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

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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Fig. 1: Polyurethanes and proposed CATCH cleavage.
Fig. 2: Preparation of acetal-containing polyurethane films.
Fig. 3: Gated degradation of polyurethane elastomers.
Fig. 4: Repurposing of polyurethane elastomers to strong adhesives.
Fig. 5: Repurposing of polyurethane elastomers to photochromic coating.
Fig. 6: Synthesis of additional CATCH-degradable polymers.

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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.

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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.

Author information

Authors and Affiliations

Authors

Contributions

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.

Corresponding author

Correspondence to Steven C. Zimmerman.

Ethics declarations

Competing interests

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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Nature Chemistry thanks Elizabeth Gillies and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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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.

Source data

Extended Data Fig. 2 Substituent effects in small molecule model degradation studies.

(a) General degradation reaction for model compounds 6A6C. 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 6A6C. 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 6A6C. Slopes from (c) were used to approximate k values, with k0 being the slope of 6A.

Source data

Extended Data Fig. 3 TGA traces of polyurethane films 10a, 10b, and 11.

All three polymers exhibit thermal stability up to approximately 250 °C.

Source data

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.

Source data

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).

Source data

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).

Source data

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 eg).

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 bd).

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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