Isomeric polyimides

This review deals with polyimides based on isomeric dianhydrides and diamines, and with chiral polyimides. First, however, a summary is presented of recent work on the synthesis of isomeric dianhydrides, the reaction of mellophanic dianhydride with diamines, and the tendency toward cyclization in reactions of some dianhydrides and diamines. Then turning to polymers, the discussion covers solubility, thermal and dielectric properties, permeability and permselectivity for gas separation, and rheology of isomeric polyimides. Several useful general rules have been found: i.e. the glass transition temperature of polyimides based on isomeric dianhydrides with a given diamine decreases in the order 3,3′->3,4′->4,4′-dianhydride if the polymers are of comparable molecular weight, whereas the thermal stability and the Tβ/Tg ratio (in absolute temperatures) increase in the order of 3,3′- <3,4′- <4,4′-dianhydride. Polyimides from 3,3′- or 3,4′-dianhydride have higher solubility than those from 4,4′-dianhydride. Polyimides from 3,4′-dianhydrides exhibit much lower melt viscosity than those from the other isomeric anhydrides. The dielectric constants of polyimides derived from m,m′-diamines are lower than those from p,p′-diamines. Polyimides based on 3,3′- or 3,4′-dianhydrides have higher permeability and slightly lower permselectivity than polyimides based on 4,4′-dianhydrides. The same tendency can be seen for the polyimides derived from p,p′-diamines and those based on the other isomeric diamines. Several optically active polyimides have been synthesized. No differences in Tg, thermal stability or solubility among chiral polyimides with R, S and racemic units in the backbone have been found.