N-chlorination and Orton Rearrangement of Aromatic Polyamides, Revisited

Abstract

A review. Polyamide membranes are widely used in water desalination. It is known that they suffer degrdn. due to the presence of free chlorine. This communication shows a detailed chem. reaction mechanism for the N-chlorination and Orton rearrangement of poly (m-phenylene isophthalamide), which is the linear arom. polyamide component of the commonly used B-9 Permasep membrane. The N-chlorination of this arom. polyamide causes the loss of hydrogen bonding. This triggers conformational changes in the polymer; the polymer becomes less rigid and void spaces open up, which decreases solute rejection and increases water flux. The N-chlorination reaction is reversible in alk. media. Therefore, if the polymer is suspected to have come into contact with hypochlorite anions or hypochlorous acid immediate cleaning with sodium hydroxide could reverse the N-chlorination. Conversely, the N-chlorination is acid catalyzed; hence, special care has to be taken during the cleaning stage, when HCl is used. Furthermore, N-chlorinated arom. polyamides can undergo an Orton rearrangement, which is also promoted in acidic media, resulting in the formation of ortho- or para-chloro substituted analogs of the arom. amide moiety. The chloro group causes a strong neg. inductive effect weakening the amide bond making it more susceptible to hydrolysis, which eventually produces chain scission. [on SciFinder(R)]