Combination of an anionic terminator multifunctional initiator and divergent carbanionic polymerization: Application to the synthesis of dendrimer-like polymers and of asymmetric and miktoarm stars

Article abstract of DOI:10.1021/ja076442t

A new and versatile synthetic strategy that provides access to precisely defined and totally soluble multicarbanionic initiators has been implemented to obtain by divergent growth dendrimer-like samples of polystyrene (PS) (up to the seventh generation) o

Full text of DOI:10.1021/ja076442t

Published on Web 01/03/2008
Combination of an Anionic Terminator Multifunctional Initiator
and Divergent Carbanionic Polymerization: Application to the
Synthesis of Dendrimer-Like Polymers and of Asymmetric
and Miktoarm Stars
Rachid Matmour and Yves Gnanou*
Laboratoire de Chimie des Polyme`res Organiques (LCPO), ENSCPB-CNRS-UniVersite´ Bordeaux
1, 16, AVenue Pey Berland, 33607 Pessac Cedex, France
Received August 27, 2007; E-mail: gnanou@enscpb.fr
Abstract: A new and versatile synthetic strategy that provides access to precisely defined and totally soluble
multicarbanionic initiators has been implemented to obtain by divergent growth dendrimer-like samples of
polystyrene (PS) (up to the seventh generation) or polybutadiene (PB) (up to the third generation) and
also asymmetric and miktoarm stars. This strategy rests on lithium-halide exchange reactions to generate
multicarbanionic species and on the design of an original reagent that can concomitantly react with living
carbanionic chains/arms to deactivate them and produce multicarbanionic sites after exchange of its
bromides against lithium. This reagent, 4,4′-dibromodiphenylethylene (1), functions as a TERminating agent
and a Multifunctional INItiator (TERMINI), according to a concept first proposed by Percec in another context.
Upon using this anionic TERMINI in living carbanionic polymerization and repeating the two steps of chain
end derivatization by 1 and divergent arm growth from the multifunctional sites generated, perfectly defined
dendrimer-like polystyrene and polybutadiene could be obtained up to the seventh generation for the former
and up to the third generation for the latter. Each step, i.e., chain end modification and arm growth, was
carefully monitored, and the dendrimer-like samples of PS and PB were all characterized by size exclusion
chromatography equipped with a multiangle laser light scattering detector (SEC/LS) and high-temperature
size exclusion chromatography equipped with a viscometric detector (HT-SEC). The viscosity behavior of
these dendrimer-like polystyrenessbell-shaped variation versus the number of generationswas found to
be similar to that reported for regular dendrimers. This chemistry, namely this anionic TERMINI, was also
exploited to derive three-arm asymmetric and miktoarm stars.
1. Introduction
the growth of the generation to come. Hedrick and co-workers
followed a similar approach to generate dendrimer-like aliphatic
polyesters of the third generation including 24 arms.4
We have been investigating the design and synthesis of
complex architectures that exhibit the shape and structure
perfection required to accomplish certain high-value functions.1
For instance, we recently reported the synthesis of dendrimer-
like poly(ethylene oxide) (PEO) that revealed anti-inflammation
properties upon modification of their outer functions with
glycosidic units.2 Likewise dendrimer-like PEO of radial sym-
metry and spherical shape were shown to expand and shrink
upon modification of their inner part with short poly(acrylic
acid) segments.3 Such dendrimers are designed to transport
encapsulated species and deliver them in high pH media.
Very convenient when the generations to be grown require
active species such as alkoxides, this divergent approach proves
also very useful in atom transfer radical polymerization (ATRP).
For instance, Hedrick and co-workers end-capped six-arm
polyester stars with 12 or 24 ATRP initiating sites and grew a
second generation of poly(methyl methacrylate) (PMMA) and
poly(hydroxyethyl methacrylate) (PHEMA) arms to eventually
obtain dendrimer-like star block copolymers.5-7 More recently
Percec et al. also resorted to a combination of “living” radical
polymerization (LRP) and chain end derivatization to prepare
by divergent growth dendritic PMMAs of the third generation.8,9
The chemistry used to construct these dendritic scaffolds was
based on a divergent iterative approach combining “living” ring-
opening polymerization of ethylene oxide (EO) from a pluri-
oxanionic initiator and subsequent branching reactions meant
to multiply by a factor of 2 the number of initiating sites for
(4) Tro¨llsas, M.; Hedrick, J. L. J. Am. Chem. Soc. 1998, 120, 4644.
(5) Hedrick, J. L.; Tro¨llsas, M.; Hawker, C. J.; Atthoff, B.; Claesson, H.; Heise,
A.; Miller, R. D.; Mecerreyes, D.; Je´rome, R.; Dubois, P. Macromolecules
1998, 31, 8691.
(6) Glauser, T.; Stancik, C. M.; Moeller, M.; Voytek, S.; Gast, A. P.; Hedrick,
J. L. Macromolecules 2002, 35, 5774.
(1) Feng, X.-S.; Taton, D.; Chaikof, E. L.; Gnanou, Y. J. Am. Chem. Soc.
2005, 127, 10956.
(2) Rele, S. M.; Cui, W.; Wang, L.; Hou, S.; Barr-Zarse, G.; Taton, D.; Gnanou,
Y.; Esko, J. D.; Chaikof, E. L. J. Am. Chem. Soc. 2005, 127, 10132.
(3) Feng, X.; Taton, D.; Borsali, R.; Chaikof, E. L.; Gnanou, Y. J. Am. Chem.
Soc. 2006, 128, 11551.
(7) Tro¨llsas, M.; Atthoff, B.; Claeson, H.; Hedrick, J. L. J. Polym. Sci., Part
A: Polym. Chem. 2004, 42, 1174.
(8) Percec, V.; Barboiu, B.; Grigoras, C.; Bera, T. K. J. Am. Chem. Soc. 2003,
125, 6503.
(9) Percec, V.; Grigoras, C.; Kim, H.-J. J. Polym. Sci., Part A: Polym. Chem.
2004, 42, 505.
9
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J. AM. CHEM. SOC. 2008, 130, 1350-1361
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Dendrimer-Like Polymers and Asymmetric/Miktoarm Stars
A R T I C L E S
In that work, these authors elaborated a new concept for the
functionalization/derivatization of growing arms. Named TER-
MINI for irreversible TERmination Multifunctional INItiator,
a specific molecule was designed by Percec and co-workers that
could interrupt the growth of “living” radical species and
generate at the same time a branching point and twice as many
initiating sites for the growth of the next generation. From
calixarenes of various functionalities (4, 6, and 8) that were
derivatized to accommodate ATRP sites, our team derived
dendrimer-like polystyrenes of the third generation by resorting
to the same iterative divergent method based on generation
growth and chain end derivatization.10 In the same vein,
amphiphilic dendrimer-like copolymers including polystyrene
(PS) and poly(acrylic acid) (PAA) generations (PSn-b-PAA2n
and PAAn-b-PS2n) were obtained by us.11 One of the difficulties
encountered while applying this divergent strategy to active
species such those of ATRP is the risk of spoiling the
architecture formed due to inevitable termination reactions:
when growing arms outward from a core, the probability of
dendrimer-dendrimer coupling is not negligible and our at-
tempts to prepare dendrimer-like polystyrene of a generation
higher than three all failed.10
This prompted us to turn toward chemistries and chain
polymerizations that are defect-free and truly “living” to
construct dendrimer-like polymers of high generation from
regular monomers such as styrene, dienes, or methacrylates.
From that standpoint anionic polymerization fulfills ideally all
the criteria of livingness and is certainly the most appropriate
chain polymerization for monomers such as those mentioned
above. However, due to the difficulty to generate pluricarban-
ionic initiators suitable for the latter monomers, and more
generally soluble species that carry a precise number of
carbanions, anionic polymerization has essentially been utilized
in a convergent manner in macromolecular engineering. Unlike
the divergent approach, the convergent synthesis entails the
coupling of “living” linear chains with multifunctional deacti-
vating agents. Dendrimer-like polymers resulting from the
convergent approach have been obtained by iteration of the two
steps of linear chain growth followed by their deactivation on
the antagonist sites of the previous generation. Knauss,12-14
Hadjichristidis,15,16 and Hirao17,18 applied this strategy of
convergent deactivation of “living” carbanionic chains, using
either chlorosilane containing electrophilic reagents or diphe-
nylethylene derivatives including masked electrophilic benzyl
bromide functions. Knauss and Hadjichristidis limited the
synthesis of their dendrimer-like samples to small generations,
but Hirao and co-workers succeeded in obtaining well-defined
polymethacrylate dendritic samples up to the seventh generation.
As multifunctional electrophilic deactivating reagents are
easier to generate and handle than multicarbanionic initiators,
as illustrated in the examples cited above, anionic polymerization
was long thought as only suitable for the convergent methodol-
ogy in the synthesis of stars and dendrimer-like polymers.
Indeed, even regular, miktoarm, and asymmetric stars have been
obtained by this deactivating convergent approach.19-23
The necessity in the latter approach to use a large excess of
“living” linear chains for a complete coupling with the elec-
trophilic moieties and thus the need to carefully fractionate the
architecture formed from the precursor is a tedious and
painstaking work that incited us to investigate an original
divergent anionic route of dendrimer synthesis. The strategy of
synthesis reported here capitalizes on a chemistry that we
recently disclosed and exploited to design perfectly defined di-
and pluricarbanionic initiators soluble in apolar media.24,25 This
chemistry is based on lithium-halide exchange reactions with
pluribromoaryls as precursors and BuLi as a lithium source.
After the first step which corresponds to the synthesis of “living”
stars including a precise number of arms, the construction of
dendrimer-like architectures requires the introduction of branch-
ing points at the end of the first generation and twice as many
active initiating sites for the next generation to grow. Inspired
by Percec’s TERMINI concept, we thus designed and report
here a compound that can interrupt the growth of “living”
carbanionic chains and concomitantly introduce at their end two
initiating sites that can get activated by lithium-halide reactions.
This report actually describes a novel method of generating on
a single (macro)molecule carbanionic species in a high and yet
precise number through an anionic TERMINI. Combining a
pluricarbanionic initiator with such an anionic TERMINI agent,
repeatedly used generation after generation, affords well-defined
dendrimer-like samples as it will be shown subsequently. The
living character of the anionic polymerization of styrene and
butadiene was undoubtedly an asset that helped us to obtain
dendrimer-like PS of the seventh generation and polybutadiene
of the third generation.
Prior to the synthesis of these dendrimer-like samples, the
TERMINI concept was tested through the synthesis of asym-
metric stars and miktoarm stars by combining the TERMINI
developed in this work with “living” linear carbanionic chains.
2. Results and Discussion
2.1. Introduction of a Branching Point at the Chain End
of Linear Carbanionic Polymers by the Means of a TER-
MINI. 4,4′-Dibromodiphenylethylene (1) is the anionic TER-
MINI designed to deactivate and functionalize “living” carban-
ionic chains. It contains a bromide in the para position of its
two phenyl rings to allow subsequently an easy lithium-
bromide exchange after its incorporation at the chain ends. The
(19) Hadjichristidis, N.; Pitsikalis, M.; Pispas, S.; Iatrou, H. Chem. ReV. 2001,
101, 3747.
(20) Hsieh, H. L.; Quirk, R. P. Anionic Polymerization. Principle and Practical
Applications; Marcel Dekker Inc.: New York, 1996.
(21) Quirk, R. P.; Yoo, T.; Lee, Y.; Kim, J.; Lee, B. AdV. Polym. Sci. 2000,
153, 67.
(22) Quirk, R. P.; Lee, B.; Schock, L. E. Makromol. Chem., Macromol. Symp.
1992, 53, 201.
(23) Quirk, R. P.; Yoo, T. Polym. Bull. 1993, 31, 29.
(24) Matmour, R.; More, A. S.; Wadgaonkar, P. P.; Gnanou, Y. J. Am. Chem.
Soc. 2006, 128, 8158.
(25) Matmour, R.; Lebreton, A.; Tsitsilianis, C.; Kallitsis, I.; He´roguez, V.;
Gnanou, Y. Angew. Chem., Int. Ed. 2005, 44, 284.
(26) Suffert, J. J. Org. Chem. 1989, 54, 509.
(10) Lepoittevin, B.; Matmour, R.; Francis, R.; Taton, D.; Gnanou, Y.
Macromolecules 2005, 38, 3120.
(11) Matmour, R.; Lepoittevin, B.; Joncheray, T. J.; El-khouri, R. J.; Taton, D.;
Duran, R. S.; Gnanou, Y. Macromolecules 2005, 38, 5459.
(12) Knauss, D. M.; Al-Muallem, H. A.; Huang, T.; Wu, D. T. Macromolecules
2000, 33, 3557.
(13) Knauss, D. M.; Al-Muallem, H. A. J. Polym. Sci., Part A: Polym. Chem.
2000, 38, 4289.
(14) Knauss, D. M.; Huang, T. Macromolecules 2003, 36, 6036.
(15) Chalari, I.; Hadjichristidis, N. J. Polym. Sci., Part A: Polym. Chem. 2002,
40, 1519.
(16) Orfanou, K.; Iatrou, H.; Lohse, D. J.; Hadjichristidis, N. Macromolecules
2006, 39, 4361.
(17) Matsuo, A.; Watanabe, T.; Hirao, A. Macromolecules 2004, 37, 6283.
(18) Hirao, A.; Matsuo, A.; Watanabe, T. Macromolecules 2005, 38, 8701.
(27) Francis, R.; Taton, D.; Logan, J.; Masse´, P.; Duran, R. S.; Gnanou, Y.
Macromolecules 2003, 36, 8253.
(28) Wolfe, J. F.; Arnold, F. E. Macromolecules 1981, 14, 909.
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A R T I C L E S
Matmour and Gnanou
Scheme 1. Synthesis of Asymmetric and “Miktoarm” Star (Co)Polymers
two bromides are also meant to impoverish the electron density
on the unsaturation and facilitate its selective attack by
nucleophiles, even weak ones. 1 is obtained in just two steps
from commercial dibromobenzophenone with an excellent yield
(see Experimental Section, Supporting Information).
two arylbromide sites of 1 (Figure S1). This demonstrates that
the polystyryl lithium chain end is too nucleophilic and therefore
not selective enough to react only with the unsaturation of the
TERMINI agent so as to generate pure ω,ω′-dibromo end-
functionalized chain polymer.
Before reacting 1 with the growing carbanionic arms of a
“living” star to generate the precursor of the second generation
dendrimer-like sample, its ability to terminate and functionalize
carbanionic stars was thus tested using “living” polystyryl
lithium chains. Upon adding an excess of 4,4′-dibromodiphe-
nylethylene to such chains, we expected to end-cap these chains
with two bromophenyl moieties and utilize subsequently the
latter to generate lithiated carbanions by exchange reaction with
a lithium source. As shown in the size exclusion chromatogram
(SEC) of the PS sample isolated (Figure S1), the reaction
between the “living” polystyryl lithium and 4,4′-dibromodiphe-
nylethylene did not occur as expected. Three different popula-
tions could actually be observed corresponding for the first peak
(1) to the expected nucleophilic addition of “living” PS chains
on the unsaturation of 1 and for the two others (2 and 3) to a
combination of both addition and coupling with either one or
For better selectivity, the reactivity of the living polystyrene
chains was reduced and their alkyl lithium chain ends were
replaced by less nucleophilic potassium alkoxide ends. To this
end, hydroxyl-terminated linear PS or PB synthesized by
deactivation of living lithiated polymers with an excess of
ethylene oxide and also hydroxyl-terminated commercial linear
1
PEO served as precursors. Upon characterization by H NMR
spectroscopy, these ω-hydroxy precursors exhibited a quantita-
tive chain end functionalization. The hydroxyl functions of these
linear polymer chains were then deprotonated in the presence
of a stoichiometric amount of diphenylmethyl potassium
(DPMK) in THF to afford potassium alkoxide living chain ends,
and then a solution containing an excess of 4,4′-dibromodiphe-
nylethylene (4 equiv per hydroxyl group) in THF was added to
the mixture (Scheme 1). The SEC analysis of the polymer
isolated after methanol quenching showed the presence of only
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Dendrimer-Like Polymers and Asymmetric/Miktoarm Stars
A R T I C L E S
Figure 1. 1H NMR spectra (CD2Cl2; 400 MHz) of PEO(Br)2 aryl bromide-terminated poly(ethylene oxide) (run 4, Table 1) and its precursor PEO-OH
hydroxyl-terminated poly(ethylene oxide) (run 3, Table 1).
one single peak slightly shifted with respect to that of the
precursor toward the high molar mass region after the func-
tionalization with 1 (Figure S2). The H NMR spectra of the
three PEO-(PhBr)2, PS-(PhBr)2, and PB-(PhBr)2 functionalized
samples revealed the total disappearance of the signal at δ )
5.4 ppm corresponding to the unsaturation of 1; in addition the
signals appearing at δ ) 6.9-8.0 ppm and at δ ) 4.2 ppm (in
the case of PEO) or δ ) 3.7 ppm (in the case of PS or PB)
assignable, respectively, to the end-standing aromatic protons
-CH2sCH(PhBr)2 and to the protons of the terminal primary
carbon groups -CH2sCH(PhBr)2 could be clearly seen (Figures
1 and S3). For the determination of the experimental functional-
ity we based our calculation on the ratios of the resonance
signals appearing at δ ) 3.3 and 0.9 ppm (methyl protons of
the initiator -CH3 in the cases of PEO and PS and PB,
respectively) to those seen at δ ) 6.9-8.0 ppm and assignable
to the aromatic protons at the chain end: all fall close to 1.
Another means of checking the functionality of the same
1
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J. AM. CHEM. SOC. VOL. 130, NO. 4, 2008 1353

A R T I C L E S
Matmour and Gnanou
Table 1. Characteristics of Asymmetric and “Miktoarm” Star (Co)Polymers
Mw
1
Mn
×
10-3 (g
‚
mol-
)
functionalitya
OH
SEC
/
LS
3
×
10-
1
run
type
theor
SEC/RI
1H NMR
(g‚
mol-
)
Mw/Mn
theor
PhBr
1
2
3
4
5
6
PS-OH
PS-(PhBr)2
PEO-OH
PEO-(Br)2
PB-OH
PB-(PhBr)2
2.4
2.8
1.1
1.4
3.0
2.4
2.7
1.0
1.3
3.1
2.3
2.7
1.1
1.4
3.1
3.5
1.04
1.06
1.05
1.06
1.03
1.0
1.0
1.0
1.0
1.0
1.9
1.9
1.0
1.0
3.4
3.5
2.0
1.9
7
8
9
10
11
12
13
(PS)-b-(PS)2(OH)2
(PS)-b-(PS)2(OH)2
(PS)-b-(PS)2(OH)2
(PB)-b-(PB)2(OH)2
(PS)-b-(PB)2(OH)2
(PS)-b-(PI)2(OH)2
(PB)-b-(PS)2(OH)2
6.0
3.9
6.2
1.06
1.03
1.02
1.04
1.01
1.05
1.04
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.9
27.8
30.0
17.8
30.0
12.5
51.0
21.0
24.5
9.7
19.7
9.6
28.2
31.8
18
30.4
12.7
52.2
1.8b
1.8b
1.8
1.9b
1.9
12.8
51.8
47.0
1.9b
1
b
a Estimated by H NMR spectroscopy. Estimated by titration of the hydroxyl end groups by DPMK ([DPMK] ) 0.521 mol‚L-1).
samples is to calculate the ratios of the resonance signals
appearing at δ ) 3.6 ppm ((OCH2CH2)n methylene protons of
the PEO chains) or at δ ) 4.9-5.8 ppm (protons of the double
bonds of the PB chains) to those assignable to the aromatic
protons of the chain end at δ ) 6.9-8.0 ppm: again the extent
of functionalization calculated by this method is close to 1. One
of the most striking features about this anionic TERMINI is
the peculiar reactivity exhibited by its disubstituted unsaturation
and in particular its faculty to undergo selective addition when
reacted with alkoxides. The presence of the two bromides on
separate phenyl rings explains the high electroaffinity exhibited
by this unsaturation. For instance in the reaction between the
potassium alkoxide carried by PEO chains with regular
1,1′-diphenylethylene, no addition on the latter compound could
be observed by 1H NMR spectroscopy. On the other hand, these
potassium alkoxides carried by these polymers are too weak
nucleophiles to react with the bromophenyl sites of 1, paving
the way for the synthesis of perfectly defined ω,ω′-dibromophe-
nyl end-functionalized PEO, PS, and PB chains, which can now
serve to generate polylithium macroinitiators by halogen-
lithium exchange reaction.
Figure 2. SEC traces (RI detector) of asymmetric star polystyrenes PS-
b-PS2 (runs 7 (B), 8 (C), and 9 (D), Table 1) and its precursor PS-(Br)2
(run 2(A), Table 1). Conditions: [Li+] ) 5 × 10-3 mol‚L-1, at RT in
benzene.
2.2. Synthesis of Miktoarm and Asymmetric Star Poly-
mers from Dicarbanionic Macroinitiators. These ω,ω′-
dibromophenyl PS and PB chains were then treated with
stoichiometric amounts of sec-butyllithium used here as a
lithium provider; this reaction was carried out in benzene or
cyclohexane and resulted in the formation of the corresponding
dilithiated species, reflected by the red color taken by the
medium, and of 2-bromobutane (Scheme 1). The two phenyl
lithiums carried by PS and PB chains were found to be totally
soluble in apolar solvents such as cyclohexane or benzene. To
neutralize 2-bromobutane, which might subsequently deactivate
the chains that are expected to grow from these carbanionic
sites upon monomer addition, 1 equiv of sec-butyllithium was
added which resulted in the formation of 3,4-dimethylhexane
(Scheme 1). This sequence of reactions, i.e., lithium-bromide
exchange reaction followed by 2-bromobutane neutralization,
was successfully brought about with the polyarylbromide
precursor.24,25 The polymerization of monomers such as styrene,
butadiene, or isoprene was then performed in cyclohexane or
benzene and at room temperature using these PS-(Ph-Li+)2, or
PB-(Ph-Li+)2 dilithiated macroinitiators. After polymerization,
the living carbanionic species were deactivated with EO to
functionalize the two grown arms with terminal hydroxyl
functions. Depending upon the monomer-macroinitiator couple
considered, either asymmetric or miktoarm star samples were
isolated and analyzed by size exclusion chromatography with
a light scattering detector (SEC/LS) and 1H NMR spectroscopy.
Table 1 gives the data pertaining to these three-arm asymmetric
and miktoarm copolymer stars grown from these dilithiated
macroinitiators. Samples of narrow molar mass distribution (1.01
< Mw/Mn < 1.15) were obtained in all cases. The complete
disappearance of the peak due to the PS-(PhBr)2 and PB-(PhBr)2
precursors from the chromatograms meant that all precursor
chains participated in the initiation process (Figures 2, S4, and
S5). The absolute molar mass values drawn from the SEC/LS
detector were found to agree almost perfectly with theoretical
values but were higher than those obtained from the refractive
index (RI) detector relative to linear standards which mirrors
their compact architecture (Table 1). The nature of the archi-
tectures formed and especially the star character was also
1
characterized by H NMR using to this end asymmetric and
miktoarm star samples of low degrees of polymerization (Figure
3 and Table 1). From the ratio of the integrals of the signal of
the methyl protons -CH3 of sec-BuLi the initiator at δ ) 0.9
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Dendrimer-Like Polymers and Asymmetric/Miktoarm Stars
A R T I C L E S
Figure 3. 1H NMR spectrum (CD2Cl2; 400 MHz) of an asymmetric star polybutadiene PB-b-PB2(OH)2 (run 10, Table 1).
ppm to that of the methylene protons due to the -CH2OH star
arm ends and -CH2-O-CH2-CH(Ph)2- branching points (δ
) 3.6 ppm), the actual functionality of several samples was
determined and values close to 2 were obtained for the number
of arms grown from each branching point of these asymmetric
and miktoarm star (co)polymers (Figure 3 and Table 1). The
functionality of the branching points was also double checked
by titration of the hydroxyl end groups carried by these
asymmetric and miktoarm stars, using a solution of diphenyl-
methyl potassium (DPMK) of known concentration ([DPMK]
) 0.521 mol‚L-1). Again functionalities close to 2 were found
in all cases (Table 1).
2.3. Synthesis of Dendrimer-Like Polystyrene and Polyb-
utadiene. As mentioned in the introduction, dendrimer-like
polymers constructed by the divergent approach described here
are grown by repeating generation after generation the two
following steps: polymerization from phenyl lithium carbanionic
sites derived by lithium-bromide exchange and end-function-
alization of the growing sites with 4,4′-dibromodiphenylethyl-
ene. The first step in such an endeavor is thus the synthesis of
living stars by divergent growth. We described recently the
preparation of a new category of tri- and tetracarbanionic
initiators obtained by halogen-lithium exchange reaction
between a tri- and tetrabromoaryl compound and sec-butyl-
lithium as the lithium source.25 The tri- and tetracarbanionic
species formed were shown to be excellent initiators for the
anionic polymerization of styrene and butadiene, and well-
defined tri- and tetraarmed polystyrene and polybutadiene stars
exhibiting monomodal and narrow molar mass distribution could
be obtained in this way.25 These pluricarbanionic initiators were
also employed to prepare (PB-b-PEO)4 amphiphilic diblock and
P(S-b-B-b-MMA)n (n ) 3 or 4) triblock star copolymers.25,29
Upon neutralizing and derivatizing the living stars described
above, in particular their four growing carbanionic sites with
EO and then 4,4′-dibromodiphenylethylene, the precursors of
the second generation would then be generated in the form of
dibromoaryl ended star polymers. The growth in a second step
of the second generation would then require a lithium-bromide
exchange before twice as many arms could be grown upon
monomer addition.
2.3.1. Synthesis of Star-Shaped (G-1) Polystyrene and
Polybutadiene Precursors (Number Following G Stands for
Number of Generation). Tetraarmed star polymers were thus
prepared from the tetrabromoaryl compound shown in Scheme
(29) Matmour, R.; Francis, R.; Duran, R. S.; Gnanou, Y. Macromolecules 2005,
38, 7754.
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J. AM. CHEM. SOC. VOL. 130, NO. 4, 2008 1355

A R T I C L E S
Matmour and Gnanou
Scheme 2. Synthesis of Third Generation Dendrimer-Like Polystyrene
2 which was first subjected to a lithium-halide exchange
reaction. The latter precursor was prepared in excellent yields
(>90%) by a Diels-Alder reaction of 2,3,4,5-tetrakis(p-
bromophenyl)cyclopentadienone and phenylacetylene with ex-
trusion of carbon monoxide; it was treated with stoichiometric
amounts of sec-butyllithium to generate the corresponding
tetraalkyllithium agent in benzene (Scheme 2).25,28 The resulting
tetralithiated species being insoluble in the latter solvent,
tetramethylethylene diamine (TMEDA) (ratio of [TMEDA]/[Li]
) 2) was utilized to solubilize the tetraphenyllithium formed,
both ligand and lithium species forming a soluble complex.
These complexes give stable species and enough steric bulk
precluding the tetralithiated initiators from aggregating. As
previously mentioned, this lithium-bromide exchange reaction
produces 2-bromobutane that has to be neutralized before
growing carbanionic chains. sec-Butyllithium (1 equiv) was thus
added next, yielding 3,4-dimethylhexane as an inert byproduct.
The anionic polymerization of styrene and butadiene could then
9
1356 J. AM. CHEM. SOC. VOL. 130, NO. 4, 2008

Dendrimer-Like Polymers and Asymmetric/Miktoarm Stars
A R T I C L E S
Table 2. Characteristics and Solution Properties of Dendrimer-Like Polystyrenes and Polybutadienes
3
Mn
×
10-
theor
1
(g‚
mol-
)
functionalitya
OH
[η]
(dL/g)
b
run
type
theor
SEC
SEC/HT
Mw/Mn
theor
PhBr
g′
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
PSG-1
PSG-1Br
PSG-2
PSG-2Br
PSG-3
PSG-3Br
PSG-4
PSG-4Br
PSG-5
PSG-5Br
PSG-6
PSG-6Br
PSG-7
PBG-1
8.0
9.4
55.9
3.08
4.3
40.3
41.7
91.2
95.6
189.9
198.5
363.3
382.3
586.0
628.2
610.4
7.6
7.9
9.3
57.0
1.1
1.1
1.07
1.1
1.08
1.1
1.05
1.07
1.04
1.08
1.04
1.08
1.04
1.03
1.03
1.05
1.05
1.05
1.05
4
8
8
3.8
0.0608
0.84
7.9
7.8
15.9c
31.8c
64.7c
127.7c
0.1848
0.2218
0.2156
0.2047
0.1679
0.1228
0.63
58.6
59.4
16
16
32
32
64
64
128
128
256
256
4
8
8
16
16
32
15.8
31.8
151.0
156.4
373.6
384.4
695.3
716.9
1212.5
1255.7
1822.0
14.0
15.9
36.7
41.3
121.9
135.0
153.6
158.9
375.3
387.4
710.5
730.6
1301.0
1345.4
1920.2
14.5
15.2
38.6
41.9
129.6
131.4
0.374
0.193
0.116
0.062
0.034
64.1
127.0
255.0
255.6c
3.8
PBG-1-Br
PBG-2
PBG-2-Br
PBG-3
7.8
28.4
29.0
72.6
7.9
15.8
31.7
7.8
15.7c
PBG-3-Br
74.0
a Estimated by H NMR spectroscopy. g′ ) [η]dendri/[η]linear; ([η] ) 1.23 × 10-2 Mn0.71). Estimated by titration of the hydroxyl end groups by DPMK
1
b
c
([DPMK] ) 0.521 mol‚L-1).
be carried out in benzene and at room temperature from this
solubilized tetralithiated initiator. After complete conversion of
the monomer, end capping was accomplished through addition
of a large excess of ethylene oxide which introduced terminal
hydroxyl functions. The characteristics of such ω-hydroxyl star
samples are listed in Table 2. The characterization of all the
samples obtained by SEC showed the disappearance of the peak
due to the tetrafunctional initiator in the low molar mass region,
implying its total consumption in the initiation of styrene and
butadiene polymerization (Figure S6). The actual molar masses
of such ω-hydroxyl polystyrene and polybutadiene stars were
determined using an SEC/LS detector; they were found to be
in good agreement with the calculated values, and their
distributions were narrow (Mw/Mn < 1.1). The tetrafunctionality
of the resulting polystyrene and polybutadiene stars was then
First, as in the case of linear chain polymers, it can be observed
that the SEC trace of these functionalized stars is unimodal and
sharp, attesting to the absence of any coupling reactions on the
bromide positions; a small shift of the peak can even be seen
toward the high molar mass region, indicating the addition of 1
at the arm ends. The functionalization of each arm end with
1
dibromodiphenylethylene was checked by H NMR on PB4-
(PhBr)8 star samples of low degrees of polymerization (Figure
S7). Indeed, the 1H NMR spectrum exhibits signals at δ ) 6.9-
8.0 ppm assignable to the aromatic protons of the star core and
of its arm tips -CH2sCH(PhBr)2 and other signals at δ ) 3.7
ppm and δ ) 3.5 ppm corresponding to the protons carried by
the terminal primary carbon groups -CH2sCH(PhBr)2 and
-CH2-O-CH2sCH(PhBr)2, respectively (Figure S7). The
experimental functionality was determined by comparing the
integration values of the signals of the aromatic protons due to
the polyphenylene core and to the dibromophenyl end groups
(δ ) 6.5-8.0 ppm; 54H) with that of the signals of the protons
carried by the terminal primary carbon groups -CH2sCH-
(PhBr)2 (δ ) 3.7 ppm; 8H) and -CH2-O-CH2sCH(PhBr)2
(δ ) 3.5 ppm; 8H). The ratio of 6.7 found between the
intensities of the signals appearing at δ ) 6.5-8.0 ppm and
the one at δ ) 3.7 ppm indicated that functionalization occurred
quantitatively (Figure S7).
2.3.3. Preparation of Pluricarbanionic Initiators for the
Synthesis of Second-Generation (G-2) Dendrimer-Like Poly-
styrene and Polybutadiene. From the eight bromophenyl sites
of PS4(PhBr)8 and PB4(PhBr)8 polystyrene and polybutadiene
stars, eight arms were grown anionically after lithium-bromide
exchange reaction and monomer addition. These stars were thus
first treated with stoichiometric amounts of sec-butyllithium in
benzene to generate the corresponding polylithiated species
PS4(Ph-Li+)8 and PB4(Ph-Li+)8, and 2-bromobutane was
neutralized into 3,4-dimethylhexane upon addition of a further
equivalent of BuLi. The presence of TMEDA (ratio of [TMEDA]/
[Li] ) 2) was necessary to solubilize the polylithiated species
formed in benzene. The polymerizations of styrene and buta-
diene were performed in a second step in benzene at room
1
demonstrated by H NMR characterization of the star samples
of low degrees of polymerization (Figure S7). The experimental
functionality values calculated from the ratios of the resonance
signals appearing at δ ) 6.8-8.0 ppm (aromatic protons of the
tetrafunctional initiators) to those seen at δ ) 3.6 ppm (-CH2-
OH chain end protons) were found to be always close to 4 for
the four-armed stars. Further evidence of the star character and
the four-arm star structure was obtained by comparing the
intrinsic viscosity [η] of our samples with that of linear
polymers, as previously demonstrated.25
2.3.2. Introduction of Branching Points at Each Arm End
by the Means of 1. The next step in the synthesis of these
dendrimer-like polymers was the introduction of a branching
point and thus of two aryl bromide groups at each arm end of
these stars. The same procedure as that used for the preparation
of asymmetric and miktoarm star polymers was followed. The
hydroxyl functions carried by the arm ends of these polystyrene
and polybutadiene stars were deprotonated in the presence of a
stoichiometric amount of DPMK in THF, and then a THF
solution of 4,4′-dibromodiphenylethylene was added. After
protonation of the functionalized chain ends by methanol
addition, the resulting functionalized star polymers were ana-
lyzed by SEC and 1H NMR spectroscopy (Figures S6 and S7).
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J. AM. CHEM. SOC. VOL. 130, NO. 4, 2008 1357

A R T I C L E S
Matmour and Gnanou
PB4PB8(OH)8 samples, two other small peaks can however be
observed next to the main peak toward the high molar mass
region whose presence results from the aggregation of the eight
hydroxyl end groups of these dendrimer-like samples in THF
(Figure S9). To break the aggregation between these peripheral
hydroxyl end groups, the different samples were characterized
by high-temperature size exclusion chromatography (HT-SEC)
running at 130 °C in dichlorobenzene as solvent and equipped
with a viscometer detector giving the absolute molar mass. In
this case, no peak due to aggregates could be seen and only
one peak exhibiting a monomodal and narrow molar mass
distribution (1.01 < Mw/Mn < 1.2) could be observed (Figures
4 and 5, Table 2). Indeed, the molar mass values determined
by HT-SEC characterization agreed well with the expected
values confirming that two PS or PB arms of well-controlled
size could be grown from each branching point. As to the
characterization by 1H NMR carried out on the second genera-
tion PB4PB8(OH)8 dendrimer-like polybutadiene (Figure S8),
it clearly revealed the presence of four signals: at δ ) 6.5-
8.0 ppm the signals of the aromatic protons of the polyphenylene
core (22H) and the branching points (32H at the branching
points) could be clearly seen as well as the signals of the
methylene protons carried by the branching points and at arm
ends -CH2sCHs(PhCH2)2s at δ ) 3.7 ppm (8H), -CH2-
O-CH2sCHs(PhCH2)2s and -CH2sOH at δ ) 3.4 ppm
(24H), and -CH2sCHs(PhCH2)2s at δ ) 2.8 ppm (4H). The
ratios of 6.8, 2.2, and 13.8 found between the intensities of the
signal appearing at δ ) 6.5-8.0 ppm (54H) and the ones at δ
) 3.7 ppm (8H), at δ ) 3.4 ppm (24H), and at δ ) 2.8 ppm
(4H), respectively, indicated that initiation occurred with
excellent efficiency (Figure S8). Indeed, the titration of the
hydroxyl end groups at the periphery of the PS and PB
dendrimer-like polymers in THF by a solution of DPMK of
known concentration ([DPMK] ) 0.521 mol‚L-1) gave in the
case of both PS or PB dendrimer-like polymers values of
functionality close to 8 (Table 2). The excellent definition of
these second generation PS4PS8(OH)8 and PB4PB8(OH)8 den-
drimer-like polymers prompted us to prepare dendritic PS and
PB of a higher generation.
Figure 4. HTSEC traces (RI detector) of dendrimer-like polystyrenes (runs
14, 16, 18, 20, 22, 24, and 26, Table 2). Conditions: [TMEDA]/[-PhLi]
) 2, at RT, in benzene.
2.3.4. Third-Generation (G-3) Dendrimer-Like Polysty-
rene and Polybutadiene. The synthetic methodology described
above which is based on an iterative divergent approach was
repeated here (i.e., chain end modification and polymerization
of styrene or butadiene) for the synthesis of dendritic PS and
PB of a higher generation. Similarly to the cases of G-1 and
G-2, the peripheral hydroxyl end groups of PS4PS8(OH)8 and
PB4PB8(OH)8 dendrimer-like polymers were deprotonated by
reaction with stoichiometric amounts of DPMK, and a solution
of 4,4′-dibromodiphenylethylene was added in excess (4 equiv
per -OH group). After protonation of the living chain ends by
addition of methanol, the resulting PS4PS8(PhBr)16 and PB4-
PB8(PhBr)16 dendrimer-like polymers were isolated and char-
Figure 5. HTSEC traces (RI detector) of dendrimer-like polybutadienes
(runs 27, 29, and 31, Table 2). Conditions: [TMEDA]/[-PhLi] ) 2, at
RT, in benzene.
temperature using PS4(Ph-Li+)8 and PB4(Ph-Li+)8 octalithiated
species, as initiators, respectively. After polymerization of the
monomer the living chain ends were deactivated by addition of
an excess of ethylene oxide; the dendrimer-like polystyrene and
polybutadiene of the second generation isolated were then
analyzed by SEC and 1H NMR spectroscopy (Figures 4, 5, S8,
and S9). First, the SEC traces of the second generation PS4-
PS8(OH)8 and PB4PB8(OH)8 dendrimer-like samples indicate
the complete disappearance of the peaks of the PS4(PhBr)8 and
PB4(PhBr)8 precursors (Figures 4, 5, and S9), confirming that
all precursor chains participated in the initiation-polymerization
process. In the SEC chromatograms of the PS4PS8(OH)8 and
1
acterized by SEC, HT-SEC, and H NMR spectroscopy. As
previously shown, no substitution reaction implying terminal
phenylbromide positions could be observed (Figures 4 and 5).
Indeed, the 1H NMR analysis of the chain ends of the PB4PB8-
(PhBr)16 dendrimer-like polybutadiene clearly indicates four
distinct signals corresponding to the aromatic protons of the
polyphenylene core and the aryl rings of the branching points
(δ ) 6.8-8.0 ppm; 118H), to the methylene protons carried by
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Dendrimer-Like Polymers and Asymmetric/Miktoarm Stars
A R T I C L E S
Figure 6. 1H NMR spectrum (CD2Cl2; 400 MHz) of an aryl bromide-terminated dendrimer-like polybutadiene PB4-b-PB8(Br)16 (run 30, Table 2).
the branching points -CH2sCHs(PhCH2)2s and appearing
at δ ) 3.7 ppm (24H), -CH2-O-CH2sCHs(PhCH2)2s at δ
) 3.4 ppm (24H), and -CH2sCHs(PhCH2)2s at δ ) 2.8 ppm
(12H) (Figure 6). In addition, the experimental -PhBr func-
tionality, determined from the ratios of the integration values
of the different signals occurring at δ ) 6.8-8.0 ppm, at δ )
3.7 ppm, at δ ) 3.4 ppm, and at δ ) 2.8 ppm were found to
correspond to the expected value of 16 groups per polymer.
Next, the resulting PS4PS8(PhBr)16 and PB4PB8(PhBr)16 den-
drimer-like polymers were treated with stoichiometric amounts
of sec-butyllithium in benzene to generate the corresponding
PS4PS8(Ph-Li+)16 and PB4 PB8(Ph-Li+)16 polylithiated species.
At this stage 16 carbanionic sites per polymer formed a physical
gel due to the stronger aggregation between these polylithiated
species in benzene. Upon addition of 2 equiv of TMEDA per
-PhLi, the polylithiated species became totally soluble in
benzene and monomerseither styrene or butadiene depending
upon the precursorswas polymerized. The living chain ends
were deactivated by addition of an excess of ethylene oxide,
and the resulting dendritic polystyrene and polybutadiene were
characterized by SEC and HT-SEC. As shown in Figures 4 and
5, the HT-SEC profiles of the PS4PS8PS16(OH)16 and PB4PB8-
PB16(OH)16 third-generation dendrimer-like polymers exhibit
only one sharp monomodal and narrow (1.05 < Mw/Mn < 1.08)
peak, corresponding, respectively, to the dendritic polystyrene
and polybutadiene, with in each case the total disappearance of
the second generation macroinitiator. As in the case of the
second generation sample, the hydroxyl end groups at the
periphery of the PS4PS8PS16(OH)16 and PB4PB8PB16(OH)16
dendrimer-like polymers were titrated in THF by a solution of
DPMK of known concentration ([DPMK] ) 0.521 mol‚L-1
)
which gave for both PS and PB dendrimer-like polymers values
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J. AM. CHEM. SOC. VOL. 130, NO. 4, 2008 1359

A R T I C L E S
Matmour and Gnanou
of functionality close to 16 indicating that initiation occurred
as expected (Table 2).
2.3.5. From the Fourth (G-4) to the Seventh Generation
(G-7) Dendrimer-Like Polystyrene. The same iterative diver-
gent approach as that used for the synthesis of the third
generation dendrimer-like polymers was applied for the prepara-
tion of the fourth (G-4), fifth (G-5), sixth (G-6), and seventh
generation (G-7) dendrimer-like polystyrenes. Successively, the
hydroxyl end groups standing at the periphery of the precursors
of these dendrimer-like polystyrenes were deprotonated by a
solution of DPMK and functionalized by reaction with 4,4′-
dibromodiphenylethylene; the dibromophenyl end-functionalized
dendrimer-like polystyrenes eventually obtained were subse-
quently used after reaction with s-BuLi as pluricarbanionic
macroinitiators for the polymerization of styrene in the presence
of TMEDA as an additive. After deactivation of the living
polystyryl lithium chain ends by an excess of ethylene oxide
and degassed methanol, all the dendritic polystyrene samples
of different generations (G-4, G-5, G-6, and G-7) were succes-
sively characterized by SEC and HT-SEC (Figures 4, 5, and
S9). As in the previous cases, the same peaks corresponding to
the aggregates were observed in the high molar mass region by
SEC. However, the latter were not observed at high temperature
as shown in the HT-SEC chromatograms of fourth (G-4), fifth
(G-5), sixth (G-6), and seventh generation (G-7) dendrimer-
like polystyrenes (Figure 4). As can be seen in Figure 4, the
HT-SEC peak of each generation of PS is monomodal and
narrow and shifts to the high molar mass region with the total
disappearance of the precursor as the iterative steps proceed.
Indeed, the experimental degree of -OH functionality was
determined by titration of hydroxyl end groups by reaction with
a solution of DPMK of known concentration ([DPMK] ) 0.521
mol‚L-1). As seen in Table 2, the results showed experimental
functionalities very close to the expected values. From the fourth
generation onward, this last method, that is the titration of
Figure 7. Evolution of the hydrodynamic radius (RH) as a function of the
number of generation (G) for dendrimer-like polystyrenes obtained from
an initial tetrafunctional initiator (runs 14, 16, 18, 20, 22, 24, and 26, Table
2).
drimers increase exponentially with g with a variation in 2g-1
,
and their hydrodynamic volume grows with g3; [η] varying
proportionally to g3/2g-1, it experiences a maximum as g
increases.
For a better comparison with the results that we previously
obtained on samples prepared by ATRP up to the third
generation, the intrinsic viscosities of our anionically prepared
dendrimer-like PS were determined at 35 °C using toluene as
solvent. First, from the experimental [η]dendritic (measured at 35
°C in toluene) and the [η]linear values (calculated from eq 1 for
linear PS of same molar mass under the same conditions),32
the g′ ratio [η]dendritic/[η]linear could be determined and is listed
in Table 2. The intrinsic viscosity of these dendritic samples is
found to be lower than that of linear PS (until 30 times lower
than that of linear PS), and the g′ parameter decreases with the
generation, confirming that the dendritic structure becomes more
and more compact as the number of generations and branched
segments increases.
1
hydroxyl end groups, was found to be more efficient than H
[η]linear ) 1.23 × 10-2 Mw
(1)
0.71
NMR spectroscopy, because of the weak intensity of the
resonance signals due to the end groups compared to that of
the aromatic protons of the styrene repeating units. Thus, upon
iteration of the two sequences of styrene polymerization and
branching reaction of this divergent approach, a seventh-
generation dendrimer-like PS (G-7) sample was synthesized with
an absolute molar mass Mn of 1920 × 103 g‚mol-1, including
no less than 256 OH end groups and 508 PS segments.
As shown by the relation between [η], which is also inversely
proportional to the hydrodynamic density, and the hydrodynamic
volume (VH)
[η] ) 2.5NaVH/M ) (10π/3)NaRH,app3/M
(2)
where RH,app is the apparent hydrodynamic radius and Na is the
Avogadro number, such a constant [η] in the particular case of
hyperbranched polymers merely reflects the fact that both VH
of the sample and its mass (M) vary approximately in the same
proportion upon increasing the number of generations.
To determine how hydrodynamic volumes vary as a function
of the number of generations (g) we resorted to the eq 3 given
below, using [η] and M determined experimentally.
2.4. Viscosity Behavior of Dendrimer-Like Polystyrenes.
In the literature, several studies deal with the viscometric
behavior of star polymers, hyperbranched polymers, dendri-
grafts, and regular dendrimers. However to the best of our
knowledge, we were the first to demonstrate, in the case of third
generation dendrimer-like polystyrenes synthesized by atom
transfer radical polymerization, that the latter dendritic archi-
tecture exhibits a behavior in solution similar to that of regular
dendrimers.10 In contrast to hyperbranched polymers, whose
intrinsic viscosity remains independent of the molar mass, the
variation of log[η] against the generation or the molar mass of
dendrimer-like PS exhibits a bell-shaped curve with an increase
of [η], followed eventually by a decrease after reaching a
maximum for the third generation as observed in the case of
regular dendrimers.30,31 In fact molar masses in regular den-
RH,app ) [3M[η]/10πNa]1/3
(3)
As shown in Figure 7, RH,app increases nearly linearly with
the number of generation for polystyrene dendrimer-like poly-
mers, indicating that VH thus grows in g3 like regular dendrimers.
(31) Mourey, T. H.; Turner, S. R.; Rubinstein, M.; Fre´chet, J. M. J.; Hawker,
C. J.; Wooley, K. L. Macromolecules 1992, 25, 2401.
(32) Polymer Handbook, 4th ed.; Wiley-Interscience: New York; p VII/1.
(30) Hult, A.; Johansson, M.; Malmstro¨m AdV. Polym. Sci. 1999, 143, 1.
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Dendrimer-Like Polymers and Asymmetric/Miktoarm Stars
A R T I C L E S
profile of [η] and that reciprocal of hydrodynamic density, the
latter being the reciprocal of [η], resemble those exhibited by
regular dendrimers indicating that the twin architectures possess
the same “signature”.
Conclusion
The methodology of synthesis of dendrimer-like samples
described in this paper is based on an original chain end
modification, involving the reaction of a potassium alkoxide
living chain/arm with the unsaturation of 4,4′-dibromodiphe-
nylethylene which affords ω,ω′-ended dibromophenyl chains/
arms. The latter bromophenyl functions carried by separate aryl
rings were then subjected to a halogen-lithium exchange
reaction to prepare the corresponding polylithiated species,
providing an efficient synthetic route to asymmetric and
“miktoarm” star (co)polymers including polystyrene, polybuta-
diene, and/or polyisoprene arms. Starting from a tetracarbanionic
initiator and upon repeating this sequence of reactions, den-
drimer-like PS and PB up to the seventh and third generations,
respectively, could be successfully synthesized. The character-
ization of dendrimer-like PS samples confirmed that regular
dendrimers and the macromolecular architectures described in
this study belong to the same family, the two types of dendritic
structures exhibiting a similar viscosity behavior with regards
to the number of generations.
Figure 8. Evolution of log[η] as a function of the number of generation
(G) for dendrimer-like polystyrenes prepared using a tetrafunctional initiator
(runs 14, 16, 18, 20, 22, 24, and 26, Table 2).
This chemistry could be successfully implemented only after
the design of an original reagent, 4,4′-dibromodiphenylethylene,
that functions successfully as a TERminator for living carban-
ionic chains and as a precursor for the Multifunctional INItiator.
Applied to styryl and dienyl carbanions, this chemistry is
versatile enough to be applied to other anionic species such as
enolates, etc.
Figure 9. Evolution of log[η] as a function of log Mw for dendrimer-like
polystyrenes prepared from a tetrafunctional initiator (runs 14, 16, 18, 20,
22, 24, and 26, Table 2).
From the variation of log[η]dendritic with the number of
generations, we observe that [η] reaches a maximum corre-
sponding to the third generation dendrimer-like PS and then
decreases gradually until the seventh generation (Figure 8). The
evolution of log[η]dendritic as a function of log Mn was also plotted
(Figure 9). The same trend as the one observed previously for
the plot of log[η]dendritic against the number of generations is
seen here, indicating that [η] is not independent of the molar
mass for dendrimer-like PS in contrast to some hyperbranched
polymers or highly dense globular structures. The peculiar
Acknowledgment. The authors are grateful to CNRS and the
French Ministry of Research for support of this research.
Supporting Information Available: Experimental details
concerning the synthesis of hydroxyl- and ω,ω′-dibromodiphe-
nyl-terminated polymers and their characterization by 1H NMR
and GPC. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA076442T
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J. AM. CHEM. SOC. VOL. 130, NO. 4, 2008 1361