Synthesis of biosourced amphiphilic phytosterols

Abstract: Phytosterols, a group of sterol compounds in plants, have interesting biological activities such as antioxidant properties, which are limited by their high melting point and poor water solubility. Current methods of chemical modifications of phytosterol to improve solubility use ethylene oxide, a harmful compound for both the environment and human health. The aim of the study was to functionalize phytosterol molecules with hyperbranched glycerol to increase the amphiphilic property of the molecule, increasing the polar moiety, thus improving the phytosterols water solubility. The amphiphilic phytosterols were prepared by ring opening multibranching polymerization of biosourced glycidol using phytosterol as the initiator. The process, which is continuous and solvent-free, is adapted to the physical properties of the reagents (melting temperature of phytosterols of 140°C and boiling temperature of glycidol of 160°C). Various parameters were studied, including reaction time, monomer addition rate, and continuous or sequential addition. The best conditions lead to functionalized phytosterols with very high hydrophilicity compared to native phytosterols, as well as surface-active properties by reducing the surface tension below 35 mN/m. MALDI-tof analysis allowed to evaluate a number-average degree of polymerization of 7, with a maximum number of glycerol branching of 18 for molecules up to MW=1700 g/mol. NMR analysis allowed to calculate a degree of branching of 0.51. Thus, this easy synthesis of amphiphilic phytosterols with biobased reactant suggests a ‘‘green” alternative to phytosterol ethoxylates. Moreover, the final amphiphilic phytosterol containing numerous hydroxyl groups allows for further functionalization which may be of interest for polymers and further applications.