Synthesis, characterization of bulky aluminium alkoxide and application in the ring-opening polymerization of ε-Caprolactone

Article abstract of DOI:10.1016/j.inoche.2011.07.012

The reaction of 2,2′-dihydroxy-3,3′,5,5′-tetra-tert- butyl-1,1′-diphenyl (TBBP-H₂) with AlMe₃ in tetrahydrofuran gives [(TBBP)Al(CH₃)(THF)] (1) in high yield. 1 further reacts with 2-propanol to afford dimeric aluminium complex [(TBBP)Al(μ-OⁱPr)]₂ (2). Catalytic studies of complex 2 indicate that 2 can efficiently initiate the ring-opening polymerization of ε-Caprolactone under a controlled manner, yielding polymers with narrow molecular weight distribution.

Full text of DOI:10.1016/j.inoche.2011.07.012

Inorganic Chemistry Communications 14 (2011) 1711–1714
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization of bulky aluminium alkoxide and application in the
ring-opening polymerization of ε-Caprolactone
b,
a,
Xiaozhe Yang a, Lei Wang a, Lihui Yao a, Jinfeng Zhang a, Ning Tang a, , Cheng Wang , Jincai Wu
⁎
⁎
⁎
a
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000,
People's Republic of China
b
The Second Affiliated Hospital of Lanzhou University, Lanzhou 730000, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butyl-1,1′-diphenyl (TBBP-H2) with AlMe3 in tetrahydro-
furan gives [(TBBP)Al(CH3)(THF)] (1) in high yield. 1 further reacts with 2-propanol to afford dimeric
aluminium complex [(TBBP)Al(μ-OiPr)]2 (2). Catalytic studies of complex 2 indicate that 2 can efficiently
initiate the ring-opening polymerization of ε-Caprolactone under a controlled manner, yielding polymers
with narrow molecular weight distribution.
Received 11 June 2011
Accepted 18 July 2011
Available online 26 July 2011
Keywords:
Aluminium complex
ε-Caprolactone
© 2011 Elsevier B.V. All rights reserved.
Catalyst
Ring-opening polymerization
Synthetic polymers have become an indispensable part of the
daily-life of human beings and the biodegradable class of polymers
holds immense value in therapeutics [1]. Of the commercially viable
polymers, poly(ε-Caprolactone) (PCL) is an important polymer due to
its mechanical properties, miscibility with a large range of other
polymers and biodegradability [2]. With regard to promising
applications of PCL in medicine and tissue engineering such as
delivery media for the controlled release of drugs, scaffolds, and the
delivery of antibodies and genes [3], the development of efficient
catalytic systems for the preparation of PCL has drawn much attention
both in academia and industry. The major method to synthesize PCL is
the ring-opening polymerization (ROP) of ε-Caprolactone (ε-CL) with
the metal alkoxides as initiators. Some well-defined metal catalysts or
initiators have been reported, including aluminium [4], titanium [5],
tin [6], zinc [7] and magnesium [8] complexes supported by various
ligands, giving polymers with high molecular weights in high yields.
Among these complexes, aluminium alkoxide based initiator systems
seem to be active and suitable for preparing the PCL due to their high
Lewis acidity and low toxicity [9]. However, when metal alkoxides are
used as initiators for ROP of lactones and lactides, they typically suffer
from undesirable back-biting and transesterificaton reactions, leading
to macrocycles, catalyst inhomogeneity, and broad or multimodal
polymer molecular weight distributions. Such side reactions can be
minimized through the use of sterically demanding ligands providing
a steric barrier to a certain extent. Bisphenolate system is attracting
considerable attention because it is easily modified and functiona-
lized. As a result, many bisphenolate ligands have been introduced
through a link of various functional groups and their metal complexes
have been proven to be effective initiators [10]. All of these encourage
us to design new aluminium complex as initiators for ROP of ε-CL.
Herein we report the synthesis and characterization of a new kind of
bulky aluminium alkoxide. The catalytic activity of [(TBBP)Al(μ-
OiPr)]2 toward ROP of ε-CL is also presented. Preliminary results show
that this aluminium alkoxide can efficiently catalyze the polymeriza-
tion of ε-CL in a living fashion.
The ligand 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butyl-1,1′-diphenyl
(TBBP-H2) was synthesized according to Ref. [11]. The tert-butyl
group was chosen as the substituent in order to favor mononuclear
complex and limit back-biting side reaction during polymerization. The
reaction of TBBP-H2 with 1.2 mol equiv of AlMe3 in tetrahydrofuran
produces a monomeric aluminium complex [(TBBP)Al(CH3)(THF)] (1)
in 67% yield, as shown in Scheme 1. Complex 1 further reacts with a
stoichiometric amount of 2-propanol, giving the 2-propanol bridged
aluminium dimeric [(TBBP)Al(μ-OiPr)]2 (2) in 75% yield. Alternatively,
the initiator 2 can also be obtained directly by the reaction of TBBP-H2
with Al(OiPr)3 in n-hexane. Complexes 1 and 2 were isolated as
colorless crystalline solids and have been fully characterized by 1H and
13C NMR spectra as well as elemental analysis. The formation of
expected complexes 1–2 were demonstrated by the disappearance of
the O–H signals of TBBP-H2 group (5.20 ppm) and the appearance of the
resonance for alkyl protons of CH3 (−0.56 ppm) for 1 and methine
protons of 2-propanol (4.39 ppm) for 2 respectively, which was further
verified by X-ray structural determination.
Single crystals of 1 suitable for X-ray structural determination
were obtained from a solution of complex 1 in a toluene/
tetrahydrofuran mixture at ambient temperature. The ORTEP drawing
⁎
Corresponding authors. Tel.: +86 931 8152583; fax: +86 931 8912582.
E-mail address: wujc@lzu.edu.cn (J. Wu).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.07.012

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X. Yang et al. / Inorganic Chemistry Communications 14 (2011) 1711–1714
Scheme 1. Preparation of complexes 1 and 2.
of the molecular structure of 1, which are crystallized in monoclinic space
group P21/n, is given in Fig. 1. The structure of 1 shows a monomeric
feature, and the immediate geometry around Al is a distorted tetrahedron
with bond distances of Al-O1 1.732(16) (phenoxy), Al-O2 1.735(17)
(phenoxy), Al-O3 1.869(18) (tetrahydrofuran), and Al-C29 1.932(3),
which are all well within the normal range for bonds reported for the
observed results for four-coordinated aluminium phenoxide containing
steric bulky substituents [12].
Single crystals of 2 suitable for X-ray structural determination
were obtained on slow cooling its saturated toluene solution, and
its ORTEP drawing is shown in Fig. 2. Complex 2 is crystallized in
monoclinic space group P21/c. The structure of 2 shows a dimeric
feature containing an Al2O2 core bridging through the oxygen atom
of 2-propanol and the geometry around Al is distorted from
tetrahedron with the bond distances of Al1-O1 1.706(3) (phenoxy),
Al1-O2 1.695(3) (phenoxy), Al1-O3 1.808(3) (2-propanol) and
Al1-O3A 1.814(3) (2-propanol).
Since several aluminium briged-phenoxides derivatives have dem-
onstrated efficient catalytic activities toward the ROP of lactones [13],
the complex 2 was expected to behave as an initiator toward the ROP of
ε-Caprolactone. Investigations have been systematically conducted at
80 °C in toluene (10 mL) employing 2 (0.02 mmol) as the initiator, and
the ROP process was monitored by taking aliquots, which were
evaluated by 1H NMR spectra. The molecular weight and polydispersity
of PCL are measured by gel permission chromatography (GPC). The
results of the polymerization of ε-CL initiated by 2 under different
reaction conditions are summarized in Table 1. The conversion yield
attained N90% in toluene within 3 h at 80 °C with the ratio [CL]0/[Al]0 in
the range of 50–200, which has almost similar activity comparing to
Table 1
Ring-opening polymerization of ε-Caprolactone catalyzed by 2a
O
O
O
O
2
n
OiPr
H
n
Entry
[M]0/[Al]0/
[iPrOH]0
Time
(h)
PDI
Mn (obsd)b
Mn
Conv
(%)d
(calcd)c
1
2
3
4
5
6
7
8
9
50:1:0
75:1:0
3
2.5
2
2
2
1.5
1.5
1.5
2
1.15
1.25
1.10
1.07
1.21
1.10
1.15
1.16
1.15
11623 (6508)
14853 (8317)
19668 (11014)
23888 (13377)
33693 (18868)
6620 (3707)
9649 (5403)
14142 (7919)
19452 (10893)
5538
8278
10446
16324
22659
2913
5767
8620
11474
96
96
91
95
100:1:0
150:1:0
200:1:0
50:1:1
100:1:1
150:1:1
200:1:1
99
N99
N99
N99
N99
a
Conditions: 0.02 mmol of complexes, 10 mL of toluene solvent, 80 °C.
Obtained from GPC analysis. Values in parentheses are the values obtained from
b
GPC times 0.56 [14].
c
Calculated from the molecular weight of ε-Caprolactone times [M]0/[Al]0 times
conversion yield divided by ([iPrOH]+1) plus the molecular weight of PrOH.
i
d
Obtained from 1H NMR analysis.
Fig. 1. Molecular structure of 1 as 30% ellipsoids (all of the hydrogen atoms are omitted
for clarity). Selected bond lengths (Å):Al-O1 1.732(16), Al-O2 1.735(17), Al-O3 1.869
(18), Al-C29 1.932(3), C2-O1 1.365(2) and C12-O2 1.360(3).
Fig. 2. Molecular structure of 2 as 30% ellipsoids (methyl carbons of the tert-butyl
groups and all of the hydrogen atoms are omitted for clarity). Selected bond lengths
(Å): Al1-O1 1.706(3), Al1-O2 1.695(3), Al1-O3 1.808(3), Al1A-O3A 1.814(3), C1-O1
1.380(5), C12-O2 1.384(4) and C29-O3 1.471(5).
Fig. 3. Polymerization of ε-CL catalyzed by 2 in toluene at 80 °C. The relationship
between Mn(□) (PDI (●)) of the polymer and the initial mole ratio [M]0/[Al]0 is shown.

X. Yang et al. / Inorganic Chemistry Communications 14 (2011) 1711–1714
1713
Fig. 4. 1H NMR of PCL-50 in CDCl3.
EDBP-Al-OR [15], and the PDIs of polyesters produced are all narrow,
ranging from 1.07 to 1.25. It is also found that there is a linear
relationship between the number-average molecular weight (Mn) and
the monomer-to-initiator ratio ([M]0/[Al]0) as shown in Fig. 3 (entries
1–5), implying the “living” character of the polymerization process,
whichmeans thegrowing chain has not terminatein the polymerization
progress. The presence of Hh (−CH2− from PCL at the hydroxyl chain
end) at 3.65 ppm and Hg (CHMe2 from PCL at 2-propanol ester end) at
5.01 ppm with an integral ratio close to 2 in the 1H NMR spectrum of
PCL-50 (the number of 50 indicates the designed [M]0/[Al]0 ratio)
demonstrates the polymer is a linear chain as shown in Fig. 4. The result
also indicates that the polymerization is initiated through the insertion
of the 2-propanoxy group from 2 into ε-CL, giving an aluminium
alkoxide intermediate, which further reacts with excess lactones giving
polyesters [16]. Additionally, the “immortal” character of polymeriza-
tion, which means the growing chain can exchange with ROH rapidly
and reversibly, is important for the ring-opening polymerization of
cyclic esters. The “immortal” character of 2 is investigated using one
additional equivalent 2-propanol as the chain transfer agent (entries 6–
9) and the Mn of the polymer in each case became nearly half comparing
to the polymers obtained without the addition of the chain transfer
agent.
Foundation for the Returned Overseas Chinese Scholars, State Education
Ministry.
Appendix A. Supplementary material
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.07.012.
1,2
References
[1] Jay Prakash Jain, Marina Sokolsky, Neeraj Kumar, A.J. Domb, Fatty acid based
biodegradable polymer, Polym. Rev. 48 (2008) 156–191.
[2] Marianne Labet, Wim Thielemans, Synthesis of polycaprolactone: a review, Chem.
Soc. Rev. 38 (2009) 3484–3504.
[3] (a) M. Okada, Chemical syntheses of biodegradable polymers, Prog. Polym. Sci.
27 (2002) 87–133;
1
a) Data for complex 1: (0.70 g, 67% Yield). 1H NMR (200 MHz, CDCl3, ppm): δ 7.28
(s, 2H, Ph), 7.07 (s, 2H, Ph), 3.81 (s, 4H, –OCH2CH2−), 1.86 (s, 4H, –OCH2CH2−), 1.44
(s, 18H, PhC(CH3)3), 1.29 (s, 18H, PhC(CH3)3), −0.56 (s, 3H, Al–CH3). 13C NMR (CDCl3,
ppm): δ 154.03, 140.31, 137.62, 131.91, 128.52, 122.61 (Ph); 72.35 (O–CH2); 35.33,
34.27, 31.76, 30.33 (t-Bu); 25.10 (–OCH2CH2−). Anal. Calcd for C33H51AlO3: C, 75.82;
H, 9.83. Found: C, 75.95; H, 9.96. b) Data for complex 2: (0.74 g, 75% Yield). 1H NMR
(300 MHz, CDCl3, ppm): δ 7.32 (s, 4H, Ph), 7.03 (s, 4H, Ph), 4.39 (m, 2H, –OCH(CH3)2,
J=6 Hz), 1.55 (s, 36H, PhC(CH3)3), 1.30 (s, 36H, PhC(CH3)3), 1.07 (d, 12H, CH(CH3)2,
J=6 Hz).13C NMR (CDCl3, ppm): δ 152.18, 141.49, 137.92, 131.04, 129.38, 123.07 (Ph);
35.60, 34.30, 31.63, 30.94 (t-Bu); 72.30 (–OCH(CH3)2); 24.13 (–OCH(CH3)2). Anal.
Calcd for C62H94Al2O6: C, 75.27; H, 9.58. Found: C, 75.72; H, 9.45.
In conclusion, a new aluminium complex has been synthesized
and fully characterized. The aluminium complex 2 is an efficient
catalyst for ring-opening polymerization of ε-Caprolactone under a
controlled manner with “living” and “immortal” characters. Fine-
tuning of the ligand and further application to the polymerization of
other monomers are currently being taken.
2
a) Crystal data for complex 1: C33H51AlO3, M=522.72, monoclinic, space group
P21/n, a=13.1560(3) Å,b=15.2095(4) Å,c=16.2525(4) Å,α=90.00, β=95.8750(1),
γ=90.00,
max =24.97, 16674 reflections collected, 5650 unique (Rint =0.0356), no. of observed
V
=3234.98 Å3, T=296(2) K, Z=4, Dc=1.073 g/cm3, F000 =1144,
θ
reflections 3909 (IN2σ(I)); R1 =0.0508, wR2 =0.1287. b) Crystal data for complex 2:
C62H94Al2O6, M=989.37, monoclinic, space group P21/c, a=12.357(2)Å, b=26.224
Acknowledgments
(5) Å, c=11.758(2)
Å ,
α=90.00, β=116.856(2), γ=90.00, V=3399.3(11) Å3,
T=296(2) K, Z=2, Dc=1.023 g/cm3, F000 =1148, θmax =28.34, 29519 reflections
collected, 8311 unique (Rint =0.1061), no. of observed reflections 3347 (IN2σ(I));
R1 =0.0953, wR2 =0.2579.
We thank the financial supports from the National Natural Science
Foundation of China (No. 21071069), and the Scientific Research

1714
X. Yang et al. / Inorganic Chemistry Communications 14 (2011) 1711–1714
(b) E. Chiellini, R. Solaro, Biodegradable polymeric materials, Adv. Mater. 8
(e) C.T. Chen, C.A. Huang, B.H. Huang, Aluminium metal complexes supported by
amine bis-phenolate ligands as catalysts for ring-opening polymerization of
ε-caprolactone, Dalton Trans. (2003) 3799–3803;
(1996) 305–313;
(c) T. Hayashi, Biodegradable polymers for biomedical uses, Prog. Polym. Sci. 19
(1994) 663–702.
(f) L.M. Alcazar-Roman, B.J. O'Keefe, M.A. Hillmyer, W.B. Tolman, Electronic
influence of ligand substituents on the rate of polymerization of ε-
caprolactone by single-site aluminium alkoxide catalysts, Dalton Trans.
(2003) 3082–3087;
[4] (a) M. Endo, T. Aida, S. Inoue, Immortal polymerization of ε-caprolactone
initiated by aluminum porphyrin in the presence of alcohol, Macromolecules
20 (1987) 2982–2988;
(b) R.C. Yu, C.H. Hung, J.H. Huang, H.Y. Lee, J.T. Chen, Four- and five-coordinate
aluminum ketiminate complexes:synthesis, characterization, and ring-opening
polymerization, Inorg. Chem. 41 (2002) 6450–6455;
(g) S. Segal, I. Goldberg, M. Kol, Zirconium and titanium diamine bis(phenolate)
catalysts for α-olefin polymerization: from atactic oligo(1-hexene) to
ultrahigh-molecular-weight isotactic poly(1-hexene), Organometallics 24
(2005) 200–202;
(h) A. Yeori, S. Groysman, I. Goldberg, M. Kol, Diastereoisomerically selective
enantiomerically pure titanium complexes of salan ligands: synthesis, structure,
and preliminary activity studies, Inorg. Chem. 44 (2005) 4466–4468;
(i) H.Y. Chen, J. Zhang, C.-C. Lin, J.H. Reibenspies, S.A. Miller, Efficient and
controlled polymerization of lactide under mild conditions with a sodium-
based catalyst, Green Chem. 9 (2007) 1038–1040;
(j) C.A. Huang, C.L. Ho, C.T. Chen, Structural and catalytic studies of lithium
complexes bearing pendant aminophenolate ligands, Dalton Trans. (2008)
3502–3510;
(k) H. Ma, T.P. Spaniol, J. Okuda, Rare-earth metal complexes supported by 1, ω-
dithiaalkanediyl-bridged bis(phenolato) ligands: synthesis, structure, and
heteroselective ring-opening polymerization of rac-lactide, Inorg. Chem. 47
(2008) 3328–3339.
(c) D. Chakraborty, E.Y.X. Chen, Chiral amido aluminum and zinc alkyls: a synthetic,
structural, and polymerization study, Organometallics 22 (2003) 769–774;
(d) C.T. Chen, C.A. Huangand, B.H. Huang, Aluminum complexes supported by
tridentate aminophenoxide ligand as efficient catalysts for ring-opening
polymerization of ε-caprolactone, Macromolecules 37 (2004) 7968–7973;
(e) P. Hormnirun, E.L. Marshall, V.C. Gibson, J.P. White, D.J. Williams, Remarkable
stereocontrol in the polymerization of racemic lactide using aluminum
initiators supported by tetradentate aminophenoxide ligands, J. Am. Chem.
Soc. 126 (2004) 2688–2689;
(f) N. Nomura, T. Aoyama, R. Ishii, T. Kondo, Salicylaldimine–aluminum
complexes for the facile and efficient ring-opening polymerization of ε-
caprolactone, Macromolecules 38 (2005) 5363–5366.
[5] (a) J. Okuda, I.L. Rushkin, Mono(cyclopentadienyl)titanium complexes as
initiators for the living ring-opening polymerization of ε-caprolactone,
Macromolecules 26 (1993) 5530–5532;
[11] J.I. Vlugt, A.C. Hewat, S. Neto, R. Sablong, A.M. Mills, M. Lutz, A.L. Spek, C. Müller, D.
Vogta, Sterically demanding diphosphonite ligands — synthesis and application in
nickel-catalyzed isomerization of 2-methyl-3-butenenitrile, Adv. Synth. Catal. 346
(2004) 993–1003.
[12] (a) A.P. Shreve, R. Mulhaupt, W. Fultz, J. Calabrese, W. Robbins, S.D. Ittel,
Sterically hindered aryloxide-substituted alkylaluminum compounds, Or-
ganometallics 7 (1988) 409–416;
(b) Y. Takashima, Y. Nakayama, K. Watanabe, T. Itono, N. Ueyama, A. Nakamura,
H. Yasuda, A. Harada, Polymerizations of cyclic esters catalyzed by titanium
complexes having chalcogen-bridged chelating diaryloxo ligands, Macro-
molecules 35 (2002) 7538–7544.
[6] (a) H. Yavuz, C. Babac, K. Tuzlako˘glu, E. Pis¸kin, Preparation and degradation of
L-lactide and ε-caprolactone homo and copolymer films, Polym. Degrad. Stab.
75 (2002) 431–437;
(b) W.D. Healy, D.A. Wierda, A.R. Barron, Sterically crowded aryloxide com-
pounds of aluminum, Orgnometallics 7 (1988) 2543–2548;
(b) A. Kowalski, J. Libiszowski, T. Biela, M. Cypryk, A. Duda, S. Penczek, Kinetics
and mechanism of cyclic esters polymerization initiated with tin(ii) octoate.
polymerization of ε-caprolactone and L,L-lactide co-initiated with primary
amines, Macromolecules 38 (2005) 8170–8176.
(c) M.D. Healy, J.W. Ziller, A.R. Barron, π-Bonding in four-coordinate aluminum
aryloxide compounds, J. Am. Chem. Soc. 112 (1990) 2949–2954;
(d) S. Schulz, H.W. Roesky, M. Noltemeyer, H.G. Schmidt, Synthesis and
structures of sterically crowded aryloxide-substituted aluminium chloride,
J. Chem. Soc., Dalton Trans. (1995) 177–180;
[7] (a) M. Vivas, J. Contreras, Ring-opening polymerization of ε-caprolactone
initiated by diphenylzinc, Eur. Polym. J. 39 (2003) 43–47;
(b) H.Y. Chen, B.H. Huang, C.C. Lin, A highly efficient initiator for the ring-opening
polymerization of lactides and ε-caprolactone: a kinetic study, Macromole-
cules 38 (2005) 5400–5405.
(e) B.T. Ko, C.C. Wu, C.C. Lin, Preparation, characterization, and reactions of
[(EDBP)Al(μ-OiPr)]2, a novel catalyst for MPV hydrogen transfer reactions,
Organometallics 19 (2000) 1864–1869.
[8] E.L. Marshall, V.C. Gibson, H.S. Rzepa, A computational analysis of the ring-
opening polymerization of rac-lactide initiated by single-site β-diketiminate
metal complexes: defining the mechanistic pathway and the origin of stereo-
control, J. Am. Chem. Soc. 127 (2005) 6048–6051.
[9] (a) O.W. Webster, Living polymerization methods, Science 251 (1991) 887–893;
(b) T. Aida, S. Inoue, Metalloporphyrins as initiators for living and immortal
polymerizations, Acc. Chem. Res. 2 (9) (1996) 39;
(c) T. Aida, Living and immortal polymerizations, Prog. Polym. Sci. 19 (1994) 469–528.
[10] (a) E.Y. Tshuva, I. Goldberg, M. Kol, Isospecific living polymerization of 1-hexene
by a readily available nonmetallocene c2-symmetrical zirconium catalyst,
J. Am. Chem. Soc. 122 (2000) 10706–10707;
[13] (a) B.-T. Ko, C.-C. Lin, Efficient “living” and “immortal” polymerization of lactones
and diblock copolymer of ε-CL and δ-VL catalyzed by aluminum alkoxides,
Macromolecules 32 (1999) 8296–8300;
(b) Y.-C. Liu, B.-T. Ko, C.-C. Lin, A highly efficient catalyst for the “living” and
“immortal” polymerization of ε-caprolactone and L-lactide, Macromolecules
34 (2001) 6196–6201.
[14] (a) T. Biela, A. Duda, S. Penczek, Control of Mn, Mw/Mn, end-groups, and kinetics
in living polymerization of cyclic esters, Macromol. Symp. 183 (2002) 1–10;
(b) M. Save, M. Schappacher, A. Soum, Controlled ring-opening polymerization of
lactones and lactides initiated by lanthanum isopropoxide, 1. General aspects
and kinetics, Macromol. Chem. Phys. 203 (2002) 889–899;
(b) Y. Nakayama, K. Watanabe, N. Ueyama, A. Nakamura, A. Harada, J. Okuda,
Titanium complexes having chelating diaryloxo ligands bridged by tellurium
and their catalytic behavior in the polymerization of ethylene, Organome-
tallics 19 (2000) 2498–2503;
(c) M.H. Chisholm, C.-C. Lin, J.C. Gallucci, B.T. Ko, Binolate complexes of lithium,
zinc, aluminium, and titanium; preparations, structures, and studies of lactide
polymerization, Dalton Trans. (2003) 406–412;
(d) C. Capacchione, A. Proto, H. Ebeling, R. M¨ulhaupt, K. M¨oller, T.P. Spaniol,
J. Okuda, Ancillary ligand effect on single-site styrene polymerization:
isospecificity of group 4 metal bis(phenolate) catalysts, J. Am. Chem. Soc.
125 (2003) 4964–4965;
(c) A. Duda, A. Kowalski, S. Penczek, Polymerization of l, L-lactide initiated by
aluminum isopropoxide trimer or tetramer, Macromolecules 31 (1998)
2114–2122.
[15] T.-C. Liao, Y.-L. Huang, B.-H. Huang, C.-C. Lin, Alcoholysis of methyl aluminium
biphenoxides: excellent initiators for the ring opening polymerisation of ε-
caprolactone, Macromol. Chem. Phys. 204 (2003) 885–892.
[16] H.-L. Chen, B.-T. Ko, B.-H. Huang, C.-C. Lin, Reactions of 2,2′-methylenebis(4-
chloro-6-isopropyl-3-methylphenol) with trimethylaluminum: highly efficient
catalysts for the ring-opening polymerization of lactones, Organometallics 20
(2001) 5076–5083.