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]