JPH0330601B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to solution polymerization, in particular, to polymerization of various polymerizable monomers such as olefins under reaction conditions such that the polymer formed in a liquid phase medium is dissolved in the liquid medium. The present invention relates to an improvement in a type of polymerization method for polymerizing polymers, and further relates to a polymerization method that allows easy control of the density and average molecular weight of the resulting polymer.

In the present invention, the term "polymerization" may be used to include copolymerization, and similarly, the term "polymer" may be used to include copolymers.

The above-mentioned type of polymerization method is known as one type in which a polymer is produced by polymerizing various polymerizable monomers. For example, taking the polymerization of olefins as an example, inert hydrocarbons and/or olefins to be polymerized are used as a medium that forms a liquid phase under reaction conditions, and the olefin polymers formed are dissolved in the liquid medium. A method of polymerizing olefins under such conditions is known. This method is particularly suitable for producing medium- and low-density grade ethylene copolymers for which slurry polymerization is difficult.

When carrying out this type of solution polymerization,
In order to obtain a polymer with good homogeneity, it is generally preferable to carry out the polymerization under conditions in a non-two-phase separation region that exhibits a uniform liquid phase between the upper cloud point and the lower cloud point. It is common to carry out polymerization. However, when attempting to produce high molecular weight polymers by these types of solution polymerization techniques, the solution viscosity of the polymerization system increases, making it difficult to remove the polymerization heat, pump the product, and process the polymerization system. Stirring and mixing cannot be performed smoothly. For this reason, it is necessary to operate in a state where the polymer concentration is diluted, and as a result, there are problems with disadvantages such as a decrease in production capacity per unit volume of the polymerization vessel and an increase in polymer separation cost.

The present inventors conducted research in order to develop an improvement method that avoids the above-mentioned disadvantages in solution polymerization. As a result, solution polymerizations of the type described above are carried out under conditions in the two-phase separation region above the upper cloud point, where polymerization uniformity would be expected to be lost, but with the exception that both phases are in a well-dispersed state of mixing. By carrying out the polymerization under sufficient stirring conditions, it is possible to create a dispersion-mixed state such as a droplet dispersion system with a higher concentration of polymer in a liquid phase with a lower concentration of polymer. This is presumed to be due to the formation of a reaction system, but it was discovered that an improvement could be achieved in which the above-mentioned troubles could be conveniently overcome without impairing the uniformity of polymerization, and
It has already been proposed in Publication No. 58-7402. This method is
To significantly increase the solution viscosity of the polymerization system by leading the produced polymerization liquid to a separation zone and separating the phases, collecting the polymer concentration liquid phase, and recycling and reusing the polymer diluted liquid phase to the polymerization tank. without pumping the product,
It was a rational process for producing polymers because the stirring and mixing of the polymerization system and the removal of polymerization heat could be carried out smoothly.

As a result of studies aimed at further improving the above-mentioned polymerization process and developing a more rational process, the present inventors found that the polymerization system inside each polymerization tank was in the two-phase separation region above the upper cloud point. When polymerizing in a polymerization process consisting of multiple stages of polymerization tanks in which both phases are dispersed and mixed, ^the initial viscosity is By adopting a method in which the molecular weight is increased to a higher molecular weight and further polymerization is continued in a subsequent polymerization tank, the density and molecular weight distribution of the resulting polymer can be adjusted much more easily than the method proposed in the above-mentioned publication. The inventors have discovered that the object can be achieved and have arrived at the present invention. According to the present invention, polymerization can be carried out in a state where the apparent viscosity of the system is lower than in conventional homogeneous solution systems, and therefore, an increase in the amount of polymer produced per reaction volume can be achieved, and phase separation is also possible. The viscosity of the polymer dilute solution phase obtained is extremely low and has excellent cooling efficiency, so that the heat of polymerization reaction can be easily and efficiently removed;
The medium can be easily and efficiently circulated and reused in the polymerization tank, and in the case of olefin polymerization, less hydrogen is used in the first stage polymerization tank to produce a high molecular weight polymer. 1st and 2nd row
There are many advantages such as eliminating the need to install a hydrogen separation device between the stages of polymerization tanks.

Since it is impossible to avoid the above-mentioned drawbacks in the conventional homogeneous solution-based multi-stage polymerization method, a low molecular weight polymer is usually produced in the first stage polymerization tank, and the molecular weight is increased in the subsequent stage polymerization tank. However, this method requires a hydrogen separation device between the polymerization tanks in order to adjust the molecular weight distribution and density of the produced polymer, and it also requires separation of copolymer components in the recycled solvent. Therefore, large-scale distillation equipment was required. In contrast, the method of the present invention is characterized by eliminating the drawbacks of conventional multistage polymerization methods and providing an excellent multistage polymerization process by employing the method described below.

To summarize the present invention, the present invention provides a method for polymerizing monomers in a multi-stage polymerization tank that satisfies the condition that the polymer formed is dissolved in the medium which forms a liquid phase under the reaction conditions. (i) The polymerization system inside each polymerization tank is in a two-phase separation region above the upper cloud point, and both phases are dispersed and mixed, and (ii) The polymerization product liquid in each polymerization tank is separated into a separation zone. The diluted polymer liquid phase is circulated and reused in the polymerization tank, and the polymer concentrated liquid phase is used in the subsequent polymerization. (iii) separating the polymer from the polymer-concentrated liquid phase obtained by two-liquid phase separation of the polymerization product liquid from the final polymerization tank; (iv) The ratio of the intrinsic viscosity of the polymer produced in the first stage polymerization tank [η ^a ] to the intrinsic viscosity [η ^z ] of the polymer obtained from the last stage polymerization tank. The gist of the invention is a polymerization method characterized in that the polymerization is carried out until the polymerization ratio is in the range of 1.1 to 4.

The above objects and many other objects and advantages of the present invention will become more apparent from the following description.

Although the method of the present invention can be advantageously applied to the polymerization of any various monomers that can be dissolved in solution and exhibit an upper cloud point, the polymerization method of the present invention will be explained in more detail below using the polymerization of olefins as an example. .

When carrying out the polymerization method of the present invention, various transition metal-containing catalysts, such as those conventionally proposed for medium and low pressure methods, can be used. As such a catalyst, for example, a transition metal-containing catalyst formed from a transition metal compound catalyst component and an organometallic compound catalyst component of a metal from Group 1 to Group 3 of the periodic table can be used.

The transition metal compound catalyst component is a compound of a transition metal such as titanium, vanadium, chromium, zirconium, etc., and may be liquid or solid under the conditions of use. These do not need to be a single compound, and may be supported on other compounds or mixed. Furthermore, it may be a complex compound or composite compound with other compounds. The above-mentioned preferred component is a highly active transition metal compound catalyst component capable of producing 5000 g or more, particularly 8000 g or more of olefin polymer per 1 mmol of transition metal, and a typical example thereof is a magnesium compound. A highly activated titanium catalyst component can be exemplified. For example, a solid titanium catalyst component containing titanium, magnesium and halogen as essential components, containing amorphous magnesium halide, and having a specific surface area of preferably about 40 m ² /g or more, particularly preferably can be exemplified by a component of about 80 m ² /g.
It may also contain electron donors such as organic acid esters, silicate esters, acid halides, acid anhydrides, ketones, acid amides, tertiary amines, phosphoric esters, phosphorous esters, ethers, and the like.
This titanium catalyst component contains, for example, approximately 0.5 titanium.
from about 1 to about 10% by weight, especially from about 1 to about 8% by weight
Contains titanium/magnesium (atomic ratio) of approximately 1/1
2 to about 1/100, especially about 1/3 to about 1/50, halogen/titanium (atomic ratio) about 4 to about 100,
Particularly preferred are those in which the electron donor/titanium (molar ratio) is in the range of about 6 to about 80, and the electron donor/titanium (molar ratio) is in the range of 0 to about 10, especially 0 to about 6.

Alternatively, as such a titanium catalyst component,
An example of a titanium catalyst component is a combination of a magnesium compound dissolved in a hydrocarbon solvent and a liquid titanium compound in the presence of an electron donor such as alcohol.

The organometallic compound catalyst component is an organometallic compound having a bond between a metal of Group 1 to 3 of the periodic law and carbon, and examples thereof include organic compounds of alkali metals and organometallic compounds of alkaline earth metals. , organic aluminum compounds, etc.
For example, alkyl lithium, aryl sodium, alkyl magnesium, aryl magnesium, alkyl magnesium halide, aryl magnesium halide, alkyl magnesium hydride, trialkyl aluminum, alkyl aluminum halide, alkyl aluminum hydride, alkyl aluminum alkoxide, alkyl lithium aluminum, mixtures thereof, etc. can be exemplified.

In addition to the above two components, for the purpose of adjusting stereoregularity, molecular weight, molecular weight distribution, etc., hydrogen, halogenated hydrocarbons, electron donor catalyst components such as organic acid esters, silicate esters, carboxylic acid halides,
Carboxylic acid amides, tertiary amines, acid anhydrides, ethers, ketones, aldehydes, etc. may also be used. During polymerization, this electron donor component may be used in the form of forming a complex compound (or addition compound) with the organometallic compound catalyst component in advance, or may be used in the form of a complex compound (or addition compound) with the organometallic compound catalyst component, or may be used in the form of a complex compound (or addition compound) with the organometallic compound catalyst component, or may be used in the form of a complex compound (or addition compound) with the organometallic compound catalyst component, or may be used in the form of a complex compound (or addition compound) with the organometallic compound catalyst component. It may be used in the form of a complex compound (or addition compound) with. The catalyst may be supplied only to the first stage polymerization reactor, or may be supplied to the first stage and each of the other polymerization reactors in parallel.

Examples of olefins used in polymerization include ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene,
1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 3-methyl-1-butene, 3-methyl-1-pentene, 4-methyl-1
-pentene, 4,4-dimethyl-1-pentene,
Butadiene, 1-isoprene, 1,4-hexadiene, dicyclopentadiene, 5-ethylidene-
Examples include 2-norbornene and 1,7-octadiene. These can be used alone or
A mixture of two or more types may be used. In particular, the present invention provides a homopolymer of ethylene or ethylene containing about 90%
It is suitable for producing resinous ethylene copolymers containing mol% or more.

The polymerization of olefins is carried out under conditions such that the olefin polymer formed is dissolved in a medium that is in a liquid phase under the reaction conditions. Examples of the medium used as a polymerization solvent include inert hydrocarbons and/or olefins used in polymerization. Examples of inert hydrocarbons include aliphatic hydrocarbons such as propane, butane, pentane, hexane, heptane, octane, nonane, decane, dodecane, and kerosene; for example, cyclopentane,
Examples include alicyclic hydrocarbons such as methylcyclopentane, cyclohexane, and methylcyclohexane; aromatic hydrocarbons such as benzene, toluene, and xylene; or a mixture of two or more of these components. .

In the method of the present invention, polymerization is carried out in a multi-stage polymerization tank that satisfies the conditions that the polymer formed forms two liquid phases and is dissolved and dispersed in a medium that forms a liquid phase under reaction conditions. At this time, the polymerization system inside each polymerization tank is in a two-phase separation region above the upper cloud point, and both phases are in a dispersed stirring mixed state. The resulting polymer solution produced in each polymerization tank is then led to a separation zone where it is separated into two liquid phases consisting of a polymer-rich liquid phase and a polymer-dilute liquid phase. The polymer is recycled and reused, and the polymer concentrated liquid phase is supplied to a subsequent polymerization tank to continue the polymerization reaction. In the method of the present invention, the polymerization liquid produced from the last stage polymerization tank is led to the separation zone in the same manner as described above, where it is separated into two liquid phases consisting of a polymer-concentrated liquid phase and a polymer-dilute liquid phase. The liquid phase is circulated to the polymerization tank and reused, and the polymer is separated and recovered from the polymer-concentrated liquid phase according to a conventional method.

In the method of the present invention, monomers, catalysts, and media are supplied individually or as a mixture of two or more to the polymerization tank constituting the above-mentioned polymerization process, and polymerized under the conditions described below. In this case, in the method of the present invention, the intrinsic viscosity of the polymer produced in the first stage polymerization tank [η ^a ] is compared to the intrinsic viscosity [η ^z ] of the polymer obtained from the last stage polymerization tank.
The ratio [η ^a ]/[η ^z ] is between 1.1 and 4, preferably
It is polymerized until it becomes 1.2 to 2.5. Also,
At this time, regarding the density of the produced polymer, the ^difference (d ^{a -} Polymerization is carried out until d ^z ) is generally -0.15 to +0.15 g/cm ³ , preferably -0.05 to +0.15 g/cm ³ . The amount of polymerization in the first-stage polymer tank and the polymerization ratio in the second-stage and subsequent polymerization tanks can be arbitrarily adjusted.

In the method of the present invention, details of the polymerization process and conditions for the polymerization reaction are as follows.

The polymerization temperature is selected in such a range that phase separation above the upper cloud point is observed. The upper cloud point is
Although it depends on the type and mutual ratio of liquid phase components in the polymerization system, experimentally, the transmitted light is measured,
It can be easily determined as the temperature at which the transmitted light intensity rapidly attenuates. At temperatures between the lower and upper cloud points, the polymer dissolves in a homogeneous liquid phase; however, at temperatures above the upper cloud point, a concentrated solution phase of the polymer and a dilute polymer phase occur. Phase separates into solution phase. In general, the higher the temperature, the higher the concentration of the polymer in the concentrated solution phase, and conversely, the lower the concentration of the polymer in the dilute solution phase. The two-phase separation region can vary depending on not only the temperature but also the type and proportion of monomers and polymers formed, the type of solvent, the pressure of the reaction system, and other conditions. Therefore, by using the above-mentioned transmitted light measurement method, the conditions of the two-phase separation region above the upper cloud point can be easily determined in advance experimentally.

From the point of view of polymerization operations, the higher the polymer concentration in the concentrated solution phase and the higher the average molecular weight of the polymer, the more viscous it becomes, so the stirring power required to uniformly disperse the concentrated solution phase in the dilute solution phase also increases. However, it also tends to adhere to the stirring blades and polymerization walls.
Trouble can be prevented by modifying the shape of the stirring blade. On the other hand, from the viewpoint of separation operations, the larger the density difference between the two phases, the better the separation efficiency, which facilitates the operations required for post-processing operations and reduces costs.

The actual polymerization temperature should be determined by taking into consideration various factors such as the advantages and disadvantages of such operations, changes in catalyst activity due to temperature, and equipment costs associated with increases and decreases in operating pressure, but in general, it is necessary to determine the actual polymerization temperature from the upper cloud point. Approximately more than that
During temperatures 200°C above the upper cloud point, especially about
Preferably, the temperature is selected between 10° C. higher and about 150° C. higher. Further, when using a titanium catalyst component highly activated by a magnesium compound as described above, it is preferable to carry out the polymerization at a temperature range of about 100 to about 300°C, particularly about 120 to about 250°C. The concentration of the olefin polymer varies depending on the molecular weight of the olefin polymer, but is adjusted to a range of about 10 to about 1000 g/, more preferably about 30 to about 200 g/in the combined state of both liquid phases. It is industrially advantageous to do so. The polymerization pressure is preferably in the range of, for example, atmospheric pressure to about 150 kg/cm ² , particularly about 10 to about 70 kg/cm ² .
Hydrogen optionally used during polymerization is, for example, about 0.0001 to about 20 mol per mol of olefin,
It is particularly preferable to use it in a range of about 0.001 to about 10 moles.

When using a transition metal compound catalyst component, an organometallic compound catalyst component, an electron donor catalyst component, etc. as described above, the transition metal compound catalyst component is approximately 0.0005 to about 1 mmol, especially about
Preferably, 0.001 to about 0.5 mmol of the organometallic compound catalyst component is used in proportions such that the metal/transition metal (atomic ratio) is from about 1 to about 2000, especially from about 1 to about 500. In addition, the electron donor catalyst component is 0 per mole of the organometallic compound catalyst component.
It is preferably used in a proportion of about 1 mol to about 1 mol, particularly about 0 to about 0.5 mol.

In the method of the present invention, polymerization is carried out not only under conditions in the two-phase separation region above the upper cloud point, but also under stirring conditions in which both phases of the polymer are in a dispersed and mixed state. If the stirring is insufficient, a dilute phase clearly appears in the upper phase portion, which impairs the uniformity of polymerization, which is not preferable. Therefore, stirring conditions are adopted such that such a separated phase does not appear. By performing polymerization in a well-dispersed state in this way, the polymerization rate is lower than when performing homogeneous phase solution polymerization at the same polymer concentration.
It is possible to carry out polymerization in a state where the viscosity is actually low, and even when producing a high molecular weight polymer, the polymerization can be carried out under relatively high concentration conditions.

The polymerization of the olefins is advantageously carried out continuously. For example, it is possible to adopt a method in which the required raw materials are continuously supplied to the polymerization vessel, while the polymerization product liquid is continuously extracted so that the volume of the polymerization vessel is constant. At this time, operating conditions such that a gas phase portion exists may be adopted, or operation may be performed such that a liquid-filled type is provided.

The extracted polymer solution is introduced into a separation zone and is separated into a polymer-concentrated liquid phase in the lower phase and a dilute polymer liquid phase in the upper phase. Phase separation can be easily performed by omitting stirring as in a polymerization vessel, and
May be heated if necessary. Of course, the separation band is
It is necessary to be under phase separation region conditions above the upper cloud point, and therefore it is advantageous to maintain conditions such as temperature, pressure, etc. similar to those of the polymerization vessel, for example.

Phase separation does not need to be performed completely; for example, both phases may be separated in a state in which a part of the dilute phase is mixed with the concentrated phase. Part or all of the polymer diluted liquid phase in the upper phase is recycled and reused in the polymerization reaction. At this time, if the material is cooled in advance before being introduced into the polymerization zone, the heat of polymerization can be effectively removed. In other words, compared to cooling the polymerization product liquid itself, the phase-separated polymer thinner phase has a lower viscosity, so the heat exchange effect in the cooler is higher, so it is less efficient in terms of thermal energy and industrial efficiency. This is extremely advantageous for practical implementation. Furthermore, since a highly concentrated polymer solution can be obtained by simply performing phase separation, operations required for polymer separation can be facilitated and separation costs can be reduced.

When recycling and reusing the separated upper phase polymer dilute liquid phase in the polymerization reaction, if multiple polymerization tanks are used, it is not necessary to circulate and reuse it in the same tank from which the polymerization product was taken. There is no need to circulate and reuse it to other polymerization tanks, and it can also be recycled to other polymerization tanks.

The concentrated phase of the polymer obtained from the final stage polymerization tank is heated, flashed, vacuum suction, etc. to remove inert hydrocarbons, dissolved olefins, etc., and then transferred to an extruder. can be fed to produce polymer pellets.

According to the present invention, polymerization and polymer separation can be carried out labor-savingly and economically by omitting operations and equipment.

Next, examples will be shown.

Example 1 <Catalyst Preparation> 10 mol of commercially available anhydrous magnesium chloride was suspended in dehydrated and purified hexane 50 in a nitrogen stream, 60 mol of ethanol was added dropwise over 1 hour with stirring, and the mixture was reacted for 1 hour at room temperature. . 28 mol of diethylaluminum chloride was added dropwise to this at room temperature, and 1
Stir for hours. Subsequently, 75 mol of titanium tetrachloride was added, and the system was heated to 80° C. and the reaction was carried out with stirring for 3 hours. The generated solid portion was separated by decantation, washed repeatedly with purified hexane, and then made into a hexane suspension. The concentration of titanium was determined by titration.

<Polymerization> Using the first stage continuous polymerization reactor A with a diameter of 50 cm and a volume of 200 as shown in Figure 1, a solvent (n-hexane containing 15 vol% of methylcyclopentane) was introduced through tube 4.
15.2/hr diethyl aluminum chloride 10 mmol/hr,
The supported catalyst is converted to Ti, 0.8 mmol/hr
is continuously supplied from tube 4, and in the polymerization vessel,
At the same time, ethylene 8.0Kg/H, hydrogen 10/hr, 1-
Butene was continuously supplied from tubes 1, 2, and 3 at a rate of 2.5 kg/hr, at a polymerization temperature of 170°C and a total pressure of 30 kg/cm ² -
G. Polymerization was carried out under conditions with a residence time of 15 minutes. The ethylene copolymer produced in polymerization reactor A was continuously extracted through tube 5 at a rate of 192 solvent/hr and fed to two-phase separator B at a temperature of 170°C and a pressure of 30 kg/cm ² -G.

The product liquid containing the ethylene copolymer supplied to the two-phase separator B is phase-separated, and the concentrated liquid phase containing most of the ethylene copolymer is discharged from the bottom through the pipe 7 at a rate of 17.6 solvent/hr. The mixture was transferred to a two-stage continuous polymerization reactor. The dilute liquid phase obtained in the two-phase separator B is passed through the tube 6 from the upper part of the separator B to the solvent.
It was extracted at a rate of 174.2/hr, cooled to an extent that the ethylene copolymer did not precipitate, and then recycled to polymer reactor A. In the second stage continuous polymerization reactor, the solvent was continuously supplied from tube 11 at a rate of 52.2/hr, and at the same time ethylene 6.5 Kg/hr, hydrogen 20/hr,
1-butene at a rate of 0.3Kg/hr, tubes 8, 9, and 1, respectively.
Polymerization was carried out under conditions of a polymer temperature of 180°C, a total pressure of 30 Kg/cm ² -G, and a residence time of 30 minutes.

When the sample polymerized in the first stage polymerization reactor was taken out from tube 7 and measured, it was found that the intrinsic viscosity was [η ¹ ]
was 3.04, and the density was 0.919.

When the sample that was continuously polymerized in the second stage polymerization reactor was taken out from the tube 14 and measured, the intrinsic viscosity [η ² ] was 1.89 and the density was 0.920. At this time, [η ¹ ]/[η ² ] is 1.6.

Pipe 5, Pipe 6, Pipe 7 and Pipe 12, Pipe 13, Pipe 14
Therefore, when we sampled solutions containing ethylene copolymer and measured the concentration of each ethylene copolymer, we found that tube 5 had 50g polymer/-solvent, tube 6 had 5g polymer/-solvent, and tube 7 had 500g polymer/-solvent. It was a solvent. Also, tube 12 contained 80 g polymer/-solvent, tube 13 contained 9 g polymer/-solvent, and tube 14 contained 250 g/-solvent. Regarding the concentration from the reactor to the two-phase separator, the first
It was confirmed that the concentration was approximately 10 times in the two-phase separator in the second stage, and approximately 3.2 times in the second stage two-phase separator.

Example 2 <Catalyst Preparation> Same as Example 1 <Polymerization> In the same apparatus as Example 1, 4 was used as a comonomer.
-Methyl-1-pentene was used to carry out the polymerization. The polymerization temperature in the first stage polymerization reactor was 170°C and the pressure was 30Kg/cm ² -G, and the polymerization temperature in the second stage polymerization reactor was 180°C and the pressure 30Kg/cm ² -G. The sample polymerized in the first stage polymerization reactor was taken out from tube 7 and measured, and the intrinsic viscosity [η ¹ ] was
4.78, and the density was 0.935. The sample that was continuously polymerized in the second stage polymerization reactor was taken out from the tube 14 and measured, and the intrinsic viscosity [η ² ] was 2.59 and the density was 0.942. At this time, [η ¹ ]/[η ² ] is
It becomes 1.84 times. tube 5, tube 6, tube 7, and tube 12,
When a solution containing an ethylene copolymer was sampled from tubes 13 and 14 and the amount of ethylene copolymer in each was measured, tube 5 contained 50 g of polymer/-solvent;
Tube 12 is 100g polymer/solvent, tube 13 is 10g
Polymer/-solvent, tube 14 contains 250g polymer/
-It was a solvent. Regarding the concentration from the reactor to the two-phase separator, the ethylene copolymer concentration is approximately 10 times higher in the first stage two-phase separator B, and approximately 2.7 times higher in the second stage two-phase separator. It has been confirmed that there is.

[Brief explanation of drawings]

FIG. 1 shows an example of an apparatus for carrying out the polymerization method of the present invention. A: 1-stage polymerization reaction tank, B: 1-stage phase separator, C: 2-stage polymerization reactor, D: 2-stage phase separator, E: hopper, F: transfer pump, G: heater, H: cooler, I: Cooler.

Claims (1)

[Claims] 1. When monomers are polymerized in a multi-stage polymerization tank that satisfies the condition that the formed polymer dissolves in a medium that forms a liquid phase under reaction conditions, (i ) The polymerization system inside each polymerization tank is in a two-phase separation region above the upper cloud point, and both phases are dispersed and mixed, (ii) The polymerization product liquid in each polymerization tank is led to the separation zone and polymerized. The polymer is separated into two liquid phases consisting of a combined concentrated liquid phase and a polymer diluted liquid phase, the polymer diluted liquid phase is circulated and reused in the polymerization tank, and the polymer concentrated liquid phase is supplied to the subsequent polymerization tank. (iii) separating the polymer from the polymer-concentrated liquid phase obtained by two-liquid phase separation of the polymerization product liquid from the final polymerization tank; (iv) the ratio of the limiting viscosity [η ^a ] of the polymer produced in the first stage polymerization tank to the intrinsic viscosity [η ^z ] of the polymer obtained from the last stage polymerization tank is 1.1 to 4. A polymerization method characterized by polymerizing until the range of .