# Novel Aromatic Estolide Esters from Biobased Resources by a Green Synthetic Approach
## 1 Introduction  
Oils and fats of vegetal and animal origin are considered essential renewable feedbacks, particularly for the chemical industry, showing a remarkable increase in the past decade. The annual worldwide production of the major vegetable oils increased by 47.8% between 2009 and 2019 [1]. Applications that use vegetable oils as raw materials include surfactants, cosmetics and lubricants [2; 3]. Efficient synthetic transformations of triglycerides and fatty acids represent an essential part of oleochemistry, a domain with increasing industrial importance which allows the sustainable majorization of easily accessible and cheap resources. A multitude of chemical modifications of carboxylic acids and their esters are possible, including transformations of the carboxylic group leading to alcohols, amines, amides, nitriles, or oxidations of the unsaturated acids and esters, yielding high value products by epoxidation, allyic oxidation, or oxidative cleavage [4]. Due to their relatively low costs, vegetable oils as well as saturated and unsaturated fatty acids derived from them are considered among the most important renewable raw materials for obtaining functional biopolymers and polymeric materials [5; 6]. Hydroxy-fatty acids (HFAs) and their derivatives represent a class of compounds of great interest. In addition to being essential molecules in biological systems, they are used in a wide variety of industrial processes and, more recently, in the field of materials chemistry. Structurally, hydroxy fatty acids may be saturated or unsaturated and may contain one or more hydroxyl groups and a long, branched chain with a terminal carboxyl group. They occur in nature as components of the cerebrosides of triacylglycerols, waves,and other lipids in animals, plants, and microorganisms [7]. Unlike non-hydroxylated fatty acids, HFAs have reactivity, miscibility in organic solvents, stability and high viscosity due to the presence of the hydroxyl group in the molecule [8]. Thus, they have utilizations in the chemical, food and cosmetic industries as raw materials for the manufacture of lubricants, emulsifiers and stabilizers [9]. HFAs also have pharmaceutical potential, including antibacterial, anti-fungal and anti-diabetic activity [10; 11]. Polymers synthesized from HFAs have several advantages over petroleum-derived polymers, such as higher heat resistance, resistance to chemicals, high flexibility, high biocompatibility, and virtually no toxicity [12]. The synthesis of estolides, oligomers of fatty acids, belongs to this sustained effort of oleochemistry development and can be achieved by (i) the addition of a fatty acid to an unsaturated one; (ii) epoxidation and epoxide ring opening; and (iii) polymerization of a hydroxy fatty acid, caused by the addition of water to the C = C double bond through enzymatic catalysis using fatty acid hydratases [13]. The interest for estolides of hydroxy fatty acids has continuously grown in the past decade, mainly due to their applications as biolubricants, but their utilizations as food additives, cosmetic ingredients or sources of polyols for polyurethanes cannot be neglected [14]. Estolide esters are synthesized mostly by chemical catalysis from ricinoleic acid (RCA), a natural hydroxy acid present in castor oil and various carboxylic acids like capric, lauric, myrstic or stearic acid [15], although oleic acid can also be the starting material [16]. At the same time, the harsh reaction conditions can result in colored products and undesired side reactions, requiring purification. To avoid these drawbacks, the enzymatic synthesis of estolides and estolide esters has been successfully investigated by several research groups [17; 18; 19; 20]. Although estolides are valuable products by themselves, additional capping of the free hydroxy and carboxyl groups by reaction with an aliphatic acid or alcohol, respectively, can add new functionalities and improved properties, as was also revealed by a recent white paper of the company Seqens [21].