Upcycling of dynamic thiourea thermoset polymers by intrinsic chemical strengthening

Thermoset polymers are indispensable but their environmental impact has been an ever-increasing concern given their typical intractability. Although concepts enabling their reprocessing have been demonstrated, their practical potential is limited by the deteriorated performance of the reprocessed materials. Here, we report a thiourea based thermoset elastomer that can be reprocessed with enhanced mechanical properties. We reveal that the thiourea bonds are dynamic which leads to the reprocessibility. More importantly, they can undergo selective oxidation during high temperature reprocessing, resulting in significant chemical strengthening within certain reprocessing cycles. This is opposite to most polymers for which reprocessing typically results in material deterioration. The possibility of having materials with inherent reprocessing induced performance enhancement points to a promising direction towards polymer recycling.


Model compound bond exchange experiments:
The model compound 1-hexyl-3-isopropylthiourea (HITU) was synthesized by the reaction of isopropyl isothiocyanate and hexylamine. 1.00 g of isopropyl isothiocyanate and 1.00 g of hexylamine were added into a glass bottle (the molar ratio of thiocyanate and amine is 1:1). After being stirred for 12 hours at room temperature, white powders were obtained, which were vacuum dried for 24 h to yield the final product. Its chemical structure was verified by 1 H-NMR analysis ( Supplementary Fig. 12).
For bond exchange experiments, a series of 50 mg HITU were added into different glass bottles and heated to desired temperatures (120 °C-150 °C). Samples were taken out of the bottles at different times (20 min, 40 min, 60 min, 90 min, and 120 min) for gas chromatography analyses.

Model compound oxidation experiments:
The model compound 1-butyl-1-ethyl-3-isopropylthiourea (BEIT) was synthesized by the reaction of isopropyl isothiocyanate and Nethylbutylamine. 1.00 g isopropyl isothiocyanate and 1.00 g of N-ethylbutylamine were added into a glass bottle (the molar ratio between thiocyanate and amine is 1:1). After being stirred for 12 hours at room temperature, a yellow liquid was obtained, which was vacuum dried for 24 hours to yield the final product. Its chemical structure was verified by 1 H-NMR analysis ( Supplementary  Fig. 13).
For the oxidation experiments, 0.40g of BEIT was heated to 140 °C in a glass bottle under oxygen atmosphere. At reaction times of 12 hours and 24 hours, about 0.10 g of the reaction product was collected for 1 H-NMR analysis.
For comparison, a non-hindered N,N'-diisopropylthiourea was also oxidized under the same condition and the product was collected after reaction for 24 hours for 1 H-NMR analysis.
Synthesis of 1-butyl-1-ethyl-3-isopropyl-urea: 1-butyl-1-ethyl-3-isopropyl-urea was synthesized as a reference for 1 H-NMR analysis of the oxidized product of BEIT. The reaction between isopropyl isocyanate and N-ethylbutylamine proceeded in an identical manner as BEIT and the molar ratio between cyanate and amine was 1:1.
Swelling tests: Weighted thiourea polymers (approximately 35 mg) were soaked in 10 mL of different solvents at room temperature until reaching the swelling equilibrium. The samples were subsequently dried under vacuum until reaching a constant value. The gel contents were calculated as: 100%×final weight/initial weight.

Supplementary Figures
Supplementary Fig. 1 | a. Representative gas chromatography spectra showing that the two new peaks appeared after the bond exchange. The peak intensity of N,N'-diisopropylthiourea was used to monitor the exchange kinetics. b. The Arrhenius curve for the self-exchange reaction of 1-hexyl-3-isopropyl-thiourea, obtained from the gas chromatographic analyses. Basic DMF b 236 88.0 a : the acidic solvent was obtained by mixing 3.65 g HCl water solution in DMF (1 L). b : the basic solvent was obtained by dissolving 4.00 g NaOH in a mixture of water (0.1 L) and DMF (0.9 L).

Supplementary Fig. 2 | Comparison of the infrared spectra (a), DSC curves (b) and TGA curves (c) of the as synthesized and post-treated thiourea networks.
The post-treatment was conducted at 140 °C for 48 hours under nitrogen flow. None of the infrared spectrum, the DSC curves and TGA curves show any notable changes after the post-treatment, suggesting that the original as synthesized network was fully cured.

Supplementary Fig. 3 | 1 H-NMR analysis of the oxidization behavior of 1-butyl-1-ethyl-3-
isopropyl-thiourea (BEIT). a, Full 1 H-NMR spectra of the original thiourea, oxidized products (24 hours in oxygen, 140 °C) and resulting urea. The comparison among these three spectra suggests the thiourea to urea is the main reaction. b, Time evolution of 1 H-NMR spectra of BEIT upon oxidation in oxygen. We emphasize here that, some unknown minor side products are also present, which highlights the complexity of oxidation reaction and the challenge for polymer reprocessing. On the flip side, it proves the effectiveness of our overall approach in countering side reactions with a constructive reaction.

Supplementary Fig. 4 | HPLC-MS analysis of the oxidized product of 1-butyl-1-ethyl-3isopropyl-thiourea (BEIT).
Each peak is labeled with its main m/z number. The starting 1-butyl-1-ethyl-3-isopropyl-thiourea and its corresponding oxidized urea (BEIU) are detected. In addition, side products corresponding to molecular weight increments of 14 from these two compounds are also identified.