from dielectric and 111 and 139 kJ/mol from stress relaxation for primary and secondary thiols, respectively. In both types of networks, the secondary thioester-based networks demonstrated slower dynamics as compared to the same primary network by at least one order of magnitude. In the anhydride network, the secondary thiol also biased the dynamics toward reversible addition.