The creation of polymer materials with precise geometries and tailored mechanical properties is crucial for diverse applications. 3D printing offers remarkable versatility in achieving complex shapes, but traditionally, the final product's geometry and mechanical properties are fixed post-printing. This limitation restricts the potential of 3D printing for advanced applications demanding adaptability and customization. Digital light processing (DLP) stands out among 3D printing technologies for its high precision and speed, but the photo-curing process often results in irreversible changes. This "one print to one product" paradigm limits the scope of 3D printing, hindering its utilization in applications requiring post-printing modifications. Dynamic covalent chemistry provides a promising avenue to overcome this limitation by introducing reversible bond formation and breaking, allowing for self-healing and reprocessing capabilities. The potential for shape reconfiguration and tunable mechanical properties through dynamic bond exchange is of particular interest. Plasticity, or the ability to permanently reconfigure shape without entropy gains due to bond exchange, allows for unique adaptability. However, existing dynamic covalent networks often lack the independent control needed for simultaneous shape and property reprogramming, and many examples are not compatible with DLP printing due to viscosity issues. This study aims to develop a DLP-printable dynamic covalent polymer network capable of independent shape reconfiguration and topology isomerization, overcoming the limitations of current 3D printing technologies and paving the way for a "one print to multiple products" approach.

Previous research has explored the use of dynamic covalent chemistries (including Diels-Alder, disulfide, imine, β-hydroxyl esters, thioester-anhydride, thiourethane, and boronate ester) in 3D printing. These advancements have resulted in geometrically complex devices with self-healing and reprocessing capabilities. However, existing dynamic network designs do not allow for independent reprogramming of shape and mechanical properties post-printing. The concept of topology isomerizable networks (TINs), where topological heterogeneity drives network transformation, has been investigated. But these TINs often rely on macromonomers, creating difficulties for DLP printing due to high viscosity and low polymerizable moiety concentrations. The current work proposes a different approach, achieving enthalpy-driven isomerization rather than relying on entropy driven changes, improving the suitability for DLP printing.

A hindered urea containing bismethacrylate (HUBM) and a hydroxyl-terminated acrylate (either 4-hydroxybutyl acrylate (HBA) or poly(propylene glycol) acrylate (PPGA)) were used as precursors in a 1:2 molar ratio. The network was synthesized via light-mediated free-radical polymerization. The reaction kinetics of hindered urea transformation to urethane bonds was studied using a model compound (*n*-hexyl-*n*-tert-butylethyl-urea) and 3-methyl-1-butanol, monitored through ¹H NMR. FTIR spectroscopy was used to investigate the exchange chemistries in the polymer network at 80°C and 120°C, monitoring the conversion from hindered urea to urethane bonds. Iso-strain stress relaxation experiments measured shape reconfigurability (quantified as shape retention ratio R_ret) at 80°C and 120°C. The glass transition temperature (Tg), modulus, and breaking stress were determined using DMA and mechanical testing to evaluate changes in mechanical properties upon isomerization. DLP 3D printing was performed using a bottom-up setup with a commercial projector. The printed samples underwent shape reconfiguration at 80°C and isomerization at 120°C. Spatio-selective isomerization was demonstrated using a non-uniform temperature field. Gel fraction, ¹H NMR, ¹³C NMR, FTIR, QTOF mass spectrometry, and mechanical testing were used for characterization.

The study successfully demonstrated a DLP-printable dynamic covalent polymer network with independent shape reconfiguration and topology isomerization capabilities. At 80°C, the homolytic exchange of hindered urea bonds enables shape reconfiguration without significantly affecting mechanical properties, evidenced by a high shape retention ratio (Rret ≈ 90%) even after multiple cycles. At 120°C, the heterolytic exchange converts hindered urea bonds to urethane bonds, allowing for reprogramming of mechanical properties. The use of PPGA as a comonomer significantly expanded the tunable range of mechanical properties upon isomerization, achieving a tenfold decrease in modulus and a substantial increase in strain-at-break. 3D printed objects demonstrated successful shape reprogrammability, producing complex shapes otherwise difficult to achieve via direct printing. Spatio-selective isomerization, by applying a non-uniform temperature field, enabled the creation of mechanically non-uniform structures with varied stiffness. The ability to independently and synergistically reprogram both shape and mechanical properties showcases the "one print to multiple products" versatility.

This research addresses the limitations of conventional 3D printing by achieving independent and synergistic control over both shape and mechanical properties of 3D printed objects. The findings demonstrate the feasibility of "one print to multiple products", significantly broadening the applications of 3D printing in areas requiring post-printing customization. The enthalpy-driven topology isomerization strategy offers advantages over entropy-driven approaches in terms of printability and control. The ability to achieve spatio-selective isomerization opens up exciting possibilities for creating functional gradient materials and complex multi-material devices. This work presents a significant advancement in materials science and 3D printing, with potential applications in areas like soft robotics, biomedical engineering, and customizable consumer products.

This study successfully demonstrates a novel approach to 3D printing that allows for on-demand reprogramming of both shape and mechanical properties post-printing. The use of a dynamic covalent network with two independent bond exchange mechanisms enables unprecedented versatility, shifting from a "one print to one product" paradigm to a "one print to multiple products" approach. Future research could explore the incorporation of other dynamic covalent chemistries and the development of more sophisticated methods for spatiotemporal control of bond exchange reactions to further enhance the material's functionality and adaptability.

While the study successfully demonstrates the concept of on-demand reprogrammability, further investigation is needed to optimize the process for large-scale manufacturing. The current method requires specific temperature control for both shape reconfiguration and property reprogramming, and the long annealing times (especially for isomerization) could be a limiting factor for some applications. Exploring alternative dynamic chemistries with faster reaction kinetics could enhance the speed and efficiency of the reprogramming process.