JPH0140044B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous method for producing 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. More specifically, in the methanol solution polymerization of vinyl acetate (hereinafter abbreviated as VAc) using a multi-stage continuous polymerization apparatus consisting of n reactors (n1), mercaptan is determined by the following formula for each stage of the reactor.
A low polymerization degree polymer having a uniform degree of polymerization distribution is supplied continuously according to (1), and the polymerization is slightly advanced after the exit of the final reactor until entering the stripping process. The present invention relates to a method for continuously producing vinyl acetate, and a method for continuously producing polyvinyl alcohol with a uniform degree of polymerization, which comprises saponifying the PVAc by a conventional method. 0[X]m/[X]m=x _n −x _n-1 /1−x _n・Cx+1− [X]m−1/[X]m ...(1) (However, [X] is mercaptan molar concentration, x
is the polymerization rate, Cx is the chain transfer constant to the mercaptan, the subscript m means the m-th reactor, and ₀ [X]m is the amount of mercaptan supplied to the first reactor that is supplied to the m-th reactor. [X] m means the molar concentration of mercaptan in the m-th reactor, m is 1 mn, and when m = 1,
x _n-1 = x ₀ = 0, [X] m _-1 = [X] ₀ = 0. ) It has been known for some time that mercaptans can be applied to VAc polymerization systems, but all of them are based on the so-called batch polymerization method, and these methods have been used to achieve low polymerization degrees with a uniform polymerization degree distribution. It is not clear whether PVAc has been obtained or not, and there are no examples yet of it being applied 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 mercaptan 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 removed by azeotroping between VAc and methanol, and the azeotropic temperature is about 60°C, which is important for the properties of the obtained PVA. It provides a 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, it must have come a little closer to the theoretical value, but it must be said that the difference from the theoretical value of 100 (or 230) 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 n-butyl mercaptan is present at the ends of the resulting polymer.
Almost no butyl mercaptan residues are present,
Therefore, approximately 80% of a polymer with a polymerization rate of 51% is a polymer that is not affected by mercaptan (it does not contain mercaptan residues at the terminals and the degree of polymerization has not decreased), and it contains only a small amount of low polymerization degree substances. It has become clear that there are only a few. It must be noted that even in the simplest polymerization system, batch polymerization, the influence of mercaptans is thus complex. 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.
PVA that contains a sufficient amount of PVA and has a narrow polymerization degree distribution
The goal is to ultimately provide a method for manufacturing. In the methanol solution polymerization of VAc using a multi-stage continuous polymerization apparatus having n reactors (n1), the present invention uses mercaptans as a chain transfer agent to produce low polymerization degree PVAc with a uniform degree of polymerization distribution. Furthermore, in a method for producing low polymerization degree PVA having a uniform degree of polymerization distribution by saponifying the PVAc by a conventional method, mercaptan is continuously added to each stage of the reactor according to the following formula (1). It is characterized in that the polymerization is carried out slightly after the exit of the final reactor and before entering the stripping process. 0[X]m/[X]m=x _n −x _n-1 /1−x _n・Cx+1−[
X]m-1/[X]m (where, [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]m 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, and [X]m means the molar concentration of the mercaptan in the m-th reactor. Then, m is 1 mm, and when m=1,
x _n-1 = x ₀ = 0, [X] m _-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
can be expressed by the following equation (2), and from this, the concentrations of mercaptans (expressed as [X] ₁ , [X] ₂ , and [X]m) can be calculated. 1/P=C _M +x _n /1-x _n C _P +1/1-x _n C _S [S]/[M
] _{0 +} 1 _/ 1 _{− x n C} S]
is the concentration of the solvent, [M] ₀ is the initial concentration of the hypothetical VAc monomer, and [X]m 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] m / [X] m = x _n -x _n-1 /1-x _n・Cx + 1- [
X]m-1/[X]m (1-3) For example, if the polymerization rate in the first reactor is 30% and that in the second reactor is 60%, and Cx = 22, then ₀ [X] ₁ / It is calculated as [X] ₁ = 10.4, ₀ [X] ₂ / [X] ₂ = 15.5 (however, [X] ₁ / [X] ₂ = 2), and determined from equation (2). It can be seen that the continuous feed rate of mercaptan to maintain the concentrations of [X] ₁ and [X] ₂ is 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 addition, in the present invention, n multi-stage continuous polymerization devices are used.
When performing methanol solution polymerization of VAc, mercaptan is continuously supplied to each stage of the reactor according to the above formula (1), and the polymerization is carried out after the exit of the final reactor until entering the stripping process. It is extremely important to make slight advances in This is to cause almost complete consumption of the mercaptan and to prevent mercaptan from contaminating the recovery rate and the product. For example, in the case of continuous polymerization with one reactor, most of the mercaptan that is continuously supplied to the reactor is consumed, but some of it is sent from the outlet to the VAc stripping process. You will enter, and something undesirable will happen. Therefore, in this case, it is necessary to allow, for example, a slightly longer residence time in the piping from the reactor outlet to the stripping device, so that polymerization can proceed slightly during this time. The polymerization rate in this case is difficult to display, but it remains.
Expressed as a polymerization rate with respect to VAc monomer, it is necessary to have a polymerization rate of 5% or more. More preferably, it is 10% or more. This treatment drastically reduces the amount of mercaptan remaining. 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. differ 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-bentyl mercaptan,
n-hexyl mercaptan, n-octyl mercaptan, n-decyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-
Hexadecyl mercaptan, n-octadecyl mercaptan, 2-mercaptoethanol, thioglycerol, thioglycolic acid and its salts, 2-
Examples include mercaptopropionic acid and its salts, 3-mercaptopropionic acid and its salts, and the like. Methyl mercaptan, which is somewhat difficult to control due to its low boiling point,
Ethyl mercaptan can also be used. 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 the above mercaptan undergoing chain transfer,
Since it is introduced as a terminal group at one end of PVAc or PVA, 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]m, ₀ [X]m are the molar concentration of mercaptan in the m-th reactor and the mercaptan in the first reactor supplied to the m-th reactor, respectively, as described above. Although it represents the molar concentration relative to the total amount of VAc and methanol to be supplied, it goes without saying that the unit may be weight. [X]m is C _M if the polymerization rate x _n is determined by equation (2),
Since C _P , C _S , and C _X are constants, they are 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. It can be calculated as a value. Therefore, [X]m is calculated as the molar ratio with this virtual VAc initial concentration, and then ₀ [X]m is similarly calculated as the virtual VAc initial concentration.
It is calculated as a molar ratio to the initial concentration of VAc, and these can of course be expressed by weight or other methods. The chain transfer constant Cx to the mercaptan used in the present invention has a small number of literature values;
Since the reliability of this value is not without problems, 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]m/[X]m 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. It is natural that the target degree of polymerization will deviate if it deviates from this calculated value, but
Furthermore, it will also affect the polymerization degree distribution.
If ₀ [X]m, 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 adopted in polymer chemistry do not have a precise meaning, so deviations in the degree of polymerization or changes in the degree of polymerization within the range of common sense should naturally be taken into consideration in the present invention. be. 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. EXAMPLES The present invention will be specifically explained below with reference to Examples, but the present invention is not limited in any way by these Examples. Example 1 In the methanol solution polymerization of VAc using a one-stage continuous polymerization apparatus, conditions were set with the aim of achieving a degree of polymerization of 200. That is, VAc 80% by weight, methanol approximately 20% by weight, AIBN 0.05% by weight (relative to
VAc), and the polymerization rate is 70% (x ₁ =0.70), then from equation (2), [X] ₁ /[M] ₀ =0.530×10 ^-4 . (However, Cx
= 22). From formula (1-1), ₀ [X] ₁ / [X] ₁ =
52.3, and the weight ratio of continuously added n-dodecyl mercaptan to VAc is 65.1×10 ⁻⁴ . The following experiment was conducted based on the above settings. VAc
200 parts/hour, methanol 50 parts/hour,
0.100 parts/hour of AIBN and 1.30 parts/hour of n-dodecyl mercaptan were continuously introduced into the reactor under reflux, and the polymerization rate was maintained at 70% while keeping the liquid volume constant. . generated
The degree of polymerization of PVAc is 186 from viscosity measurement in acetone.
It was hot. In order to remove the residual VAc outside the reactor, methanol vapor was blown into the methanol solution of PVAc. Prior to this operation, the reaction solution removed from the polymerization system was heated at approximately 60°C for 15 minutes. , the remaining n-dodecyl mercaptan was consumed. The polymerization rate during this period was approximately 4
% (13.3% based on residual VAc).
PVAc and PVA obtained by saponification by conventional methods
(degree of polymerization: 195), no mercaptan odor was detected. Comparative Example 1 Vinyl acetate was polymerized in the same manner as in Example 1, except that the continuous supply amount of n-dodecylmercaptan was increased by 1.2 times. 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 continuous supply amount of n-dodecylmercaptan was increased by 0.8 times. The results are also shown in Table 1.

[Table] Comparative Example 3 Vinyl acetate was polymerized in the same manner as in Example 1, except that instead of continuously supplying n-dodecyl mercaptan as in Example 1, the total charge was divided into 5 parts and supplied intermittently. , the degree of polymerization of the produced PVAc is
1200, which was extremely large, and a low polymerization degree PVAc of the target polymerization degree of 200 could not be obtained. Example 2 PVA with a polymerization degree of 100 using a two-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
When the polymerization rate of the reactor is set to 80% (x ₂ = 0.80) and the molar ratio of n-dodecyl mercaptan to VAc is calculated using equation (2), in the first reactor, [X] ₁ /
[M] ₀ = 2.53×10 ^-4 , [X] ₂ / in the second reactor
[M] ₀ = 0.714×10 ^-4 . From equation (1-1), the ratio ( ₀ [X] ₁ / [X] ₁ ) of n-dodecyl mercaptan supplied to the first reactor and n-dodecyl mercaptan present in the first reactor is calculated to be 15.7. is,
Similarly, from equation (1-2), the ratio of n-dodecyl mercaptan supplied to the second reactor to n-dodecyl mercaptan present in the second reactor ( ₀ [X] ₂ /
[X] ₂ ) 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 ratios), and the weight ratios are 93.3 x 10 ^-4 and 69.5 x 10 ^-4 , respectively. 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-dodecyl mercaptan were continuously introduced into the first reactor under reflux, and then sent to the second reactor while keeping the liquid volume constant. It liquefied. 1.39 parts/hour of n-dodecyl mercaptan was continuously fed to the second reactor. The degrees of polymerization (PVAc measured in acetone) at the exits of the first reactor and second reactor were 97 and 93, respectively, and were relatively stable except for the initial stage of the reaction. The reaction solution taken out from the second reactor was heated to about 60℃ for 20 minutes.
The remaining n-dodecyl mercaptan was consumed by heating for a minute. The polymerization rate during this period is approximately 3% (15% relative to the remaining VAc)
It was hot. To remove residual VAc, methanol vapor was blown into the solution to obtain a methanol solution of PVAc, which was then saponified by a conventional method to obtain PVA (degree of polymerization 103), but no mercaptan odor was detected. Example 3 In methanol solution polymerization of VAc using a single-stage continuous polymerization device, VAc was 75% by weight and methanol was 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 0.037% by weight of AIBN (relative to VAc). The polymerization rate in the first reactor was set to 70% (x ₁ =
0.70). 2-mercaptoethanol (C _x = 22 was determined in advance) in the first reactor from equation (2)
The molar ratio between Vac and VAc is calculated to be 0.078×10 ⁻⁴ (C _M = C _P = C _S = 2×10 ⁻⁴ was used). From formula (1-1), ₀ [X] ₁ / [X] ₁ = 52.3, and from this, 2-
The ratio of mercaptoethanol to that in the first reactor is determined, so 0.078×10 ^-4 ×52.3=4.1×
10 ^-4 is the molar ratio to the supplied VAc. When converted to weight, the weight ratio to VAc is 3.7×10 ^-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
0.163 parts/hour, 2-mercaptoethanol
0.25 parts/hour (supplied mixed with methanol)
The reaction solution was continuously charged at a rate of 1, and the reaction solution was drawn out of the system from the first reactor while keeping the amount of the solution constant. As a result of sampling at the outlet of the first reactor, the polymerization rate became almost constant at 70% around 1 to 1.5 times the residence time. The degree of polymerization (measured by viscosity measurement after conversion to PVA) was 530. In order to remove the remaining VAc from the reaction solution after the reaction was completed outside the reactor, methanol vapor was blown into the solution of PVAc in methanol. The remaining 2-mercaptoethanol was consumed by heating it for 20 minutes. The polymerization rate during this period was about 3% (10% based on the remaining VAc). Obtained by saponification using PVAc and conventional methods
No mercaptan odor was detected in PVA (degree of polymerization 560).

Claims (1)

[Claims] 1 In methanol solution polymerization of vinyl acetate in a multi-stage continuous polymerization apparatus consisting of n reactors (n1), mercaptan is added to each stage of the reactor according to the following formula (1). Continuous production of low degree of polymerization polyvinyl acetate having a uniform degree of polymerization distribution, characterized by supplying continuously and slightly advancing polymerization after the exit of the final reactor until entering the stripping process. Method. 0[X]m/[X]m=x _n −x _n-1 /1−x _n・Cx+1− [X]m−1/[X]m ...(1) However, [X] is the mercaptan. Molar concentration, x
is the polymerization rate, Cx is the chain transfer constant to mercaptan, the subscript m means the m-th reactor, and 0
[X]m 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, and [X]m means the molar concentration of the mercaptan in the m-th reactor. Then, m is 1mn, and when m=1,
x _n-1 = x ₀ = 0, [X] m _-1 = [X] ₀ = 0. 2. According to claim 1, the polymerization after the exit of the final reactor until entering the stripping step is 5% or more, expressed as a polymerization rate with respect to unreacted vinyl acetate monomer at the exit of the final reactor. A method for continuously producing polyvinyl acetate having a uniform degree of polymerization as described above. 3. In methanol solution polymerization of vinyl acetate in a multi-stage continuous polymerization apparatus consisting of n (n1) reactors, mercaptan is continuously supplied to each stage of the reactor according to the following formula (1), In addition, by slightly advancing the polymerization after the exit of the final reactor until entering the stripping process, low polymerization degree polyvinyl acetate having a uniform degree of polymerization distribution is obtained, and then saponification is performed by a conventional method. A method for continuously producing low polymerization degree polyvinyl alcohol having a uniform degree of polymerization distribution. 0[X]m/[X]m=x _n −x _n-1 /1−x _n・Cx+1− [X]m−1/[X]m ...(1) However, [X] is the mercaptan. Molar concentration, x
is the polymerization rate, Cx is the chain transfer constant to mercaptan, the subscript m means the m-th reactor, and 0
[X]m 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, and [X]m means the molar concentration of the mercaptan in the m-th reactor. Then, m is 1mn, and when m=1,
x _n-1 = x ₀ = 0, [X] m _-1 = [X] ₀ = 0. 4. According to claim 3, the polymerization after the exit of the final reactor until entering the stripping step is 5% or more expressed as a polymerization rate with respect to unreacted vinyl acetate monomer at the exit of the final reactor. A method for continuously producing polyvinyl alcohol having a uniform degree of polymerization as described above.