Volumetric additive manufacturing (VAM) is an emerging approach to photo polymerbased 3D printing that produces complex 3D structures in a single step, rather than from layer-by-layer assembly. [1] This paradigm holds promise because it overcomes many of the drawbacks of layerbased fabrication, such as long build times and rough surfaces. VAM also augurs a broadening of the materials available for photopolymer 3D printing, having fewer constraints on viscosity and reactivity compared to layerwise printing. Indeed, though VAM has been demonstrated with extremely soft hydrogels, [2,3] it has relied until now almost exclusively on acrylate-based chemistry. [4] This is natural, because the oxygen inhibition of acrylate polymerization provides the threshold behavior required for VAM. However, acrylate chemistry is in general limiting due to the brittle and glassy properties of the resulting materials. Accordingly, extensive efforts have been made to identify and target specific soft, elastic acrylate formulations. [5-9] Introducing alternative crosslinking chemistries to the VAM realm, as well as AM more broadly, is highly desirable as an alternative method to gain access to a wider range of mechanical, thermal, and optical performance. [10-14] Thiol-ene-based polymers are one class of materials that have drawn significant attention owing to their controllable, tunable mechanical properties. [15-17] This is generally attributed to more uniform molecular networks in thiol-ene materials, resulting from the step-growth mechanism of the polymerization reaction. [18,19] Thiol-ene materials have already shown promise for applications including use in adhesives, electronics, and as biomaterials. [20,21] This work expands the versatility of volumetric AM by introducing a new class of VAM-compatible thiol-ene resins. We demonstrate the formulation of thiol-ene resins with the nonlinear threshold-type kinetics required for VAM and show bulk-equivalent performance in the resulting 3D printed parts, confirming the advantage of the layerless whole-part process. In our volumetric AM system, a 3D distribution of light energy is delivered to the resin vat by superimposing exposures from multiple angles, a method termed computed axial litho graphy (CAL) (Figure 1a). [2] The exposures are a sequence of projections calculated from 3D CAD models using algorithms from computed Volumetric additive manufacturing (VAM) forms complete 3D objects in a single photocuring operation without layering defects, enabling 3D printed polymer parts with mechanical properties similar to their bulk material counterparts. This study presents the first report of VAM-printed thiol-ene resins. With well-ordered molecular networks, thiol-ene chemistry accesses polymer materials with a wide range of mechanical properties, moving VAM beyond the limitations of commonly used acrylate formulations. Since free-radical thiol-ene polymerization is not inhibited by oxygen, the nonlinear threshold response required in VAM is introduced by incorporating 2,2,6,6-tetrameth...
