Aiming for renewable polymers with potential recyclability and biodegradability, polyesters are very promising due to an increasing number of available biobased monomers and ester bonds that can be hydrolyzed under specific conditions. Citric acid, for example, is a biobased, nontoxic, cheap, and easily available resource. Its multifunctionality enables the synthesis of polyesters with free carboxy groups, thus providing possibilities for further functionalization and cross-linking. Citric acid and dimethylolpropionic acid were used to synthesize water-soluble polyesters via melt polycondensation at 150 °C without the need for potentially hazardous catalysts. The resulting polyesters displayed a significant amount of free carboxylic acid groups (8 mmol gpolyester–1), and reasonable number-average molar masses up to 5200 g mol–1. Via postsynthetical (partial) neutralization procedures using KOH, charged moieties could be successfully incorporated to further increase hydrophilicity. The degree of neutralization proved to be well controllable. This enables tunability of the final properties, as remaining free carboxy groups can be used for further modifications. Alternative to carboxylate moieties, the introduction of sulfonate groups promotes hydrophilicity. For this purpose, unsaturated polyesters were synthesized that contained varying amounts of maleic anhydride as a third monomer. Postsynthetical sulfonation was performed via the Michael addition of sodium sulfite, introducing a significant number of sulfonate groups. Neutralization and sulfonation were performed in aqueous solution, leading to slight decreases in molar mass due to hydrolysis. The extent of the reduction was successfully reduced by optimizing both procedures. The synthesized water-soluble polyesters carry a substantial number of functional groups. They have high potential as precondensates for cross-linked, water-absorbing materials to be used in agriculture and biomedicine, for which biodegradability is a crucial property.
Kruglhuber et al. (Thu,) studied this question.