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 hydroxide by conducting the elimination reaction in the presence of a cosolvent and certain reaction promoters.

Description: FIELD OF THE INVENTION This invention relates to a process for the preparation of 2,2-paracyclophane in relatively high yields by the Hofmann elimination of p-methylbenzyltrimethylammonium hydroxide in the presence of a cosolvent and/or 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: It 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, the 2,2-paracyclophane itself, from which the parylene coating is prepared, is made via the pyrolysis of p-xylene or Hofmann elimination reaction of p-methylbenzyltrimethylammonium hydroxide, and 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-650,947 - A - polymer formation was detected on the walls of a cooling chamber after p-xylene was vaporized and pyrolyzed. In U.S. - A - 3,149,175 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 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-p-cyclophane 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-methylbenzyltrimethylammonium hydroxide with sodium or potassium hydroxide in the presence of dimethylsulfoxide (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 dimethylformamide, N-methyl-pyrollidone or sulfolane had no effect. The earlier EP- 0 253 191-A 1 describes a process for the preparation of (2,2)-paracyclophane by the Hofmann elimination of p-methylbenzyl-trimethylammonium hydroxide, in an aqueous solution of an alkali metal hydroxide, wherein said elimination is carried out in the presence of a dialkylether or mono-and poly-ethylene glycols having the formula: R - 0 [CH₂CH₂0] n R' 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. It is therefore an object of this invention to provide an process for the preparation of the 2,2 paracyclophane used in the synthesis of parylene. Another object of the invention is to provide a process for the preparation of 2,2-paracyclophane in relative high yields a opposed to the methods disclosed in the literature to date. The 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. It is also an object of this invention to provide a process which is conducted in the presence of at least two of a colsolvent and two reaction promoters. SUMMARY OF THE INVENTION In its broad aspect, the present invention is directed to a process for the preparation of parylene, which comprises contacting an aqueous solution of p-methylbenzyltrimethylammonium hydroxide with an alkaline hydroxide in the presence of at least two of: an alkyl substituted 2-imidazolidinone cosolvent; a crown ether having from 4 to 6 oxygen atoms and from 12 to 18 carbon atoms; and a methyl alkyleneoxy ether of the formula: CH₃(-0-CH₂-CH₂-) n -0-(CH₂-CH₂0-) m -CH₃ wherein n has a value of from 1 to 18, and m is 0 or 1 to 4; and thereafter recovering the 2,2-paracyclophane. DESCRIPTION OF THE DRAWINGS A more detailed understanding of the invention will be had by reference to the drawings wherein: Figure 1 is graph showing the yield of 2,2-paracyclophane versus reaction time for a prior art process using only DMSO in varying mole ratios to the quarternary ammonium salt. Figure 2 is a graph depicting the weight percent of 2,2-paracyclophane obtained by employing dimethylsufoxide alone in accordance with U.S. - A - 4,532,369 and the weight percent of 2,2-paracyclophane obtained by employing the cosolvent of this invention in combination with certain specific reaction promoters. In Figure 2, curve C was obtained using DMSO alone; curve B was obtained using the cosolvent and a single reaction promoter, 18 crown 6 alone; and curve A obtained using the the cosolvent and the specific combination of reaction promoters hereinafter described. It is evident from the drawing that higher yields of the desired 2,2-paracyclophane can be obtained in shorter periods of time when a cosolvent and the reaction promoters are employed in the specific combination as hereinafter indicated. DETAILED DESCRIPTION OF THE INVENTION In contrast to the prior art, it was unexpectedly and surprisingly found that yields of the 2,2-paracyclophane even higher than the 70 percent reported can be obtained when a cosolvent and reaction promoter(s), as herein defined, is employed in the preparation of the 2,2-paracyclophane. It has been observed that while the use of dimethylsulfoxide in the Hofmann elimination reaction increased the yields of the desired 2,2-paracyclophane over those which had been disclosed in the literature, the yield could be increased even further if a cosolvent and certain reaction promoters were employed in the reaction system in place of DMSO. Prior to the US - A - 4 532 369, 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 said known process, the process could still not be extended to the coating of less expensive items. 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. It was also noted that attempts to increase the mole ratio of DMSO to the quarternary ammonium salt did not lead to increased yields. Moreover, it is known that DMSO in the presence of water and caustic can decompose at high temperatures and therefore the use of increased amounts was not feasible. The cosolvents which have been found to be useful in the process of the present invention are the alkyl-substituted imidazolidinones. Such compounds include those have from 2 to 4 lower alkyl groups attached to the ring carbon or nitrogen atoms. Preferably the lower alkyl groups contain from 1 to 6 carbon atoms and more preferably 1 to 2 carbon atoms. The methyl-substituted imidazolidonones are the most preferred. Illustrative compounds include 1,3-dimethyl-2-imidazolidinone, 1,3,4-trimethyl-2-imidazolidinone, 1,3,4,5-tetramethyl-2-imidazolidinone, 1,3-diethyl-2imidazolidinone, and 1,2-dimethyl-3-ethyl-2-imidazolidinone. The reaction promoters employed in the process of the present invention are those promoters which promote the Hofmann elimination reaction so as to shift the equilibrium of the reaction to favor the formation of the 2,2-paracyclophane as opposed to the competing reaction which favors polymer formation. The exact mechanism by which the reaction promoters operates is still under investigation 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 polymerization 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 should be used in a specific combination if increased yields of the 2,2-paracyclophane are to be obtained. The reaction promoter which have been found to be suitable for optimizing the yield of 2,2-paracyclophane by the process of the present invention are the crown ethers. These are, of course, 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 2 (1,4,7,10-tetraoxacyclododecane), 15 Crown 5 (1,4,7,10,13-pentaoxacyclooctadecane), 18 Crown 6 (1,4,7,10,13,16-hexaoxacyclooctadecane, 1,4,7,10-tetraoxacyclododecane), benzo-15 Crown 5, bis(benzo 15 Crown 5)methyl] pimelate, bis [(12 Crown 4) 2-ylmethyl] 2-dodecyl-2-methylmalonate, dibenzo 18 Crown 6, dibenzo 24 Crown 6, and dicyclohexano 24 Crown 8. Additional to said promoter methyl alkyleneoxy ethers of the given formula may be used. These compounds are sometimes referred to as "glymes" amd include the diglymes and the tetraglymes, particularly when the ether is capped with a methoxy group. In the given formula n is preferably from 2 to 4. 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-methylbenzyltrimethylammonium 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 dimer 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, meta-xylene, ortho-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 quarternary salt whould be within the range of from 2:1 to 20:1, and preferably from 5:1 to 12:1. b) the mole ratio of the cosolvent to the quarternary 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 quarternary 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 total raction promoters to the quarternary ammonium salt should be within the range of from 20:1 to 1:1. In practice, it has been observed that when the additional reaction promoter is a glyme it should be present within the range of from 4:1 to 8:1. The mole ratio of the crown ether as reaction promoter such as the 18 crown 6 should be within the range of from 3:1 to 1:1. e) while the mole ratio of organic solvent to the quaternary ammonium slat is not critical, good results are obtained when the mole ratio is within the range of from 10:1 to 80:1, and more preferably from 20:1 to 60:1. As indicated previously, it was unexpectedly and surprisingly found that by employing a cosolvent alone or with certain reaction promoter(s) markedly increased yields of the 2,2-paracyclophane can be obtained while polymer formation is minimized. In order to obtain optimum yields of the desired 2,2 paracyclophane the choice of reaction promoters employed is important. It has been observed that at least one of the promoters used in combination with the crown ether must be a glyme to provide the highest yields in the shortest reaction period. As indicated in the drawing, curve B which is a combination of DMI and a glyme provides greater yields than 18 crown 6 alone. 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 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, attemps to increase the yield of the 2,2-paracyclophane were for the most part failures, or at most, only small incremental increases in yields were obtained. Additionally, as previously indicated, DMSO in the presence of water and caustic decomposes at high temperatures and hence its use is not desired. 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 2,2-paracyclophane 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. 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 particularly 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 contemplated for the practice of this invention. COMPARATIVE EXAMPLE 1 PREPARATION OF PARACYCLOPHANE USING DMSO AS A REACTION PROMOTER A mixture of 509.2 gram of p-xylene, 20.3 gram of KOH in 20.3 ml of water, 12.35 gram of p-methylbenzyltrimethylammonium chloride in 8.95 ml of water, and 50.4 gram of DMSO is charged in a 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 hour. Stirrring is continued for 40-50 hours at 90-91°C throughout the reaction study. The reactor samples are taken, and measured for the [2,2]paracyclophane concentrations in the reactor by (G.C.) vapor-phase chromatographic analysis, The results are shown in Table I below: EXAMPLE 1 PREPARATION OF PARACYCLOPHANE USING COMBINATION OF REACTION PROMOTERS The process of Comparative Example 1 was repeated except that a mixture of 400.0 gram of p-xylene, 20.8 gram of KOH in 20.8 ml water, 11.6 gram of p-methylbenzyltrimethylammonium chloride in 8.4 ml of water, 80.0 gram of 1,3-dimethyl-2-imidazolidinone, 52.0 gram of tetraglyme, 51.7 gram of diglyme, and 10.7 gram of 18 crown 6 is used. The reactor samples are taken, and measured for the [2,2]paracyclophane concentrations in the reactor by (G.C.) vapor-phase chromatographic analysis, The results are shown in Table II below: COMPARATIVE EXAMPLE 2 PREPARATION OF PARACYCLOPHANE USING 18 CROWN 6 AS THE REACTION PROMOTER The process of Comparative Example 1 was repeated except that a mixture of 500.6 gram of p-xylene, 20.4 gram of KOH in 20.4 ml water, 13.0 gram of p-methylbenzyltrimethylammonium chloride in 9.4 ml of water, and 10.1 gram of 18 crown 6 is used. The reactor samples are taken, and measured for the [2,2]paracyclophane concentrations in the reactor by (G.C.) vapor-phase chromatographic analysis, The results are shown in Table III below: COMPARATIVE EXAMPLE 3 PREPARATION OF PARACYCLOPHANE USING DMSO 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 18.8 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 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 concentrations in the reactor by (G.C.) vapor-phase chromatographic analysis. The results are shown in Table IV below: COMPARATIVE EXAMPLE 4 PREPARATION OF PARACYCLOPHANE USING DMSO A mixture of 535 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 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 concentrations in the reactor by (G.C.) vapor-phase chromatographic analysis. The results are shown in Table V below: COMPARATIVE EXAMPLE 5 PREPARATION OF PARACYCLOPHANE USING DMSO 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 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 concentrations in the reactor by (G.C.) vapor-phase chromatographic analysis. The results are shown in Table VI below:

Claims: A process for the preparation of the 2,2-paracyclophane used in preparation of parylene, which comprises contacting an aqueous solution of p-methylbenzyltrimethylammonium hydroxide with an alkaline hydroxide in the presence of at least two of: an alkyl substituted 2-imidazolidinone cosolvent; a crown ether having from 4 to 6 oxygen atoms and from 12 to 18 carbon atoms; and a methyl alkyleneoxy ether of the formula: CH₃(-0-CH₂-CH₂-) n -0-(CH₂-CH₂0-) m -CH₃ wherein n has a value of from 1 to 18, and m is 0 or 1 to 4; and thereafter recovering the 2,2-paracyclophane. The process of claim 1 wherein the process iseffected in the presence of an inert water-immiscible organic solvent. The process of claim 2 wherein the solvent is selected from benzene, toluene, paraxylene, meta-xylene, ortho-xylene, hexane, octane, and methylene chloride. The process of claims 1-3 wherein the 2imidazolidinone cosolvent is 1,3-dimethyl-2-imidazolidinone. The process of claims 1-4 wherein the crown ether is 1,4,7,10,13,16-hexaoxacyclooctadecane. The process of claims 1-5 wherein the methyl alkyleneoxyether is diglyme or tetraglyme. The process of claim 6 wherein the diglyme is 2-methoxyethyl ether. The process of claim 6 wherein the tetraglyme is 2-butoxyethyl ether. The process of claims 1-8 wherein said hydroxide is sodium hydroxide or potassium hydroxide.

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