GB1592178A - Polymerization and rcovery of 2-pyrrolidone - Google Patents

Description

(54) POLYMERIZATION AND RECOVERY OF 2-PYRROLIDONE (71) We, CHEVRON RESEARCH COM- PANY, a corporation duly organised under the laws of the State of Delaware, United States of America, of 525 Market Street, San Francisco, California, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an alkaline-catalyzed process for polymerizing 2-pyrrolidone.

In a further aspect, this invention relates to such an alkaline-catalyzed process wherein the reaction product mixture of particulate poly-2-pyrrolidone is treated with acid to reduce the pH of the mixture. In a still further aspect, this invention relates to a pyrrolindone polymerization process wherein the catalyst comprises an alkaline pyrrolidone salt, prepared in situ via the reaction of an aqueous hydroxide solution with excess pyrrolidone and wherein the resulting product mixture is then maintained at an elevated temperature to reduce dimer content.

Poly-2-pyrrolidone (or Nylon-4) is produced by the alkaline-catalyzed polymerization of 2-pyrrolidone and is generally produced ;by batch bulk polymerization. The monomer and alkaline catalyst are charged to a reactor and held at polymerization temperature for 20 or more hours to obtain a hard block of polymer. Since, in general, only 30-70% of the monomer was converted to polymer, the product had to be chopped, ground and extracted with water to recover unreacted monomer. The final polymer particle size depended on the extent of grinding, as well as the percent conversion to polymer. Polymerization under agitation, such as in a stirred reactor, produces a product which is a powder or alkaline paste or slurry of particulate polypyrrolidone and polymerizate. At conversions above about 40 percent, the product is powdery. The product is conveniently washed with water to provide a particulate polypyrrolidone without grinding. However, a considerable amount of polymer degradation occurs if the alkaline product is contacted with water while still at the elevated temperature of the polymerization reactor.

If, on the other hand, the alkaline product is allowed to stand before contacting with water, it becomes a hard block of polymer requiring extensive processing and causing equipment clogging. Accordingly, the present invention concerns a novel method of substantially eliminating these problems and which facilitates both batch and continuous polymerization processes.

The catalyzed polymerization is initiated by a "dimer" formed by the condensation of two 2-pyrrolidone molecules or by other initiators such as N-acyl pyrrolidone. The dimer is systematically designated 1-(1-pyrro- lin-2-yl)-2-pyrrolidinone. It is more conveniently called pyrrolinyl-pyrrolidone, or "dimer". Dimer is present in commercially available 2-pyrrolidone in greater or lesser amounts. Because the amount of dimer in the pyrrolidone feedstock is uncertain, this can cause undesirable or too rapid polymerization, resulting in a poor-quality, lowmolecular-weight product. Hence, it would be desirable to remove the dimer from the feedstock. It is removed by fractional distillation with great difficulty or not at all.

Accordingly a preferred embodiment of the present invention includes a method for substantially eliminating such dimer and preventing uncontrolled polymerization.

In U.S. Patent 3,184,476 pyrrolinyl-pyrrolidone is removed from 2-pyrrolidone by heating with water for several hours at a temperature greater than 80"C. Slight acidification of the 2-pyrrolidone was found to accelerate the hydrolysis of the dimer. On the other hand, alkaline hydrolysis of the sensitive 2-pyrrolidone ring is said to occur even under mild conditions of prolonged heating of aqueous alkaline 2-pyrrolidone (see U.S. Patents 3,778,402 and 3,681,293).

U.S. Patent 3,213,066 discloses the production of granular polypyrrolidone by polymerization in a ball mill reactor. In the continuous polymerization process of U.S.

3,681,293 the polymerizate is extruded, and pelletized before drying and washing.

In summary, the process of the invention comprises polymerizing 2-pyrrolidone in the presence of an alkaline catalyst and treating the poly-2-pyrrolidone reaction product mixture, before the poly-2-pyrrolidone can appreciably agglomerate and in the absence of an intermediate washing step, with acid to reduce its pH to from 5-8 and thereby substantially prevent degradation and agglomeration of the poly-2-pyrrolidone.

In a preferred embodiment, the invention is applied to a continuous polymerization process, wherein excess 2-pyrrolidone is initially contacted with an aqueous hydroxide solution to form an aqueous alkaline mixture which is then maintained at an elevated temperature for the period of time necessary to reduce the dimer content of the mixture to within the desired limits for optimum rate of polymerization and product molecular weight, by hydrolysis of the dimer. The alkaline mixture is dehydrated to reduce its water content to less than about 1000 ppm and is contacted with an activator, preferably carbon dioxide, to produce the polymerizate, an activator alkaline mixture. The polymerizate is maintained at a temperature of about 20"--90"C under agitation in a polymerization zone which comprises one or more polymerization reactors to produce a product which is a powder, paste or slurry of particulate poly-2-pyrrolidone in the polymerizate.

In its broadest aspect the process of the invention for preventing ibase-catalyst polymer degradation and polymer aerlomeration is broadly applicable to virtually any alkaline-catalyzed process for polymerizing pyrrolidone. Thus, the polymerization can be conveniently effected iby polymerizing a mixture of pyrrolidone monomer alkaline lac tamate (catalyst), and optional activators and accelerators, in a suitable organic solvent. Typically, and preferably, an excess of pyrrolidone is used as the solvent: however, other inert organic solvents such as, for example, paraffinic hydrocarbons, such as hexane, heptane, and the like, could also be used. Also, as is well recognized, the poly merization is preferablv conducted under anhydrous or substantially anhydrous con ditions; i.e., the reaction mixture should contain less than about 0.2% water, based on the weight of pyrrolidone and solvent, and preferably less than about 0.05 /O. The optimum catalyst pyrrolidone ratio and polymerization temperatures will vary with the particular catalyst system used; however, typically, polymerization temperatures in the range of about from 15 to 1000C, preferably 25 C to 70"C, and polymerization times of about from 4 to 100 hours, preferably about from 6 to 72 hours, are used. Generally, best results are obtained using polymerization temperatures in the range from about 40"C to 800C and polymerization times of about from 6 to 48 hours. In a typical alkali metal pyrrolidonate catalyst system, using pyrrolidone as the solvent, a catalyst content of about from 0.5 to 30 mol percent, preferably about from 1 to 20 mol percent, based on total pyrrolidone is used.

"Total pyrrolidone" refers to all of the pyrrolidone charged to the system including that present in the catalyst as well as solvent and reactant.

In accordance with the practice of the present invention, the polymer reaction product mixture (e.g., a slurry, viscous paste, or powder, of poly-2-pyrrolidone, alkaline catalyst and solvent) is treated with sufficient acid to form a mixture having a pH of about from 5 to 8. Typically the acid treatment is conducted at temperatures in the range of about from 20 to 700C and preferably about from 30 to 60"C. In order to be effective to prevent polymer agglomeration, the acid treatment should, of course, be effected before the polymer hardens. Preferably the reaction product mixture is discharged into another vessel for the acid treatment in order to facilitate continuous use of the reactor. Polymer agglomeration is particularly undesirable, since the hardened polymer mass can only be removed from the reactor or discharge vessel with extreme difficulty, and if the hardening occurs in the reaction product conveying equipment (typically conveyor augers or screws), this results in the jamming or breakdown of this equipment.

The particular acid used is not critical, so long as it does not cause undue contamination problems or unusual polymer degradation problems in the treatment. Both inorganic and organic acids can be used. Suitable organic acids which can be used include, for example, strong organic acids such as acetic acid, propionic acid, formic acid, p-toluenesulfonic acid, and the like, and mixtures thereof. Preferably strong mineral acids such as, for example, sulfuric acid, hydrochloric acid, phosphoric acid, and the like, or mixtures thereof, are used. Conveniently, an aqueous solution of the acid is used. However, water may be added concomitantly with the acid to facilitate dispersion of the acid.

As Ibefore-noted, the amount of acid used should be adiusted to reduce the pH of the reaction product mixture such that the resulting slurry has a pH in the range of from 5 to 8. This is important, since lower pH's can cause acid degradation of the polymer and higher pH's are ineffective to prevent base degradation. The acid treatment also prevents ibase-catalyzed polymer degradation (hydrolysis), and consequently permits washing with water at higher temperatures in the range of 60-800C up to about 70--80"C.

Because the acid treatment prevents polymer agglomeration, it is especially applicable to continuous polymerization processes.

Where a continuous system is used, it is especially preferred to transfer the product reaction mixture from the reactor or polymerization zone to another vessel or zone, preferably maintained at about the same temperature as the polymerization zone, and contact the reaction product mixture with the acid in this vessel or zone as a separate operation or step in the continuous process sequence. The resulting mixture from this step is then conveniently transferred to a counter-current (water) washer to remove solvent, salts and other soluble impurities.

In a preferred continuous process, of the present invention, for the polymerization of 2-pyrrolidone, and specifically in the continuous production of catalysts for this polymerization, an aqueous hydroxide, such as 40% "by weight aqueous potassium hydroxide, is mixed with an excess of distilled pyrrolidone to form an alkaline mixture which is subjected to a rapid dehydration.

Rapid dehydration can be accomplished for example, by flash evaporation, such as in a thin-film evaporator. The dehydrated product is a substantially anhydrous solution of a pyrrolidonate salt, such as potassium 2pyrrolidonate, in 2-pyrrolidone. The 2-pyrrolidonate salt is a polymerization catalyst.

This solution is then sent to a carbonator, or the polymerization vessel, where carbon dioxide is added to the solution to form -a carbonated alkaline mixture which is then held under polymerization conditions. Rapid dehydration of the alkaline mixture is favored to avoid the base-catalyzed hydrolysis of 2-pyrrolidone to 4-aminobutyric acid which interferes with carbonation and thereby reduces the amount of carbonated catalyst available. If carbonation is substantially effected, then the yield of high-molecularweight poly-2-pyrrolidone is greatly reduced.

However, we also discovered that dimer was destroyed (hydrolyzed) by heating in the presence of base and that less dimer is destroyed by such rapid dehydration than by longer dehydration. Hence, it is necessary to balance the need to hydrolyze pyrrolinylpyrrolidone (dimer) against the danger of hydrolyzing the sensitive 2-pyrrolidone ring.

The dimer is a problem, because to achieve the high conversion of 2-pyrrolidone to polymer of high molecular weight, it is very desirable to maintain a close control of the nature and amount of polymerization initi ators and polymerization catalysts present in the polymerizate. Because each molecule of initiator is theoretically capable of giving rise to one polymer molecule, it is hypothe sized that an overabundance of initiator molecules produces a high conversion of monomer to polymer of low molecular weight in a short time. This is evidenced in a a continuous polymerization process by line plugging or build-up of low-molecular-weight solid polypyrrolidone. For example, the pre sence of 0.6 weight percent of pyrrolinyl pyrrolidone in the monomer feed was found to produce excessive line plugging by poly merization. This problem is not encountered when catalyst is prepared by batch dehydra tion of the pyrrolidone-KOH solution be cause the longer dehydration time required in batch polymerization destroys- the pyrro linyl-pyrrolidone polymerization initiator.

The continuous process, with its short con tact time between the addition of aqueous hydroxide and the charging of the polymeri zate to the reactor was found to present the unexpected risk of a runway polymerization because the amount of dimer was not suffi ciently reduced. On the other hand, the optimum selection of the amount of a selected initiator produces a high conversion of monomer to polymer of high molecular weight in a reasonable period of time. It is found that such an initiated continuous poly merization can be achieved by optimization of the amount of pyrrolinyl-pyrrolidone in the polymerizate by continuously subjecting the alkaline mixture of aqueous hydroxide and 2-pyrrolidone to a brief heat treatment, i.e., a step achieving hydrolysis of the dimer, as will be further described hereinbelow.

Typically, in the preferred operation of this process, the initial commercial 2-pyrro lidone is purified for polymerization by frac tonal distillation. The middle fraction is taken, but may still contain as much as several percent pyrrolinyl-pyrrolidone, typi cally about 1 weight percent pyrrolinyl pyrrolidone. This amount of dimer is both exceedingly difficult to remove and very deleterious to the continuous polymerization process.

Hence, in a typical process of this embodi ment, the middle fraction of 2-pyrrolidone is used for in-situ catalyst production by contacting same with an aqueous hydroxide solution. The aqueous hydroxide solution can be a solution of an alkaline metal hy droxide, an alkaline earth metal hydroxide, or a quaternary ammonium hydroxide. Pre- ferably, it is an alkali metal hydroxide such as -aqueous NaOH or preferably aqueous KOH, having a concentration of 10-60 - weight percent hydroxide. An aqueous NaH solution of about 10-25 weight per cent NaOH is also preferred, but most satis factory results have heretofore been obtained with aqueous KOH solutions of 20--60 weight percent, and usually of 3545 weight percent KOH. (See U.S. Patent 3,778,402).

The aqueous hydroxide is continuously contacted with the distilled 2-pyrrolidone in relative amounts such that an excess of 2pyrrolidone is present. The hydroxide and 2-pyrrolidone are normally fed at a rate such that hydroxide constitutes 0.5-30 mol percent, preferably about 5-20 mol percent, and most preferably about 10 mol percent of the mixture, based on total 2-pyrrolidone.

That is, after dehydration, the amount of catalysts, i.e. the amount of 2-pyrrolidonate salts, constitutes 0.5-30 mol percent, pre ferably about 5-20 mol percent, and most preferably about 10 mol percent of the dehydrated mixture based on total 2-pyrrolidone. Total 2-pyrrolidone includes both 2pyrrolidone and 2-pyrrolidonate salt.

In order to reduce the dimer content of the alkaline mixture within the desired limits by hydrolysis of the dimer, the alkaline mixture of aqueous hydroxide and 2-pyrrolidone is maintained at a temperature of from 25 60"C, preferably 30--50"C, for a period of about 3-60 minutes, preferably about 1020 minutes, at a pressure ranging from subatmospheric to superatmospheric, preferably at atmospheric pressure. The time period of hydrolysis which is selected can be varied inversely to the temperature of hydrolysis which is selected, and vice versa, to achieve hydrolysis of the dimer to within the desired limits. The hydrolysis zone can simply consist of an isolated and thermostated vessel whose volume is chosen to give a residence time corresponding to the selected period of hydrolysis under the chosen conditions of continuous feeding of the alkaline mixture and temperature. Depending on the dimer content of the distilled monomer, the alkaline mixture may enter the hydrolysis zone containing appreciable pyrrolinyl-pyrrolindone, but will exit from this zone, after the selected period of hydrolysis, containing about 0.01--0.1 weight percent dimer and preferably containing less than 0.05 weight percent dimer, based on the weight of total 2-pyrrolidone (the weight of total 2-pyrrolidone includes the weight of 2-pyrrolidone and its salt).

The alkaline mixture, containing only the desired amount of dimer, is sent to the dehydration zone consisting, for example, of a thin-film evaporator, or a vacuum distillation column, where water is rapidly removed under conditions which provide a dehydrated mixture, containing less than about 1000 ppm water, preferably less than 500 ppm water, based on the weight of total 2-pyrrolidone. Preferably the mixture is dehydrated under reduced pressure, preferably at 2-100 mm, and most preferably at 2-10 mm and about 750-1900C. The dehydrated alkaline mixture of 2-pyrrolidonate salt and 2-pyrrolidone is then sent to the carbonation zone, preferably being transferred through heated lines which maintain a temperature of about 70--90"C in order to retard polymerization.

To the substantially anhydrous mixture containing less than about 1000 ppm water, is added carbon dioxide, preferably while said dehydrated alkaline mixture is maintained at a temperature of about 60--95"C, pre ferably 7e-90"C. Sufficient carbon dioxide is absorbed by the alkaline mixture by provision of contact area and contact time such that carbon dioxide is added to the extent of about 10-50 mol percent based on the hydroxide, thus at 10 mol percent hydroxide, carbon dioxide is added to the extent of about 1-5 mol percent ibased on total 2pyrrolidone. Preferably, carbon dioxide constitutes about 1-5 mol percent, most prefera'bly about 3 mol percent of the alkaline mixture based on total 2-pyrrolidone. The carbonated alkaline mixture is sent to the polymerization zone where it is preferably mixed with additional purified monomer and additional polymerization promoters such as N-acyl compounds, tetramethylammonium halide, sulfur dioxide, acetic acid anhydride, dimer, etc.

Dimer, present in the purified monomer in known amount, is conveniently added to the polymerizate in controlled amounts by this means to initiate polymerization. Pre ferably, about 0.08-0.15 weight percent dimer, based on total 2-pyrrolidone including the added monomer, is added to the polymerizate by means of the addition of purified monomer.

The polymerization zone consists of one or more reactors, preferably used in series, wherein the temperature is maintained at 20--90"C, preferably about 40--60"C and most preferably about 45--55"C, and wherein the polymerizate is subjected to continual agitation, such as is provided by a stirred reactor mechanism. In continuous operation, the number of pounds of polymerizate in the reactor, divided by the feed rate of polymerizate in pounds/hour (which is substantially identical to the product take-off rate in continuous operation) equals the residence time in the polymerization reactor. The residence time, i.e. the reactor volume and the feed rate, is selected to provide product polymer of the desired molecular weight and to provide the desired degree of conversion of monomer to polymer. Generally, residence times are 4-36 hours, preferably 6-24 hours, depending on the temperature, the product desired, and the amounts of initiator and catalysts used to achieve that product.

The polymer exists the polymerization zone and is subjected to washing, drying, pelletiz tion, etc. as may be necessary for its ultimate use. Under these continuous polymerization conditions, the agitated reactor holds a product which is a powder, paste or slurry consisting of particulate poly2-pyrrolidone in the polymerizate, i.e. in the carbonated alkaline mixture continuously provided. Preferably the reactor holds" a powder which can then be withdrawn from the polymerization zone by an auger-like take-off or as a continuously overhead discharge at a constant rate. The product is continuously taken as an overflow discharge from the reactor and then treated with acid, as described above, preferably while maintaining the product at about polymerization zone temperature, or before substantial cooling of the product to a hard mass is allowed to occur.

The polymerization processes or treatments of the present invention are generally applicable to the production of polymers of C-alkyl-substituted pyrrolidone, such as 4methyl-2-pyrrolidone, and to the production of copolymers of 2-pyrrolidone, such as pyrrolidone/caprolactam copolymers as to the production of poly-2-pyrrolidone. Thus, in general, the processes will find use in the polymerization of 2-pyrrolidone, substituted 2-pyrrolidone, and any monomer capable of copolymerizing with 2-pyrrolidone under the stated conditions of alkaline polymerization catalysis. Preparation of polymers of 2-pyrrolidone using the process of this invention can be carried out with various amounts of monomer, catalysts, inert non-solvent liquids as in a dispersion polymerization, initiators, activators and other additives-the amount of each being properly coordinated to produce the most effective polymerization. Such polymerization initiators and catalysis aids include N-acyl lactams such as N-acetyl pyrrolidone or equivalently compounds such as acetic anhydride. Other activators can be used in replacement, or partial replacement, or in addition to, carbon dioxide. For example, sulfur dioxide can be used as a partial substitute for, or in lieu of, carbon dioxide and tetraalkyl ammonium halides such as tetramethyl ammonium chloride can find use in the polymerizate.

The accompanying diagram illustrates a non-limiting detailed embodiment of the continuous process of the present, invention.

Referring to the drawing, 2-pyrrolidone is admitted through line 1 and is joined with aqueous potassium hydroxide solution admitted through line 2. After the mixing of 2-pyrrolidone and aqueous potassium hydroxide, as in a line mixer in line 3, the alkaline mixture enters V,, a heated vessel, wherein the aqueous alkaline mixture is maintained at 250-600C for an average residence time of about 5-60 minutes. The aqueous alkaline mixture next passes via line 4 to V2 the dehydration zone, wherein a wiped-film evaporator removes water via line 5 and passes the dehydrated alkaline mixture via line 6 to the carbonator V3 wherein carbon dioxide is added through line 10. (Preferably, the dehydrated alkaline mixture passes directly from V2 to the polymerization zone V4 wherein a constant carbon dioxide pressure is maintained over the polymerizate.) The carbonated alkaline mixture is maintained at about 700-900C while passing in line 9 from V2 to the polymerization zone V4 wherein it is maintained at polymerization temperature in a continuously stirred reactor for an average residence time of about 4-36 hours. The product, having the consistency of a powder, paste or slurry, is removed from V4 by either an auger or by overflow and is then fed to the neutralizer V2 via line 8. Concentrated sulfuric acid is concurrently added to the neutralizer via line 7. The neutralized aqueous slurry formed in V is fed via line 11 to a countercurrent washer V6 concurrently with water from line 15. The used wash water from V6 containing unpolymerized 2-pyrrolidone, as well as K2SO4, is sent via line 13 to the monomer recovery zone V8, but a fraction of the used wash water passes via line 12 to neutralizer V2 wherein it is mixed with the concurrently provided acid and product to form the aqueous slurry. Recovered 2-pyrrolidone exits the monomer recovery zone V6 via line 14 while water is removed through line 19. Recovered monomer may be recycled through line 1 or taken-off for further purification. Washed polymer proceeds via line 16 to the dryer V7 from which water leaves via line 17 and dry polypyrrolidone is obtained from line 18. The polymer, so-dried,, may be obtained with some residual moisture. The moisture content of this stage depends on further processing conditions and requirements, but should preferably be less than about 40-25 weight percent water, most preferably less than 25 weight percent water. When further processing includes melt extrusion pelletization, at that stage, the water content should be reduced below about 0.1 weight percent water.

In the 2-pyrrolidone recovery process which may be employed, even where the wash solution may have a pH as low as 5, it is desirable to add acid to ensure that the pH of the wash solution is 7 or lower, and preferably about 3 to 5. The particular acid is not critical so long as it yields a salt with the alkaline catalyst which does not present unusual purification problems. Good results are obtained using strong mineral acids such as sulfuric acid, hydrochloric acid, phosphoric acid and the like, or mixtures thereof, and especially sulfuric acid. Suitable organic acids which can be used include, for example, strong organic acids such as acetic acid, propionic acid, formic acid, ptoluenesulfonic acid, and the like, and mixtures thereof.

The neutralized or acidification solution is then subjected to a first rapid evaporation, of the order of minutes to at most a few hours at atmospheric, but preferably under reduced pressure, preferably at about 25100 mm Hg (torr). The first evaporation removes a major amount, preferably about 90 weight percent or more, of the water present in the neutralized solution, and most pre ferably about 9899%. This first evaporation is preferably carried out at temperatures of about 500-1008C, but in general less than about 120 C. Preferably a forced circulation evaporator is used.

The evaporation of this large quantity of water typically precipitates a substantial amount of solids, e.g., salts such as potassium sulfate, which are relatively insoluble in 2-pyrrolidone. Where the salts are soluble in pyrrolidone, they are removed in a second evaporation step. Thus the product of the first evaporation step is normally a slurry of salts in 2-pyrrolidone and water. Furthermore, unless the evaporation is carried out substantially under these conditions, an appreciable amount, i.e. as much as 5 percent or more, of a benzene-insoluble organic residue is produced which effectively prevents the complete recovery of 2-pyrrolidone from the slurry in later steps of the process. The unsatisfactory slurry is observed to ;be "pasty". The residue is believed to comprise an oligomer or derivative of 2-pyrrolidone, such as gamma-aminobutyric acid.

The entire slurry, or the filtrate from the slurry is then directly subjected to a second evaporation at reduced pressure, i.e. most preferably about 3-5 torr, but preferably less than about 10 torr. The temperature of the slurry during the evaporative process is mer recovery facilities. The washed polymer, 1500 pounds/hr, is dried by heating at 1200C under atmospheric pressure in a fluidized dryer for 30 minutes. In this way, there is obtained 1000 pounds/hr of dry, polypyrrolidone powder having a weight average molecular weight of about 300,000.

Example 2 This example illustrates a batch process for the polymerization of pyrrolidone according to the invention.

In this example, 200 pounds (2.35 lb-mols) of purified 2-pyrrolidone is placed in a reactor equipped for vacuum distillation and fitted with a gas inlet tube. 15.4 lbs (0.235 Ib-mol, 10 mol percent based on 2-pyrrolidone) of 85.7% pure anhydrous potassium hydroxide is added and the reactor then swept with gaseous nitrogen. The mixture is then placed under reduced pressure and heated to incipient distillation of pyrrolidone to remove water formed by the reaction of pyrrolindone and potassium hydroxide. The resulting solution is then cooled to 30"C and 3.1 lbs (0.07 lb-mol), 3 mol percent based on total pyrrolidone) of carbon dioxide is bubbled through the solution, under vacuum.

The flask is then brought to atmospheric pressure by the addition of dry gaseous nitrogen. The mixture is heated and maintained at 50"C, with stirring for 12 hours and then transferred to another vessel.

Aqueous 2% sulfuric acid (700 Ibs) is then metered into the product reaction mixture with stirring, until pH-sensing instruments in the vessel indicate that the pH of the reaction product slurry is reduced to pH 7.

The slurry is then centrifuged to recover the relatively high-molecular-weight poly-2-pyrrolidone solids.

WHAT WE CLAIM IS : - 1. A process for the polymerization of 2-pyrrolidone, which comprises polymerizing a substantially anhydrous reaction mixture comprising 2pyrrolidone and an alkali line catalyst to form a product mixture comprising particulate poly-2-pyrrolidone and alkaline catalyst and, before said poly-2pyrrolidone can appreciably agglomerate and in the absence of an intermediate washing step, treating said product mixture with an acid to reduce the pH of the mixture to a value in the range from 5 to 8 and thereby substantially prevent degradation and agglomeration of the poly-2-pyrrolidone.

2. The process of Claim 1, wherein said acid is an aqueous acid.

3. The process of Claim 2, wherein said acid is selected from sulfuric acid, hydrochloric acid and phosphoric acid, and mixtures thereof.

4. The process of Claim 3, wherein the acid is sulfuric acid.

5. The process of Claim 1, 2, 3 or 4, wherein the reaction mixture contains excess 2-pyrrolidone as a solvent for the reaction mixture.

6. The process of Claim 5 and further comprising recovering unreacted 2-pyrrolidone by washing the acid-treated reaction product mixture with water and separating and recovering the resulting pyrrolidone-rich wash water and adding sufficient acid to said recovered wash water to reduce the pH of the wash water to 7 or below and removing by a first evaporation a major amount of the water from the neutral or acidified wash solution to provide a mixture comprising pyrrolidone and the substantially insoluble salt of said acid, and recovering 2-pyrrolidone from said mixture by a second evaporation under reduced pressure.

7. The process of Claim 6, wherein the acid added to the wash solution is sulfuric acid.

8. The process of any one of Claims 1 to 7, wherein the treatment of said product mixture with said acid is effected at a temperature in the range from 20 to 700C.

9. The process according to Claim 1, wherein said process is conducted as a continuous process and wherein said substantially anhydrous reaction mixture contains an activator and is prepared by the steps of: (a) containing an aqueous hydroxide solution with excess 2-pyrrolidone to form an aqueous alkaline mixture; (b) maintaining said aqueous alkaline mixture at an elevated temperature in the range from 25 to 60"C for the period of time necessary to reduce the dimer content of said mixture; (c) rapidly dehydrating said alkaline mixture to reduce its water content to less than 100 ppm; and (d) contacting the dehydrated mixture with an activator.

10. The process according to Claim 9, wherein the activator comprises caribon dioxide.

11. The process according to Claim 10, wherein said aqueous alkaline mixture is dehydrated in step (c) to reduce its water content to less than 1,000 ppm and wherein said dehydrated alkaline mixture is contacted with carbon dioxide in step (d) to produce a carbonated alkaline mixture and wherein said carbonated alkaline mixture is maintained under agitation in a polymerization zone at a polymerization temperature to form a product comprising particulate poly-2-pyrrolidone in said carbonated alkaline mixture and wherein said poly-2-pyrrolidone is withdrawn at the polymerization temperature from said polymerization zone.

12. The process of Claim 11, wherein after contacting said poly-2-pyrrolidone product with said acid to form an aqueous

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (25)

**WARNING** start of CLMS field may overlap end of DESC **. mer recovery facilities. The washed polymer, 1500 pounds/hr, is dried by heating at 1200C under atmospheric pressure in a fluidized dryer for 30 minutes. In this way, there is obtained 1000 pounds/hr of dry, polypyrrolidone powder having a weight average molecular weight of about 300,000. Example 2 This example illustrates a batch process for the polymerization of pyrrolidone according to the invention. In this example, 200 pounds (2.35 lb-mols) of purified 2-pyrrolidone is placed in a reactor equipped for vacuum distillation and fitted with a gas inlet tube. 15.4 lbs (0.235 Ib-mol, 10 mol percent based on 2-pyrrolidone) of 85.7% pure anhydrous potassium hydroxide is added and the reactor then swept with gaseous nitrogen. The mixture is then placed under reduced pressure and heated to incipient distillation of pyrrolidone to remove water formed by the reaction of pyrrolindone and potassium hydroxide. The resulting solution is then cooled to 30"C and 3.1 lbs (0.07 lb-mol), 3 mol percent based on total pyrrolidone) of carbon dioxide is bubbled through the solution, under vacuum. The flask is then brought to atmospheric pressure by the addition of dry gaseous nitrogen. The mixture is heated and maintained at 50"C, with stirring for 12 hours and then transferred to another vessel. Aqueous 2% sulfuric acid (700 Ibs) is then metered into the product reaction mixture with stirring, until pH-sensing instruments in the vessel indicate that the pH of the reaction product slurry is reduced to pH 7. The slurry is then centrifuged to recover the relatively high-molecular-weight poly-2-pyrrolidone solids. WHAT WE CLAIM IS : -

1. A process for the polymerization of 2-pyrrolidone, which comprises polymerizing a substantially anhydrous reaction mixture comprising 2pyrrolidone and an alkali line catalyst to form a product mixture comprising particulate poly-2-pyrrolidone and alkaline catalyst and, before said poly-2pyrrolidone can appreciably agglomerate and in the absence of an intermediate washing step, treating said product mixture with an acid to reduce the pH of the mixture to a value in the range from 5 to 8 and thereby substantially prevent degradation and agglomeration of the poly-2-pyrrolidone.

2. The process of Claim 1, wherein said acid is an aqueous acid.

3. The process of Claim 2, wherein said acid is selected from sulfuric acid, hydrochloric acid and phosphoric acid, and mixtures thereof.

4. The process of Claim 3, wherein the acid is sulfuric acid.

5. The process of Claim 1, 2, 3 or 4, wherein the reaction mixture contains excess 2-pyrrolidone as a solvent for the reaction mixture.

6. The process of Claim 5 and further comprising recovering unreacted 2-pyrrolidone by washing the acid-treated reaction product mixture with water and separating and recovering the resulting pyrrolidone-rich wash water and adding sufficient acid to said recovered wash water to reduce the pH of the wash water to 7 or below and removing by a first evaporation a major amount of the water from the neutral or acidified wash solution to provide a mixture comprising pyrrolidone and the substantially insoluble salt of said acid, and recovering 2-pyrrolidone from said mixture by a second evaporation under reduced pressure.

7. The process of Claim 6, wherein the acid added to the wash solution is sulfuric acid.

8. The process of any one of Claims 1 to 7, wherein the treatment of said product mixture with said acid is effected at a temperature in the range from 20 to 700C.

9. The process according to Claim 1, wherein said process is conducted as a continuous process and wherein said substantially anhydrous reaction mixture contains an activator and is prepared by the steps of: (a) containing an aqueous hydroxide solution with excess 2-pyrrolidone to form an aqueous alkaline mixture; (b) maintaining said aqueous alkaline mixture at an elevated temperature in the range from 25 to 60"C for the period of time necessary to reduce the dimer content of said mixture; (c) rapidly dehydrating said alkaline mixture to reduce its water content to less than 100 ppm; and (d) contacting the dehydrated mixture with an activator.

10. The process according to Claim 9, wherein the activator comprises caribon dioxide.

11. The process according to Claim 10, wherein said aqueous alkaline mixture is dehydrated in step (c) to reduce its water content to less than 1,000 ppm and wherein said dehydrated alkaline mixture is contacted with carbon dioxide in step (d) to produce a carbonated alkaline mixture and wherein said carbonated alkaline mixture is maintained under agitation in a polymerization zone at a polymerization temperature to form a product comprising particulate poly-2-pyrrolidone in said carbonated alkaline mixture and wherein said poly-2-pyrrolidone is withdrawn at the polymerization temperature from said polymerization zone.

12. The process of Claim 11, wherein after contacting said poly-2-pyrrolidone product with said acid to form an aqueous

slurry having a pH of 5 to 8, the aqueous slurry is washed with water and a particulate poly-2-pyrrolidone product is recovered.

13. The process for the polymerization of 2-pyrrolidone according to Claim 9, 10, 11 or 12, wherein said aqueous hydroxide solution is an aqueous alkali metal hydroxide solution.

14. The process for the polymerization of 2-pyrrolidone according to Claim 13, wherein said alkali metal hydroxide solution is a potassium hydroxide solution containing 20-60 weight percent KOH.

15. The process for the polymerization of 2-pyrrolidone according to any one of Claims 9 to 14, wherein said dimer content is reduced to- about 0.01--0.1 weight percent.

16. The process for the polymerization of 2-pyrrolidone according to any one of Claims 9 to 15, wherein said alkaline mixture is dehydrated under reduced pressure to reduce the water content to less than about 500 ppm.

17. The process for the polymerization of 2-pyrrolidone according to any one of Claims 10 to 16, wherein said dehydrated alkaline mixture is maintained at and is contacted with carbon dioxide at a temperature of about 60-950C to provide a carbonated alkaline mixture containing about 1-5 mol percent carbon dioxide based on total 2-pyrrolidone and said polymerization temperature is 400-600C.

18. The process for the polymerization of 2-pyrrolidone according to any one of Claims 9 to 17, wherein said polymerization zone comprises one or more stirred reactors in series and said polymerization temperature is 40--60"C.

19. The process for the polymerization of 2-pyrrolidone according to Claim -12, wherein said product, before substantial standing or cooling, is neutralized with aqueous sulfuric acid to form an aqueous slurry having a pH of 5-8, and said aqueous slurry is washed with water at a temperature of 60 80"C to obtain the particulate poly-2-pyrrolidone product.

20. The process of Claim 12 and further comprising (e) recovering the resulting wash waters and recovering pyrrolidone from said wash waters by adding sufficient acid, selected from the group of acids which react with said alkaline catalyst to yield a salt which is substantially insoluble in pyrrolidone to said recovered wash water to reduce the pH of the wash water to 7 or below and removing by a first evaporation a major amount of the water from the neutral or acidified wash solution to provide a slurry comprising pyrrolidone and the substantially insoluble salt of said acid, and recovering 2-pyrrolidone from said slurry iby a second evaporation under reduced pressure.

21, The prociess of Claim 12 or 20, wherein the acid employed to treat said product mixture is sulfuric acid.

22. The process of Claim 20 or Claims 20 and 21, wherein the acid used in step (e) is sulfuric acid.

23. The process of Claim 6, 8, or 20, wherein the acid added to the wash solution is an acid which produces an insoluble salt in pyrrolidone.

24. A process in accordance with Claim 1 for the polymerization of 2-pyrrolidone substantially as hereinbefore described with reference to the accompanying drawing.

25. A process in accordance with Claim 1 for the polymerization of 2-pyrrolidone, substantially as described in either of the foregoing Examples.