Stereoelective polymerization of D,L-lactide using N-heterocyclic carbene based compounds

Article abstract of DOI:10.1039/b405362a

A new Zn alkoxide catalyst supported by an N-heterocyclic carbene rapidly polymerizes D,L-lactide (D.L-LA) to heterotactic enriched poly(lactide) (PLA), while the free carbene and analogs instead yield highly isotactic enriched PLA.

Full text of DOI:10.1039/b405362a

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Stereoelective polymerization of D,L-lactide using N-heterocyclic
carbene based compounds{
Tryg R. Jensen, Laurie E. Breyfogle, Marc A. Hillmyer* and William B. Tolman*
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota,
55455, USA. E-mail: hillmyer@chem.umn.edu; tolman@chem.umn.edu; Fax: 612 624 7029;
Tel: 612 625 7834 and -4061
Received (in Columbia, MO, USA) 21st April 2004, Accepted 5th August 2004
First published as an Advance Article on the web 8th September 2004
A new Zn alkoxide catalyst supported by an N-heterocyclic
to a carbene and three benzyloxides, two bridging and one
˚
carbene rapidly polymerizes D,L-lactide (D,L-LA) to
heterotactic enriched poly(lactide) (PLA), while the free
carbene and analogs instead yield highly isotactic enriched
PLA.
terminal. The Zn–carbene bond distance (2.054 A) falls between
9
those of the two other reported examples (2.022 A and 2.096 A).
˚
˚
1
The H NMR spectrum in CD2Cl2 shows only two mesityl CH3
resonances and one benzylic resonance, suggesting that in solution
A either cleaves into C2-symmetric monomers or remains dimeric
but is highly fluxional via a process(es) that involves terminal/
bridging benzyloxide ligand exchange.
Polylactide (PLA) is an attractive alternative to petrochemically
derived polyolefins due to its utility (e.g., in biomedical and
packaging applications), biodegradation characteristics, and facile
synthesis from lactide (LA), a monomer synthesized from renew-
able resources.1 An ideal LA polymerization catalyst would exhibit
high activity, allow for precise molecular weight control, give
polymers with narrow molecular weight distributions, and show a
high degree of stereoelectivity in the polymerization of stereo-
chemically impure monomer feeds. This combination of attributes
has not been fully realized, although single site metal alkoxides are
the most promising candidates.2 For example, aluminium alkoxides
with various salen-type ligands have produced PLA from racemic
D,L-LA with tacticities ranging from highly isotactic to heterotactic,
and have yielded highly syndiotactic PLA from meso-LA.3 While
excellent stereocontrol was achieved with these Al systems, they
suffer from inherently low activity; reasonable conversions are
generally attained only at high temperatures (w 70 uC) over long
times (hours). Zinc complexes have been shown to be more active,
although less versatile from a control of tacticity standpoint.4,5
Notwithstanding these advances, our understanding of the origin
of stereocontrol and high activity in LA polymerizations by metal
alkoxides is rudimentary and improvements in catalyst versatility
are warranted. As part of an ongoing effort to address these issues,6
we explored Zn complexes of N-heterocyclic carbenes, ligands with
readily tuned steric properties important for stereoselective
reactions.7 Herein we report the preparation of such a complex,
demonstration of its efficient and selective LA polymerization
behavior, and discovery of the stereoelective polymerization of
D,L-LA by free achiral N-heterocyclic carbenes.
Complex A is an efficient catalyst for the polymerization of D,L-
LA in CH2Cl2 at 25 uC (Table 1). Polymerization control was
demonstrated by a linear relationship between conversion and Mn
(ESI).{ Kinetics measurements (ReactIR) showed a first order
dependence on [D,L-LA], with kobs ~ 1.9 6 1023 s21 for [A]0 ~
2.7 mM ([Zn]0 ~ [carbene ligand] ~ 5.3 mM). This value is only
slightly less than that of the fastest reported Zn catalyst at the same
catalyst loading (kobs ~ 1.2 6 1022 s21 at [Zn]0 ~ 5.3 mM).6a In a
reaction performed at [D,L-LA]0/[A]0 ~ 290 and terminated at 7.4%
conversion, the PLA produced had Mn ~ 2840 g mol21 by
MALDI MS in good agreement with the value predicted (Mn ~
3090 g mol21) under the assumption of initiation by all four benzyl
alkoxides. Incorporation of benzyl alcohol end groups is consistent
with the MALDI MS data (ESI, Fig. S2). 1H NMR spectroscopic
analysis10 showed heterotactic enrichment of PLA (Fig. 2, i)
consistent with chain-end control (Pr ~ 0.6).11 Similarly hetero-
tactic enriched PLA also was obtained from polymerizations of
D,L-LA performed in the melt (140–180 uC).
Although N-heterocyclic carbenes are generally considered to be
nonlabile (e.g., relative to phosphines on late transition metals),
their known efficacy as LA polymerization catalysts (in the
presence of alcohol initiator)12 raised concerns that the free carbene
B was responsible for the observed high activity. In support of
Addition of benzyl alcohol (BnOH) to the previously reported
8
complex (1,3-dimesitylimidazol-2-ylidene)ZnEt2 in toluene pro-
duced A in 92% yield (Scheme 1). The X-ray structure{ of A
(Fig. 1) shows each Zn in a distorted tetrahedral geometry bound
Scheme 1
Fig. 1 Representation of the X-ray crystal structure of A, showing non-
˚
hydrogen atoms as 50% thermal ellipsoids. Selected bond distances (A) and
angles (u): Zn1–O1, 2.0135(14); Zn1–O2, 1.9198(15); Zn1–C1, 2.054(2);
Zn1–Zn2, 3.0211(6); O1–Zn1–O2, 112.31(7); O2–Zn1–O3, 81.98(6); Zn1–
O2–Zn2, 98.02(6); O2–Z1–C1, 124.80(8); O1–Zn1–C1, 107.75(7).
{ Electronic supplementary information (ESI) available: full experimental
details and crystallographic data for A. See http://www.rsc.org/suppdata/
cc/b4/b405362a/
2 5 0 4
C h e m . C o m m u n . , 2 0 0 4 , 2 5 0 4 – 2 5 0 5
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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yield from racemic D,L-LA, showed a melting point by differential
Table 1 Polymerization of D,L-LA by compounds A–D ([LA]0 ~ 1 M,
CH2Cl2)
scanning calorimetry of 153 uC (DHfus $ 0.8 J g21) consistent with
the presence of a small amount of semicrystalline material
(supporting information).13 In addition to implicating intriguing
mechanistic differences between polymerizations by A and free B,
these findings provide motivation for the development both of new
Zn–carbene complexes and N-heterocyclic carbenes for the
controlled, rapid, and stereoelective polymerization of lactide,
particularly in view of the wide range of N-heterocyclic carbene
structures available.}
PDI
(Mw/
[LA]0/
T
t
%
Mn
Entry Catalyst [Init.]0 (uC) (min) conv.a (kg mol21 b
)
Mn)b Pm
c
1
A
130
130
150
75
25 20
25 30
220 20
25 20
25 20
96
98
71
92
95
17.2
16.9
15.9
8.42
7.01
1.25 0.40
1.23 0.59
1.26 0.75
1.39 0.60
1.36 0.55
2
Bd
Bd
Cd
Dd
3e
4
5
75
Financial support from the NSF (CHE9975357) is gratefully
acknowledged, and we thank K. A. Switek for assistance with the
MALDI-MS experiments.
a Determined by 1H NMR spectroscopy. b SEC (relative to polystyr-
ene in THF). c See ref. 11. d Benzyl alcohol added. e [LA]0 ~ 0.1 M.
Notes and references
{ CCDC 237615. See http://www.rsc.org/suppdata/cc/b4/b405362a/ for
crystallographic data in .cif or other electronic format.
§ This difference in tacticity bias between A and B persists even in the melt
polymerization of D,L-LA at 140 uC.
} A report describing N-heterocyclic carbene complexes of Zn(II) appeared
after submission of this manuscript: D. Wang, K. Wurst and
M. R. Buchmeiser, J. Organomet. Chem., 2004, 689, 2123.
1 (a) R. W. Drumwright, P. R. Gruber and D. E. Henton, Adv. Mater.,
2000, 12, 1841; (b) M. Okada, Prog. Polym. Sci., 2002, 27, 87.
2 B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc., Dalton
Trans., 2001, 2215.
3 (a) N. Spassky, M. Wisniewski, C. Pluta and A. Le Borgne, Macromol.
Chem. Phys., 1996, 197, 2627; (b) A. Bhaw-Luximon, D. Jhurry and
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6 For recent relevant examples, see: (a) C. K. Williams, L. E. Breyfogle,
S. K. Choi, W. Nam, V. G. Young, Jr, M. A. Hillmyer and
W. B. Tolman, J. Am. Chem. Soc., 2003, 125, 11350; (b) L. M. Alcazar-
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1
Fig. 2 H NMR spectra (500 MHz, CDCl3) of PLA methine resonances
with selective decoupling of PLA methyl resonances; (i) heterotactic
enriched PLA, Table 1, entry 1; (ii) isotactic enriched PLA, Table 1, entry
2; (iii) isotactic enriched PLA, Table 1, entry 3.
possible carbene ligand dissociation in A, attempts to synthesize an
analog of A with a more hindered carbene via treatment of
(C)ZnEt2 with BnOH (¢ 1 equiv.) resulted in generation of free
1
C (w 70% by H NMR spectroscopy).
To address whether free carbene B was participating in
polymerizations using A, we compared the polymerization kinetics
obtained using B (with 0.5–2 equiv. added BnOH) to the data
obtained for A. With [B]0/[BnOH]0 ~ 0.54, the PLA formation rate
was first-order in [D,L-LA], with a value (kobs ~ 3.1 6 1023 s21 at
[B]0 ~ 5.3 mM) slightly greater than for A (1.9 6 1023 s21) under
the same conditions. On the basis of this data, one cannot rule out
catalysis by some level of pure B when using A as the precatalyst.
7 (a) D. Bourissou, O. Guerret, F. P. Gabbaie and G. Bertrand, Chem.
Rev., 2000, 100, 39; (b) W. A. Herrmann, Angew. Chem., Int. Ed., 2002,
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11 Pm (Pr) is the probability of meso (racemic) linkages between monomer
units and is determined from the methine region of the homonuclear
decoupled 1H NMR spectrum (Pm 1 Pr ~ 1). See refs. 4 and 10.
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However, we discovered a striking difference in the tacticity of
the product polylactide when using B. In contrast to the
heterotactic material produced when using A (Fig. 2, i), the
combination of B and BnOH produced isotactic enriched PLA
from D,L-LA under similar conditions (CH2Cl2, 25 uC, Fig. 2, ii).§
In fact, carbenes C and D also produced PLA with isotactic
enrichment (Table 1). The highest isotacticity PLA we obtained
was with B and 0.67 equiv. BnOH at 220 uC in CH2Cl2 (Pm ~
0.75, Table 1, entry 3; Fig. 2, iii). That material, produced in 71%
13 M. Wisniewski, A. L. Borgne and M. Spassky, Macromol. Chem. Phys.,
1997, 198, 1227.
C h e m . C o m m u n . , 2 0 0 4 , 2 5 0 4 – 2 5 0 5
2 5 0 5