JPH01103604A - Improvement of property of p-vinylphenolic polymer - Google Patents

Abstract

PURPOSE:To improve economically the properties of p-vinylphenolic polymer, such as permeability to light in the ultraviolet region, solubility, and thermal cracking resistance, by bringing a p-vinylphenolic polymer into contact with hydrogen in the presence of a catalyst of a Group VIII metal of the periodic table at a prescribed temperature. CONSTITUTION:A p-vinylphenolic polymer is prepared which is, for example, a poly-p-vinylphenol obtained from crude p-vinylphenol as raw material or a copolymer obtained from p-vinylphenol and other comonomer. The p- vinylphenolic polymer is brought into contact with hydrogen in the presence of a catalyst of a Group VIII metal of the periodic table (e.g., a platinum catalyst or a nickel catalyst) and, preferably, in the presence of a solvent selected from the group consisting of lower alcohols, lower ketones, cyclic ethers and phenols (e.g., methanol) at 50-300 deg.C. Thus the properties of the p-vinylphenolic polymer can be easily improved.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention (c) uses polyp-vinylphenol or a copolymer of p-vinylphenol and other comonomers.
- It relates to a method for improving the physical or chemical properties of vinylphenol polymers, such as ultraviolet light transmittance, solubility, thermal decomposition, and reactivity. More specifically, a p-vinylphenol-based polymer is hydrogenated to reduce and remove light-absorbing impurity components in the polymer without substantially changing the basic skeleton of the polymer. In addition to improving its light transmittance, if necessary, the aromatic unsaturated bonds of the aromatic nucleus of the polymer are also hydrogenated. It also relates to methods for improving chemical properties.

(Prior art) A method of dehydrating p-acetoxyphenylmethylcarbinol, a method of decarboxylating p-hydroxycinnamic acid, a method of decomposing bisphenol ethane, and a method of decomposing p-hydroxycinnamic acid, which are conventionally known as methods for producing p-vinylphenol. Crude p- obtained by dehydrogenating p-ethylphenol, etc.
Vinyl phenol contains a large amount of phenols without unsaturated side chains such as phenol, cresol or engineered phenol, and also contains other by-products, but such crude p-vinyl phenol is purified. Polyp is economically produced by polymerizing in the presence of a polymerization accelerator without
- A method for producing vinylphenol is known (Japanese Patent Publication No. 57-47921, Japanese Patent Publication No. 61-2683).

In addition, if a comonomer such as styrene, acrylic ester or acrylonitrile is present in the polymerization reaction system during the polymerization of crude p-vinylphenol, p-vinylphenol can be
- Copolymers of vinylphenol and these comonomers are obtained. However, the p-vinylphenol polymer obtained using this crude p-vinylphenol as a raw material is
Probably due to impurities contained in the crude p-vinylphenol, it is colored and has poor UV transmittance.

Conventionally, in order to obtain p-vinylphenol-based polymers with good light transmittance in the ultraviolet region, crude p-vinylphenol was purified, and the purified p-vinylphenol was treated with boron trifluoride or aluminum trichloride. It was necessary to use it as a polymerization accelerator and polymerize at a low temperature below OC. but,
Due to its high reactivity, p-vinylphenol cannot be purified by distillation, and must be purified by repeated recrystallization from hydrocarbon solvents such as toluene and hexane. The operation is complicated and
- The loss of vinylphenol is large and it is very uneconomical.

Furthermore, it is known that when an amine is present when heating and modifying a p-vinylphenol polymer obtained from crude p-vinylphenol, the light transmittance in the ultraviolet region is improved. JP-A-60-258206). but,
The improvement effect is still not sufficient. The present inventors also discovered that p-vinylphenol obtained from crude p-vinylphenol
It has been found that treating vinylphenol-based polymers with boron hydride, etc. improves the light transmittance in the visible region, but is not very effective in improving the light transmittance in the ultraviolet region. There is.

(Problem to be solved) p-vinylphenol polymers have excellent heat resistance and alkali solubility, and because they have a phenolic hydroxyl group, they have high reactivity at the ortho position to the phenolic hydroxyl group, and the hydroxyl group can also be used in reactions. Therefore, it is useful as a functional polymer, and is being used for various purposes in addition to photosensitive materials. However, as mentioned above, it is difficult to obtain a p-vinylphenol polymer that is not colored and has good light transmittance, so its uses are limited. Particularly in the field of photosensitive materials, in recent years light sources have shifted to shorter wavelengths and ultraviolet light has become the main source of light, and materials with excellent ultraviolet light transmittance are required. Furthermore, when polyp-vinylphenol is used in the field of photosensitive materials, there is a problem in that the rate of alkali dissolution during development with alkali is excessive compared to phenol novolak and the like. Furthermore, when evaluating polyp-vinylphenol as a heat-resistant material, it is desired that its decomposition temperature be even higher. Furthermore, when a thermosetting resin is formed by the reaction between polyp-vinylphenol and an epoxy resin, it may be desired that the thermosetting speed be even faster.

Therefore, one object of the present invention is to sufficiently improve the light transmittance, especially the ultraviolet light transmittance, of a p-vinylphenol-based polymer, particularly a p-vinylphenol-based polymer obtained from crude p-vinylphenol. The purpose is to provide a method that can be easily and economically improved. Another object of the present invention is to provide a method for improving the light transmittance of a p-viniczenol polymer, as well as its various properties such as solubility, thermal decomposition, and reactivity. Still another object of the present invention is to provide a method for hydrogenating a p-vinylphenol polymer capable of achieving the above-mentioned improvements.

(l! Means to Solve the Problem) The present inventors have discovered that the above object can be achieved by bringing p-vinylphenol polymer into contact with hydrogen under certain conditions. The light transmittance, especially the light transmittance in the ultraviolet region, can be greatly and easily and economically improved, and by selecting the hydrogenation conditions, some of the phenol nuclei of the p-vinylphenol-based polymer can be improved. By hydrogenating the nuclei and converting them mainly into cyclohexanol nuclei, the physical and chemical properties of the polymer can be improved, such as controlling the alkali solubility of the polymer, improving thermal decomposition properties, and increasing the reactivity with epoxy resin. The present invention was completed after discovering that improvements could be made. That is, the gist of the present invention is to prepare a p-vinylphenol-based polymer in the presence of a Group Ⅰ metal catalyst.
The present invention relates to a method for improving the properties of a p-vinylphenol polymer, which comprises contacting it with hydrogen at a temperature of 0 to 300C.

Examples of p-vinylphenol polymers to which the method of the present invention is applied include polyp-vinylphenol and copolymers of p-vinylphenol and other comonomers. , styrenic monomers such as methylstyrene, acrylic monomers such as methyl acrylate, ethyl acrylate, methyl methacrylate, hydroxymethyl acrylate, and hydroxyethyl acrylate. These p-vinylphenol polymers are disclosed in, for example, JP-A-60-584.
Heat treatment for reforming (120-350C) as shown in No. 07 or JP-A No. 60-258206
There is no problem even if it is applied.

Furthermore, the method of the present invention can be applied to p-vinylphenol-based polymers obtained by various methods, and there is no need to particularly limit the origin of the production. It is preferably applied to p-vinylphenol-based polymers obtained from unpurified crude p-vinylphenol containing chain-free phenols and other by-products, and the effects of the method of the present invention are obtained when applied to such polymers. becomes even more pronounced. For example, p-ethylphenol is dehydrogenated by contacting it with a dehydrogenation catalyst such as iron oxide, zinc oxide, magnesium oxide, chromium oxide, tin oxide, titanium oxide, or barium oxide at high temperature, The resulting crude p-vinylphenol containing unreacted p-ethylphenol and by-products was treated with boron trifluoride,
Cationic initiators such as inorganic acids such as aluminum trichloride, sulfuric acid, hydrochloric acid, organic acids such as formic acid, chloroacetic acid, Cheryl acid, organic sulfonic acids such as methanesulfonic acid, toluenesulfonic acid, etc., or benzoyl peroxide, acetyl peroxide. ,
Polymerization in the presence of a polymerization accelerator such as a radical initiator such as dibenzoyl disulfide, azobisisobutyronitrile, etc., or thermal polymerization without using a polymerization accelerator,
If the method of the present invention is applied to polyp-vinylphenol obtained by removing p-ethylphenol and other light components from the polymerization reaction product, the light transmittance of the polyp-vinylphenol will be significantly improved, PolyJ p-vinylphenol having excellent light transmittance can be obtained efficiently and economically. Furthermore, if a styrene-based or acrylic-based comonomer is added to the crude p-vinylphenol obtained by the dehydrogenation of p-ethylphenol and polymerized in the presence of a polymerization accelerator, p-vinylphenol and these comonomers can be polymerized. A copolymer with a comonomer is obtained (see Japanese Patent Application No. 61-275307 and Japanese Patent Application Laid-Open No. 61-291606), and if the method of the present invention is applied to this polymer, a copolymer with excellent light transmittance can be obtained. It can be obtained efficiently and economically. In this case, the charging ratio of p-vinylphenol and comonomer may be appropriately selected depending on the desired physical properties and intended use of the copolymer, but is generally selected from the range of 1:9 to 9:1. .

In hydrogenating a p-vinylphenol polymer according to the method of the present invention, a solvent is generally used, and the polymer is dissolved in the solvent and brought into contact with hydrogen in the presence of a Group I metal catalyst. This solvent is preferably one that dissolves the p-vinylphenol polymer, is stable during hydrogenation treatment, and is easily removed after hydrogenation treatment, such as lower alcohols such as methanol, ethanol, propatool, butanol, etc.
Lower ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, and cyclic ethers such as tetrahydrofuran and dioxane are suitable, and phenols such as cresol and ethylphenol can also be used, with lower alcohols being particularly preferred.

The Group 1 metal of the Group 1 metal catalyst used in the method of the present invention includes nickel (Ni), copal (Co), palladium (Pd), platinum (Pt), or rhodium (Rh).
is appropriate. Its form may be either a simple metal or supported on a metal oxide carrier. Usually, Ni and Co are used as Raney type metals alone, or in silica, alumina,
It is used by being supported on a porous carrier such as carbon or diatomaceous material. Noble metals are usually used in the form of oxides or supported on carriers. The amount of catalyst used varies depending on the type of catalyst used, but generally speaking, the following amounts are appropriate. That is, the method of the present invention can be carried out either in a batch method using an autoclave or the like, or in a continuous method such as a fixed bed flow method, but in the case of a batch method, the amount of metal in the polymer to be treated is 0.01 to 10%. The weight ratio is appropriate, and in the case of a continuous flow type, the WH8V of the polymer relative to the catalyst is 0.1.
~10 ky/ky-hr is appropriate. Also,
In the case of continuous flow type, the catalyst is usually a carrier-supported catalyst.

The temperature of the hydrogenation treatment is suitably 50 to 300C, preferably 150 to 250C. Temperature affects the effect, and at low temperatures below 50C, the reaction does not proceed properly,
It is less effective, and at too high a temperature, polymer decomposition or gelation occurs. The appropriate hydrogen pressure is 10 to 200 kP/cm2, considering the reaction rate, pressure resistance of the device, etc.
Preferably it is 50 to 100 kg/lyn". The treatment time depends on the type and amount of catalyst used, treatment conditions such as treatment temperature,
Further, the time may be arbitrarily selected depending on the characteristics of the polymer to be treated, but generally speaking, about 0.1 to 10 hours is appropriate.

In the hydrogenation treatment of the p-vinylphenol-based polymer according to the present invention, the hydrogenation treatment conditions are usually selected appropriately from the range of treatment conditions described above, so that the ultraviolet rays in the polymer can be It is possible to mainly improve the light transmittance of the polymer by reducing and removing impurity components that absorb When is a copolymer with a styrene comonomer, aromatic nuclei such as benzene nuclei originating from this comonomer are hydrogenated to improve the light transmittance of the polymer, as well as improve its solubility, thermal decomposition properties, and reactivity. Furthermore, K can also adjust the degree of improvement in these characteristics. The occurrence of nuclear hydrogenation of aromatic nuclei in the hydrotreatment according to the present invention is as follows:
Although it depends on the various processing conditions employed, there are the following trends.

That is, the nuclear hydrogenation of aromatic nuclei is affected by the type of solvent used, and as a solvent, primary alcohols such as methanol, ethanol, n-propanol, acetone,
When lower ketones such as methyl ethyl ketone and methyl imbutyl ketone or phenols such as cresol and ethyl phenol are used, nuclear hydrogenation of aromatic nuclei occurs and K is difficult. On the other hand, when a secondary alcohol such as isopropanol or secondary butyl alcohol or a cyclic ether such as tetrahydrofuran or dioxane is used as a solvent, nuclear hydrogenation of aromatic nuclei tends to occur. However, by selecting the type of solvent used, it is also possible to mix a solvent that tends to cause nuclear hydrogenation and a solvent that does not easily cause nuclear hydrogenation, such as isopropanol and methanol, and use the mixture as a solvent. By changing the ratio, the degree of nuclear hydrogenation can be adjusted. In addition, the degree of nuclear hydrogenation may be appropriately selected depending on the degree of improvement of the desired properties.
It is also possible to carry out nuclear hydrogenation of about 0%.

After the hydrogenation treatment, if a catalyst is contained, it is separated and removed from the treated product, and then the solvent is separated and removed by any method to obtain the desired polymer. Examples of this method include a method in which the treated material is poured into water to precipitate a polymer, the precipitate of the polymer is separated and dried, or a method in which the solvent is evaporated from the treated material under reduced pressure. .

(Effects of the Invention) According to the present invention, p-vinylphenol is purified by repeated recrystallization and purified by cationic polymerization at extremely low temperatures, which is a complicated and lossy method. Polyp-vinylphenol having even better light transmittance can be obtained easily and with almost no loss.

Further, according to the present invention, by selecting hydrogenation treatment conditions as necessary, impurities that selectively inhibit light transmission can be obtained without substantially changing the basic skeleton of the p-vinylphenol polymer. It is better to reduce and remove the constituent components.
The light transmittance of the polymer can be improved, and by hydrogenating the phenol nucleus at the same time as the reduction and removal of the impurity components, not only the light transmittance of the polymer but also the dissolution as described above can be improved. It is also possible to improve various properties such as properties, thermal decomposition properties, and reactivity.

The p-vinylphenol polymer that has been hydrogenated according to the present invention and has improved light transmittance or light transmittance and other properties can be used for photosensitive materials such as microphotoresists, 28 plates for offset printing, and heat-sensitive materials. Alternatively, it can be suitably used as a coating material.

(Example) The following examples will specifically illustrate the invention.

In the following examples, light transmission was measured using a spectrophotometer, and molecular weight was measured using a gel permeator chromatography (GP).
C), and the hydrogenation of the phenol nucleus was determined by IRl or IR and NMR. In addition, the extinction coefficient (-) of the polymer was calculated from the 250 nm absorption spectrum data using the following equation (1) and displayed.

To: Light transmittance of ethanol only C4) T: Light transmittance of sample solution (correspondence) C: Sample concentration (17 cm3) and cell length (wound) Example 1 p-Ethylphenol was heated with steam at a temperature of 500C.
, LH8V 1. (1) Condition "t', oxidation
The crude p-vinylphenol was obtained by dehydrogenation. The composition of this crude p-vinylphenol was approximately 35% by weight of p-vinylphenol, 60% by weight of p-ethylphenol, and 5% by weight of by-products. This crude p-
Vinylphenol was heated at 150C for 2 hours to polymerize p-vinylphenol, and the resulting reaction product was
Poly p-vinylphenol (Mw 5
,000, Mn2.500). A solution prepared by dissolving this polyp-vinylphenol 10051'- in 200 K of ethanol and a nickel catalyst with diatomaceous earth as a carrier (G-49B, manufactured by Nichiwa Girdler) were added to the inner volume of 1
! After purging the autoclave with nitrogen several times and then with hydrogen several times, the temperature was raised to a hydrogen pressure of 50 kP/cm'' with stirring and maintained at 200 C for 4 hours.

During this time, there was almost no pressure drop, so hydrogen was not replenished. After cooling, the catalyst was removed from the reaction solution by filtration, and the P solution was added to 10 times the weight of water to precipitate the polymer. The precipitated polymer was collected and dried under reduced pressure at 50C. of polymer was obtained. This polymer was subjected to analysis and the results are shown in Table 1.

Table 1 Polygo 112 25 used as raw material for hydrogenation treatment
Extinction coefficient at 0 nm (same in the following examples) As is clear from Table 1, the extinction coefficient at 250 nm of the hydrogenated polymer is significantly reduced compared to the raw material polymer, and the change in molecular weight is extremely small. very few, I
No change in the R spectrum was observed.

The ultraviolet absorption spectra of the raw polymer and the hydrogenated nine polymers are shown in FIG. In FIG. 1, B is the raw material polymer, and B is the hydrogenated polymer. Both sample concentration 10"77cm"
, measured under the condition that the quartz cell length is 1 sub (the same applies to the following examples).

Example 2 Poly p-vinylphenol (Mw 6,500
, Mn 3,500) was obtained, and this was disclosed in
According to the modification method of No. 407, heat treatment was carried out at 250C for 4 hours to produce modified boiler IJ p-vinylphenol (Mw 23.00
0, Agata 4.2005 was obtained. This modified polyp-vinylphenol 1001 was treated in the same manner as in Example 1, except that a palladium catalyst with alumina as a carrier (manufactured by Engelhardt Co., Ltd., palladium loading amount: 5% by weight) was used as a catalyst. A hydrotreated polymer was obtained. Its characteristics are shown in Table 2.

Table 2 The ultraviolet absorption spectra of the raw material modified polymer and the hydrogenated polymer are shown in FIG. A and B in the figure and measurement conditions are the same as in FIG. 1.

Example 3 Crude p- obtained in the same manner as in Example 1 and having a similar composition
Vinylphenol and commercially available styrene were mixed so that the molar ratio of p-vinylphenol and styrene was 7:3, and the mixture was used as a polymerization accelerator, and 1 weight of azobisisobutyronitrile was added to the mixture. Engagement, 2 at 100C
Copolymerization of p-vinylphenol and styrene was carried out by distilling off light components such as p-ethylphenol from the reaction product under conditions of 250C and 10 fin Hg.
- Molar ratio of vinylphenol units to styrene units: 68:
32, MW 3,400, Mn 2,300) was obtained. A solution of this copolymer Zoo fP in ethanol 200F and the same palladium catalyst 2 as used in Example 2 were used. was put into a No. 11 autoclave, and after purging with nitrogen several times and hydrogen several times, the hydrogen pressure was increased to 50 ky/cm.
In step 2, the temperature was raised while stirring and maintained at 180C for 4 hours. During this time, there was almost no pressure drop, so hydrogen was not replenished. After cooling, the catalyst was separated from the reaction solution by F, and the solvent was removed from the P solution under reduced pressure using a rotary evaporator to obtain 97% of the hydrogenated copolymer as a solid content. Its properties are shown in Table 3.

Table 3 The ultraviolet absorption spectra of the raw material copolymer and the hydrogenated copolymer are shown in FIG. A and B in the figure and measurement conditions are the same as in FIG. 1.

It was confirmed from the following that the polymer obtained from the mixture of crude p-vinylphenol and styrene was a copolymer of p-vinylphenol and styrene.

The IR spectrum of the polymer has 3100-3700on
``Absorption of KOH, 1 at 690 and 750 tyn-''
Absorption of aromatic C-H was observed, and the C-NMR spectrum had absorption of C at the position shown by the following formula (In).

H1: 1l52-154pp, 2:1l12
-114pp.

3: 127 ppm, 4:
133-135ppm.

5:336-47pp, 6:1l2
4-125pp.

7.8:1l25-130pp, 9:1l42-1
45pp.

From the above results, the polymer was confirmed to be a copolymer of p-vinylphenol and styrene. Furthermore, the molar ratio of p-vinylphenol and styrene was calculated from the absorption intensity of the 13C-NMR spectrum.

Example 4 Methyl methacrylate was used instead of styrene in Example 3, and p-vinylphenol methyl methacrylate was mixed in a molar ratio of 1:1, and polymerization and post-treatment were carried out under the same conditions as in Example 3. , a copolymer of p-vinylphenol and methyl methacrylate (molar ratio of p-vinylphenol units and methyl methacrylate units: 50:
50. Mw9.800. Mn 3,700) was obtained.

100% of this copolymer was dissolved in 200% of ethanol, and a solution and 3% of the same N,i catalyst used in Example 1 were used.
The autoclave was filled with nitrogen and then replaced with hydrogen. The hydrogen pressure was set to 50 kp/cm2, the temperature was raised with stirring, and the temperature was maintained at 180 C for 4 hours.

Hydrogen replenishment was not carried out. After cooling, the catalyst was separated from the reaction solution, and the solvent was removed from the P solution under reduced pressure using a rotary evaporator to obtain a hydrogenated copolymer as a solid. Its characteristics were as shown in Table 4.

Table 4 Example 5 Crude p-vinylphenol having the same composition as that used in Example 1 was polymerized at a temperature of 80C using Scheler's acid as an accelerator, and p-ethylphenol was obtained from the reaction product obtained. Other light components were removed to obtain poly p-vinylphenol (Mwl, 800°Mn 1,200). This poly p-vinylphenol 1oOF isopropanol 200? A nickel catalyst (manufactured by Hibi Girdler, G-49B) using a solution dissolved in
) Using 3P, hydrogen was sealed and the reaction pressure was 60kll/cm.
The mixture was stirred for 3 hours at a temperature of 2001:. The polymer was then treated in the same manner as in Example 1 to obtain a hydrogenated polymer. Its properties were as shown in Table 5.

As is clear from Table 5, the hydrogenated polymer had a significantly reduced extinction coefficient at 250 nmlC compared to the raw material polymer. In addition, a decrease in solubility in alkaline solutions and a 10% increase in decomposition temperature were observed. Furthermore, an increase in the curing reaction rate with bisphenol ethane diglycidyl ether, a typical epoxy resin, was observed.

The IR spectrum of the hydrogenated polymer is shown in FIG. An absorption not seen in the raw material polymer was observed at 1040 crn-', and this was attributed to absorption based on alcohol C-0K, indicating that nuclear hydrogenation of the phenol group had occurred.

The 13C-NMR spectrum of the hydrogenated polymer is shown in FIG.

In FIG. 5, absorption based on the following structure was observed.

OHOH a, b, c, d, js js k: ~47ppm
, e: 138 ppm, f: 129 p
pmg: 116ppm, h: 156p
pm1: 71ppm.

Therefore, the polymer obtained by hydrogenation treatment is a partially hydrogenated polyp-vinylphenol having phenol groups and cyclohexanol groups, and from the absorption intensity ratio, the ratio of phenol units/cyclohexanol units is 71/29 (
molar ratio).

Example 6 Polyp-vinylphenol 100, the same raw material as Example 5
P with impropa tool 17o? and methanol 30?
A hydrogenated polymer was obtained by dissolving it in a mixed solvent and carrying out the same treatment as in Example 5.

Its characteristics are shown in Table 6.

Table 6 As is clear from Table 6, hydrogenated Bo IJ
The extinction coefficient at 250 nm was significantly reduced relative to the mother-source polymer. The IR spectrum of the hydrogenated polymer shows an absorption that is not seen in the 104104O''K raw material polymer, and this is attributed to the absorption based on alcohol C-0, indicating that nuclear hydrogenation of the phenol group has occurred. found.

The nuclear hydrogenation rate was calculated from the C-NMR spectrum of the hydrogenated polymer to be 6%.

Therefore, it was found that the nuclear hydrogenation rate was reduced by using a mixed solvent with methanol added to Improper Tool.

[Brief explanation of the drawings] Figure 1 shows the ultraviolet absorption spectra of the raw polymer and hydrogenated polymer in Example 1, and Figure 2 shows the ultraviolet absorption spectra of the raw polymer and hydrogenated polymer in Example 2. Figure 3 shows the ultraviolet absorption spectra of the raw material copolymer and the hydrogenated copolymer in Example 3, respectively. In Figures 1 to 3, A shows the ultraviolet absorption spectrum of the raw material polymer or copolymer, and B shows that of the hydrogenated polymer or copolymer. FIG. 4 is an IR spectrum of the hydrogenated polymer in Example 5, and FIG. 5 is a C-NMR spectrum of the hydrogenated polymer in Example 5. Patent applicant Maruzen Petrochemical Co., Ltd.

Claims (3)

[Claims] (1) A method for improving the properties of a p-vinylphenol polymer, which comprises contacting the p-vinylphenol polymer with hydrogen at a temperature of 50 to 300°C in the presence of a Group VIII metal catalyst. . (2) p-vinylphenol-based polymer is p-vinylphenol obtained from unpurified crude p-vinylphenol as a raw material.
The method according to claim 1, which is a vinylphenol-based polymer. (3) The method according to claim 1 or 2, wherein the method is carried out in the presence of at least one solvent selected from lower alcohols, lower ketones, cyclic ethers, and phenols.