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

摘要

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-NO_2PMI) 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-NO_2PMI) showed major weight loss in the temperature range 184-316℃ whereas the poly(N-p-ClPMI) degraded in the range 400-484℃ and the unsubstituted poly(NPMI) in 264-348℃. 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-NO_2PMI) (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 CO_2. 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.