Effect of tributyltin alkoxides chain length on the ring-opening polymerization of ε-caprolactone: Kinetics studies by non-isothermal DSC

Article abstract of DOI:10.1016/j.tca.2014.11.001

The kinetics of ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) initiated by 1.0 mol% of the tributyltin alkoxides (Bu₃SnOR; R = Me, Et, nPr and nBu) was investigated by non-isothermal DSC technique. The DSC curves showed the depende

Full text of DOI:10.1016/j.tca.2014.11.001

Thermochimica Acta 599 (2015) 1–7
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Effect of tributyltin alkoxides chain length on the ring-opening
polymerization of
DSC
e-caprolactone: Kinetics studies by non-isothermal
Wanich Limwanich, Sureerat Khunmanee, Nawee Kungwan, Winita Punyodom,
Puttinan Meepowpan *
Polymer Research Laboratory, Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 25 August 2014
Received in revised form 30 October 2014
Accepted 1 November 2014
Available online 4 November 2014
The kinetics of ring-opening polymerization (ROP) of e-caprolactone (e-CL) initiated by 1.0 mol% of the
3
tributyltin alkoxides (Bu SnOR; R = Me, Et, nPr and nBu) was investigated by non-isothermal DSC
technique. The DSC curves showed the dependency of polymerization exotherms with the heating rate.
The polymerization rate increased with increasing of heating rate for all initiating systems. The values of
activation energy (E
and Bu SnOnBu initiated ROP of
increased with increasing of alkoxy chain length but polymerization rate decreased. The variation of E
a
) obtained from the peak method of Kissinger for Bu
3 3 3
SnOMe, Bu SnOEt, Bu SnOnPr
3
e
-CL were 52.4, 70.3, 75.9 and 78.0 kJ/mol, respectively. The values of E
a
Keywords:
Kinetics
DSC
a
with monomer conversion was investigated by Friedman and Starink isoconversional methods. The
Bu SnOnBu initiator produced the highest molecular weight and %yield of poly( -caprolactone) (PCL).
3
e
e-Caprolactone
Ring-opening polymerization
Tributyltin alkoxides
ã 2014 Elsevier B.V. All rights reserved.
Alkoxy chain length
1
1
. Introduction
(Bu
found that Bu
tributyltin derivatives. Moreover, Kricheldorf and ThienBen [16]
compared the reactivity of Bu SnOEt, Bu Sn(OEt) , and BuSn(OEt)
-CL using H NMR technique. The reactivity of initiators
3
SnOPh) in the ROP of
e
-CL. From H NMR investigation, they
3
SnOMe showed the highest reactivity than others
Biodegradable poly(
e
-caprolactone), PCL, have recently become
an attractive class of aliphatic polyester for biomedical applica-
tions due to its biodegradability, biocompatibility, physical and
chemical property [1–3]. PCL with high molecular weight is
normally synthesized by ring opening polymerization (ROP) of
3
2
2
3
1
in ROP of
e
increased with increasing of alkoxy group. Our previous works
focused on the theoretical investigation of the influence of alkoxy
e
-caprolactone (
initiators such as aluminum alkoxides [4,5], titanium alkoxides
6,7], tin alkoxides [8–10] and tin caboxylates [11–13]. Among
e
-CL) using various metal alkoxide and carboxylate
3
group of tributyltin alkoxide (Bu SnOR) in ROP of D-lactide (DL)
using quantum calculations [17]. The effect of chain length of linear
alkoxy group was not significant to the values of energy barrier. In
contrast, the branch alkoxy group played an important role on the
energy barrier of transition state. The energy barrier increased
with increasing of steric hindrace of branch alkoxy group.
Furthermore, we studied the steric effect of tin(II) alkoxide (Sn
[
them, tin(IV) alkoxides with different structures have been widely
used as initiators for ROP of cyclic ester monomers [14–16].
Moreover, the tributyltin alkoxides (Bu SnOR) have been found to
3
be the attractive class of initiators due to their easy synthesis
procedure, high oxygen-moisture stability and high %yield [14].
Using this type of initiator, Kricheldorf et al. [14] reported the
comparison for the reactivity of tributyltin derivatives consist of
tributyltin chloride (Bu
tributylin acetate (Bu SnOAc), tributyltin thioacetate (Bu
tributyltin methoxide (Bu SnOMe) and tributyltin phenoxide
(OR)
From dilatometric results, it was found that the reactivity of
synthesized tin(II) t-butoxide (Sn(OtBu) ) catalyzed ROP of -CL
slower than tin(II) i-butoxide (Sn(OiBu) ) and tin(II) n-butoxide (Sn
(OnBu) ), respectively. The results from our previous studies
2
) initiator in ROP of e-CL by using dilatometry technique [18].
2
e
3
SnCl), tributyltin bromide (Bu
3
SnBr),
2
3
3
SnSAc),
2
3
clearly showed that the bulky alkoxy group decreased the
reactivity of tin(II) alkoxide initiators. Moreover, we recently
reported the computational investigation [19] on the catalytic
behavior of Sn(OnBu)
n-octoxide (Sn(OnOct)
2
, tin(II) n-hexoxide (Sn(OnHex)
2
) and tin(II)
*
Corresponding author. Tel.: +66 53 943341–5; fax: +66 53 892277.
E-mail address: pmeepowpan@gmail.com (P. Meepowpan).
2
) in ROP of -CL. We found that the
e
http://dx.doi.org/10.1016/j.tca.2014.11.001
040-6031/ã 2014 Elsevier B.V. All rights reserved.
0

2
W. Limwanich et al. / Thermochimica Acta 599 (2015) 1–7
reactivity of Sn(OnBu)
2
> Sn(OnHex)
2
> Sn(OnOct)
2
. From literature
and hermetically sealed. The samples were heated from standby
temperature of 20–300 C at heating rate of 5, 10, 15 and 20 C/min.
ꢀ
ꢀ
review, the influence of alkoxy chain length on the reactivity of
Bu SnOR has not been reported and discussed before. Moreover,
there are a few papers reported on the kinetics of ROP of cyclic
ester monomers initiated by Bu SnOR. So, the kinetic information
is still needed to describe the structure-reactivity relationship of
Bu SnOR initiators. In this study, we introduce the alternative,
3
2.4. Synthesis of poly(e-caprolactone)
3
e
-CL monomer and 0.25 mol% (molar ratio of monomer to
3
initiator ([M]/[I]) = 400) of Bu SnOR (R = Me, Et, nPr and nBu) were
3
simple and convenient thermoanalytical technique, namely
differential scanning calorimetry (DSC), which is a fast technique
to study the kinetics of cyclic esters polymerization unlike the time
consuming technique of 1H NMR. DSC normally provides the
profile of the heat released during the polymerization under the
isothermal and/or non-isothermal conditions which various useful
kinetic parameters can be obtained and analyzed. Moreover, it is an
effective and reliable technique for the determination of some
weighed into dry glass vials and capped in a controlled atmosphere
glove box under dry nitrogen. The reaction vials were immersed in
a pre-heated silicone oil bath at a constant temperature of 120 C
ꢀ
for 72 h. The obtained crude PCLs were dissolved in chloroform and
re-precipitated in cold methanol before drying in a vacuum oven at
ꢀ
45 C until constant weight. The molecular weight and molecular
weight distribution (MWD) of the synthesized PCLs were
determined by Water e2695 gel permeation chromatography
ꢀ
kinetic parameters such as rate constant (k), %conversion (%
a
) and
(GPC) at 35 C with refractive index and viscosity detectors.
activation energy (E ) of polymerization process [20–22]. Gener-
a
Tetrahydrofuran (THF) was used as an eluent with flow rate of
ꢁ
1
ally, non-isothermal DSC gives an easy interpretation of kinetics
profile by performing the experiment at different temperature
programs [23,24]. We have used this technique to investigate the
1.0 ml min . The number average molecular weights (M ) of the
n
1
synthesized PCLs were also determined by H NMR technique.
steric influence of titanium(IV) alkoxide (Ti(OR)
ROP of -CL [7]. From the calorimetric results, the more steric Ti
OR) showed the lower reactivity with higher values of E
So, the goal of this work is to compare the reactivity of the four
synthesized Bu SnOR (R = Me, Et, nPr and nBu) in the ROP of -CL
via non-isothermal DSC technique. The kinetic parameters such as
half life of polymerization (t1/2) and E are determined. The
dependency of E with monomer conversion is investigated by the
4
) initiators on the
3. Results and discussion
e
(
4
a
.
3.1. Synthesis and characterization of Bu SnOR initiators
3
3
e
The Bu SnOR can be synthesized by simple nucleophilic
3
substitution of Bu SnCl by NaOR (R = Me, Et, nPr and nBu) in
3
a
anhydrous toluene. The NaCl salt, the product from reaction, is
separated from solution by filtration to obtain the crude Bu SnOR
a
3
Friedman and Starink isoconversional methods. The results from
non-isothermal DSC experiments will be discussed to evaluate the
which was purified by vacuum distillation. The physical appear-
ance of the Bu SnOR is the colorless liquid. The %yield of the
3
nucleophilicity of Bu
3
SnOR on the ROP of
e
-CL. Furthermore, the
SnOR initiators will
synthesized Bu SnOMe, Bu SnOEt, Bu SnOnPr and Bu SnOnBu are
3
3
3
3
molecular weight of PCLs synthesized from Bu
be compared and discussed.
3
80, 60, 62 and 76%, respectively. The chemical structures of the
1
synthesized Bu SnOR are identified by FT-IR and H NMR
3
techniques. From FT-IR spectra shown in Fig. 1, the synthesized
2
. Experimental
Bu SnOR initiators show the characteristic peaks of SnO stretching
3
ꢁ1
ꢁ1
at 450 cm and SnC stretching around 590 cm which are close
to the data reported in literature [26]. Moreover, the spectra of all
initiators show CO stretching around 1070 cm , CH ,CH bending
2 3
and stretching at 1370 and 2900 cm , respectively. For more
details on the identification of Bu SnOR’s structure, the H NMR
2.1. Materials preparation
ꢁ
1
ꢁ1
Commercial
e
-caprolactone (Acros Organics, 97.0%) was puri-
1
fied by vacuum distillation before used. Tributyltin chloride (Acros
Organics, 95.0%) was used as received. Toluene (Carlo Erba, 99.8%),
methanol (Sigma, 99.8%), ethanol (Sigma, 99.5%), n-propanol
3
was used to support the results from FT-IR analysis.
1
From the H NMR spectra of the synthesized Bu SnOR (R = Me,
3
(
Sigma, 99.5%), n-butanol (Sigma, 99.5%) were purified by
Et, nPr and nBu) shown in Fig. 2. The spectra showed the multiplet
distillation over sodium metal before used.
of methyl chain end and methylene proton at 0.90 (2
0
, 3
0
, 4, 4 ) and
0
0
0
.95–1.60 ppm (2, 3, 3 ), respectively. For Bu
3
SnOMe, the methoxy
) and multiplet at 3.65 ppm
SnOEt, the quartet of methylene proton of ethoxy
group was found at 3.80 ppm (100) and multiplet at 3.65 ppm (100).
2.2. Synthesis of Bu
3
SnOR initiators
group showed singlet at 3.70 ppm (1
0
00
(
1 ). For Bu
3
Tributyltin alkoxides (Bu
synthesized by the nucleophilic substitution of tributyltin chloride
Bu SnCl) by sodium alkoxides (NaOR; R = Me, Et, nPr and nBu) in
anhydrous toluene at reflux conditions for 6 h as described in
literature [14,25]. The synthesized Bu SnOR were purified by
3
SnOR; R = Me, Et, nPr and nBu) were
(
3
3
vacuum distillation and the chemical structure was identified by
1
proton nuclear magnetic resonance spectroscopy ( H NMR) on a
Bruker Avance 400 NMR Spectrometer operating at 400 MHz in
chloroform-d (CDCl
3
) and Fourier transform infrared spectroscopy
(
FT-IR) on a Bruker Tensor 27 FT-IR spectrometer.
2.3. Non-isothermal DSC polymerizations
Kinetic studies of the ROP of
synthesized Bu SnOR (R = Me, Et, nPr and nBu) were performed on
a PerkinElmer DSC-7 under a flowing nitrogen atmosphere (20 ml/
min). The -CL and Bu SnOR initiators were placed in dry vial and
e-CL initiated by 1.0 mol% of the
3
e
3
stirred vigorously for 10 min. For each experiment, 8–10 mg of the
monomer-initiator mixture was weighed into an aluminum pan
Fig. 1. FT-IR spectra of the synthesized Bu
Bu SnOnBu.
3 3 3
SnOMe, Bu SnOEt, Bu SnOnPr and
3

W. Limwanich et al. / Thermochimica Acta 599 (2015) 1–7
3
onset (T
0
) and endset (T
f
) temperatures of polymerization shift to
lower temperature range as the alkoxy chain (OR) length and
heating rate decreased indicating the structure of initiator affects
the polymerization temperatures. The Bu
-CL at lower temperature range than Bu
Bu SnOnBu, respectively.
It is important that the DSC exotherms of polymerization for all
3
SnOMe initiated ROP of
e
3
SnOEt, Bu SnOnPr and
3
3
initiators are board related to the slow propagation of polymeri-
zation reaction which may be caused by two reasons. The first
reason was the three butyl groups connected to tin atom reduced
initiator’s nucleophilicity by inductive effect of carbon atoms. The
second was the steric interference of three butyl groups on the
coordination of initiator with monomer that reduced the
coordinating ability of initiator and monomer. From the obtained
polymerization exotherms, the monomer conversion (
a) can be
determined by Eq. (1) [20,23].
D
H
H
t
a
¼
D
(1)
p
t
where H is the heat generated during the polymerization process
and H is the total heat released from polymerization reaction.
p
From the obtained
determined. Plots of
a
, polymerization rate (d
a/dt) can also
a
and d /dt against polymerization time (t)
a
at the identical conditions (initiator concentration 1.0 mol% and a
ꢀ
heating rate of 10 C/min) are shown in Fig. 4(a) and (b),
respectively.
From the kinetic plots of conversion against temperature shown
in Fig. 4(a), the polymerization of
faster as the alkoxy chain length of Bu
showed that Bu SnOMe was reactive at lower temperature range
e
-CL occurred and completed
3
SnOR decreased. The results
3
than others initiators. Moreover, from the results shown in
Fig. 4(b), it was found that the maximum rate of polymerization
decreased with increasing of alkoxy chain length of Bu SnOR. For
3
theoretical consideration for non-isothermal DSC kinetics analysis,
the measured heat flow is related to the conversion of monomer
(a) and polymerization rate (da/dt). So, the rate equation can be
expressed as Eq. (2).
d
dt
a
¼
kðTÞfðaÞ
(2)
In non-isothermal DSC, d
a
/dt =
b
(d
a
/dT), where
b
is the heating
ꢀ
rate ( C/min) and k(T) is the rate constant expressed by the
temperature-dependent Arrhenius equation, Eq. (2) can be written
as [27]:
d
dt
a
¼ AexpðꢁE Þfð
a
Fig. 2. 1H NMR (400 MHz) spectra of the synthesized Bu
3 3
SnOR in CDCl : (a)
b
a
Þ
(3)
RT
Bu
3
SnOMe, (b) Bu
3
SnOEt, (c) Bu
3
SnOnPr and (d) Bu
3
SnOnBu.
a
where E is the activation energy, A is the frequency factor, T is the
temperature, R is the universal gas constant. The Kissinger method
[28,29] used the Murray and White approximation to determine
the p(y) value as shown in Eq. (4) [30]. When the constant
monomer conversion at the stage of maximum rate of polymeri-
zation is reached, the Kissinger method based on the relationship
For Bu
3
3
SnOnPr and Bu SnOnBu, the triplet of methylene proton
0
00
connected to SnO was found at 3.60–3.65 ppm (1 , 1 ). After
structural characterization, the four synthesized Bu SnOR were
-CL and the reactivity of them will be
3
used as initiator for ROP of
compared and discussed.
e
between the heating rate (
b
) and the temperature at maximum
3.2. Non-isothermal DSC kinetics analysis for ROP of e-caprolactone
rate of polymerization (T
m
) as shown in Eq. (5).
initiated by Bu SnOR
3
expðꢁyÞ
pðyÞ ¼
(4)
(5)
2
y
The polymerizations under non-isothermal DSC condition
provided useful kinetics and thermodynamics information which
was extracted in a short period of time. The non-isothermal DSC
b
¼ ꢁ E þ C
a
ln
curves for ROP of
in Fig. 3. The results showed that the exothermic curves from
polymerization reaction of -CL shift to higher temperature range
e
-CL initiated by 1.0 mol% of Bu
3
SnOR are shown
2
RT
m
T
m
e
as heating rate increases for all initiating systems [7,23,24]. At high
heating rate, the curves showed wider and sharper exotherms
which are related to the high polymerization rate. Moreover, the
Therefore, the value of E
a
can be obtained from the slope of a
, The plots of Kissinger method for all
initiating systems are shown in Fig. 5.
2
m m
plot of ln(b/T ) against 1/T

4
W. Limwanich et al. / Thermochimica Acta 599 (2015) 1–7
ꢀ
3 3 3 3
Fig. 3. Non-isothermal DSC curves for ROP of e-CL initiated by 1.0 mol% of Bu SnOR at heating rates of 5,10, 15 and 20 C/min: (a) Bu SnOMe, (b) Bu SnOEt, (c) Bu SnOnPr and
(
3
d) Bu SnOnBu.
The obtained kinetic parameters from non-isothermal DSC
polymerization of -CL initiated by the synthesized Bu SnOR are
summarized in Table 1. The results from Table 1 showed that the
values of T increased with increasing of heating rate and alkoxy
1.0 mol% of Bu
initiated ROP of
respectively. The results showed that the E
Bu SnOR initiators decreased with decreasing of alkoxy chain
3
SnOMe, Bu
-CL are 52.4, 70.3, 75.9 and 78.0 kJ mol
of the four synthesized
3 3 3
SnOEt, Bu SnOnPr and Bu SnOnBu
ꢁ1
e
3
e
,
a
m
3
chain length of initiator. The values of the half life (t1/2) decreased
with increasing of heating rate indicating the system required
shorter time to reach the state of 50% monomer conversion. The
length or increasing of initiator’s nucleophilic strength. These may
be caused by the increasing of alkoxy chain length reduced the
mobility of alkoxy group to attack the carbonyl carbon of
the coordination insertion mechanism resulting in the lower
reactivity and higher E . These results can be supported by the
dependency of initiator’s reactivity with alkoxy chain length of Sn
e-CL in
values of t1/2 for ROP of
Bu SnOnPr < Bu SnOnBu. These results indicated that the longer
alkoxy group of initiator increased the time required for 50% of
polymerization and decreased the reactivity of Bu SnOR. To
support this relationship, the values of E obtained from Kissinger
method was used to compare the reactivity of each initiating
systems. The values of E obtained from Kissinger method for
3 3
e-CL initiated by Bu SnOMe < Bu SnOEt
<
3
3
a
3
(OR)
2
in ROP of
e
-CL [19,31]. The results obtained from quantum
ꢀ
a
calculations showed that the rate constant at 100 C of Sn(OnBu)
2
was higher than those of Sn(OnHex)
2
and Sn(OnOct)
2
, respectively.
a
Moreover, the energy barrier of Sn(OnBu)
2
2
< Sn(OnHex) < Sn
ꢀ
Fig. 4. Plots of monomer conversion (a) and polymerization rate (b) as a function of polymerization temperature at a heating rate of 10 C/min for ROP of
e-CL initiated by
3 3 3 3
1.0 mol% of Bu SnOMe, Bu SnOEt, Bu SnOnPr and Bu SnOnBu.

W. Limwanich et al. / Thermochimica Acta 599 (2015) 1–7
5
Fig. 5. Plots of ln(
ROP of -CL initiated by 1.0 mol% of Bu
Bu SnOnBu.
b
/T
m
2) against 1000/T
m
based on the peak method of Kissinger for
e
3
SnOMe, Bu SnOEt, Bu SnOnPr and
3
3
3
(
OnOct)
chain length, Kleawkla et al. [31] reported the kinetics data of Sn
OnHex) and Sn(OnOct) initiated ROP of -CL using the classical
method of dilatometry. They found that the rate constant of
2
. For more supportive information on the effect of alkoxy
(
2
2
e
polymerization obtained from Sn(OnHex)
2
is higher than Sn
ꢀ
(
OnOct)
2
at 140 C indicating the higher reactivity of Sn(OnHex)
2
than Sn(OnOct)2. From these results, the polymerization rate of
-CL seems to depend on the size of alkoxy group of initiators. As
reported by Kricheldorf et al. [14] that the polymerization rate of
e
e
-CL with Bu
3
SnOMe was higher than of Bu
3
SnOPh ([M]/[I] = 100)
ꢀ
as observed on the plots of conversion against time at 100 C. The
polymerization of
faster than Bu SnOPh. The increasing of bulkiness of alkoxy group
of initiator reduces the polymerization rate of -CL. Therefore, the
results obtained from this work suggest that Bu SnOnBu is the
3
e-CL using Bu SnOMe as initiator completes
3
e
3
least reactive initiator due to the bulky alkoxy group as described
earlier.
To support the results from Kissinger method, the dependency
of E with monomer conversion was determined from Friedman
a
and Starink isoconversional methods. These methods allow a
single or a multiple step reaction process to be detected. For the
Fig. 6. Plots of E
isocnversional methods for the ROP of
Bu SnOEt, Bu SnOnPr and Bu SnOnBu: (a) Friedman and (b) Starink isoconversional
methods.
a
against monomer conversion (
a
) obtained from different
3
e-CL initiated by 1.0 mol% of Bu SnOMe,
3
3
3
single step reaction, the E
a
is independent from monomer
conversion (which is constant throughout polymerization). While
in the multiple step reaction, the values of E generally vary with
monomer conversion. In this work, the values of E were
determined from the monomer conversion in a range of 0.2–0.8.
The differential isoconversional method of Friedman [32–34] does
not use any mathematics approximation for temperature integral.
Table 1
a
The obtained kinetic parameters from non-isothermal DSC polymerization of
initiated by 1.0 mol% of Bu SnOR initiators.
e-CL
a
3
ꢀ
a
ꢀ
1/2b(min)
Initiators
b
( C/min)
T
m
( C)
t
a
E (kJ/mol)
Kissinger
r2
Thus, at a constant
Eq. (6).
a
, the values of E
a
are directly determined by
Bu
Bu
Bu
3
3
3
3
SnOMe
SnOEt
5
215.0
233.8
253.0
263.3
39.1
21.3
14.8
11.8
52.4 ꢂ 0.75
0.99
0.98
0.97
0.98
1
0
ꢀ
ꢁ
1
5
d
dt
a
ÞÞ ꢁ E
a;a
2
0
ln
¼ lnðA
a
fð
a
(6)
RT
a;i
a
;i
5
228.3
246.0
254.5
269.0
40.0
21.7
14.8
12.0
70.3 ꢂ 1.06
75.9 ꢂ 0.78
78.0 ꢂ 1.24
where the subscript
a
refers to the value at a particular conversion
1
0
and i refers to data for a given heating rate. Moreover, more
accurate method in estimation of E was reported by Starink
27,30] which is based on the relationship of ln(
1
5
2
0
a
1.92
[
b/T
) and 1/T as
SnOnPr
SnOnBu
5
232.0
249.3
256.5
270.5
40.9
22.0
14.9
12.2
shown in Eq. (7).
1
0
!
1
5
ꢀ
ꢁ
b
E
RT
a;
a
a
2
0
5
ln
¼ Constant ꢁ 1:0008
(7)
1
:92
T
a
;i
Bu
234.2
251.0
258.5
272.0
41.5
22.1
15.6
12.5
1
0
1
5
From the obtained values of E
a
from both methods, the
with monomer conversion is illustrated in
obtained from the
Friedman and Starink isoconversional methods for the ROP of -CL
2
0
dependency of E
a
a
Fig. 6. The results reveal that the values of E
a
Temperature at maximum rate of polymerization is reached.
Time require for 50% of monomer conversion.
b
e

6
W. Limwanich et al. / Thermochimica Acta 599 (2015) 1–7
3
Scheme 1. The coordination insertion mechanism of Bu SnOR initiated ROP of e-CL.
Table 2
Number average molecular weight (M
n
), weight average molecular (M
w
), molecular weight distribution (MWD) and %yield of PCLs from bulk polymerization of
e-CL
ꢀ
initiated by 0.25 mol% of Bu
3
SnOR initiators at 120 C for 72 h.
Initiators
[I] (mol%)
0.25
[M]/[I]a
400
Mn;NMRb
M c
Mwc
MWDc
%Yieldd (%)
n
3
4
4
4
4
4
Bu
Bu
Bu
Bu
3
3
3
3
SnOMe
SnOEt
SnOnPr
SnOnBu
5.7ꢃ10
1.1 ꢃ10
2.0 ꢃ10
2.0 ꢃ10
3.2 ꢃ10
3.7 ꢃ10
1.8
1.4
1.9
1.9
85
86
88
89
3
4
4
4
6.1 ꢃ10
8.3 ꢃ10
1.4 ꢃ10
1.7 ꢃ10
3
4
1.4 ꢃ10
2.0 ꢃ10
a
Molar ratio of monomer to initiator.
b
c
Determined from the end group analysis using 1H NMR technique, Mn;NMR ¼ ½ð1 þ 1ðICH2 OCO=ICH2 OHÞÞ ꢃ MwCLꢄ þ MwOR
ꢀ
GPC measurements in THF at 35 C calibrated with polystyrene standard.
d
Amount of polymer formed after precipitation in methanol.
initiated by Bu
Bu SnOnPr and Bu
from isoconversional kinetics analysis are similar to the peak
method of Kissinger. Furthermore, the E values obtained from
3
SnOMe are lower than those of Bu
3
SnOEt,
To investigate the efficiency of Bu
3
SnOR initiators in the
-CL was conducted
3
3
SnOnBu, respectively. The E values obtained
a
synthesis of PCL, the bulk polymerization of e
ꢀ
1
at 120 C for 72 h. From GPC and H NMR analyses listed in Table 2,
it is found that the molecular weight of PCL increases with
a
Starink isoconversional method are constant throughout polymer-
ization implying that the polymerization reaction proceeds
through a single coordination insertion mechanism. However,
3 3
increasing of alkoxy chain length of Bu SnOR. The Bu SnOnBu
produces the highest molecular weight of PCL among initiators
used in this work. However, the synthesized PCLs show MWD
values close to 2 indicating the occurrence of transesterification
the values of E
method are different from Starink method. This may be caused
by the uncertainty in determination of d /dt which is sensitive to
baseline quality [27]. In addition, for the least active Bu SnOnBu,
the E increases with monomer conversion due to the increasing
a
determined from Friedman isoconversional
reaction. Moreover, the %yields of PCLs synthesized from Bu
initiators are higher than 85%. The results demonstrate that
Bu SnOR (R = Me, Et, nPr and nBu) are the efficient initiator in the
ROP of -CL.
3
SnOR
a
3
3
a
e
viscosity of reaction mixture which reduces the coordination
ability of monomer and active center.
4. Conclusions
3.3. Bulk polymerization of e-caprolactone and mechanistic
Non-isothermal DSC technique is a convenient method to study
consideration
kinetics of ROP of
different alkoxy (OR) chain length (R = Me, Et, nPr and nBu). The
synthesized Bu SnOR can effectively initiate ROP of -CL at the
condition used in non-isothermal DSC experiments. The values of
obtained from Kissinger method for Bu SnOMe are lower than
those of Bu SnOEt, Bu SnOnPr and Bu SnOnBu, respectively. From
the Friedman and Starink isoconversional kinetics analysis, the
reactivity of Bu SnOMe is found to be highest compared to other
initiators. The results from non-isothermal DSC show the
decreasing of E with decreasing of alkoxy chain length and
increasing of the nucleophilicity of Bu SnOR. The molecular weight
of PCL synthesized from Bu SnOnBu initiator was higher than
Bu SnOnPr, Bu SnOEt, Bu SnOMe, respectively. Moreover, the
3
e-CL initiated by the synthesized Bu SnOR with
From the kinetics results, it is found that the reactivity of
3
e
Bu
efficiency of Bu
of -CL initiated by Bu
coordination insertion mechanism as depicted in Scheme 1. The
polymerization starts with the coordination of -CL and Bu SnOR
-CL with tin atom. The
-CL by the nucleophilic
-CL, then the
propagating species (—Sn—O—) is formed. The propagation step
then proceeds by the insertion of -CL into reactive Sn—O bond.
3
SnOR depends on the size of alkoxy group. In this part, the
SnOR in the synthesis of PCL is discussed. The ROP
SnOR occurs through the conventional
3
E
a
3
e
3
3
3
3
e
3
3
by the interaction of carbonyl oxygen of
second step is the acyl-oxygen cleavage of
e
e
a
attack of —OR group to carbonyl carbon of
e
3
3
e
3
3
3

W. Limwanich et al. / Thermochimica Acta 599 (2015) 1–7
[15] A. Stjerndahl, A.F. Wistrand, A.C. Albertsson, Industrial utilization of tin-
7
results from this work may be applied to describe the catalytic
initiated resorbable polymers: synthesis on a large scale with a low amount of
initiator residue, Biomacromolecules 8 (2007) 937–940.
behavior of other initiating systems in ROP of e-CL.
[
[
[
16] H.R. Kricheldorf, H.H. ThienBen, Polylactones 60: comparison of the reactivity
Acknowledgements
of Bu SnOEt, Bu Sn(OEt) , and BuSn(OEt) as initiators of
(
e
-caprolactone
2004), J. Macromol. Sci., Pure Appl. Chem. 41 (2004) 335–343.
17] N. Lawan, S. Muangpil, N. Kungwan, P. Meepowpan, S.V. Lee, W. Punyodom, Tin
IV) alkxoide initiator design for poly( -lactide) synthesis using DFT
3
2
2
3
W. Limwanich would like to thank the Research Professional
Development Project under the Science Achievement Scholarship
of Thailand (SAST) for graduate fellowship. In addition, we wish to
thanks the financial supports from the National Research Council
of Thailand (NRCT) and the National Research University Project
under Thailand’s Office of the Higher Education Commission.
Department of Chemistry, Faculty of Science and the Graduate
School of Chiang Mai University are also acknowledged.
(
D
calculations, Comp. Theor. Chem. 1020 (2013) 121–126.
18] M. Dumklang, N. Pattawong, W. Punyodom, P. Meepowpan, R. Molloy, M.
Hoffman, Novel tin(II) butoxide for use as initiators in the ring-opening
polymerization of e-caprolactone, Chiang Mai J. Sci. 36 (2) (2009) 136–148.
[19] C. Sattayanon, N. Kungwan, W. Punyodom, P. Meepowpan, S. Jungsuttiwong,
Theoretical investigate on the mechanism and kinetics of the ring-opening
polymerization of
e-caprolactone initiated by tin(II) alkoxides, J. Mol. Model.
19 (2013) 5377–5385.
[20] A.R. Hernandez, A.M. Richa, Ring opening polymerization of caprolactone
initiated by decamolybdate anion: determination of kinetic and
1
thermodynamic parameters by DSC and H NMR, J. Appl. Polym. Sci.115 (2010)
References
2
288–2295.
[
21] S.P. Huo, G.M. Wu, J. Chen, G.F. Liu, Z.W. Kong, Curing kinetics of lignin and
cardanol based novolac epoxy resin with methyl tetrahydrophthaliic
anhydride, Thermochim. Acta 587 (2014) 18–23.
[
1] A.C. Albertsson, I.K. Varma, Recent developments in ring opening
polymerization of lactones for biomedical applications, Biomacromolecules 4
(
2003) 1466–1486.
[22] I. Mondragon, L. Solar, I.B. Recalde, C.M. Gomez, Cure kinetics of a cobalt
catalysed dicyanate ester monomer in air and argon atmospheres from DSC
data, Thermochim. Acta 417 (2004) 19–26.
[
2] K.M. Stridsberg, M. Ryner, A.C. Albertsson, Controlled Ring-opening
Polymerization: Polymers with Designed Macromolecular Architecture,
Springer-Verlag, Berlin Heidelberg, 2002.
[23] P. Li, A. Zerroukhi, J. Chen, Y. Chalamet, T. Jeanmaire, Z. Xia, Kinetics study of Ti
[
3] R.M. Rasal, A.V. Jonorkar, D.E. Hirt, Poly(lactic acid) modifications, Prog. Polym.
Sci. 35 (2010) 338–356.
4] M.P.F. Pepels, M. Bouyahyi, A. Heise, R. Duchateau, Kinetic investigation on the
catalytic ring-opening (co) polymerization of (macro) lactones using
aluminum salen catalysts, Macromolecules 46 (2013) 4324–4334.
5] A.K. Sugih, F. Picchioni, H.J. Heeres, Experimental studies on the ring opening
2 4 2 4
[O(CH ) OCHCH ] initiated ring-opening polymerization of e-caprolactone
by differential scanning calorimetry, J. Appl. Polym. Sci. 110 (2008)
[
3990–3998.
[24] R. Mazarro, A.D. Lucas, I. Gracia, J.F. Rodriguez, Kinetic study of D,L-lactide and
glycolide homopolymerizations by differential scanning calorimetry,
Macromol. Chem. Phys. 209 (2008) 818–824.
[
[
[
polymerization of p-dioxanone using an Al(OiPr)
system, Eur. Polym. J. 45 (2009) 155–164.
3
-monosaccharide initiator
[25] D.B. Tkatchenko, O. Douteau, S. Stutzmann, Reactivity of carbon dioxide with
n-butyl(phenoxy)-, (alkoxy)-, and (oxo) stannanes: insight into dimethyl
carbonate synthesis, Organometallics 19 (2000) 4563–4567.
[26] D. Manral, R. Shah, M. Chandra, Mono-, di- and tributyltin derivatives of 1-
hydroxy-2-naphthoic and 3-hydroxy-2-naphthoic acids, Asian J. Chem. 6
(1994) 323–336.
[27] S. Vyazovkin, A.K. Burnham, J.M. Criado, L.A. Perez-Maqueda, C. Popescu, N.
Sbirrazzuoli, ICTAC kinetics committee recommendations for performing
kinetic computations on thermal analysis data, Thermochim. Acta 520 (2011)
1–19.
6] D. Deivasagayam, F. Peruch, Titanium complexes based on aminodiol ligands
for the ring opening polymerization of - and -lactide, Polymer 52 (2011)
686–4693.
7] W. Meelua, R. Molloy, P. Meepowpan, W. Punyodom, Isoconversional kinetic
analysis of ring-opening polymerization of -caprolactone: steric influence of
titanium(IV) alkoxides as initiators, J. Polym. Res. 19 (9799) (2012) 1–11.
8] A. Kowalski, J. Libiszowski, A. Duda, S. Penczek, Polymerization of -dilactide
initiated by tin(II) butoxide, Macromolecules 33 (2000) 1964–1971.
L
D,L
4
e
[
[
L,L
9] M. Kalmi, M. Lahcini, P. Castro, O. Lehtonen, A. Belfkira, M. Leskela, T. Repo,
Tetrakis Sn(IV) alkoxides as novel initiators for living ring-opening
polymerization of lactides, J. Polym. Sci. Part A: Polym. Chem. 42 (2004)
[28] H.E. Kissinger, Variation of peak temperature with heating rate in differential
thermal analysis, J. Res. Natl. Bur. Stand. 57 (1956) 217–221.
[29] H.E. Kissinger, Reaction kinetics in differential thermal analysis, Anal. Chem.
29 (1957) 1702–1706.
1901–1911.
[
[
10] A. Kowalski, A. Duda, S. Penczek, Kinetics and mechanism of cyclic esters
[30] M.J. Starink, The determination of activation energy from linear heating rate
experiments: a comparison of the accuracy of isoconversion methods,
Thermochim. Acta 404 (2003) 163–176.
[31] A. Kleawla, R. Molloy, W. Naksata, W. Punyodom, Ring-opening polymerization
of caprolactone using novel tin(II) alkoxide initiators, Adv. Mater. Res. 55–57
(55) (2008) 757–760.
[32] H.L. Friedman, Kinetics of thermal degradation of char-forming plastics from
thermogravimetry. Application to a phenolic plastic, J. Polym. Sci. Part C:
Polym. Symp. 6 (1964) 183–195.
polymerization initiated with tin(II) octoate. 3. Polymerization of L,L-dilactide,
Macromolecules 33 (2000) 7359–7370.
11] A. Kowalski, J. Libiszowski, T. Biala, M. Cypryk, A. Duda, S. Penczek, Kinetics and
mechanism of cyclic esters polymerization initiated with tin(II) octoate.
Polymerization of
amines, Macromolecules 38 (2005) 8170–8176.
e-caprolactone and L,L-lactide co-initiated with primary
[
12] M. Sobczak, W. Kolodziejski, Polymerization of cyclic esters initiated by
carnitine and tin(II) octoate, Molecules 14 (2009) 621–632.
[
13] M. Sobczak, Ring-opening polymerization of cyclic esters in the presence of
choline/SnOct catalytic system, Polym. Bull. (2012) 2119–2228.
14] H.R. Kricheldorf, M.V. Sumbel, I.K. Saunders, Polylactones. 20. Polymerization
of -caprolactone with tributyltin derivatives: a mechanistic study,
Macromolecules 24 (1991) 1944–1949.
[33] S. Vyazovkin, Model-free kinetics: staying free of multiplying entities without
necessity, J. Therm. Anal. Calorim. 83 (1) (2006) 45–51.
[34] J. Wang, M.P.G. Laborie, M.P. Wolcott, Comparison of model-free kinetic
methods for modeling the cure kinetics of commercial phenol-formaldehyde
resins, Thermochim. Acta 439 (2005) 68–73.
2
[
e