Living free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization): Approaches to star polymers

Article abstract of DOI:10.1021/ma021219w

The synthesis of multiarm star polymers by radical polymerization with reversible addition-fragmentation chain transfer (RAFT) is described. When the precursor RAFT agents to star polymers are trithiocarbonate derived, fragmentation of the radical intermediate can lead to different products depending on the leaving abilities of the two groups attached to sulfur. To demonstrate this, two types of RAFT agents, ones that allow growth of arms away from the core and the other attached to the core during propagation, were designed, and an example of each was synthesized. While both star RAFT agents gave excellent molecular weight control and low polydispersities (typically <1.2), the one that grows its arms away from the core offers polymers free from star-star coupled products. Thus, the direction of fragmentation should be considered when designing efficient star RAFT agents. The living nature of the arms of these polymers was demonstrated by extending the arms with a different monomer to afford a star block copolymer. The RAFT agents described are easily synthesized from commercially available reagents by a simple experimental procedure.

Full text of DOI:10.1021/ma021219w

Macromolecules 2003, 36, 1505-1513
1505
Living Free Radical Polymerization with Reversible
Addition-Fragmentation Chain Transfer (RAFT Polymerization):
Approaches to Star Polymers
Rosh a n T. A. Ma ya d u n n e,*,†,‡ J u stin e J effer y,† Gr a em e Moa d ,†,‡ a n d
Ezio Rizza r d o†,‡
CSIRO Molecular Science, Bag 10, Clayton South, Victoria 3169, Australia, and Cooperative Research
Center for Polymers, 32 Business Park Drive, Notting Hill, Victoria 3168, Australia
Received J uly 29, 2002; Revised Manuscript Received November 27, 2002
ABSTRACT: The synthesis of multiarm star polymers by radical polymerization with reversible addition-
fragmentation chain transfer (RAFT) is described. When the precursor RAFT agents to star polymers
are trithiocarbonate derived, fragmentation of the radical intermediate can lead to different products
depending on the leaving abilities of the two groups attached to sulfur. To demonstrate this, two types
of RAFT agents, ones that allow growth of arms away from the core and the other attached to the core
during propagation, were designed, and an example of each was synthesized. While both star RAFT agents
gave excellent molecular weight control and low polydispersities (typically <1.2), the one that grows its
arms away from the core offers polymers free from star-star coupled products. Thus, the direction of
fragmentation should be considered when designing efficient star RAFT agents. The living nature of the
arms of these polymers was demonstrated by extending the arms with a different monomer to afford a
star block copolymer. The RAFT agents described are easily synthesized from commercially available
reagents by a simple experimental procedure.
In tr od u ction
researchers taking on the challenge. As a consequence,
the use of anionic living polymerization for the prepara-
tion of star polymers dominates the current literature.
Regardless of the method of polymerization (ionic,
metathesis, group transfer, or radical), preparations of
star polymers described in the literature can be catego-
rized into two broad approaches, namely, (i) the arms
first and (ii) the core first approaches. In the “arms first”
approach the linear arms of the star polymer are
synthesized first followed by binding of the arms to form
the core. The binding of the arms is achieved by using
either a difunctional monomer (e.g., divinylbenzene) or
a multifunctional terminating agent (e.g., tetrachlorosi-
lane). The “core first” method involves the synthesis of
a multifunctional initiator (the core) followed by the
extension of arms by monomer addition.
The applicability of living/controlled radical polym-
erization techniques in the synthesis of star polymers
is well documented in the literature.11 Of those that use
the arms first approach, atom transfer radical polym-
erization (ATRP),12 nitroxide-mediated polymerization,13
and iniferter techniques14 with divinylbenzene as a
binder of the core are prominent. Difunctional mono-
mers have often been used to produce styrene micro-
gels,15 which are star polymers with unlimited arms and
have more than 40% mass concentrated in the core.
Methods to prepare microgels using reversible addi-
tion-fragmentation chain transfer (RAFT) polymeriza-
tion16 were reported recently.
Multiarm polymers including stars, microgels, and
dendritic polymers have attracted considerable atten-
tion in recent years.1 Much of this interest originates
from the discovery that these polymers have lower bulk
and solution viscosities compared to those of linear
analogues of the same molecular weight.2-5 The reason
for this decrease is attributed to the greater dependence
of the viscosity on the molecular weight of each arm as
compared to the total molecular weight of the star
polymer. The rheological behavior of well-defined star-
shaped polymers has been extensively studied in recent
years.6 Multiarm polymers are also capable of contain-
ing a higher degree of end group functionality compared
to linear polymers of similar molecular weight, and this
is useful in many specialized applications. Several
recent reviews have been written on the syntheses of
these types of polymers and their properties in dilute
solution.7,8
Although preparation of star polymers has been
documented as early as 1948 by Schaefgen and Flory,9
the preparation of well-defined multiarm polymers
remained a challenge until the advent of living polym-
erization techniques. This is because precise control of
the polymerization is crucial to the synthesis of such
multiarm polymers. With the discovery of living anionic
polymerization, Morton and co-workers in 195610 were
able take advantage of the method to synthesize well-
defined four-armed polystyrenes for the first time by
neutralizing living poly(styryllithium) with tetrachloro-
silane. Although the product was a mixture of four- and
three-arm stars, this work eventually led to many
The core first approach dominates the preparation of
star polymers by free radical processes. Matyjaszewski
and co-workers have explored the efficiency of a number
of multifunctional initiators in the polymerizations of
styrene, methyl acrylate, and methyl methacrylate by
atom transfer radical polymerization (ATRP).12 Star
polymers with up to eight arms and dendrimer type
multiarm polymers17-19 have been prepared using ATRP
recently. Pugh et al.,20 Sawamoto et al.,21,22 Gnanou et
† CSIRO Molecular Science.
‡ Cooperative Research Center for Polymers.
* To whom correspondence should be addressed: phone +613
9545 8138; Fax +613 9545 2446; e-mail roshan.mayadunne@
csiro.au.
10.1021/ma021219w CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/14/2003

1506 Mayadunne et al.
Macromolecules, Vol. 36, No. 5, 2003
al.,23 and Hawker et al.17 have also contributed much
to this area. Use of nitroxide-mediated living polymer-
ization in the construction of star polymers has been
recently reviewed by Hawker et al.24
mL of ethyl acetate. The organic phase was separated and the
aqueous layer extracted with ethyl acetate (2 × 20 mL). The
solution of combined organic phases was washed with brine
(20 mL), dried with MgSO4, and filtered, and the solvent was
evaporated under reduced pressure. The crude product (yellow
crystalline) was purified by flash chromatography on silica,
using 20% ethyl acetate in petroleum spirits as eluent, to afford
the title compound (0.47 g, 76%). 1H NMR: δ 2.8 (12H, SCH3),
4.6 (8H, benzyl CH2), 7.4 (2H, ArH).
Penta erythritoltetra kis(3-(S-benzyltrithioca rbonyl)propi-
onate) (2). Triethylamine (4.04 g, 40 mmol) in 10 mL of CHCl3
was added dropwise to a stirred solution of pentaerythritol (3-
mercaptopropionate) (2.44 g, 5 mmol) and carbon disulfide
(3.04 g, 40 mmol) in CHCl3 (15 mL) at room temperature. The
solution gradually turned deep yellow during the addition. The
solution was allowed to stirred for an additional 1 h. Benzyl
bromide (3.76 g, 22 mmol) dissolved in 10 mL of CHCl3 was
added dropwise, and the solution was stirred for a further 2
h. The mixture was poured into a cold solution of 10% aqueous
HCl and extracted three times with ethyl acetate to afford a
thick yellow oil. The oil was purified by column chromatog-
raphy using 30% ethyl acetate in petroleum spirits as eluent
to obtain the title compound (4.22 g, 73%). 1H NMR: δ 2.9
(8H, CH2), 3.6 (8H, CH2), 4.2 (8H, CH2), 4.6 (8H, benzyl CH2),
7.3 (20H, ArH).
In other related work, star polymers have been
synthesized by the use of multifunctional iniferters
(initiator transfer agent terminator). Kuriyama and
Otsu have described the preparation of four-arm methyl
methacrylate star polymers with the use of a tetrafunc-
tional iniferter.14 The technique relies on photolytic bond
homolysis of a weak carbon-sulfur bond. Upon initia-
tion, the carbon-centered radical propagates by addition
to monomer and is subsequently terminated by radical-
radical coupling with the original sulfur-centered radical
to afford dormant polymer. Use of mercaptans such as
pentaerythritol tetramecaptopropionate and other poly-
mercato esters as chain transfer agents is also docu-
mented.25
Limitations of current radical polymerization methods
in the preparation of multiarm polymers are numerous.
One of the most prominent drawbacks with all of the
“core first” radical polymerization techniques is star-
star coupling at relatively low monomer conversion.23
This would result in high molecular weight polymer
contaminants and may require strenuous methods of
purification of the end products. ATRP related meth-
ods,26 which rely on core first technique, can be limited
to particular types of monomers due to incompatibilities
between the initiating system (e.g., organic halide/
copper halide/2,2′-bipyridine) with monomer (e.g., (meth)-
acrylic acid). A comparison of available controlled
radical polymerization processes, highlighting the ad-
vantages and disadvantages in each, is the subject of a
recent review.27
Sodium Methyltrithiocarbonate. Carbon disulfide (9.1 g, 0.12
mol) in diethyl ether (100 mL) was added dropwise over 30
min to a suspension of sodium methanethiolate (7 g, 0.1 mol)
in diethyl ether (300 mL) at room temperature. The solution
was stirred for 2 h and the solvent removed. The residue was
extracted three times with ethyl acetate to afford the product
(13.4 g, 92%). The crude product was used without further
purification.
Styr en e P olym er iza tion s. A typical preparation (entries
5 and 6, Table 1) of star polystyrene using RAFT agents is as
follows: The RAFT agent (2) (42.19 mg, 36.6 × 10-6 mol) was
dissolved in 5.0 mL of freshly flash distilled styrene. Aliquots
of 2.0 mL of this solution were placed in glass ampules,
degassed three times with freeze, evacuate, thaw cycles, and
flame-sealed. The glass tubes were heated at 110 °C for 6 and
20 h and opened, and the contents were dissolved in THF. The
mixture was transferred to a round-bottom flask, and the
volatiles were removed to constant weight to afford star
polystyrene in 25% and 63% conversion, respectively. The
polymer samples were subsequently analyzed by GPC.
MA P olym er iza tion s. A typical preparation (entries 11-
13, Table 1) of star poly(methyl acrylate) is as follows: Two
stock solutions, (1) RAFT agent (2) (49.08 mg, 42.6 × 10-6 mol)
in 5.0 mL of methyl acrylate and (2) 3 mg of AIBN in 20.0 mL
of benzene, were prepared. Aliquots of 1.0 mL of stock solution
1 and 4.0 mL of stock solution 2 were placed in each glass
ampule, degassed three times with freeze, evacuate, thaw
cycles, and flame-sealed. The glass tubes were heated at 60
°C for 4, 8, and 16 h. The tubes were opened, the contents
were dissolved in THF and transferred to a round-bottom flask,
and the volatiles were removed to constant weight to afford
polymers with conversions 34%, 50%, and 79%, respectively.
The polymer samples were subsequently analyzed by GPC.
Sta r Block Cop olym er s. A typical preparation of star-
(PMA-block-PS)4 block copolymer is as follows. A sample four-
arm polystyrene (Mn 25 550, Mw/Mn 1.18) (107 mg, 4.19 × 10-6
mol) (see Table 3) prepared according to the above procedure
was dissolved in 2.0 mL of MA and labeled stock solution 1.
An aliquot of 1.0 mL of this solution was added to an ampule
containing 4.0 mL of stock solution 2, prepared by dissolving
AIBN (3 mg) in 20.0 mL of benzene. The mixture was degassed
three times with freeze, evacuate, thaw cycles and flame-
sealed. The glass tubes were heated at 60 °C for 4 and 8 h;
the volatiles were removed and dried to constant weight. The
polymer samples were analyzed by GPC, and the results are
shown in Table 3.
Recently, we demonstrated that thiocarbonyl thio
compounds could be used to confer living character to
radical polymerization.28-30 The thiocarbonyl thio com-
pounds reported include dithioesters,31 xanthates,32
dithiocarbamates,33,34 and trithiocarbonates.35 In this
paper we demonstrate how the process can effectively
be used to synthesize star polymers.
Exp er im en ta l Section
Gen er a l. Monomers were purified by passage through
activity I neutral alumina (to remove inhibitors) followed by
fractional distillation and then flash distilled immediately
prior to use. Gel permeation chromatography (GPC) was used
to establish the molecular weight and molecular weight
distribution (polydispersity) of the polymers. A Waters As-
sociates liquid charomatograph equipped with differential
refractometer and a set of six Ultrastyragel columns (106, 105,
104, 103, 500, and 100 Å)) was used. Tetrahydrofuran (1.0 mL
min-1) was used as eluent. The molecular weights are reported
as polystyrene equivalents. Samples for GPC analysis were
isolated by evaporation of monomer. No precipitation or
fractionation was performed prior to GPC analysis. Flash
chromatography was performed using Merck Kieselgel 60
(230-400 mesh ASTM). Glass backed Whatman MK6F pre-
coated silica microscopic slide plates with PF254 nm indicator
were used for thin-layer chromatography. The NMR spectra
were recorded on a Bruker AC 200 MHz spectrometer. Tet-
ramethylsilane (TMS) and residual CHCl3 were used as
internal standards for 1H and 31C NMR spectra, respectively.
Syn th esis of RAF T Agen ts. 2,4,5-Tris({[methylsulfanyl)-
carbonothioyl]sulfanyl}methyl)benzylmethyl Trithiocarbonate
(1). 1,2,4,5-Tetrakis(bromomethy)benzene (0.45 g, 1 mmol)
dissolved in 15 mL of THF was added over 30 min to a partially
soluble suspension of sodium methyl trithiocarbonate (see
below) (0.88 g, 6 mmol) in THF (7 mL). The solution was
allowed to stir for 12 h before adding 20 mL of water and 20
Clea va ge of Ar m s of Sta r P olym er s. A typical procedure
for entry 1 in Table 2 is as follows. A sample of star-PS (100
mg) was dissolved in 5 mL of THF and piperidine (∼100 µL)
added. The solution was stirred at room temperature for 10

Macromolecules, Vol. 36, No. 5, 2003
Star Polymers 1507
Ta ble 1. Molecu la r Weigh t/Con ver sion Da ta for Sta r P olym er s F or m ed by P olym er iza tion of Va r iou s Mon om er s in th e
P r esen ce of Com p ou n d s 1 a n d 2a
monomerb
(concn in benzene, °C)
dithio compd
initiatorc
convf
(%)
d
entry
(M × 103)
(M × 102)
time (h)
Mn
Mn(calcd)e
Mw/Mn
1
2
3
4
5
6
7
8
9
10
11
12
13
styrene (bulk, 110)
1 (7.3)
thermalg
6
22
30
48
6
20
30
48
4
24 300
54 800
57 000
70 700
25 600
63 900
77 800j
92 000j
161 000i
250 800i
37 200
53 700
73 000
30 800
89 000
103 500
117 000
31 700
78 700
102 800
119 700
1.09
1.08
1.07
1.08
1.18
1.08
1.07j
1.07j
1.64
1.94
1.10
1.08
1.14h
24
71
83
94
25
63
82
96
51
76
34
50
79
styrene (bulk, 110)
2 (7.3)
thermalg
MA (2.22 M, 60)
MA (2.22 M, 60)
1 (0.43)
2 (1.70)
AIBN (0.073)
AIBN (0.073)
16
4
8
38 900
57 700
89 400
16
a
Reaction mixtures were prepared to give the concentrations shown, degassed through three freeze-thaw-evacuate cycles, sealed
b
under vacuum, and heated in a constant-temperature bath for the stated time. Abbreviations: (monomers) MMA ) methyl methacrylate,
MA ) methyl acrylate. c Abbreviations: AIBN ) 2,2′-azobis(2-cyanopropane); VAZO-88 ) 1,1′-azobis(1-cyclohexanecarbonitrile). Molecular
d
weight data were obtained by gel permeation chromatography (GPC) with 106, 105, 104, 103, 500, and 100 Å Waters Ultrastyragel columns
connected in series. Tetrahydrofuran (1.0 mL/min) was used as eluent. Samples for GPC were isolated by evaporation of monomer and
solvent (no fractionation or precipitation was performed). System calibrated with narrow polydispersity polystyrene standards, and
molecular weights are reported as polystyrene equivalents. e The following formula was used to calculate the theoretical molecular weight:
Mn(calc) ) ([monomer]/[CTA]) × conversion × MW of monomer. The above expression does not include the small number of chains formed
from the initiator. It also requires complete consumption of the chain transfer agent. f Conversions were determined gravimetrically
following evaporation of monomer and solvent. Conversions were those obtained for the times indicated and are not a limiting conversion.
g
h
i
j
No added initiator. Includes low molecular weight tail. Trimodal distributions. The values shown are excluding the low molecular
weight hump.
Ta ble 2. Molecu la r Weigh t Da ta for Lin ea r P olym er s F or m ed by Clea vin g th e Ar m s of Sta r P olym er s Usin g P ip er id in e a t
Room Tem p er a tu r e
entry
polymer
Mn
Mn(calcd)
Mw/Mn
conv (%)
1
2
3
4
star P(Sty) 8
linear P(Sty) 10
star P(MA) 9
63 900
18 750
101 200
30 500
78700
1.08
1.19
1.19
1.37
63
4 × 18750 + 660 ) 75660
quantitative recovery
29
quantitative recovery
130000
linear P(MA) 11
4 × 30500 + 660 ) 122660
Ta ble 3. Molecu la r Weigh t/Con ver sion Da ta for sta r -(P MA-block-P S)4 P r ep a r ed Usin g 9
entry
monomer (concn in benzene, °C)
dithio compd (M × 103)
initiator (M × 102)
time (h)
Mn
Mw/Mn
conv (%)
1
2
3
first block styrene (bulk, 110)
second block MA (2.22 M, 60)
2 (7.3)
thermal
6
4
8
25 550
87 700
129 600
1.18
1.24
1.23
25
26
46
first block (0.42)
AIBN (0.073)
Sch em e 1
h, and the volatiles were removed under reduced pressure. The
crude residue after drying to constant weight was analyzed
by GPC.
Resu lts a n d Discu ssion
Syn th eses of RAF T Agen ts. The challenge in the
preparation of star polymers by radical polymerization
with RAFT rests in part on the successful design and
synthesis of the multiarmed RAFT agents. Some of the
important factors to consider are steric congestion at
the reactive sites (the thiocarbonyl group), the ways in
which fragmentation could occur, the various types of
impurity star and/or linear polymer that can result
based on the envisaged mechanism, and the ease of
synthesis of the chain transfer agents.
Taking these factors into consideration, we have
synthesized the RAFT agents 1 and 2, both having the
trithiocarbonate moiety as the active function and a
benzyl moiety as the leaving group. The methods used
for the syntheses of these chain transfer agents are
simple and use inexpensive commercial reagents, and
the reactions are high yielding. Scheme 1 illustrates the
one-pot procedure used to synthesize compound 1 in
76% yield. It is important, in carrying out this reaction,
to add the tetrabromobenzyl compound (3) to a suspen-
sion of the sodium salt of the methyl trithiocarbonate
over the period specified in the procedure, as it would
ensure all four arms being functionalized. The sodium
salt of the methyl trithiocarbonate can be either pre-
pared in situ or presynthesized as it is stable at room
temperature for extended periods of time.
Similarly, the one-pot preparation of compound 2
involves the addition of carbon disulfide to a solution
of the commercially available tetrathiol 4 and triethy-
lamine (Scheme 2). Alkylation of the resulting am-

1508 Mayadunne et al.
Sch em e 2
Macromolecules, Vol. 36, No. 5, 2003
Sch em e 4a
a
MA and MB are different monomers, and An and Bm are
Sch em e 3. RAF T P r ocessa
their respective polymers.
acteristics as they can accommodate either one or two
leaving groups attached to the sulfur atoms (Scheme
4). Depending on the leaving ability of these groups,
polymers having a trithiocarbonate moiety incorporated
in the middle of the polymer or at the terminus can be
synthesized. The ability to prepare ABA triblock copoly-
mers in two steps from those that have two good leaving
groups is one of the advantages of trithiocarbonates
reported recently.35 When used in the preparation of
star polymers, the ability to control the way in which
the RAFT agent can fragment can be used to advantage.
This allows the preparation of star polymers of con-
trolled molecular weight and narrow polydispersity, free
from star-star coupled impurities.
P olym er iza tion s. As briefly described above (see
Scheme 4), control in the direction of fragmentation can
be used to advantage in the syntheses of star polymers
with trithiocarbonates. Because each arm is tied at the
core of the molecule (compounds 1 and 2), the way in
which fragmentation of the intermediate radical occurs
is important as one would result in a propagating
radical attached and the other detached from the core
of the molecule. The direction of fragmentation of the
intermediate following radical attack at the thiocarbonyl
group is controlled by the leaving ability of the groups
attached to the two sulfur atoms of the trithiocarbonate
moiety. The leaving ability of a group is dependent on
the combination of a number of factors such as thermo-
dynamic (bond strength of the forming and the breaking
bond), steric (presence of bulky substituents), electronic
effects of the substituents directly attached to the
leaving carbon center (electron withdrawing and releas-
ing abilities of groups), and the stability of the radical
formed. From experience38,39 it is known that methyl
radicals, being primary, are poor leaving groups relative
to secondary benzyl radicals. The latter radicals also
have the effect of being stabilized by the aromatic ring
through electron delocalization; however, they are suf-
ficiently reactive to undergo addition reactions which
makes them suitable as initiators in radical polymeriza-
tions. Having detailed the mixture of complex factors
that influence fragmentation, in RAFT polymerization,
it is the relative stability/leaving ability of the adding
vs the fragmenting (leaving) radicals that is impor-
tant.32,36,37,40 As described earlier, rapid consumption of
the RAFT agent is crucial to the success of the reaction.
The first type of the proposed RAFT agents, e.g. 1
where the leaving group following fragmentation is
bound to the central core of the polymer, is shown in
Scheme 5. The resulting intermediate radical 5, formed
by addition at the sulfur of the thiocarbonyl group,
a
M is monomer; R is a radical that can initiate polymeri-
zation; Pn, Pm, and Px are polymer chains.
monium salt with benzyl bromide affords the desired
tetraarm RAFT agent (2) in good yield.
Mech a n ism of RAF T P olym er iza tion . Radical
polymerization with reversible addition-fragmentation
chain transfer (RAFT) is a living process reported
recently.28-35 The mechanism by which the process
imparts living character to the polymerization is shown
in Scheme 3 and is carried out by simply introducing a
suitable thiocarbonylthio compound to an otherwise
conventional radical polymerization. During the first
stages of the polymerization the RAFT agent (ZCdSSR)
is consumed by propagating radicals by an addition-
fragmentation mechanism. The fragmented radical (R•)
reinitiates polymerization, resulting in new propagating
radicals which then take part in the equilibrium estab-
lished between the dormant polymer and active chains.
The equilibration process allows all chains produced to
grow in a uniform manner, resulting in narrow poly-
dispersity polymers. The nature of the Z and R groups
is crucial to the success of the polymerization. The
results of an investigation on their effects will be
published shortly.36,37
Various types of effective RAFT agents have been
reported to date. These include dithioesters, dithiocar-
bamates, and trithiocarbonates.28-35 Of these, trithio-
carbonates were shown to have useful structural char-

Macromolecules, Vol. 36, No. 5, 2003
Star Polymers 1509
Sch em e 5a
a
Pn, Pm, and Pq are polymer chains.
Sch em e 6
fragments at the benzyl-sulfur bond since methyl
radicals are relatively poor radical leaving groups, as
described earlier. Upon fragmentation the benzylic
radical 6 adds to monomer, resulting in a propagating
arm attached to the core. Propagation ceases either
upon exchange of a trithiocarbonate moiety from living
linear dormant chains, as illustrated in Scheme 5 (or
from a dormant chain attached to the core, in which case
resulting in another propagating arm), or by radical-
radical termination. While the former process allows
equal chances for arms to grow to similar length, the
latter process is minimized by design by having a low
concentration of radicals in solution, as required by the
concept of “living” radical polymerization. Thus, it is
clear that at any one moment in time each polymer arm
of such a transfer agent may be “active” (propagating)
or “dormant” (capped with the trithiocarbonate moiety).
On the other hand, the second type of multiarm RAFT
agent shown in Scheme 6, e.g. 2, following fragmenta-
tion results in a benzyl radical detached from the core.
Again, fragmentation of the CH2-S bond is highly
unfavorable compared to the S-benzyl bond. Thus,
unlike with the previous case, the benzyl radicals that
reinitiate polymerization do so away from the star
polymer and result in linear propagating chains. With
the linear propagating chains in equilibrium with the

1510 Mayadunne et al.
Sch em e 7
Macromolecules, Vol. 36, No. 5, 2003
arms of the star polymer, the arms of the polymer grow
in a controlled manner. What is important to note with
these types of RAFT agents is that the arms of the star
polymer at all times are dormant, and growth of arms
always occurs away from the core as linear propagating
species.
F igu r e 1. GPC chromatograms for the polymerization of
styrene in the presence of RAFT agent 1: (a) Mn ) 24 300,
Mw/Mn ) 1.09, 6 h (Table 1, entry 1); (b) Mn ) 54 800, Mw/Mn
) 1.08, 22 h (Table 1, entry 2); (c) Mn ) 70 700, Mw/Mn ) 1.07,
48 h (Table 1, entry 4).
Mechanistically, the number of possible side reactions
that can produce impurity polymers from growing arms
attached to the core is greater than the alternative
method, i.e., growing arms away from the core. Since
termination reactions cannot be totally eliminated in
radical polymerization, most of these impurities are as
a result of such reactions, despite careful selection of
reaction conditions. With star RAFT agents similar to
1, two types of active propagating species, ones that are
attached to the core and linear chains resulting from
continuous initiation, can exist at a given moment in
time. Radical-radical termination resulting from these
species can thus produce star-star coupled polymers,
star polymers with dead arms, and linear dead polymers
(see Scheme 7). In addition, linear polymer capped with
the trithiocarbonate moiety is also a byproduct (see
Scheme 5). On the other hand, with RAFT agent 2 the
dead polymer produced as a result of termination is
limited to only linear polymer. Neither star-star coupled
products nor linear species capped with the active
functionality are possible with such RAFT agents.
Typical GPC chromatograms for the polymerization
of styrene using RAFT agent 1 are shown in Figure 1.
Initial formation of relatively broader polydispersity star
polymer (Figure 1, trace a) and the subsequent narrow-
ing of the polymer arms as RAFT equilibration takes
effect are clearly noticeable from the GPC chromato-
grams and from the results in Table 1. The polydisper-
sity, despite the multimodal distributions, remains low
at 1.07, including the leading hump, and 1.21 with both
impurities taken into account at near quantitative
conversion (48 h). The leading hump (on the high
molecular weight end) in the 48 h trace (Figure 1) is
approximately double the peak molecular weight (Mp)
of the main peak and is attributed to star-star coupled
products as discussed before. The possibility of coupling
more than two cores with one another does exist;
however, statistically this has a very low probability
and, if any occurs, may not be detected by GPC. The
tailing hump in the low molecular weight end of the
chromatogram is believed to be due to linear dormant
chains and polymer from radical-radical termination
of linear propagating radicals.
F igu r e 2. GPC chromatogram for the polymerization of
styrene in the presence of RAFT agent 2: (a) Mn ) 25 600,
Mw/Mn ) 1.18, 6 h (Table 1, entry 5); (b) Mn ) 63 900, Mw/Mn
) 1.08, 20 h (Table 1, entry 6); (c) Mn ) 92 000, Mw/Mn ) 1.07,
48 h (Table 1, entry 8). The Mn and Mw/Mn values stated are
measured excluding the low molecular weight peak.
much cleaner product polymers as expected (Figure 2).
J udging from the peak shapes, formation of star-star
coupled byproducts is not evident. The only impurity
that exists in the crude mixture is the low molecular
weight species, formed by the continuous radical-
radical termination of linear propagating radicals dur-
ing the polymerization.
Free radical polymerizations of methyl acrylate with
RAFT agents often yield good control and afford poly-
mers with low polydispersity that are free from impuri-
ties at moderate conversions. However, if the polymer-
izations are allowed to proceed to higher conversions
(over 60% approximately), the resulting polymer often
becomes contaminated with a broadly dispersed byprod-
uct, approximately twice the molecular weight of the
main narrow peak. It becomes pronounced at low
concentrations of RAFT agent with the polymerization
carried to high conversion. Increasing the concentration
of the RAFT agent has the effect of total elimination of
The GPC chromatograms resulting from polymeriza-
tion of styrene in the presence of RAFT agent 2 show

Macromolecules, Vol. 36, No. 5, 2003
Star Polymers 1511
F igu r e 4. Evolution of molecular weight with conversion for
the polymerization of styrene in the presence of RAFT agent
2 (Table 1, entries 5-8).
F igu r e 3. GPC chromatogram for the polymerization of
methyl acrylate in the presence of RAFT agent 2: (a) Mn
)
Sch em e 8
37 200, Mw/Mn ) 1.10, 4 h (Table 1, entry 11); (b) Mn ) 53 700,
Mw/Mn ) 1.08, 8 h (Table 1, entry 12); (c) Mn ) 73 000, Mw/Mn
) 1.14, 16 h (Table 1, entry 13).
this impurity at the expense of lowering molecular
weight of the resulting polymer. The mechanism of this
side reaction is yet to be defined; however, a fraction of
the byproduct is believed to result from radical-radical
termination of propagating species. In some cases it has
been established that the hump contains the dithioester
chromophore (unpublished results), indicating the pres-
ence of dormant chains among the dead chains.
The most notable observation from Figure 4 is the
marked deviation of the GPC molecular weight from the
theoretical molecular weight. The latter is calculated
based on the formula Mn(calc) ) {{[monomer]/[RAFT
agent]} × MW(monomer) × fractional conversion} +
MW(RAFT agent). From Figure 4, the difference be-
tween the two values is observed to widen with the
increase in the molecular weight of the polymer. In our
earlier publications,28-35 RAFT polymerization with
monofunctionalized RAFT agents has been demon-
strated to be “living”. RAFT polymerizations with mul-
tifunctional RAFT agents 1 and 2 also satisfy the
conditions of a living polymerization. They offer narrow
polydispersity polymers, have a linear molecular weight-
conversion profile, and have the ability to form star
block copolymers with sequential addition of monomer
(see below). The exception is that the difference between
the predicted and the measured molecular weight
increases with the molecular weight of the polymer. The
difference in physical characteristics of star-shaped
polymers (such as lower hydrodynamic volumes of the
star polymers compared to linear polymer) and the
linear polystyrene standards used to evaluate their
molecular weights is the most likely explanation for this
discrepancy.
One way to verify the absolute molecular weight of
the star polymers is to cleave the arms and characterize
the resulting linear polymer against standards. As the
trithiocarbonate link can be cleaved with strong nucleo-
philes, two samples of star polymers, one a polystyrene
8 of Mn 63 900, Mn(calc) 78 700 (Mw/Mn 1.08) and the
other a poly(methyl acrylate) 9 of Mn 101 200, Mn(calc)
130 000 (Mw/Mn 1.19), in separate experiments were
treated with piperidine (Scheme 8). The GPC molecular
weight of linear polystyrene 10 isolated from the first
of these experiments was 18 750 with Mw/Mn 1.19
With the star RAFT agent 2 we have observed the
byproduct to be on the low molecular weight side of the
chromatogram and approximately half the molecular
weight of the star polymer (or twice the molecular
weight of each arm of the star polymer). Since with
RAFT agent 2 the growth of arms always occurs as
linear species, radical-radical combination would result
in dead polymer with half the molecular weight of the
4-arm star polymer (or twice the molecular weight of
each arm). The low molecular weight impurity polymer
is minimized by increasing the concentration of the
RAFT agent while keeping the monomer and initiator
concentrations constant. The GPC chromatograms of
products obtained are shown in Figure 3, and the data
are listed in Table 1 (entries 11-13). Polymerization of
MA in the presence of RAFT agent 1 and taken to over
50% conversion affords product polymers having trimo-
dal distributions due to star-star coupling (Table 1,
entries 9 and 10).
A living polymerization is characterized by narrow
polydispersity products, linear increase in molecular
weight with conversion, predictability of molecular
weight based on the ratio of monomer consumed to the
amount of RAFT agent used, and the ability to extend
the chains by sequential addition of monomer. First, the
star polystyrene and poly(methyl acrylate) prepared
using star RAFT agents have low polydispersity which
narrows with conversion of monomer (Table 1). The
GPC chromatograms of some of the results are shown
in Figures 1-3. Second, a linear increase in molecular
weight with conversion was observed for each of the
multiarm RAFT agents 1 and 2 during polymerization
of both styrene and methyl acrylate. A typical plot for
the polymerization of styrene in the presence of star
RAFT agent 2 is shown in Figure 4.

1512 Mayadunne et al.
Macromolecules, Vol. 36, No. 5, 2003
F igu r e 5. GPC chromatograms of (a) the star polystyrene 8
F igu r e 7. GPC chromatograms of (a) the star-(PS) 9 prepared
using 2, (b) star-(PMA-block-PS)4 prepared using 9 heated with
AIBN and MA for 4 h (Table 3), and (c) star-(PMA-block-PS)4
prepared using 9 heated with AIBN and MA for 8 h (Table 3).
prepared using 2: Mn ) 63 900, Mn(calc) ) 78 700, Mw/Mn
)
1.08 (Table 2) and (b) after treatment of polystyrene 8 with
piperidine: Mn ) 18 750 (4 × Mn + Mw of core ) 4 × 18750 +
660 ) 75660 ∼ Mn(calc) of a), Mw/Mn ) 1.19 (Table 2).
In fact, the small difference in the molecular weight
distributions of the linear polymer arms, following
cleavage, compared to the star polymer can be at-
tributed to the averaging effect of small differences in
molecular weights of the arms. This was predicted in
1948 by Flory et al. and suggested the approximate
relationship Mw/Mn (of the star polymer) ) 1 + 1/f,
where f is the number of arms of the star polymer.9
Having established that all arms of the star polymer
have grown to similar molecular weight, we set out to
demonstrate that these arms can be extended by
sequential addition of monomer. A star block copolymer
consisting of an outer polystyrene segment and a inner
poly(methyl acrylate) core using the star RAFT agent
2 was synthesized for this purpose (Table 3, Figure 7).
Because, according to the mechanism, monomer inser-
tion occurs from the core in RAFT agents of the kind 2,
the outer block, polystyrene was synthesized first.
Subsequently, the arms of star polystyrene of Mn 25 550
and Mw/Mn 1.18 were extended with the insertion of
methyl acrylate from the core to afford star-(PMA-block-
PS)4 of Mn 87 700 and polydispersity 1.24 in 4 h. If the
polymerization is continued to 8 h, a star block copoly-
mer of Mn 129 600 and polydispersity 1.23 is obtained.
F igu r e 6. GPC chromatograms of (a) the star poly(methyl
acrylate) 9 prepared using 2: Mn ) 101 200, Mn(calc) )
130 000, Mw/Mn ) 1.19 (Table 2) and (b) after treatment of
poly(methyl acrylate) 9 with piperidine: Mn ) 30 500 (4 × Mn
+ Mw of core ) 4 × 30500 + 660 ) 122660 ∼ Mn(calc) of a),
Mw/Mn ) 1.37 (Table 2).
Con clu sion
(Figure 5, Table 2). The linear poly(methyl acrylate) 11
in the second experiment was of Mn 30 500 and Mw/Mn
1.37 (Figure 6, Table 2).
We have demonstrated that multiarm star polymers
of defined molecular weights can be synthesized using
radical polymerization with reversible addition-frag-
mentation chain transfer. The two ways in which such
polymers can be prepared involve the growth of arms
attached to or detached from the core. While good
molecular weight control is achieved using both types
of RAFT agents, the latter type was shown to afford star
polymers uncontaminated with high molecular weight
byproducts. The absolute molecular weights of the star
polymers were evaluated by cleaving the arms and
analyzing products against linear standards. Narrow
polydispersities following scission of arms also demon-
strates all arms of the polymer have grown to similar
chain length. The living nature of the arms of the star
polymers was demonstrated by extending the arms of
a star polymer with sequential addition of monomer
with excellent molecular weight control. The RAFT
The conclusions that can be reached from the above
experiments are twofold. First, in each case the molec-
ular weight of the linear product polymer following
treatment with piperidine multiplied by the number of
arms of the star polymer, four in the above case, is in
good agreement with the predicted molecular weight
(Mn(calc)) of the corresponding star polymer, calculated
on the basis of the consumption of monomer. This
explains the observed discrepancy between the observed
GPC molecular weight and Mn(calc) of star polymers
shown in Figure 4. Second, while one would expect the
polydispersity of a star polymer to be narrower com-
pared to a linear molecule of similar Mn (see below), the
narrow polydispersity before and after cleavage in each
experiment demonstrates that all arms of the star
polymer have grown to similar molecular weight.

Macromolecules, Vol. 36, No. 5, 2003
Star Polymers 1513
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