A New Sulfur-Containing Copolymer via Thermally Induced Radical Copolymerization of Elemental Sulfur with N²,N²-Diallylmelamine for Potential CO₂ Capture 🧪🌍



 Introduction 🌱💨

The alarming increase in atmospheric CO₂ levels — now surpassing 420 ppm — has been identified as a major driver of climate change 🌡️. To combat this, researchers are seeking efficient, cost-effective, and sustainable methods for carbon dioxide capture and storage (CCS). While traditional sorbents such as amine-based liquids are effective, they suffer from high regeneration costs, degradation, and environmental concerns.

In this context, sulfur-containing polymers have emerged as a promising class of materials. They combine low-cost feedstocks with tunable chemical functionalities for selective CO₂ capture. Here, we focus on a new sulfur-containing copolymer synthesized via the thermally induced radical copolymerization of elemental sulfur (S₈) with N²,N²-diallylmelamine (DAM) — a nitrogen-rich comonomer. The resulting polymer not only utilizes abundant elemental sulfur 🟡 but also incorporates nitrogen-rich moieties for enhanced CO₂ affinity through acid–base interactions.

Background: Why Sulfur and DAM? 🧬⚗️

2.1 Elemental Sulfur (S₈)

  • Abundance: Byproduct of petroleum refining and natural gas desulfurization.

  • Low cost: Large industrial surplus often leads to stockpiling.

  • Reactivity: Sulfur–sulfur bonds can be thermally activated to form long polymeric chains via radical pathways.

2.2 N²,N²-Diallylmelamine (DAM)

  • Structure: Melamine core with nitrogen atoms that act as Lewis bases.

  • CO₂ Affinity: Nitrogen-rich heterocycles can bind CO₂ via reversible interactions.

  • Crosslinking Ability: Allyl groups undergo radical polymerization, forming a robust network.

By combining S₈ and DAM, the resulting copolymer gains:

  • High sulfur content for polarity modulation and possible redox activity.

  • Nitrogen functionalities for CO₂ adsorption enhancement.

  • Thermal stability due to the aromatic triazine ring of melamine.

Synthesis: Thermally Induced Radical Copolymerization 🔥🧪

3.1 Reaction Principle

The inverse vulcanization technique is applied here:

  • At elevated temperatures (≥ 160 °C), S₈ undergoes ring-opening to form linear polysulfur chains with radical ends (•S–Sₙ–S•).

  • These sulfur radicals attack the allyl groups of DAM, initiating crosslinking and forming a sulfur-rich copolymer network.

3.2 Experimental Procedure

  1. Materials:

    • Elemental sulfur (purity ≥ 99%).

    • N²,N²-diallylmelamine (DAM).

    • Optional radical initiator (e.g., dicumyl peroxide) for improved kinetics.

  2. Protocol:

    • Elemental sulfur is heated to ~170 °C under an inert atmosphere (N₂) until a viscous dark-red melt forms.

    • DAM is slowly added with continuous stirring to ensure uniform dispersion.

    • The reaction mixture is maintained at the target temperature for 2–3 hours until gelation.

    • The copolymer is cooled to room temperature, forming a solid sulfur-rich network.

  3. Reaction Scheme ⚗️


S₈ (heat) → •S–Sₙ–S•  +  DAM → Sulfur–DAM copolymer

Structural Characterization 🔍📊

4.1 Fourier Transform Infrared Spectroscopy (FTIR)

  • Peaks at ~1650 cm⁻¹: C=N stretching from melamine core.

  • Peaks at ~500–550 cm⁻¹: S–S stretching vibrations.

  • Disappearance of allyl C=C stretching (~1640 cm⁻¹) confirms copolymerization.

4.2 Solid-State ¹³C NMR

  • Signals at 150–165 ppm: Triazine carbons.

  • New aliphatic carbon peaks: Crosslinked allyl units.

4.3 Scanning Electron Microscopy (SEM)

  • Rough, porous morphology — beneficial for gas adsorption.

  • Homogeneous distribution of sulfur and nitrogen elements confirmed by EDX mapping.

4.4 Thermogravimetric Analysis (TGA) 🌡️

  • Stable up to ~250 °C.

  • Multi-step weight loss profile indicating sulfur evaporation and organic decomposition.

4.5 Elemental Analysis (CHNS)

  • Confirms high sulfur content (≥50 wt%).

  • Nitrogen content from DAM directly correlates with CO₂ capture performance.

CO₂ Capture Mechanism 💨🧲

5.1 Adsorption Pathways

  • Physisorption: CO₂ interacts with polarizable sulfur atoms and van der Waals forces in the porous structure.

  • Chemisorption: Nitrogen atoms in DAM form weak Lewis acid–base adducts with CO₂, enhancing selectivity.

5.2 Performance Metrics

  • BET Surface Area: Moderate (10–50 m²/g), but functional groups compensate for low surface area.

  • CO₂ Uptake Capacity: ~1.5–3.0 mmol/g at 25 °C and 1 bar (depending on DAM ratio).

  • Selectivity: High CO₂/N₂ selectivity due to nitrogen–CO₂ affinity.

5.3 Regeneration

  • Adsorbed CO₂ can be released by mild heating (≤ 80 °C) or vacuum treatment, enabling multiple reuse cycles with minimal performance loss.

 Advantages of the Sulfur–DAM Copolymer 🌟

  1. Sustainability: Utilizes industrial sulfur waste.

  2. Cost-effectiveness: Low raw material costs.

  3. Thermal stability: Suitable for harsh operating environments.

  4. Chemical tunability: DAM content can be varied for optimal performance.

  5. Environmental benefit: Reduces sulfur stockpiles while aiding CO₂ mitigation.

Potential Applications 🏭🌍

  • Post-combustion CO₂ capture in flue gas treatment plants.

  • Direct air capture (DAC) systems for atmospheric CO₂ removal.

  • Gas separation membranes when combined with flexible polymer matrices.

  • Catalyst supports in CO₂ conversion processes (e.g., to methanol or carbonates).

Future Perspectives 🔮

  • Structural Optimization: Incorporating micro- or mesoporous templates to increase surface area.

  • Hybrid Materials: Combining with metal–organic frameworks (MOFs) for synergistic capture.

  • Functionalization: Grafting amines or ionic liquids to further boost selectivity.

  • Scale-Up: Developing continuous flow processes for industrial-scale synthesis.

Conclusion 📌

The sulfur–N²,N²-diallylmelamine copolymer represents a novel, sustainable, and effective approach to CO₂ capture. By merging abundant elemental sulfur with nitrogen-rich melamine derivatives through thermally induced radical copolymerization, researchers have developed a material that is cost-effective, thermally robust, and chemically versatile. This work not only offers a new pathway for sulfur utilization but also contributes to global CO₂ mitigation strategies 🌏♻️.





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