Reactive Decaborane Initiates Low-Temperature Polymer Decomposition and Drives Ceramic Formation

Boron carbide (B4C) is a ceramic of considerable interest due to its high hardness, elevated melting point, excellent thermal stability, low density, and chemical inertness. Decaborane-functionalized preceramic polymers (PCPs) prepared from ring-opening metathesis polymerization (ROMP) are excellent single-source precursors of B4C, but the mechanistic details for the decomposition and ceramic conversion during pyrolysis have been overlooked, preventing advances in redesigning preceramic polymers that have high char yields and desirable chemical compositions. Herein, we reveal insights into the role of decaborane during the polymer-to-ceramic transition of poly(cyclooctenyldecaborane) (PCD) to boron carbide. ROMP of the cyclooctenyldecaborane monomer enabled controllable molecular weight and produced a series of polymers with molecular weights up to 35 kg/mol and dispersities (D̵) in the range of 1.3-1.6. Thermogravimetric analysis coupled with infrared spectroscopy and mass spectrometry (TGA-IR and TGA-MS, respectively) of PCD revealed the evolution of small hydrocarbon species following an initial release of hydrogen at low decomposition temperatures around 250 °C, which is in stark contrast to the decomposition profile of unfunctionalized poly(cyclooctadiene) (PCOD) that transitions into volatile hydrocarbon species at approximately 450 °C. ReaxFF molecular dynamics (MD) simulations support the experimental findings that hydrogen release from decaborane initiates the polymer-to-ceramic conversion. MD simulations indicate that the release of H2 results in the exothermic formation of boron clusters, which promote the thermal decomposition of the hydrocarbon polymer backbone. These findings indicate that decaborane plays an active role in the polymer-to-ceramic conversion rather than serving solely as a boron source, providing mechanistic insight that will enable the rational design of next-generation PCPs with enhanced ceramic yields and tunable compositions.