Cyclic oligomers in saturated polyesters

To get an in-depth understanding of the structure–property relationship in saturated polyester resins, the reactivity of various diols and diacids as well as reaction conditions were studied. Adipic acid (AA), phthalic anhydride (PA), isophthalic acid (IPA), terephthalic acid (TPA), 2-butyl-2-ethyl-1,3-propanediol (BEPD) and 2,2-dimethyl-1,3-propanediol (NPG) were addressed. It was found that in addition to linear oligomers with carboxyl and/or hydroxyl end groups cyclic oligomers were detected. Cyclic dimers of TPA and NPG are known to migrate to the coating surface causing “blooming”. The understanding of this phenomenon and the monomers capable of forming cyclic dimers are the subject of this study, where we combined molecular modeling and analytical characterization. The probability of the formation of macrocyclic structures was shown to decrease in the order: BEPD–PA>BEPD–AA≥BEPD–IPA>BEPD–TPA. The temperature dependence was found to be small.