# Patent

## Title
Acid dianhydride, method for preparing same, and polyimide prepared therefrom

## Abstract

The present invention relates to a novel acid dianhydride, a method for preparing the same, and a polyimide prepared therefrom. More specifically, the acid dianhydride according to the present invention is useful as a colorless transparent polyimide unit exhibiting excellent thermal stability and a low dielectric ratio, and the polyimide of the present invention has excellent solubility to an organic solvent compared with the conventional polyimide.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a National Stage of International Application No. PCT/KR2014/004395, filed on May 16, 2014, which claims priority from Korean Patent Application No. 10-2013-0055391, filed on May 16, 2013, the contents of all of which are incorporated herein by reference in their entirety.
TECHNICAL FIELD
The present invention relates to an acid dianhydride, a preparation method thereof, and a polyimide prepared therefrom.
BACKGROUND ART
Polyimide finds a wide spectrum of applications in the electronic material industry including protecting materials, insulation materials, color filters, etc. for liquid crystal displays and semiconductors by virtue of its high mechanical strength, thermal resistance, and solvent resistance. In addition, expectation has recently been made of the use of polyimide as a material for optical telecommunication and as a substrate for mobile phones.
With the recent advances in these industries, materials having advanced properties are increasingly demanded. For example, not only are mechanical properties such as thermal resistance and solvent resistance needed, but also functions according to uses, such as transparency, etc., are requested in polyimide for use in this field.
Appearing with a dark amber color, wholly aromatic polyimides for general purposes, which can be obtained by the polycondensation of aromatic tetracarboxylic dianhydride with aromatic diamine, cannot be applied where high transparency is needed. In addition, since wholly aromatic polyimide is insoluble in organic solvents, its precursor polyamic acid is, in practice, employed before film formation through thermal ring-closing dehydration.
One strategy for achieving transparency is known, wherein aliphatic tetracarboxylic dianhydride is polycondensed with aromatic diamine polyimide to give a polyimide precursor, followed by conversion into polyimide that is relatively colorless, and highly transparent (Japanese Patent Unexamined Application Publication Nos. Hei 2-24294 and Sho 58-208322).
However, the polyamic acids and polyimides that are based on unsubstituted aliphatic tetracarboxylic dianhydride are almost insoluble in general organic solvents, and soluble only in polar organic solvents with high boiling points. In this context, a high temperature is employed upon the formation of the film so as to remove the solvent, exerting undesirable effects on other organic materials of organic EL devices.
In recent years, a polyimide prepared from the monomer 1,2,3,4-cyclopentane tetracarboxylic dianhydride (hereinafter referred to as “CPDA”) has been suggested for use as a gas barrier film of organic electroluminescence (hereinafter referred to as “organic EL”) (Japanese Patent Unexamined Application Publication No. 2006-232960).
However, this polyimide needs to be improved in thermal resistance and is not of sufficient solubility in organic solvents in addition to having a low degree of polymerization.
DISCLOSURE
Technical Problem
It is a primary object of the present invention to provide a novel acid dianhydride as a material monomer for a polyimide that has a low dielectric constant and exhibits excellent thermal stability, solubility in organic solvents, and light transmittance while retaining the properties of polyimide itself, and a method for preparing the same.
It is another object of the present invention to provide a polyamic acid containing the novel acid dianhydride, and a polyimide prepared through the ring-closing dehydration of the polyamic acid.
Technical Solution
According to one aspect for accomplishing the above object, the present invention provides an acid dianhydride represented by the following Chemical Formula 1:




In accordance with another aspect thereof, the present invention provides a method for preparing an acid dianhydride, represented by the following Chemical Formula 1, comprising: (a) reacting a compound represented by the following Chemical Formula 1 with piperazine to give a compound represented by the following Chemical Formula 3; (b) hydrolyzing the compound of Chemical Formula 3 in the presence of a base catalyst to obtain a compound represented by Chemical Formula 4; and (c) subjecting the compound of Chemical Formula 4 into ring-closing dehydration in the presence of a dehydrating agent.




In one preferred embodiment of the present invention, the reaction of step (a) is a Michael addition reaction.
In another preferred embodiment of the present invention, the compound of Chemical Formula 2 is reacted at a molar ratio 1:0.45 to 1:0.55 with piperazine in step (a).
In another preferred embodiment of the present invention, the base catalyst of step (b) is selected from the group consisting of potassium hydroxide, sodium hydroxide, barium hydroxide, calcium hydroxide, aluminum hydroxide, magnesium hydroxide, and a combination thereof.
In another preferred embodiment of the present invention, the hydrolysis of step (b) is conducted at 40 ̃120° C. for 1 ̃6 hrs.
In another preferred embodiment of the present invention, the dehydrating agent of step (c) is selected from the group consisting of acetic anhydride, pyridine, isoquinoline, a tertiary amine, and a combination thereof.
In another preferred embodiment of the present invention, the ring-closing dehydration of step (c) is conducted at 40 ̃100° C. for 4 ̃28 hrs.
In another preferred embodiment of the present invention, the dehydrating agent of step (c) is used in an amount of 2 ̃10 moles per mole of the compound of Chemical Formula 4.
A further aspect of the present invention provides a polyamic acid, prepared by reacting an acid dianhydride represented by the following Chemical Formula 1 with diamine:




A still further aspect of the present invention provides a polyimide, prepared from the polyamic acid of claim 10 by ring-closing dehydration.
Advantageous Effects
Provided according to the present invention are a novel acid dianhydride useful for the preparation of a colorless transparent polyimide that exhibits excellent thermal stability and has a low dielectric constant while retaining excellent properties of polyimide itself, and a method for preparing the same.
Exhibiting excellent thermal stability and having a low dielectric constant, the colorless transparent polyimide of the present invention finds applications in a variety of material industries including protecting and insulating materials in liquid crystal display devices or semiconductors, and materials for optical communication such as optical waveguides


MODE FOR INVENTION
Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
An aspect of the present invention addresses an acid dianhydride represented by the following Chemical Formula 1:




In accordance with another aspect thereof, the present invention addresses a method for preparing an acid dianhydride, represented by the following Chemical Formula 1, comprising: (a) reacting a compound represented by the following Chemical Formula 1 with piperazine to give a compound represented by the following Chemical Formula 3; (b) hydrolyzing the compound of Chemical Formula 3 in the presence of a base catalyst to obtain a compound represented by Chemical Formula 4; and (c) subjecting the compound of Chemical Formula 4 into ring-closing dehydration in the presence of a dehydrating agent.




As a rule, aliphatic polyimide exhibits higher transparency and is lower in dielectric constant, compared to aromatic polyimide, because of its low intramolecular density, polarity, and low inter- or intramolecular charge transfer. Thanks to these properties, aliphatic polyimide has attracted intensive attention for use as interlayer insulating material in optoelectronics.
For use in preparing an aliphatic polyimide that is of high transparency and low dielectric constant, a nitrogen-containing piperazine-disuccinic anhydride (an acid dianhydride represented by Chemical Formula 1) is synthesized in the present invention.
Since the acid dianhydride represented by Chemical Formula 1 in accordance with the present invention contains at least one intramolecular nitrogen atom, the lone electron pair on the nitrogen atom causes intra- or intermolecular interaction. In this mechanism, the polyamide of the present invention can be greatly improved in solubility and mechanical strength while retaining its excellent intrinsic properties.
The acid dianhydride according to the present invention may be prepared by a simple organic synthesis method, such as Michael addition and hydrolysis.
Briefly, the acid dianhydride according to the present invention is prepared by reacting a compound represented by Chemical Formula 2 with piperazine to give a compound represented by Chemical Formula 3, hydrolyzing the compound of Chemical Formula 3 in the presence of a base catalyst to obtain a compound represented by Chemical Formula 4, and ring closing by dehydration with a dehydrating agent.
This process is summarized as illustrated in the following Reaction Scheme 1.




First, a compound of Chemical Formula 2 (dimethyl fumarate) is allowed to undergo the Michael addition reaction with piperazine to form a compound of Chemical Formula 3. In this reaction, the compound of Chemical Formula 2 (dimethyl fumarate) acts as a Michael acceptor while piperazine serves as a Michael donor.
Preferably, the Michael addition reaction is conducted at 20 ̃140° C. for 4 ̃16 hrs in terms of reaction efficiency.
For use in preparing the compound of Chemical Formula 3, the compound of Chemical Formula 2 and piperazine are used at a molar ratio of 1:0.45 to 1:0.55. When piperazine is below 0.45 moles or over 0.55 moles per mole of the compound of Chemical Formula 2, the yield may decrease.
There are various ways known to prepare the compound of Chemical Formula 2. In a preferred embodiment, fumaric acid is refluxed in methanol in the presence of an acid catalyst, such as sulfuric acid, followed by neutralization with a neutralizer, such as sodium carbonate, to afford the compound.
In the present invention, the reaction substrate itself is preferably used as a solvent, but a separate solvent may be employed. No particular limitations are imposed on the reaction solvent if it does not interrupt the reaction. For example, 1,4-dioxane, toluene, NMP (N-Methyl-2-pyrrolidone), or DMAc (dimethylacetamide) may be used.
Then, the compound of Chemical Formula 3 may be hydrolyzed in the presence of a base catalyst to a compound of Chemical Formula 4. The hydrolysis is conducted at 40 ̃120° C. for 1 ̃6 hrs. At less than 40° C. or for less than 1 hr, the reaction does not proceed sufficiently, thus leaving much of the reactants unreacted. At higher than 120° C. or for more than 6 hrs, the solvent or the catalyst may evaporate, which is undesirable in terms of cost and efficiency.
The base catalyst useful for the hydrolysis reaction may be selected from the group consisting of potassium hydroxide, sodium hydroxide, barium hydroxide, calcium hydroxide, magnesium hydroxide, and a combination thereof. Preferred is potassium hydroxide or sodium hydroxide in view of cost and ease of handling.
Based on 1 mole of the compound of Chemical Formula 3, the base catalyst may be used in an amount of 5 to 10 moles. When the amount of the base catalyst is less than 5 moles per mole of the compound of Chemical Formula 3, hydrolysis is difficult to perform. On the other hand, more than 10 moles of the base catalyst leaves much hydrochloride to be deposited, which has negative effects on efficiency and productivity.
The compound of Chemical Formula 4 is ring closed by dehydration in the presence of a dehydrating agent to afford the aliphatic acid dianhydride represented by Chemical Formula 1. The ring-closing dehydration is conducted at 40 ̃100° C. for 4 ̃28 hrs. At higher than 100° C. or for longer than 28 hrs, the catalyst or the solvent is prone to evaporating, thus decreasing the yield. At lower than 40° C., the reaction should be conducted for a prolonged time, or if conducted within 4 hrs, the reaction does not proceed sufficiently, decreasing the yield.
The dehydrating agent may be at least one selected from the group consisting of acetic anhydride, and a tertiary amine such as pyridine, isoquinoline and triethylamine. In terms of efficiency, acetic anhydride and/or pyridine is preferred.
Based on 1 mole of the compound of Chemical Formula 4, the dehydrating agent may be used in an amount of 2 moles or more, and preferably in an amount of 2 to 10 moles. When the amount of the dehydrating agent is less than 2 moles per mole of the compound of Chemical Formula 4, the reaction does not proceed sufficiently with the resultant decrease of the yield. More than 10 moles of the dehydrating agent is disadvantageous in view of cost.
After completion of the above-mentioned reactions, the acid dianhydride is obtained by typical filtration and drying.
In accordance with another aspect thereof, the present invention addresses a polyamic acid prepared by reacting an acid dianhydride represented by the following Chemical Formula 1 with diamine.




Also, contemplated in accordance with a further aspect of the present invention is a polyimide obtained by ring-closing dehydration of the polyamic acid.
The acid dianhydride of Chemical Formula 1 according to the present invention can be polycondensed with diamine to give a polyamic acid that can be then subjected to ring-closing dehydration under heat or in the presence of a catalyst to afford a polyimide.
Without being particularly limited, the diamine may be one of various diamines typically used in the synthesis of polyimide. Concrete examples of the diamine include: aromatic diamines, such as p-phenylene diamine, m-phenylene diamine, 2,5-diaminotoluene, 2,6-diaminotoluene, 1,3-bis(4,4′-aminophenoxy)benzene, 4,4′-diamino-1,5-phenoxypentane, 4,4′-diaminobiphenyl, 3,3′-dimethyl-4,4′-diaminobiphenyl, 3,3′-dimethoxy-4,4′-diaminobiphenyl, 4,4′-diaminodiphenylether, 4,4′-diaminodiphenylmethane, 2,2′-diaminodiphenylpropane, bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenylsulfone, diaminobenzophenone, diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene, 1,3-bis(4-aminophenoxy)benzene, 4,4′-bis(4-aminophenoxy)diphenylsulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and 2,2′-trifluoromethyl-4,4′-diaminobiphenyl; cyclic diamines, such as 1,4-diaminocyclohexane, 1,4-cyclohexane bis(methylamine), and 4,4′-diaminodicyclohexyl methane; and aliphatic diamines, such as tetramethylene diamine, and hexamethylene diamine. These diamines may be used solely or in combination.
No particular limitations are imposed on the preparation of the polyamic acid of the present invention. It may be achieved by reacting and polymerizing the acid dianhydride of Chemical Formula 1 with diamine in a typical process. For convenience, an aliphatic dianhydride represented by Chemical Formula 1 and diamine may be mixed and reacted in an organic solvent.
Examples of the organic solvent include m-cresol, N-methyl-2-pyrrolidone (NMP), N,N-dimethyl formamide (DMF), N,N-dimethyl acetamide (DMAc), N-methyl caprolactam, dimethyl sulfoxide (DMSO), tetramethyl urea, pyridine, dimethylsulfone, hexamethyl phosphoramide, and γ-butyrolactone. These solvents may be used solely or in combination. Unable as it is to dissolve polyamic acid, an additional solvent may be used in addition to the solvent if it allows for the formation of a uniform solution.
The solution polymerization may be conducted at −20 ̃150° C., and preferably at −5 ̃100° C. In addition, the molecular weight of the polyamic acid may be controlled by adjusting the molar ratio of the acid dianhydride of Chemical Formula 1 to the diamine. A molar ratio nearer to 1 results in a higher molecular weight of the produced polyamic acid.
The polyimide of the present invention is obtained from the polyamic acid prepared above through ring-closing dehydration. Here, a conversion rate of polyimide from polyamic acid (ring-closing dehydration rate) is defined as a imidization rate. The rate of imidization for the polyimide of the present invention is not limited to 100%, and may be given a value of 1 ̃100%, as needed.
Without being particularly limited, the ring-closing dehydration of polyamic acid may be achieved typically by heating or chemically with a known ring-closing dehydration catalyst. The ring-closing dehydration by heating may be conducted at 100 ̃300° C., and preferably at 120 ̃250° C.
As for the chemical method, its ring-closing dehydration may be conducted in the presence of a catalyst such as an organic base such as pyridine or triethyl amine, or an acetic anhydride. In this regard, the reaction temperature may be at −20 ̃200° C. In this reaction, the polymerization solution of polyamic acid may be used as it is or after it is diluted. Alternatively, the polyamic acid may be dissolved in a suitable organic solvent after being recovered from the polymerization solution as described later. The organic solvent may be the same as is used for the polymerization of polyamic acid.
The polyimide (containing) solution thus obtained may be used as it is or may be added with methanol or ethanol to precipitate the polymer. This polymer may be isolated as a powder or may be re-dissolved in a suitable solvent. Any solvent can be used without limitations if it can dissolve the polymer. Selection for the solvent may be made from among, for example, m-cresol, 2-pyrrolidone, NMP, N-ethyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, DMAc, DMF(dimethylformamide), and γ-butyrolactone.
A better understanding of the present invention may be obtained through the following examples that are set forth to illustrate, but are not to be construed as limiting the present invention.
EXAMPLE 1
1-1: Synthesis of Compound of Chemical Formula 2
To a solution of 14.5 g (125 mmol) of fumaric acid in 200 ml of methanol was added 5 ml of conc. sulfuric acid, followed by refluxing at 100° C. for 1 hr. The reflux was cooled in a cooling bath and neutralized with 200 g of 10% sodium carbonate to form a white precipitate. This precipitate was filtered, washed with water, and dried at 50° C. for 12 hrs in a vacuum oven to afford the compound of Chemical Formula 2. 16.6 g (yield 92%). The preparation of the compound of Chemical Formula 2 was based on the method reported by Carr G. et al. (Carr G., Williams D. E., Di′az-Marrero A. R., Patrick B. O., Bottriell H., Balgi A. D., Donohue E., Roberge M., and Andersen R. J., J. Nat. Prod. 2010, 73, 422).
The compound of Chemical Formula 2 obtained was measured for melting point (Buchi, M-560). Its melting point (Mp) was observed to be 102° C., which is identical to that of the compound reported by Carr G. et al.
1-2: Synthesis of Compound of Chemical Formula 3
To 100 ml of 1,4-dioxane were added 14.4 g (0.1 mol) of the compound of Chemical Formula 2 obtained in Example 1-1 and 4.3 g (0.05 mol) of piperazine, followed by refluxing for 16 hrs. The reflux was cooled to form a precipitate. This precipitate was subjected to many rounds of filtration and condensation, and dried for 12 hrs in a vacuum oven to afford the compound of Chemical Formula 3 as a colorless solid. 15.3 g (yield 82%).
The compound of Chemical Formula 3 obtained was measured for melting point (Buchi, M-560), and analyzed by NMR(1H and 13C) (JEOL, JNM-LA400) and IR (AVATAR, 360 FT-IR).
m. p.: 158° C. (EtOAc)
1H NMR (400 MHz, CDCl3): δ 2.38-2.50 (m, 4H, Het-CH2CH2), 2.54-2.70 (m, 6H, β-CH2, Het-CH2CH2), 2.81 (dd, J=16.0 and 9.2 Hz, 2H, β-CH2), 3.64 (s, 6H, 2OCH3), 3.68 (dd, 2H, overlapped signals, α-CH), 3.70 (s, 6H, 2OCH3) (FIG. 4); 13C NMR (100 MHz, CDCl3): δ 171.7 and 170.9 (ester C), 63.4 (α-CH), 51.8 (OCH3), 51.5 (OCH3), 49.9 (Het-CH2CH2), 34.0 (β-CH2); Anal. Calcd. for C16H26N2O8; C: 51.33; H: 7.00; N: 7.48%. Found: C: 51.19; H: 7.09; N: 7.53%; IR (KBr, cm−1): 1734 for νC═O
1-3: Synthesis of Compound of Chemical Formula 4
In a 500 ml round-bottom flask, 11.2 g (0.03 mol) of the compound of Chemical Formula 3 obtained in Example 1-2 was mixed with 120 ml (0.24 mol) of 2 N potassium hydroxide and 120 ml of methanol. The mixture was heated at 60° C. to the degree of complete dissolution and then for an additional 3 hrs at the same temperature. After its pH was adjusted to 3.8 with conc. HCl, the solution was stirred at room temperature for 30 min to form a precipitate. This precipitate was filtered, and washed with water. Then, the precipitate was dried for 24 hrs in a vacuum oven, and re-crystallized in 2,000 ml of a mixture of 1:1 water and methanol to afford the compound of Chemical Formula 4. 8.7 g (yield 92%). Since the compound of Chemical Formula 4 is not dissolved in general organic solvents nor in water, an NMR sample was prepared by mixing the compound of Chemical Formula 4 with heavy water (D2O) in which solid potassium hydroxide was dissolved. The compound of Chemical Formula 3 obtained was measured for melting point (Buchi, M-560), and analyzed by NMR (1H and 13C) (JEOL, JNM-LA400) and IR (AVATAR, 360 FT-IR).
m.p.: 218-219° C. (H2O+MeOH)
*1H NMR (400 MHz, D2O/KOH): δ 2.46-2.54 (bd, 4H, Het-CH2CH2), 2.56-3.40 (bm, 6H, β-CH2, Het-CH2CH2), 3.44-3.54 (bm, 2H, β-CH2), 4.80 (dd, 2H, overlapped signals, α-CH); 13C NMR (100 MHz, D2O/KOH): δ 178.8 (COOH), 67.7 (α-CH), 49.1 (Het-CH2CH2), 37.4 (β-CH2); Anal. Calcd. for C12H18N2O8; C: 45.28; H: 5.70; N: 8.80%. Found: C: 45.16; H: 5.79; N: 8.83%; IR (KBr, cm−1): 1723 for νC═O.
1-4: Synthesis of Compound of Chemical Formula 1
In a 50 ml flask equipped with a magnetic stirrer, 5.4 g (17 mmol) of the compound of Chemical Formula 4 obtained in Example 1-3, 3.18 g (35.7 mmol) of pyridine, and 3.6 g (35.7 mmol) of acetic anhydride were placed, and allowed to react at 60° C. for 24 hrs. After completion of the reaction, the reaction mixture was cooled and filtered. The filtrate was washed with 200 ml of acetic anhydride and 200 ml of purified diethylether, and dried at 40° C. in a vacuum oven. Recrystallization in 100 ml of acetic anhydride produced the compound of Chemical Formula 1. 2.9 g (yield 60%).
The compound of Chemical Formula 1 obtained was measured for melting point (Buchi, M-560), and analyzed by NMR (1H and 13C) (JEOL, JNM-LA400) and IR (AVATAR, 360 FT-IR).
m. p.: 156° C. (decompose)
1H NMR (400 MHz, d6-DMSO): δ 2.39 (bd, J=6.8 Hz, 4H, Het-CH2CH2), 2.76 (bd, J=6.8 Hz, 4H, Het-CH2CH2), 3.04 (d, J=8.4 Hz, 4H, β-CH2), 4.21 (t, J=16.4 Hz, 2H, α-CH); 13C NMR (100 MHz, d6-DMSO): δ 171.6 (COOCO), 170.6 (COOCO), 63.6 (α-CH), 48.9 (Het-CH2CH2), 31.9 (β-CH2); Anal. Calcd. for C12H14N2O6; C: 51.06, H: 5.00, N: 9.93%. Found: C: 49.93, H: 5.10, N: 9.96%; IR (KBr, cm−1): 1860, 1781 (νC═O), 1210, 1127 (C—O—C)
Although the preferred embodiment(s) of the present invention have(has) been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

## Claims

1. An acid dianhydride, represented by the following Chemical Formula 1:

2. A method for preparing an acid dianhydride, represented by the following Chemical Formula 1, comprising:
(a) reacting a compound represented by the following Chemical Formula 1 with piperazine to give a compound represented by the following Chemical Formula 3;
(b) hydrolyzing the compound of Chemical Formula 3 in the presence of a base catalyst to obtain a compound represented by Chemical Formula 4; and
(c) subjecting the compound of Chemical Formula 4 to ring-closing dehydration in presence of a dehydrating agent:

3. The method of claim 2, wherein the reaction of step (a) is a Michael addition reaction.

4. The method of claim 2, wherein the compound of Chemical Formula 2 is reacted at a molar ratio 1:0.45 to 1:0.55 with piperazine in step (a).

5. The method of claim 2, wherein the base catalyst of step (b) is selected from the group consisting of potassium hydroxide, sodium hydroxide, barium hydroxide, calcium hydroxide, aluminum hydroxide, magnesium hydroxide, and a combination thereof.

6. The method of claim 2, wherein the hydrolysis of step (b) is conducted at 40 ̃120° C. for 1 ̃6 hrs.

7. The method of claim 2, wherein the dehydrating agent of step (c) is selected from the group consisting of acetic anhydride, pyridine, isoquinoline, a tertiary amine, and a combination thereof.

8. The method of claim 2, wherein the ring-closing dehydration of step (c) is conducted at 40 ̃100° C. for 4 ̃28 hrs.

9. The method of claim 2, wherein the dehydrating agent of step (c) is used in an amount of 2 ̃10 moles per mole of the compound of Chemical Formula 4.