Abstract
A major challenge in developing recyclable polymeric materials is the inherent conflict between the properties required during and after their life span. In particular, materials must be strong and durable when in use, but undergo complete and rapid degradation, ideally under mild conditions, as they approach the end of their life span. We report a mechanism for degrading polymers called cyclization-triggered chain cleavage (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 fully depolymerize the polymer backbone at room temperature. With minimal chemical modification, the resulting degradation products from a polyurethane elastomer 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.
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Data availability
The data supporting the findings in this study are available within the Supplementary Information. Source data are provided with this paper.
References
Geyer, R., Jambeck, J. R. & Law, K. L. Production, use, and fate of all plastics ever made. Sci. Adv. 3, e1700782 (2017).
Gibb, B. C. Plastics are forever. Nat. Chem. 11, 394–395 (2019).
Hong, M. & Chen, E. Y. X. Chemically recyclable polymers: a circular economy approach to sustainability. Green Chem. 19, 3692–3706 (2017).
Rahimi, A. & García, J. M. Chemical recycling of waste plastics for new materials production. Nat. Rev. Chem. 1, 0046 (2017).
Fortman, D. J. et al. Approaches to sustainable and continually recyclable cross-linked polymers. ACS Sustain. Chem. Eng. 6, 11145–11159 (2018).
Rowan, S. J., Cantrill, S. J., Cousins, G. R. L., Sanders, J. K. M. & Stoddart, J. F. Dynamic covalent chemistry. Angew. Chem. Int. Ed. 41, 898–952 (2002).
García, J. M. et al. Recyclable, strong thermosets and organogels via paraformaldehyde condensation with diamines. Science 344, 732–735 (2014).
Christensen, P. R., Scheuermann, A. M., Loeffler, K. E. & Helms, B. A. Closed-loop recycling of plastics enabled by dynamic covalent diketoenamine bonds. Nat. Chem. 11, 442–448 (2019).
Engels, H. W. et al. Polyurethanes: versatile materials and sustainable problem solvers for today’s challenges. Angew. Chem. Int. Ed. Engl. 52, 9422–9441 (2013).
Kemona, A. & Piotrowska, M. Polyurethane recycling and disposal: methods and prospects. Polymers 12, 1752 (2020).
Sheppard, D. T. et al. Reprocessing postconsumer polyurethane foam using carbamate exchange catalysis and twin-screw extrusion. ACS Cent. Sci. 6, 921–927 (2020).
Lingier, S., Spiesschaert, Y., Dhanis, B., De Wildeman, S. & Du Prez, F. E. Rigid polyurethanes, polyesters, and polycarbonates from renewable ketal monomers. Macromolecules 50, 5346–5352 (2017).
Sardon, H. & Dove, A. P. Plastics recycling with a difference. Science 360, 380–381 (2018).
Hodge, P. Recycling of condensation polymers via ring–chain equilibria. Polym. Adv. Technol. 26, 797–803 (2015).
Zhu, J.-B., Watson, E. M., Tang, J. & Chen, E. Y.-X. A synthetic polymer system with repeatable chemical recyclability. Science 360, 398–403 (2018).
DeWit, M. A. & Gillies, E. R. A cascade biodegradable polymer based on alternating cyclization and elimination reactions. J. Am. Chem. Soc. 131, 18327–18334 (2009).
Olejniczak, J., Chan, M. & Almutairi, A. Light-triggered intramolecular cyclization in poly(lactic-co-glycolic acid)-based polymers for controlled degradation. Macromolecules 48, 3166–3172 (2015).
Lv, A., Cui, Y., Du, F.-S. & Li, Z.-C. Thermally degradable polyesters with tunable degradation temperatures via postpolymerization modification and intramolecular cyclization. Macromolecules 49, 8449–8458 (2016).
McKinlay, C. J. et al. Charge-altering releasable transporters (CARTs) for the delivery and release of mRNA in living animals. Proc. Natl Acad. Sci. USA 114, E448–E456 (2017).
Liu, B. & Thayumanavan, S. Substituent effects on the pH sensitivity of acetals and ketals and their correlation with encapsulation stability in polymeric nanogels. J. Am. Chem. Soc. 139, 2306–2317 (2017).
Szycher, M. in Szycher’s Handbook of Polyurethanes 2nd edn 37–86 (2012).
Sonnenschein, M. F. Polyurethanes: Science, Technology, Markets, and Trends, Ch. 10 (John Wiley & Sons, 2015).
Segura, D. M., Nurse, A. D., McCourt, A., Phelps, R. & Segura, A. in Chemistry of Polyurethane Adhesives and Sealants 101–162 (Elsevier, 2005).
Cornille, A. et al. Promising mechanical and adhesive properties of isocyanate-free poly(hydroxyurethane). Eur. Polym. J. 84, 404–420 (2016).
Chattopadhyay, D. K. & Raju, K. V. S. N. Structural engineering of polyurethane coatings for high performance applications. Prog. Polym. Sci. 32, 352–418 (2007).
Kortekaas, L. & Browne, W. R. The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome. Chem. Soc. Rev. 48, 3406–3424 (2019).
Gernon, M. D., Wu, M., Buszta, T. & Janney, P. Environmental benefits of methanesulfonic acid: comparative properties and advantages. Green Chem. 1, 127–140 (1999).
Maisonneuve, L., Lamarzelle, O., Rix, E., Grau, E. & Cramail, H. Isocyanate-free routes to polyurethanes and poly(hydroxy urethane)s. Chem. Rev. 115, 12407–12439 (2015).
Carré, C., Ecochard, Y., Caillol, S. & Avérous, L. From the synthesis of biobased cyclic carbonate to polyhydroxyurethanes: a promising route towards renewable non-isocyanate polyurethanes. ChemSusChem 12, 3410–3430 (2019).
Acknowledgements
S.C.Z. acknowledges support from the National Science Foundation (NSF CHE-1709718). N.R.S acknowledges support from the National Science Foundation LEAP HI program (NSF CMMI 1933932). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.
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E.G.M., H.-C.W. and S.C.Z. conceived of the idea. E.G.M., M.L.P. and H.-C.W. designed and synthesized the tetrol monomers and small-molecule models. D.D., A.J. and A.R. assisted with the chemical synthesis. E.G.M. and M.L.P. fabricated the polyurethane materials and performed degradation characterization and mechanical testing of elastomers, adhesives and coatings. D.G.I. performed DMA and DSC studies of polyurethane elastomers. D.G.I. and N.R.S. analysed the data. M.L.P. designed and synthesized the monomers for the polytriazoles, fabricated the polytriazoles and polyesters and characterized their degradation products. E.G.M., M.L.P., H.-C.W. and S.C.Z. prepared the manuscript.
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The University of Illinois has filed a patent application on degradable polymers and monomers based on the hydroxyacetal chemistry described herein (US application no. 17/217,512; inventors E.G.M., H.W. and S.C.Z.). The remaining authors declare no competing interests.
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Extended data
Extended Data Fig. 1 Degradation study of 6 in 10 mol% TsOH in CD3CN.
(a) Mechanism of acid-catalyzed small molecule degradation. (b) 1H NMR spectra from t = 0 h to t = 3.5 h. Red arrow indicates disappearance of acetal proton peak in 6 (Ha), and purple arrow indicates appearance of cyclic acetal product peaks in 7 (Hb). Black arrow indicates hexamethylbenzene internal standard (IS) peak. (c) Degradation kinetics of small molecule analog 6, showing first order decay. (d,e) High resolution ESI mass spectra of expected degradation products 7 and 8.
Extended Data Fig. 2 Substituent effects in small molecule model degradation studies.
(a) General degradation reaction for model compounds 6A – 6C. All experiments were performed with 10 mM starting acetal and 0.3 mM TsOH (3 mol%) at room temperature. (b) Graph of percent starting acetal remaining over time for 6 and 6A – 6C. Table shows amount of time required for each starting acetal to reach full degradation, determined by 1H NMR. (c) Natural log of the starting acetal concentrations for the first 5 hours of each experiment. Linear plots indicate degradation is first order with respect to acetal. (d) Hammett plot of kinetics data from 6A – 6C. Slopes from (c) were used to approximate k values, with k0 being the slope of 6A.
Extended Data Fig. 3 TGA traces of polyurethane films 10a, 10b, and 11.
All three polymers exhibit thermal stability up to approximately 250 °C.
Extended Data Fig. 4 Effect of acid concentration on rate of CATCH degradation mechanism.
(a) Polymer 10a (30.6 ± 0.3 mg) in various concentrations of MSA in DCM, stirring at room temperature. (b) 1H NMR degradation studies on small molecule model 6 in varying concentrations of p-TsOH in CD3CN. Initial concentration of 6 was 5 mM in each experiment.
Extended Data Fig. 5 2D NMR evidence for hydrolysis of acetal-containing polymer degradation products.
HSQC spectra of degraded polyurethane elastomer (a) before and (b) after being subjected to hydrolysis conditions. The cross peak denoted by the purple circle in (a) indicates the presence of an acetal. The disappearance of this cross peak in (b) after hydrolysis confirms that the cyclic acetals were completely hydrolyzed to 1,2-diols.
Extended Data Fig. 6 Characterization of polyurethane elastomer degradation products.
MALDI-TOF spectra of (a) degraded elastomer 10a and (b) hexylamine-capped PPG-TDI. (c) Gel permeation chromatography of degraded elastomer (gray line) and hexylamine-capped PPG-TDI (orange line).
Extended Data Fig. 7 Greener tetrol monomer.
(a) Osmium-free synthetic route to acid-degradable tetrol monomer 21. (b) Photograph of polyurethane elastomer prepared with a 2:1 ratio of PPG-TDI to 21. (c) TGA of greener polyurethane film (99% at 251 °C).
Supplementary information
Supplementary Information
Supplementary Figs. 1–10, Supplementary Table 1, synthetic procedures for monomers and polymers and 1H and 13C NMR spectra of synthesized compounds.
Film 10a (10 mm × 10 mm × 1 mm) degrading in 1 M MSA in THF at room temperature in a 1-dram vial with stirring using a flea stir bar. Video is 20×.
Microscope slide with repurposed photochromic coating showing a purple Z generated with photomask fading under standard fluorescent laboratory lighting. Video is 20×.
Source data
Source Data Fig. 2
Raw data for DSC and DMA characterization of 10a and 11 (panel e).
Source Data Fig. 3
Raw DMA data of 11 and 10a: pristine samples, samples soaked in 1 M HCl (aq) and samples in 1 M MSA (THF) (panels e–g).
Source Data Fig. 4
Raw data for lap shear tests on aluminium substrate for cyanoacrylate and repurposed adhesive (panel d).
Source Data Extended Data Fig. 1
1H NMR kinetics calculations (panel c).
Source Data Extended Data Fig. 2
1H NMR kinetics calculations (panels b–d).
Source Data Extended Data Fig. 3
Raw TGA data of 10a, 10b and 11.
Source Data Extended Data Fig. 4
1H NMR kinetics calculations (panel b).
Source Data Extended Data Fig. 6
Raw GPC data for hexylamine-capped PPG-TDI control and 10a degradation products (panel c).
Source Data Extended Data Fig. 7
Raw TGA data for polyurethane made from 21 and PPG-TDI (panel c).
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Morado, E.G., Paterson, M.L., Ivanoff, D.G. et al. End-of-life upcycling of polyurethanes using a room temperature, mechanism-based degradation. Nat. Chem. 15, 569–577 (2023). https://doi.org/10.1038/s41557-023-01151-y
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DOI: https://doi.org/10.1038/s41557-023-01151-y
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