Converting inorganic sulfur into degradable thermoplastics and adhesives by copolymerization with cyclic disulfides

Sulfur, the fifth most abundant element, is a significant component in millions of industrial chemicals. Cyclooctasulfur (S8), the most common allotrope, is produced annually in massive quantities, yet its downstream applications remain limited compared to its production scale. This abundance makes it an attractive, sustainable alternative to fossil fuels for creating value-added materials, particularly plastics. Currently, the primary use of elemental sulfur is in rubber vulcanization, a process that crosslinks natural rubber to produce durable elastomers. However, this application is far smaller than the total sulfur production. Converting elemental sulfur into synthetic plastics presents a substantial challenge. One approach involves using inorganic sulfur as a monomer in the synthesis of sulfur-containing polymers, but this often yields polymers with low sulfur content and requires organic solvents. Another, more promising strategy, is the direct ring-opening polymerization (ROP) of S8. However, S8's low ring strain creates a thermodynamically unfavorable reaction. Prior attempts to produce poly(sulfur) often result in metastable, rubber-like materials that revert to S8 monomers within hours or days. The "inverse vulcanization" method, pioneered in 2013, uses dienes to crosslink poly(S8) at high temperatures, creating stable sulfur-rich thermosets, opening avenues for applications in various fields such as batteries, optics, and heavy metal removal. Other advancements, such as the anionic ring-opening polymerization of S8 and anionic hybrid copolymerization of S8 with acrylates, have shown progress, but creating economically viable, chemically tunable, and functional sulfur-rich thermoplastics remains elusive. Existing methodologies heavily rely on the sulfur-ene reaction. Therefore, developing an alternative, atomically efficient pathway that enables economic profitability, chemical diversity, and sustainable properties (self-repair, degradability, etc.) presents a major challenge. This paper explores a novel method to overcome these limitations by utilizing copolymerization with cyclic disulfides.

Several methods have been explored for the conversion of elemental sulfur into polymeric materials. Early work focused on the equilibrium polymerization of sulfur, revealing the inherent instability of poly(sulfur) due to the low ring strain of S8. The development of inverse vulcanization represented a major breakthrough, allowing the creation of sulfur-rich thermosets by crosslinking poly(S8) with dienes at high temperatures (185 °C). This approach successfully overcomes the thermodynamic instability of poly(sulfur) through kinetic trapping, resulting in materials with various applications. However, it relies on high-temperature processing and specific reactions. Other research has explored using inorganic sulfur sources to create sulfur-containing polymers, but this often leads to low sulfur content and requires the use of organic solvents. More recently, anionic ring-opening polymerization and anionic hybrid copolymerization of S8 with acrylates have been investigated, offering pathways to create sulfur-containing polymers with improved properties. However, these advancements have yet to fully realize the potential of elemental sulfur as a sustainable and economically viable source for high-performance thermoplastics with diverse functionalities.

The researchers synthesized a series of thioctic acid (TA)-based derivatives (TAA, TABA, TADA, TAMe, and TAH) which were characterized using NMR spectroscopy, HR-MS, and FT-IR. Copolymerization with elemental sulfur was achieved by simply mixing the two components at 120 °C for 2 hours, resulting in the formation of copolymer networks upon cooling. The resulting copolymers exhibited a range of appearances, from yellow to reddish-brown, depending on the sidechain and the monomer ratio. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), 1H NMR spectroscopy, FT-IR spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structure and bonding in the copolymers. XRD revealed the amorphous nature of the resulting polymers, while NMR and IR confirmed the presence of the expected chemical groups. Raman spectroscopy showed characteristic vibrational bands of S-S-S and S-S bonds, indicating successful ring-opening polymerization of elemental sulfur. XPS provided insights into the bonding environment of sulfur atoms, distinguishing between C-S and S-S bonds. To assess the robustness of the copolymerization, a range of radical scavengers were used; even with these, copolymerization still occurred, indicating the method's tolerance to impurities. Rheological measurements were conducted to further investigate the viscoelastic properties of the copolymers. The effect of unpolymerized monomers was investigated by purifying a sample via CS2 extraction and analyzing the purified polymer's characteristics. The mechanical properties were evaluated using uniaxial tension tests to obtain stress-strain curves, Young's modulus, tensile strength, and elongation at break. The self-healing properties were assessed by cutting a film, rejoining the fragments, and testing the recovered mechanical properties. The adhesion strength was evaluated using lap shear tests on different substrates (stainless steel, glass, and aluminum). Degradation studies involved immersing the adhesive layers in base solutions and tracking the changes in UV-Vis spectra. The synthetic methods include a detailed description of the preparation of TA-based derivatives and the procedure for the copolymerization. The methods for polymer characterization and analysis, including tensile testing, lap shear testing, and degradation studies are described in detail.

The copolymerization of elemental sulfur with cyclic disulfides, specifically thioctic acid derivatives, yielded sulfur-rich thermoplastics and adhesives with tunable properties. The resulting copolymers were amorphous and exhibited a wide range of mechanical properties, spanning from soft, gel-like materials to rigid plastics, depending on the type and ratio of monomers. The highest sulfur loading achieved was 70 wt%. The introduction of hydrogen bonding groups, such as amides and acylhydrazines, significantly influenced the mechanical properties, impacting the glass transition temperature (Tg) and enabling self-healing capabilities. The copolymers exhibited Tg values ranging from -28 °C to 27 °C, demonstrating the tunability through monomer selection and ratio. Copolymers with amide sidechains (e.g., poly(TAA-Sx)) showed elastomeric properties, while those with acylhydrazine sidechains (e.g., poly(TAH-Sx)) exhibited much higher Young's modulus, indicating greater stiffness. The self-healing capability was demonstrated by the successful re-joining and recovery of mechanical properties in a cut poly(TAA-Sx) film after 12 hours at room temperature. This self-healing was attributed to dynamic hydrogen bonding. The copolymers also showed impressive adhesion properties, with poly(TAA-Sx) and poly(TAH-Sx) exhibiting strong adhesion strength and high toughness, specifically showing a significant work of debonding. The strong and tough adhesive layers could be successfully applied to a range of surfaces (stainless steel, glass, and aluminum). Remarkably, the sulfur-rich backbone enabled the degradability of the copolymers, as demonstrated by their solubility in polar organic solvents and their depolymerization in basic solutions. The degradation process was monitored using UV-Vis spectroscopy, which showed recovery of the absorption bands associated with the cyclic disulfides monomers. This contrasts sharply with the control sample, poly(S-DIB), which showed no degradability under the same conditions. The processing ability of this new material was validated by the production of thin fibers from molten polymer which were significantly stretchable and mechanically tough, exceeding the performance of previously reported poly(sulfur)-based materials. The use of this new polymer as a strong, tough, and degradable adhesive has also been validated.

This work successfully addresses the challenge of transforming elemental sulfur into high-performance, sustainable thermoplastic materials. The use of copolymerization with cyclic disulfides provides a simple, solvent-free, catalyst-free route to synthesize sulfur-rich polymers with readily tunable mechanical properties and functionalities. The ability to control the mechanical properties over a wide range, from soft to rigid, by adjusting the monomer ratio and sidechain functionality represents a major advancement in the field. The demonstrated self-healing ability further enhances the material's practical value, indicating potential for applications requiring durability and resilience. The exceptional adhesion and toughness of these materials, coupled with their degradability under mild conditions, showcase their potential as sustainable alternatives to conventional adhesives. The high sulfur content achieved in these materials addresses sustainability concerns, maximizing the utilization of an abundant, readily available resource. The findings contribute to a more diversified approach to sulfur valorization, moving beyond the reliance on the sulfur-ene reaction. These materials may have significant implications in various industrial sectors, enabling the development of self-healing elastomers, high-performance adhesives, and degradable plastics.

This study presents a novel and efficient method for converting elemental sulfur into high-performance, degradable thermoplastics and adhesives. The copolymerization of sulfur with cyclic disulfides allows for precise control over the material's properties, producing materials with a range of mechanical properties, self-healing abilities, and excellent adhesion. The inherent degradability of these materials promotes sustainability. This research offers a significant step forward in the utilization of abundant elemental sulfur for producing functional and environmentally friendly polymers with broad applications. Future research could explore expanding the range of cyclic disulfides to further diversify functionalities and mechanical properties. Investigating the long-term stability and performance of these materials under various environmental conditions would also be important.

While this study demonstrates the effectiveness of the copolymerization method, further investigation is needed to fully assess the long-term stability of the materials under various environmental conditions, including exposure to UV light, moisture, and temperature fluctuations. The scalability of the synthesis method for industrial production should be investigated. A more detailed exploration of the mechanism of the self-healing process would enhance the understanding and further optimize the material design. Comprehensive toxicity studies of the copolymers and their degradation products are also necessary before widespread commercial applications.