Synthesis of sugar based epoxy monomers

In this chapter, the syntheses of α-D-glucopyranoside- and glucofuranoside-based epoxy monomers derived from D-glucose, an inexpensive, easily available, renewable starting material, not yet been applied as epoxy monomer precursor, are presented. Our aim was to prepare bioepoxy monomers with high functionality, whose application results in high glass temperature epoxy resins. The detailed recipes of the synthetic procedures are disclosed in [175]. Curing properties, glass transition temperature and thermal stability of the synthesized monomers are compared in order to choose the best performing ones for epoxy resin and composite preparation.

44 4.1.1.1. Synthesis of glucopyranoside-based bifunctional epoxy monomer (GPBE)

Methyl 4,6-O-benzylidene-α-D-glucopyranoside (1) was prepared by condensing methyl-α-D -glucoside (commercially available product, obtained by the condensation of ^D-glucose with methanol in the presence of cation-exchange resin as catalyst[176]) with benzaldehyde using zinc chloride as catalyst. After a reaction of 4 h at room temperature, the intermediate 1 was obtained in a yield of nearly 72% by crystallization[177]. Treatment of compound 1 with an excess of allyl bromide and solid potassium hydroxide in refluxing toluene gave diallyl ether 2, after crystallization in a yield of 90%. [178]. Diallyl ether 2 was converted by treatment with hydrogen peroxide into diglycidyl ether derivative 3 in methanol in the presence of K2CO3 and benzonitrile by the method of Holmberg[179]. The reaction temperature was kept at room temperature by external cooling. After chromatography the yield of the solid crystalline product 3 was 40% (Figure 4.1.1).

Figure 4.1.1 Synthesis of glucopyranoside-based bifunctional epoxy monomer (GPBE)

4.1.1.2. Synthesis of glucopyranoside-based trifunctional epoxy monomer (GPTE)

The synthesis of compound 7 having three glycidyl ether groups was carried out using two methods (Figure 4.1.2). Preparation of allyl-α-D-glucopyranoside (4) was performed by the reaction of D-glucose and allyl alcohol in the presence of boron trifluoride diethyl etherate (BF3.

Et2O) as catalyst in 26% yield (5 h, reflux, column chromatography)[180]. Selective protection of the 4- and 6-hydroxyl groups of the allyl-α-D-glucopyranoside with benzaldehyde dimethylacetal using p-toluenesulfonic acid (pTsOH) as catalyst was accomplished in DMF resulting in compound 5 in good yield (76%). The one pot method for preparation of compound 5 proved to be simpler [181]. D-glucose was refluxed in allyl alcohol in the presence of CF3SO3H for 48 h. After the removal of the alcohol and the acid, the residue was reacted in DMF with benzaldehyde dimethylacetal using pTsOH as catalyst (40 °C, 5 h). Mixture of α and β isomers of allyl-4,6-O-benzylidene-α- D -glucopyranoside (5) was obtained with 45% yield. The reaction of compound 5 with allyl bromide in toluene in the presence of potassium hydroxide gave the corresponding 1,2,3-tri-O-allyl derivative 6 [182]. Epoxidation of 6 with m-chloroperbenzoic acid in toluene resulted in

(2’,3’-45 epoxypropyl)-2,3-di-O-(2’,3’-epoxypropyl)-4,6-O-benzylidene-α-^D-glucopyranoside (7) after chromatography with 72% yield of the crystalline product.

Figure 4.1.2 Synthesis of glucopyranoside-based trifunctional epoxy component (GPTE)

4.1.1.3. Synthesis of glucopyranoside-based tetrafunctional epoxy monomer (GPQE)

The preparation of the tetraallyl-derivative (8) was carried out by the reaction of methyl-α-D -glucoside and allyl bromide in 1,4-dioxane in the presence of potassium hydroxide. After chromatography the yield of product 8 was 40%. The tetraepoxy-glucopyranoside-derivative (9) was obtained by the oxidation of compound 8 with m-chloroperbenzoic acid in toluene after stirring at room temperature for 24 h. The yield of the crystalline product 9 was 50% (Figure 4.1.3) [183].

Figure 4.1.3 Synthesis of glucopyranoside-based tetrafunctional epoxy component (GPQE) 4.1.1.4. Synthesis of glucofuranoside-based trifunctional epoxy monomer (GFTE)

D-Glucose was also the starting material of the isopropylidene-α-D-glucofuranoside-based epoxy monomer. The key compound was the 1,2-di-O-isopropylidene-α-D-glucofuranoside (11) (Figure 4.1.4), which could be obtained by two methods. The reaction of D-glucose with acetone (reagent and solvent) in the presence of iodine (as catalyst) lead to 1,2:5,6-di-O-isopropylidene-α-D

-46 glucofuranoside (10) in good yield (59%) after purification by crystallization [184]. Selective removal of the 5,6-O-isopropylidene group of the intermediate was carried out with diluted sulfuric acid in methanol (24 h, room temperature, yield after crystallization was 65%)[184]. The one pot method for the preparation of compound 11 proved to be more effective[185]. First, ^D -glucose was treated with acetone in the presence of sulfuric acid, then after neutralization and evaporation of the reaction mixture, the crude product (1,2:5,6-di-O-isopropylidene-α-D -glucofuranoside) was treated with hydrochloric acid giving intermediate 11 in a yield of 43% after recrystallization. The reaction of compound 11 with mixture of potassium hydroxide and allyl bromide in toluene gave the corresponding 3,5,6-tri-O-allyl derivative 12 in a yield of 74% applying a modification of Bullock’s method [186]. Epoxidation of allyl ether 12 with m-chloroperbenzoic acid in toluene resulted in 3,5,6-tri-O-(2’,3’-epoxypropyl)-1,2-O-isopropylidene-α-D -glucofuranoside (13) in a yield of 76% after purification by chromatography. The product is yellow oil, with a viscosity of 3.77 Pa·s at room temperature.

Figure 4.1.4 Synthesis of glucofuranoside-based trifunctional epoxy component (GFTE)

As among the synthesized glucose-based epoxy monomers the liquid, trifunctional glucofuranoside-based epoxy monomer provided the highest glass transition temperature, the synthesis of this product was scaled-up. During the initial synthesis both the allyl- derivative and the epoxy monomer were purified by chromatography, which is an uneconomical and extremely time-consuming procedure. During the scale-up the number of purification steps was attempted to be reduced and alternative reaction pathways were elaborated as follows:

47 1,2-di-O-isopropylidene-α-^D-glucofuranoside (11) was synthesized with the already described one-pot method. The precipitate was filtered from the mother liquor after evaporation, and after the evaporation of the secondary mother liquor, 11 was received with 55% yield. The product contains in each case minor pollutants (e.g. disaccharides), but they can be easily removed in the next step of the reaction by filtration, as they are not soluble in toluene, just the formed KBr and the eventually unreacted KOH base. The excess allyl bromide was removed from the reaction mixture during evaporation. The received product was dissolved in toluene again, washed with water to remove traces of KBr and KOH. After drying and evaporation the allyl derivative (12) was received with nearly 100% yield without purification. The crude product of the previous step was dissolved in toluene and epoxidation was carried out with m-chloroperbenzoic acid (mCPBA). The reaction by-product, m-chlorobenzoic acid (mCBA) precipitated from toluene, so it could be removed by filtration. The up-scaled synthesis required cooling to avoid side reactions above 45 °C e.g.

oxidation of the backbone or opening of the acetal ring. After filtering, the mother liquor was washed with Na2CO3 solution in order to remove the excess mCPBA and unfiltered mCBA. After evaporation, the epoxidized product, having an epoxy equivalent of 160 g/eq, was received with nearly 80% yield, without any further purification.

4.1.1.5. Preliminary testing of the synthesized sugar based bioepoxy monomers Curing properties

For investigating the applicability of the synthesized glucose-based epoxy monomers DSC measurements were carried out with 4,4’-diaminodiphenyl methane (DDM) hardener. As the synthesized components were prepared in >95% purity, their epoxy equivalents could be determined from their molecular mass (Table 4.1.2), which were in good agreement with the values determined by titration.

The onset point of the curing is about 120 °C in most cases. The peak temperatures are also in the same temperature range for GPBE, GPQE, and GFTE, while GPTE showed somewhat lower values.

From the curing enthalpy measured in DSC (J/g), the enthalpy in kJ/mol epoxy groups was calculated and compared to the theoretical value (105 kJ/mol per epoxy groups, independently from the molecular structure of epoxy resin and amine reacted [187,188]) to determine the degree of cure. In the case of the oily GFTE bioepoxy monomer, the calculated enthalpy is in good accordance with the theoretical value, similarly to the reference DGEBA – DDM system. It can be stated, that the reaction between the glucopyranoside-based epoxy components and the hardener was not complete, which can be explained by their solid state: during the mixing with solid DDM, no molecular level homogenization was reached, thus, no full curing could be

48 achieved. (Neither the solutions of the components, nor the mixtures of the melted molecules are suitable for determining the curing by the DSC method. The presence of the solvent, or the already started reaction between the components would falsify the results.)

Table 4.1.2 Curing behaviour of the synthesized bioepoxy components and the DGEBA reference

epoxy monomer GPBE GPTE GPQE GFTE DGEBA

epoxy equivalent [g/eq] 197 145 104 129 180

onset temperature of curing [°C] 127 98 118 128 121

peak temperature of curing [°C] 164 127 143 158 149

measured enthalpy of curing [J/g] 258 395 535 531 432

calculated enthalpy of curing [kJ/mol epoxy groups] 63.7 77.2 82.7 95.1 99.2

degree of cure [%] 60.7 73.5 78.8 90.6 94.5

glass transition temperature [°C] 76 154 130 177 174

The glass transition temperatures (Tg) of the glucose-based epoxy networks (Table 4.1.2) show various values. As expected, the lowest Tg was measured for the bifunctional glucopyranoside-based component (GPBE), due to the low functionality and low degree of cure. When comparing the glucopyranoside-based tri- and tetrafunctional resins, in contrast to the expectations, the lower functionality provided the higher Tg. This can be explained by the higher flexibility of GPQE structure, as the rigid bicyclic part is missing in this case, the segmental movements are less limited. The highest Tg value (177 °C), even higher than that for the reference DGEBA (174 °C), was reached using GFTE, owing to the compact structure of the molecule.

The completion of the curing in the case of GFTE was also investigated by Raman spectrometry (Figure 4.1.5).

Figure 4.1.5 Raman spectra of GFTE epoxy component, DDM curing agent and the cured resin

49 The characteristic bands of the epoxy ring can be seen at 917 and at 1257 cm^-1 in the spectrum of GFTE. At this region, the vibrations of the NH2 groups appear as weak peaks at 1317 and at 1584 cm^-1 in the spectrum of DDM. In the spectrum of the cured resin neither the epoxy component nor the amine-type hardener has characteristic peaks, which indicates the complete reaction between the two components.

Thermal stability

The thermal stability of the cured bioepoxy networks was determined by TGA (Table 4.1.3).

Table 4.1.3 TGA results of the different bioepoxy resins and DGEBA reference cured with DDM

component T-5%

The degradation of the trifunctional glucofuranoside-based resin (GFTE) starts at the lowest temperature among the investigated systems, as the 1,2-O-isopropylidene group of the molecule can easily split off, releasing acetone. The further decomposition of this sample is relatively slow.

The bi- and trifunctional glucopyranoside-based resins (GPBE and GPTE) start to degrade at about 330 °C, with the leaving of the 4,6-O-benzylidene protecting group. GPQE and DGEBA have no easily cleaveable protecting groups, so the highest thermal stability can be reached (up to 360 °C), however, their degradation rate is also high. The relatively high char yields of the synthesized bioepoxy compositions are promising in terms of flame retardancy.