Phosphite esters and metal/organophosphite salts — e.g. tris(2,4-di-tert-butylphenyl) phosphite — are industrial secondary (hydroperoxide-decomposing) antioxidants. Dispersing them as nanoparticles increases surface area and homogeneity through a polymer, so more P(III) sites reach hydroperoxide molecules before those molecules branch into new radicals. The reaction is a non-radical redox step:
P(III) + ROOH → P(V)=O + ROH (phosphite oxidized to phosphate, ROOH consumed with no new radicals)
Without enough phosphite present, autoxidation runs as a self-branching radical chain (simplified Bolland–Gee kinetics used here):
d[ROOH]/dt = kp·[R•] − kb·[ROOH] − kPhos·[ROOH]·[P(III)]active
d[R•]/dt = kI + 2·kb·[ROOH] − kt·[R•]²
d(degradation)/dt = kt·[R•]² · scission
each phosphite site: capacity −1 per ROOH turnover, inactive at 0
- Loading — how many phosphite nanoparticles (teal) are dispersed in the matrix; each is an independent [P(III)]active sink.
- Turnover capacity — ROOH molecules one particle can neutralize before its phosphorus is fully oxidized to phosphate (P(V)) and it goes inert (fades to grey).
- Thermal / UV stress — scales the background initiation rate kI that constantly seeds new radicals — heat and light both start autoxidation.
- Substrate susceptibility (kp) — how readily the surrounding polymer's C–H bonds propagate the chain into fresh hydroperoxide.
Watch orange ROOH molecules build up and occasionally flash pink as they branch into new red radicals — until the teal phosphite particles run out of capacity and turn grey, at which point the radical population escapes and the degradation index accelerates. This is exactly why real formulations dose phosphite antioxidants in proportion to expected thermal/UV exposure.