PRODUCING ISOCYANATE DIMERS USING POLYMER-BOUND CATALYSTS

Abstract: This invention relates to a process for preparing a dimer by the steps of: (a) cyclodimerizing a polyisocyanate in the presence of a polymer-bound dimerization catalyst by contacting said polyisocyanate with said catalyst at a temperature of between about 20 and about 135 DEG C in a reaction to form an uretidione-containing cyclodimerized isocyanate wherein a portion of the isocyanate moieties comprising said polyisocyanate are converted to uretidione groups, and (b) separating said catalyst from said cyclodimerized isocyanate in order to stop said reaction after a desired amount of isocyanate moieties in said polyisocyanate have been converted to uretidione moieties.

Description: PRODUCING ISOCYANATE DI ERS USING POLYMER-BOUND CATALYSTS This invention relates generally to a process for making uretidione derivatives of isocyanates, and, more specifically, to a process for preparing the isocyanate dimers using a polymer-bound catalyst. Polyuretidione derivatives of polyisocyanates (also called isocyanato uretidiones) are intermediates which can be used in the preparation of high performance urethane coatings, paints, and films. These derivatives provide reduced volatility and an associated reduced toxicity hazard during use, as compared to monomeric polyisocyanates, for example, toluene diisocyanate. In addition, because of their low viscosity, isocyanato uretidiones can be used as reactive diluents for other highly viscous or solid isocyanate-group containing coatings components or as a polyisocyanate component in solvent-free and low solvent coatings formulations. Processes for preparing these derivatives are well known. Examples illustrative of these processes can be found in U.S. Patents: 4,476,054; 4,912,210; and 4,929,724. Generally, the prior art processes involve adding a soluble catalyst which promotes the polyisocyanate to polyuretidione reaction (also known as "dimerization") to the precursor isocyanate, optionally in the presence, but usually in the absence, of a solvent, allowing the reaction to proceed to the desired extent and then stopping the reaction with a suitable quenching agent which destroys the activity of the catalyst. Alternatively, in the cases where relatively volatile catalysts are used, the reaction is stopped by distilling the catalyst along with the residual, unreacted precursor isocyanate from the product dimer. After the residual, unreacted precursor isocyanate is removed, the resulting material, in the case where the precursor isocyanate is a diisocyanate, is a mixture of oligomers composed of at least 2 (i.e., 2 , 3, 4, and the like) precursor diisocyanate molecules joined by at least one (i.e., 1, 2, 3, and the like) uretidione rings. Usually, this mixture is simply called "dimer". In the case where the precursor isocyanate is polyisocyanate, the reaction is generally stopped well before all the isocyanate groups have been converted to uretidione groups because, otherwise, the resulting product would be an unusable polymer having a very high (theoretically infinite) molecular weight and viscosity. However, the cost of equipment and energy to remove residual, unreacted precursor isocyanate dictate that the reaction not be stopped too soon. Generally, the reaction is run to more than 10% conversion but less than 50% conversion. The preferred range is between 20% and 35%. The reaction is typically stopped using a quenching agent. The reaction between conventional dimerization catalysts and quenching agents typically results in the formation of an insoluble product which is typically removed by filtration using a filter aid. Unfortunately, both the quenching agent and the filter aid increase the likelihood of introducing undesirable impurities into the product. Accordingly, new processes for producing dimers that do not employ a quenching agent and filter aid(s), and employ fewer process steps than prior art processes, would be highly desired by the dimer manufacturing community. Alternatively, in the cases where relatively volatile catalysts are used: the catalyst is contained in the recovered precursor isocyanate, making it unsuitable for any other use except recycle to the dimerization process; and, because the dimerization reaction is thermally reversible, especially in the presence of a catalyst, some of the product dimer is converted back to precursor isocyanate before the catalyst is removed at elevated temperatures. Accordingly, new processes for producing di ers that provide higher yields of product dimer, as well as catalyst-free recovered precursor isocyanate, would also be highly desired by the dimer manufacturing community. The present invention is an answer to that need. In one aspect, the present invention relates to aprocess for preparing a uretidione-containing cyclodimerized isocyanate by cyclodimerizing an isocyanate in the presence a catalytically effective amount of a polymer-bound dimerization catalyst to form said uretidione-containing cyclodimerized isocyanate. In another aspect, the present invention relates to a process for preparing and isolating a uretidione-containing cyclodimerized isocyanate by the steps of: (a) cyclodimerizing a polyisocyanate in the presence a catalytically effective amount of a polymer-bound dimerization catalyst by contacting said polyisocyanate with said catalyst at a temperature of between about 20 and about 130°C, preferably between about 20 and about 110°C, in a reaction to form said uretidione-containing cyclodimerized isocyanate wherein a portion of the isocyanate moieties comprising said -Δ - polyisocyanate are converted to uretidione groups, and (b) separating said catalyst from said cyclodimerized isocyanate in order to stop said reaction after a desired amount of said isocyanate moieties in said polyisocyanate have been converted to uretidione moieties. In yet another aspect, the present invention relates to a process for preparing a uretidione-containing cyclodimerized isocyanate by cyclodimerizing an isocyanate in the presence of a catalytically effective amount of a polymer-bound dimerization catalyst to form an uretidione-containing cyclodimerized isocyanate wherein the polymer-bound dimerization catalyst consists essentially of a polymer having an alkyene group-containing polymer backbone and having dimerization catalyst moieties chemically bound to said polymer, said catalyst moieties being selected from the group consisting of: aromatic tertiary amines, especially 4-dialkylamino pyridines; alkyl- and alkylamino-phosphines and their derivatives; and combinations thereof. These and other aspects will become apparent upon reading the following detailed description of the invention. In accordance with the present invention, it has now been surprisingly discovered that polymer-bound dimerization catalysts are suitably prepared which are then employed in a straightforward fashion to provide a facile dimerization reaction. The term "polymer-bound" as used herein is intended to designate polymer supported dimerization catalysts which are insoluble in the dimerization reaction medium by virtue of the polymer support, and thus are easily separated from the reaction medium by removal of the polymer-bound catalyst from the reaction medium after the dimerization reaction has proceeded to the desired extent of completion. The catalyst composition useful in the process of the present invention comprises a polymer which is insoluble in the reaction medium and which contains sites that promote the dimerization reaction which are bound to the polymer through ionic or, preferably, covalent bonds. Compounds which promote the reaction converting isocyanate to uretidione are well known in the art. However, heretofore it was not known to the knowledge of the present inventors whether or not these various reaction promoters would still be active dimerization catalysts when bound to a polymer to provide a polymer-bound catalyst. In accordance with the present invention, it has now been found that specific classes of functional groups are suitably employed as polymer-bound catalysts for the desired dimerization reaction. Useful moieties thus include polymer bound derivatives of the following: aromatic tertiary amines, especially 4-dialkylamino pyridines; alkyl- and alkylamino-phosphines and their derivatives; and the like. The dialkylamino pyridine containing functional groups are preferred because of their enhanced stability in the reaction medium and their ease of regeneration. The polymer support for the catalyst should be inert in the dimerization medium. Additional factors to be considered in selecting preferred polymer supports are: availability; cost; stability; ease of functionalization; and, ability to be swollen and/or "wet" by the precursor isocyanate. This last characteristic is desired in order to facilitate intimate contact between the precursor isocyanate and the active sites on the polymer and then allow the resulting uretidione to migrate away from the catalytic site, making it available for further reaction. Because of their stability to elevated temperatures and the reaction environment, polymer backbones consisting essentially of carbon to carbon bonds, derived from alkenes, are desired, such as: ethylene, propylene, isoprene, styrene, acrylates, methacrylates, and the like. Polystyrenes are most preferred because of their thermal and chemical stability and the ease with which they can be functionalized. The macroscopic form of the polymers that can be employed in the process of this invention can be varied significantly, including solid and/or liquid form. For example, polymers that, by virtue of their low molecular weight, for example, are soluble in the dimerization reaction medium, can be precipitated and then filtered from the reaction medium by the addition of an appropriate non-solvent for the polymer when the desired degree of dimer conversion is reached. However, recovering this non-solvent can entail additional costs. A preferred approach is to use a polymer which is "essentially insoluble" (i.e., not soluble to any substantial degree) in the dimerization reaction medium. The polymer can be utilized in the form of beads or powder or other relatively small particles. However, using the polymer in the form of small beads is generally preferred since this simplifies removal of the polymer bound catalyst through filtration and similar Q such techniques. The solubility of the polymer in the dimerization reaction medium is generally inversely proportional to its crosslink density. In the case where the polymer bound catalyst is based on polystyrene, the amount of crosslinking is determined by the amount of divinyl benzene co-monomer used in the preparation of the polymer. In addition to effecting the solubility of the polymer bound catalyst, the crosslink density of the polymer is an important consideration because it positively affects mechanical stability while having a negative impact on the degree of swelling and/or wettability of the polymer. Crosslink densities greater than or equal to 1% and less then 10% are preferred. Those between 1 and 5% are most preferred. It is also possible to adjust the number of catalytically active sites (i.e., functional groups) bound to the polymer. From a practical standpoint, the minimum required number of active sites on the catalyst is that amount that provides a "catalytically effective amount", i.e., an amount sufficient to catalyze the dimerization reaction. The upper limit is, in one sense, defined by the composition of the catalyst and the polymer to which it is being bound. This maximum is in practice determined by the amount that provides a catalyst that permits some control over the desired dimerization reaction. Additionally, the active site content of the polymer-bound catalyst which provides a practically useful catalyst is also a function of the activity of the catalyst that is bound to the polymer. Generally, it is found that for the types of catalytic species described above, the range of 0.01 to 10 meq of catalytic sites per gram of polymer is preferred, with levels of 0.5 to 5 meq per gram being most preferred. The catalytically active sites may be bound to the polymer support using a number of different approaches. A polymerizable- monomer containing the desired functional group, for example, N-(2-propenyl)- N-alkyl-4-amino pyridine may be co-polymerized with, for example, styrene. Alternatively, materials such as poly(styrene-co-vinylbenzylchloride) , which are commercially available as so-called Merrifield resins, or the product of the chloromethylation of polystyrene may be condensed with, for example, a 4-alkylamino pyridine. These would yield a polymer bound catalyst containing 4-dialkylamino pyridine groups. Further, for example, aroute involving the bromination of polystyrene, followed by condensation with, for example, (R 2 N)_PC1, where R is lower alkyl, using lithium, will yield a polymer bound catalyst containing phosphine amide groups. When the polymer bound catalyst is used in the form where it remains as a separate phase, i.e., where it is insoluble in the reaction medium, there are at least two options with respect to the manner in which the precursor isocyanate, optionally in the presence of asolvent, can be contacted with the catalyst, either (a) packed in a cartridge or tube, or (b) dispersed in a stirred reactor. In either case, the system can be operated in batches, e.g., where the system is charged with isocyanate, the reaction is typically run until the desired level of conversion is reached, and then the product is separated from the catalyst by filtration or similar such means. Alternatively, the system can be run as a continuous process wherein isocyanate is continuously added to the system while the product dimer having the desired level of conversion is continuously withdrawn. Potential hardware configurations include: a Continuously Stirred Tank Reactor ("CSTR") with the catalyst dispersed in the isocyanate; a CSTR which serves as a reservoir for the isσcyanate/dimer mixture that is repetitively passed, in parallel, through a battery of catalyst packed cartridges, wherein relatively low levels of conversion are achieved in each pass; or a catalyst packed tube, wherein the desired level of conversion is reached in a single pass through the tube. A range of polymer-bound catalyst concentrations may be used in the process of this invention. The factors to be considered in the selection of preferred catalyst concentrations are: the activity of the catalyst being used; the degree of conversion desired; and, the temperature at which the reaction is conducted. Generally, an amount of between 0.01 and 10 parts of polymer-bound catalyst per 100 parts of precursor isocyanate is preferred. An amount of between 0.05 and 2 parts of polymer-bound catalyst per 100 parts of precursor isocyanate is most preferred. Co-catalysts are optionally and desirably employed in the process of the present invention as a source of active hydrogens for the uretidione formation reactions. The co-catalysts may be any isocyanate reactive hydrogen containing reagents such as amines, alcohols, carbamates, ureas and the like. The preferred co-catalysts are primary and secondary alcohols, such as, for example, methanol, ethanol, 2-propanol, l,3-dihydroxy-2-hexyl propane, triethylene glycol onomethyl ether, and the like. Preferably, the co-catalyst is employed in an amount of between about 1:1 and about 10:1 molar equivalents based upon the amount of polymer-bound catalyst employed in the process of the present invention. A range of temperatures may be used in the process of this invention. The factors to be considered in the selection of preferred reaction temperatures are the amount and the activity of the catalyst being used and the degree of conversion desired. Generally, somewhat elevated temperatures are preferred because they drive the reaction at a reasonable rate. Temperatures between 20 and 130°C are preferred with temperatures between 20 and 110°C being most preferred. The time required for the process of this invention is dependent on the temperature, the amount and type of catalyst used and the degree of conversion sought. Generally, it is desirable that a combination of temperature, catalyst activity and catalyst concentration be used that achieves the required level of conversion within a period of 0.5 to 8 hours. The cyclodimerization reaction is effectively stopped by removal of the catalyst. The catalyst optionally may be deactivated prior to removal. The catalyst is deactivated by the addition of a suitable blocking agent. Such agents react preferentially with the catalytic sites and block further reaction with isocyanate functional groups. Agents such as hydrogen containing acids or salts of such acids that thermally liberate the acid, alkyl halides and the like, are employed in an amount of between about 1:1 and about 10:1 molar equivalents based upon the amount of polymer-bound catalyst employed in the process of the present invention. Once the reaction mass has been separated from the polymer bound catalyst, most of the unreacted isocyanate monomer, and any optionally used solvent, is preferably removed from the product dimer by evaporation by any convenient means including simple distillation or thin film evaporation at elevated temperatures and atmospheric or, preferably, reduced pressure, followed by a more stringent process for removal of any remaining residual solvent and precursor isocyanate monomer. This final step is preferably accomplished using a wiped film evaporator ("WFE") in which the exposure of the product stream to high temperatures is minimized, although other evaporation techniques such as a falling film evaporator ("FFE"), or a thin film evaporator ("TFE") is also suitably employed. The use of WFE, FFE and TFE are well-known in the art. Briefly, the preferred process involves passing the monomer containing feed through the WFE apparatus at elevated temperatures, 60 to 130°C, preferably between 80 and 120°C, and reduced pressure, 0.01 to 5 mm Hg, preferably between 0.1 and 2 mm Hg. The feed rate is dependant on the heated surface area of the apparatus, but should be slow enough to permit the removal of most of the residual diisocyanate monomer but fast enough to assure that the product is not exposed to high temperatures for an unnecessarily long period of time. At the end of this treatment, the residual monomer content should be less than 0.2%, preferably less than 0.1% by weight of the product. The process of the present invention is suitably employed in the production of a wide range of isocyanate dimers, including hexamethylene diisocyanate ("HDI") dimer, isophorone diisocyanate ("IPDI") dimer, H.-MDI dimer, toluene diisocyanate ("TDI") dimer, methlylene diphenylene diisocyanate ("MDI") dimer, naphthalene diisocyanate ("NDI") dimer, cyclohexylene diisocyanate ("CHDI") dimer, 1,4-phenylene diisocyanate ("PPDI") dimer, bitolyene diisocyanate ("TODI") dimer, xylyene diisocyanate ("XDI") dimer, tetramethyl xylyene diisocyanate ("TMXDI") dimer, l,3-bis(isocyanatomethyl) cyclohexane ("H g MDI") dimer, and the like, as well as, mixtures thereof. The following examples are intended to illustrate, but in no way limit the scope of, the present invention. EXAMPLE 1 A polymer-bound catalyst, namely poly(4-dimethylamino pyridine) , commercially available as POLYDMAP" 1 , a product of Reilly Industries, Inc, was employed in this example. It is supplied as crosslinked gel beads, swollen with toluene (45-47 weight percent) and contains 1.4 meq 4-dimethylamino pyridine per gram of dry resin. It was used as supplied. A-HDI Dimerization To 5 gm of POLYDMAP™ was added 100 gm HDI. The mixture was stirred and heated at 60-70°C for 6 hours. An IR spectrum of the liquid showed that at least 15% of the HDI had been converted to dimer. No absorptions due to isocyanurates were detected. B-TDI Dimerization To 0.1 gm of POLYDMAP™ was added 15 gm TDI (composed 80% 2,4-TDI and 20% 2,6-TDI). The mixture was stirred and heated at 65°C for 15 minutes. An IR spectrum of the liquid showed that least 10% of the TDI had been converted to dimer. No absorptions due to isocyanurates were detected. C-IPDI Dimerization To 1 gm of POLYDMAP™ was added 15 gm IPDI. The mixture was stirred and heated at 60-62°C for 6 hours. An IR spectrum of the liquid' showed that at least 20% of the IPDI had been converted to dimer. No absorptions due to isocyanurates were detected. EXAMPLE 2 - Another HDI Dimerization - Poly(4-dimethylamino pyridine), POLYDMAP™ is a product of Reilly Industries, Inc. It is supplied as crosslinked gel beads, swollen with toluene (45-47 weight percent) and contains 1.4 meq 4-dimethylamino pyridine per gram of dry resin. 200 gms of this material was washed with ethyl acetate and then with toluene, several times and then dried under vacuum, at 60°C, for 16 hours. The resulting amber beads were swollen with an equal weight of dry toluene for at least 2 hours prior to use. To 50 gms of this swollen resin was added 500 gms HDI. The mixture was heated and stirred at 90°C for 6 hours. An IR spectrum of the liquid showed that 30% of the HDI had been converted to dimer. Most of the supernatant liquid (446 gm) was siphoned from the catalyst using a filter stick and 527 gm fresh HDI was added. The mixture was heated and stirred at 90°C for 6 hours. An IR spectrum of the liquid showed that 32% of the HDI had been converted to dimer. This process of removing the supernatant liquid, adding fresh HDI and then heating was repeated an additional two times with- no apparent loss in activity of the catalyst. The product fractions were combined and stripped of residual monomer using a wiped film evaporator at 110°C and 0.1 mm Hg pressure. There was thus obtained HDI dimer as a very pale yellow liquid, containing 0.1% residual HDI, with a viscosity of 33 cps at 25°C. While the invention has been described above with references to specific embodiments thereof, it is apparent that many changes, modifications and variations in the materials, arrangements of parts and steps can be made without departing from the inventive concept disclosed herein. Accordingly, the spirit and broad scope of the appended claims is intended to embrace all such changes, modifications and variations that may occur to one of skill in the art upon a reading of the disclosure.

Claims: WHAT IS CLAIMED IS: 1. A process for preparing a uretidione- containing cyclodimerized isocyanate characterized by cyclodimerizing an isocyanate in the presence a catalytically effective amount of a polymer-bound dimerization catalyst to form said uretidione-containing cyclodimerized isocyanate. 2. The process of claim 1 characterized in that said polymer-bound dimerization catalyst consists essentially of a polymer backbone plus functional groups, wherein said polymer backbone consists essentially of ethylene, propylene, isoprene, styrene, acrylate, and methacrylate moieties, and combinations thereof, and wherein said functional groups consist essentially of polymer bound derivatives of the following: aromatic tertiary amines, especially 4-dialkylamino pyridines; alkyl- and alkylamino-phosphines and their derivatives; and combinations thereof. 3. The process of claim 1 characterized in that said polymer-bound dimerization catalyst comprises nitrogen containing or phosphorus containing functional groups, or a combination thereof. 4. The process of claim 1 characterized in that said polymer-bound dimerization catalyst comprises a tertiary amine. 5. The process of claim 1 characterized in that said polymer-bound dimerization catalyst has a crosslink density of between 1% and 10%. 6. The process of claim 1 characterized in that said polymer-bound dimerization catalyst has a crosslink density of between 1% and 5%. 7. The process of claim 1 characterized in that said polymer-bound dimerization catalyst a number of functional groups in- an amount of between about 0.01 and about 10 milliequivalents of catalytic sites per gram of polymer. 8. The process of claim 1 characterized in that said polymer-bound dimerization catalyst is employed in an amount of between 0.01 and 10 parts of catalyst per 100 parts of said isocyanate. 9. The process of claim 1 characterized in that said cyclodimerizing is effected in the presence of a co-catalyst being a primary or secondary alcohol employed in an amount of between about 1:1 and about 10:1 molar equivalents based upon the amount of polymer-bound catalyst employed. 10. A process for preparing and isolating a uretidione-containing cyclodimerized isocyanate characterized by the steps of: (a) cyclodimerizing a polyisocyanate in the presence of a catalytically effective amount of a polymer-bound dimerization catalyst by contacting said polyisocyanate with said catalyst at a temperature of between about 20 and about 130°C, preferably between about 20 and about 110°C, in a reaction to form said uretidione-containing cyclodimerized isocyanate wherein a portion of the isocyanate moieties comprising said polyisocyanate are converted to uretidione groups, and (b) separating said catalyst from said cyclodimerized isocyanate in order to stop said reaction after a desired amount of said isocyanate moieties in said polyisocyanate have been converted to uretidione moieties. 11. The process of claim 10 characterized in that said polymer-bound dimerization catalyst consists essentially of a polymer backbone plus functional groups, wherein said polymer backbone consists essentially of ethylene, propylene, isoprene, styrene, acrylate, and methacrylate moieties, and combinations thereof, and wherein said functional groups consist essentially of polymer bound derivatives of the following: aromatic tertiary amines, especially 4-dialkylamino pyridines; alkyl- and alkylamino-phosphines and their derivatives; and combinations thereof. 12. The process of claim 10 characterized in that said polymer-bound dimerization catalyst comprises nitrogen containing or phosphorus containing functional groups, or a combination thereof. 13. The process of claim 10 characterized in that said polymer-bound dimerization catalyst comprises a tertiary amine. 14. The process of claim 10 characterized in that said polymer-bound dimerization catalyst has a crosslink density of between 1% and 10%. 15. The process of claim 10 characterized in that said polymer-bound dimerization catalyst has a crosslink density of between 1% and 5%. 16. The process of claim 10 characterized in that said polymer-bound dimerization catalyst a number of functional groups in an amount of between about 0.01 and about 10 milliequivalents of catalytic sites per gram of polymer. 17. The process of claim 10 characterized in that wherein said polymer-bound dimerization catalyst is employed in an amount of between 0.01 and 10 parts of catalyst per 100 parts of said isocyanate. 18. The process of claim 10 characterized in that said cyclodimerizing is effected in the presence of a co-catalyst being a primary or secondary alcohol employed in an amount of between'about 1:1 and about 10:1 molar equivalents based upon the amount of polymer-bound catalyst employed. 19. A process for preparing a uretidione- containing cyclodimerized isocyanate characterized by cyclodimerizing an isocyanate in the presence a catalytically effective amount of a polymer-bound dimerization catalyst to form a uretidione-containing cyclodimerized isocyanate wherein the polymer-bound dimerization catalyst consists essentially of a polymer having an alkyene group-containing polymer backbone and having dimerization catalyst moieties chemically bound to said polymer, said catalyst moieties being selected from the group consisting of: aromatic tertiary amines, especially 4-dialkylamino pyridines; alkyl- and alkylamino-phosphines and their derivatives; and combinations thereof. 20. The process of claim 19 characterized in that said alkene group is selected from the group consisting of ethylene, propylene, isoprene, styrene, acrylates, methacrylates, and combinations thereof.

Inventors: GOLDSTEIN STEPHEN L, KRAUS THEODORE C, GOLDSTEIN, STEPHEN, L., KRAUS, THEODORE, C.
Applicants: OLIN CORP, OLIN CORPORATION
Application year: 1993
Main classification: C07D229
IPC codes: C07D229