Spatial and Temporal Control of Photomediated Disulfide–Ene and Thiol–Ene Chemistries for Two-Stage Polymerizations

Abstract

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.

Radical-disulfide exchange in thiol–ene–disulfidation polymerizations

“…This effect is exacerbated when the thiyl radical formed from hydrogen abstraction of the thiol is more stable than the radical formed by cleaving the disulfide. 17 It is hypothesized that the different thiol/disulfide pairs will impact the rates of both polymerization stages depending on the relative stability of the thiyl radicals that form the thiol and the disulfide.…”

“…12 Recently, however, a variety of disulfides have been investigated for their participation in disulfide-ene reactions in which the disulfide reacts with norbornenes 13 or vinyl ethers 14 via a step growth mechanism analogous to the thiol-ene reaction. Disulfide-ene reactions with cyclic, 15,16 and certain linear disulfides, 14,17 are efficient crosslinking chemistries that generate thioacetal linkages rather than the thioethers associated with thiol-ene, doubling the number of bonds formed per alkene. In addition, thiol-ene and disulfide-ene are highly compatible reactions.…”

“…This behavior has been attributed to increased steric hindrance during the chain-transfer step associated with a disulfide compared to a thiol. 17 Combining thiol–ene/disulfide–ene enables spatial and temporal control over network architecture by sequentially forming a thioether network via thiol–ene followed by an increase in crosslinking via thioacetal formation from disulfide–ene (Fig. 3).…”

“…In their study of the kinetics of the thiol–ene–disulfidation, Soars et al also demonstrated that a mix of different thiyl radicals can impact the rate of polymerization during the thiol–ene stage. 17 In their work, a divinyl ether monomer containing a thioglycolate disulfide core was co-reacted with two different thiols to elucidate whether a more stable thiyl radical should emerge after disulfide exchange events. That exchange process is expected to impact the rate of the thiol–ene polymerization.…”

Vat Photopolymerization Additive Manufacturing of Tough, Fully Recyclable Thermosets

“…To further extend DCB use in light-based additive manufacturing, resin systems that are compatible with photopolymerization and readily incorporate dynamic monomers are needed. Thiol–ene photopolymerization is a particularly promising synthetic method in this regard because it facilitates high monomer conversion on short timescales (≈1–10 s), is highly oxygen tolerant for use under ambient conditions, and demonstrates low shrinkage stress for high-fidelity and low-warp printing of large areas. − Furthermore, several dynamic monomers are easily incorporated into thiol–ene reactions, including thioesters, , anhydrides, and disulfides . Despite these advantages, it remains a challenge to realize desired mechanical properties such as stiffness, elongation, and toughness in thiol–ene or other single-stage vat photopolymerization processes.…”