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Synthesis and characterization of amphiphilic star copolymers of 2-(dimethylamino)ethyl methacrylate and methyl methacrylate: Effects of architecture and composition
EUROPEAN POLYMER JOURNAL
European Polymer Journal 43 (2007) 84–92
www.elsevier.com/locate/europolj
Synthesis and characterization of amphiphilic star copolymers of 2-(dimethylamino)ethyl methacrylate and methyl methacrylate: Effects of architecture and composition Efrosyni Themistou, Costas S. Patrickios
*
Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus Received 16 August 2006; accepted 12 October 2006
Abstract Six amphiphilic star copolymers comprising hydrophilic units of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and hydrophobic units of methyl methacrylate (MMA) were prepared by the sequential group transfer polymerization (GTP) of the two comonomers and ethylene glycol dimethacrylate (EGDMA) cross-linker. Four star-block copolymers of different compositions, one miktoarm star, and one statistical copolymer star were synthesized. The molecular weights (MWs) and MW distributions of all the star copolymers and their linear homopolymer and copolymer precursors were characterized by gel permeation chromatography (GPC), while the compositions of the stars were determined by proton nuclear magnetic resonance (1H NMR) spectroscopy. Tetrahydrofuran (THF) solutions of all the star copolymers were characterized by static light scattering to determine the absolute weight-average MW (M w ) and the number of arms of the stars. The M w of the stars ranged between 359,000 and 565,000 g mol 1, while their number of arms ranged between 39 and 120. The star copolymers were soluble in acidic water at pH 4 giving transparent or slightly opaque solutions, with the exception of the very hydrophobic DMAEMA10-b-MMA30-star, which gave a very opaque solution. Only the random copolymer star was completely dispersed in neutral water, giving a very opaque solution. The effective pKs of the copolymer stars were determined by hydrogen ion titration and were found to be in the range 6.5–7.6. The pHs of precipitation of the star copolymer solutions/dispersions were found to be between 8.8–10.1, except for the most hydrophobic DMAEMA10-b-MMA30-star, which gave a very opaque solution over the whole pH range. 2006 Elsevier Ltd. All rights reserved. Keywords: Star copolymers; Group transfer polymerization; 2-(Dimethylamino)ethyl methacrylate; Methyl methacrylate; Ethylene glycol dimethacrylate; Water-soluble polymers; Polyelectrolytes
1. Introduction Non-linear polymers attract much of the attention of the Polymer Community due to their unique struc*
Corresponding author. Fax: +357 22 892801. E-mail addresses: [email protected] (E. Themistou), [email protected] (C.S. Patrickios).
ture and properties. These materials include hyperbranched polymers, dendrimers and star polymers [1]. Hyperbranched polymers are usually extended structures with random branching [2]. On the other extreme is the compact and perfectly branched structure of dendrimers [3]. In between these two extremes are star polymers, having a fairly compact structure and a single branching point from which several
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E. Themistou, C.S. Patrickios / European Polymer Journal 43 (2007) 84–92
linear chains, or arms, emanate [4]. Although first reported in the 1940s [5], the synthesis of star polymers started to develop fast in the late 1950s with the advent of living anionic polymerization [6]. Since then, in addition to anionic polymerization, all other controlled polymerization techniques have also been used for the preparation of star polymers, including cationic [7], atom transfer radical (ATRP) [8], reversible addition-fragmentation chain transfer (RAFT) [9] and group transfer polymerization (GTP) [10]. Star copolymers comprise two different types of monomer repeating units, which can be distributed in the star copolymers in various ways. One possibility is that the different monomer units form linear copolymers, e.g. diblock copolymers, which constitute the arms of the star [11]. In this case, all the branches of the star copolymers are the same (‘‘homoarm’’ star copolymers). It is also possible that the two types of monomer repeating units are introduced in the star as two different types of homopolymer arms. In this case, the stars are called heteroarm or miktoarm stars [12]. There are also several reports on three-component star polymers (star terpolymers), most of which are of the ABC miktoarm type [13], but there are also some rare examples of the ABC triblock homoarm type [14]. Most of the star copolymer reports involve bulk morphology studies of water-incompatible materials, with very few investigations on water-soluble star copolymers. The purpose of the present work was to prepare and study a series of amphiphilic star copolymers. For the polymer synthesis, the controlled polymerization method on which our Research Team has expertise, GTP [15–19], was followed. GTP restricted the monomer choice to methacrylates. However, the similar reactivities of methacrylates toward GTP allowed the preparation of several star architectures, including AB and BA star block, miktoarm star, and statistical star. The star copolymer composition for the AB star block architecture was also varied.
acrylate (DMAEMA) and the neutral hydrophobic methyl methacrylate (MMA). The cross-linker used was ethylene glycol dimethacrylate (EGDMA), whereas 1-methoxy-1-trimethylsiloxy-2-methyl propene (MTS) was employed as the initiator. The polymerization catalyst was tetrabutylammonium bibenzoate (TBABB), synthesized by the reaction of tetrabutylammonium hydroxide with benzoic acid, according to the method of Dicker et al. [17]. Tetrahydrofuran (THF) was used as the polymerization solvent (reagent grade) and as the mobile phase in gel permeation chromatography (HPLC grade). 2.2. Methods The monomers and the cross-linker were passed twice through basic alumina columns to remove inhibitors and protic impurities. They were subsequently stirred over calcium hydride (to remove the last traces of moisture and protic impurities) in the presence of an added free-radical inhibitor, 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH), and stored in the refrigerator at about 5 C. They were all freshly distilled under vacuum just before their use and kept under a dry nitrogen atmosphere. The initiator was distilled once prior to the polymerization, but it was neither contacted with calcium hydride nor passed through basic alumina columns
H 2C
H 2C
H2C
O
O
CH 3
CH 2
H 3C
CH3
O
Methyl methacrylate (MMA)
N CH3
2-(Dimethylamino)ethyl methacrylate (DMAEMA)
2. Experimental section O
All chemicals were commercially available and were purchased from Aldrich, Germany. Fig. 1 shows the chemical structures and names of the main reagents used for the synthesis of the star copolymers. The two monomers used were the hydrophilic ionizable 2-(dimethylamino)ethyl meth-
CH 3
O
H3C
2.1. Materials
85
C C
CH2 O
H2C O
C C
CH2 CH2 Ethylene glycol dimethacrylate (EGDMA)
CH3
H3C H3C
O
CH3
OO H3C Si CH3 CH3
1-Methoxy-1-(trimethylsiloxy)2-methyl propene(MTS)
Fig. 1. Chemical structures and names of the main reagents used for the star copolymer synthesis: the monomers MMA and DMAEMA, the cross-linker EGDMA, and the monofunctional initiator MTS.
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E. Themistou, C.S. Patrickios / European Polymer Journal 43 (2007) 84–92
because of the risk of hydrolysis. The dried catalyst powder was stored in a round-bottom flask under vacuum until use. The polymerization solvent was THF, which was dried by refluxing it over a potassium–sodium mixture for three days and was vacuum distilled just before its use. All glassware was dried overnight at 120 C and assembled hot under dynamic vacuum prior to use. 2.3. Polymer synthesis The method used for the synthesis of the star copolymers was GTP [15–19]. The reactions were carried out at ambient temperature (20 C) without thermostating the polymerization reactor. The polymerization exotherm was monitored by a digital thermometer and was used to follow the progress of the reaction. The polymerizations were carried out in 100 ml round bottom flasks, fitted with a rubber septum. Catalytic amounts (10 mg) of TBABB were transferred to the flask, which was immediately purged with dry nitrogen. Freshly distilled THF was subsequently transferred directly into the flask via a glass syringe, followed by the addition of the initiator. Finally, the monomers and the cross-linker were added in the appropriate order. The various sequences of addition of the reagents for the synthesis of all the star structures are shown in Fig. 2. The preparation of star-block copolymers (polymers 1, 2, 3 and 6 of Table 1) required the sequential addition
of the two different monomers before the addition of the EGDMA cross-linker, while the miktoarm crosslinked star (polymer 4) was prepared when the crosslinker was polymerized between the two monomers. Finally, the statistical copolymer star (polymer 5) was prepared by the simultaneous addition of the two different monomers, which resulted in the preparation of arms of statistical architecture, before the addition of the cross-linker. A typical polymerization yielding the star-block copolymer with arms comprising linear diblock copolymers with 30 DMAEMA units and 10 MMA units is detailed next. TBABB catalyst (10 mg, 20 lmol), freshly distilled THF (37 ml) and MTS initiator (0.36 ml, 0.31 g, 1.77 mmol) were added, in this order, to a 100 ml round-bottom flask. Subsequently, DMAEMA (9.0 ml, 8.40 g, 53.2 mmol) was added slowly under stirring, giving a polymerization exotherm (27.8–40.3 C), which abated within 5 min. A sample (0.1 ml) was extracted from the reaction solution for GPC analysis, and MMA (1.9 ml, 1.78 g, 17.7 mmol) was added slowly. The reaction temperature rose from 35.6 to 38.5 C. After sampling for GPC, EGDMA (1.35 ml, 1.42 g, 7.09 mmol) was added, which produced an exotherm (35.3– 37.1 C), and a sample was again withdrawn for GPC analysis. The star copolymers formed were recovered by precipitation in n-hexane and dried for three days in a vacuum oven at room temperature.
TBABB 30 DMAEMA 10 MMA 4 EGDMA DMAEMA 30 1. THF DMAEMA30 b MMA10 (DMAEMA 30 - b - MMA10 )- star 1 MTS TBABB 20 DMAEMA 20 MMA 4 EGDMA DMAEMA 20 2. THF DMAEMA20 b MMA20 (DMAEMA20 - b -MMA 20 )- star 1 MTS TBABB 10 DMAEMA 30 MMA DMAEMA10 3. THF 1 MTS TBABB 30 DMAEMA 4 EGDMA DMAEMA30 4. THF 1 MTS
DMAEMA10 b MMA30
DMAEMA30-star
4 EGDMA
10 MMA
(DMAEMA10- b -MMA 30 )- star
DMAEMA30- star - MMA10
TBABB 30 DMAEMA + 10 MMA 4 EGDMA (DMAEMA30- co-MMA10)-star DMAEMA30-co -MMA10 5. THF 1 MTS TBABB 10 MMA 30 DMAEMA 4 EGDMA MMA10 MMA10-b-DMAEMA30 (MMA10-b-DMAEMA30)- star 6. THF 1 MTS
Fig. 2. Synthetic sequences employed for the preparation of the star copolymers of this study. TBABB = tetrabutylammonium bibenzoate, polymerization catalyst; THF = tetrahydrofuran, polymerization solvent; MTS = 1-methoxy-1-(trimethylsiloxy)-2-methyl propene, monofunctional initiator; DMAEMA = 2-(dimethylamino)ethyl methacrylate and MMA = methyl methacrylate, monomers; EGDMA = ethylene glycol dimethacrylate, cross-linker.
No.
Polymer formula
Theor. MWa
GPC results
SLS results
Mn
Mp
Mw/Mn
Mw
Composition (mol% DMAEMA) No. of arms
Theor.
By 1H NMR
1
DMAEMA30 DMAEMA30-b-MMA10 DMAEMA30-b-MMA10-star
4820 5820 N/Ab
5630 6890 707,00
6100 7430 88,700
1.09 1.10 1.19
401,000
46
75.0
72.9
2
DMAEMA20 DMAEMA20-b-MMA20 DMAEMA20-b-MMA20-star
3250 5250 N/Ab
3500 6370 72,300
3780 7020 83,800
1.10 1.12 1.22
565,000
69
50.0
46.8
3
DMAEMA10 DMAEMA10-b-MMA30 DMAEMA10-b-MMA30-star
1670 4680 N/Ab
1470 5920 84,000
2090 6630 86,200
1.10 1.12 1.27
495,000
64
25.0
24.2
4
DMAEMA30 DMAEMA30-star DMAEMA30-star-MMA10
4820 N/Ab N/Ab
5100 41,700 49,800
5600 63,200 68,800
1.09 1.12 1.28
468,000
120
75.0
71.8
5
DMAEMA30-co-MMA10 DMAEMA30-co-MMA10-star
5820 N/Ab
7330 60,800
8310 86,200
1.10 1.25
6
MMA10 MMA10-b-DMAEMA30 MMA10-b-DMAEMA30-star
1100 5820 N/Ab
1170 8380 63,700
1720 9300 91,200
1.09 1.10 1.26
515,000
49
75.0
72.3
a b
The molecular weight of the initiator fragment of 100 g mol Not applicable.
1
has also been included in the calculation.
E. Themistou, C.S. Patrickios / European Polymer Journal 43 (2007) 84–92
Table 1 Molecular weights and compositions of the star copolymers
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3. Characterization in organic solvents 3.1. Gel permeation chromatography (GPC) Molecular weights (MWs) and molecular weight distributions (MWDs) of the star copolymers and all of their precursors were determined by gel permeation chromatography (GPC) using a single Polymer Laboratories PL-Mixed ‘‘D’’ column. The mobile phase was tetrahydrofuran (THF, flow rate 1 mL min 1), delivered using a Polymer Laboratories PL-LC1120 isocratic pump. The refractive index signal was measured using an ERC-7515A refractive index detector supplied by Polymer Laboratories. The calibration curve was based on seven narrow MW (630, 4250, 13,000, 28,900, 50,000, 128,000 and 260,000 g mol 1) linear poly(MMA) standards supplied by Polymer Laboratories, which provided accurate MW calculations for the linear precursors but only qualitative estimations for the MWs of the star copolymers. In particular, the following quantities were calculated: the number-average MWs, M n s, the polydispersity indices (PDIs, M w =M n ), where M w is the weight-average MW, and the peak MWs, Mps, which are the MWs at the peak maximum. 3.2. 1H NMR spectroscopy The compositions of the star copolymers were determined by proton nuclear magnetic resonance (1H NMR) spectroscopy, using a 300 MHz Avance Bruker spectrometer equipped with an Ultrashield magnet. The NMR solvent was deuterated chloroform (CDCl3), containing tetramethylsilane (TMS) that was used as an internal reference. 3.3. Static light scattering The absolute M w s of the star copolymers were measured by static light scattering (SLS) in THF using a Brookhaven BI-MwA Molecular Weight Analyzer equipped with a 30 mW red diode laser operating at 673 nm and a multi-angle detector, measuring simultaneously the intensity of scattered light at 7 different angles, 35, 50, 75, 90, 105, 130 and 145. Polymer samples were dissolved in HPLC-grade THF at different polymer concentrations and were filtered through 0.45 lm pore size syringe filters. From the scattered intensities, the software of the instrument constructed automatically a Zimm plot from which the absolute M w
was calculated as the combined extrapolation to zero angle and zero concentration. The refractive index increments (dn/dc) of 5% w/v star copolymer solutions in THF were determined using an ABBE refractometer. 4. Aqueous solution characterization 1% w/w aqueous salt-free solutions of the star copolymers were characterized in terms of their water-solubilities and effective pKs using solubility tests, and hydrogen ion titration, respectively. 4.1. Solubility in water Five grams of deionized water were added to 50 mg of each star copolymer. The mixture was stirred and observed for complete copolymer dissolution. 4.2. Hydrogen ion titration Five grams of 1% w/w solutions/dispersions of each polymer were titrated from pH 2 to 12 using a standard NaOH 0.5 M solution under continuous stirring. The pHs were measured using a Corning PS30 portable pH meter. The pHs of precipitation were determined by visually observing the turbidity. The effective pKs were calculated as the pH at 50% ionization. 5. Results and discussion 5.1. Synthetic strategy and structure of the star copolymers The synthesis of the star copolymers was accomplished by the GTP of the appropriate monomer (DMAEMA or MMA), initiated by a monofunctional initiator, followed by the in situ cross-linking of the formed polymer at one end to a star, effected by the polymerization of a difunctional methacrylate, EGDMA. Six star copolymers with overall degrees of polymerization (DPs) of the arms of 40 were prepared using this synthetic strategy. By changing the ratio of the monomers used in the synthesis, star copolymers with different compositions resulted, whereas by changing their addition sequence, star copolymers with different structures were synthesized. Schematic representations of the structures of the six star copolymers synthesized in this report are shown in Fig. 3. Four star-block
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89
5.2. Characterization in organic solvents
DMAEMA 20-b-MMA 20-star
DMAEMA10 -b-MMA 30 -star
DMAEMA 30-star-MMA 10
DMAEMA30 -co-MMA 10 -star
MMA 10 -b-DMAEMA30 -star
Fig. 3. Schematic representation of the structures of the star copolymers of this study. The DMAEMA units are depicted in white, while the MMA units are colored black. The core of the stars, consisting of EGDMA units, is also colored black.
copolymers of different compositions, one miktoarm star, and one statistical copolymer star were synthesized. The linear ‘‘living’’ copolymer precursor to a star-diblock copolymer was produced by the sequential addition of two methacrylate monomers to the initial solution of THF, monofunctional initiator and catalyst, whereas the linear precursor to the statistical star was produced by the simultaneous addition of the two monomers. For the formation of a miktoarm star, a linear ‘‘living’’ homopolymer was formed first by the addition of only one monomer to the initial solution, followed by the addition of the dimethacrylate cross-linker to form a homopolymer star, and completed by the addition of another monomer, different from the first one, to form the final miktoarm star.
DMAEMA 20-b-MMA20 -star DMAEMA20 DMAEMA 20-b-MMA 20
refractive index signal
DMAEMA 30 -b-MMA 10 -star
5.2.1. Molecular weights of the star copolymers The MWs and MWDs of the star polymers were characterized using GPC. Fig. 4 presents the GPC chromatograms of the star copolymer DMAEMA20-b-MMA20-star and its precursors. Generally, the GPC chromatograms of the star copolymers exhibited two peaks, one due to the star polymer, and one due to some of the constituting linear copolymer, which had remained unattached (free arm). Incomplete incorporation of the linear copolymers into the stars is due to increased solution viscosity, lower chain mobility and possible chain termination/chain transfer. We wish to stress at this point that, due to the use of linear calibration standards, the MWs determined by GPC were only rough estimates of the true MWs of the star copolymers. Accurate values of the MWs of the star copolymers were determined using SLS. The apparent M n s and PDIs of all star copolymers and their precursors were calculated from the GPC chromatograms and are shown in Table 1 along with the theoretical MWs of the linear precursors. The apparent M n s of the star copolymers ranged between 49,800 and 84,000 g mol 1. The PDIs of all samples, linear and star polymers, were relatively low, below 1.3. The linear homopolymers and copolymers displayed unimodal and narrow MWDs, with PDIs equal to or lower than 1.12. Generally, with each successive addition of a monomer or the cross-linker to a ‘‘living’’ linear homo- or copolymer, or star (co)polymer, the M n s of the polymer products grew and their PDIs increased. The
0 5
6
7 elution time (min)
8
9
Fig. 4. GPC chromatograms of the star copolymer DMAEMA20-b-MMA20-star and its linear homopolymer and diblock copolymer precursors.
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M n s of the linear precursors were in most cases higher than the theoretically predicted, indicating some initiator deactivation. SLS on THF solutions of the synthesized star copolymers allowed the determination of their absolute M w s and weight-average number of arms, both presented in Table 1. The M w s of the star copolymers determined by SLS were greater than their corresponding GPC M w s calculated using a MW calibration based on linear polyMMA standards, due to the compact nature of the star structure. The weight-average numbers of arms of the star block and the statistical star copolymers were calculated by subtracting the MW corresponding to the cross-linker from the M w s of the star copolymers determined by SLS, and by dividing the calculated value by the M w s of their arms determined by GPC. The weight-average numbers of arms for these star copolymers were found to be between 39 and 69, within the same range presented by star polymers prepared by GTP [14,20–23]. The calculation of the number of arms of the miktoarm star copolymer DMAEMA30-starMMA10 was carried out in a different way because of the difference between the structure of this copolymer and the other star copolymers. While all other star copolymers were homo-arm star copolymers, composed of arms with the same DP and composition, the present miktoarm star bore two different types of arms: DMAEMA homopolymers with a DP of 30 and MMA homopolymers with a DP of 10. The ‘‘livingness’’ of GTP dictates that the number of these two different types of arms be the same. For the calculation of the number of arms for this star copolymer, first, the MW due to the EGDMA core was subtracted from its M w determined by SLS, and the resulting MW value was divided by an effective MW of a combined DMAEMA-MMA arm. This effective MW was calculated by multiplying the GPC M w of the linear polyDMAEMA by the appropriate factor, derived from the DMAEMA/ MMA composition [=(30 · 157 + 10 · 100)/ (30 · 157)]. 5.2.2. Star copolymer composition Fig. 5 shows the 1H NMR spectrum of the miktoarm star copolymer DMAEMA30-starMMA10. The characteristic peak for the MMA units was that of the three methoxy protons at 3.6 ppm (peak c), while that for the DMAEMA units was taken that of the two oxymethylene protons at 4.1 ppm (peak d). The star copolymer com-
Fig. 5. 1H NMR spectrum of the miktoarm star copolymer DMAEMA30-star-MMA10.
positions were calculated by ratioing the normalized areas of the characteristic peaks of the two monomer units, and are listed in Table 1. These compositions are close to the theoretically expected compositions calculated from the comonomer feed ratios. 5.3. Characterization in aqueous solutions Table 2 displays the results from the aqueous solution characterization of all the star copolymers, including their solubilities at various pHs and the pKs of their DMAEMA units. 5.3.1. Solubility in neutral and acidic water Table 2 shows that none of the star copolymers was readily soluble in neutral water to give a transparent solution. After stirring its slurry (50 mg) in neutral water (5 g) and leaving the mixture in the refrigerator for 2 days, the statistical star copolymer DMAEMA30-co-MMA10-star was the only one to be completely dispersed, giving, however, a very opaque solution. The star-block copolymer DMAEMA10-b-MMA30-star was not dissolved in neutral water at all due to its high hydrophobicity, whereas the MMA10-b-DMAEMA30-star was precipitated leaving the solution transparent because this star copolymer has its hydrophobic units in its periphery, making difficult its dissolution. The other star copolymers were only partially dispersed in neutral water, giving both opaque solutions and precipitates. The pH of dissolution/dispersion in neutral water for all the star copolymers was found to range between 7 and 9.
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Table 2 Dissolution and precipitation pHs, effective pKs, and solubility tests of the star copolymers at various pHs No.
Polymer formula
Solubility testsa pHdissolution
1 2 3 4 5 6
DMAEMA30-b-MMA10-star DMAEMA20-b-MMA20-star DMAEMA10-b-MMA30-star DMAEMA30-star-MMA10 DMAEMA30-co-MMA10-star MMA10-b-DMAEMA30-star
sp + os sp + os p sp + os vos p + ts
b
pH = 2
pH = 4
ts ts vos ts ts ts
ts sos vos ts sos ts
pHdissolution
pHprecipitationa
pK
8.8 8.2 7.2 8.8 9.0 8.8
10.0 10.1 vos in all pH range 9.6 8.8 9.6
6.7 6.7 6.5 7.0 7.1 7.6
a ts, transparent solution; os, opaque solution; sos, slightly opaque solution; vos, very opaque solution; p, precipitate; sp, some precipitate. b After 2 days in the refrigerator.
Most of the star copolymers were soluble in acidic water of pH 4, with the exception of DMAEMA10-b-MMA30-star, which gave a very opaque and milky solution, due to its high hydrophobicity. The equimolar star copolymer DMAEMA20-bMMA20-star and the statistical star copolymer DMAEMA30-co-MMA10-star were dissolved in water at pH 4, giving slightly opaque solutions. All the star copolymers dissolved completely in water of final pH 2 and gave transparent solutions, with the exception of the most hydrophobic star copolymer which was fully dispersed but gave a milky solution. 5.3.2. Effective pKs and precipitation pHs The results of the hydrogen ion titrations of aqueous solutions of the star copolymers are also summarized in Table 2. The effective pKs of the DMAEMA units, an ionizable tertiary amine, in all star copolymers ranged between 6.5 and 7.6, values that are close to the pK value of the DMAEMA homopolymer, of approximately 7. Precipitation occurred for all the star copolymer solutions in pHs between 9 and 10, except for the most hydrophobic star polymer, DMAEMA10-b-MMA30-star, which gave a very opaque solution over the whole pH range. For all the other star copolymers, as the pH increased, the DMAEMA units in the polymer become deprotonated and lost their charge, resulting in precipitation. This tendency for precipitation was enhanced by the presence of the hydrophobic MMA units. 6. Conclusions The successful synthesis of amphiphilic star copolymers comprising hydrophilic units of DMAEMA and hydrophobic units of MMA of various
compositions and structures (block, miktoarm and statistical) was accomplished by GTP using sequential monomer(s) and cross-linker additions. The successful synthesis of the star copolymers was confirmed by their characterization using GPC and 1H NMR. 1H NMR indicated that the star copolymer compositions were close to those theoretically expected. The absolute weight-average molecular weight determined by static light scattering in THF ranged between 359,000 and 565,000 g mol 1, while the number of arms of the star copolymers ranged between 39 and 120. The statistical copolymer star DMAEMA30-co-MMA10-star was the only one, which was fully dispersible in neutral water, giving an opaque solution. All star copolymers were soluble in acidic water at pH 4, giving transparent or slightly opaque solutions, except for DMAEMA10-b-MMA30-star, which gave a very opaque solution due to its high hydrophobicity. Their effective pKs ranged between 6.5 and 7.6 and their precipitation pHs were found to be between 9 and 10, with the precipitation pH of the most hydrophobic DMAEMA10-b-MMA30-star being indeterminate due to the high opacity of its dispersion over the whole pH range. Acknowledgments The Cyprus Research Promotion Foundation is gratefully acknowledged for providing financial support to E.T. via a PENEK 2001 Project. The A.G. Leventis Foundation is also thanked for a generous donation that enabled the purchase of the NMR spectrometer of the University of Cyprus. The University of Cyprus Research Committee (Grant 2000–2003) is thanked for providing a grant that enabled the purchase of the static light scattering spectrometer.
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[13] Iatrou H, Hadjichristidis N. Synthesis of a model 3-miktoarm star terpolymer. Macromolecules 1992;25(18):4649–51. [14] Triftaridou AI, Vamvakaki M, Patrickios CS, Stavrouli N, Tsitsilianis C. Synthesis of amphiphilic (ABC)n multiarm star triblock terpolymers. Macromolecules 2005;38(3): 1021–4. [15] Webster OW, Hertler WR, Sogah DY, Farnham WB, RajanBabu TV. Group-transfer polymerization. 1. A new concept for addition polymerization with organosilicon initiators. J Am Chem Soc 1983;105(17):5706–8. [16] Sogah DY, Hertler WR, Webster OW, Cohen GM. Grouptransfer polymerization. Polymerization of acrylic monomers. Macromolecules 1987;20(7):1473–88. [17] Dicker IB, Cohen GM, Farnham WB, Hertler WR, Laganis ED, Sogah DY. Oxyanions catalyze group-transfer polymerization to give living polymers. Macromolecules 1990;23(18): 4034–41. [18] Webster OW. The discovery and commercialization of group transfer polymerization. J Polym Sci, Polym Chem 2000; 38(16):2855–60. [19] Webster OW. Group transfer polymerization: mechanism and comparison with other methods for controlled polymerization of acrylic monomers. Adv Polym Sci 2004;167:1–34. [20] Haddleton DM, Crossman MC. Synthesis of methacrylic multi-arm star copolymers by ’’arm-first’’ group transfer polymerisation. Macromol Chem Phys 1997;198(3):871–81. [21] Vamvakaki M, Hadjiyannakou SC, Loizidou E, Patrickios CS, Armes SP, Billingham NC. Synthesis and characterization of novel networks with nano-engineered structures: Cross-linked star homopolymers. Chem Mater 2001;13(12): 4738–44. [22] Vamvakaki M, Patrickios CS. Synthesis and characterization of electrolytic amphiphilic model networks based on crosslinked star polymers: Effect of star architecture. Chem Mater 2002;14(4):1630–8. [23] Hadjiyannakou SC, Triftaridou AI, Patrickios CS. Synthesis, characterization and evaluation of amphiphilic star copolymeric emulsifiers based on methoxy hexa(ethylene glycol) methacrylate and benzyl methacrylate. Polymer 2005; 46(8):2433–42.
European Polymer Journal 43 (2007) 84–92
www.elsevier.com/locate/europolj
Synthesis and characterization of amphiphilic star copolymers of 2-(dimethylamino)ethyl methacrylate and methyl methacrylate: Effects of architecture and composition Efrosyni Themistou, Costas S. Patrickios
*
Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus Received 16 August 2006; accepted 12 October 2006
Abstract Six amphiphilic star copolymers comprising hydrophilic units of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and hydrophobic units of methyl methacrylate (MMA) were prepared by the sequential group transfer polymerization (GTP) of the two comonomers and ethylene glycol dimethacrylate (EGDMA) cross-linker. Four star-block copolymers of different compositions, one miktoarm star, and one statistical copolymer star were synthesized. The molecular weights (MWs) and MW distributions of all the star copolymers and their linear homopolymer and copolymer precursors were characterized by gel permeation chromatography (GPC), while the compositions of the stars were determined by proton nuclear magnetic resonance (1H NMR) spectroscopy. Tetrahydrofuran (THF) solutions of all the star copolymers were characterized by static light scattering to determine the absolute weight-average MW (M w ) and the number of arms of the stars. The M w of the stars ranged between 359,000 and 565,000 g mol 1, while their number of arms ranged between 39 and 120. The star copolymers were soluble in acidic water at pH 4 giving transparent or slightly opaque solutions, with the exception of the very hydrophobic DMAEMA10-b-MMA30-star, which gave a very opaque solution. Only the random copolymer star was completely dispersed in neutral water, giving a very opaque solution. The effective pKs of the copolymer stars were determined by hydrogen ion titration and were found to be in the range 6.5–7.6. The pHs of precipitation of the star copolymer solutions/dispersions were found to be between 8.8–10.1, except for the most hydrophobic DMAEMA10-b-MMA30-star, which gave a very opaque solution over the whole pH range. 2006 Elsevier Ltd. All rights reserved. Keywords: Star copolymers; Group transfer polymerization; 2-(Dimethylamino)ethyl methacrylate; Methyl methacrylate; Ethylene glycol dimethacrylate; Water-soluble polymers; Polyelectrolytes
1. Introduction Non-linear polymers attract much of the attention of the Polymer Community due to their unique struc*
Corresponding author. Fax: +357 22 892801. E-mail addresses: [email protected] (E. Themistou), [email protected] (C.S. Patrickios).
ture and properties. These materials include hyperbranched polymers, dendrimers and star polymers [1]. Hyperbranched polymers are usually extended structures with random branching [2]. On the other extreme is the compact and perfectly branched structure of dendrimers [3]. In between these two extremes are star polymers, having a fairly compact structure and a single branching point from which several
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.10.007
E. Themistou, C.S. Patrickios / European Polymer Journal 43 (2007) 84–92
linear chains, or arms, emanate [4]. Although first reported in the 1940s [5], the synthesis of star polymers started to develop fast in the late 1950s with the advent of living anionic polymerization [6]. Since then, in addition to anionic polymerization, all other controlled polymerization techniques have also been used for the preparation of star polymers, including cationic [7], atom transfer radical (ATRP) [8], reversible addition-fragmentation chain transfer (RAFT) [9] and group transfer polymerization (GTP) [10]. Star copolymers comprise two different types of monomer repeating units, which can be distributed in the star copolymers in various ways. One possibility is that the different monomer units form linear copolymers, e.g. diblock copolymers, which constitute the arms of the star [11]. In this case, all the branches of the star copolymers are the same (‘‘homoarm’’ star copolymers). It is also possible that the two types of monomer repeating units are introduced in the star as two different types of homopolymer arms. In this case, the stars are called heteroarm or miktoarm stars [12]. There are also several reports on three-component star polymers (star terpolymers), most of which are of the ABC miktoarm type [13], but there are also some rare examples of the ABC triblock homoarm type [14]. Most of the star copolymer reports involve bulk morphology studies of water-incompatible materials, with very few investigations on water-soluble star copolymers. The purpose of the present work was to prepare and study a series of amphiphilic star copolymers. For the polymer synthesis, the controlled polymerization method on which our Research Team has expertise, GTP [15–19], was followed. GTP restricted the monomer choice to methacrylates. However, the similar reactivities of methacrylates toward GTP allowed the preparation of several star architectures, including AB and BA star block, miktoarm star, and statistical star. The star copolymer composition for the AB star block architecture was also varied.
acrylate (DMAEMA) and the neutral hydrophobic methyl methacrylate (MMA). The cross-linker used was ethylene glycol dimethacrylate (EGDMA), whereas 1-methoxy-1-trimethylsiloxy-2-methyl propene (MTS) was employed as the initiator. The polymerization catalyst was tetrabutylammonium bibenzoate (TBABB), synthesized by the reaction of tetrabutylammonium hydroxide with benzoic acid, according to the method of Dicker et al. [17]. Tetrahydrofuran (THF) was used as the polymerization solvent (reagent grade) and as the mobile phase in gel permeation chromatography (HPLC grade). 2.2. Methods The monomers and the cross-linker were passed twice through basic alumina columns to remove inhibitors and protic impurities. They were subsequently stirred over calcium hydride (to remove the last traces of moisture and protic impurities) in the presence of an added free-radical inhibitor, 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH), and stored in the refrigerator at about 5 C. They were all freshly distilled under vacuum just before their use and kept under a dry nitrogen atmosphere. The initiator was distilled once prior to the polymerization, but it was neither contacted with calcium hydride nor passed through basic alumina columns
H 2C
H 2C
H2C
O
O
CH 3
CH 2
H 3C
CH3
O
Methyl methacrylate (MMA)
N CH3
2-(Dimethylamino)ethyl methacrylate (DMAEMA)
2. Experimental section O
All chemicals were commercially available and were purchased from Aldrich, Germany. Fig. 1 shows the chemical structures and names of the main reagents used for the synthesis of the star copolymers. The two monomers used were the hydrophilic ionizable 2-(dimethylamino)ethyl meth-
CH 3
O
H3C
2.1. Materials
85
C C
CH2 O
H2C O
C C
CH2 CH2 Ethylene glycol dimethacrylate (EGDMA)
CH3
H3C H3C
O
CH3
OO H3C Si CH3 CH3
1-Methoxy-1-(trimethylsiloxy)2-methyl propene(MTS)
Fig. 1. Chemical structures and names of the main reagents used for the star copolymer synthesis: the monomers MMA and DMAEMA, the cross-linker EGDMA, and the monofunctional initiator MTS.
86
E. Themistou, C.S. Patrickios / European Polymer Journal 43 (2007) 84–92
because of the risk of hydrolysis. The dried catalyst powder was stored in a round-bottom flask under vacuum until use. The polymerization solvent was THF, which was dried by refluxing it over a potassium–sodium mixture for three days and was vacuum distilled just before its use. All glassware was dried overnight at 120 C and assembled hot under dynamic vacuum prior to use. 2.3. Polymer synthesis The method used for the synthesis of the star copolymers was GTP [15–19]. The reactions were carried out at ambient temperature (20 C) without thermostating the polymerization reactor. The polymerization exotherm was monitored by a digital thermometer and was used to follow the progress of the reaction. The polymerizations were carried out in 100 ml round bottom flasks, fitted with a rubber septum. Catalytic amounts (10 mg) of TBABB were transferred to the flask, which was immediately purged with dry nitrogen. Freshly distilled THF was subsequently transferred directly into the flask via a glass syringe, followed by the addition of the initiator. Finally, the monomers and the cross-linker were added in the appropriate order. The various sequences of addition of the reagents for the synthesis of all the star structures are shown in Fig. 2. The preparation of star-block copolymers (polymers 1, 2, 3 and 6 of Table 1) required the sequential addition
of the two different monomers before the addition of the EGDMA cross-linker, while the miktoarm crosslinked star (polymer 4) was prepared when the crosslinker was polymerized between the two monomers. Finally, the statistical copolymer star (polymer 5) was prepared by the simultaneous addition of the two different monomers, which resulted in the preparation of arms of statistical architecture, before the addition of the cross-linker. A typical polymerization yielding the star-block copolymer with arms comprising linear diblock copolymers with 30 DMAEMA units and 10 MMA units is detailed next. TBABB catalyst (10 mg, 20 lmol), freshly distilled THF (37 ml) and MTS initiator (0.36 ml, 0.31 g, 1.77 mmol) were added, in this order, to a 100 ml round-bottom flask. Subsequently, DMAEMA (9.0 ml, 8.40 g, 53.2 mmol) was added slowly under stirring, giving a polymerization exotherm (27.8–40.3 C), which abated within 5 min. A sample (0.1 ml) was extracted from the reaction solution for GPC analysis, and MMA (1.9 ml, 1.78 g, 17.7 mmol) was added slowly. The reaction temperature rose from 35.6 to 38.5 C. After sampling for GPC, EGDMA (1.35 ml, 1.42 g, 7.09 mmol) was added, which produced an exotherm (35.3– 37.1 C), and a sample was again withdrawn for GPC analysis. The star copolymers formed were recovered by precipitation in n-hexane and dried for three days in a vacuum oven at room temperature.
TBABB 30 DMAEMA 10 MMA 4 EGDMA DMAEMA 30 1. THF DMAEMA30 b MMA10 (DMAEMA 30 - b - MMA10 )- star 1 MTS TBABB 20 DMAEMA 20 MMA 4 EGDMA DMAEMA 20 2. THF DMAEMA20 b MMA20 (DMAEMA20 - b -MMA 20 )- star 1 MTS TBABB 10 DMAEMA 30 MMA DMAEMA10 3. THF 1 MTS TBABB 30 DMAEMA 4 EGDMA DMAEMA30 4. THF 1 MTS
DMAEMA10 b MMA30
DMAEMA30-star
4 EGDMA
10 MMA
(DMAEMA10- b -MMA 30 )- star
DMAEMA30- star - MMA10
TBABB 30 DMAEMA + 10 MMA 4 EGDMA (DMAEMA30- co-MMA10)-star DMAEMA30-co -MMA10 5. THF 1 MTS TBABB 10 MMA 30 DMAEMA 4 EGDMA MMA10 MMA10-b-DMAEMA30 (MMA10-b-DMAEMA30)- star 6. THF 1 MTS
Fig. 2. Synthetic sequences employed for the preparation of the star copolymers of this study. TBABB = tetrabutylammonium bibenzoate, polymerization catalyst; THF = tetrahydrofuran, polymerization solvent; MTS = 1-methoxy-1-(trimethylsiloxy)-2-methyl propene, monofunctional initiator; DMAEMA = 2-(dimethylamino)ethyl methacrylate and MMA = methyl methacrylate, monomers; EGDMA = ethylene glycol dimethacrylate, cross-linker.
No.
Polymer formula
Theor. MWa
GPC results
SLS results
Mn
Mp
Mw/Mn
Mw
Composition (mol% DMAEMA) No. of arms
Theor.
By 1H NMR
1
DMAEMA30 DMAEMA30-b-MMA10 DMAEMA30-b-MMA10-star
4820 5820 N/Ab
5630 6890 707,00
6100 7430 88,700
1.09 1.10 1.19
401,000
46
75.0
72.9
2
DMAEMA20 DMAEMA20-b-MMA20 DMAEMA20-b-MMA20-star
3250 5250 N/Ab
3500 6370 72,300
3780 7020 83,800
1.10 1.12 1.22
565,000
69
50.0
46.8
3
DMAEMA10 DMAEMA10-b-MMA30 DMAEMA10-b-MMA30-star
1670 4680 N/Ab
1470 5920 84,000
2090 6630 86,200
1.10 1.12 1.27
495,000
64
25.0
24.2
4
DMAEMA30 DMAEMA30-star DMAEMA30-star-MMA10
4820 N/Ab N/Ab
5100 41,700 49,800
5600 63,200 68,800
1.09 1.12 1.28
468,000
120
75.0
71.8
5
DMAEMA30-co-MMA10 DMAEMA30-co-MMA10-star
5820 N/Ab
7330 60,800
8310 86,200
1.10 1.25
6
MMA10 MMA10-b-DMAEMA30 MMA10-b-DMAEMA30-star
1100 5820 N/Ab
1170 8380 63,700
1720 9300 91,200
1.09 1.10 1.26
515,000
49
75.0
72.3
a b
The molecular weight of the initiator fragment of 100 g mol Not applicable.
1
has also been included in the calculation.
E. Themistou, C.S. Patrickios / European Polymer Journal 43 (2007) 84–92
Table 1 Molecular weights and compositions of the star copolymers
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3. Characterization in organic solvents 3.1. Gel permeation chromatography (GPC) Molecular weights (MWs) and molecular weight distributions (MWDs) of the star copolymers and all of their precursors were determined by gel permeation chromatography (GPC) using a single Polymer Laboratories PL-Mixed ‘‘D’’ column. The mobile phase was tetrahydrofuran (THF, flow rate 1 mL min 1), delivered using a Polymer Laboratories PL-LC1120 isocratic pump. The refractive index signal was measured using an ERC-7515A refractive index detector supplied by Polymer Laboratories. The calibration curve was based on seven narrow MW (630, 4250, 13,000, 28,900, 50,000, 128,000 and 260,000 g mol 1) linear poly(MMA) standards supplied by Polymer Laboratories, which provided accurate MW calculations for the linear precursors but only qualitative estimations for the MWs of the star copolymers. In particular, the following quantities were calculated: the number-average MWs, M n s, the polydispersity indices (PDIs, M w =M n ), where M w is the weight-average MW, and the peak MWs, Mps, which are the MWs at the peak maximum. 3.2. 1H NMR spectroscopy The compositions of the star copolymers were determined by proton nuclear magnetic resonance (1H NMR) spectroscopy, using a 300 MHz Avance Bruker spectrometer equipped with an Ultrashield magnet. The NMR solvent was deuterated chloroform (CDCl3), containing tetramethylsilane (TMS) that was used as an internal reference. 3.3. Static light scattering The absolute M w s of the star copolymers were measured by static light scattering (SLS) in THF using a Brookhaven BI-MwA Molecular Weight Analyzer equipped with a 30 mW red diode laser operating at 673 nm and a multi-angle detector, measuring simultaneously the intensity of scattered light at 7 different angles, 35, 50, 75, 90, 105, 130 and 145. Polymer samples were dissolved in HPLC-grade THF at different polymer concentrations and were filtered through 0.45 lm pore size syringe filters. From the scattered intensities, the software of the instrument constructed automatically a Zimm plot from which the absolute M w
was calculated as the combined extrapolation to zero angle and zero concentration. The refractive index increments (dn/dc) of 5% w/v star copolymer solutions in THF were determined using an ABBE refractometer. 4. Aqueous solution characterization 1% w/w aqueous salt-free solutions of the star copolymers were characterized in terms of their water-solubilities and effective pKs using solubility tests, and hydrogen ion titration, respectively. 4.1. Solubility in water Five grams of deionized water were added to 50 mg of each star copolymer. The mixture was stirred and observed for complete copolymer dissolution. 4.2. Hydrogen ion titration Five grams of 1% w/w solutions/dispersions of each polymer were titrated from pH 2 to 12 using a standard NaOH 0.5 M solution under continuous stirring. The pHs were measured using a Corning PS30 portable pH meter. The pHs of precipitation were determined by visually observing the turbidity. The effective pKs were calculated as the pH at 50% ionization. 5. Results and discussion 5.1. Synthetic strategy and structure of the star copolymers The synthesis of the star copolymers was accomplished by the GTP of the appropriate monomer (DMAEMA or MMA), initiated by a monofunctional initiator, followed by the in situ cross-linking of the formed polymer at one end to a star, effected by the polymerization of a difunctional methacrylate, EGDMA. Six star copolymers with overall degrees of polymerization (DPs) of the arms of 40 were prepared using this synthetic strategy. By changing the ratio of the monomers used in the synthesis, star copolymers with different compositions resulted, whereas by changing their addition sequence, star copolymers with different structures were synthesized. Schematic representations of the structures of the six star copolymers synthesized in this report are shown in Fig. 3. Four star-block
E. Themistou, C.S. Patrickios / European Polymer Journal 43 (2007) 84–92
89
5.2. Characterization in organic solvents
DMAEMA 20-b-MMA 20-star
DMAEMA10 -b-MMA 30 -star
DMAEMA 30-star-MMA 10
DMAEMA30 -co-MMA 10 -star
MMA 10 -b-DMAEMA30 -star
Fig. 3. Schematic representation of the structures of the star copolymers of this study. The DMAEMA units are depicted in white, while the MMA units are colored black. The core of the stars, consisting of EGDMA units, is also colored black.
copolymers of different compositions, one miktoarm star, and one statistical copolymer star were synthesized. The linear ‘‘living’’ copolymer precursor to a star-diblock copolymer was produced by the sequential addition of two methacrylate monomers to the initial solution of THF, monofunctional initiator and catalyst, whereas the linear precursor to the statistical star was produced by the simultaneous addition of the two monomers. For the formation of a miktoarm star, a linear ‘‘living’’ homopolymer was formed first by the addition of only one monomer to the initial solution, followed by the addition of the dimethacrylate cross-linker to form a homopolymer star, and completed by the addition of another monomer, different from the first one, to form the final miktoarm star.
DMAEMA 20-b-MMA20 -star DMAEMA20 DMAEMA 20-b-MMA 20
refractive index signal
DMAEMA 30 -b-MMA 10 -star
5.2.1. Molecular weights of the star copolymers The MWs and MWDs of the star polymers were characterized using GPC. Fig. 4 presents the GPC chromatograms of the star copolymer DMAEMA20-b-MMA20-star and its precursors. Generally, the GPC chromatograms of the star copolymers exhibited two peaks, one due to the star polymer, and one due to some of the constituting linear copolymer, which had remained unattached (free arm). Incomplete incorporation of the linear copolymers into the stars is due to increased solution viscosity, lower chain mobility and possible chain termination/chain transfer. We wish to stress at this point that, due to the use of linear calibration standards, the MWs determined by GPC were only rough estimates of the true MWs of the star copolymers. Accurate values of the MWs of the star copolymers were determined using SLS. The apparent M n s and PDIs of all star copolymers and their precursors were calculated from the GPC chromatograms and are shown in Table 1 along with the theoretical MWs of the linear precursors. The apparent M n s of the star copolymers ranged between 49,800 and 84,000 g mol 1. The PDIs of all samples, linear and star polymers, were relatively low, below 1.3. The linear homopolymers and copolymers displayed unimodal and narrow MWDs, with PDIs equal to or lower than 1.12. Generally, with each successive addition of a monomer or the cross-linker to a ‘‘living’’ linear homo- or copolymer, or star (co)polymer, the M n s of the polymer products grew and their PDIs increased. The
0 5
6
7 elution time (min)
8
9
Fig. 4. GPC chromatograms of the star copolymer DMAEMA20-b-MMA20-star and its linear homopolymer and diblock copolymer precursors.
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M n s of the linear precursors were in most cases higher than the theoretically predicted, indicating some initiator deactivation. SLS on THF solutions of the synthesized star copolymers allowed the determination of their absolute M w s and weight-average number of arms, both presented in Table 1. The M w s of the star copolymers determined by SLS were greater than their corresponding GPC M w s calculated using a MW calibration based on linear polyMMA standards, due to the compact nature of the star structure. The weight-average numbers of arms of the star block and the statistical star copolymers were calculated by subtracting the MW corresponding to the cross-linker from the M w s of the star copolymers determined by SLS, and by dividing the calculated value by the M w s of their arms determined by GPC. The weight-average numbers of arms for these star copolymers were found to be between 39 and 69, within the same range presented by star polymers prepared by GTP [14,20–23]. The calculation of the number of arms of the miktoarm star copolymer DMAEMA30-starMMA10 was carried out in a different way because of the difference between the structure of this copolymer and the other star copolymers. While all other star copolymers were homo-arm star copolymers, composed of arms with the same DP and composition, the present miktoarm star bore two different types of arms: DMAEMA homopolymers with a DP of 30 and MMA homopolymers with a DP of 10. The ‘‘livingness’’ of GTP dictates that the number of these two different types of arms be the same. For the calculation of the number of arms for this star copolymer, first, the MW due to the EGDMA core was subtracted from its M w determined by SLS, and the resulting MW value was divided by an effective MW of a combined DMAEMA-MMA arm. This effective MW was calculated by multiplying the GPC M w of the linear polyDMAEMA by the appropriate factor, derived from the DMAEMA/ MMA composition [=(30 · 157 + 10 · 100)/ (30 · 157)]. 5.2.2. Star copolymer composition Fig. 5 shows the 1H NMR spectrum of the miktoarm star copolymer DMAEMA30-starMMA10. The characteristic peak for the MMA units was that of the three methoxy protons at 3.6 ppm (peak c), while that for the DMAEMA units was taken that of the two oxymethylene protons at 4.1 ppm (peak d). The star copolymer com-
Fig. 5. 1H NMR spectrum of the miktoarm star copolymer DMAEMA30-star-MMA10.
positions were calculated by ratioing the normalized areas of the characteristic peaks of the two monomer units, and are listed in Table 1. These compositions are close to the theoretically expected compositions calculated from the comonomer feed ratios. 5.3. Characterization in aqueous solutions Table 2 displays the results from the aqueous solution characterization of all the star copolymers, including their solubilities at various pHs and the pKs of their DMAEMA units. 5.3.1. Solubility in neutral and acidic water Table 2 shows that none of the star copolymers was readily soluble in neutral water to give a transparent solution. After stirring its slurry (50 mg) in neutral water (5 g) and leaving the mixture in the refrigerator for 2 days, the statistical star copolymer DMAEMA30-co-MMA10-star was the only one to be completely dispersed, giving, however, a very opaque solution. The star-block copolymer DMAEMA10-b-MMA30-star was not dissolved in neutral water at all due to its high hydrophobicity, whereas the MMA10-b-DMAEMA30-star was precipitated leaving the solution transparent because this star copolymer has its hydrophobic units in its periphery, making difficult its dissolution. The other star copolymers were only partially dispersed in neutral water, giving both opaque solutions and precipitates. The pH of dissolution/dispersion in neutral water for all the star copolymers was found to range between 7 and 9.
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91
Table 2 Dissolution and precipitation pHs, effective pKs, and solubility tests of the star copolymers at various pHs No.
Polymer formula
Solubility testsa pHdissolution
1 2 3 4 5 6
DMAEMA30-b-MMA10-star DMAEMA20-b-MMA20-star DMAEMA10-b-MMA30-star DMAEMA30-star-MMA10 DMAEMA30-co-MMA10-star MMA10-b-DMAEMA30-star
sp + os sp + os p sp + os vos p + ts
b
pH = 2
pH = 4
ts ts vos ts ts ts
ts sos vos ts sos ts
pHdissolution
pHprecipitationa
pK
8.8 8.2 7.2 8.8 9.0 8.8
10.0 10.1 vos in all pH range 9.6 8.8 9.6
6.7 6.7 6.5 7.0 7.1 7.6
a ts, transparent solution; os, opaque solution; sos, slightly opaque solution; vos, very opaque solution; p, precipitate; sp, some precipitate. b After 2 days in the refrigerator.
Most of the star copolymers were soluble in acidic water of pH 4, with the exception of DMAEMA10-b-MMA30-star, which gave a very opaque and milky solution, due to its high hydrophobicity. The equimolar star copolymer DMAEMA20-bMMA20-star and the statistical star copolymer DMAEMA30-co-MMA10-star were dissolved in water at pH 4, giving slightly opaque solutions. All the star copolymers dissolved completely in water of final pH 2 and gave transparent solutions, with the exception of the most hydrophobic star copolymer which was fully dispersed but gave a milky solution. 5.3.2. Effective pKs and precipitation pHs The results of the hydrogen ion titrations of aqueous solutions of the star copolymers are also summarized in Table 2. The effective pKs of the DMAEMA units, an ionizable tertiary amine, in all star copolymers ranged between 6.5 and 7.6, values that are close to the pK value of the DMAEMA homopolymer, of approximately 7. Precipitation occurred for all the star copolymer solutions in pHs between 9 and 10, except for the most hydrophobic star polymer, DMAEMA10-b-MMA30-star, which gave a very opaque solution over the whole pH range. For all the other star copolymers, as the pH increased, the DMAEMA units in the polymer become deprotonated and lost their charge, resulting in precipitation. This tendency for precipitation was enhanced by the presence of the hydrophobic MMA units. 6. Conclusions The successful synthesis of amphiphilic star copolymers comprising hydrophilic units of DMAEMA and hydrophobic units of MMA of various
compositions and structures (block, miktoarm and statistical) was accomplished by GTP using sequential monomer(s) and cross-linker additions. The successful synthesis of the star copolymers was confirmed by their characterization using GPC and 1H NMR. 1H NMR indicated that the star copolymer compositions were close to those theoretically expected. The absolute weight-average molecular weight determined by static light scattering in THF ranged between 359,000 and 565,000 g mol 1, while the number of arms of the star copolymers ranged between 39 and 120. The statistical copolymer star DMAEMA30-co-MMA10-star was the only one, which was fully dispersible in neutral water, giving an opaque solution. All star copolymers were soluble in acidic water at pH 4, giving transparent or slightly opaque solutions, except for DMAEMA10-b-MMA30-star, which gave a very opaque solution due to its high hydrophobicity. Their effective pKs ranged between 6.5 and 7.6 and their precipitation pHs were found to be between 9 and 10, with the precipitation pH of the most hydrophobic DMAEMA10-b-MMA30-star being indeterminate due to the high opacity of its dispersion over the whole pH range. Acknowledgments The Cyprus Research Promotion Foundation is gratefully acknowledged for providing financial support to E.T. via a PENEK 2001 Project. The A.G. Leventis Foundation is also thanked for a generous donation that enabled the purchase of the NMR spectrometer of the University of Cyprus. The University of Cyprus Research Committee (Grant 2000–2003) is thanked for providing a grant that enabled the purchase of the static light scattering spectrometer.
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