DFT Insights Into the Cyclization Mechanism and Thermal Vacuum Degradation of Oligosilazoxanes

ABSTRACT Silazoxanes are a class of compounds characterized by the simultaneous presence of Si─N and Si─O bonds within their molecular backbone. This unique hybrid structure confers tunable properties intermediate between silazanes and siloxanes, notably enhanced stability during pyrolysis. However, a persistent knowledge gap in establishing clear‐cut correlations between molecular architectures and pyrolysis pathways consistently poses a significant challenge to their broader application in advanced ceramic precursors and high‐performance polymeric materials. This study employs density functional theory to elucidate the cyclization mechanism of oligosilazoxanes, establishing fundamental relationships between thermostability and atomic‐level structural features. Cyclization in oligosilazoxanes proceeds through bond interchange and hydrogen abstraction mechanisms. Bond interchange manifests in three modes: Si─N bond interchange, Si─O bond interchange, and combined Si─N/Si─O bond interchange. The first two modes exhibit comparable activation energies, approximately 10 kcal/mol higher than the third mode. For trimethylsilyl‐terminated oligosilazoxanes, hydrogen abstraction via oxygen or nitrogen is thermodynamically unfavorable. This thermodynamic stability is the fundamental origin of the superior thermal stability observed in inertly terminated oligosilazoxanes relative to their oligosiloxane and oligosilazane analogs. The number of N atoms in the backbone modulates the HOMO–LUMO gap by adjusting the HOMO energy level, endowing oligosilazoxanes with superior chemical tunability.