Thermally Latent Bases in Dynamic Covalent Polymer Networks and their Emerging Applications

Abstract

A novel strategy allowing temporal control of dynamic bond exchange in covalently crosslinked polymer networks via latent transesterification catalysts is introduced. Obtained by a straightforward air‐ and water‐tolerant synthesis, the latent catalyst is designed for an irreversible temperature‐mediated release of a strong organic base. Its long‐term inactivity at temperatures below 50 °C provides the unique opportunity to equip dynamic covalent networks with creep resistance and high bond‐exchange rates, once activated. The presented thermally latent base catalyst is conveniently introducible in readily available building blocks and, as proof of concept, applied in a radically polymerized thiol–ene network. Light‐mediated curing is used for 3D‐printing functional objects, on which the possibility of spatially controlled reshaping and welding based on dynamic transesterification is illustrated. Since the catalyst is thermally activated, limitations regarding sample geometry and optical transparency do not apply, which facilitates a transfer to well‐established industrial technologies. Consequently, fiber‐reinforced and highly filled magneto‐active thiol–ene polymer composites are fabricated by a thermal curing approach. The on‐demand activation of dynamic transesterification is demonstrated by (magneto‐assisted) reshaping experiments, highlighting a wide range of potential future applications offered by the presented concept.

Spatially resolved photoactivation of dynamic exchange reactions in 3D-printed thiol–ene vitrimers

“…S4b, ESI †). 10,18 On the obtained covalently cross-linked 1 mm thick polymer sample, stress relaxation measurements were performed in the linear viscoelastic range of the material at a deformation of 3%. Amplitude sweep experiments are shown in Fig.…”

“…8 Within subsequent studies, we combined the holistic idea of latent catalysis of dynamic transesterification in vitrimer systems with digital light processing (DLP) 3D printing. [9][10][11] Whereas thermally latent catalysts can be used regardless of the absorption characteristics of the photoinitiator, photolatent catalysts require an (at least) sequence-dependent wavelength-orthogonality. 10 To achieve this fundamental prerequisite, the emission range of the light source used for DLP 3D printing must be adequately separated from the excitation window of the photolatent catalyst.…”

“…[9][10][11] Whereas thermally latent catalysts can be used regardless of the absorption characteristics of the photoinitiator, photolatent catalysts require an (at least) sequence-dependent wavelength-orthogonality. 10 To achieve this fundamental prerequisite, the emission range of the light source used for DLP 3D printing must be adequately separated from the excitation window of the photolatent catalyst. Thus, the latency of the light-sensitive catalyst persists in the cured network, whilst a second range of wavelengths allows spatiotemporal activation.…”

Thermally Reversible Organocatalyst for the Accelerated Reprocessing of Dynamic Networks with Creep Resistance

“…Specifically, quaternary ammonium salts exhibit distinct catalytic behavior by displaying basic properties upon liberation of a tertiary amine. , Originally employed to achieve temporal and thermal control of the curing reaction processes of diverse formulations, − and more recently to enhance the durability and stability of PU adhesives, this type of organocatalyst has garnered attention in polymer chemistry due to their facile and scalable synthesis, making their use suitable for industrial applications. More recently, this concept has been extended to trigger the dynamic exchange in CANs: Schlögl and co-workers showed the promotion of transesterification reactions by the thermal activation of a latent base, 1,5,7-triazabicyclo[4.4.0]­dec-5-enylcyanoacetate (TBD-CA), in a cross-linked thiol–ene polymer network . The organic salt dissociation occurred at 130 °C, which also triggered the decarboxylation of the acid, leading to the irreversible release of the catalytic TBD.…”

“…More recently, this concept has been extended to trigger the dynamic exchange in CANs: Schlogl and co-workers showed the promotion of transesterification reactions by the thermal activation of a latent base, 1,5,7-triazabicyclo[4.4.0]dec-5-enylcyanoacetate (TBD-CA), in a cross-linked thiol−ene polymer network. 23 The organic salt dissociation occurred at 130 °C, which also triggered the decarboxylation of the acid, leading to the irreversible release of the catalytic TBD. This group has also shown that a latent catalyst activation can be achieved by light.…”