Journal of Inorganic and Organometallic Polymers and Materials, Vol. 17, No. 1, March 2007 ( 2007) DOI: 10.1007/s10904-006-9094-z
New Preparation and Purification Methods for MetalloSupramolecular Block Copolymers Christina Ott,1 Daan Wouters,1 Hanneke M. L. Thijs,1 and Ulrich S. Schubert1,2
Submitted: September 7, 2006; Accepted: October 20, 2006
The bis-complex formation of two terpyridine-modified polymers using microwave assisted reaction conditions was investigated in detail. In order to construct a heteroleptic block copolymer polystyrene, which was prepared by nitroxide-mediated radical polymerization (NMRP), and a commercial available poly(ethylene oxide) were used. These studies include the influence of temperature, reaction time, concentration and the use of different solvent mixtures on complexation. The conversion was determined by GPC measurements. The amphiphilic metallo-supramolecular block copolymers were investigated towards their formation of micellar aggregates in water. Other characterization techniques include 1H-NMR, UV/vis spectroscopy, dynamic light scattering (DLS), atomic force spectroscopy (AFM) and transmission electron microscopy (TEM) techniques. KEY WORDS: Terpyridine; supramolecular polymers; metal complexes; block copolymer; microwave; Ruthenium(III) monocomplex; nitroxide-mediated radical polymerization (NMRP); preparative GPC; fractionation; micelles
and macromolecular chemistry because of its high binding affinity towards transition metal ions [3, 4]. The well-known tridendate chelating ligand forms stable 2:1 octahedral geometries with a large variety of transition metal ions in low oxidation states, such as Ru(II), Fe(II), Zn(II), Co(II) and Cu(II) due to strong metal to ligand dp-pp* back donation. The most convenient metal ion for coordination polymers is ruthenium because it allows the direct synthesis of homoleptic and heteroleptic complexes. Furthermore, ruthenium complexes show interesting optical and photophysical properties [5–7]. The commercially available 4-chloro-2,2¢:6¢,2¢¢terpyridine allows the introduction of the ligand moiety into monomers or as endgroup modification onto polymers. A broad range of supramolecular polymers with various architectures, e.g. dendrimers, rod-like structures, copolymers, block copolymers and graft copolymers, are easily accessible [8–17]. Nowadays, a broad range of functional well-defined block copolymers are available using living radical polymerization techniques such as nitroxide-medi-
1. INTRODUCTION In the last decade, supramolecular chemistry has attracted special interest and represents an important area of todayÕs material research. Non-covalent interactions, including hydrogen bonding, metal coordination, van der Waals forces, electrostatic effects and hydrophilic–hydrophobic interactions, are used to self assemble molecules into supramolecular materials. As a consequence, the materials are expected to reveal new properties with respect to film formation, surface activity, and reversibility of the complexation, which can result in ‘‘switchable’’ systems under certain conditions [1, 2]. Since the first isolation of the 2,2¢:6¢,2¢¢-terpyridine in 1932 by Morgan and Burstall, the unit integrated well as a building block in supramolecular 1
2
Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, PO Box 5135600 MB, Eindhoven, Netherlands. To whom correspondence should be addressed. E-mail:
[email protected]
241 1574-1443/07/0300-0241/0 2007 Springer Science+Business Media, LLC
242 ated radical polymerization (NMRP) [18], reversible addition-fragmentation chain-transfer (RAFT) polymerization [19, 20], and transition-metal-mediated living radical polymerization (e.g. ATRP) [21]. Amphiphilic diblock copolymers are able to selfassemble into polymeric micelles or aggregates in selective solvents [22]. Stimuli-responsive materials are sensitive to external conditions such as temperature, pH, light, ionic strength, electric and magnetic field. Block copolymers containing these defined properties in micellar structures are promising candidates for distinct applications, such as drug release, templating nanoparticles, pigment stabilization, etc [23–25]. In this contribution we describe the optimization of a complexation reaction with ruthenium ions based on two commonly used polymers functionalized with terpyridine moieties at the chain ends and its purification via SEC/GPC. Successful fractionation is presented for high molecular weight block copolymers of the type PSn–[Ru]–PEG70. Block copolymers of this composition are known to form micelles which were further on investigated by DLS, AFM and TEM.
2. EXPERIMENTAL Chemicals were received from Aldrich, Fluka and Acros. All monomers were freshly purified on an AlOx-filtration column prior to use in order to remove the inhibitor. Microwave-assisted reactions were performed in capped reaction vials especially designed for the single-mode microwave system Emrys Liberator (Biotage, formerly Personal Chemistry). The pressure and the temperature inside the vial are monitored by the bowing of the septum and an IR temperature sensor, respectively. All reactions were carried out under temperature control with variable microwave power (maximal 300 W). 1 H-NMR and 13C-NMR were recorded on a Varian Mercury spectrometer with frequencies of 400 and 100 MHz and on a Varian Gemini spectrometer with frequencies of 300 and 75 MHz at 25 C, respectively. Chemical shifts are given in ppm downfield from TMS. Gel permeation chromatography (GPC) was measured either on a Shimadzu system with a SCL-10A controller, a LC-10AD pump, a RID-10A refractive index detector, and a PLgel 5 lm mixed-D column using a chloroform:triethylamine: 2-propanol (94:4:2) mixture as eluent at a flow rate of 1 mL min)1 at 50 C or on a Waters system with a
Ott, Wouters, Thijs, and Schubert 1515 pump, a 2414 refractive index detector, and a Waters Styragel HT4 column utilizing a N,N-dimethylformamide and 5 mM NH4PF6 solution as eluent at a flow rate of 0.5 mL min)1 at 50 C. Preparative size exclusion chromatography was performed on an Agilent system consisting of a Agilent 1100 series Control Module, a Agilent 1100 series Isocratic Pump, A Agilent 1100 series RID refractive index detector, a Agilent 1100 series Manual Injector and a PSS Gram preperative 100 A˚ column utilizing THF and 5 mM NH4PF6 as eluent at a flow rate of 3 mL/min. UV/Vis spectra were recorded on a Perkin Elmer Lambda 45P spectrophotometer. 2.1. Terpyridine-functionalized Polystyrene The initiator [26] was dissolved in purified styrene and a degree of polymerization of 80, 150 and 300 was targeted. Three freeze-pump-thaw cycles were applied for removal of oxygen before the reaction vessels were immersed in an oilbath of 125 C. The polymerization was carried out for a certain amount of time and then stopped according to the kinetic data obtained previously. The polymers were precipitated twice from CH2Cl2 into cold methanol. 1H-NMR (CDCl3): d = 8.68 (m, 2 H; H6:6¢¢), 8.62 (m, 2 H; H3:3¢¢), 8.21 (m, 2 H; H3¢:5¢), 7.93 (m, 2 H; H4:4¢¢), 7.47–6.32 (m, HPS backbone aromatic; Haromatic, H5,5¢¢), 5.34 (m, 2 H; tpyOCH2), 4.27–4.07 (broad, 1 H; HC-ON), 3.50–3.15 (m, 1 H; ON-CH), 2.45–0.53 (m, HPS backbone aliphatic; C(CH3)3; CH3CH CH3; CH3 initiating fragment). GPC (UV): Mn (PDI): 4,600 g mol)1 (1.13) PS50-[, 69% yield. GPC (UV): Mn (PDI): 12,700 g mol)1 (1.13) PS130-[, 77% yield. GPC (UV): Mn (PDI): 23,900 g mol)1 (1.17) PS285-[, 75% yield. 2.2. Terpyridine end-functionalized poly(ethylene oxide) Powdered KOH (0.56 g, 10 mmol) and a-methoxy-x-hydroxy-poly(ethylene oxide) with Mn = 3,000 g/mol (10 g, 3.33 mmol) were stirred under argon in dry DMSO at 70 C [27]. After 30 min a two times excess of 4¢-chloro-2,2¢:6¢,2¢¢-terpyridine (1.78 g, 6.7 mmol) was added. The mixture was stirred for 24 h at the given temperature, then poured into cold water (precipitation) and extracted with CH2Cl2. The combined organic layers were dried over Na2SO4 and removed in vacuo. The polymer was purified by a
New Preparation and Purification Methods double precipitation from THF into diethyl ether. Yield: 8.72 g (78%). 1 H-NMR (400 MHz, CDCl3): d = 8.68 (dd, 2 3 H, J = 4.8, 4J = 1.6 Hz; H6,6¢¢), 8.61 (dd, 2 H, 3 J = 7.6 Hz, 4J = 1.2 Hz ; H3,3¢¢), 8.04 (s, 2 H, H3¢,5¢), 7.85 (td, 2 H, 3J = 8 Hz, 4J = 2 Hz; H4,4¢¢), 7.34 (dd, 2 H, 3J = 4.8 Hz, 4J = 0.8 Hz; H5,5¢¢), 4.40 (m, 2 H; tpyOCH2), 3.93 (m, 2 H; tpyOCH2CH2), 3.82–3.45 (m, PEO backbone), 3.38 (s, 3 H, OCH3). UV-vis (H2O): kmax () = 278 (13,200), 234 (17,000) nm (L mol)1 cm)1). GPC (UV): Mn (PDI): 6,960 g mol)1 (1.07). 2.3. Synthesis of RuCl3 Poly(ethylene oxide) Mono-complexes A three fold excess of anhydrous RuCl3 (0.12 g, 0.58 mmol) with respect to the terpyridine end functionalized polymer was heated in dry degassed DMA (6 mL) to 130 C. After the color of the suspension turned brown, a solution of the poly(ethylene oxide) (0.62 g, 0.19 mmol) in dry degassed DMA was added dropwise. Stirring continued overnight at 130 C at inert conditions and then the solution was allowed to cool to room temperature. The resulting mixture was partitioned between dichloromethane and water. The organic layer was separated, dried over Na2SO4, filtered and evaporated in vacuo. The brown residue was taken up in a minimum amount of THF and precipitated twice in ice-cold diethyl ether. Yield: 0.53 g (78%). 1 H-NMR and 13C-NMR (CDCl3): only polymer backbone visible because of paramagnetic nature of Ru(III)-complex. GPC (UV): Mn (PDI): 3,420 g mol)1 (1.20). UV-vis (CHCl3): kmax () = 401 (8,700), 311 (16,500), 276 (31,000) nm (L mol)1 cm)1). 2.4. Synthesis of the PSn–[Ru]–PEG70 Block Copolymers Terpyridine-functionalized polystyrene and the RuCl3 poly(ethylene oxide) mono-complex were reacted in a 1:1 molar ratio in a 4:1 solvent mixture of degassed tetrahydrofuran and methanol for 1 h at 90 C in a sealed vial. The reaction mixture was stirred for 10 min at room temperature after a 10-fold excess of NH4PF6 was added. The solution was poured into water, and the aqueous layer was
243 extracted twice with chloroform. The combined organic layers were dried over Na2SO4, filtered, and evaporated in vacuo. The crude product was purified by fractionation via SEC/GPC. 1 H-NMR (PS130–[Ru]–PEG70, 400 MHz, CD2Cl2): d = 8.60–8.37 (m, 8 H; H3¢:5¢¢ ,H3:3¢¢), 7.85 (m, 4 H; H4:4¢¢), 7.35 (m, 4 H; H6:6¢¢), 7.34–6.32 (m, 661 H; HPS backbone aromatic; Haromatic, H5,5¢¢), 5.34 (m, 2 H; tpyOCH2), 4.27–4.07 (broad, 1 H; HC-ON), 3.90–3.15 (m, 281 H; ON–CH, OCH2 PEG backbone ), 2.62–0.40 (m, 409 H, HPS backbone aliphatic; C(CH3)3; CH3CHCH3; CH3 initiating fragment). GPC (UV): Mn (PDI): 39,800 g mol)1 (1.05) PS50–[Ru]–PEG70; Yield: 20 mg (35%). GPC (UV): Mn (PDI): 46,800 g mol)1 (1.05) PS130–[Ru]–PEG70; Yield: 30 mg (38%). GPC (UV): Mn (PDI): 56,700 g mol)1 (1.06) PS285–[Ru]–PEG70; Yield: 10 mg (19%). UV-vis (CH2Cl2) kmax () = 305 (20,500), 485 (5,250) nm (L mol)1 cm)1). 2.5. Preparation of the Micelles The block copolymers were dissolved in N,NÕdimethylformamide (DMF) at a concentration of 1 g L)1. Subsequently, drops of water were added stepwise to induce aggregation of the insoluble poly(styrene) block. After that an equal amount of water was added in one shot to ‘‘freeze’’ the micelles. Finally, the DMF/water solution was dialyzed against water for 24 h, replacing the water at least three times (Spectra-Por dialysis bags, cutoff 1,000 Da). Dynamic light scattering (DLS) experiments were performed on a Malvern CGS-3 equipped with a He-Ne laser (633 nm) at a 90 angle and at room temperature (25 C). Transmission microscopy measurements were performed on a FEI Tecnai 20, type Sphera TEM operating at 200 kV with a LaB6 filament and a bottom mounted 1 k x 1 k Gatan CCD. Samples were prepared by blotting a diluted solution of the respective micelle solution on a 200 mesh carbon coated grid followed by overnight drying, after which the grids were examined in the TEM. TEM grids were hydrophilized directly before use by 40 s plasma treatment. The samples for TEM measurements were not stained. Samples for AFM were prepared by drop casting the micelle solution onto freshly cleaved mica. Imaging was performed in intermittent contact mode on multimode SPM (Digital Instruments, Santa Barbara, CA) using OLTESPA-type tips.
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3. RESULTS AND DISCUSSION
1.0 RI signal normalized
Controlled radical polymerization (CRP) processes are nowadays frequently used to prepare welldefined polymers with predetermined, unimodal, and narrow molecular weight distributions. The tolerance to many functional groups present in the monomers is provided using these advanced polymerization techniques. Furthermore, CRP features good control over composition, chain architecture, and end-functionalization (telechelics). In the present research, a terpyridine-functionalized unimolecular initiator, based on HawkerÕs initiator system [18, 28], was used for the nitroxide-mediated radical polymerization (NMRP) of styrene. It was shown that this initiator is perfectly suitable for the synthesis of homopolymers as well as block copolymers [26, 29]. Terpyridine-functionalized polystyrene (PS-[) was prepared in three different chain lengths (Fig. 1) according to the kinetic data [30]. The degrees of polymerization were calculated by integrating the 1H-NMR signals of the 3¢,5¢-protons of the terpyridine ligand that are clearly resolved between 9 and 7 ppm in the spectrum. The nitroxide is attached to the polystyrene as can be observed from the weak resonances between 4.6 and 4.3, at 3.3 and at 0.5 ppm (Fig. 2). Commercial available poly(ethylene oxide) served as basis for the second block amongst others because of its ability to dissolve in water. Hydroxy-functionalized poly(ethylene oxide) was reacted with 4¢-chloroterpyridine in DMSO at 70 C. The terpyridine-modified PEO was obtained in fairly good yields and was subsequently reacted with RuCl3. The formed mono-terpyridine-ruthenium complex was analyzed using 1H-NMR spectroscopy where only the PEO-backbone was visible due to the paramagnetic nature of the Ru(III)-complex. Moreover, the combination of GPC with a photo diode array detector (PDA) revealed the formation of the mono-complex since such complexes show a characteristic metal-to-ligand charge transfer (MLCT) band at around 390 nm, as can be seen in Fig. 3. This mono-complex was converted into a heteroleptic bisterpyridine ruthenium complex with the terpyridinemodified polystyrene under reducing conditions (Scheme 1). However, this process still poses a challenge because harsh conditions are required to reduce the Ru(III) to Ru(II). This usually results in low yields (35–50%) of the bis-terpyridine ruthenium complex due to an incomplete conversion unless highly optimized reaction conditions are employed [13, 31]. Above all, the following purification of the
3.
]-PS50 ]-PS130 ]-PS285
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0.0 5
6
7 8 elution volume / mL
9
Fig. 1. GPC traces of terpyridine-functionalized polystyrene with varying molecular weights.
9
8
7
6
5 4 (ppm)
3
2
1
0
Fig. 2. 1H-NMR spectrum of terpyridine-functionalized polystyrene (PS130-[) in CDCl3.
product is rather difficult when using high molecular weight polymers. For that reason, we were aiming at the optimization of the reaction conditions and the purification of the crude product. Typically, a catalytic amount of N-ethylmorpholine is added to the reaction mixture which is stirred under reflux for several hours. The effectiveness of N-ethylmorpholine was checked thoroughly. Two reactions were started with exactly the same conditions (temperature, volume and concentration); one with the addition of the catalyst and the other without, leading to the conclusion that N-ethylmorpholine has no real influence on the reaction in this particular case. Therefore, all further reactions were performed without the addition of N-ethylmorpholine. In order to compare the following results all reactions were performed by using a
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Fig. 3. GPC-PDA diagram of PEG70-[RuCl3.
n
O
n
N
Cl O
+
Cl
Ru
N
O
O
O 70
THF / MeOH (4:1)
O 2+
90 °C
N N
N
N
Cl N
O
N
N N
N 2 PF6-
Ru N
N N
O
O
O 70
Scheme 1. Schematic representation of the synthesis of the PSn–[Ru]–PEG70 block copolymer.
microwave synthesizer to ensure a better control over all reaction parameters, such as reaction time and temperature. A very important advantage is that a uniform heating throughout the reaction mixture takes place and that a greater reproducibility is guaranteed since all process parameters are controlled and stored. Sealed microwave vials were used for the reaction which can be heated above the boiling temperature of the solvent. First of all, the influence of the applied temperature was investigated.
The progress of the reaction was determined by using GPC as can be seen in Fig. 4. It should be mentioned here that it was crucial for these investigations to utilize an optimized GPC system that suppresses the interaction of the charged supramolecular analytes with the column material [32]. By increasing the temperature the bis-complex was formed to a greater extent. However, it can be also seen that the trace of the starting material was shifting to lower molecular weight which might be explained by decomposition at
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Ott, Wouters, Thijs, and Schubert 110°C, 1 h, 8:1( CHCl3 : MeOH) (1)
PEG70-[RuCl3
110°C, 1 h, 4:1( CHCl3 : MeOH) (2)
microwave, 1 h, 80 °C microwave, 1 h, 90 °C microwave, 1 h, 110 °C
0.8
110°C, 1 h, 8:1( CHCl3 : MeOH), high conc. (3) 110°C, 1 h, 4:1( CHCl3 : MeOH), high conc. (4)
1.2
0.6 normalised RI signal
normalized RI signal
1.0
0.4 0.2 0.0 15
16
17
18 19 time / min
20
21
22
Fig. 4. GPC traces of the block copolymer formation at varying temperatures (CHCl3).
too high temperatures. The efficiency of the complexation reaction was determined by UV-vis spectroscopy as well. By increasing the reaction temperature an increasing MLCT band at around 490 nm (formation of the bis-complex) and a decreasing MLCT band at 390 nm (starting material) were observed (Fig. 5). The GPC curves do not reveal more bis-complex formation when applying longer reaction times, only enhanced decomposition of the terpyridine ruthenium monocomplex. The concentration of the sample plays a significant role; the higher the concentration the better the yield of the biscomplex. For the previous discussed reaction a solvent mixture of chloroform and methanol (8:1) was utilized. We have found that an increased amount of methanol, which is functioning as reducing agent, also promotes the bis-complex formation. However, the amount of methanol cannot be increased too much because methanol is a precipitant
normalized RI signal
2.0
microwave, 1 h, 80 °C microwave, 1 h, 90 °C microwave, 1 h, 110 °C
1.5
1.0
0.5
350
400
450 500 wavelength / nm
550
600
Fig. 5. UV-vis spectra of the block copolymer formation at varying temperatures.
1.0 0.8 0.6 0.4 0.2 0.0 15
16
17
18
19 20 time / min
21
22
23
Fig. 6. GPC traces taken during the optimization of the reaction parameters (DMF).
for a wide range of polymers. With the optimized parameters the conversion was raised up to 73% (Fig. 6). Instead of chloroform also THF can be used in a mixture with methanol (4:1) which provides slightly better conversions. All reactions up to this point were carried out at 110 C; however, later on the temperature was reduced to 90 C because the GPC trace of the starting material shifted less to lower molecular weight. Unfortunately, the standard purification of the obtained polymers by preparative size exclusion chromatography (Bio-beads) did not work well due to the maximum exclusion limit of these materials of 14,000 g mol)1. Also several re-precipitations and extractions were unsuccessful and silica columns could not be applied because of too strong interactions with the column material. We finally succeeded to purify the PSn–[Ru]–PEG70 block copolymers by using preparative size exclusion chromatography with automated fractionation. As it has already been described in the literature [32] the GPC analysis of bis-terpyridine metal-complexes is rather difficult which is mainly due to strong interactions with the column material (caused by the nitrogen atoms and the charged complex). Pure solvents often lead to fragmentation of the metal–ligand bond. Sometimes even oxidation to the Ru(III) monocomplex has been observed. But with the adjustment of temperature, flow rate and usage of additives like NH4PF6 nearly all column interactions were suppressed. However, N,NÕ-dimethylformamide (DMF) is not a recommendable candidate for the fractionation via SEC/GPC because of its low volatility. For
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this reason, the fractionation was carried out with tetrahydrofuran (THF). 50 mg of the corresponding block copolymer were dissolved in 1 ml of THF and injected onto a PSS Gram preparative 100 A˚ column. Fractions were taken in equal time intervals and collected with a fraction collector. The collected fractions were then measured with a conventional GPC with DMF as eluent. An example of the GPC traces before and after fractionation can be seen in Fig. 7. Due to the amphiphilic character of the obtained block copolymers, micelles were prepared and investigated by means of dynamic light scattering (DLS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). In AFM the diameter of the micelle can be estimated from the observed height [33]. The metallo-supramolecular block copolymer is insoluble in water. Therefore, it was first dissolved in the unselective solvent DMF and afterwards an excess of deionized water was added dropwise to the solution. The solubility of the polystyrene block decreases with increasing amount
(a)
before fractionation after fractionation PS130-[Ru]-PEG70
RI signal normalized
1.0 0.8 0.6 0.4 0.2 0.0
8
normalized RI signal
(b) 1.0
9 10 elution volume / mL
11
of water leading to the formation of aggregates. In the beginning there is a thermodynamic equilibrium between single polymer chains and micelles. However, the structure becomes kinetically frozen when more water is added [34]. Ammonium hexafluorophosphate (NH4PF6), which is an additive in the eluent of the GPC, and the residual unselective solvent were then effectively removed by dialysis. DLS was applied to characterize the micelles in solution. It has been already shown that micelles prepared from metallo-supramolecular block copolymers tend to aggregate [34, 35]. In fact, two different populations were observed with the CONTIN size distribution analysis: the first peak corresponds to single micelles and small aggregates and the second very broad one is attributed to large aggregates. A mean hydrodynamic radius (Dh) of 93 nm was observed for the ‘‘crew-cut’’ behaving PS285–[Ru]– PEG70 micelle. With decreasing polystyrene length, also decreasing values for the mean Dh were found. However, these analyzed values are not conclusive enough due to aggregation and the swollen condition of the micelle. Therefore, we used AFM and TEM imaging to characterize the micelles. For AFM measurements, the imaging was performed in dry state and the micellar solution was deposited on mica. The height of the measured micelles approximates the size of the core for two reasons: firstly, the flexible PEG chains of the corona should collapse upon itself and secondly, the short length of the PEG chains is not expected to have an influence on the size of the micelle, especially for larger PS chains. For TEM imaging, no contrasting agent was used to visualize the micelles. Spherical micelles were observed for the
PS285-[ PS285-[Ru]-PEG70
0.8 0.6 0.4 0.2 0.0 8
9
10
11
elution volume / mL
Fig. 7. (a) GPC traces of PS130–[Ru]–PEG70 before and after fractionation (DMF); (b) GPC traces of PS285-[ and PS285–[Ru]– PEG70 before and after the complexation reaction (DMF).
Fig. 8. AFM height images of PS285–[Ru]–PEG70 (a), PS130–[Ru]– PEG70 (b) and PS50–[Ru]–PEG70 (c) drop-casted on mica (blown dry under a stream of nitrogen).
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Fig. 9. TEM images of the unstained PS285–[Ru]–PEG70 micelles in water (left) andTEM images of the unstained PS130–[Ru]–PEG70 micelles in water.
three PSn–[Ru]–PEG70 block copolymers. Figure 8a and 9 (left) represent AFM and TEM images of the PS285–[Ru]–PEG70 block copolymer. The AFM image exemplifies clustering of single micelles that exhibit an average height of 55 nm. The left TEM picture shows once more the evidence that metallosupramolecular micelles form large aggregates. The size of the micelles is rather uniform; however also much larger micelles are formed as can be seen in the right TEM image. The micelle size determined by TEM is approximately 55 nm. For PS130–[Ru]– PEG70 a size of 40 nm by TEM (see Fig. 9) and 35 nm by AFM could be detected, respectively. The aggregation numbers for the above discussed micelles have been determined using the density of amorphous polystyrene (0.95 g/cm3). PS285–[Ru]–PEG70 reveals an aggregation number of 1650, and PS130–[Ru]– PEG70 1360, respectively. The values are in good agreement with those described in literature [36]. For the sample with the smallest poly(styrene) block length (PS50–[Ru]–PEG70) no stable micelles were observed on the hydrophilic TEM-grid nor on the hydrophilic mica support; no reliable results were obtained with these techniques, although hints of spherical objects were found. Apart from this aspect the data obtained by both techniques are in good agreement.
terpyridine-functionalized polymers. The optimization of this complexation reaction with ruthenium was successful. Three PSx–[Ru]–PEG70 block copolymers have been obtained with conversions up to 73% using the optimized parameters (1 h, 90 C, chloroform or THF and methanol (4:1)). It has been shown that fractionation via SEC/GPC is a powerful tool to purify metallo-supramolecular polymers. In the future we will apply the preparative GPC to more complicated systems such as A–B–[Ru]–C block copolymers. On the basis of different solubilities of the two blocks, micellar structures have been formed and measured by AFM, TEM and DLS. The measured sizes of the micelles by AFM are comparable with those obtained by TEM. Depending on the length of the poly(styrene) block the size of the micelle could be varied. We are currently focusing on the preparation of block copolymers which differ in their volume fraction for morphology studies. ACKNOWLEDGMENTS The authors thank the Dutch Counsel for Scientific Research (NWO, VICI award for USS) and the Fonds der Chemischen Industrie for funding. This research has been carried out with the support of the Soft Matter Cryo-TEM Research Unit, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology.
4. CONCLUSIONS Terpyridine-functionalized polystyrene with varying molecular weights (Mn = 5,200; 13,500 and 29,600 g mol)1) have been prepared by NMRP. The well-known chelating properties of the terpyridine– ligand allowed the controlled introduction of other
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