Synthesis of degradable miktoarm star copolymers via atom transfer radical polymerization

Article abstract of DOI:10.1021/ma0503099

Both nondegradable and degradable miktoarm star copolymers were synthesized via ATRP using the "in-out" method. The synthesis consisted of three steps: synthesis of a linear macroinitiator (MI) using the first monomer, a core-forming reaction with a divinyl compound, and then further chain extension with the second monomer. Various characterization methods, including GPC, NMR, and GC, were used to analyze the initial star polymers and to identify the optimal experimental conditions for the synthesis of the multifunctional star initiators and the miktoarm star copolymers. During the synthesis of miktoarm star copolymers, both interstar and intrastar arm-arm couplings were observed. The initiating efficiency of the alkyl bromide sites in the core of the star polymers was determined by analysis of the linear block copolymers, obtained after cleavage of the degradable star and miktoarm star copolymers. The results confirmed that the initiation efficiency was reduced by the highly cross-linked star core. In the case of degradable miktoarm star copolymers, only 19% of the total Br functionality incorporated into the first star copolymer initiated the polymerization of the second monomer.

Full text of DOI:10.1021/ma0503099

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Macromolecules 2005, 38, 5995-6004
5995
Synthesis of Degradable Miktoarm Star Copolymers via Atom Transfer
Radical Polymerization
Haifeng Gao, Nicolay V. Tsarevsky, and Krzysztof Matyjaszewski*
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue,
Pittsburgh, Pennsylvania 15213
Received February 13, 2005; Revised Manuscript Received May 10, 2005
ABSTRACT: Both nondegradable and degradable miktoarm star copolymers were synthesized via ATRP
using the “in-out” method. The synthesis consisted of three steps: synthesis of a linear macroinitiator
(MI) using the first monomer, a core-forming reaction with a divinyl compound, and then further chain
extension with the second monomer. Various characterization methods, including GPC, NMR, and GC,
were used to analyze the initial star polymers and to identify the optimal experimental conditions for
the synthesis of the multifunctional star initiators and the miktoarm star copolymers. During the synthesis
of miktoarm star copolymers, both interstar and intrastar arm-arm couplings were observed. The
initiating efficiency of the alkyl bromide sites in the core of the star polymers was determined by analysis
of the linear block copolymers, obtained after cleavage of the degradable star and miktoarm star
copolymers. The results confirmed that the initiation efficiency was reduced by the highly cross-linked
star core. In the case of degradable miktoarm star copolymers, only 19% of the total Br functionality
incorporated into the first star copolymer initiated the polymerization of the second monomer.
Introduction
CRP permits the synthesis of star polymers by one of
three methods: “core-first”,17-21 “arm-first”,22-24 or their
combination.25,26 The combination of “core-first” and
“arm-first” methods is particularly useful in the syn-
thesis of miktoarm star copolymers.1,19 The procedure
involves the synthesis of the first type of polymer chain
polyA, which can be used as a macroinitiator (MI) in a
subsequent cross-linking reaction using a divinyl com-
pound to produce a (polyA)n-polyX star polymer, where
polyX represents the core of the star polymer and n is
the number of polyA arms. Because of the “living”
nature of the CRP, the initiating sites are preserved in
the core of the star polymer (i.e., alkyl halide groups in
ATRP), and the star polymer can be used as a multi-
functional initiator in the chain extension reaction with
a different monomer, B, to yield a miktoarm star
copolymer, (polyA)n-polyX-(polyB)m. This combination
method for synthesizing miktoarm star copolymers was
termed the “in-out” method.1,25
One important challenge in the synthesis of miktoarm
star copolymers using this method is to determine the
initiation efficiency for the chain extension reactions
with the second monomer, B, i.e., the percent of initiat-
ing sites that actually initiated the polymerization of
B. Because of the high cross-linking density in the star
core, it can be envisioned that not all of the dormant
sites would be efficiently activated in a chain extension
reaction. Thus, evaluation of initiation efficiency be-
comes a significant challenge because the presence of a
highly cross-linked core hinders the accurate analysis
of star polymers using NMR or GPC.
Star polymers consist of several linear polymer chains
connected at one point.1 Their compact structure and
globular shape predetermine their unique properties,
such as low solution viscosity, and enable potential
applications, including use as drug carriers.2 Star
polymers can be divided into two structural categories:
homoarm and miktoarm star polymers. In the former
case, the arms are identical in chemical composition,
while in the latter case, two or more than two different
types of arms build the star molecule. Typically, star
polymers are prepared by anionic polymerization be-
cause it allows one to control the arm size and size
distribution. Drawbacks of this method include strin-
gent reaction conditions, such as high sensitivity to CO2
and moisture.
In contrast, free radical polymerization is more toler-
ant to protic impurities and is capable of polymerizing
a vast variety of vinyl monomers. However, because of
slow initiation and fast radical-radical termination
reactions, the resulting materials are polydisperse, and
the control over molecular weight and functionality is
very difficult. However, recently developed controlled/
“living” radical polymerization (CRP) processes allow for
the synthesis of well-designed (co)polymers.3-8 All CRP
methods are based on establishing a dynamic equilib-
rium between a low concentration of active propagating
chains and a large number of dormant chains, which
are unable to propagate or terminate but can be
reactivated. Thus, the fraction of chains that undergo
radical-radical termination is reduced, and the radical
polymerization system attains a “living” character. One
of the most efficient and popular CRP methods is atom
transfer radical polymerization (ATRP),7,9,10 which al-
lows for the synthesis of (co)polymers with predeter-
mined degree of polymerization, narrow molecular
weight distribution (low polydispersity, Mw/Mn),11 high
functionality,12 and desired microstructure.13-16
The strategy proposed here is to degrade the star
polymers into the constituent linear polymer chains.
Through analysis of the cleaved linear chains by stan-
dard techniques, it should be possible to determine the
initiation efficiency in the second chain extension reac-
tion. Decomposition of a star polymer requires a degrad-
able cross-linker used in the formation of the star. It is
well-known that the disulfide group can be easily
cleaved in the presence of various reducing agents,27,28
* Corresponding author. E-mail: km3b@andrew.cmu.edu.
10.1021/ma0503099 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/14/2005

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5996 Gao et al.
Macromolecules, Vol. 38, No. 14, 2005
Scheme 1. Scheme of the Synthetic Procedure of (PolyBA)n-PolyDVB-(PolySt)m Miktoarm Star Copolymers
Scheme 2. Proposed Synthetic Procedure of (PolyMMA)n-PolySS-(PolyBA)m Miktoarm Star Copolymers and
Their Cleavage by a Reducing Agent
such as thiols29 and phosphines.30,31 Both initiators and
monomers containing disulfide groups were successfully
used in ATRP for the preparation of reversibly degrad-
able polymeric materials via a redox process,32,33 for
cleavage of polymers attached to gold surfaces,34 and
for polymers useful in protein modification.35 Introduc-
ing the disulfide group into a divinyl compound should
provide a cross-linker with the needed degradability.
This article reports the syntheses of both nondegrad-
able and degradable miktoarm star copolymers via
ATRP using the “in-out” method. Two different cross-
linkers were used in the synthetic procedures, as shown
in Scheme 1 and Scheme 2. For the synthesis of
nondegradable miktoarm star copolymers (Scheme 1),
ATRP was used to produce a poly(n-butyl acrylate)
(polyBA) MI with narrow molecular weight distribution
(MWD) and high degree of Br functionality. This was
accomplished by employing ethyl 2-bromopropionate
(EBrP) as initiator and CuBr/N,N,N′,N′′,N′′-penta-
methyldiethylenetriamine (PMDETA) as catalyst. Using
ATRP, this polyBA MI was then chain extended, cross-
linked with divinylbenzene (DVB) to form nondegrad-
able (polyBA)n-polyDVB star polymers by the “arm-
first” method. Finally, polymerization of a second
monomer, styrene, initiated by the active alkyl halide
initiating sites remaining within the core of the star,
formed polystyrene chains (polySt) and thereby a (po-
lyBA)n-polyDVB-(polySt)m miktoarm star copolymer.
Degradable miktoarm star copolymers were synthe-
sized using a similar strategy: a poly(methyl methacry-
late) MI was prepared employing ethyl 2-bromoisobu-
tyrate (EBiB) as initiator and CuBr/2,2′-bipyridine (bpy)
as catalyst followed by chain extension with a divinyl
monomer containing an internal degradable disulfide
link, bis(2-methacryloyloxyethyl) disulfide (SS), produc-
ing degradable star polymers. Subsequent polymeriza-
tion of the second monomer, BA, resulted in a degrad-
able (polyMMA)n-polySS-(polyBA)m miktoarm star
copolymer. The initiating efficiency of the alkyl halide
sites within the core of the (polyMMA)n-polySS star
polymer in the chain extension with BA was evaluated
by GPC and NMR analysis of the linear block copoly-
mers formed when the star and miktoarm star copoly-
mers were cleaved by a reducing agent, tributylphos-
phine (Bu3P).
Experimental Section
Materials. Styrene (St, 99%, Aldrich), n-butyl acrylate (BA,
99%, Aldrich), methyl methacrylate (MMA, 99%, Aldrich), and
divinylbenzene (DVB, 99%, Aldrich) were purified by passing
the neat liquid through a column filled with basic alumina to
remove the inhibitor. CuBr (98%, Acros) was purified using a
modified literature procedure,36 namely washing with glacial
acetic acid followed by filtration and rinsing with 2-propanol.
All other reagentssbis(2-hydroxyethyl) disulfide (99%,
TCI), methacryloyl chloride (98%, Aldrich), pyridine, ethyl
2-bromopropionate (EBrP), ethyl 2-bromoisobutyrate (EBiB),
N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), 2,2′-
bipyridine (bpy), tributylphosphine (Bu3P), and CuBr2sand all
solvents were used as received without further purification.
Synthesis of the Disulfide Cross-Link Agent Bis(2-
methacryloyloxyethyl) Disulfide (SS). A clean, dry round-
bottom flask containing 200 mL of methylene chloride (dried
overnight with MgSO4) was cooled to 0 °C in an ice-water
bath. The solvent was purged with N2 for 30 min before
pyridine (15.8 mL, 0.195 mol) and bis(2-hydroxyethyl) disulfide
(10.00 g, 0.065 mol) were added. The heterogeneous mixture
was magnetically stirred for 10 min before dropwise addition
of methacryloyl chloride (18.8 mL, 0.195 mol) over a period of
30 min. The flask was then removed from the ice-water bath,
and the reaction was allowed to proceed for 24 h at room
temperature. During the reaction, the initially heterogeneous
mixture slowly turned into a yellowish solution. The methylene
chloride solution was washed successively with 300 mL of 1
M HCl, 300 mL of 1 M NaOH, and 300 mL of deionized water
and was then dried over anhydrous MgSO4 overnight. After
removing the solvent via rotary evaporation, the product was
dissolved in 200 mL of ethyl ether and passed through a
column filled with anhydrous Na2CO3 and basic alumina (two

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Macromolecules, Vol. 38, No. 14, 2005
Degradable Miktoarm Star Copolymers 5997
bands, 30/70 by volume). The absorbent was washed with 50
mL of ethyl ether, and the final product was obtained after
removing the ethyl ether from the combined filtrate. The yield
of SS cross-linker was 11.9 g (63%), and the monomer structure
for GC measurement of MMA conversion. After removal of
acetone by evaporation, the sample was dissolved in THF for
measurement of molecular weight using GPC. The reaction
was stopped at 60% MMA conversion after 2 h by exposure to
air and dilution with acetone. Polymers were purified via
filtration through a column filled with neutral alumina before
precipitation in hexane. The final product was dried under
vacuum at 60 °C for 2 days. Mn ) 8800 g/mol, Mw/Mn ) 1.40.
Synthesis of (PolyMMA)n-PolySS Star Polymer Using
PolyMMA as MI. The synthetic procedure of (polyMMA)n-
polySS star polymers was similar to that of (polyBA)n-
polyDVB star polymers. With polyMMA of Mn ) 8800 g/mol
(1.0 g, 114 µmol) as MI, degradable cross-linker SS (1.15 g,
3.97 mmol) was polymerized with CuBr (16.3 mg, 114 µmol)/
bpy (35.5 mg, 227 µmol) as catalyst in acetone (8.0 mL)
containing 0.4 mL of anisole. The reaction temperature was
50 °C. At timed intervals, samples were withdrawn for the
analysis of SS conversion and polymer molecular weight via
1H NMR and GPC, respectively. After 10 h, the reaction was
stopped at about 64% conversion of SS by exposure to air and
dilution with acetone. The solution was filtered through a
column filled with neutral alumina, and the final star copoly-
mer was obtained after precipitation into hexane and drying
under vacuum at 60 °C for 2 days.
Synthesis of (PolyMMA)n-PolySS-(PolyBA)m Mik-
toarm Star Copolymer Using (PolyMMA)n-PolySS Star
Polymer as MI. The synthetic procedure of (polyMMA)n-
polySS-(polyBA)m miktoarm star copolymers was similar to
that employed for the (polyBA)n-polyDVB-(polySt)m mik-
toarm star copolymers. The polymerization was carried out
at 60 °C using (polyMMA)n-polySS star MI (0.38 g, 24.8 µmol
of -Br initiating sites), BA (1.61 mL, 11.2 mmol), PMDETA
(10.4 µL, 49.8 µmol), CuBr (6.4 mg, 44.6 µmol), CuBr2 (1.1 mg,
4.92 µmol), and DMF (4 mL) as reactants. The reaction was
stopped after 20 h by exposure to air and dilution with THF,
ca. 25% BA conversion. To purify the polymer, the copper
complex was removed by filtration through a column filled with
neutral Al2O3, and then the polymer was precipitated in
hexane and dried under vacuum at 60 °C for 2 days.
Cleavage of Star Polymers and Miktoarm Star Co-
polymers with Disulfide Core by Reducing Agent. In a
typical procedure, 0.1 g of the star polymer or 0.05 g of the
miktoarm star copolymer was mixed with 5 mL of 0.08 M Bu3P
in THF. The solution was stirred magnetically at room
temperature. Samples were periodically withdrawn to deter-
mine the molecular weights of the product of the reductive
cleavage using GPC.
Characterization. Monomer conversions were determined
from the concentration of the unreacted monomer in the
samples periodically removed from the reactions using a
Shimadzu GC-14A gas chromatograph, equipped with a capil-
lary column (DB-Wax, 30 m × 0.54 mm × 0.5 µm, J&W
Scientific). The conversion of SS was measured using 1H NMR
(with CDCl3 as the solvent). In both techniques, DMF or
anisole was used as an internal standard for the calculation
of monomer conversion. The molecular weights and MWD of
the samples were measured by GPC (Polymer Standards
Services (PSS) columns (guard, 105, 103, and 102 Å), with THF
eluent at 35 °C, flow rate ) 1.00 mL/min, and differential
refractive index (RI) detector (Waters, 2410)). Toluene was the
internal standard; the apparent molecular weights were
determined with a calibration based on linear polySt or
polyMMA standards using WinGPC 6.0 software from PSS.
The detectors employed to measure the absolute molecular
weights were a RI detector (Wyatt Technology, Optilab REX)
and a multiangle laser light scattering (MALLS) detector
(Wyatt Technology, DAWN EOS) with the light wavelength
at 690 nm. The same RI detector was used to determine the
refractive index increment (dn/dc) of the star and miktoarm
star samples in THF at 35 °C with the light wavelength at
690 nm. Absolute molecular weights were determined using
ASTRA software from Wyatt Technology. NMR spectra of the
polymer solutions in CDCl3 or THF-d8 were collected on Bruker
Avance 300 MHz spectrometer at 27 °C.
1
was verified by H NMR spectroscopy (δ, CDCl3 as solvent):
4.40 ppm (t, 2H, CH2S), 2.95 ppm (t, 2H, CH2O), 6.12 ppm (s,
1H, CH2dC(CH3)), 5.57 ppm (s, 1H, CH2dC(CH3)), and 1.94
ppm (s, 3H, CH2dC(CH3)). On the basis of the NMR and IR
spectral analysis, the final product contained no unreacted
methacrylic acid.
Synthesis of PolyBA MI by ATRP. A clean and dry
Schlenk flask was charged with BA (24 mL, 0.167 mol),
PMDETA (0.291 mL, 1.4 mmol), and DMF (4.0 mL). The flask
was degassed by five freeze-pump-thaw cycles. During the
final cycle the flask was filled with nitrogen and CuBr (0.200
g, 1.4 mmol) was quickly added to the frozen mixture. Special
care was not taken to avoid moisture condensation. The flask
was sealed with a glass stopper and then evacuated and
backfilled with nitrogen five times before it was immersed in
an oil bath at 60 °C. Finally, the N2-purged initiator EBrP
(0.181 mL, 1.4 mmol) was injected into the reaction system,
via a syringe, through the sidearm of the Schlenk flask. At
timed intervals, samples were withdrawn via a N2-purged
syringe fitted with stainless steel needle and diluted with THF.
The samples were used to measure the monomer conversion
and polymer molecular weights by GC and GPC, respectively.
The reaction was stopped after 1.5 h, at about 60% BA
conversion, via exposure to air and dilution with THF. The
solution was filtered through a column filled with neutral
alumina to remove the copper complex before the polymer was
precipitated by addition to a methanol/water mixture (1/1 by
volume). The precipitant was dried under vacuum at 60 °C
for 2 days. The polyBA MI had Mn ) 9000 g/mol and Mw/Mn )
1.12.
Synthesis of (PolyBA)n-PolyDVB Star Polymer Using
PolyBA as MI. PolyBA MI (1.0 g, 111 µmol), DVB (0.237 mL,
1.67 mmol), PMDETA (23.2 µL, 111 µmol), and anisole (4.0
mL) were charged to a Schlenk flask. The flask was degassed
by five freeze-pump-thaw cycles and filled with nitrogen.
CuBr (15.9 mg, 111 µmol) was quickly added to the frozen
mixture. The flask was sealed with a glass stopper and then
evacuated and backfilled with nitrogen five times before being
immersed in a 110 °C oil bath. At timed intervals, samples
for GC and GPC analysis were withdrawn via syringe. After
20 h (ca. 86% DVB conversion by GC), the reaction mixture
was exposed to air and diluted with THF. The solution was
filtered through a column filled with neutral alumina to
remove the copper complex before the polymer was precipi-
tated in a methanol/water mixture (1/1 by volume). Finally,
the polymer product was dried under vacuum at 60 °C for 2
days.
Synthesis of (PolyBA)n-PolyDVB-(PolySt)m Mik-
toarm Star Copolymer Using (PolyBA)n-PolyDVB Star
Polymer as a Multifunctional MI. The synthesis and
postpurification procedures for the (polyBA)n-polyDVB-
(polySt)m miktoarm star copolymers were similar to those of
(polyBA)n-polyDVB star polymers. Assuming there was no
loss for Br functionality during the synthesis of the star MI,
the number of initiating sites per star molecule would be the
same as the number of arms per star. A typical reaction
mixture composition was (polyBA)n-polyDVB star MI (0.233
g, 21.8 µmol of alkyl bromide initiating sites), St (1 mL, 8.7
mmol), PMDETA (9.1 µL, 43.6 µmol), DMF (3 mL), CuBr (5.6
mg, 39.0 µmol), and CuBr2 (1.0 mg, 4.5 µmol). The reaction
was conducted at 110 °C. Samples for GC and GPC measure-
ments were periodically withdrawn, and the reaction was
stopped after 21.5 h at approximately 14% St conversion.
Synthesis of PolyMMA MI by ATRP. The synthetic
procedure for the preparation of polyMMA MI was similar to
that for the polyBA MI. MMA (20 mL, 0.187 mol) was
polymerized using EBiB (0.274 mL, 1.87 mmol) as the initiator
and CuBr (0.134 g, 0.93 mmol)/bpy (0.292 g, 1.87 mmol) as
the catalyst. The reaction was carried out at 50 °C in acetone
(12.0 mL), containing 0.8 mL of anisole as internal standard
for GC analysis. At timed intervals, samples were withdrawn

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5998 Gao et al.
Macromolecules, Vol. 38, No. 14, 2005
rated into the star polymers increased, which was
confirmed by the decreasing RI signal corresponding to
the unreacted linear polyBA MI. After a 20 h reaction
period nearly all the MI molecules were incorporated
into the star polymers, and the peak representing
unreacted linear polyBA chains became undetectable.
The GPC elution volume of the product, (polyBA)n-
polyDVB star polymers, gradually decreased due to the
increasing hydrodynamic volume and star formation.
Polymers with a complex structure, such as stars or
brush-like polymers, have a more compact structure
than the corresponding linear polymers of the same
molecular weight. Therefore, when a mass-sensitive
detector (e.g., RI detector) is used in GPC measure-
ments, the apparent molecular weight values deter-
mined with a calibration based on linear polymer
standards are smaller than the true molecular weight.39
A more advanced and accurate technique is to measure
the molecular weights of star polymers using light
scattering techniques. A MALLS detector was used in
conjunction with GPC for this purpose. The absolute
molecular weights of (polyBA)n-polyDVB star polymers
were calculated with the assumption that all of the star
polymers (Table 1, entries 2-8) had the same dn/dc
value (0.072 ( 0.003 mL/g), which was equal to the
measured dn/dc for the (polyBA)n-polyDVB star poly-
mer in Table 1, entry 5. Because of the similarity of
repeating unit structure, it is assumed that the dn/dc
of the polyDVB segment is equal to that of polySt
homopolymer (0.195 mL/g).38 However, the actual mea-
sured dn/dc value of the star polymer was closer to that
for polyBA homopolymers in THF (0.065 mL/g)38 be-
cause the weight fraction of polyBA arms in the star
polymer (e.g., 85 wt % for Table 1, entry 5) was
dominant. Therefore, the (polyBA)n-polyDVB star poly-
mers listed in Table 1 should have the dn/dc values close
to 0.072 ( 0.003 mL/g. The peak molecular weights (Mp)
of the star polymers were obtained when the [DVB]/
[polyBA-Br] ratio was 15/1. Although the Mp values
using either RI detector or MALLS detector increased
with DVB conversion, their ratio (Mp,MALLS/Mp,RI) also
changed gradually. As a general trend, this ratio
increases as the star polymer becomes more compact.
When the DVB conversion reached ca. 86%, after 20 h
reaction, the molecular weight of (polyBA)n-polyDVB
star polymers obtained from the MALLS detector was
more than 3 times larger than that determined from
the RI detector (Table 1, entry 5). The number-average
Figure 1. Dependence of DVB conversion and ln([M]0/[M])
on reaction time (A) and GPC traces (B) during the synthesis
of (polyBA)n-polyDVB star polymers. Experimental condi-
tions: [DVB]/15 ) [polyBA-Br] ) [CuBr] ) [PMDETA] ) 0.018
M in anisole at 110 °C. GPC conditions: RI detector, linear
polySt as standard.
Results and Discussion
Synthesis of (PolyBA)n-PolyDVB-(PolySt)m Mik-
toarm Star Copolymers by ATRP. Generally, three
steps are needed to synthesize (polyA)n-polyX-(polyB)m
miktoarm star copolymers using ATRP via the “in-out”
method. First, a highly functionalized mono-Br-termi-
nated linear polymer, polyA, is synthesized via ATRP.
Then, a divinyl compound (X) is employed to chain
extend and cross-link the polyA MI into (polyA)n-polyX
star polymer. Finally, polymer chains of a second
monomer, polyB, are grown from the alkyl halide
initiating sites located in the core of (polyA)n-polyX star
polymer to produce the desired (polyA)n-polyX-
(polyB)m miktoarm star copolymer. DVB was selected
as the cross-linker for the synthesis of nondegradable
miktoarm star copolymers with polyBA and polySt arms
(Scheme 1) because the chain extension from polyBA
to DVB is easily accomplished in ATRP and the chemi-
cal structure of DVB is similar to the subsequent
monomer, St. In other words, the initiation efficiency
in the two chain extension steps is expected to be
sufficiently high.37
value of the number of arms per star molecule (Narm
)
was calculated using the following equation:
Mp,star × armwt%
Narm
)
)
Mp,polyBA
Synthesis of (PolyBA)n-PolyDVB Star Poly-
mers. Linear polyBA chains (Mn ) 9000 g/mol, Mw/Mn
) 1.12) were synthesized via ATRP by employing EBrP
as initiator and CuBr/PMDETA as catalyst at 60 °C.
With this polyBA polymer as MI, a ratio of [DVB]/
[polyBA-Br]/[CuBr]/[PMDETA] ) 15/1/1/1 (Figure 1A)
was employed for the synthesis of the (polyBA)n-
polyDVB star polymer, and a semilogarithmic kinetic
plot was linear up to ca. 60% DVB conversion. A possible
reason for the curvature at higher DVB conversion was
that some alkyl bromide initiating sites inside the highly
cross-linked polyDVB core became less accessible and
less likely to be activated and react with DVB. The GPC
curves in Figure 1B indicate that as the conversion of
DVB increased, the amount of polyBA chains incorpo-
Mp,star
Mp,polyBA + MDVB × convDVB × [DVB]/[polyBA-Br]
(1)
where Mp,star and Mp,polyBA are the peak molecular
weights of (polyBA)n-polyDVB star and polyBA MI
from GPC-MALLS measurement; armwt% is the weight
fraction of polyBA arm in the star polymer, MDVB is the
molecular weight of DVB, and [DVB]/[polyBA-Br] is the
molar ratio of the added DVB to polyBA before polym-
erization. The conversion of DVB (convDVB) was deter-
mined using GC. The results listed in Table 1 (entries
1-5) show that the number-average arm number per
star increased with DVB conversion and reached 25