Chemical oxidative polymerization of safranines

Abstract

Phenosafranine and safranine have been oxidized with ammonium peroxydisulfate in acidic aqueous solution. The oxidative coupling of both safranines was proved by gel permeation chromatography demonstrating the presence of oligomeric chains of mass-average molar masses 6500 and 4500 g mol-1 for polyphenosafranine and polysafranine, respectively. A theoretical study of the mechanism of safranine and phenosafranine polymerization was based on the MNDO-PM3 semiempirical quantum chemical computations of the heat of formation of dimeric reaction intermediates, taking into account solvation effects. The study of the redox properties of the hydrated safranines and their reactive species shows that nitrenium cations are the main reactive species generated by the oxidation of the parent safranines with a two-electron oxidant, peroxydisulfate, in the initiation phase. The dominant dimers of safranines are formed by N-C coupling reactions between nitrenium cations and the parent safranines. The main coupling reactions of phenosafranine are N-C2 (C8) and N-C4 (C6); N-C4 (C6) is the dominant coupling mode for safranine. The molecular structure of oligosafranines has been studied by FTIR, Raman, and UV-vis spectroscopies. Besides prevalent unoxidized monomeric units, polymerization products of safranines contain also the iminoquinonoid and newly formed fused phenazine units.