Polymerization of lactide with zinc and magnesium β-diiminate complexes: Stereocontrol and mechanism

Article abstract of DOI:10.1021/ja003851f

A series of zinc(II) and magnesium(II) alkoxides based upon a β-diiminate ligand framework has been prepared. [(BDI-1)ZnOⁱPr]₂ [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene] exhibited the high

Full text of DOI:10.1021/ja003851f

J. Am. Chem. Soc. 2001, 123, 3229-3238
3229
Polymerization of Lactide with Zinc and Magnesium â-Diiminate
Complexes: Stereocontrol and Mechanism
Bradley M. Chamberlain, Ming Cheng, David R. Moore, Tina M. Ovitt,
Emil B. Lobkovsky, and Geoffrey W. Coates*
Contribution from the Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell UniVersity, Ithaca, New York 14853-1301
ReceiVed NoVember 1, 2000
Abstract: A series of zinc(II) and magnesium(II) alkoxides based upon a â-diiminate ligand framework has
been prepared. [(BDI-1)ZnOiPr]2 [(BDI-1) ) 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)-
imino)-2-pentene] exhibited the highest activity and stereoselectivity of the zinc complexes studied for the
polymerization of rac- and meso-lactide to poly(lactic acid) (PLA). [(BDI-1)ZnOiPr]2 polymerized (S,S)-
lactide to isotactic PLA without epimerization of the monomer, rac-lactide to heterotactic PLA (Pr ) 0.94 at
0 °C), and meso-lactide to syndiotactic PLA (Pr ) 0.76 at 0 °C). The polymerizations are living, as evidenced
by the narrow polydispersities of the isolated polymers in addition to the linear nature of number average
molecular weight versus conversion plots and monomer-to-catalyst ratios. The substituents on the â-diiminate
ligand exert a significant influence upon the course of the polymerizations, affecting both the degree of
stereoselectivity and the rate of polymerization. Kinetic studies with [(BDI-1)ZnOiPr]2 indicate that the
polymerizations are first order with respect to monomer (rac-lactide) and 1.56 order in catalyst. Polymerization
experiments with [(BDI-1)MgOiPr]2 revealed that this complex is extremely fast for the polymerization of
rac-lactide, polymerizing 500 equiv in 96% yield in less than 5 min at 20 °C.
Introduction
properties, as well as their rates of chemical and biological
degradation, the synthesis of new microstructures may lead to
new applications for PLA.15 Prior to work in our laboratory, a
range of metal alkoxide initiators was reported to polymerize
lactide (LA) with retention of configuration by cleaving the
acyl-oxygen bond via a coordination-insertion mechanism.16-20
Nonetheless, these catalysts were limited to producing only
atactic, partially heterotactic, or isotactic microstructures (Figure
1). For example, polymerization of rac- or meso-lactide with
aluminum tris(alkoxides)19 or tin bis(carboxylates)20 yields
amorphous, atactic polymers with random placements of either
-(RR)- and -(SS)- stereosequences for rac-lactide or -(RS)-
and -(SR)- stereosequences for meso-lactide. Kasperczyk has
reported that LiOtBu produces predominantly heterotactic PLA
from rac-lactide.21,22 In contrast, in the absence of epimerization
reactions, polymerization of optically active (S,S)-lactide (or
(R,R)-lactide) affords crystalline, isotactic polymers. More
recently, we have reported routes to syndiotactic4 and stereo-
block isotactic23 PLAs using chiral aluminum catalysts.
Although remarkable advances have been reported concerning
the development of single-site metal catalysts for olefin po-
lymerization,1 relatively few well-defined metal catalysts are
available for the ring-opening polymerization of heterocycles
such as lactide.2-14 Because the stereochemistry of poly(lactic
acids) (PLA) determines both their mechanical and physical
(1) Coates, G. W. Chem. ReV. 2000, 100, 1223-1252.
(2) Single-site catalysts are those polymerization catalysts where en-
chainment of monomer occurs at a metal center (M, the active site) which
is bound by an organic ligand (L). This ancillary ligand remains bound
throughout the catalytic reaction, modifying the reactivity of the metal center.
Typical single-site catalysts for lactone polymerization are of the form
LnMOR, where the alkoxide group (OR) is capable of propagation. These
complexes are of a different class than typical lactone polymerization
catalysts of the form M(OR)n, where all of the ligands are capable of forming
a polymer chain and no permanent ligand is present. For recent examples
of single-site catalysts for lactone polymerization, see refs 3-14.
(3) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J.
Am. Chem. Soc. 1999, 121, 11583-11584.
(4) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072-
4073.
(15) PLA materials have already found many medical, agricultural, and
packaging applications. For recent examples, see: (a) Chiellini, E.; Solaro,
R. AdV. Mater. 1996, 29, 1375-1381. (b) Ikada, Y.; Tsuji, H. Macromol.
Rapid Commun. 2000, 21, 117-132. (c) Vert, M.; Li, S. N.; Spenlenhauer,
G.; Guerin, P. J. Pure Appl. Chem. 1995, A32, 787-796.
(16) Kricheldorf, H. R.; Lee, S. R.; Bush, S. Macromolecules 1996, 29,
1375-1381.
(5) Chamberlain, B. M.; Sun, Y. P.; Hagadorn, J. R.; Hemmesch, E.
W.; Young, V. G.; Pink, R.; Hillmyer, M. A.; Tolman, W. B. Macromol-
ecules 1999, 32, 2400-2402.
(6) Spassky, N.; Wisniewski, M.; Pluta, C.; Le Borgne, A. Macromol.
Chem. Phys. 1996, 197, 2627-2637.
(7) Chisholm, M. H.; Eilerts, N. W. Chem. Commun. 1996, 853-854.
(8) Radano, C. P.; Baker, G. L.; Smith, M. R. J. Am. Chem. Soc. 2000,
122, 1552-1553.
(17) Stevels, W. M.; Dijkstra, P. J.; Feijen, J. Trends Polym. Sci. 1997,
5, 300-305.
(9) Chamberlain, B. M.; Jazdzewski, B. A.; Pink, R.; Hillmyer, M. A.;
Tolman, W. B. Macromolecules 2000, 33, 3970-3977.
(18) Dubois, P.; Jacobs, C.; Je´roˆme, R.; Tessie´, P. Macromolecules 1991,
24, 2266-2270.
(10) Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29, 39-48.
(11) Beckerle, K.; Hultzsch, K. C.; Okuda, J. Macromol. Chem. Phys.
1999, 200, 1702-1707.
(19) Kowalski, A.; Duda, A.; Penczek, S. Macromolecules 1998, 31,
2114-2122.
(20) Kowalski, A.; Duda, A.; Penczek, S. Macromolecules 2000, 33,
689-695.
(12) Yasuda, H.; Ihara, E. AdV. Polym. Sci. 1997, 133, 53-101.
(13) Ko, B. T.; Lin, C. C. Macromolecules 1999, 32, 8296-8300.
(14) Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C.; Iyer, S. S.; Pacold,
M.; Phomphrai, K. J. Am. Chem. Soc. 2000, 122, 11845-11854.
(21) Bero, M.; Dobrzynski, P.; Kasperczyk, J. J. Polym. Sci., Part A
1999, 37, 4038-4042.
(22) Kasperczyk, J. E. Macromolecules 1995, 28, 3937-3939.
10.1021/ja003851f CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/16/2001

3230 J. Am. Chem. Soc., Vol. 123, No. 14, 2001
Chamberlain et al.
Results and Discussion
Complex Synthesis and Analysis. The isostructural zinc
amides (BDI-1)ZnN(SiMe3)2, (BDI-2)ZnN(SiMe3)2, and (BDI-
3)ZnN(SiMe3)2 were prepared by treatment of the appropriate
ligand with stoichiometric Zn[N(SiMe3)2]2 in toluene at 80 °C
(Scheme 1). Complexes (BDI-1)ZnN(SiMe3)2 and (BDI-2)ZnN-
(SiMe3)2 were isolated as crystalline solids in 53% and 62%
yield, respectively. Complex (BDI-3)ZnN(SiMe3)2 was isolated
as an oil in quantitative yield; attempts to crystallize (BDI-3)-
ZnN(SiMe3)2 from all common solvents were unsuccessful.
X-ray diffraction analysis was performed on crystals of (BDI-
1)ZnN(SiMe3)2, and the structure was found to be a rare three-
coordinate species.31 1H NMR spectroscopic data (C6D6, 300
MHz) for (BDI-1)ZnN(SiMe3)2, (BDI-2)ZnN(SiMe3)2, and
(BDI-3)ZnN(SiMe3)2 show one symmetrical species each in
solution; on the basis of the similarity of their 1H NMR spectra
to (BDI-1)ZnN(SiMe3)2, (BDI-2)ZnN(SiMe3)2 and (BDI-3)-
ZnN(SiMe3)2 are thus also assigned a three-coordinate struc-
ture.
Treatment of the zinc amides with stoichiometric 2-propanol
in toluene resulted in the release of HN(SiMe3)2 along with
concomitant formation of the three isostructural zinc alkoxides
[(BDI-1)ZnOiPr]2, [(BDI-2)ZnOiPr]2, and [(BDI-3)ZnOiPr]2 in
54%, 67%, and 54% isolated yield, respectively (Scheme 1).
X-ray diffraction analysis had previously revealed the molecular
structure of [(BDI-1)ZnOiPr]2 to be a dimeric species wherein
isopropoxide ligands bridge distorted tetrahedral zinc centers.3
X-ray diffraction analyses of crystals of [(BDI-2)ZnOiPr]2 and
[(BDI-3)ZnOiPr]2 grown from toluene revealed analogous
dimeric structures, with no significant perturbations of the Zn2O2
core across the ligand series. For example, the Zn‚‚‚Zn
separation in [(BDI-1)ZnOiPr]2 [3.099 Å] is slightly larger than
that in [(BDI-3)ZnOiPr]2 [2.990 Å].32 1H NMR spectroscopic
data (C6D6, 300 MHz) for [(BDI-1)ZnOiPr]2, [(BDI-2)ZnOiPr]2,
and [(BDI-3)ZnOiPr]2 show one symmetrical species each in
solution; on the basis of solution exchange experiments between
related methoxide complexes,33 we propose that all three
alkoxides retain their dimeric structures in solution.
Treatment of the zinc amide (BDI-1)ZnN(SiMe3)2 with
stoichiometric rac-methyl lactate in toluene resulted in the
isolation of the novel complex rac-(BDI-1)ZnOCH(Me)CO2-
Me as colorless blocks in 41% yield (Scheme 1). The molecular
structure of rac-(BDI-1)ZnOCH(Me)CO2Me, as determined by
X-ray diffraction, is a monomeric species wherein the zinc center
is ligated in a distorted tetrahedral array by one â-diiminate
ligand and one methyl lactate moiety (Figure 2).32 Both the
alkoxide and carbonyl oxygen atoms of the methyl lactate ligand
coordinate the zinc center, although the distance of the dative
Zn-carbonyl bond [Zn(1)-O(2) ) 2.189 Å] is significantly
longer than that of the Zn-alkoxide bond [Zn(1)-O(1) ) 1.879
Å]. 1H NMR spectroscopic data (C6D6, 300 MHz) for rac-(BDI-
1)ZnOCH(Me)CO2Me exhibit four inequivalent CHMe2 shifts;
we tentatively propose that the monomeric species is retained
in solution.
Figure 1. Lactide stereochemistry and PLA microstructures.
On the basis of the observation of earlier classes of polym-
erization catalysts,24 we anticipated that zinc complexes25 ligated
by bulky â-diiminate ligands (BDI) might be capable of
stereochemical control in the polymerization of rac- and meso-
lactide via a chain-end control mechanism. In such a reaction,
the bulky ligands increase the influence of the stereogenic center
of the last inserted monomer, which in turn determines the
relative sequence of stereocenters in the main chain. Although
stereocontrol of this type is conceptually simple, typically only
a modest degree of chain-end control during lactide polymer-
ization has been reported.21,22,26-29 However, we recently
communicated the synthesis of a zinc alkoxide complex that
acted as a single-site catalyst for the preparation of heterotactic
PLA;3 herein we report the kinetics of polymerization for this
catalyst, along with the effect of initiating groups and ligand
substituents upon the rate and stereoselectivity of lactide
polymerization.30
(23) Ovitt, T. M.; Coates, G. W. J. Polym. Sci., Part A 2000, 38, 4686-
4692.
(24) For some recent examples of polymerization catalysts with bulky
ligands that control stereochemistry by a chain-end control mechanism,
see: (a) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120-
2130. (b) Resconi, L.; Franciscono, G. Macromolecules 1992, 25, 6814-
6817.
(25) For some examples of zinc-based catalysts for lactone polymeri-
zation, see: (a) Watanabe, Y.; Yasuda, T.; Aida, T.; Inoue, S. Macromol-
ecules 1992, 25, 1396-1400. (b) Barakat, I.; Dubois, P.; Jerome, R.; Teyssie,
P. Macromolecules 1991, 24, 6542-6545. (c) Nijenhuis, A. J.; Grijpma,
D. W.; Pennings, A. J. Polym. Bull. 1991, 26, 71-77. (d) Noltes, J. G.;
Verbeek, F.; Overmars, H. G.; Boersma, J. J. Organomet. Chem. 1970, 24,
257-267. (e) Kricheldorf, H. R.; Damrau, D. O. Macromol. Chem. Phys.
1998, 199, 1747-1752. (f) Schwach, G.; Coudane, J.; Engel, R.; Vert, M.
Polym. Int. 1998, 46, 177-182.
Treatment of (BDI-1)H with ZnEt2 in toluene at 70 °C
followed by removal of solvent in vacuo resulted in the
formation of (BDI-1)ZnEt as a viscous yellow-brown oil in
(26) Thakur, K. A. M.; Kean, R. T.; Hall, E. S.; Kolstad, J. J.; Lindgren,
T. A.; Doscotch, M. A.; Siepmann, J. I.; Munson, E. J. Macromolecules
1997, 30, 2422-2428.
(27) Thakur, K. A. M.; Kean, R. T.; Hall, E. S.; Kolstad, J. J.; Munson,
E. J. Macromolecules 1998, 31, 1487-1494.
(31) For other examples of three-coordinate zinc complexes, see: (a)
Gorrel, I. B.; Looney, A.; Parkin, G.; Rheingold, A. L. J. Am. Chem. Soc.
1990, 112, 4068-4069. (b) Gruff, E. S.; Koch, S. A. J. Am. Chem. Soc.
1989, 111, 8762-8763. (c) Al-Juaid, S. S.; Buttrus, N. H.; Eaborn, C.;
Hitchcock, P. B.; Roberts, A. T. L.; Smith, J. D. S.; Sullivan, A. C. J. Am.
Chem. Soc. 1986, 108, 908-909.
(28) Wisniewski, M.; Le Borgne, A.; Spassky, N. Macromol. Chem. Phys.
1997, 198, 1227-1238.
(29) Coudane, J.; Ustariz-Peyret, C.; Schwach, G.; Vert, M. J. Polym.
Sci., Part A 1997, 35, 1651-1658.
(30) (BDI)MOR (M ) Mg, Zn) catalysts are also active for the
polymerization of other lactones, such as ꢀ-caprolactone and â-butyrolactone
(manuscript in preparation).
(32) See Supporting Information.
(33) Coates, G. W. et al. Manuscript in preparation.

Polymerization of Lactide
J. Am. Chem. Soc., Vol. 123, No. 14, 2001 3231
Scheme 1
Table 1. Effect of Initiating Group: (BDI-1)ZnX with rac-Lactidea
time
(h)
conversionb
(%)
Mn (kg/mol)
Mw/Mn
entry
catalyst
(GPC)c
(GPC)c
1
2
3
4
5
(BDI-1)ZnN(SiMe3)2
[(BDI-1)ZnOiPr]2
rac-(BDI-1)ZnOCHMeCO2Me
(BDI-1)ZnEt
10
0.33
0.33
20
70
97
95
97
97
92
33.6
37.9
30.5
63.3
61.4
2.95
1.10
1.14
1.83
2.07
[(BDI-1)ZnOAc]2
a [LA]/[Zn] ) 200; Trxn ) 20 °C; [LA] ) 0.4 M in CH2Cl2. b As determined via integration of the methyl resonances of LA and poly-LA
(CDCl3, 300 MHz). c Determined by gel-permeation chromatography, calibrated with polystyrene standards in tetrahydrofuran.
is apparent that the identity of the initiating group significantly
affects both the molecular weight and the polydispersity of the
polymers. In particular, -N(SiMe3)2, -Et, and -OAc are clearly
inferior initiating units, as they yield polymers with broad
polydispersities and molecular weights that compare poorly with
theoretical values. Unlike -OiPr and -OCH(Me)CO2Me,
initiation with these three species most likely does not occur
by direct insertion of the moiety into the acyl-oxygen bond.
Rather, these groups likely slowly react with monomer impuri-
ties (e.g., lactic acid, hydrolyzed lactide, water) to form a new
initiating group. Consequently, the rate of initiation (ki) is slower
than the propagating rate of polymerization (kp), resulting in
polymers with broad molecular weight distributions and higher
than expected molecular weights. In contrast, the isopropoxide
and methyl lactate initiating groups closely mimic the putative
propagating groups of the presumed active species, and com-
plexes with these moieties produce PLA of predictable molecular
weight and narrow molecular weight distribution. As [(BDI-
1)ZnOiPr]2 is both the easiest and least expensive to access, all
complexes discussed subsequently will possess isopropoxide
propagating units.
Stereochemistry of Lactide Polymerization with (BDI)-
ZnOiPr Catalysts. According to Bernoullian statistics, PLA
derived from both rac- and meso-lactide possesses five tetrad
sequences in relative ratios determined by the ability of the
catalyst to control racemic [r-dyad] and meso [m-dyad] con-
nectivity of the monomer units (Schemes 2 and 3, Table 2).35
Determination of the stereochemical microstructures of PLA is
achieved through inspection of the methine region of homo-
nuclear decoupled 1H NMR spectra of the polymers.26,36,37 Thus,
Figure 2. Molecular structure of rac-(BDI-1)ZnOCH(Me)CO2Me.
Selected bond distances (Å) and angles (deg): Zn(1)-N(1) 1.977(3),
Zn(1)-N(2) 1.952(2), Zn(1)-O(1) 1.879(2), Zn(1)-O(2) 2.189(2),
C(1)-O(2) 1.226(4), C(2)-O(1) 1.391(4), N(1)-Zn(1)-N(2)
99.67(1), O(1)-Zn(1)-O(2) 82.44(9), N(1)-Zn(1)-O(1) 124.67(10),
N(2)-Zn(1)-O(2), 112.62(9), Zn(1)-O(1)-C(2), 114.7(2), Zn(1)-
O(2)-C(1) 105.3(2).
quantitative yield,33 while treatment of (BDI-1)H with n-
butyllithium followed by Zn(OAc)2 resulted in the isolation of
[(BDI-1)ZnOAc]2 (Scheme 1).34
Effect of Initiating Group upon Lactide Polymerization.
Complexes (BDI-1)ZnN(SiMe3)2, [(BDI-1)ZnOiPr]2, rac-(BDI-
1)ZnOCH(Me)CO2Me, (BDI-1)ZnEt, and [(BDI-1)ZnOAc]2
were examined for polymerization activity with rac-lactide
([LA]/[Zn] ) 200; Table 1). From the polymerization data, it
(35) Bovey, F. A.; Mirau, P. A. NMR of Polymers; Academic Press:
San Diego, 1996.
(34) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
1998, 120, 11018-11019.
(36) Thakur, K. A. M.; Kean, R. T.; Zell, M. T.; Padden, B. E.; Munson,
E. J. Chem. Commun. 1998, 1913-1914.

3232 J. Am. Chem. Soc., Vol. 123, No. 14, 2001
Chamberlain et al.
Scheme 2
Figure 3. Homonuclear decoupled 1H NMR spectrum of the methine
region of heterotactic PLA prepared from rac-LA with [(BDI-1)Zn-
OiPr]2 at 0 °C (500 MHz, CDCl3).
Scheme 3
°C is substantially heterotactic with a Pr of 0.90 (entry 1, Table
3).39 In other words, the probability of a (R,R)-lactide unit to
be enchained after a (S,S)-lactide unit (or vice versa) is 90%.40
Upon cooling the reaction mixture to 0 °C, Pr can be improved
to 0.94 (entry 4, Table 3). Furthermore, the polymerization data
indicate that the substituents on the â-diiminate ligand signifi-
cantly affect the ability of the catalyst to control monomer
enchainment (entries 1-3, Table 3). For instance, changing the
ligand substituents from isopropyl to ethyl groups results in a
decrease in heterotacticity (Pr ) 0.79). Similarly, substitution
with n-propyl groups lowers the heterotacticity (Pr ) 0.76).
In contrast, polymerization of meso-lactide with [(BDI-1)-
ZnOiPr]2 affords syndiotactic PLA (Scheme 3). According to
Bernoullian statistics, syndiotactic polymers may be prepared
from meso-lactide provided that the propagating chain end shows
a propensity for r-dyad placement between monomer units
(Table 2). Interestingly, [(BDI-1)ZnOiPr]2 produces syndiotactic
PLA from meso-lactide, while [(BDI-2)ZnOiPr]2 produces
moderately heterotactic PLA. The origins for the dramatic
change in tacticity are not understood at the current time,
although we anticipate a possible variance in their detailed
mechanisms of enchainment. From the polymerization data
(entry 5, Table 3), it is apparent that PLA prepared from meso-
lactide with [(BDI-1)ZnOiPr]2 (76% racemic linkages between
monomer units) is less syndiotactic than that prepared with a
chiral Schiff base complex of aluminum (96% racemic linkages
between monomer units).4 Nonetheless, [(BDI-1)ZnOiPr]2 il-
lustrates novel chain-end stereocontrol during the polymerization
of meso-lactide (Figure 4). Therefore, [(BDI-1)ZnOiPr]2 provides
access to syndiotactic PLA microstructures without the use of
expensive chiral ligands.
Table 2. Tetrad Probabilities Based on Bernoullian Statistics
probability
tetrad
rac-lactide
meso-lactide
[mmm]
[mmr]
[rmm]
[rmr]
[rrr]
[rrm]
[mrr]
[mrm]
Pm2 + PrPm/2
0
0
0
PrPm/2
PrPm/2
Pr2/2
(Pm2 + PrPm)/2
Pr2 + PrPm/2
PrPm/2
0
0
0
PrPm/2
(Pr2 + PrPm)/2
Pm2/2
1
although atactic PLA exhibits five resonances in its H NMR
spectrum, perfectly heterotactic PLA (Scheme 2) shows only
rmr and mrm tetrads. [(BDI)ZnOiPr]2 complexes polymerize
(S,S)-lactide to isotactic PLA without observable epimerization.
Microstructural analysis of PLA formed from rac-lactide with
[(BDI-1)ZnOiPr]2 revealed that the complex exerts a significant
influence on the tacticity of the growing polymer chain (Figure
3). Two scenarios are possible: if a chain end of R stereochem-
istry selects (R,R)-lactide (meso enchainment; kR/RR . kR/SS),
then isotactic PLA forms; if this chain end selects (S,S)-lactide
(racemic enchainment; kR/SS . kR/RR), then heterotactic38 PLA
forms (Scheme 2). PLA produced by [(BDI-1)ZnOiPr]2 at 20
Mechanism and Kinetics of Lactide Polymerization with
[(BDI-1)ZnOiPr]2. Aliquots periodically withdrawn from a
polymerization of rac-lactide with [(BDI-1)ZnOiPr]2 were used
to construct a plot of Mn versus conversion ([Zn] ) 0.8 mM;
[LA] ) 0.4 M; [LA]/[Zn] ) 500; Figure 5). The linear nature
of this plot, in conjunction with the narrow polydispersities
(Mw/Mn < 1.10), suggests that the polymerization is living and
proceeds by a coordination-insertion mechanism.41 Furthermore,
(37) Chisholm, M. H.; Iyer, S. S.; Matison, M. E.; McCollum, D. G.;
Pagel, M. Chem. Commun. 1997, 1999-2000.
(38) Heterotactic macromolecules are a rare type of polymer that have
alternating pairs of stereogenic centers in the main chain (i.e., only mr/rm
triads are observed). See: (a) Hirano, T.; Yamaguchi, H.; Kitayama, T.;
Hatada, K. Polym. J. 1998, 30, 767-769. (b) Yamada, K.; Nakano, T.;
Okamoto, Y. Macromolecules 1998, 31, 7598-7605. (c) Kitayama, T.;
Hirano, T.; Hatada, K. Tetrahedron 1997, 53, 15263-15279. (d) Nakahama,
S.; Kobayashi, M.; Ishizone, T.; Hirao, A. J. Macromol. Sci., Pure Appl.
Chem. 1997, A34, 1845-1855. (e) Fossum, E.; Matyjaszewski, K.
Macromolecules 1995, 28, 1618-1625.
(39) Even when the initiating group perfectly mimics a putative lactide
chain end (as in rac-(BDI-1)ZnOCH(Me)CO2Me), Pr values are identical
to those produced by (BDI-1)ZnOiPr at 20 °C.
(40) Pr can also be expressed in terms of the enchainment rate
constants: Pr ) kR/SS/(kR/SS + kR/RR) ) kS/RR/(kS/RR + kS/SS).
(41) Stevels, W. M.; Ankone´, M. J. K.; Dijkstra, P. J.; Feijen, J.
Macromolecules 1996, 29, 3332-3333.

Polymerization of Lactide
J. Am. Chem. Soc., Vol. 123, No. 14, 2001 3233
Table 3. Effect of Temperature and Ligand on Stereochemistry: [(BDI)ZnOiPr]2 with Various Lactidesa
temp
(°C)
time
(h)
conversionb
(%)
Mn (kg/mol)
Mw/Mn
d
entry
catalyst
monomer
(GPC)c
(GPC)c
Pr
1
2
3
4
5
6e
[(BDI-1)ZnOiPr]2
[(BDI-2)ZnOiPr]2
[(BDI-3)ZnOiPr]2
[(BDI-1)ZnOiPr]2
[(BDI-1)ZnOiPr]2
[(BDI-2)ZnOiPr]2
rac-LA
rac-LA
rac-LA
rac-LA
meso-LA
meso-LA
20
20
20
0
0
0
0.33
97
97
97
97
82
97
37.9
42.5
35.8
38.8
22.4
13.8
1.10
1.09
1.18
1.09
1.07
1.02
0.90
0.79
0.76
0.94
0.76
0.37
8
19
2
4
4
a [LA]/[Zn] ) 200; Trxn ) 20 °C; [LA] ) 0.4 M in CH2Cl2. b As determined via integration of the methyl resonances of LA and poly-LA
(CDCl3, 300 MHz). c Determined by gel-permeation chromatography, calibrated with polystyrene standards in tetrahydrofuran. d Pr is the probability
of racemic linkages between monomer units and is determined from the methine region of the homonuclear decoupled 1H NMR spectrum. e [LA]/
[Zn] ) 99.
envelope centered at m/z 840 and spaced by m/z 72 is due to
unknown species that we believe form during quenching due
to a zinc-catalyzed methanolysis of the oligomers. Analysis of
oligomers formed by [(BDI-1)ZnOiPr]2 at [LA]/[Zn] ) 9 after
reaction overnight revealed that transesterification is catalyzed
by these complexes at longer reaction times, as oligomers of
+
the formula H(OCHMeCO)nOiPr‚NH4 were found.32 This
suggests that transesterification could degrade both the molecular
weight distributions as well as stereochemistry of these poly-
mers.21 These data, along with fair agreement between observed
and theoretical values of Mn, strongly support our hypothesis
that [(BDI-1)ZnOiPr]2 acts as a single-site catalyst wherein the
bulky â-diiminate ligand remains bound to the zinc center while
the isopropoxide ligand initiates polymerization.
Conversions of rac-lactide with time at various concentrations
of [(BDI-1)ZnOiPr]2 in methylene chloride at 25 °C were
monitored by 1H NMR spectroscopy until monomer consump-
tion was complete ([LA]o ) 1.026 M; [Zn] ) 2.10 to 5.46 mM;
[LA]/[Zn] ) 488 to 188). In each case, first-order kinetics in
monomer were observed; the appropriate semilogarithmic plots
for several of these polymerizations are shown in Figure 7. Thus,
the polymerization of rac-lactide by [(BDI-1)ZnOiPr]2 presum-
ably proceeds according to:
Figure 4. Homonuclear decoupled 1H NMR spectrum of the methine
region of syndiotactic PLA prepared from meso-LA with [(BDI-1)-
ZnOiPr]2 at 0 °C (500 MHz, CDCl3).
-d[LA]/dt ) kapp[LA]1
(1)
where kapp ) kp[Zn]x, and where kp is the propagation rate
constant. To determine the order in zinc (x), we plotted ln kapp
versus ln [Zn] (Figure 8). From this plot, the order in initiator
(slope) is 1.56 ( 0.06. Therefore, the polymerization of rac-
lactide by [(BDI-1)ZnOiPr]2 follows an overall kinetic law of
the form:
-d[LA]/dt ) kp[Zn]1.56[LA]
(2)
Fractional dependencies upon catalyst have been previously
observed for ring-opening lactone polymerizations with related
aluminum18,42-45 and yttrium alkoxides;9 in these systems,
studies indicate that the observed kinetics result from aggrega-
tion of the active species in the polymerization medium.44,45
We are currently investigating the nature of the active species
in these polymerization systems, as they appear to be more
complex than originally anticipated.
Figure 5. Plot of PLA Mn (versus polystyrene standards) and molecular
weight distribution as a function of conversion with rac-LA and [(BDI-
1)ZnOiPr]2 (CH2Cl2, 20 °C, [LA]/[Zn] ) 500).
Conversion versus time data were also collected for the
polymerization of meso- and (S,S)-lactide by [(BDI-1)ZnOiPr]2
(CH2Cl2; 25 °C; [LA]o ) 1.023, 1.005 M, respectively; [Zn] )
analysis of oligomers formed by [(BDI-1)ZnOiPr]2 ([LA]/[Zn]
) 9) after quenching immediately with methanol) by electro-
spray-ionization mass spectrometry exhibited oligomers of the
(42) Duda, A.; Penczek, S. Macrmol. Rapid Commun. 1994, 15, 559.
(43) Duda, A.; Penczek, S. Polym. Prepr. (Am. Chem. Soc., DiV. Polym.
Chem.) 1990, 31, 12.
(44) Ouhadi, T.; Hamitou, A.; Jerome, R.; Teyssie, P. Macromolecules
1976, 9, 927.
(45) Penczek, S.; Duda, A.; Biela, T. Polym. Prepr. (Am. Chem. Soc.,
DiV. Polym. Chem.) 1994, 35, 508.
+
formula H(OCHMeCO)2nOiPr‚NH4 (n ) 3 to 12) as well as
molecular ions corresponding to the methanolyzed ligand ((BDI-
1)H2+) (Figure 6). Small peaks in the spectrum with masses
intermediate to those of the primary species are due to
transesterification during catalysis, while the other small

3234 J. Am. Chem. Soc., Vol. 123, No. 14, 2001
Chamberlain et al.
Figure 6. Electrospray MS of the crude reaction product of rac-lactide and [(BDI-1)ZnOiPr]2 ([LA]/[Zn] ) 9), quenched with methanol immediately
after reaction is complete. Region m/z 430 to 1900.
Figure 8. Plot of ln kapp versus ln [Zn] for the polymerization of rac-
lactide with [(BDI-1)ZnOiPr]2 as initiator (CH2Cl2, 25 °C, [LA]o )
1.026 M).
Figure 7. Semilogarithmic plots of rac-lactide conversion with time
in CH2Cl2 at 25 °C with [(BDI-1)ZnOiPr]2 as initiator ([LA]o ) 1.026
M: I, [Zn] ) 5.46 mM, [LA]/[Zn] ) 188; II, [Zn] ) 4.78 mM, [LA]/
[Zn] ) 215; III, [Zn] ) 3.78 mM, [LA]/[Zn] ) 271; IV, [Zn] ) 3.18
polymerization of (S,S)-lactide should proceed significantly
mM, [LA]/[Zn] ) 323; V, [Zn] ) 2.57 mM, [LA]/[Zn] ) 399; VI,
slower than that of rac-lactide. Specifically, the value of Pr )
0.90 for the polymerization of rac-lactide with [(BDI-1)Zn-
OiPr]2 at 20 °C forecasts a relative rate difference of nine, which
closely agrees with the observed value.
[Zn] ) 2.10 mM, [LA]/[Zn] ) 488).
2.07, 5.39 mM, respectively; [LA]/[Zn] ) 494, 186, respec-
tively). In both cases, first-order kinetics in monomer were
observed; the appropriate semilogarithmic plots for these
polymerizations relative to rac-lactide are shown in Figures 9
and 10. Interestingly, the first-order rate constant (kapp) for the
polymerization of meso-lactide (0.037 min-1) is slightly smaller
than that observed under identical conditions with rac-lactide
(0.054 min-1) ([LA]/[Zn] ≈ 490, [Zn] ≈ 2.1 mM; Figure 9).
In contrast, the polymerization of (S,S)-lactide exhibits a kapp
(0.031 min-1) approximately seven times smaller than the
observed kapp (0.22 min-1) for the analogous polymerization
with rac-lactide ([LA]/[Zn] ≈ 188, [Zn] ≈ 5.4 mM; Figure 10).
These results agree well with our measured stereoselectivities.
In particular, from the polymerization of rac-lactide, we
observed that kR/SS . kR/RR (or kS/RR . kS/SS). Thus, the
Finally, conversion versus time data were collected for the
polymerization of rac-lactide with [(BDI-2)ZnOiPr]2 and [(BDI-
3)ZnOiPr]2 (CH2Cl2; 25 °C; [LA]o ) 1.023, 1.02 M, respec-
tively; [Zn] ) 5.44, 5.51 mM, respectively; [LA]/[Zn] ) 188,
185, respectively). In both cases, first-order kinetics in monomer
were observed; the appropriate semilogarithmic plots for these
polymerizations are shown in Figure 11. From the kinetic data,
it is apparent that the substituents on the â-diiminate ligand
significantly affect the rate of polymerization. For instance,
changing the ligand substituents from isopropyl to ethyl groups
results in a decrease in kapp from 0.22 min-1 to 0.017 min-1
.
Similarly, substitution with n-propyl groups lowers kapp to 0.0066
min-1. These results reinforce recent reports that subtle ma-

Polymerization of Lactide
J. Am. Chem. Soc., Vol. 123, No. 14, 2001 3235
Figure 11. Semilogarithmic plots of rac-lactide conversion with time
in CH2Cl2 at 25 °C (I: [(BDI-1)ZnOiPr]2, [LA]o ) 1.026 M, [Zn] )
5.46 mM, [LA]/[Zn] ) 188; II: [(BDI-2)ZnOiPr]2, [LA]o ) 1.023 M,
[Zn] ) 5.44 mM, [LA]/[Zn] ) 188; III: [(BDI-3)ZnOiPr]2, [LA]o )
1.02 M, [Zn] ) 5.51 mM, [LA]/[Zn] ) 185).
Figure 9. Semilogarithmic plots of meso- and rac-lactide conversion
with time in CH2Cl2 at 25 °C with [(BDI-1)ZnOiPr]2 as initiator ([LA]o
) 1.023, 1.026 M, respectively; [Zn] ) 2.07, 2.10 mM, respectively;
[LA]/[Zn] ) 494, 488, respectively).
Figure 10. Semilogarithmic plots of (S,S)- and rac-lactide conversion
with time in CH2Cl2 at 25 °C with [(BDI-1)ZnOiPr]2 as initiator ([LA]o
) 1.005, 1.026 M, respectively; [Zn] ) 5.39, 5.46 mM, respectively;
[LA]/[Zn] ) 186, 188, respectively).
Figure 12. X-ray crystal structure of [(BDI-1)Mg(OiPr)]2. Selected
bond lengths (Å) and bond angles (deg): Mg(1)-N(1) 2.123(3),
Mg(1)-N(2) 2.114(3), Mg(1)-O(1) 1.978(2), Mg(1)-O(1A)
1.987(2), N(1)-Mg(1)-N(2) 91.27(10), O(1)-Mg(1)-O(1A)
80.95(9), N(1)-Mg(1)-O(1) 126.19(10), N(2)-Mg(1)-O(1)
123.44(10), Mg(1)-O(1)-Mg(1A) 99.05(9).
nipulations of ligand architecture can lead to drastic changes in
lactide polymerization activity.9,46
Lactide Polymerization with [(BDI-1)MgOiPr]2 Catalysts.
Due to the successful implementation of tris(pyrazolyl)borato
magnesium alkoxides as single-site catalysts for lactide polym-
erization,7 we decided to investigate the synthesis of discrete
â-diiminate magnesium alkoxides as catalysts for lactone
polymerization.47 On the basis of the synthesis of â-diiminate
zinc alkoxides by the alcoholysis of complexes of the form
(BDI)ZnR (R ) alkyl, amido), we synthesized the analogous
magnesium complexes. Reaction of (BDI-1)H with Mg[N-
(SiMe3)2]2 yields (BDI-1)MgN(SiMe3)2 in an isolated yield of
84% following crystallization. We have not analyzed this
compound by X-ray diffraction, but presume that it is a
monomer analogous to (BDI-1)ZnN(SiMe3)2. Reaction with
2-propanol produces (BDI-1)MgOiPr in 65% isolated yield. The
molecular structure of the complex was determined by X-ray
diffraction to be an alkoxide-bridged dimer, virtually isostruc-
tural to [(BDI-1)Zn(OiPr)]23 (Figure 12). The Mg-Mg separa-
tion is 3.01 Å, while Mg-O bonds are 1.98 and 1.99 Å, well
within the acceptable range.47 The six-membered chelate is
puckered with a deviation of 0.78 Å from the plane of N(1)-
N(2)-C(6). [(BDI-1)MgOiPr]2 is highly active for the polym-
erization of rac-lactide; in 2 min at 20 °C, it polymerized rac-
lactide to complete conversion to yield atactic PLA ([Mg] ) 2
mM; [LA] ) 0.4 M; [LA]/[Mg] ) 200). Nonetheless, gel-
(46) Bhaw-Luximon, A.; Jhurry, D.; Spassky, N. Polym. Bull. 2000, 44,
31-38.
(47) For recent reports of magnesium complexes with â-diiminate ligands,
see: (a) Bailey, P. J.; Dick, C. M. E.; Fabre, S.; Parsons, S. J. Chem. Soc.;
Dalton Trans. 2000, 10, 1655-1661. (b) Gibson, V. C.; Segal, J. A.; White,
A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2000, 122, 7120-7121.

3236 J. Am. Chem. Soc., Vol. 123, No. 14, 2001
Chamberlain et al.
Table 4. Polymerization of rac-Lactide using [(BDI-1)MgOiPr]2/
or standard Schlenk line techniques. NMR spectra were recorded on a
Bruker AF300 (1H, 300 MHz; 13C, 75 MHz) or a Varian INOVA400
(1H, 400 MHz; 13C, 100 MHz) spectrometer, and are referenced versus
shifts of solvents containing residual protic impurities. Gel permeation
chromatography (GPC) analyses were carried out on a Waters instru-
ment (M510 pump, U6K injector) equipped with Waters UV486 and
Milton Roy differential refractive index detectors, and four 5 µm PL
Gel columns (Polymer Laboratories; 100 Å, 500 Å, 1000 Å, and Mixed
C porosities) in series. The GPC columns were eluted with tetra-
hydrofuran at 45 °C at 1 mL/min and were calibrated with 23
monodisperse polystyrene standards. Crystallographic data were col-
lected with a SMART CCD Area Detector System (Mo KR, λ )
0.71073 Å), and frames were integrated with the Siemens SAINT
program. Electrospray-ionization mass spectrometry was carried out
on a Micromass Quattro I mass spectrometer operated in positive-ion
mode. Samples were dissolved in acetonitrile with a trace of ammonium
hydroxide and were infused with a syringe pump at 5µL/min (source
temperature 80 °C; cone voltage 19 V).
2-Propanola
time conversionb Mn (kg/mol) Mw/Mn
entry [LA]:[Mg]:[iPrOH] (min)
(%)
(GPC)c
(GPC)c
1
2
3
4
5
50:1:1
100:1:1
200:1:1
300:1:1
500:1:1
1
2
2
5
5
97
97
97
98
96
11.0
17.7
29.7
39.8
55.3
1.20
1.28
1.29
1.33
1.35
a Trxn ) 20 °C; [LA] ) 0.4 M in CH2Cl2. b As determined via
integration of the methyl resonances of LA and poly-LA (CDCl3, 300
MHz). c Determined by gel-permeation chromatography, calibrated with
polystyrene standards in tetrahydrofuran.
permeation chromatography (GPC, versus polystyrene standards)
revealed an Mn of 64 000 g/mol (Mn theory ) 28 800 g/mol)
and a molecular weight distribution of 1.59. We tentatively
attribute the broad molecular weight distribution of this polymer
to slow initiation (relative to propagation) of the complex as
well as transesterification reactions. However, polymerization
of rac-lactide with [(BDI-1)MgOiPr]2 and 1 equiv of 2-propanol
led to the formation of narrow molecular weight distribution
PLA with molecular weights that agree well with theoretical
values (Table 4). These results show that 2-propanol can interact
with and modify the reactivity of [(BDI-1)MgOiPr]2. Regardless
of the mechanism of polymerization, these complexes exhibit
polymerization rates that are among the highest reported.48 It
is currently unclear why the structurally similar zinc complex
exhibits heterospecific chain-end control in the polymerization
of rac-lactide while the magnesium complex does not exhibit
chain-end control under the conditions employed.
Materials. Hexanes and methylene chloride were distilled from CaH2
under nitrogen, while toluene was distilled from sodium ketyl under
nitrogen. rac- and (S,S)-lactide were purchased from Purac USA and
sublimed prior to use. meso-lactide was prepared by published
procedures.4 2-((2,6-Diisopropylphenyl)amino)-4-((2,6-diisopropylphe-
nyl)imino)-2-pentene (BDI-1)H and (BDI-2)H were prepared by
procedures analogous to those described in the literature.49 2,6-
Diallylaniline was prepared according to the published procedure.50 Zinc
bis(trimethylsilyl)amide and rac-methyl lactate were purchased from
Aldrich and used as received. Magnesium bis(trimethylsilyl)amide was
prepared as described in the literature.51 Anhydrous 2-propanol was
purchased from Acros and used as received. (BDI-1)ZnN(SiMe3)2,3
[(BDI-1)ZnOiPr]2,3 and [(BDI-1)ZnOAc]2 were prepared according
34
to published procedures.
2,6-Di-n-propylaniline. 2,6-Diallylaniline (3.76 g, 21.7 mmol) was
dissolved in absolute ethanol and transferred to a Fischer-Porter bottle
equipped with a pressure fitting rated to 100 psi. After addition of 10%
palladium on carbon (0.100 g), the reactor was pressured to 100 psi
with H2. The reaction was then stirred at room temperature, with
periodic refilling of H2, until the pressure remained constant. Following
the release of pressure, the solvent was removed in vacuo to afford a
dark orange oil. This oil was dissolved in diethyl ether (20 mL) and
treated with gaseous HCl, resulting in a flocculent white precipitate.
This precipitate was collected by filtration, suspended in diethyl ether
(20 mL), and then washed with saturated aqueous bicarbonate (3 × 10
mL) followed by brine (1 × 25 mL). The organic layer was separated,
dried over Na2SO4, and filtered. Following removal of the solvent in
vacuo, the resulting oil was distilled (bp 110 °C/60 mmHg) to afford
Summary and Perspective
In conclusion, we report a series of zinc alkoxide complexes
that act as single-site, living initiators for the polymerization
of (S,S)-lactide to isotactic PLA, rac-lactide to heterotactic PLA,
and meso-lactide to syndiotactic PLA. Microstructural analysis
of the polymers formed by these complexes reveals that the
â-diiminate ligands exert significant influence on the tacticity
of the growing polymer chains. Furthermore, kinetic analysis
suggests that, although polymerizations of rac-lactide by these
complexes are first order in monomer, the active initiating
species are much more complex than previously anticipated in
the polymerization medium. Finally, microstructural and kinetic
data indicate that the substituents on the â-diiminate ligand
significantly affect catalytic activity as well as the ability of
the zinc-based complexes to control the stereochemistry of
monomer enchainment, while interestingly we do not observe
stereochemical control with the magnesium analogues. Future
work will be directed toward understanding the significant
differences between these structurally similar zinc and magne-
sium complexes, as well as to discover new metal-based lactide
polymerization catalysts employing the â-diiminate ligand
structure. Our studies not only represent important progress in
understanding the core issues surrounding the selectivity and
mechanism of lactide polymerization but also provide a firm
foundation for the further development of single-site catalysts
for the construction of PLA and other new polyester architec-
tures.
1
the title compound as a light brown oil (3.08 g, 80% yield). H NMR
(C6D6, 300 MHz): δ 6.93 (2H, d, J ) 7.5 Hz, ArH), 6.77 (1H, t, J )
7.5 Hz, ArH), 3.10 (2H, br, NH2), 2.27 (4H, t, J ) 7.5 Hz, CH2CH2-
CH3), 1.52 (4H, sextet, J ) 7.5 Hz, CH2CH2CH3), 0.86 (6H, t, J ) 7.3
Hz, CH2CH2CH3) ppm.
2-((2,6-Di-n-propylphenyl)amino)-4-((2,6-di-n-propylphenyl)-
imino)-2-pentene (BDI-3)H. 2,6-Di-n-propylaniline (2.36 g, 13.3
mmol), 2,4-pentanedione (0.668 g, 6.67 mmol), and concentrated HCl
(0.59 mL) were combined in absolute ethanol (10 mL) and heated at
reflux for 3 days. After being cooled to room temperature, the orange
solution was concentrated and hexanes (50 mL) were added. The
resulting precipitate was collected by filtration, suspended in diethyl
ether (20 mL), and washed with saturated aqueous bicarbonate (3 ×
10 mL) followed by brine (1 × 25 mL). The organic layer was
separated, dried over Na2SO4, and filtered. Removal of the solvent in
vacuo afforded (BDI-3)H as a light brown oil (1.50 g, 54% yield). 1H
NMR (C6D6, 300 MHz): δ 12.20 (1H, s, NH), 7.08 (6H, br, ArH),
4.83 (1H, s, â-CH), 2.60 (8H, m, CH2CH2CH3), 1.64 (14H, br, R-Me,
CH2CH2CH3), 0.97 (12H, t, J ) 7.5 Hz, CH2CH2CH3) ppm.
Experimental Section
(49) Feldman, J.; McLain, S. J.; Parthasaranthy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514-1516.
(50) Arestat, G. Synth. React. Inorg. Met. Org. Chem. 1979, 9, 377-
390.
General Considerations. All reactions with air- and/or water-
sensitive compounds were carried out under dry nitrogen with drybox
(48) McLain, S. L.; Ford, T. M.; Drysdale, N. E. Polym. Prepr. (Am.
Chem. Soc., DiV. Polym. Chem.) 1992, 33 (2), 463-464.
(51) Allan, J. F.; Henderson, K. W.; Kennedy, A. R. Chem. Commun.
1999, 1325-1326.

Polymerization of Lactide
J. Am. Chem. Soc., Vol. 123, No. 14, 2001 3237
(BDI-2)ZnN(SiMe3)2. To a solution of (BDI-2)H (3.54 g, 9.77
mmol) in toluene (15 mL) was added zinc bis(trimethylsilyl)amide (3.95
mL, 9.79 mmol) in toluene (5 mL). After being stirred for 18 h at 70
°C, the clear yellow solution was dried in vacuo, giving (BDI-2)ZnN-
(SiMe3)2 in quantitative yield (5.74 g). The light yellow solid was
recrystallized from hexanes at -5 °C to yield colorless blocks (3.59 g,
80 °C, the clear solution was dried in vacuo giving (BDI-1)ZnEt as a
viscous yellow oil in quantitative yield (0.612 g). This oil crystallized
1
as large blocks upon standing over the course of several weeks. H
NMR (C6D6, 300 MHz): δ 7.07 (6H, m, ArH), 4.98 (1H, s, â-CH),
3.18 (4H, m, CHMe2), 1.69 (6H, s, R-Me), 1.25 (12H, d, J ) 7.0 Hz,
CHMeMe), 1.14 (12H, d, J ) 7.0 Hz, CHMeMe), 0.89 (3H, t, J ) 8.0
Hz, CH2CH3) ppm. 13C{1H} NMR (C6D6, 75 MHz): δ 167.03, 144.67,
141.17, 125.61, 123.46, 95.03, 28.16, 23.88, 23.14, 22.93, 11.74, -1.53
ppm.
1
62% yield). H NMR (C6D6, 300 MHz): δ 7.11 (6H, br, ArH), 4.87
(1H, s, â-CH), 2.73 (4H, m, J ) 7.5 Hz, CH2CH3), 2.60 (4H, m, J )
7.5 Hz, CH2CH3), 1.58 (6H, s, R-Me), 1.21 (12H, t, J ) 7.5 Hz,
CH2CH3), 0.03 (18H, s, CH3) ppm. 13C{1H} NMR (C6D6, 75 MHz): δ
169.21, 145.86, 136.99, 125.99, 125.85, 95.62, 24.79, 23.43, 13.59,
5.17 ppm.
(BDI-1)MgN(SiMe3)2. To a solution of (BDI-1)H (2.045 g, 4.883
mmol) in toluene (20 mL) was added a solution of magnesium bis-
(trimethylsilyl)amide (1.707 g, 4.947 mmol) in toluene (20 mL). After
the mixture was stirred for 18 h at 80 °C, the solvent was removed in
vacuo to give (BDI-1)MgN(SiMe3)2 in quantitative yield. The light
yellow solid was recrystallized from toluene at -30 °C to yield colorless
(BDI-3)ZnN(SiMe3)2. To a solution of (BDI-3)H (0.523 g, 1.25
mmol) in toluene (5 mL) was added zinc bis(trimethylsilyl)amide (0.505
mL, 1.25 mmol) in toluene (5 mL). After being stirred for 18 h at 80
°C, the clear yellow solution was dried in vacuo, giving (BDI-3)ZnN-
1
blocks (2.48 g, 84% yield). H NMR (C6D6, 300 MHz): δ 7.12 (6H,
1
(SiMe3)2 in quantitative yield (0.804 g). H NMR (C6D6, 300 MHz):
br, ArH), 4.80 (1H, s, â-CH), 3.20 (4H, m, J ) 7.0 Hz, CHMe2), 1.62
(6H, s, R-Me), 1.36 (12H, d, J ) 7.0 Hz, CHMeMe), 1.14 (12H, d, J
) 7.0 Hz, CHMeMe), 0.01 (18H, s, CH3) ppm. 13C{1H} NMR (C6D6,
75 MHz): δ 170.43, 144.19, 141.86, 125.72, 123.95, 95.29, 28.69,
24.57, 24.39, 24.29, 5.05 ppm.
δ 7.11 (6H, br, ArH), 4.90 (1H, s, â-CH), 2.69 (4H, t, J ) 7.5 Hz,
CH2CH2CH3), 1.64 (14H, s, R-Me, CH2CH2CH3), 0.99 (12H, t, J )
7.5 Hz, CH2CH2CH3), 0.02 (18H, s, CH3) ppm.
[(BDI-2)ZnOiPr]2. To a solution of (BDI-2)ZnN(SiMe3)2 (0.449 g,
0.764 mmol) in toluene (6 mL) was added 2-propanol (58 µL, 0.763
mmol). After the solution was stirred at room temperature for 1 h, the
resulting slurry was dried in vacuo. The white solid was recrystallized
from toluene (10 mL) at -5 °C to yield [(BDI-2)ZnOiPr]2 as colorless
blocks (0.25 g, 67% yield). 1H NMR (CD2Cl2, 300 MHz) δ 7.11 (6H,
br, ArH), 4.74 (1H, s, â-CH), 3.46 (1H, m, J ) 5.9 Hz, OCHMe2),
2.45 (4H, m, J ) 7.5 Hz, CH2CH3), 1.94 (4H, m, J ) 7.5 Hz, CH2-
CH3), 1.41 (6H, s, R-Me), 0.98 (12H, t, J ) 7.5 Hz, CH2CH3), 0.81
(6H, d, J ) 5.9 Hz, OCHMe2) ppm. 13C{1H} NMR (C6D6, 75 MHz):
δ 167.51, 147.72, 137.61, 125.52, 124.42, 94.78, 65.04, 27.79, 23.53,
23.49, 13.60 ppm. X-ray analysis of the crystals revealed that the
complex exists as a µ-isopropoxide-bridged dimer in the solid state
(see Supporting Information).
[(BDI-1)MgOiPr]2. To a solution of (BDI-1)MgN(SiMe2)2 (0.558
g, 0.926 mmol) in toluene (8 mL) was added 2-propanol (78 µL, 1.03
mmol). After the mixture was stirred for 4 h at room temperature, the
resulting white slurry was dried in vacuo. The white solid was then
recrystallized from toluene (30 mL) at room temperature to yield [(BDI-
1)MgOiPr]2 as colorless blocks (0.302 g, 65% yield). 1H NMR (toluene-
d8, 300 MHz): δ 7.07 (6H, m, ArH), 4.78 (1H, s, â-CH), 3.87 (1H, m,
J ) 6.0 Hz, OCHMe2), 3.45 (2H, m, J ) 7.0 Hz, CHMe2), 3.11 (2H,
m, J ) 7.0 Hz, CHMe2), 1.45 (6H, s, R-Me), 1.36 (6H, d, J ) 7.0 Hz,
CHMeMe), 1.23 (6H, d, J ) 6.0 Hz, OCHMe2), 1.16 (6H, d, J ) 7.0
Hz, CHMeMe), 1.07 (6H, d, J ) 7.0 Hz, CHMeMe), 0.40 (6H, d, J )
7.0 Hz, CHMeMe) ppm. 13C{1H} NMR (toluene-d8, 75 MHz):
δ
169.25, 161.15, 146.49, 142.42, 140.89, 137.51, 136.05, 128.95, 128.19,
125.49, 125.32, 123.65, 123.23, 123.02, 93.90, 53.60, 28.28, 28.00,
24.13, 23.40, 23.09, 22.77, 21.05, 20.42, 20.14 ppm. X-ray analysis of
the crystals revealed that the complex exists as a µ-isopropoxide-bridged
dimer in the solid state (see Supporting Information).
General Polymerization Procedure. A Schlenk flask was charged
with a 34 mM solution of the catalyst in methylene chloride. To this
solution was added a 0.40 M solution of monomer in methylene chloride
([monomer]:[Zn] ) 200:1). The reaction was stirred at the desired
temperature for the desired reaction time. After a small sample of the
crude material was removed for characterization, the reaction was
quenched with methanol (1 mL), the solution was concentrated in vacuo,
and the polymer was precipitated with excess methanol. The polymer
was then dried in vacuo to constant weight.
General Kinetic Procedure. A solution of catalyst in methylene
chloride (5 mL) was added to monomer (13 mL of a 1.39 M solution
in methylene chloride; resultant [LA]o ) 1.0 M). The mixture was then
stirred at 25 °C under N2. At appropriate time intervals, 0.5 mL aliquots
were removed and quenched with methanol (3 drops). The aliquots
were then dried to constant weight in vacuo and analyzed by 1H NMR.
X-ray Crystallography. Suitable crystals of each sample were
attached to a glass fiber with heavy-weight oil and mounted in the
Siemens SMART system for data collection at 173(2) K. Initial sets of
cell constants for each sample were calculated from reflections harvested
from three sets of 20 frames. These initial sets of frames were oriented
such that orthogonal wedges of reciprocal space were surveyed. This
produced orientation matrices from 248 reflections. In each case, final
cell constants were calculated from a set of no less than 5000 strong
reflections from the actual data collections. For each sample, a randomly
oriented region of reciprocal space was surveyed to the extent of 1.3
hemispheres to a resolution of 0.84 Å. Three major swaths of frames
were collected with 0.30° steps in ω.
[(BDI-3)ZnOiPr]2. To a solution of (BDI-3)ZnN(SiMe3)2 (0.80 g,
1.24 mmol) in toluene (6 mL) was added 2-propanol (95 µL, 1.24
mmol). After the mixture was stirred at room temperature for 1 h, the
resulting slurry was dried in vacuo. The white solid was recrystallized
from hexanes (10 mL) at 20 °C to yield [(BDI-3)ZnOiPr]2 as colorless
blocks (0.360 g, 54% yield). 1H NMR (C6D6, 300 MHz): δ 7.17 (6H,
br, ArH), 4.72 (1H, s, â-CH), 3.83 (1H, septet, J ) 5.9 Hz, OCHMe2),
2.71 (4H, m, CH2CH2CH3), 2.04 (4H, m, CH2CH2CH3), 1.67 (8H, m,
CH2CH2CH3), 1.46 (6H, s, R-Me), 1.18 (6H, d, J ) 5.9 Hz, OCHMe2),
0.99 (12H, t, J ) 7.2 Hz, CH2CH2CH3) ppm. 13C{1H} NMR (C6D6, 75
MHz): δ 167.90, 148.17, 136.85, 128.45, 128.32, 128.00, 127.87,
127.81, 127.69, 126.61, 124.33, 118.93, 118.86, 94.68, 65.22, 32.88,
24.06, 23.10, 14.69 ppm. X-ray analysis of the crystals revealed that
the complex exists as a µ-isopropoxide-bridged dimer in the solid state
(see Supporting Information).
rac-(BDI-1)ZnOCH(Me)CO2Me. To a solution of (BDI-1)ZnN-
(SiMe3)2 (0.452 g, 0.703 mmol) in toluene (6 mL) was added rac-
methyl lactate (74 µL, 0.704 mmol). After the mixture was stirred for
5 h at room temperature, the resulting slurry was dried in vacuo.
Hexanes (20 mL) were added, and the precipitate was filtered through
a pad of Celite. The filtrate was then concentrated and cooled to -5
°C, whereupon rac-(BDI-1)ZnOCH(Me)CO2Me crystallized as colorless
blocks (0.167 g, 41% yield). 1H NMR (C6D6, 300 MHz): δ 7.08 (6H,
m, ArH), 4.94 (1H, s, â-CH), 4.29 (1H, quartet, J ) 7.0 Hz;
OCHMeCO2Me), 3.41 (4H, m, J ) 7.0 Hz, CHMe2), 3.14 (3H, s, CO2-
Me), 1.73 (6H, s, R-Me), 1.41 (6H, d, J ) 7.0 Hz, CHMeMe), 1.32
(6H, d, J ) 7.0 Hz, CHMeMe), 1.19 (6H, d, J ) 7.0 Hz, CHMeMe),
1.16 (6H, d, J ) 7.0 Hz, CHMeMe), 0.70 (3H, d, J ) 7.0 Hz,
OCHMeCO2Me) ppm. 13C{1H} NMR (C6D6, 75 MHz): δ 168.79,
144.43, 142.75, 142.56, 125.83, 123.89, 123.78, 93.98, 70.64, 52.88,
28.22, 28.17, 24.66, 24.48, 24.43, 23.72, 23.56 ppm (one resonance
missing possibly due to low resolution). X-ray analysis of the crystals
revealed that the complex exists as a monomer in the solid state (see
Supporting Information).
Space groups for each sample were determined on the basis of
systematic absences and intensity statistics. In each case, successful
direct-methods solutions were calculated which provided most non-
hydrogen atoms from the E-map. Several full-matrix least-squares/
difference Fourier cycles were then performed to locate the remainder
of the non-hydrogen atoms. For each sample, all non-hydrogen atoms
(BDI-1)ZnEt. To a solution of diethyl zinc (0.61 mL, 5.95 mmol)
in toluene (10 mL) was slowly added (BDI-1) (0.501 g, 1.196 mmol)
in toluene (10 mL) at 0 °C. After the mixture was stirred for 18 h at

3238 J. Am. Chem. Soc., Vol. 123, No. 14, 2001
Chamberlain et al.
were refined with anisotropic displacement parameters, and all hydrogen
atoms were placed in ideal positions and refined as riding atoms with
individual (or group if appropriate) isotropic displacement parameters.
Complete drawings and full details of the X-ray structure determina-
tions, including tables of bond lengths and angles, atomic positional
parameters, and final thermal parameters for [(BDI-2)ZnOiPr]2, [(BDI-
3)ZnOiPr]2 rac-(BDI-1)ZnOCHMeCO2Me and [(BDI-1)MgOiPr]2 are
provided in the Supporting Information.
Facility (supported by the NSF; CHE-9700441). G.W.C. grate-
fully acknowledges a Camille and Henry Dreyfus New Faculty
Award, a Research Corporation Research Innovation Award,
an Alfred P. Sloan Research Fellowship, an Arnold and Mabel
Beckman Foundation Young Investigator Award, a Camille
Dreyfus Teacher-Scholar Award, a 3M Untenured Faculty Grant,
an IBM Partnership Award, and a Union Carbide Innovation
Recognition Award. We thank Dr. Athula Attygalle for as-
sistance with obtaining mass spectroscopy data, and Joseph
Reczek for assistance with ligand synthesis.
Acknowledgment. This work was generously supported by
the NSF (Career Award CHE-9875261). We thank the Cornell
University Center for Biotechnology, a New York State Center
for Advanced Technology supported by the New York State
Science and Technology Foundation, and industrial partners for
partial support of this research. This work made use of the
Cornell Center for Materials Research Shared Experimental
Facilities, supported through the National Science Foundation
Materials Research Science and Engineering Centers program
(DMR-0079992), and the Cornell Chemistry Department X-ray
Supporting Information Available: Polymer analysis data
(MS/NMR spectra) and crystal structure data for [(BDI-2)Zn-
OiPr]2, [(BDI-3)ZnOiPr]2, rac-(BDI-1)ZnOCHMeCO2Me, and
[(BDI-1)MgOiPr]2 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA003851F