Atom transfer radical polymerizations of styrene and butadiene as ...

J Polym Res (2011) 18:41–48
DOI 10.1007/s10965-010-9389-1
REVIEW PAPER
Atom transfer radical polymerizations of styrene
and butadiene as well as their copolymerization initiated
by benzyl chloride / 1-octanol-substituted MoCl 5 / PPh 3
Jing Hua & Haibing Xu & Jieting Geng & Zhifeng Deng &
Ling Xu & Yong-liang Yu
Received: 10 November 2008 /Accepted: 4 January 2010 /Published online: 29 January 2010
# Springer Science+Business Media B.V. 2010
Abstract A novel initiator system, benzyl chloride / 1-
octanol-substituted MoCl 5 / triphenyl phosphine (PPh 3 ),
was applied to the atom transfer radical polymerizations
(ATRP) of both butadiene and styrene as well as their
copolymerization. Characterization revealed a linear increase
in the number average molecular weight with
monomer conversion and rather wide molecular weight
distributions of the polymerization products. Increasing the
polymerization temperature promoted the reaction rate and
narrowed the polydispersity index of polystyrene proportionally.
The polymerization rule for butadiene catalyzed by
the above Mo-based system catalyst is similar to that of
styrene. The microstructure of the butadiene was investigated
by IR and 1 H NMR. IR,
13 C NMR and DSC
measurements showed that the butadiene and styrene
copolymer was a random copolymer. The chlorine atom at
the ω end group of the polymer and the change in the
valence state of molybdenum, as explored by UV-Vis
spectroscopy, revealed that the polymerization proceeded
in a manner closest to the mechanism of ATRP.
Keywords Atom transfer radical polymerization . Styrene .
Butadiene . Molybdenum base catalyst . Copolymerization
Introduction
Obtaining well-defined macromolecular architectures using
controlled polymerization techniques has been one of the
J. Hua (*) : H. Xu : J. Geng : Z. Deng : L. Xu : Y.-l. Yu
Key Laboratory of Rubber-Plastics, Ministry of Education,
Qingdao University of Science and Technology,
Zhengzhou No. 53,
Qingdao 266042, China
e-mail: Huajing72@qust.edu.cn
most important goals of both academic and industrial
laboratories. ATRP is a robust and versatile technique that
accurately controls the chain length and polydispersity
index (PDI=M w /M n ) of the polymer, and can also be used
to synthesize well-defined copolymers. The reactive system
of ATRP, a multicomponent system, consists mainly of the
monomer, the initiator, the catalyst, and some additives.
ATRP has recently received a great deal of attention from
researchers because of its easy experimental setup and its
utilization of readily accessible and inexpensive catalysts
and simple, commercially available or easily prepared
initiators [1–3].
A number of low oxidation state metal complexes [3],
such as ruthenium(II), copper(I), iron(II) and nickel(II), are
known to be effective ATRP catalysts. Among them, the
most commonly utilized catalyst systems are Cu(I)-based
systems, which were first developed by Matyjaszewski [4].
On the other hand, there are only a few reports on Mo(V)-
based systems. A series of lithium molybdate(V) complexes
of general formula [LiMo(NAr) 2 (C–N)R], where (C–N)=
C 6 H 4 (CH 2 NMe 2 )-2, and R=(C–N), Me, CH 2 SiMe 3 , or
p-tolyl, have been used in the ATRP of styrene with benzyl
chloride as the initiator [5]. However, the products had
relatively high polydispersities (M w /M n =1.5–1.7) and the
efficiency of the benzyl chloride initiator was rather poor
(6–18%). In recent contributions, a number of Mo(III)/Mo
(IV) complexes, including cyclopentadienyl complexes
[6–8] and phosphine-containing chloride complexes [9],
have been reportedly used for the controlled radical polymerization
of styrene, methyl acrylate and butyl acrylate.
To date, ATRP has been applied to a wide variety of
monomers, including styrenics, (meth)acrylates and many
functional monomers [10]. Recently, Ayusman Sen [11–14]
and others [15, 16] demonstrated that acrylates can be
radically copolymerized with 1-alkenes by ATRP. However,

42 J. Hua et al.
the authors found only one work [17] in the literature
referring to the ATRP of butadiene using MoO 2 Cl 2 /
triphenyl phosphine as the catalyst and organic halide
compounds such as methyl 2-chloropropionate, CCl 4 , 1,4-
dichloromethyl benzene, 1-phenylethyl chloride and benzyl
chloride as initiators.
Several kinds of polystyrene copolymer have been
synthesized by ATRP. Syntheses of forced gradients of
styrene and acrylonitrile as well as spontaneous gradients of
styrene and n-butyl acrylate by ATRP have been reported
[18]. Block copolymers were synthesized by ATRP with a
yield of 30% and low polydispersity from the polystyrene
with Br end-group macroinitiators and the azo monomer,
catalyzed by copper(II) chloride [19]. The preparation of
polystyrene-grafted magnesium hydroxide nanoparticles via
surface-initiated atom transfer radical polymerization in a
toluene system with 2,2′-bipyridine and Cu(I)Br catalysts
has also been reported [20].
In this work, 1-octanol-substituted MoCl 5 [i.e., MoCl 3
(OC 8 H 17 ) 2 ] and triphenyl phosphine (PPh 3 ) were used as
catalyst and ligand, respectively, in the ATRPs of styrene
and butadiene and in their copolymerization. This is an
easily accessible catalyst system with good solubility in the
organic solvent. We are aware that there are already many
methods for synthesizing polybutadiene (PB) and the
copolymer of butadiene (Bd) and styrene (St), such as
radical polymerization and anionic polymerization, but the
development of other new and effective techniques for
synthesizing these polymers is still very important. Therefore,
the primary aim of this study was to investigate
whether the ATRPs of butadiene and its styrene copolymer
can be realized using the system mentioned above. The
effects of the reaction parameters on the monomer yields,
molecular weights and polydispersity indices of the
polymerized butadiene, styrene and their copolymer are
investigated, so that the structural unit compositions and
yields of the copolymers can be effectively controlled. It is
expected that polybutadiene and the copolymer of St and
Bd will have suitable molecular weights and copolymer
compositions.
Experimental
Materials
Butadiene was distilled using a 4 Å molecular sieve.
Styrene (AR, Shanghai Chemical Reagent Co.) was
distilled under reduced pressure from calcium hydride
powder before use. Toluene (AR, Shanghai Chemical
Reagent Co.) was purified by refluxing over sodium metal
under a nitrogen atmosphere. Triphenyl phosphine (AR,
Shanghai Chemical Reagent Co.) was recrystallized from
ethanol. Benzyl chloride (AR, Beijing Chemical Industrial
Co.) was purified by distillation under reduced pressure.
1-Octanol-substituted MoCl 5 , MoCl 3 (OC 8 H 17 ) 2 , was
prepared by reacting MoCl 5 (AR, Beijing Chemical
Industrial Co.) with 1-octanol (AR, Shanghai Chemical
Reagent Co.) at 30°C under nitrogen. The molar ratio of
MoCl 5 to 1-octanol was 1:4; the hydrochloric acid
produced by the reaction was removed under vacuum, and
the number of chlorines replaced by 1-octanol in MoCl 5 was
detected by titration.
General procedure for polymerization
The flask was sealed with a rubber septum and cycled
between vacuum and nitrogen at 120°C for 2 h using a
high-purity nitrogen gas (99.99%). After that, the solution
containing the monomer, initiator, ligand and solvent was
degassed by purging with nitrogen for 15 min before it was
injected into the reaction flask using a syringe. The reaction
flask was then placed in a preheated oil bath at a desired
temperature. At a given time, a given volume of the
reaction solution was removed by syringe. The polymer
was precipitated into a large amount of ethanol. The dried
product was then characterized by gravimetry and GPC.
Measurements and analysis
Molecular weights and their distributions were measured
using GPC with a Shimadzu system consisting of a set of
KF-1, KF-2, KF-3, KF-4, and KF-6 microstyragel columns
in tetrahydrofuran with polystyrene standard calibration.
Theoretical molecular weight was calculated using the
formula: M th =([monomer]/[initiator])×molecular weight
of polymer repeating unit×monomer conversion.
FT-IR spectra were recorded on a Nicolet FT-IR Magna
750 spectrophotometer using KBr pellets. The content of
each type of polybutadiene structural unit was calculated by
the following equations [21]:
Content of cis 1; 4PB¼ 17667D 738 =A;
Content of trans 1; 4PB¼ 4741:4D 967 =A;
Content of 1; 2PB ¼ 3673:8D 911 =A;
where A=17667D 738 +4741.4D 967 +3673.8D 911 , D=logI 0 /I,
I 0 is the intensity of the incident light and I is the intensity
of the transmitted light at that wavelength.
13 C NMR spectra were measured in CDCl 3 with a
Bruker MSL-300 NMR spectrometer using 13 C as probe
and deuterated acetone as reference standard.
1 H NMR
spectra were measured in CDCl 3 using tetramethylsilane as
internal reference. The UV-vis spectra were obtained with a
Tu-1800PC spectrophotometer. Thermal properties were
measured using a Netzsch 204 DSC (differential scanning

Atom transfer radical polymerizations of styrene and butadiene as well as their copolymerization 43
the rate of reversible oxidation and reduction of MoCl 3
(OC 8 H 17 ) 2 and the rate constant of polymerization with
polymerization temperature.
Figure 3 shows the 1 H NMR spectrum of the PSt
obtained with this catalyst system. The signal at δ 4.56 ppm
arises from the terminal proton adjacent to the chlorine
atom in the ω-end group, i.e., –CH 2 C–(C 6 H 5 )H–Cl. This
further verified that the mechanism of the synthesis of the
polystyrene had ATRP character. A chloride atom was
transferred between the polymer radical and the dormant
species.
Butadiene polymerization
Fig. 1 Plot of Ln([M] 0 /[M]) vs polymerization time for the ATRP of
styrene in toluene. [C 6 H 5·CH 2 Cl] 0 :[MoCl 3 (OC 8 H 17 ) 2 ]:[P(Ph) 3 ]:[St] 0 =
1:1:3:135; [St] 0 =1. 4 M; T=90°C
calorimeter) under a nitrogen atmosphere. The heating rate
was 10°C/min.
Results and discussion
Styrene polymerization
A novel initiator system, C 6 H 5·CH 2 Cl /MoCl 3 (OC 8 H 17 ) 2 /
PPh 3 , was used for the ATRP of styrene. Figure 1 presents
the kinetics of the polymerization of St in toluene at 90°C.
As shown, ln([M 0 ]/[M]) increased continuously with time,
and a semilogarithmic plot of the kinetics could be
approximated by a line, thus indicating that there was an
approximately constant concentration of the growing
radical. Coordination between the benzene ring and the
Mo catalyst may be one of the reasons for the induction
period of styrene ATRP.
Figure 2 shows a linear dependence of M n on conversion
and a decrease in M w /M n with conversion. The results
shown in Figs. 1 and 2 together demonstrate a “living”
radical polymerization process. The rather wide polydispersities
(M w /M n ≈1.9) observed may be attributed to many
factors, such as the different chemical structures of benzyl
chloride and styrene and the low rate of reversible oxidation
and reduction of MoCl 3 (OC 8 H 17 ) 2 .
Table 1 shows the conversion, molecular weight, and
polydispersity index of the polymerized styrene as a
function of reaction temperature and reaction time.
The polymerization rate increased and the polydispersity
index of the product narrowed with increasing polymerization
temperature. This can be attributed to the increases in
Figure 4 presents plots of the kinetics of butadiene polymerization
when using C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 as
initiator. The curve of ln([M 0 ]/[M]) versus time was almost
linear, suggesting that it is first order in the rate. Figure 5
shows a linear dependence of M n on conversion, while the
polydispersity index (M w /M n ≈1.7) was rather wide and
remained almost constant during the polymerization.
The reaction time had an important influence on the
monomer conversion and the molecular weights. Both
factors increased with reaction time. As can be seen, the
measured molecular weights were much greater than
those of the theoretical value M th at a low monomer
conversion. This fact is due to the period needed to reach
the equilibrium between the active propagation species
and the dormant species and thus the high concentration of
the active propagation species at the beginning of
polymerization. The efficiency of the benzyl chloride
initiator was about 20%, a little higher than that of a
lithium molybdate complex catalytic system [4]. The
polydispersity index of the polymer was about 1.7, which
MnX10 -3
14
12
10
8
6
4
2
0
1.5
0 20 40 60 80 100
Conversion (%)
Fig. 2 Dependences of M n,GPC and M w /M n on the conversion during
the polymerization of styrene. The conditions were the same as in
Fig. 1. Squares, M n ; circles, M w /M n
2.0
1.9
1.8
1.7
1.6
Mw/Mn

44 J. Hua et al.
Table 1 Effects of reaction time and reaction temperature on the molecular weight and polydispersity index of the synthesized PSt
Temperature (°C) Time (h) Monomer conversion (%) M th ×10 −4 M n,GPC ×10 −4 M w /M n
120 6 63.9 1.33 1.61 1.71
120 27 94.2 1.95 1.79 1.55
90 23 57.5 1.20 0.88 1.96
90 39 98.2 2.04 1.86 1.89
Conditions: [C 6 H 5·CH 2 Cl] 0 :[MoCl 3 (OC 8 H 17 ) 2 ]:[P(Ph) 3 ]:[St] 0 =1:1:3:200; theoretical molecular weight (M th )=([St]/[initiator])×formula weight of
PSt repeating unit× conversion
was rather wide and remained almost constant during the
polymerization.
It was believed that the ideal initiator should have a
similar chemical structure to the monomer. Unfortunately, a
suitable initiator for butadiene is yet to be found. In this
work, we attempted to polymerize butadiene with CCl 4 ,1-
phenylethyl chloride and 2-chloropropionic acid methyl
ester. However, no significant improvement in terms of
polymerization control was observed. These results indicate
that some side reactions participate in the ATRP of Bd. The
poor polymerization control observed in this work may be
associated with the effects of the reaction parameters on the
rates of the activation, deactivation and propagation steps,
which were inherently variable.
In order to verify the “living polymerization” character
of this polymerization system, the thermal polymerization
of butadiene, the polymerization of butadiene initiated by
C 6 H 5·CH 2 Cl and that initiated by the C 6 H 5·CH 2 Cl/
MoCl 3 (OC 8 H 17 ) 2 /PPh 3 system were investigated, respectively,
in toluene at 120°C. The influences of the initiator
system on the conversion, molecular weight and polydispersity
index are shown in Table 2.
When the butadiene polymerization was initiated by heat
and C 6 H 5·CH 2 Cl at 120°C, only a trace amount of product
was obtained. Moreover, the molecular weights and the
polydispersities were higher than those obtained with the
C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 initiator system.
That means that the polymerization that resulted from using
the first two types of initiator system was ordinary radical
polymerization. However, the product yield of the
C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 initiator system
reached 23.6% after 15.5 h, so M n achieved with this
system was far closer to M th (assuming “living” conditions)
than the M n values obtained with the other two systems.
M w /M n was also narrower for this system than for the other
two initiator systems, which shows that the “living”/
controlled radical polymerization of butadiene can be
carried out in toluene at 120°C with the C 6 H 5·CH 2 Cl /
MoCl 3 (OC 8 H 17 ) 2 / PPh 3 system.
It is worth comparing the above results with those
obtained in the literature concerning the ATRP of isoprene
with an CuBr/N,N,N′,N′,N″-pentamethyl diethylenetriamine
catalyst and an ethyl bromopropionate initiator. The
reported ATRP of isoprene in a bulk system resulted in
Fig. 3 1 H NMR spectrum of the PSt obtained with C 6 H 5·CH 2 Cl /
MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3
Fig. 4 Plot of ln([M 0 ]/[M]) vs polymerization time for the ATRP of
butadiene in toluene. [C 6 H 5·CH 2 Cl] 0 /[MoCl 3 (OC 8 H 17 ) 2 ]/[P(Ph) 3 ]/
[Bd] 0 =1/1/3/200; [Bd] 0 =2.19 M; T=120°C

Atom transfer radical polymerizations of styrene and butadiene as well as their copolymerization 45
Fig. 5 Dependences of M n , GPC and M w /M n on the conversion in the
ATRP of butadiene.The conditions were the same as in Fig. 3. Unfilled
circles, M n ; filled circles, M w /M n ; line, theoretical value M th
(assuming “living” conditions)
only a trace amount of product, irrespective of the reaction
parameters used [22]. At the same time, the polydispersity
of the product was around 2.0. In this paper, the conversion
of polybutadiene reached 23.6% after 15.5 h at 120°C. The
polydispersity of this system was lower than 2.0.
The contents of cis-1,4-, trans-1,4- and 1,2-butadiene
structural units were 22.1%, 58.4% and 19.5%, as elucidated
using the FTIR spectrum of the polybutadiene. These
results were similar to those obtained from 1 H NMR of the
PBd (the content of 1,4-butadiene was 83.1% and that of
1,2-butadiene was 16.9%, as calculated according to [23]).
The microstructure of the PBd obtained here was similar to
that of PBd obtained by conventional free radical polymerization
[24].
Copolymerization of styrene and butadiene
It was interesting to investigate whether styrene and
butadiene copolymer can be obtained through ATRP
initiated by this novel initiator system, C 6 H 5·CH 2 Cl /
MoCl 3 (OC 8 H 17 ) 2 / PPh 3 . The copolymerization of Bd and
St was studied in this regard.
Table 2 The influence of the initiator system on conversion, M n , GPC
and M w /M n in the polymerization of butadiene
Conditions C 6 H 5·CH 2 Cl /
MoCl 3 (OC 8 H 17 ) 2 :P(Ph) 3 :
Bd=1:1:3:200
C 6 H 5·CH 2 Cl :
Bd=1:200
Polymerization 15.50 62.27 62.27
time (h)
Conversion (%) 23.60 4.80 4.80
M n ×10 −4 1.24 9.88 8.86
M w /M n 1.75 2.44 2.58
Conditions: [Bd] 0 =2.19 M; T=120°C; solution was toluene
Bd
Fig. 6 Relationship between ln([M 0 ]/[M]) and polymerization time
for the copolymerization of St and Bd in toluene. [St+Bd] 0 =4.69 M;
T=90°C
The kinetics of the copolymerization of St and Bd in
toluene at 90°C, initiated by the C 6 H 5·CH 2 Cl / MoCl 3
(OC 8 H 17 ) 2 / PPh 3 initiator system, are shown in Fig. 6.
The molar ratio of C 6 H 5·CH 2 Cl/MoCl 3 (OC 8 H 17 ) 2 /PPh 3 /
(St+Bd) was 1/1/3/200, and that of St/Bd was 2/1. It was
found that a semilogarithmic kinetics plot of ln([M 0 ]/[M])
versus time gave a straight line. Figure 7 shows that there
is a linear dependence of M n on the monomer conversion
and a decrease in M w /M n with monomer conversion.
Figure 7.
The microstructure of the copolymer was characterized by
IR, 13 C NMR and 1 H NMR. In the IR spectrum of the
copolymer (Fig. 8), absorptions at 738, 911 and 967 cm −1
were characteristic of cis-1,4-, 1,2- and trans-1,4 butadiene
structural units, respectively, and the peaks at 3023∼3085 cm −1
were assigned to the aromatic C–H stretching of styrene units
in the copolymer. It was also found that the characteristic
Mn
25000
20000
15000
10000
5000
0
0 10 20 30 40 50 60
Conversion%
Fig. 7 Dependence of M n , GPC and M w /M n on conversion during the
copolymerization of St and Bd in toluene. The conditions were the
same as in Fig. 1. Squares, M n ; triangles, M w /M n
2.4
2.2
2.0
1.8
1.6
Mw/Mn

46 J. Hua et al.
8 6 4 2 0 ppm
Fig. 10 1 H NMR spectrum of poly(butadiene-styrene)
Fig. 8 FTIR spectra of PSt (a), St/Bd copolymer (Bd/St=1/4) (b), St/
Bd copolymer (Bd/St=1/3) (c), St/Bd copolymer (Bd/St=1/2) (d), St/
Bd copolymer (Bd/St=1/1) (e), PBd (f)
absorption peaks of the styrene units became stronger and
those of the butadiene structure unit became weaker as the
molar ratio of styrene in the copolymer increased. At the
same time, the characteristic peaks of the 1,2-butadiene
structural units increased while those of the cis-1,4- and
trans-1,4-butadiene structural units decreased.
Figure 9 shows 13 C NMR spectra of the styrene and
butadiene copolymer. The peaks between 40.9 and
45.5 ppm (40.9, 42.6, 42.8, 43.0, 43.2, 45.5 ppm) in
Fig. 9 correspond to the carbons of the S*V, S*c, V*S, and
S*t chain structures (here, S, V, c and t denote styrene and
1,2-, cis-1,4- and trans-1,4-butadiene structural units,
respectively); the peak at 145.15 ppm corresponds to the
carbon of the S unit between c and t [25].
There is no absorption peak over 6.800 in the 1 HNMR
spectrum of the copolymer (Fig. 10), which shows that
there is no block styrene chain segmental structure in this
copolymer [26]. This verifies that the copolymer is a
random copolymer.
Figure 11 shows the single glass transition temperature
in the DSC curve of the styrene and butadiene copolymer.
The T g of the copolymer dropped from 16.2°C to −16.6°C
as the molar ratio of Bd/St increased from 1/3 to 1/2. These
results further prove that the copolymer is a random
copolymer.
Analysis of the polymerization mechanism
Figure 12 shows UV-vis spectra of toluene solutions of
MoCl 3 (OC 8 H 17 ) 2 , MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 ,
MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl, and St /
MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl. In the UV-vis
spectrum of MoCl 3 (OC 8 H 17 ) 2 , characteristic absorption
bands were observed for Mo 5+ near 430 nm and 730 nm
and for Mo 4+ near 510 nm [27]. Mo 4+ was probably present
because the reduction of MoCl 3 (OC 8 H 17 ) 2 took place to a
small degree when creating the 1-octanol-substituted MoCl 5
solution. The absorption peak at 730 nm was caused by a
d–d transition in the Mo atom in MoCl 3 (OC 8 H 17 ) 2 .After
Fig. 9 13 C NMR spectra of the styrene and butadiene copolymer. The
molar monomer ratio of St/Bd was 3/1
Fig. 11 DSC curves for styrene and butadiene copolymers with
different monomer ratios

Atom transfer radical polymerizations of styrene and butadiene as well as their copolymerization 47
Abs
3
2
1
4
0
200 400 600 800 1000
wavelength(nm)
Fig. 12 UV-vis spectra of MoCl 3 (OC 8 H 17 ) 2 (1), MoCl 3 (OC 8 H 17 ) 2 /P
(Ph) 3 (2), MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl (3) and St /
MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 /C 6 H 5·CH 2 Cl (4) in toluene
2
1
3
PPh 3 was added, the absorption at 730 nm became weaker,
perhaps because of competition between the PPh 3 and 1-
octanol ligands to coordinate with the Mo atom. At the same
time, the absorption at 430 nm increased, which indicated an
increase in the Mo 4+ content. After the solution of St /
MoCl 3 (OC 8 H 17 ) 2 / P(Ph) 3 / C 6 H 5·CH 2 Cl in toluene was
placed in a oil bath at 90°C for 5 h, the absorption at 730 nm
became smooth, and that at 430∼460 nm became weaker. At
the same time, a yellow deposit appeared in the solution,
which accounted for the increased content of Mo 6+ . These
results reveal that Mo 4+ , Mo 5+ and Mo 6+ were present
together in this reaction system. The valence state of
molybdenum changed during the reaction. All of these
results imply that the oxidation and reduction reaction of
MoCl 3 (OC 8 H 17 ) 2 occurred according to the ATRP mechanism.
A similar rule also applies to the Bd and St/Bd
copolymerization systems.
Therefore, we can deduce that the mechanism for the
polymerization of styrene or butadiene or their copolymerization
using a C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3
initiation system is that shown in Scheme 1. When
MoCl 3 (OC 8 H 17 ) 2 reacts with C 6 H 5·CH 2 Cl, an electron
transfer reaction can occur, where the Mo(V) or Mo(IV)
is oxidized to Mo(VI) or Mo(V), respectively, with the
concomitant formation of a benzyl radical. This reaction is
typical of the initiation step of an ATRP. The benzyl
radicals formed react with the alkene (diolefin) to produce
polymer chain radicals. Both of the radicals can then react
with Mo n+1 Cl n+1–2 (OC 8 H 17 ) 2 / PPh 3 to give Mo n Cl n−2
(OC 8 H 17 ) 2 / PPh 3 and the corresponding alkyl halide
(dormant species, Scheme 1). The occurrence of more than
one kind of reversible oxidation and reduction reaction in
this reaction system may be another reason for the rather
wide molecular weight polydispersity index.
Conclusions
The atom transfer radical polymerizations of butadiene and
styrene and their copolymerization were successfully
initiated by C 6 H 5·CH 2 Cl / MoCl 3 (OC 8 H 17 ) 2 / PPh 3 system.
The almost linear kinetics plot, suggesting a first-order rate,
and the linear increase in the number average molecular
weight with conversion demonstrate that it has “living”
radical polymerization character. However, the rather wide
molecular weight distributions observed indicate that
control over this polymerization system needs to be
improved. PBd was an atactic polymer, as detected by IR.
The microstructure of the butadiene and styrene copolymer,
as characterized by IR,
13 C NMR,
1 H NMR and DSC,
indicates that it is a random copolymer. The primary
reaction mechanism was shown to proceed according to
the ATRP mechanism using UV-vis spectroscopy.
Scheme 1 The mechanism for
the polymerization of styrene or
butadiene or their copolymerization
using the C 6 H 5·CH 2 Cl /
MoCl 3 (OC 8 H 17 ) 2 / PPh 3 initiation
system
Initiation
Ka
CH-Cl
n CH . n + 1
2 + Cl (OC8H17
+
Mo Cl (OC8H17 )
n-2
) / 2
PPh3 2
Mo
Kd
n +1-2
2 / PPh3
monomer
Propagation
1
R
1
Ka
R
1
R
n n + 1
CH- ( CH C )
2 Cl + + Cl (OC8H17 p Cl (OC8H17 n-2
) CH- ( CH C
) / PPh3
2 Mo
2 / PPh3
2
)
Kd
p CH2
C Mo
2
n +1-2
2
R2
R2
R2
Kp
1
R
monomer
n = 4, 5 ;
CH2
C
was monomer = styrene or butadiene
R2

48 J. Hua et al.
Acknowledgements The authors are thankful to the National Natural
Science Foundation of China (No. 50603009) for financial support.
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