Low temperature modification of polymers by triazolinediones, 2. On the mechanism of the electrophilic aromatic substitution with 4‐substituted 1,2,4‐triazoline‐3,5‐diones

Abstract 4‐Phenyl‐1,2,4‐triazoline‐3,5‐dione (2, PTD), like other triazolinediones, reacts with electron‐rich aromatic compounds to give an electrophilic substitution. The reaction of PTD with poly(oxy‐2,6‐dimethyl‐1,4‐phenylene) (1, PPE) is studied by UV and IR spectroscopy. Based on the kinetic measurements, three stages of reaction can be distinguished: a slow induction period; a period with strongly increasing reaction rate associated with the formation of a strongly absorbing species, probably a charge‐transfer complex, and conversion of most of the PTD; and finally a period with a slow decay of the previously formed absorbing species. Experiments with urazole or trifluoroacetic acid added at the beginning of the reaction prove that the strong increase in the reaction rate is due to autocatalysis by the urazole groups formed during the course of the reaction. A mechanism is proposed to account for the experimental findings.