4.5.2.1 PREPARATION AND CHARACTERIZATION OF TPG

Vegetable oils such as soybean oil and tung oil (TO) and bio-based phenols such as cardanol (CD) and pyrogallol (PG) are promising raw materials for the preparation of
flexible bio-based phenolic epoxy hardeners. TO is a triglyceride extracted from the seeds of the tung tree (Aleurites fordii), in which approximately 80% of the fatty acid chains is a-eleostearic acid, that is, 9-cis, n,13-trawsoctadecatrienoic acid.6869 Therefore, TO with the conjugated triene moiety shows a characteristic reactivity which is not seen in the convention soybean oil and linseed oil, etc. From the past studies, it was found that the reaction of soybean oil and phenol in the presence of a super acid such as trifluoromethanesulfonic acid or tetrafluoroboric acid pro­duce a complex mixture of phenolated soybean oils oligomerized by Diels-Alder reaction.7071 In contrast, the reaction of TO and phenol smoothly proceed in a mild acidic condition without the formation of oligomerized materials to produce a de­sired TO-phenol resin.7275 PG is obtained by decarboxylation of gallic acid which is a basic component of hydrolysable tannin. In the past studies, PG-formaldehyde resin76 and TO-PG resin77 (TPG) have been successfully synthesized and applied for a thermosetting wood adhesive and a positive photoresist developed by alkaline solutions, respectively. We carried out the preparation and structural analysis by 1H NMR spectroscopy of TPG and used TPG as an epoxy-hardener.25 The reaction of TO and PG in the presence of />-toluenesulfonic acid in dioxane at 80 °C for 3 h gave TPG as a brown viscose liquid in 37% yield (Fig. 4.28). The fact that a considerable amount of TPG is lost during the repeated washing with hot water for the removal of unreacted PG is a reason for the low yield. So, there is a possibility that the yield is improved by the optimization of purification method. As PG has a high reactiv­ity at both the 4- and 6-positions to an electrophile, it is supposed that crosslinking reaction should occur in the reaction with a multifunctional reagent such as TO. Actually, the reaction at a higher temperature than 80 °C or the use of other acid catalysts such as hydrochloric acid and borontrifluoride diethyl etherate resulted in a formation of gelatinous materials. The obtained TPG was soluble to ethanol, acetone, ethyl acetate, tetrahydrofuran, diethyl ether, ^A^-dimethylformamide and dimethylsulfoxide, and insoluble to water, chloroform and hexane.

Figure 4.29 shows FT-IR spectra of TO, TPG and PG. The band at 3375 cm1 due to O-H stretching vibration and that at 1623 cm-1 due to benzene ring frame­work stretching vibration in addition to the band at 1714 cm-1 due to C=O stretch­ing vibration and those at 2950-2840 cm-1 due to sp3C-H stretching vibration were observed for the spectrum of TPG, indicating that pyrogallol moiety and tung oil moiety certainly bonded. Also, the fact that the bands at 993 cm-1 and 732 cm-1 due to =C-H out-of plane bending vibrations of trans and c/s-olefinic moieties, respec­tively observed for TO considerably diminished for TPG, suggesting that the addi­tion reaction of PG to the olefinic moieties of TO certainly proceeded.

TPG

O-H

stretching

1,2,3-trisubstituted

benzene

O-H

stretch і n$

benzene ring Framework ‘ stretching 1623 cm"

Figure 4.30 shows the 1H-NMR spectra of TO and TPG measured in d6-acetone. The 1H signal of methine proton of glyceride unit (Ha and Ha,) in TPG and TO was observed at 5.29 ppm (s) and 5.27 ppm (s), respectively. The integral values of other proton signals were evaluated relative to those of Ha and Ha, signal (1H). Because we could not specify the phenolic hydroxy groups of PG unit in TPG, the H-D exchange reaction was performed by the addition of D2O in a NMR tube. As a result, the 1H signals from 7.92 to 6.71 ppm (6.8H) in d6-acetone disappeared in the spectrum of TPG in d6-acetone/D2O, indicating 2.3 pyrogallol units are added to a TO triglyc­eride moiety. This number is in agreement with the integral values of 1H signals at 6.45 ppm (d, 2.3H, Hf, J = 8.3 Hz) and 6.34 ppm (d, 2.2 H, Hg, J = 8.3 Hz) in the pyrogallol ring of TPG. The fact that coupling constant of two protons (Hf and Hg)

of the pyrogallol ring is 8.3 Hz indicates that electrophilic substitution reaction of the TO-derived carbocation occurred at 4-position of pyrogallol (1,2,3-trihydroxy — benzene), because the product obtained by the reaction at 5-position should have the coupling constant at around 3 Hz. The PG-substituted methine proton (H) is also observed at 3.65 ppm (m, 2.3H). The number of olefins of TO per triglyceride is estimated to be 7.6 from the integral value of the olefinic 1H signals at 6.45-5.41 ppm relative to that of Ha,. Similarly, the number of olefins of TPG is estimated to be 4.3 from the integral value of the olefinic 1H signals at 6.07-5.35 ppm relative to that of H. From their values, the number of diminished olefins of TPG relative to TO is estimated to be 3.3, which is a little higher than the degree of addition of PG (2.3). This discrepancy may be attributed to the possibility that some components of TO with lower olefinic number are eliminated by the purification. Four structural formulae (R) of TPG in Fig. 4.28 are capable as the structures of the pyrogallol — substituted hexadiene moiety of TPG, considering the stability of the carbocation of reaction intermediate, if the horizontally flipped structures of R are omitted. In addition, there is a possibility that the original cis-trans configuration of triene part of TO is transformed to other configurations by the migration of п-bond in the car­bocation intermediates. Among the olefinic proton signals of TPG, the signal at a lower magnetic field (6.07 ppm) is assigned to inner protons (-С^СЯ-СЯ^^) of conjugated diene moiety, and that at a higher magnetic field (5.35 ppm) is related to the protons of isolated olefin moiety. However, we could not assign the olefinic proton signals of TPG more precisely because many structural and configurational isomers are contained.