Living ring-opening homo- and copolymerisation of ε-caprolactone and l-lactide by cyclic β-ketiminato aluminium complexes

Article abstract of DOI:10.1039/c3dt52712c

A series of novel aluminium complexes containing cyclic β-ketiminato ligands of type Me₂Al{O-[(ArNCHC₄H₄(C ₆H₄))]} (3a, Ar = 2,6-ⁱPr₂C ₆H₃; 3b, Ar = C₆H₅; 3c, Ar = C ₆F₅) have been prepared in high yields. These complexes were identified by ¹H, ¹³C NMR spectroscopy and elemental analysis. X-ray structural analyses for 3a-c revealed that these complexes have a distorted tetrahedral geometry around Al, and both bond distances and bond angles were considerably influenced by the ligand structure. These complexes were tested as catalyst precursors for ring-opening polymerisation of ε-caprolactone (ε-CL) and l-lactide (l-LA) in the presence of 2-propanol as an initiator. Complex 3a could polymerize ε-CL in a controlled manner with high efficiency. Based on the living characteristics, the preparation of well-defined block copolymers PCL-b-PLLA via sequential addition of monomers was performed by 3a. Note that complex 3c exhibited rather high catalytic activity for the ROP of l-LA with narrow molecular weight distribution. The monomer conversion reached completion only in 4 h when the l-LA/Al molar ratio was 100 at 80 °C. PLLA-b-PCL copolymers were thus easily produced by 3c.

Full text of DOI:10.1039/c3dt52712c

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Living ring-opening homo- and copolymerisation
of ε-caprolactone and L-lactide by cyclic
β-ketiminato aluminium complexes†
Cite this: Dalton Trans., 2014, 43,
2244
Yan Liu,a,b Wei-Shi Dong,a Jing-Yu Liu*a and Yue-Sheng Lia
A series of novel aluminium complexes containing cyclic β-ketiminato ligands of type Me2Al{O-
[(ArNvCHC4H4(C6H4))]} (3a, Ar = 2,6-iPr2C6H3; 3b, Ar = C6H5; 3c, Ar = C6F5) have been prepared in high
yields. These complexes were identified by 1H, 13C NMR spectroscopy and elemental analysis. X-ray struc-
tural analyses for 3a–c revealed that these complexes have a distorted tetrahedral geometry around Al,
and both bond distances and bond angles were considerably influenced by the ligand structure. These
complexes were tested as catalyst precursors for ring-opening polymerisation of ε-caprolactone (ε-CL)
and L-lactide (L-LA) in the presence of 2-propanol as an initiator. Complex 3a could polymerize ε-CL in a
controlled manner with high efficiency. Based on the living characteristics, the preparation of well-
defined block copolymers PCL-b-PLLA via sequential addition of monomers was performed by 3a. Note
that complex 3c exhibited rather high catalytic activity for the ROP of L-LA with narrow molecular weight
distribution. The monomer conversion reached completion only in 4 h when the L-LA/Al molar ratio was
100 at 80 °C. PLLA-b-PCL copolymers were thus easily produced by 3c.
Received 30th September 2013,
Accepted 22nd October 2013
DOI: 10.1039/c3dt52712c
www.rsc.org/dalton
transition metals have been used to synthesize ε-CL/LA block
Introduction
copolymers.12–20 Note that the order of monomer addition was
The ring-opening polymerisation (ROP) of cyclic esters, such very important to obtain well-defined block polymers. When
as ε-caprolactone (ε-CL) and lactide (LA), promoted by metal the ε-CL monomer was polymerized first, the living PCL*
catalysts via
a coordination–insertion mechanism is an macromolecules were then able to initiate LA polymerisation
effective method for the synthesis of biodegradable and bio- and give rise to diblock copolymer PCL-b-PLA effectively.
compatible materials due to the advantages of well controlled However, reports of the living PLA* initiating ε-CL polymeri-
molecular weights and low polydispersity indices.1–4 The sation in a living manner are limited so far.
physical properties and functional parameters of poly(ε-capro-
Aluminium alkoxide-based initiator systems seem to be
lactone) (PCL) are quite different from those of polylactide active and suited for the ROP of cyclic esters on account of
(PLA). For instance, PCL has notable drug permeability, elas- their high Lewis acidity and low toxicity. Many four coordi-
ticity and thermal properties, while PLA exhibits excellent nation aluminium complexes have proven to initiate the living
mechanical properties but poor elasticity.5 The ε-CL/LA copoly- ROP of ε-caprolactone, producing polymers with well-
merisation thus allowed the properties of the resultant poly- controlled molecular weights and narrow molecular weight
esters to be tuned by changing the composition and the distributions.21–30 Nevertheless, almost all low coordinated
distribution of the monomer repeat units along the copolymer Schiff-base aluminium catalysts only exhibit relatively low
chain.6–20 A variety of complexes of aluminium, tin, and efficiency in L-LA polymerisation.31–37 Our aim is to obtain
highly efficient aluminium catalysts not only for ε-CL poly-
merisation but also for L-LA polymerisation, to prepare well-
aState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
defined block polymers (PCL-b-PLA and PLA-b-PCL). Recently,
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China.
we have reported a new class of neutral nickel complexes
E-mail: ljy@ciac.jl.cn
based on cyclic β-ketiminato ligands, which displayed highly
active for ethylene polymerisation without an activator. As part
of our investigation of β-ketiminato ligands in homogeneous
polymerisation catalysis, we were interested in preparing a new
family of aluminium complexes containing cyclic β-ketiminato
ligands. Herein, we thus described the synthesis and charac-
terization of some novel four coordination aluminium
bCollege of Environment & Chemical Engineering, Yanshan University, Qinhuangdao
066004, China
†Electronic supplementary information (ESI) available: Crystal data and struc-
ture refinements of complexes 3a–c; the structures for 3b–c and X-ray diffraction
data for 3a–c as cif; 13C NMR spectra of PCL-b-PLLA and PLLA-b-PCL; DSC curve
of PCL-b-PLLA; GPC curves of PLLA and PLLA-b-PCL. CCDC 946613–946615.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52712c
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Scheme 1
complexes containing cyclic β-ketiminato ligands of type
Me2Al{O-[(ArNvCHC4H4(C6H4))]} (3a, Ar = 2,6-iPr2C6H3; 3b,
Ar = C6H5; 3c, Ar = C6F5) (Scheme 1), and explored their appli-
cation for the ROP of ε-CL and L-LA. Activated by 2-propanol as
an initiator, these complexes proved to be highly active
towards the polymerisation of ε-CL and L-LA, allowing a well-
controlled chain growth. We also explored CL/LA copolymeri-
sation with these complexes to prepare block copolymers.
Table 1 Selected bond distances (Å) and angles (°) for complexes 3a–c
3a
3b (A)
3b (B)
3c
Bond distances in Å
1.8021(13) 1.8028(17) 1.7943(17) 1.8121(18)
Al(1)–O(1)
Al(1)–N(1)
Al(1)–C(1)
Al(1)–C(2)
N(1)–C(4)
O(1)–C(11)
1.9366(15) 1.948(2)
1.946(2)
1.957(3)
1.955(3)
1.439(3)
1.310(3)
1.974(2)
1.950(3)
1.945(3)
1.414(3)
1.304(3)
1.958(2)
1.960(2)
1.454(2)
1.310(2)
1.957(3)
1.954(3)
1.436(3)
1.307(3)
Bond angles in °
93.85(8) 94.26(8)
115.23(10) 118.15(12) 116.93(12) 120.86(12)
O(1)–Al(1)–N(1)
C(1)–Al(1)–C(2)
93.44(6)
92.47(8)
Results and discussion
Al(1)–O(1)–C(11) 128.77(12) 129.32(16) 130.45(16) 123.48(16)
Al(1)–N(1)–C(3)
Al(1)–N(1)–C(4)
O(1)–Al(1)–C(1)
O(1)–Al(1)–C(2)
N(1)–Al(1)–C(1)
N(1)–Al(1)–C(2)
121.07(12) 122.05(16) 122.32(16) 117.10(16)
123.42(11) 120.16(15) 119.48(16) 125.03(16)
Synthesis of aluminium complexes
A series of novel aluminium complexes containing cyclic
β-ketiminato ligands of the type, Me2Al{O-[(ArNv
CHC4H4(C6H4))]} (3a, Ar = 2,6-iPr2C6H3; 3b, Ar = C6H5; 3c, Ar =
C6F5), were effectively prepared via the reaction of AlMe3
with 1.0 equiv. of the corresponding neutral ligand
[ArNvCHC4H4(C6H4)]OH in toluene, as shown in Scheme 1.
These reactions took place along with the evolution of
methane, and the analytically pure samples were collected
from the chilled concentrated mixture of toluene and n-hexane
solution containing Al complexes cooled in the freezer
(−20 °C). The resultant complexes were identified by 1H and
13C NMR spectra and elemental analysis. The formulations of
alkyl aluminium complexes 3a–c are consistent with the NMR
spectroscopic data and elemental analyses. A sharp single reso-
nance assigned to the Al-Me protons and the resonances
corresponding to the ligands were observed. Crystals of 3a–c
suitable for X-ray crystal analysis were grown from the chilled
concentrated CH2Cl2 and n-hexane mixture solution. The
crystallographic data together with the collection and refine-
ment parameters are summarized in Table S1,† and the
selected bond distances and angles are summarized in
Table 1.
113.88(8)
109.45(8)
109.39(8)
113.52(8)
110.87(11) 112.59(10) 110.01(10)
110.76(10) 109.16(10) 105.34(11)
109.74(11) 107.24(10) 113.87(10)
110.82(11) 114.47(11) 110.17(11)
The structure of 3a is shown in Fig. 1. Complex 3a adopted
a distorted tetrahedral geometry around the Al metal centre, as
Fig. 1 Molecular structure of complex 3a with thermal ellipsoids at 30%
seen in the bond angles for C(1)–Al–C(2) (115.23(10)°), O(1)– probability level. Hydrogen atoms are omitted for clarity.
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Table 2 Ring-opening polymerisation of ε-caprolactone and L-lactide by the 3a–c/iPrOH systema
Entry
Complex
Time
Monomer
Conversion (%)
Mn,calcd b,c (10−4
)
Mn d (10−4
)
Mw/Mn
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3b
3c
3a
3b
3c
3c
3c
3c
3c
C1
C2
20 s
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
ε-CL
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
27
63
92
>99
86
94
58
71
35
64
88
96
>99
Trace
17
0.31
0.72
1.05
1.15
0.99
1.08
0.84
1.03
0.51
0.93
1.27
1.39
1.45
—
0.39
0.87
1.22
1.31
0.95
1.28
0.65
0.85
0.39
0.72
1.00
1.12
1.20
—
1.06
1.07
1.09
1.10
1.15
1.17
1.05
1.05
1.05
1.06
1.07
1.08
1.12
—
1 min
3 min
5 min
10 min
10 min
4 h
4 h
0.5 h
1 h
2 h
3 h
4 h
4 h
9
10
11
12
13
14
15
4 h
0.25
0.28
1.08
i
a Reaction conditions: 20–50 μmol Al catalyst in toluene, PrOH 1.0 equiv. to Al, monomer 5.0 mmol, monomer/Al (molar ratio) = 100, 80 °C.
i
i
b Calculated by ([CL]0/[OH]0) × 114.14 × conv. (%) + PrOH. c Calculated by ([LA]0/[OH]0) × 144.13 × conv. (%) + PrOH. d GPC data in THF vs.
polystyrene standards, using a correcting factor 0.56 for PCL and 0.58 for PLA.
Al–C(1) (113.88(8)°), and O(1)–Al–C(2) (109.45(8)°), although
the O(1)–Al–N(1) bond angle was somewhat small (93.44(6)°).
These bond angles are somewhat analogous to those in Me2Al-
[O-2-tBu2-6-{(2,6-iPr2C6H3)NvCH}C6H3] (C1).22,23 The Al–O(1)
bond length is 1.8021(13) Å, indicating O atom formed a
σ-bond with Al. The Al–N(1) bond distance is 1.9366(15) Å in
complex 3a, indicative of significant coordination of nitrogen
atom to the metal center. In the solid state, the six-membered
Al–N–C–C–C–O ring is nearly planar. The structures of 3b and
3c, determined by X-ray crystallography are shown in Fig. S1
and S2,† with selected bond distances and angles summarized
in Table 1. Crystals of 3b consist of two crystallographically
independent molecules in the unit cell, with only minor differ-
ences from each other. These complexes have a distorted tetra-
hedral geometry around Al. The Al–N bond distances in a
■
□
) vs. monomer conversion in the ROP of
Fig. 2 Mn
(
) and Mw/Mn
(
series of 3a–c increased in the order: 3a (1.9366(15) Å) < 3b ε-CL initiated by the 3a/iPrOH system.
(1.948(2), 1.946(2) Å) < 3c (1.974(2) Å). The C(1)–Al–C(2) bond
angles increased in the order: 3a (115.23(10)°)
< 3b
(118.15(12), 116.93(12)°) < 3c (120.86(12)°). These results
indicated that the bond distances and angles in Me2Al{O–
[(ArNvCHC4H4(C6H4))]} are influenced by the substituents in
the imino groups.
Homopolymerisation of ε-caprolactone and L-lactide
Homopolymerisation of ε-CL and L-LA were explored in the
i
presence of PrOH (1.0 equiv. to Al), and typical results are
summarized in Table 2. At a monomer/metal molar ratio of
100, the 3a/iPrOH system displayed rather high activity for the
ROP of ε-CL, the monomer conversion nearly reached com-
pletion after 5 min (entries 1–4). A linear relationship was
observed between the monomer conversions and the number
of average molecular weight (Mn) values with narrow molecular
weight distributions (MWDs) (Fig. 2). Moreover, the polymeris-
ation rate of the ROP was first-order dependent on the
monomer concentration (Fig. 3), suggesting no deactivation of
Fig. 3 Time course plots of Ln[CL]/[CL]0 ([CL] = concentration of CL in
active species during the ROP of ε-CL. It was therefore clear mmol mL−1).
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that the ROP by the 3a/iPrOH system proceeded in a living
manner under these conditions.
The ROP of ε-CL catalyzed by 3b, without any substituent,
took place less efficiently than that by catalyst 3a (entry 5), and
the monomer conversion only reached 86% in 10 min.
Complex 3c with a fluorinated ligand exhibited improved cata-
lytic performance for the ROP of ε-CL compared with 3b,
affording PCLs with narrow MWD (entry 6). Taking these
results into account, it is therefore clear that the catalyst struc-
ture significantly influenced the polymerisation behaviors. The
signals assigned to the hydroxy end group (–CH2OH) and the
isopropyl ester end-group ((CH3)2CHO(CO)) were observed in
1H NMR spectra of polylactones prepared by 3a, indicating
that the polymerisation involved the selective rupture of the
acyl–oxygen bond of the monomer and the insertion into the
alkoxide–aluminium bond.
Fig. 5 Time course plots of Ln[LA]/[LA]0 ([LA] = concentration of LA in
mmol mL−1).
The ROP of L-lactide initiated by complexes 3a–c was also
investigated, as shown in Table 2. The catalytic activity in the
ROP of L-LA with a series of Me2Al{O-[(ArNvCHC4H4(C6H4))]}
increased in the order: 3a < 3b < 3c. Complex 3a showed the
lowest activity for L-LA polymerisation among these catalysts
under similar conditions. The observed result was an interest-
ing contrast to that found for ε-CL polymerisation, in which 3a
exhibited highest catalytic activities. It should be noted that
using analogue 3c brought about obvious improvements in the
catalytic performances for ROP of L-LA. The monomer conver-
sion nearly reached completion only in 4 h (entry 13). The Mn
aluminium complex (C2) were prepared for comparison. The
results are also listed in Table 3. Neither C1/iPrOH nor
C2/iPrOH was an efficient catalyst system for the ROP of L-LA,
only a tiny amount of polymer was obtained after 4 h at 80 °C
(entries 14 and 15). These results provided further evidence of
the advantage of using these novel four coordination alu-
minium complexes bearing a cyclic β-ketiminato ligand in the
ROP of cyclic esters.
values linearly increased for longer reaction time with consist- Copolymerisation of ε-caprolactone and L-lactide
ently low PDI values (Fig. 4). The same first-order relationship
between the Mn values and the yields was observed under
these conditions (Fig. 5). These results indicated that the cata-
The living characters of both ε-CL and L-LA homopolymerisa-
tion catalyzed by new aluminium complexes allowed the
preparation of well-defined block copolymers via sequential
addition of the two monomers.
lyst 3c could polymerize L-LA in
a controlled manner.
The resulting PLA macromolecular chain is capped with the
i
hydroxyl group at one end, and with the PrO- group at the
Synthesis of PCL-b-PLLA diblock copolymers
other end, suggesting probably a coordination insertion
mechanism.
The L-LA monomer was added after the first step polymeri-
The salicylaldiminato aluminium analogue Me2Al[O-2-tBu- sation of ε-CL had gone to completion. A typical result is sum-
6-{(2,6-iPr2C6H3)NvCH}C6H3] (C1) and β-enaminoketonato marized in Table 3. As expected, the Mn value of the copolymer
increased with the growth of the chain segment, and the MWD
of the copolymer only changed slightly. For example, the Mn
increased from 23 400 (PDI = 1.10) for the first chain segment
to 33 400 (PDI = 1.17) for the second chain segment with a
unimodal peak (Fig. 6).
In the 1H NMR spectrum (Fig. 7), the signals due to methyl-
ene protons of the hydroxyl end groups (–CH2–OH) of the PCL
at 3.65 ppm completely disappeared and a new signal corres-
ponding to –CH(CH3)OH, methyne proton of the hydroxyl end
group of the copolymer appeared at 4.25 ppm, indicating that
the PCL reactive chain end initiated the polymerisation of
L-LA. Besides, the 13C NMR spectra of the copolymers exhibited
two carbonyl resonances at 173.3 and 169.4 ppm corres-
ponding to the PCL and PLA blocks (Fig. S3†). The thermal
analysis of the copolymers by DSC shows two melting peaks
(Tm) corresponding to PCL and PLLA blocks, respectively
(Fig. S4†). Taking these results into account, it is therefore
■
□
) vs. monomer conversion in the ROP of
Fig. 4 Mn
(
) and Mw/Mn
(
L-LA initiated by the 3c/iPrOH system.
clear that the block copolymer PCL-b-PLLA was formed.
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Table 3 Synthesis of diblock copolymers from cyclic ester monomersa
Mn,SEC b (10−4
)
Mn c,d(10−4
)
Mw/Mn
b
Entry
Complex
Time
Conversion (%)
1 PCL-b-PLLAe
2 PLLA-b-PCLf
3a
3c
5 min (CL) + 4 h (LA)
4 h (LA) + 1.5 h (CL)
73
91
3.34
3.91
1.89
2.23
1.17
1.32
a Reaction conditions: 50 μmol complex in toluene, iPrOH 1.0 equiv. to Al, monomer 5.0 mmol, Cl/LA/Al = 100/100/1, 80 °C. b GPC data
determined by SEC in THF relative to polystyrene standards. c,d GPC data determined by SEC in THF relative to polystyrene standards corrected
by the Mark–Houwink correction factor (c Mn,SECcorr = Mn,SEC raw data × 0.56 × %PCL + Mn,SEC raw data × 0.58 × %PLLA d Mn,SECcorr = Mn,SEC raw data
, ×
0.58 × %PLLA + Mn,SEC raw data × 0.56 × %PCL). e After reaction with CL for the prescribed time, LA was added and reacted for 4 h. f After reaction
with LA for 4 h, CL was added and reacted for the prescribed time.
Fig. 6 GPC profiles (in THF at 25 °C) of PCL and PCL-b-PLLA obtained
by the 3a/iPrOH system.
Fig. 8 1H NMR spectrum of PLLA-b-PCL copolymer by catalyst 3c
(entry 2, Table 3).
Fig. 9 DSC curve of PLLA-b-PCL prepared by catalyst 3c.
Fig. 7 1H NMR spectrum of PCL-b-PLLA copolymer by catalyst 3a
(entry 1, Table 3).
The appearance of less intense signals at 2.4 and 4.1 ppm in
Synthesis of PLLA-b-PCL
1
the H NMR spectrum and at 172.8 and 170.9 ppm in the 13C
To further explore the formation of diblock copolymers, the NMR spectrum indicated the occurrence of few transesterifica-
“poly(lactide) block first route” was examined with 3c by tion side reactions.
sequential ROP of L-LA and CL in toluene to yield PLLA-b-PCL.
Thermal analysis of the copolymer PLLA-b-PCL was carried
The result is also summarized in Table 3. SEC plots showed out by DSC and showed two melting peaks at 50.3 °C (PCL)
that the block copolymer molar mass was higher than that of and 168.9 °C (PLA). Moreover, PLLA-b-PCL also crystallized
the pure PLLA homopolymer (Fig. S5†). The expected signals during cooling from the melt (Tc = 78.2 °C), as shown in Fig. 9.
1
of the copolymer were observed from H NMR (Fig. 8) and 13C A similar observation was also reported in the literature.38,39
NMR spectra (Fig. S6†), similar to those of PCL-b-PLLA. The These results indicated that the PLLA chain end was able to
peaks corresponding to the isopropyl ester and hydroxyl initiate PCL chain growth, with reasonably controlled mole-
methylene chain ends were also seen in the 1H NMR spectrum. cular weight distribution.
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Ar–H), 7.09 (d, J = 7.6 Hz, 2H, Ar–H), 7.04 (t, J = 7.6 Hz, 1H,
Ar–H), 2.94 (t, J = 6.4 Hz, 2H, CH2), 2.71 (t, J = 7.2 Hz, 2H,
Conclusions
A series of novel aluminium complexes containing cyclic CH2). 13C NMR (CDCl3): δ 186.62, 140.95, 140.55, 139.54,
β-ketiminato ligands of type Me2Al{O-[(ArNvCHC4H4(C6H4))]} 134.04, 130.82, 128.62, 126.83, 125.75, 125.54, 121.86, 114.81,
(3a, Ar = 2,6-iPr2C6H3; 3b, Ar = C6H5; 3c, Ar = C6F5) have been 104.17, 28.74, 26.71.
effectively prepared and characterized. With 2-propanol as an
C6F5NvCHC4H4(C6H4)OH (2c). Yield: 70%. 1H NMR
initiator, these well-defined complexes proved to be highly (CDCl3): δ 11.95 (d, J = 10.3 Hz, 1H, –OH), 8.06 (d, J = 7.6 Hz,
active towards ring-opening polymerisation of L-lactide and 1H, NvC–H), 7.49–7.42 (m, 2H, Ar–H), 7.37 (t, J = 10.4 Hz, 1H,
ε-caprolactone in a controlled manner. The preparation of Ar–H), 7.23–7.26 (m, 1H, Ar–H), 2.94 (t, J = 4.4 Hz, 2H, CH2),
well-defined block copolymers PCL-b-PLLA via sequential 2.69 (t, J = 5.6 Hz, 2H, CH2). 13C NMR (CDCl3): δ 189.26,
addition of two monomers was performed by catalyst 3a. Note 142.41, 141.74, 140.09, 139.58, 137.64, 137.13, 134.45, 132.70,
that the aluminium complex bearing a fluorinated ligand pos- 128.09, 127.04, 117.36, 108.71, 29.54, 27.88.
sessed excellent catalytic performance for the ROP of L-LA. L-LA
polymerisation by 3c can reach near completion only in 4 h.
To the best of our knowledge, this novel complex exhibited the
highest catalytic efficiency for the ROP of L-LA among the low
coordinated Schiff-base aluminium catalysts reported. Using NvCHC4H4(C6H4)O]Al(CH3)2. Into
Synthesis of aluminium complexes 3a–c
Synthesis of aluminium complex 3a [(2,6-iPr2C6H3)-
stirred solution of
a
catalyst 3c, furthermore, the PLA chain end was able to initiate (2,6-iPr2C6H3)NvCHC4H4(C6H4)OH (0.71 g, 2.15 mmol) in
PCL chain growth, PLLA-b-PCL was thus easily produced via toluene (10 mL), AlMe3 (1 M n-hexane solution, 2.2 mL) was
sequential addition. The present catalyst system is thus a rare added drop-wise over a 10 min period at −20 °C. The solution
example for affording PLA-b-PCL with significant catalyst was allowed to warm to room temperature and was stirred for
efficiency.
3 h. The reaction mixture was concentrated in vacuo. The
chilled-concentrated toluene and n-hexane mixture solution
was placed in the freezer (−20 °C) and afforded complex 3a
(0.76 g, 95% yield) as yellow microcrystals. 1H NMR (CDCl3):
δ 8.02 (d, J = 8.8 Hz, 1H, NvC–H), 7.42–7.32 (m, 4H, Ar–H),
7.25 (d, J = 3.2 Hz, 2H, Ar–H), 7.22–7.18 (m, 1H, Ar–H),
3.18–3.12 (m, 2H, –CH), 2.92 (t, J = 7.6 Hz, 2H, CH2), 2.57 (t, J =
7.6 Hz, 2H, CH2), 1.26 (d, J = 6.8 Hz, 6H, –CH3), 1.09 (d, J =
6.8 Hz, 6H, –CH3), −0.80 (s, 6H, Al–CH3). 13C NMR (CDCl3):
δ 171.78, 168.82, 143.54, 142.82, 140.81, 133.16, 131.60,
127.42, 126.89, 126.49, 124.04, 103.74, 28.87, 28.05, 25.90,
25.85, 22.91, −9.84. Anal. calcd for C25H32AlNO: C, 77.09;
H, 8.28; N, 3.60. Found: C, 77.18; H, 8.24; N, 3.72.
Experimental
General procedures and materials
All manipulations of air- and/or moisture-sensitive compounds
were carried out under a dry argon atmosphere using standard
Schlenk techniques or under a dry argon atmosphere in an
MBraun glovebox unless otherwise noted. All solvents were
purified from an MBraun SPS system. The NMR data of the
ligands and complexes used were obtained using a Bruker
400 MHz spectrometer (400 MHz for H, 75.5 MHz for 13C) at
1
ambient temperature, with CDCl3 as the solvent (dried by MS
4 Å). Elemental analyses were recorded on an elemental Vario
EL spectrometer. Gel permeation chromatographic (GPC)
measurements were carried out using a Waters instrument
(515 HPLC pump) equipped with a Wyatt interferometric
refractometer, eluted with THF at 25 °C at 1 cm3 min−1. The
molecular weights were calibrated against polystyrene stan-
dards. The melting temperatures (Tms) of the resultant copoly-
mers were measured using a Perkin-Elmer Pyris 1 Differential
Scanning Calorimeter at a rate of 10 °C min−1, and Tm values
were collected after the second heating cycle. Reagent grade
AlMe3 in n-hexane was purchased from Acros and stored in a
bottle in the drybox and was used as received.
Synthesis of aluminium complex 3b [C6H5NvCHC4H4-
(C6H4)O]Al(CH3)2. Synthesis of 3b was carried out according
to the same procedure as that of 3a, except (C6H5)-
NvCHC4H6(C6H4)OH (0.53 g, 2.15 mmol) was used. Yield
0.58 g (88%). 1H NMR (CDCl3): δ 7.99 (d, J = 8.8 Hz, 1H, NvC–
H), 7.75 (s, 1H, Ar–H), 7.42–7.37 (m, 4H, Ar–H), 7.34–7.31 (m,
3H, Ar–H), 7.25–7.20 (m, 2H, Ar–H), 2.92 (t, J = 6.8 Hz, 2H,
CH2), 2.64 (t, J = 6.8 Hz, 2H, CH2), −0.72 (s, 6H, Al–CH3).
13C NMR (CDCl3): δ 172.29, 165.19, 147.68, 140.97, 132.96,
131.67, 129.56, 127.39, 126.82, 126.46, 126.37, 121.87, 104.87,
28.84, 26.12, −9.21. Anal. calcd for C19H20AlNO: C, 74.74; H,
6.60; N, 4.59. Found: C, 74.61; H, 6.47; N, 4.54.
Synthesis of aluminium complex 3c [C6F5NvCHC4H4(C6H4)-
O]Al(CH3)2. Synthesis of 3c was carried out according to the
Synthesis of aluminium complexes
same procedure as that of 3a, except (C6F5)NvCHC4H6(C6H4)-
1
Synthesis of ligands 2a–c. Various cyclic β-ketiminato OH (0.73 g, 2.15 mmol) was used. Yield 0.75 g (88%). H NMR
ligands [(ArNvCHC4H4(C6H4))]OH (2a, Ar = 2,6-iPr2C6H3; 2b, (CDCl3): δ 8.03 (d, J = 7.6 Hz, 1H, NvC–H), 7.47–7.43 (m, 2H,
Ar = C6H5; 2c, Ar = C6F5) were prepared according to the litera- Ar–H), 7.37–7.33 (m, 1H, Ar–H), 7.25–7.23 (m, 1H, Ar–H), 2.96
ture procedures.40
(t, J = 6.8 Hz, 2H, CH2), 2.64 (t, J = 6.8 Hz, 2H, CH2), −0.79 (s,
C6H5NvCHC4H4(C6H4)OH (2b). Yield: 85%. 1H NMR 6H, Al–CH3). 13C NMR (CDCl3): δ 177.53, 167.49, 142.10,
(CDCl3): δ 11.96 (d, J = 11.3 Hz, 1H, –OH), 8.04 (d, J = 7.6 Hz, 141.07, 133.10, 132.37, 127.77, 127.34, 127.10, 126.22,
1H, NvC–H), 7.43–7.30 (m, 4H, Ar–H), 7.23–7.21 (m, 2H, 125.69, 123.04, 106.23, 28.79, 26.12, −10.74. Anal. calcd for
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 2244–2251 | 2249

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Dalton Transactions
C19H15AlF5NO: C, 57.73; H, 3.82; N, 3.54. Found: C, 57.61; H, bath pre-heated at 80 °C, and the solution was stirred for the
3.87; N, 3.54.
prescribed time (4 h). The polymerisation mixture was then
quenched by adding methanol (1.0 mL), and the resultant
solution was then poured into methanol (400 mL). The ring
Synthesis of aluminium complexes 1 and 2
Synthesis of aluminium complex 1. Complex 1 was prepared opened polymer was then collected as the methanol insoluble
according to literature procedures.22
white precipitates; the resultant polymer was then collected on
Synthesis of aluminium complex 2. Synthesis of 2 was a filter paper and was dried in vacuo.
carried out according to the same procedure as that of 3a,
Synthesis of diblock copolymers
except
(2,6-iPr2C6H3)NC(CH3)C(H)C(C6H5)OH
(0.69
g,
2.15 mmol) was used. Yield 0.77 g, 95%. 1H NMR (CDCl3): δ
7.96–7.94 (m, 2H, Ar–H), 7.48–7.42 (m, 3H, Ar–H), 7.32–7.28
(m, 2H, Ar–H), 7.21–7.19 (m, 2H, Ar–H), 6.08 (s, 1H, CH),
The same procedure is followed for all diblock copolymer
syntheses. Typical polymerisation procedures (entry 1, Table 3)
are as follows. Into a sealed Schlenk tube containing a toluene
i
i
3.02–2.96 (m, 2H, Pr–CH), 1.91 (s, 3H, CO–CH3), 1.22 (d, J =
solution of 3a (0.050 mmol), PrOH (0.5 mL, 0.050 mmol) was
i
i
6.8 Hz, 6H, Pr–CH3), 1.13 (d, J = 6.8 Hz, 6H, Pr–CH3), −0.86
(s, 6H, Al–CH3). 13C NMR (CDCl3): δ 175.75, 173.30, 141.79,
138.21, 136.27, 130.08, 127.25, 126.19, 123.26, 96.57, 26.98,
23.60, 23.43, 22.83, −12.14. Anal. calcd for C24H32AlNO: C,
76.36; H, 8.54; N, 3.71. Found: C, 76.18; H, 8.62; N, 3.77.
added in a dry box at room temperature. The solution was
stirred for 10 minutes, and then ε-CL (5.0 mmol) was added to
the solution. The reaction mixture was then placed into an oil
bath pre-heated at 80 °C, and the solution was stirred for the
prescribed time (5 min). Subsequently, the second monomer
L-LA (5.0 mmol) was added to the polymerisation mixture and
heated at the same temperature. The polymerisation was con-
tinued for the prescribed time (4 h). The polymerisation
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 on a filter paper and was dried in vacuo.
X-Ray crystallography
Single crystals of complexes 3a–c suitable for X-ray structure
determination were grown from a concentrated CH2Cl2–hexane
mixture solution at −30 °C in a glove box, thus maintaining a
dry, O2-free environment. The intensity data were collected
with the ω scan mode (186 K) on a Bruker Smart APEX
diffractometer with a CCD detector using Mo Kα radiation (λ =
0.71073 Å). Lorentz, polarization factors were made for the
intensity data and absorption corrections were performed
using the SADABS program. The crystal structures were solved
using the SHELXTL program and refined using full matrix
least squares. The positions of hydrogen atoms were calculated
theoretically and included in the final cycles of refinement in
a riding model along with attached carbons.
Acknowledgements
The authors are grateful for financial support provided by the
National Natural Science Foundation of China (no. 20923003).
Ring-opening polymerisation of ε-caprolactone
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