In situ elucidation of PO radical formation pathways in phosphonate flame retardants with different P–C linkage motifs

The conditions under which phosphorus-containing radicals can already form during low-temperature degradation of flame-retardant-treated textiles remain insufficiently understood. Here, we investigate the degradation and flame-retardant action of phosphonate-functionalized silanes featuring distinct P–C linkage motifs: a classical phosphonate (TRI/DPTS(1:3)), a triazine-linked derivative (2DTA), and a carbamoylphosphonate (ID). The compounds were studied both as neat materials and as cotton finishes using a multimethod approach combining standardized flame testing and thermal analysis with steady-state in situ molecular beam mass spectrometry employing mild electron-impact ionization under pyrolytic and oxidative conditions, complemented by transient in situ MS as well as spectroscopic and elemental characterization of evolved gases and residues. Cotton treated with 2DTA showed the best flame retardancy, consistent with a comparatively stronger condensed-phase contribution. In neat decomposition experiments, no detectable volatilization of phosphorus-containing species was observed for 2DTA, whereas TRI/DPTS(1:3) showed minor and ID pronounced volatilization, mirroring the trend in condensed-phase action on cotton. Notably, PO radical formation on cotton under pyrolytic conditions was detected only for 2DTA. Under oxidative conditions 2DTA also exhibited the highest PO emission, indicating competing oxidative and pyrolytic formation pathways. Overall, PO formation correlates inversely with phosphorus volatilization, supporting the hypothesis that low-temperature radical activity (<500°C) may be strongly influenced by condensed-phase collision and transfer reactions.These findings provide a new perspective on the flame-poisoning mechanism of organophosphorus flame retardants.