On the hunt for the PO radical: How substrate interactions shape the gas-phase mechanism of phosphoramidate flame retardants

• Three novel custom-built in-situ setups for mechanistic studies of flame retardants • Detailed decomposition mechanisms of phosphoramidate flame retardant and transformation products • Transformation of phosphoramidates reduces their flame-retardant efficacy • Gaseous P(V) species do not form PO radicals • Substrate alters decomposition mechanism—PO radicals form exclusively on it Phosphorus-centered radicals such as PO • are often proposed as key species in gas-phase flame inhibition, yet their formation under realistic thermal decomposition conditions remains unclear. This uncertainty arises from the coupling of transport, pyrolysis, and mixing processes, as well as from fragmentation of radical ions in conventional mass spectrometry. Here, steady-state in situ 18 eV electron-ionization molecular-beam mass spectrometry is combined with TG–IR, thermogravimetric analysis, microcombustion calorimetry, standardized flame tests, and quantum-chemical/statistical-rate modeling to elucidate the mode of action of a tris-phosphoramidate flame retardant (TD). TD is studied neat and when applied to cotton, between 200 and 500°C. Control pyrolysis of triethyl phosphate (TEP) and diethyl phosphoramidate (DEPAm) is performed. Neat TD decomposes to TEP and ethene without forming PO radicals. In contrast, TD-coated cotton yields DEPAm, ethanol, and PO radicals above ∼400°C. Neither isolated TEP nor DEPAm forms PO • , and calculations show unimolecular PO • formation to be unfavorable. Removing TEP by brief curing worsens flame performance, and TG–IR/TGA/MCC indicate that formation of a cross-linked phosphorodiamidate network lowers condensed-phase acidity and FR-effectiveness. Thus, suggesting that PO • formation is substrate-mediated, most likely following bimolecular pathways involving substrate released radicals, without being the principal driver of FR-efficacy.