VINYLAROMATIC AND CYCLIC ALIPHATIC MONOMER PURIFICATION PROCESS

Abstract: A process for purification of vinylaromatic monomers or cyclic aliphatic monomers containing contaminating quantities of indene and/or phenylacetylene comprising contacting the impure monomer with a dialkylmagnesium compound containing from 1 to 10 carbons in each alkyl group, removing reaction byproducts and recovering the purified monomer.

Description: VINYL AROMATIC AND CYCLIC ALIPHATIC MONOMER PURIFICATION PROCESS The present invention relates to an improved process for preparing vinylaromatic and cyclic aliphatic monomers in a highly pure form suitable for use in a Ziegler-Natta polymerization process. More particularly the present invention relates to such a process for the facile removal of indene and other impurities from an impure monomer stream. It has been long known to be advantageous to purify vinylaromatic monomers, especially styrene, prior to use in an anionic polymerization process. Generally, distillation, deaereation, contacting with a desiccant, contacting with a molecular sieve or careful titration with a solution of a lithium hydrocarbyl compound have been satisfactorily used to purify monomers prior to an anionic polymerization. Generally, contaminants that must be removed from a monomer stream prior to an anionic polymerization include oxygenated species, such as aldehydes or acids and acetylenic species, such as phenylacetylene. The polymerization of styrene monomer to prepare useful polymers by means of a Ziegler-Natta type polymerization, that is, by the use of a catalyst comprising a transition metal complex, has been previously disclosed in the art. In US-A-4,680,353 the preparation of vinylaromatic polymers having a high syndiotactic configuration utilizing such a catalyst is disclosed. Ethylene-styrene copolymers wherein the styrene is in a pseudo-random configuration prepared using Ziegier-Natta catalysts comprising Group 4 metal complexes are disclosed in EP-A-416,815. Copolymers containing cyclic aliphatic monomers, especially vinylcyclohexane or ethylidene norbornene, are also disclosed in the foregoing publication. In the practice of these processes, a certain portion of the catalyst is normally exhausted by reaction with impurities during the polymerization. Because the catalyst is relatively expensive, commercially viable processes require improved yields of polymer. Thus it would be desirable to provide a process for polymerization of vinylaromatic monomers and cyclic aliphatic monomers having improved catalyst efficiency and lifetime. In US-A-5,064,918 a process for phenylacetylene removal by hydrogenation is disclosed. Several purification processes are disclosed in EP-A-410,361 and elsewhere, including: 1) treatment with an absorbent substance such as alumina, activated alumina, silica gel, activated charcoal, calcium oxide, aluminum silicate or diatomaceous earth, 2) degassing under reduced pressure by bubbling a dried inert gas therethrough, 3) treatment with an organoaluminum compound, 4) distillation, 5) treatment with an alkali metal, lithium hydrocarbyl or Grignard reagent, 6) hydrogenation, and 7) combinations of the foregoing processes. These processes are often effective in removing oxygenates and acetylenic contaminates from the monomer stream. However, indene, which is also often found in such monomer streams, has now also been found to adversely affect polymerizations using Ziegier-Natta catalysts based on Group 4 metal complexes. Disadvantageously, the previously mentioned processes have been found to be ineffective in removal of indene, especially from vinylaromatic monomer or cyclic aliphatic monomer streams, without concomitant undesired reaction with the monomer. Consequently, there remains a desire to devise a cost effective means to purify vinylaromatic or cyclic aliphatic monomers for use in Ziegler-Natta polymerization processes. In Eisch et al., J. Organomet. Chem.. 296 C27-C31 (1985) the magnesiation of carbon Bronsted acids, including indene, with dialkylmagnesium compounds including n-butyl s- butyl magnesium was disclosed. It has now discovered that indene, if present in vinylaromatic or cyclic aliphatic monomers adversely affects Ziegler-Natta polymerization catalysts to reduce catalyst lifetime and efficiency. Furthermore, the present inventor has now discovered that indene, as well as other contaminants such as phenylacetylene, may be removed from such monomers by contacting the same with a dialkyl magnesium compound having from 1 to 10 carbons in each alkyl group and thereafter separating reaction byproducts and recovering the purified monomer. The process provides a quick and economical method for purifying a vinyl aromatic monomer, cyclic aliphatic monomer, or a mixture thereof prior to polymerization under Ziegler-Natta polymerization conditions. Preferably, the process lowers the indene content and the phenylacetylene content of such monomer stream to 10 ppm or less, more preferably to 1 ppm or less. Polymerization processes wherein the purification process according to the present invention may be employed include the previously described known procedure for preparing syndiotactic polystyrene, as illustrated in US-A-4,680,353. In addition to such processes for preparing syndiotactic polyvinylaromatic monomers, monomers purified according to the present invention may be utilized in a mixture of one or more vinylaromatic monomers or cyclic aliphatic monomers and one or more noncyclic aliphatic olefins, especially ethylene or propylene, to prepare interpolymers therefrom. Such a process is disclosed in EP-A-416,815. Suitable catalysts for such process include the combination of (N-t-butylamido)-(dimethyl)(η 5 - 2,3,4,5-tetramethylcyclopentadienyl)silane titanium dimethyl with an activating cocatalyst, especially trispenta-fluorophenyl borane. In addition to aliphatic olefins such as ethylene, propylene, and 1-octene, polymerizations for which a monomer stream prepared according to the present invention may be used may additionally include diolefins such as butadiene or isoprene. Ziegler-Natta catalysts for use according to the present invention preferably comprise: 1) an active catalyst species which is a metal complex of a Group 3, 4 or Lanthanide metal of the Periodic Table of the Elements, and, optionally, 2) an activating cocatalyst. Such catalysts may be employed in either a supported or an unsupported manner. Suitable supports include MgCI 2 , silica and alumina. Especially desired supports are silica or alumina that have been calcined, or pretreated with an alumoxane. The presence of phenylacetylene and indene in monomers used according to the present invention may be detected by standard analytical techniques. One suitable technique is gas chromatography (GC). Vinyl aromatic and cyclic aliphatic monomer streams for use herein may have levels of phenylacetylene as high as 500 parts per million or higher and indene contents of 200 ppm or higher. According to the present technique it is possible to purify such styrene monomer containing high levels of phenylacetylene or indene rendering the same suitable for use in an anionic polymerization process or in a Ziegier-Natta polymerization. The process may be employed as part of a multistep purification whereby the monomer is first treated (primary treatment) to remove some or all contaminants such as water, oxygen, oxygenates and acetylenic compounds. Suitable primary treatments include deareation, contacting with a molecular sieve, contacting with activated alumina, hydrogenation, or a combination of the foregoing techniques. Preferred dialkyl magnesium compounds are di n-butyl magnesium, di n-octyl magnesium, or ethyl n-butyl magnesium. The dialkyl magnesium compound may be used in the form of a solution in an organic solvent, such as a C 8 alkane, or the monomer to be treated. It also may be added directly in neat form to the monomer stream. Suitable compounds may aiso comprise adducts of dialkylmagnesium compounds with aluminum trialkyl compounds having 1 to 6 carbons in each alkyl group. However, it should be noted that aluminum trialkyl compounds are by themselves incapable of reacting with indene. Accordingly, when dialkylmagnesium adducts of aluminum trialkyls are employed herein, the benefits of the invention are believed to be due to the presence of the dialkyl magnesium compound rather than the trialkyl aluminum compound. It may also be stated that secondary alkyl substituted magnesium compounds, particularly, sec butyl magnesium, have been found to generate undesired byproducts with certain monomers, especially styrene. Such byproducts, particularly 1 -pheny l-3-methylpentane, reduce overall monomer yield and are not preferred for use with such monomers. The method of contacting is not critical. The dialkyl magnesium compound may be in a solid form such as deposited or absorbed on an inorganic support in a bed through which the monomer stream in passed. Alternatively, the dialkyl magnesium compound may be in a solution which is contacted with the monomer stream. Preferred dialkyl magnesium solutions are from 0.001 to 5.0 M in concentration, preferably from 0.01 to 2.0 M. Most preferably the monomer stream and a solution of the dialkyl magnesium compound in the monomer to be treated are contacted and intermixed for a time from one second to 10 hours, preferably from 10 seconds to 1 hour. Generally the amount of dialkyl magnesium compound used is from 0.5 to 10,000 moles per mole of indene contaminant to be removed. More preferably from 1 to 100 moles of dialkyl magnesium compound per mole of indene contaminant is used. After contacting of the dialkyl magnesium compound and monomer stream, the monomer is separated by filtration or distillation, especially when a solid dialkyl magnesium compound is used or by passing the solution of monomer and dialkyl magnesium compound through a bed of an absorbent, especially when a solution of dialkyl magnesium compound is used. Suitable absorbents include alumina, activated alumina, silica gel, activated charcoal, calcium oxide, aluminosilicates and diatomaceous earth. Preferred absorbents have a surface area from 1 to 10,000 M 2 /g, more preferred from 100 to 5,000 M /g. A most preferred absorbent is activated alumina. Further purification by distillation or other means may also be employed, but is generally unnecessary. Suitable temperatures for the process are from -20°C to 45°C, preferably from 20°C to 30°C. After substantial removal of any phenylacetylene and/or indene has occurred, the monomer stream is ready for use in a polymerization process. Utilizing the present technique, improved catalyst efficiencies, measured by the amount of polymer which results per weight catalyst, on the order of 50 to 1000 percent, are obtained, compared to the use of styrene monomer which has not been treated to remove indene. In an anionic polymerization, the present process allows extremely narrow molecular weight distribution polymers to be formed. Preferred vinylaromatic monomers adapted for use herein especially include styrene, vinyltoluene (all isomers, especially p-vinyltoluene), divinylbenzene, vinylbenzocyclobutene, and mixtures thereof. Having described the invention, the following examples are provided to further illustrate the invention. Unless stated to the contrary, ali parts and percentages are based on weight. Example 1 Four-hundred milliliters of styrene containing 103 ppm of phenylacetylene and 113 ppm of indene was sparged with nitrogen and placed in an inert atmosphere glove box. The styrene was split into two samples of approximately 200 mL each. One was treated with 800 μL of 1.0 M dibutylmagnesium in hexane, the other with 3,200 μL of 1.0 M dibutylmagnesium in hexene. After an hour, two 25 mL aliquots were taken from each sample and placed in a glass flask containing 10 g of activated alumina (activated by heating under flowing nitrogen at 375°C). Samples for analysis were collected by filtration after one and four hours contact with the alumina. Likewise, four hours after addition of the magnesium reagent, two more aliquots of each styrene sample (four total aliquots) were removed and treated with alumina for one and four hours respectively. Analysis of the four samples from the batch treated with 0.8 mmol of di-n-butylmagnesium indicated nearly identical levels of phenylacetylene (-5 ppm) and indene (85 ppm). The four samples of styrene treated with 3.2 mmol of dialkyl magnesium all had phenylacetylene and indene levels that were undetectable (< 2 ppm). Example 2 Two 250 mL portions of styrene containing 67 ppm of phenylacetylene and 60 ppm of indene were placed in pressure equalizing addition funnels equipped with nitrogen spargers and sparged for fifteen minutes. To one funnel, 2 mL of 1 M dibutylmagnesium in hexane was added. The mixture was agitated for one hour with slow nitrogen sparging. Both liquids were then dripped down packed columns of activated alumina (-3 X 40 cm). Analysis of the sample treated only with alumina showed it contained 47 ppm of phenylacetylene and 59 ppm of indene. The sample treated with dibutylmagnesium and alumina contained 4 ppm of phenylacetylene and 10 ppm of indene. Polymerization The two differently treated styrene samples were then subjected to polymerization with a catalyst system comprising one equivalent of pentamethylcyclopentadienyltrimethoxytitanium, 25 equivalent of methylalumoxane and 75 equivalent of triisobutylaluminum. Polymerizations were run at 50°C for 30 minutes. Using a mole ratio of styrene monomer : titanium of 200,000:1 , the alumina treated styrene gave <1 percent conversion to syndiotactic polystyrene. The dialkylmagnesium treated styrene gave 44.5 percent conversion. At a styrene : titanium molar ratio of 350,000:1 , the alumina treated styrene gave <1 percent conversion. The dialkylmagnesium and alumina treated monomer gave an average conversion of 23.5 percent. Example 3 Three -130 mL samples of a mixture of styrene and p-methylstyrene (4 percent by weight) were passed through activated alumina then through a palladium on alumina bed in the presence of hydrogen. The resulting monomer mixture contained 0 ppm phenylacetylene and 5 ppm indene. An additional quantity of indene (5 μL), was added to two of the samples. These samples were then treated by passing through activated alumina or by addition of 1 mL of 1 M dibutylmagnesium in mixed hexanes and, after stirring for 1 hour, by passage through activated alumina. Indene levels remained essentially constant in the sample treated only by contacting with alumina (58 ppm before, 61 ppm after). Indene levels declined significantly in the sample treated first by dibutylmagnesium followed by contacting with alumina (56 ppm before, 4 ppm after). Polymerization At molar ratios of styrene to titanium of 200,000:1 and 350,000:1, using the same catalyst mixtures as described in Example 2, under one atmosphere of hydrogen, the sample lacking added indene gave average conversions of 63.7 percent and 56.6 percent, respectively, to styrene/p-methylstyrene syndiotactic copolymer. The monomer mixture treated only by contacting with alumina gave average conversions of 49.5 percent and 38.5 percent respectively. The dibutylmagnesium and alumina treated sample gave 59.6 percent and 46.7 percent conversions respectively.

Claims: WHAT IS CLAIMED IS: 1. A process for purifying a vinylaromatic monomer or cyclic aliphatic monomer mixture contaminating quantities of indene, said process comprising contacting said -, 5 monomer mixture with a dialkylmagnesium compound having from 1 to 10 carbons in each alkyl group removing reaction byproducts, and recovering the resulting purified monomer. 2. A process according to Claim 1 wherein the dialkylmagnesium compound is in the form of a solution in an organic solvent. 3. A process according to Claim 1 wherein the solution of dialkylmagnesium 10 compound is from 0.001 to 5 M. 4. A process according to Claim 1 wherein after contacting with the dialkylmagnesium compound the reaction byproducts are removed by contacting the vinylaromatic monomer with an absorbent. 5. A process according to Claim 4 wherein the absorbent is activated alumina. 15 6. A process according to Claim 1 wherein the monomer is styrene. 7. A process according to Claim 4 wherein the monomer is styrene.

Inventors: LAPOINTE ROBERT E, LAPOINTE, ROBERT, E.
Applicants: DOW CHEMICAL CO, LAPOINTE ROBERT E, LAPOINTE, ROBERT, E., THE DOW CHEMICAL COMPANY
Application year: 1996
Main classification: C07C7
IPC codes: C07C7, C07C15