Scheme 1. Synthetic way to a bio-based dimethacrylate starting by esterification of oleic acid with a diol (e. g. ethylene glycol) catalyzed by p -toluene sulfonic acid (1. step), epoxidation of the double bonds of the diester with hydrogen peroxide in the presence of formic acid (2. step), and epoxy ring opening reaction with methacrylic acid catalyzed by 1,4-diazabicyclo[2.2.2]octane (DABCO, 3. step) resulting in three isomers (i1, i2, and i3) of the bio-based dimethacrylate. Photoinitiated radical polymerization of the dimethacrylate alone or in the presence of bio-based methacrylates comprising aromatic structures resulted in crosslinked polymer films showing water-repellent properties as detected by contact angle measurements.[12]
SOURCES FOR THE STARTING MATERIAL
Plant oil and fat represent main resources for triglycerides that are hydrolyzed into fatty acids and glycerol by chemical or enzymatic processes.[4,15] Though industrial use of plant oil and fat e. g. for biodiesel production from triglycerides as alternative to petrochemical diesel or for monomer synthesis compete with food production, only cooking oil available as waste from industry, restaurants or household should be utilized for fatty acid manufacturing. Furthermore, the glycerol formed as by-product in the biodiesel production may be useful for esterification of oleic acid instead of using ethylene glycol as shown in Scheme 1.[12] Ethylene glycol is obtained in two different ways; that is a petrochemical and a bio-based way. The latter is described as hydrolytic hydrogenolysis reaction of cellulose.[16,17] Lignin available in bark, wood and plants provides compounds comprising aromatic moieties derived from natural resources.[18,19]
STRUCTURAL VARIABILITY OF THE CROSSLINKABLE BIO-BASED MONOMER
Various structural modifications of the bio-based dimethacrylate may be possible by choosing different diol compounds for esterification with oleic acid resulting in variation of the bridge between the oleate structures on the one hand and by selection of different reagents for ring opening reaction of the epoxidized di-oleates that give a structural variation of the substituents R1, R2, R3, and R4 on the other hand (Scheme 1). The latter may contain either an OH-group or aromatic structure and two (meth)acrylate groups. As a result, the structure depicted in Scheme 2 represents only some possible structural modifications of the dimethacrylate derived from oleic acid and ethylene glycol, which was firstly described in 2021.[ 12]