Chemical modification of monolithic poly (styrene-divinylbenzene) PolyHIPE@ materials Neil R. Cameron,"+ David C. Sherrington,*" Isao Andob and Hiromichi Kurosub "University of Strathclyde, Dept. Pure & Applied Chemistry, 295 Cathedral St., Glasgow, UK, GI 1XL bDept.of Polymer Chemistry, Tokyo Institute of Technology, 0-okayama, Meguro-ku, Tokyo 152, Japan Monolithic samples of highly porous poly(styrene/DVB) PolyHIPE@ have undergone a number of electrophilic aromatic substitution reactions, namely sulfonation, nitration and bromination. Mild, hydrophobic reagents and homogeneous reaction conditions were sought in an effort to achieve uniform chemical modification, to a reasonable degree of substitution, throughout the large polymeric structures. Thus, sulfonation was carried out with lauroyl sulfate in cyclohexane, nitration with tetrabutylammonium nitrate-trifluoroacetic anhydride (TBAN-TFAA) in dichloromethane and bromination with bromine- stannic chloride in dichloromethane. An average degree of sulfonation of 2.4 mmol g-1 was achieved, with a drop in sulfonic acid content of approximately 1mmol g-' from surface to centre. Nitration occurred to a lesser extent, with similar differences in substitution between surface and centre being observed.PolyHIPE@ monolithic samples were brominated to an extent of 3.6 mmol g-' furthermore, this was uniform across the entire substrate. The differences in extent of each reaction are explained by consideration of such factors as the nature of the solvent, polarity of the reagents and compatibility between the reagents and the polymer matrix throughout the reactions.Polymerisation of the continuous phase of a high internal phase emulsion (HIPE), in which the dispersed phase droplets occupy greater than 74% of the total emulsion volume and the continuous phase contains one or more, typically vinyl, monomer, leads to a novel, monolithic, macrocellular material known as PolyHIPE@ 1*2(Fig. 1). These materials are highly porous, with a void volume fraction generally of 0.9, but which can be as high as 0.99. In addition, they usually possess an open cellular structure, in which each cavity is connected to its neighbours. This results in a polymer material of very low density. Styrene-divinylbenzene (DVB) PolyHIPE@ polymers have been studied fairly extensively, and conditions for the control of their cellular str~cture,~ cell size,4 porosity and surface area5 are well documented.The preparation of polymer-supported reagents and catalysts is most commonly achieved via the chemical modification of preformed polymers, and numerous examples have been reported in the chemical literature. Crosslinked polystyrene t Present address: Eindhoven University of Technology, Laboratory of Polymer Chemistry and Technology, Den Dolech 2, PO Box 513, 5600 MB Eindhoven, The Netherlands. Fig. 1 Scanning electron micrograph of poly(styrene-DVB) PolyHIPE@ resins, which have been extensively employed as polymeric support systems, can be chemically modified by e.g.direct electrophilic aromatic substitution of the styrene residues, nucleophilic substitution of chloromethylated polystyrene resins$ and electrophilic substitution of lithiated polystyrene. A vast range of polymer-supported species have been synthesised via these routes.6 There are, however, experimental problems involved in the chemical modification of crosslinked polymer resins. Since the vast majority (>99%) of reactive sites are located in the interior of the polymer particles, the diffusion of reactants to these sites is of paramount importance if a high degree of modification is to be achieved. In the case of gel-type resins, this necessitates the use of reaction solvents which will swell the crosslinked polymer beads.It should also be noted that the polymer must remain swollen throughout the entire reac- tion to ensure continued access of the reagents to the reactive sites. As the reaction proceeds, the swelling properties of the polymer may change drastically. An example of this is in the chemical modification of chloromethylated polystyrene with tertiary amines, generating quaternary ammonium salts. The polymer changes, therefore, from being relatively hydrophobic to possessing highly polar, ionic groups. Even with macrop- orous resins, care can be required in selecting the solvent with which to achieve high levels of functionalisation. In the case of monolithic PolyHIPE@ materials, these diffusion-related problems are greatly exacerbated.The diffusion path-length from external solution to the interior of a polymeric rod can typically be as high as 20-25 mm. Additionally, the relatively large dimensions of the polymer samples require that the reactant solution should equilibrate fully throughout the crosslinked matrix prior to the reaction occurring to any significant extent, in order to achieve uniform chemical modification. A number of conclusions regarding the experimental con- ditions required for uniform chemical modification of mono- lithic PolyHIPE@ samples can therefore be made. First, the $ Owing to the extreme carcinogenicity of chloromethyl ether, nucleophilic substitution of poly(vinylbenzy1 chlonde) resins is more preferable. J. Muter. Chem., 1996, 6(5), 719-726 719 reaction solvent should swell the starting polymer material and, where possible, the polymer should remain swollen during the reaction Second, the reagents themselves should be com- patible with the polymer matrix, and therefore relatively hydro- phobic Third, the reaction conditions should be such that the rate of reaction is low at ambient or ice temperature, allowing equilibration of the reagent solution throughout the polymer, while moderate heating of the system produces a reasonable rate of chemical modification This third point implies that relatively mild reagents are required The aim of this research was to achieve the uniform chemical modification of poly- (styrene-DVB) PolyHIPE@ monolithic samples via electro-philic aromatic substitution, the particular reactions chosen being sulfonation, nitration and bromination With the above requirements in mind, suitable reagents with which to achieve these goals were sought Experimenta1 Materials and instrumentation Styrene (Fisons), DVB (Aldrich, tech grade, 55%), potassium persulfate (Fisons, 97 + YO),calcium chloride hexahydrate (Fisons), sulfuric acid (Fisons, 96%), chlorosulfonic acid (BDH, %'YO), lauric acid (Aldrich, 99 + YO),acetic anhydride (BDH, 98%0), nitric acid (BDH GPR, 69%), trifluoroacetic anhydride (Aldrich, 99 + %), ammonium nitrate (Aldrich, 98 + YO), bromine (BDH, 99"/), stannic chloride (Aldrich, 99%0) and pyridine (Aldrich, 99+%) were used as received Tetrabutylammonium nitrate (Fluka) was recrystallised from ethanol and dichloromethane (Hays Ltd ) was distilled under nitrogen from calcium hydride The surfactant sorbitan monooleate (Span 80, Koch-Light) and all other solvents were used as received Infra-red spectra were recorded on a Mattson 1000 FTIR spectrometer, samples were prepared by crushing and com- pressing into KBr discs C, H and N elemental analyses were carried out simultaneously with a Perkin Elmer 2400 Analyser Halogen and sulfur contents were determined by titration methods Solid state 13C NMR spectra were recorded on a JEOL GSX-270W spectrometer with cross-polarisation and magic angle spinning (CP-MAS) accessories, and employing a total suppression of spinning side-band (TOSS) pulse sequence The peaks were referenced to the higher field peak of ada- mantane, which was set at 29 5 ppm from tetramethylsilane (TMS) All solid state 13C NMR spectra were proton-decoupled and recorded at 67 8 MHz NMR measurements were performed in the Department of Polymer Chemistry, Tokyo Institute of Technology, Japan PolyHIPE@ coding system The PolyHIPE@ matenals described in this article are classified by a code, which is dependent on the crosslink density and pore volume of the polymer The codes for the poly (styrene-DVB) systems have the general form XaPV b, where X is the nominal crosslink density (% of actual divi- nylbenzene isomers used) and PV is the pore volume (YOV/V of aqueous solution used as the internal phase during PolyHIPE@ preparation) Thus X2OPV90 would represent a poly(styrene-DVB) PolyHIPE@ material of approx 20% crosslinker content and 90% pore volume PolyHIPE@ preparation The example given here is for a poly(styrene-DVB) material of 90% porosity and crosslink density of approximately 5% These values are changed by altering the aqueous to organic phase ratio, and the DVB to styrene ratio, respectively Styrene (22 5 ml, 0 2 mol), DVB (2 5 ml, ca 10 mmol) and Span 80 (50 g) were placed in a 300 ml wide-necked polyethyl- 720 J Muter Chem , 1996, 6(5), 719-726 ene bottle The mixture was stirred with a glass rod fitted with a D-shaped PTFE paddle, connected to an overhead stirrer motor, at approx 300 rpm Plastic film was stretched over the neck of the container to reduce monomer evaporation The aqueous phase was prepared separately by dissolving potassium persulfate (0 5 g, 18 mmol) and calcium chloride hexahydrate (2 5 g, 10 mmol) in distilled water (225 ml) Ths was added, dropwise, with constant mechanical stirring, to the organic solution As the aqueous phase was added, the bottle was lowered to maintain stirring just below the surface of the developing HIPE, ensuring that no water pockets formed Once all the aqueous phase had been added, stirring was continued for a further 5 min, to produce as uniform an emulsion as possible The stirrer was then removed and the bottle was sealed The HIPE was polymerised by immersing the plastic bottle in a water bath, thermostatted at 65 "C, for 48 h The container was then cut away from the resulting polymer monolith, which was extracted in a Soxhlet apparatus with water, for 24 h, to remove inorganic materials, followed by a lower alcohol, for a further 48 h The bulk of the liquid was removed from the porous material zn vacuo at room temperature, and drying was completed zn uucuo at 50 "C for 24 h PolyHIPE@ preparation in polymerisation mould To eliminate unnecessary wastage of plastic bottles, a cylindri- cal PVC PolyHIPE@ polymerisation mould was designed This came in the form of two halves, which were screwed together, with separate screw-on base and lid When assembled, the mould had an internal height of 14cm and an internal diameter of 4 5 cm Pnor to use, the inner surfaces were sprayed with PTFE spray to prevent the polymer from adhering to the mould The HIPE was prepared as above, in a plastic bottle, and was subsequently poured into the assembled mould, no sealing of the seams was required The mould was then immersed in a water bath as above, to form the PolyHIPE@ The polymer monolith could easily be retrieved from the disassembled vessel, and was washed and dried as above Sulfonation Small scale reactions.Acetyl suEfate Acetyl sulfate (1) was prepared prior to each experiment Acetic anhydride (15 4 ml, 0 16 mol) was added to 1,2-dichloroethane (DCE) (39 6 ml), and the solution was cooled to below 10°C in an ice bath Sulfuric acid (5 6 ml, 0 10mol) was then added, resulting in a clear solution of 16 mol 1-1 acetyl sulfate in DCE (assuming quantitative reaction) A small cube (approx 2cm per side) of poly(styrene-DVB) PolyHIPE@ (X2OPV90) (0 59 g, ca 3 4 mmol Ph groups) was placed in a three-necked 100 ml round-bottomed flask fitted with a rubber septum This was evacuated, to aid liquid penetration into the porous polymer, and acetyl sulfate in DCE (6 0 ml) was introduced uza syringe The impregnated PolyHIPE@ cube was heated at 70-75 "C for 24 h, after which it was extracted with isopropyl alcohol in a Soxhlet apparatus for 24 h It was dried in uacuo at 50°C for a further 24 h Mass of product =O 79 g Microanalysis surface sample C, 63 4, H, 6 0, S, 6 9%, gives 2 2 mmol (S0,H) 8-l Centre sample C, 678, H, 63, S, 62%, gives 19mmol (S03H)g-', FTIR v/cm-' 3387 (OH str), 3030, 2940, 1600, 1191 (S=O str), 706 (C-S str) Lauroyl sulfate Lauroyl sulfate (2) was also prepared prior to each sulfonation Lauric acid (8 01 g, 40 0 mmol) was dis- solved in cyclohexane (10 8 ml) in a three-necked 100 ml round-bottomed flask with magnetic stirring, under nitrogen Chlorosulfonic acid (17 ml, 25 0 mmol) was added, and stirring was continued at 25 "C for 1h A 2 cm side cube of poly(styrene-DVB) PolyHIPE* (X5PV90) (0 85 g, ca 73mmol Ph groups) was evacuated in a three-necked 100 ml round-bottomed flask, fitted with a rubber septum.The lauroyl sulfate solution was subsequently transferred to the flask via syringe, and the flask was heated at 50°C for 24 h. The PolyHIPE@ was then removed and extracted with light petroleum (bp 60430°C) in a Soxhlet apparatus for 48 h and dried in uucuo at 50°C for 24 h.Mass of product = 1.66 g. Microanalysis: surface sample: C, 36.8; H, 4.4; S, 13.0% gives 4.0 mmol (S03H) 8-l. Centre sample: C, 48.2; H, 5.5; S, 10.2% S, gives 3.2 mmol (SO,H) 8-l. Large scale reactions. Lauroyl sulfate. Lauroyl sulfate (2) in cyclohexane was prepared as above. Chlorosulfonic acid (16.0m1, 0.24mol) was added to a solution of lauric acid (78.6 g, 0.39 mol) in cyclohexane (134 ml) at 25 "C, and the resulting solution was stirred at 25°C for 1 h. A PolyHIPE@ monolith (X2OPV90) (15.2 g, ca. 0.1 mol Ph groups), of 10.3 cm height and 4.5 cm diameter, was placed in a round-bottomed glass reactor vessel fitted with a ground-glass flange.The reactor was evacuated for approx. 1 h then sealed, after which the solution of lauroyl sulfate in cyclohexane was introduced with the vessel still under vacuum. The solution was allowed to equilibrate throughout the PolyHIPE@ porous structure; following this, the vessel was immersed in a water bath thermostatted at 55°C for 48 h. The monolithic product was then extracted with cyclohexane in a large Soxhlet apparatus for 96 h, and was dried under reduced pressure at room temperature, and finally in uucuo at 50 "C for 24 h. Mass of product =23.0 g. Microanalysis: surface sample: C, 50.2; H, 6.0; S, 8.6%, gives 2.7 mmol (S03H) 8-l; FTIR: v/cm-' 3413 (OH str.), 3030, 2928, 1626, 1191 (S=O str.), 706 (C-S str.); 6, 17.1 (CH,), 29.8 (CH,), 40.6 (CH, CH,), 128.2 (aryl-CH), 138.0 (aryl-C-SO,H), 145.6 (aryl-C).Nitration Small scale reactions. Nitric acid-sulfuric acid. Two small cubes (approx. 5 mm per side) of poly(styrene-DVB) PolyHIPE@ (X5PV90) (0.15g, ca. 1.3 mmol Ph groups) were placed in a 50 ml round-bottomed flask, which was cooled in an ice bath. A 2: 1 mixture of sulfuric and nitric acids (3.5 ml), previously cooled to ice temperature, was added at atmospheric pressure, and the flask was maintained at ice temperature for a further 2 h. It was gradually allowed to warm to room temperature and left for a total of 24 h. Following this, the polymer sample was immersed in approximately 200 ml of ice- water and left for 16 h, then extracted with de-ionised water in a Soxhlet apparatus until the pH of the washings was neutral (72 h).The product was further extracted with ethanol for 3 h, then diethyl ether for 16 h, and was dried in uacuo at 50 "C for 24 h. Mass of product =0.24 g. Microanalysis: C, 52.9; H, 3.5; N, 12.1, gives 8.6 mmol (NO,) 8-l; FTIR: v/cm-' 3089, 2935, 2870, 1615, 1534 (NO, asym. str.), 1355 (NO, sym. str.), 856 (aryl C-N str.), 834, 747, 704. Nitric acid-sulfuric acid-DMF. Poly(styrene-DVB) PolyHIPE@ (X5PV90) (0.14 g, ca. 1.2 mmol Ph groups), in the form of small cubes (approx. 5 mm per side), was placed in a 50 ml round-bottomed flask, and DMF (3 ml) was added at atmospheric pressure. The flask was placed in an ice bath, and the polymer was left to swell for about 1 h.The nitrating mixture (H,SO,-HNO,, 2: 1) (3.5 ml), pre-cooled in an ice bath, was added, and the temperature was allowed to increase to ambient. The reaction was left at room temperature for 60 h, after which the polymer was washed and dried as above. Mass of product =0.14 g. Microanalysis: C, 73.0; H, 4.9; N, 5.9%, gives 4.2 mmol (NO,) 8-l. Nitric acid-sulfuric acid-Kinetic Study. Poly(styrene-DVB) PolyHIPE* (X5PV90) (0.39 g, ca. 3.4 mmol Ph groups), in the form of small cubes (approx. 5 mm per side), was placed in a 100 ml round-bottomed flask, which was cooled in an ice bath. A mixture of sulfuric and nitric acids (2 :1) (9 ml), pre-cooled to ice temperature, was added at atmospheric pressure, and the flask was allowed to come to room temperature over a period of about 2 h.A small cube of PolyHIPE@ was removed at certain time intervals (1, 2, 4, 10 and 24 h) and immersed in a 1 moll-' NaOH solution for 6 h. Each polymer sample was then placed in a beaker of distilled water; the water was decanted and replaced until the pH remained neutral. Samples were then immersed in ethanol for at least 6 h, after which they were dried in uucuo at 50 "C for 24 h. The extent of nitration was determined by microanalysis. Total mass of product =0.61 g. Ammonium nitrate-triJluoroacetic anhydride. Small cubes (approx. 5 mm per side) of poly(styrene-DVB) PolyHIPE@ (X5PV90) (0.51 g, ca. 4.4 mmol Ph groups) and ammonium nitrate (0.36 g, 4.5 mmol) were placed in a three-necked 100 ml round-bottomed flask, to which trifluoroacetic anhydride (3.3 g, 15.7 mmol) and chloroform (20 ml) were added at atmospheric pressure. The flask was fitted with a reflux con- denser and anhydrous calcium chloride drying tube, and the reaction mixture was kept at room temperature for 24 h, then refluxed for 24 h.The polymer was removed, extracted with chloroform for 24 h, then ethanol for a further 24 h, and was dried under reduced pressure at room temperature, and finally in uucuo at 50°C for 24 h. Mass of product=0.56 g. Microanalysis: C, 79.0; H, 4.9; N, 3.0%, gives 2.1 mmol (N0,)g-'; FTIR: v/cm-l 3063, 3037, 2922, 2858, 1604, 1529 (NO, asym. str.), 1355 (NO, sym. str.), 856 (aryl C-N str.), 758. 703. Tetrabutylammonium nitrate-trijluoroacetic anhydride.Poly(styrene-DVB) PolyHIPE@ (X5PV90) (0.57 g, ca. 4.9 mmol Ph groups), again as cubes of approximately 5mm per side, was placed in a three-necked 100ml round- bottomed flask. Tetrabutylammonium nitrate ( 1.61 g, 5.3 mmol) was dissolved in dichloromethane (DCM) (20 ml); the resulting solution, along with trifluoroacetic anhydride (3.3 g, 15.7mmol), was added to the flask at atmospheric pressure, which was subsequently fitted with a reflux condenser. This was carried out under a stream of N, gas. The flask was heated at 30°C for 24 h, under N,, after which the polymer was removed and immersed in fresh DCM and left for 16 h. The product was then extracted in a Soxhlet apparatus with ethanol for 24 h, and was dried in uucuo at 50°C for 24 h.Mass of product=0.68 g. Microanalysis: C, 65.3; H, 5.5; N, 5.9, gives 4.2mmol (NO,) g-'; FTIR: v/cm-' 3086, 3028, 2924, 2855, 1600, 1523 (NO, asym. str.), 1352 (NO, sym. str.), 859 (aryl C-N str.), 755, 704; 6, 40.9 (CH, CH?), 128.1 (aryl-CH), 146.8 (aryl-C-NO,), 153.0 (aryl-C). Large scale reactions. Nitric acid-sulfuric acid. A 2 :1 mixture of sulfuric and nitric acids (l00ml) was added to a round- bottomed glass reactor with a ground-glass flange, fitted with a vacuum outlet and a rubber septum. A poly(styrene-DVB) monolithic sample (X5PV90) (2.35 g, ca. 0.02 mol Ph groups), of 45 mm diameter and 22 mm height, was suspended above the nitrating solution via a knife-blade attached to a glass rod, which had been pushed through the rubber septum.The reactor was then immersed in an ice bath and evacuated for 1 h. The PolyHIPE@ sample was then lowered into the acid mixture by pushing the glass rod through the rubber septum, and the vessel was quickly repressurised. In this manner, the reagents would be forced into the evacuated interior of the porous material. The reactor was initially left in the ice bath, which was allowed to warm to room temperature, and was further left at ambient temperature for 24 h. The PolyHIPE@ macrosample was then extracted in a large Soxhlet apparatus with water, until the pH of the washing water was neutral, J. Muter. Chem., 1996, 6(5),719-726 721 then with ethanol for 48 h It was allowed to dry on the open bench, and was further dried zn uucuo at 50°C for 72 h Mass of product =3 19 g Microanalysis surface sample C, 56 3, H, 4 1, N, 11 1% N, gives 7 9 mmol (NO,) g-', FTIR v/crn-' 3101,2935,2870,1615, 1534 (NO, asym str), 1349 (NO, sym str ), 863 (aryl C-N str ), 747, 712 Tetrabutylammonium nitrate-trzjcluoroacetic anhydride Tetrabutylammonium nitrate (12 3 g, 0 04 mol) was dissolved in DCM (200 ml), and the solution was placed in a round- bottomed glass reactor with a ground-glass flange, fitted with a nitrogen gas inlet Trifluoroacetic anhydride (17 8 g, 0 08 mol) was added under a flow of N, gas, and the solutions were mixed A monolithic sample of poly(styrene-DVB) PolyHIPE@ (X5PV90) (3 93 g, ca 0 03 mol Ph groups), of similar dimen- sions to that used above, was added at atmospheric pressure, and the vessel was heated at 30°C for 24 h under N, gas The polymer cylinder was subsequently removed, cut in half along its diameter, and the pieces were extracted in a Soxhlet apparatus with DCM for 48 h, followed by ethanol for 24 h The product was then dried on the open bench and in uucuo at 50 "C for 24 h Mass of product =4 11 g Microanalysis surface sample C, 80 2, H, 6 7, N, 2 8%, gives 2 0 mmol (NO,) g-', FTIR v/crn-' 3089, 2935, 2858, 1606, 1526 (NO, asym str), 1354 (NO, sym str), 860 (aryl C-N str), 756, 710 Bromination Small scale reactions.Bromine-pyridzne Bromine (093 g, 5 8 mmol) and pyridine (3 drops) were added to chloroform (10 ml) in a 25 ml round-bottomed flask, which had been pre- cooled to ice temperature A red crystalline precipitate quickly formed To this solution was added poly(styrene-DVB) PolyHIPE@ (X5PV90) (0 14 g, ca 1 2 mmol Ph groups), in the form of small cubes of side 5 mm, at atmospheric pressure, and the flask was allowed to warm to room temperature over about 2 h The reaction mixture was then refluxed for 24 h, after which the polymer was extracted in a Soxhlet apparatus with chloroform for 72 h and ethanol for 20 h It was dried in uucuo at 50 "C for 24 h Mass of product =0 16 g Microanalysis C, 67 0, H, 3 2, Br, 26 4%, gives 3 3 mmol (Br) g-', FTIR v/cm-' 3063, 3030, 2922, 2858, 1606, 699 The experiment was repeated (mass of starting polymer= 0 14 g) with chlorobenzene (10ml) as solvent Initially the mixture was homogeneous but a crystalline precipitate appeared after a short while The reaction was warmed to room temperature, then heated at 120 "C for 24 h The polymer product was washed and dried as above Mass of product= 0 13 g Microanalysis C, 69 4, H, 3 7, Br, 24 0%, gives 3 0 mmol (Br) g-' Bromine-stannzc chloride Poly(styrene-DVB) PolyHIPE* (X5PV90) (0 66 g, ca 5 7 mmol Ph groups), as cubes of side 5 mm, was placed in a three-necked 100 ml round-bottomed flask fitted with condenser, nitrogen gas inlet and gas bubbler The system was purged with N, gas for 10 min, after which a solution of bromine (4 0 g, 25 mmol) and stannic chloride (0 04 g, 0 15 mmol) in DCM (30 ml) was added at atmospheric pressure The flask was heated at 35 "C for 24 h, following this, the polymer was extracted in a Soxhlet apparatus with chloro- form for 24 h, then ethanol for 6 h It was dried zn uucuo at 50 "C for 72 h Mass of product =O 87 g Microanalysis C, 59 2, H, 45, Br, 340%, gives 43mmol (Br) g-', FTIR v/cm-' 3055, 3030,2928,2864, 1600,834,760,706,6,40 5 (CH, CH2), 120 2 (aryl-C-Br), 129 8 (aryl-CH), 144 7 (aryl-C) Large scale reactions. Bromine-stannic chloride A mono-lithic sample of poly(styrene-DVB) PolyHIPE'@ (X5PV90) (4 48 g, ca 0 04 mol Ph groups), with a height of 30 mm and a diameter of 45 mm, was placed in a round-bottomed glass 722 J Muter Chem, 1996,6( 5), 719-726 reactor with a ground-glass flange, fitted with a nitrogen gas inlet The vessel and polymer were cooled in an ice-salt bath to between -5 and -10"C, then were evacuated for 1 h and purged with N, gas, twice A solution of bromine (27 9 g, 0 18 mol) and stannic chloride (031 g, 1 19 mmol) in DCM (200ml) was added, a condenser and gas bubbler were fitted, and the reaction mixture was maintained at ice-salt tempera- ture for 2 h, warmed to room temperature over 1 h then heated at 35 "C for 24 h, all under a steady flow of N, The polymer material was then extracted in a large Soxhlet apparatus with chloroform for 20 h and ethanol for 24 h, then dried at reduced pressure at room temperature for 8 h and, finally, in uacuo at 50 "C for 24 h Mass of product = 5 11 g Microanalysis surface sample C, 62 9, H, 4 6, Br, 29 8, gives 3 7 mmol (Br) g-l, FTIR v/cm-' 3063, 3028, 2924, 2855, 1606, 824, 765, 702 Determinationof the chemical modification profiles of monolithic PolyHIPE@ samples The dry polymer cylinders were cut in half to give a circular cross-section Small samples (15-20 mg) were taken radially, at regular intervals, from the centre to the surface, these were submitted for microanalysis to enable the construction of a chemical modification profile across the radius of the PolyHIPE@ monolith Results and Discussion PolyHIPE@ preparation Poly (styrene-DVB)-based PolyHIPE@ materials have received the most attention in the literature The procedure for their preparation described in the Experimental section is the result of optimisation studies previously carried out in this research groups7 In principle, any divinyl crosslinker can be used as long as it is not excessively hydrophilic This property may destabilise the HIPE or cause the species to be solubilised by the aqueous phase The PolyHIPE@ polymerisation mould was designed to prevent wastage of polyethylene bottles and to facilitate mono- lith isolation, as it was envisaged that a large quantity of monoliths would be prepared The internal surfaces of the mould were sprayed with PTFE to allow easy removal of the material after polymerisation Preparation of the HIPE inside the mould was not possible as the joins were not sealed and so leakage would occur However, if formed in a plastic bottle then poured into the mould, the concentrated emulsions are sufficiently viscous to prevent escape Sulfonation The conventional method for the sulfonation of polystyrene macroporous resins is treatment with concentrated sulfuric acid' However, this was anticipated as being unsuitable for uniform sulfonation of large PolyHIPE@ monoliths since H,S04 is a very strong sulfonating reagent, and is not particu- larly compatible with crosslinked polystyrene Other sulfonation reagents previously employed include chlorosulfonic acid in chlorinated solventsg and sulfur triox- ide Mild sulfonation of polystyrene has been achieved with acetyl sulfate (1) in halogenated solvents,12 l3 and Thaleri4 has reported the mild, homogeneous sulfonation of polystyrene with hydrocarbon-soluble acyl sulfates, such as lauroyl sulfate (2) (Scheme 1) Both acetyl and lauroyl sulfate appeared attractive reagents with which to achieve uniform sulfonation of poly (styrene-DVB) PolyHIPE@ macrosamples, and so were investigated further Sulfonation was initially attempted with acetyl sulfate on a small cube of the material, of approximately 2 cm per side The reaction was performed in 1,2-dichloro- SO,H CH,(CH,),CO,H ACH,(CI-4)&O2SO,H 2 Scheme 1 Sulfonation of poly(styrene-DVB) PolyHIPE@ with hydro- philic and hydrophobic reagents.Reagents and conditions: i, H,SO,, DCE, 0-10 "C; ii, ClSO,H, cyclohexane, room temp., 1 h. ethane (DCE), causing the polymer to swell; this would hopefully lead to a high degree of modification. The presence of aromatic sulfonic acid groups in the product is confirmed by a broad -OH stretch peak at 3387 cm-' and the -S=O stretch signal at 1191 cm-' in the FTIR spectrum. The microanalytical data indicate an average sulfonic acid content of 2 mmol g-', almost uniformly throughout the sample.However, this represents a 'yield' of only approximately 35%, based on the quantity of styrene residues (neglecting DVB and ethylvinylbenzene isomers) in the substrate. It was believed that this figure could be improved by employing a more hydrophobic sulfonating reagent which would have a higher compatibility with the polymer matrix. Such a reagent is lauroyl sulfate, which is a member of the same homologous series and possesses a long (Cll) hydro-carbon chain.A sulfonation level of 4 mmol g-' was obtained, which corresponds to about 70% substitution. However, sul- fonation is less homogeneous, with a drop of almost 1 mmol g-' in sulfonic acid content between surface and centre of the cube. This may be due to the lower swelling ability of cyclohex- ane compared to DCE. The higher overall level of sulfonation may also accentuate the differences between interior and exterior sulfonic acid content. Sulfonation of monolithic poly (styrene-DVB) PolyHIPE@ polymers was performed with lauroyl sulfate in cyclohexane. The highly porous material was evacuated for 1 h prior to addition of the reagent solution. It was hoped that this would allow rapid equilibration of the solution throughout the poly- mer substrate, reducing the differences in extent of sulfonation between surface and interior. Aromatic sulfonation was confirmed by FTIR spectroscopy, and additionally by solid state 13C NMR spectroscopy.A peak in the latter spectrum at 6 138, which appeared as a shoulder on the high field side of the aryl quaternary carbon peak at 6 145.6, is attributed to the aryl sulfonic acid groups. The sulfonic acid contents, as determined by microanalysis, indicate that there is a drop in sulfonation level of approximately 1mmol g-' between the surface and the centre of the structure. Overall sulfonic acid content is somewhat lower than was obtained in the small scale experiment, but still respectable at an average of 2.4 mmol g-' (ca. 42% conversion).The sulfonation profile is shown in Fig. 2. The difference in extent of sulfonation between interior and exterior is simply due to diffusion of the reagent solution across a relatively large distance (radius of sample is 23 mm). As the reaction proceeds, the concentration of reagent in solution is depleted throughout the sample. However, unchanged lauroyl sulfate in the bulk solution can diffuse quickly to polymer chains closer to the monolith surface, hence maintaining the local reagent concentration, whereas polymer near the centre of the monolith would be expected to experience a more serious depletion in reagent levels. This is likely to lead ,,,,,g2 0.0 l 0 5 10 15 20 25 distance from centrdmm Fig. 2 Sulfonation profile of monolithic poly(styrene-DVB) PolyHIPE@ with lauroyl sulfate (55 "C, 48 h) ultimately to different reaction rates throughout the polymer monolith, since the reaction kinetics are dependent on c~ncentration.'~ Another factor which may affect the uniformity of sulfon- ation is the viscosity of the reagent solution, which was quite high.This would increase the time taken for equilibration of the solution throughout the entire sample and would also hinder transport of reagent molecules from the external solu- tion to reactive sites in the interior of the polymer monolith. Nitration Polystyrene can be nitrated using a mixture of concentrated nitric and sulfuric acids;15 however, degradation of the polymer can occur. Several milder reagents for general aromatic nitration have been developed, including acetyl nitrate in CC14,15316which gave low degrees of substitution with poly- styrene.Aromatic nitration can also be performed with N-nitropyrazole, in the presence of an acid ~atalyst,'~with N-nitropyridinium salts" and with nitric acid in the presence of either trifluoromethanesulfonic acid" or anhydride.20 Several inorganic reagents have also been used, such as ceric ammonium nitrate2' and the system NaN0,-C1SiMe3-AlC1, .22 However, these methods involve either heterogeneous con-ditions or polar solvents, making them unsuitable for PO~YHIPE@nitration. Crivel10~~has described a method for mild, efficient nitration of various aromatics, including polystyrene, with ammonium nitrate and trifluoroacetic anhydride (TFAA).Hodge et ~1.~~ have successfully used this system to nitrate polyacenaphthy- lene to high degrees of substitution. A more hydrophobic reagent was prepared with te trabu tylammonium nitrate (TBAN) and TFAA, which was used to nitrate aromatic compounds in chlorinated solvents at ambient tempera-ture~~'~~~(Scheme 2). Crosslinked polystyrene PolyHIPE@ materials were initially nitrated with a 1 :2 mixture of concentrated nitric and sulfuric acids at room temperature. On a small scale, this proved quite successful, with a high level of nitration being achieved; 8.6 mmol g -'represents essentially complete monosubstitution of aromatic rings. Peaks at 1534 cm-' (NO2 asymmetric stretch) and 1355 cm-I (NO2 symmetric stretch) in the FTIR spectrum of the product confirmed that aromatic nitration has occurred.HNO,-&S 0, ;8 -2\\ Bu,"O,-(CF,CO),O Scheme 2 Nitration of poly(styrene-DVB) PolyHIPE@ J. Muter. Chem., 1996, 6(5), 719-726 723 The reaction was repeated, with the polymer preswollen in DMF and subsequent addition of the acid mixture However, it can be seen from the microanalytical data that this procedure does not lead to a high degree of substitution The reaction is extremely non-uniform In the some areas, the PolyHIPE'@ cubes are highly coloured, whereas others are completely white It is evident that the nitrating reagents did not mix sufficiently with DMF in the interior of the polymer matrix The progress of the nitration reaction involving nitric and sulfuric acid was followed by removing a small cube of PolyHIPE@ from the reaction flask, at various time intervals, quenching in dilute aqueous sodium hydroxide and determin- ing the nitrogen content of the polymer by microanalysis of the crushed PolyHIPE@ cube The concentration of nitro groups with time is plotted in Fig 3 Almost complete monosubstitution is achieved after 4 h, and the subsequent increase in NOz content is only slight However, since a significant extent of reaction occurs after only 1h at slightly below room temperature, this method of nitration may not be suitable for the uniform chemical modification of large, monolithic samples Nevertheless, nitration of a PolyHIPE@ macrosample was attempted with the above reagent system A monolith was vacuum-filled with a 2 1 mixture of sulfuric and nitric acids, pre-cooled to ice temperature After washing and drying the product the nitro-group content along the radius of the cylindrical sample was determined from the nitrogen microana- lytical data The results are plotted in Fig 4 Degrees of nitration are evidently very low in the interior of the porous polymeric matnx, this was also apparent from the white colour in the core of the sample Obviously, the hydro- philic acid mixture is incapable of fully penetrating the pore structure of the hydrophobic polymer, leading to low levels of substitution towards the centre The highly nitrated surface sample possessed an identical FTIR spectrum to the PolyHIPE@ product previously nitrated with nitric and sulf- uric acids A more hydrophobic nitrating system was required The combination of ammonium nitrate and trifluoroacetic anhy- dnde, which is homogeneous in chlorinated solvents, was 1 I 1 I I 5 10 15 20 25 bh Fig.3 Progress of poly(styrene-DVB) PolyHIPE@ nitration with H,SO,-HNO, (3 1, room temp ) I 1 I I J 5 10 15 20 25 distance from centre/mm Fig. 4 Nitration profile of monolithic poly (styrene-DVB) PolyHIPE@ with H,SO,-HNO, (2 1, room temp, 24 h) 724 J Muter Chem , 1996,6(5), 719-726 employed It was envisaged that this would lead to a more uniform modification of PolyHIPE@ monoliths On a small scale, however, a relatively low degree of modification is obtained, 2 1 mmol g-' is equivalent to only 25% conversion Exchanging the salt for the more hydrophobic tetrabutylammonium nitrate results in a two-fold increase in the degree of substitution (42 mmol g-l, 50% substitution), on a small scale, which was deemed satisfactory for the monolith modification The polymeric products from both reactions have identical FTIR spectra to the previous nitrated PolyHIPE@ samples In addition, a sharp signal at 6 146 8 in the solid state I3C NMR spectrum of the latter product indicates that aromatic nitration has occurred The active species in these reactions is thought to be trifluoroacetyl nitrate, which forms from the nitrate salt and trifluoroacetic anhydride (Scheme 3) 23 Trifluoroacetyl nitrate can dissociate into its component ions, however, it is not known whether reactions involve intact tnfluoroacetyl nitrate, associated ions or free ions as the electrophilic species (Scheme 4) 26 In non-polar solvents such as those employed in the present study, it is probable that both undissociated trifluoroacetyl nitrate and tightly bound ions are the nitrating species The tetrabutylammonium nitrate-trifluoroacetic anhydnde (TBAN-TFAA) system was used to nitrate a monolithic sample of poly(styrene-DVB) PolyHIPE@, producing a material with the same FTIR spectrum as before The nitration profile results are shown in Fig 5 These indicate that a much lower overall degree of substitution was achieved compared with the small scale experiment However, the difference in NO2group content between surface and interior is not so great and is certainly an improvement relative to the sample nitrated with the acid mixture It is thought that the relatively low extent of nitration is due to incompatibility between the nitrate salt and the non- polar polymer matrix This problem may be accentuated with large PolyHIPE@ samples since the diffusion distance is increased Also, it was evident that the nitrated polymer did not swell in DCM Therefore, after a certain degree of substi- tution, the remaining reactive sites of the polymer might not 0 0 I/ //(CF&O),O + M+N03-====== CF3C\ + CF&\ ON02 0-M+ M = NH4+,NBu: Scheme 3 0 -CF,< -CF3CO;NOg CF3COz-+ NO2+ ON02 I / Scheme 4 4q,l,c O OO 5 10 15 20 25 distance from centre/mm Fig.5 Nitration profile of monolithic poly(styrene-DVB) PolyHIPE@ wlth TBAN-TFAA (30 "C,24 h) be as accessible to the reagents as at the start of the reaction. However, the overall ability of DCM to swell the starting material produces a more uniformly nitrated porous material. Bromination Bromination of polystyrene resins is carried out with bromine C-Br range (6 20-30) indicates that neither heterolytic bro- mination nor addition across residual double bonds in the starting polymer has occurred to any appreciable extent. This system therefore appeared to be an ideal one with which to achieve uniform chemical modification of a PolyHIPE* macrosample.Treatment of a cylinder of poly (styrene-DVB) PolyHIPE@ under similar conditions did and a Lewis acid catalyst, such as FeC1, or T~(OAC)~.~~indeed lead to bromination of the material to an almostHowever, with the former catalyst the level of bromination is completely homogeneous extent (Fig. 6). The results indicate difficult to control and reproduce. The latter is costly and that the concentration of bromine in the centre of the sample requires an excess to achieve high degrees of substitution. is 3.6 mmol g-l, whereas at the surface it is 3.7 mmol g-'. Another aromatic bromination method involves the use of bromine with pyridine.,' The bromine adds to the pyridine, forming the N-bromopyridinium bromide salt (3).The electro- philic activity of bromine is increased in this way.Additionally, Camps et have used stannic chloride as the Lewis acid catalyst in the bromination of polystyrene with bromine. The reaction is homogeneous in CH,Cl,, and is performed at room temperature. Bromination of poly (styrene-DVB) PolyHIPE@ was carried out with these latter two systems (Scheme 5). Reaction with bromine and pyridine in chloroform on a small scale caused the rapid precipitation of what was pre- sumed to be N-bromopyridinium bromide salt. Despite this, refluxing the heterogeneous mixture for 24 h gave a reasonable level of bromination. The salt may have a limited solubility in CHC1, at higher than ambient temperatures, which allows the transport of small quantities to the reactive sites of the polymer.Pyridine is regenerated upon reaction with polymer aromatic groups, and this can reform the catalyst salt in close proximity to the polymer internal surface. Thus, a relatively high concen- tration of brominating reagent may be present in the interior of the polymer matrix. This experiment was repeated with chlorobenzene, in which it was found that no precipitate had formed when bromine and pyridine were added in a previous solubility test. In addition, this chlorinated aromatic solvent causes extensive swelling of the polymer. The reaction mixture was indeed homogeneous initially, however, a red precipitate formed after about 1 h. Nevertheless, the flask was heated at 120°C for 24 h, affording a polymer product containing 3.0 mmol g-' of bromine.Rather curiously, the mass of the product was slightly lower than that of the starting material, implying that some polymer degradation had occurred. Perhaps the high tempera- ture, in the presence of bromine and air, is the cause of this. Despite achieving reasonable levels of bromination with pyridine and bromine, it was believed that improvements could be made in a completely homogeneous system, preferably with a swelling solvent. Therefore, the combination of bromine with the Lewis acid catalyst stannic chloride was investigated. On a small scale, after 24 h at 35 "C under nitrogen, a polymer material containing 4.3 mmol g-' of bromine was obtained. Aromatic bromination was confirmed by the appearance of a signal at 6 120.2 in the 13C solid state NMR spectrum of the product.Furthermore, the absence of peaks in the aliphatic Br Scheme 5 Bromination of poly (styrene-DVB) PolyHIPE@ The constant degree of substitution throughout the monolith can be attributed to a combination of factors such as a homogeneous reaction mixture, a good swelling solvent for the polymer, high compatibility between polymer and reagents and the ability of the solvent to swell the product. This last property is important if a high level of substitution is to be achieved as it allows continued access of the polymer active sites to the reagent species. The preparation of uniformly sulfonated, nitrated and brominated monolithic PolyHIPE@ materials is an import- ant advance in PolyHIPE@ chemistry.Sulfonated poly-(styrene-DVB) PolyHIPE@ polymers may find applications as strong acid catalysts, which are important in a number of industrial processes, or as the basis of highly porous ion exchangers. Nitrated PolyHIPE@ could be reduced to the amino-functionalised species, which would provide a route to further chemical modification via diazotisation of the amino group. Finally, the brominated polymer could be lithiated by lithium-bromide exchange,22 allowing further specific electro- philic addition reactions to be carried out. The materials described in this publication now offer access to a wide variety of novel, functionalised, highly porous monolithic polymers. Conclusions The chemical modification of poly (styrene-DVB) PolyHIPE@ materials was carried out with the aim of producing uniformly functionalised monolithic samples.Thus, the sulfonation, nitration and bromination of cylinders of poly(styrene-DVB) PolyHIPE@ was investigated employing mild, organic-soluble reagents. Sulfonation with lauroyl sulfate in cyclohexane affords reasonably uniform modification to an average level of 2.4 mmol g- '. Nitration was performed with the tetrabutyl- ammonium nitrate-trifluoroacetic anhydride system in DCM at room temperature, which gives a product possessing a moderate level of nitration (1.8 mmol g-' average) with a slight drop between centre and surface. Bromination was conveniently carried out with bromine and the Lewis acid catalyst SnC14, again in DCM at room temperature.This results in completely uniform substitution, to a degree of 3.7 mmol g- '. The products have additionally been character- ised by I3C solid state NMR spectroscopy. From the studies, a number of conclusions have been made regarding the requirements to achieve a high and uniform *--0 -3.0 3--2.0 8-E 1.0 -.-5 I-c 1 I I I I0.0; 5 10 15 20 25 distance from centrdmm Fig. 6 Bromination profile of monolithic poly(styrene-DVB) PoIyHIPE@ with Br,-SnC1, (35 "C, 24 h) J. Muter. Chem., 1996, 6(5),719-726 725 degree of substitution These include a good swelling solvent for the starting polymer, homogeneous reaction conditions, swellability of the product in the solvent and high compatibility between reagents and polymer Continued and easy access of the polymer reactive sites should be assured under these 11 12 13 14 H R W Ansrink and H Cerfontain, Recl Trav Chim Pays-Bas, 1992,111,183 R A Weiss, A Sen, C L Willis and L A Pottick, Polymer, 1991, 32,1867 Z Su, X Li and S L Hsu, Macromolecules, 1994,27,287 W A Thaler, Macromolecules, 1983, 16, 623 conditions 15 A Philippides, P M Budd, C Price and A V Cuncliffe, Polymer, 1993,34,3509 16 E S Rudakov, V L Lobachev and 0 B Savsunenko, Kinet N R C is grateful to the EPSRC for the provision of a CASE studentship and to Unilever Research, Port Sunlight, the Cooperating Body The authors would also like to thank Dr Robert Sowden of The British Council in Tokyo for funding the visit of N R C to Tokyo Institute of Technology 17 18 19 Catal, 1990,31,938 G A Olah, S C Narang and A P Fung, J Org Chem, 1981, 46,2706 E K Kim, K Y Lee and J K Kochi, J Am Chem SOC, 1992, 114,1756 C L Coon, W G Blucher and M E Hill, J Org Chem, 1973, 38,4243 20 G A Olah, V P Reddy and G K S Prakash, Synthesis, 1992, References 21 1087 S DinGturk and J H Ridd, J Chem SOC, Perkin Trans 2, 1982, 1 2 3 4 5 6 7 8 9 D Barby and Z Haq, Eur Pat 0060 138 (to Unilever), 1982 N R Cameron and D C Shernngton, Adv Polym Sci, 1995, in the press J M Williams and D A Wrobleski, Langmuir, 1988,4,656 J M Williams, A J Gray and M H Wilkerson, Langmuir, 1990, 6,437 P Hainey, I M Huxham, B Rowatt, D C Sherrington and L Tetley, Macromolecules, 1991,24, 117 P Hodge, in Syntheses and Separations Using Functional Polymers, ed D C Sherrington and P Hodge, Wiley, UK, 1988 P Hainey, BSc Thesis, 1988, University of Strathclyde J R Millar, D G Smith, W E Marr and T R E Kressman, J Chem SOC,1963,218 B Chakravorty, R N Mukherjee and S Basu, J Membr Sci, 1989,41,155 22 23 24 25 26 27 28 29 30 31 965 G A Olah, P Ramaiah, G Sandford, A Orlinkov and G K S Prakash, Synthesis, 1994,468 J V Crivello, J Org Chem, 1981,46,3056 P Hodge, B J Hunt and I H Shakhshier, Polymer, 1985,26,1701 B Masci, J Org Chem, 1985,50,4081 B Masci, Tetrahedron, 1989,45,2719 M J Farrall and J M J Frechet, J Org Chem, 1976,41,3877 B S Furniss, A J Hannaford, P W G Smith and A R Tatchell, Vogels Textbook of Practical Organic Chemistry, Longman, Harlow, UK, 5th edn 1989, p 860 M Camps, J P Montheard, F Benrokia, J M Camps and Q T Pham, Eur Polym J, 1990,26,53 M Camps, J P Montheard and F Benrokia, Eur Polym J, 1991, 27,389 M Camps and F Benrokia, Polym Commun ,1991,32,433 10 A G Theodoropoulos, V T Tsakalos and G N Valkanas, Polymer, 1993,34, 3905 Paper 5/06569K, Received 15th October, 1995 726 J Muter Chem, 1996, 6(5),719-726