Synthesis of Bio-Based Photo-Cross-Linkable Polyesters Based on Caffeic Acid through Selective Lipase-Catalyzed Polymerization

Abstract

Caffeic acid was esterified and then successfully converted with ethylene carbonate into a new potentially fully bio-based aromatic diol containing a pendant α,β-unsaturated ester. The lipase B from Candida antarctica allowed the synthesis of polyesters from this caffeic-based diol and diethyl adipate. 1,6-Hexanediol was also used as a second diol monomer to study the impact of the aromatic diol proportion on the enzymatic activity and final material properties. For caffeic diol (CD) ratios of 10% and above, linear polyesters were obtained with Mw up to 28 000 g·mol–1. Increasing the ratio of CD tended to reduce the polyester final molar masses, probably due to inhibition of the enzyme. However, increasing the proportion of CD in the polyester improved its thermal properties, with Tg reaching up to 12 °C. Finally, the pendant double bonds of the polyester were shown to undergo [2 + 2] cycloaddition under light irradiation, enabling its UVA-activated cross-linking. This cross-linking was also shown to be partially reversible, thanks to the UVC-induced splitting of the cyclobutane rings in the network.