JPH0322401B2 - - Google Patents

Description

[Detailed description of the invention]

A. Industrial Application Field The present invention relates to a method for continuous production of low polymerization degree polyvinyl acetate (hereinafter abbreviated as PVAc) and low polymerization degree polyvinyl alcohol (hereinafter abbreviated as PVA) having a uniform polymerization degree distribution. For more details n
Mercaptan is used as a chain transfer agent in the methanol solution polymerization of vinyl acetate (hereinafter abbreviated as VAc) in a multi-stage continuous polymerization apparatus consisting of 2 reactors (n2), and the mercaptan is continuously transferred to each stage of the reactor. In the method for producing low polymerization degree PVAc having a uniform degree of polymerization distribution by supplying the mercaptans to for each stage of reactor,
The present invention relates to a method characterized in that each of the following formulas (1) is continuously supplied. ₀ [X] _n / [X] _n =X _n -X _n-1 /1-X _n・C _x +1-[X
] _n-1 / [X] _n …(1) (However, [X] is the molar concentration of mercaptan,
x represents the polymerization rate, Cx represents the chain transfer constant to the mercaptan, the subscript m means the m-th reactor, and ₀ [X] _n represents the amount of mercaptan supplied to the m-th reactor in the first reactor. [X] _n means the molar concentration of mercaptan in the m-th reactor, m is 1 mn-1, and when m = 1, x _n-1 =x ₀ =0, [X] _n-1 = [X] ₀ =0. ) B Prior Art It has been known that mercaptans are applied to VAc polymerization systems, but all of them are based on so-called batch polymerization methods, and these methods have a uniform degree of polymerization distribution. It is not clear whether PVAc with a low degree of polymerization has been obtained or not, and there is still no example of its application to a multistage continuous polymerization method. The concept of supplying mercaptan to such a continuous polymerization system cannot be said to be particularly new, but in what proportion should memercaptan be supplied to each stage in order to obtain the desired degree of polymerization and a uniform distribution? This cannot be easily set even by those skilled in the art. In addition, in continuous solution polymerization in a solvent with a small chain transfer constant such as methanol used in the present invention, the degree of polymerization is less than 500.
It is extremely difficult to efficiently produce PVAc, and it is common knowledge that the concentration of VAc must be lowered to 50% or less, or that the concentration of PVAc in the reaction solution must be significantly increased. This was not desirable because it caused a decline in productivity. Even if such a method is used, the degree of polymerization is less than 300, especially less than 200.
It is no exaggeration to say that manufacturing PVAc, and by extension PVA, is extremely difficult and industrially impossible. As a polymerization solvent for VAc, the chain transfer constant is 20×
10 ^-4 or higher (e.g. ethanol,
A method has been proposed for producing low polymerization degree PVA by using isopropanol, etc.) However, using methanol as a polymerization solvent is extremely advantageous for the following reasons, and it is difficult to industrially use anything other than methanol. This will significantly increase the manufacturing cost of PVA. That is, the role of methanol is primarily to control the polymerization temperature, and the heat of polymerization is gradually removed by the azeotrope of VAc and methanol, and the azeotropic temperature is about 60°C, which is important for the properties of the obtained PVA. gives the preferred temperature.
Second, a large amount of unpolymerized VAc remains in the methanol solution of PVAc after polymerization.
Azeotropy with methanol is also effectively used to remove VAc. The third and most important point is that when PVA is produced from PVAc by saponification, a methanol solution of PVAc is an essential condition. This is because the saponification reaction is methanolysis of the acetyl group in PVAc with methanol, and the alkali only plays the role of a catalyst. Therefore, if a solvent other than methanol is used as the polymerization solvent, complicated operations such as replacing the solvent with methanol will be required during saponification. in this way
When producing PVA industrially, it is clearly disadvantageous to use solvents other than methanol during the polymerization of VAc. It is a well-known fact that when a substance that causes severe chain transfer, such as a mercaptan, is added to a polymerization system, the degree of polymerization is generally drastically reduced.
However, in the case of polymerization of VAc, the inventors have quite surprisingly found that the degree of polymerization does not decrease as calculated. For example, to produce PVAc with a degree of polymerization of 100 using n-butyl mercaptan in a batch polymerization method, if the chain transfer constant (hereinafter abbreviated as Cx) of n-butyl mercaptan is 50 (literature value), the mercaptan and VAc The molar ratio of
Theory teaches that it is sufficient to set the value to 10 ^-4 . So, with that concentration of n-butyl mercaptan,
In addition to VAc and methanol (70/30 weight ratio), 0.016% by weight of azobisisobutyronitrile (hereinafter abbreviated as AIBN) was added and polymerization was carried out at 60°C.
After 3 hours, the polymerization was stopped, and the polymer was taken out by a conventional method (polymerization rate: 51%), and the degree of polymerization was measured, and it was found to be 1772. The inventors have measured that the Cx of n-butyl mercaptan is more correct when it is 22 than 50, but even taking this into account, the degree of polymerization should be about 230 under the above conditions.
It is true that the degree of polymerization is lower than the degree of polymerization of 1900 in the absence of mercaptan, but the degree is small, and the difference from the target degree of polymerization of 100 (or 230) is too large to be a problem. When comparing theory and practice in radical polymerization, it is common practice to keep the polymerization rate below 10%. Therefore, the present inventors also shortened the polymerization time to 30 minutes and measured the degree of polymerization in the same manner, and found that it was 1382. In this case, the theoretical value is 100 (or
230), it must be said that the difference is too large. The present inventors investigated the polymer thus produced, and found that a polymer having a polymerization degree of up to 2%, which is quite close to the theoretical level, and having n-butyl mercaptan residues at the terminals was obtained. , when the polymerization rate exceeds 10%, almost no n-butyl mercaptan exists in the polymerization system, and almost no n-butyl mercaptan residues are present at the ends of the resulting polymer; therefore, the polymerization rate is 51%. About 80
It has become clear that approximately 5% of polymers are unaffected by mercaptans (no mercaptan residues at the ends and the degree of polymerization has not decreased), and that they only contain a small amount of low polymerization degree. . It must be noted that even in the simplest polymerization system, batch polymerization, the influence of mercaptans is thus complex. C Problems to be Solved by the Invention The present invention applies a mercaptan that exhibits such complicated behavior to a continuous polymerization system, has a desired degree of polymerization, and has mercaptan residues at the terminals in a predetermined proportion.
The present invention ultimately provides a method for producing PVAc and PVA containing sufficient amounts of PVAc and PVA and having a narrow polymerization degree distribution. D Means for Solving the Problems The present invention uses mercaptans as a chain transfer agent in the methanol solution polymerization of VAc in a multistage continuous polymerization apparatus having n reactors (n2), and transfers the mercaptans to the reactor. Low polymerization degree with uniform polymerization degree distribution by continuously supplying
By saponifying PVAc, and further saponifying the PVAc by a conventional method, a low degree of polymerization with a uniform degree of polymerization distribution can be obtained.
The method for producing PVA is characterized in that the mercaptans are continuously supplied to each reactor according to the following formula (1). ₀ [X] _n / [X] _n =X _n -X _n-1 /1-X _n・C _x +1-[X
] _n-1 / [X] _n …(1) (However, [X] is the molar concentration of mercaptan,
x represents the polymerization rate, Cx represents the chain transfer constant to mercaptan, the subscript m means the m-th reactor, and ₀
[X] _n means the molar concentration of the mercaptan supplied to the m-th reactor relative to the total amount of vinyl acetate and methanol supplied to the first reactor, [X] _n means the molar concentration of the mercaptan in the m-th reactor death,
1mn-1, and when m=1, x _n-1 =
x ₀ =0, [X] _n-1 = [X] ₀ =0. ) Now, in continuous polymerization, the polymerization rate in the first reactor is X ₁ ,
If the polymerization rate in the second reactor is expressed as X ₂ and the polymerization rate in the m-th reactor is expressed as X _n , then the degree of polymerization P of PVAc produced by continuous polymerization in each reactor is expressed as follows Since it can be expressed as (2), the concentrations of mercaptans (expressed as [X] ₁ , [X] ₂ , and [X] _n ) can be calculated from this. 1/P=C _M +x _n /1-X _n Cp+1/1-x _n Cs [S]/[M
_{[ S ] _ _}
is the concentration of the solvent, [M] ₀ is the initial concentration of the hypothetical VAc monomer, and [X] _n is the concentration of mercaptan in the mth reactor. ) However, even if the concentration in each reactor is known, it is not easy to determine the conditions for supplying mercaptan to each reactor in order to keep the concentration constant. As a result of intensive research on this point, the present inventors have discovered that the object can be achieved by supplying mercaptan according to formula (1), and have arrived at the present invention.
Equation (1) can be rewritten more specifically as follows. First reactor ₀ [X] ₁ / [X] ₁ = x ₁ / 1-x ₁・Cx+1 (1-1) Second reactor ₀ [X] ₂ / [X] ₂ = x ₂ - x ₁ / 1−x ₂・Cx
+1-[X] ₁ / [X] ₂ (1-2) mth reactor ₀ [X] _n / [X] _n =x _n -x _n-1 /1-x _n・Cx
+1- [X] _n-1 / [X] _n (1-3) For example, if the polymerization rate in the first reactor is 30%, that in the second reactor is 60%, and Cx = 22, then ₀ [X ] ₁ / [X] ₁ = 10.4, ₀ [X] ₂ / [X] ₂ = 15.5
(However, [X] ₁ / [X] ₂ = 2), and the continuity of mercaptan to maintain the concentration of [X] ₁ and [X] ₂ determined from equation (2). It can be seen that the quantities supplied are 10.4 and 15.5 times these values, respectively. In order to achieve the target degree of polymerization, the continuous supply amount of mercaptan is determined by formula (1),
Low polymerization degree PVAc with uniform polymerization degree distribution or
It was completely unknown in the past to produce PVA by a multi-stage continuous polymerization method. In the present invention, by using n multi-stage continuous polymerization apparatuses.
When performing methanol solution polymerization of VAc, it is necessary to continuously add mercaptan up to the (n-1)th reactor and not to add mercaptan to the nth reactor. This causes almost complete consumption of the mercaptan in the last reactor,
This is to prevent mercaptan from getting into the recovery system and from getting into the product. In the continuous polymerization apparatus used in the method of the present invention, it is important that the inside of the reactor be sufficiently stirred. Although the stirring speed etc. vary depending on the capacity and form of the apparatus, it is not normally carried out industrially. In the method described above, the reaction solution is sufficiently refluxed at a sufficient stirring speed, so it can be considered that the stirring is sufficient. Mercaptans used in the present invention include alkyl mercaptans and substituted alkyl mercaptans. Examples of these include n-propyl mercaptan, sec-propyl mercaptan, n-butyl mercaptan, sec-butyl mercaptan, t-butyl mercaptan, n-pentyl mercaptan,
n-hexyl mercaptan, n-octyl mercaptan, n-decyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-
Hexadecylmercaptan, n-octadecylmercaptan, 2-mercaptanethanol, thioglycerol, thioglycolic acid and its salts, 2
-Mercaptopropionic acid and its salts, 3-mercaptopropionic acid and its salts, and the like. Methyl mercaptan and ethyl mercaptan can also be used, although they are somewhat difficult to control due to their low boiling points. These may be used alone or in combination of two or more. These mercaptans are continuously supplied alone or diluted with an inert solvent such as a polymerization solvent. Continuity here does not necessarily have to be continuous in the strict sense, but also includes being added as a pulsating flow. As a result of chain transfer, the mercaptan is introduced as a terminal group at one end of PVAc or PVA, so the introduced mercaptan residue can be effectively utilized. From this point of view, it may be advantageous to use an alkyl chain having as long a length as possible. Furthermore, when the mercaptan residue must not impair the properties of the final target product, PVA, a mercaptan with a short alkyl chain length or a substituted alkyl mercaptan substituted with a hydrophilic group is advantageously used. As mentioned above, methanol is used as a solvent in the present invention because it is easy to control the polymerization heat due to the latent heat of vaporization due to azeotropy with VAc, and because methanol vapor is blown in during the VAc removal step after polymerization,
This is because it is advantageous for saponification reaction as a methanol solution of PVAc. VAc
Although the ratio of VAc and methanol is not particularly specified, it is usually advantageous to continuously supply VAc at 50% by weight or more. Although the polymerization initiator is not particularly limited, an azo type initiator such as azobisisobutyronitrile (hereinafter sometimes abbreviated as AIBN) is advantageous. Naturally, the appropriate amount of initiator is used depending on the polymerization rate, but it can be supplied not only to the first reactor but also continuously to the second and subsequent reactors depending on the polymerization rate. good. In equation (1), [X] _n , ₀ [X] _n is, as mentioned above,
They respectively represent the molar concentration of mercaptan in the m-th reactor and the molar concentration of the mercaptan fed to the m-th reactor relative to the total amount of VAc and methanol fed to the first reactor, and this unit is weight. It goes without saying that this is a good thing. [X]
If the polymerization rate x _n is determined using equation (2), _n is Cm, Cp, Cs, Cx
Since is a constant, it is expressed as a ratio to the virtual initial concentration of VAc monomer. The hypothetical initial concentration of VAc is the concentration of VAc when the polymerization rate is 0 (this does not exist in continuous polymerization), and this is essentially the concentration of VAc when considering a solution consisting of VAc and solvent. can be calculated as a value. Therefore, [X] _n is calculated as the molar ratio with this virtual initial VAc concentration,
Subsequently, ₀ [X] _n is similarly calculated as a molar ratio with the virtual initial VAc concentration, and these can of course be expressed in terms of weight or other methods. Chain transfer constant to mercaptans used in the present invention
Since there are only a few literature values for Cx and the reliability of the values cannot be said to be problem-free, it is desirable to obtain it in advance using a conventional method, and this task is not very difficult for those skilled in the art. . In the method of the present invention, ₀ [X] _n /[X] _n is calculated and converted into a numerical value, but the scope of the present invention does not necessarily apply only when this numerical value is employed extremely accurately. If it deviates from this calculated value, it is natural that the target degree of polymerization will deviate, but it will also affect the distribution of the degree of polymerization. If ₀ [X] _n , which should be supplied at a constant rate, is changed intentionally or by chance, the degree of polymerization and its distribution will similarly change. However, the degree of polymerization and the distribution of the degree of polymerization used in polymer chemistry do not have a strict meaning, so deviations in the degree of polymerization or changes in the degree of polymerization within the range of common sense are naturally taken into consideration in the present invention. It is possible. In the present invention, PVAc obtained as a result of polymerization is saponified by a known method to become PVA. Saponification may be either complete saponification or partial saponification, and is not particularly limited. E Function and Effects of the Invention The present invention provides PVAc and PVA having a desired degree of polymerization and having mercaptan residues at the terminals in a predetermined ratio.
containing a sufficient amount of PVAc and having a narrow polymerization degree distribution
A method of manufacturing PVA may be provided. EXAMPLES The present invention will be specifically explained below with reference to Examples, but the present invention is not limited to these Examples in any way. Example 1 In methanol solution polymerization of VAc using a two-stage continuous polymerization device, 75% by weight of VAc and approx.
An experiment was conducted to obtain PVA with a degree of polymerization of 550 under continuous charging conditions of 25% by weight of AIBN and 0.033% by weight of AIBN (relative to VAc). The polymerization rate in the first reactor was set to 53% (x ₁ =
0.53), and that of the second reactor was 82% (x ₂ =0.82). The molar ratio of 2-mercaptoethanol (Cx=22 was determined in advance) and VAc in the first reactor is calculated from equation (2) to be 0.216×10 ^-4 (where CM=CP=Cs=2 ×10 ^-4 ). (1
-1) From the formula, ₀ [X] ₁ / [X] ₁ = 25.8, and from this, the ratio of 2-mercaptoethanol that is continuously charged to the first reactor and that in the first reactor can be found, so 0.216 × The molar ratio to the supplied VAc is 10 ^-4 × 25.8 = 5.57 × 10 ^-4 . When converted to weight
The weight ratio to VAc is 5.06×10 ^-4 . In accordance with the above settings, an experiment was conducted as follows. Add 440% VAc monomer to the first reactor.
parts/hour, methanol 147 parts/hour, AIBN
0.145 parts/hour, 2-mercaptoethanol
0.222 parts/hour (supplied mixed with methanol)
The reaction liquid is continuously fed from the first reactor to the second reactor while keeping the liquid volume constant, and the reaction liquid is continuously supplied to the second reactor while keeping the liquid volume constant in the second reactor. The reaction solution was drained out of the system. Vigorous boiling of the reaction liquid occurred inside each reactor, and combined with mechanical stirring, mixing was considered complete. As a result of sampling at the respective outlets of the first reactor and the second reactor, the residence time of 1 to
The polymerization rate becomes almost constant around 1.5 times, and each
53% and 82%. The degree of polymerization (by viscosity measurement after conversion to PVA) was approximately 630 and 510, respectively. VAc remaining from reaction solution after completion of reaction
By removing azeotropically with methanol to obtain a methanol solution of PVAc, and saponifying it by a conventional method,
PVA with a polymerization degree of 580, which is almost in line with the target.
I got it. Example 2 PVA with a polymerization degree of 100 using a three-stage continuous polymerization device
The following experiments were conducted with the aim of obtaining the following. First, the conditions are VAc 70% by weight, methanol approximately 30% by weight, polymerization rate of the first reactor 40% (x ₁ = 0.40), second reactor
Setting the polymerization rate of the reactor as 80% (x ₂ = 0.80) and the polymerization rate of the third reaction as 94% (x ₃ = 0.94), using equation 2, n
- Calculating the molar ratio of dodecyl mercaptan to VAc, in the first reactor, [X] ₁ / [M] ₀ = 2.53
×10 ^-4 , in the second reactor [X] ₂ / [M] ₀ = 0.714×
10 ^-4 . From equation (1-1), the ratio of n-dodecyl mercaptan supplied to the first reactor to n-dodecyl mercaptan present in the first reactor ( ₀
[X] ₁ / [X] ₁ ) is calculated as 15.7, and also (1
-2) From the formula, the ratio ( ₀ [X] ₂ / [X] ₂ ) of n-dodecyl mercaptan supplied to the second reactor and n-dodecyl mercaptan present in the second reactor is
Calculated as 41.5. From these, ₀ [X] ₁ / [M] ₀ =
39.7×10 ^-4 , ₀ [X] ₂ / [M] ₀ = 29.7×10 ^-4 (both molar ratio), and the weight ratio is 93.3×10 ^-4
and 69.5×10 ^-4 . Note that the third reactor has n
-Dodecyl mercaptan shall not be supplied. Based on the above setting conditions, VAc200 copies/hour,
85.7 parts/hour of methanol, 0.05 parts/hour of AIBN, and 1.87 parts/hour of n-dodecylmercaptan are continuously introduced into the first reactor under reflux, and the liquid is sent to the second reactor while keeping the liquid volume constant. did. 1.39 parts/hour of n-dodecyl mercaptan was continuously fed to the second reactor. 1st reactor, 2nd reactor, 3rd reactor
Degree of polymerization at the reactor outlet (measured in acetone)
) of PVAc were 97.93 and 104, respectively, and were relatively stable except at the beginning of the reaction. After the completion of the third reactor, PVAc was made into a methanol solution by a conventional method and further saponified to obtain PVA (degree of polymerization 106). Comparative Example 1 Vinyl acetate was polymerized in the same manner as in Example 1, except that the amount of 2-mercaptoethanol continuously supplied to the first reactor was 1.2 times that in Example 1. The results are shown in Table 1. Comparative Example 2 Vinyl acetate was polymerized in the same manner as in Example 1, except that the amount of 2-mercaptoethanol continuously supplied to the first reactor was 0.8 times that in Example 1. The results are also shown in Table 1.

[Table] Comparative Example 3 Except for supplying 2-mercaptoethanol to the first reactor in Example 1 not continuously but in 5 parts and intermittently charging 0.0444 parts per batch every 12 minutes. Polymerization of vinyl acetate was carried out in the same manner as in Example 1. As a result, the degree of polymerization was 980, and PVA with the target degree of polymerization could not be obtained.

Claims (1)

[Claims] 1 In the methanol solution polymerization of vinyl acetate in a multi-stage continuous polymerization apparatus consisting of n (n2) reactors, from the first reactor (n-1)
Mercaptan is continuously supplied to each reactor up to the nth reactor according to the following formula (1), and mercaptan is not newly supplied to the nth reactor and polymerization is continued. A method for continuously producing polyvinyl acetate with a low degree of polymerization having a uniform degree of polymerization distribution. ₀ [X] _n / [X] _n =X _n -X _n-1 /1-X _n・C _x +1-[X
] _n-1 / [X] _n …(1) However, [X] is the molar concentration of mercaptan, x
is the polymerization rate, Cx is the chain transfer constant to mercaptan, the subscript m means the m-th reactor, and ₀
[X] _n means the molar concentration of the mercaptan supplied to the m-th reactor relative to the total amount of vinyl acetate and methanol supplied to the first reactor, [X] _n means the molar concentration of the mercaptan in the m-th reactor death,
1mn-1, and when m=1, x _n-1 =
x ₀ =0, [X] _n-1 = [X] ₀ =0. 2 In methanol solution polymerization of vinyl acetate in a multi-stage continuous polymerization apparatus consisting of n (n2) reactors, mercaptan is added to each reactor from the first reactor to the (n-1)th reactor. By continuously supplying mercaptan according to the following formula (1) and continuing polymerization without newly supplying mercaptan to the n-th reactor, low polymerization degree polyvinyl acetate having a uniform polymerization degree distribution can be obtained. , and then saponification by a conventional method. ₀ [X] _n / [X] _n =X _n -X _n-1 /1-X _n・C _x +1-[X
] _n-1 / [X] _n …(1) However, [X] is the molar concentration of mercaptan, x
is the polymerization rate, Cx is the chain transfer constant to mercaptan, the subscript m means the m-th reactor, and ₀
[X] _n means the molar concentration of the mercaptan supplied to the m-th reactor relative to the total amount of vinyl acetate and methanol supplied to the first reactor, [X] _n means the molar concentration of the mercaptan in the m-th reactor death,
1mn-1, and when m=1, x _n-1 =
x ₀ =0, [X] _n-1 = [X] ₀ =0.