مرکزی صفحہ Polymer Bulletin Chemoenzymatic synthesis of Y-shaped diblock copolymer

Chemoenzymatic synthesis of Y-shaped diblock copolymer

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english
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DOI:
10.1007/s00289-009-0050-2
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May, 2009
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Polym. Bull. (2009) 62:643–655
DOI 10.1007/s00289-009-0050-2
ORIGINAL PAPER

Chemoenzymatic synthesis of Y-shaped diblock
copolymer
Bao Zhang Æ Yapeng Li Æ Wei Wang Æ Liang Chen Æ
Shuwei Wang Æ Jingyuan Wang

Received: 9 January 2009 / Revised: 5 February 2009 / Accepted: 8 February 2009 /
Published online: 19 February 2009
Ó Springer-Verlag 2009

Abstract Y-shaped diblock copolymer polycaprolactone-block-(polystyrene)2
[PCL-b-(PSt)2] was synthesized successfully by the combination of enzymatic ringopening polymerization (eROP) and atom transfer radical polymerization (ATRP).
CH3O-terminated PCL was synthesized firstly by eROP of e-caprolactone (e-CL) in
the presence of biocatalyst Novozyme 435 and initiator CH3OH, subsequently the
resulting PCL was converted to macroinitiator by the esterification of it with
2,2-dichloro acetyl chloride (DCAC). PCL-b-(PSt)2 diblock copolymers were synthesized in an ATRP of the styrene with CuCl/2,20 -bipyridine as the catalyst system.
The kinetic analysis of ATRP indicated a controlled/‘living’ radical polymerization.
The structure and composition of obtained polymers were characterized with NMR,
GPC and FTIR. The thermal behavior was characterized by differential scanning
calorimetry (DSC).
Keywords Atom transfer radical polymerization (ATRP)  Enzymatic
polymerization  Ring-opening polymerization  Y-shaped block copolymers

Introduction
Star-shaped polymers has been received great interest since Schaefgen and Flory
first proposed this concept in 1948 [1] because of its unique properties such as
impact-resistant plastics, different phase behavior, thermoplastic elastomers, variety
of morphologies, polymeric emulsifiers, sol–gel states, and gas permeation
B. Zhang  Y. Li (&)  W. Wang  S. Wang  J. Wang
Alan G. MacDiarmid Institute, Jilin University, 130023 Changchun, People’s Republic of China
e-mail: liyapeng_jlu@yahoo.com.cn
L. Chen
The First Hospital of JiLin University, 130021 Changchun, People’s Republic of China

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me; mbranes[2]. Y-shaped block copolymer is a type of star-shaped polymer, which
is typically synthesized by living anionic polymerization, [3–5] cationic polymerization [6] and controlled/’living’ radical polymerization[7]. Teyssié et al. [8] firstly
reported the synthesis of Y-shaped block copolymers. PS-(PEO)2 was obtained by
end-capping a living polystyryl carbanion by a naphthalene derivative followed by
polymerization of ethylene oxide using naphthalene sodium radical ion as the
initiator. However, the synthesis of block copolymers is particularly suited to
investigate the combination of fundamentally different synthetic techniques.
Because various combinatorial approaches enables those polymers obtained from
different types of polymerizations. The new polymers obtained can have variable
compositions and architectures, thus they may have astonishing properties [9–11].
Many groups investigated the synthesis of Y-shaped block copolymers by the
combination of different modes of polymerizations [12–16].
For instance, Hizal et al. [12] synthesized an ABC-type miktoarm star polymer
using a core-out method via a combination of ROP, SFRP and ATRP. Gnanou et al.
[13] prepared AA0 2-type asymmetric star and AB2-type miktoarm star polymers by
the combination of ATRP and chemical modification of the termini of ATRPderived polymers. Armes et al. [14] synthesized two bifunctional ATRP macroinitiators via Michael addition, followed by the synthesis of Y-shaped block
copolymers from these macroinitiators by polymerizing various hydrophilic
methacrylic monomers via ATRP. Shortly after, a series of well-defined Y-shaped
(AB2-type) zwitterionic block copolymers were synthesized by Armes with the
same method [15]. Dumas et al. [16] synthesized well-defined (PCL)2-arm-PtBuMA
and (PCL)2-arm-PS star block copolymers from a heterotrifunctional initiator
bearing two hydroxyl groups able to initiate ROP of e-CL [with AlEt3 or Sn(Oct)2 as
coinitiator] and a bromide function group able to initiate ATRP of tBuMA or
styrene.
The enzymatic polymerization in vitro is evaluated as a new, environmental
friendly methodology in polymer science. ‘‘Green’’ biocatalyst enzyme has many
special properties, such as its nontoxicity, recyclability, (enatio-, regio- and chemo-)
selectivity, biocompatibility and ability to operate under mild conditions [17, 18].
Moreover, enzymatic polymerization can prepare useful polymers which are often
difficult to be synthesized by conventional polymerization. Consequently, the above
merits of biocatalytic polymerizations motivated more researchers to study the
chemoenzyme-catalyzed route to block copolymer synthesis.
Heise et al. [19–22] first made use of this strategy to carry out the consecutive/
one-step cascade/simultaneous synthesis of the diblock copolymer from a dualinitiator, which contained a primary alcohol and single a-bromoester. Our group
also demonstrated the feasibility of 2,2,2-trichloroethanol as another novel dualinitiator, [23, 24] which permits a sequential two-step synthesis combining eROP
and ATRP. In addition, our group introduced another synthetic technique. It usually
requires an intermediate transformation step to convert the end group of the PCL
into an active initiating site for the polymerization of the second monomer [25–28].
It has also been employed successfully in the preparation of block copolymers,
where the esterification of the end hydroxyl groups of preformed polyester PCL

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645

from eROP of e-CL with halogenated acyl halide proved to be an excellent method
for producing macroinitiators suitable for the block-ATRP of styrene.
In this work, we reported the synthesis of Y-shaped block copolymer by the
combination of eROP and ATRP. The kinetic analysis of ATRP indicated a
controlled/‘living’ radical polymerization. The structure and composition of
obtained polymers were characterized with NMR, GPC and FTIR. The thermal
behavior was investigated by differential scanning calorimetry (DSC). We believe
that Y-shaped block copolymers will exhibit peculiarity in morphology and have
important applications in the fields such as controlled drug delivery, advanced
materials.

Experimental
Materials
CH3OH was refluxed for 6 h in the presence of magnesium which was activated by
iodine and distilled. Novozyme-435 (activity approximately 7,000 PLU/g) was a
gift from Novo Nordisk A/S and employed without further purification. e-CL were
obtained from Aldrich Chemical Co. and distilled over calcium hydride (CaH2)
under vacuum before use. CuCl (Beijing Chemical Co.) was purified by
precipitation from acetic acid to remove Cu2?, filtrated and washed with ethanol
and then dried. 2,20 -bipyridine (Beijing Chemical Co.) was used without further
purification. 2,2-dichloro acetyl chloride (DCAC, 99%) was purchased from Aldrich
Chemical Co. Styrene (Beijing Chemical Co.) was distilled over CaH2 under
reduced pressure. Toluene (Tianjin Chemical Co.) and dichloromethane (Tianjin
Chemical Co.) were dried with CaH2 and distilled. Triethylamine (Beijing Chemical
Co.) was refluxed for 12 h in the presence of CaH2 and distilled under vacuum. All
the reagents used in this study were of analytic grade.
Synthesis of CH3O-terminated polyester (CH3O-PCL)
Novozyme-435 (0.216 g, 10% w/w of the monomer weight), dried in a desiccator
with P2O5 as desiccant under vacuum (0.1 mmHg, 25 °C, 24 h), was transferred
into an oven-dried 50 ml reaction vial under dry argon atmosphere, and the vial was
immediately sealed with a rubber septum. The monomer e-CL (2.156 g,
1.89 9 10-2 mol), solvent toluene (4.3 mL, twice v/w of the monomer weight),
and initiator CH3OH (0.025 mL, 6.3 9 10-4 mol) were transferred into the reaction
vial via a gastight syringe under argon. The vial was then placed into a constant
temperature (70 °C) oil bath with magnetic stirring for 4 h. The reaction was
terminated by pouring excess cold chloroform into the reactants and the enzyme was
removed via filtration. The filtrate was concentrated by rotary evaporation. The
crude products was precipitated in methanol and dried in a vacuum oven. The yield
is 90%. Mn.NMR = 5,500, Mn,GPC = 10,200, Mw/Mn = 1.24. 1H-NMR (CDCl3, d):
4.1 (m, CH2O in PCL), 3.65 (t, terminal CH2O in PCL), 3.67 (s, CH3O in methanol),
2.30 (m, COCH2 in PCL), 1.6 (m, CH2 in PCL), 1.4 (m, CH2 in PCL).

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Synthesis of macroinitiator [CH3O-PCL(Cl)2]
The resulting PCL (1 g, 1.8 9 10-4 mol) was dissolved in 5 mL of dry
dichloromethane and then cooled in an ice bath (0 °C). To this solution was added
1 mL (7.2 9 10-3 mol) of triethylamine. After 5 min of stirring, 5 mL of a
dichloromethane containing 0.6 ml of 2,2-DCAC (0.92 g, 6 9 10-3 mol) was
added dropwise to the solution over a period of 1 h. The reaction was carried out at
0 °C for 2 h and then at room temperature for 22 h. The solution was filtrated to
remove the quaternary ammonium halide (CH3CH2)3NH?Cl-. The filtrate was
concentrated and then precipitated in methanol. The yield is 80%. Mn,GPC = 11,700,
Mw/Mn = 1.21. 1H-NMR (CDCl3, d): 5.95 (s, Cl2HCCO), 4.28 (t, terminal CH2O in
PCL), 4.1 (m, CH2O in PCL), 2.30 (m, COCH2 in PCL), 1.6 (m, CH2 in PCL), 1.4
(m, CH2 in PCL).
Synthesis of Y-shaped diblock copolymers [PCL-b-(PSt)2]
A dry flask equipped with a magnetic stirrer was charged with CuCl (0.018 g,
1.8 9 10-4 mol), bpy (0.084 g, 5.4 9 10-4 mol), and macroinitiator (0.2 g,
3.6 9 10-5 mol). The reaction vial was sealed and degassed three times by
freeze-pump-thaw cycles. Solvent toluene (2 mL) and monomer styrene (2 g,
2 9 10-2 mol) degassed by inert dry argon were introduced into the flask via an Arwashed syringe. After PCL macroinitiator was completely dissolved, the reaction
flask was placed into a constant temperature (120 °C) oil bath with magnetic stirring
for a predetermined time. Aliquots (about 0.8 mL of reaction mixture) were
removed from the reaction mixture at selected time intervals to monitor the reaction
progress. The reaction was rapidly terminated in an ice bath. The catalyst was
removed by passage of the polymer solution through an aluminum oxide column.
The crude polymer was precipitated in methanol, and then dried under vacuum
overnight. The GPC data are listed in Table 1. 1H-NMR (CDCl3, d): 6.3–7.0
(m, aromatic protons), 4.1 (m, CH2O in PCL), 2.30 (m, COCH2 in PCL), 0.90–2.18
(m, CH and CH2 in PSt), 1.6 (m, CH2 in PCL), 1.4 (m, CH2 in PCL).
Characterization
The monomer conversion was determined gravimetrically. 1H and 13C nuclear
magnetic resonance (NMR) spectra were recorded on a Bruker ARX-500 NMR
spectrometer with CDCl3 as solvent at 500 and 125 MHz, respectively. Chemical
shifts (ppm) were reported downfield from 0.00 ppm with trimethylsilane (TMS) as
internal standard. The molecular weights and molecular weight distributions were
measured on a Waters 410 gel permeation chromatography (GPC) apparatus
equipped with a 10-lm Styragel HT6E column (300 9 7.8 mm) with linear
polystyrene standards. THF was used as the eluent at a flow rate of 1 mL/min. The
infrared spectra (IR) of polymers were recorded on a NICOLET Impact 410 at room
temperature. Dried samples (20 mg) were mixed with 100 mg of dry KBr and
pressed into disk (100 kg cm-2). Differential scanning calorimetry (DSC) was
carried out on a DSC-7 (Perkin-Elmer) to study the thermal properties of the

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1,0,200

720/1

720/1

720/1

720/1

720/1

720/1

4

5

6

7

8

9

The conversion was determined gravimetrically

1,200

780

600

480

360

270

180

Time
(min)

66.7

46.3

40.2

33.1

27.3

22.1

15.2

Monomer
conversion (%)

59,600

43,200

38,300

32,600

27,800

23,600

17,900

58,700

44,300

38,000

33,000

28,050

22,400

17,500

(g/mol)

(g/mol)

37,600

27,200

25,000

22,300

20,800

18,800

16,400

(g/mol)

Men,GPC

1.29

1.15

1.19

1.19

1.20

1.21

1.25

Mw/Men

1.21

Mw/Men

1.24

Mw/Men

e

d

Determined by GPC measurements

EI represents the efficiency of initiator, EI = Mn.th/Mn.nmr

The theoretical molecular weights (Mn.th) calculated from the ratio of the monomer to the initiator [M]0/[I]0 and the monomer conversion
Mn;th ¼ ½M0 =½I0  Mmonomer  concentration% þ Mn ðmacroÞinitiator

c

b

Determined by 1H-NMR analysis

720/1

3

a

[M]0/[I]0

Copolymer

Man.nmr

Mcn,th

11,700

56%

Men,GPC
(g/mol)

[98%

5,500

EId

2

3,100

Mdn.nmr
(g/mol)

Men,GPC
(g/mol)

b

90%

\2%

Mcn,th
(g/mol)

The degree of end
functionalization (mol%)

30/1

1

Monomerb
conversion

Carboxyl terminala
chains (mol%)

Macroinitiator

[M]0/[I]0

PCL

Table 1 Results for PCL, macroinitiator and block copolymers

Polym. Bull. (2009) 62:643–655
647

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polymers at a heating and cooling rate of 10 °C/min under a nitrogen flow of
200 ml/min. A polymer (about 3.0 mg) was loaded in a cell, and the heat exchange
was recorded during the heating and cooling cycles.

Results and discussions
Synthesis of CH3O-terminated polyester
The methanol-initiated eROP of lactones was investigated in previous reports [29].
Lipase porcine pancreatic lipase (PPL) and lipase PS 30 (pseudomonas, Ceracia
lipase) were used as biocatalysts; their poor catalytic activity enabled the
preparation of low-molecular-weight oligomers with a rather long polymerization
time (about several weeks). Therefore, our group has made use of the more active
biocatalyst Novozyme 435 (lipase CALB immobilized on an acrylic resin) to carry
out eROP of e-CL at 70 °C in toluene (twice w/v of monomer) where methanol was
used as the initiator. The general synthetic route used for the preparation of the
Y-shaped block copolymers is shown in Scheme 1.
Control of the PCL structure (polydispersity, end-group structure, and molecular
weight) strongly depends on the frequency of the side reactions caused by the water
reactivity because of competitive initiation reactions between water and initiator
CH3OH. Water is used as an acyl acceptor, and it cannot only induce the
nucleophilic initiation but also cause the hydrolysis. Both of the reactions will
broaden the molecular weight distribution and result in the polyester chains
terminated with a carboxylic acid groups other than the initiator segment. So it is
crucial to dry the reaction components as much as possible to minimize the water
initiation.
Figure 1a shows the 1H-NMR spectrum of CH3O-PCL 1. The multiplet signals,
centered at 1.4, 1.6, 2.3, and 4.1 ppm, represented the PCL main chain protons, the
triplet signal a at 3.65 ppm corresponded to the methylene protons of the terminal
hydroxyl groups. The characteristic signals g of the initiator segment (CH3O–) at the
end of the chain could be pointed out at 3.67 ppm, which clarified methanol
initiated successfully eROP of e-CL. Also, if a fraction of the PCL chains were

Scheme 1 Synthesis route of the Y-shape block copolymer

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649

Fig. 1 1H-NMR spectrum of PCL 1 (a) macroinitiator 2 (b) and Y-shaped PCL-b-(PSt)2 3 (c) were
recorded at room temperature in CDCl3

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initiated by water, it would result in terminal carboxyl acid groups, thus, the
methylene protons linked to it should appear in the region of about 2.4 ppm.
The absence of any resonance at 2.4 ppm in Fig. 1a suggested that the initiation of
the PCL chains was carried out quantitatively by methanol, and the amount of
water-initiated PCL could reduce to less than 2% (the limitation of detection by
NMR analysis). Combining GPC analysis, it was clear that the Mn,th 1 (3,100 g/mol)
was lower than those (10,200 g/mol) obtained by GPC. The discrepancy could
mainly be resulted from the GPC technique underestimating Mn because of different
hydrodynamic volumes of the PCL and the linear polystyrene standards. The Mn.nmr
determined by 1H-NMR was higher than the expected molecular weight (Mn.th) of
PCL 1 (Table 1), which was possibly caused by the low efficiency of initiation
(about 56%) of the methanol due to the partial volatilization of the initiator at the
initial stage of the reaction at 70 °C.
Synthesis of macroinitiator
The macroinitiator 2 was prepared by an esterification reaction between terminal
OH group of the resulting PCL 1 and 2,2-DCAC. During the process, triethylamine
was used as the catalyst and absorbed HCl from the solution to generate a
precipitate of quaternary ammonium halide (CH3CH2)3NH?Cl-, which benefited
the esterification. The structure of macroinitiator is determined by 1H-NMR. Due to
the ester formation, the methylene protons a experienced a shift from 3.65 to
4.28 ppm. The new signal h at 5.95 ppm assigned to the [CH– protons close to the
active chloride (Fig. 1b), which indicated that the 2,2-dichloro acetyl group was
attached to the PCL chain end. The existence of the signal g revealed that the
esterification didn’t interfere with the terminal CH3O– group. Based on the above
result, it was obvious that the macroinitiator had been prepared. The peak area ratio
of a and g is 1:1, which confirmed the complete substitution of the terminal
hydroxyl groups. As shown in Table 1, it was noted that the polydispersity after the
esterification reaction was lower than those of the starting PCL, whereas number
average molecular weight (Mn) was slightly higher. This could be due to inevitable
fractionation of macroinitiator during the course of precipitation after esterification.
Synthesis of Y-shaped block copolymer
The ATRP of St from macroinitiator 2 was carried out in toluene at 120 °C with
CuCl/bpy as the catalyst system. GPC-determined Mn, theoretical molecular weight
(Mn,th), and polydispersity index (Mw/Mn) versus monomer conversion for ATRP
are shown in Fig. 2. Mn increased linearly with conversion while the polydispersity
index varied only a few degrees and was relatively low (\1.30). The Mn,th values
were higher than the experimental ones (Mn), which resulted from the GPC
technique underestimating Mn because of different hydrodynamic volumes of the
copolymers and the linear polystyrene standards. Figure 3 shows the time
dependence of ln([M]0/[M]t). The linear relationship indicated that the polymerization was first-order with respect to monomer concentration, and the number of
active species remained constant throughout the course of reaction. The kinetic

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651

Fig. 2 Dependence of Mn.th (open circle), Mn (filled circle) and polydispersity index (triangle)on monomer
conversion for ATRP of St initiated by PCL macroinitiator (I) [St]:[I]:[CuCl]:[bpy] = 720:1:5:15, reaction
temperature: 120 °C. Mn and polydispersity were determined by GPC calibrated with polystyrene. The
theoretical molecular weights (Mn,th) were calculated from Eq. 1 (Table 1)

Fig. 3 ln([M]0/[M]t) versus time for ATRP of St initiated by PCL macroinitiator. [M]0 and [M]t
represent the initial monomer concentration and the monomer concentration after time t, respectively

behavior of ATRP proves that polymerization of St is a ‘living’/controlled radical
process.
From the 1H-NMR spectra of the Y-shaped diblock copolymer 3 [PCL-b-(PSt)2]
(Fig. 1C), we could see that besides the dominant PCL signals b–f, the occurrence
of the signals at 6.5–7.0 ppm were due to aromatic protons D and E of the PSt
block. The GPC traces of the starting PCL 1, macroinitiator 2 and the final block
copolymer 3 were present in Fig. 4. It was clear that the ATRP of styrene using
macroinitiator 2 resulted in an increase in molecular weight. The unimodal and
symmetrical shape on the GPC plot of the block copolymer suggested the absence of
a homopolymer composed of either styrene or e-CL and the complete initiation of

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Fig. 4 GPC traces of PCL 1
(Mn = 10,200 g/mol,
polydispersity = 1.24),
macroinitiator 2 (11,700 g/mol,
1.21) and Y-shaped diblock
copolymer (30,700 g/mol, 1.29).
The molecular weight and
polydispersity were determined
by GPC calibrated with PSt

Fig. 5 IR spectra of PCL 1 A macroinitiator 2 B and Y-shaped block copolymer PCL-b-(PSt)2 3 C

the macroinitiator during the ATRP process. Combing the GPC analysis and
1
H-NMR results indicated the formation of the Y-shaped block copolymer [PCL-b(PSt)2].
Figure 5 shows the FTIR spectra of the obtained polymers. For PCL, the
characteristic absorption bands appeared in the wave number region of 1,740 cm-1
assigned to the ester carbonyl group of the PCL main chains. Compared with PCL,
PCL-b-(PSt)2 copolymers showed the new peaks at the wave numbers of
approximately 3,030, 1,450, and 690 cm-1, which were ascribed to the ring
vibration of the aromatic group of PSt. The variance of the IR spectroscopic results
confirmed the formation of the PSt block.

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653

Fig. 6 DSC thermogram of the
PCL homopolymer A and
the block copolymer
PCL-b-(PSt)2 3 B

Table 2 DSC results for PCL and copolymers PCL-b-(PSt)2
Sample

Tc (°C)

Tm (°C)

(DHm) (J g-1)

Xc (%)a

PCL 1

32.31

53.87

67.86

50.3

Copolymer 3

19.55

52.18

27.26

20.2

a

Xc =

(DHm/DH*m)

9 100%

DSC was carried out to investigate the melting and crystallization behaviors of
the Y-shaped block copolymers. DSC analysis was performed in a range from -50
to 160 °C at a rate of 10 °C/min under nitrogen. To minimize the effect of
recrystallization from the solution, the evaluation of thermal properties was
performed on the second thermal scan. Figure 6 shows a single melting peak and a
single crystallization peak of PCL homopolymer (A) at Tm = 53.87 °C and
Tc = 32.31 °C, respectively. Since PSt is an amorphous material (without any
crystallinity), the crystallinity of the block copolymers were attributed to the PCL
block. Obviously, the introduction of PSt segments changed Tc gradually from 32.31
to 19.55 °C. The melting enthalpy (DHm) reflects the amount of crystallinity
developed in each sample. The degree of crystallinity (Xc) was determined by the
equation Xc = (DHm/DH*m) 9 100%, where DHm is the heat of enthalpy of polymer
and DH*m is the theoretical heat of enthalpy of PCL at 100% crystallinity
(135 J g-1). The Xc values in the polymers declined gradually from 50.3 to 20.2%
with increasing PSt content. As shown in Table 2, the lower Tc and Xc values of
copolymer could be attributed to the crystalline imperfections because the
introduction of PSt segments rendered crystallization more difficult. The melting
temperature (Tm) of the PCL block in copolymer decreased in comparison with that
of PCL; however, Tm varied little, from 53.87 to 52.18 °C. The DSC results
indicated the difference in the thermal properties between PCL and copolymers,
which confirmed the formation of PSt blocks from PCL block.

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Conclusions
The eROP and ATRP was combined to synthesize the Y-shaped block copolymer.
The ATRP macroiniitiator effectively initiated ATRP of styrene with CuCl/2,20 bipyridine as the catalyst system. Linear first-order kinetics, linearly increasing
molecular weight with conversion, and low polydispersities (\1.29) were observed
in this process. The structures and composition of the Y-shaped copolymer were
well characterized by means of NMR, IR and GPC measurement. DSC analysis
showed that the crystallinity of the copolymer decreased with the introduction of the
PSt block. Chemoenzymatic synthesis of Y-shaped block copolymer is a novel
technique, which not only allows the variation of the polymer composition by
adjusting the ratios between the macroinitiator and monomer, but also can control
the structure of polymer exactly. In addition, the research for the morphology of
Y-shaped block copolymers is in progress.
Acknowledgments This work was supported by the Natural Science Foundation of China
(No. 20574028) and Natural Science Foundation for Youth.

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