NOVEL METHOD FOR THE POLYMERIZATION OF SUGARS

Abstract

The present invention relates to a method for preparing a polysaccharide comprising a step of the polymerization of a saccharide monomer by non-thermal plasma treatment.

Claims

1. A method for preparing a polysaccharide comprising a step of non-thermal plasma polymerization of a saccharide monomer, wherein the polymerization step is not carried out in the presence of a solid support.

2. The method according to claim 1, wherein the saccharide monomer is a monosaccharide or a disaccharide.

3. The method according to claim 1, wherein the saccharide monomer is a monosaccharide.

4. The method according to claim 1, wherein the polymerization step is carried out at a temperature below the melting temperature of the saccharide monomer.

5. The method according to claim 1, wherein the polymerization step is carried out at a temperature between 0 C. and 140 C.

6. The method according to claim 1, wherein the polymerization step is carried out for a period of less than 30 minutes.

7. The method according to claim 1, wherein the polysaccharide has a molar mass of from 1000 g/mol to 100,000 g/mol.

8. The method according to claim 1, characterized in that it does not include a subsequent purification step.

9. The method according to claim 1, comprising the steps of: placing a saccharide monomer between two electrodes insulated from one another by a dielectric material; application of an electric field greater than 5.Math.10.sup.5 V/m between the two electrodes to generate a plasma discharge between the two electrodes; and formation of a polysaccharide by the polymerization of the saccharide monomer.

10. The method of claim 1, wherein the polymerization step is carried out for a period between 5 and 20 minutes.

Description

EXAMPLES

Example 1 Polymerization of Mannose

[0052] The mannose was placed in the solid state between two copper electrodes of 25 cm.sup.2 arranged in parallel and isolated from one another by a dielectric (called a DBD reactor). In order to maintain the formation of an optimal plasma, the gap between the two electrodes was set at 4 mm. The plasma was created using a bipolar generator at a voltage of 9.5 kV and a frequency of 2.2 KHz. The air flow is 100 mL/min. During the plasma treatment, mannose samples were taken after 10, 15 and 30 min and then analyzed by steric exclusion chromatography (SEC). It was found that mannose is completely consumed after only 15 minutes of plasma treatment and that products of higher molecular weight are formed.

[0053] The polymerization of mannose may also be observed indirectly by X-ray diffraction (XRD) analysis and by .sup.1H and .sup.13C NMR. In fact, after plasma treatment, a significant broadening of the signals is observed in both types of analysis which is often the sign of an anarchic (or disordered) polymerization.

[0054] Interestingly, it may be observed on the MALDI-TOF spectra that the polymerization actually starts after 7 min of plasma treatment. The conversion of mannose as a function of the plasma treatment time has been studied by SEC in order to obtain more information on this aspect. In agreement with the MALDI-TOF analyzes, an induction period of 7 min was observed by SEC after which the mannose is rapidly polymerized in only 3 min. This induction period is related to the increase in the temperature of the plasma reactor (provided by the dissipated energy). In particular, this induction period corresponds to the time required for the reactor to reach 40 C. In order to support this hypothesis, the plasma reactor was initially cooled to 23 C. In this case, the time required for the reactor to reach 40 C. passed from 7 to 15 min, which coincides with an extension of the induction period from 7 to 15 min. Similarly, when the reactor is at 65 C. or 75 C. when placed at the start of treatment, there is no induction period and polymerization begins almost instantaneously. Finally, when the plasma reactor is successively started and then stopped in order to avoid an increase in temperature above 40 C., no polymerization takes place and the mannose remains unconverted. All of these results suggest that mannose polymerization begins when the external temperature of the reactor reaches 40 C.

[0055] Analysis of Mannose Polymers

[0056] The mannose polymers were analyzed by various techniques. At first, IR and RAMAN spectroscopy was used. No characteristic signal of a CO or CC group was determined thus again confirming the stability of the mannose units during the plasma treatment. Solid or liquid NMR analysis (.sup.1H and .sup.13C) confirms this observation and no characteristic peak of a CO group was observed. These results are surprising considering that the species generated by the plasma are often used for oxidation reactions. In order to obtain further information, the mannose polymers were analyzed by X-ray photoelectron spectrometry (XPS) which provides information on the chemical composition of a surface in a 10 nm layer. Interestingly, XPS reveals oxidation of the surface of the mannose polymer particles with the presence of OCO groups with about one CO for three mannose units. The oxidation of the surface of the mannose particles is also supported by the measurement of the pH (at 10 g/L) which decreases from 6 to 4.2 after plasma treatment in agreement with the production of a small amount of CO.sub.2H group. It should be noted that when the mannose is impregnated with an acetic acid solution in order to lower its pH to 4.2 and then treated at 50 C. for 15 min in the solid state, no polymerization takes place which suggests that the acidic species formed on the surface of the mannose are not responsible for the polymerization observed. Moreover, the mannose conversion rate remains similar, regardless of the initial plasma reactor temperature which suggests that the activation energy is very low, which is in agreement with a radical mechanism.

[0057] In order to collect more information at a molecular level, the mannose polymers were analyzed by GC/MS using commercial standards for assignment of different peaks. More particularly, we focused on the disaccharide fraction in order to determine the different positions of the mannose involved in the polymerization. Disaccharide fraction analysis was performed at a mannose conversion of 43% so that the signals could be more accurately quantified. These analyses reveal that all the hydroxyl groups are involved in the polymerization of mannose. However, the link between two mannose units is primarily between positions 1 and 6 (71% probability). Selectivity between -1,6 and -1,6 bonds is 27% and 44%, respectively. It is clear that the polymerization of mannose takes place in a disordered manner which rationalizes the signal expansions observed by XRD and NMR.

[0058] The mannose polymers were analyzed by SEC/MALS to obtain information on the mass distribution and conformation of the mannose polymers. Elution profiles show at least three different types of populations that differ in their hydrodynamic volume, reflecting a strong polydispersity. These analyses reveal that the molar masses of the mannose polymer range from 210.sup.3 to 910.sup.6 g/mol with a hydrodynamic radius ranging from 1.2 to 37.2 nm. More generally, the mannose polymers are characterized by a mean molar mass (M.sub.w) of 95.590 g/mol, an intrinsic viscosity () of 7.7 ml/g and a hydrodynamic radius (Rh) of 3.3 nm. The mannose polymers also exhibit a high polydispersity (M.sub.w/M.sub.n) of 15 which, again, is consistent with disordered mannose polymerization.

[0059] The conformation of mannose polymers was then studied by plotting Rh as a function of M.sub.w. Rh and M.sub.w are bonded together and obey equation (1) where Rh and M.sub.w are respectively the hydrodynamic radius and the molar mass, v.sub.h is the hydrodynamic coefficient and K.sub.h is a constant.

RhK.sub.hM.sub.w.sup..sup.hequation (1)

[0060] The hydrodynamic coefficient depends on the general shape of the macromolecules, the temperature and the macromolecule-solvent interactions. A theoretical v.sub.h of 0.33 is obtained for a sphere, 0.5-0.6 for a coil shape and 1 for a rod. The v.sub.h obtained is 0.43. A linear relationship between Rh and M.sub.w is obtained, meaning that the mannose polymers have similar conformations regardless of the degree of polymerization. A value of v.sub.h of 0.43 means that the mannose polymers adopt a conformation close to a sphere, which means that the mannose polymers have compact and/or hyperbranched structures. This statement is supported by the high solubility of mannose polymers in water (500 g/L).

Example 2: Polymerization of Other Mono- and Disaccharides

[0061] Three mono- and four disaccharides were tested. Because induction periods vary with carbohydrate, plasma treatment was arbitrarily set at 30 min in all cases. The results are summarized in Table 1 below. Remarkably, the plasma is able to polymerize all the carbohydrates tested. Only the carbohydrates liquefying in the reactor (for example, fructose) could not be polymerized but a cooling of the plasma reactor should allow their polymerization. A difference in the induction period was observed between the different carbohydrates, which is related to a different activation temperature. When the disaccharides were used, it was observed by MALDI/TOF that the disaccharide unit was the basic unit of the polymer which suggests that the glycosidic bonds are not broken. As observed in the case of mannose, a white powder is obtained in all cases.

[0062] The structural parameters of the recovered polymers were determined as before. The average molar mass remains similar in all cases and ranges from 2,000 to 5,500 g/mol with a polydispersity ranging from 2 to 11. These values are however lower than those obtained from mannose. This result is not surprising and stems from the fact that the plasma has been optimized for mannose, while the parameters applied are certainly not the optimal parameters for each carbohydrate. This is the reason why the M.sub.w and the conversions presented in Table 1 differ from those of mannose. Nevertheless, Table 1 clearly illustrates the plasma potential for the polymerization of carbohydrates under dry conditions. As previously carried out with the mannose polymers, the conformation of the polymers presented in Table 1 was studied by plotting Rh as a function of M.sub.w. Again, a linear correlation was obtained. In particular, a v.sub.h of around 0.40 was obtained (values ranged from 0.37 to 0.44) indicating that the polysaccharides have very similar macromolecular structures. As previously mentioned, a v.sub.h of 0.40 indicates a compact and/or hyperbranched organization of polysaccharides. The formation of hyperbranched polysaccharides also confirms a disordered polymerization of carbohydrates. However, it is interesting to note that from the isomaltulose and turanose, the v.sub.h are lower suggesting an even more compact and/or hyperbranched appearance for the corresponding polysaccharides in agreement with the mass distribution profile.

TABLE-US-00001 TABLE 1 Polymerization of various plasma-induced carbohydrates [00001] [00002] [00003] Induction Polymerization Conv Mw Mn Rh Saccharides period (min) temp. ( C.) (%) (g/mol) (g/mol) DP (nm) (ml/g) .sub.h Glucose 15 66 70 5051 909 5.6 33 1.4 5.0 0.41 Xylose 10 56 90 5368 1121 5.3 45 1.4 5.3 0.42 Galactose 20 72 62 3378 302 11.2 28 0.8 3.5 0.44 Maltulose 7 40 92 3723 1746 2.1 26 1.2 4.2 0.4 Maltose 10 56 91 2325 835 2.8 15 1.0 4.2 0.42 Isomaltulose 15 64 87 3409 1207 2.8 25 1.0 3.5 0.38 Turanose 30 80 80 2214 581 3.8 18 0.8 3.2 0.37