Synthesis and characterization of dendrimer-star polymer using dithiobenzoate-terminated poly(propylene imine) dendrimer via reversible addition-fragmentation transfer polymerization

Article abstract of DOI:10.1021/ma050455e

Two new reversible addition-fragmentation transfer (RAFT) agents with 8 and 16 terminal dithiobenzoate groups on the surface of poly(propylene imine) dendrimers (generation 2.0 and 3.0, respectively) were prepared, and successively they were used in the RAFT polymerization to produce well-defined dendrimer-star polymers. The polymerization kinetics were confirmed to pseudo-first-order behavior. The polymerizations displayed excellent molecular weight control and produced polymer with low polydispersities (<1.3). The synthetic method was also suitable for the preparation of dendrimerstar block copolymers.

Full text of DOI:10.1021/ma050455e

Macromolecules 2005, 38, 6841-6848
6841
Synthesis and Characterization of Dendrimer-Star Polymer Using
Dithiobenzoate-Terminated Poly(propylene imine) Dendrimer via
Reversible Addition-Fragmentation Transfer Polymerization
Quan Zheng and Cai-yuan Pan*
Department of Polymer Science and Engineering, University of Science and Technology of China,
Hefei, 230026, Anhui, People’s Republic of China
Received March 3, 2005; Revised Manuscript Received June 3, 2005
ABSTRACT: Two new reversible addition-fragmentation transfer (RAFT) agents with 8 and 16 terminal
dithiobenzoate groups on the surface of poly(propylene imine) dendrimers (generation 2.0 and 3.0,
respectively) were prepared, and successively they were used in the RAFT polymerization to produce
well-defined dendrimer-star polymers. The polymerization kinetics were confirmed to pseudo-first-order
behavior. The polymerizations displayed excellent molecular weight control and produced polymer with
low polydispersities (<1.3). The synthetic method was also suitable for the preparation of dendrimer-
star block copolymers.
Introduction
outward growth of arms from the core (core is a part of
R-group), following “arm-first” and “core-first” ap-
proaches, respectively. These two approaches are comple-
mentary with their respective merits and drawbacks.12-15
For the synthesis of star polymers with dendritic or
hyperbranched cores, the RAFT agent was connected
to the core via Z-group.16-18 Gnanou et al. recently
described the synthesis of hybrid dendrimer-star poly-
mers via the RAFT process by the “arm-first” approach.
In this case, the core acts as the Z-group of the RAFT
agent. The growth occurs at the nexus of the core and
the arm. As the molecular weight of the arm builds up,
a remarkable “shield effect” was found, which prevents
formation of a well-defined dendrimer-star polymer.16
Different from the “arm-first” approach reported, in this
article, we present a “core-first” RAFT approach to
prepare well-defined dendrimer-star polymers and block
copolymers using dithiobenzoate-terminated poly(pro-
pylene imine) (PPI) dendrimer as multifunctional RAFT
agent that is the attachment of the RAFT agent on the
dendrimer via R groups.
Dendrimers are monodispersive molecules with a
globular shape, well-defined, and perfectly branched
structure, and they possess a precise number of terminal
functional groups which provide high surface reactivity.1
Dendrimer-star polymers, in which many linear homo
or block copolymer chains are attached to the dendrim-
ers, have been developed because they combine the
properties of star polymers with those of dendrimers.
They can be used in controlling the guest reactivity in
dendrimer host in response to the change of external
stimuli,2,3 extraction applications,4 stabilizers for col-
loids,5 DNA delivery, and electrokinetic capillary chro-
matography.6 Two general methods have been used to
prepare dendrimer-star polymers. One is to link mono-
functional linear polymers onto the dendrimer
surface.2,5-7 The other is the growth of arm polymer
chains from the surface of dendrimer by “controlled/
living” polymerizations. Surprisingly, only a few den-
drimer-star polymer examples have been synthesized
via living polymerizations, including anionic polymer-
ization,8 ring-opening polymerization (ROP),9 and atom
transfer radical polymerization (ATRP).10 However,
anionic and cationic polymerizations suffer from rigor-
ous requirements, and some functional monomers can-
not be used by ATRP method.
In the past five years, a significant development in
controlled/living radical polymerization came about with
the use of thiocarbonylthio compounds ZC(S)SR as
chain-transfer agents (CTAs) in the so-called reversible
addition-fragmentation transfer (RAFT) polymeriza-
tion.11 The controlled/living growth of the chains is
achieved by the alternation of the activation and
deactivation of the CTA between dormant and active
moieties. Among the various types of RAFT agents
prepared, only a few of them have been applied for star
polymer synthesis.12-15 Two types of multifunctional
CTAs can be employed in RAFT process: those involving
the reaction of the linear chains with the functional core
(core is a part of Z-group) and those implying an
Experimental Part
Materials. Polypropylenimine octaamine dendrimer
(DAB-8-Am, generation 2.0) and polypropylenimine hexade-
caamine dendrimer (DAB-16-Am, generation 3.0) were pur-
chased from Aldrich. 4,4′-Azobis-(4-cyanopentanoic acid)
(Fluka, > 98%), N-hydroxysuccinimide) (Fluka, >97%), and
dicyclohexylcarbodiimide (DCC) (>98%, Shanghai First Re-
agent Co.) were used without further purification. Methyl
acrylate (MA, >99%) and styrene (St, >99%) were purchased
from the Shanghai First Reagent Co. were distilled under
reduced pressure to remove the inhibitor. Azobisisobutylnitrile
(AIBN, Shanghai First Reagent Co, 95%) was purified by
recrystallization from ethanol. Tetrahydrofuran (THF) was
refluxed for 24 h over sodium and distilled prior to use.
Dichloromethane (DCM) and ethyl acetate (EA) were dried
over CaH2 and distilled prior to use. All other reagents were
of analytical grade purchased from Shanghai First Reagent
Co. and used as received without purification. Dithiobenzoic
Acid (DTBA) was prepared according to the method described
in ref 19 in 67% yield. 1H NMR (300 MHz, δ, ppm, CDCl3):
8.05 (d, 2H, o-ArH), 7.60 (t, 1H, p-ArH), 7.40 (t, 2H, m-ArH).
Synthesis of 4-(4-Cyanopentanoic Acid) Dithiobezoate
(1). The target compound was prepared according to a modi-
* To whom correspondence may be ddressed. E-mail: pcy@
ustc.edu.cn.
10.1021/ma050455e CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/13/2005

6842 Zheng and Pan
Macromolecules, Vol. 38, No. 16, 2005
Table 1. Results and Conditions of the Reversible Addition-fragmentation Chain Transfer (RAFT) Polymerization of
Styrene Using Dendritic RAFT Agent 3a
time conversion
entry (h)
(%)
Mn(linearchain) (GPC)b Mn(star) (GPC)b PDIc Mn(arm) (NMR)d Mn(star) (NMR)e Mn(star) (th)f Mn(arm)g hydrolysis
1
2
3
4
5
5
10
16
24
40
13.6
37.8
46.2
53.6
69.0
8 300
20 100
22 200
26 800
32 900
36 600
79 800
1.07
1.09
1.14
1.21
1.25
6 800
20 300
23 900
25 400
33 100
57 300
165 300
194 000
206 000
267 700
59 500
160 000
195 000
226 000
290 000
7 500
20 000
23 100
26 300
32 500
93 600
109 000
131 000
a Conditions: in tetrahydrofuran (THF), temperature ) 120 °C, [St]/[RFAT agent 3] ) 4000 ([St]/[dithiobenzoate functions] ) 500).
b Calculated based on the GPC method, in which narrow PDI polystyrene standards were used in the calculation. c Evaluated from GPC
in THF. d The number-average molecular weight of the arm in the dendrimer star polystyrene Mn(arm) (NMR), calculated from 1H NMR
based on eq 1. e The number-average molecular weight of dendrimer-star polystyrenes, calculated from 1H NMR based on eq 2. f Calculated
from the following equation: Mn (star) (th) ) [St]/[RAFT agent] × MSt × conversion + MRAFT agent, where [St] and [RAFT agent] are initial
concentrations of St and RAFT agent and MSt and MRAFT agent are molecular weights of St and RAFT agent. g GPC measurement of the
hydrolysis product.
Table 2. RAFT Polymerization of Styrene using 3 or 4 as
fication method of Thang et al.20 The DTBA (4.62 g, 30 mmol)
RAFT Agents with Different Ratios of [St]/[RAFT Agent]a
and a catalytic amount of I2 (50 mg) were dissolved in 15 mL
conversion Mn(star)
Mn(star)
of ethyl acetate in a 100-mL two-neck flask. Into this solution,
a solution of dimethyl sulfonyl oxide (DMSO) (1.2 g, 15 mmol)
in 5 mL of ethyl acetate was added slowly while stirring
vigorously. The reaction mixture was stirred in the dark for
10 h. Without further purification, the crude disulfide was used
directly in the next step. 4,4′-Azobis-(4-cyanopentanoic acid)
(6.3 g, 23 mmol) and 15 mL of ethyl acetate was added into
the flask. The reaction solution was heated at reflux for 18 h.
The ethyl acetate was removed under reduced pressure. The
crude product was isolate by silica gel column chromatograph
using ethyl acetate: hexane (1:2) as eluent. Those fractions
in pink red were combined. The solvent was removed in vacuo
to give a red solid 1 (5.0 g, yield ) 44% for two steps). 1H NMR
(300 MHz, δ, ppm, CDCl3): 7.91 (d, 2H, o-ArH), 7.58 (t, 1H,
p-ArH), 7.41 (t, 2H, m-ArH), ∼2.40-2.80 (m, 4H, -CH2CH2-),
1.95 (s, 3H, -CH3).
entry [St]/[RAFT agent]
(%)
(GPC)b PDIc
(th)d
6e
2 400
800
24.5
21.3
22.7
20.4
23.6
28 000 1.07 64 100
13 600 1.18 21 200
8 900 1.26 11 700
35 300 1.17 96 300
13 000 1.29 43 500
7e
8e
400
9f
4 800
1 600
10f
a Conditions: in tetrahydrofuran, temperature ) 120 °C. b Cal-
culated based on the GPC method, in which narrow PDI polysty-
rene standards were used in the calculation. c Evaluated from GPC
in THF. d Calculated from the following equation: Mn (star) (th) )
[St]/[RAFT agent] × MSt conversion + MRAFT agent, where [St]
×
and [RAFT agent] are initial concentrations of St and RAFT agent
and MSt and MRAFT
are molecular weights of St and RAFT
agent
agent. e Dendritic RAFT agent 3 as CTA. f Dendritic RAFT agent
4 as CTA.
Synthesis of 4-Cyano-4-((thiobenzoyl)sulfanyl) Pen-
tanoic Succinimide Ester (2). The compound 1 (2.04 g,
7.3 mmol) and N-hydroxyl succinimide (0.84 g, 7.3 mmol) were
dissolved in 15 mL of anhydrous DCM. Dicyclohexylcarbo-
diimide (DCC) (1.51 g, 7.3 mmol) was added in one portion to
the solution. The reaction mixture was stirred at room tem-
perature in the dark for 16 h. A white byproduct was filtrated
out. The filtrate was concentrated, and the concentrated liquid
was purified through a silica gel column with ethyl acetate:
hexane (1: 4, v/v) as eluent. A red solid 2 (2.5 g) was obtained
1
in a yield of 91%. H NMR (300 MHz, δ, ppm, CDCl3): 7.93
(d, 2H, o-ArH), 7.58 (t, 1H, p-ArH), 7.41 (t, 2H, m-ArH), 3.20
(m, 2H, -OC-CH2-CH2-C(CN)(CH3)-), 2.86 (s, 4H, -OC-
CH2-CH2-CO-), ∼2.31-2.74 (m, 2H, -CH2-CH2-C(CN)-
(CH3)-),1.93 (s, 3H, -CH3).
Synthesis of Dendritic RAFT Agent 3. A solution of
DAB-8-Am (0.23 g, 0.30 mmol) in 5 mL of anhydrous DCM
was added slowly to the solution of 4-cyano-4-((thiobenzoyl)-
sulfanyl) pentanoic succinimide ester 2 (1.02 g, 2.7 mmol) in
10 mL of DCM. The reaction mixture was stirred at room
temperature in the dark for 48 h. After DCM was removed,
the residue was purified by pouring the dendritic RAFT agent
in DCM into hexane three times. The small molecular byprod-
uct and the unreacted starting material were removed. After
dried in a vacuum oven at 40 °C for 24 h, dendritic RAFT agent
Figure 1. 1H NMR spectrum of dendritic RAFT agent 3.
(s, 48H, -CH3), ∼1.70-1.88 (m, 60H, -N-CH2-CH2-CH2-
N-).
Polymerization. A typical polymerization procedure for
synthesizing the dendrimer-star polymer is as follows. A glass
tube was charged with a solution of dendritic RAFT agent 3
(12 mg, 3.8 × 10-6 mol) and freshly distilled St (1.6 g, 1.5 ×
10-2 mol) in 1.5 mL of THF. After the polymerization mixture
was degassed by three freeze-evacuate-thaw circles, the tube
was sealed under a vacuum. The polymerization was carried
out at 120 °C without AIBN for 10 h. The reaction was stopped
by plunging the tube into ice water. The dendrimer-star PSt
was precipitated by adding the polymer solution in THF into
petroleum ether (bp ∼30-60 °C) and then collected by filtra-
tion. The polymer obtained was dried in a vacuum oven at
40 °C for 24 h in a yield of 37.8%. Mn(GPC) ) 79 800;
Mw/Mn ) 1.09. 1H NMR (300 MHz, δ ppm, CDCl3): 7.9 (o-ArH
1
3 (1.04 g) was obtained in 95% yield. H NMR (300 MHz, δ,
ppm, CDCl3): 7.88 (d, 16H, o-ArH), 7.55 (m, 8H, p-ArH),
7.38 (m, 16H, m-ArH), 3.28 (m, 16H, -CO-NH-CH2-),
∼2.70-2.90 (m, 32H, -C-CH2-CH2-CO-NH-), ∼2.45-2.60
(m, 36H, -N-CH2-CH2-CH2-N-), 1.92 (s, 24H, -CH3),
∼1.76-1.88 (m, 28H, -N-CH2-CH2-CH2-N-).
Synthesis of Dendritic RAFT Agent 4. The dendritic
RAFT agent 4 was synthesized from DAB-16-Am in 94% yield
by the same method used to prepare dendritic RAFT agent 3.
1H NMR (300 MHz, δ ppm, CDCl3): 7.90 (m, 32H, o-ArH), 7.62
(m, 16H, p-ArH), 7.40 (m, 32H, m-ArH), ∼3.15-3.35 (m, 32H,
-CO-NH-CH2-), ∼2.81-3.02 (m, 64H, -C-CH2-CH2-CO-
NH-), ∼2.50-2.75 (m, 84H, -N-CH2-CH2-CH2-N-), 1.93

Macromolecules, Vol. 38, No. 16, 2005
Dendrimer Star Polymer 6843
Scheme 1. Two Types of Multifunctional RAFT Agents Used in the Synthesis of Star Polymers
of dithiobenzoate group), 7.5 (p-ArH of dithiobenzoate group),
∼6.4-7.3 (ArH of St units, and m-ArH of dithiobenzoate
group), 4.9 (-CH2-CH-S-), 3.1 (-CO-NH-CH2-), 2.7 (-C-
CH2-CH2-CO-NH-), 1.7 (-CH2-CH- of St unit).
star polymers (Scheme 1): (1) The multifunctional
RAFT agents with the Z-group as a part of the core will
allow the growth of arms away from the core during
polymerization (arm-first growth). (2) For those RAFT
agents with the R-group as a part of the core, chain
growth must occur on the surface of core (core-first
growth). Gnanou group used a 12-armed dendritic RAFT
agent in the preparation of dendrimer-star polystyrene
via arm-first growth.16 They found that, with the
progress of polymerization, the growing linear chains
experienced a higher probability of irreversible termina-
tions occurring by couplings of polystyryl radicals,
forming dead linear polymers free of any thiocarbonyl-
thio groups. Therefore we selected the dithiobenzoyl-
terminated dendrimers 3 and 4 as RAFT agents, in
which the R-group is a part of the core.
Hydrolysis of the Dendrimer-Star PSt, [Den(PSt)8].
The dendrimer-star PSt, Den(PSt)8, with Mn ) 93 600, PDI )
1.14 (0.20 g, entry 3 in Table 1), was dissolved in anhydrous
THF (10 mL), and then the solution was treated with 1 mL of
sodium methoxide solution (0.46 g, 20 mmol Na reacted with
10 mL anhydrous MeOH). The mixture was heated at 65 °C
for 1 day. Subsequently water (0.5 mL) was added, and the
mixture was reheated at 65 °C for an additional day. Diluted
HCl was added to neutralize the reaction mixture. The
hydrolyzed linear polymer was then precipitated in methanol.
After it was filtrated and dried in a vacuum oven for 24 h,
white powder (0.16 g) was obtained with Mn ) 23 100, PDI )
1.40 (GPC).
Preparation of Dendrimer-Star Diblock Copolymer,
[Den(PSt-b-PMA)8]. A typical polymerization procedure for
synthesizing dendrimer-star AB diblock copolymer is as fol-
lows. A glass tube was charged with the dendritic polymer
Den(PSt)8 with Mn ) 28 000, PDI ) 1.07 (0.21 g, entry 6 in
Table 2), AIBN (1.0 mg, 6.0 × 10-6 mol), fresh MA (1.0 g,
1.2 × 10-2 mol) and THF (1.0 mL), then the polymerization
mixture was degassed by three freeze-evacuate-thaw cycles.
The tube was sealed under a vacuum. The block polymeriza-
tion was carried out at 90 °C for 5 h. The dendrimer-star
diblock copolymer Den(PSt-b-PMA)8 was precipitated by add-
ing the mixture into petroleum ether (bp ∼30-60 °C) and
collected by filtration. The polymer obtained was dried in a
The dendritic RAFT agents 3 and 4 were synthesized
according to Scheme 2. The PPI dendrimers with the
terminal dithiobenzoate groups were prepared by cou-
pling reaction of 4-cyano-4-((thiobenzoyl)sulfanyl) pen-
tanoic succinimide ester 2 with amine groups on the
surface of DAB-8-Am or DAB-16-Am. We selected
compound 2 as a coupling reagent for two reasons: (1)
compound 2 contains dihiobenzoate group which is a
suitable RAFT agent for St and (2) the succinimide ester
group of compound 2 is easier to react with amine group.
Generally, compound 1 was prepared via a two-step
procedure from DTBA.21 We omitted the separation
procedure after the oxidization reaction of DTBA, and
the yield of compound 1 was not affected.
vacuum oven at 40 °C for 24 h in yield of 36% with Mn
)
15 6000, PDI ) 1.35. 1H NMR (300 MHz, δ ppm, CDCl3): 7.9
(o-ArH of dithiobenzoate group), ∼6.4-7.3 (ArH of St unit),
4.9 (-CH2-CH-S-), 3.7 (-O-CH3 of MA units), ∼1.3-2.3
(-CH2-CH- of PSt and PMA unit).
To ensure complete transformation of all eight amine
groups in DAB-8-Am into dithiobenzoyl groups, an
excess of the compound 2 was used. The unreacted
compound 2 and byproduct N-hydroxyl succinimide
could be removed by precipitation from hexane. Figure
Characterization. 1H NMR (300 MHz) spectra were
recorded on a Bruker 300 nuclear magnetic resonance (NMR)
instrument, using CDCl3 as a solvent and tetramethylsilane
(TMS) as an internal reference. The molecular weight, Mn
(GPC) and molecular weight distribution (MWD) were deter-
mined at 30 °C on a Waters 515 gel permeation chromatog-
raphy (GPC) equipped with microstyragel columns, 103, 104,
and 105 Å. Standard narrow polydispersity polystyrenes were
used in the calibration of molecular weight and molecular
weight distribution and THF as eluent at a flow rate of
1.0 mL/min. Infrared spectra were recorded on a Bruker
VECTOR-22 IR spectrometer.
1
1 shows the typical H NMR spectrum of the resulting
dithiobenzoate-teminated dendrimer CTA 3. The signals
at δ ) 7.88 (a′), 7.58 (b′), and 7.38 (c′) ppm are assigned
to the ortho, para, and meta aromatic protons relative
to the dithiobenzoate group, respectively. Compared to
1
the H NMR spectrum of the starting PPI dendrimer
DAB-8-Am, the peak (g) of the methylene protons next
to carbimide on the surface of dendrimer obtained from
the reaction with compound 2 moves from 2.78 to
3.28 ppm. In addition, the signals at δ ) 2.50 (h) and
1.80 ppm (i) are ascribed to the methylene protons in
the PPI dendrimer. The integration ratio of the peaks
a′: b′: c′: d′: e′: f′: g: h: i is almost equal to 16:8:16:
Results and Discussions
Synthesis of Dendritic RAFT Agents 3. Afore-
mentioned in the introduction, there are two types of
multifunctional RAFT agents used in the synthesis of

6844 Zheng and Pan
Macromolecules, Vol. 38, No. 16, 2005
Scheme 2. Synthesis of Two Dendritic RAFT Agents
16:16:24:16:36:28. This evidence shows that all of the
eight amine groups have turned to imide groups,
indicating the formation of eight dithiobenzoate-termi-
nated dendritic RAFT agent 3. With the similar reaction
condition, we synthesized the 16 dithiobenzoate-termi-
nated dendritic RAFT agent 4 from DAB-16-Am. The
FT-IR spectra of the dithiobenzoate-terminated den-
dritic CTA 3 and 4 are shown in Figure 2. The
characteristic bands for the RAFT agents 3 and 4 are
weight of star is as expected.12 We therefore selected a
polymerization temperature of 120 °C, with no addition
of radical initiator for better control of molecular weight
of the star polymers. The results and conditions of the
RAFT polymerization are listed in Table 1. The molec-
ular weight increased with increasing conversion up to
69%, which is different from that observed by Gnanou.16
With the core-first growth process, the RAFT agent 3
reacted rapidly with a propagating radical (Pn•) to form
a linear thiocarbonylthio polymer chain [SdC(Ph)S-Pm]
which will leave from the core, and the fragmented star
radicals (R•) on the surface of the core can initiate the
polymerization of St. The star radicals will not be
shielded by PSt because the radicals and RAFT sites
are consistently on the surface of the dendrimer-star
polymers.
clearly observed in the IR spectra: CdS, 1048 cm-1
CdO, 1649 cm-1; CtN, 2230 cm-1; N-H, 3430 cm-1
;
.
RAFT Polymerization of St Using Multifunc-
tional Dendritic RAFT Agents. The dendrimer-star
Den(PSt)8 was prepared by RAFT polymerization of St
in the presence of dendritic RAFT agent 3 (Scheme 3).
Because the dendritic RAFT agent 3 cannot dissolve in
St, toluene, and anisole, all polymerizations were carried
out in THF. It was reported that, at the higher tem-
perature and without a radical initiator, the molecular
The pseudo-first-order kinetic plot of the polymeri-
zation of St in the presence of dendritic RAFT agent 3
is shown in Figure 3, indicating the concentration of

Macromolecules, Vol. 38, No. 16, 2005
Dendrimer Star Polymer 6845
Scheme 3. RAFT Polymerization and Block Copolymerization with Dendritic RAFT Agent and Hydrolysis of the
Dendrimer-Star
chain radicals is constant during the polymerization.
However, the straight line does not pass through zero.
This inhibition of polymerization may be ascribed to
slow fragmentation and/or slow reinitiation of the
leaving group in the early stage of polymerization.22-24
A well-controlled molecular weight is confirmed by
linear increase of the molecular weight of the den-
drimer-star PSt obtained with conversion as shown in
Figure 4 , and the molecular weight distribution is
narrow (PDI ) 1.07-1.25).
Although this RAFT polymerization displayed living
character, linear PSt always exists along with propaga-
tion of the dendrimer-star PSt. This can be observed
by following the polymerization by GPC. Figure 5 shows
the GPC curves of the polymers obtained at different
conversions. The GPC curves of all the polymer samples
in Figure 5 demonstrate two peaks: the high molecular
weight peak is dendrimer-star PSt, Den(PSt)8, and the
lower molecular weight peak is linear PSt, which will
be verified later. Both molecular weights increase with
conversion, and the relative amount of linear PSt in the
polymers increases as polymerization progresses. On the
basis of the integral values of the two peaks, the
contents of linear PSt were calculated; they are 0.8%
for the polymers obtained at 13.6% conversion, 4.8% at
conversion of 37.8%, and 12.4% at 69%. These phenom-
ena should be related to the RAFT polymerization
mechanism with multifunctional dendrimer RAFT agent.
The livingness of RAFT polymerization is due to estab-
lishing the equilibrium between the propagating radical
(Pn•) and dormant chain [SdC(Z)S-Pm]. Different from
ATRP, in this equilibrium reaction, split of the adduct
formed by the attack of chain radical on dormant chain
will release a new dormant chain and a new propagating
chain.11,20
For the polymerization initiated by multifunctional
RAFT agents, we propose a mechanism of “core-first”
approach (Scheme 4) based on the mechanism with a
monofunctional RAFT agent. At the initial stage, the
attack of a propagating radical (Pn•) on the multifunc-
tional RAFT agent will produce a radical on star core,
and it will initiate the polymerization. As the reaction
proceeds, the equilibrium of the active species on the
linear and dendrimer-star chains with dormant linear
and dendrimer-star chains is established in Scheme 4.
Thus the star polymer growth might be accompanied

6846 Zheng and Pan
Macromolecules, Vol. 38, No. 16, 2005
Figure 2. IR spectra of dendritic RAFT agents 3 (a) and
4 (b).
Figure 5. Evolution of GPC chromatograms for the RAFT
polymerization of styrene at 120 °C in tetrahydrofuran in the
presence of dendritic RAFT agent 3 ([St]/[RAFT agent 3] )
4000): (a) entry 1 in Table 1; (b) entry 2 in Table 1; (c) entry
5 in Table 1.
Scheme 4. Mechanism of RAFT Polymerization with
Dendritic RAFT Agent via “Core-First” Approach
Figure 3. Pesudo-first-order kinetic plot of the preparation
of dendrimer-star polystyrene at 120 °C in tetrahydrofuran
in the presence of dendritic RAFT agent 3. ([St]/[RAFT agent
3] ) 4000).
Figure 4. Evolution of the molecular weight and PDIs with
conversion for the RAFT polymerization of styrene at 120 °C
in tetrahydrofuran in the presence of dendritic RAFT agent 3
([St]/[RAFT agent 3] ) 4000).
at δ ) 7.9 (a), 7.5 (b), 3.1 (d), 2.7 (e + f) ppm originated
from the dendritic RAFT agent 3 appear still. The other
signals from the dendrimer are overlapped with the
characteristic signals of the PSt. While a new signal
around δ ) 4.7 ppm (c) ascribed to the methine proton
of St next to dithiobenzoyl group appears in Figure 6,
which might result from the cleavage of C-S bond, and
the formation of a new C-S bond between St unit and
dithiobenzoyl group in the RAFT process. The number-
average molecular weight of each arm, Mn (arm)(NMR)
can be calculated according to eq 1
with a parallel polymerization of single chains. It is
reasonable that the linear polymer should have an
identical molecular weight to each of the arms attached
to the dendrimer. The linear PSt propagated with the
propagation of dendrimer-star PSt was shown in Figure
5.
For estimating the molecular weights of each arm and
1
the star, the H NMR spectra of dendrimer-star PSts,
Den(PSt)8 obtained after removal of the linear PSt were
1
measured. Figure 6 is a typical H NMR spectrum of
the dendrimer-star PSt, Den(PSt)8 (entry 1 in Table 1).
Except for the characteristic signals of PSt, those signals
Mn(arm)(NMR) ) [ 2I1.7/( 3I3.1)]MSt
(1)

Macromolecules, Vol. 38, No. 16, 2005
Dendrimer Star Polymer 6847
Figure 7. GPC chromatograms of polystyrene dendrimer-star
(solid line) and the resulting linear polystyrene after hydrolysis
(dash line): (a) Mn (star)(GPC) ) 93 600, PDI ) 1.14 (Table 1,
entry 3); (a′) Mn ) 23 100, PDI ) 1.40; (b) Mn(star)(GPC) )
131 000, PDI ) 1.25 (Table 1, entry 5); (b′) Mn ) 32 500,
PDI ) 1.45.
Figure 6. 1H NMR spectrum of the dendrimer-star polysty-
rene with Mn (star)(GPC) ) 36 600 (Table 1, entry 1) using
dendritic agent 3 as the CTA.
with I1.7 and I3.1 relative integrations of the -CH2-CH-
protons (δ ) ∼1.3-2.0 ppm) of the PSt backbone and of
the -CH2-CONH- protons (δ ) 3.1 ppm) of arm ends
of PPI dendrimers. The number-average molecular
weight of dendrimer-star PSts, Mn (star)(NMR), can be
calculated based on eq 2
Mn (star)(NMR) ) Mn (arm)(NMR)n + MdendriticRAFTagent
(2)
with MdendriticRAFTagent the molecular weight of dendritic
RAFT agent and n the number of branches of the star.
The calculated results are listed in Table 1 and also
shown in Figure 4. The Mn(star)(GPC) of dendrimer-star
PSt is less than their Mn (star)(th) and Mn (star)(NMR)
values because the molecular weights are calibrated
based on the linear polystyrene standards and they have
different hydrodynamic characters. Mn(star)(th) values
were calculated based on the feed molar ratio of
St/dendritic RAFT agent and conversion of monomer.
The Mn (star)(NMR) values of the dendrimer-star PSts
agree with Mn(star)(th) at lower conversions. But at
higher conversions, such as at 53.6 and 69% conver-
sions, the Mn(star)(NMR) values are lower than the
correspondent Mn(star)(th)s because of the higher per-
centage of linear PSt (Figure 5).
To confirm that the linear chain has an identical
molecular weight to each of the arms attached to the
core of the star, we hydrolyzed the dendrimer-star PSt
in sodium methanol. The amide bond of the dendrimer-
star PSt was cleaved to afford the linear PSt. The
molecular weight of each arm can be directly obtained
from its GPC curve (Figure 7). The GPC traces of the
hydrolyzed product showed exactly the same molecular
weight of the linear products. A shoulder on higher
molecular weight was observed, and it may be due to
incomplete hydrolysis or occurrence of some irreversible
terminations, including coupling reactions between star
and linear chain radicals, as shown in Scheme 4. The
exact reason is not clear at present.
Figure 8. GPC chromatograms of dendimer-star diblock
copolymer PSt-b-PMA (solid line) using dendrimer-star poly-
styrene (dash line) as macro CTA: (a) Mn(GPC) ) 28 000,
PDI ) 1.07 (Table 2, entry 6); (b) Mn(GPC) ) 156 000,
PDI ) 1.35.
with the molar ratio increase of St/dendritic RAFT
agents in both cases.
Dendrimer-Star Block Copolymer, Den(PSt-b-
PMA)8. One key aspect of living radical polymerization
is its application in the preparation block copolymers.
The GPC curve of the dendrimer-star block copolymer
obtained shown in Figure 8 demonstrates clearly the
occurrence of the chain extension because its curve is
shifted to a higher molecular weight position, and a
shoulder at the low molecular weight appearing in
Figure 8b may be ascribed to linear block copolymer
formed during the block copolymerization based on the
RAFT polymerization mechanism discussed above. In
addition, a small amount of dead polymers in the macro
RAFT agent may not excluded. To confirm further the
formation of S(PSt-b-PMA)8, the 1H NMR spectra were
measured and a typical spectrum is shown in Figure 9.
In addition to the characteristic signals of PSt, the
signal at δ ) 3.7 (D) ppm is ascribed to the ester methyl
protons of MA units, confirming the formation of Den-
(PSt-b-PMA)8. We can also find the signal at δ ) 7.9 (a)
assigned to the ortho aromatic protons of dithiobenzoyl
groups, which must result from RAFT block polymeri-
zation of MA.
The influence of the molar ratio of RAFT agent 3 to
St on RAFT polymerization was investigated. The
results are summarized in Table 2. The 16-armed
dendritic RAFT agent 4 was also used in the RAFT
polymerization of St, and the results are also listed in
Table 2. We can see that the molecular weights of the
dendrimer-star PSts, Den(PSt)16, obtained increased
Conclusion
Two multifunctional dithiobenzoate-terminated PPI
dendrimer RAFT agents were synthesized, and they

6848 Zheng and Pan
Macromolecules, Vol. 38, No. 16, 2005
(2) Roovers, J.; Zhou, L. L.; Toporowski, P. M.; Vanderzwan, M.;
Iatrou, H.; Hadjichristidis, N. Macromolecules 1993, 26,
4324-4331.
(3) Kimura, M.; Kato, M.; Muto, T.; Hanabusa, K.; Shirai, H.
Macromolecules 2000, 33, 1117-1119.
(4) Cooper, A. I.; Londono, J. D.; Wignall, G.; McClain, J. B.;
Samulski, E. T.; Lin, J. S.; Dobrynin, A.; Rubinstein, M.;
Burke, A. L. C.; Frechet, J. M. J.; DeSimone, J. M. Nature
1997, 389, 368-371.
(5) Hedden, R. C.; Bauer, B. J.; Smith, A. P.; Grohn, F.; Amis,
E. Polymer 2002, 43, 5473-5481.
(6) Luo, D.; Haverstick, K.; Belcheva, N.; Han, E.; Saltzman, W.
M. Macromolecules 2002, 35, 3456-3462.
(7) Hedden, R. C.; Bauer, B. J. Macromolecules 2003, 36, 1829-
1835.
(8) Comanita, B.; Noren, B.; Roovers, J. Macromolecules 1999,
32, 1069-1072.
(9) Zhao, Y. L.; Shuai, X. T.; Chen, C. F.; Xi, F. Chem. Mater.
2003, 15, 2836-2843.
Figure 9. 1H NMR of dendrimer-star block copolymer
PSt-b-PMA (Mn(GPC) ) 156 000, PDI ) 1.35).
(10) Heise, A.; Diamanti, S.; Hedrick, J. L.; Frank, C. W.; Miller,
R. D. Macromolecules 2001, 34, 3798-3801.
(11) Moad, G.; Chiefari, J.; Chong, Y. K.; Krstina, J.; Mayadunne,
R. T. A.; Postma, A.; Rizzardo, E.; Thang, S. H. Polym. Int.
2000, 49, 993-1001.
were successfully used in the preparation of well-defined
dendrimer-star polymers via “core-first” RAFT ap-
proach. The polymerization displays living characters.
With the propagation of the dendrimer-star polymer, the
linear polymer grows in parallel, and its relative amount
to dendrimer-star polymer increases also (12.4% at 69%
conversion). Since each arm in dendrimer-star polymers
and linear polymers both have the same possibility to
propagate, they have similar molecular weights. This
was confirmed by the hydrolysis of the dendrimer star
polymer. No shielding effect was observed in the po-
lymerization system because the chain radicals and
RAFT sites existed consistently on the surface of den-
drimer-star polymer. The dendrimer-star block copoly-
mer, Den(PSt-b-PMA)8, was successively prepared by
the RAFT polymerization of MA using macro RAFT
agent, and the linear block copolymer was observed
during the block copolymerization.
(12) Stenzel-Rosenbaum, M.; Davis, T. P.; Chen, V.; Fane, A. G.
J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2777-2783.
(13) Stenzel, M. H.; Davis, T. P. J. Polym. Sci., Part A: Polym.
Chem. 2002, 40, 4498-4512.
(14) Mayadunne, R. T. A.; Jeffery, J.; Moad, G.; Rizzardo, E.
Macromolecules 2003, 36, 1505-1513.
(15) Dureault, A.; Taton, D.; Destarac, M.; Leising, F.; Gnanou,
Y. Macromolecules 2004, 37, 5513-5519.
(16) Darcos, V.; Dureault, A.; Taton, D.; Gnanou, Y.; Marchand,
P.; Caminade, A. M.; Majoral, J. P.; Destarac, M.; Leising,
F. Chem. Commun. 2004, 2110-2111.
(17) Hao, X.; Nilsson, C.; Jesberger, M.; Stenzel, M. H.; Malm-
strom, E.; Davis, T. P.; Ostmark, E.; Barner-kowollik, C. J.
Polym. Sci., Part A: Polym. Chem. 2004, 42, 5877-5890.
(18) Jesberger, M.; Barner, L.; Stenzel, M. H.; Malmstrom, E.;
Davis, T. P.; Barner-kowollik, C. J. Polym. Sci., Part A:
Polym. Chem. 2004, 41, 3847-3861.
(19) Bai, R. K.; You, Y. Z.; Pan C. Y. Polym. Int. 2000, 49, 898-
902.
(20) Chong, Y. K.; Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S.
H. Macromolecules 1999, 32, 2071-2074.
(21) Mitsukami, Y.; Donovan, M. S.; Lowe, A. B.; McCormick, C.
L. Macromolecules 2001, 34, 2248-2256.
Acknowledgment. This work is supported by the
National Natural Science Foundation of China under
Contract No. 20374048.
(22) Monteiro, M. J.; de Brouwer, H. Macromolecules 2001, 34,
349-352.
(23) Kwak, Y.; Goto, A.; Tsujii, Y.; Murata, Y.; Komatsu, K.;
Fukuda, T. Macromolecules 2002, 35, 3026-3029.
(24) Chong, Y. K.; Krstina, J.; Le, T. P. T.; Moad, G.; Postma, A.;
Rizzardo, E.; Thang, S. H. Macromolecules 2003, 36, 2256.
References and Notes
(1) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev.
1999, 99, 1665-1688.
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