LETTER https://doi.org/10.1038/s41586-018-0054-x Rapid energy-efficient manufacturing of polymers and composites via frontal polymerization Ian D. Robertson 1,2,7 , Mostafa Yourdkhani 1,7 , Polette J. Centellas 1,3 , Jia En Aw 1,3 , Douglas G. Ivanoff 1,4 , Elyas Goli 1,5 , Evan M. Lloyd 1,6 , Leon M. Dean 1,4 , Nancy R. Sottos 1,4 , Philippe H. Geubelle 1,3 , Jeffrey S. Moore 1,2 & Scott R. White 1,3 * 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 1–5 . 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 6 . 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 6,7 . 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. Present technologies for manufacturing high-performance thermo- set and fibre-reinforced polymer composite (FRPC) parts rely on curing in large, expensive autoclaves or ovens. For example, curing a small section of the Boeing 787’s carbon fibre/epoxy fuselage is estimated to require 350 gigajoules (GJ) of energy during its eight-hour cure cycle, producing more than 80 tons of carbon dioxide 7 . Consequently, there has been much interest in producing these materials with less energy, reducing their cost and environmental impact and furthering their application in commercial markets 6,8–11 . Frontal polymerization is a promising curing strategy that substantially reduces manufactur- ing burdens by using the enthalpy of polymerization to provide the energy for materials synthesis, rather than requiring an external energy source 12 . In frontal polymerization, a solution of a monomer and a latent initiator is heated locally until the initiator is activated for polym- erization of the monomer, producing heat from the polymerization that further drives the reaction. The autoactivation process produces a propagating reaction wave that rapidly transforms the available mono- mer into polymer. Frontal polymerization has been used to synthesize a variety of polymeric materials, including functionally graded polymers, nanocomposites, hydrogels, sensory materials and FRPCs 13–21 . Most of the materials used in frontal polymerization to date, however, are unsuitable for high-performance applications. For example, although acrylate monomers possess the requisite energy density and reactivity to frontally polymerize, the mechanical properties of the resulting polymers are greatly inferior to those used in structural FRPCs. By con- trast, epoxy monomers produce mechanically robust polymers, but are challenging to frontally polymerize because of their lower reactivity 20,22 . Moreover, it is essential for successful manufacturing of FRPCs that the liquid monomer be stable and essentially free of background polymeri- zation at room temperature. These requirements motivate the develop- ment of frontal-polymerization chemistry with a controllable and stable processing window, a high energy density and reactivity, and a mechanically and thermally robust polymer product. Here, we demonstrate that well-controlled frontal polymerization facilitates the rapid production of high-performance thermoset and FRPC parts with minimal energy input. Furthermore, the process is compatible with commonly used manufacturing techniques and pro- duces high-quality thermoset materials. Frontal curing of FRPCs is challenging because a high volume fraction of fibres is necessary to produce a composite material with good mechanical properties, yet the corresponding reduction in resin content reduces the exother- mic energy density available for frontal polymerization. As such, the frontal-polymerization chemistry must have a high molar enthalpy of polymerization and sufficiently high rate of polymerization to prevent frontal quenching. Fabricating small components with frontal polym- erization is similarly challenging because much of the heat of polym- erization is lost to the environment through air or tooling surfaces 23,24 . The frontal ring-opening metathesis polymerization (FROMP) of dicyclopentadiene (DCPD) using a thermally activated ruthenium catalyst exhibits the high energy density, high reactivity and low vis- cosity required for the synthesis of high-performance thermosets (Fig. 1a). The resulting polydicyclopentadiene (pDCPD) is a cross- linked thermoset polymer that is suitable for the fabrication of durable resin and FRPC parts, owing to its high fracture toughness, impact resistance, stiffness and chemical resistance 25–27 . However, FROMP chemistry has been severely limited in the past by its short pot life of less than 30 minutes 28,29 . Recently, we demonstrated that inhibitors of the alkyl phosphite family substantially extend the room-temperature liquid-processing window for FROMP of DCPD up to 30 hours 30 . Here, we use phosphite-inhibited FROMP of DCPD to efficiently fabricate neat pDCPD and carbon FRPC structures. Compared with conven- tional curing, our frontal-polymerization strategy reduces energy requirements by more than ten orders of magnitude for large compo- nents (Fig. 1b). Phosphite-inhibited DCPD containing second-generation Grubbs’ catalyst (GC2) slowly transforms at room temperature from a liquid to a viscoelastic gel. Remarkably, the gelation of the monomer does not result in concomitant spontaneous polymerization, as observed with previous FROMP chemistries. Tuning the inhibitor concentration allows access to a range of rheological profiles between the low-viscosity 1 Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL, USA. 2 Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL, USA. 3 Department of Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA. 4 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA. 5 Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA. 6 Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA. 7 These authors contributed equally: Ian D. Robertson, Mostafa Yourdkhani. *e-mail: swhite@illinois.edu 10 MAY 2018 | VOL 557 | NATURE | 223 © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.