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Atomic-Scale Mechanistic Insights into the Ring-Opening Polymerization of Elemental Sulfur

Summary

Molten elemental sulfur undergoes a viscosity jump near the λ-transition (~159 °C) associated with ring-opening polymerization (ROP), but experimental characterization of polymeric sulfur species is difficult. This paper introduces a ReaxFF parameterization aimed specifically at sulfur’s ROP chemistry, trained against extensive quantum-mechanical reference data, and applies it to large-scale reactive MD (systems exceeding 10,000 atoms) at temperatures relevant to polymerization. The simulations provide an atomistic picture of liquid sulfur composition and mechanism, including the report that very large macrocyclic sulfur rings appear at the onset of polymerization—a motif contrasted with the long-standing “chain model” picture dominated by S₈ rings and linear chains.

Methods

  • Reactive model: ReaxFF reactive force field reparameterized for elemental sulfur and its polymerization chemistry, with parameters fit to quantum mechanical datasets spanning relevant sulfur bonding environments and reaction energetics (exact training sets and validation benchmarks are given in the article and Supporting Information).
  • Simulations: Classical reactive molecular dynamics at molten-state temperatures bracketing the λ-transition region, using large systems (>10⁴ atoms) to sample polymerization kinetics and composition in the melt.
  • Software context: Large-scale MD consistent with group practice (e.g. LAMMPS-class engines for ReaxFF); the article and SI list integrator, thermostat, time step, equilibration vs production segments, and temperature windows around the λ transition.

1 — MD application (atomistic dynamics). Reactive MD in the melt for >10,000 atom composition; N/A in this short summary: exact NPT/NVE/NVT label, fs time step value, full ps/ns production run length, and PBC discussion—confirm in the version-of-record PDF. N/A — no NPT barostat parameters stated here. N/A — no static electric field protocol in the ROP survey as summarized. N/A — no metadynamics or replica exchange; N/A — umbrella sampling is not part of the stated ROP production plan on this page.

2 — Force-field training. Parent: ReaxFF reparameterized for sulfur ROP chemistry, extending prior S/O/H training where applicable. QM reference: DFT (and broader QM) data sets across relevant S bonding and reaction energetics, per the article. Training set / optimization: structures, reaction energetics, and related targets listed in the main text and Supporting Information; parameters reoptimized to match the reference set. Validation uses DFT/QM and literature comparisons as stated.

3 — Static QM / DFT-only. N/A as a stand-alone DFT production study—the paper centers on reactive MD with QM-trained ReaxFF.

4 — Reviews / non-simulation — N/A.

Findings

  • Reactive MD with the new sulfur-focused ReaxFF reproduces a temperature-dependent composition of the melt consistent with the view that small rings and polymeric species coexist across the λ-transition, going beyond prior classical models that cannot form/break bonds.
  • Simulations report the emergence of large macrocyclic sulfur rings during early polymerization stages, challenging older mechanistic pictures that emphasize only small rings and linear chains.

  • The work positions reactive MD as a route to mechanism-level insight in molten sulfur Ring-opening polymerization and related chemistry contexts, with temperature near the λ transition a key lever (see the article for the reported window).

  • Corpus honesty: ROP barriers and exact kinetic rates in the melt must be quoted from the PDF/SI, not this summary.

Limitations

Force-field fidelity is bounded by the QM training coverage and classical MD approximations; longest relaxation times and nucleation rare events may still require enhanced sampling or larger trajectories than reported. Local corpus text is extract-limited for every numerical MD setting—confirm timesteps and run lengths in the PDF/SI.

Relevance to group

Led in part by Adri C. T. van Duin with Penn State and Arizona collaborators; foregrounds purpose-built ReaxFF training for polymerizing sulfur.

Citations and evidence anchors

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