Coordinating effect in ring-opening polymerization of ε-caprolactone using aluminum complexes bearing bisphenolate as catalysts

Article abstract of DOI:10.1039/c5ra10753a

A series of Al complexes bearing diphenolate ligands was synthesized and their application for the ring-opening polymerization of ε-caprolactone was studied. Positional variation of the substituent on the aryl ring of the RCH(4,6-di-t-butylphenol)₂ ligand was shown to have a considerable influence on the catalysis result. Complexes with a ortho-substituent showed greater catalytic activity than those with a para-substituent. Substitutions of an aryl moiety by H or methyl groups resulted in the catalytic activity falling between that of the ortho-substitution Al complexes and that of the para ones. Our results demonstrate that the coordinated functional group in the ortho-position of the phenyl ring could increase the catalytic activity. Moreover, X-rays of the structure and DFT analysis revealed that the coordinated functional group in the ortho-position could bridge two Al centers resulting in the transformation of a dinuclear Al complex with bridging benzyl alkoxide into a complex with terminal benzyl alkoxide, further promoting the efficacy of the initiator.

Full text of DOI:10.1039/c5ra10753a

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Coordinating Effect in Ring-opening Polymerization of ε-Caprolactone Using Aluminum
Complexes Bearing Bisphenolate as Catalysts
HsingꢀYin Chen,a YingꢀHsien Lee,a Michael Y. Chiang,ab WeiꢀYi Lu,a HsiꢀChing Tseng, HsinꢀYi
Tsai,a YuꢀHsieh Chen,a YiꢀChun Lai,a HsuanꢀYing Chen*a
a
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung,
Taiwan, 80708, R.O.C.
b Department of Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan, 80424, R.O.C.
Abstract
A series of Al complexes bearing diphenolate ligands were synthesized and their application
for the ringꢀopening polymerization of εꢀcaprolactone was studied. Positional variation of the
substituent on the aryl ring of RC*H(4,6ꢀdiꢀtꢀbutylphenol)2 ligand was shown to have a
considerable influence on the catalysis result. Complexes with orthoꢀsubstituent showed greater
catalytic activity than those with paraꢀsubstituent. Substitutions of aryl moiety by H or methyl
groups resulted in a catalytic activity falling between that of the ortho-substitution Al complexes
and that of the para ones. Our results demonstrate that the coordinated functional group in the
orthoꢀposition of the phenyl ring could increase the catalytic activity. Moreover, Xꢀrays of the
structure and DFT analysis revealed that the coordinated functional group in the orthoꢀposition
could bridge two Al centers resulting in the transformation of a dinuclear Al complex with bridging
benzyl alkoxide into a complex with terminal benzyl alkoxide, further promoting the efficacy of the
initiator.
Keywords: Aluminum complex, εꢀCaprolactone, Ringꢀopening polymerization
________________________________________________________________________________
* Corresponding Author: HsuanꢀYing Chen, Eꢀmail: hchen@kmu.edu.tw
Tel.: +886ꢀ7ꢀ3121101ꢀ2585, Fax: +886ꢀ7ꢀ3125339
Introduction
Poly(εꢀcaprolactone) (PCL) has established its practicability in a diversity of fields due to its
biodegradability, biocompatibility, and permeability1. The common method of synthesizing PCL is

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using metal complexes as catalysts for ringꢀopening polymerization (ROP) of
ε
ꢀcaprolactone (CL) 2.
Aluminum complexes are commonly used as catalysts for ROP due to their ease of synthesis and
the low cost of precursor materials.3ꢀ4 However, when metal alkoxides were used as initiators for
ROP, macrocycles from backbiting reactions always occur as side reactions commonly referred to
as “transesterification” shown in Scheme 1. The undesired backbiting reactions can be reduced by
using a suitable sterically bulky ligand to coordinate with the active center and therefore provide a
steric barrier to prevent larger polymer from coordination to minimize the side reactions but smaller
monomers still could interact coordinatively with metal center and further be activated. It was also
mentioned in the literature4i,4o that diphenol ligands can be useful in protecting the metal center
against transesterification, thereby maintaining well controlled polymer weight and narrow
polydispersity. Example like [(EDBP)Al(ꢀ
ꢀOBn)]24l was used by Lin et al. as successful catalyst to
carry out “living” or “immortal” ROP of lactones (EDBP =
2,2′ꢀethylideneꢀbis(4,6ꢀdiꢀtertꢀbutylphenol)). Further research revealed that even higher catalytic
activity was found when the ligand was changed form EDBP to
2,2’ꢀ(2ꢀmethoxybenzylidene)bis(4ꢀmethylꢀ6ꢀtertꢀbutylphenol) (MEBBP).4o It was reported that the
steric effect of ligands dictated the catalytic efficiency of the aluminum complexes. This appears to
be a reasonable explanation noting that the hydrogen on the tertiary carbon (C* in Figure 1) was
close to the Al atom as revealed in the crystal structure of the MEBBP aluminum complex (Figure
1). 4l,4o,5 This means the methoxy group of MEBBP is unable to coordinate with Al atom and
therefore had no coordinating effect on catalytic activity. However, 2ꢀmethoxyphenyl group is far
from Al center and cannot hinder the monomers. To prove that steric effect is the actual reason for
the superior performance of MEBBP in the catalysis of diphenolate Al complexes compared with
EDBP, a series of diphenolate Al complexes were synthesized and subjected to kinetic analysis in
order to examine the catalytic role of MEBBP moiety in polymerization of εꢀcaprolactone.

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Scheme 1. Transesterification
O
C*
H
O
O
O
Al
tBu
Figure 1. the structure of MEBBP Al complex
Figure 2. Synthesis of various Al complexes bearing diphenolate ligands
Results and Discussion
Synthesis and Characterization of Al Complexes.
MEBBPꢀlike diphenolate ligands, RC*H(4,6ꢀtꢀbutylphenol)2 (shown in Figure 2), were synthesized
following the literature report4o. The varying R groups on the diphenolate ligands were designed to
probe their different electronic or steric influence on catalytic behavior. All ligands reacted with a
stoichiometric quantity of trimethylaluminum in THF to produce a moderate yield of Al compounds
(Figure 2). The formula and structure were confirmed by 1H and 13C NMR spectra, elemental
analysis, and Xꢀray crystal analysis. The Xꢀray structure of FCl-AlMe (Figure 3) illustrates the
distorted tetrahedral geometry of the Al complex with THF and methyl group. The angles of
O(1)ꢀAlꢀO(2) and O(3)ꢀAlꢀC(36) are 115.35(9), and 103.87(12)°, respectively. The distances
between the Al atom and O(1), O(2), O(3), and C(36) are 1.7174(19), 1.7188(19), 1.886(2), and

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1.939(3) Å, respectively. Crystalꢀrelated data indicated the same situation that the hydrogen in C(7)
was close to Al atom. The distance between Al and H in C(7) atom is 2.466 Å.
Figure 3. Molecular structures of FCl-AlMe as 30% probability ellipsoids. CCDC number:
1032158 (hydrogen atoms were omitted for clarity)
Polymerization of
Polymerization of
as an initiator (Table 1). All aluminum complexes exhibited catalytic activity in polymerization of
ꢀcaprolactone and the substituent R in the bridging carbon (C* in Figure 2) highly influenced the
ε
-caprolactone.
εꢀcaprolactone using Al complexes was investigated using one equivalent BnOH
ε
catalytic activity. As shown in entries 1, 2, 4, 5, and 7 of Table 1, the coordinating groups in the
orthoꢀposition of the phenyl ring in the bridging carbon C* presented more pronounced catalytic
activity than the coordinating groups in the paraꢀposition (entries 3, 6, and 8). The polymerization
result using [Al(MEBBP)(
greater polymerization control with accurate molecular weight of PCL and narrow PDI but lower
polymerization rate. The possible reason may be that [Al(MEBBP)( ꢀOBn)]2 is pure Al alkoxide
ꢀ
ꢀOBn)]24o a catalyst was listed in entry 11 of Table 1 and it revealed the
ꢀ
but the mixture of Al alkyl complex and BnOH produces a few side product influenced the
polymerization result. In addition, higher concentration of [CL] and Al complex also increase the

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polymerization rate. A kinetic analysis was conducted to accurately compare the details of catalytic
activities exerted by these Al complexes, as shown in Table 2 and Figure 4.
Table 1. Results on polymerization of ε
ꢀcaprolactone catalyzed by different Al complexes at 50 oC
b
a
c
Entry
Cat.
Time
(min)
30
Conv.a
(%)
96
MnCal MnNMR MnGPC PDIc
1
2
FClꢀAlMe
oꢀBrAlMe
pꢀBrAlMe
oꢀClꢀAlMe
oꢀFꢀAlMe
11100 15200
11200 15300
10700 17800
11000 19500
10400 18900
10600 10600
11000 14100
15600 1.43
60
97
9500
9300
5900
6300
8000
1.27
1.30
1.33
1.24
1.63
3
170
80
93
4
95
5
80
90
6
pꢀFꢀAlMe
130
70
92
7
oꢀOMeꢀAlMe
pꢀOMeꢀAlMe
MeꢀAlMe
96
10500 1.28
8
130
90
77
8900
15300
9200
7400
1.30
1.27
9
90
10400 10800
10100 10700
11300 10800
10
11d
HꢀAlMe
90
88
10300 1.61
11100 1.09
[Al(MEBBP)
180
98
(ꢀꢀOBn)]2
Reaction condition: toluene (5 mL), [M]0/[Cat.]0/[BnOH]0 = 100:1:1, [CL] = 2.0 M, at 50 oC. a Obtained from 1H NMR analysis. b Calculated from the
molecular weight of monomer x [monomer]0/ [Cat]0 x conversion yield + Mw(BnOH). c Obtained from GPC analysis and calibration based on the
polystyrene standard. Values in MnGPC column are the values obtained from GPC times 0.56. d Reaction condition: toluene (30 mL),
[M]0/[Cat.]0/[BnOH]0 = 100:1:0, [CL] = 0.67 M, at 50 oC.4o

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Table 2. Kinetic study of polymerization of CL using each of the Al complexes as the catalysts with
1 equivalent of BnOH in toluene
Entry
Catalyst
FClꢀAlMe
oꢀBrꢀAlMe
oꢀClꢀAlMe
oꢀOMeꢀAlMe
oꢀFꢀAlMe
MeꢀAlMe
kobs (error)
0.1298 (49)
0.0659 (3)
0.0319 (15)
0.0304 (12)
0.0279 (23)
0.0270 (5)
0.0245 (4)
0.0197 (5)
0.0114 (2)
0.0109 (2)
1
2
3
4
5
6
7
HꢀAlMe
8
pꢀFꢀAlMe
pꢀOMeꢀAlMe
pꢀBrꢀAlMe
9
10
4.0
FCl-AlMe
o-BrAlMe
o-OMeAlMe
o-ClAlMe
o-FAlMe
MeAlMe
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
HAlMe
p-FAlMe
p-OMeAlMe
p-BrAlMe
-40
-20
0
20
40
60
80
100
120
140
Time (min)
Figure 4. Firstꢀorder kinetic plots of CL polymerizations with various Al complexes plotted against
time (■ FClꢀAlMe, ● oꢀBrꢀAlMe, ▲oꢀOMeꢀAlMe, ▼oꢀClꢀAlMe, ⊳ oꢀFꢀAlMe, ꢀ MeꢀAlMe, ◆
HꢀAlMe, ꢀ pꢀFꢀAlMe, ꢁ pꢀOMeꢀAlMe, ★ pꢀBrꢀAlMe,)

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Results of the kinetic study revealed a firstꢀorder dependency on [CL] and a descending order of
catalytic activity for different Al complexes as follows: FClꢀAlMe > oꢀBrꢀAlMe > oꢀClꢀAlMe ≧
oꢀOMeꢀAlMe > oꢀFꢀAlMe ≧ MeꢀAlMe ≧ HꢀAlMe > pꢀFꢀAlMe > pꢀOMeꢀAlMe ≧
pꢀBrꢀAlMe. These results indicated that the coordinating group in the orthoꢀposition of the phenyl
ring in bridging carbon C* indeed increased the catalytic activity. The tendency was discovered that
increasing the size of the coordinating group in the orthoꢀposition of the phenyl ring in bridging
carbon C* also increased the catalytic activity (oꢀBrꢀAlMe > oꢀClꢀAlMe ≧ oꢀOMeꢀAlMe >
oꢀFꢀAlMe). In addition, it can be deduced that the steric effect was not a major factor because
HꢀAlMe was not the weakest catalyst. The above results strongly implied there was another effect
such as coordinating effect influenced the catalytic activity of Al complexes except steric effect.
However, the crystal structures of these Al complexes presented no indication of coordination
between Al and the coordinating group in the orthoꢀposition of phenyl ring in bridging carbon C*.
To confirm the possibility of coordinating effect between the coordinating group and Al atom, DFT
calculation of mononuclear form of oꢀOMeAlOMe and dinuclear form of (oꢀOMeAlꢀꢀꢀOMe)2
(Figure 5) was studied and the benzyl oxide and tertꢀbutyl groups were replaced by methoxide and
hydrogen atoms, respectively, in order to reduce computational costs. The results revealed that
(oꢀOMeAlꢀꢀꢀOMe)2, was more stable than the oꢀOMeAlOMe about 41.3 kcal/mol in energy,
indicating that the coordinating group in the orthoꢀposition of phenyl ring should not be possible to
coordinate Al atom.
Figure 5. Models of mononuclear form of oꢀOMeAlOMe and dinuclear form of
(oꢀOMeAlꢀ
ꢀꢀOMe)2 for DFT calculation
To elucidate the relationship between polymerization activity and the coordinating group in the

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orthoꢀposition, a triphenol ligand (TriO-H) and its Al complex were synthesized (Figure 6) to
determine how the coordinated functional group influenced the catalytic center. The crystal data of
TriO2Al2 showed the unidentified solvent which should be THF observed from 1NMR spectrum
(Figure S1) of TriO2Al2 and ignored by using SQUEEZE. Our results revealed the formation of an
unexpected dinuclear Al complex (TriO2Al2) comprising two triphenolate ligands, two Al atoms,
and two terminal THFs (Figure 7). The two Al atoms were bridged by two phenolate groups from
two different triphenolate ligands. This indicates that the coordinating group in the orthoꢀposition of
the phenyl ring in bridging carbon C* could very likely coordinate with Al atom to form dinuclear
complex. Compared with Al complex with MEBBP in Figure 1, the coordinating group in the
orthoꢀposition could not coordinate Al atom in monomeric form because of far distance between the
coordinating group and Al atom. A survey of the structures of diphenolate Al alkoxide4l,4m~4o,6
revealed that they prefer dinuclear form (A in Figure 8) with bridging alkoxides and terminal donor
solvent molecule (benzyl aldehyde or THF). It is a clear indication that the dinuclear form would
not decompose during CL coordination or alkoxide initiation (B in Figure 8). However, bridging
alkoxides were weaker initiators than terminal alkoxides because bridging alkoxides are unable to
rotate freely to initiate CL attack and the volume of the lone pair electrons is less than that of the
terminal alkoxide (Figure 8, left reaction). The coordinating functional group (X in Figure 8) could
allow bridging alkoxides (A in Figure 8) to transform into terminal alkoxides (C in Figure 8, right
reaction) and thus increase the initiating rate.
Figure 6. Synthesis of TriOꢀH and TriO2Al2

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Figure 7. Molecular structure of TriO2Al2 as 30% probability ellipsoids. CCDC number: 1032157
(all of the hydrogen atoms were omitted for clarity)
Figure 8. Possible mechanisms associated with CL polymerization between Al complexes with
coordinating group X
Determining the role of coordination in polymerization using DFT calculation
To determine the likelihood of exchange between two dinuclear forms (A and C in Figure 8),
DFT calculation of two dinuclear forms of (oꢀOMeAlOMe)2 (Figure 9) was studied and the benzyl
oxide and tertꢀbutyl groups were replaced by methoxide and hydrogen atoms, respectively, in order
to reduce computational costs. The results revealed that the (oꢀOMeAlꢀtꢀOMe)2, in where each
OMe group of the phenyl ring bridges both Al centers, is not a stable structure. Geometry
optimization starting from the initial structure of (oꢀOMeAlꢀtꢀOMe)2 resulted in a spontaneous
collapse to a structure of (oꢀOMeAlꢀtꢀOMe)2* in which each OMe group coordinates with only one

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Al atom. The conformation in which the methoxide serves as a bridging group, namely
(oꢀOMeAlꢀꢀꢀOMe)2, was found to be slightly more stable than the (oꢀOMeAlꢀtꢀOMe)2* about 2.4
kcal/mol in energy, indicating that a transformation between these two conformations should be
possible. Figure 10 illustrates a possible mechanism based on experiment data and DFT
calculations. Mononuclear methyl Al complexes reacted with BnOH to form dinuclear Al benzyl
alkoxide (A). If a third coordinating group exited in the diphenolate ligand, then A would transfer to
C with the terminal benzyl alkoxide, which would initiate CL (right side of Figure 10). However,
the dinuclear Al benzyl alkoxide (A) without a third coordinating group maintained the form of a
bridging benzyl alkoxide throughout the process of CL coordination and benzyl alkoxide initiation.
Due to its lack of free rotation and lone electron pairs, the initiating ability of briding benzyl
alkoxide was weaker than that of terminal benzyl alkoxide. That explains why Al complexes with a
third coordinating group in the dipheolate ligand produce greater activity than do those without a
third coordinating group. This mechanism also could explain the catalytic trend of CL
polymerization. FClꢀAlMe presented the best catalytic activity due to the existence of two
coordinated atoms in the orthoꢀposition of the phenyl ring, which enhanced the likelihood of the
coordination with Al. The catalytic activity increased according to an increase of the size of the
halide in the orthoꢀposition of the phenyl ring in bridging carbon C* (oꢀBrꢀAlMe > oꢀClꢀAlMe >
oꢀFꢀAlMe) due to the fact that Br has the lone pairs in larger shells where they are farther from the
nucleus, less tightly held, and consequently more coordinately reactive than Cl and F. In addition, it
can be deduced that the steric effect was not a major factor because HꢀAlMe was not the weakest
catalyst.
Figure 9. Models of two dinuclear forms for DFT calculation

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Figure 10. Possible mechanism of ROP using Al complexes bearing diphenolate ligand as catalysts
Conclusions
The present work describes a variety of synthetic procedures used to access Al species bearing
diphenolate ligands and their application in CL polymerization. Polymerization results reveal that
the coordinating functional group in the orthoꢀposition of the phenyl ring in the bridging
methanetriyl carbon increased the catalytic activity and proved that coordination between the third
coordinating group and Al enhanced the catalytic activity of Al complexes. The crystal structure of
TriO2Al2 provided a possible coordination model between Al and the third coordinated group. DFT
calculations resulted in a more stable dinuclear Al intermediate with terminal benzyl alkoxide in a
distorted tetrahedral form. Based on experiment data and DFT calculations, a possible mechanism
of CL polymerization was proposed and the coordinating effect of the third coordinated group
allowed the transfer of dinuclear Al complexes with bridging benzyl alkoxide to Al complexes with
terminal benzyl alkoxide, resulting in greater ability to initiate CL polymerization.
Experimental Section
Standard Schlenk techniques and a N2ꢀfilled glovebox were used throughout the isolation and

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handling of all the compounds. Solvents,
prior to use. EDBP was purchased from ALDRICH. Deuterated chloroform, paraformaldehyde,
2,4ꢀdiꢀtertꢀbutylphenol, and ꢀcaprolactone were purchased from Acros. Benzyl alcohol,
εꢀcaprolactone, and deuterated solvents were purified
ε
trimethylaluminum, 2ꢀfluorobenzaldehyde, 2ꢀchlorobenzaldehyde, 2ꢀbromobenzaldehyde,
2ꢀmethoxybenzaldehyde 2ꢀmethoxybenzaldehyde, 4ꢀmethoxybenzaldehyde, 4ꢀbromobenzaldehyde,
2ꢀchloroꢀ6ꢀfluorobenzaldehyde, 2,5ꢀdimethoxybenzaldehyde, and pꢀtoulenesulfonic acid were
purchased from Alfa Aesar. 1H and 13C NMR spectra were recorded on a Varian Gemini2000ꢀ200
(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 performed using a Heraeus
CHNꢀOꢀRAPID instrument. GPC measurements were performed on a Jasco PUꢀ2080 PLUS HPLC
pump system equipped with a differential Jasco RIꢀ2031 PLUS refractive index detector using THF
(HPLC grade) as an eluent (flow rate 1.0 mL/min, at 40 °C). The chromatographic column was
JORDI Gel DVB 103 Å, and the calibration curve was made by primary polystyrene standards to
calculate Mn(GPC). MeꢀAlMe4l, HꢀAlMe7b, oꢀOMeꢀdiOH8, pꢀOMeꢀdiOH13, and HꢀdiOH7 were
prepared following literature procedures.
Synthesis of FCl-diOH
A mixture of 2,4ꢀdiꢀtertꢀbutylphenol (4.12 g, 20 mmol) and 2ꢀchloroꢀ6ꢀfluorobenzaldehyde (1.58 g,
10 mmol) with catalytic amount of benzenesulfonic acid was refluxed for one day in hexane (30
mL). The solution was at ꢀ20 °C for 2 days and yellow crystalline solids were obtained. Yield : 3.92
1
g (63 %). H NMR (CDCl3, 200 MHz) δ 7.33~7.19 (7H, m, HꢀAr), 6.14 (1H, s, CH), 4.71 (2H, s,
OH), 1.38 (18H, s, oꢀC(CH3)3), 1.17 (18H, s, pꢀC(CH3)3). 13C NMR (CDCl3, 50 MHz) δ 164.92
(COHꢀPh(tBu)2), 159.91, 150.67, 142.37, 136.19, 134.97, 129.02, 127.11, 125.92, 124.22, 123.18,
115.39, 114.92 (CꢀAr), 40.51 (CH), 34.83 (oꢀC(CH3)3), 34.25 (pꢀC(CH3)3), 31.42 (oꢀC(CH3)3),
29.92 (pꢀC(CH3)3). Elemental Anal. Found (Calcd.) for FCl-diOH: C35H46ClFO2: C, 75.89 (75.99) ;
H, 8.02 (8.38) %.
Synthesis of o-Br-diOH

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Using a method is similar to that for FClꢀdiOH except 2ꢀbromobenzaldehyde was used in place of
2ꢀchloroꢀ6ꢀfluorobenzaldehyde. Yield : 2.21g (76 %). 1H NMR (CDCl3, 200 MHz) δ 7.64 (1H, d, J
= 7.8 Hz, 2ꢀHꢀAr(oꢀBr)), 7.20 (4H, s, HꢀPh(tBu)2), 6.96 (1H, d, J = 7.8 Hz, 5ꢀHꢀAr(oꢀBr)) , 6.57
(2H, m, 3,4ꢀHꢀAr(oꢀBr)), 6.94 (1H, s, CH), 4.72 (2H, s, OH), 1.38 (18H, s, oꢀC(CH3)3), 1.16 (18H,
s, pꢀC(CH3)3). 13C NMR (CDCl3, 50 MHz) δ 150.63 (COH), 142.66, 140.49, 136.56, 133.21,
130.85, 128.79, 127.71, 126.48, 126.35, 125.65, 124.16, 123.05 (CꢀAr), 46.85 (CH), 34.91
(oꢀC(CH3)3), 34.22 (pꢀC(CH3)3), 31.42 (oꢀC(CH3)3), 29.84 (pꢀC(CH3)3). Elemental Anal. Found
(Calcd.) for o-Br-diOH: C35H47BrO2: C, 72.63 (72.52) ; H, 8.05 (8.17) %.
Synthesis of o-Cl-diOH
Using a method is similar to that for FClꢀdiOH except 2ꢀchlorobenzaldehyde was used in place of
1
2ꢀchloroꢀ6ꢀfluorobenzaldehyde.. Yield : 1.96 g (73 %). H NMR (CDCl3, 200 MHz) δ 7.47, 7.34,
6.98, 6.60 (8H, m, HꢀPh), 6.00 (1H, s, CH), 4.67 (2H, s, OH), 1.38 (18H, s, oꢀC(CH3)3), 1.16 (18H,
s, pꢀC(CH3)3). 13C NMR (CDCl3, 50 MHz) δ 150.56 (COHꢀPh(tBu)2), 142.70, 138.77, 136.89,
134.75, 130.89, 124.37, 129.82, 128.55, 127.11, 126.30, 124.09, 123.11 (CꢀAr), 44.16 (CH), 34.85
(oꢀC(CH3)3), 34.24 (pꢀC(CH3)3), 31.42 (oꢀC(CH3)3), 29.85 (pꢀC(CH3)3). Elemental Anal. Found
(Calcd.) for o-Cl-diOH: C35H47ClO2: C, 78.42 (78.55) ; H, 8.95 (8.85) %.
Synthesis of o-F-diOH
Using a method is similar to that for FClꢀdiOH except 2ꢀfluorobenzaldehyde was used in place of
1
2ꢀchloroꢀ6ꢀfluorobenzaldehyde. Yield : 1.89 g (73 %). H NMR (CDCl3, 200 MHz) δ 7.32~6.65
(8H, m, HꢀAr), 5.88 (1H, s, CH), 4.42 (2H, s, OH), 1.38 (18H, s, oꢀC(CH3)3), 1.16 (18H, s,
13
pꢀC(CH3)3). C NMR (CDCl3, 50 MHz) δ 150.53 (COH), 142.81, 136.63, 130.60, 129.14, 128.97,
126.18, 124,39, 123.93, 123.27, 120.37, 115.76, 115.32 (CꢀAr), 40.37 (CH), 34.86 (oꢀC(CH3)3),
34.29 (pꢀC(CH3)3), 31.42 (oꢀC(CH3)3), 29.84 (pꢀC(CH3)3). Elemental Anal. Found (Calcd.) for
o-F-diOH: C35H47FO2: C, 81.25 (81.04) ; H, 8.99 (9.13) %.
Synthesis of p-Br-diOH

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Using a method is similar to that for FClꢀdiOH except 4ꢀbromobenzaldehyde was used in place of
2ꢀchloroꢀ6ꢀfluorobenzaldehyde.. Yield : 1.84 g (64 %). 1H NMR (CDCl3, 200 MHz) δ 7.48 (2H, d,
J = 6.6 Hz, oꢀHꢀAr(pꢀBr)), 7.26 (2H, s, HꢀPh(tBu)2), 7.06 (2H, d, J = 6.6 Hz, mꢀHꢀAr(pꢀBr)), 6.65
(2H, s, HꢀPh(tBu)2), 5.62 (1H, s, CH), 4.75 (2H, br, OH), 1.38 (18H, s, oꢀC(CH3)3), 1.16 (18H, s,
pꢀC(CH3)3). 13C NMR (CDCl3, 50 MHz) δ 150.34 (COH), 142.93, 140.35, 136.70, 131.86, 131.17,
127.58, 124.31, 123.95, 123.08, 121.10, 115.86 (CꢀAr), 46.36 (CH), 34.32 (oꢀC(CH3)3), 34.22
(pꢀC(CH3)3), 31.48 (oꢀC(CH3)3), 29.92 (pꢀC(CH3)3). Elemental Anal. Found (Calcd.) for
p-Br-diOH: C35H47BrO2: C, 72.53 (72.52) ; H, 8.12 (8.17) %.
Synthesis of TriO-H
A mixture 2ꢀ(bis(3,5ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxyphenyl)methyl)phenyl 4ꢀmethylbenzenesulfonate8 (6.7
g, 10 mmol) and NaOH (0.4 g, 10 mmol) in ethanol (50 mL) was refluxed for one day and the
solvent was removed from the mixture under vacuum. The residue was extracted with hexane (3 ×
100 mL). The organic layer was dried over MgSO4, filtered, and concentrated to 20 mL. The
solution was at ꢀ20 °C for 2 days and white powder was obtained. Yield : 2.73g (53 %). 1H NMR
(CDCl3, 200 MHz) δ 7.25ꢀ6.99, 6.94ꢀ6.82 (4H, m, ArOH) 7.28, 6.74 (4H, s, But2ArOH), 5.79 (1H,
s, CH), 5.00 (1H, s, OH) 4.83 (2H, s, OH), 1.38 (18H, s, oꢀC(CH3)3), 1.67 (18H, s, pꢀC(CH3)3). 13C
NMR (CDCl3, 50 MHz) δ 164.92 (COHꢀPh(tBu)2), 159.91, 150.67, 142.37, 136.19, 134.97, 129.02,
127.11, 125.92, 124.22, 123.18, 115.39, 114.92 (CꢀAr), 40.51 (CH), 34.83 (oꢀC(CH3)3), 34.25
(pꢀC(CH3)3), 31.42 (oꢀC(CH3)3), 29.92 (pꢀC(CH3)3). Elemental Anal. Found (Calcd.) for TriO-H:
C35H48O3: C, 81.51 (81.35) ; H, 9.19 (9.36) %.
Synthesis of FCl-AlMe
A mixture of FClꢀdiOH (3.11 g, 5 mmol) and AlMe3 (3 mL, 2.0 M, 5.5 mmol) in THF (20 mL), was
o
stirred for 3 hr at 0 C. Volatile materials were removed under vacuum to give light yellow powder
and then hexane (30 mL) was transferred to be the suspension. The white powder was obtained after
filtering. Yield: 2.1 g (63 %). 1H NMR (CDCl3, 200 MHz) δ 7.52~6.92 (7H, m, HꢀAr), 6.79 (1H, s,

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CH), 4.08 (2H, br, OCH2CH2), 1.99 (2H, br, OCH2CH2), 1.41 (18H, s, oꢀC(CH3)3), 1.23 (18H, s,
pꢀC(CH3)3), ꢀ0.57 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ 152.90 (COCH3ꢀAr(pꢀOMe)),
139.00, 136.41, 130.85, 127.53, 127.32, 126.01, 124.94, 124.71, 121.22, 116.20, 115.65 (CꢀAr),
71.11 (OCH2CH2), 25.26 (OCH2CH2), 39.44 (CH), 35.23 (oꢀC(CH3)3), 34.16 (pꢀC(CH3)3), 31.63
(oꢀC(CH3)3), 30.26 (pꢀC(CH3)3), ꢀ13.36 (AlCH3). Elemental Anal. Found (Calcd.) for FCl-AlMe:
C40H55AlClFO3: C, 71.89 (71.67) ; H, 7.96 (8.61) %. Mp：184 oC.
Synthesis of o-Br-AlMe
Using a method is similar to that for FClꢀAlMe except o-Br-diOH was used in place of FCl-diOH.
Yield: 2.7 g (77 %). 1H NMR (CDCl3, 200 MHz) δ 7.70~6.98 (8H, m, HꢀPh), 6.02 (1H, s, CH), 4.21
(4H, br, OCH2CH2), 2.05 (4H, br, OCH2CH2), 1.39 (18H, s, oꢀC(CH3)3), 1.21 (18H, s, pꢀC(CH3)3),
ꢀ0.60 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ 153.10 (COAl), 143.99, 139.12, 136.77, 133.71,
131.35, 130.78, 127.20, 125.97, 124.15, 120.66, 120.35 (CꢀAr), 71.49 (OCH2CH2), 25.29
(OCH2CH2), 42.83 (CH), 35.12 (oꢀC(CH3)3), 34.09 (pꢀC(CH3)3), 31.63 (oꢀC(CH3)3), 30.14
(pꢀC(CH3)3), ꢀ13.56 (AlCH3). Elemental Anal. Found (Calcd.) for o-Br-AlMe: C40H56AlBrO3: C,
69.40 (69.45) ; H, 8.39 (8.16) %. Mp：208 oC.
Synthesis of o-Cl-AlMe
Using a method is similar to that for FClꢀAlMe except o-Cl-diOH was used in place of FCl-diOH.
Yield: 2.0 g (62 %). 1H NMR (CDCl3, 200 MHz) δ 7.67~7.00 (8H, m, HꢀPh), 6.16 (1H, s, CH), 4.32
(4H, br, OCH2CH2), 2.08 (4H, br, OCH2CH2), 1.39 (18H, s, oꢀC(CH3)3), 1.23 (18H, s, pꢀC(CH3)3),
ꢀ0.59 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ 152.95 (COAl), 142.42, 139.05, 136.79, 135.09,
130.23, 134.28, 131.31, 130.40, 130.09, 126.98, 125.82, 120.70 (CꢀAr), 71.36 (OCH2CH2), 25.26
(OCH2CH2), 40.57 (CH), 35.12 (oꢀC(CH3)3), 34.10 (pꢀC(CH3)3), 31.64 (oꢀC(CH3)3), 29.87
(pꢀC(CH3)3), ꢀ13.64 (AlCH3). Elemental Anal. Found (Calcd.) for o-Cl-AlMe: C40H56AlClO3: C,
73.92 (74.22) ; H, 8.99 (8.72) %. Mp：216 oC.

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Synthesis of o-F-AlMe
Using a method is similar to that for FClꢀAlMe except o-F-diOH was used in place of FCl-diOH.
Yield: 1.7 g (53 %). 1H NMR (CDCl3, 200 MHz) δ 7.64 (1H, t, J = 8.4 Hz, 2ꢀHꢀAr(oꢀF)), 7.19~7.07
(7H, m, HꢀAr), 6.17 (1H, s, CH), 4.33 (4H, br, OCH2CH2), 2.07 (4H, br, OCH2CH2), 1.39 (18H, s,
oꢀC(CH3)3), 1.22 (18H, s, pꢀC(CH3)3), ꢀ0.60 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ 152.57
(COCH3ꢀAr(oꢀF)), 139.08, 136.97, 132.31, 131.29, 130.34, 127.42, 123.74, 121.01, 120.35, 115.28,
114.84 (CꢀAr), 71.30 (OCH2CH2), 20.93 (OCH2CH2), 36.17 (CH), 35.15 (oꢀC(CH3)3), 34.12
(pꢀC(CH3)3), 31.63 (oꢀC(CH3)3), 30.07 (pꢀC(CH3)3), ꢀ14.04 (AlCH3). Elemental Anal. Found
(Calcd.) for o-F-AlMe: C40H56AlFO3: C, 75.89 (76.16) ; H, 8.65 (8.95) %. Mp：226 oC.
Synthesis of o-OMe-AlMe
Using a method is similar to that for FClꢀAlMe except o-OMe-diOH was used in place of
FCl-diOH. Yield: 2.1 g (64 %). 1H NMR (CDCl3, 200 MHz) δ 6.78~7.59 (8H, m, HꢀAr), 6.01 (1H,
s, CH), 4.31 (4H, br, OCH2CH2), 3.15 (3H, s, OCH3), 2.00 (4H, br, OCH2CH2, THF), 1.39 (18H, s,
oꢀC(CH3)3), 1.20 (18H, s, pꢀC(CH3)3), ꢀ0.60 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ 170.11
(COCH3), 158.44 (COAl), 152.81, 138.74, 136.60, 135.25, 132.58, 129.31, 126.79,123.77, 120.31,
114.12 (CꢀAr), 70.92 (OCH2CH2,THF), 25.27 (OCH2CH2,THF), 56.39 (OCH3), 37.27 (CH), 35.15
(oꢀC(CH3)3), 34.10 (pꢀC(CH3)3), 31.67 (oꢀC(CH3)3), 30.10 (pꢀC(CH3)3), ꢀ13.87(AlCH3). Elemental
Anal. Found (Calcd.) for o-OMe-AlMe: C41H59AlO4: C, 76.99 (76.60) ; H, 8.98 (9.25) %. Mp：202
oC.
Synthesis of p-Br-AlMe
Using a method is similar to that for FClꢀAlMe except p-Br-diOH was used in place of FCl-diOH.
Yield: 2.8 g (82 %). 1H NMR (CDCl3, 200 MHz) δ7.32 (2H, m, oꢀHꢀAr(pꢀBr)), 7.08 (2H, m,
mꢀHꢀAr(pꢀBr)), 5.85 (1H, s, CH), 4.12 (4H, br, OCH2CH2), 1.88 (4H, br, OCH2CH2), 1.38 (18H, s,
oꢀC(CH3)3), 1.21 (18H, s, pꢀC(CH3)3), ꢀ0.60 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ 153.39
(COAl), 145.24, 139.82, 137.09, 132.02, 131.09, 130.49, 124.13, 121.28, 119.06 (CꢀAr), 71.34

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(OCH2CH2), 25.09 (OCH2CH2), 41.11 (CH), 35.18 (oꢀC(CH3)3), 34.21 (pꢀC(CH3)3), 31.66
(oꢀC(CH3)3), 30.04 (pꢀC(CH3)3), ꢀ14.05 (AlCH3). Elemental Anal. Found (Calcd.) for p-Br-AlMe:
C40H56AlBrO3: C, 69.48 (69.45) ; H, 8.51 (8.16) %. Mp: 228 oC.
Synthesis of p-F-AlMe
pꢀFꢀdiOH was synthesized using a method is similar to that for FClꢀdiOH; however, it is brown oil
and it could not be crystallized in hexane. pꢀFꢀAlMe was synthesized using a method is similar to
that for FClꢀAlMe with impure pꢀFꢀdiOH. Yield: 1.9 g (61 %). 1H NMR (CDCl3, 200 MHz) δ
7.12~6.85 (8H, HꢀAr), 5.88 (1H, s, CH), 4.12 (4H, br, OCH2CH2), 1.88 (4H, br, OCH2CH2), 1.40
(18H, s, oꢀC(CH3)3), 1.21 (18H, s, pꢀC(CH3)3), ꢀ0.60 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ
153.37 (COAl), 141.52, 139.75, 137.03, 132.36, 130.79, 130.64, 124.22, 121.17, 120.94, 114.31,
113.90 (CꢀAr), 66.50 (OCH2CH2), 25.14 (OCH2CH2), 41.78 (CH), 35.18 (oꢀC(CH3)3), 34.19
(pꢀC(CH3)3), 31.66 (oꢀC(CH3)3), 30.04 (pꢀC(CH3)3), ꢀ14.09 (AlCH3). Elemental Anal. Found
(Calcd.) for p-F-AlMe: C40H56AlFO3: C, 76.60 (76.16) ; H, 9.45 (8.95) %. Mp：226 oC.
Synthesis of p-OMe-AlMe
Using a method is similar to that for FClꢀAlMe except p-OMe-diOH was used in place of
FCl-diOH. Yield: 2.2 g (67 %). 1H NMR (CDCl3, 200 MHz) δ 7.17 (2H, d, J = 8.6 Hz, HꢀPh(t
Bu)2), 6.89 (2H, d, J = 8.6 Hz, oꢀHꢀAr(pꢀOMe)), 6.89 (2H, d, J = 8.6 Hz, pꢀHꢀAr(pꢀOMe)), 5.84
(1H, s, CH), 4.24 (4H, br, OCH2CH2), 1.89 (4H, br, OCH2CH2), 3.79 (3H, s, Ar(pꢀOCH3)), 1.40
(18H, s, oꢀC(CH3)3), 1.22 (18H, s, pꢀC(CH3)3), ꢀ0.61 (3H, s, AlCH3). 13C NMR (CDCl3, 50 MHz) δ
157.06 (COCH3ꢀAr(pꢀOMe)), 153.39 (COAlꢀPh(tBu)2), 158.44 (COAl), 139.50, 137.83, 136.82,
132.64, 130.23, 124.37, 120.91, 112.74 (CꢀAr), 71.46 (OCH2CH2), 24.98 (OCH2CH2), 55.09
(Ar(pꢀOCH3)), 40.54 (CH), 35.13 (oꢀC(CH3)3), 31.68 (pꢀC(CH3)3), 31.60 (oꢀC(CH3)3), 30.02
(pꢀC(CH3)3), ꢀ14.08 (AlCH3). Elemental Anal. Found (Calcd.) for p-OMe-AlMe: C41H59AlO4: C,
76.87 (76.60) ; H, 9.60 (9.25) %. Mp：210 oC.

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Synthesis of TriO2Al2
Using a method is similar to that for FClꢀAlMe except TriO-H was used in place of FCl-diOH;
however the product was not pure from the NMR spectrum. The crystal was obtained into the NMR
tube 0.05 g of TriO2Al2in 1 mL CDCl3 was set with a cover for 1 month at room temperature.
General procedures for the polymerization of ε-caprolactone
A typical polymerization procedure was exemplified by the synthesis of entry 1 (Table 1) using
complex FCl-AlMe as a catalyst. The polymerization conversion was analyzed by 1H NMR
spectroscopic studies. Toluene (5.0 mL) was added to a mixture of complex FCl-AlMe (0.1 mmol),
BnOH (0.1 mmol), and ε
ꢀcaprolactone (10 mmol) at 50 oC. At indicated time intervals, 0.05 mL
aliquots were removed, trapped with CDCl3 (1mL), and analyzed by 1H NMR. After the solution
was stirred for 30 min, the reaction was then quenched by adding a drop of iso-propanol, and the
polymer precipitated as white solid when pouring into nꢀhexane (30.0 mL). The isolated white solid
was dissolved in CH2Cl2 (5.0 mL) and then nꢀhexane (70.0 mL) was added to give purified
crystalline solid. Yield: 1.00 g (88 %).
General procedures for the X-ray experiment
Xꢀray experimental procedure of FCl-AlMe was that 0.10 g of FCl-AlMe in 1 mL CH2Cl2 was set
into the NMR tube with a cover. The colorless crystal appeared after CH2Cl2 in the NMR tube
evaporated. Xꢀray experimental procedure of TriO2Al2 was that 1.0 g of impure TriO2Al2 in 10 mL
THF was set into 20 mL vial and frozen at ꢀ10 oC. The colorless crystal of TriO2Al2 was obtained
after a week.
Computational Methods
All the DFT calculations were accomplished by the Gaussian 09 package.9 The B3LYP hybrid
functional10,11 in combination with the 6ꢀ31G(d) basis sets was employed in the present DFT
calculations. In addition, empirical correction for dispersion was taken into account by using the D3
version of Grimme’s dispersion correction12.

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Electronic supplementary information (ESI) available: Polymer characterization data, and
details of the kinetic study.
Acknowledgment This study is supported by Kaohsiung Medical University “Aim for the top 500
universities grant" under Grant No. KMUꢀDT103007, NSYSUꢀKMU JOINT RESEARCH
PROJECT (NSYSU KMU 103ꢀI004), and the Ministry of Science and Technology (Grant NSC
101ꢀ2113ꢀMꢀ037 ꢀ009ꢀMY2). We thank Center for Research Resources and Development at
Kaohsiung Medical University for the instrumentation and equipment support.
References
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DOI: 10.1039/C5RA10753A