Non-isocyanate Polythiourethane Network from Biowaste: Achieving Circularity via Multidimensional Chemical Recycling with Valuable Small-Molecule Recovery and Reprocessability by Understanding the Dynamic Chemistry

Abstract

We studied and established the dual nature of nonisocyanate polythiourethane (NIPTU) dynamic chemistry and capitalized on our understanding to achieve multidimensional chemical recycling of a cross-linked NIPTU, also known as poly(mercapto-thiourethane). This NIPTU chemical recycling includes the first demonstration of recovery of valuable small molecules in addition to reprocessability with full cross-link density recovery. In particular, we performed the first investigation of NIPTU dynamic chemistry using small-molecule analogues. We identified two types of dynamic chemistry: reversible cyclic thiocarbonate aminolysis, where the non-isocyanate thiourethane (NITU) groups in NIPTU reversibly dissociate into cyclic thiocarbonates and amines, and trans(thio)carbamoylation, where the thionourethane linkages within the NITU groups undergo exchange reactions with alcohol. We synthesized a renewable glycerol-based NIPTU (GNIPTU) with a high biowaste-derivable content. Capitalizing on trans(thio)carbamoylation, we recovered pure di(thiocarbamate) small molecules with a 94 mol % yield, one of the highest yields among reported studies of chemical recycling of polymers. The GNIPTU network exhibited full property recovery after reprocessing, providing another effective method of chemical recycling. With robust properties, high biowastederivable content, the capability to undergo multidimensional chemical recycling with excellent small-molecule recovery, and full reprocessability, the GNIPTU network exemplifies how low-cost, renewable, non-isocyanate polyurethane-like materials can be developed with both high-performance characteristics and the potential to contribute meaningfully to polymer circularity.

Techno-economic Analysis and Life Cycle Assessment of Biomass-Derived Polyhydroxyurethane and Nonisocyanate Polythiourethane Production and Reprocessing

“…A flow diagram for reprocessing used NIPU materials is depicted in Figure . Purple boxes indicate process modeling relied on experimental data. ,− , The downstream polycondensation step is colored red because the polycondensation reactions modeled here are yet to be tested experimentally, and they have not been fully investigated in the literature. Instead, the polycondensation steps are modeled stoichiometrically in Aspen Plus V12 based on the general temperature profiles of carbamate monomers polymerizing with diols using a TBD catalyst. − This step was necessary to enable consistent comparison with baseline polymers and, although imperfect, allows for a general understanding of the relative cost and environmental performance.…”

“…We modeled these processes in Aspen Plus V12 leveraging published models and experimental data. − Box colors in Figure depict the origin of the data. Blue boxes denote data derived from literature and patents. , Purple boxes indicate data arose from experiments. ,− , Detailed process flow diagrams and stream summary tables are available in Figures S.1, S.2, Tables S.1, and S.2 in the Supporting Information (SI). Biosynthesis of PHU begins with the epoxidation of 1,3-butadiene (1,3-BD) with 2 equiv of hydrogen peroxide to form 1,3-butadiene diepoxide (1,3-BDE).…”

“…Life cycle inventory data for PHU, NIPTU, and 2° NIPU are shown in Tables S.7–S.9 of the SI. Life cycle inventory data for ECH, t -BuOK, XDA, TTEA, and SnCl 2 (Tables S.10–S.14, respectively) were not available through GREET and were obtained from literature, openLCA software, and patents. − Data for the epoxidation and separation steps of PHU and NIPTU production were obtained from the literature. , Empirical data were used in evaluating PHU and NIPTU production as well as carbamate monomer recovery processes. , Experimental data to inform material and energy flows for the polycondensation reactions modeled in this study are limited for 2° PHUs and absent for 2° NIPTUs. To enable a consistent comparison with baseline polymers, we modeled these reactions stoichiometrically in Aspen Plus V12 based on the general temperature ranges for polycondensation reactions between carbamates and 1,4-BD. − Baseline data for the fossil energy use, greenhouse gas (GHG) emissions, and water consumption of fossil-based PU foams were sourced from the GREET model .…”

“…Progress exists on each of these three fronts. Polyhydroxyurethane (PHU) and nonisocyanate polythiourethane (NIPTU) are two types of nonisocyanate polyurethane (NIPU) materials that can be synthesized from biomass and reprocessed into secondary (2°) NIPU materials. − In addition, NIPU materials eliminate the need for toxic and potentially harmful diisocyanates. ,, For instance, catalytic coupling of epoxide residues with either carbon dioxide or carbon disulfide affords 5-membered cyclic carbonates or 5-membered cyclic dithiocarbonates, respectively. Subsequent ring-opening polyaddition with an amine affords PHU from the cyclic carbonates or NIPTU from the cyclic dithiocarbonates.…”

Reprocessable Covalent Adaptable Networks via Free-Radical Polymerization with an Aromatic Disulfide Cross-Linker

“…Covalent adaptable networks, or CANs, combine the robust mechanical properties of thermosets with the recyclability of thermoplastics. While thermoset materials contain permanent cross-links, which cannot be broken after formation without degrading the material as a whole, CANs contain dynamic covalent cross-links which provide chemical resistance, dimensional stability, and robust properties at use temperatures while also allowing for exchangeability and reconfiguration at processing temperatures. − These dynamic cross-links can take a multitude of forms, with most classified as either associative or dissociative chemistries. − Associative dynamic chemistries are described as maintaining a constant cross-link density during bond rearrangement, as they involve exchange reactions with essentially simultaneous bond formation and breakage (enabled by the brief existence of intermediate states with three or more participating functional groups). , These chemistries include transesterification − and boronic ester exchange, − with materials incorporating associative chemistries sometimes referred to as vitrimers. , In contrast, dissociative dynamic chemistries exhibit a reduction in cross-link density during the rearrangement process, as they involve reversible bond breakage/reformation resulting from the application/removal of a stimulus such as heat. − Notable examples of these chemistries include Diels–Alder reactions ,− and hindered urea exchange. − Furthermore, some CANs (notably, those incorporating urethane or quasi-urethane linkages) can exhibit both associative and dissociative dynamic chemistries. − …”