Salicylaldimine-aluminum complexes for the facile and efficient ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1021/ma050606d

A facile and efficient catalytic system of salicylaldimine-aluminum complexes for the ring-opening polymerization (ROP) of ε-caprolactone (CL) was investigated. The polymerization of Cl was examined at 25 °C in the presence of 1 mol % of benzyl alcohol (B

Full text of DOI:10.1021/ma050606d

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enhanced the polymerization (entries 1-5, Table 1), and
2,4,6-tri-tert-butylphenylimine gave the best result (en-
try 5). The electron-withdrawing substituents also
somewhat enhanced the polymerizations in combination
with steric effects (entries 6-9), and these results are
in contrast to the bis(phenolato)bis(amine)-Al system
reported by Hillmyer and Tolman,4i where the unprec-
edented electronic effects of the substituents were
reported. The steric effects of the salicylidene moiety
appeared sensitive. The methyl substituent at the
5-position of the salicylidene moiety (entry 10) had only
a little effect on the reactivity compared to that of the
nonsubstituted ligand (entry 5). On the other hand, the
introduction of methyl or isopropyl substituents at the
3-position of the salicylidene moiety led to a high activity
of the catalyst (entries 11-13). These results were in
sharp contrast with those of the lactide polymerization
using the salen- and homosalen-Al complexes,12 in
which bulky substituents hindered the polymerization.
In the presence of sterically demanding substituents,
more than a methyl group broadened the polydispersi-
ties (entries 13 and 14) or hindered the polymerization
(entry 15). The Br substituent in the 3-position presum-
ably might have shown steric effects (entry 16) like a
methyl substituent rather than electronic effects. The
most reactive and efficient catalyst was obtained by the
ligand with the sterically demanding 2,4,6-tri-tert-
butylphenylimine moiety15 and a methyl substituent at
the 3-position of the salicylidene moiety. Since the
preparative ortho-formylation of 2,4-dimethylphenol for
the synthesis of 3,5-dimethylsalicylaldehyde was more
practical than that of 2-methylphenol,16 we used the
complex in entry 12 (complex 1) in additional experi-
ments.
We then examined the role of BnOH in the ROP of
CL using the most efficient complex 1. No reaction took
place in the absence of BnOH (entry 17). In the case of
a half amount of BnOH (0.50 mol %) to 1 (1.0 mol %),
the DPn value of the polymer gave a 2-fold number of
the DPn in entry 12 although the polydispersity index
number (PDI) of PCL increased (entry 18). The polym-
erization with 2 equiv of BnOH (2.0 mol %) to 1 (1.0
mol %) afforded PCL of nearly half the DPn (∼50) with
a narrow polydispersity (entry 19) as does the immortal
polymerization.17 The results in entries 12, 18, and 19
suggested two mechanistic insights: (1) BnOH reacted
with 1 to afford LnAl-OBn, and the broad PDI in entry
18 was due to coexistence of ∼0.50 mol % of 1 (entries
12 and 18); (2) the alkoxide (-OR) exchanges between
LnAl-OR and H-OR should be faster than the propa-
gation rate (entries 12 and 19). The opposite design of
the ligand, which was prepared from aniline and bulky
3,5-di-tert-butylsalicylaldehyde, was not promising (en-
try 20). It is surprising that the salicylaldimine ligands
derived from 2,4,6-tri-tert-butylaniline have been previ-
ously hardly utilized as ligands.18,19
Salicylaldimine-Aluminum Complexes for
the Facile and Efficient Ring-Opening
Polymerization of E-Caprolactone
Nobuyoshi Nomura,* Takuji Aoyama,
Ryohei Ishii, and Tadao Kondo
Laboratory of Polymer Chemistry,
Graduate School of Bioagricultural Sciences,
Nagoya University, Nagoya 464-8601, Japan
Received March 23, 2005
Revised Manuscript Received May 17, 2005
Polyesters have provided significant benefits to our
daily life, and some aliphatic ones are particularly
receiving attention due to their practical biodegrad-
ability via our recent concerns with the environment as
well as their biocompatibility for medical and pharma-
ceutical applications.1 If certain criteria for a living/
controlled polymerization are met, the ring-opening
polymerization (ROP) of lactones is a powerful tool for
synthesizing polyesters with the desired molecular
weights and narrow polydispersities, both of which are
important factors for controlling the polymer properties.
Therefore, a number of initiators/catalysts have been
developed using various combinations of ligands and
metals,2-7 and aluminum is one of the most studied
metals among them. The initiators/catalysts in the
living polymerization systems are isolated before use
in most cases, while the isolation of metal complexes is
often a time-consuming process in order to find a new
well-defined molecular catalysis,8,9 especially when
screening various ligands to create the most appropriate
environment around the metal center.10 We examined
the ROP of ꢀ-caprolactone (CL) using a diphenyl-
substituted homosalen complex,11 which could be pre-
pared in situ and was the most reactive catalyst for the
polymerization of racemic lactide at 70 °C in our
previous studies.12 The initiation reaction immediately
took place at 25 °C, while the propagation required 70
°C. These results prompted us to study the polymeri-
zation of CL using the Al-salicylaldimine ligand com-
plexes without the linking of two salicylaldimines.
Although isolation of some salicylaldimine-Al com-
plexes13 and their applications in the polymerization14
have been reported, systematic investigations of ligand
substituents for the polymerizations have been rarely
reported.14b,c We now report a facile and efficient system
of the ROP of CL at room temperature using the
salicylaldimine-Al complex.
The polymerization of CL was examined at 25 °C in
the presence of 1 mol % of benzyl alcohol (BnOH) using
the complex prepared from 1 mol % of AlEt3 and 2 mol
% of the ligand in situ (70 °C for 1 h). In each case, the
1
number-average degree of polymerization (DPn) by H
NMR was quite consistent with the calculated one on
the basis of the amount of BnOH, and the number-
average molecular weight (Mn) by size exclusion chro-
matography (SEC) was estimated to be greater than
those by 1H NMR. The 1H NMR spectra of all the PCLs
indicated the presence of a benzyl ester group at 5.11
ppm (singlet, CH2Ph) as the initiating terminus. The
ligands with a sterically demanding imine moiety
PCL with a higher Mn (DPn ∼ 300) was synthesized
by the polymerization of 0.33 mol % of 1-BnOH at 25
°C, and an almost quantitative conversion of CL was
observed after 60 min (entry 1, Table 2).20 The linear
relationship between the monomer conversion and Mn
with a narrow polydispersity indicated that the polym-
erization proceeded in a living/controlled manner. To
obtain PCL in a short time, the reaction temperature
* Corresponding author. E-mail nnomura@nagoya-u.jp.
10.1021/ma050606d CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/01/2005

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5364 Communications to the Editor
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Table 1. Substituent Effects of Ligandsa
entry
R1
R2
R3
R4
BnOH, mol %
1.0
time
conv,b %
DPn by NMR
Mn,c ×103
8.9
4.5
19.6
9.2
26.6
22.4
30.5
26.1
6.5
29.6
6.7
8.5
25.8
10.4
27.4
30.8
29.2
29.8
29.7
33.1
18.1
27.3
PDIc,d
1
2
H
H
H
H
H
24 h
31
19
80
34
97
69
92
86
25
99
31
28
94
38
98
94
95
96
94
89
47
72
0
34
18
81
33
97
71
91
92
25
104
30
27
97
37
97
96
96
96
97
104
47
80
1.06
1.07
Me
iPr
Ph
3 h
e
24 h
1.27
3
4
H
3 h
1.07
1.19
e
24 h
Ph
1 h
1.17
1.16
1.18
1.08
2 h
5
6
tBu
F
tBu
F
1 h
3 h
e
24 h
1.22
7
8
H
Cl
Cl
H
24 h
1.05
1.08
1.17
1.09
1.14
1.16
1.16
1.16
1.28
1.26
1.16
1.14
3 h
24 h
9
Cl
Cl
3 h
24 h
10
11
12
13
14
15
16
17
18
19
20
tBu
tBu
H
Me
H
Me
H
H
tBu
Br
Me
1 h
Me
Me
iPr
Ph
tBu
Br
10 min
10 min
10 min
10 min
10 min
10 min
10 min
30 min
10 min
4 h
Me
0
0.5
2.0
1.0
97
98
<1
210
53
56.1
13.5
1.36
1.11
H
H
tBu
tBu
a Polymerization conditions: AlEt3, 0.020 mmol; ligand, 0.040 mmol; toluene, 2.0 mL; CL, 2.0 mmol; temp, 25 °C; a N2 atmosphere.
b The crude samples were analyzed by 400 MHz 1H NMR every 0.5-6 h (entries 1-9) or 10-30 min (entries 10-20). c The crude samples
were analyzed by SEC (polystyrene standards in CHCl3) without purification. d Polydispersity index number (Mw/Mn) of the crude samples.
e The SEC traces had small shoulders in the higher molecular elution volume.
Table 2. Effects of Polymerization Temperature and
Initial Concentration of CLa
Scheme 1. Ring-Opening Polymerization of VL and
BL
b
entry temp, °C [CL]0 time, min conv,c % Mn,d ×103 PDId
1
25
0.90
30
60
10
2
85
98
98
81
83
91
62.9
77.1
72.4
61.4
68.9
73.5
1.14
1.16
1.25
1.18
1.16
1.19
2
3
4
5
50
50
70
70
0.90
4.7
0.90
4.7
2.5
1
a 1 (prepared in situ), 0.020 mol (0.33 mol %); BnOH, 0.020
mmol (0.33 mol %); CL, 5.96 mmol; toluene, 6.0 mL (entries 1, 2,
and 4) or 0.60 mL (entries 3 and 5); a N2 atmosphere. After the
indicated reaction time, the reaction mixture was cooled at 0 °C
and then immediately exposed to air to decompose the Al complex.
reached 60%. The PDI value became higher at the
higher conversions; 6 days, 82% conversion, Mn 8600,
PDI 1.20; 9 days, 88% conversion, Mn 9000, PDI 1.25.
The structure of complex 1 was examined. A clean
1H NMR spectrum of the complex prepared from 1 equiv
of the ligand (L-H) to AlEt3 in toluene-d8 indicated the
formation of LAlEt2 in which the phenolic proton (ArO-
H) was completely consumed.21 In addition to the
assigned peaks of the LAlEt2 complex, some other minor
peaks were also detected.22 Free rotation of ArO-Al was
restricted by coordination of the pendant Schiff base
nitrogen to the Al center, which made the methylene
protons of the ethyl groups diastereotopic (Figure 1a).
This complex (1 mol %) in the presence of BnOH (1 mol
%) needed 1 h to reach >90% conversion of CL. It
suggested that complex 1 is formed by the reaction of 2
equiv of the ligand to AlEt3, although it is known that
the second equivalent of the analogous ketimine (L′-
H) reacts very slowly with L′AlR2.23,24 Using the 2 equiv
of the ligand to AlEt3, the peaks of LAlEt2 were not
detected. To our surprise, all ten Csp2-H’s, six tBu’s,
and four Csp2-CH3’s individually appeared in 1H NMR,
1
b The initial concentration of CL. c Conversion of CL by H NMR
(400 MHz) of the crude samples. d The crude samples were
analyzed by SEC (CHCl3, polystyrene standards).
was elevated. At 50 °C, the monomer conversion reached
98% after 10 min at the same initial concentration of
CL ([CL]0) with a larger PDI (entry 2), probably due to
the transesterification after the high conversion of CL.
For the higher [CL]0, a shorter polymerization time was
sufficient (entry 3). The same tendency was observed
at 70 °C (entries 4 and 5). Notably, PCL with DPn ∼
300 could be synthesized in a few minutes at 50-70 °C
in the present system.
Complex 1 was also effective for the polymerization
of δ-valerolactone (VL) as shown in Scheme 1. Although
the polymerization of â-butyrolactone (BL) was much
slower even at 70 °C, poly(BL) with a narrow polydis-
persity was obtained until the monomer conversion

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Macromolecules, Vol. 38, No. 13, 2005
Communications to the Editor 5365
Figure 1. 1H NMR assignment (400 MHz) of Et groups and imine protons: (a) LAlEt2; (b) L2AlEt.
Willem, R.; Dubois, P. Chem.sEur. J. 2003, 9, 4346. (c)
and two imine protons are under a very different
environment (8.02 and 9.14 ppm). These suggested that
one ligand chelated the Al center but the other was
monodentate (Figure 1b), and the conformation is highly
restricted. It is known that the chemical shifts of the
27Al NMR correspond to the coordination number of the
Al center.25 Unfortunately, no peak of 1 was obtained
in the 350 ppm range from -70 to 280 ppm, probably
due to the extremely low symmetric property around
the Al center. So far we have not succeeded in obtaining
single crystals of 1 for an X-ray diffraction analysis but
only a powdery complex. The structure of 1 in the
presence of 1 equiv of BnOH was examined, and the free
ligand (L-H) and unidentified complex compounds in
addition to complex 1 were detected in 1H NMR (toluene-
d8). The active species of the present system is still
under investigation.
In conclusion, a facile and efficient catalytic system
of salicylaldimine-aluminum complexes for the ROP of
CL was realized through a systematic exploration of the
ligand substituents. Such studies of designing ligands
will be informative for the development of finely tuned
catalysts/initiators. The in-situ reaction between the
ligand and AlEt3 is clean enough to directly use the
complex to achieve the living/controlled ROP of CL.
Further details of the polymerizations of CL, the active
species of the Al complex in the presence of BnOH, and
applications to other monomers using 1 are now under
investigation in our laboratory.
Mo¨ller, M.; Kange, R.; Hedrick, J. L. J. Polym. Sci., Part A:
Polym. Chem. 2000, 38, 2067.
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A. V.; Arndt, P.; Baumann, W.; Spannenberg, A.; Fischer,
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46, 51. (c) Zhong, Z.; Schneiderbauer, S.; Dijkstra, P. J.;
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(9) A number of aluminum Lewis acids prepared in situ have
been used in (asymmetric) organic synthesis. (a) Maruoka,
K.; Yamamoto, H. In Catalytic Asymmetric Synthesis; Ojima,
I., Ed.; VCH: New York, 1993; Chapter 9. (b) Yamamoto,
H. In Organometallics in Synthesis A Manual; Schlosser,
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(10) Wasserman, E. P.; Annis, I.; Chopin, L. J., III,; Price, P. C.
Macromolecules 2005, 38, 322.
(11) See the Supporting Information for the structure of the
complex and the reaction scheme. A part of it was reported
in the 53rd JPSJ Symposium on Macromolecules 2004:
Ishii, R.; Nomura, N.; Yamamoto, Y.; Kondo, T. Polym.
Prepr., Jpn 2004, 53, 2Pb024. The details of polymerization
of CL using 1 and its analogues, some of which were
characterized by X-ray crystallographic studies, will be
reported in due course.
Acknowledgment. A grant from Sumitomo Foun-
dation (2002) (N.N.) and a Grant-in-Aid for Young
Scientists (No. 16750094) from the Ministry of Educa-
tion, Science, Sports, and Culture, Japan, are gratefully
acknowledged. We are also thankful for a JSPS Re-
search Fellowship for Young Scientists (R.I.). We also
thank Mr. Yutaka Maeda (Research Center for Material
Science, Nagoya University) for his assistance with the
27Al NMR measurement.
Supporting Information Available: Experimental de-
tails, NMR data of the LAlEt2 (1H, 13C, and 1H-13C (HMQC)),
1
complex 1 (1H, 13C, H-13C (HMQC), and 27Al), and 1 in the
presence of 1 equiv of BnOH (1H). This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
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5366 Communications to the Editor
Macromolecules, Vol. 38, No. 13, 2005
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measured by low-angle laser light scattering (LALLS): Mw-
(SEC) ) 86 000; calculated Mw ) 36 800 (114.14 × 300 ×
95%/100 × 1.13 (by SEC); Mw (LALLS) ) 34 000. It is well-
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(22) The minor peaks detected in the synthesis of LAlEt2 turned
out to be those of L2AlEt. The concentration of AlEt3 that
we used should have been slightly lower (∼4%) than we
calculated. It was consistent with 4% of L2AlEt in the
synthesis of LAlEt2 and the free ligand L-H (∼8%) in the
synthesis of L2AlEt.
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bution of (N-phenylsalicylaldiminato)AlMe2 by 4-methylpy-
ridine; see ref 13f.
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MA050606D