Polymer bromination process in solution

Abstract: An improved process is provided for the bromination of isoolefin-conjugated diolefin polymers by brominating said polymer in solution in selected halogen-containing hydrocarbons as solvent. There is also provided an improved product of said process wherein the brominated polymer contains not less than 80 per cent of the bound bromine atoms in a form suitable to participate in the vulcanization of the polymer and wherein not less than 70 per cent of the bound conjugated diolefin units are similarly so brominated.

Description: This invention relates to an improved process for the bromination of isoolefin polymers. Isoolefin polymers have been known for many years and are commercially available as polymers of an isoolefin and a conjugated diolefin, especially of isobutylene and isoprene. While such polymers have a wide range of desired properties they are not readily covulcanizable with highly unsaturated polymers, such as polymers containing high proportions of one or more conjugated diolefin. In order to overcome such a problem, isoolefin-conjugated diolefin polymers, and especially isobutylene-isoprene polymers which are also known as butyl polymers, have been halogenated. The halogenation, especially chlorination or bromination, is achieved by treating the butyl polymer in solution in an inert organic solvent with controlled quantities of chlorine or bromine. The resulting halogenated butyl polymer has the inherent satisfactory properties of the butyl polymers while also being covulcanizable with the highly unsaturated polymers. The halogenation process is well known to be rather inefficient. One reason for the inefficiency is that for every atom of chlorine or bromine that is incorporated into the polymer a molecule of hydrogen chloride or hydrogen bromide is formed. Another reason is that some of the hydrogen chloride or hydrogen bromide may add to the polymer forming a chemically undesirable group. A further reason is that the actual efficiency of utilization of the chlorine or bromine is quite low, generally being of the order of about 25 to about 40 per cent by weight. DESCRIPTION OF THE PRIOR ART The preparation of butyl polymers is well known wherein a mixture of an isoolefin, preferably isobutylene, and a conjugated diolefin, preferably isoprene, in an inert diluent, preferably methyl chloride, is reacted at a temperature of from about-80°C to about -120°C in the presence of a Friedel-Crafts catalyst, preferably aluminum chloride. The butyl polymer so produced contains about 95 to about 99.5 mole per cent of isobutylene and from about 0.5 to about 5 mole per cent of isoprene. Such a polymer is dissolved in an organic solvent and reacted, at about 10°C to about 60°C, with, preferably, chlorine or bromine for sufficient time to yield a polymer containing not more than 1 atom of combined chlorine per double bond in the polymer or not more than 3, and preferably not more than 1, atoms of combined bromine per double bond in the polymer - see for example U.S. Patents 2 944 578 and 3 011 996. U.S. Patent 3 018 275 describes a process for the halogenation of butyl polymers wherein an oxidizing agent, including a material selected from hydrogen peroxide, sodium peroxide, sodium chlorate or bromate and sodium hypochlorite, is present during the halogenation process to increase the utilization of the halogenating agent in the process. US-A-3,042,662 discloses a process for the bromination of copolymers in which a mixture of an inert solvent and alcohols and/or ethers is used. Said mixture of an inert solvent may be itself a mixture of various inert solvents. It is silent with respect to the benefits obtainable by the use of a halogenated solvent and does not teach the purposeful selection of a halogen-containing hydrocarbon. US-A-2,631,984 discloses a process for producing brominated butyl rubber high in primary allylic bromine in a hydrocarbon or halogenated derivatives thereof. It is silent about the benefits achievable by a careful and purposeful selection of halogenated hydrocarbons as solvent during the bromination process but in fact teaches that bromination may be carried out by preparing a solution, dispersion or cement of the interpolymer "in a liquid organic solvent or diluent such as a hydrocarbon or halogenated derivative thereof". SUMMARY OF THE INVENTION It is an objective of this invention to provide an improved process for the bromination of isobutylene polymers by increasing the utilization of the bromine in the process. Accordingly, one aspect of the invention provides an improved process for the bromination of a C 4 -C 6 isoolefin-C 4 -C 6 conjugated diolefin polymer which comprises preparing a solution of said polymer in a solvent, adding to said solution bromine and reacting said bromine with said polymer at a temperature of from 10°C to 60°C and separating the brominated isoolefin-conjugated diolefin polymer, the amount of bromine being from 0.3 to 1.0 moles per mole of conjugated diolefin in said polymer, characterised in that said solvent consists essentially of either a halogenated C 2 to C 6 paraffinic hydrocarbon having a dielectric constant of at least 2.19 at 45°C or a halogenated aromatic hydrocarbon having a dielectric constant of at least 2.19 at 45°C, and that the solvent further contains up to 20 volume per cent of water or up to 20 volume per cent of an aqueous solution of an oxidising agent that is soluble in water and is suitable to oxidise the hydrogen bromide to bromine in the process substantially without oxidising the polymeric chain. The inert halogen-containing hydrocarbon may be selected from the group consisting of ethyl bromide, propyl chloride, n-butyl chloride and monochlorobenzene. In another aspect, the solvent further contains up to 20 volume per cent of water. One aspect of the invention provides for the process wherein the solvent further contains up to 20 volume per cent of an aqueous solution of an oxidising agent. The oxidising agent is soluble in water and is suitable to oxidise the hydrogen bromide to bromine in the process substantially without oxidising the polymeric chain. In a particular aspect, the oxidising agent is an oxygen containing oxidising agent selected from the group comprising sodium hypochlorite, hydrogen peroxide, sodium peroxide, sodium chlorate, bromate, or other suitable oxidising agents. In yet another aspect, the solvent further contains up to 20 volume per cent of an aqueous solution of sodium hypochlorite, the amount of sodium hypochlorite being equivalent to not more than about 200 per cent of the moles of bromine added. The process of this invention provides a brominated isobutylene-isoprene polymer wherein the brominated polymer contains from 1 to 4 weight per cent of bromine based on the brominated polymer and not less than 80 per cent of the bound bromine atoms are present in a form suitable to participate in the vulcanization of said polymer and not less than 70 per cent of the 1,4-isoprene units in the original isobutylene-isoprene polymer are converted in the brominated polymer into a form suitable to participate in the vulcanization of said polymer. DETAILED DESCRIPTION OF THE INVENTION Isoolefin-conjugated diolefin polymers are well known in the art as also is the process for the manufacture of such polymers. Isoolefins are selected from the C 4 to C 6 isoolefins with isobutylene being the preferred isoolefin. Conjugated diolefins are selected from the C 4 to C 6 conjugated diolefins with isoprene being the preferred conjugated diolefin. Such polymers comprise from 95 to 99.5 mole per cent of the isoolefin, preferably from about 97 to 99.5 mole per cent of isobutylene, and from 0.5 to 5 mole per cent of conjugated diolefin, preferably from 0.5 to about 3 mole per cent of isoprene. The polymer is prepared by the cationic polymerization of the isoolefin and conjugated diolefin, in an inert diluent which is preferably methyl chloride or ethyl chloride, at a temperature of from about -80°C to about -120°C in the presence of a Friedel-Crafts catalyst which is preferably aluminum chloride. For the prior art bromination process, the polymer is dissolved in an inert hydrocarbon solvent such as pentane, hexane and heptane and the solution is fed to a halogenation reactor. The halogenation reactor is typically a vessel equipped with inlet and outlet lines and an agitator. Bromine is also fed to the halogenation reactor at a controlled rate in relation to the amount of polymer and the double bond content of the polymer. The material from the reactor is treated with an aqueous alkaline solution, such as sodium hydroxide, to neutralize the hydrogen bromide formed in the halogenation reaction and to react with residual bromine and then contacted with hot water and steam to remove the solvent and produce a slurry of brominated polymer in water which is then handled in a conventional manner to yield the essentially dry brominated polymer. Stabilizers for the brominated polymer may be added during the recovery process. Such prior art processes exhibit poor utilization of the bromine in the halogenation process, due to the formation of one molecule of hydrogen bromide for each atom of bromine incorporated into the polymer, due to the addition of a small amount of the hydrogen bromide into the polymer and due to the need to use more bromine than is actually incorporated into the polymer. We have now discovered that the halogenation process can be significantly improved by the use of a solvent as defined in claim 1. Preferably, the inert halogen-containing hydrocarbon is selected from the group consisting of ethyl bromide, propyl chloride, n-butyl chloride and monochlorobenzene. The solvent also contains up to 20, preferably from 3 to 15, volume per cent, based on the total solvent, of water. Alternatively the solvent also contains up to 20, preferably from 3 to 15, volume per cent of an aqueous solution of an oxidising agent such as an oxygen containing oxidising agent selected from the group comprising sodium hypochlorite, hydrogen peroxide, sodium peroxide, sodium chlorate or bromate. The oxidising agent is soluble in water and is suitable to oxidise the hydrogen bromide to bromine in the process substantially without oxidising the polymeric chain. Weak oxidising agents at low concentration, such as sodium hypochlorite, are preferred in order to prevent or minimize oxidation of the polymer. Preferably, the oxidising agent will comprise an aqueous solution of sodium hypochlorite, the amount of sodium hypochlorite being equivalent to not more than about 200, preferably from about 100 to about 140, per cent of the moles of bromine added to the process. Using such a solvent for the polymer leads to an increase in the amount of bromine which is incorporated into the polymer to form chemical structures that participate in the vulcanization of said polymer. We have found that for a fixed reaction time in the process of the present invention in excess of about 85 mole per cent of the bromine supplied to the process is incorporated into the polymer to form chemical structures that participate in the vulcanization of the polymer. In contrast, in the prior art only about 60 to 70 mole per cent of the bromine is so incorporated. When the solvent includes water, the improvement in the utilization of bromine is believed to be due to the hydrogen bromide formed in the reaction being preferably soluble in the water and thus not so readily available to form hydrogen bromide addition structures. When the solvent includes aqueous sodium hypochlorite, the improvement in the utilization of bromine to form the desired chemical structures may be as high as about 190 mole per cent of the bromine supplied to the process - this is believed to be due to the hydrogen bromide formed in the reaction being oxidized by the sodium hypochlorite to form bromine which may then further react with the polymer. Without intending to limit the scope of the invention, the bromine utilisation improvement is believed to be achieved by the increase of the dielectric constant of the reaction medium via the use of select halogen-containing hydrocarbons as solvent for the C 4 -C 6 isoolefin-C 4 -C 6 conjugated diolefin polymers instead of an inert hydrocarbon solvent. It is believed that the dielectric constant of the halogen-containing hydrocarbon could be used as a guide for the selection of proper reaction medium for the process of this invention. Table A lists the dielectric constant of some halogen-containing hydrocarbons and hexane. It is apparent from Table A that not all the halogen-containing hydrocarbons will be suitable for the process of this invention. For example, the dielectric constant of carbon tetrachloride is just slightly higher than that of hexane, therefore only marginal improvement can be expected. In contrast use of solvents with high dielectric constant, such as monochlorobenzene, n-butyl chloride, ethyl bromide, should lead to exceptional results. To achieve desired results, the dielectric constant will be greater than that of carbon tetrachloride. In some of the halogenated solvents listed in Table A, solubility of the polymer may be limited. In such cases addition of some paraffinic hydrocarbon is recommended to aid dissolution of the polymer. Dielectric Constant of Some Solvents Halogenated Solvent Dielectric Constant @ 45 o C Hexane 1.85 Carbon tetrachloride 2.19 Chloroform 4.38 Bromobenzene 5.11 Chlorobenzene 5.30 n-Butyl chloride 6.42 Bromoethane 8.16 Methylene chloride 8.23 Methyl chloride 8.64 Dichloroethane 9.17 Such a discovery means that the amount of bromine supplied to the process may be reduced, that the amount of excess bromine leaving the halogenation reactor with the brominated polymer may be reduced which in turn reduces the quantity of aqueous alkaline solution required to react with it, that the amount of bromine chemically bound in the polymer may be more effectively used in the subsequent vulcanization process and that the unsaturation level in the polymer (that is the amount of bound conjugated diolefin) may be reduced because with the more complete utilization of the bromine more of the unsaturated double bonds in the polymer are effectively brominated than in the prior art processes. The desired chemical structures in the brominated polymer include the exo allylic bromide structure, the endo allylic bromide structure and the re-arranged exo allylic bromide structure. In all of these cases the bromine atom is present attached to a carbon atom which is in an allylic configuration with a carbon-carbon double bond (i.e. C=C-CBr) wherein exo and endo have the conventional meanings. In these cases, the bromine atom is chemically very active and participates in the subsequent vulcanization process. The quantities of these various chemical structures may be readily determined by 500 MHz HNMR with high levels of accuracy. The exo allylic bromide structure is the predominant one and usually forms about 75 to about 85 per cent of the desired chemical structures. The total primary structure thus refers to the total of the exo allylic bromide, the endo allylic bromide and the re-arranged exo allylic bromide and the product of the present process contains not less than 80 per cent of the bound bromine atoms in these configurations which are the form to actively participate in the subsequent vulcanization process. The process of the present invention leads to a high conversion of the bound 1,4-isoprene units into the primary structure. The present process leads to such conversions of not less than 70 and up to about 90 or more per cent into the primary structure whereas the prior art processes lead to such conversions of only about 50 to about 60 per cent. The total primary structure as a percentage of the bound 1,4-isoprene units clearly shows the above effect. The halogenation process is operated at a temperature of from 10°C to 60°C, preferably from about 20°C to about 50°C and the reaction time may be from about 1 to about 10 minutes, preferably from about 1 to about 5 minutes. The pressure in the halogenation reactor may be from about 0.8 to about 10 bar. The brominated polymer recovered from the halogenation process typically has a molecular weight, expressed as the Moorey viscosity (ML 1+8 at 125°C), of from 25 to 55. Brominated polymer such as bromobutyl polymer ccntains from 0.5 to 3, most preferably from about 1 to about 2, mole per cent of isoprene and from 97 to 99.5, most preferably from about 98 to about 99, mole per cent of isobutylene based on the hydrocarbon content of the polymer, and from about 1 to about 4, preferably from about 1.5 to about 3, weight per cent of bromine based on the bromobutyl polymer. Further, the product of the present process contains not less than about 80 per cent of the bound halogen atoms in a form suitable to participate in the vulcanization of the polymer. In the product of the prior art, a small amount of the bound halogen is present in the polymer as a result of the addition of a hydrogen bromide molecule across a carbon-carbon double bond thereby forming a saturated group in which the bromine atom is essentially inert in the vulcanization process. In the product of the present process, the amount of bound bromine present in the polymer as a result of the addition of a hydrogen bromide molecule across a carbon-carbon double bond is reduced from that of the prior art. Analysis of the polymer by 500 MHz HNMR was the method used to determine the structural composition of the brominated polymers. In isobutylene-isoprene polymers, the isoprene bound in the 1,4-configuration in the HNMR spectrum exhibits a resonance at about 5.1 ppm. In the brominated isobutylene-isoprene polymers, the HNMR spectrum exhibits resonances at about 5.4, 5.05 and 4.35 ppm which correspond to the exo allylic bromide structure, at about 5.6 ppm which correspond to the endo allylic bromide structure, at about 4.08 and 4.10 ppm which correspond to the re-arranged exo allylic bromide structure. The hydrobrominated structure is calculated from the mass balance. Also in the product of the present process, the proportion of the carbon-carbon double bonds that have been reacted with a bromine atom (to form an unsaturated bromine containing group which is active in the vulcanization process) is much higher than is the case for the product of the prior art processes. Hence the product of the present process contains a higher proportion of the bound bromine atoms in a form suitable to participate in the vulcanization process. The brominated polymer is used to make vulcanizates. The vulcanization of polymers is well known. Carbon black is well known in the art for the reinforcement of vulcanizates and is added to the polymer during the compounding process. Hydrocarbon extender oils are also well known materials used in the compounding process. Generally the carbon black will be selected from the furnace and channel carbon blacks and may be used in amounts of from about 20 to about 90 parts by weight per 1)0 parts by weight of polymer. Hydrocarbon extender oils may be selected from the paraffinic, naphthenic and aromatic oils, preferably from the paraffinic and naphthenic oils, and may be used in amounts from about 5 to about 40 parts by weight per 100 parts by weight of polymer. The vulcanization systems for use with the present product are those already known in the art for use with bromobutyl polymers and generally will include a metal oxide, at least one sulphur based accelerator and, optionally, elemental sulphur. A suitable metal oxide is zinc oxide used in an amount of from about 1 to about 7 parts by weight per 100 parts by weight of polymer. Suitable sulphur based accelerators may be selected from the thiuram sulphides, the thiocarbamates, the thiazyl compounds and the benzothiazyl compounds. The amounts of such accelerators may be from about 0.3 to about 3 parts by weight per 100 parts by weight of polymer. Elemental sulphur may be present in an amount up to about 2 parts by weight per 100 parts by weight of polymer. Various stabilizers, antioxidants, tackifiers, etc. may also be added during the compounding process. The compounding itself will be by conventional methods using a rubber mill or an internal mixer, controlling the temperature to keep it below about 80°C, in one or two stages, generally with the cure active components being added last. The so-formed compounds are then shaped and vulcanized by heating for from about 5 to about 60 minutes at temperatures of from about 150°C to about 200°C. EXAMPLE 1 To a 0.5 L glass reactor, equipped with a stirrer and two reagent addition ports, was added a solution of 20 g of butyl polymer (an isobutylene-isoprene polymer containing 1.77 mole per cent of isoprene) in 270 mL of solvent. The solvent used is shown in Table I. The polymer solution, with the agitator in operation, was heated to 45°C following which 18 mL of water or an aqueous sodium hypochlorite solution was added and dispersed thoroughly throughout the solution. The reaction mixture was protected from light to avoid light induced bromination of the polymer or the solvent. Through one of the reagent addition ports was added 0.18 mL of bromine and agitation of the mixture was continued. After a reaction time of two minutes, the reaction was stopped by the addition, through the second reagent addition port, of 20 mL of a 6% solution of sodium hydroxide. Stabilizers for the polymer were added, the solution was washed with distilled water until neutral and the brominated polymer was recovered by removing the bulk of the solvent in a rotary evaporator followed by final drying in a vacuum oven. Analysis of the polymers by 500 MHz HNMR gave the results shown in Table I. Experiment #1 was a control because hexane was used as the solvent. The improvements in the nature of the product are clearly seen in the results. EXAMPLE 2 Using the procedures described in Example 1, further solvents were evaluated as shown in Table II, the Experiments #6 and 7 being controls. DEFINITIONS The term inert in relation to halogenated solvents means solvents that are inert with respect to reaction with bromine under the conditions of bromination. It is of course, understood that the above examples are submitted merely to illustrate the invention and there is no intention to limit the invention to them.

Claims: An process for the bromination of a C 4 -C 6 isoolefin-C 4 -C 6 conjugated diolefin polymer which comprises preparing a solution of said polymer in a solvent, adding to said solution bromine and reacting said bromine with said polymer at a temperature of from 10°C to 60°C and separating the brominated isoolefin-conjugated diolefin polymer, the amount of bromine being from 0.30 to 1.0 moles per mole of conjugated diolefin in said polymer, characterized in that said solvent consists essentially of either a halogenated C 2 to C 6 paraffinic hydrocarbon having a dielectric constant of at least 2.19 at 45°C, or a halogenated aromatic hydrocarbon having a dielectric constant of at least 2.19 at 45°C and that the solvent further contains up to 20 volume per cent of water or up to 20 volume per cent of an aqueous solution of an oxidising agent that is soluble in water and is suitable to oxidise the hydrogen bromide to bromine in the process substantially without oxidising the polymeric chain. The process of claim 1 wherein said solvent comprises an inert halogen-containing hydrocarbon selected from the group consisting of ethyl bromide, propyl chloride, n-butyl chloride and monochlorobenzene. The process of claim 1 or 2 wherein the oxidising agent is selected from the group comprising sodium hypochlorite, hydrogen peroxide, sodium peroxide, sodium chlorate or bromate. The process of claim 1 or 2 wherein the oxidising agent is sodium hypochlorite, the amount of sodium hypochlorite being equivalent to not more than 200 per cent of the moles of bromine added. The processs of claim 1, 2, 3 or 4 wherein the isoolefin-conjugated diolefin polymer is an isobutylene-isoprene polymer comprising from about 97 to about 99.5 mole per cent of isobutylene and from 0.5 to 3 mole per cent of isoprene. The process of claim 5 wherein the isoolefin-conjugated diolefin polymer is an isobutylene-isoprene polymer comprising from 97 to 99.5 mole per cent of isobutylene and from 0.5 to 3 mole per cent of isoprene, the solvent selected from the group consisting of ethyl bromide and n-butyl chloride the reaction is at a temperature of from 20° to 50°C and the reaction time is from 1 to 5 minutes. The process of claim 1 wherein the isoolefin-conjugated diolefin polymer is an isobutylene-isoprene polymer comprising from 97 to 99.5 mole per cent of isobutylene and from 0.5 to 3 mole per cent of isoprene, the solvent is a halogene-containing hydrocarbon selected from the group consisting of ethyl bromide and n-butyl chloride and water forming from 3 to 15 volume per cent of the solvent, the reaction is at a temperature from 20°C to 50°C and the reaction time is from 1 to 5 minutes. The process of claim 1 wherein the isoolefin-conjugated diolefin polymer is an isobutylene-isoprene polymer comprising from 97 to 99.5 mole per cent of isobutylene and from 0.5 to 3 mole per cent of isoprene, the solvent is a halogene-containing hydrocarbon selected from the group consisting of ethyl bromide and n-butyl chloride and an aqueous solution of sodium hypochlorite forming from 3 to 15 volume per cent of the solvent, the reaction is at a temperature from 20°C to 50°C and the reaction time is from 1 to 5 minutes.

Inventors: BAADE WOLFGANG DR, KASZAS GABOR, KOENINGSHOFEN HEINRICH DR, BAADE, WOLFGANG, DR., KASZAS, GABOR, KOENINGSHOFEN, HEINRICH, DR., Köningshofen, Heinrich, Dr.
Applicants: BAYER AG
Application year: 1997
Main classification: C08F8
IPC codes: C08F8