AbstractA spectral method for determining the imidization from maleamic acid to maleimide was described. The formation condition and mechanism of polymaleimide compared withN‐phenyl‐maleimide (I) and 4,4′‐bis(N‐maleimido)‐diphenylmethane (II) were investigated. A series ofN‐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 ofN‐phenylmaleamic acid with Ac2O was determined at a mole ratio 0.1:1 of NaOAc toN‐phenyl‐maleamic acid and a second‐order constantKof 1.38 × 10−2(mL/mol s) was obtained. The activation energies for the ring‐closure reaction ofN‐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 ofN‐substituted maleimide formation wasN‐(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 spectr