Synthesis and functionalisation of polyHIPE® beads

Reactive & Functional Polymers 53 (2002) 183–192www.elsevier.com/ locate/ react

S ynthesis and functionalisation of polyHIPE beadsa,b a a ,*´Alexandre Desforges , Marianne Arpontet , Herve Deleuze ,

bOlivier Mondain-Monvala ´ ´Laboratoire de Chimie Organique et Organometallique, UMR 5802—CNRS, 351 Cours de la Liberation, 33405Talence, France

bCentre de Recherche Paul Pascal, UPR 8641—CNRS, Avenue A. Schweitzer, Domaine Universitaire, 33600Pessac, France

Received 19 December 2001; received in revised form 6 September 2002; accepted 20 September 2002

Abstract

Discrete spheroidal particles of interconnected microcellular foams were synthesised by suspension polymerisation of ahigh internal phase emulsion. These particles have been surface-grafted with functionalised macromolecular chains bycontrolled radical polymerisation using a covalently-bond TEMPO initiator. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Microcellular foams; polyHIPE s; Concentrated emulsions; TEMPO; Controlled polymerisation

1 . Introduction are known as TentaGels [3]. The main draw-back of these supports is their low functional

The use of polymer supports in organic group loading as these one are located only atchemistry has increased tremendously in the last the terminal end of the hydrophilic chain. Thedecades [1]. The ease of separation of the third alternative involves use of highly cross-grafted moieties from the species in solution linked macroreticular resins. These supportsand the resulting possibilities of automation of posess a permanent porosity and, therefore, canthe syntheses explain, in part, this development. be used in a large range of solvents. However,The majority of the solid-supported applications due to the small size of their porosity, thehave been performed on lightly cross-linked diffusion of the solvent may be difficult inside(1–2%) polystyrene beads called gel-type the beads [4].beads. These supports have to be swollen by Highly porous microcellular foams are wellgood solvents of polystyrene in order to allow a known structures [5]. The more efficient way togood diffusion of the reagents from solution to obtain a fully interconnected morphology con-the polymeric matrix [2]. In the case of more sists in the polymerisation of the continuouspolar solvents, often used in organic synthesis, phase of a highly internal phase emulsiongel-type beads grafted with oligomeric poly- (HIPE). These materials, known as

(ethylene glycol) arms have been designed andpolyHIPE s, have been widely studied by dif-ferent groups during the last 2 decades [6].However, their application in chemistry has*Corresponding author. Fax:133-5-684-6994.

E-mail address: [email protected](H. Deleuze). always been made either in the form of mono-

1381-5148/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S1381-5148( 02 )00172-4

mailto:[email protected]

184 A. Desforges et al. / Reactive & Functional Polymers 53 (2002) 183–192

liths having the shape of the mold in which they divinylbenzene (DVB) was used: monomerhave been prepared or under irregular granular content of 80 wt.% DVB and 20 wt.% ethyl-form [7]. vinylbenzene (EVB).

Even if small discs or other geometricalforms of macroporous polymers have been 2 .2. Characterizationproposed as an alternative to classical beads [8],

2 .2.1. Elemental analysisthe spherical shape remains the more desiredSamples of functional polymers were sent topresentation for supports in solid phase organic

´ ´the ‘‘Service Central d’Analyse Elementaire’’synthesis (SPOC). Furthermore, large size beadsCNRS (Vernaison, France) where elementalwith an increased loading capacity would bemicroanalytical data were obtained with an errordesirable for the automated one bead–one com-of 0.3% for S and N and 0.5% for Br.pound approach [9].

The only work reported in the literature2 .2.2. Surface area measurement and poreconcerning the preparation of beads of mac-size estimationrocellular foams synthesized via the concen-

To determine the specific surface area, Ntrated emulsion process (polyHIPE ) was dis- 2

adsorption measurements were performed on aclosed recently in a series of patents [10], andMicromeritics ASAP 2010 V4 analyser, usingthe resulting material is commercialized underthe Micromeritics 2100 SA software, and thethe name Magnapore [11]. However, theresultant data were subjected to the Brunauer,Magnapore beads are only styrene–di-Emmet and Teller (BET) treatment [12].vinylbenzene copolymers without any function-

Intrusion/extrusion mercury measurementalisation.were performed using a Micromeritics Auto-We present in this work our preliminaryPores IV apparatus.results concerning the preparation of function-

alised polyHIPE beads by a somewhat differ-2 .2.3. Scanning electron microscopyent approach. Two different methods have been

In order to obtain a more direct insight intoused for the functionalisation: (i) the copoly-the structure of the polyHIPE materials, scan-merisation of a functional monomer with

ning electron microscopy (SEM) was carriedstyrene and divinylbenzene during the out. Specimen preparations were as follows: apolyHIPE formation, (ii) the grafting of a

dried polyHIPE beads were mounted on stubsfunctionalised macromolecular chain on the and sputter-coated from a gold/palladium elec-wall of a previously prepared polyHIPE bead.

trode, in some experiment, in order to visualiseThis new kind of support is expected tothe inside of the bead, this one was cut with acombine the large porosity of the open-cell

razor blade. Micrographs were taken on a JEOLstructure of the polyHIPE allowing a good840 ME SEM instrument.diffusion of the dissolved species into the

polymer matrix with the bead geometry widelyused in SPOC. 2 .2.4. Preparation of a styrene–divinylbenzene

HIPE P3

Divinylbenzene (80% commercial solution,0.1 g) styrene (0.9 g 8.8 mmol) and sorbitan2 . Experimental monooleate (Span 80), (0.2 g) were placed ina reactor. The mixture was stirred with a rod2 .1. Materialsfitted with a D-shaped paddle, connected to an

Unless otherwise noted, all materials were overhead stirrer motor, at approx. 250 rpm. Thepurchased from Aldrich Chemical Company and aqueous phase was prepared separately by dis-used as received. A technical grade commercial solving the initiator potassium persulphate

A. Desforges et al. / Reactive & Functional Polymers 53 (2002) 183–192 185

K S O (1.0 g) and sodium chloride NaCl (1.5 graduated cylinder (10 mm I.D.). The apparent2 2 8

g) in distilled water (100 ml). A 90-ml volume volume of the dry material was measured (V ).d

of this solution was added, dropwise, in 30 min, Toluene was added to the tube with gentleunder constant mechanical stirring to the or- shaking to remove every entrapped air. Theganic solution in order to obtain a thick white beads were then allowed to swell withouthomogeneous emulsion without apparent free entrapping bubbles at room temperature for 24 hwater. Once all the aqueous phase has been and the apparent swollen volume measured (V ).s

added, stirring was carried on for a further 15 The swelling level was calculated as:S (ml21min to produce an emulsion having a size g )5 (V 2V ).s d

distribution as narrow as possible. The otherHIPEs have been prepared using the same2 .2.7. Preparation of TEMPO-grafted

protocol changing the relative amounts of polyHIPE beads P8 –10monomers. The grafting of TEMPO onto chloro-

methylated polyHIPE beadsP has been5–72 .2.5. Synthesis of the polyHIPE beads by performed according to the literature [14].suspension polymerisation

The polyHIPE beads were prepared by a 2 .2.8. Living free radical polymerisation ontoconventional aqueous suspension polymeriza- the TEMPO-grafted polyHIPE beads

tion method using a parallel-side flanged glass In a typical experiment, TEMPO-graftedreactor specially designed as previously de- polyHIPE beadsP (50 mg, 0.01 mmol of7

scribed [13]. The HIPE was placed in a 50 ml TEMPO), were placed in a 50 ml round-bot-polyethylene syringe and added, through a tomed flask. TMI (1 g, 5.3 mmol) and styreneneedle, to the stirred aqueous suspension phase,(0.5 g, 4.8 mmol) were added. The swollenpre-heated at 808C, using a syringe pump at beads, without apparent free liquid, were heated

21about 70 ml h . The suspension system used at 1308C for 20 h. After cooling, the resultingconsists of a 3.5% solution of poly polymeric mass was vigorously shaken with(diallyldimethylammonium chloride) (PDDAC) dichloromethane until the resin beads float free.of high molecular weight as stabilizer. The resulting beads were filtered, washed five

The stirring speed was adjusted between 250 times with a dichloromethane–methanol mix-to 300 rpm in order to obtain, visually, droplets ture and then dried under vacuum at 608C.of satisfactory size. Stirring was carried on 15 Isolated wasP (506 mg).12min after total addition of the HIPE. Thepolymerization was then carried on, withoutstirring, at 808C for 6 more hours. After 3 . Results and discussioncooling, the reaction mixture was filtered and

the beads copiously washed with water and The first step of the synthesis of a polyHIPEethanol, then continuously extracted with tetra- is the preparation of a stable HIPE. This HIPEhydrofuran (THF) in a soxhlet apparatus for 24 is a water-in-oil (W/O) emulsion where theh. After a last washing cycle with ethanol, the continuous phase is composed of a polymeris-beads were dried under vacuum at 608C for 24 able medium. The conditions of obtaining ah. The dry beads were sieved and the very fine stable HIPE with the styrene–divinylbenzeneand large particles were discarded. system are well established [15]. The organic

phase contains a surfactant (usually the hydro-2 .2.6. Measurement of the swelling properties phobic sorbitan monooleate, Span 80). The

The swelling levels of the beads were ob- dispersed (aqueous) phase contains mineral saltstained as follows: about 200 mg of dry and an hydrosoluble radical initiator such as

polyHIPE beads was accurately weighed into a potassium persulphate. The proportion of aque-


ous phase has to be over 74% and, more commercial DVB (80%) using a conventionalgenerally, over 90%. The final polyHIPE procedure. This HIPE then has to be added to

material is generally obtained by a radical the stirred suspension medium in order topolymerisation thermally initiated at 808C. The generate droplets which, by heating, will be

best way to prepare beads of polymer with the polymerised into polyHIPE beads.size suitable for synthetic applications is to Direct addition of the HIPE to the stirredperform a suspension polymerisation [16]. In suspension medium did not give access to

order to obtain beads of polyHIPE , droplets of droplets whatever the stirring speed used: a highW/O emulsion have to be suspended in an stirring rate leading to the destruction of theaqueous medium and polymerised therefrom. If HIPE by coalescence onto the external aqueouswe consider a W/O/W multiple emulsion, that medium. Therefore, the HIPE had to be addedis droplets constituted of a W/O emulsion to the suspension phase already in the shape ofdispersed in an aqueous phase, there will be a droplets. For that, a syringe pump connectedlipophilic surfactant in the primary W/O emul- with an iron needle was used (Fig. 1).sion and an hydrophilic agent in the external However, the viscosity of an 80% DVB HIPEaqueous phase of the multiple emulsion [17]. In appears to be too high to allow the productionour case, we want to obtain a suspension of of discrete droplets using this approach. Thedroplets of inverse emulsion of about 0.5 to 1 HIPE was rather more flowing out of the needlemm in diameter. Therefore, the chosen inner as a continuous ribbon. In an attempt to reducesurfactant was the Span 80, known to be the the viscosity of the starting HIPE, we decided tobest compound to obtain stable HIPE [18], add a low boiling point solvent such as petro-whereas the external surfactant employed was leum ether (PE, b.p.540–608C) to the concen-the poly(diallyldimethyl) ammonium chloride, trated emulsion. The idea was that the PE wouldan hydrosoluble synthetic polymer, that we have be quickly evaporated at the polymerisationpreviously used successfully as stabilisant for temperature (808C). Thus, by adding 20% (v/v)suspension polymerisations [13]. of PE to the 80% DVB HIPE, it become

possible to inject this emulsion under the shape of droplets of about 1 mm in diameter to the3 .1. Preparation of polyHIPE beads

stirred, preheated (808C), suspension medium.First of all, a HIPE was prepared with a After 1 h under stirring at 250 rpm, followed by

Fig. 1. Device for the synthesis of polyHIPE beads.


6 h at 808C to complete the polymerisation, a amount of DVB (1%), beads could not besatisfactory amount of rather spherical beads obtained in a satisfactory size using thesewere obtained which could be filtered and dried. experimental conditions (P ).4

The concentration of the stabilisant was The resistance to a magnetic stirring in afound to have an important effect on the pro- round-bottom flask is a parameter of importanceduction of beads. Experiments were made using in the case of beads to be used in supporteddifferent concentrations (0.7, 1.5 and 3%) of organic synthesis. Therefore, we imagined astabilisant, the last one giving the best results. qualitative test to estimate the resistance of our

An increase of this value did not improve the polyHIPE toward magnetic stirring (see Ex-yield or the regularity of the obtained beads. perimental section). In these specified condi-

We now had in hand a procedure to prepare tions, supportsP and P turned into powder0 1beads of polyHIPE from commercial DVB. within 10 to 20 min, whereas it takes about 40

However, the use of PE as dilutant was not fully min for theP beads to be destroyed. On the2

satisfactory as it gave access to a rather brittle contrary,P beads could be stirred more than 33

material. Another approach to reduce the vis- h without apparent damage. Therefore, wecosity of the starting HIPE consists to replace a selected the S–DVB (90:10) compositionP ,3

part of the DVB with styrene. Indeed, we prepared without PE, as the best formulation toobserved that the viscosity of the HIPE was obtain robust spherical beads in good yields.lowered by reducing the amount of DVB (at that Solvent uptake of cross-linked supports aretime, we have no clear explanation for this generally considered as a dominant criteria for abehaviour). Table 1 reports the main results solid-phase resin [19]. Solvent penetration intoobtained using this approach. the polymer resin being a prerequisite for any

Polymerisation of a HIPE made with only reaction to occur within a polymeric support.commercial (80%) DVB needs the addition of Therefore, we decided to estimate the solvent

PE as previously mentioned in order to obtain uptake of ourP , P andP polyHIPE beads in0 1 3

satisfactory beads (P ). The behaviour was the some representative solvents. The main results0

same for a mixture of styrene–DVB (50:50) obtained are reported in Table 2.(P ). In both cases, the obtained beads were The obtained values are in agreement with1

very brittle. In the case of a ratio S–DVB of the expected rules: the uptake is more important(90:10), the presence of PE was no longer in good solvents of polystyrene such as toluenenecessary. Beads could be obtained with or and THF than in methanol. The solvent uptakewithout its presence with comparable yields and decreases with the cross-linking level. However,size (P and P ). In the case of a very low the obtained values are much larger than those2 3

Table 1Preparation of (S–DVB) polyHIPE beads

a bpolyHIPE Styrene weight (%) DVB weight (%) Petroleum ether Yield of beads (%) Beads aspect

P 100 Y 70 Spherical0

|3 mmP 50 50 Y 95 Spherical1

|2.5 mmP 90 10 Y 65 Spherical2

|1 mmP 90 10 N 75 Spherical3

|1 mmP 99 1 N 20 Powder4

a 80% commercial DVB.b 20% (v/v) in the continuous phase where appropriate.


Table 2Solvent uptake of polyHIPE beads in different solvents

21polyHIPE Nominal Solvent uptake (ml g )

cross-linking (%) Toluene THF Methanol

P 80 18 19 80

P 40 26 24 121

P 8 50 46 153

reported for macroporous or even gel-type least to some extend, the role of a porogen [21],resins [20]. These high uptake values suppose a thus explaining the more brittle character of thelarge swelling of the matrix in addition to the resulting beads and the higher specific surfacefilling of the permanent porosity of the area.

polyHIPE structure. For example, forP , the The mercury intrusion/extrusion porosimeter3

nominal porosity is around 90%, which would has been recorded in the case of sampleP (Fig.321 2).represent an uptake of 0.9 ml g , whereas the

21 The chart shows a rather narrow distributionmeasured value in toluene is 50 ml g . Thisof pore sizes (that is, in fact, the size of thebehaviour means that the uptake is mainly dueconnecting ‘‘windows’’ between the connectedto the swelling.cells). This result is quite similar to thoseThe specific BET surface area of the

obtained with monolithic materials [21]. Fig. 3polyHIPE beads prepared with 10% DVB withreports the SEM micrographs of a polyHIPEPE (P ) or without it (P ) were recorded. The2 3

2 21 bead at different magnitudes: (i) the wholeresults obtained areS 5 20 m g for P and32 21 bead, (ii) the surface and (iii) the internal124 m g for P . The value forP is the2 3

morphology.expected result for a polyHIPE structure with-These photographs show a rather sphericalout porosity on the walls, whereas the higher

geometry with a smooth surface due to abrasionvalue obtained forP shows that PE plays, at2

Fig. 2. Mercury porosimetry pore size distribution of sampleP .3


Fig. 3. SEM photography of polyHIPE bead sampleP . (i) Whole bead, (ii) surface of the bead, (iii) inside of the bead.3

during the suspension stirring. The inner struc- using two different functional monomers: theture is similar to the one of the monolithic 4-(chloromethyl)styrene, CMS, and the 3-iso-materials [14]. propenyl-a,a9-dimethylbenzyl isocyanate, TMI

(Scheme 1). The use of CMS will give access to a3 .2. Functionalisation of polyHIPE beads

polyHIPE equivalent to the Merrifield resinIn order to be usable in solid-phase synthesis, which could be further modified by nucleophilic

these polyHIPE beads have to be grafted with substitution. The insertion of TMI ontofunctional groups that could be modified after- polyHIPE beads gives access to an insoluble

ward. The first way to graft a functional group isocyanate useful as scavenger [24]. In bothonto a support is through the chemical modi- cases, the functional monomer replaces part offication of a non-functionalised precursor. An the styrene, with no other changes in the HIPEefficient approach of this type concerns the formulation.functionalisation of the remaining pendant vin- The different materials obtained by inserting

ylic bonds of polyHIPE monoliths [22]. CMS or TMI on the HIPE formulation areAnother method consist of replacing a part of reported in Table 3.the styrene by a functional styrenic monomer in In all the experiments reported in Table 3, thethe continuous phase of the HIPE [23]. This yield and geometrical aspect of the beads aremethod is straightforward but could be limited satisfactory. The insertion of CMS into theby the destabilisation of the emulsion with too emulsion at a level as high as 45% was possiblepolar monomers. We have chosen this approach with no destabilisation of the emulsion. This

result confirms those previously reported byBarbetta et al. [23a]. As for the HIPEs madewith a S–DVB mixture, the viscosity of theconcentrated emulsion is mainly dictated by theproportion of DVB. Thus, for a level of 88%DVB, there is a need of PE in order to obtainbeads (P ), while, for 10% DVB, PE was no5

longer required whatever the proportion of CMSused (P and P ). Also, polyHIPE beads6 7

Scheme 1. Preparation of functionalised polyHIPE beadsP . prepared with 10% DVB are more robust to-5–8


Table 3Synthesis of functional polyHIPE beads

b 21polyHIPE Monomer feed composition PE Yield of beads Loading (mmol g )a c d(wt.%) FM–S–DVB (%) Expected Observed

CMSP 12:0:88 Y 40 0.79 0.675

P 45:45:10 N 70 2.95 2.606

P 12:78:10 N 60 0.79 0.637

TMIP 60:30:10 N 90 3.1 3.08

a FM5Functional monomer (CMS or TMI), S5 stryene, DVB5divinylbenzene.b PE5Petroleum ether (see text).c From monomer feed composition.d From elemental analysis: Cl (CMS) and N (TMI).

ward magnetic stirring than those containing ture. Therefore, it seemed important to increase88% DVB. Concerning TMI, as high as 60% by the loading by volume of the polyHIPE beads

weight of functional monomer could be inserted as well as to improve their accessibility.in the HIPE and as expected from the results of A way to obtain this result consists in graft-the literature [25]. ing functionalised macromolecular chains on the

The copolymerisation of a functional mono- inner surface of the polyHIPE structure. Themer gives, efficiently, access to polyHIPE method used consists in the grafting of a

beads with a high weight loading (2.6 and 3.1 TEMPO species onto the beads by the method21mmol g for CMS and TMI, respectively). previously described by Hodge et al. [14]

However, the apparent density of the (Scheme 2). 23 Starting with polyHIPE beadsP , P andP ,polyHIPE being about 0.1 g cm , the corre- 5 6 7

the grafting of TEMPO was performed in yieldssponding loading by volume are quite low. Thisin the 55–65% range, which is satisfactory for abehaviour, increased by the high solvent uptakegrafting reaction on such a structure (Table 4).of these supports implies the use of large

The next step consists in the controlledquantities of solvent and this could be a prob-radical polymerisation of a functional monomerlem in some applications. Furthermore, all the

using the polyHIPE -grafted TEMPO beads. Infunctions may not be accessible to reagents inthis context, the main advantage of using thesolution as too buried in the wall of the struc-controlled radical polymerisation instead of theconventional one [26], is that there will be nonon-grafted polymerisation.

The results obtained with the polymerisationsconducted with CMS and TMI are reported inTable 5.

The weight increase due to the polymeri-sation (i.e., weight of final support divided byweight of initial support), is quite significant(8.6 and 10.1 for CMS and TMI, respectively).The final functional loading is very high in both

Scheme 2. Functionalisation of polyHIPE beads. cases. The values obtained are quite close from


Table 4Grafting of TEMPO onto CMS-functionalised polyHIPE beads

21polyHIPE Initial loading (Cl) Final TEMPO loading (mmol g )21 a bInitial Final (mmol g ) Expected Observed

P P 0.67 0.58 0.375 8

P P 2.60 1.57 0.856 9

P P 0.40 0.36 0.207 10

a From initial chlorine loading.b From elemental analysis: N.

Table 5Polymerisation from polyHIPE -grafted TEMPO beads

21Initial Initial TEMPO Final Bead Final loading (N) (mmol g ) Functional Average a c d epolyHIPE loading polyHIPE weight Expected Observed group loading chain

21 b 21 fbeads (mmol g ) beads increase (mmol g ) length

CMSP 0.85 P 8.6 0.10 0.16 5.8 589 11

TMIP 0.2 P 10.1 0.02 4.52 22610 12

a From elemental analysis of starting support: N.b Weight of final support /weight of initial support.c From initial loading and weight increase.d From elemental analysis of final polymer: N.e From elemental analysis of final polymer: N or Cl.f Loading of grafted monomer/final expected TEMPO loading.

the maximum possible values of the corre- performed using two approaches. The first onesponding soluble homopolymers (6.55 and 4.96 involves the insertion in the HIPE of a func-

21 tional monomer and its copolymerisation duringmmol g for CMS and TMI, respectively). Thethe polyHIPE preparation. In the second ap-estimate of the TEMPO loading before and after

proach, functional macromolecular chains havepolymerisation has been conducted when pos-been grafted onto the inner surface of thesible (CMI). The close values obtained is anmaterial by a controlled radical polymerisationindication of the low level of chain terminationusing a covalently-bonded nitroxide species.during the polymerisation. The average chainWhereas, the former way is more straightfor-length was estimated from the weight increase,ward, the latter one allows the expectation of athe monomer molecular weight and the amountbetter accessibility of the grafted moieties to-of TEMPO grafted.ward reagents in solution. Indeed, in this case,we have obtained a beaded composite supporthaving a rigid matrix whose open cells are

4 . Conclusion partly filled by a non-cross-linked functionalpolymeric chain. Either approach chosen, the

In this paper, we have reported the first large pore size of the polyHIPE material andattempt described in the open literature to the high loading obtained makes these new kindsynthesize functionalised spheroidal beads of of beaded supports, good candidates to be aopen-cells microcellular foams from a HIPE. viable alternative to gel-type or macroporousThe functionalisation of these beads has been beads presently used in SPOC.


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Synthesis and functionalisation of polyHIPE® beads