Lipase-catalyzed solution polycondensation of 1,4-butanediol and diethyl succinate: Effect of diphenyl ether concentration on enzyme stability, reuse and PBS molar mass distribution

Abstract

Polybutylene Succinate (PBS) is a biodegradable polyester which can be produced from renewable monomers (provided by fermentation routes) and has been widely spread in value-added applications, such as in the biomedical sector. Regarding the synthesis routes of PBS for biomedical applications, enzymes have been gaining prominence, due to their high selectivity, milder operating temperature, absence of toxic metals and the possibility of reuse (when immobilized). Therefore, this work seeks to investigate the enzymatic polycondensation in solution, using diethyl succinate and 1,4-butanediol as monomers, Novozym 435 as biocatalyst, and diphenyl ether as solvent. The possibility of reusing the immobilized biocatalyst was also addressed. Solvent-free reactions, or containing 5 and 50 wt% of diphenyl ether were tested, at 70, 80 and 90 °C. Average molar masses were measured by Gel Permeation Chromatography to evaluate the enzyme performance in PBS synthesis. Furthermore, the kinetic behaviour of the synthesis was monitored by the formation of the by-product. The reactions containing 5 wt% of solvent at 80 °C and 90 °C proved to be more advantageous, resulting in Mw between 2.500 and 3.350 g.mol−1, in reaction times between 60 and 90 min. In this case, small additions of diphenyl ether presented diffusional advantages when compared to solvent-free processes. In addition, reuse tests of the biocatalyst were performed at 70 °C with 5 wt% of solvent. A pronounced decrease in the enzymatic activity was observed in the N435 reuse cycles, indicating that diphenyl ether affected the stability of the immobilized biocatalyst. Therefore, additional investigation is still needed in using green solvents that keep the immobilised biocatalyst's stability, providing subsidies for future works aimed at scale-up and commercial/industry production of PBS.