Polymerizing Tetrahydrofuran to Produce Polytetramethylene Ether Glycol Using a Modified Fluorinated Resin Catalyst Containing Sulfonic Acid Groups

The present invention relates to an improved process for producing polytetramethylene ether glycol from tetrahydrofuran using a catalyst which is a fluorinated resin containing both sulfonic acid groups and carboxylic acid groups or a blend of fluorinated resin containing sulfonic acid groups and a fluorinated resin containing carboxylic acid groups. An important aspect of the present invention involves activating the sulfonic acid groups containing fluorinated resin prior to its use as catalyst. This activation involves a partial dehydration of the catalyst to contain from not more than 1,000 to 2,000 ppm (0.1 to 0.2 wt. %) water. Further, the catalytic resin may be blended with one or more other resins to control its catalytic activity.

Poly(tetramethylene ether) glycol (PTMEG) is a commodity in the chemical industry, widely used in the manufacture of block copolymers in combination with polyfunctional urethanes and polyesters such as polybutylene terephthalate. It is commonly prepared by reaction of tetrahydrofuran (THF) with a strong acid catalyst such as fluorosulfonic acid and then quenching and hydrolyzing the product with water.

While this process has proved to be quite satisfactory, it is not as efficient as desired because the acid catalyst cannot be recovered and reused again. Moreover, disposal of the spent acid is a problem because of its toxicity and corrosiveness.

U.S. Pat. No. 4,120,903 describes the preparation of PTMEG by the polymerization of THF using, as catalysts, perfluorinated ion-exchange polymers with pendant sulfonic acid groups (PFIEP-SO.sub.3 H) which have a low enough solubility to allow easy separation from the product, thus permitting their reuse. A method for avoiding the disposal problem traditionally associated with the strong acid catalyst was therefore possible based on the reaction equations (1) through (4) of U.S. Pat. No. 4,120,903, Column 1, where P-SO.sub.3 H has the same meaning as PFIEP-SO.sub.3 H herein. The PFIEP-SO.sub.3 H catalyst is further described by F. J. WALLER and R. W. Van Scoyoc, CHEMTECH, Jul., 1987, pages 438-441.

There are two drawbacks to implementation of this technology, however, in that (1) the polymerization is sensitive to moisture necessitating that both the catalyst and the THF be specially dried before use and (2) the PTMEG made using this process had a number average molecular weight 10,000 or greater.

One approach to circumvent these drawbacks by Heinson et al using the PFIEP-SO.sub.3 H catalyst of U.S. Pat. No. 4,120,903 above is disclosed in U.S. Pat. No. 4,163,115. This patent discloses maintaining an anhydrous reaction environment by carrying out the polymerization of terahydrofuran in the presence of an acylium ion precursor , i.e. a carboxylic acid anhydride such as acetic anhydride and optionally the corresponding carboxylic acid. Esters of the PTMEG were formed with facile control of molecular weight in the desired commercial range or up to very high molecular weights. Recovery of the PTMEG, however, required saponification or methanolysis, and thereby diminishing the advantages of this process.

An insoluble catalyst, less sensitive to moisture in the polymerization system, and which could yield commercial grade PTMEG in the desired molecular weight ranges of 250 to 4,000 would have a distinct advantage.

The present invention is an improvement in the process for PTMEG manufacture within the commercially important molecular weight range of 250 to 4,000 in which THF containing the amount of water usually found in commercial polymerization-grade material, and an insoluble catalyst are brought together under conditions suitable for polymerization. After the desired polymerization has taken place, the catalyst may be separated from the bulk of the reaction mass for reuse and the PTMEG isolated by the usual methods. Part of the water introduced into the system either with the catalyst or the THF, acts as the chain terminator for the polymerization and, as such, is consumed. The catalyst of this invention is a blend of a perfluorinated ion exchange polymer containing pendant sulfonic acid groups (PFIEP-SO.sub.3 H) with a perfluorinated ion exchange polymer containing pendant carboxylic acid groups (PFIEP-CO.sub.2 H). The PFIEP-SO.sub.3 H/PFIEP-CO.sub.2 H may be used as is or it may be further blended with other, inert polymers before use. Ordinarily, these catalysts would not yield commercial grade PTMEG in the desired molecular weight range from 250 to 4,000 largely due to the poor access of the active sulfonic acid end groups to THF. The accessibility is considerably improved by swelling the catalyst in THF after activation by partial dehydration through (1) heating to 90.degree. C. or (2) by leaching the catalyst with dry THF or (3) drying the catalyst using a dehydrating agent like trimethyl orthoformate or (4) by distilling off the water contained in the catalyst or through any combination of the above methods.

10 The catalyst of this invention is a blend of perfluorinated ion exchange polymer containing pendant sulfonic acid groups (PFIEP-SO.sub.3 H) with a perfluorinated ion exchange polymer containing pendant carboxylic acid groups (PFIEP-CO.sub.2 H). Activating this catalyst is an extremely important step in preparing the desired low molecular weight PTMEG. The activity of the catalyst is improved sufficiently by partial dehydration by either (1) heating to 90.degree. C. for a period of time sufficient to reduce water content to a critical level, (2) leaching the catalyst with dry THF, or (3) drying the catalyst using a dehydrating agent, trimethyl orthoformate for example, or (4) by distilling off the water contained in the catalyst or through any combination of the above methods.

The polymerization process may be run either batchwise or continuously. Water input is monitored since it is a chain terminating agent and its levels affect the molecular weight of the product PTMEG. After the reaction mass is removed from the reactor, the unreacted THF and catalyst are removed by conventional means to yield the PTMEG product.

The catalyst of this invention is a blend of PFIEP-SO.sub.3 H and PFIEP-CO.sub.2 H formed by blending a melt-fabricable first polymer having sulfonyl functional groups and a melt-fabricable second polymer having carboxylic functional groups, hydrolyzing the functional groups to ionizable functional groups, and finally, if base has been used for the hydrolysis, acidifying the ionizable functional groups to the corresponding sulfonic and carboxylic acid groups.

The melt-fabricable first polymer having sulfonyl functional groups is typically a polymer having a fluorinated hydrocarbon backbone chain to which are attached the functional groups or pendant side chains which in turn carry the functional groups. The pendant side chains can contain, for example, ##STR1## groups, wherein R.sub.f is F, Cl, or a C-1 to C-10 perfluoroalkyl radical and n is an integer from 1 to 5. Ordinarily, the functional group in the side chains of the polymer will be present in terminal ##STR2## groups where R.sub.f and n have the meanings defined above. Examples of fluorinated polymers of this kind are disclosed in U.S. Pat. Nos. 3,282,875, 3,560,568, 3,718,627, 4,417,969 and 4,610,762.

The melt-fabricable second polymer having carboxylic functional groups is typically a polymer having a fluorinated hydrocarbon backbone chain to which are attached the functional groups or pendant side chains which carry the functional groups. The pendant side chains can contain, for example, ##STR3## groups, wherein Z is F or CF.sub.3, m is 1 to 12, and W is --COOR, --CF.sub.2 COOR or --CN, wherein R is lower alkyl. Ordinarily, the functional group in the side chains of the polymer will be present in terminal ##STR4## groups. Examples of fluorinated polymers of this kind are disclosed in British Patent No. 1,145,445 and U.S. Pat. No. 3,506,635.

The first and second polymers are then blended as described for example in U.S. Pat. No. 4,176,217, Col. 7, lines 43-55 and Example 1. The resulting blend is then extruded into film or pellet form, hydrolyzed with base, and then acidified to give a blend of PFIEP-SO.sub.3 H and PFIEP-CO.sub.2 H. Techniques for hydrolyzing and subsequently acidifying the ionizable functional groups of these polymers are described in U.S. Pat. No. 4,176,215, Col. 9 lines 25-45 and Example 1.

The catalysts of this invention can be used as is or blended with one or more inert polymers prepared from ethylenically unsaturated monomers. Examples of such monomers are ethylene, styrene, vinyl chloride, vinyl fluoride, vinylidene fluoride, chlorotrifluoroethylene, vinyl ethers, perfluoroalkyl vinyl ethers, tetrafluoroethylene, hexafluoropropylene, or combinations thereof. The solid blends of the polymers of this invention, containing preferably about 20 to 80% of the inert diluent polymer, can be prepared by techniques familiar in the art. Powders, granules or pellets of the individual polymers can first be mixed together. Such a mixture is then subjected to heat and pressure by various means, such as pressing, extruding in a screw extruder, or working on a roll mill or rubber mill. To insure formation of an intimate uniform blend, the steps can be repeated two or more times. For example, pressed films can be flaked or cut into small pieces and repressed into film. Extruded polymer can be chopped into pellets as it is extruded, and then re-extruded. Powders for blending can be made by grinding in a mill; cold grinding in a freezer mill is also a useful technique.

A suitable catalyst for use in this invention is a blend of 40-95% PFIEP-SO.sub.3 H and 60-5% PFIEP-CO.sub.2 H. The preferred catalyst is a blend of 70-95% PFIEP-SO.sub.3 H and 30-5% PFIEP-CO.sub.2 H. The PFIEP-CO.sub.2 H component can also be replaced with an inert filler like polytetrafluoroethylene, polychlorotrifluoroethylene or copolymers of either of these with hexafluoropropylene or a perfluoroalkyl vinyl ether wherein the alkyl groups contain from 1 to 6 carbon atoms, or any combination thereof.

Catalyst activation is an extremely important step in preparing the desired low molecular weight PTMEG. Unactivated catalyst, when used for polymerizing THF, usually results in production of PTMEG having molecular weight greater than 10,000. The catalyst can be activated by any one or a combination of the following methods:

(1) The catalyst is first dried in an oven at about .about.100.degree. C. for about 8 hours under reduced pressure ranging between 0.1-100 mm mercury. The catalyst is then mixed with THF containing 50 to 300 ppm water and the slurry heated under nitrogen pressure at 85.degree. C. or higher for at least three hours. The preferred range of activation temperatures is 85-150.degree. C. and the hold-up time should be at least 3 hours. This type of activation is useful because THF does not polymerize under these conditions. It is thermodynamically impossible to polymerize THF at temperatures greater than 85.degree. C. One plausible explanation for catalyst activation is the fact that the bulk of water present in the system is vaporized at these temperatures thereby dehydrating the catalyst. (2) The catalyst is leached with dry THF (water concentration <100 ppm) several times at temperatures ranging between 0.degree. and 90.degree. C. Each time the catalyst is leached with THF, the water concentration in the catalyst decreases. The most active catalyst is produced by leaching with THF until water concentration in THF is reduced to below 100 ppm. The estimated water concentration in the leached catalyst is about .about.2000 ppm. (3) The catalyst is dried in an oven at 140-180.degree. C. in a vacuum oven maintained at 0.1-10 mm mercury in the presence of trimethyl orthoformate (TMOF). The ratio of trimethyl orthoformate to the catalyst ranges between 0.5-50 by weight. The trimethyl orthoformate reacts with water present in the catalyst to produce methyl formate and methanol both of which are highly volatile. The catalyst is treated in several stages with TMOF, (Total amount of TMOF used is 0.5-50 times the amount of catalyst). The catalyst is then dried under the same conditions as above in a vacuum oven for 8 hours to ensure the unreacted TMOF along with the volatile reaction product are removed. The water concentration in the dehydrated catalyst is about 1000-2000 ppm. The dehydrated catalyst is cooled and ready to use for polymerizing THF. (4) The catalyst is dried by distilling off the water in the presence of THF. The wet catalyst is mixed with THF and the THF/water mixture distilled off either by heating at the normal boiling point of the solution at atmospheric pressure or by vacuum distillation at lower temperatures ranging between 0.degree.-60.degree. C. The amount of THF distilled and used in this process ranges from 10 to 20 times the weight of the catalyst, depending on initial moisture content of the catalyst. The catalyst contains from 1000 to 2000 ppm of residual water following this treatment and is active enough to be used as is without further activation.

The THF used for polymerization can be any of those commercially available for polymerization. Its water content usually varies between 10 and 1000 ppm. The peroxide content is normally less than 0.002% by weight. The THF may optionally contain a oxidation inhibitor such as butylated hydroxytoluene to prevent formation of undesirable color or excess peroxides. The type and concentration of the inhibitor have little or no effect on the polymerization.

The water contained in the system of this invention acts as the chain terminator for the polymerization and, as such, its concentration is important since it has an effect on the number-average molecular weight of the product PTMEG. The less water present the higher the average molecular weight. The more water present, the lower the molecular weight. If too much water is present the propagation rate is very slow. For these reasons, attention must be given to the water content of the system. Water enters the system either with the THF or the catalyst. As stated above, polymerization-grade THF usually contains about 0.001 to 0.1% water. The catalyst also contains water presumably as tightly held hydrates of the strongly acidic sulfonic acid groups. Even after the catalyst has been vacuum dried under stringent conditions, it is estimated that it still may contain up to about 0.2% water by weight. During reaction, water is removed from the system by being consumed as the chain terminator. Because of the above factors, it may be necessary, as processing proceeds and equilibrium conditions are approached, to adjust the water content of the THF being fed to the reactor system to maintain a relatively steady molecular weight in the desired range, from batch to batch.

The process of this invention can be run batchwise or continuously:

When run batchwise, a closed reaction vessel purged with a slow flow of dry nitrogen and fitted with an agitator is charged with THF and about 5 to 45%, preferably about 25 to 40%, by weight of the THF, of catalyst which has been activated by one or several combinations of the activation methods described above. The reaction mass is then held, with stirring or agitation, at a temperature ranging from ambient to the boiling point of THF, preferably at 0.degree. to 65.degree. C. If higher temperatures are desired, the reaction can be run under pressure. When the desired degree of polymerization has been reached, generally after 2 to 60 hours, the catalyst can be removed by filtration, centrifugation or decantation, and again dried, if necessary, before reuse.

The progress of the polymerization can be monitored by periodic sampling and analysis by, for example, gel permeation chromatography or by determination of the hydroxyl number.

When run continuously, the THF containing small amounts of chain-terminating water, and optionally the catalyst, also containing small amounts of water, are continuously fed into a reactor at rates which provide the requisite concentrations and suitable residence time, and product and excess reactants are continuously removed. Preferably, the catalyst is charged initially and is held in the reaction zone by suitable screens or filters, and can remain in continuous use. Reactor hold-up-times used in the continuous system range from 4 to 40 hours, operating at temperatures ranging from 5.degree. to 70.degree. C., using a feed rate to the reactor of 0.1 to 1.0 g THF per g of catalyst per hour.

After the reaction mass is withdrawn from the reactor, whether the reactor be batch or continuous, the unreacted THF and catalyst are recovered for recycle by conventional techniques to yield the PTMEG product.

If the catalyst loses its activity over a period of time it may be reactivated by one or several methods of activation outlined above.

Examples of batch and continuous polymerization experiments follow.

(a) A batch of pelletized (approximately 0.2 mm diameter .times. 0.5 mm long cylindrical) ion exchange resin blend designated NR-55 containing 78% PFIEP-SO.sub.3 H and 22% PFIEP-CO.sub.2 H by weight was activated by leaching with THF repeatedly until the water content of the THF was <80 mcg (micrograms water/mL THF). Leaching was carried out at 20.degree. C. and the activated catalyst was kept soaked in THF for 5 hours.

(b) A 45 g portion of the virgin catalyst as prepared in (a) above and 90.0 g THF were added to a glass-ware batch reactor, and heated to 85.degree. C. for 14 hours to initiate the polymerization. The reactor was then cooled to 2.degree. C. to allow propagation. After 1 hour at 2.degree. C. there was 7.0% conversion to PTMEG with a number average molecular weight (M.sub.n) of 550. At the end of 2nd hour the conversion reached 8.2% at M.sub.n 911. At the end of the 3rd hour conversion increased to 10.6% at M.sub.n 933. After 4 hours and 14.05% conversion to M.sub.n 940, the water concentration had increased to 99 mcg.

The catalyst was rinsed with dry THF, 0.15 g water was added to the reactor, and on the next day Recycle 1 of the same catalyst charge with 90 g new THF was started after heating to 85.degree. C. for 4.5 hours to reinitiate the polymerization.

The above data are reported in the following TABLE 1 extended through four recycles of the same catalyst charge:

Another sample of 78% PFIEP-SO.sub.3 H, 28% PFIEP-CO.sub.2 H as used in EXAMPLE 1 (a) above was acid treated and repeatedly rinsed with THF until the water concentration was 92 mcg. This virgin catalyst (39 g) and 77.0 g of dry THF were charged to the batch reactor and heated to 85.degree. C. for 20.0 hours to initiate the polymerization. Next, the reactor and its contents were cooled to 2.degree. C. for the propagation step of the reaction. After the 1st hour, 2.0% conversion to PTMEG with M.sub.n 2300 was found; after the 2nd hour, 7.9% conversion and M.sub.n 2300 were measured; and after the 3rd hour, 6.0% conversion and M.sub.n 2227.

A 31 .0 g sample of 78% pFIEp-SO.sub.3 H, 28% PFIEP-CO.sub.2 H catalyst as used in EXAMPLES 1 (a) and 2 above was soaked in water, dried at 20.degree. C., and leached with dry THF. The virgin catalyst was added to 63.4 g THF, heated to 85.degree. C. for 19.5 hours to initiate the polymerization, and cooled to 7.degree. C. for propagation. After 1 hour, 10.0% conversion to PTMEG of 1400 M.sub.n resulted.

A 32.0 g portion of 78% PFIEP-SO3H, 28% PFIEP-CO2H catalyst as used in the above EXAMPLES was leached with dry THF until its water content was mcg. The virgin catalyst and 64.0 g of THF were heated to 86.degree. C. for 12.0 hours to initiate the polymerization and cooled to 54.degree. C. for propagation. Results are reported in the following TABLE 2:

EXAMPLES 5 through 8 used a catalyst made from a blend of 80% by weight PFIEP-SO.sub.3 H and 20% PFIEP-CO.sub.2 H. The PFIEP-SO.sub.3 H precursor resin used had a melt index of 11. EXAMPLES 5 through 7 were normal particle size: 0.1 diameter .times. 0.5 mm long cylindrical pellets. The particle size of the catalyst used in EXAMPLE 8 was reduced by approximately one-half. The catalyst of EXAMPLE 9 was made from a blend of 80% by weight PFIEP-SO.sub.3 H and 20% fluorinated ethylene propylene polymer (FEP) of the normal particle size, and dried by extraction with THF in the usual manner.

The catalyst for EXAMPLES 5 and 6 started with 70.0 g of resin leached with dry THF at 45.degree. to 50.degree. C. several times until the water concentration in the THF was 166 mcg. The catalyst slurry was divided into approximately equal portions containing 35 g of catalyst and 70 g of THF. The same 35:70 weight ratio of Catalyst to THF was also maintained in EXAMPLE 7. The ratio varied from 35:100 to 35:72-76 for EXAMPLE 8, and was 32:65 in EXAMPLE 9.

In EXAMPLE 5 the catalyst slurry was charged to a glass reactor; in EXAMPLE 6 the reactor was made of steel.

In EXAMPLES 7 and 8 the catalyst was prepared by acid exchange followed by rinsing with dry THF at 21.degree. C. until water in the THF was 74 mcg.

Performance of these catalysts in polymerization of THF to PTMEG is reported in TABLE 3 below.

Reactor temperature was 68.degree. C. at 257 hours. Reactor temperature dropped to 61.degree. C. by 262 hours and maintained at that temperature until hour 689 at termination of run.