Ring-opening polymerization of various cyclic esters by Al complex catalysts containing a series of phenoxy-imine ligands: Effect of the imino substituents for the catalytic activity

Article abstract of DOI:10.1016/j.molcata.2008.06.009

Ring-opening polymerizations (ROPs) of various cyclic esters [ε-caprolactone (CL), δ-valerolactone (VL), rac-lactide (LC)] using a series of Al complexes containing phenoxy-imine ligands of type, Me₂Al[O-2-^tBu-6-(RN{double bond, long}CH)C₆H₃], [R = ^tBu (1a), cyclohexyl (1b), adamantyl (1c), C₆H₅ (1d), 2,6-Me₂C₆H₃ (1e), 2,6-ⁱPr₂C₆H₃ (1f), 2,4,6-Me₃C₆H₂ (1g), 2,4,6-^tBu₃C₆H₂ (1h), C₆F₅ (1i)], have been explored in the presence of ⁿBuOH. Synthesis and identification of 1g and 1h including structural analysis of 1g by X-ray crystallography have also been explored. Both the catalytic activity and the catalyst efficiency in the ROPs were found to be highly affected by the imino substituents (R) employed. The 2,4,6-^tBu₃C₆H₂ analogue (1h) showed the notable catalytic activity in spite of its low catalyst efficiency in the ROP of CL, and the C₆F₅ analogue (1i) was most effective in terms of both the activity and the efficiency. The C₆F₅ analogue (1i) also showed the highest catalytic activity for the ROP of VL, and the polymerization proceeded in a living manner. The C₆F₅ analogue also showed relatively high catalytic activity in the ROP of LC, but the resultant polymer had no stereo-regularity. Although the Al complexes containing alkyl substituents (1a-c) exhibited low or negligible catalytic activities in the ROPs of CL and VL, the cyclohexyl analogue (1b) showed moderate catalytic activity for the ROP of LC. Attempts for the ROP of β-butyrolactone (β-BL) and γ-butyrolactone (γ-BL) using 1e,h,i-ⁿBuOH catalyst systems afforded negligible amount of polymers under the similar conditions.

Full text of DOI:10.1016/j.molcata.2008.06.009

Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ring-opening polymerization of various cyclic esters by Al complex catalysts
containing a series of phenoxy-imine ligands: Effect of the imino
substituents for the catalytic activity
Naruhito Iwasaa,b, Michiya Fujikia, Kotohiro Nomuraa,∗
a Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan
b Research Laboratories, Daiso Co., Ltd., 9 Otakasu, Amagasaki, Hyogo 660-0842, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2008
Received in revised form 7 June 2008
Accepted 19 June 2008
Available online 27 June 2008
Ring-opening polymerizations (ROPs) of various cyclic esters [␧-caprolactone (CL), ␦-valerolactone (VL),
rac-lactide (LC)] using a series of Al complexes containing phenoxy-imine ligands of type, Me2Al[O-2-tBu-
6-(RN CH)C6H3], [R = tBu (1a), cyclohexyl (1b), adamantyl (1c), C6H5 (1d), 2,6-Me2C6H3 (1e), 2,6-iPr2C6H3
(1f), 2,4,6-Me3C6H2 (1g), 2,4,6-tBu3C6H2 (1h), C6F5 (1i)], have been explored in the presence of nBuOH.
Synthesis and identification of 1g and 1h including structural analysis of 1g by X-ray crystallography have
also been explored. Both the catalytic activity and the catalyst efficiency in the ROPs were found to be
highly affected by the imino substituents (R) employed. The 2,4,6-tBu3C6H2 analogue (1h) showed the
notable catalytic activity in spite of its low catalyst efficiency in the ROP of CL, and the C6F5 analogue (1i)
was most effective in terms of both the activity and the efficiency. The C6F5 analogue (1i) also showed
the highest catalytic activity for the ROP of VL, and the polymerization proceeded in a living manner. The
C6F5 analogue also showed relatively high catalytic activity in the ROP of LC, but the resultant polymer
had no stereo-regularity. Although the Al complexes containing alkyl substituents (1a–c) exhibited low
or negligible catalytic activities in the ROPs of CL and VL, the cyclohexyl analogue (1b) showed moderate
catalytic activity for the ROP of LC. Attempts for the ROP of ␤-butyrolactone (␤-BL) and ␥-butyrolactone
(␥-BL) using 1e,h,i-nBuOH catalyst systems afforded negligible amount of polymers under the similar
Keywords:
Al complex
Ring-opening polymerization
Lactone
Lactide
conditions.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
We recently reported a study for ring-opening polymerization
(ROP) of ␧-caprolactone (CL) using various Al complexes containing
Organoaluminum compounds, especially Al alkoxides have been
known to be initiators for ring-opening polymerization (ROP) of
cyclic esters [1] such as lactides [2], lactones [2j,n,3]. These aliphatic
polyesters possess promising characteristics as biodegradable and
bioassimilable materials not only due to their practical biodegrad-
ability, but also due to their biocompatibility for medical and
pharmaceutical applications [4]. Design of monomeric or dimeric
Al complex catalysts for the efficient polymerization thus attracts
considerable attention [5]. Although it is known that an introduc-
tion of extremely bulky aryloxide ligands to aluminum generates
monomeric Lewis acid catalysts that can be used in various organic
reactions [6], reports concerning ligand effect toward the catalytic
activity in the ROP of cyclic esters using a series of ‘isolated’ Al
complexes were rare.
a
series of phenoxy-imine ligands of type, Me2Al[O-2-R1-6-
(R2N CH)C6H3], [R1 = Me, tBu; R2 = tBu, cyclohexyl, adamantyl,
C6H5, 2,6-Me2C6H3, 2,6-iPr2C6H3, C6F5], in the presence of nBuOH
(1.0 equiv.) [7,8]. We demonstrated that the imino substituent
(R2) rather than the aryloxo substituent (R1) strongly affected
the catalytic activity (turnover number, TON) in the ROP. The
activity increased in the order: R2 = C6F5 ꢀ 2,6-Me2C6H3 > 2,6-
iPr2C6H3 > C6H5 > cyclohexyl > tBu ꢀ adamantyl, and the ROP using
the C6F5 analogue proceeded in a living manner with high initia-
tion efficiency [8]. This fact is an interesting contrast to a previous
assumption that the electrophilicity of the metal center in the ROP
of lactones is much less important [3g,i]. In contrast, the observed
facts are analogous to the facts that the catalytic activity, molecular
weight for the resultant polymers, and the polymerization behav-
ior for ethylene (and/or propylene) polymerization using various
zirconium complexes containing a series of bis(phenoxy)imine lig-
ands were highly affected by the substituents on both the phenoxy
and the imino groups [9,10].
∗
Corresponding author. Fax: +81 743 72 6049.
E-mail address: nomurak@ms.naist.jp (K. Nomura).
1381-1169/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2008.06.009

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N. Iwasa et al. / Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
(a), cyclohexyl (b), adamantyl (c), C6H5 (d), 2,6-Me2C6H3 (e),
2,6-iPr2C6H3 (f), C6F5 (i)], were prepared according to our pre-
vious reports [7,8]. In order to explore effect of the aromatic
substituents in the imino group toward the catalytic activity,
as described above, the 2,4,6-tBu3C6H2 analogue (1h) has been
chosen, because a mixed catalyst system consisting of AlEt3, 2-
{(2,4,6-tBu3C6H2)N CH}C6H4OH (2 equiv. to Al), and PhCH2OH
(1 equiv. to Al) was known to exhibit high catalytic activity
for ROP of CL [3j]. The 2,4,6-Me3C6H2 analogue (1g) and the
2,4,6-tBu3C6H2 analogue (1h) were prepared by the analogous
procedures (Scheme 2) and were identified based on 1H, 13C
NMR spectra and elemental analysis. The structure of 1g was
determined by X-ray crystallography (Fig. 1) [11], and the struc-
ture showed that 1g folds a distorted tetrahedral geometry
around the Al metal center. This can be seen from the bond
angles for C(1)–Al–C(2) [117.58(12)◦], O(1)–Al–C(1) [111.75(10)◦],
O(1)–Al–C(2) [110.76(11)◦], although the O(1)–Al–N(1) bond angle
is somewhat small [93.37(8)◦]. The Al–N bond distance [1.965(2) Å]
is somewhat short but within the range of those in the series of
Me2Al[O-2-tBu-6-(RN CH)C6H3] [1a–d,f,g,i: 1.965–1.9896 Å] [7,8].
The Al–O bond distance [1.7748(19) Å] is also close to those in the
other Al complexes [1a–d,f,g,i; 1.7592–1.7792 Å].
2.2. Ring-opening polymerization of ␧-caprolactone
Ring-opening polymerizations (ROPs) of ␧-caprolactone (CL)
were conducted at 60 ◦C in the presence of Me2Al[O-2-tBu-6-
(RN CH)C6H3] (1a–i) upon the addition of nBuOH (1.0 equiv. to Al),
and the results are summarized in Table 1. The results in our pre-
vious report by 1a–f,i [8] conducted under the same conditions are
also placed for comparison. As demonstrated previously, presence
of nBuOH (1.0 equiv. to Al) was prerequisite and the polymerization
did not take place in the absence of nBuOH [7,8].
The catalytic activity [TON values, TON = CLconsumed (␮mol)/Al
(␮mol), TON after 60 min at 60 ◦C, Al 20 ␮mol, initial molar ratio of
[CL]/[Al] = 250] in the ROP of CL with a series of Me2Al[O-2-tBu-
6-(RN CH)C6H3] increased in the order: R = C6F5 [1i, TON > 248 (or
430, Al 10 ␮mol) after 30 min, run 12 (run 14)] > 2,4,6-tBu3C6H2 [1h,
TON > 235 (or 330, Al 10 ␮mol) after 30 min, run 10 (run 11)] ꢀ 2,6-
Me2C6H3 (1e, 185 after 30 min, run 5) > 2,4,6-Me3C6H2 (1g, 223, run
8) > 2,6-iPr2C6H3 (1f, 209, run 6) > C6H5 (1d, 113, run 4). The Al com-
plexes containing alkyl substituents in the imino group showed low
catalytic activities (runs 1–3), as reported previously [7,8]. The ROP
by both the C6F5 analogue (1i) and the 2,4,6-tBu3C6H2 analogue
(1h) completed even after 30 min (Al 20 ␮mol, 60 ◦C, runs 10 and
12), and the C6F5 analogue (1i) showed the higher activity than the
2,4,6-tBu3C6H2 analogue (1h) when the ROP was conducted under
low Al concentration conditions (Al 10 ␮mol, run 14).
Scheme 1.
In this paper, we thus conducted ROP of various cyclic
esters [␧-caprolactone (CL), ␦-valerolactone (VL), rac-lactide
(LC), ␤-butyrolactone (␤-BL) and ␥-butyrolactone (␥-BL)] using
Me2Al[O-2-tBu-6-(RN CH)C6H3], [1, R = tBu (a), cyclohexyl (b),
adamantyl (c), C6H5 (d), 2,6-Me2C6H3 (e), 2,6-iPr2C6H3 (f), 2,4,6-
Me3C6H2 (g), 2,4,6-tBuC6H2 (h), C6F5 (i)] upon the presence of
nBuOH (1.0 equiv.). In order to explore effect of the aromatic sub-
stituents in the imino group, we prepared and identified the
2,4,6-Me3C6H2 analogue (1g) and the 2,4,6-tBu3C6H2 analogue
(1h) in the present study including the structure analysis for 1g
by X-ray crystallography. Through these experiments, we tried to
explore the ligand effect, especially effect of the aromatic sub-
stituent in the imino ligand, as well as monomers toward the
catalytic activity in the ROP of various cyclic esters (Scheme 1).
2. Results and discussion
2.1. Synthesis of Me2Al[-2-tBu-{(2,4,6-Rꢁ3C6H2)N CH}C6H3]
(Rꢁ = Me, tBu)
The ROP by 1i in the presence of PhCH2OH in place of nBuOH
was also conducted and the resultant ring-opened polymer con-
tained polymer chain end derived from PhCH2OH (Fig. 2a). The
Mn value (4.42 × 104) estimated by the 1H NMR spectrum was
Various Al complexes containing a series of phenoxy-imine
ligands of the type, Me2Al[O-2-tBu-6-(RN CH)C6H3] [1, R = tBu
Scheme 2.

N. Iwasa et al. / Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
69
Fig. 1. ORTEP drawing for Me2Al[O-2-tBu-6-{(2,4,6-Me3C6H2)N CH}C6H3] (1g). Thermal ellipsoids are drawn at the 50% probability level and H atoms are omitted for
clarity. Selected bond distance (Å): Al(1)–O(1) 1.7748(19), Al(1)–N(1) 1.965(2), Al(1)–C(1) 1.953(2), Al(1)–C(2) 1.956(2), N(1)–C(9) 1.296 (2), N(1)–C(14) 1.452 (2). Selected
bond angles (◦): O(1)–Al(1)–N(1) 93.37(8), C(1)–Al(1)–C(2) 117.58(12), Al(1)–O(1)–C(3) 129.37(16), Al(1)–N(1)–C(9) 121.45(16), Al(1)–N(1)–C(14) 119.84(13), O(1)–Al(1)–C(1)
111.75(10), O(1)–Al(1)–C(2) 110.76(11), N(1)–Al(1)–C(1) 110.45(10), N(1)–Al(1)–C(2) 110.30(11).
very close to the exact value (4.23 × 104, run 13) calculated, cor-
showed the low efficiency (run 10, 61%). The results thus suggest
that the actual propagation rate by 1h should be higher than that
by 1i.
rected based on the Mn values by the GPC measurement versus
polystyrene standards [12]. The result thus suggests that the ROP
was initiated with Al-OR via coordination insertion mechanism as
proposed [1,13]. Moreover, the catalyst efficiency for the ROP by
1i in the presence of PhCH2OH was estimated based on both the
exact Mn value corrected by the value measured by GPC [12] and
the polymer yields. The calculated value of 65% (run 13) was lower
than that by the 1i-nBuOH system (88%, run 12), and the results
thus clearly explain the fact that the Mn value in the resultant
ring-opened polymer by the 1i-PhCH2OH system was higher than
that by the 1i-nBuOH system due to the difference in the catalyst
efficiency.
As shown in Table 2, the observed activities (by 1e,f,h,i)
decreased upon decreasing the polymerization temperature (from
60 to 50, 40 ◦C), and no significant differences in the order toward
the activity in the ROP using Al complexes with a series of the aro-
matic substituents in the imino group were seen. The C6F5 analogue
(1i, run 19, 86%) thus showed the highest catalytic activity (TON
values) among these Al complexes when the ROPs were conducted
at 50 ◦C. Rather notable differences in the TON values were seen
between the C6F5 analogue (1i) and the 2,4,6-tBu3C6H2 analogue
(1h), when the ROPs were performed under low Al concentration
conditions (10 ␮mol, runs 11, 14, 21–24); the polymerization by
1h did not proceed or exhibited negligible activity at 40 ◦C. The
observed differences, especially the tendency in the ROPs using 1h
is due to the low catalyst efficiency under these conditions. There-
fore, the C6F5 analogue is concluded to be suited as the catalyst
precursor in terms of both the catalytic activity and the catalyst effi-
ciency. As reported previously, the ROP of CL by 1i proceeded in a
living manner with high initiation efficiency under optimized con-
The catalyst efficiencies were thus estimated [based on N values
(in ␮mol) vs. Al used (␮mol)] in all polymerization runs at 60 ◦C
according to the above procedure (Table 1, runs 1–10, 12), and the
Al complexes containing alkyl substituents and 2,4,6-tBu3C6H2 in
the imino group were found to be low (runs 1–3, 10, rather low N
values). Note that the observed activity (TON value) by the 2,4,6-
tBu3C6H2 analogue (1h) estimated based on the polymer yield was
almost equal to the C6F5 analogue (1i, run 12, 88%), whereas 1h
Table 1
Ring-opening polymerization of ␧-caprolactone (CL) by Me2Al[O-2-tBu-6-(RN CH)C6H3] (1a–i) [R = tBu (1a), cyclohexyl (1b), adamantyl (1c), C6H5 (1d), 2,6-Me2C6H3 (1e),
2,6-iPr2C6H3 (1f), 2,4,6-Me3C6H2 (1g), 2,4,6-tBu3C6H2 (1h), C6F5 (1i)]a
c
c
Run
Complex (R)
Al (␮mol)
Time (min)
Yield (mg(%))
TONb
Mn × 10−4
Mw/Mn
Nd (␮mol)
1e
2e
1a (tBu)
1b (cyclohexyl)
1c (adamantyl)
1d (C6H5)
1e (2,6-Me2C6H3)
1f (2,6-iPr2C6H3)
1f (2,6-iPr2C6H3)
1g (2,4,6-Me3C6H2)
1g (2,4,6-Me3C6H2)
1h (2,4,6-tBu3C6H2)
1h (2,4,6-tBu3C6H2)
1i (C6F5)
20
20
20
20
20
20
20
20
20
20
10
20
20
10
60
60
60
60
30
60
30
60
30
30
30
30
30
30
46 (8)
156 (28)
Trace
20
70
–
113
185
209
85
223
78
235
330
248
245
430
1.64
2.07
–
1.10
1.19
–
5.0
13.5
–
3e
4e
258 (45)
413 (74)
472 (84)
192 (34)
499 (89)
171 (31)
524 (94)
375 (66)
551 (99)
550 (98)
481 (86)
2.83
4.84
4.35
2.87
4.92
2.03
7.66
10.6
5.60
7.56
9.34
1.38
1.64
1.61
1.24
1.93
1.35
1.58
1.68
1.64
1.66
1.64
16.3
15.2
19.4
11.9
18.1
15.0
12.2
5.9
5e
6e
7e
8
9
10
11
12e
13e,f
14e
17.6
13.0
9.2
1i (C6F5)
1i (C6F5)
a
Conditions: Al 10 or 20 ␮mol, nBuOH 1.0 equiv. to Al (in toluene 50 ␮L), CL 5.0 mmol, 60 ◦C.
TON = (molar amount of CL reacted)/(molar amount of Al).
GPC data in THF vs. polystyrene standards.
b
c
d
e
f
Estimated number of polymer chain (␮mol) = polymer yield (mg)/{0.56 × Mn(GPC)} [12].
Cited from our preliminary article [8].
PhCH2OH was used in place of nBuOH [Mn = 4.42 × 104 estimated by 1H NMR spectrum].

70
N. Iwasa et al. / Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
Fig. 2. (a) 1H NMR spectrum (in C6D6 at 25 ◦C) for poly(CL) (run 13 in Table 1, CL = ␧-caprolactone), Mn(NMR) = 4.42 × 104. (b) 1H NMR spectrum (in C6D6 at 25 ◦C) for poly(VL)
(run 33 in Table 3), Mn(NMR) = 3.10 × 104. Peak marked with * is due to a resonance ascribed to impurities.
ditions, affording the ring-opened polymer with narrow molecular
weight distributions [8].
run 32 (run 36)] > 2,6-Me2C6H3 [1e, 215 (or 350, Al 10 ␮mol), run
29 (run 34)] > 2,4,6-tBu3C6H2 [1h, 205 (or 235, Al 10 ␮mol) run 31
(run 35)] > 2,6-iPr2C6H3 (1f, 178, run 30) > C6H5 (1d, 133, run 28).
The observed trend concerning effect of the aromatic substituent
toward the activity in the ROP of VL was similar to that in the ROP of
CL, except that the 2,6-Me2C6H3 analogue (1e) showed the higher
activity than the 2,4,6-tBu3C6H2 analogue (1h); the C6F5 analogue
(1i) showed the highest activity and the ROP reached to completion
even if the polymerization was performed with low Al concentra-
tion conditions (Al 10 ␮mol, run 36).
2.3. Ring-opening polymerization (ROP) of ı-valerolactone and
rac-lactide
ROPs of ␦-valerolactone (VL) using Me2Al[O-2-tBu-6-
(RN CH)C6H3] (1a–f,h,i)-nBuOH (1.0 equiv. to Al) catalyst systems
were conducted under the similar conditions for the ROPs of CL,
and the results are summarized in Table 3.
The ROP using the C6F5 analogue (1i) was conducted upon
PhCH2OH instead of nBuOH, and the resultant ring-opened
polymer possessed polymer chain end derived from PhCH2OH con-
firmed by 1H NMR spectrum (Fig. 2b). Moreover, the Mn value
[Mn(NMR) = 3.10 × 104], which was estimated by the 1H NMR spec-
trum based on methylene protons at the polymer chain end
derived from PhCH2OH, was relatively close to the theoretical value
[Mn(calcd) = 2.26 × 104] calculated based on VL/Al molar ratio (and
90% yield). The catalyst efficiency (73%, run 33) for the ROP by 1i-
PhCH2OH catalyst system was estimated based on these Mn values
The polymerizations did not proceed or exhibited negligible
catalytic activities, when the Al complexes containing alkyl sub-
stituents in the imino group [R = tBu, cyclohexyl, adamantyl (1a–c)]
were employed. In contrast, as seen in the ROPs of CL, the ROPs
proceeded with high catalytic activities, when the Al complexes
containing aromatic substituents in the imino group (1d–f,h,i)
were employed as the catalyst precursors. The activity [TON val-
ues, TON after 60 min at 60 ◦C, Al 20 ␮mol, initial molar ratio of
[VL]/[Al] = 250] using a series of Me2Al[O-2-tBu-6-(RN = CH)C6H3]
increased in the order: R = C6F5 [1i, TON = 230 (or 450, Al 10 ␮mol),

N. Iwasa et al. / Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
71
Table 2
Effect of temperature in ring-opening polymerization of ␧-caprolactone (CL) by Me2Al[O-2-tBu-6-(RN CH)C6H3] [R = 2,6-Me2C6H3 (1e), 2,6-iPr2C6H3 (1f), 2,4,6-tBu3C6H2
(1h), C6F5 (1i)]a
c
c
Run
Complex (R)
Al (␮mol)
Temperature (◦C)
Time (min)
Yield (mg(%))
TONb
Mn × 10−4
Mw/Mn
Nd (␮mol)
6e
15e
5e
1f (2,6-iPr2C6H3)
1f (2,6-iPr2C6H3)
1e (2,6-Me2C6H3)
1e (2,6-Me2C6H3)
1h (2,4,6-tBu3C6H2)
1h (2,4,6-tBu3C6H2)
1h (2,4,6-tBu3C6H2)
1i (C6F5)
20
20
20
20
20
20
20
20
20
20
10
10
10
10
10
10
60
50
60
50
60
50
40
60
50
40
60
50
40
60
50
40
60
60
30
60
30
30
30
30
30
30
30
30
30
30
30
30
472 (84)
198 (35)
413 (74)
449 (80)
524 (94)
457 (82)
274 (50)
551 (99)
480 (86)
331 (59)
375 (66)
179 (32)
Trace
209
88
4.35
2.58
4.84
5.39
7.66
7.15
4.86
5.60
5.27
4.33
10.6
5.79
–
9.34
5.58
1.62
1.61
1.27
1.64
1.74
1.58
1.57
1.46
1.64
1.40
1.28
1.68
1.47
–
19.4
13.7
15.2
14.9
12.2
11.4
10.1
17.6
16.3
13.7
5.9
185
200
235
205
125
248
215
148
330
160
–
16e
10
17
18
12e
19e
20
11
1i (C6F5)
1i (C6F5)
1h (2,4,6-tBu3C6H2)
1h (2,4,6-tBu3C6H2)
1h (2,4,6-tBu3C6H2)
1i (C6F5)
21
5.5
–
9.2
7.3
22
14e
23
24
481 (86)
228 (41)
45 (8)
430
205
40
1.64
1.39
1.10
1i (C6F5)
1i (C6F5)
5.0
a
b
c
Conditions: Al 10, 20 ␮mol, nBuOH 1.0 equiv. to Al (in toluene 50 ␮L), CL 5.0 mmol.
TON = (molar amount of CL reacted)/(molar amount of Al).
GPC data in THF vs. polystyrene standards.
Estimated number of polymer chain (␮mol) = polymer yield (mg)/{0.56 × Mn(GPC)} [12].
Cited from our preliminary article [8].
d
e
Table 3
Ring-opening polymerization of ␦-valerolactone (VL) by Me2Al[O-2-tBu-6-(RN CH)C6H3] (1a–f,h,i) [R = tBu (1a), cyclohexyl (1b), adamantyl (1c), C6H5 (1d), 2,6-Me2C6H3
(1e), 2,6-iPr2C6H3 (1f), 2,4,6-tBu3C6H2 (1h), C6F5 (1i)]a
c
c
Run
Complex (R)
Al (␮mol)
Time (min)
Yield (mg(%))
TONb
Mn × 10−4
Mw/Mn
25
26
27
28
29
30
31
32
33d
34
35
36
1a (tBu)
1b (cyclohexyl)
1c (adamantyl)
1d (C6H5)
1e (2,6-Me2C6H3)
1f (2,6-iPr2C6H3)
1h (2,4,6-tBu3C6H2)
1i (C6F5)
20
20
20
20
20
20
20
20
20
10
10
10
60
60
60
30
30
30
30
30
30
30
30
30
Trace
Trace
Trace
–
–
–
–
–
–
–
–
–
264 (53)
430 (86)
355 (71)
411 (82)
462 (92)
452 (90)
349 (70)
234 (47)
453 (90)
133
215
178
205
230
225
350
235
450
1.30
1.95
1.61
1.72
1.94
2.01
1.93
1.39
2.54
1.21
1.37
1.33
1.30
1.38
1.32
1.21
1.24
1.39
1i (C6F5)
1e (2,6-Me2C6H3)
1h (2,4,6-tBu3C6H2)
1i (C6F5)
a
b
c
Conditions: Al 10, 20 ␮mol (in toluene 85 ␮mol), nBuOH 1.0 equiv. to Al, VL 5.0 mmol, 60 ◦C.
TON = (molar amount of VL reacted)/(molar amount of Al).
GPC data in THF vs. polystyrene standards.
PhCH2OH was used in place of nBuOH [Mn = 3.10 × 104 estimated by 1H NMR spectrum].
d
[Mn(calcd)/Mn(NMR)], and the result would suggest that the present
ROP was initiated with Al-OR via coordination insertion mechanism
as proposed [1,13].
deactivation. The Mn values in the resultant polymer increased lin-
early upon increasing the TON values (polymer yields) consistently
with low Mw/Mn values (Mw/Mn = 1.13–1.17, Fig. 4). These results
clearly indicate that the ROP by the C6F5 analogue (1i) proceeds in
The ROPs of VL using the C6F5 analogue (1i)-nBuOH catalyst
system were conducted in toluene at 50 ◦C, and the results are
summarized in Table 4. The ROP proceeded at significant rate at
beginning and the rate gradually decreased upon consumption of
VL. As shown in Fig. 3, the rates were first order dependent upon
the VL concentration, suggesting that the ROP proceeded without
Table 4
Ring-opening polymerization of ␦-valerolactone (VL) by Me2Al[O-2-tBu-6-
{(C6F5)N CH}C6H3] (1i) in toluenea
c
c
Run
Time (min)
Yield (mg(%))
TONb
Mn × 10−4
Mw/Mn
37
38
39
40
30
45
60
75
199 (40)
309 (62)
367 (73)
417 (83)
100
155
183
208
0.77
1.06
1.16
1.33
1.14
1.13
1.15
1.17
a
Conditions: Al 20 ␮mol (in toluene 100 ␮L), n-butanol 1.0 equiv. to Al (in toluene
85 ␮L), VL 5.0 mmol, toluene 4.35 mL, initial conc. VL = 1.0 mmol/mL, 50 ◦C.
b
TON = (molar amount of VL reacted)/(molar amount of Al).
GPC data in THF vs. polystyrene standard.
Fig. 3. Time course plots of log[VL]/[VL]0. [VL] = concentration of ␦-valerolactone
(VL) in mmol/mL. Detailed conditions are described in Table 4.
c

72
N. Iwasa et al. / Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
enhanced by placing electron-releasing group as the imino sub-
stituent.
1H NMR spectra of methine region of the resultant poly(LC) indi-
cated that the resultant polymers prepared by the Al complexes
(1a–f,h,i)-nBuOH catalyst systems had no tacticity (stereo regular-
ity), and atactic polymers were thus collected (Fig. 5).
2.4. Attempted ring-opening polymerizations of ˇ-butyrolactone
and ꢀ-butyrolactone
Ring-opening polymerizations (ROPs) of ␤-butyrolactone and ␥-
butyrolactone using the Me2Al[O-2-tBu-6-(RN CH)C6H3] (1e,h,i)-
nBuOH catalyst systems were attempted under bulk (neat)
conditions at 80 ◦C for 24 h (Al 50 ␮mol, monomer/Al molar
ratio = 100). However, the attempted ROPs gave negligible amount
of polymers under these conditions. These results thus suggest that
the present catalytic systems are limited to be effective for cyclic
esters consisting of six or seven membered ring.
Fig. 4. Plots of Mn and Mw/Mn values for poly(CL) vs. TON for ring-opening polymer-
ization of ␦-valerolactone (VL) catalyzed by Me2Al[O-2-tBu-6-{(C6F5)N CH}C6H3]
(1i) in toluene. Detailed conditions are described in Table 4.
a living manner, as seen in the ROP of CL. Taking into account these
results, we would also conclude that the ROP of VL by 1i-nBuOH
(and PhCH2OH) catalysts system proceeded without side reaction
like transesterification [14].
Ring-opening polymerizations (ROPs) of rac-lactide (LC) using
1a–f,h,i were conducted at 80 ◦C in the presence of nBuOH, and
the results are summarized in Table 5. The catalytic activity [TON
values, TON after 24 h at 80 ◦C, Al 50 ␮mol, initial molar ratio
of [LC]/[Al] = 100] in the ROP of LC using a series of Me2Al[O-2-
tBu-6-(RN CH)C6H3] increased in the order: R = C6F5 [1i, TON = 94
(36 after 8 h), run 49 (run 50)] > 2,4,6-tBu3C6H2 [1h, 91 (24 after
8 h), run 47 (run 48)] > cyclohexyl (1b, 72, run 43) > 2,6-Me2C6H3
(1e, 46, run 45) > 2,6-iPr2C6H3 (1f, 23, run 46) > C6H5 (1d, 19, run
44) ꢀ adamantyl (1c, 5, run 42), tBu (1b, 4, run 41). The ROPs using
the 2,4,6-tBu3C6H2 analogue (1h), the C6F5 analogue (1i) were also
conducted at 70 ◦C in the presence of nBuOH, but the observed activ-
ities (runs 51, 52) were significantly lower than those performed at
80 ◦C (runs 47, 49). Both the activity and the catalyst efficiency by
1i seemed to be more sensitive to temperature than those by 1h.
Although the Al complexes containing alkyl substituent in the
imino group (1a–c) were not efficient for the ROP of both CL and VL,
the cyclohexyl analogue (1b) was found to be effective for the ROP
of LC (run 43). The activity of both the tert-butyl analogue (1a) and
the adamantyl analogue (1c) were significantly lower than that by
1b probably due to the steric bulk. The moderate catalytic activity
by 1b would be caused by an electronic effect of alkyl substituent,
because the catalytic activity by the phenyl analogue (1d) possess-
ing the same steric bulk was about quarter of that by 1b (run 44).
The results thus suggest that the nucleophilicity of OnBu would be
2.5. Concluding remarks
We have conducted ring-opening polymerization of ␧-
caprolactone (CL), ␦-valerolactone (VL), and rac-lactide (LC) using
a series of Al complexes containing phenoxy-imine ligands of
type, Me2Al[O-2-tBu-6-(RN CH)C6H3] [1, R = tBu (a), cyclohexyl
(b), adamantyl (c), C6H5 (d), 2,6-Me2C6H3 (e), 2,6-iPr2C6H3 (f),
2,4,6-Me3C6H2 (g), 2,4,6-tBuC6H2 (h), C6F5 (i)] upon the presence
of nBuOH (1.0 equiv.). It turned out that the imino substituent
strongly affect toward both the catalytic activity and the cata-
lyst efficiency, and the C6F5 analogue (1i) is a suitable catalyst
precursor for the ROPs. The polymerization of CL and VL by 1i pro-
ceeded in a living manner. The 2,4,6-tBu3C6H2 analogue (1h) also
exhibited remarkable catalytic activities for ROPs of CL and VL, but
the catalyst efficiency of 1h was apparently lower than 1i. Both
the C6F5 analogue (1i) and the 2,4,6-tBu3C6H2 analogue (1h) also
showed moderate catalyst activity in the ROP of LC, but the resultant
polymer possessed no stereo regularity (atactic). The attempts for
polymerization of ␤-butyrolactone and ␥-butyrolactone afforded
negligible amount of polymers, suggesting that the present cata-
lyst systems are limited to be effective for cyclic esters consisting
of six or seven membered ring.
Our present study in this project focuses on more details
for the ROPs using the C6F5 analogue, Me2Al[O-2-tBu-6-
{(C6F5)N CH}C6H3] (1i), including not only synthesis of various Al
Table 5
Ring-opening polymerization of rac-lactide (LC) using Me2Al[O-2-tBu-6-(RN CH)C6H3] (1a–f,h,i) [R = tBu (1a), cyclohexyl (1b), adamantyl (1c), C6H5 (1d), 2,6-Me2C6H3 (1e),
2,6-iPr2C6H3 (1f), 2,4,6-tBu3C6H2 (1h), C6F5 (1i)]a
c
c
Run
Complex (R)
Temperature (◦C)
Time (min)
Yield (mg(%))
TONb
Mn × 10−4
Mw/Mn
Nd (␮mol)
e
e
41
42
43
44
45
46
47
48
49
50
51
52
1a (tBu)
1c (adamantyl)
1b (cyclohexyl)
1d (C6H5)
1e (2,6-Me2C6H3)
1f (2,6-iPr2C6H3)
1h (2,4,6-tBu3C6H2)
1h (2,4,6-tBu3C6H2)
1i (C6F5)
80
80
80
80
80
80
80
80
80
80
70
70
24
24
24
24
24
24
24
8
24
8
24
24
27 (4)
36 (5)
4
5
–
–
–
–
–
e
e
–
517 (72)
140 (19)
333 (46)
168 (23)
657 (91)
164 (24)
668 (94)
264 (36)
77 (11)
72
19
46
23
91
24
94
36
11
5
1.71
1.08
1.69
1.35
2.37
1.43
2.13
1.3
1.22
1.11
1.15
1.10
1.18
1.08
1.30
1.09
1.14
1.20
52.1
22.3
34.0
21.5
47.8
19.8
54.1
35.0
12.2
8.1
1i (C6F5)
1h (2,4,6-tBu3C6H2)
1i (C6F5)
1.09
0.7
33 (5)
Conditions: Al 50 ␮mol (in toluene 85 ␮L), nBuOH 1.0 equiv. to Al, LC 5.0 mmol, initial conc. of LC = 1.0 mmol/mL, toluene 4.9 mL.
TON = (molar amount of LC reacted)/(molar amount of Al).
GPC data in THF vs. polystyrene standards.
Estimated number of polymer chain (␮mol) = polymer yield (mg)/{0.58 × Mn(GPC)} [12].
Resultant polymer was insoluble in THF.
a
b
c
d
e

N. Iwasa et al. / Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
73
Fig. 5. 1H NMR spectra (in CDCl3 at 25 ◦C) of the methine region of the resultant poly(LC) using Me2Al[O-2-tBu-6-{RN CH}C6H3] [R = cyclohexyl (1b, top left, run 43), phenyl
(1d, center left, run 44), 2,6-Me2C6H3 (1e, bottom left, run 45), 2,6-iPr2C6H3 (1f, center right, run 46 in Table 4), C6F5 (1i, bottom right, run 49)].
complexes containing a series of fluorinated aromatic substituents
as the imino substituent, but also isolation of the initiating species
for the ROP. These results will be introduced in the near future.
25 ◦C unless otherwise noted, and chemical shifts are given in
ppm and are referenced to SiMe4 (ı 0.00, 1H, 13C). Molecular
weights and the molecular weight distributions of resultant poly-
mers were measured by gel-permeation chromatography (GPC).
GPC were performed at 40 ◦C on a Shimazu SCL-10A using a RID-10A
detector (Shimazu Co. Ltd.) in THF (containing 0.03 wt.% 2,6-di-
tert-butyl-p-cresol, flow rate 1.0 mL/min). GPC columns (ShimPAC
GPC-806, 804 and 802, 30 cm × 8.0 mm ꢁ, spherical porous gel
made of styrene/divinylbenzene copolymer, ranging from <102
to 2 × 107 MW) were calibrated versus polystyrene standard
samples.
3. Experimental
3.1. General procedures
All experiments were carried out under a nitrogen atmosphere
in a Vacuum Atmospheres drybox or using standard Schlenk tech-
niques. Anhydrous-grade n-hexane, toluene (Kanto Kagaku Co.,
Ltd.) were transferred into a bottle containing molecular sieves (a
mixture of 3A 1/16, 4A 1/8, and 13X 1/16) in the drybox under
N2 stream, and were passed through a short alumina column
under N2 stream before use. All chemicals used were of reagent
grade and were purified by the standard purification procedures.
Reagent-grade AlMe3 in n-hexane (Kanto Kagaku Co. Ltd.) were
stored in the drybox and were used as received. Various sali-
cylaldimines (imino-phenols) containing different substituent on
the imino groups, 2-tBu-6-(RN CH)C6H3OH [R = tBu, cyclohexyl,
adamantyl, Ph, 2,6-Me2C6H3, 2,6-iPr2C6H3, 2,4,6-tBu3C6H2, C6F5]
were prepared according to the reported procedures [9]. Synthe-
ses of Al complexes (1a–f,i) were referred to our previous report
[7,8]. Elemental analyses were performed by using a PE2400II Series
(PerkinElmer Co.). All 1H, and 13C NMR spectra were recorded on
a JEOL JNM-LA400 spectrometer (399.65 MHz for 1H, 100.40 MHz
for 13C). All spectra were obtained in the solvent indicated at
3.2. Synthesis of
Me2Al[O-2-tBu-6-{(2,4,6-Me3C6H2)N CH}C6H3] (1g)
Into
a
stirred
solution
containing
2-tBu-6-{(2,4,6-
Me3C6H2)N CH}C6H3OH (1.41 g, 4.00 mmol) in n-hexane
(10.0 mL), AlMe3 (1.1 M n-hexane solution, 4.67 mL, 4.20 mmol Al)
was added dropwise over 10 min period at −20 ◦C. The solution
was allowed to warm to room temperature and was stirred for 3 h
(Scheme 2). The mixture was then concentrated in vacuo, and the
resultant solid was dissolved in a minimum amount of n-hexane.
The chilled solution (−20 ◦C) afforded colorless microcrystals of
1g (1.39 g, 99% yield). 1H NMR (C6D6): ı −0.28 (s, 6H, AlMe2),
t
1.56 (s, 9H, Bu), 1.98 (s, 6H, Me), 2.05 (s, 3H, Me), 6.59–6.67 (m,
4H, aromatic), 7.31 (s, 1H, CH N), 7.42 (d, J = 7 Hz, 1H, aromatic).
13C NMR (C6D6): ı −8.6, 18.4, 20.7, 29.5, 35.4, 117.3, 119.8, 129.7,

74
N. Iwasa et al. / Journal of Molecular Catalysis A: Chemical 292 (2008) 67–75
131.4, 133.7, 135.0, 136.8, 141.8, 142.7, 164.8, 174.8. Anal. calcd for
C22H31AlNO: C, 75.18; H, 8.60; N, 3.99. Found: C, 75.34; H, 8.76; N,
3.97.
solution was stirred for 10 min, and the solution was then added
toluene (4.9 mL), rac-lactide (5.0 mmol). The reaction mixture was
then placed into an oil bath preheated at 60 ◦C, and the solution was
stirred for the prescribed time (24 h). The polymerization mixture
was then quenched by adding methanol (1.0 mL), and the resul-
tant solution was then poured into cold methanol (300 mL). The
ring-opened polymer was then collected as the methanol insoluble
white precipitates; the resultant polymer was then collected onto
a filter paper and was dried in vacuo. 1H NMR (CDCl3): ı 1.54–1.58
(m, 6H), 5.13–5.24 (m, 2H). 13C NMR (CDCl3): ı 16.6–16.7 (m, CH3),
69.0–69.2 (m, CCH3), 169.1–169.6 (m, C O).
3.3. Synthesis of
Me2Al[O-2-tBu-6-{(2,4,6-tBu3C6H2)N CH}C6H3] (1h)
Into
a
stirred
solution
containing
2-tBu-6-{(2,4,6-
tBu3C6H2)N CH}C6H3OH (0.905 g, 2.15 mmol) in n-hexane
(3.0 mL), AlMe3 (1.1 M n-hexane solution, 1.62 g, 2.37 mmol Al)
was added dropwise over 10 min period at −20 ◦C. The solution
was allowed to warm to room temperature and was stirred for 3 h
(Scheme 2). Removal of the solvent from the mixture by using a
rotary evaporator in vacuo gave complex 1h (0.88 g, 85.8% yield)
as yellow powder. 1H NMR (C6D6): ␦ −0.18 (s, 6H, AlMe2), 1.26
3.7. Crystallographic analysis
The crystallographic analysis for Me2Al[O-2-tBu-6-{(2,4,6-
Me3C6H2)N CH}C6H3] (1g) was made on a Rigaku RAXIS-RAPID
imaging plate diffractometer with graphite-monochromated Mo
K␣ radiation. All structures were solved by direct methods and
expanded using Fourier techniques [15], and the non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were included
but not refined. All calculations were performed using the Crys-
tal Structure crystallographic software package [16]. The selected
crystal collection parameters: crystal color, habit = red, block;
formula = C22H30AlNO; crystal system, space group = triclinic,
Pna21 (#33); a = 8.8176(4) Å; b = 25.7758(9) Å; c = 9.3767(3) Å;
V = 2131.15(14) Å3; Z = 4; Dcalcd = 1.095 g/cm3; F0 0 0 = 760.00; no.
of reflections measured = 20,121; no. of observations (I > 2.00ꢂ
(I)) = 2213; R1 = 0.0342; wR2 = 0.0956; goodness of fit = 1.000.
The crystallographic data (including CIF files) for 1g was
also deposited in Cambridge Crystallographic Data Centre as
CCDC 687433. The data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html or e-mail
deposit@ccdc.cam.ac.uk. Detailed analysis results such as CIF file,
the structure report including crystal data and crystallographic
parameters are shown in Supplementary Material.
t
t
t
(s, 9H, Bu), 1.42 (s, 18H, Bu), 1.55 (s, 9H, Bu), 6.59 (t, J = 7.72 Hz,
1H, aromatic), 6.72 (d, J = 8.08 Hz, 1H, aromatic), 7.39 (d, J = 7.32 Hz,
1H, aromatic), 7.59 (s, 1H, aromatic), 7.90 (s, 1H, CH N). 13C NMR
(C6D6): ı −6.7, 29.6, 31.3, 34.4, 34.8, 35.4, 37.6, 117.8, 120.2, 125.1,
133.0, 134.8, 142.3, 143.5, 143.7, 148.5, 164.7, 177.1. Anal. calcd for
C31H48AlNO: C, 77.94; H, 10.13; N, 2.93. Found: C, 77.60; H, 10.27;
N, 3.02.
3.4. Ring-opening polymerization (ROP) of ␧-caprolactone (CL)
Typical polymerization procedures (Table 1, run 11) are as
follows. Into a sealed Schlenk tube containing a toluene solu-
tion containing 1i (0.010 mmol/0.085 mL of toluene), nBuOH
(0.010 mmol) was added in the drybox at room temperature. The
solution was stirred for 10 min, and the solution was then added
␧-caprolactone (5.0 mmol). The reaction mixture was then placed
into an oil bath preheated at 60 ◦C, and the solution was stirred for
the prescribed time (30 min). The polymerization mixture was then
quenched by adding methanol (1.0 mL), and the resultant solution
was then poured into methanol (400 mL). The ring-opened polymer
was then collected as the methanol insoluble white precipitates;
the resultant polymer was then collected onto a filter paper and
was dried in vacuo. 1H NMR (CDCl3): ı 1.33 (m, 2H), 1.51–1.63 (m,
4H), 2.25 (t, 2H), 4.00 (t, 2H). 13C NMR (CDCl3): ı 24.5, 25.5, 28.3,
34.1, 64.1, 173.5 (C O).
Acknowledgements
N.I. and K.N. express their thanks to Daiso Co., Ltd. for support,
and to Mr. Shohei Katao (NAIST) for structural analysis.
3.5. Ring-opening polymerization (ROP) of ı-valerolactone (VL)
Appendix A. Supplementary data
Typical polymerization procedures (Table 3, run 32) are as
follows. Into a sealed Schlenk tube containing a toluene solu-
tion containing 1i (0.020 mmol/0.085 mL of toluene), nBuOH
(0.020 mmol) was added in the drybox at room temperature. The
solution was stirred for 10 min, and the solution was then added
␦-valerolactone (5.0 mmol). The reaction mixture was then placed
into an oil bath preheated at 60 ◦C, and the solution was stirred for
the prescribed time (30 min). The polymerization mixture was then
quenched by adding methanol (1.0 mL), and the resultant solution
was then poured into methanol (400 mL). The ring-opened polymer
was then collected as the methanol insoluble white precipitates;
the resultant polymer was then collected onto a filter paper and
was dried in vacuo. 1H NMR (C6D6): ı 1.39–1.59 (m, 4H), 2.07 (t,
2H), 3.95 (t, 2H). 13C NMR (CDCl3): ı 21.7, 28.4, 33.7, 63.8, 64.1, 172.6
(C O).
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.molcata.2008.06.009.
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