Quantum chemical study of cyclization mechanism in polyamic acids

Abstract

The mechanism of cyclization has been studied by the MINDO quantum-chemical method in neutral and ionic forms of model compounds: maleamic and phthalamic acids as well as para-substituted N-phenylmaleamic acids. The activation energy of cyclization is shown to be higher in the neutral than in the ionic forms of all amic acids investigated. Cyclization of amic acids deprotonated in the amide group leads primarily to imides, while those protonated in the carboxyl group yield mostly isoimides. Formation of a mixed anhydride, which precedes the cyclization in the case of amic acids in their ionic form, facilitates not only the cyclization itself, but also the decomposition of cyclic tetrahedral intermediates to the final cyclization product. The results show that thermal imidization involves the neutral while chemical imidization the ionic form of the amic acid.