Influence of Brønsted acid ionic liquid structure on hydroxyacid polyesterification

Abstract

Brønsted acid ionic liquids (BAILs) based on the 4-(3′-butyl-1′-imidazolio)-1-butanesulfonic acid cation were found to be very efficient polyesterification solvents and catalysts. Only 5–30 min at 90–110 °C was required to obtain high molar mass poly(12-hydroxydodecanoic acid) (M_w up to 40000 g mol⁻¹). The polyesterification was faster in BAILs with the bis(trifluoromethylsulfonyl)imidide anion (Tf₂N), but small amounts of ethers and double bonds arising from side reactions were detected in the final polymer. On the other hand, no side reactions took place in the BAIL with the hydrogen sulfate anion, except for the formation of a sulfonate ester intermediate that can further react with carboxylic acid groups to yield the expected ester. This intermediate, not observed in Tf₂N-based BAILs, might be involved in the protection of hydroxy end groups from etherification side reactions in HSO₄⁻-based BAILs. To explain the different behaviors of these BAILs, and since the acidity of H₂SO₄ is much higher than that of Tf₂NH, it is suggested that the structure of these BAILs could be different: alkylsulfonic acid-substituted imidazolium for the former, while the latter could be just a mixture of imidazolium–sulfonate zwitterion and Tf₂NH. The influence of reaction temperature, water elimination method and BAIL concentration on polyesterification are also discussed.