Well-Defined Uncharged Polymers with a Sharp UCST in Water and in

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DOI: 10.1021/ma102677k

Well-Defined Uncharged Polymers with a Sharp UCST in Water and in Physiological Milieu Stefan Glatzel,† Andr
e Laschewsky,†,‡ and Jean-Franc- ois Lutz*,†,§ †

Fraunhofer Institute for Applied Polymer Research, Geiselbergstrasse 69, Potsdam 14476, Germany, and Department of Chemistry, University of Potsdam, Potsdam, Germany. §New address: Institut Charles Sadron, UPR-22 CNRS, 23 rue du Loess, BP 84047, 67034 Strasbourg cedex 2, France. ‡

Received November 24, 2010; Revised Manuscript Received December 13, 2010

Thermoresponsive macromolecules have become very important in academic and applied polymer science during the past two decades.1 In particular, water-soluble polymers capable of changing their solution properties at useful temperatures (e.g., room temperature, body temperature, fever temperature) are broadly studied for applications in nanomedicine, surface science, and cosmetics.1 There are typically two basic types of thermoresponsive behaviors in aqueous medium: polymers with a lower critical solution temperature (LCST) precipitate upon heating, whereas polymers with an upper critical solution temperature (UCST) dissolve upon heating.1 However, LCST polymers have been by far the most studied ones. Indeed, their aqueous behavior is rather peculiar (i.e., heat does not favor dissolution but phase separation) and useful. Yet, although counterintuitive, the LCST behavior is actually quite common. In fact, virtually all uncharged water-soluble macromolecules seem to exhibit a LCST in water. The explanation of this behavior is, after all, quite simple. A polymer dissolves in water because it forms favorable interactions with the solvent. In the case of uncharged macromolecules, polymer-water interactions are mostly hydrogen bonds. At elevated temperatures, these weak polymer-solvent interactions are easily disrupted, thus resulting in phase separation. Hence, LCST behavior has been observed for a wide variety of water-soluble nonionic polymers, including poly(ethylene oxide), poly(vinyl alcohol), and poly(hydroxyethyl methacrylate).2,3 However, only some polymers exhibit LCST values in a convenient range of temperature. So far, poly(N-substituted acrylamide)s such as poly(N-isopropylacrylamide),4 polymers with short oligo(ethylene glycol) side chains,5 poly(alkyloxazolines),6 and elastinlike polypeptides7 are the most studied thermoresponsive polymers for practical applications.8 On the other hand, there are only a few known polymers exhibiting a controllable UCST in aqueous medium. The UCST behavior is more commonly observed in organic solvents or in water/organic solvent mixtures.9 In pure water, this behavior is rather atypical. The reason for that is simple to understand: UCST in water implies that strong polymer-polymer interactions are formed in aqueous medium but that these interactions are weak enough to be disrupted by temperature. In a sense, the heat treatment of biopolymer solutions (e.g., protein thermal denaturation and DNA melting) follows this scheme. However, these events occur at relatively high temperatures and generally do not result in observable and controllable phase transitions. The UCST behavior is even more unlikely in the case of synthetic macromolecules. Indeed, typical polymer-polymer interactions such as hydrophobic aggregation (e.g., hydrophobic polymers or *Corresponding author. E-mail: [email protected]. r 2010 American Chemical Society

macrosurfactants) or electrostatic interactions (e.g., polyampholites or polyelectrolyte complexes) are generally not affected by temperature. Hence, UCST behavior in water has been only reported for some specific homopolymers,3,10,11 copolymers,12 cross-linked networks,13 and polymer complexes.14 In the latter case, Kataoka and co-workers described an interesting series of hydrogen-bonding polymer complexes.14 It was shown that complementary homopolymers containing either H-bond donor or H-bond acceptor functions exhibit a UCST behavior in water. Such bicomponent systems require relatively concentrated polymer solutions and are therefore difficult to exploit for applications in materials science and life science. In fact, the most studied UCST polymers to date are sulfobetaine-based (meth)acrylic polymers.10 These zwitterionic homopolymers usually exhibit a UCST in water. Moreover, they can be polymerized using controlled radical polymerization (CRP) techniques such as atom transfer radical polymerization (ATRP)15 or reversible additionfragmentation chain transfer (RAFT) polymerization16 and can therefore be exploited for macromolecular engineering (e.g., synthesis of block copolymers or modification of surfaces).17 Still, these polymers bear ionic groups and are therefore relatively sensitive to ionic strength and specific ion-ion interactions.18 In this context, the synthesis of well-defined uncharged macromolecules exhibiting a defined UCST in water is a pertinent topic. Ohnishi and co-workers19 reported that some (co)polymers based on N-acryloylglycinamide (NAGAM) may exhibit a UCST in water, as more recently extended by Agarwal and co-workers.20 In these studies, the polymers were prepared by conventional free radical polymerization (RP). We studied lately the RAFT homopolymerization of NAGAM in water and demonstrated that well-defined polymers with a controlled chain-length and a narrow molecular weight distribution can be synthesized.21 However, poly(NAGAM) polymers prepared by RAFT polymerization did not exhibit a UCST behavior in aqueous medium. Instead, these polymers exhibit gel-sol thermogelation in water and in physiological media.21,22 The UCST behavior described for RP polymers was not obvious for the RAFT samples. Only weak turbidities were observed for dilute solutions of low molecular weight samples (i.e., DPn ∼ 100). Generally speaking, poly(NAGAM) polymers led to transparent physical gels in aqueous medium. In this Note, we describe the RAFT homopolymerization of N-acryloylasparaginamide (NAAAM, 1). The molecular structure of this monomer resembles that of NAGAM as it also possesses both H-bond donor and acceptor amide sites.19 However, NAAAM contains two primary amide functions instead of one in the case of NAGAM. NAAAM was synthesized in one step by reacting acryloyl chloride with L-asparaginamide Published on Web 12/30/2010

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hydrochloride. Because of its particular molecular structure, NAAAM is not soluble in most of the conventional organic solvents but can be dissolved in DMSO and water. Interestingly, this monomer starts to dissolve only above 22 °C in pure water (Figure S1). Thus, NAAAM was an attractive option for exploring the synthesis of well-defined polymers with a UCST behavior in aqueous medium. A series of NAAAM homopolymers of variable chain length were synthesized by RAFT polymerization (Table 1). Because of the limited solubility of the monomer in organic solvents, the polymerization was conducted in deionized water. The watersoluble chain transfer agent 2 (Scheme 1) and the water-soluble radical initiator VA-044 were used to generate a controlled RAFT process. We previously reported that this combination allows a good control of the radical polymerization of NAGAM in water.21 In the present study, homogeneous NAAAM polymerizations were observed at 60 °C in all cases. The formed polymers were purified by dialysis against pure deionized water, lyophilized, and analyzed by 1H NMR and size exclusion chromatography (SEC) (Table 1 and Figure 1). These characterizations Table 1. Properties of the Homopolymers Prepared by RAFT Polymerization DPn,tha

Mnb [g mol-1]

Mw/Mnc

DPn

cloud pointf [°C]

100 17 300 1.26 79d 4.3f 200 27 800 1.26 161d 10.6g 300 60 800 1.27 326e 18.9g 400 100 400 1.22 540e 26.2g 500 118 700 1.26 639e 24.3g a Theoretical average chain-length DPn,th = [1]/[2]. b Number-average molecular weight measured by SEC in DMSO. c Molecular weight distribution measured by SEC in DMSO. d Estimated from 300 MHz 1H NMR spectra in D2O. e Not measurable in D2O at room temperature. f Cloud point of a 1 wt % solution measured visually. g Cloud point of a 1 wt % solution measured by turbidimetry.

Scheme 1. Molecular Structures of the Monomer and the ChainTransfer Agent Used

showed that well-defined poly(NAAAM) samples with controlled chain lengths, chain ends, and molecular weight distributions were synthesized in all cases. For instance, Figure 1A shows the SEC traces obtained in DMSO for this series of polymers. For all samples, monomodal chromatograms with a narrow molecular weight distribution (i.e., Mw/Mn ∼ 1.25) were obtained. We previously reported that poly(NAGAM) homopolymers prepared by RAFT have a tendency to aggregate in DMSO and that therefore bimodal SEC chromatograms can be observed in some cases in this solvent.21 The RAFT homopolymers of 1 obviously behave differently in DMSO, thus suggesting different modes of self-association for poly(NAAAM) and poly(NAGAM) in polar solvents. Besides SEC analysis, 1H NMR measurements confirmed that the homopolymers of 1 are prepared via a controlled RAFT mechanism. Typical chain-end signals due to the initiating (i.e., R-group) and transfer (i.e., Z-group) moieties of the RAFT chain transfer agent are visible in the spectra of the purified polymers (Figure 1). The integration of these signals confirmed that the polymers have a controlled degree of polymerization (Table 1). Additionally, the properties of the formed homopolymers in dilute aqueous solutions were studied by turbidimetry. These measurements evidenced an obvious UCST behavior for all samples (Table 1). Yet, this phase transition was found to depend on chain length (Figure S2). Short poly(NAAAM) samples (i.e., DPn,th < 300) exhibited UCSTs below room temperature, whereas longer chains (i.e., DPn,th > 300) exhibited higher, and therefore more practical, values. Besides, the phase transition was also found to be dependent on concentration for a given homopolymer. For instance, Figure 1 and Figure S3 show the cloudpoint measurements obtained for a homopolymer with a DPn,th of 500 at different concentrations in water. In dilute conditions (i.e., polymer concentration below 1 wt %), the UCST behavior was found to depend on concentration. In addition, small hystereses of about 0.5-0.8 °C were noticed in this range of concentration. These particular aspects should be taken into account for potential applications. However, at higher concentrations (i.e., above 2 wt %), the UCST behavior was found to be less sensitive to concentration. In addition, no significant hysteresis was observed for more concentrated solutions (Figure 1). Furthermore, it is important to specify that below UCST a macroscopic phase separation can be observed. At concentrations around 2-3 wt %, the polymer solutions appear as stable milky colloidal dispersions at first, but after some hours, they typically flocculate into filterable or centrifugable polymer-rich

Figure 1. Molecular characterization of homopolymers of 1 prepared by RAFT polymerization: (left) SEC chromatograms recorded at 70 °C in DMSO for samples of different DPn. From left to right: DPn,th = 100, 200, 300, and 500; (middle) 1H NMR spectrum recorded in D2O for a homopolymer with an average degree of polymerization of 100; (right) turbidimetry measurements recorded for a homopolymer with an average degree of polymerization of 500 in pure deionized water at different concentrations. From left to right: concentration = 0.5, 1, 1.5, 2, and 3 wt %. Solid lines: cooling curves; dotted lines: heating curves. The raw experimental data were smoothened for clarity.

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phases. Thus, above a critical DPn, poly(NAAAM) homopolymers seem very interesting macromolecules for applications in materials science and biosciences. For instance, samples with DPn,th in the range 400-500 exhibited optimal UCST values in between room and body temperature. This interesting behavior was also observed in physiological medium, e.g., in phosphate buffered saline (PBS) solutions. The phase transitions measured in PBS were roughly similar to those observed in pure water (Figure S4). In conclusion, we demonstrated that RAFT polymerization allows a successful control of the radical polymerization of NAAAM in aqueous medium. The formed polymers exhibit a well-defined molecular structure and unprecedented UCST properties in water and in physiological milieu. These thermoresponsive macromolecules devoid of any ionic groups open new avenues for the design of waterborne stimuli-responsive materials. Acknowledgment. This research was supported by the Fraunhofer Society (Bioactive Surfaces Project) and the German Federal Ministry of Education and Research (BMBF, IZIB project 03IS2201B). Additionally, the authors thank Dr. Michael P€ ach (Fraunhofer IAP) for providing the RAFT CTA and Dr. Helmut Schlaad and Marlies Gr€awert (MPI-KGF) for SEC measurements in DMSO. Supporting Information Available: Full experimental part and Figures S1-S4. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Gil, E. S.; Hudson, S. M. Prog. Polym. Sci. 2004, 29, 1173–1222. (b) Nath, N.; Chilkoti, A. Adv. Mater. 2002, 14 (17), 1243–1247. (c) de las Heras Alarc
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