End-of-life upcycling of robust polyurethanes using a room temperature, mechanism-based degradation

A major challenge in developing recyclable polymeric materials is the inherent conflict between the properties required during and after its life span. In particular, materials must be strong and durable when in use, but undergo complete and rapid degradation upon end-of-life. We report a new mechanism for degrading polyurethanes called CyclizAtion-Triggered CHain (CATCH) cleavage that achieves this duality. CATCH cleavage features a simple glycerol-based acyclic acetal unit as a kinetic and thermodynamic trap for gated chain-shattering. Thus, an organic acid induces transient chain breaks with oxocarbenium ion formation and subsequent intramolecular cyclization to depolymerize fully the polyurethane backbone at room temperature. With minimal chemical modification, the resulting degradation products can be repurposed into strong adhesives and photochromic coatings demonstrating the potential for upcycling. The CATCH cleavage strategy for low-energy input breakdown and subsequent upcycling may be generalizable to a broader range of synthetic polymers and their end-of-life waste streams. The global challenge to reduce polymeric waste entering oceans and landfills has increased in urgency as the scale of problem has come into focus.1,2 Of the many approaches to make polymeric materials more sustainable, closed loop recycling have been especially interesting where depolymerization produces the original monomer.3,4 The process can be quite efficient for some materials, however others, especially thermosets require harsh conditions and most often the breakdown is insufficiently clean and too energy intensive to be practical.5 Dynamic

covalent chemistry 6 offers one promising strategy to make entirely new polymeric materials that contain chemical linkages that are easily broken. 7,8 A complementary approach uses existing polymer chemistry but new degradation processes to generate either the original monomers for recycling or alternative breakdown products for upcycling applications. [3][4][5] In this regard, a particularly challenging polymer is polyurethane.
The widespread commercial success of polyurethane is attributable to both its ease of synthesis via polyaddition polymerization of polyols and polyisocyanates and especially their superior materials properties. 9 Indeed, polyurethanes are found in a wide range of consumer products from durable foams, rubbers, and adhesives to hard plastics and coatings. The high stability of the urethane linkage provides outstanding durability but makes polyurethanes particularly difficult to recycle and, thus, more likely to accumulate in oceans and landfills. Given the low reactivity of the urethane linkage, one degradation strategy is to integrate a more readily cleavable bond into either the isocyanate or polyol monomer. However, a key property of polyurethane is its hydrophobicity, making hydrolysis significantly more challenging. For example, a polyurethane containing ketal groups was reported to be resistant to hydrolysis even at pH 1 and elevated temperature for two weeks (Fig. 1a). 10 These studies underscore the central paradox of the plastic recycling problem. 11 Whereas long-lived, robust properties are a requirement for the life-span of the material, a rapid and complete degradation under mild conditions is desirable for their disposal and possible recycling/upcycling. Herein we report one strategy to achieve this conflicting duality with a gated polyurethane that responds to two signals with transient breaks that are kinetically trapped leading to full breakdown. Further, we show the upcycling of the degradation products into strong adhesives and coatings.

Results and Discussion
Design of polyurethanes with gated cleavage using a CATCH mechanism. Ring-chain equilibria have proven a useful strategy for depolymerizing suitable condensation polymers. 12 Clever tuning of the monomer structure can produce high molecular weight polymers with useful properties that can be repeatedly cycled between monomer and polymer. 13 Our design was also influenced by stimuli responsive degradable polymers that undergo chain cleavage through iterative cyclization processes. 14,15,16,17 Thus, the need for water was obviated by pendant alcohol groups proximal to an acetal (see 1) that undergo cyclization upon induction of transient oxocarbenium ion chain breaks (2, see Fig. 1b,c). The resulting cyclic acetal (3) serves as a kinetic and thermodynamic trap for chain cleavage in a process we refer to as CATCH cleavage. As outlined in Fig. 1d, an organic solvent might permeabilize the polyurethane allowing entry of an organic acid that would catalyze CATCH cleavage. The resultant gated system would avoid premature breakdown. We describe here a simple acetal-containing polyol (5, Fig. 2) that crosslinks commercial diisocyanates to form robust polyurethanes 1 that are capable of undergoing this type of gated CATCH cleavage degradation to 3 with the derived polyol 4 capable of upcycling into other polyurethane materials. Thus, polyurethanes derived from 5 are robust but can be degraded with minimal energy input and given a useful second life through repurposing.
Polyol synthesis and mechanistic verification in a model. The synthesis of tetrol 5 was accomplished in two steps using a preparation that was conveniently performed on a 50 g scale. Thus, butyraldehyde and allyl alcohol reacted into the corresponding acetal that underwent Upjohn dihydroxylation (Fig. 2a). To test the potential for the CATCH cleavage mechanism in a model system, 5 was reacted with butylisocyanate to give 6 ( Fig. 2b). A small amount of product from reaction at a secondary alcohol group was observed and this was removed during purification. With 1 H NMR monitoring, a solution of 6 with 0.5 mol% p-toluenesulfonic acid in anhydrous acetonitrile-d3 was shown to transform smoothly over a 6 h period (t1/2 ∼40 min, Supplementary Fig. 1) to at least two new species. The 1 H NMR and MS were fully consistent with formation of cyclic acetal 7 and diol 8, the former a mixture of diastereomers. In addition to supporting the Fig. 1c cleavage mechanism, the small molecule model demonstrated the significantly greater reactivity of the primary vs. secondary alcohol with alkyl isocyanates.
Synthesis and degradation of polyurethane elastomers. Following the successful verification of the proposed cleavage mechanism, we sought to demonstrate an analogous acetal degradation in a crosslinked polyurethane. Thus, elastomeric polyurethane film 10 was prepared from tetrol 5 and toluene 2,4-diisocyanate terminated poly(propylene glycol) (PPG-TDI, Mn ∼2,300) by mixing 5 and PPG-TDI in a 1:2 ratio (Fig. 2c,d).
Different ratios of 5 with PPG-TDI were tested with a 1:2 ratio providing the lowest amount of 5 while maintaining the desired CATCH cleavage degradation. An analogous elastomeric film (11), physically indistinguishable from that shown in Fig. 2d, was prepared from tetrol 9 (Fig. 2c). Polyurethane film 11 serves as a control because 9 lacks the acetal group and therefore the ability to degrade by CATCH cleavage. Beyond the two films having identical physical appearances, their thermal and viscoelastic properties as measured by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are similar (Fig. 2e). More importantly, thermal gravimetric analysis (TGA) of both films showed similar onset degradation temperatures (>200 °C), indicating their comparable thermal stability ( Supplementary Fig. 2). The TGA results provide evidence that thermal degradation is not impaired by the degradable bonds of the acetal moiety. Film 10a with a 2:1 PPG-TDI to tetrol ratio was further characterized by FT-IR which showed a clear alcohol stretch at ca. 3400 cm -1 but no detectable residual isocyanate peaks. In contrast, film 10b with a 5:1 PPG-TDI to tetrol ratio showed a residual isocyanate band at ca. 2270 cm -1 but a negligible alcohol absorption consistent with nearly full reaction of all four alcohol groups. Based on the characterization and degradation data, the polyurethane elastomer prepared with a 2:1 PPG-TDI to tetrol ratio was used for further study because each acetal unit was likely to have at least one residual unreacted alcohol group to participate in CATCH cleavage.
To test the potential degradation of the bulk polyurethane elastomeric materials, 40 x 10 x 1 mm films 10a and We performed tensile DMA studies to test the thermomechanical response of 10a and 11 to gain more insight about their relative stability in aqueous acid. Films 10a and 11 were soaked in 1 M HCl for 24 h and compared to their pristine counterparts. We observed that DMA studies on 11 indicated direct overlap of the tan δ and E' curves for the pristine and soaked films (Fig. 4a). Although 10a has a slight shift in E′ and tan δ after soaking in 1 M HCl, the rubbery plateau for the soaked films indicates that the bulk material remained crosslinked providing evidence that bulk polymer degradation is limited under aqueous acidic condition (Fig. 4b). Additional studies must be performed to determine the cause of these shifts in tan δ and storage modulus but from a practical point of view, such durability in aqueous acid suggests that polyurethane materials derived from tetrol 5 perform well under realworld conditions. To further quantify the degradation and relative stability of these crosslinked polyurethane materials, we performed in situ storge modulus measurements while the films were immersed in a 1 M MSA in THF solution. We observed a rapid decrease of E′ of 10a after addition of 1 M MSA in THF at 100 s, indicating rapid degradation of the film, whereas the E′ of 11 remained unchanged providing further evidence that films without acetal groups were stable in THF/MSA solutions (Fig. 4c). The DMA results indicate degradation for acetal-containing polyurethane 10a whereas 11 maintains its integrity.
Indirect support for the importance of the hydroxyl groups in the breakdown of polyurethane 10a comes from studies of 10b prepared with a 5:1 PPG-TDI to tetrol ratio. Analogous to 11, these elastomeric films minimally changed with the THF/MSA treatment, at least in part because nearly all the hydroxyl groups were converted to urethane linkages and thus not able to participate in CATCH cleavage. More direct support for the Fig. 1c mechanism was provided by NMR. Thus, the HSQC spectrum of the degradation mixture showed a correlation between large and small 1 H resonances at δ 4.8 ppm and 13 C signals at 104 ppm, consistent with the acetal protons and carbon of 1,3-dioxolane (major) and 1,3-dioxane (minor) units of 12 (Fig. 5a and Supplementary Fig. 5).
Importantly, this signal disappeared upon hydrolysis with aqueous HCl to give 13. The extent and nature of the degradation was examined by MALDI-TOF MS and GPC on 12 to 13. To better grasp how the degraded material compared to the starting material, we reacted PPG-TDI with two equivalents of hexylamine to give 14 (Fig. 5b) as a control. MALDI-TOF MS data indicated m/z values for the main mass series seen in Supplementary Fig. 6 are consistent with structure 13 where n ∼35-45 PPG units, the same mass series present in 14 (Supplementary Fig.   6a). One other higher mass series is observed for 13 that corresponds to two PPG-TDI segments tethered together by a glycerol unit. The origin of this degradation product can be traced to the reaction of two PPG TDI units with a single 1,2-diol unit of 5 during the polymerization. This higher mass series is derived from the cross-linking that occurs during curing and is not seen in hexylamine-capped PPG-TDI control (Supplementary Fig. 6b). The GPC provided Ð = 2.31 and Mn = 7.7 kDa for 13 and Ð = 1.93 and Mn = 2.85 kDa for 14 ( Supplementary Fig. 6c). The GPC data further support complete degradation with generation of the original PPG-TDI length and a smaller amount of higher molecular weight material derived from cross-linking. We predict that this product distribution should not affect the upcycling into new materials.

Repurposing polyurethane elastomer into strong adhesives and photochromic coatings.
In addition to their utility as elastomers, polyurethanes are frequently applied as coatings, adhesives, and sealants. Indeed, they are strong and desirable adhesives with an ability to adhere to various substrates, water-repellant, and as thermosets, they have a wide range of operating temperatures. 18,19 Whereas facile degradation of crosslinked polyurethane materials into processable components is an important first step towards a sustainable polyurethane lifecycle, the utility of the post-degradation materials for reuse or recycling is equally crucial.
To demonstrate the reusability of the degraded materials generated from 10a via CATCH cleavage, we chose to re-formulate the post-degradation polyol urethane prepolymer, 13, as an adhesive. Polyol 13 was polymerized with 15 wt% of industrially relevant poly[(phenylisocyanate)-co-formaldehyde] (Mn ~340), PAPI 2027 (15, Fig.   5c). Adhesives fabricated with PAPI 2027 exhibited strong adhesion to both aluminum and glass substrates. The adhesives withstood a 20 lb weight for 24 h and 3 h for glass and aluminum, respectively (Fig. 5c). The significantly stronger adhesion to glass likely arises from the hydrogen bonding between the polyurethane adhesive and the hydroxylated silicate surface. 20 Additionally, lap shear tests were carried out on the PAPI-based adhesive for both substrates. Our repurposed polyurethane adhesive showed comparable magnitude of shear strength on the aluminum substrate as super glue (Figure 5d). For the glass substrate, adhesion of cyanoacrylate glue (super glue) was measured to have a shear strength of 0.8 ± 0.3 MPa, but no measurement was obtainable with the available instrument for the repurposed polyurethane adhesive because the glass substrates fractured before any failure in the adhesive could be observed (Fig. 5e). That is, the adhesion was sufficiently strong that the glass substrate would break during the lap shear test. Thus, we were able to degrade and repurpose our polyurethane material into a practical product that demonstrates competitive performance and potential commercial value.
In addition to forming adhesives, polyol prepolymer 13 can also be repurposed for use in functional coatings. 18,21 Polyurethane coatings are ubiquitous, and their properties can vary depending on the desired application but are often hard, chemically resistant, and provide appealing high-gloss finishes. As a proof of concept, we focused on demonstrating the utility and versatility of the polyol prepolymer 13 to formulate a photochromic coating as a light responsive coating, loosely analogous to smart window coatings. Thus, we blended a neat mixture of 13, 2,4,6-trioxotriazine-1,3,5(2H,4H,6H)tris(hexamethylene)isocyanate (16), and the well-studied spiropyran dye 22 17 to generate a UV-responsive coating material. This mixture was applied onto a glass slide using a paintbrush and cured (Fig. 4a). The photochromic activity of the repurposed polyurethane coating was tested by incorporating a Z-shaped mask on top of the coating and irradiated with 365 nm for 3 min which generated a purple Z on the coated section of the glass slide corresponding to formation of the merocyanine dye 18. The Z entirely disappeared in 30 min when exposed to white light. The process was repeated for four cycles (Fig. 6, Supplementary Movie 2).
We have demonstrated that simple polyol monomer 5 forms thermally and aqueous stable polyurethane elastomeric films that can be degraded into new polyols that can be reused to make different polyurethane materials.
From a sustainability standpoint, one disadvantage of 5 is its preparation utilizes the toxic osmium tetroxide for dihydroxylation. To test the generality of the CATCH cleavage mechanism and provide a greener alternative to 5 we prepared a new tetrol acetal from trimethylolethane as starting material. The new acetal-containing polyol monomer was conveniently prepared in three steps ( Supplementary Fig. 7). Despite the alcohol groups all being primary and the CATCH cleavage mechanism exclusively forming six membered 1,3-dioxane rings, model polyurethane elastomeric films still degraded within 20 min under anhydrous acidic conditions and the TGA exhibited a similar onset degradation temperature (280 °C) to 10a.

Conclusions
Because bulk polyurethane waste is made up of highly durable thermosets, it is usually incinerated or simply discarded in the environment. With the demand for polyurethane growing unabated, the safe and economically viable management of polyurethane waste will remain a key challenge facing modern society. We developed a new approach to this challenge by designing an acetal-containing polyol that is compatible with the current industrially relevant isocyanate-polyol polyurethane chemistry. The water resistant, crosslinked hydroxyacetal polyurethane demonstrated rapid acid-catalyzed depolymerization in various organic solvents at room temperature, whereas the acetal-free control polyurethane and the hydroxyl-free acetal polyurethane analog were largely unaffected by the same treatment. Additionally, the recovered polyol from the degraded waste product was successfully repurposed in two practical applications: a polyurethane adhesive with comparable performance to super glue and a photochromic coating. Again, both processes employed commercially available, industrial polyisocyanates as co-reagents. The generality of hydroxyacetal approach and compatibility with existing polyurethane technologies suggests its application to other polyurethane systems that include isocyanate-free polyurethane chemistries, 23,24 and thereby opening a practical pathway to a new generation of sustainable polyurethanes. More broadly, the CATCH cleavage strategy should be applicable to other polymer chain functionality allowing selective and facile breakdown of other polymeric materials.

Supplementary Information
Supplementary figures as well as detailed experimental procedures, NMR spectra, additional characterization data, model studies, kinetic data, material characterization, and fabrication and degradation studies of polyurethane materials.

Data Availability
The data supporting the findings in this study are available within the article and the supplementary information.