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Polymer International 42 (1997) 373È379
Preparation, Characterization and
Biodegradable Characteristics of
Poly(D,L-lactide-co -1,3-trimethylene
carbonate)
Cai Jie & K. J. Zhu*
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PeopleÏs Republic of China
(Received 23 July 1996 ; accepted 23 October 1996)
Abstract : Poly(D,L-lactide-co-1,3-trimethylene carbonate) (PLCA) has been synthesized by ring-opening polymerization of 1,3-trimethylene carbonate (CA) and
D,L-lactide (LA) using stannous octoate as catalyst. The copolymers were characterized by 1H nuclear magnetic resonance (NMR), 13C NMR and di†erential
scanning calorimetry. Water content and static contact angle of distilled water
on the polymer surface were used to evaluate hydrophobicity of the polymers. It
was found that the hydrophobicity increased with increasing CA fractions in the
copolymers. Biodegradation experiments were conducted in vitro and in vivo. The
results indicated that the biodegradation behaviour changed from surface to bulk
degradation when the LA content exceeded 30 mol% in the copolymers. These
properties of PLCA may be useful in protein delivery systems.
Key words : poly(D,L-lactide-co-1,3-trimethylene carbonate), polymer biodegradation, protein-controlled release matrix
chemical bonds in the polymer main-chains may be
useful in protein delivery systems. When the polymer is
mixed with protein and made into a device, the matrix
degrades from the surface in vivo, and the protein of the
surface layer is released simultaneously. Since there is
low permeability of water in the highly hydrophobic
polymer, the protein inside the matrix can be kept in a
stable solid form. Thus the protein bioactivity may be
maintained in the release process. For many polymers,
degradation is a very complicated process that depends
on many factors, including polymer chain scission, crystallinity, polymer chain length and water di†usivity.
Polyanhydrides and polyorthoesters, as surface degradation polymers, have been studied extensively.10h15
Zhu et al.16 reported that poly(1,3-trimethylene
carbonate) (PCA) showed surface biodegradable character in vivo, but degradation was very slow. In order to
improve this material, a series of CA-containing copolymers have been prepared and the degradation characteristics have been explored in our laboratory recently.
In this paper, we report some preliminary results on the
INTRODUCTION
There have been many papers concerning polymers
which can be applied in drug delivery systems.1,2
Among them, biodegradable polymers have received
more attention, their main advantage being that no
retrieval of the materials is needed.3h7 With the development of biotechnology, many peptides and proteins
have been reÐned, which are very useful in livestock
farming and human therapy.8 However, these drugs are
usually unstable in the body. Maintaining their bioactivities during the release process has become a key
problem in protein-controlled release systems.9 Polylactide (PLA) and poly(lactide-co-glycolide) (PLGA) have
been studied by many researchers. The bulk degradation characteristics of these materials, however, are
unfavourable for maintaining protein bioactivity in the
release process. On the other hand, surface biodegradable polymers with high hydrophobicity and unstable
* To whom all correspondence should be addressed.
373
Polymer International 0959-8103/97/$09.00 ( 1997 SCI. Printed in Great Britain
C. Jie, K. J. Zhu
374
preparation and biodegradation properties
poly(lactide-co-1,3-trimethylene carbonate) (PLCA).
of
EXPERIMENTAL
Differential scanning calorimetry (DSC )
measurements
Samples of 10È15 mg were heated to melting in aluminium pans with inverted lids on a Perkin Elmer
DSC-7 Thermal Analyzer with a heating rate of
20¡C min~1. The instrument was calibrated with cyclohexane and indium prior to use.
Monomers
1,3-Trimethylene carbonate (CA) was synthesized
according to the literature.17 1,3-Propanediol (60É8 g),
diethyl carbonate (114 g) and a small cube of sodium
metal were warmed until the sodium had completely
dissolved, and then the mixture was reÑuxed at 130¡C
for 4 h. Ethanol and residual diethyl carbonate were distilled out and the oil residue was dissolved in benzene,
washed with water and then dried over calcium chloride. After removal of the solvent, the crude product
was obtained by vacuum distillation (160È165¡C/
6 mmHg) and then recrystallized from absolute ether
(melting point 46È47¡C).
D,L-Lactide (LA) was prepared by a method similar
to that described in Ref. 18. Five-hundred grams of
lactic acid was mixed with 2È5 wt% zinc oxide in a
three-necked Ñask, and the temperature raised to 120¡C.
As the rate of water elimination fell, the temperature
increased to 180¡C and the pressure reduced gradually
from 760 to 20 mmHg over a period of 4È6 h. When no
more steam was evolved, the vacuum increased to 0É5È
1É0 mmHg. The crude product was distilled out and
then recrystallized from ethyl acetate. White crystalline
LA with a melting point of 125È127¡C was obtained.
Copolymerization
CA and LA were copolymerized in bulk. Monomers
and catalyst/petroleum ether solution were introduced
to a thoroughly cleaned and dried vessel. The solvent
was removed in vacuum and the vessel was heated to
140¡C for 36 h. The product was dissolved in methylene
chloride and precipitated from methanol. The molecular
weight of the polymers was determined by gel permeation chromatography (GPC ; Waters 208 with 103,
104 and 105 ultrastyragel columns). Samples were eluted
in tetrahydrofuran (THF) at a Ñow rate of 1É5 ml min~1
at 25¡C.
Polymer hydrophobicity evaluation
Water content and static contact angle of distilled water
on the polymer surface were used to evaluate the hydrophobicity. Water content was deÐned as percentage of
water in saturated polymer. It was measured gravimetrically after polymer had been immersed in distilled
water to equilibrium at room temperature. Static
contact angles were measured by a contact angle meter
(JY-82). The polymer Ðlms were made on silanized glass
microscopy slides. The coating thickness was about
2 km. Static contact angles were measured at 25¡C on
proÐles of sessile drops using a microscope with a Ðxed
goniometer eyepiece, magniÐcation 20]. Readings were
taken within 10È15 s ; average drop size was 0É05 ml.
Angles were measured on six di†erent regions of each
polymer surface and the results were averaged.
Polymer degradation
Samples with cylindrical shape (2É5 mm in diameter,
7 mm in length) were made by compression in a mould
at 40¡C for 5 min under a pressure of 100 kg cm~2.
Hydrolysis of the polymers in vitro was measured by
immersion of samples in phosphate bu†er solution (pH
7É4) at 37¡C. The samples were recovered periodically
and the molecular weight changes were determined by
GPC.
In vivo degradation was tested in rats. Polymer
samples were weighed and sterilized by dipping in 70%
ethanol solution before implanting subdermally in adult
kung ming rats in the scapular area lateral to the dorsal
midline. At time intervals animals were sacriÐced and
the polymers were recovered. The samples were freed
from adhering tissues, then rinsed with distilled water
and dried. Weight loss of the matrix was determined
gravimetrically and the molecular weight changes of
inner bulk and outer surface layer were measured by
GPC.
Nuclear magnetic resonance (NMR ) measurements
A JEOL 90Q NMR instrument was employed for
NMR measurements using CDCl as solvent. 1H NMR
3
measurements were conducted in 5 mm o.d. sample
tubes with Me Si as internal shift reference at
4
89É55 MHz and in 10 mm o.d. sample tubes at
22É49 MHz for 13C NMR measurements.
RESULTS AND DISCUSSION
Polymer synthesis
The polymerization of aliphatic polyesters, such as
polylactide (PLA) and poly(caprolactone) (PCL), has
POLYMER INTERNATIONAL VOL. 42, NO. 4, 1997
Properties of PL CA
375
TABLE 1. Some data of 1,3-trimethylene carbonate (CA) and D,Llactide (LA) copolymerizationa
Catalyst conc.
(M/I, mol)
Temperature
(¡C)
Time
(h)
Yield
(%)
M1
n
(Ã104)
M1 /M1
w n
100
250
500
1000
500
500
500
500
140
140
140
140
140
120
160
140
36
36
36
36
20
36
36
48
84
85
80
75
65
73
86
87
1·85
2·02
1·95
1·73
1·78
1·86
1·72
1·65
2·0
2·1
2·0
1·8
1·9
1·9
2·0
1·9
a CA : LA ¼ 85·15 (by mol) in feed, Sn(oct) as catalyst. M1 and M1 /M1
n
w n
2
were measured by GPC using THF as solvent at 25¡C.
been studied extensively.19 However, the copolymerization of CA and LA has not been reported so far. We
synthesized CA and LA copolymers at 140¡C over 36 h
using stannous octoate as catalyst ; some data are
shown in Table 1. It can be seen that the yield and
molecular weight (MW) increased with increasing reaction time, but over 40 h MW decreased. When the reaction temperature was higher than 140¡C, MW also
decreased and the product became brown. The yield
and MW decreased slightly with increasing catalyst
concentration in the range examined (M/I, molar ratio,
from 100 to 1000). All the copolymers (PLCA15,
PLCA30, PLCA50 and PLCA60, the numbers representing the approximate molar percentage of LA in
the copolymer) were soluble in CH Cl , CHCl , THF
2 2
3
and dioxane, but insoluble in alcohol and ether. The
GPC proÐles of the copolymers showed symmetrical
single peaks.
In comparison with 1H and 13C NMR spectra of
PCA and PLA,16,17 the peaks at 2É10 and 4É25 ppm in
the 1H NMR spectrum of PLCA can be assigned to
wCH w and wOwCH w of CA units, and the peaks
2
2
at 1É65 and 5É20 ppm to wCH and xCHw of LA
3
units. In the 13C NMR spectrum of PLCA, the peaks at
16É7, 70É0 and 169É4 ppm correspond to wCH ,
3
xCHw and CO of LA units, respectively, and 28É0,
64É5 and 155É0 ppm to wCH w, wOCH w and CO of
2
2
CA units, respectively.
Fig. 1. The relationship between hydrophobicity and composition of PLCA copolymers. (A) Water content, which was measured
gravimetrically after samples were immersed in distilled water to equilibrium at room temperature. (B) Static contact angle of
distilled water on the polymer surface, which was measured by a contact angle meter at 25¡C ; the average drop size was 0É05 ml
and the readings were taken within 10È15 s.
POLYMER INTERNATIONAL VOL. 42, NO. 4, 1997
C. Jie, K. J. Zhu
376
Fig. 2. Di†erence in molecular weight between inner bulk and surface layers of PLCA copolymers for in vitro test (M1 and M1
n
no
were determined by GPC in THF at 25¡C ; M1 \ initial molecular weight of samples). (a) PLCA15 and PLCA30 copolymers ; (b)
no
PLCA50 and PLCA60 copolymers.
TABLE 2. DSC data of poly(D,L-lactide-co -1,3trimethylene) (PLCA)a
Sample
Composition
(LA mol%)
T (¡C)
g
PCA
PLCA15
PLCA30
PLCA50
PLCA60
PLA
0
15·3
30·2
49·7
60·4
100
É 26
É 18
É8
9
18
56
a T s were obtained by DSC measurements with a heating
g
rate of 20¡C minÉ1 using a Perkin Elmer DSC-7 instrument.
The compositions were determined by 1H NMR.
The reactivity ratios of LA and CA (r and r ) were
L
C
calculated according to the following equation :18
M/N \ (r m/n ] n)/(r n/m ] n)
(1)
L
c
where m and n are feed compositions of LA and CA,
and M and N are compositions of the copolymers
determined by 1H NMR. The values of r and r
L
C
obtained were 1É03 and 0É76, respectively.
Polymer hydrophobicity
The water content and static contact angles of PLCA
copolymers are shown in Fig. 1. Both have good linear
relationships with copolymer composition, that is, the
POLYMER INTERNATIONAL VOL. 42, NO. 4, 1997
Properties of PL CA
377
Fig. 3. Di†erence in molecular weight between inner bulk and surface layers of PLCA copolymers for in vivo test (M1 and M1
n
no
were determined by GPC in THF at 25¡C ; M1 \ initial molecular weight of samples). (a) PLCA15 and PLCA30 copolymers ; (b)
no
PLCA50 and PLCA60 copolymers.
hydrophobicity decreases with increasing LA fraction in
the copolymers. DSC studies indicate that all of the
copolymers are amorphous and have only one glass
transition temperature, T (miscible material) (Table 2).
g
Thus, the change of water content and static contact
angle of the copolymers can be attributed to the di†erent hydrophobicity of the LA and CA segments, since
the water absorption of PLA is 5É2%, which is higher
than that of PCA (1É8%).
Polymer degradation
The degradation of PLCA copolymers in vitro and in
vivo was evaluated by changes in molecular weight of
POLYMER INTERNATIONAL VOL. 42, NO. 4, 1997
the inner bulk and outer surface layers, and in the
weight loss of the polymers. The low dimensional stability and deformation of the samples recovered (because
of their low T ) prevented precise sampling of surface
g
and bulk. Therefore, the recovered samples were
extracted with CHCl for 5 min to selectively separate
3
the surface fraction (about 3 wt% of sample). Control
experiments showed that CHCl , a poor solvent for
3
PLCA copolymers, slowly dissolved the surface layer of
the polymer and did not selectively leach oligomeric
components. It was found that the di†erence in molecular weight between inner bulk and surface layers
increased with decreasing LA content in the copolymers
both in vitro and in vivo (Figs 2 and 3). There are large
C. Jie, K. J. Zhu
378
di†erences in molecular weight for smaller LA content
of PLCA, such as PLCA15 and PLCA30, but almost
identical results for higher LA content in the copolymers, such as PLCA50 and PLCA60. Thus, the former
show surface degradation character, and the latter bulk
degradation character over the test period. The critical
transition composition for surface to bulk degradation
is around 40 mol% LA in the copolymers.
The degradation of the polymer matrix can be
divided into surface and bulk degradation depending on
factors such as matrix hydrophobicity, water penetration rate and polymer chain scission rate.13,19 In the
case of PLCA degradation, the copolymers containing
more LA have less hydrophobicity and water di†usion
is relatively easy. Considering the degradation mechanism of PLA and PCA, as shown in kinetic eqns (2) and
(3),16,20 an autocatalysation by degraded acid products
existed in PLA degradation, but this e†ect was not
important in PCA because of the much lower amount
of acid from degraded products (CO ). These factors are
2
favourable for bulk degradation of copolymers containing more LA.
PLCA60 (16% for PLCA15, 0É4% for PLCA60). The
change of composition of PLCA15 in the degradation
process indicates that the CA content decreased from
85 mol% at the initial stage to 80 mol% at 40 days for
the in vivo test, but there was no apparent change for
the in vitro test. This enzyme attack on the surface of
CA segments additionally favours the surface degradation of PLCA copolymers with greater CA content.
Figure 4 shows weight loss of PLCA copolymers in
the in vivo test. It can be seen that PLCA15 and
PLCA30 lost weight gradually and almost linearly. This
is another characteristic of surface degradation of polymers. For PLCA50 and PLCA60, the matrix weight
changed slightly initially, but after 40 days, weight loss
increased rapidly. This phenomenon is consistent with
the fact that bulk degradation chain cleavage of the
polymer backbone proceeds until a critical MW is
reached, after which weight loss occurs by the di†usion
of water-soluble cleavage fragments out of the matrix.21
[d[ester]/dt \ k[H O][COOH][ester]
(2)
2
[d[carbonate]/dt \ k[H O][carbonate]
(3)
2
Each PLCA copolymer has a similar degradation
pattern in vitro and in vivo, but the degradation rate in
vivo is higher than that in vitro. This may be attributed
to enzyme attack on CA segments of the matrix.16 The
evidence in our experiments is that by comparing
M1 /M1 data of the outer surface layers of the copolyn no
mers for in vitro and in vivo degradation at 90 days, the
discrepancy for PLCA15 is much larger than that for
The copolymer, PLCA, can be synthesized by ringopening polymerization of LA and CA with stannous
octoate. Hydrophobicity of the copolymers decreased
with increasing LA fraction in the copolymers. It was
found that the copolymers containing 30 mol% or less
LA showed surface degradation characteristics in both
in vitro and in vivo tests. For LA contents exceeding
30 mol% the copolymers had bulk degradation behaviour. Enzyme e†ects may play an important role in the
degradation process in vivo for the copolymers containing more CA.
CONCLUSIONS
Fig. 4. Weight loss of PLCA15, PLCA30, PLCA50 and PLCA60 copolymers for in vivo test, which was determined gravimetrically
with recovered samples.
POLYMER INTERNATIONAL VOL. 42, NO. 4, 1997
Properties of PL CA
ACKNOWLEDGEMENTS
We are indebted to the National Natural Science Foundation of China for their support of this work. We also
thank Ms Qiu Liyan for her assistance in the animal
tests.
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