Ligand-Free Magnesium Catalyst System: Immortal Polymerization

Article pubs.acs.org/Macromolecules

Ligand-Free Magnesium Catalyst System: Immortal Polymerization of L‑Lactide with High Catalyst Efficiency and Structure of Active Intermediates Yang Wang,†,‡ Wei Zhao,†,‡ Xinli Liu,† Dongmei Cui,*,† and Eugene Y.-X. Chen*,§ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China § Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States S Supporting Information *

ABSTRACT: A simple, inexpensive, and convenient catalyst system consisting of supporting ligand-free MgnBu2 in combination with an alcohol, isopropanol (iPrOH), benzyl methanol (PhCH2OH), diphenylmethanol (Ph2CHOH), or triphenylmethanol (Ph3COH), generates a convenient catalyst system to promote the polymerization of L-LA. In particular, the binary system MgnBu2/Ph2CHOH demonstrates an unprecedentedly high activity in the presence of a large excess amount of Ph2CHOH with the [OH]0/[Mg]0 ratio varying from 2 to 500, producing up to 500 polylactide (PLA) chains per Mg center and thus showing a typical nature of immortal polymerization. The molecular weights of the obtained PLAs with a broad range of monomer-to-metal ratios ([L-LA]0/[Mg]0 = 200−5000) are rather accurately controlled by the Ph2CHOH loading, relative to [Mg]0, while the molecular weight distributions remain nearly constant with polydispersity index (PDI) = 1.08−1.18. Moreover, the active polymerization intermediate has been isolated from the stoichiometric reaction between MgnBu2 and Ph2CHOH and structurally characterized as a tetranuclear complex, Mg4(Ph2CHO)8(THF)2 (1). Complex 1 remains the tetranuclear structure in solution or in the presence of excess Ph2CHOH as determined by 2D DOSY. On the basis of structural information about the active intermediates and polymerization kinetics, a coordination−insertion polymerization mechanism is proposed.

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INTRODUCTION

drawbacks (high catalyst loading and low catalyst efficiency) found in most of the traditional coordination−insertion ROP; precise control over polymer molecular weight and PDI values; and simultaneous chain end-capping with an alkyl or hydroxyl functionality that allows for constructing block or other topological polymer-based materials in a one-pot fashion. However, an IMP does not mean forever living, which depends on the nature of CTAs2c−e and the tolerance of the catalyst toward CTAs, particularly the protic ones. Consequently, in many IMP systems, the CTA loading is restricted to only a small amount to avoid diminishing the polymerization

The immortal polymerization (IMP) was first introduced by Inoue et al. while studying the ROP of epoxides with the porphyrin−aluminum/alcohol catalytic system.1 The most remarkable characteristic of the IMP is the rapid and reversible exchange reaction between the active species and the protic compound which endows the resultant polymers with unimodal and narrow molecular weight distributions and as well as controlled molecular weights (by the ratio of monomer to the protic compound). Later on, the protic compound was extended to various types of Lewis acids and their functional derivatives as chain transfer agents (CTAs); accordingly, the IMP became the catalytic living chain-transfer polymerization.2,3 Several advantages of the IMP are apparent: low catalyst loading and high catalyst efficiency that overcomes the © XXXX American Chemical Society

Received: April 14, 2012 Revised: July 17, 2012

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dx.doi.org/10.1021/ma3007625 | Macromolecules XXXX, XXX, XXX−XXX

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Table 1. ROP of L-LA by MgnBu2 in Combination with Four Different Alcohols ROH

[OH]0/[Mg]0

time (min)

convb (%)

Mn,calcdc × 10−4

Mn,expd × 10−4

Mw/Mnd

0 1 2 6 1 2 6 1 2 6 1 2

5 5 10 10 5 10 10 5 5 5 5 10

63 >99 30 15 >99 60 18 >99 >99 >99 86 27

2.73

PrOH i PrOH i PrOH PhCH2OH PhCH2OH PhCH2OH Ph2CHOH Ph2CHOH Ph2CHOH Ph3COH Ph3COH

2.10 6.60/1.67 1.03 0.25 6.83/1.69 2.31 0.30 6.52/1.41 3.97 1.27 6.87/1.58 1.01

1.77 1.32/1.12 1.25 1.27 1.22/1.18 1.66 1.31 1.32/1.24 1.56 1.31 1.53/1.23 1.47

entrya 1 2 3 4 5 6 7 8 9 10 11 12

i

1.30 0.22 2.60 0.27 4.30 1.45 1.19

Polymerizations were carried out in THF at 25 °C, [LA]0 = 2.08 M, [LA]0/[Mg]0 = 600. Determined by 1H NMR spectroscopy. cWhen [ROH]0/ [Mg]0 = 0, Mn,calcd = 1/2 × 600 × 144.13 × conv (%) + 71.15(−nBu); when [ROH]0/[Mg]0 ≥ 2, Mn,calcd = ([LA]0/[ROH]0) × 144.13 × conv (%) + MROH. dDetermined by size exclusion chromatography (SEC) against polystyrene standard. Mn values were obtained using a correcting factor for polylactides (0.58).27 a

b

large-scale PLA production and minimizes metal residue in the polymer product. The obtained PLAs possess tunable molecular weights and narrow molecular weight distributions. Especially, the polymerization active intermediate was isolated and structurally characterized as a homoleptic magnesium alkoxide. Although tin homoleptic complexes,18 aluminum alkoxides,19 calcium alkoxides,20 zinc lactates,21 and yttrium22 as well as lanthanide homoleptic alkoxides22a,c,e,23 have been synthesized using a similar procedure; tin octates have been applied in industry to produce PLA materials, few of them have defined molecular structures, and many of them have complicated cluster structures such as Y5(μ-O)(OiPr)1324 and Fe5(μ-O)(OEt)1325 or exist in equilibrium forms such as Al(OiPr)3 and its multinuclear derivatives.19 Consequently, no such complexes were investigated to catalyze immortal polymerization of LA in the presence of a large amount of protic compounds due to their low activity and propensity to be multisite under such conditions.

rate and broadening the polymer molecular weight distribution. In general, the protic CTAs are poisons to the typical electrophilic metal catalysts, thus requiring bulky supporting ligands to provide steric shielding to the metal centers. This general strategy has been utilized to develop various metal complexes to effect the ROP of cyclic esters such as βbutyrolactone,4 ε-caprolactone,5 lactide,4a,5g,h,6 and cyclic carbonates.7 A large number of bulky ligand-supported metal complexes of magnesium,5g,h,8 aluminum,6e,9 calcium,8a,c zinc,8,9b,10 and rare-earth metals4a,6a,c,d,f,11,12 have been reported to be efficient, some of which are also living, toward the ROP of LA, but only limited success has been achieved for the immortal ROP of LA. Success in the development of the efficient immortal ROP of LA will allow for the production of the biodegradable and biocompatible PLA materials that have found applications in biomedical and pharmaceutical industries and tissue engineering,13 in a highly catalytic fashion so that both the metal catalyst cost for large-volume manufacturing of PLA and metal residue in the resulting polymer materials can be minimized. To this end, Carpentier et al. reported the use of an alkoxy-amino-bis(phenolate) (a Salan ligand) yttrium complexes to initiate the stereocontrolled immortal ROP of rac-LA in the presence of 50 equiv of alcohol.14 We found that amino-amino-bis(phenolate) (a Salan ligand) yttrium alkyl complexes in combination with triethoxyamine (TEA) initiate the heteroselective immortal ROP of rac-LA with the TEA loading up to 81 equiv per yttrium metal.15 Recently, a welldefined cationic amino-ether phenolate zinc complex and amino-ether fluoroalkoxide ligated alkaline-earth metal complexes were also reported to show immortal behavior for the ROP of LA with the highest 50 equiv of alcohol loading per metal at high temperature via the activated monomer mechanism.16 Despite their IMP utilities, these complexes require delicate, complicated supporting ligands, the synthesis of which may reduce their practice application values. Herein, we report an immortal LA polymerization catalyst system based on a simple, supporting ligand-free, commercially available dibutylmagnesium precursor, Mgn Bu 2 (Mg2+ participates in human metabolism17), which can take up to 500 equiv of Ph2CHOH alcohol (i.e., up to 500 PLA chains produced per metal center), thereby rendering extremely high catalyst efficiency; such high catalyst efficiency significantly lowers the catalyst cost in the

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RESULTS AND DISCUSSION Immortal Polymerization of Lactide. MgnBu2 alone can initiate the ROP of lactide but in an uncontrolled manner. The resultant PLAs had molecular weights higher than the theoretical values, coupled with very broad molecular weight distributions, due to transesterification (Table 1, entry 1).26 As metal alkoxides, which are usually presumed to be the true propagating active species in the polymerization of LA, exhibit superior performances to their alkyl analogues, alcohols are typically chosen to combine with metal alkyl precursors to generate in situ the metal alkoxide initiators via alcoholysis.5a,6a,e Accordingly, alcohols with different steric bulk and acidity, including iPrOH, PhCH2OH, Ph2CHOH, and Ph3COH, were employed to activate MgnBu2. Addition of an equimolar amount of such alcohols to MgnBu2 did not bring about improvement in polymerization control (Table 1, entries 2, 5, 8, 11); the SEC (size exclusion chromatography) traces of the isolated PLA samples were broad and bimodal (Figure 1a), indicating the presence of at least two initiating species Mg−C/ Mg−O in the system as a result of partial alcoholysis (Figure S1). Improvement was achieved by increasing the loading of the alcohol iPrOH, PhCH2OH, or Ph3COH to 2 equiv, but unfortunately the polymerization became sluggish when the amount of the alcohol reached 6 equiv (Table 1, entries 3, 4, 6, B


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Table 2. Ring-Opening Polymerization of L-Lactide Catalyzed by MgnBu2/Ph2CHOH

Figure 1. SEC (size exclusion chromatography at 40 °C using THF as the eluent with a flowing rate of 0.35 mL/min) traces of the resulting PLAs under various ratios of [LA]0:[OH]0:[Mg]0. (a) [LA]0:[OH]0: [Mg]0 = 600:1:1 (Table 1, entries 2, 5, 8, 11); (b) [LA]0: [Ph2CHOH]0:[Mg]0 = 1000:10:1, 1000:40:1, and 1000:100:1 (Table 2, entries 3, 5, 8).

entrya

[LA]0/ [Mg]0

[OH]0/ [Mg]0

time (min)

convb (%)

Mn,calcdc × 10−4

Mn,expd × 10−4

Mw/Mnd

1e 2e 3 4 5 6 7 8 9 10 11 12 13f 14f 15g

200 1000 1000 1000 1000 1000 1000 1000 1500 2000 2000 3000 5000 5000 1000

2 5 10 20 40 50 70 100 150 10 200 300 10 500 10

5 5 5 5 10 15 20 30 30 20 30 40 60 60 5

>99 >99 >99 >99 >99 >99 >99 >99 95 94 93 98 79 91 >99

1.46 2.90 1.45 0.73 0.38 0.30 0.22 0.16 0.16 2.73 0.15 0.16 5.71 0.15 1.46

1.26 3.12 1.41 0.82 0.47 0.33 0.28 0.16 0.22 2.73 0.19 0.22 4.04 0.17 0.88

1.15 1.07 1.09 1.08 1.09 1.09 1.09 1.08 1.14 1.06 1.15 1.18 1.03 1.12 1.12

Polymerizations were carried out in THF at 25 °C, [LA]0 = 1.28 M. Determined by 1H NMR spectroscopy. cMn,calcd = ([LA]0/[OH]0) × 144.13 × conv.(%) + 184.23. dDetermined by size exclusion chromatography (SEC) against polystyrene standard. Mn values were obtained using a correcting factor for polylactides (0.58).27 e[LA]0 = 0.64 M. fPolymerizations were carried out in THF at 70 °C. grac-LA as the monomer. a b

7, and 12). On the other hand, the behavior of Ph2CHOH, the steric bulk of which is between those of PhCH2OH and Ph3COH, caught our attention. When the [Ph2CHOH]0/ [Mg]0 ratio was below 6, the polymerization behavior was similar to those observed with iPrOH, PhCH2OH, and Ph3COH, affording PLA with molecular weights higher than the theoretical values, but dif ferently and signif icantly; regardless of the Ph2CHOH-to-Mg ratio employed, the polymerization rate was not affected (Table 1, entries 9 and 10). The deviation of the molecular weight could be attributed to the high viscosity of the polymerization system that suppresses the monomer diffusion, a result of high rate of polymerization and high molecular weight of polymer under the conditions of high monomer concentration ([LA]0 = 2.08 M) and low alcohol loading. Thus, we performed the polymerization under dilute conditions ([LA]0 = 0.64 M), which, to our delight, became controlled (Table 2, entries 1 and 2). On the basis of the above results, a higher loading of Ph2CHOH was added to the polymerization system. When the [Ph2CHOH]0/[Mg]0 ratio was 10 and above (Table 2, entries 3−12), the resultant PLAs had molecular weights very close to the theoretical values and the SEC traces of these PLAs samples were unimodal and narrow (Figure 1b), showing the characteristics of a living and immortal polymerization. Apparently, the molecular weight of PLA obtained was strongly dependent on the amount of Ph2CHOH, which decreased inversely with the increase of the Ph2CHOH loading (Figure 2; Table 2, entries 10 vs 11, 13 vs 14). Moreover, the polymerization carried out smoothly under a broad range of [Ph2CHOH]0/[Mg]0 ratios varying from 2 to 300 with high monomer loadings up to 3000 per metal to achieve high conversions within 5−40 min (Table 2, entries 3− 12), giving a high catalyst efficiency of 30 000%. Under much a higher ratio of [LA]0/[Mg]0 = 5000 and [OH]0/[Mg]0 = 500, quantitative conversion can also be reached albeit at higher temperature and longer time (70 °C, ∼60 min), achieving, to be the best of our knowledge the highest catalyst efficiency of 50 000% (Table 2, entry 14) among magnesium complexes.5g,h,8,16 When rac-LA was employed as the monomer, the result of the polymerization was quite similar to that with L-LA,

Figure 2. Dependence of molar mass Mn,SEC (PDI indicated in parentheses) on the [Ph2CHOH]0/[Mg]0 ratio. Conditions: [LA]0/ [Mg]0 = 1000, [LA]0 = 1.28 M, THF, Tp = 25 °C.

which indicated that the polymerization rate was independent of the stereochemistry of the LA monomer (Table 2, entry 15). Structure of Active Intermediate. The stoichiometric reaction between MgnBu2 and Ph2CHOH (2 equiv) was performed in a mixed solution of toluene and THF (Scheme 1). Concentration of the above solution and cooling at −35 °C over 2 days afforded the colorless crystalline solids of complex 1 in quantitative yield. NMR spectroscopic analysis in CDCl3 showed three resonances centered at 5.89, 5.70, and 5.22 ppm attributed to the methine protons of the newly formed Mg− OCHPh2 groups in complex 1 (Figure 3), which correlate with the signals in 13C NMR spectrum at 79.10, 77.88, and 77.63 ppm (Figures S2 and S3), suggesting that the Ph2CHO− groups are bound to Mg in three different coordination modes. Meanwhile, there are two THF molecules coordination to the C


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Scheme 1. Stoichiometric Reaction between MgnBu2 and Ph2CHOH (2 equiv vs [Mg]0)

Mg−O bond lengths are comparable to those reported in the literature.28 To confirm whether complex 1 remains its tetranuclear structure in the polymerization medium or not, further 1H NMR spectroscopy analysis was carried out in THFd8, which also displayed three signals at 5.89, 5.55, and 5.23 ppm for the methine protons of complex 1 (Figure S4). In addition, in the 2D DOSY spectrum (Figure S5), all the peaks on three horizontal lines correlate with chemical shifts attributed to complex 1, tetrahydrofuran, and hexane (impurity), respectivelyamong which the chemical shifts from complex 1 correlate with the log D = −8.4 × m2 s−1 line, indicating that complex 1 is stable and remains its tetranuclear structure in THF at room temperature. Hence, the solid-state structure of complex 1 is consistent with that of the solution state. Based on these results, the behavior of complex 1 in the presence of excess Ph2CHOH in CDCl3 was monitored by 1H NMR as shown in Figure 5: with the [Mg]0/[OH]0 ratio being

Figure 3. 1H NMR spectrum of complex 1 (400 MHz, CDCl3, 25 °C).

magnesium center giving resonances around 2.30 and 0.88 ppm. The molecular structure of 1, as shown in Figure 4, was confirmed by X-ray diffraction analysis. Complex 1 is

Figure 4. ORTEP diagram of the molecular structure of complex 1. Thermal ellipsoids were drawn at the 35% probability level. All of the hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Mg(1)−O(1) 1.939(6), Mg(1)−O(3) 1.964(5), Mg(1)−O(2) 1.972(6) Mg(2)−O(4) 1.856(5), Mg(2)−O(1) 1.954(6), Mg(2)−O(2) 1.969(7); O(1)−Mg(1)−O(2) 83.1(3), O(3)−Mg(1)−O(2) 115.9(2), Mg(1)−O(1)−Mg(2) 97.8(3), Mg(2)−O(2)−Mg(1) 96.2(3).

Figure 5. Reactions of complex 1 with excess diphenylmethanol in CDCl3 monitored by the 1H NMR technique (300 MHz, 25 °C).

changed from 1:1 to 1:4, the typical peak b stays at the same position while peaks a and c overlap with those from free Ph2CHOH molecules.29 Furthermore, the DOSY spectrum of complex 1 and 2 equiv of Ph2CHOH showed that there are just two diffusion coefficients which belong to complex 1 and alcohol, respectively, suggesting that 1 does not collapse into other species in the presence of Ph2CHOH (Figure S8). Furthermore, the ROP of L-LA using complex 1 directly with [LA]0/[OH]0/[Mg]0 = 1000/8/1 achieved 100% conversion within 5 min, affording PLA with Mn = 1.69 × 104 g/mol and PDI = 1.04, which is similar to the result achieved by the in situ

tetranuclear, featuring four tetracoordinated Mg2+ ions in which two adjacent Mg2+ ions are doubly bridged by the two Ph2CHO groups; it has a C2 molecular symmetry, with the inner Mg2+ ions bound to four bridging Ph2CHO fragments and the two outer Mg2+ ions bound to two bridging and one terminal Ph2CHO groups and additionally a THF molecule. All D


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Scheme 2. Proposed Mechanism for the Immortal ROP of L-LA by MgnBu2 and Excess Ph2CHOH

formation of C is evidenced by the following observations through monitoring the model polymerization in a very low ratio of [LA]0/[Ph2CHOH]0/[1]0 = 2/2/1 (Figure 6): (a) the three typical signals of the methine protons arising from the Mg−OCHPh2 fragments in complex 1 disappeared immediately; (b) a new singlet showed up at a much lower field around 6.87 ppm, attributable to the methine protons of −COOCHPh2 from the active and dormant species (a + a′); (c) the two quartets at 5.23 and 5.13 ppm arose from the methine protons b and b′, the two quartets at 4.26 and 4.17 ppm (d + d′) attributed to −CH−OMg (active) and −CH− OH (PLA), and the two doublets at 1.71 and 1.38 ppm

generated system. These results are consistent with complex 1 being the starting active intermediate. Mechanistic Pathway. Based on the above results, a possible polymerization mechanistic scenario is depicted in Scheme 2, comprising the following four key features. First, MgnBu2 reacts with excess Ph2CHOH to afford the tetranuclear intermediate active species 1. Although complex 1 is stable to excess Ph2CHOH, upon coordination of two L-LA molecules it quickly collapses into a monomeric active species (A). Subsequent insertion of the two LA molecules into the Mg− OCHPh2 bonds gives a ring-opened intermediate of eight metallocycle B, which transforms to the active species C. The E


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Figure 6. 1H NMR spectrum of the oligomeric propagating species derived from the reaction with [LA]0/[Ph2CHOH]0/[Mg]0 = 2/2/1 (400 MHz, THF-d8, 25 °C).

Figure 7. 1H NMR spectrum of PLA-20 (300 MHz, CDCl3, 25 °C).

attributed to the methyl groups (c + c′ and e + e′) from the ring-opened LA molecules; and (d) a widened peak at 4.76 ppm belongs to the OH group of the dormant PLA or free Ph2CHOH. C repeatedly adds L-LA via coordination insertion into the Mg−OPLA bonds to propagate into polymeric metal alkoxide species D. Second, rapid and reversible exchange reaction of alcohol Ph2CHOH with D gives the magnesium alkoxide propagating species (A), along with dormant hydroxylcapped polymer E. Third, the dormant E can get back into the propagation cycle after activation with (A) or other polymeric metal alkoxide propagating species F leading to D and another dormant chain G. Fourth, upon further L-LA addition and insertion, all Mg-based dialkoxide species propagate into polymeric chain H with high monomer units. Moreover, the

unimodal and narrow molecular weight distribution (PDI = 1.08−1.18) of the isolated PLA implies the exchange reactions involving the active species (A), C, D, F, or H, and the CTAs Ph2CHOH and dormant hydroxyl-ended chains E and G are reversible and much more rapid than the chain initiation and propagation steps, ensuring that the rapid growing/dormant inter conversion goes on over the entire lifetime of the polymerization process. The above coordination−insertion mechanism was further confirmed by characterization of the resulting PLA macromolecular chain ends. The resonances at 2.60 ppm are hydroxyl chain ends (if measured in THF-d8, the OH group shows at 4.76 ppm), a typical feature of the polymer obtained by an IMP; the resonance at 6.81 ppm is for the other chain ends, F


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Synthesis of Magnesium Alkoxide Mg4(Ph2CHO)8(THF)2 (1). In the glovebox, diphenylmethanol (368 mg, 2.00 mmol) in 5.00 mL of toluene−tetrahydrofuran (VTol:VTHF = 1:1) was added to a 5.00 mL toluene−tetrahydrofuran (VTol:VTHF = 1:1) solution of MgnBu2 (1.00 mL, 1.0 M in n-heptane, 1.00 mmol) in a 25 mL vial. The reaction mixture was stirred at room temperature for 2 h and concentrated to ∼2 mL; the residue was cooled to −35 °C over 2 days to afford a colorless crystalline solid (340 mg, 87.0% yield). 1H NMR (400 MHz, CDCl3, 25 °C): δ = 7.56−7.02 (m, t, 80 H, Ph−H), 5.89 (s, 2 H, Ph2CHO−), 5.70 (s, 2 H, Ph2CHO−), 5.22 (s, 4 H, Ph2CHO−), 2.30 (s, 8 H, THF−H), 0.88 (s, 8 H, THF−H). 13C NMR (100 MHz, CDCl3, 25 °C): δ = 148.38, 148.08, 147.27 (16 C, Ph−C−CH), 128.96−125.28 (80 C, Ph−CH), 79.14 (2 C, Ph2CHO−), 77.87 (4 C, Ph2CHO−), 77.63 (2 C, Ph2CHO−), 67.89 (4 C, THF−C), 24.32 (4 C, THF−C). 1H−13C H MQC (400 MHz, CDCl3, 25 °C) spectrum of complex 1 (Figure S3). 1H NMR (400 MHz, THF-d8, 25 °C): δ = 7.56−7.03 (m, t, 80 H, Ph−H), 5.89 (s, 2 H, Ph2CHO−), 5.55 (s, 4 H, Ph2CHO−), 5.23 (s, 2 H, Ph2CHO−). X-ray Diffraction Analysis. A suitable single crystal of complex 132 was sealed in a thin-walled glass capillary, and the data collection was performed at −88.5 °C on a Bruker SMART diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). The SMART program package was used to determine the unit-cell parameters. The absorption correction was applied using SADABS.33 The structure was solved by direct methods and refined on F2 by fullmatrix least-squares techniques with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were placed at calculated positions and were included in the structure calculation without further refinement of the parameters. All calculations were carried out using the SHELXS-97 program.34 The molecular structure was generated using ORTEP program.35 Typical Polymerization Procedure. A typical polymerization procedure (Table 2, entry 1) was described as follows. Under a nitrogen atmosphere a Schlenk flask was charged with a solution of MgnBu2 and Ph2CHOH, prepared previously by addition of Ph2CHOH (11.8 mg, 0.0638 mmol) to MgnBu2 (6.38 μL, 1.0 M in n-heptane, 0.006 38 mmol) in 5 mL of THF. Next, L-LA (0.92 g, 6.38 mmol, 1000 equiv) was added, and the reaction mixture was stirred vigorously at 25 °C for 5 min. After a small sample of the crude material was removed with a pipet for characterization by 1H NMR, the reaction was quenched with acidified ethanol (0.5 mL of a 1.0 M HCl solution in EtOH). The polymer was precipitated with excess ethanol (80 mL) and dried under a vacuum to a constant weight. Synthesis of Dibenzyl Ester End-Functionalized Poly(L-LA). A typical polymerization procedure was exemplified by the synthesis of PLLA-20 (the number 20 indicates the [LA]0/[Ph2CHOH]0 ratio). To a rapidly stirred THF (5 mL) solution of MgnBu2 (0.10 mL, 0.10 mmol) and Ph2CHOH (36.8 mg, 0.20 mmol) was added L-LA (0.58 g, 4.0 mmol). The reaction mixture was stirred at room temperature for 1 h. The acidified ethanol (0.5 mL of a 1.0 M HCl solution in EtOH) was added to the system to terminate the reaction. The dibenzyl ester end-functionalized PLLA sample used for NMR analysis (Figure 7) was purified by washing repeatedly with a THF/n-hexane mixture and dried under vacuum at 40 °C for 2 days (0.50 g, 86% yield).

Ph2CHOOC− groups, which are formed by LA insertion into the Mg−OCHPh2 bond, indicative of a coordination−insertion mechanism (Figure 7).13f,18e,f,19c,30 Preliminary investigation of the kinetics of the polymerization by complex 1 and Ph2CHOH showed the overall rate law of −d[ L -LA]/dt = kapp[Ph2CHOH]1.0[L-LA]2.0 (Figures S9 and S10).

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CONCLUSION In contrast to the commonly adopted strategy for catalyst design that employs electronically and sterically tailored supporting ligands for achieving controlled polymerization, through this study we have demonstrated that the ligand-free, simple, and readily available dibutylmagnesium and an appropriate alcohol (Ph2CHOH) can mediate the rapid immortal ROP of L-LA with high catalyst activity and remarkable catalyst efficiency. The choice of an alcohol with a suitable bulky substituent is crucial to establish an effective immortal catalyst system. When less steric alcohol is used, the generated active metal alkoxide species might be complicated metal alkoxide clusters with unequal initiator groups. While with more bulky alcohol, it is difficult for the monomer insertion into the crowded central metal−oxygen bond. Preliminary mechanistic study reveals that dibutylmagnesium in combination with Ph2CHOH generates the tetranuclear magnesium diphenyl methoxide as the initial active intermediate, which transforms selectively into the mononuclear active species upon insertion of LA molecules. This mononuclear active species initiates LA polymerization via an immortal coordination−insertion mechanism, producing up to 500 PLA polymer chains having the molecular weights accurately controlled by the [Ph2CHOH]0 loading relative to [Mg]0, while the molecular weight distributions remain narrow and nearly constant. This low cost and nontoxic catalytic IMP system provides a new approach toward design of catalysts for large-scale industrial production of PLA materials.

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EXPERIMENTAL SECTION

General Methods. All operations were carried out under an atmosphere of argon using standard Schlenk techniques or in a nitrogen gas filled MBraun glovebox. Solvents were reagent grade, dried by standard methods,31 and distilled under nitrogen prior to use. Toluene, tetrahydrofuran, and n-hexane were dried over Na. Deuterated NMR solvents were purchased from Cambridge Isotopes, dried over Na (for C6D6) and molecular sieve (for CDCl3), and stored in the glovebox. MgnBu2 was purchased from Sigma-Aldrich. L-Lactide (from Aladdin) was recrystallized with dry toluene and then sublimed three times under vacuum at 80 °C. Benzyl alcohol, isopropanol, biphenylmethanol, and triphenylmethanol, all purchased from Aladdin, were dried over by calcium hydride or their tetrahydrofuran solution dried over by anhydrous magnesium sulfate prior to distillation. Glassware and vials used in the polymerization were dried in an oven at 115 °C overnight and undergone the vacuum−argon cycle three times. Measurements. Organometallic samples for NMR measurements were prepared in NMR tubes and sealed with paraffin film in the glovebox. 1H and 13C NMR spectra were recorded on a Bruker AV300, a Bruker AV400, or a Bruker AV600 (FT, 300 MHz for 1H, 75 MHz for 13C, 400 MHz for 1H, 100 MHz for 13C, or 600 MHz for DOSY) spectrometer. NMR peak assignments were confirmed by the 1H−1H COSY and 1H−13C HMQC experiments when necessary. The number-average molar mass (Mn) and polydispersity index (PDI) of the polymer were measured by size exclusion chromatography (SEC) on a TOSOH HLC-8220 SEC instrument (column: Super HZM-H × 3) at 40 °C using THF as eluent with a flowing rate of 0.35 mL/min; the values were relative to polystyrene standards.

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ASSOCIATED CONTENT

S Supporting Information *

Texts including 1H NMR, 13C NMR, 1H−13C HMQC spectra, DOSY spectrum, ORTEP diagram, cif file for complex 1, 1H NMR and DOSY spectra of complex 1 with 2 equiv of diphenylmethanol in THF-d8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax +86-431-85262773; e-mail [email protected] (D.C.), [email protected] (E.Y.-X.C.). G


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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. 20904051, 51021003) and the U.S. National Science Foundation (NSF-1012326) for financial support. D.C. thanks the State Key Lab of Rare-Earth Sources Utilization for financial support.

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Ligand-Free Magnesium Catalyst System: Immortal Polymerization

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