Alkali‐Catalyzed Synthesis of Acylglycerol Estolides Exclusively from Castor Oil

ABSTRACT

Estolides are oligomers composed of fatty acid (FA) monomers linked through secondary ester bonds located along the mid‐chain region. They occur naturally or can be produced by chemical modification of double bonds, epoxides, or hydroxyl moieties in FAs. Estolides may be synthesized from triacylglycerols (TAGs), free FAs, or FA monoalkyl esters, and their structural and physicochemical properties depend on the degree of oligomerization, chain length, and number and nature of functional groups present. Traditionally, when vegetable oils are used as feedstock, estolides are obtained by reacting TAGs containing hydroxy groups on their fatty chains with free FAs in the presence of mineral acids or enzymes. However, previous work by our research group demonstrated that estolides can also be formed during typical alkali‐catalyzed interesterification conditions in the presence of just castor oil, with the C12‐OH of the ricinoleate moiety acting as alcohol in transesterification reactions. In the present study, this process was further investigated through a series of solvent‐free reactions between castor oil and potassium methoxide used in catalytic amounts, varying reaction time (5–150 min) and temperature (80°C–120°C). Reaction conditions were correlated with the properties of the resulting estolides. ¹³ C NMR analyses revealed that diacylglycerol‐ and monoacylglycerol‐derived estolides (DAG‐ and MAG‐estolides) were the predominant products. The maximum conversion of the C12‐OH groups into ester functionalities reached 76%, and the kinetic viscosity at 40°C increased to 635 mm ² s ⁻¹ , approximately 150% higher than that of neat castor oil. Estolide formation followed apparent second‐order kinetics with respect to the C12‐OH conversion, with an estimated activation energy of 33.1 kJ mol ⁻¹ .