JPH1087706A - Production of methacrylic resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To industrially very stably obtain a methacrylic resin which is excellent in resistance to thermal decomposition, suppressed in runaway reaction caused by the gel effect and of high purity by the continuous solution polymerization and specifying the amount of the solvent to less than a certain amount. SOLUTION: In order to polymerize (A) monomers mainly comprising methyl methacrylate, (B) a solvent (suitably an alkylbenznen) is used in an amount of 0.1-10wt.% based on the whole mixture on the polymerization. In more detail, a recycled liquid mixture containing unreacting components A and B and a newly sopplied component A are continuously fed to a distillation column and the distillate monomers and component B are continuously fed into a polymerization reactor and polymerized. Then, the polymerization reaction mixture is evaporated to take out the polymer and simultaneously unreacting components A and B are recycled. At this time, the amount of the component B is preferably adjusted to 0.1-10wt.% based on the whole weight of the mixture on the polymerization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a methacrylic resin by a solution polymerization method.

[0002]

2. Description of the Related Art Methacrylic resins can be used, for example, in lighting equipment, signboards, various decorative articles, nameplates, tail lamps, etc. due to their excellent transparency, good mechanical properties, workability, and beautiful appearance of molded articles. Widely used for automobile parts and tableware. Recently, it has been widely used in optical fields such as various lenses, optical disks, and light guide plates. Therefore, advanced optical characteristics are being demanded.

[0003] As a method for producing a methacrylic resin, a suspension polymerization method is widely employed. However, the greatest disadvantage of this production method is that the suspension stabilizer used remains partially in the polymer, and the polymer properties, especially the optical properties, are degraded. As methods for improving the disadvantages of the suspension polymerization method, bulk polymerization methods (JP-A-49-37993, JP-A-3-111408, etc.) and solution polymerization methods (JP-A-63-57613, Kaihei 1-1724
No. 01 publication) has been proposed. According to these methods, unlike the suspension polymerization method, a methacrylic resin having excellent optical properties can be produced because no suspension stabilizer is used. However, bulk polymerization has a problem that it is difficult to control a stable polymerization reaction, and sometimes the diffusion of polymer molecules is restricted as the viscosity of the polymerization system increases, and the termination reaction is reduced. There is.

[0004] Furthermore, when this bulk polymerization is continuously produced on an industrial level, there is another major problem. That is, in general, when methacrylic resin is continuously industrially produced by bulk polymerization, polymerization is generally carried out using a complete mixing type polymerization reactor, but when the polymerization conversion rate is 100%, the viscosity of the polymerization liquid is increased. Is too high to stir, so that polymerization is usually carried out with the polymerization conversion rate kept at 70% or less. Therefore, a recycled liquid composed of unreacted methyl methacrylate after polymerization is reused. However, since the recycle liquid contains impurities and the like generated during polymerization, it is purified by a distillation column or a partial condenser. In the case of methacrylic resins requiring optical characteristics, it is necessary to purify the recycle solution in order to remove impurities generated during polymerization, particularly, colored impurities.

[0005] As described above, a distillation column or a partial condenser is used to purify the recycle solution.
Since it is difficult to remove impurities having a low boiling point from a partial condenser, a method using a distillation column is usually used. When purifying in a distillation column, impurities are not distilled because they have a higher boiling point than monomers such as methyl methacrylate and remain at the bottom of the distillation column, and the main component is a methyl methacrylate monomer at the bottom of the distillation column. Remove by withdrawing with bottom liquid. However, because the residence time of the bottom liquid containing methyl methacrylate monomer as a main component is long and the temperature is high, polymerization of methyl methacrylate monomer proceeds at the bottom of the distillation column, and the heat exchange efficiency in the reboiler decreases. And clogging of the piping by the generated polymer occurs. Or, in an extreme case, there is a large problem in industrially stable production such as polymerization progressing and solidifying in the entire bottom of the distillation column.

On the other hand, in the solution polymerization, since the viscosity of the reaction system is reduced by the solvent, the gel effect can be suppressed and the runaway reaction can be suppressed. Also, if a solvent having a boiling point higher than that of a monomer such as methyl methacrylate is used as a solvent, the bottom of the distillation column is mainly composed of the solvent, and the polymerization here hardly occurs, and impurities generated by the polymerization can be extracted together with the solvent. A methacrylic resin can be produced extremely stably. However, there is another major problem. It is a decrease in the thermal decomposition resistance of the methacrylic resin to be produced. When the heat decomposition resistance is reduced, decomposition occurs due to the zipper reaction of the polymer when molding and processing the methacrylic resin, and due to the decomposition products mainly composed of this methyl methacrylate monomer, silver streaks called silver streaks are formed. Occurs
There is a problem that it becomes a product defect.

[0007]

The object of the present invention is to provide a method for stably producing methacrylic resins, which are excellent in optical properties and thermal decomposition resistance, and are industrially stable.

[0008]

The present inventors have conducted intensive studies to find a method for solving the above-mentioned problems. As a result, the amount of the solvent in the continuous solution polymerization method was determined based on the weight of the whole mixture at the time of polymerization. By setting the content to 10% by weight or less, preferably 5% by weight or less, excellent thermal decomposition resistance, a runaway reaction due to a gel effect is suppressed, and in the case of a process for purifying a recycled solution using a distillation column, a distillation column is used. It has been found that impurities generated during polymerization can be stably taken out without producing a polymer at the bottom, and a methacrylic resin having a high optical purity can be produced industrially extremely stably to complete the present invention. Reached.

That is, the present invention is characterized in that in polymerizing a monomer mainly composed of methyl methacrylate, a solvent is used in an amount of 0.1 to 10% by weight based on the weight of the whole mixture at the time of polymerization. And a method for producing a methacrylic resin. Further, (A) a new monomer mainly composed of methyl methacrylate and a recycled liquid containing a monomer mainly composed of unreacted methyl methacrylate after polymerization and a solvent are continuously supplied to the distillation column. , The distilled monomer and solvent are continuously supplied to the polymerization reactor, polymerization is performed, and then the polymerization liquid is devolatilized to remove the polymer, and at the same time, the unreacted vinyl monomer and solvent are recycled and used. A continuous solution polymerization process or (B) after the polymerization, a recycle liquid containing a monomer containing unreacted methyl methacrylate as a main component and a solvent is continuously supplied to a distillation column, and the distilled methyl methacrylate is used as a main component. The monomer and solvent and the new monomer containing methyl methacrylate as the main component are continuously supplied to the polymerization reactor for polymerization, and then the polymerization solution is devolatilized to remove the polymer. Reaction methyl methacrylate In a continuous solution polymerization process in which a monomer and a solvent containing as main components are recycled, the amount of the solvent used is 0.1 to 10% by weight, preferably 0.1 to 5% by weight based on the weight of the whole mixture at the time of polymerization. The present invention relates to a method for producing a methacrylic resin, which is characterized in that it is a weight percent.

The monomer containing methyl methacrylate as a main component comprises methyl methacrylate alone or a monomer copolymerizable with methyl methacrylate. Monomers copolymerizable with methyl methacrylate are ethyl methacrylate,
Butyl methacrylate, alkyl methacrylates such as cyclohexyl methacrylate, methyl acrylate,
Acrylic acid alkyl esters such as ethyl acrylate and butyl acrylate; aromatic vinyl compounds such as styrene, vinyl toluene and α-methylstyrene; vinyl cyanides such as acrylonitrile and methacrylonitrile; N-phenylmaleimide; N-cyclohexyl Examples include maleimides such as maleimide, unsaturated carboxylic anhydrides such as maleic anhydride, and unsaturated acids such as acrylic acid, methacrylic acid, and maleic acid. Among these monomers copolymerizable with methyl methacrylate, acrylates in particular have good thermal decomposition resistance, and methacrylic resins obtained by copolymerizing acrylates have fluidity during molding. High and preferred.

The amount of the acrylate to be copolymerized with methyl methacrylate is preferably 15% by weight or less. If the content exceeds 15% by weight, the effect of improving thermal decomposition resistance, fluidity and the like is high, but the heat resistance, that is, the heat deformation temperature, is undesirably reduced. Among the acrylates, particularly, methyl acrylate and ethyl acrylate have the most remarkable improvement effect even when a small amount thereof is copolymerized with methyl methacrylate. The copolymerizable monomers can be used alone or in combination of two or more.

The solvent used in the present invention dissolves a methyl methacrylate monomer and a monomer mixture copolymerizable with the methyl methacrylate monomer and impurities to be removed at the bottom of the distillation column and inside the distillation column; Although it is not particularly limited as long as it has a higher boiling point than the methyl methacrylate monomer and the monomer copolymerizable with methyl methacrylate, usually, toluene, xylene, ethylbenzene, aromatic compounds such as diethylbenzene Octane, aliphatic compounds such as decane, alicyclic compounds such as decalin, ester compounds such as butyl acetate and pentyl acetate, 1,1,1,2-tetrachloroethane, 1,1,2,2-tetrachloroethane And the like can be used. The boiling point of the solvent used is desirably higher than the boiling points of the methyl methacrylate monomer and the monomer copolymerizable with methyl methacrylate, preferably 15 ° C. or higher, more preferably 30 ° C. or higher. Among them, toluene, xylene and ethylbenzene are particularly preferable because they can be industrially obtained at low cost, do not adversely affect the polymerization, and have high solubility of impurities generated by the polymerization.

The amount of the solvent used is 0.1 to 10% by weight, preferably 0.1 to 5% by weight, based on the weight of the whole mixture at the time of polymerization.
% By weight. When the amount of the solvent is in this range, the thermal decomposition resistance is good and there is no great difference from the bulk polymerized product. The reason for this is that one of the factors that usually lowers the thermal decomposition resistance is a polymer terminal double bond structure generated by stopping the disproportionation reaction of two polymer-grown radicals (Japanese Patent Laid-Open No. 3-294307).
No.). However, it is presumed that the terminal double bond becomes a hydrogen terminal structure due to the termination of hydrogen abstraction due to the presence of the solvent, and the generation of the terminal double bond structure due to the disproportionation reaction is suppressed.

The runaway reaction due to gelation can be suppressed even with this amount of solvent. The reason for this is that the runaway reaction due to the gel effect occurs when the viscosity of the polymerization system is increased, as described above, whereby the diffusion of polymer molecules is restricted and the termination reaction is reduced. It is estimated that the termination reaction is promoted by hydrogen abstraction. Furthermore, polymerization at the bottom of the distillation column is prevented with this amount of solvent.

Of course, there is no problem if the amount of the solvent is large,
Even if 0.1 wt% is added, if the solvent used in the bottom of the distillation column has a higher boiling point than the monomer to be distilled, the bottom liquid mainly contains the solvent. The amount of the monomer in the bottom liquid is small, so that the polymerization can not be performed there and the operation can be stably performed. If the amount of the solvent exceeds 10% by weight, the thermal decomposition resistance is greatly reduced, which is not preferable. If the amount is less than 0.1% by weight, a runaway reaction due to the gel effect is likely to occur, which is not preferable. When the amount of the solvent is set to 10% by weight or less, the viscosity of the polymerization reaction system becomes high, and in some cases, it becomes difficult to control the polymerization reaction. However, this problem can be solved by slightly lowering the polymerization conversion. I do.

The continuous solution polymerization process of the present invention includes:
A reactor equipped with a mixing device, a temperature control device, and having a supply port and a discharge port capable of continuously supplying and discharging the raw material and having a single or a plurality of containers connected in series is used as a raw material in a reactor. The raw material mainly composed of a monomer and a solvent is continuously supplied, and at the same time, the reaction liquid is continuously discharged from the final reactor, and the discharged reaction liquid is continuously introduced into the devolatilization apparatus through the pre-heater. A process in which a polymer is separated by evaporating unreacted monomers and a solvent under high temperature and reduced pressure.

Each polymerization reactor has a structure in which a uniform reaction liquid composition is obtained by being uniformly stirred by a stirring blade, and examples of the reactor include a double helical ribbon, a pitched paddle, a turbine, an anchor type stirring blade, and the like. And a complete mixing type reactor capable of stirring uniformly. In these individual complete mixing reactors, even when the amount of the solvent is 10% by weight or less, when the polymerization conversion exceeds 70%, it becomes difficult to control the polymerization reaction, or the polymerization reactor is continuously used. The pressure loss in the piping to the equipment becomes large, which makes it difficult to transport the polymerization liquid, which is not preferable. Therefore,
It is necessary that the polymerization conversion of the complete mixing type reactor be 70% or less.

If the polymerization conversion in the complete mixing type reactor is less than 40%, the load of the devolatilization step due to volatile components is large, and for example, devolatilization becomes insufficient due to the restriction of the heat transfer efficiency of the preheater. In some cases, this is not preferable. On the other hand, a reactor other than the complete mixing type reactor can be used singly or after a plurality of complete mixing type reactors. In such a reactor, uniform mixing in the reactor is not required, so that the stirring state of the system is not destabilized due to local progress of polymerization, and the polymerization conversion rate is stable. Any range is possible as long as it can be transferred.

The polymerization temperature of the methacrylic resin is preferably in the general range of 120 to 160 ° C. If the polymerization temperature is lower than 120 ° C., the polymerization is too slow, and
If the temperature exceeds 60 ° C., it becomes difficult to adjust the conversion, which is not preferable. One example of the continuous solution polymerization process of the present invention will be described with reference to FIG. (A) A new monomer 1 mainly containing methyl methacrylate and a recycled liquid 9 containing a monomer mainly containing unreacted methyl methacrylate after polymerization and a solvent are continuously fed to the distillation column 2. Fed and distilled monomers and solvents,
Further, a polymerization initiator and a molecular weight regulator are continuously added to the polymerization reactor 5.
Then, the polymerization solution is devolatilized in a devolatilization tank 8 to take out a polymer 10, and at the same time, a continuous solution polymerization is carried out by recycling 9 an unreacted vinyl monomer and a solvent. Or (B) a distillation column in which a new monomer 1 mainly containing methyl methacrylate and a recycled liquid 9 containing a monomer mainly containing unreacted methyl methacrylate after polymerization and a solvent are directly and continuously mixed. And then continuously polymerized in the same manner.

The distillation is carried out, for example, by a packed column type, a tray type distillation column or the like. In the distillation method, for example, a recycle solution containing impurities to be removed or a mixed solution of this and a new monomer is supplied from the middle or upper stage of the distillation column, and the bottom solution of the distillation column is heated by a heater such as a reboiler. The distillation is performed while condensing a vapor of a monomer and a solvent containing methyl methacrylate as a main component and distilled off from the top of the distillation column with a condenser. In this case, if the solvent used has a higher boiling point than the monomer, impurities generated during polymerization, for example, impurities such as methyl methacrylate dimer, oligomer or metal foreign matter, burned foreign matter of the polymer, and storage stability of the recycle solution are required. Impurities, such as a polymerization inhibitor to be added or a polymerization inhibitor contained in the raw material, which deteriorate optical properties are concentrated at the bottom of the distillation column. These impurities can be continuously or intermittently taken out from the bottom of the distillation column and can be continuously removed by adding the removed amount of the solvent. The distillation is
As described above, there are a method in which a new monomer and a recycled liquid containing an unreacted monomer and a solvent after polymerization are distilled together, and a method in which only the recycled liquid is distilled.The former is a new method. Foreign substances contained in the monomer can be removed, which is more preferable.

A monomer and a solvent containing methyl methacrylate as a main component distilled as described above, and optionally a new monomer containing methyl methacrylate as a main component, a polymerization initiator,
A molecular weight modifier and the like are individually or mixed and supplied to a polymerization reactor for polymerization. This monomer solution is continuously supplied to, for example, a countercurrent contact tower to improve the optical characteristics, and is replaced with an inert gas to reduce the dissolved oxygen in the monomer solution to 1 ppm or less. Further, in order to remove foreign substances such as metal foreign substances, it is preferable to filter the monomer solution with a filter having a size of 0.5 μm or less.

At this time, the polymerization initiator used is one which is decomposed actively at the polymerization temperature to generate radicals, for example, di-tert-butyl peroxide, di-cumyl peroxide, methyl ethyl ketone peroxide, di-tert.
-Butyl perphthalate, di-tert-butyl perbenzoate, tert-butyl peracetate, 2,5
-Dimethyl-2,5-di (tert-butylperoxy) hexane, 1,1-bis (t-butylperoxy)
-3,3,5-trimethylcyclohexane, di-ter
Organic peroxides such as t-amyl peroxide, benzoyl peroxide, cumene hydroperoxide and lauryl peroxide, azobisisobutanol diacetate, 1,1′-azobiscyclohexanecarbonitrile, 2-phenylazo 2,4 An azo compound such as -dimethyl-4-methoxyvaleronitrile, 2-cyano-2-propylazoformamide and 2,2'-azobisisobutyronitrile can be used. These can be used alone or in combination of two or more. The use amount of these polymerization initiators is preferably in the range of 0.0010 to 0.03% by weight based on the weight of the whole reaction mixture.

Further, as the molecular weight modifier used at this time, mercaptans are mainly used. Examples of the mercaptans include n-butyl mercaptan, isobutyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan, sec-dodecyl mercaptan, te
Primary or secondary and tertiary mercaptans having an alkyl group or a substituted alkyl group such as rt-butyl mercaptan, aromatic mercaptans such as phenyl mercaptan, thiocresol, thioglycolic acid and its esters, ethylene thioglycol, etc. Can be used. These can be used alone or in combination of two or more. The amount of use of these molecular weight regulators is appropriately determined according to the molecular weight of the polymer to be produced, but is usually selected in the range of 0.01 to 0.5% by weight based on the weight of the whole reaction mixture.

The polymerization solution obtained by such a polymerization reaction is devolatilized to take out a polymer, and at the same time, a monomer and a solvent mainly composed of unreacted methyl methacrylate, which are volatile components, are separated. The volatile components are distilled and reused as a recycle liquid to continuously produce a methacrylic resin. As the devolatilizing device, an extruder with a multi-stage vent, a devolatilizing tank, or the like is used. Preferably, the polymerization solution is placed in a preheater or the like for 200 hours.
Heat to a temperature of ~ 290 ° C, have enough space on top,
The polymer is fed to a devolatilization tank under vacuum at a temperature of 200 to 250 ° C. and a temperature of 20 to 100 torr, and at the same time, a volatile component consisting of a monomer and a solvent containing unreacted methyl methacrylate as a main component is separated. And reuse it as a recycle liquid. The volatile matter remaining in the polymer is 1% by weight or less,
It is preferably at most 0.5% by weight, more preferably at most 0.3% by weight. This method of introducing a polymerization solution into a devolatilization tank held under reduced pressure causes instantaneous volatilization of volatile components and foaming thereby, forming an extremely large evaporation area, and using a high boiling point solvent. This is a preferred devolatilization method in which volatile components are efficiently removed in a short time, the amount of solvent or residual monomer remaining in the polymer is small, the coloring of the polymer is small, and the optical properties are excellent.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The method for measuring the physical properties of a polymer is as follows. (1) Measurement of intrinsic viscosity of polymer 150 mg of polymer was dissolved in 50 ml of chloroform.
It was measured with an Ostwald viscometer at a temperature of 5 ° C. (2) Measurement of total light transmittance It was measured by the ASTM D-1003 method. (3) Measurement of heat decomposition resistance Using a 3 oz. Injection molding machine, and setting the temperature of the molding machine to 290
Set to ° C. After weighing the polymer, 10
Stayed for a minute. The polymer is thermally decomposed by staying in the molding machine, and the polymer proportional to the amount of the decomposed gas is discharged from the nozzle. The discharged amount was measured and used as a measure of the thermal decomposability.

[0026]

Example 1 Recycle liquid after polymerization of 91.6% by weight of methyl methacrylate, 2.0% by weight of methyl acrylate and 6.4% by weight of ethylbenzene / methyl methacrylate 9
A fresh monomer solution of 8.0% by weight and 2.0% by weight of methyl acrylate was continuously fed to the distillation column in a weight ratio of 47/53. The distilled monomer solution was 95% by weight of methyl methacrylate, 2.0% by weight of methyl acrylate, and 3.0% by weight of ethylbenzene. 1,1-bis (t-butylperoxy) -3,3,5-
An amount of trimethylcyclohexane of 105 ppm and an amount of n-octyl mercaptan of 1950 ppm were continuously added and continuously supplied to a polymerization reactor, where polymerization was performed at a polymerization temperature of 155 ° C. and a residence time of 2.0 hours. At this time, the polymerization conversion of the monomer was 53%. The polymerization solution was continuously taken out of the polymerization reactor, and then heated at 260 ° C.
And cast and dropped into the devolatilization tank through the gap between the heating plates. The polymer was separated from the unreacted monomer and the solvent while maintaining the temperature in the devolatilization tank at 30 Torr and 230 ° C. The polymer was extruded from an extrusion die, and the unreacted monomer and solvent were distilled and reused as a recycle liquid, and a continuous operation was carried out for about 5 days. The resulting methacrylic resin pellets had a methyl acrylate content of 1.9% by weight and a residual monomer content of 2300 p.
pm. In addition, the intrinsic viscosity is 56 ml / g, the total light transmittance is 93%, and the discharge amount, which is a measure of thermal decomposition resistance, is 17%.
g. Also, no polymer was precipitated at the bottom of the distillation column.

[0027]

Examples 2 to 5, Comparative Examples 1 and 2 The same operation as in Example 1 was carried out except that the conditions in Example 1 were shown in Table 1.
Table 1 shows the results. Example 1 is also described. In addition, in Comparative Example 1, precipitation of a part of the polymer was observed in the reboiler hoe portion during operation for about 5 days. However, the example is
No polymer precipitation was observed in any case.

[0028]

[Table 1]

[0029]

Example 6 A methacrylic resin was produced in the same manner as in Example 1 except that methyl acrylate used was changed to ethyl acrylate. Ethyl acrylate content in the obtained methacrylic resin pellets 2.0% by weight, residual monomer 230
0 ppm, intrinsic viscosity 54 ml / g, total light transmittance 93
%, Which was a discharge amount of 17 g, which is a measure of thermal decomposition resistance.
Also, no polymer was precipitated at the bottom of the distillation column.

[0030]

Example 7 A methacrylic resin was produced in the same manner as in Example 1 except that the solvent used was changed to mixed xylene. The obtained methacrylic resin pellets had a methyl acrylate content of 2.0% by weight, a residual monomer of 2,300 ppm, an intrinsic viscosity of 58 ml / g, a total light transmittance of 93%, and a discharge amount of 16 g, which is a measure of thermal decomposition resistance. there were. Also, no polymer was precipitated at the bottom of the distillation column.

[0031]

According to the present invention, in continuous solution polymerization, the amount of solvent is adjusted to 10% by weight based on the weight of the total mixture during polymerization.
By setting the content to 5% by weight or less, excellent thermal decomposition resistance is obtained, a runaway reaction due to a gel effect is suppressed, and impurities generated during polymerization without polymer formation at the bottom of the distillation column are taken out. Therefore, a methacrylic resin having high optical purity can be industrially extremely stably produced.

[Brief description of the drawings]

FIG. 1 shows an example of a continuous solution polymerization apparatus in a solution polymerization process of the present invention.

[Explanation of symbols]

 1, new monomer supply line 2, distillation column 3, impurity extraction line 4, condenser 5, polymerization reactor 6, metering pump 7, heating plate 8, devolatilization tank 9, volatile matter recycling line 10, devolatilization tank 11. Polymer dispensing line

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C08F 220: 12) (C08F 220/14 220: 18)

Claims (8)

[Claims] 1. A methacrylic resin characterized by containing a solvent in an amount of 0.1 to 10% by weight, based on the weight of the entire mixture during polymerization, in polymerizing a monomer containing methyl methacrylate as a main component. Manufacturing method. 2. A continuous distillation of a (A) unreacted methyl methacrylate-based monomer after polymerization and a recycle liquid containing a solvent and a new methyl methacrylate-based monomer continuously. Supply to the column, continuously supply the distilled monomer and solvent to the polymerization reactor, polymerize, then devolatilize the polymerization solution to take out the polymer, and simultaneously remove the unreacted vinyl monomer and solvent. 2. The methacrylic resin production according to claim 1, wherein the amount of solvent used in the continuous solution polymerization process using recycling is 0.1 to 10% by weight based on the weight of the whole mixture at the time of polymerization. Method. And (B) continuously supplying a recycle liquid containing a monomer and a solvent mainly composed of unreacted methyl methacrylate after polymerization to a distillation column, and using distilled methyl methacrylate as a main component. The monomer and solvent to be added and the new monomer containing methyl methacrylate as the main component are continuously supplied to the polymerization reactor, polymerized, and then devolatilized to remove the polymer and unreacted at the same time. In a continuous solution polymerization process in which a monomer and a solvent containing methyl methacrylate as a main component are recycled, the amount of a solvent used is 0.1 to 10% by weight based on the weight of the whole mixture at the time of polymerization. The method for producing a methacrylic resin according to claim 1. 4. The method for producing a methacrylic resin according to claim 1, wherein the amount of the solvent is 0.1 to 5% by weight based on the weight of the whole mixture at the time of polymerization. 5. The method according to claim 1, wherein the monomer containing methyl methacrylate as a main component comprises methyl methacrylate alone or a methyl methacrylate monomer and 15% by weight or less of an acrylate monomer. 4. The method for producing a methacrylic resin according to 1, 2, or 3. 6. The method for producing a methacrylic resin according to claim 5, wherein the acrylate monomer is a methyl acrylate monomer or an ethyl acrylate monomer. 7. The method for producing a methacrylic resin according to claim 1, wherein the solvent is an alkylbenzene. 8. The method according to claim 7, wherein the solvent is selected from toluene, xylene, and ethylbenzene.