JPH0140043B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to low polymerization polyvinyl acetate (hereinafter abbreviated as PVAc) and low polymerization polyvinyl alcohol (hereinafter abbreviated as PVA) having a uniform polymerization degree distribution.
Relating to a manufacturing method. More specifically, mercaptans are used as chain transfer agents in the batch solution polymerization of vinyl acetate (hereinafter abbreviated as VAc) in methanol, and a specific amount of the mercaptans is added to the polymerization system before the start of polymerization. Once substantially initiated, VAc is polymerized while continuously feeding the mercaptan into the polymerization system at a constant rate.
Method for producing PVAc and obtained by the method
Regarding a method for producing low polymerization degree PVA having a uniform degree of polymerization distribution by saponifying PVAc by a conventional method, it is particularly suitable for producing PVAc and PVA having a degree of polymerization of 700 or less and having a uniform degree of polymerization distribution. The aim is to provide a method for
PVAc and PVA, especially PVA, are hydrophilic polymers that are used in large quantities in the form of aqueous solutions for pasting fibers, paper processing, and as protective colloids for vinyl acetate (hereinafter abbreviated as VAc) emulsions. As is well known. Furthermore, an aqueous solution of PVA is used for a wide range of purposes, such as spinning into vinylon fibers and forming into vinylon films, and various brands of PVA are manufactured by combining various degrees of polymerization and saponification. being produced. PVAc and PVA generally tend to have a high degree of polymerization, and it is quite difficult to industrially produce those with a degree of polymerization of less than 700, especially less than 400. It is no exaggeration to say that it is extremely difficult to industrially produce materials with a degree of polymerization of 100 or less. Despite the fact that there are various demands for a low degree of polymerization in terms of PVA applications, the current situation is that these demands have not yet been fully met. There are several proposals for producing PVA with a low degree of polymerization. For example, there is a method of cleaving the 1,2-glycol bonds present in PVA using periodic acid to produce PVA with a polymerization degree of about 100 or 60, but in this method, aldehyde is attached to both ends of PVA with a low polymerization degree. The manufacturing method itself cannot be said to be cheap, as it has disadvantages such as the aldehyde group reacting with other PVA molecules to form branches, or in extreme cases, becoming three-dimensional. . Another proposal is to use an alcohol with a chain transfer constant of 20 × 10 ^-4 or higher (e.g., ethanol, isoparopanol) as a polymerization solvent for VAc, thereby reducing the degree of polymerization by chain transfer to the solvent.
There are methods to produce PVAc and PVA. However, when producing PVA industrially by polymerizing VAc, the polymerization solvent is limited to methanol for the following reasons, and it must be said that it is extremely difficult to use a solvent other than methanol. . In other words, methanol is not only used as a diluent for VAc and a solvent for PVAc, but also because of the azeotrope between VAc and methanol, the heat of polymerization during polymerization is released outside the polymerization system, where it is cooled, and then returned to the inside of the system. There is a meaning to return, and the azeotropic temperature is about 60
℃ has the advantage of being a preferable temperature for the properties of the PVA obtained. Furthermore, since a large amount of unpolymerized VAc remains in the methanol solution of PVAc after polymerization, azeotropy with methanol is effectively used to remove this VAc. What is even more important is that
When PVA is produced from PVAc by saponification, a methanol solution of PVAc is an essential condition. Because the saponification reaction
The reason for this is that the acetyl group in PVAc is methanol-mediated, and the alkali only serves as a catalyst. Therefore, in the above proposal, after the polymerization is completed, a method is adopted in which, for example, an ethanol solution (containing a large amount of VAc) of PVAc is evaporated to remove ethanol and VAc from PVAc, and then dissolved in methanol again. As a third proposal, in the methanol polymerization of VAc,
There is a method of producing low polymerization degree PVAc and PVA by lowering the concentration of VAc to actively cause chain transfer to methanol. However, in this method, for example, in order to obtain PVA with a degree of polymerization of about 350, it is necessary to have a charging composition of 20% by weight of VAc and 80% by weight of methanol, and if the degree of polymerization is less than 200,
In order to obtain PVA, it is necessary to use less than 10% VAc and more than 90% methanol by weight. such low
Polymerization at a VAc concentration requires a large amount of initiator, has low production efficiency, and furthermore requires too much energy, so it is difficult to say that it can be industrially produced. It is known to reduce the degree of polymerization by adding to the polymerization system a substance that undergoes rapid chain transfer, such as a mercaptan. There are many examples in which it has been applied to the polymerization of VAc, and the degree of polymerization certainly appears to be decreasing. The present inventors also attempted to synthesize low polymerization degree PVAc and PVA by adding mercaptan to the polymerization system of VAc, but quite surprisingly, they discovered that it was not possible to obtain the targeted low polymerization degree PVA. . For example, if we use the chain transfer constant (hereinafter abbreviated as C _x ) of n-butyl mercaptan (hereinafter abbreviated as C
In order to synthesize PVAc with a degree of polymerization of 100, the molar ratio of mercaptan and VAc (expressed as [X]/[M] _p )
Theory teaches that it is sufficient to set the value to 2×10 ^-4 . Therefore, the concentration of n-butyl mercaptan was mixed with VAc and methanol (70/30 weight ratio).
In addition to azobisisobutyronitrile (hereinafter
0.016% by weight of AIBN (abbreviated as AIBN) was added, polymerization was carried out at 60°C, the polymerization was stopped after 3 hours, the polymer was taken out by a conventional method (the polymerization yield was 51%), and the degree of polymerization was measured. In fact, it was 1772. As mentioned below, the chain transfer constant of n-butyl mercaptan is more correct than 50, but
Even taking this into consideration, the degree of polymerization should be about 230 under the above conditions. Indeed, when the same experiment is conducted in the absence of mercaptan, the degree of polymerization of PVAc obtained is approximately 1900, so it can be said that the degree of polymerization is certainly lower. However, the difference in the obtained degree of polymerization of 1772 is too large compared to the target degree of polymerization of 100 (or 230), which is problematic. When comparing theory and practice in radical polymerization, it is often attempted to conduct the polymerization at a polymerization rate of not exceeding 10%, so the inventors also shortened the polymerization time to 30 minutes under the above conditions, and conducted the same experiment. When the degree of polymerization was measured, it was 1382
It was hot. 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 have arrived at the present invention as a result of intensive studies regarding this large difference between theory and experiment. The present invention will be explained in detail below. In the batch solution polymerization of VAc in methanol, the present invention processes mercaptan using the following formula (1) 2×10 ^-4 (1+[S]/[M] _p )+C _x [X]/[M] _p
=1/P...(1) (However, [S] and [X] are the concentrations of methanol and mercaptan, respectively, [M] _p is the initial concentration of VAc, and the unit is mol/, and C _x is the concentration of mercaptan. (chain transfer constant, P indicates the target degree of polymerization) is added to the polymerization system before the start of polymerization, and once the polymerization has substantially started, the mercaptan is continuously added to C _x ] / [M] _p R _pp (where R _pp is the initial polymerization speed, the unit is mole//second), which has a uniform polymerization degree distribution by polymerizing vinyl acetate while supplying it to the polymerization system at a rate of Low degree of polymerization
Method for producing PVAc and obtained by the method
The present invention provides a method for producing low polymerization degree PVA having a uniform polymerization degree distribution, which comprises saponifying PVAc by a conventional method. Generally, PVAc and
Theory teaches that the degree of polymerization of PVA is determined, but as mentioned above, in methanol solution polymerization of VAc with mercaptan added, it does not work at all as theory as a practical matter. It has been found that it is extremely important to add mercaptan to the polymerization system at a rate of C _x [X]/[M] _p R _pp as the polymerization progresses. The importance of this will be explained in more detail by showing experimental results. Table 1 shows n-
This shows the results for dodecyl mercaptan. Polymerization was carried out with a charging composition of 80% by weight of VAc and 20% by weight of methanol, and the polymerization yield was measured by sampling every 30 minutes, and the degree of polymerization of each PVAc was determined.

【table】

[Table] This is what was measured. Experiment No. I is the result in the absence of n-dodecyl mercaptan, but the degree of polymerization of PVAc slightly increases from about 2500 to 2800 as the polymerization time increases. From experiment number onwards, n-dodecyl mercaptan was added to the polymerization system (C _x =22 was adopted). The experiment number is the one in which 0.225g of n-dodecyl mercaptan was initially charged into the polymerization system, and the left side of equation (1) is calculated as 25.4×
10 ^-4 , so theoretically PVAc with a degree of polymerization of 394
This is what is supposed to be. However, as is clear from the table, the degree of polymerization for the experiment number was 1631 even for the polymerization time of 0.5 hours (polymerization yield 6.0%), which is actually more than four times the theoretical value of 394. It is clear that as the polymerization time increases further, the degree of polymerization reaches about 2500, approaching the result of experiment number. On the other hand, in the experiment number, the amount of n-dodecyl mercaptan charged at once before the start of polymerization is the same as the experiment number, but the amount of n-dodecyl mercaptan charged continuously after the start of polymerization is C _x ] / [M] _p・R
Calculated according to _pp , dripping from Piuret
It was added over a period of 2.5 hours (R _pp was previously determined to be 3.26 x 10 ^-4 mol//sec from a preliminary experiment and was used as a value converted to the scale of the experiment). As is clear from Table 1, the degree of polymerization of PVAc obtained by sampling every 0.5 hours is between 395 and 414, which is extremely close to the theoretical value of 394. The experiment number is a known method, and the experiment number is a method of the present invention. In addition, the absolute amount of n-dodecyl mercaptan in the experiment number is 8.7 times as large as that in the experiment number, so there is a suspicion that the product with a low degree of polymerization was successfully produced due to the influence of this. The experiment was conducted by charging the same amount as the total amount of mercaptan before starting polymerization. As is clear from Table 1,
In the experiment number, the degree of polymerization of PVAc sampled at a polymerization time of 0.5 hours is 1053, which is overwhelmingly larger than the theoretical value (59 in this case), and is also very large compared to the experiment number. big. It is also clear that as the polymerization time increases (that is, as the polymerization yield increases), the degree of polymerization increases further and approaches the degree of polymerization of the experimental number in the absence of mercaptan. As mentioned above, the present inventors found that even if mercaptan is added only at the initial stage of polymerization (before the start of polymerization) in VAc batch solution polymerization, a product with a low polymerization degree as theoretically expected cannot be obtained. As a result of study, the following facts were discovered, and as a result, the present invention was arrived at. In other words, mercaptan added to the polymerization system before the start of polymerization is consumed extremely quickly once the initiator is charged and polymerization begins, and as a result, a polymer with a low degree of polymerization close to the theoretical one is produced at the very early stage of polymerization. In response to a sudden decrease in the concentration of mercaptan in the polymerization system, a polymer with a high degree of polymerization is produced. Table 2 shows the degree of polymerization of the polymer obtained in a short time (ie, low polymerization rate) after the start of polymerization. Table 2 is essentially the same as the experimental conditions in Table 1, but the scale has been reduced to 1/4. (purified and used). Table 2
Than,

[Table] Polymerization time 5 minutes (polymerization rate 1.4%) for experiment number in which n-dodecyl mercaptan was added before the start of polymerization (this is the same experiment number and experimental conditions in Table 1)
The polymerization rate was 503, which is quite close to the theoretical value of 394, and the polymerization rate rapidly increased to 1216 for the polymerization time of 10 minutes (polymerization rate 2.9%).
It is clear that the degree of polymerization behaves almost the same as in the experiment number. The results in Table 2 clearly show that the mercaptan is consumed at the very beginning of the polymerization. It is a surprising fact that in the radical polymerization of VAc, the effect of mercaptan as a polymerization degree regulator has already expired at a low polymerization rate of 2.9%, as mentioned above. It can be said that sufficient effects have been achieved for the first time through this method. One of the major characteristics of the method of the present invention is that PVAc and PVA with a narrow distribution of polymerization degree can be obtained, and this is clear from the above-mentioned experimental results. Determining the chain transfer constant C _x is important in the present invention. The measurement of C _x is based on the relationship between the degree of polymerization and the polymerization rate in the absence of mercaptan.
The degree of polymerization P _p is first determined, and then the degree of polymerization P at a polymerization rate of 0 is determined from the relationship between the degree of polymerization and the polymerization rate in the presence of mercaptan, and is calculated from the following formula. Polymerization rate 0
_{C _ _} A method of extrapolating a curve of reciprocal and polymerization rate to a polymerization rate of 0 may be adopted. In the present invention, the rate of continuous addition of mercaptan is
As mentioned above, it is extremely important to set the speed to C _x [X]/[M] _p ·R _pp , but this point will be further explained. It is important that the continuous addition rate is adjusted to the consumption of mercaptan, and if the polymerization rate changes significantly, it is important to change the addition rate in accordance with the polymerization rate to achieve a narrow polymerization degree distribution and the target. This is important in order to obtain a high degree of polymerization. However, as long as the initiator concentration does not change during the polymerization or the polymerization temperature changes, the continuous addition rate does not need to be changed. This is because _Rpp /[M] _p in the continuous addition rate equation is the ratio between the initial rate of polymerization and the initial concentration of VAc, but since the polymerization rate is usually linearly proportional to the VAc concentration, as polymerization progresses, VAc This is because even if the concentration decreases and the polymerization rate decreases, the ratio of the polymerization rate to the VAc concentration remains constant. However, if a so-called gel effect appears during polymerization (such as when the degree of polymerization is relatively high)
Therefore, it is desirable to slightly increase the continuous addition rate after the gel effect. Further, in the present invention, it is important to add a predetermined amount of mercaptan to the polymerization system before the start of polymerization. The amount varies depending on the target degree of polymerization, but
It is calculated and determined so that the left side of equation (1) is equal to the reciprocal of the target degree of polymerization. Polymerization is initiated at the concentration of mercaptan thus determined, and once polymerization has substantially started, mercaptan is continuously supplied to the polymerization system. Table 3-(A) shows the case where the calculated amount of mercaptan was added to the polymerization system before the start of polymerization (experiment number), and the case where about 10 times the amount of mercaptan was added to the polymerization system before the start of polymerization (experiment number). ) and when approximately 1/15th the amount of mercaptan was added to the polymerization system before the start of polymerization (experiment number),
The graph shows the change in the polymerization degree of PVAc as the polymerization time increases (the amount of n-dodecyl mercaptan that is continuously charged is the target polymerization degree of 100, R _pp = 2.79 ×
(calculated from 10 mol//sec).

[Table] Table 3-(A) shows that the degree of polymerization distribution is narrower and closer to the target degree of polymerization in the experiment number in which the initial amount of mercaptan (i.e., before the start of polymerization) is set to the target degree of polymerization. It turns out that. In the experiment numbers prepared so that the initial degree of polymerization was 1/10 of the target value, the degree of polymerization approached the target value as the polymerization progressed. In the experiment number in which the initial degree of polymerization was set to be 10 times the target value, the degree of polymerization was slightly higher than the target value during the short polymerization time, but as the polymerization progressed, the degree of polymerization approached the target value. It is clear that in these examples, the degree of polymerization distribution is wider than that prepared as calculated. Furthermore, in the present invention, it is extremely important to continuously feed mercaptan to the polymerization system at a rate of C _x [X]/[M] _p R _pp once the polymerization has substantially begun. No matter how much mercaptan is added in the predetermined amount determined by equation (1) above before the polymerization starts, after the polymerization has substantially started, the amount of mercaptan continuously supplied to the polymerization system is C _x [X ]/[M] _p R _pp If the amount is other than that determined by p R pp , the object of the present invention cannot be achieved. Table 3-(B) shows that a predetermined amount of mercaptan calculated by the above formula (1) is added to the polymerization system before the start of polymerization, and C _x [X]/[M] _p R _pp When a predetermined amount determined by the above is continuously supplied after the start of polymerization (experiment number Although the calculated predetermined amount of mercaptan is added to the polymerization system before the start of polymerization, the amount of mercaptan continuously supplied after the start of polymerization is determined by C _x [X]/[M] _p R _pp . (Experiment No. Although added to the system,
The amount of mercaptan that is continuously supplied after the start of polymerization is a predetermined amount determined by C _x [X]/[M] _p R _pp (
The change in the degree of polymerization of PVAc with increasing polymerization time was shown for the case (experiment number XII) where the amount was 1.2 times the amount used in experiment number X).

[Table] From Table 3-(B), it can be seen that the polymerization degree distribution was narrowest and closest to the target polymerization degree in the experiment number. In the case of Experiment Nos. XI and XII, it is clear that the target degree of polymerization was not achieved and the distribution of degree of polymerization was also wide. One of the features of the present invention is the obtained PVAc or
This is because the terminal groups of PVA are modified with mercaptan residues. Mercaptan residues introduced as terminal groups impart unique properties to polymers (especially PVA) depending on the type of original mercaptan, such as increasing the surfactant ability of PVA or increasing its hydrophilicity. It brings about. It cannot be said that this effect is sufficiently expressed by the conventional method of initially adding mercaptan to a VAc polymerization system. The reason for this is that, as can be seen from the experimental results explained so far, in conventionally known methods, mercaptan is consumed at the very early stage of polymerization, and after most of the mercaptan is consumed, the resulting polymer remains in the polymer. This is because it is impossible to introduce a mercaptan residue as a terminal group. As a result, the conventionally known method results in a composition of a low polymerization degree polymer having a mercaptan residue at its terminal and a high polymerization degree polymer having no terminal mercaptan residue. The two do not necessarily have good compatibility; for example, in the process of creating a film by evaporating water from an aqueous solution, a concentrated aqueous solution may undergo phase separation, or the created film may exhibit an opaque white color. Something happens. These phenomena mean that low polymerization degree PVA, which has mercaptan residues at the ends, is not completely compatible with high polymerization degree PVA, which does not have mercaptan residues at the ends, and this phase separation phenomenon is important for the applications of PVA. becomes significantly narrower. In the present invention, the polymer has a narrow distribution of degree of polymerization and most
Since mercaptan residues are introduced at the ends of the PVA molecules, such drawbacks do not exist. In addition
Contains a mercaptan residue at the end of the PVA molecule
If the fraction of PVA is defined as blocking efficiency, the blocking efficiency in the present invention depends on the degree of polymerization, and is approximately 85% and 94% at degrees of polymerization of 700, 300, and 100, respectively.
%, about 98%, and it can be said that the mercaptan is introduced to the terminal end extremely efficiently. Mercaptans used in the present invention include alkyl mercaptans and substituted alkyl mercaptans. Examples 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-decylmercaptan, n-dodecylmercaptan, t-dodecylmercaptan, n-hexadecylmercaptan, n-octadecylmercaptan, 2-mercaptoethanol, thioglycerol, thioglycolic acid and its salts, 2-mercaptopropionic acid and its salts, Examples include 3-mercaptopropionic acid and its salts, and methyl mercaptan and ethyl mercaptan may 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 introduced into the polymerization system alone or diluted with methanol or an inert solvent. In the present invention, the word "continuous" does not necessarily mean continuous in a strict sense. Naturally, this includes being added dropwise and being added as a pulsating flow using a pump or the like. However, based on the idea of the present invention, polymerization is 1%.
Addition methods that result in no mercaptan replenishment during near-term progress should be avoided. Methanol is advantageously used as the polymerization solvent in the present invention. There is nothing that cannot be done with bulk polymerization, but it is important to control the polymerization heat and to
There is not much advantage when considering VAc removal, saponification reaction, etc. A large molar ratio of methanol to VAc is not preferred industrially because it reduces production efficiency, and it is preferable that the molar ratio does not exceed 2.7 (approximately 50/50 in terms of weight ratio). In the method of the present invention, the molar ratio is 1.45.
(approximately 35/65 by weight) or less with low polymerization degree PVAc or
PVA can be manufactured. In the method of the present invention listed in the table in the main text, the molar ratio is 0.67 (weight ratio 20/80), which shows that the method of the present invention is superior. In the present invention, the batch polymerization method refers to a method in which the main portions of VAc and methanol are charged before the start of polymerization, and refers to a so-called batch polymerization method. The polymerization initiator used in the present invention is not particularly limited, and ordinary radical polymerization initiators can be used. Among them, an azo type initiator such as azobisisobutyronitrile (hereinafter sometimes abbreviated as AIBN) is advantageously used. It is advantageous to carry out the polymerization at the azeotropic temperature of VAc and methanol, which is about 60°C, in order to ensure that the heat of polymerization can be removed. However, it is possible to carry out the polymerization under pressure at higher temperatures or at lower temperatures.
However, in these cases, the term 2×10 ^-4 on the left side of equation (1) needs to be slightly changed to take into account the activation energy, but since the activation energy is only a few kilocalories, the effect is not that large. . The polymerized PVAc can be saponified to a desired degree of saponification by a known method if necessary, and can be made into partially saponified PVA or completely saponified PVA. In the present invention, the degree of polymerization is measured by a conventional viscosity method.
When the degree of polymerization is small as in the present invention, the degree of polymerization of PVAc and the degree of polymerization of PVA do not necessarily correspond. This is because the viscosity-degree of polymerization equation is difficult to apply to low degrees of polymerization. In the present invention, the degree of polymerization determined by measuring the viscosity of PVAc in an acetone solution is employed. In general, the degree of polymerization of PVAc immediately after polymerization and the reacetylation after saponification of this into PVA are
It does not necessarily match the degree of polymerization of PVAc. This is because a branching reaction (a type of grafting reaction) occurs on PVAc during polymerization, and the branch point is cleaved by the saponification reaction, and the degree of polymerization of PVAc immediately after polymerization is slightly higher than that of re-acetyl PVAc. It is well known that it is large. However, when the degree of polymerization is small, the effect of branching becomes small, especially when the degree of polymerization is small.
When it is 200 or less, the degree of polymerization of PVAc immediately after polymerization almost completely matches the degree of polymerization of PVAc re-acetylated after saponification. Low polymerization degree PVAc obtained by the method of the present invention and
PVA is useful in paints, textile processing, emulsion polymerization, paper processing, and more. EXAMPLES Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto. Hereinafter, unless otherwise specified, parts represent parts by weight. Example 1 Polymerization of VAc in the presence of 2-mercaptoethanol was preliminarily carried out and C _x =22 was determined. Under the polymerization conditions described below, R _pp = 2.79×10 ^-4 mol//
This was determined from another preliminary experiment. Degree of polymerization
VAc 960 parts and methanol with a goal of 200
After charging 240 parts into a reaction vessel and thoroughly purging the inside with nitrogen gas, the external temperature was set to 67°C. When the reaction solution started to reflux and the internal temperature reached approximately 59℃,
- Added 0.185 parts of mercaptoethanol to the reaction solution (left side of equation (1) is 2.00×10 ^-4 (1+240×86/960×32)
+22× 0.185×86/960×78=50.1×10 ^-4 ). Subsequently, 0.174 parts of AIBN was added to the reaction solution. Since preliminary experiments confirmed that there is a 3-minute induction period in this method, it was assumed that polymerization had substantially started 3 minutes after the addition of AIBN, and from this point on, continuous addition of 2-mercaptoethanol was started. . Continuous addition 2
-Mercaptoethanol C _x・[X]/[M] _p・R _pp = 22
× 0.185×86/960×78×2.79×10 ^-4 ×1.2/0.9×78=1.35
It is calculated as 6 x 10 ^-4 parts/second (however, 0.9 is the specific gravity of the mixture of VAc and methanol), so 2.44 parts (= 1.356 x
10 ^-4 parts x 3600 seconds x 5) diluted with methanol to 60
volume, and 12 volumes of this was added dropwise over an hour. That is, it was added dropwise at a constant rate of 0.2 volume/minute (more specifically, the time from when one drop was added until the next one was added was about 0.35 seconds). The contents were sampled every hour and added to hexane, the precipitated polymer was purified by a conventional method, and its degree of polymerization was determined from the following equation by measuring the viscosity at 30° C. using acetone as a solvent. [n] = 7.94×10 ^-3 ×P ^-0.62 (However, the unit of [n] is dl/g). The hourly polymerization degrees thus determined are 203, 226, 218, 224, and 219, and the hourly polymerization rates are 11.4, 22.5, 33.8, 44.6, and
It was 52.7%. After 5 hours, the reaction solution was cooled to 35° C., the remaining VAc was distilled out of the system together with methanol under reduced pressure, methanol was further added, and this operation was repeated twice to form a solution of PVAc in methanol. PVA was obtained by saponifying a portion of it using a conventional method. One portion was taken and re-acetylated using pyridine and acetic anhydride in a conventional manner, and the degree of polymerization of the resulting PVAc was measured to be 218. Examples 2 to 6 Various degrees of polymerization were carried out in the same manner as in Example 1, except that the amount of 2-mercaptoethanol was changed.
PVAc and PVA were obtained. The results are summarized in Table 4. As is clear from Table 4, PVAc was obtained almost as expected in each case, and the degree of polymerization was almost the same regardless of the polymerization time. These were converted into PVA by saponification using a conventional method. The obtained PVA had no coloration at all and was pure white. Examples 7 to 11 Polymerization was carried out in the same manner as in Example 1, with different types of mercaptan, aiming at a degree of polymerization of 100.
The mercaptans used were all from preliminary experiments.
C _x =22, and the initial polymerization rate was the same as in Example 1. The results are summarized in Table 5.

【table】

[Table] As is clear from Table 5, the degree of polymerization was almost as expected.
PVAc was obtained, and a polymer with a substantially constant degree of polymerization was obtained regardless of the polymerization time. These were saponified by a conventional method to obtain PVA having an alkyl group at the end. The obtained PVA had no coloration and was pure white. Example 12 Polymerization of VAc in the presence of thioglycerol was preliminarily carried out, and Cx=40 was determined. It was determined from another preliminary experiment that under the polymerization conditions described below, R _pp =2.23×10 ^-4 mol//sec. Polymerization was carried out in the same manner as in Example 1, aiming at a degree of polymerization of 80.
However, the initial addition amount of thioglycerol is 0.365
The left side of equation (1) is calculated as 12.44×10 ^-4 .
The continuous addition amount is calculated as 3.89 x 10 ^-4 parts/second, so dilute 7.00 parts with methanol to make 60 volumes.
Continuous addition was carried out over 5 hours in the same manner as in Example 1. As a result of hourly sampling, PVAc
The degree of polymerization is 89, 88, 90, 83, and 79, and the polymerization rate is
They were 7.9, 17.9, 26.3, 32.6, and 39.4%.
After completion of the polymerization, a portion of the PVAc produced was taken and saponified by a conventional method to obtain pure white PVA. Examples 13 to 15 Polymerization was carried out in the same manner as in Example 1 using n-butyl mercaptan, sec-butyl mercaptan and t-butyl mercaptan. However, C _x
22, 35, and 40 were determined separately, and a degree of polymerization of 80 was targeted. The initial polymerization rates were 3.26×10 ^-4 , 2.79×10 ^-4 and 2.79, respectively, as a result of preliminary experiments.
It was calculated to be ×10 ^-4 mol//sec. The polymerization results are shown in Table 6. As is clear from Table 6, PVAc having a degree of polymerization almost as expected was obtained, and the variation in degree of polymerization due to polymerization time (polymerization rate) was also small. these
Pure white PVA was obtained by saponifying PVAc using a conventional method.

【table】

Claims (1)

[Claims] 1. In a method of polymerizing vinyl acetate in methanol by a batch solution polymerization method, a mercaptan is used as a chain transfer agent, and the mercaptan satisfies the following formula (1) before the start of polymerization. 2×10 ^-4 (1+[S]/[M] _p )+C _x [X]/[M] _p
=1/P...(1) (However, [S] and [X] are the concentrations of methanol and mercaptan, respectively, [M] _p is the initial concentration of vinyl acetate, and the units are mol/, and C _x is (The chain transfer constant of mercaptan, P indicates the target degree of polymerization) Once the polymerization has substantially started, the mercaptan is further continuously transferred to C _x '[X]/[M] _p R _pp (where R _p
A method for producing low polymerization degree polyvinyl acetate having a uniform degree of polymerization distribution, characterized by polymerizing vinyl acetate while supplying it to a polymerization system at a rate ( _p is the initial polymerization rate and the unit is mol//second). . 2. In a method of polymerizing vinyl acetate in methanol by a batch solution polymerization method, mercaptans are used as a chain transfer agent, and the mercaptans are added to the polymerization system in an amount that satisfies the following formula (1) before the start of polymerization. In addition, 2×10 ^-4 (1+[S]/[M] _p )+C _x [X]/[M] _p
=1/P...(1) (However, [S] and [X] are the concentrations of methanol and mercaptan, respectively, [M] _p is the initial concentration of vinyl acetate, and the units are mol/, and C _x is The chain transfer constant of mercaptan, P indicates the target degree of polymerization.) Once the polymerization has substantially started, the mercaptan is further continuously transferred to C _x '[X]/[M] _p R _pp (however,
Homogeneous polymerization characterized by polymerizing vinyl acetate while supplying it to the polymerization system at a rate (R _pp is the initial polymerization rate in moles//second), and then saponifying the polymerization product by a conventional method. A method for producing low polymerization degree polyvinyl alcohol having a degree distribution.