Tris(hydroxymethyl)phosphine oxide – synthesis, chemistry, and applications

Tris(hydroxymethyl)phosphine oxide, (HOCH₂)₃PO (THPO), is recognized as an efficient flame-retardant polyol and a derivative of PH₃ – an environment-friendly, halogen-free source for phosphorus compounds and polymers. Synthesis and industrial production of THPO are based on straightforward oxidation of tris(hydroxymethyl)phosphine, (HOCH₂)₃P (THP), or neutralized tetrakis(hydroxymethyl)phosphonium chloride (THPC), or sulfate (THPS), by air/O₂ or H₂O₂. In alkaline aqueous media, THP is oxidized by water with liberation of H₂. As an alcohol, THPO readily reacts with isocyanates, epoxides, aziridines, and carboxylic acid derivatives, and is widely used as crosslinker or chain-extender to produce flame-retardant polyurethanes, polyethers, polyesters, and composite materials. Similarly, with compounds containing E–Cl bonds (E = P, Si, and S), THPO forms a variety of flame-retardant P/E-compounds or polymers. Condensation of THPO with phenols proceeds via cleavage of a P–C bond of THPO and liberation of CH₂O that leads to flame-retardant THPO-phenol resins. With functionalized alkyl halides, THPO forms tris(alkoxymethyl)phosphine oxide monomers containing, for example, pendant allyl, propargyl, or silane groups. Halogenation of THPO leads to tris(chloromethyl)phosphine oxide, (ClCH₂)₃PO, or tris(bromomethyl)phosphine oxide, (BrCH₂)₃PO, useful for syntheses of multifunctional organophosphorus compounds, for example, tris(aminomethyl)phosphine oxide, (NH₂CH₂)₃PO, and phosphorus-containing podands. Similar to THPO, tris(hydroxymethyl)phosphine sulfide, (HOCH₂)₃PS, is used in preparation of flame-retardant components for polymers, and, in synthesis of asymmetric phosphorus compounds.