No abstract

Photopolymerization is a powerful tool in materials science with many applications, including coatings, adhesives, inks, and 3D printing. Until now, the majority of photoinitiating systems have been suitable only for radical photopolymerization, which automatically excludes the use of light to trigger a great number of polymerization reactions. For instance, the preparation of polyurethanes via photopolymerization from isocyanates remains a real challenge since it requires a catalyst able to mediate nucleophilic substitution reactions. In this context, this study reports the successful synthesis of three new photobase generators based on a thioxanthone chromophore functionalized with a protonated 1,8diazabicyclo[5.4.0]undec-7-ene as a latent base for the direct synthesis of polyurethanes from commercially available polyols and polyisocyanates. The catalytic activity of the photobase is modulated by introducing different functional groups at the α-position of the carboxylate which act as a photocleavable link between the chromophore and the latent base. A direct correlation between the steric hindrance of such groups and more efficient release of the base is observed by 1 H NMR. DFT studies have been performed to shed some light on the base release mechanism and to further confirm this evidence. To demonstrate their use, the ability of these photobases to mediate the nucleophilic substitution between isocyanates and alcohols has been proven by using bifunctional and trifunctional monomer mixtures by 1 H NMR, FTIR, and rheology experiments. To further exploit the full potential of the thioxanthone-based photobase generators, polyurethane coatings as well as 3D printed figures have been prepared at room temperature by using light as an external trigger.

The use of three-dimensional (3D) printable hydrogels for biomedical applications has attracted considerable attention as a consequence of the ability to precisely define the morphology of the printed object, allowing patients’ needs to be targeted. However, the majority of hydrogels do not possess suitable mechanical properties to fulfill an adequate rheological profile for printability, and hence, 3D printing of cross-linked networks is challenging and normally requires postprinting modifications to obtain the desired scaffolds. In this work, we took advantage of the crystallization process of poly­(ethylene glycol) to print non-isocyanate poly­(hydroxyurethane) hydrogels with tunable mechanical properties. As a consequence of the crystallization process, the hydrogel modulus can be tuned up to 3 orders of magnitude upon heating up to 40 °C, offering an interesting strategy to directly 3D-print hydrogels without the need of postprinting cross-linking. Moreover, the absence of any toxicity makes these materials ideal candidates for biomedical applications.

High energy photons (λ < 400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle‐reinforced composite networks since deep light penetration of short‐wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, the unconventional, yet versatile, multiexciton process of triplet–triplet annihilation upconversion (TTA‐UC) is employed for curing opaque hydrogel composites created by direct‐ink‐write (DIW) 3D printing. TTA‐UC converts low energy red light (λmax = 660 nm) for deep penetration into higher‐energy blue light to initiate free radical polymerizations within opaque objects. As proof‐of‐principle, hydrogels containing up to 15 wt.% TiO2 filler particles and doped with TTA‐UC chromophores are readily cured with red light, while composites without the chromophores and TiO2 loadings as little as 1–2 wt.% remain uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D‐printed composite polymer networks.