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Hydrogen fluoride (HF) is a versatile reagent for material transformation, with applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. Here, we report the mechanochemically coupled generation of HF from alkoxy-gem-difluorocyclopropane (gDFC) mechanophores derived from the addition of difluorocarbene to enol ethers. Production of HF involves an initial mechanochemically assisted rearrangement of gDFC mechanophore to α-fluoro allyl ether whose regiochemistry involves preferential migration of fluoride to the alkoxy-substituted carbon, and ab initio steered molecular dynamics simulations reproduce the observed selectivity and offer insights into the mechanism. When the alkoxy gDFC mechanophore is derived from poly(dihydrofuran), the α-fluoro allyl ether undergoes subsequent hydrolysis to generate 1 equiv of HF and cleave the polymer chain. The hydrolysis is accelerated via acid catalysis, leading to self-amplifying HF generation and concomitant polymer degradation. The mechanically generated HF can be used in combination with fluoride indicators to generate an optical response and to degrade polybutadiene with embedded HF-cleavable silyl ethers (11 mol %). The alkoxy-gDFC mechanophore thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms. 

While Si-containing polymers can often be deconstructed using chemical triggers such as fluoride, acids, and bases, they are resistant to cleavage by mild reagents such as biological nucleophiles, thus limiting their end-of-life options and potential environmental degradability. Here, using ring-opening metathesis polymerization, we synthesize terpolymers of (1) a “functional” monomer ( e.g. , a polyethylene glycol macromonomer or dicyclopentadiene); (2) a monomer containing an electrophilic pentafluorophenyl (PFP) substituent; and (3) a cleavable monomer based on a bifunctional silyl ether . Exposing these polymers to thiols under basic conditions triggers a cascade of nucleophilic aromatic substitution (S N Ar) at the PFP groups, which liberates fluoride ions, followed by cleavage of the backbone Si–O bonds, inducing polymer backbone deconstruction. This method is shown to be effective for deconstruction of polyethylene glycol (PEG) based graft terpolymers in organic or aqueous conditions as well as polydicyclopentadiene (pDCPD) thermosets, significantly expanding upon the versatility of bifunctional silyl ether based functional polymers. 

Thermoset polymers and fiber-reinforced polymer composites possess the chemical, physical, and mechanical properties necessary for energy-efficient vehicles and structures, but their energy-inefficient manufacturing and the lack of end-of-life management strategies render these materials unsustainable. Here, we demonstrate end-of-life deconstruction and upcycling of high-performance poly(dicyclopentadiene) (pDCPD) thermosets with a concurrent reduction in the energy demand for curing via frontal copolymerization. Triggered material deconstruction is achieved through cleavage of cyclic silyl ethers and acetals incorporated into pDCPD thermosets. Both solution-state and bulk experiments reveal that seven- and eight-membered cyclic silyl ethers and eight-membered cyclic acetals are incorporated efficiently with norbornene-derived monomers, permitting deconstruction at low comonomer loadings. Frontal copolymerization of DCPD with these tailored cleavable comonomers enables energy-efficient manufacturing of sustainable, high-performance thermosets with glass transition temperatures of >100 °C and elastic moduli of >1 GPa. The polymers are fully deconstructed, yielding hydroxyl-terminated oligomers that are upcycled to polyurethane-containing thermosets having a higher glass transition temperatures than that of the original polymer upon reaction with diisocyanates. This approach is extended to frontally polymerized fiber-reinforced composites, where large-fiber volume fraction composites (Vf = 65%) containing a cleavable comonomer are deconstructed and the reclaimed fibers are used to regenerate composites via frontal polymerization that display properties nearly identical to those of the original. This work demonstrates that the use of cleavable monomers, in combination with frontal manufacturing, provides a promising strategy to address sustainability challenges for high-performance materials at multiple stages of their lifecycle. 

Convenient strategies for the deconstruction and reprocessing of thermosets could improve the circularity of these materials, but most approaches developed to date do not involve established, high-performance engineering materials. Here, we show that bifunctional silyl ether, i.e., R′O–SiR2–OR′′, (BSE)-based comonomers generate covalent adaptable network analogues of the industrial thermoset polydicyclopentadiene (pDCPD) through a novel BSE exchange process facilitated by the low-cost food-safe catalyst octanoic acid. Experimental studies and density functional theory calculations suggest an exchange mechanism involving silyl ester intermediates with formation rates that strongly depend on the Si–R2 substituents. As a result, pDCPD thermosets manufactured with BSE comonomers display temperature- and time-dependent stress relaxation as a function of their substituents. Moreover, bulk remolding of pDCPD thermosets is enabled for the first time. Altogether, this work presents a new approach toward the installation of exchangeable bonds into commercial thermosets and establishes acid-catalyzed BSE exchange as a versatile addition to the toolbox of dynamic covalent chemistry.