Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2007
Publication Date: 3/1/2008
Citation: Zerkowski, J.A., Nunez, A., Solaiman, D. 2008. Structured Estolides: Control of Length and Sequence. Journal of the American Oil Chemists' Society. 83:277-284. Interpretive Summary: Estolides are short polymers that can be constructed from fatty acids. They are biodegradable and have been proposed as functional fluids such as lubricants. It has been known for several years that they can be prepared readily from vegetable oils and a strong acid. The drawback to this method of preparation, however, is that the product estolides come in a wide variety of sizes at the molecular level; furthermore, the strong acid is incompatible with many molecular units that could impart useful functions to the estolides. This paper describes ways to overcome these problems. We have been able to control the size of the estolides by using protecting groups; in this way, lengthening of the chain occurs at precisely one site. The size of the polymer can be built up with precise control by removing the protecting groups and repeating the reaction. More importantly, the bond-forming reaction we use is mild and compatible with a wide array of chemical groups, so that we have been able to incorporate new units into estolides that have not been possible previously. In one example, one of these added units can itself be used to create polymers; in other words, our estolides can be converted into polymers of polymers. Alternatively, polar or charged groups can be incorporated into the chain, which will alter the estolides’ water solubility. We expect that these new varieties of estolides will contribute to fundamental studies on their structure/function properties, as well as allow their use in new areas such as macromolecular surfactants, thin films, and biocompatible coatings.
Technical Abstract: Using ester-forming reactions such as carbodiimide coupling and a modified Yamaguchi symmetrical anhydride method, a variety of estolides from 17-hydroxy oleic and 17-hydroxy stearic acid have been prepared. These hydroxy fatty acids are produced in good yields from hydrolysis of sophorolipids, which are in turn derived from fermentation of fats and oils. Since the estolides are formed one unit, or ester bond, at a time, their length and sequence can be precisely controlled. The key to this control is the use of protecting groups at either the carboxylic or hydroxy end of the starting hydroxy fatty acids. This methodology opens the way to functionalized estolides, and several variants were prepared: hybrid estolides, containing non-fatty acid moieties such as amino acids; polymerizable estolides, containing a norbornene unit; and non-linear estolides that extend from a branched core such as glycerol or pentaerythritol.