# Novel Aromatic Estolide Esters from Biobased Resources by a Green Synthetic Approach
## 3 Results and Discussion
### Structural Characterization of the Estolide Ester Products
#### 3.3.2 MALDI-TOF MS Analysis  
The formation of reaction products was monitored by MALDI-TOF MS spectrometry. Examples of MALDI-TOF MS spectra are represented in Figures 3-6, for the products ob  
Figure 2: Overlaid FT-IR spectra of caffeic acid (red), ricinoleic acid (green) and the oligoestolide ester product (blue). tained in toluene, using commercial _Candida antarctica_ B immobilized lipase (Novozyme 435). In all spectra, the identified products were linear copolymers. Thus, the series of peaks attributed to the potassium adducts of the oligomer series ([M + K]+) indicate the formation of linear copolymers containing RCA and aromatic moieties. Being new products, the spectra of all three oligoricinoleate esters will be presented and discussed. A table with the selected molecular weights of the reaction products identified in the MALDI-TOF MS spectrum and the theoretical values of the masses are included in Table S1. Figure 4: MALDI-TOF spectrum of the oligoesters obtained from caffeic acid and RCA (2:1 molar ratio) synthesized at 80 \({}^{\circ}\)C and 72 h, using the lipase from _Candida antarctica_ as a biocatalyst, in toluene. Inset is the 550–2000 \(m/z\) region. Figure 3: MALDI-TOF spectrum of the RCA estolides synthesized at 80 \({}^{\circ}\)C and 72 h, using the lipase from _Candida antarctica_ as a biocatalyst, in toluene. Inset is the 600–2400 \(m/z\) region. As a control reaction, ricinoleic acid was used as monomer in the absence of aromatic moieties, in similar reaction conditions. The resulting oligoesters exhibited a degree of polymerization ranging from 2 to 8 units, demonstrating the successful synthesis of linear copolymers. The correspondent MALDI TOF MS spectrum is presented in Figure 3. As an example, the peaks with \(m/z\) 937.2, 1217.4, 1498.4 correspond to the 2K\({}^{+}\) adducts of linear estolides with 3 \(\div\) 5 RCA units. The MALDI-TOF MS spectra of the reaction products obtained from caffeic acid and RCA is presented in Figure 4. The peaks with \(m/z\) 818.4, 1099.4, 1380.7 correspond to  
Figure 5: MALDI-TOF spectrum of the oligoesters obtained from sinapic acid and RCA (2:1 molar ratio) synthesized at 80 \({}^{\circ}\)C and 72 h, using the lipase from _Candida antarctica_ as a biocatalyst, in toluene. Inset is the 550–1800 \(m/z\) region. Figure 6: MALDI-TOF spectrum of the oligoesters obtained from cinnamyl alcohol and RCA (2:1 molar ratio) synthesized at 80 \({}^{\circ}\)C and 72 h, using the lipase from _Candida antarctica_ as a biocatalyst, in toluene. Inset is the 600–1600 \(m/z\) region. the 2K\({}^{+}\) adducts of linear oligoesters with 1 \(\div\) 3 RCA units and one caffe acid unit. The highest polymerization degree for these products was 6. The MALDI-TOF MS spectra of the reaction products obtained from sinapic acid and RCA are presented in Figure 5. The peaks with \(m/z\) 583.0, 862.7, 1144.2 correspond to the 2K\({}^{+}\) adducts of linear oligoesters with 1 \(\div\) 3 RCA units and one sinapic acid unit. The maximum polymerization degree for these products was 3. The spectrum also shows visible adducts with 1K\({}^{+}\). The MALDI-TOF MS spectra of the reaction products obtained from cinnamyl alcohol and RCA is shown in Figure 6. The peaks with \(m/z\) 735.0, 1015.4, 1296.6 correspond to the 2K\({}^{+}\) adducts of linear co-oligoesters with 1 \(\div\) 3 RCA units and one cinnamyl alcohol unit. The maximum polymerization degree for these products was 4. Interestingly, the formation of estolides was also observed in this case. The medium molecular weights and dispersity values are presented in Table 1. Compared to the estolides, the values obtained for the aromatic oligoesters were lower. The M\({}_{\rm n}\) of the oligoester with caffeic acid was about 9.2% lower compared to the simple estolides, while the M\({}_{\rm w}\) was about 5.9% lower and the dispersity (D\({}_{\rm M}\)) was slightly higher by 3.5%.