Blocked Phosphates as Photolatent Catalysts for Dynamic Photopolymer Networks

Abstract

While latent catalysts are a well-established strategy for initiating and controlling the rate of polymerization reactions, their use in dynamic polymer networks is still in its infancy. The ideal latent catalyst should be thermally stable and release a highly active species in response to an external trigger. Here, we have synthesized a temperature resistant (> 200 °C) organic phosphate with a photolabile o-nitrobenzyl protecting group that can be cleaved by UV light. Introduced in a visible light curable thiol-click photopolymer, the sequence-dependent λ-orthogonality of the curing and cleavage enables an efficient network formation at 451 nm, without premature release of the catalyst. Once cured, irradiation at 372 nm spatiotemporally activates the phosphate, which catalyzes transesterifications at elevated temperature. The formed catalyst has no effect on the thermal stability of the polymeric network and allows the activation of bond exchange reactions in selected domains of printed 3D objects.

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

“…Due to outstanding curing properties with regard to the DLP printing technology, most commonly dior multifunctional acrylates are applied as (co)monomers. [21,22,46] However, this category of polymer networks suffers from internal stresses resulting from a pronounced cure shrinkage arising during photopolymerization. [47] As a consequence, for the generated covalently crosslinked latent vitrimer networks, an undesired stress relaxation, i.e., creep, can be observed even before their actual activation, reflecting a lack in dimensional stability.…”

“…[47] As a consequence, for the generated covalently crosslinked latent vitrimer networks, an undesired stress relaxation, i.e., creep, can be observed even before their actual activation, reflecting a lack in dimensional stability. [21,22] Owing to the herein introduced photocurable monomer combination between a multifunctional thiol and a bifunctional alkene, the described constraint is overcome. 3D-printed creep-resistant objects equipped with spatially resolved on-demand activatable vitrimeric properties (Figure 4) were produced and exploited for reshaping and welding (Figure 5) by the dynamic exchange of covalent bonds within the crosslinked network topology.…”

“…Overall, the vast majority of vitrimeric polymers enabling a temporal control in terms of their dynamic-bond-exchange behavior are based on photolatent catalysis. [18][19][20][21][22][23] Substantial shortcomings appear in their practical application as the photolatent effect can be only maintained under the exclusion of (day)light. In addition, the geometry of the components to be produced is seriously limited as often, material thicknesses beyond the penetration depth of the light, essential to activate the photolatent catalyst, are required.…”

“…[19] Apart from photolatent bases, we also demonstrated the suitability of various photolatent acids and a photolatent organophosphate for the spatially resolved catalysis of transesterification reactions within a covalently crosslinked thiol-acrylate network structure. [21,22] While photolatent systems benefit from a spatiotemporal catalyst activation in high resolution, considerable shortcomings appear in their practical application. [18,23] Resulting from a limited penetration depth of the light necessary for activation, the spatial object geometry as well as the use of fillers and colorants is severely limited.…”

Recent Progress on the 3D Printing of Dynamically Cross‐Linked Polymers

“…However, it is not always desired given that the reconfigurability is at the expense of network stability (creep or relaxation resistance, etc.). By utilizing latent catalysts, the cross-linked network can be transformed into a dynamic one through exposure to an external stimulus at specific regions, [75] thereby achieving on-demand reconfiguration and optimizing the balance between network stability and recyclability. [76][77][78] In 2018, Bowman et al [79] introduced a photolatent base catalyst that can be activated by light stimulation while remaining stable under ambient conditions.…”