Ultrasound-driven frontal polymerization: direct and remote activation approaches for the low-energy fabrication of engineering plastics and composites

This work advances ultrasound-initiated frontal ring-opening metathesis polymerization (U-FROMP) as a two-mode curing platform that uses acoustic cavitation to activate the catalyst and initiate self-propagating fronts. Building on Direct U-FROMP in neat dicyclopentadiene (DCPD), we establish Remote U-FROMP, a fully non-contact configuration in which ultrasonic waves pass through water or air to produce cavitation. After optimizing both modes, we demonstrate two applications. First, Remote U-FROMP yields homogeneous p-DCPD/MMA blends at MMA up to 50 wt%, overcoming boiling-induced front quenching and phase separation that limit thermal FROMP, and suppresses fingering instabilities while supporting three-dimensional spherical front propagation, expanding frontal polymerization to previously inaccessible monomers. Second, optimized Direct and Remote U-FROMP cure carbon-fiber-reinforced polymers (CFRPs) within minutes, requiring over four orders of magnitude less external energy than oven processing. Resulting CFRPs show 15–50% higher tensile modulus, 25–30% higher flexural strength, and superior impact resistance, including penetration-resistant performance at 60 J, versus oven-cured counterparts. U-FROMP reduced void content while raising tensile and flexural moduli by 67% and 37% compared with Th-FROMP, despite a 7.1% drop in tensile strength. Ultrasound thus emerges as a distinct frontal polymerization trigger, offering clear advantages over oven-cured and thermally initiated systems.