Remarkable Stereocontrol in the Polymerization of Racemic Lactide Using Aluminum Initiators Supported by Tetradentate Aminophenoxide Ligands

Article abstract of DOI:10.1021/ja038757o

A new family of aluminum complexes bearing tetradentate bis(aminophenoxide) ligands is reported and shown to initiate the living ring-opening polymerization of rac-lactide. The microstructures of the polylactide products are found to be highly dependent upon the ancillary ligand substituents, ranging from highly isotactic (P_m = 0.79) to very highly heterotactic (P_r = 0.96). Copyright

Full text of DOI:10.1021/ja038757o

Published on Web 02/12/2004
Remarkable Stereocontrol in the Polymerization of Racemic Lactide Using
Aluminum Initiators Supported by Tetradentate Aminophenoxide Ligands
Pimpa Hormnirun, Edward L. Marshall, Vernon C. Gibson,* Andrew J. P. White, and
David J. Williams
Imperial College London, South Kensington Campus, London SW7 2AY, U.K.
Received September 27, 2003; E-mail: v.gibson@imperial.ac.uk
There is currently much interest in the development of new
catalyst systems for the production of polylactide (PLA), a relatively
new commodity polymeric material derived from biorenewable
feedstocks such as corn starch.1 The mechanical properties of PLA
are intimately linked to its microstructure, and hence, the stereo-
regular polymerization of lactide has become a key focus of
attention. Some of the most significant advances in stereocontrolled
polymerization have been seen using aluminum alkoxides stabilized
by salen-type ligands (Scheme 1). For example, I was found to
polymerize rac-LA with a pronounced isotactic bias,2 while
enantiopure (-)-II afforded a tapered block copolymer p(D-LA)-
p(L-LA).3 The enantiomorphic site selectivity of (-)-II has more
recently been exploited to prepare highly syndiotactic PLA from
meso-LA.4 The racemic form of this initiator, (()-II, converts rac-
LA into an isotactic stereoblock material which exhibits a higher
softening temperature than p(L-LA);5 each enantiomer of II
preferentially consumes either D-LA or L-LA, with low levels of
stereochemical imperfections arising from the exchange of propa-
gating chains between Al centers of opposing chirality.5b,c Isotactic
PLA stereocopolymers have also been prepared in the melt phase
Figure 1. Molecular structure of 2. Selected bond lengths (Å) and angles
(deg): Al-C 1.955(3), Al-O(1) 1.7930(16), Al-N(8) 2.0550(18), Al-
using III,6 and highly isotactic PLA has been produced from rac-
LA using a mixture of achiral IV and benzyl alcohol (Scheme 1).7
N(11) 2.4532(19), Al-O(18) 1.7570(15), C-Al-N(8) 119.93(12), C-Al-
O(18) 122.84(12), N(8)-Al-O(18) 113.86(8), O(1)-Al-C 100.23(10),
O(1)-Al-N(8) 90.95(7), O(1)-Al-O(18) 96.69(8), O(1)-Al-N(11)
167.97(7).
Here, we report a new family of aluminum catalysts stabilized
by tetradentate phenoxyamine (salan-type8) ligands which shows
an hitherto unprecedented degree of stereocontrol in the polymer-
ization of racemic lactide. Such ligands have previously been
attached to zirconium and exploited by Kol and co-workers to
catalyze the living polymerization of 1-hexene.9 In general, they
can be accessed by a modified Mannich condensation reaction.10
However, this route is not applicable to the unsubstituted phenoxy
derivatives which we have therefore synthesized via the stepwise
condensation of an N,N-disubstituted ethylenediamine with the
appropriate salicylaldehyde, followed by reduction.11
Scheme 1
The aminophenols reacted smoothly with Me3Al to afford the
desired methyl complexes, 1-8 (Scheme 2). Unlike the highly
conjugated salen ligands, which typically form colored metal
complexes, the N-alkylated bis(aminophenoxide) complexes are
white solids, an important factor for the commercial production of
polymers since costly decolorization (catalyst removal) steps may
be avoided.
Scheme 2
Crystals of 2 were grown from toluene and a single-crystal X-ray
analysis revealed a distorted trigonal bipyramidal coordination
geometry at the aluminum center (Figure 1). C, N(8) and O(18)
form the equatorial plane from which the metal atom deviates by
ca. 0.20 Å in the direction of the axial atom O(1); a weakly
associated fifth donor atom [Al-N(11), 2.4532(19) Å] occupies
the other axial site. With the exception of this long axial contact,
the coordination distances are unexceptional, the axial Al-O(1)
bond length [1.7930(16) Å] being, as expected, noticeably longer
than its equatorial counterpart [Al-O(18), 1.7570(15) Å].
Alkoxide initiators for lactide polymerization were generated by
in situ alcoholysis of 1-8 using PhCH2OH. Polymerizations were
carried out at 70 °C in toluene; the results are collected in Table 1.
All of the initiator systems exhibit molecular weights in close
agreement with calculated values [Mn(calcd) PLA100 ) 14 400] and
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J. AM. CHEM. SOC. 2004, 126, 2688-2689
10.1021/ja038757o CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Polymerization Data for Complexes 1-8a
the unsubstituted phenoxide derivative 5 affords a higher isotactic
content than 1. Examination of the X-ray structure of the precatalyst
1 (Figure 1) reveals a possible explanation: the N(8)-C(21) bond
of the equatorial amine donor, and especially the N-Me [N(1)-
C(22)] bond of the axial amine unit, eclipses the Al-C bond,
showing that the alkylamino backbone substituents can closely
approach the site of polymer chain growth and thereby influence
monomer selectivity.
In conclusion, we have found quite remarkable stereocontrol in
the polymerization of rac-lactide by aluminum initiators supported
by tetradentate aminophenoxide ligands. The polymerizations are
well-controlled and living, affording PLA materials that range from
highly isotactic to highly heterotactic, depending upon the ligand
substitution pattern. To our knowledge, this is the first time
aluminum-based systems have been found to give heterotactic PLA
and the first time a dramatic switch in tacticity has been observed
for a lactide polymerization system upon small changes to the
remote ligand substituents. Work is continuing in our laboratories
to understand the origin of stereocontrol in this new family of lactide
polymerization initiators.
convb (%)
M c
M /M c
kapp (10-6 s-1
182
25.4
0.28
)
P /Pm
r
d
time (hr)
n
w
n
1
2
3e
4
5
6
7
8
23
24
1464
24
21
24
97
87
66
94
98
75
77
94
18 920
12 725
4730
12 500
21 180
13 350
11 290
17 770
1.04
1.09
1.08
1.11
1.08
1.06
1.05
1.06
0.32:0.68
0.80:0.20
0.42:0.58
0.88:0.12
0.21:0.79
0.83:0.17
0.61:0.39
0.96:0.04
38.2
79.8
12.4
3.4
120
21
37.8
a
b
[LA]/[Al] ) 100, toluene, 70 °C. Monomer conversion determined
by 1H NMR spectroscopy. Determined by GPC. Pr and Pm are the
c
d
1
probability of hetero- and iso-tactic enchainment determined by H NMR
e
spectroscopy.13a
[LA]/[Al] ) 50.
Acknowledgment. The Royal Thai Government is thanked for
a studentship (P.H.) and EPSRC for a postdoctoral fellowship
(E.L.M.).
Supporting Information Available: Full experimental details for
the synthesis of complexes 1-8, including crystallographic data for 2
and polymerization data (Mn vs conversion plots, homonuclear
decoupled NMR spectra for Pr/Pm determination, and kinetic data)
(PDF, CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
1
Figure 2. Homonuclear decoupled H NMR spectra (methine region) of
PLA generated from (a) 5/PhCH2OH and (b) 8/PhCH2OH (CDCl3, 298 K,
500 MHz).
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narrow molecular weight distributions, characteristic of well-
controlled living propagation. The high level of control afforded
by these initiators is further exemplified by linear correlations
between Mn and conversion for all eight complexes (see Supporting
Information).
Kinetic analyses showed that, as the size of the phenoxide alkyl
t
substituent increases (from H to Me to Bu), the rate of polymer-
ization decreases. Further, the 3,5-dichlorophenoxides, 4 and 8, are
appreciably more active than their dimethyl analogues, most likely
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(11) See Supporting Information.
Table 1 also reveals the remarkable range of microstructures
accessible using these initiators. Where the phenoxide groups are
unsubstituted (1 and 5), high levels of isoselectivity are observed
(as high as 79% in the case of 5; see Figure 2a). However, when
the phenoxide units contain substituents in the 3 and 5 positions,
the tacticity is changed dramatically to a strong heterotactic bias.
In the case of 8, 96% heterotactic PLA is obtained (Figure 2b).12,13
This is lowered to 83% for the sterically comparable 3,5-dimethyl
derivative, 6, implying that the chloro substituents exert an influence
beyond a straightforward steric effect.
(12) For other examples of highly heteroselective initiators for the polymer-
ization of rac-LA, see: (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E.
B.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 11583. (b) Chamberlain,
B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates,
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J.; Phomphrai, K. Inorg. Chem. 2002, 41, 2785. (d) Chisholm, M. H.;
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E. Macromolecules 1995, 28, 3937.
(13) 1H NMR assignments made according to: (a) Kricheldorf, H. R.;
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Padden, B. E.; Paterick, A. J.; Thakur, K. A. M.; Kean, R. T.; Hillmyer,
M. A.; Munson, E. J. Macromolecules 2002, 35, 7700.
It can also be seen that the tacticity is significantly influenced
by the substituents (R1) attached to the amino nitrogen donors. For
example, the benzylamine derivatives 6 and 8 afford higher
heterotacticities than their methylamine analogues 2 and 4, while
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