Synthesis, characterization, and catalytic activity of titanium iminophenoxide complexes in relation to the ring-opening polymerization of L -lactide and ε-caprolactone

Article abstract of DOI:10.1002/pola.26381

This study synthesized a series of titanium iminophenoxide complexes and investigated their suitability as catalysts for the ring-opening polymerization of L-lactide (L-LA) and ε-caprolactone (CL). Complexes with bidentate ligands demonstrate higher catal

Full text of DOI:10.1002/pola.26381

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Synthesis, Characterization, and Catalytic Activity of Titanium
Iminophenoxide Complexes in Relation to the Ring-Opening
Polymerization of L-Lactide and e-Caprolactone
Hsuan-Ying Chen, Wei-Yi Lu, Yen-Jen Chen, Sodio C. N. Hsu, Siou-Wei Ou,
Wei-Te Peng, Nai-Yuan Jheng, Yi-Chun Lai, Bo-Sheng Wu, Hsuan Chung,
Yun Chen, Ta-Chou Huang
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, 100, Shih-Chuan 1st Road,
Kaohsiung, 80708, Kaohsiung, Taiwan, Republic of China
Correspondence to: H.-Y. Chen (E-mail: hchen@kmu.edu.tw)
Received 8 August 2012; accepted 8 September 2012; published online 12 October 2012
DOI: 10.1002/pola.26381
ABSTRACT: This study synthesized a series of titanium imino-
phenoxide complexes and investigated their suitability as
catalysts for the ring-opening polymerization of L-lactide (L-LA)
and e-caprolactone (CL). Complexes with bidentate ligands
demonstrate higher catalytic activity than their tridentate coun-
terparts since the third coordination atom needs to contend
with L-LA and CL. Differences in the geometric framework of
bidentate ligands also influence the catalytic activity. Type II
ligands (N, N-trans form of Ti complex) prevent the coordina-
tion of monomers to Ti thereby decreasing the initiation rate.
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KEYWORDS: e-caprolactone; L-lactide; biomaterials; catalysts;
polyesters; ring-opening polymerization; Schiff base; titanium
complex
INTRODUCTION Ligands are crucial to organometallic cataly-
sis, like olefin polymerization and asymmetric synthesis, due
to their ability to improve reactivity and selectivity. Schiff base
ligands are commonly selected due to their diversity and sim-
plicity of preparation, and Schiff base metal-supported com-
plexes can be found in a wide range of applications.1 Schiff
base complexes such as Zn,2 Mg,3 Ca,4 and alkali metals5 have
been used in the ring-opening polymerization (ROP) of cyclic
esters, such as rac-lactide (rac-LA) and e-caprolactone (CL)
and the effectiveness of Schiff base ligands in protecting the
catalytic center from trans-esterification and enhancing reac-
tivity has been widely demonstrated. Biodegradable polyesters
are commonly employed in the encapsulation and delivery of
proteins, the formation of hydrogels, and various medical
applications including the development of microspheres and
drug delivery systems.6 Metal residing within the polymer
must be taken into consideration in applications involving bio-
materials. As a result, non-cytotoxic metal complexes such as
titanium complexes7 have been widely researched in connec-
tion with lactide (LA) and CL polymerization. In the following,
we provide a recent survey of cyclic ester polymerization
using Ti complexes with a Schiff base.
could be reversibly oxidized/reduced. The reduced form
demonstrated a propagation rate 30-fold greater than that of
the oxidized form, following the addition of AgOTf. The elec-
tron-donating substituent within the salen structure
increases the rate of LA propagation.
The same group synthesized 1,10-ferrocenediyl salicylaldi-
mine ligands and their titanium alkoxide complexes [Fig.
1(b)]. The Ti complex with N-2,6-diisopropylphenyl-substi-
tuted ligand showed slightly greater activity than with N-
phenyl-substituted ligand in rac-LA polymerization. The Ti
complex supported with 1-phosphino-10-hydroxyferrocene
demonstrated greater activity than 1,10-ferrocenediyl salicy-
laldimine due to the relatively low coordination number of
the titanium in this complex.
Do and coworkers10 synthesized chiral tridentate Schiff base
ligands and their chlorotitanium complexes [Fig. 1(c)]. Their
results demonstrated a high degree of activity in the con-
trolled polymerization of L-LA, despite the lack of normal ini-
tiators such as alkoxides or amides. Titanium complex sup-
ported by the tert-butyl-substituted ligand was more active
than the hydrogen-substituted ligand and trichlorotitanium
complex showed slightly greater activity than dichlorotita-
nium complex.
Gibson et al.8 synthesized salen ligated titanium alkoxide
complex using two ferrocene molecules [Fig. 1(a)], which
Additional Supporting information may be found in the online version of this article.
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FIGURE 1 Titanium or zirconium alkoxide complexes supported by Schiff base ligands. (a) Salen ligand with two ferrocene mole-
cules.8 (b) 1-Phosphino-10-hydroxyferrocene and 1,10-ferrocenediyl salicylaldimine.9 (c) chiral tridentate Schiff base ligands.10 (d)
chiral bi- and tridentate Schiff base ligands.11
Davidson and coworkers11 synthesized chiral bidentate Schiff
base ligands and their titanium and zirconium complexes
[Fig. 1(d)] The titanium complexes were inactive for the ROP
of rac-LA; however, the zirconium complexes demonstrated a
high degree of activity in toluene at both 20 ꢀC and 80 ꢀC.
Polymerization is insensitive to ligand substitution and chir-
ality. In a solvent-free system, both the titanium and zirco-
nium complexes demonstrated activity; the former displayed
greater control in polymerization than the latter, but lacked
selectivity.
FIGURE 2 Titanium alkoxide complexes supported by various bi- and tri-dentate Schiff base ligands.
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FIGURE 3 Preparation of (L1–L6)2TiOiPr2 compounds.
Johnson et al.12 synthesized a variety of bidentate and tri-
dentate Schiff base ligands and their titanium alkoxide com-
plexes. It was revealed that four types of titanium alkoxide
complexes are dependent on the steric effect of ligands
(Fig. 2). However, the author did not report the ROP of CL or
LA using these complexes as catalysts.
with a stoichiometric quantity of titanium isopropoxide in
toluene to produce a moderate yield of Ti compounds
(Fig. 3). According to Johnson’s description,12 the titanium
alkoxide complexes of L1–L5 are Type I and (L6)2Ti(OiPr)2 is
Type II.
Polymerization of e-Caprolactone and L-Lactide
Researchers have demonstrated that the various steric
effects of Schiff base ligands can alter the coordinated form
of titanium alkoxide complexes; however, no studies have
dealt with the relationship between the rate of ROP and
these coordinated forms. Gibson and coworkers9 indicated
that coordination numbers can influence the activity of LA
ROP; however, the evidence was inconclusive because not all
of the ligands were Schiff base ligands. This study synthe-
sized six Schiff base ligands and their titanium alkoxide com-
plexes, and investigated the influence of the coordinated
forms and coordination numbers on the ROP of CL and LA
(Fig. 3).
This study investigated the polymerizations of LA and CL in
toluene using Ti complexes as initiatoꢀrs. The experiments
were conducted under nitrogen at 100 C (Table 1). Table 1
illustrates the degree of catalytic activity in the polymeriza-
tion of LA and CL associated with various Ti complexes and
their related ligands. As shown in Table 1 Ti complexes with
bidentate ligands (Entries 3 and 9) demonstrated the greater
activity than tridentate ligands (Entries 1, 2, 7, and 8) and
complexes with N-alkyl-substituted tridentate ligands were
more active than N-aryl-substituted tridentate ligands (Entry
1 vs. 7, 2 vs. 8, and 3 vs. 9). The molecular weight (NMR) of
polymers catalyzed using these Ti complexes was lower than
the expected value and Ti complexes with bidentate ligands
(entries 3 and 9) demonstrated poor controllability with the
broad polydispersity index (PDI). The reason that bidentate
ligands demonstrated a less pronounced chelation effect may
be due their lack of a third coordinated atom, which
RESULTS AND DISCUSSION
Synthesis and Characterization of Ti Complexes
All L1-H to L6-H ligands were prepared according to the pro-
cedures outlined by Johnson et al.12 L1-H to L6-H reacted
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TABLE 1 Polymerization of e-Caprolactone and L-Lactide Using Each of the Ti Complexes as an Initiator at 100 8C
Conv.a (%)
Mn(Cal)
Mn(NMR)
Mn(GPC)
Pd
PDIc
b
a
c
Entry
Cat. L¼
Time (h)
1e
2e
3f
4g
5h
6i
L1
L2
L3
L3
L3
L3
L4
L5
L6
L1
L2
L3
L4
L5
L6
64.33
39.75
0.30
0.33
0.58
0.75
7.00
2.55
1.00
68.00
71.67
0.33
7.50
2.25
1.75
93
92
93
95
89
91
89
90
92
90
62
90
87
91
89
5,400
5,300
5,400
5,500
5,100
5,200
5,100
5,200
5,300
6,500
4,500
6,500
6,300
6,600
6,500
e
5,400
3,200
2,000
1,200
3,700
1,300
4,600
4,100
2,700
4,100
6,300
4,700
4,900
5,700
6,000
6,300
6,000
2,100
2,000
5,500
6,900
7,600
15,800
0.16
1.17
1.23
1.72
1.69
1.40
1.28
1.17
1.25
1.46
1.26
1.27
1.19
1.26
1.26
1.39
0.26
35.34
32.81
17.49
13.83
1.45
7e
8e
9e
10j
11j
12j
13j
14j
15j
a
4.02
4,300
k
10.49
0.19
k
k
k
k
k
0.13
39.27
1.67
5.82
7.32
Obtained from 1H NMR analysis.
Conditions: toluene (5 mL), [CL] ¼ 4.0 M, [CL]:[Cat] ¼ 100:1.
Conditions: toluene (20 mL), [CL] ¼ 1.0 M, [CL]:[Cat] ¼ 100:1.
Conditions: toluene (25 mL), [CL] ¼ 0.8 M, [CL]:[Cat] ¼ 100:1.
Conditions: toluene (40 mL), [CL] ¼ 0.5 M, [CL]:[Cat] ¼ 100:1.
Conditions: toluene (50 mL), [CL] ¼ 0.4 M, [CL]:[Cat] ¼ 100:1.
Conditions: toluene (5 mL), [LA] ¼ 2.0 M, [LA]:[Cat] ¼ 100:1.
Unavailable
b
f
Calculated from the molecular weight of monomer ꢁ [monomer]0/
g
h
i
2[Cat]0 ꢁ conversion yield þ Mw(O Pr)
i
.
c
Obtained from GPC analysis and calibration based on the polystyrene
standard. Values in parentheses are the values obtained from GPC
times 0.58 for PLA13 and 0.56 for PCL.14
j
d
k
The productivity ¼ the polymer weight/mol of cat. ꢁ time (kg/mol h).
increased the space available for the monomer to associate
with the metal center. However, this also made it easier for
the polymer chain to backbite to the metal causing trans-
esterification with broad PDI. This situation was resolved by
reducing the concentration of the catalyst and CL (Entries 4,
5, and 6). Results similar to those of LA polymerization by Ti
complexes are shown in Entries 10–15; however the PDI of
the polymers can only be controlled up to less than 1.39.
The molecular weights of polymers catalyzed by Ti com-
plexes with N-aryl-substituted ligands (Entries 10, 11, and
12) were higher than those of N-alkyl-substituted ligands
(Entries 13, 14, and 15). L3H and L6H are both stereo-bulky
bidentate ligands; however, the Ti complex with L3 showed
greater activity than the complex with L6. This is probably
due to the framework of Ti complexes. (L3)2Ti(OiPr)2 is Type
I whereas (L6)2Ti(OiPr)2 is Type II, as shown in Figure 4.
The middle structure in Figure 4 is the L2Ti(OiPr)2 model, in
which three positions are available for the monomer to coor-
dinate to Ti. However, OiPr initiators are unable to initiate
the monomer in position 3; therefore, it is of no benefit to
the polymerization. tButyl groups are capable of blocking
positions 2 and 3 in (L3)2Ti(OiPr)2, while isopropyl groups
are capable of blocking positions 1 and 2 in (L6)2Ti(OiPr)2.
This decreases the effectiveness with which the monomer
coordinates to Ti, which reduces catalytic activity. The 1H
NMR spectra of PCL and PLA are shown in Supporting Infor-
mation Figures S6 and S7.
the resulting polymers (Supporting Information Table S1 and
Fig. S1). The results showed that the Ti complexes with L1
and L2 provide better control with narrow PDI (less than
1.21) than other Ti complexes; however, this is accompanied
by a drop in catalytic activity. It appears that ligands with
multi-dentate atoms are capable of protecting the metal c-
enter against the trans-esterification. N-alkyl-substituted
ligands are more flexible than N-aryl-substituted ligands in
their ability to coordinate to the metal; however, this
decreases the likelihood of monomer coordination.
The control of LA polymerization using (L4)2Ti(OiPr)2 is evi-
dent in the linear relationship between Mn(GPC) and
To compare the polymerization control using these Ti com-
plexes, polymerization using various ratios of CL and cata-
lysts were studied, respectively, to identify the properties of
FIGURE 4 Spatial structure of Type I and Type II Ti complexes.
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FIGURE 5 Linear plot of Mn(GPC) vs. [LA]0
ꢁ
conv./
[(L2)2Ti(OiPr)2]0, with polydispersity indexes indicated by closed
circles (Table 2Entries 5–8).
[monomer]0 ꢁ conv./[(L2)2Ti(OiPr)2]0 (Table 2 Fig. 5), as
well as the polymers with low PDIs, ranging from 1.22 to
1.34.
Kinetic Studies of the Polymerization of CL and LA
Catalyzed by (L4)2Ti(OiPr)2
ꢀ
Kinetic studies were performed at 100 C using the following
ratios: [M]0/[(L4)2Ti(OiPr)2] ([CL] ¼ 4 M in toluene 5 mL
and [LA] ¼ 2 M in CDCl3 1 mL) shown in Supporting Infor-
mation Tables S2 and S3 and Figures S2 and S4. Preliminary
results indicate a first-order dependency on [CL] and [LA]
(Supporting Information S2 and S4). By plotting lnkobs vs. ln
[(L4)2Ti(OiPr)2], (L4)2Ti(OiPr)2 orders of both 1, kappa val-
ues of 7.51 and 3.41 were discovered for CL and LA (Sup-
porting Information Fig. S3 and S5), respectively. The poly-
merization of CL and LA using (L4)2Ti(OiPr)2 at 100ꢀC
demonstrated the following rate law:
ÂÀ
Á
À
Á Ã
1
1
d½CLꢂ=dt ¼ 7:51 ꢁ ½CLꢂ L4 2Ti OiPr
2
ÂÀ
Á
À
Á Ã
1
1
d½LAꢂ=dt ¼ 3:41 ꢁ ½LAꢂ L4 2Ti OiPr
2
TABLE 2 Controlled L-Lactide Polymerizations of Using
(L4)2Ti(OiPr)2 as an Initiator at 100 8C.
Time Conv.a
FIGURE 6 Possible mechanisms underlying the polymerization
Entry [M]/[Ti] (h)
(%)
Mn(Cal) Mn(GPC) Pd
PDIc
b
c
by hexa-coordinated Ti complex.
1e
2e
3e
4e
a
100/1
150/1
200/1
250/1
7.50
87
5,000
7,800
6,000
9,200
1.67 1.22
2.16 1.25
1.53 1.32
13.33 91
17.33 92
17.33 90
Mechanistic Studies of Polymerization
10,600 11,100
13,000 14,000
According to the kinetic characteristics, one monomer is con-
sumed in every polymerization cycle and the Ti complexes
retain a mononuclear form because the order of the mono-
mer and Ti complex is 1. There are two possible polymeriza-
tion mechanisms. One involves a monomer associating at the
position of dissociation of nitrogen in the ligand forming the
hexa-coordinated form (Mechanism A in Fig. 6). The other is
a monomer bonding to Ti from position 1 or 2 (the middle
structure in Fig. 4), forming the hepta-coordinated form
3.6
1.34
Obtained from 1H NMR analysis.
b
Calculated from the molecular weight of monomer ꢁ [monomer]0/
2[Cat]0 ꢁ conversion yield þ Mw(O Pr).
i
c
Obtained from GPC analysis and calibrated using the polystyrene
standard. Values in parentheses are the values obtained from GPC
times 0.58 for PLA.
d
The productivity ¼ the polymer weight / mol of cat. ꢁ time (kg/mol h).
e
Condition: toluene (5 mL), [(L4)2Ti(OiPr)2] ¼ 0.02 M.
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(Mechanism B in Fig. 6). Assuming that Mechanism A is the
correct, the left side of Figure 6 shows the intermediates of
(L3)2Ti(OiPr)2 and (L6)2Ti(OiPr)2 during polymerization.
(L6)2Ti(OiPr)2 should have greater catalytic activity than
(L3)2Ti(OiPr)2 because there are two isopropoxide groups in
the cis-position of the monomer in (L6)2Ti(OiPr)2, which
determines the ring-opening rate. However, data related to
polymerization returned the opposite result, thereby disqual-
ifying Mechanism A as a candidate. The actual mechanism is
most likely Mechanism B, as previously reported.15 The
crowded situation enhanced the initiation of isopropoxide to
monomer, releasing in the unstable energy of repulsion, the
repeated coordination of monomers, and the initiation of alk-
oxide to produce the polymer. L1, L2, L4, and L5 Ti complexes
displayed low activity due to the coordination of the mono-
mer having to compete against the third coordinated atom of
the ligand, which decreased the polymerization rate.
Synthesis of (L2)2Ti(OiPr)2
A mixture of L2-H (4.185 g, 20 mmol) and Ti(OiPr)4 (2.843
g, 10 mmol) in THF (10 mL), was stirred for 12 h. Volatile
materials were removed under vacuum to give yellow pow-
der and then it was washed with hexane (20 mL) and a light
yellow powder was obtained after filtration. Yield: 4.11 g
(58 %). 1H NMR (CDCl3, 200 MHz): d 8.06 (2H, s, CH¼¼N),
7.25–7.40, 6.72–6.83 (8H, m, ArH), 4.63 (2H, sept, J ¼ 6.0
Hz, OCH(CH3)2), 4.59 (2H, t, J ¼ 4.4 Hz, CH(OCH3)2), 3.47
(4H, d, J ¼ 4.4 Hz, CH2CH(OCH3)2), 3.23 (12H, s, CH(OCH3)2),
1.06 ppm (12H, br, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): d
164.19 (C¼¼N), 167.92, 134.27, 133.77, 122.05, 119.03,
117.12 (Ar), 103.82 (CH2CH(OCH3)2), 78.21 (OCH(CH3)2),
63.39 (CH2CH(OCH3)2), 55.29 (CH2CH(OCH3)2), 55.10
(OCH3), 25.27 (OCH(CH3)2). Anal. Calcd (found) for
C
28H42N2O8Ti (582.78): ꢀC, 57.73 (56.90); H, 7.27 (7.36); N,
4.81 (4.98)%. m.p.: 116 C.
Synthesis of (L5)2Ti(OiPr)2
EXPERIMENTAL
Using a method similar to that for (L2)2Ti(OiPr)2. Yield: 4.55
g (59%). 1H NMR (CDCl3, 200 MHz): d 7.80 (2H, s, CH¼¼N),
7.43, 7.10–6.90 (4H, m, ArH), 6.61 (2H, t, J ¼ 7.4 Hz, ArH),
6.14 (2H, d, J ¼ 7.8 Hz, ArH), 4.90 (2H, sept, J ¼ 6.2 Hz,
OCH(CH3)2), 2.10 (6H, s, SCH3), 1.21 ppm (12H, d, J ¼ 6.2
Hz, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): d166.77 (COH),
151.43 (C¼¼N), 164.79, 134.09, 125.41, 123.90, 118.85,
116.83 (Ar), 78.22 (OCH(CH3)2), 25.50 (OCH(CH3)2), 15.85
(SCH3). Anal. Calcd (found) for C34H38N2O4S2Ti (650.96): C,
Standard Schlenk techniques and a N2-filled glovebox were
used in all the isolation and treatment of all the compounds.
Solvents, CL, L-LA, and deuterated solvents were purified prior
to use. Sodium hydride, salicylaldehyde, tert-butylamine, N, N-
dimethylethylenediamine, o-anisidine, 2,6-diisopropylaniline,
and 2-(methylthio)aniline were purchased from Alfa. 2,2-
Dimethoxyethylamine and benzyl alcohol were purchased
from Aldrich. 1H and 13C NMR spectra were recorded on a
Varian Gemini2000-200 (200 MHz for 1H and 50 MHz for
13C) spectrometer with chemical shifts given in ppm from the
internal TMS or center line of CDCl3. Microanalyses were per-
formed using a Heraeus CHN-O-RAPID instrument. The gel
permeation chromatography (GPC) measurements were per-
formed on a Waters 1515 Isotratic HPLC pump system
equipped with a differential Waters 2414 refractive index de-
tector using THF (HPLC grade) as the eluent. The chromato-
graphic column was a Water Styragel Column (HR4E), and
the calibration curve was made by polystyrene standards to
calculate Mn(GPC). Ligands of L1-H to L6-H were prepared by
acid-catalyzed condensation following literature procedures.5
(L3)2Ti(OiPr)2, (L4)2Ti(OiPr)2, and (L6)2Ti(OiPr)2 were pre-
pared following literature procedures.12
ꢀ
63.01 (62.76); H, 5.99 (5.89); N, 4.25 (4.31) %. M.p.: 120 C.
General Procedures for the Polymerization of
e-Caprolactone and L-lactide
A typical polymerization procedure was exemplified by the
synthesis of Entry 7 (Table 1) using complex (L4)2Ti(OiPr)2
as a catalyst. The polymerization conversion was analyzed
by 1H NMR spectroscopic studies. Toluene (5.0 mL) was
added to a mixture of complex (L4)2Ti(ꢀOiPr)2 (0.1238 g, 0.2
mmol) and CL (2.2 g, 20 mmol) at 100 C. After the solution
was stirred for 7 h, the reaction was then quenched by add-
ing to a drop of ethanol, and the polymer was precipitated
pouring into n-hexane (30.0 mL) to give white solids. The
white solid was dissolved in CH2Cl2 (5.0 mL) and then n-hex-
ane (70.0 mL) was added to give white crystalline solid.
Yield: 1.25 g (57%).
Synthesis of (L1)2Ti(OiPr)2
A mixture of L1-H (3.845 g, 20 mmol) and Ti(OiPr)4 (2.843
g, 10 mmol) in THF (10 mL), was stirred for 12 h. Volatile
materials were removed under vacuum to give red oil and
then it was washed with hexane (20 mL) and a deep orange
oil was obtained. Yield: 3.890 g (58%). 1H NMR (CDCl3, 200
MHz): d 8.06 (2H, s, CH¼¼N), 7.21–7.39, 6.71–6.85, (8H, m,
ArH), 4.68 (sept, 2H, J ¼ 5.8 Hz, OCH(CH3)2) 3.48 (4H, br,
NCH2CH2N(CH3)2), 2.54 (2H, br, NCH2CH2N(CH3)2), 2.38 (2H,
br, NCH2CH2N(CH3)2), 2.07 (12H, s, N(CH3)2), 1.19 (d, 12H,
J ¼ 5.8 Hz, OCH(CH3)2). 13C NMR (CDCl3, 50 MHz): 163.13
(C¼¼N), 166.92, 133.24, 132.76, 121.29, 118.18, 116.29 (Ar),
77.17 (OCH(CH3)2), 59.17 (NCH2CH2N(CH3)2), 58.18 (NCH2
CH2N(CH3)2), 44.66 (NCH2CH2N(CH3)2), 24.62 (OCH(CH3)2).
Anal. Calcd (found) for C28H44N4O4Ti (548.8): C, 61.31
(60.50); H, 8.08 (7.83); N, 10.21 (10.05)%.
CONCLUSIONS
This study synthesized a series of iminophenol ligands (L1-H
to L6-H) and their titanium complexes. Changing the coordi-
nation number and size of the N-substituted group altered
the polymerization of CL and LA. Generally, if a Ti complex
with a bidentate ligand is changed to a tridentate one, the
degree of activity decreases dramatically, due to the third
coordination atom having to contend with the monomer. The
geometric framework also influences the catalytic activity for
ROP. Compared with the Type I Ti complex, the steric bulky
N-substituted group in the Type II Ti complex forms blocks
in more positions. This increases the effectiveness in pre-
venting the coordination of monomers to Ti and decreases
the initiation rate.
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ACKNOWLEDGMENTS
Financial support is from the National Science Council (Grant
NSC 99-2113-M-037 -012 -MY2). We thank Center for Research
Resources and Development at Kaohsiung Medical University
for the instrumentation and equipment support.
8 Gregson, C. K. A.; Gibson, V. C.; Long, N. J.; Marshall, E. L.;
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