JPS6214165B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing stable polyacetals. In more detail, the number average molecular weight is
10,000 or more and a compound selected from the group consisting of alkylene oxides and cyclic formals are reacted in the presence of a coordination compound of a Lewis acid and a specific dialkyl ether to improve thermal stability and base stability. The present invention relates to a method for producing a stable polyacetal having excellent stability in terms of properties and the like. Japanese Patent Publication No. 43-26872 describes a method of reacting polyoxymethylene with 1,3-dioxolane, and Japanese Patent Publication No. 48-3711 describes a method of reacting polyoxymethylene with 1,3,6-trioxokene. A method is described. Furthermore, Japanese Patent Publication No. 44-27667 discloses a method of reacting polyoxymethylene and ethylene oxide in an acidic reaction medium. In these documents, a Lewis acid catalyst is used for the reaction of polyoxymethylene. In the reaction of polyoxymethylene using a typical Lewis acid, the following main chain scission reaction called hydride shift occurs frequently, and the molecular weight of the polymer usually decreases. Hydride shift reaction (hydrogen abstraction reaction) (〓〓 indicates an oxymethylene chain.) In other words, a polymer consisting of an oxymethylene chain is formed by a Lewis acid with a methoxy group (-
OCH ₃ ) and a formate group (-OCHO) at the end of the polymer. Since this hydride shift reaction is difficult to suppress, it has become extremely difficult to obtain high molecular weight polymers in the reaction of polyoxymethylene using a Lewis acid catalyst. As a result of a deep study on the reaction of polyoxymethylene, the present inventors found that hydride shift during the reaction can be suppressed only when a coordination compound of a Lewis acid and a certain dialkyl ether is used as a catalyst. This discovery led to the completion of the present invention. That is, the present invention provides 100 parts by weight of polyoxymethylene having a number average molecular weight of 10,000 or more, and

[Formula] (R ₀ ; hydrogen, alkyl group, substituted alkyl group, aryl group, substituted aryl group, each of which may be the same or different; m = 2 to 6); alkylene oxide; 0.01-200 parts by weight and general formula Compound selected from the group consisting of cyclic formals represented by (R ₁ ; alkyl group, aryl group) 0.01
~200 parts by weight, Lewis acid, general formula R ₁ OR ₁ (R ₁ : carbon number 1 ~
6 alkyl groups, each of which may be the same or different. ), and the pKa value at 20℃ is between -3.7 and -5.5.
React in the presence of 0.0005 to 2 parts by weight, or 1 x 10 ^-7 to 1 x 10 ^-1 mol/based on Lewis acid when using an organic medium, at a temperature of -20°C to 230°C and a time of 5 seconds to 300 minutes. To provide a method for producing stable polyacetal characterized by The present invention will be specifically explained below. The polyoxymethylene used in the present invention is a polymer having repeating oxymethylene units (-CH ₂ O-), and is a polymer in which both ends of the polymer are hydroxyl groups, or one or both ends of the polymer are This is a polymer blocked using a known method. Esterification methods, urethanization methods, etherification methods, and the like are known as methods for blocking polymer terminals. In addition, the molecular weight (number average molecular weight) of polyoxymethylene used as a starting material is at least
It is necessary that it is 10,000 or more, but considering the practical performance of the polyacetal obtained as a result of the reaction,
More preferably, it is 30,000 or more. The upper limit of the molecular weight is mainly determined by the desired molecular weight of the polyacetal and, if a molecular weight regulator is used, the amount of the molecular weight regulator used, but it is more preferably 500,000 or less. The compound to be reacted with polyoxymethylene in the present invention is selected from alkylene oxides and cyclic formals. As alkylene oxide, the general formula (R ₀ : hydrogen, alkyl group, substituted alkyl group, aryl group, substituted aryl group, each of which may be the same or different; m = 2 to 6) This comes first. For example, ethylene oxide, propylene oxide, butylene oxide, trans-2,3-epoxybutane, cis-2,3-epoxybutane, isobutylene oxide, oxetane, 3,3-bis(chloromethyl)oxetane, tetrahydrofuran, 2-methyltetrahydrofuran, oxepane , styrene oxide, p-chlorostyrene oxide, epichlorohydrin, etc. are included in this group. In addition to these compounds, cyclohexene oxide and the like can also be used. Among these alkylene oxides, ethylene oxide, propylene oxide, butylene oxide, styrene oxide, and cyclohexene oxide are preferred, and ethylene oxide is particularly preferred. As a cyclic formal, the general formula The first examples include cyclic alkylene glycol formals having the structure represented by (p=2 to 10), such as ethylene glycol formal, 1,3-propanediol formal, 1,4-butanediol formal, and 1,5-pentanediol. Formal, 1,6-hexanediol formal,
1,7-heptanediol formal, 1,8-
Octanediol formal and 1,10-decanediol formal are included in this group. Among these cyclic formals, ethylene glycol formal, 1,4-butanediol formal, 1,5-pentanediol formal and 1,6-hexanediol formal are particularly preferred. As a cyclic formal, the general formula The second example is cyclic polyethylene glycol formal having a structure represented by (q=2 to 15), such as diethylene glycol formal, triethylene glycol formal, tetraethylene glycol formal, hexaethylene glycol formal, decaethylene glycol formal, and polyethylene glycol formal. −200 formal etc. are included in this group. Among these cyclic formals, diethylene glycol formal, triethylene glycol formal and tetraethylene glycol formal are particularly preferred. In addition, in the present invention, the general formula A cyclic formal having a structure represented by (R ₁ :alkyl group, aryl group) can also be used.
For example, propylene glycol formal, butylene glycol formal, styrene glycol formal, etc. are included in this group. In the present invention, a coordination compound of a Lewis acid and a dialkyl ether is used as a catalyst. Here, the coordination compound is a dialkyl ether in which the oxygen serves as an electron donor.
On the other hand, a Lewis acid is defined as a compound that serves as an electron acceptor. Formation of a coordination compound can be confirmed using means such as infrared absorption spectroscopy, ultraviolet absorption spectroscopy, nuclear magnetic resonance spectroscopy, and the like. Further, the coordination compound used in the present invention is synthesized by adding a Lewis acid to a dialkyl ether or to an organic medium containing a dialkyl ether. The first group of Lewis acids that can form coordination compounds with dialkyl ethers include tin tetrachloride,
tin tetrabromide, titanium tetrachloride, aluminum trichloride,
Examples include Friedel-Crafts type compounds such as zinc chloride, vanadium trichloride, phosphorus pentafluoride, antimony trichloride, antimony pentafluoride, boron trifluoride, boron tribromide, and boron trichloride. The second group of Lewis acids includes perchloric acid, acetyl perchlorate, t-butyl perchlorate, hydroxyacetic acid, trichloroacetic acid,
There are inorganic acids and organic acids such as trifluoromethanesulfonic acid and p-toluenesulfonic acid. A third group of Lewis acids includes composite salt compounds such as triphenylmethylhexafluoroantimonate, allyldiazonium hexachlorophosphate, and triethyloxonium tetrafluoroborate. A fourth group of Lewis acids includes organometallic compounds such as triethylaluminum, diethylaluminum chloride, aluminum isopropoxide, diethylzinc, and triethoxyboron. In addition to these four groups, molybdenum oxide acetylacetonate and the like can also be used as Lewis acids. Among these Lewis acids, Friedel-Crafts type compounds are preferred, and boron trifluoride is particularly preferably used. The dialkyl ether to form a coordination compound with a Lewis acid has the general formula R ₁ OR ₁ (R ₁ : carbon number 1 to 6
and may be the same or different. ) at 20℃
Compounds with pKa values between -3.7 and -5.5. Here, the pKa value of the dialkyl ether was measured based on the method of Edward MA _RNETT and CHIN _{YOUG WU} , and the measurement _method and pKa value were each published in the Journal of It is disclosed in American Chemical Society, Vol. 72, p. 4999 (1960), and Vol. 84, p. 1680 (1962). When a dialkyl ether with a pKa value between −3.7 and −5.5 is used, it is possible to suppress the hydride shift during the reaction of polyoxymethylene to a low level. Specific compounds of the dialkyl ether used in the present invention are as follows. (The numbers in [ ] indicate pKa values.) Dimethyl ether [-3.83], methyl ethyl ether [-3.82], methyl n-propyl ether [-3.79], ethyl n-butyl ether [-
4.12], di-n-propyl ether [-4.40], diisopropyl ether [-4.30], di-n-butyl ether [-5.40], dialkyl ether with a pKa value of pKa>-3.7,
For example, diethyl ether [-3.59], methyl isopropyl ether [-3.47], and methyl n-butyl ether [-3.50] have a large hydride shift and are difficult to use for the purpose of the present invention. On the other hand, ethers with a pKa value of pKa<-5.5, such as anisole [-6.54], phenethole [-
6.44], phenyl n-butyl ether [-6.99],
In this case, the activity of the catalyst produced by coordination with the Lewis acid decreases. For this reason, it is necessary to increase the amount of catalyst. With increased amounts of catalyst,
Increasing the amount of catalyst is never desirable because it induces (1) an increase in hydride shift and (2) an increase in the amount of catalyst remaining in the polymer. It is an unexpected phenomenon that good results are produced only when dialkyl ethers with pKa values between -3.7 and -5.5 are used. In the present invention, dialkyl ethers having a pKa value between -3.7 and -5.5 are used as described above, but among these dialkyl ethers, di-n-
Butyl ether is a preferred compound. Furthermore, among the coordination compounds of the aforementioned Lewis acid and dialkyl ether, a boron trifluoride di-n-butyl ether coordination compound is particularly preferred. In the present invention, the molecular weight of the polymer can also be arbitrarily adjusted using a molecular weight regulator. Molecular weight modifiers that can be used in the present invention include acetal compounds, orthoformates, alcohols, carboxylic acids, carboxylic acid anhydrides, and water. The first group of acetal compounds are:
There are formal compounds and hemiformal compounds represented by R ₂ OCH ₂ OR ₂ and HOCH ₂ OR ₂ (R ₂ :alkyl group). Specifically, methylal, diethoxymethane, di-iso-propoxymethane, di-n-
Examples include butoxymethane and butoxyhydroxymethane. The second group of acetal compounds are:
There is a polyacetal compound represented by _R2O ( _CH2O ) _nR2 (n=2-20). Specific examples include dioxymethylene dimethoxide, tetraoxymethylene dioctoxide, decaoxymethylene diisopropoxide, dioxymethylene diethoxide, and the like. These compounds are often difficult to isolate as single substances and are usually used as a mixture. The degree of polymerization (n) of these compounds is
It can be determined using methods such as nuclear magnetic resonance spectroscopy, gas chromatography, and liquid chromatography. The third group of acetal compounds is

There is an acetal compound represented by the formula: Specific examples include dimethyl acetal, diethyl acetal, di-iso-propyl acetal, di-t-butyl acetal, dihexyl acetal, and dioctyl acetal. Orthoformate is a compound represented by HC(OR' ₂ ) ₃ (R' ₂ : alkyl group, aryl group). Specific examples include methyl orthoformate, ethyl orthoformate, diethylpropyl orthoformate, propyl orthoformate, and phenyl orthoformate. Alcohol is a compound represented by R′ ₂ OH. Specifically, there are aliphatic alcohols such as methanol, ethanol, n-propanol, iso-butanol, lauryl alcohol and stearyl alcohol, and aromatic alcohols such as benzyl alcohol and 2-phenylethanol. Carboxylic acid is a compound represented by R ₃ COOH (R ₃ : hydrogen, alkyl group, aryl group). Specifically, there are aliphatic carboxylic acids such as formic acid, acetic acid, propionic acid, lauric acid, and stearic acid, and aromatic carboxylic acids such as benzoic acid and iso-phthalic acid. Carboxylic acid anhydride is

【formula】 【formula】

【formula】

It is a compound represented by the formula (R ₄ : hydrogen, alkyl group). Specifically, aliphatic carboxylic anhydrides such as acetic anhydride, propionic anhydride, stearic anhydride, succinic anhydride, dodecenylsuccinic anhydride, and maleic anhydride; aromatic carboxylic anhydrides such as phthalic anhydride and 4-methylphthalic anhydride; There are acid anhydrides. Among these molecular weight modifiers, formal compounds represented by R ₂ OCH ₂ OR ₂ , R ₂ O (CH ₂ O) nR A polyacetal compound represented by ₂ , and

An acetal compound represented by the formula is preferable, and methylal and diethyl acetal are particularly preferable from the viewpoint of ease of purification and availability. The reaction between polyoxymethylene, alkylene oxide, and cyclic formal is carried out without a solvent or in an organic medium. The reaction without a solvent is usually carried out by adding a catalyst to a mixture of polyoxymethylene, an alkylene oxide, and a cyclic formal, using the raw material polyoxymethylene in a solid powder state or a molten state. The reaction in an organic medium is usually carried out by adding a catalyst to the organic medium containing the raw material polyoxymethylene, alkylene oxide, and cyclic formal. The reaction without solvent includes the case where no organic medium is used at all and the case where the organic medium is used in an arbitrary ratio of 50 parts or less to 100 parts of polyoxymethylene. The reaction without a solvent is usually carried out using an extruder, a kneader such as a co-kneader, a screw conveyor, a high-speed rotation mixer, a stirring reaction tank, or the like. On the other hand, the reaction in an organic medium refers to the case where the organic medium is used in an arbitrary ratio of 50 parts or more to 100 parts of polyoxymethylene, and polyoxymethylene is often handled in the form of a slurry. The reaction in an organic medium is usually carried out using a stirred reaction tank, screw conveyor, or the like. Organic media that can be used in the present invention include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane, n-octane, cyclohexane, and cyclopentane, benzene, toluene,
Examples include aromatic hydrocarbons such as xylene, halogenated aliphatic hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride, ethylene chloride, and trichloroethylene, and halogenated aromatic hydrocarbons such as chlorobenzene and o-dichlorobenzene. These organic media may be used alone or in combination of two or more. When using a molecular weight regulator, the molecular weight regulator is uniformly dissolved or dispersed in the reaction system. The concentration of the molecular weight regulator in the system is
Depending on the requirements of the desired molecular weight of the polyacetal, it can be easily determined by experiment. The alkylene oxide and cyclic formal are used in an amount of 0.01 to 200 parts by weight, preferably 0.05 to 100 parts by weight, per 100 parts by weight of raw material polyoxymethylene. When the coordination compound of Lewis acid and dialkyl ether is used without a solvent, the raw material polyoxymethylene
0.0005 to 2.0 parts by weight per 100 parts by weight, and when using an organic medium, 1 x 10 ^-7 to 1 x based on Lewis acid
Used at a concentration of 10 ^-1 mol/- organic medium. The reaction temperature is usually set between -20 and 230°C, but preferably between 20 and 210°C in the case of no solvent, and more preferably between 0 and 120°C when using an organic medium. . There is no particular limit to the reaction time, but from 5 seconds
Set between 300 minutes. In the case of no solvent, a basic substance such as sodium hydroxide, ammonia, amine, or quaternary ammonium salt is added to the system after a predetermined period of time has passed, and the reaction is completed. It is possible to add a stabilizer etc. to the stabilized polymer as it is and use it for practical use, or after heating the stabilized polymer with water, alcohol, etc.
It is also possible to add stabilizers and the like for practical use. When an organic medium is used, the polymer is separated from the organic medium after a predetermined period of time, and a basic substance such as sodium hydroxide, ammonia, amine, or quaternary ammonium salt is added to complete the reaction. It is also possible to add a stabilizer to the stabilized polymer as it is for practical use. Alternatively, after heating the stabilized polymer with water, alcohol, etc., a stabilizer etc. can be added for practical use. By using the method of the present invention described in detail above, it has become possible to obtain stable polyacetal. The features of the present invention are listed below. It is possible to produce polyacetal with excellent stability such as thermal stability and base stability. There is little hydride shift and little decrease in molecular weight during the reaction. The molecular weight of polyacetal can be controlled arbitrarily by using a molecular weight regulator. By using a specific molecular weight regulator, it is possible to simultaneously control the molecular weight and improve stability such as thermal stability and base stability. Measured values in the following examples are based on the following measurement method. △-OOCH ₃ : Increase rate of terminal methoxy groups, determined using the modified Zeisel method, and is a measure of the occurrence of hydride shift. △-OHO: Increase rate of terminal formate group, quantification using infrared absorption spectrum, hydride
It is a measure of the occurrence of a shift. Reduced viscosity: 0.5gr/in p-chlorophenol/tetrachlorethylene (1:1 weight ratio) solution
Polymer concentration of dl, measured at 60℃. It is a measure of molecular weight. Base stabilization yield: at 5% polymer concentration in benzyl alcohol containing 1% tributylamine
Polymer recovery rate when heat treated at 150℃ for 60 minutes.
It is a measure of stability. Rv222: The amount of polymer remaining after heating at 222°C under vacuum for 50 minutes. It is a measure of stability. The present invention will be explained below with reference to some examples.
The present invention is not limited to these examples. Example 1 50 gr of fully dehydrated polyoxymethylene with a number average molecular weight of 61500 and a reduced viscosity of 3.08, substantially anhydrous toluene distilled over sodium metal.
500ml and 4mg of methylal as molecular weight regulator
was added to the flask. propylene oxide
After charging 3.8 gr into the flask, the contents of the flask were heated to 60°C. In this flask,
- Added 85 mg of boron trifluoride di-n-propyl ether coordination compound prepared by blowing boron trifluoride gas into propyl ether (coordination compound of 34 mg of boron trifluoride and 51 mg of di-n-propyl ether) The reaction was started by doing this. After maintaining the internal temperature of the flask at 60° C. for 65 minutes, 5 ml of tributylamine was added to stop the reaction. After separating the polymer,
The polymer was washed 5 times with ethanol and then vacuum dried at 60°C for 5 hours to recover 50g of polymer. The reduced viscosity of this polymer was 1.78, which was a desired value. Moreover, the base stabilization yield of this polymer is 94.5
%, and the Rv of the polymer after base stabilization was 99.2%, indicating good stability. The terminal methoxy group of this polymer was 25×10 ⁻⁵ mol/CH ₂ O 1 mol, and the starting material had 0 methoxy group. Therefore, the amount of methoxy groups (Δ-OCH ₃ ) generated by hydride shift during the reaction was 25×10 ⁻⁵ mol./mol., which was a very low level. On the other hand, when we investigated the increase in the terminal formate group, we obtained a value of △−OCHO=24×10 ^-5 mol./mol.
Due to hydride shift, −OCH ₃ and −
It was confirmed that OCHO was generated at a ratio of 1:1. Example 2 A flask was charged with 200 gr of polyoxymethylene diacetate having a reduced viscosity of 3.28, 13 gr of ethylene oxide, and 1000 ml of cyclohexane. The reaction was started by adding to the flask 330 mg of a boron trifluoride di-n-butyl ether coordination compound synthesized by bubbling boron trifluoride into di-n-butyl ether. After maintaining the contents of the flask at 72 °C for 40 min,
The reaction was stopped by adding 10 ml of IN KOH ethanol solution. Separately, washing and drying were performed to obtain 201gr of polymer. The reduced viscosity of this polymer is 2.88, the base stabilization yield is 90.8%, and the Rv of the polymer after base stabilization is 99.3.
%, and a polyacetal with good stability was obtained. △−OCHO of this polymer is 21×
10 ^-5 mol./mol., which was a very low level. Examples 3 to 13 A flask was charged with 200 gr of polyoxymethylene hydroxide having a reduced viscosity of 2.88, the alkylene oxide shown in Table 1, a cyclic formal, a molecular weight modifier if used, and 1000 ml of toluene. A catalyst solution was prepared by adding the Lewis acid shown in Table 1 to methylene chloride containing the dialkyl ether shown in Table 1, and the catalyst solution was mixed with toluene at a catalyst concentration of 1.5×10 ^-3 mol. was added to the flask to start the reaction. After maintaining the contents of the flask at 60 °C for 45 minutes,
The reaction was stopped by adding 15 ml of 10% ammonia aqueous solution. The sample was then separated, washed and dried, and the results shown in Table 1 were obtained. In all Examples, stable polyacetals having the desired molecular weight and excellent thermal stability and base stability were obtained. Furthermore, in all Examples, the hydride shift was at a low level. Comparative Example 1 All operations were performed in the same manner as in Example 2, except that 237 mg of boron trifluoride diethyl ether coordination compound was used in place of the boron trifluoride di-n-butyl ether coordination compound used in Example 2. The obtained results are the first
It is also shown in the table. When diethyl ether is used as the ether, the occurrence of hydride shift increases. Comparative Example 2 All operations were performed in the same manner as in Example 2, except that 580 mg of boron trifluoride anisole coordination compound was used in place of the boron trifluoride di-n-butyl ether coordination compound used in Example 2. The results are also shown in Table 1. When anisole is used as the ether, an increased amount of catalyst is required, and the increased amount of catalyst also increases the hydride shift.

【table】

[Table] Example 14 350 gr of polyoxymethylene diacetate, which had a reduced viscosity of 2.30 and which had both ends capped with acetic anhydride, and 58 gr of ethylene oxide were charged into an autoclave, the contents were stirred at high speed, and the temperature was raised to 80°C. tin tetrachloride
140 mg was added to 100 ml of methylene chloride in which 25 mg of dimethyl ether was dissolved to prepare a catalyst solution. The reaction was started by adding 58 ml of catalyst solution to the autoclave using a high pressure injection pump. Twenty minutes after the start of the reaction, 15 ml of tributylamine was added using a high pressure injection pump to stop the reaction. After washing the polymer with acetone, it was dried to recover 342 gr of polymer. The reduced viscosity of this polymer was 1.88. Also △−
OCHO is surprisingly low at 30×10 ^-5 mol./mol.
Hydride shift was at a low level. 2,2-methylene-bis(4-
methyl-6-tert-butylphenol) 0.25 parts,
When 0.50 parts of a polycaprolactam/polyhexamethylene adipamide/polyhexamethylene sebamide terpolymer was added and the mixture was molded, a molded product with strong physical properties was obtained. Example 15 A slurry having the following composition was poured into a screw conveyor with an inner diameter of 2 inches and a length of 4 m, which had a jacket heated to 105°C by a heating medium.
It was continuously supplied at a rate of Kg/hr. Polyoxymethylene dihydroxide (reduced viscosity
2.28) 40.0wt% 1,4-butanediol formal 4.5wt% Toluene 55.5wt% Boron trifluoride diisopropyl ether coordination compound
A sample of the polymer was sampled from the exit of the 0.01wt% screw conveyor and subjected to analysis, and the following results were obtained. Reduced viscosity 1.86 Group-stabilized yield 91.5% Rv of polymer after base-stabilized 99.3% △−OCH ₃ 33×10 ^-5 mol./mol. Hydride shift maintained at a low level even at high temperatures of 100°C or higher has been done. Example 16 Polyoxymethylene with a reduced viscosity of 3.48 (all terminals
After mixing 2500gr (45% is methoxy group, 55% is hydroxyl group), 125gr diethylene glycol formal, and 0.15gr diethyl acetal inside a sufficiently dried hopper, a 30mmφ L/D=35 equipped with a vent port is used.
It was extruded using an extruder. The time required for extrusion was 32 minutes, and the temperature of the molten resin at the die exit was 188°C. boron trifluoride
150 mg was added to 100 ml of benzene in which 300 mg of di-n-propyl ether had been dissolved to prepare a catalyst solution. This catalyst solution was forcibly added through the vent port using a metering pump at a rate of 2 ml per minute over the 32 minutes required for extrusion. The average residence time of the resin from the vent port to the die exit was 24 seconds. The molten resin coming out of the die was introduced into a 1.5% Na ₂ CO ₃ aqueous solution, and 2460 gr of polymer was recovered as a strand. The reduced viscosity of this polymer was 1.88. Moreover, △-OCHO is 45×10 ^-5 mol./mol., and △-OCHO of the polymer obtained under the same conditions using boron trifluoride diethyl ether coordination compound is 85×10 ^-5 mol./mol.
The value was low compared to mol.

Claims (1)

[Scope of Claims] 1. 100 parts by weight of polyoxymethylene having a number average molecular weight of _10,000 or more; , each may be the same or different. 0.01 to 200 parts by weight of alkylene oxide represented by m = 2 to 6) and the general formula Compound selected from the group consisting of cyclic formals represented by (R ₁ ; alkyl group, aryl group) 0.01
~200 parts by weight, Lewis acid, general formula R ₁ OR ₁ (R ₁ : carbon number 1 ~
6 alkyl groups, each of which may be the same or different. ), and the pka value at 20℃ is between -3.7 and -5.5.
React in the presence of 0.0005 to 2 parts by weight, or 1 x 10 ^-7 to 1 x 10 ^-1 mol/based on Lewis acid when using an organic medium, at a temperature of -20°C to 230°C and a time of 5 seconds to 300 minutes. A method for producing stable polyacetal that is characterized by