Well-defined synthetic polymers with a protein-like gelation behavior in water

Article abstract of DOI:10.1039/c0cc00038h

Homopolymers of N-acryloyl glycinamide were prepared by reversible addition-fragmentation chain transfer polymerization in water. The formed macromolecules exhibit strong polymer-polymer interactions in aqueous milieu and therefore form thermoreversible physical hydrogels in pure water, physiological buffer or cell medium.

Full text of DOI:10.1039/c0cc00038h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Well-defined synthetic polymers with a protein-like gelation behavior
in waterw
a
a
a
ab
¸
ois Lutz*a
¨ ´
Stefan Glatzel, Nezha Badi, Michael Pach, Andre Laschewsky and Jean-Franc
Received 19th February 2010, Accepted 12th April 2010
First published as an Advance Article on the web 18th May 2010
DOI: 10.1039/c0cc00038h
Homopolymers of N-acryloyl glycinamide were prepared by
reversible addition-fragmentation chain transfer polymerization
in water. The formed macromolecules exhibit strong polymer–
polymer interactions in aqueous milieu and therefore form
thermoreversible physical hydrogels in pure water, physiological
buffer or cell medium.
Scheme 1 Molecular structures of the monomer and the chain-
Thermoreversible hydrogels are hydrophilic macromolecular
networks exhibiting a thermo-induced sol–gel (i.e. the gel
forms upon heating) or gel–sol (i.e. the gel melts upon heating)
transition. Both types of gels have found wide applications in
materials science, food science, cosmetics and medicine.1,2
However, so far, the large majority of synthetic thermogels
exhibit a sol–gel transition in water.2 This interesting phenomenon
can be obtained, for example, by using amphiphilic block
copolymers3 or macromolecular architectures containing
thermoresponsive segments (i.e. polymers exhibiting a lower
critical solution temperature in water).4
transfer agent used in the present work.
physiological media. Polymers of N-acryloyl glycinamide
(NAGAM, 1, Scheme 1) are known to form strong self-
interactions in aqueous medium, even at very low concentrations.8
This is a very special situation implying that, while hydro-
philic, this polymer has a higher tendency to form interactions
with itself than with water.9 This feature is exceptional among
the class of uncharged water-soluble polymers. In most other
cases (e.g. poly(ethylene oxide), poly(vinyl alcohol), poly-
(N-isopropylacrylamide)), polymer–water hydrogen bonding
predominates.
In contrast, biopolymer gels (e.g. protein-based or poly-
saccharide-based hydrogels) often exhibit a gel–sol behavior in
aqueous medium.5 Although quite common in our daily lives,
this gelation mode is far from being trivial in terms of
macromolecular science. It implies that (i) strong polymer–
polymer interactions are formed at room temperature in
water, (ii) the macromolecular network remains overall highly
hydrophilic, and (iii) the physical crosslinks are weak enough
to be disrupted by heating. In biopolymer gels, this subtle
balance of parameters is due to the formation of highly-
ordered crystallites. For instance, the gel–sol transition in
protein gels frequently relies on the thermal disruption of
helical aggregates.6 This complex aggregation/disaggregation
mechanism is based on specific peptide domains with a defined
primary and secondary structure.
The controlled radical polymerization of 1 was studied.
After some preliminary investigations, it became obvious that
the synthesis and characterization of poly(N-acryloyl glycinamide)
are far from trivial. Indeed, the unconventional molecular
structure of this polymer significantly limits the use of
common organic solvents. Typically, homogeneous reaction
conditions can only be obtained in water or in DMSO. Thus,
the polymerization of 1 was investigated in pure deionized
water. The reversible addition-fragmentation chain transfer
(RAFT) process was selected among the different methods of
controlled radical polymerization, as it is certainly the
most adapted approach for aqueous polymerization.10 The
polymerizations were performed at 60 1C in the presence of the
RAFT chain transfer agent 2 and a water-soluble azo radical
initiator. A series of polymers with an average degree of
polymerization ranging from 100 to 500 was synthesized
(Table S1w). After synthesis and purification, the polymers
formed were characterized by size exclusion chromatography
(SEC) and NMR spectroscopy (Fig. 1).
1H NMR spectra in D2O clearly indicated that the polymers
are end-functional structures obtained via a controlled RAFT
process. Typical a- and o-chain-end signals due to the initiating
and control moieties of the chain transfer agent are seen in all
spectra. The presence of defined chain-ends was also confirmed
by Heteronuclear Single Quantum Coherence (Fig. S1w) and
Heteronuclear Multiple Bond Coherence (data not shown)
experiments. Moreover, a comparison of the integrals of the
Comparable gel–sol transitions are rarely observed in
synthetic hydrogels, which typically exhibit a less controlled
macromolecular structure. Although this behavior has been
observed in water for some polymer complexes or mixtures,7
such multicomponent systems are usually difficult to tune for
practical applications in materials science or life science.
In the present communication, we report that a simple
homopolymer, prepared via controlled radical polymerization,
exhibits an adjustable gel–sol transition in water and in
a Fraunhofer Institute for Applied Polymer Research, Geiselbergstrasse
69, Potsdam 14476, Germany. E-mail: lutz@iap.fhg.de;
Fax: +49 331 568 3000; Tel: +49 331 568 1127
b University of Potsdam, Department of Chemistry, Potsdam,
Germany
chain-end and backbone protons indicated
a precisely
controlled chain-length in all cases (Table S1w). This aspect
was confirmed by the SEC measurements in hot DMSO.
w Electronic supplementary information (ESI) available: Experimental
part, Table S1 and Fig. S1–S3. See DOI: 10.1039/c0cc00038h
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4517–4519 | 4517

View Article Online
Fig. 2 Evolution of the storage modulus (G0, blue symbols) and the
loss modulus (G0 0, red symbols) as a function of temperature for a
homopolymer of 1 (DPn = 500) in phosphate buffered saline solution
(concentration B 5.5 wt%). These measurements have been recorded
at a frequency of 1 Hz. The inset shows the evolution of the critical
gelation concentration as a function of DPn for the polymers listed in
Table S1w.
strong gel at room temperature but fully melts at physiological
temperature.11
In conclusion, well-defined thermoreversible hydrogelators
can be obtained via simple RAFT homopolymerization in
aqueous medium. Similarly to proteins such as gelatin, these
polymers form strong self-associations in water, which can be
disrupted at elevated temperatures. These macromolecules are
not only relevant for hydrogel applications, but more generally
for the whole field of aqueous polymer self-assembly. Indeed,
using controlled radical polymerization techniques,10,12 a wide
variety of self-associating polymer architectures may be
envisioned and prepared.
Fig. 1 Molecular characterization of homopolymers of 1 prepared by
RAFT polymerization: (top) SEC chromatograms recorded at 70 1C
in DMSO for samples of different DPn; (bottom) H NMR spectrum
1
recorded in D2O for a homopolymer with an average chain length
of 100.
Chromatograms with a narrow molecular weight distribution
were measured for each targeted degree of polymerization.
Yet, it should be noted that a careful sample preparation is
needed to molecularly dissolve the polymers (Fig. S2w). Due
to the strong aggregation tendency of these macromolecules
(Fig. S3w), high molecular weight shoulders may be observed
in the SEC chromatograms. For high molecular weight
samples (i.e. DPn > 300), these aggregate peaks could be
minimized but not completely suppressed (Fig. S2w). Never-
theless, both NMR and SEC indicated an efficient control of
the macromolecular structure.
This research was supported by the Fraunhofer Society
(bioactive surfaces project) and the Federal Ministry of
Education and Research (IZIB project). Moreover, the
authors thank Dr Matthias Heydenreich (Universitat
¨
Potsdam) for the HSQC and HMBC measurements and Dr
Helmut Schlaad and Marlies Grawert (MPI-KGF) for the
¨
SEC measurements in DMSO.
Notes and references
The properties of the obtained polymers have been investigated
in aqueous milieu. All polymers formed physical gels at room
temperature in pure water but also in phosphate buffered
saline (PBS) solution and in cell medium (the last two media
were selected in order to show the broad applicability of these
materials). However, the critical gelation concentration (CGC)
depended on polymer chain length (Fig. 2, inset). For instance,
short polymer chains (i.e. DPn B 100–200) did not form gels in
PBS at concentrations lower than 10 wt%. On the other hand,
samples with longer chains (i.e. DPn B 300–500) formed gels
at relatively low concentrations. For example, a homopolymer
with an average chain length of 500 formed gels in PBS at
concentrations as low as 5 wt%. Furthermore, a protein-like
thermoreversible gelation behavior was observed in all cases.
For example, Fig. 2 shows the rheological measurements of a
PBS solution of a homopolymer with a DPn of 500. A clear
gel–sol transition is observed at 27 1C. This material forms a
1 (a) J.-M. Guenet, Thermoreversible Gelation of Polymers
and Biopolymers, Academic Press, London, 2nd edn, 1992, 280;
(b) C. de las Heras Alarcon, S. Pennadam and C. Alexander, Chem.
´
Soc. Rev., 2005, 34, 276–285; (c) B. Jeong, H. Y. Bae, D. S. Lee and
S. W. Kim, Nature, 1997, 388, 860–862; (d) C. Wang, R. J. Stewart
and J. Kopecek, Nature, 1999, 397, 417–420.
´
2 (a) E. Ruel-Gariepy and J.-C. Leroux, Eur. J. Pharm. Biopharm.,
2004, 58, 409–426; (b) L. Yu and J. Ding, Chem. Soc. Rev., 2008,
37, 1473–1481.
3 (a) A. Cabana, A. AitKadi and J. Juhasz, J. Colloid Interface Sci.,
1997, 190, 307–312; (b) B. Jeong, Y. H. Bae and S. W. Kim,
Macromolecules, 1999, 32, 7064–7069.
4 (a) C. Li, J. Madsen, S. P. Armes and A. L. Lewis, Angew. Chem.,
Int. Ed., 2006, 45, 3510–3513; (b) S. E. Kirkland, R. M. Hensarling,
S. D. McConaughy, Y. Guo, W. L. Jarrett and C. L. McCormick,
Biomacromolecules, 2008, 9, 481–486; (c) N. Fechler, N. Badi,
K. Schade, S. Pfeifer and J.-F. Lutz, Macromolecules, 2009, 42,
33–36; (d) N. Badi and J.-F. Lutz, J. Controlled Release, 2009, 140,
224–229; (e) A. P. Vogt and B. S. Sumerlin, Soft Matter, 2009, 5,
2347–2351.
5 A. Clark and S. Ross-Murphy, Adv. Polym. Sci., 1987, 83, 57–192.
ꢀc
This journal is The Royal Society of Chemistry 2010
4518 | Chem. Commun., 2010, 46, 4517–4519

View Article Online
6 (a) P. J. Flory and E. S. Weaver, J. Am. Chem. Soc., 1960, 82,
4518–4525; (b) W. A. Petka, J. L. Harden, K. P. McGrath,
D. Wirtz and D. A. Tirrell, Science, 1998, 281, 389–392.
7 (a) M. Shibayama, Y. Hiroyuki, K. Hidenobu, F. Hiroshi and
N. Shunji, Polymer, 1988, 29, 2066–2071; (b) I. Akiba and
S. Akiyama, J. Macromol. Sci., Part B: Phys., 2001, 40, 157–169.
8 (a) H. C. Haas and N. W. Schuler, J. Polym. Sci., Part B: Polym. Lett.,
1964, 2, 1095–1096; (b) H. C. Haas, R. D. Moreau and N. W. Schuler,
J. Polym. Sci., Part A-2: Polym. Phys., 1967, 5, 915–927.
9 (a) O. Marstokk, B. Nystrom and J. Roots, Macromolecules, 1998,
31, 4205–4212; (b) D. Ostrovskii, P. Jacobsson, B. Nystrom,
O. Marstokk and H. B. M. Kopperud, Macromolecules, 1999,
32, 5552–5560.
E. Rizzardo and S. H. Thang, Polymer, 2008, 49, 1079–1131;
(c) C. Barner-Kowollik and S. Perrier, J. Polym. Sci., Part A:
Polym. Chem., 2008, 46, 5715–5723; (d) M. Mertoglu,
A. Laschewsky, K. Skrabania and C. Wieland, Macromolecules,
2005, 38, 3601–3614.
11 A similar behavior was also observed in pure water. However, the
gel–sol transition in pure water is slightly higher than in PBS
(i.e. around 28 1C).
12 (a) K. Matyjaszewski and N. V. Tsarevsky, Nat. Chem., 2009, 1,
276–288; (b) L. Tao, G. Mantovani, F. Lecolley and
D. M. Haddleton, J. Am. Chem. Soc., 2004, 126, 13220–13221;
(c) J. Du, Y. Tang, A. L. Lewis and S. P. Armes, J. Am. Chem.
Soc., 2005, 127, 17982–17983; (d) J.-F. Lutz, Polym. Int., 2006, 55,
979–993; (e) G. Pasparakis, N. Krasnogor, L. Cronin, B. G. Davis
and C. Alexander, Chem. Soc. Rev., 2010, 39, 286–300.
10 (a) C. L. McCormick, B. S. Sumerlin, B. S. Lokitz and
J. E. Stempka, Soft Matter, 2008, 4, 1760–1773; (b) G. Moad,
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4517–4519 | 4519