Investigation of the kinetics of chemical imidization

The cyclization of poly(4,4′-oxydiphenylene) pyromellitamido acid in the solid phase under the effect of acetic anhydride in the presence of pyridine has been investigated. From the IR spectra of the products of cyclization, the changes with time in the optical density of the absorption bands characteristic of o-carboxyamide groups and of imide and iso-imide rings have been followed. The process of chemical imidization is described by a scheme that includes the closing of o-carboxyamide groups to form imide and iso-imide rings as two parallel first order reactions as well as the isomerization of iso-imide rings to form imide rings as a reversible first order reaction. A probable mechanism for the chemical imidization of the polyamic acid is proposed on the basis of the kinetic data.