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Apr 19, 2013 - Lactide Cyclopolymerization Kinetics, X‑ray Structure, and Solution. Dynamics of ( t. Bu-SalAmEE)Al and a Cautionary Tale Of Polymeta...
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Lactide Cyclopolymerization Kinetics, X‑ray Structure, and Solution Dynamics of (tBu-SalAmEE)Al and a Cautionary Tale Of Polymetalate Formation Solomon H. Reisberg,§ Harry J. Hurley,§ Robert T. Mathers,‡ Joseph M. Tanski,† and Yutan D. Y. L. Getzler*,§ §

Department of Chemistry, Kenyon College, Gambier, Ohio 43022, United States Deparment of Chemistry, The Pennsylvania State University, New Kensington, Pennsylvania 15068, United States † Department of Chemistry, Vassar College, 124 Raymond Avenue, Box 601, Poughkeepsie, New York 12604, United States ‡

S Supporting Information *

ABSTRACT: The complex (tBu-SalAmEE)Al (tBu-SalAmEEH3 = N , N - b i s ( 3 , 5 - d i - t e r t - bu t y l - 2 - h y d r o x y b e n z y l ) - 2 - ( 2 aminoethoxy)ethanol, 1) catalyzes the ring-expansion polymerization of lactide to form cyclic poly(lactide) (cPLA). The X-ray structure of 1 was determined, its polymerization kinetics were examined and its interactions with Lewis bases were observed. The data from these experiments are consistent with a coordination−insertion mechanism whose rate-determining step is catalyst rearrangement by loss of a hemilabile, datively bound, bridging ligand ether. cPLA was examined by thermogravimetric analysis and found more stable than its linear counterpart. In the course of these studies, we unexpectedly observed the formation of polymetalate (AlMe(tBu-SalAmEE)AlMe2)2 (6), which was characterized (X-ray, EA, and 1H and 13C NMR). as lactide polymerization catalysts.38,44−46 Jones and co-workers noted the formation of a dinuclear salalen complex when the metalation reaction was conducted at a concentration below ∼60 mM, even with only one equivalent of AlMe3.44 More commonly, 2 equiv of AlMe3 are reported for dinuclear metalate synthesis.38,45,46A mixture of species is reported at intermediate stoichiometries38,46 and concentrations.44 In most cases, the authors clearly intended to form the mononucleate, but found the dinuclear species through careful observation.38,44,46 Where tested, the binuclear complexes initiated lactide polymerization, in some cases achieving comparable activities under analogous reaction conditions. The ubiquity of salen and salan ligands as mononucleates20,25,31,39 in lactide polymerization catalysis, makes their use by Lin and Ma, respectively, to form dinucleates active for lactide polymerization especially notable. It is noteworthy that higher order nucleates from salen ligands and AlMe3 are well documented.47,48 We recently disclosed the synthesis of cyclic poly(lactic acid) (cPLA) using (N,N-bis(3,5-di-tert-butyl-2-benzyloxy)-2-(2aminoethoxy)ethoxy)aluminum [(tBu-SalAmEE)Al, 1],49 a homogeneous, five-coordinate aluminum complex and one of

I. INTRODUCTION A five-coordinate aluminum compound seems ideally suited for acid catalysis as its coordinative unsaturation may allow for substrate binding while its high coordination number may sterically shield against the aggregation common to aluminum complexes.1 Unsurprisingly, homogeneous five-coordinate aluminum complexes have been implicated in diverse catalytic reactions. Most of these complexes are made with tetradentate ligands, presumably because chelation allows greater control of catalyst sterics and maintains ligand attachment to the metal. Homogenous five-coordinate aluminum complexes act as Lewis acids in a broad array of catalytic transformations.2−17 Of particular interest to this work, five-coordinate aluminum complexes appear widely in lactide polymerization, operating as Lewis acids in a coordination−insertion mechanism.18−24 The profusion of aluminum complexes has been supported by the commercial availability of easily handled solutions of aluminum alkyls. Metalation protocols most commonly add an equivalent25−36 or slight excess37,38 of aluminum alkyl to a solution of the proligand. Other aluminum precursors, including alkoxides,33,39,40 amides,41 halides,42 and alkyl halides,26,33,43 are also used for metalation. However, aluminum alkyls are buttressed as the precursors of first resort because the generation of alkane gas obviates an explicit byproduct removal step. Several recent reports using aluminum alkyls for metalation have revealed dinuclear complexes and their activity © 2013 American Chemical Society

Received: January 7, 2013 Revised: March 29, 2013 Published: April 19, 2013 3273

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generate cPLA and regenerate the active catalyst (1). The reaction can thus be represented by eq 1.

only three catalyst systems capable of achieving high molecular weight cPLA.50−53 Catalyst design was guided by three ideas. First, the initiating alkoxide should be permanently tethered to the catalyst, as illustrated by the Grubbs system for ringexpansion polymerization (REP) of octene.54 Second, to combat the frequently irreversible aggregation common to alumatranes and their derivatives,19,55,56 the ligand should be pentadentate. Finally, to make a catalytic site available and potentially increase Lewis acidity while preventing aggregation, the ligand should contain a hemilabile component. In this report we confirm the proposed monomeric structure of (tBuSalAmEE)Al using X-ray crystallography. Furthermore, kinetic studies show the REP of lactide is zero-order in monomer and first-order in catalyst, consistent with a structural change in the catalyst such as loss of a hemilabile component. Changes in the chemical shifts of ligand resonances in the presence of several Lewis bases are also consistent with ligand hemilability, as is crystallographic data (vide inf ra). In the course of these investigations, we unexpectedly discovered a polymetalated dimer, which is also active for lactide polymerization and has been characterized by X-ray crystallography.

kobs = k[1]a [lactide]b

(1) 1

Monomer conversion by H NMR was used as an indirect measure of kobs. The polymerizations were run to constant reaction time and at constant temperature while catalyst or monomer concentration was independently varied from 7.1 to 23 mM and from 0.283 to 1.11 M, respectively. These ranges provided reproducible measurements at low conversion and avoiding viscosity induced deviations from linearity at high conversion. Regression analyses of the data reveal linearity in both cases (Figure 1), showing that the ring-expansion

II. RESULTS AND DISCUSSION (tBu-SalAmEE)Al Lactide Polymerization Kinetics. Previous work showed that 1 was active for the REP of lactide to form high molecular weight cPLA. We found linear increases in cPLA molecular weight with both reaction time and catalyst loading along with generally low polydispersity indices, consistent with a well-defined catalyst system, and proposed a simple polymerization mechanism based on the coordination− insertion paradigm.49 In a more detailed proposed mechanism (Scheme 1), the catalyst (1) is presumed to engage in a Lewis acidic activation of the monomer carbonyl (2) toward nucleophilic attack by the pendant ligand alkoxide (3). Repeated monomer insertion generates macrometallacycle 4, which in turn undergoes intramolecular chain transfer to

Figure 1. Plots of log(percent lactide conversion), by 1H NMR, vs log([1]) and log([lactide]), toluene, 130 °C, 3 h.

polymerization of lactide by 1 is first order in catalyst and zero order in monomer. Regression analysis of lactide conversion vs [1] yields an intercept of zero (Supporting Information). We also determined the activation energy (Ea = 90 ± 10 KJ/mol) and frequency factor (A = 5.1 × 1010), which were within the range of those previously determined for lactide polymerizations (Supporting Information).57 These kinetic data are consistent with intramolecular catalyst rearrangement to allow substrate binding. As such, the ratedetermining step (rds) would be an intermediate between 1 and 2 (Scheme 1) and could take two forms that would preserve the observed high symmetry (Scheme 2). Loss of the datively bound ether oxygen would give tetrahedral complex 1a,

Scheme 1. Proposed Mechanism for the Ring-Expansion Polymerization of Lactide by 1

Scheme 2. Possible Rearrangements of 1 Consistent with Observed Kinetics and Symmetry

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1, entries 1−5), heating the solution decreased the magnitude of Δδ, consistent with the equilibrium shifting back toward free 1 and L: (Supporting Information). These data do not allow an unambiguous distinction between the putative compounds 5a and 5b (Table 1). However, the Δδ seen for resonances adjacent to the tBu-SalAmEE nitrogen (b and e) but not for the alkoxide-adjacent resonance (a) best matches proposed compound 5b, as electron density could flow from the bound base to the nitrogen through an aluminum p orbital. Furthermore, where Δδ was observed, the magnitude was most commonly greatest for resonances c and d. These data are also most consistent with proposed compound 5b, where breaking the dative Al−O bond would be expected to significantly change the magnetic environment of proximate atoms. X-ray Structure of (tBu-SalAmEE)Al. X-ray quality crystals of 1 were isolated from a slowly cooled solution of 1 in a toluene/hexane mixture. Crystallography revealed two molecules in the asymmetric unit, both monomers as initially proposed (Figure 2), establishing 1 as the first five-coordinate

with a lower coordination number and presumably higher Lewis acidity, while opening the angle between the pseudoequatorial arenes would give square pyramidal complex 1b with less steric crowding at the putative site for monomer coordination. While 27Al NMR is ideally suited for determination of coordination number, the limited solubility of 1 prevented the use of 27Al NMR as a tool for even ground state structure determination. However, if catalyst rearrangement is the rds, the resting state of the catalyst should be 1. Lewis Base-(tBu-SalAmEE)Al Interaction. To determine if (tBu-SalAmEE)Al was capable of behaving as a Lewis acid, we set out to systematically investigate its interaction with a diversity of Lewis bases (L:), including lactide. Regardless of the base, only four resonances displayed significant shifts. Of the many possible modes of base coordination to 1, we envision two with the high symmetry capable of producing the observed 1 H NMR spectra (Supporting Information), tentatively proposed as 5a or 5b (Table 1). A series of 35 mM solutions of 1 in C6D6 were prepared, different bases were added and changes in the 1H chemical shifts (Δδ) of tBu-SalAmEE resonances were observed. Table 1. Interactions of Lewis Bases with 1, by 1H NMR.a

entry

Lewis base

Δb (ppm)

Δc (ppm)

Δd (ppm)

Δe (ppm)

1 2 3 4 5 6 7 8 9 10

γ-butyrolactone Et2O cyclopentanone acetone MeCN (±)-lactide THF pyridine t BuOMe DME

b 0.11 0.06 0.05 0.08 0.01 0.01 0.01 0.00 b

0.19 0.16 0.08 0.09 0.07 0.00 0.02 0.02 0.01 b

0.33 0.27 0.09 0.18 0.05 0.01 0.02 0.02 0.02 0.02

0.18 0.15 0.05 0.10 0.03 0.00 0.01 0.01 0.01 0.02

Figure 2. ORTEP of 1 at the 50% probability level. Hydrogen atoms are omitted for clarity.

N2, 5 mg 1 in 250 μL of C6D6, 1 drop of L.; 26 °C. bResonances masked by L.

a

group 13 compound with a fully coordinated pentadentate ligand.58 Bond angle analysis (Table 2) of aluminum’s

Added lactide induced no Δδ in the 1H NMR spectrum (Table 1, entry 6). Raising the temperature to 80 °C resulted in slow production of polymer, but no evidence of lactide coordination, thus implicating catalyst rearrangement as the rds in the REP of lactide by 1. Many bases did induce a significant Δδ in the 1H NMR spectrum of 1, consistent with the proposal that 1 can behave as a Lewis acid (Table 1, entries 1−5). The largest changes were observed with γ-butyrolactone (GBL) (Table 1, entry 1). For GBL itself, the largest Δδ was observed for the α-protons, consistent with GBL-1 coordination via the carbonyl oxygen. Significant Δδs were also observed with the addition of cyclopentanone, acetone and acetonitrile, all sterically uncongested bases. The large shifts caused by Et2O (Table 1, entry 2) were a surprise, especially when compared with THF, which caused no significant change (Table 1, entry 7). Considering the negligible differences in pKa between the THF and Et2O, and the former’s smaller ligand steric profile, we have no satisfying explanation for this curious observation. Other acyclic ethers such as DME or tert-butyl methyl ether show no evidence of coordination to 1. For all Lewis bases that showed evidence of coordination to 1 (Table

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for Compound 1 Bond Lengths Al(1)−O(11) Al(1)−O(12) Al(1)−N(1) Al(1)−O(14) Al(1)−O(13)

1.7483(18) Al(2)−O(21) 1.7478(18) Al(2)−O(22) 2.112(2) Al(2)−N(2) 1.9700(19) Al(2)−O(24) 1.7806(18) Al(2)−O(23) Bond Angles

O(11)−Al(1)−O(12) O(11)−Al(1)−O(13) O(12)−Al(1)−O(13) O(11)−Al(1)−N(1) O(12)−Al(1)−N(1) O(14)−Al(1)−N(1) O(13)−Al(1)−O(14) O(13)−Al(1)−N(1) O(12)−Al(1)−O(14) O(11)−Al(1)−O(14) 3275

119.30(9) 105.11(9) 93.94(8) 92.35(8) 90.14(8) 77.92(8) 82.64(8) 157.11(9) 131.84(9) 107.79(9)

O(21)−Al(2)−O(22) O(21)−Al(2)−O(23) O(22)−Al(2)−O(23) O(21)−Al(2)−N(2) O(22)−Al(2)−N(2) O(24)−Al(2)−N(2) O(23)−Al(2)−O(24) O(23)−Al(2)−N(2) O(22)−Al(2)−O(24) O(21)−Al(2)−O(24)

1.7474(19) 1.7602(18) 2.053(2) 2.128(2) 1.740(2) 106.84(9) 121.32(11) 94.18(9) 98.34(9) 92.93(8) 77.97(8) 80.02(9) 135.32(11) 159.16(10) 93.12(9)

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Figure 3. ORTEP of 6 at the 50% probability level. Hydrogen atoms and solvent are omitted for clarity.

geometry using Addison’s method shows that, in the solid state, 1 exists near the midpoint between trigonal bipyramidal and square pyramidal (Al1: τ = 0.42; Al2: τ = 0.40).59 The similarity of these values should not mask the differences between the two independent molecules. For example, the largest bond angles in each molecule involves different ligand atoms, alkoxide and nitrogen in one case (O(13)−Al(1)−N(1) = 157.11(9)) and phenol and ether in the other (O(22)−Al(2)− O(24) = 159.16(10)). Bond lengths are generally unremarkable, although substantial variation between the two independent molecules is again noted. In each of the independent molecules the longest Al−O bond is to the bridging ligand ether (Al(1)−O(14) = 1.9700(19); Al(2)−O(24) = 2.128(2)), consistent with this being the most labile of the ligand oxygens. Finally, the latter bond is substantially longer than dative Al−O distances in comparable molecules.19,60−65 These data are consistent with the proposed hemilability of the ligand ether and VT-NMR data showing multiple stable conformers for 1 at 26 °C that average to trigonal bipyramidal at 80 °C (Supporting Information). Polymetalate Discovery and Structure. In the course of this investigation we noticed the 1H NMR spectrum of the ligand metalation reaction showing changes in successive syntheses, despite the use of an established protocol. The presence of resonances below 0 ppm indicated a metal alkyl, leading us to determine solvent evaporation from our stock solution of AlMe3 in heptane had increased its concentration giving an unintended excess of AlMe3 during metalation. An intentional gross excess of AlMe3 allowed isolation of X-ray quality crystals of the dimeric polymetalate 6 (Figure 3). One aluminum (Al(1)) is four coordinate in a pseudotetrahedral geometry, with deviations from ideal bond angles of no more than 10° while the other (Al(2)) is five coordinate with substantial bond-angle deformation placing it midway between trigonal bipyramidal and square pyramidal (Al(2): τ = 0.49).59 Bond lengths are unremarkable (Table 3). Polymerization Activity and Polymer Characterization. As could be expected of an aluminum alkoxide, lactide is polymerized in the presence of 6. The polymer formed is clearly distinct from that produced by 1, including a significant early eluting peak and decidedly non-Gaussian peak profiles, as seen by GPC (Figure 4). The polymer produced by 6 is also distinct for the broadened molecular weights distribution of the high molecular weight fraction, which represents a significant percent of the polymer produced (Table 4). Previously, Waymouth et al. postulated that cPLA was more thermally stable than linear analogues based on isothermal TGA data at 225 °C.50 After examining a series of linear and cyclic PLA’s with TGA over a broad temperature range (25−

Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for Compound 6 Bond Lengths Al(1)−O(1) Al(1)−O(2) Al(1)-N Al(2)−O(3) Al(2)−O(4)

1.7407(9) Al(1)−C(37) 1.7423(9) Al(2)−C(35) 2.0055(11) Al(2)−C(36) 2.2552(9) Al(2)−O(4)i 1.8403(10) Al(2)−Al(2)i Bond Angles

O(1)−Al(1)−O(2) O(1)−Al(1)−C(37) O(2)−Al(1)−C(37) O(4)−Al(2)−O(3) O(3)−Al(2)−O(4)i C(35)−Al(2)−O(4)i C(36)−Al(2)−O(4) C(35)−Al(2)−C(36) C(35)−Al(2)−O(4)i

114.24(5) 117.62(5) 109.80(5) 75.32(4) 151.12(4) 122.01(6) 115.27(6) 121.24(6) 101.59(6)

O(1)−Al(1)−N O(2)−Al(1)−N C(37)−Al(1)−N C(36)−Al(2)−O(4)i C(35)−Al(2)−O(3) C(36)−Al(2)−O(3) O(4)−Al(2)−O(4)i Al(2)−O(4)- Al(2)i

1.9511(13) 1.9593(14) 1.9673(15) 1.8945(9) 2.9383(7) 98.04(4) 99.01 116.30(5) 103.36(5) 89.21(5) 93.31(5) 76.25(4) 103.75(4)

Figure 4. GPC trace of polymer produced in the presence of 1 (solid orange) or 6 (dotted black), detected by viscometry.

450 °C), the onset of decomposition and the maximum for the first derivative of weight loss indicated cPLA has better thermal stability. The additional thermal stability afforded by complex 1 correlates reasonably well to the intrinsic viscosity of the polyester (Figure 5). Although the presence of Sn(Oct)2 has been shown to promote decomposition of linear PLA,66 the variation in Td for a range of [lactide/Sn(Oct)2] values (10:1− 500:1) was minor compared to the larger difference in thermal stability observed for linear and cPLA. A control experiment involving intensive removal of Sn (II) from a linear PLA sample also confirmed the substantial differences seen in Figure 5 (Supporting Information). 3276

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Table 4. Characterization of PLA from 1 or 6.a entry 1 2

catalyst 1 6

[monomer]/ [catalyst] 190 200

convn (%)b 81 68

low Mw (%)c >99.6 94.1

low Mw (kg/mol)d

low Mw/Mnd

high Mw (kg/mol)d

24 19

1.19 1.08

418

e

f

high Mw/Mnd f 1.42

a Polymerizations conducted under N2, 130 °C, 1.5 h. bConversion calculated from the 1H NMR spectrum of the crude reaction mixture. cLow Mw is the peak eluting from 12 to 16 mL; high Mw is the peak elutingfrom 6 to 12 mL. dWeight-average (Mw) molecular weights and molecular weight distributions (Mw/Mn) were calculated by GPC using light scattering in THF. eRemaining 24 h. Teflon lined caps were purchased from Qorpak. NMR was performed on a Bruker Unity 300 NMR, processed in SpinWorks 2.1 or iNMR 2.0 and referenced to solvent residual peaks. NMR solvents were purchased from Cambridge Isotope Laboratories. Polymer molecular weight values were measured by gel-permeation chromatography (GPC) at 35 °C. The two columns (Polymer Lab, Mixed C) were eluted with tetrahydrofuran (THF) (1.0 mL/min). Polymer solutions (200 μL) in THF were filtered and injected at a concentration of 5−8 mg/mL. The GPC system was equipped with a three-angle Wyatt MiniDawn light scattering detector (λ = 690 nm, 30 mW Ga−As laser), a Wyatt ViscoStar viscometer and a Wyatt Optilab rEX differential refractive index detector (λ = 690 nm). The light scattering detector was calibrated with the known Rayleigh ratio for toluene. The specific refractive increments (dn/dc) were calculated with Wyatt Technology’s Astra V software assuming 100% mass recovery. The thermal stability was determined with a TA Instruments TGA Q500 at 20 °C/min under nitrogen. The decomposition temperatures (Td) were obtained from the maximum of the first derivative of weight loss. X-ray Crystallography. Single crystals suitable for X-ray analysis were obtained using the following procedures: Compound 1 was recrystallized from a minimum of boiling hexanes; compound 6 was isolated from the crude evaporated product when 3 eq. of AlMe3 was used in metalation. X-ray diffraction data were collected on a Bruker APEX 2 CCD platform diffractometer (Mo Kα (λ = 0.71073 Å)) equipped with an Oxford liquid nitrogen cryostream. Crystals were mounted in a nylon loop with Paratone-N cryoprotectant oil. The structures were solved using direct methods and standard difference map techniques, and were refined by full-matrix least-squares procedures on F2 with SHELXTL (Version 6.14).68 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon were included in calculated positions and were refined using a riding model. Crystal data and refinement details are presented in the Supporting Information for all species while selected bond distances and angles are listed in Tables 2 and 3. Images for publication were rendered using Mercury 2.4.69 Kinetics Polymerizations. In a representative reaction, in a glovebox, a 1 dram vial fitted with a stir bar was charged with 1 (3.3 mg, 0.0058 mmol, 1.0 equiv), racemic lactide (56.1 mg, 0.389 mmol, 67 equiv), and toluene (0.58 mL). The vial was sealed with a Teflon-lined cap and placed in a preheated aluminum block (130 °C), stirred for 3 h, and cold-quenched in a 3 °C refrigerator. The thoroughly cooled reaction was warmed to room temperature and solvent was evaporated under a stream of air, leaving the polymer as a white solid. Percent conversion was determined from the 1H NMR spectrum, and was calculated from the average of the integration ratio of both pairs of monomer/polymer resonances. 1H NMR (300 MHz, CDCl3: δ 1.524 (m, 3H, polymer), 1.618 (d, J = 6.6 Hz, 3H, monomer), 5.017 (quartet, J = 6.6, 13.5 Hz, 1H, monomer), (5.131, m, 1H, polymer). (N,N-Bis(3,5-di-tert-butyl-2-benzyloxy)-2-(2-aminoethoxy)ethoxy)aluminum [(tBu-SalAmEE)Al, 1]. In a glovebox, a 50 mL round-bottom flask was charged with tBu-SalAmEEH3 (136 mg, 0.251

Figure 5. Intrinsic viscosity data versus maximum of the first derivative of weight loss (Td) from TGA (20 °C/min) for a) cPLA (green ○) made with complex 1 and b) linear PLA made using Sn(Oct)2 (blue −).

III. CONCLUSION We have established important mechanistic details for the ringexpansion polymerization of lactide by 1 to form cPLA through solution polymerization kinetics, examination of the catalyst solid state structure and the study of Lewis base coordination. In particular, these data are consistent with loss of the hemilabile, datively bound ligand ether as the rds. As such, decreasing the Lewis basicity of this oxygen should increase reaction rate. This line of research is currently under investigation in our laboratory. Thermogravimetric analysis revealed greater stability for cPLA relative to linear PLA, consistent with prior work. As with others in the polymer synthesis community, we were initially surprised by the formation of a polymetalate from what we presumed was a mononucleating ligand. This surprise was significantly dampened upon examination of the substantial literature precedence for the formation of higher order nucleates. We hope this report, in combination with other recent publications, will raise awareness in the polymer community of the possibility of polymetalate formation when using aluminum alkyls for metalation. IV. EXPERIMENTAL SECTION General Considerations. Standard air-free techniques were performed under N2 using an mBraun Unilab glovebox and Schlenk lines powered by Edwards RV12 pumps. Reaction temperatures were maintained by IKA C-MAG stirring hot plates controlled by ETS-D5 electronic contact thermometers. All air-free glassware was made by Quark Inc. using Kontes valves. Unless otherwise specified, purchased materials were used without purification, with anhydrous materials first being brought into the glovebox or cannulated into a Strauss bomb. Racemic lactide (Acros) was twice recrystallized from anhydrous toluene and then sublimed. Anhydrous chemicals were purchased as dry, under nitrogen (toluene, hexanes, THF, acetone, acetonitrile and pyridine from Acros; DME from Sigma) and used as received or dried over 20% w/v activated 3 Å molecular sieves,67 distilled and degassed by freeze/pump/thaw cycles (Et2O from Pharmco-AAPER; GBL from Sigma; cyclopentanone from Acros). The ligand (tBu-SalAmEEH3) was prepared according to the literature procedure.49 AlMe3 (2.0 M in 3277

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mmol, 1.0 equiv) and Et2O (∼25 mL). Dropwise addition of AlMe3 (2.0 M in toluene, 0.13 mL, 0.26 mmol, 1.0 equiv) to the vigorously stirred solution induced ebullition. The mixture was stirred at room temperature for 1 h and solvent was removed in vacuo yielding a sticky yellow solid. On a Schlenk line, the solid was dissolved in a minimum of boiling toluene (∼2 mL) and cooled slowly to 0 °C to give a white precipitate. The mother liquor was decanted via cannula, the solid was washed with cold toluene (2 × ∼1 mL) and dried in vacuo to yield the title compound (109 mg, 77%) as a white solid, pure by 1H NMR49 and suitable for polymerization experiments. [(AlMe(tBu-SalAmEE)AlMe2)2, 6]. In a glovebox, a 50 mL roundbottom flask was charged with tBu-SalAmEEH3 (137 mg, 0.253 mmol, 1.0 equiv) and Et2O (∼25 mL). Dropwise addition of AlMe3 (2.0 M in toluene, 0.26 mL, 0.52 mmol, 2.1 equiv) to the vigorously stirred solution induced ebullition. The mixture was stirred at room temperature for 1 h and solvent was removed in vacuo. The resultant white solid was washed with pentane (3 × ∼1 mL) and dried in vacuo to yield the title compound (141 mg, 85%) as a white solid suitable for polymerization experiments. Material was prepared for elemental analysis by dissolving crude product in a minimum of freshly distilled pentane, removing solvent in vacuo until a precipitate formed, decanting the mother liquor by pipet and drying the resultant white solid in vacuo. 1H NMR (300 MHz, C6D6): δ 7.59 (4H), 6.83 (4H), 3.72 (4H), 3.32 (12H), 2.74 (8H), 1.70 (36H), 1.41 (36H), −0.25 (6H), −0.60 (12H). 13C NMR (75 MHz, C6D6): δ 155.6, 140.4, 139.0, 125.3, 124.7, 121.1, 70.1, 64.8, 59.8, 58.0, 52.5, 35.5, 34.4, 32.1, 30.0, −10.1, −14.2. Anal. Calcd for C74H122Al4N2O8 (%): C, 69.67; H, 9.64; Al, 8.46; N, 2.20. Found (avg. of two trials) (%): C, 69.53; H, 9.44; Al, 9.21; N, 3.75.

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ASSOCIATED CONTENT

S Supporting Information *

Lactide polymerization kinetics data, 1H and 13C NMR spectra of 6, data for Δδ as a function of temperature for Lewis base coordination, polymerization protocols and crystallographic data (.cif file) for 1 and 6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (Y.D.Y.L.G.)[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.T.M. acknowledges the Alcoa Foundation for funding Grant NSF-0521237. J.M.T. and P.I. for the X-ray diffraction facility at Vassar.

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REFERENCES

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