A study on the effect of para substitution on the thermal degradation of poly N-arylmaleimides

Abstract

With a view to study the effect of electron releasing and withdrawing groups present in the para position on the N-phenyl ring on the degradation mechanism of poly(N-arylmaleimides), the monomers N-phenylmaleimide (NPMI), N-p-chlorophenylmaleimide (N-p-ClPMI) and N-p-nitrophenylmaleimide (N-p-NO2PMI) were synthesized and polymerized using benzoyl peroxide as the initiator. The structural characterization of the monomers and the polymers was done using infrared (IR) and 1H NMR. Thermogravimetric study of poly(N-p-NO2PMI) showed major weight loss in the temperature range 184-316 °C whereas the poly(N-p-ClPMI) degraded in the range 400-484 °C and the unsubstituted poly(NPMI) in 264-348 °C. The energy of activation value (calculated using Dharwadkar and Kharkhanawala equation) for the degradation of poly(NPMI) (192 kJ mol-1) lies between those for poly(N-p-NO2PMI) (138 kJ mol-1) and poly(N-p-ClPMI) (217 kJ mol-1). The two mechanistic pathways through which the poly(N-arylmaleimide) undergoes degradation are (a) the random chain scission followed by inter- and intra-molecular hydrogen radical transfer leading to various poly N-aryl substituted succinimides and (b) the imide-isoimide rearrangement followed by the ejection of CO2. The effect of the para substituents on the phenyl ring on the first mechanism is minimal, whereas the substituents have marked effect on the imide-isoimide rearrangement. © 2003 Elsevier Science B.V. All rights reserved.