Synthesis of levulinic acid–glycerol ketal–ester oligomers and structural characterization using NMR spectroscopy

Oligomerizations of renewable resources based monomers levulinic acid, and glycerol were studied using three different catalysts, Sb2O3, p-toluenesulfonic acid, and 1-(1-propylsulfonic)-3-methylimidazolium chloride. The highest average degree of oligomerization of 9.8 was achieved by heating an equimolar mixture of levulinic acid, and glycerol first from 23 to 210 °C under a nitrogen atmosphere for 1 h, and then at 210 °C under a vacuum for 9 h, in the presence of 1.0 mol% Sb2O3 catalyst. p-Toluenesulfonic acid, and 1-(1-propylsulfonic)-3-methylimidazolium chloride catalyzed reactions gave oligomers with relatively lower molecular weights. These levulinic acid–glycerol oligomers are shown to have ketal and ester links in a linear oligomer chain by using 1H and 13C NMR spectroscopy. Three different types of terminal units: keto, glycerol-ketal, and glycerol-ester were identified with the help of model compounds, and their relative percentages were calculated using quantitative 1H and 13C NMR data.