GB2474356A - Preparation of bromine-containing copolymers - Google Patents

Abstract

Bromine-containing copolymers are prepared by copolymerising a vinyl aromatic monomer, such as styrene, and a cyclic polyene monomer, such as cyclododecatriene (CDT) to form copolymerization product, and subsequently brominating the copolymerization product. Also disclosed is a method of preparing bromine-containing copolymers by copolymerising a vinyl aromatic monomer, such as styrene and a brominated unsaturated cyclic monomer such as tetrabromocyclododecene.

Description

Bromine-containing copolymers Bromine-containing compounds are useful in the flame retardation of various polymers. Recently, it has been proposed to brominate styrene/butadjene copolymers, and to use the resulting bromine-containing copolymers as flame retardant additives (see, for example, WO 2008/0214 17) The present invention is directed to bromine-containing copolymers obtainable by either one of the following processes: 1) copolymerizing a vinyl aromatic monomer and a cyclic polyene monomer, to form a copolymerization product, and subsequently brominating said copolymerization product; 2) copolymerizing a vinyl aromatic monomerand a brominated unsaturated cyclic monomer.

The reaction scheme for preparing the bromine-containing copolymers of the present invention can be illustrated by the following flow chart (Scheme 1): CH=CH2 + IIm COP OlymenzatiOn bromination em-k copoiymerization Ri In scheme 1, the vinyl aromatic monomer is represented by the formula CH=CB2 wherein R1 is selected from the group consisting of hydrogen, Cl-C5 straight or branched alkyl groups and halogens (specifically chlorine and bromine) . Thus, the vinyl aromatic monomer to be used according to the invention may be styrene (R1 is H) or ring-substituted styrene, such as the ortho-, meta-and para-, methyl, ethyl, propyl, butyl styrene. The ring may be substituted by one or more of said R1 groups (which may be the same or different) . Mixtures of two or more of such optionally substituted styrene monomers may also be used.

In scheme 1, the cyclic polyene monomer is schematically represented by the symbol: The cyclic polyene is preferably an optionally-substjue cycloalkene which possesses two or more carbon-carbon double bonds. All of the carbon-carbon double bonds may be endocyclic double bonds, forming part of the ring structure.

However, the term "cyclic polyene", as used herein, embraces also cycloalkenes having one or more carbon-carbon double bonds which are external to the ring structure. As may be appreciated, one of the carbon-carbon double bonds of the cyclic polyene monomer takes part in the copolymerization reaction, whereas the remaining double bonds are available for the bromine addition. For the purpose of clarity, the carbon-carbon double bond which is reactive in the copolymerizatjon reaction is indicated by an asterisk. The remaining m carbon-carbon double bonds, wherein m is an integer in the range between 1 and 4, are bonds to which bromine may be added. Preferably, the cyclic polyene is a diene (m equals 1) or triene (m equals two), with the latter being especially preferred. The cyclic polyenes of the invention may be either conjugated or non-conjugated. The number of carbon atoms in the ring of the cyclic polyene preferably varies between 5 and 14, with all the carbon-carbon double bonds preferably being endocyclic bonds. A particularly preferred cyclic polyene to be used according to the invention is cyclododecatriene. The name "cyclododecatriene", as used herein, indicates any possible isomer of said compound or a mixture of such isomers, e.g., the ciS-trans-trans_cyc1ododecatrjen, 5, 9, the trans-trans-trans-cyclododecatrjene_1,5,g or any mixture of these isomers. Examples for cyclic polyene monomers, which contain carbon-carbon double bonds which are external to the ring structure include l-vinyl-cyclohexene and 2-vinyl-l,3--cyclohexadiene.

In scheme 1, the brominated unsaturated cyclic monomer is represented by the symbol: Br2k wherein k is an integer in the range between 1 and m. Most preferably, k equals m, such that the brominated cyclic unsaturated monomer contains only one carbon-carbon double bond (marked by the asterisk), which bond is available for the subsequent copolymerization. Thus, as shown by scheme 1, the addition of bromine to the cyclic polyene monomer, which contains a multiplicity of rn-fl carbon-carbon double bonds, most preferably gives brominated cycloalkene which is substituted with 2m bromine atoms. Accordingly, when the cyclic polyene to be used according to the invention is cyclododecatriene, then said cyclododecatriene undergoes a bromination reaction to form a brominated cycloalkene which is most preferably tetrabromocyclododecene.

In scheme 1, the intermediate, non-brominated copolymer and the final, brominated copolymer are represented by the following illustrative formulas, respectively: Wherein Ri, m and k have the meanings set forth above, and n is the average degree of polymerization. it should be understood, however, that the repeating units depicted above are only illustrative, and in the copolymers of the invention the monomeric units are not necessarily distributed in alternating fashion, but may be also distributed randomly or in blocks. For convenience, using the acceptable source-based names for copolymers, the copolymers obtained by means of the first synthetic route are named herein brominated copoly(styrene/cyclic polyene), indicating that the bromination succeeds the copolymerizatjon. Copolymers prepared from the monomers styrene and brominated cycloalkene are named herein copoly(styrene/brominated cycloalkene) . For the sake of simplicity, the final, bromine-containing copolyrners are represented throughout the description using the same structural formula.

Accordingly, the preferred copolymers provided by the invention are brominated copoly (styrene/cyclododecatrjene) and copoly(styrene/tetrabromocyclododecene), in which case scheme 1 can be reduced to the following form (Scheme 2): Procs A PrOCS! B

CDT CDT

3 TEABIBr2 H2OIMC Br\Br,B/Br L6J /TBCD =?\ The brominated copoly(styrene/cyclododecatrjene) is prepared according to route A of Scheme 2, namely, by a process comprising the copolymerization of the monomers styrene and cyclododecatriene to form the copolymer styrene/cyclododecatrjene, and subsequent brominat ion of said copolymer.

The copolymer styrene/cyclododecatriene may be prepared by combining the monomers at any suitable molar ratio in the presence of a free radical initiator. The free radical polymerization may be accomplished in solution, bulk (also under pressure) or in emulsion. The weight ratio styrene: cyclododecatrjene may be in the range between 9:1 and 1:9, more preferably in the range between 5:1 and 1:5.

Regarding solution polymerization, the monomers styrene and cyclododecatriene are dissolved in a suitable organic solvent under heating, in the presence of a free radical initiator belonging to the class of peroxides (ROOR, wherein R is the organic residue) . The weight concentration of the mixture of monomers in the solution may be in the range between 10 and 60%. The weight concentration of the initiator in the solution may be in the range from 1 to 10%.

The copolymerization reaction is preferably carried out at the reflux temperature. Upon completion of the copolymerizatjon reaction, the precipitation of the copolymer is induced by techniques known in the art, which techniques may involve partial evaporation of the reaction solvent to form a concentrated solution, cooling of the reaction mixture, the addition of an-anti solvent to the reaction mixture (namely, a solvent in which the copolymer exhibits poor solubility), or any combination of said techniques. The copolymerization product is then separated from the liquid phase (e.g., by filtration or centrifugation), optionally washed and dried. The preparation of the styrene/cyclododecatrien copolyrner by means of solution copolymerization was reported by Tokarzewskj and al in Polyrnery, 1969, 23, using benzene as a solvent and benzoyl peroxide as initiator. The inventors have found that the solution copolymerization may be more conveniently accomplished by reacting the monomers in a non-polar saturated hydrocarbon solvent such as cyclohexane, in the presence of a suitable amount of the initiator (e.g. benzoyl peroxide), followed by adding a polar anti-solvent to the reaction mixture, in order to induce the precipitation of the copolymerization product. A preferred anti-solvent capable of inducing the crystallization of the copolymer is a lower alcohol, such as methanol.

Regarding bulk polymerization, it has been found that the copolymerization of styrene and cyclododecatriene may be also effectively accomplished in the melt in the presence of a suitable amount of the peroxide initiator (e.g. benzoyl peroxide) . The reaction may be also carried out in a closed vessel under pressure.

Regarding the emulsion polymerization, the monomers styrene and cyclododecatriene are dispersed in an aqueous phase in the presence of an emulsifying agent and a suitable initiator, preferably of the redox type. Operative emulsifying agents include, but are not limited to, aniolnic surfactants, such as sodium dodecyl sulfate. A redox system operative according to the present invention comprises a water soluble oxidant in combination with a water soluble reductant. A particularly useful combination is that of a persulfate salt as the oxidant, together with a water soluble metabisulfite salt as the reductant, the two together forming a redox pair. Utilizable persulfates include potassium persulfate, sodium persulfate and amrnonium persulfate. The metabisulfites include potassium metabisulfite and sodium rnetabisulfite.

Having prepared and collected the copoly(styrene/cyclododecatriene), *the subsequent brornination reaction is carried out by reacting said copolymer with a brominating agent in a suitable organic solvent, which is preferably halogenated hydrocarbon (or a mixture of halogenated hydrocarbons) . The bromination reaction may be carried out at a temperature in the range from -10 to about 60°C, in accordance with the brominating agent used, as discussed in more detail hereinafter.

Halogenated hydrocarbons which may be suitably used as solvents for the brominat ion reaction of the styrene/cyclododecatriene copolymer include methylene chloride or dibromomethane. Other solvents and diluents suitable for bromiriation reactions may be used.

The brominating agent is added into a reaction vessel containing the solution of the copolymer styrene/cyclododecatriene in the halogenated hydrocarbon(s) solvent. The concentration of the copolymer in the solution may be in the range between 10 and 60% by weight. The brorninating agent may be provided either in a neat form, or in the form of a feed mixture (or a feed solution) comprising said halogenated hydrocarbon(s) solvent and a suitable amount of the brominating agent.

Elemental bromine (Br2) may of course be used as the brominating agent, in which case the reaction is preferably accomplished under heating. However, a particularly preferred brominating agent to be used for the bromination of the styrene/cyclododecatriene copolymer is a quaternary arnnioniurn tribromide salt. This class of brominating agents includes salts of the tribromjcje anion wherein the cation is a quaternary amrnonium which is preferably selected from the group consisting of tetraalkylariurionium, such as tetramethylammonium, tetraethylarnxnonjum and tetrabutylammonium. Other suitable quaternary ammonium tribromide salts include benzyltrimethylarnrnonium tribromide and cetyltrimethylarnmonium tribromide. These brominating agents are commercially available or can be made by methods known in the art (see, for example, US 7,005,548).

When the brominating agent is a quaternary arnmoniurn tribromide salt as set forth above, then the bromination reaction is preferably carried out at a temperature within the range from -10 to +10°C. Thus, a solution of the copolymer in the organic solvent is prepared in a reaction vessel, and is cooled to a temperature in the range between -10 and +10°C, preferably between 0 and 5°C. The quaternary amrnonium tribromide salts are normally available as solid, crystalline materials. It is therefore more convenient to premix the salt in a mixture of water and halogenated hydrocarbon solvent, to form a feed mixture, which feed mixture is subsequently charged into the reaction vessel containing the solution of the styrene/cyclododecatriene copolymer in the organic solvent. The weight ratio of the water to the halogenated hydrocarbon in the feed mixture is preferably between 2:1 and 1:1, and the quaternary amrnoniurn tribromide salt may be loaded into said feed mixture in a concentration in the range between 10% and S0%(w/w) . The molar ratio between the brominating agent and the copolymer participating in the reaction is preferably in the range between 2:1 and 2.2:1.

Having completed the addition of the brominating agent to the reaction vessel, the reaction mixture is maintained under stirring at a temperature within the range indicated above, for at least one hour (and preferably for at least 3 hours), in order to complete the brominatjon reaction, following which the mixture is heated to room temperature.

The reaction mixture is separated into aqueous and organic phases, and the bromine-containing copolymer is then recovered from the organic phase using known techniques.

More specifically, the. organic phase is combined with an aqueous solution of sodium sulfite, and after a short mixing period, the aqueous and organic layers are allowed to separate. The organic layer is subsequently treated to remove the solvent (for example, by steam stripping) and to recover the crude bromine-containing copolymer.

The poly (styrene/tetrabromocyclododecene) is prepared according to route B of Scheme 2, namely, by a process comprising the bromination of the compound cyclododecatriene to give tetrabromocyclododecene, and subsequent copolymerization of the monomers styrene and tetrabromocyclododecene.

The monomer' tetrabromocyclododecene may be prepared by brominating cyclododecatriene according to methods known in the art. For example, tetrabromocyclododecene can be made by the addition of molecular bromine to cyclododecatrjene in propionic acid, as described in detail in US 3,274,266. More specifically, according to said publication, cyclododecatriene is dissolved in propionic acid, following which a solution of bromine in propionic acid is gradually added to the reaction vessel under stirring at a temperature in the range from -25°C to -15°C. Tetrabromocyclododecene precipitates from the reaction mixture and is separated therefrom by means of suction filtration. The crude product is successively washed with propionic acid, water and aqueous methanol. The yield reported by US 3,274,266 is about 70%.

It has now been found that tetrabromocyclododecene may be conveniently and selectively prepared under mild conditions by reacting cyclododecatriene with a bromiriating agent selected from the group consisting of quaternary ammonium salts of tribromide in water-immiscible organic solvent, which is preferably halogenated hydrocarbon. The reaction may be carried out in the presence of water (water is used to deliver the salt into the reaction vessel, as described in more detail below) . In this case, the reaction mixture consists of organic and aqueous phases, and therefore upon completion of the reaction, the reaction mixture is separated into the aqueous and organic phases, and tetrabromocyclododecene is isolated from the organic phase.

Preferably, a solution of the cyclododecatrjene starting material in the water immiscible organic solvent is prepared in a reaction vessel, and is cooled to a temperature in the range between -10 to +10°C, preferably between 0 to 5°C. The concentration of cyclododecatriene starting material in the solution may be in the range between 5 and 50% by weight, preferably between 7 and 15% by weight. The quaterriary ammonium salt of tribromide is then gradually added to the reaction vessel containing the solution of starting material. The brominating agent is preferably charged into the reaction mixture in the form of a feed mixture, which comprises water and a water-immiscible organic solvent (e.g., halogenated hydrocarbon such as methylene chloride) The weight ratio between the organic and aqueous components in the feed mixture is preferably between 1:2 and 1:1, and the concentration of the brominating agent in said mixture is in the range from 10% to 50%(w/w). The brominating agent, such as tetraethylammonjum tribrornjde (TEATB), is used in a slight molar excess relative to the cyclododecatriene starting material. The molar excess may be up to 10% (that is, from 2 to 2.2 part of TEATB for 1 part of cyclododecatriene) The addition of the brominating agent into the reaction vessel containing the solution of the starting material is preferably carried out over a period of time, wherein the rate of the addition is adjusted in order to maintain the reaction temperature within the range indicated above (preferably between 0 and 5°C) . More specifically, the gradual addition of the brominating agent into the reaction vessel may be accomplished either continuously, over a period of time of not less than 120 minutes at an approximately constant rate or in a portion-wise manner, such that approximately equal quantities of the brominating agent are sequentially charged into the reaction mixture over a period of time, at intervals of about 2 to 5 minutes.

The rate of addition may depend on the reaction scale.

After the completion of the addition of the brominating agent to the reaction vessel, the bromination reaction is allowed to continue under stirring for an additional period of time, generally for at least 1 hour, or even more (e.g., for at least 3 hours) . The bromination reaction may be monitored by means of gas chromatography. When the reaction is over, the reaction mixture is separated into aqueous and organic phases, and the tetrabromocyclododecene product is isolated from the organic phase, preferably by removing the organic liquid by means of evaporation. The isolated tetrabromocyclododecene may be optionally purified, for example, by recrystallization.

Having prepared and collected the tetrabromocyclododecene monomer, the subsequent copolymerization reaction with the styrene monomer may be carried out by the methods set forth above. The weight ratio of the monomers styrene and tetrabromocyclododecene may vary from 9:1 to 1:9, preferably from 5:1 to 1:5.

The crude bromine-containing copolymers obtained by the processes set forth above may be purified by known methods, which involve washing and recrystallization.

Thermogravimetric analysis (TGA) of the bromine-containing copolymers of the invention indicates that they exhibit high thermal stability. Therefore, the bromine-containing copolymers may be suitably processed in the melt compounding with, for example, vinyl aromatic polymer such as polystyrene, with or without a foaming agent. The bromine-containing copolymers of the invention may be used for reducing the flammability of expanded polystyrene foams (EPS) and extruded polystyrene foams (XPS) . More specifically, the TGA indicates that a sample of the bromine-containing copolymers exhibits a weight loss of 5%, relative to its initial weight, at a temperature which is at least 230°C, preferably at least 250°C, more preferably at least 260°C and even more preferably at least 270°C. The thermogravimetric analysis reported herein was obtained using Mettler-Toledo instrument model 850; the temperature range scanned was 25°C-400°C and the rate of scanning was 10°C/mm under air.

The incorporation of the bromine-containing copolymers flame retardants of the invention into the flammable polymer may be carried out by methods known in the art. The various ingredients of the formulation to be prepared are blended together according to their respective amounts. In general, the ingredients are first dry blended using suitable mixing machines, or may be dosed directly to an extruder. The bromine-containing copolyrner of the invention is used in an amount sufficient to provide bromine concentration within the range from 0.l%-15% preferably 0.3% to 5% (w/w, relative to the total weight of the final polymeric formulation) The powder mixture may then be processed and compounded to form homogeneous pellets, for example, by using a twin-screw extruder. The granular polymer obtained is suitable for feed to an article shaping process such as injection or compression molding and extrusion. Other fabrication techniques can also be applied, for example, when polymeric foams are contemplated.

Examples

Example 1

Brominated copoly (styrene/cyclododecatriene) Step 1: Copolymerization of the monomers styrene and cyclododecatriene cyclododecatriene (10 g; abbreviated CDT) and styrene (40 g) are mixed in cyclohexane (50 g) at a temperature of 95°C for 24h, in the presence of 2g of benzoyl peroxide. Methanol is then slowly added to the reaction mixture in an amount sufficient for precipitating the copolymer from the liquid phase. The solid copolymer is then separated by filtration and dried overnight in a vacuum oven operating at a set point temperature of 50°C.

Step 2: Bromination of the copolymer The copolymer (4 g) from step 1 was dissolved in rnethylene chloride (40 g) in a round bottom flask equipped with overhead stirrer, condenser, an addition funnel and a nitrogen inlet. The solution is cooled to 0°C. A mixture of water and methylene chloride (15g/llg), which contains 10.8 g of triethylammonium tribromide (TEATB), is added to the flask. The reaction mixture is stirred at said temperature for 3h, following which the mixture is heated to room temperature. The layers are separated. The organic layer is washed with aqueous sodium sulfite. After mixing for 10 mm, the layers are allowed to separate. The organic layer is subsequently steam stripped to isolate the brominated.

copolymer. TGA indicates the following weight loss profile of the product: Percent weight loss 2% 5% 10% L Temperature 231°c 272°c 337°c Exantple 2 Poly (styrerle/tetrabromocyclododecene) Step 1: Brominatjon of cyclododecatrjene to form tetrabrornocyclododecene -Cyclododecatrj (16.2g) and methylene chloride (150 g) were placed in a round bottom flask equipped with overhead stirrer, condenser, an addition funnel and a nitrogen inlet.

The reaction mixture is cooled to 0°C. Tetraethylammonium tribromide (97.7g), in water/methylene chloride mixture (13Og/80g), is slowly added via the addition funnel such that the temperature is kept below 5°C. The reaction mixture is stirred at said temperature for three hours.

The water phase is then separated. The organic phase is washed twice with water and the organic solvent is evaporated affording to the expected tetrabromocyc1ododec (TBCD) product with 97.5% yield. %Br: 67 % (calculated 66.4%) . The melting point of the product was 112-115°C.

Step 2: Copolymerization of the styrene and TBCD monomers Tetrabromocyclododecene from step 1 (10 g) and styrene (40 g) are mixed and heated at 95°c in the presence of 2g of benzoyl peroxide for a period of 8h. The bromine content of the product is 13.4% (calculated 13.27%) . TGA of the product indicates the following weight loss profile: Percent weight loss 2% 5% 10% L Temperature 225°c 261°c 284°C

Example 3

Poly (styrene/tetrabromocyclododecene) This example illustrates the bulk copolymerization of styrene and TBCD in a closed vessel under pressure.

TBCD (2g) and styrene (2g) were placed in a parrbomb equipped with a magnetic stirrer. Benzoyl peroxide (2 g, 75% purity) was added and the mixture was stirred at 90°C for about 23 hours. The copolymer obtained had a yellow-brown color. HPLC indicated the presence of about 1.4% of an un-reacted TBCD.

The crude copolymer (a sample of 2 g) was grinded and slurried in methanol (20 ml) for one hour at room temperature. The solid product was then collected by filtration and dried overnight in a vacuum oven at 50° C to give a dry copolymer (1.7 g) with a bromine content of 27.4%. TGA indicates the following weight loss profile: Percent weight loss 2% 5% 10% L Temperature 212°c 234°c 252°C