H-1 NMR study of the kinetics of substituted aniline polymerization. I. Homopolymerization of 2-methoxyaniline

We studied the kinetics of the oxidative chemical homopolymerization of 2-m ethoxyaniline (OMA) in aqueous acid solutions by monitoring OMA depletion w ith H-1 NMR spectroscopy. We used the same semiempirical kinetic model used for aniline (ANI) homopolymerization to evaluate the experimental data. Th e reaction kinetics of OMA homopolymerization was similar to that of ANI, a lthough we obtained longer induction and propagation times for OMA, This wa s attributed to steric hindrance of the bulky methoxy substituent during th e coupling reaction. Furthermore, it was suggested that a lower OMA polymer ization rate could also be related to a lower concentration of nonprotonate d OMA molecules in the reaction solution due to a higher pK(a) value for OM A than for ANI, This may also explain the lower OMA end conversion (90%) co mpared with that of ANI (96%). The OMA end conversion was not influenced su bstantially by reaction conditions; it was lower than 90% only when high ac id or low oxidant (oxidant-deficient oxidant/OMA ratio) concentrations were applied. Because the oxidant took an active part in polymerization, it mar kedly influenced the polymerization rate, especially the initiation rate. T he OMA initiation and propagation rates increased with increasing oxidant a nd initial monomer concentrations and with the reaction temperature, but th ere was no uniform trend in the correlation between the homopolymerization rate and acid concentration. The activation energies of the OMA initiation and propagation were 57 and 10 kJ/mol, respectively. (C) 2001 John Wiley & Sons, Inc.