Solid-State and Solution Stability of Poly(anhydrides) and Poly(esters)

The stability of poly (anhydrides) composed of the diacids sebacic acid (SA), suberic acid (SuA), adipic acid (AA), bis(p-carboxyphenoxy)methane (CPM), l,3-bis(p-carboxyphenoxy)propane (CPP), 1,6-bis(p-carboxyphenoxy)hexane (CPH), and phenylenedipropionic acid (PDP) in solid state and in organic solutions is reported. Aromatic poly (anhydrides) (poly (CPH) and poly(CPM)) maintained their original molecular weight in both solid state and solution while aliphatic poly(anhydrides) (poly(SA) and poly(PDP)) decreased in molecular weight over time. The decrease in molecular weight shows first-order kinetics with activation energies of 7.5 kcal.mol^-1K^-1. The decrease in molecular weight was explained by an internal anhydride interchange mechanism as revealed from elemental and spectral analysis (IR and ¹³C and ¹H NMR). The depolymerization in solution can be catalyzed by metals. Among several metals tested, copper and zinc were the most effective. Studies on the stability of aliphatic poly (esters) in chloroform showed that poly (orthoesters) depolymerized over time while poly(a-esters) and poly(ethylenesuccinate) remained intact. The solid-state and organic solution stability of the polymers do not correlate with their hydrolytic stability.