Method for producing polyacetal copolymer

Abstract

The object of the present invention is to provide: a method for manufacturing a polyacetal copolymer having a good appearance of a molded product formed by a polyacetal copolymer and small differences between each product. The object of the present invention is achieved by the following. A method for manufacturing a polyacetal copolymer, which has at least the following steps in sequence: step (A), using trioxane as the main monomer (a), using at least one selected from a cyclic formal having at least one carbon-carbon bond and a cyclic ether as a comonomer (b), adding a protonic acid polymerization initiator (c) and a molecular weight regulator (d1) according to the situation, and polymerizing a polyacetal copolymer in a polymerization machine; step (B), taking the polyacetal copolymer obtained in the aforementioned step (A) out of the polymerization machine, adding the aforementioned liquid molecular weight regulator (d2) having a boiling point lower than 200°C, and adjusting the molecular weight of the aforementioned polyacetal copolymer; and, step (C), immediately following the step of adjusting the molecular weight, deactivating the aforementioned polymerization initiator (c).

Description

Technical Field

[0001] The present invention relates to a method for producing a polyacetal copolymer which provides a molded article having excellent surface appearance. Background Art

[0002] Conventionally, as a method for producing a polyacetal copolymer, a cationic copolymerization is known in which trioxane is used as a main monomer and a compound selected from a cyclic ether having at least one carbon-carbon bond and a cyclic formal is used as a comonomer. As a cationic active initiator used in these copolymerizations, various substances such as Lewis acids and protonic acids are known.

[0003] Industrially, the above-mentioned main monomer, comonomer, polymerization initiator, and an appropriate amount of molecular weight modifier are supplied from one end of a polymerization apparatus, and a polyacetal copolymer is discharged from the other end in the form of powder. The polyacetal copolymer thus obtained is purified and then blended with stabilizers / additives for commercialization.

[0004] Polyacetal copolymers have an excellent balance of mechanical properties, chemical resistance, sliding properties, etc., and are easy to process. Therefore, they are widely used as a representative process plastic, mainly in electrical and electronic parts, automobile parts, and other various mechanical parts.

[0005] In recent years, as the scope of application of polyacetal copolymers has expanded, they are often used in thin-walled parts. However, although polyacetal copolymers have excellent fluidity and moldability, they sometimes suffer from surface roughness due to poor flow to the thin-walled part of the molded product and poor filling at the end of the cavity. The poor appearance of the molded product significantly reduces its commercial value, which is a major problem. So far, this has been addressed by designing the molding die and adjusting the molding conditions, but it is not sufficient.

[0006] As a countermeasure to the self-polymerization, a method of increasing the amount of a molecular weight modifier during polymerization of a polyacetal copolymer to improve the melt index (hereinafter also referred to as melt flow rate, MFR) has been proposed. However, the above method leads to a decrease in mechanical properties, particularly a decrease in impact resistance, and therefore, a preferred result cannot be obtained.

[0007] Patent Document 1 describes a polyacetal resin composition which, by using a mixture of two components of polyoxymethylene copolymers having different melt indexes, has high fluidity during injection molding and can produce a molded article having an improved surface condition.

[0008] In addition, Patent Document 2 describes a bulk polymerization method, in which a cationic polymerization catalyst is supplied to a mixture of trioxane and a cyclic ether and / or a cyclic formal, and then all or the remaining part of a molecular weight modifier is supplied in one or more stages. In Patent Document 2, the surface fluidity and filling degree during molding are improved, so that a good appearance and impact resistance can be obtained at the same time.

[0009] However, Patent Documents 1 and 2 require a large number of steps for blending polyacetal copolymers and precise improvement of polymerization equipment, which are difficult to implement industrially because they vary from product to product.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 50-30949

[0013] Patent Document 2: Japanese Patent Application Laid-Open No. 11-12337 Summary of the invention

[0014] Problem that the invention aims to solve

[0015] An object of the present invention is to provide a method for producing a polyacetal copolymer in which a molded article formed of the polyacetal copolymer has a good appearance and small differences among products.

[0016] Solutions for solving problems

[0017] The present inventors have conducted intensive studies to achieve the above-mentioned problems, and as a result, have found that: after a step of polymerizing a polyacetal copolymer in a polymerizer (polymerization step), the polyacetal copolymer obtained in the polymerization step is taken out of the polymerizer, and a liquid molecular weight regulator is added before the polymerization initiator is deactivated, and the molecular weight of the polyacetal copolymer is adjusted. Thus, the above-mentioned purpose can be achieved, and the present invention has been completed.

[0018] The present invention is as follows.

[0019] 1. A method for producing a polyacetal copolymer, comprising at least the following steps in sequence:

[0020] Step (A), using trioxane as a main monomer (a), at least one selected from a cyclic formal having at least one carbon-carbon bond and a cyclic ether as a comonomer (b), adding a protonic acid polymerization initiator (c) and, if necessary, a molecular weight regulator (d1), and polymerizing in a polymerization machine to produce a polyacetal copolymer;

[0021] Step (B), taking out the polyacetal copolymer obtained in the step (A) from the polymerization machine, adding a liquid molecular weight regulator (d2) having a boiling point lower than 200° C., and adjusting the molecular weight of the polyacetal copolymer; and

[0022] Step (C), which is a step of adjusting the molecular weight, is to deactivate the polymerization initiator (c).

[0023] 2. The method for producing a polyacetal copolymer according to 1 above, wherein the mass ratio of the molecular weight modifier (d1) added in the polymerization step (A) to the liquid molecular weight modifier (d2) added in the step (B) is 0:100 to 80:20.

[0024] 3. The method for producing a polyacetal copolymer according to 1 or 2 above, wherein the molecular weight modifier (d1) and the liquid molecular weight modifier (d2) are at least one selected from linear methylals, alcohols and water.

[0025] Effects of the Invention

[0026] According to the present invention, there can be provided a method for producing a polyacetal copolymer in which a molded article formed of the polyacetal copolymer has a good appearance and small differences among products. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram showing an example of a polymerization apparatus used in the production method of the present invention. DETAILED DESCRIPTION

[0028] Hereinafter, specific embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments at all and can be implemented with appropriate modifications within the scope of the purpose of the present invention.

[0029] <Materials used in the method for producing a polyacetal copolymer>

[0030] 《Main monomer (a)》

[0031] The main monomer in the present invention is trioxane. Trioxane is a cyclic trimer of formaldehyde and is generally obtained by reacting a formaldehyde aqueous solution in the presence of an acidic catalyst and then purified by distillation or the like before use.

[0032] Here, the main monomer refers to the monomer used in the largest amount among all monomer substances, preferably 80% by mass or more.

[0033] 《Comonomer (b)》

[0034] Comonomers refer to cyclic formals and cyclic ethers having at least one carbon-carbon bond and are compounds commonly used in the production of polyacetal copolymers.

[0035] Examples of the cyclic formal include 1,3-dioxolane, 1,3,6-trichloromethane, 1,4-butanediol formal, and 1,3,5-trioxepane.

[0036] Examples of the cyclic ether include ethylene oxide.

[0037] In the present invention, the amount of the comonomer (b) is preferably 0.01 to 20 parts by mass, more preferably 0.05 to 5 parts by mass, based on 100 parts by mass of trioxane.

[0038] <Protonic acid polymerization initiator (c)>

[0039] As the polymerization initiator, a protonic acid polymerization initiator known in cationic copolymerization using trioxane as a main monomer can be used. Examples of the protonic acid polymerization initiator include heteropolyacids, isopolyacids, perfluoroalkanesulfonic acid and derivatives thereof.

[0040] Heteropolyacid refers to the polyacid generated by dehydration condensation of heterogeneous oxyacids, which is a compound having a mononuclear or polynuclear complex ion formed by condensation of a condensed acid group with a specific heterogeneous element at the center and a shared oxygen atom. Homopolyacids are also referred to as isopolyacids, homonuclear condensed acids, and homogeneous polyacids, and refer to compounds of high molecular weight inorganic oxyacids formed by condensates of inorganic oxyacids of a single type of metal with a V valence or a VI valence.

[0041] Examples of the heteropolyacid include phosphomolybdic acid, phosphotungstic acid, phosphomolybdic tungstic acid, phosphomolybdic vanadic acid, phosphomolybdic tungstic vanadic acid, phosphotungstic vanadic acid, silicotungstic acid, silicomolybdic acid, silicotungstic acid, silicotungstic vanadic acid, etc. Among them, from the viewpoint of polymerization activity, the heteropolyacid is preferably selected from silicomolybdic acid, silicotungstic acid, phosphomolybdic acid, and phosphotungstic acid.

[0042] Examples of the isopolyacid include isopolytungstic acid exemplified in paratungstic acid, metatungstic acid, etc., isopolymolybdic acid exemplified in paramolybdic acid, metamolybdic acid, etc., metapolyvanadic acid, isopolyvanadic acid, etc. Among them, isopolytungstic acid is preferred from the viewpoint of polymerization activity.

[0043] Examples of perfluoroalkanesulfonic acid include trifluoromethanesulfonic acid, pentafluoroethanesulfonic acid, heptafluoropropanesulfonic acid, nonafluorobutanesulfonic acid, undecafluoropentanesulfonic acid, tridecafluorohexanesulfonic acid, pentafluoroheptanesulfonic acid, and heptadecafluorooctanesulfonic acid. In addition, examples of its derivatives include esters and anhydrides such as methyl trifluoromethanesulfonate and trifluoromethanesulfonic anhydride. Among these, trifluoromethanesulfonic acid is preferred from the viewpoint of polymerization activity.

[0044] The amount of the polymerization catalyst added is not particularly limited, but is preferably 0.1 ppm to 50 ppm, more preferably 0.1 ppm to 30 ppm, and further preferably 0.1 ppm to 15 ppm, based on the total amount of all monomers.

[0045] <Molecular weight regulator>

[0046] In the method for producing the polyacetal copolymer of the present invention, a molecular weight modifier is used to adjust the molecular weight. As the molecular weight modifier, a chain transfer agent known in cationic polymerization can be used.

[0047] 《Molecular weight regulator (d1)》

[0048] There is no particular limitation on the molecular weight modifier (d1) added in the step (A) of polymerizing the polyacetal copolymer in a polymerizer (hereinafter also referred to as the polymerization step or the step (A)), and one or more chain transfer agents known in cationic polymerization can be suitably used. In this step, the molecular weight modifier (d1) is not necessarily required and may not be added depending on the circumstances.

[0049] Examples of molecular weight modifiers (d1) include linear methylals such as methylal, acetal, and dibutoxymethane, alcohols such as methanol, ethanol, and ethylene glycol, and water. Among them, chain transfer agents that do not form unstable terminals, i.e., methylal, acetal, and dibutoxymethane, are preferred. The amount of molecular weight modifier (d1) added is preferably more than 0 ppm and less than 1500 ppm, more preferably 300 to 1200 ppm, relative to the total monomer amount.

[0050] 《Liquid Molecular Weight Regulator (d2)》

[0051] (B) In the step of taking out the polyacetal copolymer obtained in the aforementioned step (A) (hereinafter also referred to as a crude polyacetal copolymer) from the polymerization machine and adjusting the molecular weight of the obtained crude polyacetal copolymer (hereinafter also referred to as a molecular weight adjustment step or step (B)), the liquid molecular weight regulator (d2) used is a liquid having a boiling point (bp) of less than 200°C at normal pressure (1 atmosphere).

[0052] The liquid molecular weight regulator (d2) may be the same as or different from the molecular weight regulator (d1). Examples of the liquid molecular weight regulator (d2) include methylal (bp42°C), methanol (bp65°C), ethanol (bp78°C), acetal (bp88°C), water (bp100°C), dibutoxymethane (bp180°C), etc. By making the boiling point of the liquid molecular weight regulator (d2) lower than 200°C, the causes of residual odor and useless side reactions in the polyacetal copolymer can be suppressed. It should be noted that the liquid molecular weight regulator (d2) can be used alone or in combination of two or more.

[0053] The liquid molecular weight regulator (d2) can be diluted with a diluent solvent. As the diluent solvent, there is no restriction other than the boiling point, and aromatics such as benzene and toluene, ethers such as ether and dibutyl ether, and esters such as methyl formate and ethyl acetate are preferred. The concentration after dilution can be appropriately selected.

[0054] The amount of the liquid molecular weight modifier (d2) added is preferably 100 to 2000 ppm relative to the amount of the crude polyacetal copolymer. The mass ratio of the molecular weight modifier (d1) to (d2) is preferably 0:100 to 80:20, more preferably 20:80 to 60:40.

[0055] <Inactivating agent (e)>

[0056] In the process of deactivating the protonic acid polymerization initiator of the present invention (hereinafter also referred to as the deactivation process or the (C) process), a deactivator (e) can be used. The deactivator is not particularly limited, and preferably comprises at least one selected from amine compounds, compounds having a quaternary ammonium group, triazine ring-containing compounds having an amino group or a substituted amino group, carbonates, bicarbonates or carboxylates of alkali metal elements or alkaline earth metal elements, or hydrates thereof. In addition, two or more kinds can also be used. By using the above-mentioned deactivator, melt kneading is carried out under heating, thereby providing for the deactivation of the polymerization initiator and stabilization of the unstable ends of the crude polyacetal copolymer.

[0057] Examples of the amine compound include primary amine compounds such as methylamine, ethylamine, propylamine, isopropylamine, and butylamine, secondary amine compounds such as dimethylamine, diethylamine, dipropylamine, diisopropylamine, and dibutylamine, and tertiary amine compounds such as trimethylamine, triethylamine, tripropylamine, triisopropylamine, and tributylamine. Compounds having a quaternary ammonium group include choline hydroxide and the like.

[0058] Examples of the triazine ring-containing compound having an amino group or a substituted amino group include melamine, methylolated melamine, methoxymethyl melamine, benzoguanamine, and the like.

[0059] Examples of carbonates, bicarbonates or carboxylates of alkali metal elements or alkaline earth metal elements or hydrates thereof include sodium formate, sodium acetate, potassium carbonate, sodium carbonate, sodium bicarbonate, disodium succinate, sodium laurate, sodium palmitate, sodium stearate or calcium stearate. It should be noted that it is preferred to contain at least one selected from them.

[0060] The amount of the deactivator added is not particularly limited, but is preferably 0.002 to 5.0% by mass based on the crude polyacetal copolymer.

[0061] <Method for producing polyacetal copolymer>

[0062] The method for producing a polyacetal copolymer of the present invention is characterized in that it comprises at least the following steps in sequence: step (A), using trioxane as a main monomer (a), using at least one selected from cyclic formal and cyclic ether having at least one carbon-carbon bond as a comonomer (b), adding a protonic acid polymerization initiator (c) and, if necessary, a molecular weight regulator (d1), and polymerizing the polyacetal copolymer in a polymerizer; step (B), taking out the polyacetal copolymer obtained in the step (A) from the polymerizer, adding a liquid molecular weight regulator (d2) having a boiling point lower than 200° C., and adjusting the molecular weight of the polyacetal copolymer; and step (C), immediately after the molecular weight adjustment step, deactivating the polymerization initiator (c).

[0063] 《(A) Process》

[0064] Step (A) can be carried out in the same equipment and method as the conventionally known copolymerization of trioxane. That is, it can be a batch type, a continuous type, or a semi-continuous type. It is common to use a liquid monomer and obtain a solid powdery polymer while the polymerization is in progress. In the present invention, a continuous type is preferred.

[0065] As the polymerization apparatus used in the present invention, a reaction tank with a stirrer commonly used in the intermittent type can be used. As a continuous type, a co-kneader, a twin-screw continuous extruder mixer, a twin-screw paddle type continuous mixer, and a continuous polymerization apparatus for trioxane proposed so far can be used. In addition, a combination of two or more types of polymerization apparatuses can be used.

[0066] 〔Crushing process〕

[0067] Before entering step (B), a pulverizing step may also be provided. In the pulverizing step, the crude polyacetal copolymer taken out from the polymerization machine is pulverized using a dry pulverizer. In the pulverizing step, it is preferred that the crude polyacetal copolymer taken out has a constant particle size, for example, it is preferred that the proportion of particles under the sieve when sieving using a sieve with an aperture of 11.2 mm is 90 parts by weight or more. As a pulverizer, a general dry pulverizer may be used, specifically, there are: jet type, hammer type, ball type, roller type, rod type, horizontal type, etc.

[0068] 《(B) Process》

[0069] Step (B) is a step of allowing a liquid molecular weight regulator (d2) having a boiling point of less than 200°C to act on the powdered crude polyacetal copolymer taken out of the polymerization machine. There are methods such as using a liquid pump to drip or spray the crude polyacetal copolymer, or dripping or liquid spraying followed by homogenization in a mixer having a stirring and mixing function. When the amount of the liquid molecular weight regulator (d2) is small, it may be diluted with a solvent having a boiling point of less than 200°C.

[0070] In this process, the surface of the crude polyacetal copolymer particles obtained in the (A) process contacts the liquid molecular weight regulator (d2), so that the reaction with the molecular weight regulator is promoted, causing a decrease in molecular weight. On the other hand, the molecular weight is not reduced inside the particles that do not contact the liquid molecular weight regulator (d2) and do not cause a reaction. In addition, there is a distribution of sizes in the particle size, so the molecular weight of small particles is easy to reduce, and the molecular weight of large particles is not easy to reduce. As a result, the crude polyacetal copolymer after the (B) process has a molecular weight distribution different from that of the crude polyacetal copolymer obtained in the (A) process.

[0071] 《(C) Process》

[0072] The process is as follows: a deactivator (e) is added to the crude polyacetal copolymer after adjusting the molecular weight obtained in the step (B), and melt-kneading treatment is performed to deactivate the non-volatile protonic acid polymerization initiator. This process can be performed by a conventionally known method. There is no particular limitation on the melt-kneading treatment device, which has the function of kneading the molten copolymer, preferably has a vent function, for example, a single-screw or multi-screw continuous extruder kneader, a co-kneader, etc. having at least one vent. The melt-kneading treatment is preferably performed in a temperature range of above the melting point of the copolymer to 260°C.

[0073] In the present invention, the above-mentioned melt kneading treatment is preferably carried out in the presence of an antioxidant. As the antioxidant, a known substance used as a stabilizer for existing polyacetal resins, such as various hindered phenol antioxidants, is used. The antioxidant is preferably added in an amount of 0.05 to 0.5 mass % of the crude polyacetal copolymer.

[0074] Furthermore, in this stage, known substances as stabilizers for various polyacetal resins may be added as needed. Inorganic fillers such as glass fibers, crystallization accelerators (nucleating agents), mold release agents, antioxidants, etc. may also be added.

[0075] As described above, the deactivator (e) is added to the crude polyacetal copolymer subjected to molecular weight adjustment in step (B), and melt-kneading is performed. Formaldehyde gas, unreacted monomers, oligomers, molecular weight regulators, etc. generated by decomposition are usually removed from the exhaust port of the extruder under reduced pressure, and the copolymer is molded into pellets, etc., to obtain a product for resin processing. The pellets may be dried as needed. The drying conditions for the pellets are, for example, 135° C. for 4 hours.

[0076] Example

[0077] Hereinafter, the present invention will be specifically described based on Examples, but the present invention is not limited thereto.

[0078] The polyacetal copolymers of Examples and Comparative Examples were prepared by the following polymerization steps. The materials and amounts used are shown in Table 1. The abbreviations representing the materials shown in Tables 1 and 2 refer to the following compounds.

[0079] Main Monomer

[0080] Trioxane: TO

[0081] 《Comonomer》

[0082] 1,3-Dioxolane: DO

[0083] Butanediol Methylal: BDF

[0084] 《Polymerization Initiator》

[0085] Phosphomolybdic acid: PMA

[0086] Phosphotungstic acid: PWA

[0087] Trifluoromethanesulfonic acid: TFMSA

[0088] Boron trifluoride ether complex: BF3 / OEt2

[0089] 《Molecular weight regulator》

[0090] Methylal: DMM

[0091] Acetal: DEM

[0092] Dibutoxymethane: DBM

[0093] Methanol: ME

[0094] Acetic acid: AA

[0095] Water: H2O

[0096] PD: 1,5-pentanediol (bp242℃)

[0097] 《Dilution solvent》

[0098] Toluene: TOL

[0099] <Manufacturing process>

[0100] <<(A) Polyacetal copolymer polymerization process>>

[0101] The polymerizer uses a continuous twin-shaft paddle screw extruder. A jacket is provided on the outside of the main body for passing a medium for heating or cooling. The main body is divided into an upper and lower structure, and the upper part is open. Inside the polymerizer, two rotating shafts with a large number of paddles for stirring and propulsion are provided along the length direction.

[0102] In the jacket, the temperature is adjusted by a medium at 80°C, and a mixed solution containing the main monomer (a), comonomer (b) and molecular weight regulator (d1) in the mass parts shown in Table 1 is continuously supplied, and a polymerization initiator (c) is added thereto (relative to all monomers, unit: mass ppm). Phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) as polymerization initiators are added in the form of methyl formate solution, and trifluoromethanesulfonic acid (TFMSA) is added in the form of cyclohexane solution. The polymerization reaction is confirmed to be in a steady state by stabilizing the temperature and discharge amount of each part of the reaction device.

[0103] 《(B) Molecular weight adjustment step》

[0104] A liquid molecular weight regulator (d2) is added to the crude polyacetal copolymer discharged from the polymerization machine by a liquid pump and then introduced into a mixer ( Figure 1 2: stirring and mixing device), mixed with the crude polyacetal copolymer and reacted.

[0105] 《(C) Inactivation process》

[0106] 0.1 mass % of melamine as a deactivator (e) and 0.3 mass % of triethylene glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)propionate] as an antioxidant were added to the crude polyacetal copolymer whose molecular weight was adjusted in step (B), and the mixture was melt-kneaded and extruded continuously using a twin-screw extruder with a vent at a temperature of 220° C. and a vacuum degree of 5 mmHg at the vent portion to prepare pellets of the polyacetal copolymer resins of Examples and Comparative Examples.

[0107] <Evaluation>

[0108] The pellets of the polyacetal copolymers of Examples and Comparative Examples were dried at 135° C. for 4 hours, and then the appearance of the molded products was evaluated as described below. The results are shown in Tables 1 and 2.

[0109] [Evaluation of molded product appearance]

[0110] Using the injection molding machine "J75SS2A (Φ35)" (manufactured by Japan Steel Works, LTD.), a 50 mm square 3 mm t flat plate with a Φ1.5 mm central pin-shaped gate was injection molded under the following conditions. Afterwards, the size of the flow mark near the gate of the molded product was measured and the appearance was evaluated visually. The level of the product that was better than the best was recorded as A, the level that was worse than the best but usable was recorded as B, and the level that was unusable was recorded as C.

[0111] Molding conditions: Barrel temperature: Nozzle-C1-C2-C3

[0112] 200-200-170-150℃

[0113] Mold temperature: 90℃

[0114] Holding pressure: 750kg / cm 2

[0115] Injection time 4.5 seconds

[0116] Injection conditions: Measuring: 25-20-8mm

[0117] Speed: 25-2.5mm / sec

[0118] [Table 1]

[0119]

[0120] [Table 2]

[0121]

[0122] As can be seen from the evaluation results in Tables 1 and 2, the molded products were excellent in appearance within the technical scope of the present invention.

[0123] Description of Reference Numerals

[0124] 1 Aggregation Machine

[0125] 2. Stirring and mixing device

[0126] 3 Feeder

[0127] 4 Melt mixing extruder

[0128] 5 pellets

[0129] (a) Trioxane

[0130] (b) Comonomer

[0131] (c) Polymerization initiator

[0132] (d1) Molecular weight regulator

[0133] (d2) Liquid molecular weight regulator

[0134] (e) Inactivating agent

[0135] AO antioxidant.

Claims

1. A method for producing a polyacetal copolymer, comprising at least the following steps in sequence: Step (A), using trioxane as a main monomer (a), at least one selected from a cyclic formal having at least one carbon-carbon bond and a cyclic ether as a comonomer (b), adding a protonic acid polymerization initiator (c) and, if necessary, a molecular weight regulator (d1), and polymerizing in a polymerization machine to produce a polyacetal copolymer; Step (B), taking out the polyacetal copolymer obtained in the step (A) from the polymerization machine, adding a liquid molecular weight regulator (d2) having a boiling point lower than 200° C., and adjusting the molecular weight of the polyacetal copolymer; and Step (C), following the step of adjusting the molecular weight, deactivating the polymerization initiator (c), The mass ratio of the molecular weight regulator (d1) added in the step of polymerization (A) to the liquid molecular weight regulator (d2) added in the step of polymerization (B) is 0:100 to 80:

20.

2. The method for producing a polyacetal copolymer according to claim 1, wherein The molecular weight regulator (d1) and the liquid molecular weight regulator (d2) are at least one selected from linear methylals, alcohols and water.

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