Novel N,O,N-supported tetracoordinate aluminum complexes for the highly controlled and immortal ROP of trimethylene carbonate (TMC) under mild conditions: Access to narrowly disperse poly-TMC and derived copolymers

Article abstract of DOI:10.1021/om200703d

The novel N,O,N-supported tetracoordinate amidoaluminum complexes {η³(N,O,N}-(C₅H₉)N-C₆H ₄}₂OAlNMe₂ (2a, R = C₅H₉; 2b, R = C₆H₁₁) have been synthesized and structurally characterized. In the solid state, as determined from X-ray crystallographic studies, complex 2a consists of a four-coordinate Al species in which the Al center adopts a distorted-trigonal-monopyramidal (tmp) geometry with the nitrogens of the three amido groups (defining the pyramidal base) being nearly coplanar with Al. Such Al species, when combined with an alcohol source such as benzyl alcohol, effectively polymerize trimethylene carbonate (TMC) at room temperature in a highly controlled manner to yield narrowly disperse poly(trimethylene carbonate), as deduced from polymer characterizations with various kinetic studies. The high degree of molecular chain length control of the present system was further exploited to access narrowly dispersed PEG-functionalized amphiphilic copolymers. The attractive features of the system lie in the combination of an excellent activity, a high level of chain length control, and mild reaction conditions.

Full text of DOI:10.1021/om200703d

ARTICLE
pubs.acs.org/Organometallics
Novel N,O,N-Supported Tetracoordinate Aluminum Complexes for
the Highly Controlled and Immortal ROP of Trimethylene Carbonate
(TMC) under Mild Conditions: Access to Narrowly Disperse poly-TMC
and Derived Copolymers
Frꢀedꢀeric Hild,† Lydia Brelot,‡ and Samuel Dagorne*,†
†Laboratoire DECOMET and ‡Service de Radiocristallographie, Institut de Chimie de Strasbourg (CNRS-Universitꢀe de Strasbourg),
1 rue Blaise Pascal, 67000 Strasbourg, France
S Supporting Information
b
ABSTRACT: The novel N,O,N-supported tetracoordinate
amidoaluminum complexes {η3(N,O,N}-(C5H9)N-C6H4}2O-
AlNMe2 (2a, R = C5H9; 2b, R = C6H11) have been synthesized
and structurally characterized. In the solid state, as determined
from X-ray crystallographic studies, complex 2a consists of a
four-coordinate Al species in which the Al center adopts a
distorted-trigonal-monopyramidal (tmp) geometry with the
nitrogens of the three amido groups (defining the pyramidal
base) being nearly coplanar with Al. Such Al species, when
combined with an alcohol source such as benzyl alcohol, effectively polymerize trimethylene carbonate (TMC) at room temperature
in a highly controlled manner to yield narrowly disperse poly(trimethylene carbonate), as deduced from polymer characterizations
with various kinetic studies. The high degree of molecular chain length control of the present system was further exploited to access
narrowly dispersed PEG-functionalized amphiphilic copolymers. The attractive features of the system lie in the combination of an
excellent activity, a high level of chain length control, and mild reaction conditions.
Chart 1
iodegradable aliphatic polyesters and polycarbonates such as
poly(lactic acid) (PLA) and poly(trimethylene carbonate)
B
(PTMC) have received increased interest in recent years due to
their important biomedical and pharmaceutical applications as
well as their being viable alternatives to petrochemical-based
plastics.1 In this area, the ring-opening polymerization (ROP) of
cyclic esters (lactide, ε-caprolactone for example) by discrete
metal-based initiators has undoubtedly established itself as the
method of choice to access well-defined and narrowly disperse
polyesters through precise chain length control.2 In contrast,
despite interest in the resulting polymer, the ROP of cyclic
carbonates such as trimethylene carbonate (TMC), a monomer
that may be readily prepared from glycerol, by well-defined metal
species has been relatively less studied and the reported systems
thus far, some of which display high catalytic activity, typically
require heating and yield poly(trimethylene carbonate) (PTMC)
with moderately narrow polydispersity (PDI > 1.5) whether in
bulk or solution polymerization.3 In general, apart from their
intrinsic specific properties, narrowly disperse polymers are also
of interest as well-defined building blocks for their subsequent
incorporation in, for instance, copolymer block structures to
produce various tailor-made materials with precise chain length,
featuring improved mechanical properties versus their ill-defined
counterparts. In this regard, PTMC appears as an interesting
candidate for subsequent access to well-defined biomaterials of
current interest such as copolymer PTMC-PLA, a material more
flexible and acid resistant than PLA, and PTMC-PEG, an amphi-
philic polymer that may be used as a drug-delivery agent.1,4,5
Albeit little studied, constrained tetracoordinate Al(III) com-
plexes supported by an appropriately designed LX2-dianionic
ligand forcing the Al center into a trigonal-pyramidal coordination
geometry (versus its classically preferred tetrahedral geometry)
have been shown to be effective Lewis acid catalysts for the
mediation of various asymmetric transformations (B, Chart 1).6
On one occasion, such Al complexes (A, Chart 1) were also
shown to initiate the ROP of rac-lactide and propene oxide, yet
with low activity and control. The increased reactivity of such Al
Received: July 29, 2011
Published: September 19, 2011
r
2011 American Chemical Society
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Organometallics
ARTICLE
complexes (vs tetrahedral Al systems) arises from a destabilizing
ligand-defined geometry distortion, resulting in a more Lewis
acidic Al metal center.6c Yet, in comparison to usually reactive Al
systems, which typically require a low-coordinate metal center
(<4) for Lewis acid activation and whose limited stability and
tendency to aggregate is frequently problematic,7 distorted tetra-
coordinate aluminum chelates may be readily accessible (in a
mononuclear form) and provide a superior steric protection of
the Lewis acidic metal center and, hence, an increased stability.
Such entities may thus well represent a reasonable reactivity/
stability balance, which is of interest in catalysis.
On these basis and within the above context, we are interested
in the development of novel families of tetracoordinate group 13
Lewis acids in order to probe their potential in the polymerization
catalysis of polar monomers. As part of this work, we report here
on the synthesis and structural characterization of novel N,O,N-
supported tetracoordinate aluminum species (2a,b; Scheme 1)
and their subsequent use as initiators (in the presence of an
alcohol source) for the highly controlled and immortal ROP of
TMC. Also, as shown below, the attractive features of the des-
cribed system, which combines an excellent activity, a high level
of chain length control, and mild reaction conditions, were
further exploited to access PTMC-PEG amphiphilic polymers
in a well-defined manner.
’ RESULTS AND DISCUSSION
As an entry to novel constrained tetracoordinate Al species,
the diamino ether proligands 1a,b (Scheme 1) were picked as
supporting ligands for their backbone structure, thought to be
susceptible for imposing the desired trigonal-monopyramidal
geometry (tmp) to the coordinated Al center; also, compounds
1a,b are readily available in good yield via a one-step procedure
from commercially available 2,20-oxidianiline and have already
been shown to be suitable for coordination to oxophilic metals.8
As illustrated in Scheme 1, the derived N,O,N-supported Al
amido species {η3(N,O,N)-RN-C6H4}2OAlNMe2 (2a, R = C5H9;
2b, R = C6H11; Scheme 1) may be directly prepared in good
yield via an amine elimination route by reaction of ligands 1a,
with 1 equiv of Al(NMe2)3. Compounds 2a,b were isolated as
highly air- and moisture-sensitive colorless solids, and their mol-
ecular structures were confirmed by X-ray crystallography analysis.
As depicted in Figure 1, complex 2a indeed consists of a four-
coordinate Al species effectively η3(N,O,N)-chelated by the dia-
nionic diamido amino [{(C5H9)N-C6H4}2O]2ꢀ ligand, forcing
the Al center to adopt a distorted-tmp geometry. The nitrogens of
the three amido groups (defining the pyramidal base) are thus
nearly coplanar with Al (sum of NꢀAlꢀN angles 356.83°). All
bond lengths in 2a are rather as expected with, in particular, all
AlꢀN amido bond distances (see Figure 1) lying within the typical
range(1.78ꢀ1.86 Å) for terminal Alꢀamido bonds.10 Importantly,
these solid-state data for 2a indicate the presence of an apical vacant
site ideally disposed for coordination to the Lewis acid Al center.
NMR solution data for 2a,b are consistent with a Cs-symmetric
structure for both complexes under the studied conditions (room
temperature, C6D6), in agreement with solid-state structural data.
Compounds 2a,b were then tested as ROP initiators of TMC
in both the absence and presence of an alcohol source such as
BnOH. As expected, species 2a,b exhibit very similar ROP
activities and control; thus, these will be discussed only for 2a.
Scheme 1
Figure 1. Molecular structure of the Al complex 2a: (a) front view; (b) side view. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Al(1)ꢀN(1) = 1.834(1), Al(1)ꢀN(2) = 1.828(1), Al(1)ꢀN(3) = 1.784(1), Al(1)ꢀO(1) = 1.964(1); N(1)ꢀAl(1)ꢀN(2) = 125.00(6),
N(1)ꢀAl(1)ꢀN(3) = 116.11(6), N(3)ꢀAl(1)ꢀN(2) = 115.73(7), N(3)ꢀAl(1)ꢀO(1) = 115.72(6).
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ARTICLE
Table 1. ROP of Trimethylene Carbonate by the Al Complex
2a in the Presence/Absence of BnOHa
e
f
g
entry TMCb BnOHb t (min)c conversn (%)d Mn,theor Mn,corr Mw/Mn
1
2
3
4
5
6
100
100
0
5
10h
5
30h
30h
30h
30
100
90
10 200 23 521 2.29
1 940 1 514 1.12
10 200 9 492 1.18
20 400 20 678 1.12
11 220 11 973 1.04
7 140 8 120 1.08
300
3
100
100
100
70
600
3
1100
3000i
10
30
a Polymerization conditions: [TMC]0 = 1 M, CH2Cl2, room tempera-
ture. b Amount in equiv versus Al initiator 2a. c Reaction time. d Monomer
conversion. e Calculated using Mn,theor = [TMC]0/[BnOH]0 ꢁ MTMC
ꢁ
conversn. f Measured by GPC in THF (30 °C) using PS standards and
corrected by applying the appropriate correcting factor (0.57 for entry 1,
0.88 for entries 2ꢀ5).3c g Measured by GPC in THF (30 °C). h The
reaction time is not optimized. i [TMC]0 = 2 M.
Figure 3. Plot of the molecular weight number (Mn) of the formed
PTMC as a function of [2a]0/[BnOH]0 in the ROP of TMC by a 2a/
BnOH initiating system. Conditions: 1100 equiv of TMC (vs 2a),
[TMC]0 = 1 M, CH2Cl2, room temperature.
present ROP system is further substantiated by the linear
correlation between the Mn(corrected) value of the formed
PTMC and monomer conversion during the polymerization
reaction (Figure 2). Also, as depicted in Figure 3, varying the
initial Al initiator/BnOH ratio (for a given amount of monomer)
was found to be linearly related to the molecular weight number
(Mn) of the resulting polymers, this being consistent with the
alcohol-mediated chain transfer reactions proceeding in a con-
trolled manner during these polymerization reactions.
Whether it be with regard to catalytic performance or polymer
chain length control, the ROP activity of initiators 2a,b compares
very favorably with that of higher coordinate Al(III) complexes
such as, for instance, salen-supported Al alkoxides; at best, the
latter have been reported to quantitatively polymerize 350 equiv
of TMC within 2 h at 95 °C to yield PTMC with moderately
narrow polydispersity (PDI = 1.6).3b
Figure 2. Plot of the molecular weight number (Mn) of the formed
PTMC as a function of TMC conversion in the ROP of TMC by a 2a/
BnOH initiating system. Conditions: 100 equiv of TMC (vs 2a), 5 equiv
of BnOH (vs 2a), [TMC]0 = 1 M, CH2Cl2, room temperature.
End-group analysis of the obtained PTMCs (entries 2ꢀ5,
Table 1) by 1H NMR and MALDI-TOF spectrometry (see the
Supporting Information, Figure S1) unambiguously establish the
presence of a OBn moiety at the ester end and the absence of
amido-capped PTMCs, suggesting that the Alꢀamido group in
2a may not act as an initiating moiety at all in these polymeri-
zation reactions as might have been thought. This, along with
control experiments showing that 2a does not readily convert to
the corresponding AlꢀOBn derivative in the presence of BnOH
at room temperature, disfavor a coordinationꢀinsertion me-
chanism proposal similar to that taking place in the ROP of TMC
by salen-bearing Al alkoxide complexes.3a Rather, it appears likely
that the present polymerization may occur through a sequential
Lewis acid monomer activation/alcohol nucleophilic attack and
subsequent chain transfer reactions,3g in which complex 2a
would act as a well-defined Lewis acid component (Scheme 2).
The apparent attractive features of the 2a/ROH initiating
system prompted us to further exploit its possibilities to access
various well-defined biomaterials, such as PTMC-PEG amphi-
philic polymers, through the use of a monocapped methoxy ether
PEG-OH(5000) polymer as an alcohol source. Remarkably, the
ROP of TMC (from 100 to 600 equiv) initiated by a 2a/mPEG-
OH (5000) two-component initiator was found to proceed at
room temperature with efficiency and high level of chain length
control similar to that observed with 2a/BnOH (Table 2,
Scheme 3). In all cases, the ROP reactions were complete within
30 min to yield narrowly disperse PTMC-PEG block copolymers
As a starting point, the Al amido species 2a was found to readily
mediate the ROP of TMC with the quantitative conversion of
100 equiv of TMC within a few minutes at room temperature to
PTMC (entry 1, Table 1), yet the resulting polymer features a
broad polydispersity, indicative of a likely poorly controlled
polymerization reaction. In contrast, the ROP of TMC initiated
by a 2a/BnOH mixture was much more successful (entries 2ꢀ5,
Table 1). Thus, using various amounts of BnOH and monomers,
the 2a/BnOH system may quantitatively polymerize from 100
up to 1100 equiv of TMC at room temperature within 30 min to
yield narrowly disperse PTMC, as deduced from NMR and GPC
data (see the Supporting Information, Figures S2 and S3).
Carrying out the ROP catalysis using 3000 equiv of TMC in
the presence of 30 equiv of BnOH (vs species 2a) yields a 70%
conversion to PTMC after 30 min at room temperature
(corresponding to a TON of 4200 hꢀ1), thereby showing the
excellent activity of the present 2a/BnOH system. All data
support that the ROP polymerization by 2a/BnOH proceeds
in an immortal manner, with BnOH acting as a chain transfer
agent.9 In particular, as shown in Table 1, all observed Mn values
(after correction) closely match the initial [TMC]0/[BnOH]0
ratio with, for instance, up to 30 polymer chains being generated
per Al center (entry 6, Table 1). The controlled character of the
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ARTICLE
Scheme 2
Table 2. ROP of Trimethylene Carbonate by the Al Complex
2a in the Presence of mPEG-OH (5000)a
frequently resulting in a broadening of the material’s molecular
distribution. The present 2a/ROH initiating systems might well
apply to the synthesis of various well-defined PTMC-containing
block copolymers via the use of appropriate polymer mono-
blocks as alcohol sources.
entry TMCb mPEG-OHb Mn,theor Mn,obs(GPC)d Mn,corr Mw/Mn
c
e
f
1
2
3
4
100
200
400
600
5
5
5
5
7040
10979
11739
15328
25418
6627
1.08
1.20
1.09
1.11
In conclusion, novel constrained tetracoordinate aluminum
complexes 2a,b supporting a N,O,N diamido-amino dianionic
ligand have been prepared in a two-step procedure from simple
reagents. When they are combined with an alcohol source, such
Al entities afford a highly effective two-component initiator for
the ROP of TMC. Importantly, the reported 2a,b/ROH systems
combine catalytic efficiency, a high level of molecular weight
control for the resulting homo-/copolymers, and mild polymer-
ization conditions, allowing a straightforward access to well-
defined PTMC and associated block copolymers such as PTMC-
PEG. Such Al-based initiators may well be useful for the ready
preparation of various tailor-made and narrowly disperse copo-
lymeric biomaterials.
9080
9278
13160
13525
17240
19020
a Polymerization conditions: [TMC]0 = 1M, CH2Cl2, room tempera-
ture, 30 min. All conversions are quantitative after 30 min (the reaction
time is not optimized). b Amount in equiv versus initiator 2a. c Measured
by GPC using standard PS calibration in THF (30 °C). d Calculated
using Mn,theo r= [TMC]0/[mPEG-OH]0 ꢁ MTMC + 5000. e Measured
by GPC in THF (30 °C) using PS standards. f Measured by GPC in THF
(30 °C) using PS standards and corrected by applying the appropriate
correcting factors X (X1 = 0.61 for mPEG(5000) and X2 = 0.57 or 0.88
for PTMC) using Mn,corr = Mn,obs(GPC) ꢁ X1 ꢁ (mPEG fraction in
1
PTMC-PEG based on H NMR) + Mn,obs(GPC) ꢁ X2 ꢁ (PTMC
fraction in PTMC-PEG based on 1H NMR).11
’ EXPERIMENTAL SECTION
Scheme 3
General Procedures. All experiments were carried out under N2
using standard Schlenk techniques or in a Mbraun Unilab glovebox.
Toluene and pentane were collected after being passed through drying
columns (SPS apparatus, MBraun) and stored over activated molecular
sieves (4 Å) for 24 h in a glovebox prior to use. Tetrahydrofuran was
distilled over Na/benzophenone and stored over activated molecular
sieves (4 Å) for 24 h in a glovebox prior to use. CH2Cl2, CD2Cl2, and
C6D6 were distilledfrom CaH2, degassed under a N2 flow, and stored over
activated molecular sieves (4 Å) in a glovebox prior to use. All deuterated
solvents were obtained from Eurisotop (CEA, Saclay, France). All other
chemicals were purchased from Aldrich and were used as received, with
the exception of trimethylene carbonate, purchased from either TCI
Europe Corp. or Boehringer: it was recrystallized twice from dry Et2O
prior to use. The NMR spectra were recorded on Bruker AC 300 and
400 MHz NMR spectrometers in Teflon-valved J. Young NMR tubes at
ambient temperature. 1H and 13C chemical shifts are reported vs SiMe4
with molecular weight numbers in agreement with the initial
[TMC]0/[mPEG-OH]0 ratio, as deduced from H NMR and
1
GPC data (see the Supporting Information, Figures S4ꢀS6).
Also, indicative of their amphiphilic nature, the 1H NMR
spectrum in D2O of all prepared PTMC-PEG copolymers only
contains the PEG resonances, which is consistent with a biphasic
medium under the conditions studied.4 Notably, as a compar-
ison, PTMC-PEG copolymers (with moderate to broad molec-
ular distributions) are typically prepared via the ROP of TMC at
130 °C using a Sn(Oct)2/mPEG-OH initiator.5a For the ROP of
TMC by 2a/mPEG-OH, it appears likely that the mild reaction
conditions account for the precise molecular control of the
resulting copolymers; in that regard, higher ROP temperatures
are, for instance, known to favor polymer chain transfer reactions,
1
and were determined by reference to the residual H and 13C solvent
peaks. Elemental analyses for all compounds were performed at the
Service de Microanalyse of the Universitꢀe de Strasbourg (Strasbourg,
France). GPC analyses were performed on a system equipped with a
Shimadzu RID10A refractometer detector using HPLC-grade THF as
an eluant. Molecular weights and polydispersity indices (PDIs) were
calculated using polystyrene standards. In the case of molecular weight
number (Mn), these were corrected with appropriate correcting factors
for the Mn values. MALDI-TOF mass spectroscopic analyses were
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performed at the Service de Spectromꢀetrie de Masse de l’Institut de
Chimie de Strasbourg and run in a positive mode: samples were prepared
by mixing a solution of the polymers in CH2Cl2 with a 0.5 mg/100 mL
concentration, and 2,5-dihydroxybenzoic acid (DHB) was used as the
matrix in a 5/1 volume ratio. Al(NMe2)3 was prepared according to a
literature procedure.12
SEC chromatograms of PTMC and PTMC-PEG polymers,
and crystal data and refinement details for complex 2a. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
(RNH-C6H4)2O (1a, R = C5H9; 1b, R = C6H11). The bis-amino-
ether ligands were synthesized in good yields via a one-step procedure from
2,20-oxidianiline, (H2N-C6H4)2O, according to a literature procedure.8
{η3(N,O,N)-(C5H9)N-C6H4}2OAlNMe2 (2a). In a nitrogen-filled
glovebox, the ligand 1a (4.26 mmol) was charged in a Schlenk flask and a
toluene solution (10 mL) of Al(NMe2)3 (677 mg, 4.26 mmol) was added
to yield a colorless solution. The reaction mixture was then heated for 24 h
at 90 °C in an oil bath to yield a pale yellow solution that was sub-
sequently cooled to room temperature and evaporated to dryness in
vacuo, affording an off-white solid residue. The latter was washed with
pentane and further dried under vacuum to afford the Al complex 2a as
an analytically pure colorless solid (1.02 g, 60% yield). 1H NMR (300 MHz,
Corresponding Author
*E-mail: dagorne@unistra.fr.
’ ACKNOWLEDGMENT
We thank the CNRS and The University of Strasbourg for
financial support.
’ REFERENCES
(1) (a) Lindblad, M. S.; Liu, Y.; Albertsson, A.-C.; Ranucci, E.;
Karlsson, S. Adv. Polym. Sci. 2002, 157, 139. (b) Vert, M. Biomacromol-
ecules 2005, 6, 538. (c) Nair, L. S.; Laurencin, C. T. Prog. Polym. Sci.
2007, 32, 762.
(2) (a) O’Keele, B. J.; Hillmeyer, M. A.; Tolman, W. B. Dalton Trans.
2001, 2215. (b) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D.
Chem. Rev. 2004, 104, 6147. (c) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C.
Coord. Chem. Rev. 2006, 205, 602. (d) Dove, A. P. Chem. Commun.
2008, 6446. (e) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polym.
Rev. 2008, 48, 11. (f) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165.
(g) Arbaoui, A.; Redshaw, C. Polym. Chem. 2010, 1, 801.
(3) (a) Yang, J.; Yu, Y.; Li, Q.; Li, Y.; Cao, A. J. Polym. Sci. A: Polym.
Chem. 2005, 43, 373. (b) Darensbourg, D. J.; Ganguly, P.; Billodeaux, D.
Macromolecules 2005, 38, 5406. (c) Palard, I.; Schappacher, M.; Belloncle,
B.; Soum, A.; Guillaume, S. M. Chem. Eur. J. 2007, 13, 1511. (d) Helou,
M.; Miserque, O.; Brusson, J.-M.; Guillaume, S. M.; Carpentier, J.-F.
Chem. Eur. J. 2008, 14, 8772. (e) Darensbourg, D. J.; Choi, W.;
Karroonnirun, O.; Bhuvanesh, N. Macromolecules 2008, 41, 3493.
(f) Helou, M.; Miserque, O.; Brusson, J.-M.; Carpentier, J.-F.; Guillaume,
S. M. Adv. Synth. Catal. 2009, 351, 1312. (g) Helou, M.; Miserque, O.;
Brusson, J.-M.; Guillaume, S. M.; Carpentier, J.-F. ChemCatChem 2010,
2, 306. (h) Koller, J.; Bergman, R. G. Organometallics 2011, 30, 3217.
(4) (a) P^ego, A. P.; Poot, A. A.; Grijpma, D. W.; Feijen, J. J. Controlled
Release 2003, 87, 69. (b) Tyson, T.; Finne-Wistrand, A.; Albertsson, A.-C.
Biomacromolecules 2009, 10, 149.
(5) (a) Zhang, Z.; Grijpma, D. W.; Feijen, J. J. Controlled Release
2006, 112, 57. (b) Zhang, H.-H.; Huang, Z.-Q.; Sun, B.-W.; Guo, J.-X.;
Wang, J.-L.; Chen, Y.-Q. J. Polym. Sci. A: Polym. Chem. 2008, 46, 8131.
(c) Tyrrell, Z. L.; Shen, Y.; Radosz, M. Prog. Polym. Sci. 2010, 35, 1128.
(6) Only a few N,N,N-supported Al complexes have been reported
and used in catalysis; see: (a) Emig, N.; Nguyen, H.; Krautscheid, H.;
Rꢀeau, R.; Cazaux, J.-B.; Bertrand, G. Organometallics 1998, 17, 3599.
(b) Nelson, S. G.; Peelen, T. J.; Wan, Z. J. Am. Chem. Soc. 1999, 121,
9742. (c) Nelson, S. G.; Kim, B.-K.; Peelen, T. J. J. Am. Chem. Soc. 2000,
122, 9318.
CD2Cl2): δ 7.47 (dd, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 2H), 7.10 (dt, 3JHH
=
8.1 Hz, 4JHH = 1.5 Hz, 2H), 6.73 (dd, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 2H),
6.50 (dt, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 2H), 3.83 (q, 3JHH = 7.5 Hz, 2H),
2.56 (s, 6H, AlꢀNMe2), 1.40ꢀ2.40 (m, 16H). 13C NMR (300 MHz,
CD2Cl2): δ 146.4 (Cipso), 146.3 (Cipso), 127.9 (Ar), 117.5 (Ar), 113.0
(Ar), 56.6 (CH-C5H9), 40.2 (Al-NMe2), 34.2 (C5H9), 34.1 (C5H9),
25.1 (C5H9), 24.9 (C5H9). Anal. Calcd for C24H32AlN3O: C, 71.08; H,
7.95; N, 10.36. Found: C, 70.96; H, 8.07; N, 9.57.
{η3(N,O,N)-(C6H11)N-C6H4}2OAlNMe2 (2b). The Alamidocom-
plex 2b was synthesized by following a procedure identical with that used
for compound 2a using equimolar amounts of ligand 1b (550 mg, 1.51
mmol) and Al(NMe2)3 (240 mg) to afford the Al complex 2b in a pure
1
form as a colorless solid (445 mg, 68% yield). H NMR (300 MHz,
CD2Cl2): δ 7.43 (dd, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 2H), 7.05 (dt, 3JHH
=
8.1 Hz, 4JHH = 1.5 Hz, 2H), 6.70 (dd, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 2H),
6.49 (dt, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, 2H), 3.26 (q, 3JHH = 7.5 Hz, 2H),
2.56 (s, 6H), 1.10ꢀ2.30 (m, 20H). 13C NMR (300 MHz, CD2Cl2):
δ 146.4 (Cipso), 145.5 (Cipso), 127.9 (Ar), 117.9 (Ar), 112.9 (Ar), 112.6
(Ar), 54.1 (CH-Cy), 40.7 (Al-NMe2), 34.2 (Cy), 34.0 (Cy), 26.7 (Cy),
26.5(Cy), 26.1 (Cy). Anal. Calcd for C24H32AlN3O: C, 72.03; H, 8.37;N,
9.69. Found: C, 71.85; H, 8.14; N, 9.04.
Typical Procedure for Trimethylene Carbonate Polymer-
ization by Al Complexes 2a,b. In a glovebox, an appropriate
amount of species 2a,b was charged in a vial equipped with a Teflon-
tight screw cap and a TMC/ROH dichloromethane solution (prepared
in such a way that [TMC]0 = 1 M) was quickly added via a syringe all at
once. The resulting colorless solution was vigorously stirred at room tem-
perature for 30 min. The reaction mixture was then quenched with
MeOH, provoking the precipitation of the polymer. After filtration
through a glass frit, the latter material was washed several times with
MeOH, dried in vacuo to constant weight, and subsequently analyzed by
1H NMR, SEC, and MALDI-TOF spectrometry.
Determination of the Correcting Factor X for m-PEG(5000)
Correlating Mn,obs(GPC) (using PS Standards Calibration)
with the Real Mn. This was performed according to a well-established
literature procedure,3c,11a,11b as follows: the real molecular weight
number for the monocapped methoxy ether PEG-OH(5000) (m-PEG-
(5000)) was determined via MALDI-TOF spectrometry (Mn = 5030),
while GPC (PS standards) analysis of m-PEG(5000) yielded Mn = 8225
along with PDI = 1.04. The factor was thus determined to be 0.61 (i.e.,
5030/8225).
(7) (a) Dagorne, S. Low-Coordinated Group 13 Compounds. In
Encyclopedia of Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; Wiley:
New York, 2005; p 2714. (a) Dagorne, S.; Atwood, D. A. Chem. Rev.
2008, 108, 4037.
(8) (a) Hild, F.; Haquette, P.; Brelot, L.; Dagorne, S. Dalton Trans.
2010, 39, 533. (b) Baumann, R.; Stumpf, R.; Davis, W. M.; Liang, L.-C.;
Schrock, R. R. J. Am. Chem. Soc. 1999, 121, 7822.
(9) For a perspective on immortal metal-catalyzed ROP of cyclic
esters and carbonates, see: Ajellal, N.; Carpentier, J.-F.; Guillaume, C.;
Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton
Trans. 2010, 39, 8363.
(10) ForrepresentativeX-ray-characterizedcompoundsfeaturing term-
inal AlꢀN amido bonds, see: (a) Krossing, I.; N€oth, H.; Schwenk-Kircher,
H. Eur. J. Inorg. Chem. 1998, 927. (b) Wehmschulte, R. J.; Power, P. P.
Inorg. Chem. 1998, 37, 6906. (c) Liang, L.-C.; Huang, M.-H.; Hung, C.-H.
’ ASSOCIATED CONTENT
S
Supporting Information. Figures, tables, and a CIF file
b
giving the MALDI-TOF spectrum of PTMC, representative
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Organometallics
ARTICLE
Inorg. Chem. 2004, 43, 2166. (d) Olson, J. A.; Boyd, R.; Quail, J. W.;
Foley, S. R. Organometallics 2008, 27, 5333.
(11) See the Experimental Section and ref 3c; see also: (a) Save, M.;
Schappacher, M.; Soum, A. Macromol. Chem. Phys. 2002, 203, 889.
(b) Delcroix, D.; Martin-Vaca, B.; Bourissou, D.; Navarro, C. Macromolecules
2010, 43, 8828.
(12) Benaissa, M.; Gonsalves, K. E.; Rangarajan, S. P. Appl. Phys. Lett.
1997, 71, 3685.
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