Full text of DOI:10.1007/BF03218299

Macromolecular Research, Vol. 10, No. 2, pp 108-114 (2002)
Preparation and Characterization of High Molecular
†
Weight Poly(butylene succinate)
Yang-Kyoo Han*, Sung-Rim Kim, and Jinyeol Kim
Department of Chemistry, Hanyang University, Seoul 133-791, Korea and
Center for Advanced Functional Polymers, KAIST, Taejon 305-701, Korea
Received Jan. 17, 2002 ; Revised Feb. 21, 2002
Abstract: Poly(butylene succinate) (PBS) prepolymers were prepared by the condensation polymerization of 1,4-
butanediol (1,4-BD) and succinic acid (SCA) in the presence of titanium (VI) isoproxide(TIP) catalyst. The PBS pre-
polymers reacted with 1,4-BD or SCA to obtain hydroxyl or carboxylic acid group terminated PBS. High molecular
weight linear or branched PBS was synthesized by a coupling reaction between hydroxyl and carboxylic acid group
terminated PBS, or by a branching reaction between carboxylic acid group terminated PBS and glycerol as a branching
agent. The weight average molecular weight of the prepared linear or branched PBS was in the range of 100,000-
2
20,000. Both melting point and thermal stability of the high molecular weight linear and branched PBSs were
somewhat higher than those of general PBS. From a tensile behavior by Instron test, modulus, tensile strength and
elongation at break improved with increase in the molecular weight of the prepared PBS through the coupling or the
branching reaction. In particular, the high molecular weight linear PBS had about 2.5 times higher value in modulus
than the branched one.
Keywords : high molecular weight, aliphatic polyester, branched polymer, poly(butylene succinate), branching
reaction.
Introduction
matrix resins for biomedical materials or drug careers.
These limitations in application are due to their low melt
4
Every year, several hundred thousand tons of nondegrad-
able plastic products are discarded and filled into marine
and earth environments. This affects very serious ecological
and environmental problems.1,2 Therefore, recently biode-
gradable polymers have received much attention because
they can replace nondegradable polymers. A number of bio-
degradable polymers such as poly(hydroxy alcanoates),
poly(ε-caprolactone), aliphatic polyesters, and so forth have
strength and viscosity. These rheological properties associated
with melt processing are influenced not only by molecular
weight and molecular weight distribution but also by
branched structures of the polymers. In particular, rheology
related to elongation is greatly affected by long-chain
1
0
branching.
Thus, many researchers have focused on the development
of high molecular weight aliphatic polyesters with linear or
branched structures to improve their low rheological proper-
ties from an industrial point of view. As a result, high
molecular weight aliphatic polyesters such as polyethylene
succinate, PBS and polybutylene adipate, which are known
as BIONOLLE, have been developed first by Showha High-
3-6
been reported. Among them, especially, biodegradable
aliphatic polyesters derived from aliphatic diols and dicar-
boxylic acids have been extensively studied since they are
relatively easy to be prepared and also much cheaper than
the other degradable polymers such as poly(hydroxy alca-
7-9
11
noates) and poly(ε-caprolactone).
polymer Co., Japan. The weight-average molecular weights
On the other hand, the aliphatic polyesters have not been
applied to blow-molded or extruded products, but just to
limited articles such as disposable plastic bags and films and
of the polymers were in the range of 50,000 to the maximum
1,000,000 and they showed excellent properties in mechanical
strength and melt processing. Consequently, they could be
utilized in various applications such as film, foamed sheet,
blown bottles, or highly expanded foam. However, the method
for synthesizing these high molecular weight polymers has
been reported only in the patents, and detailed experimental
†Dedicated to Dr. Un Young Kim on the occasion of his retire-
ment.
*
e-mail : ykhan@hanyang.ac.kr
1
2
1
598-5032/04/108-07 2002 Polymer Society of Korea
conditions have not been disclosed. This led us to attempt
108

High Molecular Weight PBS
our work.
connected with gas inlet and outlet adapters. The mixture
was immersed in a silicon oil bath and then reacted at
In this article we report the synthetic conditions for the
preparation of high molecular weight linear and branched
PBSs through coupling and chain-branching reactions and
also describe their thermal and mechanical properties.
o
200 C for 100 min under nitrogen atmosphere. The viscous
o
liquid was heated to 220 C and then polymerized at 2.0 torr
for 2 h with the removal of water generated during the first
polymerization. At the end of the first-stage polymerization,
the pressure in the reaction vessel was reduced to 0.5 torr,
Experimental
o
whereas the temperature was raised to 230 C, and then the
Materials. 1,4-Butanediol (99%) and TIP (99.99%) were
purchased from Aldrich Chemical Company and used without
further purification. Succinic acid (SCA: 99%) was purified
by sublimation under a reduced pressure. Glycerol (99%), a
branching agent, and triethylamine (TEA: 99.8%) were pur-
chased from TEDIA Chemical Company and used without
further purification. In addition, chloroform solvent, an
industrial grade, was purified by fractional distillation.
Measurements. A fourier transform infrared (FT-IR)
second polymerization proceeded for 2-6 h to increase the
molecular weight of the resulting linear PBS. After the poly-
merization was completed, the temperature in the reaction
vessel was cooled down to room temperature. The PBS pre-
polymer was dissolved with 80 mL of chloroform, and then
1,4-butanediol (5.52 g, 0.06 mol) and TIP (0.03 mL, 0.1
mmol) were added again into the solution. The homogeneous
o
solution was stirred at 80 C and atmospheric pressure under
nitrogen purging for 1 h to remove the chloroform solvent.
The PBS prepolymer was then reacted with the added 1,4-
spectrum was obtained with a KBr pellet on a Brucker IFS
1
o
4
8 spectrometer. A H-NMR spectrum was recorded on a
butanediol at 200 C for 100 min under nitrogen atmo-
Varian Gemini 400 MHz spectrometer for determination of
polymer structure. CDCl was used as a solvent for H-
sphere. After the end-capping reaction, about 120 mL of
chloroform were used to dissolve the product, and then fil-
tered to remove impurities. The filtrate was precipitated into
methanol in excess to obtain a white powdery polymer. The
filtered polymer was washed with methanol and dried at
1
3
NMR spectra of PBSs. The inherent viscosity (I.V.) of pre-
pared polymers was measured at concentration of 0.5 g/dL
o
in chloroform at 25 C using a Cannon-Fenske viscometer.
o
Thermal properties of synthesized polymers were measured
by DSC and TGA 1000 plus (Polymer Laboratories Ltd.) at
25 C for 24 h in a vacuum oven in order to get a prepolymer
with hydroxyl groups at both ends of the PBS chains
(Scheme I).
o
a heating rate of 10 C/min under nitrogen atmosphere.
Molecular weights of the polymers were determined by a
Waters gel permeation chromatographer (GPC) using HR4E
and HR5E styragel columns with chloroform as solvent and
Shodex standard polystyrenes (Showa Denko K. K.) for a
calibration curve. Mechanical properties (ASTM D882) of
the films 20~30 μm thick were studied on Instron tensile
tester (Model TTD) at a strain rate of 20 mm/min. Surfaces
of degraded films after hydrolysis in a buffer solution were
measured by a scanning electron microscope (SEM: Hitachi
S-25000C).
In addition, another kind of PBS prepolymer with carboxylic
acid groups at both ends was prepared by the same method
Synthesis of Hydroxyl or Carboxylic Acid Group Ter-
minated Linear PBS. SCA (58.0 g, 0.48 mol), 1,4-butanediol
(
55.20 g, 0.6 mol), TEA (0.224 mL, 1.56 mmol) as a co-cata-
lyst, and TIP (0.30 mL, 1.0 mmol) as a catalyst were added
into a 250 mL two-necked round-bottom flask, which was
Scheme I. Synthesis of hydoxyl or carboxylic acid group termi-
nated linear PBS.
a
Table I. Preparation of Hydroxyl or Carboxylic Acid Group Terminated Linear PBS
b
Prepolymer
Sample No.
Reaction Time (h)
Conversion (%)
Mn
Mw
6,600
PDI
2.03
2.12
2.05
1.98
P1
P2
P3
P4
-
73
88
82
95
3,300
58,300
33,500
84,600
HO-PBS-OH
5
2
123,400
68,700
167,700
HOOC-PBS-COOH
6
a
o
o
The melt polymerization was carried out at 200 C for 2 h under an ambient pressure, 220 C for 2 h under a reduced pressure of 2.0 torr, and
o
finally 230 C for 2-6 h under reduced pressure of 0.5 torr.
b
o
Polymerization time was carried out at 230 C under reduced pressure of 0.5 torr.
Macromol. Res., Vol. 10, No. 2, 2002
109

Y.-K. Han et al.
described above except for adding SCA (5.8 g, 0.048 mol)
mer (sample P3 in Table I) was put in a 100 mL one-necked
round-bottom flask and dissolved in 40 mL of chloroform.
0.125 mL (0.015 g of pure glycerol) of 10% (v/v) glycerol
ethanol solution, a branching agent, was added into the
flask, which was immersed in a silicon oil bath, with
0.25 mL (0.025 g of pure TIP) of 10% (v/v) TIP chloroform
instead of 1,4-butanediol.
Synthesis of High Molecular Weight linear PBS. 4.0 g
of the hydroxyl group (sample P1 in Table I) and carboxylic
acid group (sample P3) terminated PBA prepolymers of was
put in a 100 mL one-necked round-bottom flask, respectively,
and dissolved with 50 mL of chloroform. 0.10 mL of 10%
o
solution. The mixture was heated to 60 C and stirred for 1 h
(
v/v) TIP chloroform solution was added into the flask,
under nitrogen atmosphere to remove the solvents. The
pressure in the reaction vessel was reduced to 0.5 torr,
which was immersed in a silicon oil bath. The mixture was
o
o
heated to 60 C and stirred for 40 min under nitrogen atmo-
whereas the temperature was heated to 210 C for 2 h for a
sphere to remove the solvents. The pressure in the reaction
vessel was reduced to 0.5 torr, whereas the temperature was
branching reaction between the carboxylic acid group ter-
minated PBS and glycerol. After the reaction, 60 mL of
chloroform were added into the flask and stirred for 2 h.
The solution was filtered to remove some insoluble gels,
cross-linked polymers that were generated during the
branching reaction. The filtrate was precipitated into metha-
nol in excess to obtain a white powdery polymer. The fil-
o
heated to 230 C for 3 h for a coupling reaction between the
hydroxyl group and carboxylic acid group terminated pre-
PBA polymers. After the reaction, 60 mL of chloroform
were added into the flask and stirred for 2 h. The solution was
filtered to remove impurities. The filtrate was precipitated
into methanol in excess to obtain a white powdery polymer.
The filtered polymer was washed with methanol and dried
o
tered polymer was washed with methanol and dried at 25 C
for 24 h in a vacuum oven in order to get a high molecular
weight of branched PBS (sample P3G6 in Table III) as
shown in Scheme II.
Measurement of Insoluble Gel. The high molecular
weight branched PBS obtained through a branching reaction
o
at 25 C for 24 h in a vacuum oven in order to get a high
molecular weight linear PBS (sample P6 in Table II).
Synthesis of High Molecular Weight Branched PBS.
.0 g of the carboxylic acid group terminated PBS prepoly-
5
a
Table II. Preparation of High Molecular Weight Linear PBS by a Coupling Reaction
Prepolymer
ReactionTemp.
Sample No.
o
Conversion (%)
Mn
Mw
PDI
(
C)
HO-PBS-OH HOOC-PBS-COOH
P5
P6
210
230
210
230
210
230
86
73
92
43
91
85
102,500
108,800
64,500
96,300
63,600
96,300
205,000
213,300
142,100
192,500
134,400
192,500
2.00
1.96
2.20
2.00
2.11
2.00
P1
P2
P2
P3
P3
P4
P7
P8
P9
P10
aCoupling reaction was carried out at 0.5 torr for 3 h.
a
Table III. Effect of Glycerol Concentration on Molecular Weight of Branched PBS by a Branching Reaction
Branched
PBS
Glycerol
Reaction
Conversion
(%)
Gelation
(%)
o
η
M
n
M
w
PDI
(wt% to PBS)
Temp.( C)
PG1
PG2
PG3
PG4
PG5
PG6
PG7
PG8
0.2
0.3
0.4
0.5
0.2
0.3
0.4
0.5
200
88
90
81
70
89
83
48
46
0.86
0.86
1.27
0.90
1.04
1.28
1.96
1.66
35,700
36,200
59,200
37,300
36,300
60,300
113,700
95,500
102,100
102,500
146,100
106,800
126,100
147,600
219,600
196,700
2.87
2.83
2.47
2.86
3.47
2.45
1.93
2.06
-
-
-
-
210
2.94
8.3
35
30
a
Carboxylic acid group terminated prepolymer P3 was used as a substrate polymer for a branching reaction for 2 h. The concentration of TIP as
a catalyst was 0.5 wt% in terms of the concentration of the prepolymer P3.
110
Macromol. Res., Vol. 10, No. 2, 2002

High Molecular Weight PBS
Scheme II. Preparation of high molecular linear PBS.
Figure 1. IR spectrum of the hydroxyl group terminated PBS.
reacted with 1,4-butanediol or succinic acid to produce the
corresponding prepolymers with hydroxyl groups or car-
boxylic acid groups at both ends of the prepolymer chains.
Such end-capping reaction showed the best results when the
succinic acid (or 1,4-butanediol), an end-group substituting
agent, was used to be 10 mol% in terms of the concentration
of the monomer used for the synthesis of linear PBS. On the
other hand, when used over 10 mol% of succinic acid, we
observed that the molecular weights of the prepolymers
with carboxylic acid groups decreased because of the trans-
esterification reaction between the linear polymer chains
Scheme III. Preparation of high molecular weight branched
PBS.
was dissolved in chloroform. The polymer solution was
filtered under reduced pressure by using a filter paper. The
4
and succinic acid molecules.
o
filtered gel, insoluble polymer, was dried at 25 C for 24 h in
In addition, the structures of the hydroxyl and carboxylic
acid groups terminated PBS prepolymers were confirmed
a vacuum oven, and the dried gel was weighed to calculate
the ratio of the branched polymer to the gel generated during
the branching reaction.
1
by FT-IR and H-NMR spectra. Figure 1 is FT-IR spectrum
of the hydroxyl group terminated PBS prepolymer having a
low molecular weight (P1). The spectrum showed strong and
broad stretching bands of methylene, carbonyl, and ether
Preparation of Polymeric Thin Films. The PBA thin
films 20-30 µm thick were cast from 10% chloroform sol-
ution. Mechanical tests were performed on five specimens
-
1
groups around 3000, 1700, and 1200 cm , respectively,
indicating typical characteristic bands of aliphatic polyester.
On the other hand, weak and sharp stretching band of the
hydroxyl groups at both ends of the low molecular weight
(
5
60 mm) for each sample. Their tensile properties were
measured by an Instron tensile tester.
-1
Results and Discussion
prepolymer (P1) chain appeared at 3550 cm , but the
hydroxyl group in the prepolymer P2 with a higher molecular
weight was not observed because of its low concentration.
Synthesis of Hydroxyl Group or Carboxylic Acid
Group Terminated Linear PBS. To prepare high molecular
weight linear and branched PBSs, we first synthesized linear
PBS prepolymers with hydroxyl groups or carboxylic acid
groups at both ends of the polymer chains through a two-
step process shown in Scheme I. In the first stage, low
molecular weight linear PBS (P1 in Table I) was prepared
from the polyesterification of 1,4-butanediol and succnic
1
H-NMR spectrum of the hydroxyl group terminated PBS
prepolymer had three kinds of methylene peaks existing in
the polymer backbone at 4.1, 2.6, and 1.7 ppm, respectively:
one is due to two methylene groups next to the ether linkage
in the polymer and the other to two methylene groups next
to the ketone and another to two methylene groups in the
middle of the 1,4-butanediol monomer. Their hydrogen
integrations matched well with the structure of the PBS.
o
acid in the presence of TIP as a catalyst at 220 C under
reduced pressure of 2.0 torr. In the second polymerization
Glass transition temperature (T ) and melting temperature
g
o
o
stage proceeding at 230 C under reduced pressure of 0.5
(T ) of the prepolymers were 40 and 112 C, respectively.
m
torr, raising the polymerization time from 2 to 6 h increased
the weight-average molecular weight of linear PBS prepoly-
mers from 68,700 (P3) to 167,700 (P4) with the time, which
are listed in Table I.
The prepolymers having weight-average molecular weight s
of below 100,000 showed similar transition temperatures.
Based on these results, we used the four kinds of hydroxyl
and carboxylic acid groups terminated linear PBS prepoly-
mers with different molecular weights as a substrate polymer
After the melt polymerization, the linear prepolymers
Macromol. Res., Vol. 10, No. 2, 2002
111

Y.-K. Han et al.
for the synthesis of high molecular weight PBSs with a linear
or branched structure.
(1.0% weight loss) temperature of the P6 improved from
o
244 of P1 to 289 C.
Preparation of High Molecular Weight Linear PBS. To
prepare PBSs with the weight-average molecular weights of
over 100,000, we carried out a coupling reaction between
the prepolymers with hydroxyl groups or the carboxylic
acid groups at both ends. That is, we examined the influence
of the temperature and the molecular weight of the prepoly-
mers on the molecular weight of linear PBS, as shown in
Table II. The weight-average molecular weight of the linear
PBS increased from 134,400 to 213,300 with the increase in
Preparation of High Molecular Weight Branched
PBS. Recently, branched polymers have been reported to
have superior melt strength and melt viscosity compared with
linear polymers. In particular, the Introduction of branching
to the linear polymers proved effective in enhancing pro-
cessability based on elongational flows. A few of these pro-
cesses include fiber spinning, film blowing and casting, and
foaming. Branching increases the melt strength and confers
tension-hardening properties on the polymer, which help to
maintain extension in the polymer processing in which a
o
the temperature of the coupling reaction from 210 to 230 C.
1
3,14
From Table II, we found that the molecular weight of the
prepared linear PBS closely related to the molecular weight
of the prepolymers selected for the coupling reaction. In
other words, when the prepolymers P1 and P3 were used as
substrate polymers, synthesized linear PBS had a high
molecular weight of over 200,000. However, the molecular
weight of the prepared PBS decreased rather than the sum
of the molecular weight of each of the prepolymers, when
the prepolymers P2 and P4 with a high molecular weight
were chosen. It is most likely that such difference is due to
mobility of the prepolymers as well as relatively high con-
centrations of the hydroxyl and carboxylic acid groups in
the prepolymers. That is, the prepolymer P1 of a low molec-
ular weight plays a role of a chain extender in the coupling
reaction between P1 and P3 prepolymers. On the other
hand, the prepolymers P2 and P4 with a relatively high
molecular weight have not only lower concentration of the
functional groups at both ends of the prepolymers but also
slower mobility than P1 and P3. They cause decrease of the
molecular weight of the resulting linear PBS. In addition,
transesterification reaction between the prepolymers also
gets reduced the molecular weight of the linear PBS.
high degree of orientation is required.
On the basis of these merits, it is proposed that branched
aliphatic polyesters may be useful in preparing many blow-
molded, extruded, or foamed products, by themselves or as
an additive to the linear polyesters. For an example, the lin-
ear polymers may be modified by blending with the
branched polymers to improve their properties. However,
few researches have been conducted on the synthesis and
1
5-17
characterization of branched polyesters.
In the present work we tried to synthesize the branched
PBS polymers with weight-average molecular weight of
over 100,000 through a branching reaction between the car-
boxylic acid group terminated PBS prepolymer (P3) and
branching agent. As a branching agent, we used glycerol
that has three hydroxyl groups. In addition, we examined
the influence of the reaction parameters such as concentration
of glycerol, reaction temperature, and concentration of cata-
lyst on the molecular weight of branched polymers.
Effect of Concentration of Glycerol. To establish the
influence of the branching agent on the synthesis of high
molecular weight branched PBS, we measured the change
of the molecular weight of the branched polymer with the
concentration of glycerol as a branching agent. As shown in
Table III, the molecular weight of the branched PBA in-
creased from 102,100 (PG1) to 146,100 (PG3) with increase
in the glycerol concentration up to 0.4 wt% in terms of the
From a DSC measurement, the high molecular weight lin-
o
o
ear PBS had higher T (115 C) and T (45 C) than the pre-
m
g
polymer P1. Figure 2 shows TGA curves of the linear
polymer P6 and the P1. As a result, the initial decomposition
o
prepolymer P3. When we used more than 0.4 wt% at 200 C,
however, the molecular weight remarkably decreased to
1
06,800 (PG4).
Such phenomenon appeared even at the condition of the
o
branching temperature of 210 C: On the other hand, raising
the reaction temperature to 210 C increased not only the
o
insoluble gel content generating during the branching reaction
but also the molecular weight from 106,900 to 219,600 with
increase in the glycerol content up to 0.4 wt%. However,
increasing the glycerol concentration to 0.5 wt% decreased
the molecular weight as well as the gel creation. The reason
is not clear yet, but their drastic decrease seems to be attrib-
utable to the increase of the number of glycerol molecules
added as a branching agent. In other words, the number of
glycerol molecules compared with the carboxylic acid group
terminated PBS chains was too many, and the intermolecular
Figure 2. TGA curves of different molecular weight linear PBSs.
112
Macromol. Res., Vol. 10, No. 2, 2002

High Molecular Weight PBS
Table IV. Effect of Catalyst Concentration on Molecular Weight of Branched PBS by a Branching Reactiona
Branched
PBS
TIP
Conversion
(%)
Gelation
(%)
η
Mn
Mw
PDI
(wt% to PBS)
PG 9
PG10
PG 6
PG11
PG12
PG13
0.3
0.4
0.5
0.6
0.7
0.8
84
80
83
70
84
34
0.83
1.24
1.28
1.74
1.54
1.28
30,500
53,500
60,300
102,100
84,500
60,300
93,000
126,800
147,600
206,000
181,400
147,600
3.05
2.37
2.45
2.01
2.15
2.45
-
-
8.3
22
32
45
a
Carboxylic acid group terminated prepolymer P3 was used as a substrate polymer for a branching reaction. The branching reaction was carried
o
out at 210 C for 2 h in the presence of glycerol of 0.3 wt% to the prepolymer P3.
reaction between low molecular weight branched polymer
chains necessary for preparing high molecular weight bran-
ched PBS hardly occurred. Such intermolecular branching
reaction gives rise to the gel creation. These phenomena are
ascertained by the fact that the polydispersity of the pre-
pared branched PBS decreases from 3.47 to 2.06 as the
glycerol concentration increases from 0.2 to 0.5 wt%.
Thus, we found that the concentration of the glycerol and
the branching reaction temperature has a strong effect on the
molecular weight of the branched PBS and gelation
Effect of Concentration of Catalyst. We performed the
branching reaction by varying the concentration of catalyst
TIP to the carboxylic acid group terminated prepolymer P3,
keeping the glycerol concentration and reaction temperature
constant to investigate the influence of the catalyst concen-
tration on the molecular weight of the resulting branched
polymer. As summarized in Table IV, the molecular weight
of the branched PBS increased with increase in the concen-
tration of TIP up to 0.6 wt%. However, when more than 0.6
wt% of TIP was used, the molecular weight drastically
decreased, meanwhile, the color of the branched polymer
became more and more yellowish. At 0.8 wt%, the insoluble
gel, crosslinked PBS, was obtained up to 45% during the
branching reaction. This phenomenon is due to the increased
reactivity between the carboxylic acid group linked to the
end groups of the PBS prepolymer and the hydroxyl group
of the branching agent. Therefore, it is advantageous to fix
the concentration of TIP of below 0.6 wt% to avoid the
color change of the polymer and repress the gel creation.
Thermal Properties of Linear and Branched PBAs.
Figure 3. DSC thermograms of high molecular weight linear
and branched PBSs.
DSC and TGA measurements were employed to observe
the thermal behavior and stability of high molecular weight
linear and branched PBS polymers. As shown in Figure 3,
o
the branched PBS had higher by about 10 C T than the lin-
g
ear one, whereas its T was almost the same as the linear
m
PBS. The thermal stability of the high molecular weight
polymers showed no obvious difference regardless of the
linear or branched structure and their molecular weights.
The initial decomposition temperature of 1.0% weight loss
o
was in the range of 320-324 C.
Mechanical Properties of Linear and Branched PBS
Polymers. Tensile properties of the linear and branched
PBSs are given in Table V. In stress-strain curves, all the
a
Table V. Tensile Properties of PBS Films
PBS
P2
I.V
Thickness (μm)
M
w
Modulus (MPa) Tensile Strength (MPa) Elongation at Break (%)
1.01
1.93
1.04
1.96
25
25
25
30
123,400
213,300
126,100
219,600
204
508
170
180
14
22
17
24
55
111
55
P6
PG5
PG7
120
a
o
Tensile test were performed on a strain rate of 10 mm/min with a load cell of 10 N at 20 C.
Macromol. Res., Vol. 10, No. 2, 2002
113

Y.-K. Han et al.
linear and branched PBSs were observed to decrease in
the linear PBS with similar molecular weight.
tensile strength at the yield point and thereafter elongate.
This demonstrates that the synthesized PBSs have tough-
ductile mechanical properties. The tensile strength at break
as well as modulus increased as the molecular weight of
PBS increased in both linear and branched structures. Com-
paring the moduli of the PBSs, however, we observed that
the modulus of the linear polymers (P2 and P6) before and
after the coupling reaction was always higher than that of
the branched resulting polymers (PG5 and PG7) although
their molecular weights had a very similar value. In particu-
lar, the high molecular weight linear PBS (P6) had about 2.5
times higher value in modulus than the branched one (PG7).
As can be imagined, the branched PBS has a lower packing
density because branched points produce more free volume.
Hence, the branched PBS gives rise to lower tensile modulus
Acknowledgement. The authors acknowledge financial
support from the Center for Advanced Functional Polymers,
KAIST. This work also was supported in part by the
research program of 2000, Hanyang University.
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than the linear one.
(
The elongation at break of the high molecular weight
branched PBS (120%) had a slightly greater value than that
of the linear polymer (111%). Thus, the branched polymer
had better toughness than the linear one.
2
Conclusions
We synthesized high molecular weight linear and
branched PBS polymers through a coupling reaction between
the PBA prepolymers with hydroxyl groups and carboxylic
acid groups at both ends or a branching reaction between
the hydroxyl group terminated prepolymer and glycerol, a
branching agent. The molecular weight of the linear PBS
polymers was dependent not only on the molecular weight
of the hydroxyl and carboxylic acid groups terminated pre-
polymers but also the temperature for the coupling reaction.
The molecular weight of the branched PBSs also was gov-
erned by the concentration of catalyst and branching agent
as well as by the reaction temperature of the branching
reaction. Among the conditions, especially, the reaction
temperature greatly affected the molecular weight of the
branched polymer and the gel content of generation during
the branching reaction. The mechanical properties improved
with the introduction of the branching into the linear PBS
chain. In particular, the branched PBS showed better prop-
erties in the tensile strength and the elongation at break than
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5
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(
(
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(
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5
229 (1999).
(
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143 (2001).
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114
Macromol. Res., Vol. 10, No. 2, 2002