PROCESS FOR THE PREPARATION OF (2,2)-PARACYCLOPHANE

Abstract: An improved process is provided for the preparation of the dimer, 2,2-paracyclophane, which is useful as the starting material for parylene conformal coatings used in the electronics industry for the protection of various sensitive electronic components. The process comprises optimization of the normally low yield of dimer formed by the Hofmann elimination reaction of p-methylbenzyltrimethyl­ ammonium halide by conducting the elimination reaction in the presence of dimethylsulfoxide and certain reaction promoters.

Description: FIELD OF THE INVENTION This invention relates in general to a process for the preparation of 2,2-paracyclophane in relatively high yields by the Hofmann elimination of p-methylbenzyltri­ methylammonium hydroxide in the presence of dimethyl­ sulfoxide (DMSO) and certain reaction promoters. BACKGROUND OF THE INVENTION Parylene is a generic term applied to a class of poly-p-xylylenes which are derived from a dimer of the structure: Parylene is an inert, transparent, conformal coating which has excellent barrier properties and can be used at relatively high temperatures. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, it is ideally suited for use as a conformal coating in a wide variety of fields, particularly in the electronics industry. However, 2,2-paracyclophane from which the parylene coating is prepared, is made via the pyrolysis of p-xylene or Hofmann elimination reaction of p-methylbenzyltrimethylammonium hydroxide, and is usually obtained in relatively low yields. Accordingly, the overall process for the application of parylene as a conformal coating is expensive and severely restricts its application where it might otherwise be used. The preparation of p-xylylene polymers by various routes has been reported in the patent literature. For example, U.S.-A- 2,719,131 disclosed a process for preparing poly-p-xylene wherein the vapors of p-xylene were pyrolyzed in the presence of chlorine gas. Also in GB-A-650,947, polymer formation was detected on the walls of a cooling chamber after p-xylene was vaporized and pyrolyzed. In U.S.-A- 3,109,175 a process was reported for the preparation of di-para-xylylenes in yields of 10 percent and higher. The process involved pyrolyzing a mixture of steam and p-xylene at a temperature between about 800°C and 1000°C to generate a free radical and condensing the reactive diradical in a fluid medium. More recently, U.S.-A- 4,532,369 discloses a process for the preparation of 2,2-paracyclophane from p-methylbenzyltrimethylammonium hydroxide. It is indicated in the patent that known processes which existed prior to the invention disclosed gave only low yields or the starting materials were not readily accessible. By contacting aqueous p-methylbenzyl­ trimethylammonium hydroxide with sodium or potassium hydroxide in the presence of dimethytsulfoxide (DMSO) the patentee indicated that yields as high as 70 percent were obtained, It was also indicated at column 1, lines 55-58, that the resulting high yields were surprising since the addition of other comparable aprotic solvents such as dimethyl­ formamide, N-methyl-pyrollidone or sulfolane had no effect. In the older EP 0 253 191-A1 which is no pre-publica­ tion a process for the preparation of 2,2-paracyclophane by the Hofmann elimination of p-methylbenzyl-triethyl­ ammonium hydroxide or derivatives thereof, in an aqueous solution of an alkali metal hydroxide, wherein said elimination is carried out in the presence of a di­ alkylether or mono-and poly-ethylene glycols having the formula wherein R and R′, which may be the same or different, represent alkyl groups having from 1 to 5 carbon atoms, and n is an integer from 1 to 5 is suggested. It is therefore an object of this invention to provide an process for the preparation of 2,2 paracyclo­ phane used in the synthesis of parylene in relative high yields as opposed to the methods disclosed in the literature to date. The inventive process is simple and efficient and hence is effective in reducing the overall cost in the preparation of 2,2-paracyclophane and various substituted derivatives thereof. SUMMARY OF THE INVENTION The present invention is directed to a process for the preparation of the 2,2-paracyclophane used in the preparation of parylene comprising contacting an aqueous solution of p-methylbenzyl-trimethylammonium halide with sodium or potassium hydroxide in the presence of dimethylsulfoxide and optionally tetraglyme and a crown ether having from 4 to 6 oxygen atoms and from 12 to 18 carbon atoms as reaction promoter. DESCRIPTION OF THE DRAWING A more detailed understanding of the invention will be had by reference to the drawing wherein the Figure is a graph which depicts the weight percent of 2,2-paracyclophane obtained by employing dimethyl­ sulfoxide alone and in combination with certain specific reaction promoters of the present invention. Curve C was obtained using DMSO alone; curve B was obtained using DMSO in combination with glyme; and curve A obtained using DMSO in combination with glymes and 18 crown 6. It is evident from the drawing that higher yields of the desired dimer can be obtained in shorter periods of time when reaction promoters are employed in combination with DMSO. DETAILED DESCRIPTION OF THE INVENTION As indicated above, the present invention provides a simple and efficient method for the preparation of the 2,2-paracyclophane in relatively high yields and from readily accessible starting materials. In contrast to the teachings of the aforementioned U.S.-A- 4,532,369 it was unexpectedly and surprisingly found that yields of the 2,2-paracyclophane higher than the 70 percent reported can be obtained when a reaction promoter, as herein defined, is employed in conjunction with dimethylsulfoxide. It has been observed that while the use of dimethylsulfoxide in the Hofmann elimination reaction increased the yields of the desired product over those which had been disclosed in the literature, the yield could be increased even further if certain reaction promoters were also employed in the reaction system. Prior to the U.S.-A- 45 32 369, only very small increases in yields were obtainable and hence the cost of the process remained exceedingly high and therefore the application of parylene conformal coatings was limited to the preparation of relatively expensive materials. However, even with the advent of the U.S.-A- 45 32 369, the process could still not be extended to the coating of less expensive items because of the longer reaction times needed to prepare the 2,2-paracyclophane. For the most part the use of parylene had been confined to the military or those applications in the electronics industry where the cost of the final product could justify the use of the expensive coating process. The reaction promoters employed in the process of the present invention are those promoters which, when employed with dimethylsulfoxide, promote the Hofmann elimination reaction so as to shift the equilibrium of the reaction to favor the formation of the dimer as opposed to the competing reaction which favors polymer formation. The exact mechanism by which the reaction promoters operates is not readily apparant but it is believed that the presence of the promoters plays a critical part in promoting the reaction to 2,2-paracyclophane formation while retarding the reaction of the 2,2-paracyclophane so formed from polymerizing to parylene. It has been found that a limited number of specific reaction promoters are suitable for use in the process of the present invention, and that such promoters must be used in a specific combination if increased yields of the dimer are to be obtained. The compounds which can be employed as reaction promoters in the process of the present invention are the crown ethers. These are the cyclic ethers composed of carbon, oxygen and hydrogen. In practice, crown ethers containing from 4 to 6 oxygen atoms and from 12 to 18 carbon atoms can be employed in the process of the present invention. Particularly preferred is 18 crown 6. Illustrative crown ethers which are suitable for use in the process of this invention are ethers such as 12 Crown 4 (1, 4, 7, 10-tetraoxacyclododecane), 15 Crown 5 (1, 4, 7, 10, 13-pentaoxacyclopentadecane), 18 Crown 6 (1, 4, 7, 10, 13, 16-hexaoxacyclooctadecane), benzo-15 Crown 5, bis(benzo 15 Crown 5)methyl]pimelate, bis[(12 Crown 4) 2-ylmethyl]2-dodecyl-2methyl malonate, dibenzo 18 Crown 6, dibenzo 24 Crown 6, dicyclohexano 18 Crown 6, dicyclohexano 18 Crown 6, and dicyclohexano 24 Crown 8. In practice, the process of the present invention is conducted under conditions suitable for the Hofmann elimination reaction to proceed and wherein the formation of the 2,2-paracyclophane is favored over the formation of the polymerized product, i.e., parylene. It has been found that best results are obtained when the reaction is conducted in a two phase system comprised of water and an organic phase. The starting material, i.e., the p-methylbenzyl­ trimethylammonium hydroxide is preferentially soluble in the aqueous phase whereas the 2,2-paracyclophane is preferentially soluble in the organic phase. Any undesirable polymer formation usually occurs at the boundary between the two phases. Thus, there is always the undesirable competing reaction for polymer formation. The reaction as indicated is conducted in an aqueous phase and an organic phase. The organic phase is one in which the 2,2-paracyclophane is soluble and which the starting p-methylbenzyltrimethylammonium hydroxide is largely insoluble. A wide variety of organic materials can be employed as the organic medium and include such compounds as benzene, toluene, the xylenes, such as para-xylene, ortho-­ xylene, meta-xylene, hexane, octane and methylene chloride. It was noted that in order to achieve optimum yields of the desired 2,2-paracyclophane on a consistent basis, the mole ratio of the components in the reaction mixture should be within certain specific ranges as indicated below: a) the mole ratio of alkaline hydroxide to the quaternary ammonium salt should be within the range of from 2: 1 to 20: 1, and preferably from 2: 1 to 12: 1. b) the mole ratio of dimethylsulfoxide to the quaternary ammonium salt should be within the range of from 2: 1 to 30: 1, and preferably from 8: 1 to 20: 1. c) the mole ratio of water to the quaternary ammonium salt should be within the range of from 20: 1 to 70: 1 and preferably from 40: 1 to 50: 1. d) the mole ratio of organic solvent to the quaternary salt is not necessarily critical, but good results are obtained when the mole ratio of solvent to the quaternary salt is within the range of from 10: 1 to 80: 1 and more preferably from 20: 1 to 60: 1. e) the mole ratio of the total reaction promoters to the quaternary salt should be within the range of from 20: 1 to 1: 1. The mole ratio of the 18 crown 6 to the quarternary salt should be within the range of from 3:1 to 1:1. As indicated previously, it was unexpectedly and surprisingly found that by employing reaction promoters in conjunction with the dimethylsulfoxide, markedly increased yields of the dimer can be obtained while polymer formation is minimized. In practice, it has been observed that the concentration of the dimethylsulfoxide is important and not the feed configuration. Thus, in contrast to the prior art, all of the components in the present invention can be added together at one time and none need to be added dropwise. This can be a great saving in time over the process disclosed in the US-A- 4,532,369. It is also quite evident that the mole ratio of the DMSO to quarternary salt is important if optimum yields are to be obtained. Additionally, it has been noted that DMSO in the presence of water an caustic can decompose at high temperatures and hence it is preferred to employ lesser amounts of DMSO when the reaction promoters of the present invention are used. In actual practice of the process of the present invention, the mixture is stirred to insure intimate mixing of the oganic and aqueous phases. Thereafter the phases are separated and the 2,2-paracyclophane recovered from the organic phase and purified according to known techniques. As indicated above, the process of the present invention is preferably conducted at a temperature between 50 and 130°C, and more preferably between 90 and 120°C. Temperatures above and below this range can be employed but are less preferred. Pressure is not critical and the reaction can be conducted at atmospheric, subatmospheric or super atmospheric pressures. The p-methylbenzyltrimethylammonium hydroxide is a well known compound and has been reported in the literature. If desired, it can be prepared in situ by reaction of a base on the corresponding quarternary ammonium salt such as the halide. As also indicated, the prior art process provided relatively low yields of the desired 2,2-paracyclophane largely due to the existence of competing reactions which resulted in relatively large amounts of polymer formation. Hence, until the process as described in the aforementioned U.S.-A- 4,532,369, attempts to increase the yield of the 2,2-paracyclo­ phane were for the most part failures, or at most, only small incremental increases in yields were obtained. Due to the difficulties in finding an acceptable process for preparing the 2,2-paracyclophane, to date, it has been possible to achieve only minor increases in yield of the desired product. In many instances when high yields have been indicated in the literature, it has not been possible to reproduce them with the degree of consistency necessary for an efficient commercial operation. Moreover, as indicated above, DMSO is known to decompose at high temperatures, and thus attempts to increase yields of the 2,2-paracyclophane by increasing the concentration of DMSO were found to be unsatisfactory. As indicated above, parylene is an excellent, inert, transparent conformal coating which, due to its unusual properties, renders it particularly suitable for a wide variety of applications particulaly in the electronics industry. Methods of applying the coatings starting from the 2,2-paracyclophane, and equipment for effecting such applications are all well known and readily available. The following examples illustrate the best mode presently contemptated for the practice of this invention. COMPARATIVE EXAMPLE A A mixture of 509.2 gram of p-xytene, 20.3 gram of KOH in 20.3 ml of water, 12.35 gram of p-methyl­ benzyltrimethylammmonium chloride in 8.95 ml of water, and 50.4 gram of DMSO is charged to a one liter glass reactor equipped with stirrer and condenser. Nitrogen is swept slowly through the system. The solution is heated to 90°C over a period of 1 h. Stirring is continued for 40-50 hours at 90-91°C, throughout the reaction. Reactor samples are taken, and measured for the 2,2-­ paracyclophane concentrations in the reactor by (G.C.) vapor-phase chromatographic analysis. Product yields are also calculated based on the G.C. analysis. The results obtained are shown in Table 1 below: EXAMPLE The process of Comparative Example A is repeated except that a mixture of 402.3 gram of p-xylene, 20.3 gram of KOH in 20.3 ml of water, 12.35 gram of p-methylbenzyl­ trimethylammonium chloride in 8.95 ml of water, 50.4 gram of DMSO, 51.8 gram of tetraglyme, 50.1 gram of diglyme and 10.1 gram of 18 crown 6 is is used. The yields are obtained by measuring the 2,2-paracyclophane content of residues by G.C. analysis. The results obtained are shown in Table II below: COMPARATIVE EXAMPLE B A mixture of 457.7 grams of p-xylene, 32.32 grams of NaOH in 48.48 ml of water, 29.0 grams of p-­ methylbenzyltrimethylammonium chloride in 18.1 ml of water and 118.5 grams of DMSO is charged in a one liter stainless steel reactor equipped with stirrer. Nitrogen is used to regulate reaction pressure at 2.4 bar (35 psi.) the solution is heated to 120°C over a period of 1 hour. Stirring is continued for 10-19 hours at 120-121°C throughout the reaction study. The reactor samples are taken and measured for the 2,2-paracyclophane concentration in the reactor by (G.C.) vapor-phase chromatographic analysis. The results are shown in table III below: COMPARATIVE EXAMPLE C A mixture of 539 grams of p-xylene, 33.6 grams of NaOH in 50.4 ml of water, 24.9 grams of p-­ methylbenzyltrimethylammonium chloride in 18.01 ml of water and 21.7 grams of DMSO is charged in a one liter stainless steel reactor equipped with stirrer. Nitrogen is used to regulate reaction pressure at 2,4 bar (35 psi.) the solution is heated to 120°C over a period of 1 hour. Stirring is continued for 10-19 hours at 120-121°C throughout the reaction study. The reactor samples are taken and measured for the 2,2-paracyclophane concentration in the reactor by (G.C.) vapor-phase chromatographic analysis. The results are shown in table IV below: COMPARATIVE EXAMPLE D A mixture of 490.2 grams of p-xylene, 32.8 grams of NaOH in 49.2 ml of water, 24.94 grams of p-­ methylbenzyltrimethylammonium chloride in 18.06 ml of water and 64.8 grams of DMSO is charged in a one liter stainless steel reactor equipped with stirrer. Nitrogen is used to regulate reaction pressure at 2,4 bar (35 psi.) the solution is heated to 120°C over a period of 1 hour. Stirring is continued for 10-19 hours at 120-121 degrees C. throughout the reaction study. The reactor samples are taken and measured for the 2,2-paracyclophane concentration in the reactor by (G.C.) vapor-phase chromatographic analysis. The results are shown in table V below:

Claims: 1. A process for the preparation of the 2,2-paracyclo­ phane used in the preparation of parylene, which comprises contacting an aqueous solution of p-methylbenzyltrimethyl­ ammonium halide with an alkali hydroxide in the presence of a promoter and dimethylsulfoxide and optionally tetra­ glyme and recovering the 2,2-paracyclophane, characterized in that the promoter is a crown ether having from 4 to 6 oxygen atoms and from 12 to 18 carbon atoms. 2. The process of claim 1 wherein the reaction is effected in the presence of an inert water-immiscible organic solvent. 3. The process of claim 2 wherein the solvent is selected from benzene, toluene, para-xylene, ortho-xylene, meta-­ xylene, hexane, octane, and methylene chloride. 4. The process of claims 1 to 3 wherein the crown ether is 1, 4, 7, 10, 13, 16-hexaoxacyclooctadecane. 5. The process of claims 1 to 4 wherein the hydroxide is sodium or potassium hydroxide.

Inventors: BASSETT, DAVID ROBINSON, LEE, CHINSOO
Applicants: UNION CARBIDE CORPORATION
Application year: 1988
Main classification: C07C13
IPC codes: C07C1, B01J31, C07C67, C07B61, C07C13