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

Elementary sulfur (S8) is abundant yet underutilized relative to its production scale. Direct valorization into plastics is challenging because S8 has minimal ring strain, rendering its ring-opening polymerization thermodynamically unfavorable and resulting poly(sulfur) unstable. Prior approaches include inverse vulcanization to yield crosslinked thermosets and anionic ROP or hybrid copolymerizations that introduce C–S bonds, but these either produce thermosets, polymers with lower sulfur content, or rely on solvents and specific comonomers. The research question is whether elementary sulfur can be converted into value-added, sulfur-rich thermoplastics with tunable properties via a simple, solvent-free, and catalyst-free process. The authors hypothesize that combining the ROP of S8 with the ROP of strained cyclic disulfides (e.g., 1,2-dithiolanes), both mediated by disulfide exchange, will provide thermodynamic driving force and dynamic covalent behavior to stabilize poly(sulfur) segments, yielding functional, degradable thermoplastics and adhesives with tunable mechanics and self-healing.

- Poly(sulfur) can be formed by rapid cooling of molten sulfur but reverts to S8 due to metastability at room temperature (Tobolsky and Eisenberg). - Inverse vulcanization (Chung et al., 2013) uses dienes to kinetically trap sulfur into sulfur-rich thermosets with applications in Li–S batteries, optics, and heavy metal removal; subsequent work developed catalytic and photoinduced variants and mechanochemical routes. - Anionic ROP of S8 (Penczek et al., 1978) and recent anionic hybrid copolymerization of sulfur with acrylates (Yang et al., 2023) incorporate sulfur into polymer backbones but often yield materials that are not thermoplastics or require specific monomers/conditions. - Multicomponent polymerizations of sulfur with organic substrates yield sulfur-containing polymers but often with lower sulfur content and use organic solvents. - Poly(disulfide)s are known dynamic covalent polymers enabling recyclability and self-healing; cyclic disulfides like 1,2-dithiolanes possess ring strain that can drive ROP. The present work seeks a distinct, atom-efficient, solvent- and catalyst-free route using cyclic disulfides to copolymerize with S8, enabling tunable thermoplastics and adhesives that are degradable and self-healing.

- Monomer synthesis: Thioctic acid (TA) was converted to TA-based cyclic disulfide derivatives with varied sidechains: TAA (amide), TABA (n-butyl amide), TADA (diethyl amide), TAMe (methyl ester), and TAH (acylhydrazide) via activation with disuccinimidyl carbonate in acetonitrile and subsequent amine/ammonia/hydrazine substitution. Monomers were characterized by NMR, HR-MS, and FT-IR. - Copolymerization: Solid S8 was melted at 120 °C to generate viscous sulfur (Sγ/Ss). A given TA-derivative was added to molten sulfur and stirred at 120 °C for 2 h (no solvent, no inert atmosphere, no catalyst). The melt was cast and cooled to obtain free-standing copolymer networks. Sulfur loadings up to 70 wt% were achievable; higher loadings led to sulfur crystallites. - Characterization: • XRD showed amorphous copolymers; FESEM showed smooth films without sulfur phase separation. • 1H/13C NMR (solution in DMSO-d6) indicated ring-opened 1,2-dithiolane and preserved carbonyls (no sulfuration of carbonyls). • Raman displayed S–S–S and S–S bands (450–520 cm−1) indicative of sulfur chain formation. • XPS: C 1s components at 284.6 (C–C), 284.8 (C–S), 285.3 (C–O), 288.2 eV (C=O); S 2p deconvolution into C–S and S–S at 163.7/165.0 and 164.0/165.1 eV. • MALDI-TOF-MS supported copolymerization between cyclic disulfides and Sx oligomers. • Thermal analysis: TGA decomposition 180–240 °C; DSC Tg tunable −28 to 27 °C. Rheology indicated thermoplastic behavior (Tg ~23 °C; flow ~90 °C for representative networks). • Robustness to radical inhibitors: Polymerizations conducted with TEMPO, p-toluidine, chloranil, 5-hexen-1-ol, and hydroquinone still yielded predominantly amorphous copolymers (minor sulfur precipitation in some cases), indicating impurity tolerance. • Purification study: CS2 extraction (3 days) removed soluble monomer/oligomer (~10% residues by 1H NMR). Post-wash samples showed reduced Sx UV–vis bands (450, 520 nm), similar TGA, slightly decreased Tg. - Mechanical testing: Uniaxial tensile tests on films (Instron 34TM-5, 50 mm/min; n≥3) with various Sx:monomer ratios; fiber tensile tests at 10 mm/min (n≥10). Activation energies from stress relaxation via rheology. - Adhesion testing: Hot-melt application at 120 °C between substrates (stainless steel, glass, aluminum). Lap shear measured on Instron with 5 kN load cell; shear strength = max force/overlap area. Work of debonding from integrating force–extension curves (energy per area). Degradability assessed via solvent/base exposure and UV–vis monitoring of cyclic disulfide bands.

- Process: Copolymerization of S8 with strained cyclic disulfides proceeds at 120 °C for 2 h without catalyst, solvent, or inert atmosphere, producing sulfur-rich, amorphous thermoplastics. Reaction is impurity tolerant to common radical quenchers. - Composition/structure: Sulfur loading up to 70 wt% achievable; higher yields sulfur crystallites. XRD amorphous; FESEM smooth morphology without phase separation. Raman displays S–S–S and S–S bands (450–520 cm−1). XPS S 2p shows C–S and S–S doublets at 163.7/165.0 and 164.0/165.1 eV. 13C NMR shows unchanged C=O at 173 ppm (no carbonyl sulfuration). MALDI-TOF-MS supports Sx incorporation. - Thermal properties: TGA decomposition 180–240 °C. DSC Tg tunable from −28 to 27 °C depending on sidechain and composition; representative network Tg ~23 °C, flow temperature ~90 °C, consistent with thermoplastic supramolecular crosslinking. - Mechanical tunability: • Young’s modulus tunable across three regimes: soft (0.9–9.1 MPa), elastomeric (68.1–214.9 MPa), rigid (224.9–949.1 MPa) depending on sidechain and S:monomer ratio. • Increasing sulfur content stiffens networks: poly(Sx/TAA) modulus from 68 MPa (1:5) to 215 MPa (1:1); poly(Sx/TABA) from 4.2 MPa (1:3) to 9.1 MPa (1:1). • poly(TAA–Sx): elastomeric with high ductility (135.0–1153.1%) and toughness (6.5–14.6 MPa) due to abundant H-bond sacrificial crosslinks. • poly(TAH–Sx): high modulus (224.9–949.1 MPa) and maximum stress 21.1 MPa owing to dense H-bonding. • poly(TABA–Sx) and poly(TADA–Sx): soft, gel-like with creep at room temperature. • Fibers from poly(Sγ/TAH=1/3): Young’s modulus >0.8 GPa, >100% strain at break, strength ~15 MPa, toughness ~9.4 MPa. - Self-healing: poly(TAA–Sx) films self-repair at room temperature within 12 h with recovery of tensile properties, attributed mainly to H-bonding at the interface. - Adhesion: As hot-melt adhesives on stainless steel, glass, and aluminum, poly(S–TAA) and poly(S–TAH) show high shear strengths (>10 MPa). Debonding work and strength are high: poly(S–TAA) work ~2.93 kN/m with shear strength 6.4 MPa; poly(S–TAH) work up to 5.36 kN/m on steel (reported highest). H-bonding provides energy dissipation; sulfur–metal interactions contribute to adhesion (control poly(Sx–DIB) shows moderate adhesion). - Degradability/removal: Adhesive layers dissolve in polar solvents (DMSO, DMF, HMPA) and depolymerize in base at room temperature to monomers/oligomers, confirmed by re-emergence of cyclic disulfide UV–vis bands. Control poly(S–DIB) is not degradable under the same conditions. - Network interactions: FT-IR shows H-bonding (amide νC=O ~1650 cm−1; νN–H ~3300 cm−1) and evidence of S-mediated H-bonds varying with S:monomer ratios; rheology indicates supramolecular crosslinking governs flow and relaxation; higher activation energies for TAA/TAH vs TABA/TADA confirm stronger H-bond networks.

The study addresses the intrinsic thermodynamic limitation of S8 ROP by coupling it with the ROP of strained 1,2-dithiolane rings, both mediated by disulfide exchange. This dual-ROP strategy provides sufficient thermodynamic and entropic driving force to stabilize sulfur-rich chains in an amorphous, dynamically crosslinked network, overcoming reversion to S8. The use of TA-derived cyclic disulfides with tunable sidechains introduces abundant, designable supramolecular (H-bond) crosslinks that govern mechanics, processability, self-healing, and adhesion. Consequently, a simple, catalyst- and solvent-free melt process at 120 °C yields thermoplastic materials with properties ranging from soft gels to rigid plastics, self-healing elastomers, strong and exceptionally tough adhesives, and degradable systems that can be removed or recycled under mild conditions. The robust impurity tolerance, high sulfur loading, and dynamic covalent backbone collectively demonstrate a sustainable and versatile pathway for valorizing abundant sulfur into functional materials, complementing and extending beyond inverse vulcanization thermosets and solvent-based approaches.

Copolymerizing elementary sulfur with TA-derived cyclic disulfides via disulfide-exchange-mediated dual ROP affords sulfur-rich thermoplastics and adhesives with tunable mechanical properties, self-healing, strong and tough adhesion, and degradability. The solvent-free, catalyst-free, and impurity-tolerant process enables high sulfur loadings (up to 70 wt%) and produces amorphous networks whose performance can be engineered through sidechain design and S:monomer ratios. This provides a sustainable route to sulfur-based functional materials and broadens sulfur valorization beyond traditional inverse vulcanization. Future work could expand monomer chemical space for property tuning (e.g., conductive, optical, or bio-derived sidechains), optimize processing and fiber spinning, quantify long-term stability and recyclability, and explore applications in self-healing elastomers, high-performance and removable adhesives, and green thermoplastics.

- Residual unreacted species: Approximately 10% unpolymerized monomers/oligomers remain after bulk copolymerization (per 1H NMR); although not necessary for use, removal requires solvent extraction (e.g., CS2). - Sulfur loading ceiling: Above ~70 wt% sulfur, excess S8 crystallizes, leading to phase-separated mixtures rather than uniform copolymers. - Thermal limits: Thermal decomposition onset is 180–240 °C, which may constrain high-temperature applications; flow around 90 °C for some formulations necessitates consideration of service temperature. - Processing temperature: Requires heating to ~120 °C to melt/process and cure in situ. - Material variability: Certain sidechains (e.g., TABA, TADA) yield very soft, creeping materials at room temperature, which may limit structural applications without additional reinforcement. - Chemical stability: Networks are intentionally degradable in basic media and soluble in strong polar solvents (DMSO, DMF, HMPA), which may limit use in such environments unless protected.