# Novel Aromatic Estolide Esters from Biobased Resources by a Green Synthetic Approach
## 3 Results and Discussion
### Structural Characterization of the Estolide Ester Products
#### 3.3.3 Characterization of the Estolide Ester Products by NMR Spectroscopy  
\({}^{1}\)H NMR spectroscopy has been utilized for the structural analysis of the reaction products of ricinoleic acid with the aromatic acids and alcohol, respectively, to demonstrate the formation of the targeted aromatic estolide esters. As new compounds, the most significant signals that enable the structural identification will be discussed below for each of the three product types.  
In the \({}^{1}\)H NMR spectrum of oligo (ricinoleyl) caffeine shown in Figure 7, the most relevant signals were attributed as follows: 6 (ppm): 4.30-4.00 ppm (m) to the methylene protons (COO-CH\({}_{2}\)) adjacent to the ester linkage; aromatic protons of the caffeic acid moiety are seen at 6 (ppm): 7.00-6.80 (m) and 6 (ppm): 6.70-6.50 (m); olefinic protons in the caffeic acid moiety are at 8 (ppm): 6.50-6.20 (m); olefinic protons of the ricinoleic acid moiety are found at 8 (ppm): 5.50-5.30 (m). In addition, the hydroxymethyl protons (CH\({}_{2}\)OH) in the ricinoleic acid moiety correspond to the signals at 8 (ppm): 3.60-3.40 (m), while the methine proton (CH-OH) resonates at 5 (ppm): 3.40-3.20 (m). The signals at 8 (ppm): 2.60-2.20 (m) are assigned to the allylic protons (CH\({}_{2}\)) in the ricinoleic acid chain.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Endcapping Compound** & **M\({}_{\rm n}\) [g/mole]** & **M\({}_{\rm w}\) [g/mole]** & **D\({}_{\rm M}\)** \\ \hline - & 1104 & 1258 & 1.14 \\ Caffeic acid & 1003 & 1184 & 1.18 \\ Sinapic acid & 821 & 898 & 1.09 \\ Cinnamyl alcohol & 965 & 1035 & 1.07 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The average molecular weights and dispersity values calculated based on the MALDI-TOF MS analysis of the oligomeric esters.  
The assignment of the most relevant NMR signals for oligo (ricinoleyl)-cinnamate depicted in Figure S4 is \(\delta\) (ppm): 7.46-7.22 ppm (m) and corresponds to the five aromatic protons on the benzene ring from cinnamyl alcohol; \(\delta\) (ppm): 6.59 ppm (d) is attributed to the olefinic proton adjacent to the phenyl group; \(\delta\) (ppm): 6.39-6.41 ppm (d) represents the olefinic proton near the ester linkage; \(\delta\) (ppm): 5.38-5.30 ppm covers the olefinic protons in the ricinoleic chain; \(\delta\) (ppm): 4.78-4.69 ppm (t) relates to the methylene protons next to the ester linkage; and \(\delta\) (ppm): 2.78-2.69 ppm is the allylic protons adjacent to the double bond in the ricinoleic chain.  
The \({}^{1}\)H NMR spectrum of the reaction product of ricinoleic acid and sinaptic acid, Figure S5, and the most relevant signals are \(\delta\) (ppm): 6.90-6.80, attributed to the aromatic protons from sinaptic acid; \(\delta\) (ppm): 6.60-6.50 ppm, attributed to the olefinic protons in the sinaptic acid moiety; \(\delta\) (ppm): 5.50-5.30, attributed to the olefinic protons in the ricinoleic acid moiety (shifted compared to the previous spectra); \(\delta\) (ppm): 4.20-4.00, attributed to the methylene protons (CH\({}_{2}\)) adjacent to the ester linkage; \(\delta\) (ppm): 3.60-3.40 ppm, attributed to the hydroxymethyl protons (CH\({}_{2}\)OH) from ricinoleic acid; \(\delta\) (ppm): 3.40-3.20, attributed to the methine proton (CH) adjacent to the hydroxyl group in the ricinoleic acid moiety; \(\delta\) (ppm): and 2.50-2.20, attributed to the allylic protons adjacent to the double bond in the ricinoleic chain.