Studies on the imidization of N‐substituted polymaleamic acids

Abstract

AbstractA spectral method for determining the imidization from maleamic acid to maleimide was described. The formation condition and mechanism of polymaleimide compared with N‐phenyl‐maleimide (I) and 4,4′‐bis(N‐maleimido)‐diphenylmethane (II) were investigated. A series of N‐substituted maleimides were synthesized by the reaction of their corresponding maleamic acids with acetic anhydride and sodium acetate as a catalyst. The rate of dehydration reaction of N‐phenylmaleamic acid with Ac2O was determined at a mole ratio 0.1:1 of NaOAc to N‐phenyl‐maleamic acid and a second‐order constant K of 1.38 × 10−2 (mL/mol s) was obtained. The activation energies for the ring‐closure reaction of N‐phenylmaleamic acid and polyphenylene‐methylenemaleamic acid were 1.26 × 104 (cal/g mol) and 1.32 × 104 (cal/g mol), respectively. The decreasing order for the rate of N‐substituted maleimide formation was N‐(p‐methylphenyl)maleimide > I ≒ II > N‐(p‐chlorophenyl)maleimide. The result was attributed to the para‐substituent effect. These imidization reaction were observed qualitatively by IR spectra and analyzed quantitatively by 1H‐NMR spectra.