Thermal oxidation of model molecules to reveal vegetable oil
polymerization studied by NMR spectroscopy and self-diffusion
Abstract
Oxidative polymerization of plant oils and lipids is poorly understood
yet widely encountered. Oil oxidation is accelerated at high
temperatures, typically above 110°C, where tri-acylglycerides are
converted into toxic compounds and viscous deleterious polymers.
Polymerization of mono-unsaturated oil (210°C, 3h, open to air) was
investigated by comparing four similar sized molecules with different
functional groups: oleic acid, methyl oleate, trans-7-tetradecene and
stearic acid. Non-volatile products identified by NMR spectroscopy are
minor ketones for saturated fatty acid (stearic acid), epoxides for acyl
chains without acid groups (methyl oleate, tetradecane) and copious
oligomerization, through ester cross-links, for acyl chains with acid
and olefinic groups (oleic acid). Long range C-H coupling clearly shows
ester (not ether) cross-links, contradicting long held beliefs. Chain
fragmentation also occurs as revealed by species with methylene groups
bonded to oxygen, -CH2-O-C(=O)-R. Large size (slow diffusion) of the
first oligomer (trimer) formed by thermal oxidation of oleic acid,
(representing hydrolyzed vegetable oil) was evidenced by DOSY (diffusion
ordered spectroscopy). Since the first oligomers formed still have
reactive groups (olefin, carboxylic acid), poly-ester formation is
inevitable at longer oxidation times. Model oil reactions monitored by
NMR spectroscopy are important for resolving the complex chemistry of
vegetable oil polymerization.