Functionalisation of polyHIPE Materials by ATRP Surface Grafting

Downloaded by NORTH CAROLINA STATE UNIV on January 11, 2013 | http://pubs.acs.org Publication Date: August 13, 2009 | doi: 10.1021/bk-2009-1023.ch022

Chapter 22

Functionalisation of polyHIPE Materials by ATRP Surface Grafting David M. Cummins1 Pieter C. M. M. Magusin1, and Andreas Heise*1,2 1

Technische Universiteit Eindhoven, Den Dolech 2, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands 2 Dublin City Univerity, Glasnevin, Dublin 9, Ireland

Photopolymerization of a high internal phase emulsion (HIPE) containing a polyemrizable ATRP initiator resulted in polyHIPE with ATRP initiator groups on the surface available for polymer grafting reactions. This was initially shown for the grafting of MMA. Further functionalization of the polyHIPE was obtained by the grafting of glycidyl methacrylate (GMA). The macroporous morphology and the up to 800 nm thick grafted pGMA layer was clearly visible in SEM micrographs. Epoxide ring-opening reactions of the epoxide groups with sodium azide yielded near quantitative azidation of the PGMA layer. Under optimized conditions, azide conversions of around 80 % were estimated from IR. Further modification was achieved by Huisgens-type ‘click’ chemistry with various alkynes, for example amino acids and a fluorescent alkyne. Moreover, amino functional silica microparticles were attached to the PGMA modified pHIPE.

© 2009 American Chemical Society In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction Atom transfer radical polymerization (ATRP) is one of the most versatile methods for polymer grafting from solid supports (1-8) due to its tolerance towards many reaction conditions and the large array of monomers that can be used (9-11). We investigated ATRP for the surface modification of macroporous polymers obtained from high internal phase emulsions (HIPE). These HIPE materials are defined by an internal or droplet phase volume ratio of 0.74 or higher (i.e. the minimum volume of the emulsion comprised of droplets is 74%). Polymerization of a monomeric continuous phase of a HIPE leads to a highly porous cross-linked polymer material resulting in foams that have a well structured morphology, void size and interconnecting window size. Generally, the system is composed of an organic (continuous) phase containing a monomer such as styrene, a suitable emulsifier, an aqueous (dispersed) phase containing the radical initiator and a cross-linker, e.g. divinyl benzene. The addition of droplets of the aqueous phase into the organic phase during constant stirring results in a dilute reverse emulsion. Increasing the amount of water in the organic phase results in a concentrated emulsion with thin monomer films surrounding the water droplets (HIPE). Polymerization of HIPE was initially reported by Bartl and von Bonin (12, 13) and considerable advances in this technology were made by Hainey, Sherrington and Cameron ( 14 , 15 ). A number of articles have discussed how structural features vary as a function of the synthetic conditions (16-20). The predominant work carried out in the synthesis of pHIPE involves thermal curing of styrenic systems (21). More recently, Pierre et al. have reported photopolymerization as a beneficial method for a faster and more benign polymerization (22). Some investigated application areas for pHIPEs include ion-exchange resins (23), membrane filters for the removal of particulates from aerosol (24), solid phase peptide supports (25), supports for cells and enzymes (26, 27), and as materials for the removal of heavy metals (28, 29). All these applications require surface functionalization of the pHIPE, which can be achieved by the addition of a functional monomer to a HIPE formulation or by surface modification after cross-linking. While the first approach is limited by the stability of the HIPE (hydrophilic monomers destabilize the emulsion), the second approach has to rely on the chemistry possible with the pHIPE building blocks. Both approaches have also limitations in the density of functional groups which can be introduced. The ability to conveniently modify pHIPE surfaces with a high density of functional groups is crucial to opening new application areas. An effective and versatile approach to this is surface grafting of polymer chains. Stable polymer brushes covalently attached to a surface possess excellent chemical and mechanical robustness as well as the flexibility to introduce a large variety of other functional monomers. Our strategy to highly functional pHIPE is shown in Figure 1. An ATRP initiator with polymerizable group (inimer) is added to the HIPE formulation. After UV-curing, ATRP is initiated from the pHIPE surface to obtain densely packed polymer brushes. By the right choice of functional monomers, further functionalization by a secondary reaction will be possible.

In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.


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Figure 1. Synthesis of functional pHIPE in two steps: (1) formation of stable high internal phase emulsion (HIPE) in the presence of the inimer and UV curing to obtain pHIPE with ATRP initiator groups (I-pHIPE). (2) Grafting of MMA and GMA by ATRP from I-pHIPE surface. (3) Secondary reaction on the functional surface grafts.

Experimental The synthesis of the ATRP initiator functionalized pHIPE, the conditions for the ATRP grafting and the click reactions has been described previously (30, 31).

Modification of pHIPE-g-PGMA with silica microparticles Samples of control (non-functionalized) pHIPE and pHIPE-GMA were added to vials containing 15% w/w modified silica microparticles in methanol and THF. 10 drops of conc. HCl were added to each vial and the reactions were allowed to run for 16 hours. Two types of silica particles were used: (1) 3 μm spherical Si-Amine (Si(C3H6)NH2), and (2) 3 μm spherical Si-Thiol (Si(C3H6)SH). After the reaction time the samples were placed in a sonicator for 1 h to remove any unbound Si particles.


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Results and Discussion


Synthesis of polyHIPE materials with ATRP initiator The pHIPE material used was prepared in a bulk amount by photopolymerization of a HIPE containing the ATRP inimer (denoted I-pHIPE) (30). To enable comparison studies, a control pHIPE was also prepared which did not contain any ATRP initiator. This control was used in all experiments. SEM images recorded after the polymerization of the HIPE (Figure 2) show the highly porous material (approximately 90% porosity based on the formulation). From these images, the average diameter of the pores was estimated to be approximately 25 μm. Adjacent pores are interconnected by windows which were visually estimated to have an average diameter of 8 μm. What is also apparent here is the presence of extremely small pores in an almost uniform pattern around the larger windows (white arrow in Figure 2). The reason for the formation of these pores is unknown yet but they are important for three reasons. Firstly, they give an indication of the thickness of the wall; secondly, they give an indication as to the strength of the wall as they are not consumed by the larger windows; and thirdly, due to the almost perfect alignment around the windows, they are most likely a result of the polymerization. Also observed is a divot like effect on the surface of the matrix walls which is a consistent character of the pHIPE samples prepared.

Figure 2. Scanning electron microscopy (SEM) images of I-pHIPE. Scale bar (left) = 20μm, (middle) = 5 μm, (right) = 2 μm.



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Figure 3. SEM images of p(HIPE-g-MMA); showing a distinct difference in the surface morphology of the matrix. Scale bar (left) = 5 μm, (middle) = 1 μm, (right) = 500 nm. (Reproduced from reference 30. Copyright 2007 American Chemical Society.) ATRP from pHIPE Inspired by the work of Moine et al. who reported initial work on ATRP from thermally cured styrene based pHIPE (32), we investigated whether the advantages of fast acrylate UV-curing for pHIPE formation could be combined with ATRP surface grafting. Efficient surface ATRP in polar solvents like methanol and water has been reported by Huck et al. (33, 34). They proposed that the activity of the catalyst is increased by the high dielectric constant of the solvent. The conditions allow for rapid polymerizations and lead to high layer thicknesses without the addition of sacrificial initiator (1). We adopted this method for our ATRP reactions from the I-pHIPE surfaces allowing us to maintain reaction control while rapidly grafting water insoluble polymers from the pHIPE. In order to exhibit the availability of the ATRP groups for grafting, we first used MMA as the monomer. Typically soft and rubbery I-pHIPE samples (0.5-1.0 cm3) were added to the reaction mixture. Upon completion, the recovered pHIPE was brown in colour due to the absorption of the copper catalyst. Intensive washing with suitable solvents to remove ungrafted polymer and EDTA to remove all catalyst, yielded a white polymer functionalised pHIPE. The first indication of a successful modification is that the material had become more brittle. Characteristic signals of the PMMA could also be identified in the IR spectra of the obtained product (30). SEM analysis provided stronger evidence for the grafting success. Figure 3 shows the images of the change in surface morphology due to the presence of polymer on the surface. When compared with the corresponding image in Figure 2, the overall porosity of the monolith has been preserved with little change between these samples and the precursors. This shows that the pHIPE can be washed thoroughly after grafting and that the monolith porosity remains intact. Notably, a change in surface roughness is observed and the divots present in the blank sample are now surrounded by smaller mounds. Also notable is that the small pores circling the windows are still visible. However, it is not possible from these images to determine the homogeneity and thickness of the grafted polymer layer.


332 (a): p(HIPE-g-MMA)

(b): p(HIPE-g-(MMA-b-HEMA))

(c): p(HEMA))


(d): (b) – (a)

ν (cm-1)

Figure 4. Infrared absorption spectra showing in descending order: (a) p(HIPE-g-MMA); (b) p(HIPE-g-MMA) after ATRP of HEMA (block copolymerization); (c) p(HEMA); and (d) spectrum resulting from the subtraction of (a) from (b). (Reproduced from reference 30. Copyright 2007 American Chemical Society.)

Further evidence for the grafting of the detected pMMA was provided by the fact that: (i) polymerization of MMA was not observed when I-pHIPE was used in the absence of ATRP catalysts; (ii) polymerization of MMA was not observed when the same reaction was carried out using control pHIPE without ATRP initiator, and (iii) when an ATRP reaction was conducted with free ATRP initiator in solution in the presence of a pHIPE, polymerization occurred in solution but not in the pHIPE. After polymerization of the MMA, the monolith went through the same washing procedure and although the reaction clearly produced PMMA, no corresponding signal was detected by IR for the pHIPE. This confirms the efficiency of the washing process and supports the conclusion that the PMMA is grafted from the pHIPE surface. The ability to re-initiate further polymerization from the p(HIPE-g-MMA) was investigated with hydroxyethylmethacrylate (HEMA) to yield p(HIPE-g(PMMA-b-PHEMA)). IR analysis revealed the presence of an O-H vibrational stretch at 3400 cm-1 corresponding to the hydroxy group of the HEMA and confirming the presence of p(HEMA) on the surface (Figure 4). When compared to the precursor, changes in the fingerprint region of this sample provide further evidence for the grafting of the HEMA and compare well with results reported by Brantley (35).


333 (a): p(HIPE-g-GMA)

(b): I-pHIPE


(c): p(GMA)

(d): (a) – (b)

ν (cm-1)

Figure 5. Infrared absorption spectra showing in descending order: (a) spectrum of pHIPE after GMA grafting; (b) I-pHIPE; (c) p(GMA) (from unbiased data base); and (d) spectrum resulting from the subtraction of (b) from (a). (Reproduced from reference 30. Copyright 2007 American Chemical Society.)

Functionalization of pHIPE by ATRP grafting of glycidyl methacrylate (GMA) A major benefit of the grafting of GMA from the pHIPE is the high density of reactive functionalities available for ring-opening of the epoxide group, e.g. carboxylic acids, alcohols, water or amines all yield a versatile platform for the further attachment of other functional materials. After the ATRP of GMA, the modified pHIPE became even more brittle than that with PMMA verifying that a reaction had occurred. IR analysis of the resulting p(HIPE-g-GMA) revealed the presence of the epoxide groups in the fingerprint region at around 900-800 cm-1. The presence of the band at 904 cm-1 confirms that the epoxide rings remain closed confirming that they are unaffected by the reaction conditions used (Figure 5). As no free monomer or polymer existed in the network after the washing process, the change in the IR spectrum is attributed to the grafted pGMA polymer.



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Figure 6. SEM images showing the polyGMA growth on the surface of pHIPE after(a) 3,(b) 5, (c) 16 and (d) 24 hours polymerization times. In order to determine the change of surface morphology due to the GMA layer growth as a function of reaction time, samples were removed from the reaction after 1, 3 5, 16 and 24 hours. The samples were worked up in the standard manner and then examined using SEM (Figure 6). The results clearly show that up to the first three hours there is no evidence of polymer growth. However between three and five hours, small pockets of islands begin to appear. These islands suggest that the ATRP initiator is not uniformly distributed across the surface. After 16 hours the polymer layer has grown and is shown to cover the entire surface. The small windows that surrounded the larger windows before polymerization have now disappeared due to being covered and filled with PGMA. After 24 hours a 600 – 800 nm thick PGMA layer is clearly visible as a homogenous, uniform layer. The control pHIPE (without ATRP initiator) was added to a solution based ATRP reaction of GMA, under the same conditions as the functionalisations reaction but remained unmodified with polymerization of the GMA in solution only and not in the matrix. The rapid polymer growth of the GMA, in comparison to the MMA is in agreement with reports from Huck, who attributed this effect to the copper catalyst coordination with the epoxide group of the GMA, resulting in the displacement of the ligand and increasing the activity of the catalyst (1). While further functionalization experiments using the glycidyl groups confirm that they are fully intact after the polymerization, it cannot be excluded that partial cross-linking of the PGMA via the epoxy groups occurs in the graft layer. Secondary functionalization of p(HIPE-g-GMA) by ‘click’ reactions Huisgen 1,3-dipolar cycloaddition (click reaction) between terminal alkynes and azides is a highly efficient approach for the post modification of polymers. This technique is tolerant to a wide rage of reaction conditions and functional groups allowing fast coupling reactions under simple reaction conditions with


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high yields and high selectivity (36-39). The use of Cu(I) catalysts further accelerates the process leading to quantitative coupling yields at 25 °C (40). These reactions have been highlighted in a number of material science applications recently ( 41 , 42 ) e.g., the preparation of highly functionalized macromolecules ( 43 - 48 ) as well as in the surface immobilization of biomacromolecules ( 49 - 55 ). and the functionalization of self-assembled monolayers (SAMs) (56-60).

Figure 7. Schematic representation of the modification of macroporous pHIPE by click reactions; (1) formation of azide modified pHIPE by nuceophilic ring opening of epoxide groups on pGMA grafted pHIPE. (2) Click reactions with alkyne functional molecules. (Adapted from reference 31 by permission of The Royal Society of Chemistry.) The azide functionalised pHIPE-N3 was obtained by nucleophilic ringopening of the epoxy-rings on the pHIPE by sodium azide (44). The appearance of a typical azide IR band at 2100 cm-1 confirms the presence of the azide on the pHIPE. Click reactions were then conducted with different alkyne functionalized molecules (31). Special attention was given to assure that the reaction occurred throughout the three-dimensional morphology of the microporous substrates. Optimization of the reaction conditions for the ‘click’ reactions was performed using the small, non-bulky propargyl alcohol molecule 1 (Figure 7). The best results were obtained when the click reaction was carried out in the presence of Cu(I) at 55 °C in which case an almost equal reduction of the azide band of the internal and external pHIPE surface of 75 - 83 % was observed (with respect to carbonyl band). While the reaction also proceeds at room temperature, lower conversion and in the absence of Cu (I) even a distinct difference between the interior (33 %) and the exterior (67 %) of the pHIPE was observed. In the absence of propargyl alcohol, under otherwise identical conditions with pHIPEN3, no reduction of the azide band was observed (31). The high conversions of the azide under optimized conditions suggest that functionalization must have occurred throughout the layer of the original pGMA surface layer on the pHIPE. It further shows that a homogeneous functionalization depends on geometry related parameters by its influence on mass transport through the macroporous material. This will be more significant for the functionalization of larger samples like monoliths for bioseparation.



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Figure 8. Left: ATR-FTIR transmittance spectra of pHIPE-N3 before and after ‘click’ reaction with 1 (catalyst: Cu (I); 55 °C). Spectra show the reduction of the azide peak (ν (N3)) in the exterior (click exterior) and the interior (click interior) of the pHIPE normalized against the polyacrylate of the pHIPE (ν (C=O)). Right: Fluorescent microscope image of control-pHIPE (left, no azide groups) and pHIPE-N3 (right) after reaction with 2 and washing. The latter is cut in half showing the interior of the pHIPE. (Reproduced from reference 31 by permission of The The Royal Society of Chemistry.)

The success of the click reaction was further visualized by attachment of 3,4-difluorophenylacetylene (DFA) 2 at 55°C. Fluorescent microscope pictures taken of the DFA modified pHIPE monolithic samples cut in half are shown in Figure 8 (right). The image clearly shows that the DFA modified material is highly fluorescent throughout the sample, while the control is not. This also confirms the efficiency of the washing procedure removing all unbound DFA. SEM analysis shows that the surface roughness of the samples has changed following the reaction (Figure 9).

Figure 9. SEM images showing the surface morphology of pHIPE-N3 before (left) and after (right) the ‘click’ reaction with 1. (Reproduced from reference 31 by permission of The Royal Society of Chemistry.)



337 Moreover, we attached a number of amino acids via a pendant alkyne group. Commonly the attachment of amino acids to solid substrates is done via amide bond formation using the amino group of the amino acid. Since these amide bonds are highly susceptible to enzymatic degradation, a nonproteinogenic form of amino acids was prepared. Incorporation of an alkyne group enabled the reaction with an azide, effectively mimicking the amide bond (amide isostere) and forming a triazole ring. It has been shown that the biological activity of these amino acids, for example in glycopeptides, remained the same while the proteolytic degradation of the biological systems was inhibited (61). Using this method to successfully attach amino acids results in a biofunctional pHIPE with improved long term stability with both N- and Cterminus available for further modification or interaction.

Figure 10. Deprotection of amino acid 3 attached to pHIPE. (Reproduced from reference 31 by permission of The Royal Society of Chemistry.) As an example the attachment of 3 is shown. From IR analysis a 70 % azide conversion was estimated in the click reaction. Subsequently, the amino acid functionalized pHIPE sample was exposed to hydrochloric acid/THF, with the aim to selectively remove the N-terminal (BOC) protecting group (Figure 10). The success of the click reactions between the pHIPEs and the amino acids and the subsequent deprotection was monitored by 13C MAS NMR (31). Spectrum 1 in Figure 11 shows the 13C solid state NMR spectrum of a pHIPEN3. Signals of the pHIPE acrylate matrix and the azide functional pGMA (b, c, d) can be assigned between 10 and 80 ppm and at 180 ppm. In the spectrum of the amino acid modified pHIPE, additional peaks can be identified (Figure 11, spectrum 2). After subtraction of the pHIPE-N3 spectrum these signals can be assigned to the triazole ring formed upon click coupling and the deprotected amino acid (Figure 11, spectrum 3). For example, the olefinic carbon signals at 125 (e) and 148 ppm (f) are consistent with the triazole structure. Moreover, peaks g-k and m can be assigned to the amino acid attached to the pHIPE. Notably, the carboxy signal in the amino acid (m, 171 ppm) is slightly shifted upfield as compared to the carboxy signal of the polyacrylate matrix (a, 178 ppm), which indicates that the methyl ester protecting group has not been removed. In contrast, the fact that no -NH-C(O)-O- signal at ca. 160 ppm, nor the quaternary 13C signal of the t-butoxy group expected around 80 ppm are observed in the spectrum, provides evidence that the BOC protection group has been removed in this reaction step.



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Figure 11. Solid-state CPMAS 13C NMR. 1: spectrum of pHIPE-N3; 2: pHIPE modified with 6 after deprotection of amino acid (Scheme 2); 3: Difference spectrum. *: spinning sideband. (Reproduced from reference 31 by permission of The Royal Society of Chemistry.) While these results clearly show, that the triazole ring is formed upon click reaction and that it is stable under amino acid deprotection conditions, further measurements are necessary to quantify the yield of the deprotection reaction. Secondary functionalization of pHIPE-PGMA with silica microparticles The increase in surface area of the pHIPE is important to open up new application areas. One approach is to attach spherical particles to the surface of the monolithic material. This has been, for example achieved by electrostatic interaction between charged latex nanoparticles and a monolithic surface (62). In preliminary experiments we investigated if functional silica microparticles could be covalently adsorbed to the p(HIPE-g-GMA) surface so as to produce stable, functional pHIPE with an increased surface area.

Figure 12. SEM images of pHIPE without PGMA graft layer(left: 100 μm ) and p(HIPE-g-GMA) after reaction with amine functionalized Si particles ( middle: scale bar 100 μm; right: scale bar 10 μm).



339 To achieve this, samples of unmodified pHIPE (without PGMA) and p(HIPE-g-GMA) were reacted with 3 μm spherical Si-amine (Si(C3H6)NH2) functionalized particles. After the reaction, the samples were placed in a sonicator for 1 h to remove any unbound Si particles. SEM analysis of the Siamine reaction clearly shows the difference between the control pHIPE and the p(HIPE-g-GMA) after the reaction. In the control almost all silica particles have been removed as they are not covalently bound to the surface (Figure 12). However in the functionalized pHIPE sample there is a high density of SiAmine particles in the pores of the pHIPE which must be attached via the epoxy group. It should also be noted that the images have been made of the interior of the pHIPE samples after the reaction samples were sliced. This shows that the mass transport has occurred. The image on the right furthermore suggests that a monolayer of silica particles on the inside wall of the pore has been formed. Similar results were observed for the Si-thiol functionalized particles (not shown here). Currently measurements to determine the surface area are under way. While the modification with the functional particles needs to be further optimized, this approach has the advantage that not only the surface area of the pHIPE can be increased but at the same time highly useful functional groups (NH2 and SH) are introduced.

Conclusion We have shown that an ATRP initiator group can be chemically incorporated into an acrylate based pHIPE formulation without compromising the emulsion stability. UV-curing of this formulation lead to pHIPE with ATRP initiator groups on the surface available for polymer grafting reactions. The latter was demonstrated by first grafting of pMMA and re-initiation of HEMA in a block copolymerization approach. Moreover, functionalized polyHIPE was obtained by the grafting of GMA. This resulted in a 600-800 nm smooth and homogeneous surface coverage of the pHIPE surface with a high density of reactive epoxy groups. These materials can be used as a platform for further functionalization as has been demonstrated by the attachment of alkynes via ‘click’ chemistry and amine functional silica microparticles.

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