Thermoset polymers and composite materials are integral to today's aerospace, automotive, marine and energy industries and will be vital to the next generation of lightweight, energy-efficient structures in these enterprises, owing to their excellent specific stiffness and strength, thermal stability and chemical resistance. The manufacture of high-performance thermoset components requires the monomer to be cured at high temperatures (around 180 °C) for several hours, under a combined external pressure and internal vacuum . Curing is generally accomplished using large autoclaves or ovens that scale in size with the component. Hence this traditional curing approach is slow, requires a large amount of energy and involves substantial capital investment. Frontal polymerization is a promising alternative curing strategy, in which a self-propagating exothermic reaction wave transforms liquid monomers to fully cured polymers. We report here the frontal polymerization of a high-performance thermoset polymer that allows the rapid fabrication of parts with microscale features, three-dimensional printed structures and carbon-fibre-reinforced polymer composites. Precise control of the polymerization kinetics at both ambient and elevated temperatures allows stable monomer solutions to transform into fully cured polymers within seconds, reducing energy requirements and cure times by several orders of magnitude compared with conventional oven or autoclave curing approaches. The resulting polymer and composite parts possess similar mechanical properties to those cured conventionally. This curing strategy greatly improves the efficiency of manufacturing of high-performance polymers and composites, and is widely applicable to many industries.

Complex patterns integral to the structure and function of biological materials arise spontaneously during morphogenesis. In contrast, functional patterns in synthetic materials are typically created through multistep manufacturing processes, limiting accessibility to spatially varying materials systems. Here, we harness rapid reaction-thermal transport during frontal polymerization to drive the emergence of spatially varying patterns during the synthesis of engineering polymers. Tuning of the reaction kinetics and thermal transport enables internal feedback control over thermal gradients to spontaneously pattern morphological, chemical, optical, and mechanical properties of structural materials. We achieve patterned regions with two orders of magnitude change in modulus in poly­(cyclooctadiene) and 20 °C change in glass transition temperature in poly­(dicyclopentadiene). Our results suggest a facile route to patterned structural materials with complex microstructures without the need for masks, molds, or printers utilized in conventional manufacturing. Moreover, we envision that more sophisticated control of reaction-transport driven fronts may enable spontaneous growth of structures and patterns in synthetic materials, inaccessible by traditional manufacturing approaches.

Fully-cycled depolymerization and repolymerization of a low ceiling temperature polymer, cyclic poly(phthalaldehyde) (cPPA), yielding high performance structural polymer is demonstrated. The facile conditions for cPPA depolymerization circumvent the extreme conditions required to break down and recycle traditional thermoplastics and thermosets. cPPA depolymerizes in as little as 14 min at 120 °C, with concurrent evaporation and quantitative recovery of the monomer. Polymerization of the recovered monomer produces cPPA with molecular and mechanical properties identical to the original material. Depolymerization of cPPA is also demonstrated in the presence of various carbon fiber reinforcements. Continuous carbon fibers retain 100% of their moduli and tensile strength through multiple generations of recycling, while fully recycled cPPA/carbon nanofiber composites exhibit mechanical properties equivalent to the original composite and show no degradation with cycling.

Over the past 20 years, the field of polymer mechanochemistry has amassed a toolbox of mechanophores that translate mechanical energy into a variety of functional responses ranging from color change to small-molecule release. These productive chemical changes typically occur at the length scale of a few covalent bonds (Å) but require large energy inputs and strains on the micro-to-macro scale in order to achieve even low levels of mechanophore activation. The minimal activation hinders the translation of the available chemical responses into materials and device applications. The mechanophore activation challenge inspires core questions at yet another length scale of chemical control, namely: What are the molecular-scale features of a polymeric material that determine the extent of mechanophore activation? Further, how do we marry advances in the chemistry of polymer networks with the chemistry of mechanophores to create stress-responsive materials that are well suited for an intended application? In this Perspective, we speculate as to the potential match between covalent polymer mechanochemistry and recent advances in polymer network chemistry, specifically, topologically controlled networks and the hierarchical material responses enabled by multinetwork architectures and mechanically interlocked polymers. Both fundamental and applied opportunities unique to the union of these two fields are discussed.

Transient polymers rapidly and controllably depolymerize in response to a specific trigger, typically by a chain-end unzipping mechanism. Triggers, such as heat, light, and chemical stimuli, are generally dependent on the chemistry of the polymer backbone or end groups. Single electron transfer (SET), in contrast to other triggering mechanisms, is achievable by various means including chemical, electrochemical, and photochemical oxidation or reduction. Here, we identify SET and subsequent mesolytic cleavage as the major thermal triggering mechanism of cyclic poly(phthalaldehyde) (cPPA) depolymerization. Multimodal SET triggering is demonstrated by both chemical and photoredox-triggered depolymerization of cPPA. Redox-active small molecules (p-chloranil and 1,3,5-trimethoxybenzene) were used to tune the depolymerization onset temperature of cPPA over the range 105−135 °C. Extending this mechanism to photoredox catalysis, N-methylacridinium hexafluorophosphate (NMAPF 6 ) was used to photochemically degrade cPPA in solution and thin films. Finally, we fabricated photodegradable cPPA monoliths with a storage modulus of 1.8 GPa and demonstrated complete depolymerization within 25 min of sunlight exposure. Sunlight-triggered depolymerization of cPPA is demonstrated and potentially useful for the manufacture of transient devices that vanish leaving little or no trace. Most importantly, this new mechanism is likely to inspire other SET-triggered transient polymers, whose development may address the ongoing crisis of plastic pollution.