Kinetics of the polymerization of aromatic amines by ferric chelates in aqueous solutions

Abstract

AbstractIt was found that the oxidative polymerization of aromatic amines in the presence of oxygen with ferric chelate in aqueous solutions proceeds via a mechanism similar to that of the MICHAELIS‐MENTEN reaction, the steady state being maintained during the polymerization. On the basis of this mechanism, a kinetic analysis was carried out.The rate constant of the polymerization of p‐phenylenediamine (p‐PD) with Fe(III)EDTA (ferric ethylendiaminetetraacetic acid) (k2) reaches its maximum at pH 7–8, but K1 (the reciprocal of the MICHAELIS constant (Km)) exhibits a minimum in this pH region. From the fact that the rate constant of the oxidation reaction of Fe(II)EDTA alone with oxygen (k0) is very small compared with k2, it was assumed that oxygen is part of the activated radical complex composed of the iron chelate and the monomer. When different ferric chelates are used, the k2 values vary in this order: Nitrilotriacetic acid (NTA) < N‐hydroxyethylethylenediamine‐N.N′.N′‐triacetic acid (HEDTA) < ethylenediaminetetraacetic acid (EDTA) > diethylenetriaminepentaacetic acid (DTPA) > cyclohexylenediaminetetraacetic acid (CyDTA), but K1 decreases in the following order: NTA > HEDTA ≈ EDTA > CyDTA > DTPA. The order of the K1 values for different amines is in accordance with their basicity. Comparing with ferric and cupric chelates, both k2 and K1 exhibit this order: Fe(III)EDTA > Cu(II)EDTA. The above results suggest k2 to be separated into an electron‐transfer step (ke) and a dissociation step of the activated monomer (kd), and that the k2 value is close to that of kd, the latter being the rate‐determining step.