Interlayer Bonding through Olefin Metathesis in 3D-Printed Poly(dicyclopentadiene) Thermosets

The direct ink writing (DIW) of thermosetting polymers is fundamentally constrained by weak interlayer bonding, leading to mechanical anisotropy and limiting their use in structural applications. Existing strategies to mitigate this challenge typically rely on thermal postprocessing, which increases energy consumption and processing complexity while yielding incomplete interlayer bonding. Herein, we introduce a simple and scalable reactive-interface engineering strategy that overcomes this limitation by incorporating phenolic antioxidants into a dicyclopentadiene-based ink formulated for frontal ring-opening metathesis polymerization (FROMP). The implementation of this strategy produced DIW 3D-printed poly(dicyclopentadiene) (pDCPD) samples exhibiting near-isotropic mechanical properties, closing an almost 50% deficit in ultimate tensile strength relative to cast samples. The optimized formulation maintains frontal polymerization kinetics and bulk thermomechanical performance. By suppressing surface oxidation during printing, this approach preserves interfacial chemical reactivity and enables catalyst-mediated covalent bonding between the deposited layers without postcuring. Calorimetric and deposition-sequence experiments further establish that suppressing oxidation on the first-layer surface is a prerequisite for interlayer bonding and that the preserved interfacial reactivity, comprising both residual catalyst activity and available olefins, enables olefin metathesis between the deposited layers. These results collectively offer a mechanistic understanding of a scalable strategy that eliminates postcuring requirements and bridges the performance gap between DIW 3D-printed and cast pDCPD thermosets, thereby advancing the additive manufacturing of high-performance structural polymers.