PREVENTION OF GEL FORMATION DURING USE OF AND PRESERVING/REGENERATING ACTIVITY OF NAFION-TYPE POLYMERIZATION CATALYST

Abstract: A method of suspending the manufacture of diesters of polytetramethylene ethers by polymerizing THF or the like in a reactor utilizing a solid catalyst containing sulfonic acid groups in a liquid reaction medium containing a carboxylic acid with carboxylic acid anhydride (e.g., acetic acid/acetic anhydride) as molecular weight control agents involving the stopping of the addition of reactant THF and carboxylic acid anhydride and simultaneously increasing the concentration of carboxylic acid in said reactor to sustain and preserve catalyst activity. Also, a spent solid acid catalyst containing sulfonic acid catalytic reactive sites deactivated from sustained use during such ring-opening polymerization of THF can be regenerated by contacting the spent catalyst with a solution of THF and carboxylic acid for a time sufficient to convert any alkylated sulfonic acid reactive site to the reactive sulfonic acid site. In both the suspending of polymerization and regeneration of spent catalyst, a 7 weight percent or greater solution of acetic acid in THF has been found to be effective in preserving/restoring catalyst activity.

Description: PREVENTION OF GEL FORMATION DURING USE OF AND PRESERVING/REGENERATING ACTIVITY OF NAFION -TYPE POLYMERIZATION CATALYST Technical Field The present invention relates to an improved method of suspending catalytic ring-opening polymerization of tetrahydrofuran, THF, or the like employing a solid catalyst containing sulfonic acid groups without inducing (i.e., preventing) gel formation in the polymerization reactor while preserving the catalytic activity as well as method of regenerating such catalyst after being deactivated from sustained use during ring-opening polymerization. More specifically but not by way of limitation, the present invention relates to the use of a carboxylic acid dissolved in THF or the like (e.g., THF containing 7 weight % or greater acetic acid) to preserve and regenerate catalyst activity. Background Art Polytetramethylene ether glycol (PTMEG) is a commodity in the chemical industry which is widely used to foπn segmented copolymers with poly¬ functional urethanes and polyesters. PTMEG is commercially prepared by reacting tetrahydrofuran (THF) with fluorosulfonic acid and then quenching the product with water. While this process has proved to be quite satisfactory, it is not as efficient as desired because the acid cannot be recovered and reused. Moreover, disposal of the spent acid is a problem because of its toxicity and corrosiveness. U.S. Pat. No. 4, 120,903, discloses the polymerization of THF using a polymer containing alpha-fluorosulfonic acid groups as a catalyst and water or 1,4-butane diol as a chain terminator. The nature of the catalyst permits its reuse, thereby eliminating the disposal problems and the catalyst's lack of solubility in the reaction mass makes it easy to separate the catalyst from the product at the end of the polymerization reaction. This very low solubility also minimizes loss of catalyst as the reaction proceeds. However, this process produces a polytetramethylene ether glycol having a molecular weight of 10,000 or more, while the commercial products generally have molecular weights of less than 4,000 with the majority of commercial products having a number average molecular weight from 650 to 3,000. U.S. Pat. No. 4, 163, 1 15 discloses the polymerization of THF and/or THF with comonomers to polytetramethylene ether diester using a fluorinated resin catalyst containing sulfonic acid groups, in which the molecular weight is regulated by addition of an acylium ion precursor to the reaction medium. The patent discloses the use of acetic anhydride and acetic acid being used with the solid acid catalyst. The polymeric product is isolated by stripping off the unreacted THF and acetic acid/acetic anhydride for recycle. The product isolated is the diacetate of polymerized tetrahydrofuran (PTMEA) which must be converted to the corresponding dihydroxy product, polytetramethylene ether glycol (PTMEG), to find application as a raw material in most urethane end use applications. Disclosure of Invention It has now been recognized that during the sustained polymerization of THF and/or THF with comonomer, particularly during copolymerization of THF and 3-methyl tetrahydrofuran (3-MeTHF), using the method as described in U.S. Pat. No. 4, 163, 115 the catalyst can become deactivated over time. It has also been discovered that during periods temporary suspension or interruption of the polymerization, the reactor vessel containing reactant, catalyst and polymer becomes susceptible to run-away polymerization producing high molecular weight polymer and/or gel formation reminiscent of the above mentioned problem associated with the process disclosed in U.S. Pat. No. 4, 120,903. In view of the above, the present invention provides in a process for producing diesters of polytetramethylene ethers by polymerizing THF optionally with one or more substituted THF or alkylene oxide comonomers in a reactor utilizing a solid catalyst containing sulfonic acid groups in a liquid reaction medium containing a carboxylic acid with carboxylic acid anhydride as molecular weight control agents, wherein the improvement comprises the steps of: suspending polymerization in the reactor by stopping the addition of reactant THF and carboxylic acid anhydride and simultaneously increasing the concentration of carboxylic acid in the reactor to sustain and preserve catalyst activity. Also, the present invention provides a method of regenerating a spent solid acid catalyst containing sulfonic acid catalytic reactive sites deactivated from use during ring- opening polymerization of THF optionally with one or more substituted THF or alkylene oxide comonomers and carboxylic acid with carboxylic acid anhydride comprising the step of contacting the spent catalyst with a solution comprising THF and carboxylic acid for a time sufficient to convert any alkylated sulfonic acid reactive site to the reactive sulfonic acid site. In one embodiment of the suspending polymerization process and of the preserving and/or regenerating catalyst activity process, a solution of THF containing at least 7 weight percent acetic acid is used. Thus, it is an object of the present invention to provide a method of inhibiting gel formation and preventing catalyst deactivation during periods of temporary shut-down as well as a method of regenerating the highly active solid acid catalyst if and when deactivation from sustained use may take place. Fulfillment of these objects and the presence and fulfillment of additional objects will become apparent upon complete reading of the specification and attached claims. Modes of Carrying Out the Invention The overall process for producing diesters of polytetramethylene ether compositions within which the present invention is found to be useful includes broadly any such polymerization employing a highly acid solid phase catalyst containing sulfonic acid groups capable of ring-opening polymerization of cyclic ethers and the like as generally known in the art. This includes by way of example but not by limitation, polymeric catalysts which contain sulfonic acid groups and optionally carboxylic acid groups. Particularly preferred solid acid catalyst are catalysts whose polymer chains are copolymers of tetrafluoroethylene or chlorotrifluoroethylene and a perfluoroalkyl vinyl ether containing sulfonic acid group precursors (again with or without carboxylic acid groups) as disclosed and taught in U.S. Pat. Nos. 4, 163, 1 15 and 5, 1 18,869 and as supplied commercially by E. I. duPont de Nemours and Company under the tradename Nafion®. In presenting the following description and examples reference to the Nafion® catalyst will predominate since benefits and advantages of the improved process according to the instant invention are felt to be optimum for such highly reactive catalyst. However, other heterogeneous catalysts wherein the catalytic site involves the sulfonic acid group, as mentioned above, are felt to be equivalent to Nafion® for purposes of the present invention in that one or more of the advantages and/or benefits of the instant process will be derived or realized. The diesters of polytetramethylene ether compositions being produced by the process within which the present invention is found to be useful are generally any such polyether as generally known in the art typically produced via an acid catalyzed ring opening polymerization reaction of a cyclic ether or mixture in die presence of a carboxylic acid and carboxylic acid anhydride wherein tetrahydrofuran is the major and/or dominant reactant; i.e., substantial THF being incorporated into the PTMEA product. More specifically, the polyether diesters are derived from the polymerization of THF with or without an alkyl substituted tetrahydrofuran comonomer (preferably for example 3-MeTHF) as well as the copolymerization of THF with or without 3-MeTHF and with an alkylene oxide or equivalent comonomer. As such, the following description and examples will predominantly refer to the polymerization of THF and/or THF with 3-MeTHF with the understand that the other comonomers may optionally be present. Also, for purposes of describing and claiming the present invention the term "polytetramethylene ethers" generically include both the homopolymerized THF polyether backbone as well as the corresponding copolymerized polymers. The THF used in the process of the invention can be any of those commercially available. The THF will preferably have a water content of less than about 0.001%, by weight, and a peroxide content of less than 0.002%, by weight, and optionally contains an oxidation inhibitor such as butylated hydroxytoluene to prevent formation of undesirable byproducts and color. If desired, from about 0.1 to about 50% by weight of the THF of the alkyl substituted tetrahydrofuran capable of copolymerizing with THF can be used in combination with the THF. A particularly preferred alkyl substituted THF is 3-MeTHF. Similarly the acetic acid and acetic anhydride used in the process of the invention can be any of those commercially available. Importance of Acetic Acid in the Feed to the Polymerization Reactor: It is necessary to have some acetic acid in the feed to the polymerization reactor. The key quality parameter of commercial grade PTMEG is molecular weight. The ability of a manufacturing operation to produce a product with consistent molecular weight and molecular weight distribution is of great value in PTMEG Industry. PTMEG customers are very sensitive to batch-to- batch molecular weight variations as this adversely affects the ratio of ingredients that are used in their processes. The variable ratio in turn has an adverse effect on their product quality. Even though the acetic anhydride concentration in the reactor feed is a key parameter that determines the molecular weight of PTMEA produced, acetic acid has a key role in aiding excellent molecular weight control in the overall process. In reality, a feed having no acetic acid will perform very poorly in the polymerization reactor, producing products with variable molecular weights in the reactor. The variability in molecular weights can be so great that it adversely affects the molecular weight distribution. Normally, a small variation in molecular weight distribution is acceptable but variations similar to the ones encountered with a no acetic acid in the reactor feed will result in products with very high molecular weight distributions. Separation of the various molecular weight species is not a cost effective alternative. The reason for this can be explained in simple batch experiments. In a batch polymerization of THF in the presence of acetic anhydride the molecular weight will initially, rapidly increase and reach a maximum value which depends upon the initial acetic anhydride concentration. This will be followed by a gradual decline in molecular weight which will plateau at a steady value. Molecular weight control of the polymer in a "non-acetic acid" system such as this will be impractical. In the presence of acetic acid in the reactor feed, however, a gradual molecular weight increase to a steady state value is observed. The batch molecular weight profiles for the two systems are included in the following Table. Table Basis: 10% by weight acetic anhydride/acetic acid mixture in THF at 10% by weight Nafion® catalyst loading (equivalent weight 1,080) with a reaction temperature of 50°C. Case I: Ratio of acetic anhydride/acetic acid = 9 (Virtually acetic acid free system) Time, hrMol Wt 1 hr 15 min 2800 3 hr 30 min 1275 19 hr 30 min 950 Case II: Ratio of acetic anhydride/acetic acid = 0.5 Time. hrMol Wt 50 min 1250 3 hr 30 min 1550 19 hr 30 min 1750 From these experiments one can clearly rationalize that acetic acid greatly accelerates the scission of PTMEA chains. Although again it is felt that the present invention should not be confined or limited to any single mechanistic explanation, one mechanism consistent with these observations involves initial formation of an acylium ion from acetic anhydride by acid (H + ) protonation and cleavage of the anhydride bond resulting in an acylium ion and one molecule of acetic acid. Acylation of the ether oxygen in the polyether chain by the acylium ion produces an acylated oxonium ion intermediate. Nucleophilic attack by acetic acid at the (alpha) methylene carbon adjacent to the acyl bearing oxygen leads to chain ether bond cleavage. This addition of the acetic acid molecule during ether bond cleavage produces two diacetate end-capped polymer fragments of lower molecular weight. Thus the probability of bond cleavage is at least in part dependent on and/or influenced by the presence of acetic acid. Thus according to this proposed mechanism, the relative concentration of acetic acid as well as the acetic anhydride in the THF reactant medium will play a critical role in polymer scission and ultimate molecular weight of the product. Literally the ether cleavage mechanism can be viewed as competing with the oxonium ion ring-opening propagation step occurring at the catalyst site and thus advantageously controlling molecular weight. Yet clearly the fact that the initiation of this bond cleavage involves an acetic anhydride that produces in situ an acetic acid molecule, no net make-up of acetic acid in principle is necessary during steady state polymerization and only acetic anhydride concentration (i.e., acetic anhydride make-up) is necessary to control the product molecular weight while bulk THF make-up is independently related the rate of production of polymer. This view of the role of the acetic acid is also consistent with an observed advantages associated with using increased acetic acid concentrations in the polymerization reactor during periods of temporary down¬ time to inhibit catalyst deactivation and prevent uncontrolled reaction run-away and polymer gelation problems as will explained in more detail below. The mechanistic view is also not inconsistent with the proposed use of acetic acid in THF to regenerate spent catalyst again as explained below. Acetic Acid in Prevention of Gel Formation and Catalyst Activation/Regeneration: Acetic acid is helpful in preventing gel formation in the polymerization reactor. In fact a one-step runaway reaction could occur in the polymerization reactor in the absence of acetic acid. Typically, THF can polymerize very rapidly in the presence of Nafion® producing extremely high molecular weight PTMEA directly. The potential for this to occur in the polymcrization reactor is high in a commercial reactor system as there is always a likelihood of losing fresh acetic anhydride flow to the reactor. These high molecular weight polymer produced by one-step process can have molecular weights as high as 1 million units. No good method has been identified to separate the Nafion® catalyst from such high molecular weight polymer. During the acetic anhydride Nafion® polymerization of THF, active sites on the catalyst can become deactivated. This will also occur for generally any solid acid catalyst wherein the reactive site is the sulfonic acid group. While not intending to limiting this observed catalyst deactivation to any given mechanism, it is proposed that the deactivation is caused, at least in part, by polymer chains becoming tightly associated with the catalyst active site via an ion pair to alkyl sulfonate covalent bond equilibrium reaction. Thus in the case of Nafion® as the catalyst the deactivation equilibrium reaction can be represented as follows (where NfS0 3 ( ~ ) represents the reactive ionized sulfonic acid site (less the H + ) attached to the fluoroethylene/perfluoroalkyl vinyl ether copolymer backbone, Nf): O O NfS0 3 (-) [ θ (+) -[(CH 2 ) 4 -0] n -C-CH 3 ] «→ NfS0 3 [(CH 2 )4-0] n+1 -C-CH 3 Ion Pair alkyl sulfonate This deactivation is especially associated with a high molecular weight campaign, in which low amounts of acetic acid are co-fed to the reactor. The active sites are presumably tied up in an equilibria between ion-pair and alkyl sulfonate. The ion pair is the reactive species, while the formation of an alkyl sulfonate deactivates the site. Washing the catalyst with a mixture of acetic acid in THF will regenerate the catalyst to full activity by converting any ion pairs to PTMEA and the corresponding acid form of the catalyst (thus tending to shift the above deactivation equilibrium to the left) as follows: O NfS0 3 (-) [ [^ o(+) -[( CH 2)4-0]n-C-CH 3 ] + CH 3 COOH <τ- Ion Pair acetic acid O NfS0 3 H + CH 3 -C-[(CH 2 ) 4 -0] n+1 -C-CH 3 acid catalyst PTMEA Acetic acid reactivation (removing alkyl sulfonate groups) solves the problems of reliably making high molecular weight PTMEA while reusing the same catalyst, of idling the polymerization reactor for long periods of time, and of restoring activity of spent catalyst. To further illustrate the use of acetic acid according to the present invention in order to both regenerate spent catalyst as well as preserve catalytic activity during periods of sustained shut-down of a reactor, the following examples are presented. As such, the examples are felt to be only illustrative of the improved processes according to the present inventions and should not be unduly limiting of the claimed improvements. Example 1 To regenerate the catalyst, a polymer solution is drained from the Nafion® catalyst. A solution of organic acid in THF (typically 7% acetic acid in THF) is added to Nafion® catalyst and briefly heated to 50°C. The organic acid solution is drained off and polymerization resumed. Alternatively, the catalyst may be stored in this solution indefinitely. Example 2 A glass lined, water cooled (jacketed), continuously stirred tank reactor having a nominal production capacity of about 100 pounds of PTMEA per hour was employed in combination with a feed tank to premix the THF, acetic acid and acetic anhydride introduced to the reactor. A Nafion® perfluorosulfonic acid resin powder with an equivalent weight of 1, 100 and an average particle size 1/16" Diameter and 1/8" to 1/4" long was used as the solid phase acid catalyst to intentionally produce four standard commercial grade PTMEA polymer product. The catalyst concentration in the polymerization reactor was 10% by weight and the hold-up time was 60 minutes. The rate of THF addition to the reactor was based on the production rate of polymer (i.e., about 100 lbs./hr.) while th rate of addition of acetic anhydride was determined by the desired molecular weiglit of polymer. More specifically, the four runs targeted commercial products with molecular weights of 650, 1,000, 1,800, and 2, 100 which corresponded to operating the reactor with an acetic anhydride concentrations of 7.3, 4.5, 2.3 and 1.7 percent by weight, respectively. All runs were conducted at 50°C and at an acetic acid concentration of 3% by weight. A solution of polymer was continuously withdrawn from the reactor and product recovered therefrom indicated a percent conversion to PTMEA of from 39 to 33% by weight. Continuous polymerization at the nominal capacity of the reactor established that an acetic acid concentration controlled at 3 weight percent in the feed regardless of the product molecular weight insured the elimination of any one-step runaway reaction. However, when the reactor was shut down for periods greater than 2 days, an increase in the acetic acid concentration from 3% to 7% by weight was required to prevent any side reaction resulting in formation of gel in the reactor. This prevention of gel formation resulted in the maintaining of the active catalyst sites available for the polymerization process and had no adverse effect on the long-term catalyst activity. At acetic acid concentrations above 7% the rate of the reaction is slowed to the point where the material in the reactor would not gel in 20 days. Industrial Applicability The advantages and benefits of the improved process according to the present invention are felt to be numerous and significant. For example, the improved method, according to the present invention, of suspending catalytic ring- opening polymerization of tetrahydrofuran or the like employing a solid catalyst containing sulfonic acid groups without inducing gel formation in the polymerization reactor while preserving the catalytic activity is a significant value during the sustained polymerization of THF and or THF with comonomer, particularly during copolymerization of THF and 3-methyl tetrahydrofuran. Similarly the method of regenerating such catalyst after being deactivated from sustained use during ring-opening polymerization represents a significant savings in commercial applications.

Claims: CLAIMS We Claim: 1. In a process for producing diesters of polytetramethylene ethers by polymerizing THF optionally with one or more substituted THF or alkylene oxide comonomers in a reactor utilizing a solid catalyst containing sulfonic acid groups in a liquid reaction medium containing a carboxylic acid with carboxylic acid anhydride as molecular weight control agents, wherein the improvement comprises the steps of: suspending polymerization in said reactor by stopping the addition of reactant THF and carboxylic acid anhydride and simultaneously increasing die concentration of carboxylic acid in said reactor to sustain and preserve catalyst activity. 2. A process of Claim 1 wherein said carboxylic acid is acetic acid and wherein the concentration of the acetic acid in said liquid reaction medium is increased to at least 7 weight percent. 3. Λ method of regenerating a spent solid acid catalyst containing sulfonic acid catalytic reactive sites deactivated from use during ring-opening polymerization of THF optionally with one or more substituted THF or alkylene oxide comonomers and carboxylic acid with carboxylic acid anhydride comprising the step of contacting the spent catalyst with a solution comprising THF and carboxylic acid for a time sufficient to convert any alkylated sulfonic acid reactive site to the reactive sulfonic acid site. 4. A method of Claim 3 wherein said solution of THF and carboxylic acid is a solution containing at least 7 weight percent acetic acid. 5. A method of Claim 4 wherein said catalyst comprises copolymers of tetrafluoroethylene or chlorotrifluoroethylene and a perfluoroalkyl vinyl ether containing a sulfonic acid group.

Inventors: DORAI SURIYANARAYAN, SCHULTZ JAMES A, DORAI, SURIYANARAYAN, SCHULTZ, JAMES, A.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1996
Main classification: C08G65
IPC codes: C08G65