Efficiency of liquid tin(ii): N -alkoxide initiators in the ring-opening polymerization of l-lactide: Kinetic studies by non-isothermal differential scanning calorimetry

Article abstract of DOI:10.1039/d0ra07635j

Novel soluble liquid tin(ii) n-butoxide (Sn(OnC4H9)2), tin(ii) n-hexoxide (Sn(OnC6H13)2), and tin(ii) n-octoxide (Sn(OnC8H17)2) initiators were synthesized for use as coordination-insertion initiators in the bulk ring-opening polymerization (ROP) of l-lac

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Efficiency of liquid tin(II) n-alkoxide initiators in the
ring-opening polymerization of L-lactide: kinetic
studies by non-isothermal differential scanning
calorimetry†
Cite this: RSC Adv., 2020, 10, 43566
abc
abd
Montira Sriyai,
and Winita Punyodom
Tawan Chaiwon,ac Robert Molloy,d Puttinan Meepowpan
abd
*
Novel soluble liquid tin(II) n-butoxide (Sn(OnC4H9)2), tin(II) n-hexoxide (Sn(OnC6H13)2), and tin(II) n-octoxide
(Sn(OnC8H17)2) initiators were synthesized for use as coordination–insertion initiators in the bulk ring-
opening polymerization (ROP) of L-lactide (LLA). In order to compare their efficiencies with the more
commonly used tin(II) 2-ethylhexanoate (stannous octoate, Sn(Oct)2) and conventional tin(II) octoate/n-
alcohol (SnOct2/nROH) initiating systems, kinetic parameters derived from monomer conversion data
were obtained from non-isothermal differential scanning calorimetry (DSC). In this work, the three non-
isothermal DSC kinetic approaches including dynamic (Kissinger, Flynn–Wall, and Ozawa);
isoconversional (Friedman, Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW)); and
Borchardt and Daniels (B/D) methods of data analysis were compared. The kinetic results showed that,
under the same conditions, the rate of polymerization for the 7 initiators/initiating systems was in the
order of liquid Sn(OnC4H9)2 > Sn(Oct)2/nC4H9OH > Sn(Oct)2 y liquid Sn(OnC6H13 2
)
> Sn(Oct)2/
nC6H13OH y liquid Sn(OnC8H17 2
)
> Sn(Oct)2/nC8H17OH. The lowest activation energies (Ea ¼ 52, 59, and
Received 6th September 2020
Accepted 11th November 2020
56 kJ molꢀ1 for the Kissinger, Flynn–Wall, and Ozawa dynamic methods; Ea ¼ 53–60, 55–58, and 60–
62 kJ molꢀ1 for the Friedman, KAS, and OFW isoconversional methods; and Ea ¼ 76–84 kJ molꢀ1 for the
B/D) were found in the polymerizations using the novel liquid Sn(OnC4H9)2 as the initiator, thereby
showing it to be the most efficient initiator in the ROP of L-lactide.
DOI: 10.1039/d0ra07635j
rsc.li/rsc-advances
devices.1–5 Poly(lactic acid) (PLA) can be produced from lactic acid
derived from the fermentation of renewable starch-containing
resources such as corn, sugar beet, and cassava.6–8 Lactide (L)
monomer is prepared by condensing lactic acid to low molecular
weight PLA which is then thermally cracked to yield the six-
membered ring lactide monomer. The crude lactide monomer
can be puried by repeated recrystallization before being poly-
merized in bulk to form high molecular weight PLA.9
Tin(II) 2-ethylhexanoate or stannous octoate (Sn(Oct)2) is the
most common initiator used in the ROP of lactones and lactides
due to its effectiveness and versatility. It is also easy to handle
and is soluble in most common organic solvents and mono-
mers.10–12 Moreover, it has been approved for use as a food
additive by the US Food and Drug Administration (FDA).
However, in order to improve its effectiveness, Sn(Oct)2 is
usually used in combination with an alcohol co-initiator (ROH)
as the initiating system.
1. Introduction
Biodegradable aliphatic polyesters have attracted much interest
as replacements for petroleum-based materials due to their
environmentally friendly properties and derivability from
renewable resources. The most popular and important biode-
gradable aliphatic polyesters are poly(L-lactide) (PLLA), poly-
glycolide (PGA), poly(3-caprolactone) (PCL), and their other high
molecular weight co- or terpolymers. Among these, PLA-based
materials are of the most interest due to their biocompati-
bility and biodegradability in a broad range of applications,
especially biomedical applications such as absorbable surgical
sutures, controlled drug delivery systems, and bone xation
aDepartment of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai
50200, Thailand
bCenter of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science,
Mechanistically, Kricheldorf and co-workers proposed the
coordination of the Sn(Oct)2 and the alcohol with the cyclic
ester monomer followed by ROP.13 In this mechanism, the
Sn(Oct)2 serves as a catalyst while the alcohol is an initiator, as
depicted in Fig. 1(a). However, Penczek et al. later suggested
Chiang Mai University, Chiang Mai 50200, Thailand
cGraduate School, Chiang Mai University, Chiang Mai 50200, Thailand
dCenter of Excellence in Materials Science and Technology, Chiang Mai University,
Chiang Mai 50200, Thailand. E-mail: winitacmu@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra07635j
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Fig.
1 Comparison of the old (a) and the new (b) mechanistic
proposals using Sn(Oct)2: (a) complexation of the monomer and
alcohol prior to ROP and (b) formation of tin(II) alkoxide before
ROP.13,14
that the Sn(Oct)2 and the alcohol react together resulting in the
formation of a tin(II) monoalkoxide ((Oct)Sn(OR)) and tin(II)
dialkoxide (Sn(OR)2) which then become the ‘true’ initiators as
shown in Fig. 1(b).14 This latter mechanism is now widely
accepted as the true initiation pathway. Therefore, in order to
produce high molecular weight polyesters via ROP, it is vitally
important to know the exact concentration of the tin(II) alkoxide
initiator which should be easily and completely soluble in the Fig. 3 Chemical structures of: tin(II) octoate; tin(II) n-butoxide; tin(II) n-
hexoxide; and tin(II) n-octoxide initiators.
cyclic ester monomer.
As previously mentioned, it is now widely accepted that the
tin(II) monoalkoxide, (Oct)Sn(OR), and/or the dialkoxide,
The synthesis of solid tin(II) alkoxides has been known for
over 50 years.15,16 However, they have been found to have their
drawbacks as initiators, in particular their difficult solubility in
cyclic ester monomers and common organic solvents and their
instability in contact with air and moisture which is caused by
Sn(OR)2, are the true initiators formed in situ via the reaction
between Sn(Oct)2 and ROH. Since the coupled reactions in
Fig. 1(b) are both reversible and interdependent, the exact
initiator concentrations will be unknown. Moreover, stannous
octoate is well known to be an effective transesterication
catalyst, ROH can act as a chain transfer agent, and the octanoic
acid (OctH) by-product can also catalyze other unwanted side-
reactions. Thus, the kinetics of the ROP and the nal molec-
ular weight of the polymer cannot be accurately predicted.
Therefore, in order to overcome this problem, it is logical to
synthesize the tin(II) dialkoxide (Sn(OR)2) separately so that it
can be used directly in an accurately known concentration.
Fig. 2 Molecular self-aggregations in solid tin(II) alkoxides: (a) angular Fig. 4 Idealized DSC curve for kinetic parameters determination by
aggregation and (b) linear aggregation.
using Borchardt and Daniels (B/D) method.
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Table 1 Polymer codes and conditions used in the ROP of L-lactide
using liquid Sn(OnC4H9)2 as an initiator
[Sn(OnC4H9)2]
(mol%)
Temperature
(ꢁC)
Polymerization time
(h)
Polymer code
PLLA 1
PLLA 2
PLLA 3
PLLA 4
PLLA 5
PLLA 6
PLLA 7
PLLA 8
0.01
0.01
0.01
0.01
0.01
0.01
0.05
0.05
0.05
0.05
0.05
0.05
0.10
0.10
0.10
0.10
0.10
0.10
0.50
0.50
0.50
0.50
0.50
0.50
1.00
1.00
1.00
1.00
1.00
1.00
100
110
120
130
140
150
100
110
120
130
140
150
100
110
120
130
140
150
100
110
120
130
140
150
100
110
120
130
140
150
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
PLLA 9
PLLA 10
PLLA 11
PLLA 12
PLLA 13
PLLA 14
PLLA 15
PLLA 16
PLLA 17
PLLA 18
PLLA 19
PLLA 20
PLLA 21
PLLA 22
PLLA 23
PLLA 24
PLLA 25
PLLA 26
PLLA 27
PLLA 28
PLLA 29
PLLA 30
their solid-state molecular self-aggregations as shown in
Fig. 2(a) and (b). According to their low solubility in most
organic solvents and cyclic ester monomers of these solid tin(II)
alkoxides, the polymerizations of monomers such as L-lactide,
D-lactide, DL-lactide, 3-caprolactone and other cyclic esters are
relatively slow and ineffective.
Consequently, in this work, some novel liquid soluble tin(II)
alkoxides were prepared for use in the ROP of L-lactide in order
to overcome the molecular self-aggregation difficulties found in
previous works with solid tin(II) alkoxides. Meepowpan and co-
workers have proposed a method for synthesizing soluble liquid
Sn(OR)2 using stoichiometric amounts of anhydrous tin(II)
chloride (SnCl2), diethylamine ((C2H5)2NH), and an alcohol
(ROH).17 Because the liquid Sn(OR)2 is completely soluble, it
leads to more reproducible results which, in turn, leads to
a more detailed understanding of the kinetics and mechanism
of the ROP reaction. Using the Sn(OR)2 initiator directly instead
of generating it in situ should give more reproducible and
predictable results in terms of both kinetic and molecular
weight control compared with the Sn(Oct)2/ROH initiating
system or Sn(Oct)2 alone.
Fig. 5 DSC thermograms of normalized heat flow (W gꢀ1) against
temperature (ꢁC) for the ROP of L-lactide using: (a) 1.0 mol% Sn(Oct)2;
(b) 1.0 mol% liquid Sn(OnC4H9)2; and (c) 1.0/2.0 mol% Sn(Oct)2/
nC4H9OH as initiators and initiating system at heating rates of (ꢀ) 5, (ꢀ)
10, (ꢀ) 15, and (ꢀ) 20 ꢁC minꢀ1
.
as a function of time. This can be done by a variety of methods
such as dilatometry, gravimetry, proton-nuclear magnetic reso-
nance (1H-NMR) spectroscopy, infrared (IR) spectroscopy, Raman
spectroscopy, and differential scanning calorimetry (DSC).18–23
Among those aforementioned methods, DSC has been found
to be a fast and convenient method for studying polymerization
In order to investigate the efficiency of the liquid tin(II)
alkoxide initiators, the kinetics of the ROP of LA were investi-
gated by measuring the decrease in the monomer concentration
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liquid and solid cyclic ester monomers. Furthermore, with the
advent of convenient soware-based data analysis programs,
the ability to obtain such kinetic information has become more
practical compared to other techniques.24–27
DSC kinetic experiments can be performed under either
isothermal or non-isothermal conditions. In isothermal DSC, the
polymerization is conducted at a constant temperature while in
non-isothermal DSC, polymerization occurs during a tempera-
ture scan at a constant heating rate. The conversion of monomer
to polymer can be determined from the amount of heat released
from the reaction at any time t (DHt) divided by the total heat of
reaction (DHm). Then, from the monomer conversion, various
kinetic parameters such as rate of polymerization (da/dt), acti-
vation energy (Ea), and rate constant (k) can be determined.
However, due to the overlap of the endothermic lactide
melting peak and the exothermic peak from its polymerization
in isothermal DSC, as reported in previous work,24 only non-
isothermal DSC studies of the ROP of LLA polymerization in
bulk were carried out in this work.
2. Experimental
2.1. Purication of L-lactide monomer
Crude L-lactide (Bioplastic Production Laboratory for Medical
Applications, ISO 13485:2016 Accredited Laboratory, Chiang
Mai University) was puried by repeated (approximately three
times) recrystallization from distilled ethyl acetate to yield pure
L-lactide as a white, needle-like, crystalline solid. It was then
dried to constant weight in a vacuum oven at 55 ꢁC prior to
being transferred to a controlled atmosphere glove box (Lab-
conco) for use in synthesis. The nal product of >99.9% purity
was obtained in a percentage yield of 40–55%.
2.2. Purication of Sn(Oct)2 initiator
Sn(Oct)2 (Sigma-Aldrich) was puried by bulb-to-bulb vacuum
distillation. Firstly, Sn(Oct)2 was stirred under reduced pressure
(high vacuum) at room temperature to remove residual water.
Then, the 2-ethylhexanoic acid impurity was removed under
vacuum distillation at 120–126 ꢁC/15 Torr (boiling point ¼ 140
ꢁC/23 Torr).28 The puried Sn(Oct)2 remaining in the heating
ask was obtained as a colorless viscous liquid and was stored
˚
over molecular sieves type 4 A.
Fig. 6 Ea determinations based on dynamic methods of: (a) Kissinger;
(b) Flynn–Wall; and (c) Ozawa for the ROP of L-lactide using 1.0 mol%
liquid Sn(OnC4H9)2 as an initiator.
2.3. Synthesis of liquid tin(II) n-alkoxide initiators
In this work, the synthesis of liquid tin(II) alkoxides for use as
initiators in the ROP of L-lactide was as described in the novel
modied method by Meepowpan et al.17 and shown in eqn (1):
kinetics and for determining kinetic parameters such as
monomer conversion (a), rate of polymerization (da/dt), order
of reaction (n), and activation energy (Ea) of the ROP of both
The synthesis process employed anhydrous tin(II) chloride
(SnCl2, Sigma-Aldrich) dissolved in n-heptane (nC7H16, Sigma-
SnCl2 þ 2ROH þ 2ðC2H5Þ NH/SnðORÞ þ 2ðC2H5Þ NH$HClY
2
2
2
(1)
ð5% excess of the ROH in n-heptane as solventÞ
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Aldrich) mixed with dry diethylamine ((C2H5)2NH, Panreac). An atmosphere (20 mL minꢀ1). Prior to measurement, the instru-
alcohol, n-ROH, in which the R group was either n-C4H9, n-C6H13, ment was calibrated using high purity (>99.999%ꢀ) 1indium and
or n-C8H17 (Labscan) was added to the reaction mixture and tin standards (Tm ¼ 156.60 ꢁC, DHf ¼ 28.45 J g and Tm
¼
stirred for 12 hours. The reaction mixture of the diethylamine 231.88 ꢁC, DHf ¼ 60.46 J gꢀ1 respectively).29,30 The data obtained
hydrochloride salt, the n-heptane solvent plus any residual were analyzed by using Pyris 1 soware. For each experiment, 5–
alcohol, ROH, and diethylamine was then ltered under 10 mg of the well-mixed monomer-initiator sample was weighed
a nitrogen atmosphere before being evaporated to dryness. All of into
a 50 mL aluminium pan and hermetically sealed.
the three tin(II) alkoxides, namely: tin(II) n-butoxide (Sn(OnC4- Measuremꢀen1 ts were performed at heating rates of 5, 10, 15, and
ꢁ
ꢁ
H9)2), tin(II) n-hexoxide (Sn(OnC6H13)2), and tin(II) n-octoxide 20 C min over the temperature range of 20 to 240 C. The
(Sn(OnC8H17)2) were obtained as viscous, dark yellow liquids experimental data were analyzed by the three so-called dynamic;
which were readily soluble in most common organic solvents isoconversional; and B/D approaches.
such as chloroform, toluene, and n-heptane. Moreover, they
could all be stored under an inert atmosphere for long periods i.e.
2.4.1. Non-isothermal kinetic analyses of ROP of L-lactide
2.4.1.1. Dynamic methods. In general, the polymerization
up to 1 month without any signicant change in their reactivity rate can be described by the following rate eqn (2):
and, therefore, in their efficiency as initiators for the ROP of L-
lactide. Fig. 3 shows chemical structures of stannous octoate and
the three liquid tin(II) n-alkoxides synthesized in this work.
da
¼ kf ðaÞ
(2)
dt
where da/dt is the rate of the polymerization reaction; a is the
fractional conversion at a specic time t; k is the rate constant;
and f(a) is the concentration dependence function of L-lactide or
the conversion dependence function.
The dependence of a rate constant (k) on temperature (T) is
given by the Arrhenius eqn (3):31
2.4. Kinetic studies by DSC
Non-isothermal DSC kinetic studies of the ROP of L-LA using the
Sn(Oct)2; Sn(OnR)2 initiators and Sn(Oct)2/nROH initiating
systems were performed on a PerkinElmer DSC 7 Differential
Scanning Calorimeter under
a
owing nitrogen (N2)
Table 2 Summary of Tp, T50%, and Ea values from the DSC dynamic methods of Kissinger; Flynn–Wall; and Ozawa for L-lactide ROP using various
initiators/initiating systems at heating rates of: 5, 10, 15, and 20 ꢁC minꢀ1
Ea (kJ molꢀ1
Kissingera
57
)
Heating rate,
Initiators/initiating systems
Sn(Oct)2
b (ꢁC minꢀ1
)
Tp (ꢁC)
T50% (ꢁC)
Flynn–Wallb
Ozawac
61
5
142.0
157.8
167.3
176.0
128.9
145.3
150.8
160.3
139.2
152.3
161.5
170.0
144.8
157.5
170.0
177.0
148.8
160.0
174.3
179.3
152.3
168.5
175.5
185.3
160.3
169.3
177.5
187.0
141.8
157.0
166.0
173.0
131.1
147.2
152.5
161.7
138.8
151.7
160.3
169.0
144.2
157.1
167.3
174.3
148.5
159.0
173.3
178.0
152.8
167.3
175.8
184.0
158.3
168.0
176.5
186.7
65
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
Liquid Sn(OnC4H9)2
Sn(Oct)2/nC4H9OH
52
60
57
61
65
71
59
67
60
67
71
74
56
64
61
66
69
72
Liquid Sn(OnC6H13 2
)
Sn(Oct)2/nC6H13OH
Liquid Sn(OnC8H17 2
)
Sn(Oct)2/nC8H17OH
10
15
20
a
Kissinger: d[ln (b/Tp2)]/d(1/Tp) ¼ ꢀEa/R ¼ slope. b Flynn–Wall: log g(a) ¼ log (Af(cat)Ea/R) ꢀ log b ꢀ 2.315 ꢀ 0.457 (Ea/RT50%), slope ¼ ꢀ0.457 (Ea/
c
R). Ozawa: log b ¼ constant ꢀ 0.4567 (Ea/RTp), slope ¼ ꢀ0.4567 (Ea/R).
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ꢀ
ꢁ
where b is the constant heating rate which equals dT/dt
(K minꢀ1) and Tp is the peak temperature at which the
maximum polymerization rate occurs. Therefore, Ea and A can
be obtained from the slope and intercept of the linear t from
a plot of ln (b/TP2) against 1/Tp.
The Flynn–Wall method is an integral method for deter-
mining the activation energies without any reaction order.36
This method combining with Doyle's approximation leads to
eqn (5):37
Ea
k ¼ A exp
ꢀ
(3)
RT
where A is a pre-exponential or “frequency” factor (minꢀ1); Ea is
the activation energy (J molꢀ1); R is the gas constant ¼ 8.314 J
molꢀ1 Kꢀ1; and T is the temperature (K).
The activation energy (Ea) of the polymerization reaction can
be determined by using peak methods which consider using the
peak temperature at maximum rate (Tp) such as the Kissinger
and Ozawa methods and using the temperature at 50%
conversion (T50%) such as the Flynn–Wall method. The Kis-
singer, Flynn–Wall, and Ozawa methods are DSC dynamic
methods which rely on approximating the so-called tempera-
ture integral and require data on temperature only.32,33
The Kissinger method uses the temperature at which the rate
of polymerization is at the maximum (Tp) and the activation
energy, Ea, is obtained from the maximum reaction rate where
d(da/dt)/dt is zero under a constant heating rate condition
leading to eqn (4) as follows:34,35
ꢀ
ꢁ
ꢀ
ꢁ
b
AR
Ea
Ea
ln
¼ ln
ꢀ
(4)
2
RTP
TP
Fig. 7 Plots of (a) fraction of conversion (a) and (b) rate of polymeri-
zation (da/dt, minꢀ1) against temperature (ꢁC) for the ROP of L-lactide Fig. 8 Determination of Ea based on isoconversional methods of: (a)
using 1.0 mol% liquid Sn(OnC4H9)2 as an initiator at heating rates of: ( ) Friedman; (b) KAS; and (c) OFW for the ROP of L-lactide using 1.0 mol%
5; ( ) 10; ( ) 15; and ( ) 20 ꢁC minꢀ1
.
liquid Sn(OnC4H9)2 as an initiator.
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Table 3 Summary of Ea ranges from DSC isoconversional methods for
ROP of L-lactide using various initiators/initiating systems at heating
rates of 5, 10, 15, and 20 ꢁC minꢀ1
2.4.1.2. Isoconversional methods. Isoconversional methods
are amongst the most reliable methods for the calculation of
activation energy of thermally activated reactions. Isoconver-
sional methods can be categorized into two main groups of
methods.33 The rst group are considered as Type A methods (or
rate-isoconversional methods). A well-known method of this
type is the Friedman method which makes no mathematical
approximations. The second group are considered as Type B
methods which apply a range of approximations for the
temperature integral p(x) such as the Kissinger–Akahira–Sunose
(KAS) method and the Flynn–Wall–Ozawa method.36,38–40
These isoconversional methods employ multiple tempera-
ture programs (e.g., different heating rates) in order to obtain
data from various rates at a constant fraction of conversion
(extent of reaction), a. In other words, isoconversional methods
allow complex (i.e., multi-step) processes to be determined via
a variation in activation energy (Ea) with conversion a.41 This
means that, if a signicant variation in Ea occurs with conver-
sion a, the process is complex. On the other hand, if Ea is
independent of conversion a, then the reaction is a single-step
Ea range (kJ molꢀ1
)
Initiator/initiating system
Friedmana
KASb
OFWc
Sn(Oct)2
Sn(OnC4H9)2
Sn(Oct)2/nC4H9OH
Sn(OnC6H13 2
Sn(Oct)2/nC6H13OH
Sn(OnC8H17 2
58–63
53–60
56–63
56–61
60–63
66–76
68–81
59–74
55–58
56–64
56–60
63–71
66–72
65–74
63–77
60–62
60–67
60–64
67–74
70–76
69–78
)
)
Sn(Oct)2/nC8H17OH
a
Friedman: ln (da/dt) ¼ ln (Af(a)) ꢀ (Ea/RT), slope ¼ ꢀEa/R. b KAS: ln (b/
T2) ¼ ln [(AR)/Ea] ꢀ ln g(a) ꢀ Ea/RT, slope ¼ ꢀEa/R. OFW: ln b ¼ ln
c
[(AE)/R] ꢀ ln g(a) ꢀ 5.331 ꢀ 1.052 (Ea/RT), slope ¼ 1.052 (ꢀEa/R).
ꢂ
ꢃ
Af ðcatÞEa
Ea
log g ðaÞ ¼ log
ꢀ log b ꢀ 2:315 ꢀ 0:457
(5)
R
RT50%
where g(a) is an integral conversion dependence function. From
eqn (5), Ea can then be obtained from a plot of log b against 1/
T50% at a constant g(a). Therefore, activation energy Ea deter-
mination from dynamic DSC polymerization only requires
determining the temperature at which 50% polymerization is
achieved.
Similar to the Flynn–Wall method, the Ozawa method uses
the temperature at which the rate of polymerization is at the
maximum which, when combined with Doyle's approximation,
leads to eqn (6):38
Ea
log b ¼ constant ꢀ 0:4567
(6)
RTp
In this method, plots of log b against 1/Tp are used to
determine the energy of activation (Ea) from the slopes.
Fig. 9 Dependence of activation energy (Ea, kJ molꢀ1) on the fraction
of conversion (a) from the data obtained from the: ( ) Friedman; ( ) Fig. 10 Plots of (a) monomer conversion (a) and (b) rate (da/dt) against
KAS; and ( ) OFW isoconversional methods for the ROP of L-lactide temperature (ꢁC) for L-lactide ring-opening polymerization using
using 1.0 mol% liquid Sn(OnC4H9)2 as an initiator.
various initiators/initiating systems at a heating rate of 5 ꢁC minꢀ1
.
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process. Isoconversional methods are based exclusively on
dynamic DSC analysis.
The Ozawa–Flynn–Wall (OFW) isoconversional method uses
Doyle's approximation leading to eqn (9):43,44
ꢀ
ꢁ
ꢀ
ꢁ
Based on eqn (2) and (3), the kinetic approach proposed by
Friedman expresses the logarithm of reaction rate, ln (da/dt), as
a function of the reciprocal temperature, as shown in eqn (7).42
This enables the activation energy Ea to be determined for each
fraction of conversion, a:
AE
R
Ea
ln b ¼ ln
ꢀ ln gðaÞ ꢀ 5:331 ꢀ 1:052
(9)
RT
2.4.1.3. Borchardt and Daniels method.45–48 The Borchardt
and Daniels (B/D) approach gives the calculation of activation
energy (Ea), pre-exponential factor (A), heat of reaction (DH),
reaction order (n), and rate constant (k) from a single DSC
scan.45–48 This approach assumes that the reaction follows nth
ꢀ
ꢁ
da
dt
Ea
ln
¼ ln ðAf ðaÞÞ ꢀ
(7)
RT
This equation implies that the reaction rate is only a func- order kinetics and obeys the general rate eqn (10):
tion of temperature at a constant value of a. It is obvious that if
the function f(a) is constant for a particular value of a, then
ln (Af(a)) is constant as well. Therefore, by plotting ln (da/dt)
against 1/T, a value for ꢀEa/R, and hence Ea, can be obtained.
The Kissinger–Akahira–Sunose (KAS) method is based on
eqn (8) as follows:
da
n
¼ kðTÞð1 ꢀ aÞ
(10)
dt
where da/dt ¼ reaction rate (minꢀ1); a ¼ fractional conversion;
k(T) ¼ specic rate constant at temperature T; and n ¼ reaction
order. The B/D approach also assumes Arrhenius behavior as
mentioned previously in eqn (3). Substituting eqn (3) into eqn
(10), rearranging, and taking logarithms yields:
ꢀ
ꢁ
ꢀ
ꢁ
b
T2
AR
Ea
Ea
ln
¼ ln
ꢀ ln gðaÞ ꢀ
(8)
RT
ꢀ
ꢁ
da
dt
Ea
n
¼ A exp
ð1 ꢀ aÞ
(11)
RT
The activation energy Ea can then be obtained from the slope
of a semi-log plot of ln (b/T2) against 1/T at constant conversion.
Table 4 Summary of activation energy (Ea), Arrhenius pre-exponential factor (A), and apparent polymerization rate constant at 150 ꢁC (kapp, 150
ꢀ1
C) estimated from Borchardt and Daniels (B/D) approach using various initiators/initiating systems at heating rates of 5, 10, 15, and 20 ꢁC min
ꢁ
Kinetic parameters
Heating rate,
Initiators/initiating systems
Sn(Oct)2
b (ꢁC minꢀ1
)
Eaa (kJ molꢀ1
)
Ab (minꢀ1
)
kapp, 150
(minꢀ1
)
c
ꢁC
5
79
89
89
91
76
78
80
84
78
84
87
88
80
87
87
87
89
90
92
94
90
91
96
97
97
98
98
100
5.42 ꢂ 1010
2.35 ꢂ 109
1.49 ꢂ 109
6.32 ꢂ 108
3.40 ꢂ 1010
2.65 ꢂ 1010
2.19 ꢂ 1010
2.87 ꢂ 1010
7.82 ꢂ 109
2.59 ꢂ 1010
3.05 ꢂ 1010
2.57 ꢂ 1010
2.63 ꢂ 1011
3.40 ꢂ 1011
2.19 ꢂ 1011
2.07 ꢂ 1011
3.64 ꢂ 1011
3.79 ꢂ 1011
4.63 ꢂ 1011
1.13 ꢂ 109
3.52 ꢂ 1011
4.36 ꢂ 109
9.56 ꢂ 109
9.48 ꢂ 109
7.66 ꢂ 108
3.44 ꢂ 109
7.84 ꢂ 1010
1.27 ꢂ 1010
0.3150
0.3162
0.3747
0.4163
0.5057
0.5742
0.6729
0.7696
0.4230
0.4322
0.4346
0.4936
0.2323
0.2352
0.2458
0.2773
0.1710
0.1722
0.1975
0.2172
0.1446
0.1475
0.1617
0.1639
0.0372
0.0943
0.1077
0.1082
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
10
15
20
5
Liquid Sn(OnC4H9)2
Sn(Oct)2/nC4H9OH
Liquid Sn(OnC6H13 2
)
Sn(Oct)2/nC6H13OH
Liquid Sn(OnC8H17 2
)
Sn(Oct)2/nC8H17OH
10
15
20
B/D: multiple linear regression of eqn (12), ꢀEa/R ¼ slope. b ln (A) ¼ intercept. c kapp ¼ apparent rate constant at 150 ꢁC ¼ da/dt/(1 ꢀ a)n.
a
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ꢂ
ꢃ
polymer products obtained were white tough solid and were
characterized by the combination of instrumental methods of:
FTIR (Bruker Tensor 27 FTIR spectrometer), 1H-NMR (400 MHz
Bruker Avance II NMR spectrometer), DSC (PerkinElmer DSC 7
Differential Scanning Calorimeter) and dilute-solution viscom-
da
dt
Ea
ln
¼ ln ðAÞ þ n ln ð1 ꢀ aÞ ꢀ
(12)
RT
Eqn (12) can be solved with a multiple linear regression of
the general form: z ¼ a + bx + cy (where z y ln [da/dt]; ln (A) y
a; b y n; x y ln [1 ꢀ a]; c y ꢀEa/R; and y y 1/T). The values for
da/dt; a; and T are experimental parameters obtained from
a single linear heating rate DSC experiment scanning through
the temperature region of the reaction exotherm as shown in
Fig. 4.
¨
etry (Schott-Gerate AVS 300, Ubbelohde viscometer no. 532 00
0c) respectively. Table 1 shows the synthesized polymer codes
and conditions used in this work.
3. Results and discussion
3.1. Synthesis of liquid tin(II) n-alkoxides
In this B/D method, assume a value for n ¼ 1, then the value
for ln [k(T)] can be calculated using eqn (13):
All three synthesized liquid tin(II) n-alkoxides products namely:
tin(II) n-butoxide (Sn(OnC4H9)2), tin(II) n-hexoxide (Sn(OnC6-
H13)2), and tin(II) n-octoxide (Sn(OnC8H17)2) were obtained as
viscous, dark yellow liquids of approximately 70–85% yield. The
physical appearances, solubility test results, and structural
conrmations by various techniques of these three liquid
initiator products were stated elsewhere.17
ln [k(T)] ¼ ln [da/dt] ꢀ n ln [1 ꢀ a]
(13)
Then Ea can be obtained from the slope of a plot of ln [k(T)]
against 1/T at constant conversion (a ¼ 0.1–0.9).
2.5. Synthesis and characterization of poly(L-lactide) using
liquid tin(II) n-butoxide as an initiator
3.2. Kinetic analyses
In this work, ring-opening polymerization of L-lactide using
liquid Sn(OnC4H9)2 initiator was conducted in the bulk phase.
Prior to polymerization, L-lactide monomer of a purity of
>99.5% was dried under vacuum at 55 ꢁC for at least 8 h. All
3.2.1. Dynamic methods. In dynamic methods, DSC ther-
mograms of normalized heat ow (W gꢀ1) against temperature
(ꢁC) for L-lactide polymerization using Sn(Oct)2; liquid Sn(OnC4-
H9)2; and Sn(Oct)2/nC4H9OH are compared in Fig. 5(a)–(c). Fig. 6
ꢁ
glasswares and apparatus used were dried at 120 C overnight
and cooled in a controlled atmosphere glove box prior to use.
For each polymerization, approximately 3 g of puried and
dried L-lactide monomer were accurately weighed into a 10 mL
round bottom ask with a magnetic stirring bar. Then required
concentration of tin(II) n-butoxide, Sn(OnC4H9)2, initiator in
toluene was added. The reaction ask was then sealed and
immersed in a silicone oil-bath for 24 h. At the end of the
polymerization period, the reaction ask was removed from the
oil-bath and allowed to cool to room temperature. Aer that, the
polymerisate was dissolved in chloroform as solvent and the
polymer precipitated from solution by dropwise addition with
efficient stirring into ice-cooled absolute methanol. The
Fig. 12 DSC thermograms for (a) 1st run and (b) 2nd run of the purified
Fig. 11 Visualization of the meaning of the value of activation energy, poly(L-lactide) (PLLA 1) using 0.01 mol% liquid Sn(OnC4H9)2 as an
Ea.
initiator at 100 ꢁC for 24 h.
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shows activation energy (Ea) determinations based on dynamic
Furthermore, the Ea values for the L-lactide polymerizations
methods of: (a) Kissinger; (b) Flynn–Wall; and (c) Ozawa for L-lac- using the seven initiators/initiating systems can be obtained
tide polymerization using liquid tin(II) n-butoxide (Sn(OnC4H9)2) as from the slopes of the plots of (a) ln (b/Tp2) as a function of 1/Tp,
an initiator. Additionally, the peak temperature at the maximum (b) log b as a function of 1/T50% and (c) log b as a function of 1/
polymerization rate (Tp), the temperature at 50% conversion Tp based on the Kissinger, Flynn–Wall and Ozawa methods
(T50%), as well as the Ea values from these three dynamic respectively. The Ea (kJ molꢀ1) values obtained from linear
approaches of L-lactide ring-opening polymerization using all the equation tting for these three dynamic methods are compared
seven initiators/initiating systems are summarized in Table 2.
in Table 2 as previously mentioned.
From the original DSC thermograms of heat ow
From the results obtained, the Ea values are seen to be in the
(normalized, W gꢀ1) against temperature (ꢁC) at the four order of liquid Sn(OnC4H9)2 < Sn(Oct)2/nC4H9OH < Sn(Oct)2 y
different heating rates of 5, 10, 15, and 20 ꢁC minꢀ1, it was liquid Sn(OnC6H13)2 < Sn(Oct)2/nC6H13OH y liquid Sn(OnC8-
found that the temperature at the maximum peak (Tp) and the H17)2 < Sn(Oct)2/nC8H17OH.
temperature of 50% L-lactide monomer conversion (T50%) were
found to increase with increasing heating rate.
3.2.2. Isoconversional methods. For the isoconversion
methods, selected plots of the fraction of conversion (a) and
However, when considering the initial temperatures (i.e. rate of polymerization (da/dt, minꢀ1) against temperature (ꢁC)
onset temperatures, Tonset), it was found that Tonset only slightly for the ROP of L-lactide using liquid Sn(OnC4H9)2 as an initiator
increased when increasing the heating rate. This observation at heating rates of 5, 10, 15, and 20 ꢁC minꢀ1 are shown in
was seen for almost every initiator/initiating system. Interest- Fig. 7(a) and (b). It can be seen from Fig. 7 that both the
ꢁ
ingly, the lowest Tonset polymerization temperature of ꢃ108 C conversion and the rate of polymerization plots against
was found using the liquid Sn(OnC4H9)2 initiator (Fig. 5(b)) temperature are shied to higher temperature with increasing
indicating its potential as an initiator to synthesize the PLA at heating rate.
low temperature and yielding the polymer with controlled
molecular weight and narrow molecular weight distribution in of L-lactide were determined from the non-isothermal DSC data
a short period of time. by the three methods of Friedman, KAS, and OFW. The plots of
According to isoconversional methods, values of Ea of the ROP
Table 5 Summary of values on DSC thermal transition temperatures (Tg, Tc, and Tm), heats of crystallisation (DHc), and heats of melting (DHm) of
all synthesized PLLAs using liquid Sn(OnC4H9)2 as an initiator for 24 h
Tg (ꢁC)
Tc (ꢁC)
Tm (ꢁC)
DHc (J gꢀ1
)
DHm (J gꢀ1
)
Polymer code
1st
2nd
1st
2nd
94.2
98.8
103.0
106.8
108.8
105.3
95.2
118.0
106.7
108.7
106.7
107.5
90.2
1st
2nd
1st
2nd
1st
2nd
PLLA 1
PLLA 2
PLLA 3
PLLA 4
PLLA 5
PLLA 6
PLLA 7
PLLA 8
68.5
67.0
69.9
68.6
63.7
—
54.2
69.9
68.6
72.3
—
71.0
68.3
67.0
69.4
71.7
70.3
69.0
62.6
60.4
68.1
74.7
70.7
—
56.6
54.3
55.6
59.8
58.7
59.1
51.6
55.2
55.8
60.1
57.6
59.1
43.4
51.7
54.8
60.2
57.1
58.2
42.1
39.6
40.2
48.8
45.4
43.9
36.5
37.0
39.0
38.8
43.6
41.9
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
181.0
178.0
179.8
177.8
176.0
176.5
178.5
178.7
178.2
178.8
174.3
176.0
176.2
177.2
176.0
179.5
171.7
174.2
166.5
169.3
175.0
175.5
173.0
173.3
164.3
165.0
171.5
172.3
171.8
171.2
178.3
175.5
179.5
177.7
175.7
176.8
174.8
177.0
178.7
178.8
174.5
176.5
168.2
173.9
176.5
180.2
171.0
176.2
160.8
163.7
167.3
169.0
166.5
166.0
157.7
157.0
164.2
162.3
163.0
163.2
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
41.0
40.4
38.6
34.2
40.5
36.2
38.6
53.6
39.3
32.1
35.9
36.6
40.0
37.8
43.2
35.1
42.4
37.0
37.6
40.4
37.2
38.5
34.6
31.6
33.7
34.9
33.6
27.3
31.0
29.1
64.1
64.5
46.4
38.0
43.3
42.2
72.1
52.5
52.3
34.1
41.2
43.2
60.5
64.0
44.8
39.0
43.5
51.6
67.7
63.4
56.0
50.5
49.8
49.3
63.6
67.8
60.1
49.3
51.0
50.6
76.0
70.5
56.1
45.9
47.1
50.9
68.1
47.5
46.4
36.9
43.2
41.5
55.7
56.1
46.6
45.6
38.5
40.2
56.8
47.5
53.8
48.1
43.0
45.2
47.2
50.3
53.9
45.3
45.2
46.8
PLLA 9
PLLA 10
PLLA 11
PLLA 12
PLLA 13
PLLA 14
PLLA 15
PLLA 16
PLLA 17
PLLA 18
PLLA 19
PLLA 20
PLLA 21
PLLA 22
PLLA 23
PLLA 24
PLLA 25
PLLA 26
PLLA 27
PLLA 28
PLLA 29
PLLA 30
99.3
106.2
109.3
112.2
108.2
80.2
86.3
85.2
95.0
91.7
87.3
73.0
77.0
78.8
78.8
83.5
80.8
—
61.9
—
75.6
—
69.4
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ln (da/dt), ln (b/dT2), and ln b against 1/T (Kꢀ1) based on these
methods for the ROP of L-lactide using 1.0 mol% liquid
Sn(OnC4H9)2 as an initiator are shown in Fig. 8(a)–(c). In general,
linear plots were obtained for each of the initiating systems
although the Friedman plots showed more scattering of the
points than those of the KAS and OFW. The values of Ea at
a particular conversion, a, can be obtained from the slopes of
these linear plots and the values are summarized in Table 3.
Again, the Friedman plots showed more variation in Ea with
a than the KAS and OFW plots, indicating that the isoconver-
sional KAS and OFW methods are more suitable for Ea determi-
nation in L-lactide polymerization. From Fig. 9, similar smaller Ea
variations with a were observed for the KAS and OFW methods
Table 6 Dilute-solution viscosity data and calculated viscosity terms
in chloroform at 25 ꢄ 0.1 ꢁC for purified poly(L-lactide) (PLLA 9) ob-
tained using 0.05 mol% liquid Sn(OnC4H9)2 as an initiator at 120 ꢁC for
24 h
Concentration
Flow-time
(sec)
(g dLꢀ1
)
hrel
hsp
hred (dL gꢀ1
)
0
132.4
207.6
301.8
423.6
562.0
—
—
—
0.2012
0.4120
0.6056
0.8012
1.567
2.278
3.198
4.243
0.567
1.278
2.198
3.243
2.819
3.103
3.630
4.048
with the former being consistently <5 kJ molꢀ1 lower than the at various conditions were conrmed by FTIR and 1H-NMR
latter. The consistency of the Ea values over almost the complete respectively.
range of conversion is an indication that the coordination–
3.3.1. Temperature transitions and heat of melting by DSC.
insertion mechanism ring-opening polymerization under the For the temperature transition determination, all DSC analyses
conditions is the sole mechanism which is operational in L-lac- of the poly(L-lactide) were conducted at a heating rate of
tide employed in this study.
10 ꢁC minꢀ1 under dry nitrogen (N2) using a Perkin Elmer DSC7
From Table 3, Ea values were found to be the lowest for the Differential Scanning Calorimeter (Pyris 1 Soware). For each
Sn(OnC4H9)2 initiator (Ea ¼ 53–60, 55–58, and 60–62 kJ molꢀ1 measurement, a polymer sample was weighed in the range of 3–
for Friedman; KAS; and OFW respectively) and the highest for 5 mg and hermetically sealed in a 50 mL sample pan. Typical
Sn(Oct)2/nC8H17OH (Ea ¼ 68–81, 65–74, and 69–78 kJ molꢀ1 for DSC thermograms (1st run and 2nd run) for poly(L-lactide)
Friedman, KAS, and OFW respectively). When comparing the synthesized in this work are illustrated in Fig. 12(a) and (b)
results at the same heating rate of 5 ꢁC minꢀ1 (see Fig. 10(a) and respectively showing the polymer's glass transition temperature
(b)), the conversion and rate plots showed different values of (Tg), crystallisation temperature (Tc) and melting temperature
Tonset with different conversions or rates of polymerization in (Tm). The appearance of the Tc peak in the 2nd run is a result of
the order of: liquid Sn(OnC4H9)2 > Sn(Oct)2/nC4H9OH > Sn(Oct)2 the slow crystallisability of the PLLA during the intermediate
y
liquid Sn(OnC6H13)2
>
Sn(Oct)2/nC6H13OH
y
liquid fast cooling step. Consequently, whereas the PLLA at the state of
the 1st run was semi-crystalline, at the start of the 2nd run it was
Sn(OC8H17)2 > Sn(Oct)2/nC8H17OH.
3.2.3. Borchardt and Daniels (B/D) method. For B/D largely amorphous, hence the appearance of the cold crystal-
method, kinetic parameters Ea and frequency factor A can be lisation Tc peak.
obtained from multiple linear regression from the plot of ln
Information on the DSC thermal transition temperatures (i.e.
[k(T)] versus 1/T. Table 4 shows values obtained from ROP of L- the glass transition temperature (Tg), the crystallisation temper-
lactide using all seven initiators/initiating systems at heating ature (Tc), and the melting temperature (Tm)) as well as the heat
rates of 5, 10, 15, and 20 C minꢀ1 respectively.
of crystallization (DHc) and the heat of melting (DHm) from all
ꢁ
Similar to previous dynamic and isoconversional synthesized PLLA polymer products were summarized in Table 5.
approaches, liquid tin(II) n-butoxide shown to be the most effi-
cient initiator due to its lowest activation energy range (76–
84 kJ molꢀ1) with highest apparent rate constant values.
What the Ea value for L-lactide ring-opening polymerization in
bulk means essentially is the Ea value for the propagation step
since the number of propagation steps far outnumber the initi-
ation and termination steps. Therefore, the actual meaning of Ea
can be visualized as shown in the energy diagram in Fig. 11.
3.3. Synthesis and characterization of poly(L-lactide) using
liquid tin(II) n-butoxide as an initiator
From the DSC kinetic studies, liquid tin(II) n-butoxide,
Sn(OnC4H9)2, has shown to be the most efficient initiator in
terms of rate, therefore, it was chosen for a more detailed study
of how it can be used to control molecular weight in the ring-
opening polymerizaition in bulk of L-lactide by varying its
concentration and also the polymerization temperature (Table
1). From the results obtained, structures of all synthesized
poly(L-lactide) products using liquid Sn(OnC4H9)2 as an initiator Sn(OnC4H9)2 as an initiator at 120 ꢁC for 24 h.
Fig. 13 Plots of reduced viscosity, hred ( ) and inherent viscosity, hinh
( ) in CHCl3 at 25 ꢄ 0.1 ꢁC against concentration (g dLꢀ1) for purified
poly(L-lactide) (PLLA 9) obtained from using 0.05 mol% liquid
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3.3.2. Molecular weight determination by dilute-solution
viscometry. The viscometric ow-time data and derived
viscosity parameters relating to poly(L-lactide) sample syn-
thesised using 0.05 mol% Sn(OnBu)2 at 120 ꢁC for 24 h (PLLA 9)
are given in Table 6. The measurement was performed using
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4
ꢀ
Mv ¼ 9.03 ꢂ 10
ꢀ
Therefore, the number-average molecular weight (Mn), can
be calculated from the gamma function50 eqn (15); assuming an
approximately “most probable” molecular weight distribution,
as:
¨
a Schott-Gerate Ubbelohde viscometer (type no. 532 00 0c) in
¨
conjunction with the Schott-Gerate AVS 300 Automatic Viscosity
Measuring System. A polymer sample was dissolved in CHCl3
Mn
Mv
1
ꢁ
¼
(15)
solution at 25 ꢄ 0.1 C.
1=a
½ð1 þ aÞGð1 þ aÞꢅ
Fig. 13 depicts plots of reduced viscosity (hred) and inherent
viscosity (hinh) against concentration (g dLꢀ1). Double extrapo- where a ¼ 0.73 (for PLLA in CHCl3 at 25 C) and G ¼ gamma
ꢁ
4
ꢀ
lating to zero concentration giving a value of intrinsic viscosity function, from Mv ¼ 9.03 ꢂ 10 , therefore giving:
[h] of the PLLA sample of 2.26 dL gꢀ1 was obtained.
Mn ¼ 4.80 ꢂ 104 g molꢀ1
ꢀ
From the value of [h] ¼ 2.26 dL gꢀ1 (Fig. 13), the polymer's
ꢀ
viscosity-average molecular weight, Mv, can be calculated from
the Mark–Houwink–Sakurada49 eqn (14) for PLLA in chloroform
ꢀ
ꢀ
The values of [h], Mv, and Mn obtained from dilute-solution
ꢁ
at 25 ꢄ 0.1 C:
viscometry for PLLA products were provided in Table 7.
ꢀ0.73
[h] ¼ 5.45 ꢂ 10ꢀ4 Mv
dL gꢀ1
(14)
ꢀ
4. Conclusions
In conclusion, these kinetic studies of the ring-opening poly-
merization of L-lactide in bulk by dynamic (Kissinger; Flynn–
Wall; Ozawa), isoconversional (Friedman, KAS, and OFW) and
Borchardt and Daniels (B/D) methods have indicated that the
efficiency of the various initiators/initiating systems used in this
work is in the order of liquid tin(II) n-butoxide (Sn(OnC4H9)2) >
[2.26] ¼ 5.45 ꢂ 10ꢀ4 Mv
dL gꢀ1
ꢀ0.73
ꢀ
ꢀ
ꢀ
Table 7 Summary on the values of: [h], Mv and Mn obtained from
dilute-solution viscometry for all synthesized PLLAs using liquid tin(II)
n-butoxide as an initiator
tin(II) octoate/n-butanol (Sn(Oct)2/C4H9OH)
> tin(II) octoate
Intrinsic
Viscosity-average
Number-average
(Sn(Oct)2) y liquid tin(II) n-hexoxide (Sn(OnC6H13)2) > tin(II)
octoate/n-hexanol (Sn(Oct)2/nC6H13OH) y liquid tin(II) n-octoxide
(Sn(OnC8H17)2) > tin(II) octoate/n-octanol (Sn(Oct)2/nC8H17OH).
Therefore, liquid tin(II) n-butoxide (Sn(OnC4H9)2) is regarded
as being the most efficient initiator for ring-opening polymeri-
zation of L-lactide via coordination–insertion mechanism as
conrmed by the non-isothermal DSC kinetic studies which
gave the lowest Ea values and the fastest rates of polymerization
at the lowest temperature.
The results also conrm that the use of liquid tin(II) n-
alkoxide (Sn(OR)2) initiator directly is more efficient than
generating it in situ via the Sn(Oct)2/ROH reaction. It also has
the important advantages of (a) knowing the [Sn(OR)2]
concentration accurately for the purpose of being able to predict
polymerization rates and polymer molecular weights and (b) to
avoid any unwanted side-reactions due to the use of Sn(Oct)2
alone and/or Sn(Oct)2/ROH system.
Polymer viscosity,
molecular weight,
molecular weight,
ꢀ
ꢀ
code
[h] (dL gꢀ1
)
Mv (g molꢀ1
)
Mn (g molꢀ1
)
PLLA 1 0.66
PLLA 2 1.05
PLLA 3 2.15
PLLA 4 1.69
PLLA 5 1.92
PLLA 6 1.90
PLLA 7 0.98
PLLA 8 1.67
PLLA 9 2.26
PLLA 10 2.35
PLLA 11 1.84
PLLA 12 2.17
PLLA 13 0.85
PLLA 14 1.01
PLLA 15 2.38
PLLA 16 2.02
PLLA 17 1.58
PLLA 18 1.65
PLLA 19 0.31
PLLA 20 0.48
PLLA 21 0.84
PLLA 22 1.05
PLLA 23 0.89
PLLA 24 0.85
PLLA 25 0.23
PLLA 26 0.29
PLLA 27 0.63
PLLA 28 0.73
PLLA 29 0.64
PLLA 30 0.52
2.22 ꢂ 104
3.15 ꢂ 104
8.42 ꢂ 104
6.09 ꢂ 104
7.20 ꢂ 104
7.12 ꢂ 104
2.89 ꢂ 104
5.98 ꢂ 104
9.03 ꢂ 104
9.51 ꢂ 104
6.83 ꢂ 104
8.52 ꢂ 104
2.36 ꢂ 104
2.99 ꢂ 104
9.67 ꢂ 104
7.73 ꢂ 104
5.51 ꢂ 104
5.90 ꢂ 104
6.06 ꢂ 103
1.07 ꢂ 104
2.31 ꢂ 104
3.17 ꢂ 104
2.53 ꢂ 104
2.35 ꢂ 104
4.06 ꢂ 103
5.48 ꢂ 103
1.56 ꢂ 104
1.91 ꢂ 104
1.61 ꢂ 104
1.42 ꢂ 104
1.18 ꢂ 104
1.68 ꢂ 104
4.48 ꢂ 104
3.24 ꢂ 104
3.83 ꢂ 104
3.79 ꢂ 104
1.54 ꢂ 104
3.18 ꢂ 104
4.80 ꢂ 104
5.05 ꢂ 104
3.63 ꢂ 104
4.53 ꢂ 104
1.25 ꢂ 104
1.59 ꢂ 104
5.14 ꢂ 104
4.11 ꢂ 104
2.93 ꢂ 104
3.14 ꢂ 104
3.22 ꢂ 103
5.71 ꢂ 103
1.23 ꢂ 104
1.68 ꢂ 104
1.35 ꢂ 104
1.25 ꢂ 104
2.16 ꢂ 103
2.91 ꢂ 103
8.32 ꢂ 103
1.02 ꢂ 104
8.55 ꢂ 103
6.86 ꢂ 103
Conflicts of interest
The authors declare no conict of interest.
Acknowledgements
The authors wish to thank the Center of Excellence for Inno-
vation in Chemistry (PERCH CIC), the Center of Excellence in
Materials Science and Technology, Basic Research Fund and the
Graduate School of Chiang Mai University, Thailand for funding
this research work. The authors would also like to thank the
This journal is © The Royal Society of Chemistry 2020
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