Molecular dynamics simulation of the solid-state topochemical polymerization of S2N2

Area of Science:

• Solid-state chemistry
• Polymer science
• Computational materials science

Background:

• The solid-state topochemical polymerization of tetrasulfur tetranitride (S2N2) rings into polymeric sulfur nitride ((SN)x) is a key reaction in materials science.
• Understanding the reaction mechanism and resulting polymer structure is crucial for controlling material properties.

Purpose of the Study:

• To investigate the mechanism of solid-state topochemical polymerization of S2N2 rings using molecular dynamics (MD) simulations.
• To explore the influence of pressure and temperature on the polymerization rate and pathway.
• To compare simulated polymer structures and packing with experimental observations.

Main Methods:

• Molecular dynamics (MD) simulations incorporating Density Functional Theory (DFT) methods and periodic functions.
• Application of isotropic pressure compression and elevated temperature to overcome activation barriers.
• Analysis of bond cleavage, fragment attack, and reaction propagation along crystal lattice axes.

Main Results:

• Polymerization initiated by single bond cleavage in an S2N2 ring, followed by rapid attack on neighboring rings.
• Energetically favorable propagation primarily along the a-axis, forming polymer chains consistent with experimental structures.
• Observed changes in molecular packing from herringbone in S2N2 to layered in (SN)x, showing qualitative similarity to experimental data.
• Simultaneous propagation along a and c axes observed in some simulations, leading to complex networks and supporting experimental findings of polymorphs and disorder.

Conclusions:

• MD simulations provide a detailed mechanism for S2N2 solid-state polymerization.
• Simulated results qualitatively agree with experimental observations regarding chain structure and packing transformations.
• The simulations highlight the potential for complex network formation and disorder, consistent with experimental observations of (SN)x polymorphs.