pH responsiveness of dendrimer-like poly(ethylene oxide)s

Article abstract of DOI:10.1021/ja0631605

Poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA), two polymers known to form pH-sensitive aggregates through noncovalent interactions, were assembled in purposely designed architecture -a dendrimer-like PEO scaffold carrying short inner PAA chains-

Full text of DOI:10.1021/ja0631605

Published on Web 08/12/2006
pH Responsiveness of Dendrimer-like Poly(ethylene oxide)s
Xiaoshuang Feng,† Daniel Taton,*,† Redouane Borsali,† Elliot L. Chaikof,‡ and
Yves Gnanou*,†
Contribution from the Laboratoire de Chimie des Polyme`res Organiques, Ecole Nationale
Supe´rieure de Chimie et Physique de Bordeaux-UniVersite´ Bordeaux-1-CNRS 16, AVenue Pey
Berland, 33607 Pessac Cedex, France, and Laboratory for Biomolecular Materials Research,
Department of Surgery, Emory UniVersity School of Medicine, Atlanta, Georgia 30322
Received May 5, 2006; E-mail: gnanou@enscpb.fr; taton@enscpb.fr
Abstract: Poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA), two polymers known to form pH-sensitive
aggregates through noncovalent interactions, were assembled in purposely designed architecture sa
dendrimer-like PEO scaffold carrying short inner PAA chainssto produce unimolecular systems that exhibit
pH responsiveness. Because of the particular placement of the PAA chains within the dendrimer-like
structure, intermolecular complexation between acrylic acid (AA) and ethylene oxide (EO) unitssand thus
macroscopic aggregation or even mesoscopic micellizationscould be avoided in favor of the sole
intramolecular complexation. The sensitivity of such interactions to pH was exploited to generate dendrimer-
like PEOs that reversibly shrink and expand with the pH. Such PAA-carrying dendrimer-like PEOs were
synthesized in two main steps. First, a fifth-generation dendrimer-like PEO was obtained by combining
anionic ring-opening polymerization (AROP) of ethylene oxide from a tris-hydroxylated core and selective
branching reactions of PEO chain ends. To this end, an AB2C-type branching agent was designed: the
latter includes a chloromethyl (A) group for its covalent attachment to the arm ends, two geminal hydroxyls
(B2) protected in the form of a ketal ring for the growth of subsequent PEO generations by AROP, and a
vinylic (C) double bonds for further functionalization of the interior of dendrimer-like PEOs. Reiteration of
AROP and derivatization of PEO branches allowed us to prepare a dendrimer-like PEO of fourth generation
with a total molar mass of 52,000 g‚mol-1, containing 24 external hydroxyl functions and 21 inner vinylic
groups in the interior. A fifth generation of PEO chains was generated from this parent dendrimer-like PEO
of fourth generation using a “conventional” AB2-type branching agent, and 48 PEO branches could be
grown by AROP. The 48 outer hydroxy-end groups of the fifth-generation dendrimer-like PEO obtained
were subsequently quantitatively converted into inert benzylic groups using benzyl bromide. The 21 internal
vinylic groups carried by the PEO scaffold were then chemically modified in a two-step sequence into
bromoester groups. The latter which are atom transfer radical polymerization (ATRP) initiating sites thus
served to grow poly(tert-butylacrylate) chains. After a final step of hydrolysis of the tert-butyl ester groups,
double, hydrophilic, dendrimer-like PEOs comprising 21 internal junction-attached poly(acrylic acid) (PAA)
blocks could be obtained. Dynamic light scattering was used to determine the size of these dendrimer-like
species in water and to investigate their response to pH variation: in particular, how the pH-sensitive
complexation of EO and AA units affects their overall behavior.
Introduction
that linear PEO chains exhibit very limited attachment capacitys
only one or two reacting sites at the most. The solution might
be to arrange PEO chains into branched architectures that could
be functionalized with many terminal reactive sites.3,4 Starlike,5
Poly(ethylene oxide) (PEO), often referred to as poly(ethylene
glycol) (PEG), exhibits unique properties such as chemical
stability, water solubility, nontoxicity, ion-transporting ability,
nonrecognition by the immune system (stealth effect), and
presence of functional groups that permit the covalent attach-
ment of biologically active molecules (PEGylation reactions).1,2
In particular, its nonrecognition by the immune system has been
exploited to conjugate biologically active molecules and increase
the in ViVo stability and therapeutic efficacy of the latter. The
main limitation in these applications, however, lies in the fact
(2) (a) Merrill, E. W. In Poly(ethylene glycol) Chemistry: Biotechnical and
Biomedical Applications; Harris, J. M., Ed.; Plenum Press: New York,
1992; p 199. (b) Poly(ethylene glycol): Chemistry and Biomedical
Applications, Harris, J. M., Zaplisky, S., Eds.; ACS Symposium Series 680;
American Chemical Society: Washington, DC, 1997. (c) Roberts, M. J.;
Bentley, M. D.; Harris, J. M. AdV. Drug DeliVery ReV. 2002, 54, 459. (d)
Greenwald, R. B.; Pendri, A.; Bolikal, D. J. Org. Chem. 1995, 60, 331. (e)
Borona, G. M.; Ivanova, E.; Zarytova, V.; Burcovich, B.; Veronese, F. M.
Bioconjugate Chem. 1997, 8, 793. (f) Ooya, T.; Lee, J.; Park, K. J.
Controlled Release 2003, 93, 121. (g) Lecolley, F.; Tao, L.; Mantovani,
G.; Durkin, I.; Lautru, S.; Haddleton, D. M. Chem. Commun. 2004, 2026.
(h) Pasut, G.;Veronese, F. M. AdV. Polym. Sci. 2006, 192, 95.
† Ecole Nationale Supe´rieure de Chimie et Physique de Bordeaux-
Universite´.
(3) Fishman, A.; Elmi Farrah, M.; Zhong, J.-H.; Paramanantham, S.; Carrera,
C.; Lee-Ruff, E. J. Org. Chem. 2003, 68, 9843.
(4) Rele, S. M.; Cui, W.; Wang, L.; Hou, S.; Barr-Zarse, B.; Taton, D.; Gnanou,
Y.; Esko, J. D.; Chaikof, E. L. J. Am. Chem. Soc. 2005, 127, 10132.
‡ Emory University School of Medicine.
(1) Woodle, M. C. AdV. Drug. DeliVery ReV. 1998, 32, 139.
9
10.1021/ja0631605 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 11551-11562
11551

A R T I C L E S
Feng et al.
hyperbranched,6 arborescent,7 but also dendrimer-like PEOs8
have thus been synthesized with a view of augmenting the
loading capacity of PEOs. The latter were obtained by reiteration
of two steps that are the living anionic chain polymerization of
ethylene oxide from multihydroxylated precursors and the
derivatization of chain ends in order to introduce the branching
points and the initiating sites for the growth of the next
generation.9 On this basis, we could prepare samples of eighth-
generation PEOs carrying not less than 384 hydroxyls and could
control not only the chain length of polymeric arms between
the branching points but also the chemical nature of terminal
functions. Such dendrimer-like PEOs thus resemble regular
dendrimers10 by the presence of a central core, a precise number
of branching points, and outer terminal functions but differ from
the latter by the macromolecular size of their generation.11,12
In this way, the advantages provided by the dendritic structures
multiplicity of reactive sitesscould be combined with the unique
features of PEO, namely its stealth effect. In a recent contribu-
tion, we have indeed described the synthesis of PEOs with a
dendrimer-like architecture and glycosidic end units,4 which
were found to exhibit better antiinflammatory activity in ViVo
than their linear and even their starlike glycosidic counterparts.
to pH, which is perceived as a limitation in applications requiring
that the scaffoldsone of the main uses of PEOsvaries in size
with the pH of the medium. On the other hand, PEO forms
pH-sensitive aggregates when associated with poly(methacrylic
acid) (PMAA) or with poly(acrylic acid) (PAA).13 Complexation
of EO units with AA ones occurs below pH 8, the morphology
and the size of the aggregate formed, depending on the
copolymer architecture and the composition. For instance, pH-
responsive mesospheres of PMAA-g-PEO graft copolymers
were designed for drug-release purposes.14 To obtain dendrimer-
like scaffolds that could be pH responsive, we contemplated
the possibility of functionalizing the interior of dendrimer-like
PEOs with PAA chains. In such a dendritic structure, indeed,
intermolecular interactions between the two copolymers would
be shielded, and hydrogen bonding between the two blocks
could only be intramolecular, entailing pH responsiveness of
the whole dendrimer-like architecture. The incorporation of
functional groups at the interior of dendrimers has already been
exploited to finely tune their physical properties15 and target
specific applications in nanoscience such as light harvesting16
or catalysis.15,17 Introduction of functional groups inside the
dendrimer was also found useful for probing the local microen-
vironment or for boosting intramolecular reaction by a concen-
tration effect. This was illustrated, for instance, by Zimmermann
and colleagues in the case of the ring-closing metathesis reaction
of dendrimers containing inner allylic functions.18 Finally,
dendrimers functioning as microreactors with outer hydrophilic
functions to ensure water solubility and inner hydrophobic
functions to accommodate hydrophobic guests offer other
illustrative examples of the unique possibilities opened by
dendritic structures. In this respect, Newkome and colleagues
reported the first example of hydrocarbon dendrimers externally
functionalized with carboxylates and internally with boron
clusters.19 In this contribution, we describe for the first time
how the interiors of dendrimer-like PEOs of fifth generation
were derivatized and PAA chains were internally grown. To
obtain such PAA-carrying dendrimer-like PEOs, we relied on
a divergent approach similar to that developed for the synthesis
of dendrimer-like PEOs of the eighth generation.9 Starting from
a triol as the inner core, steps of the polymerization of ethylene
oxide and chain-end branching were repeated; as the branching
points are the only sites within these dendritic architectures from
which polyacrylate chains could be grown, an original branching
agent carrying a vinylic function was designed. Dendrimer-like
PEOs of the fourth generation including 21 vinyl-fitted branch-
ing points were thus obtained. A fifth generation of PEO chains
was grown after introducing at the tip of the fourth generation
branches an AB2-type, allyl free, branching agent. After
derivatization of the 21 inner branching points into atom transfer
radical polymerization (ATRP) initiating sites, poly(tert-buty-
lacrylate) chains were grown and modified into PAA. Then the
In the continuity of our effort to assemble PEO chains in
various architectures for biomedical applications, we recently
directed our work toward generating dendrimer-like PEOs that
could be sensitive to variations of pH. PEO in itself is insensitive
(5) See for instance: Taton, D.; Saule, M.; Logan, J.; Duran, R.; Hou, S.;
Chaikof, E. L.; Gnanou, Y. J. Polym. Sci., Part A: Polym. Phys. 2003, 41,
1669 and references therein.
(6) Hawker, C. J.; Chu, F.; Pomery, P. J.; Hill, D. J. T. Macromolecules 1996,
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G.; Penczek, S. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1576. (c)
Walach, W.; Trzebicka, B.; Justynska, J.; Dworak, A. Polymer 2004, 45,
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(8) Hou, S.; Taton, D.; Saule, M.; Logan, J. L.; Chaikof, E. L.; Gnanou, Y.
Polymer 2003, 44, 5067.
(9) Feng, X. S.; Taton, D.; Chaikof, E. L.; Gnanou, Y. J. Am. Chem. Soc.
2005, 127, 10956.
(10) For a review on dendrimers, see for instance: (a) Hawker, C. J. AdV. Polym.
Sci. 1999, 147, 113. (b) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int. Ed.
1999, 38, 884. (c) Roovers, J.; Comanita, B. AdV. Polym. Sci. 1999, 142,
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Grayson, S. M.; Fre´chet, J. M. J. Chem. ReV. 2001, 101, 3819. (f) Inoue,
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A., Eds.; John Wiley & Sons: New York, 2001. (h) Tomalia, D. A. Prog.
Polym. Sci. 2005, 30, 294. (i) Smith, D. K. Chem. Commun. 2006, 34.
(11) For a review on dendrimers with macromolecular generations, see Teerstra,
S. J.; Gauthier, M. Prog. Polym. Sci. 2004, 29, 277.
(12) Examples of dendrimer-like polymers described in the recent literature:
(a) Six, J.-L.; Gnanou, Y. Macromol. Symp. 1995, 95, 137. (b) Tro¨llsas,
M.; Hedrick, J. L. J. Am. Chem. Soc. 1998, 120, 4644. (c) Taton, D.; Cloutet,
E.; Gnanou, Y. Macromol. Chem. Phys. 1998, 199, 2501. (d) Angot, S.;
Taton, D.; Gnanou, Y. Macromolecules 2000, 33, 5418. (e) Francis, R.;
Taton, D.; Logan, J.; Masse´, P.; Duran, R. S.; Gnanou, Y. Macromolecules
2003, 36, 8253. (f) Matmour, R.; Lepoittevin, B.; Joncheray, T. J; El
Khoury, R. J.; Taton, D.; Duran, R. S.; Gnanou, Y. Macromolecules 2005,
38, 5459. (g) Lepoittevin, B.; Matmour, R.; Francis, R.; Taton, D.; Gnanou,
Y. Macromolecules 2005, 38, 3120. (h) Tro¨llsas, M.; Hedrick, J. L. J. Am.
Chem. Soc. 1998, 120, 4644. (i) Tro¨llsas, M.; Atthoff, B.; Claeson, H.;
Hedrick, J. L. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1174. (j)
Tro¨llsas, M.; Atthoff, B.; Claesson, H.; Hedrick, J. L.; Pople, J. A.; Gast,
A. P. Macromolecules 2000, 33, 6423. (k) Hedrick, J. L.; Tro¨llsas, M.;
Hawker, C. J.; Atthoff, B.; Claesson, H.; Heise, A.; Miller, R. D.;
Mecerreyes, D.; Je´rome, R.; Dubois, Ph. Macromolecules 1998, 31, 8691.
(l) Tro¨llsas, M.; Claesson; Atthoff, B.; Hedrick, J. L. Angew. Chem., Int.
Ed. 1998, 37, 3112. (m) Hedrick, J. L.; Magbitang, T.; Connor, E. F.;
Glauser, T.; Volksen, W.; Hawker, C. J.; Lee, V. Y.; Miller, R. D. Chem.
Eur. J. 2002, 8, 3309. (n) Stancik, C. M.; Pople, J. A.; Tro¨llsas, M.; Lindner,
P.; Hedrick, J. L.; Gast, A. P. Macromolecules 2003, 36, 5765. (o) Chalari,
I.; Hadjichristidis, N. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1519
(p) Percec, V.; Barboiu, B.; Grigoras, C.; Bera, T. K. J. Am . Chem. Soc.
2003, 125, 6503. (q) Percec, V; Grigoras, C.; Kim, H.-J. J. Polym. Sci.,
Part A: Polym. Chem.. 2004, 42, 505. (r) Matsuo, A.; Watanabe, T.; Hirao,
A. Macromolecules 2004, 37, 6283. (s) Hirao, A.; Matsuo, A.; Watanabe,
T. Macromolecules 2005, 38, 8701.
(13) Khousakoun, E.; Gohy, J.-F.; Je´roˆme, R. Polymer 2004, 45, 8303.
(14) Robinson, D. N.; Peppas, N. A. Macromolecules 2002, 35, 3668.
(15) Hecht, S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1047.
(16) Fre´chet, J. M. J. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 3713.
(17) (a) Piotti, M. E.; Rivera, F., Jr.; Bond, R.; Hawker, C. J.; Fre´chet, J. M. J.
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Sci., Part A: Polym. Chem. 2001, 39, 821.
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1389. (b) Schultz, L. G.; Zhao, Y.; Zimmerman, S. C. Angew. Chem., Int.
Ed. 2001, 40, 1962. (c) Zimmerman, S. C.; Wendland, M. S.; Rakow, N.
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G. H. Angew. Chem., Int. Ed. Engl. 1994, 33, 666-668.
9
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pH Responsiveness of Dendrimer-like Polymers
A R T I C L E S
Table 1. Molecular Characteristics of Functionalized Dendrimer-like PEOs and Related Copolymers
N0 ofg
N0 ofg
double
bonds
Mn(theo)
(103)
Mn(SEC)
(103)
peripheral
hydroxyls
samples
Mn(NMR)(103)
PDI
PEOG1(OH)3
PEOG2(ene)3(OH)6
PEOG3(ene)9(OH)12
PEOG4(ene)21(OH)24
3.10a
11.0a
24.6a
52.7a
150a
171b
209b
260b
166c
3.00
10.0d
24.1d
61.8d
154f,d
174e
241e
274e
165f
3.60
11.8
24.9
36.3
54.6
36.6
61.5
63.8
nd
1.09
1.13
1.20
1.25
1.48
1.47
1.35
1.44
3
6
0
3
9
12
24
48
-
-
-
-
-
-
21
21
-
-
-
-
-
-
PEOG5(ene)21(OH)48
PEOG5(Pt-BA6)21(OBn)48
PEOG5(Pt-BA31)21(OBn)48
PEOG5(Pt-BA43)21(OBn)48
PEOG5(PAA5)21(OBn)48
PEOG5(PAA25)21(OBn)48
PEOG5(PAA37)21(OBn)48
-
-
-
203c
196f
nd
nd
221c
214f
a Theoretical molar mass. b ATRP of t-BA was performed at 80 °C in toluene (25 vol %) using PEOG5(Br)21(OBn)48 as macroinitiator (Mn ) 158 000
g‚mol-1), feed ratio: [Br]/[CuBr]/[PMDETA]/[t-BA])1/1/2/234; theoretical molar mass was calculated according to the coversion Mn(theo))([t-BA]/[Br] ×
conv × 128) + 158k, where 128 and 158k are the molar masses of t-BA and macroinitiator, respectively. c Calculated based on the complete removal of
tert-butyl groups in the Pt-BA block. d Calculated by 1H NMR using the peak of alkene protons according to formula in the text. e Determined by 1H NMR
in CDCl3 using the molar mass (158k) of the macroinitiator as reference. f Determined by 1H NMR in MeOD supposing the molar mass of PEO (158k) block
remains the same. g Theoretical values.
Figure 1. (A) Representation of all starlike and dendrimer-like PEOs described in this work. (B) Structure of constitutive units in starlike and dendrimer-
like PEOs described in (A).
and then vacuum distilled before polymerization. All PEO precursors
were dried by freeze-drying from a dioxane solution. The solvents used
for polymerization or chemical reaction, tetrahydrofuran (THF), CH2-
Cl2, toluene, and dimethyl sulfoxide (DMSO) were distilled over CaH2
prior to use. 5,5-Dimethyl-2-hydroxymethyl-1,3-dioxane (ket-OH)12a
and PEOG1(OH)3 (Mn(NMR) ) 3000 g/mol) were synthesized as
reported. All other chemicals were from Aldrich and used without
further purification.
pH responsiveness of these PAA-carrying dendrimer-like PEOs
was investigated; for the first time, spherically shaped PEOs
were demonstrated to shrink and expand as a function of pH.
Experimental Section
9
Materials. Ethylene oxide (EO) (Fluka, 99.8%) was distilled over
sodium into a buret. Diphenylmethylpotassium (DPMK) was prepared
in THF and titrated with acetanilide according to well-known proce-
dures.9 tert-Butylacrylate (t-BA) (Aldrich, 99%) was stored over CaH2
Synthesis of 2-(2′-Chloromethyl-propenyloxymethyl)-2-methyl-
9
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A R T I C L E S
Feng et al.
Figure 2. 1H NMR spectra (DMSO-d; 400 MHz) of three-arm PEO star derivatives: PEOG1(OH)3, PEOG1(ene)3(ket)3, and PEOG1(ene)3(OH)6.
5,5-dimethyl-1,3-dioxane (2 or DBBA, AB2C-Type Branching Agent
with Allylic Double Bond). To a 100-mL round flask containing
2-hydroxymethyl-2-methyl-5,5-dimethyl-1, 3-dioxane (8 g, 50 mmol)
in dry THF (50 mL) solution was added NaH (3.0 g). Then methallyl
dichloride (9.4 g, 75 mmol) was added under nitrogen atmosphere at
room temperature, and the whole mixture was stirred overnight at 50
°C; after removal of the precipitate by filtration and evaporation of the
solvent, DBBA (10 g, 80%) could be recovered as colorless oil after
vacuum distillation. 1H NMR (δppm, CDCl3): 5.23 (d, 2H, dCH2), 4.08
(s, 4H, ClCH2CCH2O-), 3.72-3.43 (m, 6H, -OCH2CCH3(CH2O)2-
(ene)3(ket)3 (3.5 g) was added 10 mL (0.1 M) of HCl in MeOH under
nitrogen atmosphere. After the solution was stirred at room temperature
overnight, the MeOH was evaporated under reduced pressure. The
residue was extracted with dichloromethane two times. The organic
layers were dried over anhydrous MgSO4 and concentrated. The solution
was precipitated in cold diethyl ether. The product (3.0 g, 86%) was
obtained after filtering and drying in a vacuum at room temperature.
1H NMR (δppm, DMSO-d6): 5.10 (s, 6H, CdCH2), 4.30 (t, 6H, OH),
3.88 (d, 12H, OCH2CCH2O), 3.51 (PEO, broad peak), 0.84 (s, 3H,
CH3), 0.76 (s, 9H, CH3).
Representative Procedure for Polymerization of EO with Their
Respective PEO Precursors: Synthesis of PEOG2(ene)3(OH)6. To
a two-neck 250-mL flask charged with the lyophilized dry precursor
PEOG1(ene)3(OH)6 (2.1 g, 3.6 mequiv OH) was added dry DMSO (60
mL) under vacuum. DPMK (1.1 mmol) was introduced at -20 °C,
and the temperature was slowly raised to room temperature and stirred
until the red color of DPMK disappeared and homogeneous solution
was formed. The flask was again chilled, and EO (4.5 mL, 90 mmol)
was added. The polymerization was carried out at room temperature
for 3 days. The alkoxides were deactivated with methanol. The solvent
was distilled under vacuum, and the polymer (5.8 g) was obtained by
double precipitation with diethyl ether from a THF solution. 1H NMR
(δppm, DMSO-d6): 5.11 (s, 6H, CdCH2), 4.60 (t, 6H, OH), 3.90 (d,
12H, OCH2CCH2O), 3.51 (PEO, broad peak), 0.85 (s, 12H, CH3).
Mn(NMR) ) 10000, Mw/Mn ) 1.13.
Hydroboration-Oxidation of Interior Alkene, Synthesis of
PEOG5(OH)21(OBn)48. To a 50-mL dry flask were added PEOG5-
(ene)21(OBn)48 (2.3 g, 0.32 mequiv CdC) and 10 mL of dried THF
under N2. After the solution was cooled to 0 °C in an ice bath, 3.2 mL
(1.6 mmol) of 0.5 M 9-BBN of THF solution was added slowly and
stirred for 24 h. The mixture was quenched with 2.2 mL of 3 M NaOH
solution followed by a dropwise addition of 0.6 mL (4.8 mmol) of
30% H2O2 aqueous solution. THF was evaporated under reduced
pressure after stirring for 3 h. The residue was extracted with
dichloromethane. The organic layers were dried over anhydrous MgSO4
and concentrated. The solution was precipitated in diethyl ether. The
), 1.39 (d, 6H, (CH3)2CO2), 0.87 (s, 3H, CH3). 13C NMR (δppm
,
CDCl3): 142, 116, 97.8, 73.1, 71.4, 66.5, 45.1, 34.3, 26.7, 20.7, 18.2.
Synthesis of 2-(3′-Chloromethybenzyloxymethyl)-2-methyl-5,5-
dimethyl-1,3-dioxane (AB2-Type, Aromatic Branching Agent). The
procedure was similar to that mentioned above. The targeted compound
was obtained as colorless oil in 56% yield after vacuum distillation.
1H NMR (δppm, CDCl3): 7.31 (m, 4H, aromatic protons), 4.56 (s, 4H,
ClCH2PhCH2O-), 3.72-3.43 (m, 6H, -OCH2CCH3(CH2O)2-), 1.40
(d, 6H, (CH3)2CO2), 0.90 (s, 3H, CH3). 13C NMR (δppm, CDCl3): 139,
137, 128, 127.5, 127.4, 127.3, 97.8, 73.1, 72.8, 66.5, 46.2, 34.3, 26.6,
20.8, 18.2.
Representative Procedure for Etherification of Terminal Hy-
droxyls of Dendrimer-like PEO (with Branching Agent or Benzyl
Chloride): Synthesis of PEOG1(ene)3(ket)3. To a solution of tetrabu-
tylammonium bromide (TBAB) (135 mg, 0.42 mmol) and NaOH (1.68
g, 42 mmol) in 1.7 mL of water were added PEO-G1(OH)3 (4.2 g, 4.2
mequiv OH) and THF (4 mL). After stirring 30 min at 50 °C, DBBA
(2.1 g, 8.5 mmol) was added under N2. The solution was kept for 24
h at 50 °C under vigorous stirring. The volatiles were removed, and
the residues were extracted with dichloromethane. The solution was
dried and concentrated. The product was obtained by precipitation with
1
excess cold diethyl ether (3.8 g, 90%). H NMR (δppm, CDCl3): 5.14
(s, 6H, CdCH2), 3.97 (d, 12H, OCH2CCH2O), 3.64 (PEO, broad peak),
1.37 (d, 18H, (CH3)2CO2), 0.87, 0.85 (two s, 12H, CH3).
Representative Procedure for Deprotection of Ketal Group:
Synthesis of PEOG1(ene)3(OH)6. To a round flask containing PEOG1-
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pH Responsiveness of Dendrimer-like Polymers
A R T I C L E S
Scheme 1. Overall Synthetic Strategy for Dendrimer-like PEOs with Inner Double Bonds; the Synthesis of the AB2C Branching Agent Is
Shown in Scheme 2
product (1.8 g, 78%) was obtained after filtering and drying in a vacuum
at room temperature. 1H NMR (δppm, DMSO-d6): 7.33-7.24 (m, 336H,
aromatic protons), 4.49, 4.44 (two s, 192H, OCH2Ph), 4.34 (t, 21H,
OH), 3.51 (PEO, broad peak), 0.88, 0.84 (two s, 138H, CH3).
Synthesis of ATRP Macroinitiator PEOG5(Br)21(OBn)48. Under
N2 atmosphere, to a dried, two-neck, 100-mL flask equipped with a
magnetic bar, 2.1 g (0.3 mequiv OH) of PEOG5(OH)21(OBn)48 was
first dissolved in 50 mL of dried CH2Cl2 before triethylamine (4.2 mL,
30 mmol) and 2-bromopropionyl bromide (3.1 mL, 30 mmol) were
added dropwise at room temperature. After 2 days of reaction, the
precipitated salts were removed through filtration. To remove the
ammonium salts contained in the crude product, it was dissolved in 10
mL of toluene, and the mixture was centrifuged to separate the salts
from the polymer. Such procedure was repeated until no salts appeared
in toluene solution (two or three times were needed). Finally, 1.9 g of
pure product was obtained after drying in a vacuum at room temper-
ature. 1H NMR (δppm, DMSO-d6): 7.33-7.24 (m, 336H, aromatic
protons), 4.67 (q, 21H, CH3CHBrCOO), 4.49, 4.44 (two s, 192H, OCH2-
Ph), 4.16-4.20 (m, 42H, CH2OCOCHBr), 3.51 (PEO, broad peak),
2.18 (m, 21H, CH(CH2O)3-), 1.71 (d, 63H, CHBrCH3), 0.87, 0.84
(two s, 138H, CH3).
macroinitiator, PEOG5(Br)21(OBn)48 (0.42 g, 0.057 mequiv Br), tert-
butylacrylate (2.0 mL, 13.6 mmol), distilled toluene (6 mL), and
N,N,N,N,N-pentamethyldiethylenetriamine (PMDETA) (24 µL, 0.11
mmol) and degassed before CuBr (8 mg, 0.057 mmol) under nitrogen.
The solution was degassed three times with freeze-thaw cycles. For a
given reaction time the flask was immersed in an oil bath preheated at
80 °C. The crude polymer was recovered directly through precipitation
in pentane. Then the polymer was dissolved in dichloromethane and
washed with dilute ammonium hydroxide solution until no blue color
appeared in solution. The copolymer was recovered by precipitation
in pentane after the organic solution was dried over magnesium sulfate
and concentrated. The related data are shown in Table 1 and SI6 in the
Supporting Information.
Hydrolysis of PEOG5(Pt-BA)21(OBn)48 Copolymers. Into 3 mL
of dichloromethane solution of the diblock polymer (300 mg), trifluo-
roacetic acid (0.9 mL, 11.7 mmol) was slowly added at 0 °C with
vigorous stirring. The reaction was kept at 0 °C for 3 h and then
overnight at room temperature. The reaction mixture was carefully
evaporated and then redissolved in dioxane for purification by freeze-
drying.
Preparation of Solution for Dynamic Light Scattering (DLS).
Thirty milligrams of ATRP macroinitiator was dissolved in 10 mL of
DMF, and the solution stirred for 3 days at room temperature. Then
Synthesis of PEOG5(Pt-BA)21(OBn)48 Block Copolymers by
ATRP. In a typical polymerization, a Schlenk flask was charged with
9
J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11555

A R T I C L E S
Feng et al.
the solution was filtered with 0.2 µm Millipore filter for DLS analysis.
As for the characterization of the PEOG5(PAA)21(OBn)48 copolymer
samples in water, the following method was applied. First, 20 mg of
sample was dissolved in 10 mL of LiCl water solution (1 M, pH )
11). After it was stirred at 50 °C for one week, the solution was filtered
with 0.1 µm Millipore filter (only one unimer population was obtained
based on the DLS result). The salt LiCl was removed through dialysis
against deionized water with a membrane (Spectra/Por membrane with
cutoff of 3500 Da) for another week. Then the solution was again
filtered with 0.1 µm Millipore filter and adjusted to different pHs for
DLS analysis with concentrated acid or base (prefiltered with 0.1 µm
Millipore filter).
Characterization. 1H NMR spectra were recorded on a Bruker AC
200 spectrometer. The molar masses were determined by size exclusion
chromatography (SEC) equipped with a PSS column (8 mm × 300
mm, 5 µm), a refractive index detector (Varian RI-4), and with
tetrahydrofuran (THF) as eluent (1 mL/min) at 25 °C and calibration
using linear polystyrene samples.
MALDI-TOF mass spectrometry was performed using a Micromass
TofSpec E spectrometer equipped with a nitrogen laser (337 nm), a
delay extraction, and a reflector. The MALDI mass spectra represent
averages of over 100 laser shots. This instrument operated at an
accelerating potential of 20 kV. The polymer solutions (10 g‚L-1) were
prepared in THF. The matrix solution (1,8-dithranol-9(10H)-anthra-
cenone, dithranol) was dissolved in THF. The polymer solution (2 µL)
was mixed with 20 µL of the matrix solution, and 2 µL of a sodium
iodide solution (10 g‚L-1 in methanol) was added to favor ionization
by cation attachment. The final solution (1 µL) was deposited onto the
sample target and dried in air at room temperature.
Dynamic light scattering (DLS) experiments were performed using
an ALV Laser Goniometer, which consists of a 22 mW HeNe linear
polarized laser with 632.8 nm wavelength and an ALV-5000/EPP
Multiple Tau Digital Correlator with 125-ns initial sampling time. The
samples were kept at constant temperature (25.0 °C) during all
experiments. The accessible scattering angular range varied from 40°
up to 150°. The solutions were introduced into the 10-mm diameter
glass cells. The minimum sample volume required for the experiment
was 1 mL. The data acquisition was done with the ALV-Correlator
Control software, and the counting time varied for each sample from
300 s up to 600 s. Millipore water was thoroughly filtered through 0.1
µm filters and directly used for the preparation of the solutions. All
the solutions showed a monomodal distribution with a translational
diffusive mode. The hydrodynamic radius (RH) of each dendrimer-like
sample was then calculated from the diffusion coefficient using the
Stokes-Einstein relation
sites from which poly(tert-butylacrylate) chains can be grown
before being hydrolyzed into PAA chains. For the synthesis of
such dendrimer-like PEOs, we relied on an iterative divergent
method combining living anionic ring-opening polymerization
(AROP) of ethylene oxide (EO) and chain-end functionalization/
branching reactions. As discussed in detail in a previous work,9
the conditions the best suited to polymerize EO from multi-
functional hydroxylated precursors require that the latter be
partially deprotonated (below 30%) by a solution of DPMK and
that the AROP be carried out in dimethyl sulfoxide (DMSO).
By preventing the aggregation of propagating alkoxides and
promoting a rapid exchange of protons with dormant hydroxy-
lated species, such conditions permitted AROP of EO to proceed
in a controlled way, yielding PEO star samples of targeted molar
masses and low polydispersities.9 A sketch of the 18 starlike
and dendrimer-like PEO compounds synthesized in the present
work is displayed in Figure 1.
The trifunctional precursor, 1,1,1-tris(hydroxymethyl)ethane
(1), thus afforded well-defined, hydroxy-ended, three-arm PEO
stars, PEOG1(OH)3, following the procedure already reported.9
Because inner poly(tert-butylacrylate) chains are to be subse-
quently grown from the branching points separating each
generation of PEO, an original branching agent of AB2C-type
was designed. The latter contains (i) a chloromethyl group (A)
for the hooking reaction with the hydroxyl groups of PEO arms,
(ii) two hydroxyls (B2) masked under a ketal form to be released
for the growth of the next generation by AROP of EO, and (iii)
an allylic group (C) for derivatization purposes into ATRP sites.
Allyls were chosen as C groups because they are insensitive to
alkoxides present in both AROP of EO and during chain-end
branching. Moreover, allyl functions readily undergo chemical
modification into hydroxyls or carboxylic groups. For this
purpose, methallyl dichloride (MDC) was used as starting
material to derive the AB2C-type branching agent, namely, 2-(2′-
chloromethyl-propenyloxymethyl)-2-methyl-5,5-dimethyl-1,3-
dioxane (DBBA) (2), as described in Scheme 1. As shown by
Fre´chet and colleagues, MDC proved an efficient building block
in Williamson-type reactions for the synthesis of dendritic
aliphatic polyethers.20 In this work, a large excess of MDC was
reacted with 5,5-dimethyl-2-hydroxymethyl-1,3-dioxane under
basic conditions to ensure that only DBBA was formed as a
monoadduct. The 1H and 13C NMR spectra of DBBA confirmed
the expected structure (see SI1 in the Supporting Information).
This branching agent was then reacted with PEOG1(OH)3 in
a mixture of water and THF and in the presence of a phase
transfer catalyst (TBAB), using an etherification procedure
similar to that already reported:9 a three-arm PEO star, noted
PEOG1(ene)3(ket)3, carrying three double bonds and two
geminal hydroxyls protected in the form of a ketal ring (ket) at
each arm end could be obtained. The release of hydroxyl groups
was readily accomplished by acidic hydrolysis, giving access
to a three-arm PEO star, PEOG1(ene)3(OH)6, possessing three
double bonds attached to the outer branching points and six
terminal hydroxyls (Scheme 1). The effectiveness of this two-
step branching reaction was monitored by 1H NMR spectroscopy
and by MALDI-TOF mass spectroscopy. Figure 3 shows the
1H NMR spectra of three-arm PEOG1(OH)3, PEOG1(ene)3-
(ket)3, and PEOG1(ene)3(OH)6. After treatment with DBBA,
the terminal OH protons entirely disappeared, whereas the
kT
6ΠηRH
D )
/
where η is the viscosity of the medium (water) and
Γ
D )
q2
when πq f 0 (q being the wave vector and
4π
λ‚sin(θ/2)
q )
Results and Discussion
Synthesis of Dendrimer-like PEOs of Fifth Generation
Containing 21 Inner Double Bonds. Two main sequences are
to be distinguished in the synthesis of the targeted PAA-carrying
dendrimer-like PEOs of fifth generation: first, the formation
of dendrimer-like PEOs containing inner allylic double bonds
and second, the derivatization of the latter into ATRP-initiating
(20) Grayson, S. M.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122, 10335.
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pH Responsiveness of Dendrimer-like Polymers
A R T I C L E S
Figure 3. MALDI-TOF MS of PEOG1(OH)3, PEOG1(ene)3(ket)3, and PEOG1(ene)3(OH)6.
protons due to the allylic double bonds and to the six methyl
protons of the ketal ring of PEOG1(ene)3(ket)3 were detected
at 5.14 ppm and at 1.37 ppm, respectively. In addition, the
methyl protons belonging to the core of the star at 0.87 ppm
and those of the branching entities appearing at 0.85 ppm could
be clearly distinguished. As shown in Figure 2, the relative
intensity of this series of peaks is in good agreement with the
expected values: I0.87:I0.85:I5.14:I1.37 ) 1:3:2:6, which indicates
that the etherification reaction occurred quantitatively. Protons
of primary hydroxyls of PEOG1(ene)3(OH)6 could be clearly
identified at δ ) 4.30 ppm after the hydrolysis step, and the
signals at 0.84 and 0.76 ppm ascribed to the two types of methyl
groups (core and branching points) mentioned above are now
well resolved. More importantly, the protons of double bonds
are still observed, meaning that they were not affected by the
acidic deprotection.
MALDI-TOF mass spectroscopy is another powerful char-
acterization means to monitor the derivatization of polymer
chain ends and, more generally, to determine the quality of the
dendrimer samples prepared.21 Figure 3 shows the MALDI-
TOF mass spectra of PEOG1(OH)3, PEOG1(ene)3(ket)3, and
PEOG1(ene)3(OH)6. In all cases, only one single distribution
is observed with a peak-to-peak mass increment of 44.05
g‚mol-1 corresponding to the molar mass of one EO unit. The
distribution of chain could be perfectly accounted for, taking
into account the molar mass of both the core and chain ends.
(21) Van Renterghem, L. M.; Feng, X.; Taton, D.; Gnanou, Y.; Du Prez, F. E.
Macromolecules 2005, 38, 10609.
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The reiteration of the same divergent method allowed us to
obtain the third and the fourth generation of dendrimer-like
PEOs, the latter compound containing 24 external hydroxyls
and 21 internal allylic groups; its molar mass determined by
1H NMR was found to be 61,800 g‚mol-1. Characterization of
1
these samples by H NMR afforded values of molar masses
very close to the targeted ones (Table 1). For these calculations,
the dendritic samples synthesized were assumed to be free of
any defects and the two-step branching reaction to be quantita-
tive, as shown by the MALDI TOF MS results for the first-
generation sample. The integration value of the allylic protons
was taken as reference and the following formula:
I3.51
Mn(NMR) ) 44f ×
2I5.10
was used to calculate the sample molar masses, where I3.51 and
I5.10 are the integral values of peaks at 3.51 and 5.10 ppm,
respectively, and f is the theoretical number of double bonds
present in the polymer (see Table 1). As shown in Figure 5,
the SEC traces of the four generations of these dendritic PEOs
reflect symmetrical and relatively narrow molar mass distribu-
tions (polydispersities were lower than 1.15) free of any side
population in the low molar mass region. A slight broadening
of the peaks as the generation number increases is, however,
observed, likely due to interactions of these OH-ended PEOs
with the chromatographic support in the presence THF used as
the eluent. These data confirm that well-defined and well-
functionalized dendrimer-like PEOs were obtained, which was
also supported by results obtained using dynamic light scattering
(see below).
Figure 4. SEC traces (THF, RI detector) of dendrimer-like PEOs from
generations 1-5 (see also Table 1).
The peaks, indeed, appeared at
m/z ) 44.05n + Mtermi + 23
where n is the degree of polymerization, 23 is the molar mass
of the sodium ion generated during the ionization process, and
Mtermi is the molar mass of end groups and of the core. These
results bring further evidence that PEOG1(OH)3 could be
quantitatively modified with DBBA and the branching points
selectively introduced at arm ends of three-arm PEO stars.
The same sequence of reactions (i), (ii), and (iii) was applied
using PEOG1(ene)3(OH)6 as precursor to produce the second-
generation dendrimer-like PEO, denoted PEOG2(ene)3(OH)6.
Again, only 30% of the six OH groups were deprotonated using
DPMK, and DMSO was the solvent of polymerization in step
(i). The SEC trace of the PEO derivative obtained, PEOG2-
(ene)3(OH)6, showed a symmetrical and unimodal shape with a
marked shift to the higher molar mass region with regard to
that of the PEOG1(OH)3 precursor (Figure 4).
A fifth generation of PEO chains with a molar mass of
approximately 2000 g‚mol-1 per arm was subsequently grown
from the parent dendrimer-like PEOG4(ene)21(OH)24 of fourth
generation, according to the four-step sequence shown in
Scheme 3. In the present case, a “conventional” AB2-type
branching agent, namely 2-(3′-chloromethybenzyloxy)-5,5-di-
methyl-1,3-dioxane (3), free of any allylic double bond was
hooked at the tip of PEO branches of fourth generation by
etherification reaction. As it is essential that AA and EO units
interact only intramolecularly in the final PAA-carrying den-
drimer-like PEO for the latter to exhibit pH responsiveness, the
branching points fitted with polyacrylate-initiating sites were
to be confined in the interior of the architecture. Consequently,
only branching points of the three first generations included
derivatizable allylic double bonds. To offer a better shielding
to possible intermolecular interactions between PAA and PEO,
the branching agent of the fourth generation was thus designed
so as to provide only two initiating sites for AROP of EO; it
was synthesized by reaction of 1,3-dichloromethylbenzene with
5,5-dimethyl-2-hydroxymethyl-1,3-dioxane under basic condi-
tions (see the Experimental Section for its NMR characteriza-
tion). After etherification, the PEO derivative denoted PEOG4-
(ene)21(ket)24 was obtained. Next, the deprotection of the 48
terminal hydroxyls by acidic treatment of PEOG4(ene)21(ket)24
afforded the corresponding PEOG4(ene)21(OH)48 (Figure 1 and
Scheme 2). The presence of the characteristic protons of the
1
The H NMR spectrum of PEOG2(ene)3(OH)6 revealed all
the expected signals, including the signal of both types of methyl
protons (core and branching points) at 0.8 ppm, that of the
terminal hydroxyls (CH2OH) at 4.6 ppm and also that due to
the protons of EO units around 3.5 ppm (see SI2 in Supporting
Information). From the integration ratio of the two latter signals
the molar mass of this dendrimer-like PEO of second generation
could be evaluated, Mn(NMR) ) 10,000 g‚mol-1 (Table 1), a value
in relative good agreement with the theoretical one calculated
from the molar ratio of [ethylene oxide] to the [PEOG1(ene)3-
(OH)6] precursor (Mn(theo) ) 11,000 g‚mol-1). Mn values could
also be calculated from the protons of allylic groups taken as
reference, as discussed below. Then PEOG2(ene)3(OH)6 was
subjected to reactions (ii) and (iii), which afforded the second-
generation dendrimer-like PEOG2(ene)9(OH)12 containing a total
of nine allylic branching points and 12 (6 × 2) gemini-type
terminal hydroxyls. The transformation of the six primary
hydroxyls of PEOG2(ene)3(OH)6 into six peripheral ketal rings
by reaction with DBBA and the cleavage reaction that released
the 12 hydroxyl functions of PEOG2(ene)9(OH)12 were moni-
1
AB2-type branching agent was checked by H NMR (see SI3
1
tored by H NMR ((see SI2 in the Supporting Information).
in the Supporting Information); aromatic protons and those of
the methylene group bonded to the aromatic ring appeared at
7.20 and 4.50 ppm, respectively. Because MALDI-TOF mass
Again, all peaks could be assigned, and integration ratios were
in rather good concordance with theoretical values.
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pH Responsiveness of Dendrimer-like Polymers
A R T I C L E S
Figure 5. MALDI TOF MS spectrum of the reaction product between a linear R-methoxy-ω-hydroxy PEO and the AB2-type branching agent 3.
Scheme 2. Synthesis of AB2C (2) and AB2 (3) Branching Agents
iamson reaction using benzyl bromide; this afforded PEOG5-
(ene)21(OBn)48, a sample whose NMR characterization con-
firmed the expected structure (see SI5 in the Supporting
Information). Blocking the hydroxyl termini with benzyl groups
induced a decrease of the SEC apparent molar mass from 54,-
600 to 44,000 and of the PDI from 1.48 to 1.20. As illustrated
in Schemes 1 and 3 (see also Figure 1) the overall synthetic
strategy which combined AROP of EO and selective derivati-
zation of PEO chain ends thus afforded dendrimer-like PEO
samples whose chain lengths for each generation and whose
double-bond positionings could be precisely controlled.
Derivatization of the Interior of Dendrimer-like PEOs and
Generation of PAA Chains. The presence of the 21 double
bonds inside these dendrimer-like PEOs provided us with the
possibility of derivatizing them for subsequent growth of poly-
spectrometry could not be used to characterize such a high molar
mass sample, a model branching reaction was carried out to
check the efficiency of the etherification reaction using 3 as
branching agent and a linear R-methoxy,ω-hydroxy PEO sample
(tert-butylacrylate) chains by the “grafting from” technique.
[or poly(ethylene glycol methyl ether), Mn ) 2000 g‚mol-1].
After hydrolysis of their tert-butyl ester groups, short pH-
Characterization by both 1H NMR (see SI4 in Supporting
sensitive PAA would be dangling from the branching junctions
Information) and MALDI TOF mass spectrometry attested to a
of dendrimer-like PEOs. Such chemical transformations could
quantitative derivatization of the OH end group of this PEO
actually be achieved in a four-step sequence, as depicted in
precursor (Figure 5). Indeed, the integration ratio of the signal
Scheme 4. Primary hydroxyls were first generated by treatment
at 3.36 ppm due to the methyl of the methoxy end group to
of PEOG5(ene)21(OBn)48, using 9-9-borabicyclo[3.3.1]nonane
those characteristics of the branching agent matched the
(9BBN) and H2O2 in a hydroboration-oxidation reaction of
expected value. In addition, the MALDI TOF mass spectrum
double bonds, according to a known procedure;22 this resulted
of the functionalized PEO sample shows a single population
in the formation of PEOG5(OH)21(OBn)48. The efficiency of
corresponding to the expected molar mass distribution, which
is
1
this first step was monitored by H NMR: signals at 1.88 and
4.30 ppm respectively characteristic of -CHCH2OH and
-CHCH2OH protons are clearly discerned in the corresponding
spectrum, while that due to allylic protons completely vanished.
M ) 294 + 44.05n + 23
Next, bromoester functions were readily introduced by esteri-
fication of the hydroxyls of PEOG5(OH)21(OBn)48 with 2-bro-
mopropionyl bromide in the presence of triethylamine, yielding
PEOG5(Br)21(OBn)48. 1H NMR spectroscopy (see SI5 in
Supporting Information) showed the appearance of a signal
around 4.13 ppm due to the protons of -CHCH2OCOCCH-
(CH3)Br group. This series of chemical modifications were also
reflected in the SEC traces, which showed slight changes of
the apparent molar masses and PDIs.
where 23 is the molar mass of the sodium counterion generated
by the MALDI TOF process and 294, the molar mass of the
end groups.
The dendrimer-like PEOG4(ene)21(OH)48 then served as a
precursor for AROP of EO; PEO branches of about 2000
g‚mol-1 were grown as the fifth generation of the dendrimer-
like polymer, denoted PEOG5(ene)21(OH)48, whose molecular
characteristics are reported in Table 1. To prevent any interfer-
ence of these terminal hydroxyls during the subsequent steps
of functionalization of the interior (see below), they were
quantitatively converted into apolar benzylic groups by Will-
(22) Brown, H. C.; Zweifel, G. J. Am. Chem. Soc. 1960, 82, 4708.
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A R T I C L E S
Feng et al.
Scheme 3. Synthesis of a Dendrimer-like PEO of Fifth Generation Possessing 21 Inner Double Bonds and 48 Peripheral Benzyl Groups;
the Synthesis of the AB2 Branching Agent Is Shown in Scheme 2
Scheme 4. Derivatization of the Inner Allylic Groups for the Synthesis of PAA-Carrying Dendrimer-like PEOs
Atom transfer radical polymerization (ATRP) of tert-buty-
lacrylate (t-BA) was then carried out at 80 °C in toluene using
PEOG5(Br)21(OBn)48 as macroinitiator, in the presence of CuBr/
pentamethyldiethylenetriamine as the catalytic system.23 Three
copolymer samples, noted PEOG5(Pt-BA)21(OBn)48, were pre-
pared by varying the chain length of poly(t-BA) blocks.
Characterization by SEC showed symmetrical traces shifting
to higher molar masses, compared to the SEC trace of the
PEOG5(Br)21(OBn)48 precursor (Figure 6), which attests to the
well-defined character of these amphiphilic systems. A final
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11560 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006

pH Responsiveness of Dendrimer-like Polymers
A R T I C L E S
Figure 7. CONTIN analysis obtained from correlation function measured
from the DMF solution for PEOG5(Br)21(Bn)48 (3 mg/mL) at the scattering
angle θ ) 90°.
Figure 6. SEC traces (THF, RI detector) of macroinitiator PEOG5(Br)21-
(OBn)48 and PEOG5(Pt-BA)21(OBn)48 dendrimer-like copolymers of dif-
ferent compositions.
step of selective hydrolysis of the tert-butyl esters of PEOG5-
(Pt-BA)21(OBn)48 was performed using trifluoroacetic acid in
dichloromethane, which afforded PEOG5(PAA)21(OBn)48. Typi-
1
cal H spectra (see SI6 in the Supporting Information) of the
dendritic copolymers obtained before and after hydrolysis are
shown with assignments of all peaks. In particular, the tert-
butyl ester protons of Pt-BA blocks disappeared after hydrolysis,
and the integration ratio of the signal ascribed to the methine
(2.2 ppm) to that of methylene protons (1.9-1.4 ppm) of PAA
is consistent with the value (1:2) of quantitative hydrolysis.
Investigation by Dynamic Light Scattering of the Size of
PAA-Functionalized Dendrimer-like PEOs as a Function of
pH. To demonstrate the impact of PAA pH sensitivity and of
its complexation with PEO on the overall size of the dendrimer-
like PEO, dynamic light scattering (DLS) is the most appropriate
technique. Experiments were performed on dilute solutions of
dendrimer-like PEO samples of the fifth generation. From the
analysis of the normalized intensity autocorrelation functions
C(q,t), q being the scattering vector, both the z-average
hydrodynamic radius (RH) and the distribution of hydrodynamic
size could be determined using the cumulant and CONTIN
method (Figure 7). All the measurements were carried out at
different scattering angles (q) ranging from 40° to 150°. The
autocorrelation functions were better described by a single-
exponential decay whose relaxation time or frequency Γ varies
linearly with q2, indicating diffusive behavior and a spherical
shape of the dendrimer-like particles. First, the size of the parent
dendrimer-like PEOG5(Br)21(OBn)48 was determined in dim-
ethylformamide (DMF), a good solvent for PEO chains, the
terminal groups as well as the branching points. No aggregation
phenomenon was observed as shown in Figure 7, which shows
one single population of narrow distribution corresponding to
a size of 2RH ) 18.4 nm (diameter) attributable to the individual
dendritic unimers in a good solvent.
Figure 8. Autocorrelation functions measured at the scattering angle of
90° and CONTIN analysis showing the time distribution.
sample in water. It was only under these conditions that one
single distribution was observed and no aggregates were formed.
A value of 30.8 nm was determined at pH ) 11 for the diameter
(2RH) of the copolymer whose total molar mass was equal to
158,000 g‚mol-1 in PEO and 38,700 g‚mol-1 in PAA. Com-
pared to the diameter of the precursor PEOG5(Br)21(OBn)48
(18.4 nm), the increase in size suggests that the presence of
PAA chains forces the overall dendritic architectures to expand.
In a second step, the solutions were dialyzed against deionized
water to remove LiCl. Then the response to pH of these water
solutions was investigated by DLS. Typical CONTIN analyses
of the DLS correlation functions at different pH are shown in
Figures 8 and 9. The results show that one single relaxation
describes the ongoing dynamical behavior at all investigated
pHs, including acidic condition (low pH). Significant changes
of the size of these double hydrophilic, dendrimer-like samples
were observed as a function of the pH, in particular for those
with the highest content in PAA. At low pH, the PEO branches
are forced to shrink due to the contraction of PAA and their
mutual complexation through hydrogen bonding. In contrast,
Before investigating the variation in size of PAA-carrying
dendrimer-like PEOG5(PAA)21(OBn)48 with the pH in water,
their solutions were carefully prepared: LiCl was added to avoid
the formation of aggregates during the solubilization of the PEO
(23) For an overview on ATRP, see for instance: (a) Matyjaszewski, K.; Xia,
J. Chem. ReV. 2001, 101, 2921. (b) Kamigaito, M.; Ando, T.; Sawamoto,
M. Chem. ReV. 2001, 101, 3689.
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A R T I C L E S
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Figure 9. Autocorrelation functions measured at the scattering angle of 90° and CONTIN analysis showing the time distribution.
at higher pH no such complexation exists; the neutralized PAA
chains then tend to expand by polyelectrolyte effect, compelling
in turn the entire dendrimer-like sample to expand accordingly.
The higher the content in PAA, the larger the changes in size:
for example, the diameter varied from 23 to 28.2 nm for PAA
with Mn ) 38,700, and from 33.4 to 51.8 nm for PAA with Mn
) 56.700. Because of the dendritic structure the interactions
between PAA and PEO PEOG5(Br)21(OBn)48 could be confined
at the intramolecular level, entailing pH responsiveness on the
part of these PEO-based dendrimer-like samples. Such a size
variation with the pH breaks the ground for new developments
in the biomedical area.
as the size of each generation and the position of functional
groups at the periphery or at the branching junctions could be
precisely controlled. After derivatization of the 21 allylic double
bonds (C) thus introduced, atom transfer radical polymerization
of tert-butylacrylate could be triggered from the 21 interior
branching points; internal poly(acrylic acid) were then generated
by simple hydrolysis. The spherical, double hydrophilic, den-
drimer-like copolymers thus formed demonstrated pH respon-
siveness at the nanometer-size range with a capacity to shrink
at low pH and expand at high pH. This remarkable feature is
the direct result of the particular dendritic arrangement of PAA
and PEO, which prevented their intermolecular complexation
and the formation of large aggregates. Instead, only intramo-
lecular complexation could develop in such architecture, hence
accomplishing the pH sensitivity of these dendrimer-like PEOs.
Conclusion
Dendrimer-like poly(ethylene oxide)s functionalized internally
with poly(acrylic acid) were successfully synthesized following
an iterative divergent approach that combined “living” anionic
ring-opening polymerization (AROP) of ethylene oxide (EO),
selective branching reactions of PEO chain ends, and post
modification of the interior of the dendritic macromolecules.
For this purpose, two different types of branching agents, one
of AB2C-typescarrying one hooking site (A), two protected
sites for AROP of EO (B), and a derivatizable allylic function
(C)sand the other of AB2-type were designed and successfully
used so that functional groups could be introduced at the interior
of dendrimer-like PEOs. In this way, molecular features such
Acknowledgment. We are grateful to the National Institute
for Health for the financial support of this work (program no.
5R01 RR14190-04). We also thank Nicolas Guidolin for his
technical assistance.
Supporting Information Available: Additional figures. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA0631605
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11562 J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006