Bis(pyrrolidene) Schiff Base Aluminum Complexes as Isoselective

Article pubs.acs.org/Macromolecules

Bis(pyrrolidene) Schiff Base Aluminum Complexes as IsoselectiveBiased Initiators for the Controlled Ring-Opening Polymerization of rac-Lactide: Experimental and Theoretical Studies Sittichoke Tabthong,† Tanin Nanok,† Pattarawut Sumrit,† Palangpon Kongsaeree,§ Samran Prabpai,§ Pitak Chuawong,‡ and Pimpa Hormnirun*,† †

Laboratory of Catalysts and Advanced Polymer Materials, Department of Chemistry, Faculty of Science, and ‡Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, and Special Research Unit for Advanced Magnetic Resonance (AMR), Kasetsart University, Bangkok 10900, Thailand § Department of Chemistry and Center for Excellence in Protein Structure and Function, Faculty of Science, Mahidol University, Bangkok 10400, Thailand S Supporting Information *

ABSTRACT: A series of bis(pyrrolidene) Schiff base aluminum complexes (1−7) were synthesized and characterized by NMR spectroscopy and elemental analysis. All complexes were efficient initiators for the ring-opening polymerizations of L-LA and racLA in toluene at 70 °C. Kinetic studies revealed first-order kinetics in monomer and the rates of L-LA and rac-LA polymerizations decreased in the order of 1,2-benzylene (4) ≫ 1,3-propylene (2) > 2,2-dimethyl-1,3-propylene (3) > 1,4-butylene (5) > rac-1,2cyclohexylene (7) > 1,2-ethylene (1) ≫ 1,2-phenylene (6). Microstructure analyses of the resulting polylactides by homonuclear decoupled 1H NMR spectroscopy disclosed the isotactic-biased stereocontrol of all synthesized complexes, except 5. Isotactic stereoblock polylactide with a high Pm value of 0.80 was produced by 3. A systematic DFT study on the rac-lactide ring-opening mechanism initiated by the initiators synthesized in this study revealed the correlation between the structure of backbone linker and the polymerization activity and stereoselectivity.

■

INTRODUCTION Polylactide (PLA) has become one of the most important aliphatic polyesters due to its biodegradable and biocompatible properties and to the possibility of deriving the monomer from renewable resources.1 PLA and its copolymers have found interest in many applications ranging from controlled drug delivery systems, resorbable sutures, artificial tissue matrices, food packaging to clothing products.2 Even though several methods have been exploited to prepare PLA, the most convenient and efficient one is the ring-opening polymerization (ROP) of lactide (LA), the cyclic diester of lactic acid. Various metal alkoxide complexes, including the metals aluminum,3 zinc,4 tin,5 yttrium,6 magnesium,7 calcium,8 and the lanthanides,9 have been studied in order to achieve better control, higher activity, and stereoselectivity of the ROP of LA. Recently, the research on metal alkoxide complexes has been extensively reviewed.10 © XXXX American Chemical Society

Among the large variety of metal-coordination initiators known to catalyze the ROP of lactide, aluminum alkoxide complexes modified by ancillary ligands have in particular attracted interest over the past 20 years. Aluminum complexes supported by N,N,O,O-tetradentate ligands have been extensively studied as efficient initiators for the ROP of lactide. The most important breakthrough was the discovery of the chiral Salen aluminum complexes for the stereoselective polymerization of rac-LA by Spassky and co-workers.11 Some other chiral Salen aluminum catalysts were reported to exhibit excellent stereocontrol for the polymerization of rac-LA to give isotactic stereoblock PLAs via an enantiomorphic-site control mechanism (SCM).12 Highly stereoselective polymerizations Received: June 24, 2015 Revised: September 6, 2015

A

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

■

via a chain-end control mechanism (CEM) were also achieved by a number of achiral Salen aluminum complexes.13 The discovery concerning an efficient stereocontrol using aluminum complexes with achiral and chiral Salen ligands has stimulated extensive studies on the development of new catalyst systems. Gibson and co-workers discovered the closely related achiral aluminum complexes bearing N,N,O,O-tetradentate aminophenoxy (Salan) ligands.14 Stereoselectivity in the polymerization of rac-lactide was remarkably controlled by subtle change of ancillary ligand substituents, leading to the production of polylactide with microstructures ranging from highly isotactic (Pm = 0.79) to highly heterotactic (Pr = 0.96). A recent study by Kol and co-workers described the switch of stereochemical control from isoselectivity to hetereoselectivity employing alkylaluminum complexes bearing chiral 2,2′bipyrrolidine-based Salan ligands.15 A highly heterotactic PLA with a Pr ≥ 0.98 was demonstrated to be produced by an insertion/autoinhibition/exchange mechanism. Feijen and coworkers reported the use of chiral Salan aluminum complexes as efficient catalysts for the ROP of rac-LA and meso-LA.16 Polylactide materials with a Pm value of 0.66 and a Pr value of 0.73 were obtained from rac-LA, whereas PLA with a Pr value of 0.70 was produced from meso-LA. Further investigations on the ROP of lactide using aluminum complexes bearing symmetric13f and asymmetric17 Salan ligands demonstrated that changes in ligand substitution pattern can lead to different degrees of complex fluxionality that have dramatic effects on the rate and stereoselectivity of the polymerization. Recently, aluminum complexes of N,N,O,O-tetradentate Salalen ligands were experimentally and theoretically studied for the ROP of rac-LA by Jones and co-workers.18 PLAs with microstructure ranging from medium heterotactic to medium isotactic were obtained. Theoretical study revealed that the ortho substituent of the ligand is critical for the positioning of the new incoming LA, leading to different polymer microstructures.18b Very recently, Kol and Lamberti reported the use of aluminum complexes containing enantiomerically pure aminomethylpyrrolidine-based Salalen ligands for the stereoselective polymerization of rac-LA.19 A new gradient isotactic multiblock microstructure was proposed to operate by a combination of enantiomorphic-site and chain-end control mechanism. In the search of new ancillary ligand systems for the ROP of lactide, the N,N,N,N-tetradentate ligands have found less popular for the complexation with aluminum precursor. Only few studies concerning the aluminum complexes of N,N,N,Ntetradentate ligands were reported.20 Chen and Feijen reported the synthesis of achiral aluminum alkyl complexes bearing bis(pyrrolidine) Schiff base ligands.20a These complexes were shown to produce highly isotactic PLLAs from L-LA without epimerization of the monomer, isotactic-biased PLA from racLA, and atactic PLA from meso-LA. However, the effect of catalyst architecture on the stereocontrol found in this initiator system remains elusive at present. Also, an understanding of the relationship between the catalyst structure and reactivity has yet to be elucidated. In this work, we experimentally investigated the catalytic performance for rac-lactide and L-LA polymerizations of aluminum methyl complexes supported by bis(pyrrolidene) Schiff base ligands with a wide range of backbone linkers. In addition, density functional theory (DFT) calculations were conducted to reveal the ROP mechanism and to highlight the effect of the catalyst structure on the polymerization activity and the stereoselectivity found in this system.21

Article

RESULTS AND DISCUSSION

Synthesis and Analysis of Complexes. The bis(pyrrolidene) Schiff base ligands with different backbone linkers, H2L1−H2L7, were synthesized via a condensation reaction with 2 equiv of pyrrole-2-carboxaldehyde and 1 equiv of an α,ω-diamine in ethanol at room temperature according to the published procedure.22 All ligands were obtained as white solids in high yields. Treatment of the ligands with stoichiometric trimethylaluminum (AlMe3) in anhydrous toluene at 110 °C afforded the corresponding complexes 1−7 as shown in Scheme 1. The complexes were formulated on the basis of 1H and 13C NMR spectroscopy, elemental analysis, and, for 3, X-ray crystallography. Scheme 1. Synthesis of Complexes 1−7

Solution 1H NMR studies revealed the formation of aluminum complexes 1−7 from the disappearance of the NH signal of the free ligands and the appearance of protons of the methyl groups bound to the aluminum center in the high field region of the 1H NMR spectra.23,24 For example, the 1H NMR spectrum of 1 revealed complex second-order multiplets arising from the diastereotopic methylene protons of the ligand backbone around δ 4.00−3.66 ppm. The signal ascribed to the imine and the methyl protons of the aluminum methyl group appeared at δ 8.33 and −0.97 ppm, respectively, with an integral ratio of 2:3. This ratio indicated the formation of the five-coordinate aluminum complex. The spectra of complexes 1−3, 5, and 6 contained only one imine signal (NCH) and a symmetric pattern of the pyrrolic protons. In the case of 7, the conformation of the rac-cyclohexyldiimino unit rendered the two pyrrole rings nonequivalent. Hence, two imine resonances at δ 8.48 and 8.18 ppm and two sets of pyrrolic protons were observed. The purity of 1−7 was ensured through correct elemental analysis. Single crystals of 3 suitable for X-ray diffraction analysis were grown from a saturated toluene solution at −20 °C. The molecular structure of 3 features a monomeric molecule with a five-coordinate aluminum center in a geometry best described as distorted square-based pyramidal (Figure 1). The amount of distortion can be quantified using the geometric criterion τ = (β − α)/60 proposed in the literature.24,25 The τ value ranges from 0 (perfectly square pyramidal) to 1 (perfectly trigonal bipyramidal). In this case, the τ value is 0.18, indicating the distorted square-based pyramidal geometry around the aluminum center. The apical position of the pyramid is occupied by the methyl group, and the basal plane is constituted of the four nitrogen atoms. Selected bond lengths and angles are listed in Figure 1 (see Tables S1 and S2 in the Supporting Information for the crystallographic details). Ring-Opening Polymerization of Lactide. The series of complexes 1−7 allowed the systematic investigation of the B

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Molecular structure of 3. Selected bond lengths (Å) and angles (deg) are as follows: Al(1)−C(20), 1.967(2); Al(1)−N(11), 2.0535(14); Al(1)−N(1), 1.9759(14); Al(1)−N(17), 1.952(15); Al(1)−N(7), 2.0334(13); N(11)−C(12), 1.291(2); N(7)−C(6), 1.289(2); C(20)−Al(1)− N(1), 108.96(17); C(20)−Al(1)−N(17), 115.40(7); C(20)−Al(1)−N(7), 106.28(7); C(20)−Al(1)−N(11), 102.07(7); N(1)−Al(1)−N(17), 92.86(6); N(1)−Al(1)−N(7), 80.44(5); N(2)−Al(1)−N(11), 148.16(6); N(17)−Al(1)−N(7), 137.59(6); N(17)−Al(1)−N(11), 79.92(6); N(7)−Al(1)−N(11), 84.27(5).

Table 1. Polymerization of rac-LA and L-LA Using Complexes 1−7 in the Presence of Benzyl Alcohola entry

complex

monomer

time (h)

convb (%)

Mn(theory)c (g mol−1)

Mn(GPC)d (g mol−1)

Mw/Mnd

Pme

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 6 7 1 2 3 4 5 6 7

rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA rac-LA L-LA L-LA L-LA L-LA L-LA L-LA L-LA

48 1 1 0.5 24 200 24 48 1 1 0.5 24 200 24

77 98 95 99 82 86 82 73 92 83 99 94 82 87

11 200 14 200 13 800 14 400 11 900 12 500 11 900 10 600 13 400 12 100 14 400 13 700 11 900 12 600

8 500 10 400 12 800 13 700 9 900 11 600 10 900 10 100 10 600 11 200 12 800 12 700 12 500 11 200

1.06 1.05 1.03 1.07 1.02 1.11 1.09 1.08 1.04 1.06 1.07 1.05 1.15 1.02

0.61 0.69 0.80 0.74 0.52 0.64 0.73 1 1 1 1 1 1 1

a [LA]0/[Al] = 100, [Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, toluene, 70 °C. bAs determined via integration of the methine resonances (1H NMR) of LA and PLA (CDCl3, 400 MHz). cCalculated by ([LA]0/[Al]) × 144.13 × conversion + 108.14. dDetermined by gel permeation chromatography (GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA. ePm is the probability of meso linkage between monomer units and was calculated from the homonuclear decoupled 1H NMR spectra of the obtained poly(rac-LA): [mmm] = Pm2 + (1 − Pm)Pm/2; [mmr] = [rmm] = (1 − Pm)Pm /2; [rmr] = (1 − Pm)2/2; [mrm] = [(1 − Pm)2 + (1 − Pm)Pm]/2.

respectively, in 1 h (entries 2 and 3, Table 1). Complex 4 was much more active than 2 and 3 and showed high monomer conversion (99%) in 30 min (entry 4, Table 1). Polymerizations using 5 and 7 proceeded to 82% conversion (entries 5 and 7, Table 1) in 24 h, whereas 6 polymerized rac-LA to 86% conversion after 200 h (entry 6, Table 1). In the cases of L-LA, all polymerizations also proceeded in a controlled fashion, which led to PLAs with narrow molecular weight distributions and molecular weights in good agreement with the Mn values calculated from the monomer conversions (entries 8−14, Table 1). Similar to rac-LA, complex 4 was the most active (entry 11, Table 1) and the least active catalyst was complex 6 (entry 13, Table 1). Aliquots periodically withdrawn from the polymerization reactions of rac-LA with 1−7 were used to construct plots of molecular weight versus percent conversion of monomer (with

effects of bis(pyrrolidene) Schiff base ligands on the polymerization activities. Polymerizations of rac-LA and L-LA using 1−7 in the presence of 1 equiv of benzyl alcohol were carried out at 70 °C in toluene. The molar ratio of monomer to initiator was fixed at 100:1 ([LA]0/[Al] = 100; [LA]0 = 0.83 M; [Al] = 8.33 mM; Mn(theory) = 14 400). All complexes were effective for the polymerizations of rac-LA and L-LA, as shown in Table 1. The molecular weights and PDIs (Mw/Mn) were determined by gel permeation chromatography (GPC) using the Mark− Houwink correction factor of 0.58.26 For rac-LA polymerizations, the corrected Mn values (entries 1−7, Table 1) are closed to the theoretical values, and narrow PDIs were observed, indicative of living characteristic. rac-LA polymerization using 1 proceeded to 77% conversion after 48 h (entry 1, Table 1); this polymerization was found to be slower than the one using 2 or 3, which reached 98% and 95% conversion, C

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules [LA]0/[Al] = 50; [Al] = 8.33 mM; [LA]0 = 0.42 M). A linear correlation between molecular weight and percent conversion, in conjunction with a narrow PDI, was observed in all cases (Figure 2 and Figures S1−S6), indicative of a well-controlled

Figure 3. Plot of PLA Mn (versus polystyrene standards) and molecular weight distribution versus the molar ratio of monomer to initiator ([LA]0/[Al]) for the polymerization of rac-LA using 3/ PhCH2OH ([Al] = 8.33 mM, toluene, 70 °C). Figure 2. Plot of PLA Mn (versus polystyrene standards) and molecular weight distribution as a function of monomer conversion for a rac-LA polymerization using 7/PhCH2OH ([LA]0/[Al] = 50, toluene, 70 °C).

logarithmic plots of the rac-LA conversion (ln[LA]0/[LA]t) versus time, as shown in Figure 4. In all cases, first-order kinetics in monomers were observed, as evidenced from the linear relationship between ln([LA]0/ [LA]t) and time. Compared with the closely related bis(pyrrolidene) Schiff base aluminum ethyl/2-propanol catalytic system,20a the rac-lactide polymerization using the aluminum methyl/benzyl alcohol initiating system showed no induction period, indicating that the active species form instantaneously by an in situ alcoholysis reaction utilizing benzyl alcohol. Thus, the polymerization proceeds according to the rate law in eq 1.

living polymerization and a single site of reaction.9d,12b In addition, the observation of relatively narrow and constant PDI values (ca. 1.1) throughout the course of polymerization suggests that no significant degree of transesterifications operates in this initiating system. Further evidence for the well-controlled character of polymerization is provided by changing the molar ratio of monomer to initiator ([LA]0/[Al] and the concentration of aluminum complex was fixed at 8.33 mM. As indicated in Table 2, complex 3 was able to polymerize rac-LA at a high loading of monomer to initiator ratio ([LA]0/[Al] = 1600:1). The linear relationship of Mn to [LA]0/[Al] for 3 is depicted in Figure 3. The number-average molecular weights Mn were close to the theoretical values, indicative of a well-controlled polymerization process. The living characteristic was further highlighted by the low molecular weight distributions observed. In addition, a high degree of isotacticity was observed up to high monomer-toinitiator ratios. The Pm values of the resulting polymers remained unchanged in the range of Pm = 0.78−0.80 (entries 1−5, Table 2). Kinetic Studies. To determine the reaction kinetics of the rac-LA polymerizations with 1−7, all reactions were carried out under the same conditions at 70 °C in toluene ([LA]0/[Al] = 50; [Al] = 8.33 mM; [LA]0 = 0.42 M). The progress of polymerizations was monitored by 1H NMR spectroscopy. The apparent rate constants (kapp) were obtained from semi-

−d[LA]/dt = kapp[LA]

(1)

x

where kapp = kp[Al] and kp is the propagation rate constant. To determine the order in aluminum (x), the concentration of 3 was varied from 4.16 to 24.96 mM under otherwise identical conditions ([LA]0 = 0.42 M). The appropriate semilogarithmic plots are shown in Figure 5. The order in aluminum was obtained from the slope of the plot of ln kapp versus ln[Al]. As can be seen in Figure 6, the plot of ln kapp versus ln[Al] is linear with the noninteger slope of ca. 1.0 (1.17), suggesting the proportionally intriguing −d[LA]/dt ∝ [Al]. The propagation rate constant (kp) can be alternatively obtained from the gradient of the kapp versus [Al] plot, as depicted in Figure 7. The linear relationship between kapp and [Al] indicates a firstorder kinetics in aluminum concentration, with a kp value of (56 ± 2) × 10−6 s−1 mol−1 L at 70 °C. Therefore, the polymerization of rac-LA initiated by 3 follows the overall rate equation of the form −d[LA]/dt = kp[LA][3].

Table 2. Polymerization of rac-Lactide Using Complex 3 in the Presence of Benzyl Alcohola entry

[LA]0/[Al]

time (h)

convb (%)

Mn(theory)c (g mol−1)

Mn(GPC)d (g mol−1)

Mw/Mnd

Pme

1 2 3 4 5

100 200 400 800 1600

1 6 12 48 60

95 93 96 93 95

13 800 26 900 55 500 107 300 219 200

12 800 26 100 59 300 106 400 233 600

1.03 1.02 1.18 1.11 1.17

0.80 0.78 0.80 0.78 0.78

a General polymerization conditions: [Al] = 8.33 mM, toluene, 70 °C. bAs determined via integration of the methine resonances (1H NMR) of LA and PLA (CDCl3, 400 MHz). cCalculated by ([LA]0/[Al]) × 144.13 × conversion + 108.14. dDetermined by gel permeation chromatography (GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA. ePm is the probability of meso linkage between monomer units and was calculated from the homonuclear decoupled 1H NMR spectra of the obtained poly(rac-LA): [mmm] = Pm2 + (1 − Pm)Pm/2; [mmr] = [rmm] = (1 − Pm)Pm /2; [rmr] = (1 − Pm)2/2; [mrm] = [(1 − Pm)2 + (1 − Pm)Pm]/2.

D

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 5. Semilogarithmic plots of the rac-lactide conversion versus time in toluene at 70 °C with complex 3/PhCH2OH as an initiator ([LA]0 = 0.42 M: I, [Al] = 24.96 mM, [LA]0/[Al] = 17; II, [Al] = 18.72 mM, [LA]0/[Al] = 22; III, [Al] = 12.50 mM, [LA]0/[Al] = 34; IV, [Al] = 10.41 mM, [LA]0/[Al] = 40; V, [Al] = 8.33 mM, [LA]0/ [Al] = 50; VI, [Al] = 6.24 mM, [LA]0/[Al] = 67; VII, [Al] = 4.16 mM, [LA]0/[Al] = 101).

Figure 6. Plot of ln kapp versus ln[Al] for the polymerization of raclactide with complex 3/PhCH2OH as an initiator (toluene, 70 °C, [LA]0 = 0.42 M).

Figure 4. Semilogarithmic plots of rac-lactide conversion versus time in toluene at 70 °C with (a) complexes 1 (●) and 7 (■), (b) complexes 2 (●), 3 (⧫), 4 (▲), and 5 (▼), and (c) complex 6 (⧫) ([LA]0/[Al] = 50, [Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, [Al] = 8.33 mM).

The first-order rate constants (kapp) for the polymerizations of rac-LA and L-LA with 1−7 are listed in Table 3 (see also Figure S7). For the rac-LA polymerization, changing the diimine backbone from ethylene to propylene resulted in an increase in kapp from (36 ± 10) × 10−6 to (787 ± 32) × 10−6 s−1. It was proposed that the C2 backbone could provide more steric hindrance during the coordination and/or ring-opening of the lactide, leading to the lower rate.27 The polymerization initiated by 3 with the gem-methyl groups on the propylene backbone exhibited a kapp of (368 ± 11) × 10−6 s−1, which was approximately 2 times slower than the rate of PLA formation from 2, which was in contrast with the results reported by

Figure 7. Plot of kapp versus [Al] for the polymerization of rac-lactide with complex 3/PhCH2OH as an initiator (toluene, 70 °C, [LA]0 = 0.42 M).

Gibson,13e Nomura,28 and Chen13i,29 for the Salen aluminum catalysts and the results obtained by Chen30 for the enolic E

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 3. Kinetic Results for the Polymerizations of rac-LA and L-LA Using Complexes 1−7 in the Presence of Benzyl Alcohola entry

initiator

1 2 3 4 5 6 7

1 2 3 4 5 6 7

kapp (106 s−1) rac-LA 36 788 368 2843 303 3 105

± ± ± ± ± ± ±

10 3 11 113 4 0.01 4

some extent during the polymerization of rac-LA. In the case of a racemic complexes 4 and 7, the polymerization rate of L-LA was faster than that of rac-LA, suggesting catalyst inhibition by the misinsertion behavior during the course of rac-LA polymerization.31 The slower polymerization rates of rac-LA compared to L-LA using racemic catalyst were also reported in other systems.12e,31,32 Stereochemistry of rac-Lactide Polymerization. The stereochemical microstructures of the PLA samples produced by 1−7 were determined by analyzing the resultant 1H NMR signal intensities of the methine region using Bernoullian statistics.33 All homonuclear decoupled 1H NMR spectra produced by complexes 1−7 are shown in Figure 8 and

kapp (106 s−1) L-LA 37 1309 510 3422 361 4 211

± ± ± ± ± ± ±

2 23 27 96 12 0.1 10

General polymerization conditions: toluene as a solvent, 70 °C, [LA]0/[Al] = 50, [Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, [Al] = 8.33 mM.

a

Schiff base aluminum complexes: the complex with a 2,2dimethyl-1,3-propylene backbone was more active than that with a propylene backbone. Changing the backbone to benzylene (4) gave rise to substantial enhancement of the polymerization rate (kapp = (2843 ± 113) × 10−6 s−1), which was ca. 8 times faster than the kapp value of 3. This observation was in contrast with the closely related Salen aluminum system in which catalyst containing a benzylene backbone was less active than that containing a 2,2-dimethyl-1,3-propylene backbone.13e,i Lengthening the backbone to a butylene linker (5) led to a kapp ((303 ± 4) × 10−6 s−1) comparable to the one observed for 3. Comparing 6 with 1 showed a 10-fold rate decrease when changing the C2 linker from ethylene to phenylene. However, the replacement of the C2 ethylene linker with a rac-1,2-cyclohexyl bridge (7) resulted in an increase in kapp from (36 ± 10) × 10−6 to (105 ± 4) × 10−6 s−1. In comparison with the series of unsubstituted phenoxy Salen aluminum complexes under the same conditions ([LA]0/[Al] = 50 and [Al] = 8.3 mM), the kapp value of the most active complex 4 with the 1,2-benzylene unit was ca. 3 times higher than that of the most active Salen aluminum complex with 2,2dimethyl-1,3-propylene linker (kapp = 1073 × 10−6 s−1).13e Such a marked influence of the subtle change of backbone linker on the polymerization rate has also been observed in other systems.13e,i,27 In the series of complexes, the rates of polymerization decrease in the order 1,2-benzylene ≫1,3propylene > 2,2-dimethyl-1,3-propylene > 1,4-butylene > rac1,2-cyclohexylene > 1,2-ethylene ≫ 1,2-phenylene. The results revealed that the balance of backbone flexibility has a remarkable influence on the rate of rac-LA polymerization. In order to obtain the polymerization rate without the stereocontrol factor involved, the polymerizations of L-LA polymerizations were carried out. As shown in Table 3, the same order of polymerization rates as for rac-LA polymerizations was observed. If the polymerization proceeds via a chain-end control mechanism and without stereoselectivity control, the rate of L-LA and rac-LA polymerizations should be identical due to the rates of addition of L- and D-monomers to the growing polymer chain ends are not affected by the configuration of the last enchained unit.13i It was found that there were no significant difference in the rates of L-LA polymerizations initiated by C2 complexes (1 and 6) and C4 complex (5) compared to rac-LA polymerizations. In the cases of C3 complexes (2 and 3), the polymerizations of L-LA were much faster than those of rac-LA. The difference in the rates between L-LA and rac-LA polymerizations suggested that the more flexible C3 complexes show stereoselectivity control to

Figure 8. Homonuclear decoupled 1H NMR spectra of the methine region of PLA prepared from rac-lactide at 70 °C in toluene (500 MHz, CDCl3) with (a) 3/PhCH2OH and (b) 7/ PhCH2OH.

Figures S8−S12. According to the data shown in Table 1, complexes 1−4, 6, and 7 exhibited isotactic selectivity (the Pm values are in the range of 0.61−0.80). PLA produced by 1 was moderately isotactic with a Pm value of 0.61 while complex 2 resulted in a slightly higher isoselectivity (Pm = 0.69). Complexes 3 and 4 showed high isotactic selectivity: Pm = 0.80 for 3 and Pm = 0.74 for 4. The Tm value of 153 °C was obtained for the highly isotactic PLA produced by 3. The F

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 2. Ring-Opening Insertion Mechanism for the First (S,S)-LA Monomer by 3 (H Atoms on the Ligand Are Omitted for Clarity: Pink = Al; Red = O; Blue = N; Gray = C; White = H)

groups on the pyrrole rings resulted in a decreased isotactic enchainment (Pm = 0.60).20a The modification of the backbone and the phenoxy substituent group of the Salen ligands shows a dramatic effect on the stereoselectivity of the aluminum Salen complexes.11−13 The highly isotactic PLA with the Pm value of 0.97 was produced by the aluminum complex with 2,2dimethyl-1,3-propylene linkage and the ortho tBuMe2Si phenoxy substituents.13d The dichloroaluminum−Salen complex with the biphenyl linker, the only derivative in the Salen− aluminum family, was found to produce heterotactic-biased PLA (Pr = 0.63).13e In the cases of Salan− and Salalen− aluminum catalysts, the reverse stereoselectivity from isoselectivity to hetereoselectivity can be controlled by subtle change of ligand substituents.14−19 However, the more pronounced influence from ligand substituents on the level of stereocontrol was observed in the Salan−aluminum complexes. The homonuclear decoupled 1H NMR analysis of the stereosequence distribution of PLA produced by the most selective achiral catalyst 3 shows a large mmm tetrad peak, indicating the formation of an isotactic biased PLA with a Pm value of 0.80 (Figure 8a). The highly isoselective ROP of racLA by 3 was proposed to proceed via a chain-end control mechanism (CEM) due to achiral nature of the catalyst. Figure 8b shows the homonuclear decoupled 1H NMR spectrum of PLA prepared by the racemic aluminum complex 7. The isotactic PLA with the Pm value of 0.73 was obtained. It is worthy to note that the isoselectivity control exerts by 7 via a site-control mechanism may include the chain-end control after the misinsertion of the unfavorable enantiomer as previously

isoselectivity difference between 3 and 4 can be attributed to the flexibility of the backbone, in which 2,2-dimethyl-1,3propylene backbone is more flexible than the 1,2-benzylene linker.13h Furthermore, when the butylene backbone substituted complex 5 was examined, almost no stereoselectivity was observed for the rac-LA polymerization (Pm = 0.52). In the cases of other C2 linker complexes (6 and 7), the isotacticbiased polymers were produced (Pm = 0.64 and 0.73, respectively). It can be seen that the C3 backbone complexes (2 and 3) afforded a higher isoselectivity than the C2 ethylene derivative (1). A similar trend was observed in the unsubstituted phenoxy Salen aluminum catalysts system studied by Gibson,13e Nomura,13d and Chen.13i Nomura and co-workers demonstrated that the more stereoselectivity control in C3 complexes was attributed to the role of inversion of ligand; i.e., the chiral geometry of the complex can be inverted by the introduction of the opposite chiral sense of the polymer terminus during the propagation reactions if the chiral environment of the complex is flexible, resulting in the enhance of the efficiency of the stereodifferentiation of D-LA and L-LA.13d Hence, the higher degree of stereoselectivity control was accordingly observed in the more flexible C3 complexes. In comparison with the closely related tetradentate Salen−, Salan−, and Salalen−aluminum catalyst systems, the bis(pyrrolidene) Schiff base aluminum complexes produced only isotactic-biased PLAs, and the level of isoselectivity control was moderately high with the highest Pm value of 0.80 (from 3). Chen and Feijen reported that the introduction of the methyl G

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules reported by Chisholm 13j and recently by Kol 19 and Lamberti.19,32 Computational Investigations. Density functional theory (DFT) calculations were carried out in order to gain further insight into the reaction mechanism of this catalytic system. The underlying influences of the backbone structure on the energetics and competing reaction pathways reflecting the catalytic activity and stereoselectivity of the catalysts were established. All calculations were performed at the M06-2X level of theory34 with the 6-311G(d,p) basis set.35 This methodology has been extensively validated for organic and main-group inorganic systems and has recently been used to study the polymerization mechanism of ε-caprolactone catalyzed by Salen−aluminum complexes.21e It was found that a good agreement in terms of bond length and bond angle between the X-ray and the optimized structures of 3 is observed with the standard deviations of the bond lengths and the bond angles around the aluminum center in the range of 0.06−1.02% and 0.9−6.18/%, respectively (see also Figure S13 for the optimized structure of complexes 1−7 with their selected structure parameters collected in Table S3). To address the different catalytic activities of complexes 1−7, the ring-opening insertion of the (S,S)-lactide monomer was first examined. In order to reduce the computational time, the benzyloxide initiating group (−OCH2Ph) was replaced by the methoxide unit (−OCH3). All sequential steps in the ringopening insertion of the first lactide monomer proposed for 3 are applied to all initiators in this study, and only the mechanistic demonstration for 3 is provided for simplification. The key species proposed in the coordination−insertion mechanism of the first (S,S)-lactide monomer initiated by 3 is illustrated in Scheme 2, and the calculated free energy profile is presented in Figure 9 (see Figures S14−S19 for the free energy profiles of 1, 2, and 4−7).

set as a reference point of the overall reaction. At the transition state, the activated carbonyl carbon of LA is attacked by the methoxy species with the O(1)···C(1) separation of 2.08 Å, and the bond between the O(2) of LA and the aluminum center is simultaneously formed (see Scheme 2 and Table S4). In this configuration, the aluminum center adopts a distorted octahedral geometry in which the O(2) of LA is located in the cis-equatorial position to the methoxy O(1) donor atom with an O(2)···Al separation of 2.06 Å and an Al···O(1) distance of 1.84 Å. With respect to the separated lactide monomer and initiator, RA1, the formation of the first intermediate (INT1) has the activation free energy of 4.4 kcal mol−1, and this process is exergonic by 2.4 kcal mol−1 (see Figure 9). Before the ring-opening step, the INT1 is reorganized by the rotation of the lactide ring about the O(2)−C(1) bond to form the second intermediate (INT2). This results in the close interaction between an acyl oxygen atom of LA and the aluminum center. The second step of the coordination insertion mechanism corresponds to the ringopening of the inserted lactide to create the new alkoxy initiating group, also denoted as a growing polymer chain, bound to the aluminum center. This step proceeds through the second transition state (TS2) at which the lactide acyl bond (C(1)−O(3)) is elongated (2.08 Å) with respect to a free LA (1.35 Å) in concomitance with the formation of a new aluminum−alkoxide bond (Al−O(3) = 1.89 Å) (see Scheme 2 and Table S5). After the reorganization of the growing polymer chain, a five-membered metallocyclic product, INT3, is afforded with a weak interaction between Al and Ocarbonyl (2.11 Å). The formation of the five-membered chelate was also observed for aluminum−Salalen initiators for the ROP of rac-LA.18b With respect to the most stable intermediate (INT1), the calculated activation free energy for the ring-opening is 9.1 kcal mol−1. Although the deinsertion of INT1 to RA1 is more likely than the ring-opening to INT3, the latter process is favored under thermodynamic control. Overall, the conversion of RA1 into INT3 is exergonic by 12.0 kcal mol−1. From a geometric point of view, the key transition states, TS1 and TS2, involve in the nucleophilic attack and the ringopening steps, and hence, their geometries might account for the rate of polymerization. A close-up on both transition state geometries using 3 is presented in Figure 10. It can be seen that the calculated four-membered transition states (TS1 and TS2) engage in a simultaneous bond cleavage and formation in the

Figure 9. Calculated free energy profile for the ring-opening insertion of the first (S,S)-LA monomer by 3.

As shown in Scheme 2, the first step of the coordination− insertion mechanism involves the insertion of the coordinated LA into the Al−OCH3 bond of the initiator to form the tetrahedral intermediate (INT1) via a four-membered transition state, TS1. Unlike the diketiminate metal initiators, where the lactide monomer strongly coordinates to the positively charged metal centers,21a,b the direct coordination of this monomer to the five-coordinate aluminum center is found to be unstable on the free energy surface. Therefore, the sum of free energies between isolated complex and free lactide, RA1, is

Figure 10. Optimized geometries of TS1 and TS2 in the ring-opening insertion of the first (S,S)-LA monomer with 3 (H atoms on the ligand are omitted for clarity: pink = Al; red = O; blue = N; gray = C; white = H). θ is the cis-quatorial angle N(1)−Al−N(4). H

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

89.9° (TS1) and 89.0° (TS2). In comparison between C2 and C3 backbone complexes, the calculated free energy barriers of the C2 complexes (1, 6, 7) are higher than that of the C3 complexes (2, 3, 4), suggesting the lower polymerization rates. It is apparent that lengthening the backbone from C2 to C3 linker results in an increase of the bite angle, which allows the initiator to better accommodate the key transition states involved in the ROP process. In the case of C2 complexes, the highest ΔG‡TS1 and ΔG‡TS2 values were observed for 6, which are in agreement with the lowest experimental polymerization rate. For the group of complexes containing C3 linker, the lower ΔG‡TS1 and ΔG‡TS2 values of 2 compared with 3 are also in line with the higher observed polymerization rate of 2. In this case, the greater flexibility of the propylene linkage is responsible for the enhancement of the polymerization rates. However, the correlation between the bite angles and the observed polymerization rate cannot be applied in the cases of initiators 4 and 5. A similar contradiction is also observed for 7, in which the experimental polymerization rate of 7 is higher than that of 1 although the DFT calculations reveal virtually the same values of bite angles in both TS1 and TS2. These confounding observations are also reported in the aluminum complexes featuring Salen-type ligands with different backbones,13e in which the polymerization rate cannot be explained solely by the bite angle effect. Therefore, the contributions of unknown factors are expected during the propagation reactions. The origin of stereocontrol for rac-lactide polymerization was investigated by inspecting the insertion of the second lactide monomer into the first enchained (S,S)- or (R,R)-LA monomer unit. For simplicity, the (SS,RR) notation was used to represent the insertion of (R,R)-LA into the first enchained (S,S)-LA monomer unit. Similar notation patterns are applied for other assembly modes of insertion. In each initiator system, we examined the free energy surfaces of the four sets of different assembly modes leading to isotactic enchainments (SS,SS and RR,RR sequences) and heterotactic enchainments (SS,RR and RR,SS sequences).18b,21a,c,e It should be noted that the (RR,RR) and (RR,SS) assembly modes of insertion are equivalent to the (SS,SS) and (SS,RR) enantiomeric analogues in the initiator systems for 1, 2, 3, 5, and 6 due to achiral nature of the complexes. Therefore, the (RR,RR) and (RR,SS) assembly modes of insertion in these initiator systems are redefined as i(SS,SS) and i-(SS,RR), corresponding to the interconversion of ligand configuration. Similar to the first monomer insertion mechanism (see Scheme S1), the second monomer insertion

equatorial plane of the octahedral geometry. Therefore, the geometric constraints imposed by the cis-equatorial θN(1)−Al−N(4) angle of the ligand framework is expected to have a strong effect on the stability of the transition states. The magnitude of cis-equatorial θN(1)−Al−N(4) angle is found the be strongly correlated with the bite angle, N(3)−Al−N(4), of the N,N chelate of the backbone; i.e., the increase of the bite angle leads to a significant reduction of the cis-equatorial angle. Since the cis-equatorial angle is vertically opposite to the O(1)methoxy− Al−O(2)carbonyl angle in TS1 and to the O(2)carbonyl−Al− O(3)acyl angle in TS2, its narrowing results in the reduction of the steric congestion at the metal center and, thus, weakens the constraints in the transition state complexes. This causes a reduction of the free energy barriers of the transition states TS1 and TS2 (ΔG‡TS1 and ΔG‡TS2), leading to higher polymerization activities. Table 4 shows the calculated bite angles, the cis-equatorial angles, and the free energy barriers of TS1 and TS2 using Table 4. Activation Free Energy, Bite Angle, and cisEquatorial Angle Values for the Ring-Opening Insertion of the First Lactide Monomer Using 1−7 TS2

TS2

complex

ΔG‡TS1a

bite angleb

θc

ΔG‡TS2d

bite angleb

θc

1 2 3 4 5 6 7

5.9 2.1 4.4 4.2 2.3 10.0 5.2

76.3 83.2 85.6 83.5 89.9 75.9 76.9

107.7 101.5 107.6 104.7 98.3 112.3 108.5

10.0 7.4 9.1 8.5 6.9 15.6 12.1

76.6 83.1 85.4 83.6 89.0 76.0 76.9

105.8 99.8 105.5 102.7 96.1 109.0 106.7

Activation free energy (kcal mol−1) for the insertion transition state (TS1). bAngle between donor atom−metal−donor atom, N(3)−Al− N(4). cThe cis-equatorial angle N(1)−Al−N(4). dActivation free energy (kcal mol−1) for the ring-opening transition state (TS2) with respect to the most stable intermediate (either INT1 or INT2). Bond angles are in deg. a

initiators 1−7. The results can be clearly divided into three groups according to the number of carbon atoms in the backbone (C2, C3, and C4 backbones). In view of the molecular structure, the C2 complexes have bite angles in the range of 75.9°−76.9°, whereas the C3 complexes hold bite angles in the range of 83.1°−85.6°. The C4 complex 5 has the bite angles of

Figure 11. Free energy profile for the insertion of the second lactide into the first enchained (S,S)-LA complex, INT3, in different assembly modes mediated by 3 (a) before and (b) after the conformational inversion of the ligand. I

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

to take place. These results suggest that the initiator prefers to continuously polymerize the (S,S)-LA monomer after the first (S,S)-LA enchainment, leading to the experimentally observed isotactic PLA. A closer examination of the insertion transition state structures (TS1′) of these two assembly modes (Scheme 3) reveals that the relatively high free energy of activation in the slow step of the (SS,RR) insertion corresponds to the repulsive interactions between the stereogenic center of the inserting monomer and the α-methyl group of the first enchained unit while similar steric repulsions are absent in the fast step of the (SS,SS) insertion. However, the calculations elucidate a totally opposite scenario when the ligand conformation is in its inverted form (Scheme 3 and Figure 11). In this case, the enchainment in the i-(SS,RR) sequence (ΔG‡TS1′ = 10.2 kcal mol−1 and ΔG‡TS2′ = 8.8 kcal mol−1) is apparently more favorable than that of the i(SS,SS) sequence (ΔG‡TS1′ = 13.3 kcal mol−1 and ΔG‡TS2′ = 16.3 kcal mol−1). It should be noted that the inverted form of the ligand favors the enchainment in the (SS,RR) sequence, and thus, it would also favor the (RR,RR) enchainment. However, this conformation of the ligand disfavors the enchainment in the (SS,SS) assembly mode. A closer inspection of the insertion transition state (TS1′) of the i-(SS,SS) sequence (Scheme 3a) reveals that the higher ΔG‡TS1′ of the insertion in the i-(SS,SS) mode relative to that in the (SS,SS) mode by 5.3 kcal mol−1 results from the repulsive interactions between the methyl group of the inserting monomer and the ligand framework. In the case of the i-(SS,RR) insertion (Scheme 3b), it is found that the α-methyl group of the first enchained unit is able to position itself away from the stereogenic center of the inserting monomer. This reorganization of the complex results in the minimization of the repulsive interactions during the insertion process. As a result, the free energy of activation in this step (ΔG‡TS1′) is substantially reduced from that of the (SS,RR) sequence by 5.3 kcal mol−1. In the ring-opening step, the stability of the transition state (TS2′) is determined by two crucial interaction factors as shown in Figure S21. The TS2′ of the i-(SS,RR) sequence is the most stable configuration while that of the i-(SS,SS) is the least stable one. The repulsive interactions between the α-methyl group of the first enchained unit and the stereogenic carbon center of the ring-opening monomer and between the methyl group of the ring-opening monomer and ligand framework are absent in the i-(SS,RR) mode while these two repulsive interactions are present in the i-(SS,SS) sequence. Nevertheless, in this study, the stability of the INT1 and INT2 is also taken into account for determining the rate of the ring-opening step. Even though the i-(SS,RR) insertion is the most kinetically favorable pathway, this mode of insertion seems less compelling in the overall reaction. The less stable INT3′ of the i-(SS,SS) mode of enchainment (Figure 11) by 5.2 kcal mol−1 relative to that of the (SS,SS) mode indicates that the ligand inversion, probably through the reorganization of the ring-opened intermediate to form INT3′, is unlikely to take place during the propagation in the (SS,SS) sequence. Instead, the conformational inversion of the ligand is more likely to occur after the misinsertion of the opposite enantiomeric monomer, (R,R)-LA, into the growing (S,S)-polymer chain during the propagation reactions. In this process, the last-inserted (R,R)LA would induce the formation of the inverted ligand via mechanistic crossing from INT2′ of the (SS,RR) insertion mode to TS2′ of the i-(SS,RR) insertion mode, which has lower free energy of activation. Therefore, the initiator would

begins with the insertion of LA into the terminal Al−OR bond of INT3. This step proceeds through the transition state TS1′ to afford the intermediates INT1′ and INT2′. Subsequently, the ring-opening occurs through TS2′ in which the intracyclic C−Oacyl and the Ocarbonyl−Al bonds are broken to give the new alkoxide species which reorganizes to form the intermediate INT3′. From an analysis of the free energy profiles (Figure 11), before and after the conformational inversion of the ligand, it can be concluded that the combined effects of the lastenchained monomer unit and ligand conformation are two important key factors for the mechanistic interpretation of the isoselective ring-opening polymerization of rac-LA by 3. Because of the achiral property of the initiator, it should be expected that the first LA enchainment takes place without preferential selection of one enantiomer over another. However, the ligand conformation is induced by the chirality of the first enchained monomer to produce the chiral environment around the aluminum center. This results in the enhancement of the enantiomeric differentiation ability of the initiator. As can be seen in Figure 11 and Table 5 (initiator 3), Table 5. Calculated Activation Free Energies (kcal mol−1) for the Insertion (TS1′) and the Ring-Opening (TS2′) Steps of rac-LA in Different Assembly Modes Using 1−7 initiator

assembly mode

ΔG‡TS1′

ΔG‡TS2′

1

SS,SS SS,RR i-SS,RR i-SS,SS SS,SS SS,RR i-SS,RR i-SS,SS SS,SS SS,RR i-SS,RR i-SS,SS SS,SS SS,RR RR,SS RR,RR SS,SS SS,RR i-SS,RR i-SS,SS SS,SS SS,RR i-SS,RR i-SS,SS SS,SS SS,RR RR,SS RR,RR

9.0 17.5 12.4 13.9 9.9 16.7 11.7 13.6 8.0 15.5 10.2 13.3 9.5 14.6 11.8 12.3 9.0 15.5 10.7 12.3 13.0 18.9 15.0 13.5 10.4 17.1 11.0 12.0

14.5 11.5 19.8 15.1 11.6 9.9 7.3 13.3 13.4 11.0 8.8 16.3 10.5 14.9 16.1 15.0 11.1 15.2 9.9 18.8 21.1 21.7 23.9 23.6 10.8 11.6 12.4 16.5

2

3

4

5

6

7

the insertion step in the (SS,SS) mode (ΔG‡TS1′ = 8.0 kcal mol−1) is more kinetically favorable than that in the (SS,RR) mode (ΔG‡TS1′ = 15.5 kcal mol−1). However, the ring-opening step of the former sequence (ΔG‡TS2′ = 13.4 kcal mol−1) is slightly less favored than that of the latter one (ΔG‡TS2′ = 11.0 kcal mol−1). Taking both ΔG‡TS1′ and ΔG‡TS2′ into account, the enchainment in the (SS,SS) route is more statistically likely J

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 3. Insertion Transition States (TS1′) in the Second Lactide Insertion by 3 and Their Corresponding Activation Free Energies before and after the Conformational Inversion of the Ligand: (a) for the (SS,SS) and (b) for the (SS,RR) Modes of Insertiona

a

Double-headed arrows represent the repulsive interactions between two atomic centers; H atoms of the ligand are omitted for clarity: pink = Al; red = O; blue = N; gray = C; white = H.

activation free energies of the four assembly modes of insertion (complexes 4 and 7 in Table 5 and Figures S24 and S27), it was found that the (SS,SS) mode is the most favorable insertion pathway in both initiator systems with ΔG‡TS1′ and ΔG‡TS2′ of 9.5 and 10.5 kcal mol−1, respectively, for 4 and of 10.4 and 10.8 kcal mol−1, respectively, for 7. These computational results are in line with the experimental data in which the isotactic-biased PLAs were produced by initiators 4 and 7 (Pm = 0.74 for 4 and Pm = 0.73 for 7). The next most favorable insertion pathway was found to be the (RR,RR) mode for 4 and the (RR,SS) mode for 7. These results suggest that after a single misinsertion of the opposite enantiomer (the (SS,RR) mode) the initiator 4 would prefer to polymerize (R,R)-LA from this point onward or until a new enantiomeric misinsertion occurs while the catalyst 7 would prefer to retain the (SS,SS) mode of insertion. It is worth mentioning that these calculation results did not provide the information on the polymeryl exchange process12b,13j,32 that occurred during the course of polymerization which is also responsible for stereoselectivity enchainment of rac-LA by the chiral initiators.

preferentially polymerize the (R,R)-LA from this point onward or until the misinsertion and ligand inversion take place once again. A similar mechanistic interpretation has also been proposed by Nomura and co-workers for the stereoselective ROP of rac-LA initiated by achiral Salen−aluminum complexes.13d Because the subsequent insertion of a new monomer is controlled by the configuration of the last enchained monomer unit, the experimentally observed stereoselectivity of rac-LA polymerization mediated by 3 can be rationalized by assuming a chain end control mechanism.4b,13d Selected bond lengths and angles of the insertion (TS1′) and ring-opening (TS2′) transition state structures initiated by 3 are provided in Tables S7 and S11. By taking both ΔG‡TS1′ and ΔG‡TS2′ (see Table 5) into account, the (SS,SS) assembly mode of enchainment is found to be the most preferential pathway for all other achiral aluminum initiators (1, 2, 5, and 6), suggesting the formation of isotactic-biased PLAs. A decreased level of isoselectivity in these initiator systems might be attributed to the degree of chiral environment created around the aluminum center of these initiators as well as the frequent occurrence of the ligand inversion during the propagation reactions. The extent of the latter factor is related to the comparable stabilities of INT3′ in the (SS,SS) and i-(SS,SS) modes of enchainment (Figures S22, S23, S25, and S26). On the other hand, the stereoselectivity of the ring-opening polymerization of initiators 4 and 7 could be explained by the enantiomorphic-site control mechanism3a,12a,b,36 because the inversion of ligand configuration in these initiators does not occur during the polymerization process. By considering the

■

CONCLUSION In summary, bis(pyrrolidene) Schiff base aluminum complexes, 1−7, with a series of backbone linkers have been synthesized and characterized on the basis of NMR spectroscopy, elemental analysis, and X-ray crystallography for 3. All complexes are effective initiators for the polymerization of rac-lactide; the polymerization proceeds efficiently in a living manner. Studies of the polymerization kinetics in the presence of 1−7 consistently reveal a first-order behavior in monomers. The K

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules flexibility of backbone structure is demonstrated to exert a significant influence on the polymerization rate and the polymer microstructure. In the series of complexes, the rates of polymerization decrease in the order 1,2-benzylene (4), 1,3propylene (2) > 2,2-dimethyl-1,3-propylene (3) > 1,4-butylene (5) > rac-1,2-cyclohexylene (7) > 1,2-ethylene (1) ≫ 1,2phenylene (6). These results revealed that rate of polymerization generally increases with increasing the flexibility of the backbone structure, although the steric shielding of the aluminum center by the long butylene linker may have an additional role in complex 5. Complexes 1−3 and 6 afford the isotactic-biased stereoblock polylactide via a chain-end control mechanism with the highest Pm value of 0.80 observed for 3. In the case of the chiral aluminum complexes 4 and 7, the isotactic-biased stereoblock polylactides were proposed to form by a combination of enantiomorphic-site and chain-end control mechanism. DFT studies show that the activation free energies (ΔG‡TS1 and ΔG‡TS2) for the monomer insertion are inversely related to the complex bite angles of the key transition states (TS1 and TS2). Complexes bearing C3 and C4 backbone linkers (2−5) exhibit lower activation free energies than those containing C2 backbone linkers (1, 6, and 7). The correlations between activation free energies and bite angles of complexes indicate that the structure of the linker units has a significant influence on the polymerization activity. In general, the predicted catalytic activities of most complexes (1−3 and 6) are in good agreement with the experimentally observed polymerization rate constants. The inconsistency of the predicted results of complexes 4, 5, and 7 with the experimental observations suggests that unknown factors not contemplated by these calculations may contribute. The studies of the second monomer insertion in different assembly modes demonstrate that the steric interactions between the last enchained unit of the monomer and a stereogenic center of the incoming monomer together with the conformational interconversion of ligand are the key factors determining the stereoselectivity of insertion. We believe that these findings are important for the development of future catalyst systems through a rational design approach.

■

experiments were performed on a Bruker Advance 500 MHz spectrometer. The 1H and 13C NMR spectra were referenced internally to the residual proton impurity peaks according to the literature.37 Elemental analysis data (C, H, N) were obtained from a LECO elemental analyzer CS-932H. Differential scanning calorimetry (DSC) analysis was performed under N2 atmosphere with a NETZSCH DSC 204 F1 Phoenix instrument. The sample was first heated to 200 °C at a rate of 10 °C min−1 and cooled to 20 °C for two cycles. Then, the sample was cooled to −100 °C, followed by heating to 200 °C at a heating rate of 5 °C min−1 to determine the thermal properties. Gel permeation chromatography (GPC) measurements were conducted on a Polymer Laboratories PL-GPC-220 instrument equipped with PLgel 5 μm MIXED-D 300 × 7.5 mm column, and tetrahydrofuran (THF) was used as the eluent (flow rate: 1 mL min−1 at 40 °C). The number-average molecular weights (Mn) and polydispersity indice (PDIs) were calibrated against polystyrene (PS) standards. The Mn values of PLAs were corrected with a Mark−Houwink factor of 0.58 to account for the difference in hydrodynamic volume of polystyrene and polylactide. General Protocol for the Synthesis of H2L1 to H2L7. The reaction was performed according to the procedure previously reported.22 To a stirred solution of pyrrole-2-carboxaldehyde (3.00 g, 31.6 mmol) in ethanol (50 mL) was slowly added the corresponding diamine (15.8 mmol). The reaction mixture was stirred, and a catalytic amount of formic acid was added. The reaction mixture was stirred at room temperature for 4 h and during which time a white precipitate formed. The white solid was collected by filtration and dried under vacuum. N,N′-Bis((1H-pyrrol-2-yl)methylene)ethane-1,2-diamine (H2L1). Yield: 3.01 g, 89%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 11.42 (br s, 2H, NH), 8.11 (s, 2H, NCH), 6.90−6.87 (m, 2H, pyrrole-H), 6.45 (dd, 4JHH = 1.4 Hz, 3JHH = 3.5 Hz, 2H, Pyrrole-H), 6.12 (dd, 3JHH = 2.6 Hz, 3JHH = 3.5 Hz, 2H, Pyrrole-H), 3.76 (s, 4H, NCH2). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 152.6 (N = CH), 130.0 (Pyrrole-C), 121.9 (Pyrrole-CH), 113.4 (PyrroleCH), 108.8 (Pyrrole-CH), 61.4 (NCH2). Elemental analysis for C12H14N4: C, 67.27; H, 6.59; N, 26.15. Found: C, 67.07; H, 6.68; N, 26.47. N,N′-bis((1H-pyrrol-2-yl)methylene)propane-1,3-diamine (H2L2). Yield: 3.32 g; 92%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 11.38 (br s, 2H, NH), 8.11 (s, 2H, N = CH), 6.91−6.88 (m, 2H, Pyrrole-H), 6.46 (dd, 4JHH = 1.5 Hz, 3JHH = 3.5 Hz, 2H, Pyrrole-H), 6.14 (dd, 3JHH = 2.6 Hz, 3JHH = 3.5 Hz, 2H, Pyrrole-H), 3.56 (dt, 4JHH = 0.8, 3JHH = 6.5 Hz, 4H, NCH2), 1.93 (q, 3JHH = 6.9 Hz, 2H, CH2). 13 C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 151.8 (NCH), 130.0 (pyrrole-C), 121.9 (pyrrole-CH), 113.2 (pyrrole-CH), 108.7 (pyrrole-CH), 58.1 (NCH2), 32.5 (CH2). Elemental analysis for C13H16N4: C, 68.39; H, 7.06; N, 24.54. Found: C, 68.25; H, 7.32; N, 24.77. N,N′-Bis((1H-pyrrol-2-yl)methylene)-2,2-dimethylpropane-1,3-diamine (H2L3). Yield: 1.17 g, 57%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 11.25 (br s, 2H, NH), 8.08 (s, 2H, NCH), 6.91−6.89 (m, 2H, pyrrole-H), 6.46 (dd, 4JHH = 1.5 Hz, 3JHH = 3.5 Hz, 2H, pyrrole-H), 6.14 (dd, 3JHH = 2.6 Hz, 3JHH = 3.5 Hz, 2H, pyrroleH), 3.38 (d, 4JHH = 1.0, 4H, NCH2), 0.97 (s, 6H, C(CH3)2). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 151.8 (NCH), 130.1 (pyrrole-C), 121.7 (pyrrole-CH), 112.9 (pyrrole-CH), 108.7 (pyrroleCH), 69.4 (NCH2), 36.7 (C(CH3)2), 24.3(C(CH3)2). Elemental analysis for C15H20N4: C, 70.28; H, 7.86; N, 21.86. Found: C, 70.20; H, 7.95; N, 22.18. N,N′-Bis((1H-pyrrol-2-yl)methylene)-2-aminobenzylamine (H2L4). Yield: 2.49 g, 83%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 11.69 (br S, 1H, NH), 11.41 (br S, 1H, NH), 8.25 (s, 1H, NCH), 8.23 (s, 1H, NCH), 7.33−7.36 (m, 1H Ar-H), 7.25−7.31 (m, 1H Ar-H), 7.13−7.19 (m, 1H Ar-H), 7.07−7.09 (m, 1H, pyrrole-H), 7.01−7.05 (m, 1H, Ar-H), 6.84−6.88 (m, 1H, pyrrole-H), 6.69−6.74 (m, 1H, pyrrole-H), 6.36−6.40 (m, 1H, pyrrole-H), 6.21−6.25 (m, 1H, pyrrole-H), 6.07−6.12 (m, 1H, pyrrole-H), 4.88 (s, 2H, ArCH2). 13 C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 152.9 (NCH), 150.3 (ArC), 150.1 (NCH), 133.5 (ArCCH2), 130.9 (pyrrole-C),

EXPERIMENTAL SECTION

Materials and Methods. All manipulations with air- and/or water-sensitive compounds were carried out under a dry nitrogen atmosphere using standard Schlenk and cannula techniques in ovendried glassware or in a glovebox. Toluene was distilled from sodium benzophenone before use. Hexane and pentane were distilled from CaH2 prior to use. Benzyl alcohol was refluxed over sodium and then freshly distilled onto activated 4 Å molecular sieves. All solvents were degassed prior to use unless stated otherwise. NMR solvents were dried over 4 Å molecular sieves and degassed prior to use. A 2.0 M solution of trimethylaluminum in toluene (Aldrich) was used without purification. 1,2-Ethylenediamine (99%), 1,3-propanediamine (99%), 2,2-dimethyl-1,3-propanediamine (99%), 2-aminobenzylamine (98%), 1,4-butanediamine (99%), trans-1,2-diaminocyclohexane (99%), 1,2phenylenediamine (99.5%), pyrrole-2-carboxaldehyde (98%),and trimethylaluminum (2.0 M solution in toluene) were purchased from Aldrich and used as received. The monomers rac-lactide (Aldrich) and L-lactide (Purac) were recrystallized from dry toluene and then sublimed three times prior to use. All other chemicals are commercially available and were used as received unless otherwise stated. The 1H (399.87 MHz) and 13C (100.55 MHz) nuclear magnetic resonance (NMR) spectra were recorded on a Varian Unity Inova 400 MHz spectrometer at 300 K. The homonuclear decoupled 1H NMR L

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

data (100.55 MHz, [D6]DMSO, 300 K): δ 160.8 (NCH), 135.1 (pyrrole-C), 134.9 (pyrrole-CH), 116.6 (pyrrole-CH), 113.8 (pyrroleCH), 66.8 (NCH2), 35.8 (C(CH3)2), 25.9 (C(CH3)2), 22.0 (C(CH3)2). Elemental analysis for C16H21N4Al: C, 64.35; H, 7.14; N, 18.91. Found: C, 64.26; H, 7.35; N, 18.72. L4AlMe (4). Yield: 0.67 g, 59% (pale yellow solid). 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 8.57 (s, 1H, NCH), 8.49 (s, 1H, NCH), 7.42 (s, 1H pyrrole-H), 7.41 (s, 1H pyrrole-H), 7.32− 7.38 (m, 2H, Ar-H), 7.31−7.33 (m, 1H, Ar-H), 7.23−7.30 (m, 1H ArH), 6.93−6.94 (m, 2H pyrrole-H), 6.95 (d, 4JHH = 1.5 Hz, 2H, pyrroleH), 6.80 (d, 3JHH = 1.5 Hz, 2H, pyrrole-H), 6.35−6.36 (m, 2H, pyrrole-H), 6.30−6.33 (m, 2H, pyrrole-H), 4.94 (d, 2JHH = 16.3 Hz, 1H, N(ArCHH)N), 4.80 (d, 2JHH = 16.3 Hz, 1H, N(ArCHH)N), −1.21 (s, 3H, AlCH3). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 161.4 (CNH), 153.5 (CNH), 144.2 (ArC), 136.4 (pyrrole-C), 135.8 (ArCH), 134.6 (pyrrole-C), 134.5 (ArCH), 130.2 (ArCCH2), 129.0 (pyrrole-CH), 128.5 (pyrrole-CH), 126.2 (ArCH), 119.8 (ArCH), 119.2 (pyrrole-CH), 117.3 (pyrrole-CH), 114.2 (pyrroleCH), 113.8 (pyrrole-CH), 56.1 (ArCH2). Elemental analysis for C18H17AlN4: C, 68.34; H, 5.42; N, 17.71. Found: C, 67.58; H, 5.55; N, 18.02. L5AlMe (5). Yield: 0.53 g, 81% (white solid). 1H NMR data (399.87 MHz, [D6]DMSO 300 K): δ 8.32 (s, 2H, NCH), 6.92−6.88 (m, 2H, pyrrole-H), 6.66 (dd, 4JHH = 1.0, 3JHH = 3.3, 2H, pyrrole-H), 6.22 (dd, 3JHH = 3.4, 3JHH = 1.8, 2H, pyrrole-H), 3.80−3.67 (m, 2H, NCH2), 3.57−3.46 (NCH2), 2.05−1.92 (m, 2H, CH2), 1.89−1.77 (m, 2H, CH2), −0.81 (s, 3H, AlCH3). 13C NMR data are not available due to the limited solubility of complex. Elemental analysis for C15H19N4Al: C, 63.81; H, 6.78; N, 19.85. Found: C, 64.11; H, 6.72; N, 9.63. L6AlMe (6). Yield: 0.33 g, 26% (white solid). 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 8.94 (s, 2H, NCH), 7.80 (dd, 4JHH = 3.4, 3JHH = 5.9, 2H, ArH), 7.69 (m, 2H, pyrrole-H), 7.33 (dd, 3JHH = 5.9, 4JHH = 3.3, 2H, ArH), 7.05−7.01 (m, 2H, pyrrole-H), 6.45 (dd, 3 JHH = 3.6, 3JHH = 1.4, 2H, pyrrole-H), −1.13 (s, 3H, AlCH3). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 151.0 (NCH), 138.3 (pyrrole-C), 137.4 (ArC), 137.2 (pyrrole-CH), 127.4 (ArCH), 120.5 (ArCH), 117.2 (pyrrole-CH), 115.7 (pyrrole-CH). Elemental analysis for C17H15N4Al: C, 67.54; H, 5.00; N, 18.53%. Found: C, 67.42; H, 5.24; N, 18.32%. L7AlMe (7). Yield: 0.14 g, 20% (pale yellow solid). 1H NMR data (399.87 MHz, [D6]DMSO, 298 K): δ 8.48 (s, 1H, NCH), 8.18 (s, 1H, NCH), 7.43−7.41 (m, 1H, pyrrole-H), 7.22−7.21 (m, 1H, pyrrole-H), 6.75−6.73 (m, 1H, pyrrole-H), 6.67−6.65 (m, 1H, pyrrole-H), 6.30−6.27 (m, 1H, pyrrole-H), 6.22−6.20 (m, 1H, pyrrole-H), 3.11−3.01 (m, 2H, cyclohexyl-CH), 2.70−1.40 (m, 8H, cyclohexyl-CH2), −0.95 (s, 3H, AlCH3). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 150.8 (NCH), 129.9 (pyrrole-C), 121.6 (pyrrole-CH), 113.1 (pyrrole-CH), 108.6 (pyrrole-CH), 73.7 (cyclohexyl-CH), 34.4 (cyclohexyl-CH2), 24.2 (cyclohexyl-CH2). Elemental analysis for C17H21N4Al: C, 66.22; H, 6.86; N, 18.17%. Found: C, 66.51; H, 7.11; N, 17.90%. General Polymerization Procedure. In a nitrogen-filled glovebox, lactide (720 mg, 5.0 mmol) and benzyl alcohol (5.2 μL, 0.05 mmol) were placed in a polymerization ampule. To this ampule was added a solution of initiator (0.05 mmol) in toluene (6.00 mL) ([monomer]:[Al] = 100:1). The reaction was stirred for the desired reaction time at 70 °C. At the desired reaction time, the reaction was quenched with methanol (2−3 drops). The polymer was precipitated from excess methanol, collected by filtration, and dried in vacuo to a constant mass. Conversions were determined by integration of the monomer versus polymer methine resonances in the 1H NMR spectrum of the crude product (in CDCl3). General Procedure for Kinetic Studies. The polymerizations were carried out at 70 °C in a glovebox. The molar ratio of monomer to initiator was fixed at 50:1. At appropriate time intervals, 0.5 mL aliquots were removed and quenched with methanol. The solvent was removed in vacuo, and the percent conversion was determined by 1H NMR in CDCl3.

130.2 (pyrrole-C), 128.9 (ArCH), 127.9 (ArCH), 124.9 (ArCH), 123.8 (pyrrole-CH), 122.1 (pyrrole-CH), 117.8 (ArCH), 116.1 (pyrroleCH), 113.7 (pyrrole-CH), 10.9.8 (pyrrole-CH), 109.0 (pyrrole-CH), 59.5 (ArCH2). Elemental analysis for C17H16N4: C, 73.89; H, 5.84; N, 20.27. Found: C, 73.92; H, 6.06; N, 20.74. N,N′-Bis((1H-pyrrol-2-yl)methylene)butane-1,4-diamine (H2L5). Yield: 1.89 g, 99%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 11.37 (br s, 2H, NH), 8.09 (s, 2H, NCH), 6.89−6.86 (m, 2H, pyrrole-H), 6.45 (dd, 4JHH = 1.5 Hz, 3JHH = 3.5 Hz, 2H, pyrrole-H), 6.12 (dd, 3JHH = 2.6 Hz, 3JHH = 3.5 Hz, 2H, pyrrole-H), 3.56−3.50 (m, 4H, NCH2), 1.68−1.63 (m, 4H, CH2). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 152.2 (NCH), 130.8 (pyrrole-C), 122.5 (pyrrole-CH), 113.8 (pyrrole-CH), 109.4 (pyrrole-CH), 60.8 (NCH2), 29.2 (CH2). Elemental analysis for C14H18N4: C, 69.39; H, 7.49; N, 23.12. Found: C, 69.29; H, 7.66; N, 23.41. N,N′-Bis((1H-pyrrol-2-yl)methylene)benzene-1,2-diamine (H2L6). Yield: 1.11 g, 26%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 11.62 (br s, 2H, NH), 8.25 (s, 2H, NCH), 7.20−7.17 (m, 2H, ArH), 7.08−7.05 (m, 2H, ArH), 7.04−7.01 (m, 2H, pyrrole-H), 6.69− 6.67 (m, 2H, pyrrole-H), 6.24−6.21 (m, 2H, pyrrole-H). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 152.2 (NCH), 145.2 (ArC), 131.3 (pyrrole-C), 125.7 (ArCH), 124.0 (ArCH), 121.5 (pyrrole-CH), 116.2 (pyrrole-CH), 110.0 (pyrrole-CH). Elemental analysis for C16H14N4: C, 73.26; H, 5.38; N, 21.36. Found: C, 72.85; H, 5.70; N, 21.40. N,N′-Bis((1H-pyrrol-2-yl)methylene)cyclohexane-1,2-diamine (H2L7). Yield: 0.77 g, 55%. 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 11.19 (br s, 2H, NH), 7.98 (s, 2H, NCH), 6.81−6.79 (m, 2H, pyrrole-H), 6.34 (dd, 4JHH = 1.4, 3JHH = 3.5, 2H, pyrrole-H), 6.05 (dd, 3JHH = 3.5, 3JHH = 2.6, 2H, pyrrole-H), 3.24−3.18 (m, 2H, cyclohexyl-CH), 1.84−1.40 (m, 8H, cyclohexyl-CH2). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 151.6 (NCH), 130.6 (pyrrole-CH), 122.3 (pyrrole-C), 113.9 (pyrrole-CH), 109.4 (pyrrole-CH), 74.4 (cyclohexyl-CH), 34.1 (cyclohexyl-CH2), 25.0 (cyclohexyl-CH2). Elemental analysis for C16H20N4: C, 71.61; H, 7.51; N, 20.88. Found: C, 71.47; H, 7.81; N, 20.58. General Protocol for the Synthesis of L1AlMe (1) to L7AlMe (7). To a stirred suspension of H2L (4.67 mmol) in toluene (30 mL) was slowly added AlMe3 (2.33 mL of a 2.0 M solution in toluene, 4.67 mmol). The reaction mixture was stirred at 110 °C for 24 h and during which time a solid precipitated. For L2AlMe (2), L3AlMe (3), L6AlMe (6), and L7AlMe (7), the solid was formed after subsequent cooling at −20 °C. The solid was collected by filtration and dried under vacuum. The synthetic procedures of L2AlMe−L7AlMe are reported in the Supporting Information. L1AlMe (1). Yield: 0.86 g, 83% (white solid). 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 8.33 (s, 2H, NCH), 7.30 (s, 2H, pyrrole-H), 6.62 (d, 3JHH = 3.3, 2H, pyrrole-H), 6.18 (dd, 3JHH = 3.3, 3 JHH = 1.9, 2H, pyrrole-H), 4.00−3.93 (m, 2H, NCH2), 3.74−3.66 (m, 2H, NCH2), −0.97 (s, 3H, AlCH3). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 158.6 (NCH), 136.2 (pyrrole-C), 133.7 (pyrrole-CH), 115.6 (pyrrole-CH), 112.8 (pyrrole-CH), 46.9 (N(CH2). Elemental analysis for C13H15N4Al: C, 61.41; H, 5.95; N, 22.03. Found: C, 61.38; H, 6.14; N, 22.00. L2AlMe (2). Yield: 0.33 g, 28% (colorless crystals). 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 8.28 (s, 2H, NCH), 7.34− 7.32 (m, 2H, pyrrole-H), 6.67 (dd, 4JHH = 0.8, 3JHH = 3.4, 2H, pyrroleH), 6.25 (dd, 3JHH = 3.4, 3JHH = 1.9, 2H, pyrrole-H), 3.88−3.76 (m, 4H, NCH2), 2.12−2.01 (m, 1H, CH2), 1.86−1.74 (m, 1H, CH2), −0.90 (s, 3H, AlCH3). 13C NMR data (100.55 MHz, [D6]DMSO, 300 K): δ 160.3 (NCH), 135.5 (pyrrole-C), 134.8 (pyrrole-CH), 116.7 (pyrrole-CH), 113.8 (pyrrole-CH), 55.6 (NCH2), 30.2 (CH2). Elemental analysis for C14H17N4Al: C, 62.67; H, 6.39; N, 20.88. Found: C, 62.05; H, 6.35; N, 20.30. L3AlMe (3). Yield: 0.47 g, 57% (colorless crystals). 1H NMR data (399.87 MHz, [D6]DMSO, 300 K): δ 8.24 (s, 2H, NCH), 7.39− 7.36 (m, 2H, pyrrole-H), 6.68 (dd, 4JHH = 0.8, 3JHH = 3.4, 2H, pyrroleH), 6.26 (dd, 3JHH = 3.4, 3JHH = 1.9, 2H, pyrrole-H), 3.76 (d, 2JHH = 12.0, 2H, NCH2), 3.45 (d, 2JHH = 12.2, 2H, NCH2), 1.05 (s, 3H, C(CH3)2), 0.84 (s, 3H, C(CH3)2), −0.89 (s, 3H, AlCH3). 13C NMR M

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Computational Details. All geometry optimizations were carried out in the gas phase without molecular symmetry constraints using the hybrid meta exchange-correlation functional with double the amount of nonlocal exchange34b (M06-2X) level of theory as implemented in the Gaussian09 program.38 The all-electron basis set 6-311G(d,p) was applied to all atoms in the systems. Frequency calculations in the harmonic approximation performed at the same level of theory were used to verify that the optimized transition states possess only one imaginary vibrational frequency corresponding to the relevant reaction coordinate. The free energies were computed at 298 K. Crystal Structure Determination. X-ray diffraction data were measured on a Bruker-Nonius kappaCCD diffractometer with graphite monochromated Mo Kα radiation (λ = 0.710 73 Å) at 150(2) K. The structure was solved by direct methods with SIR9739 and refined with full-matrix least-squares calculations on F2 using SHELXL-97.40 Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre under the reference numbers CCDC 909527. Copies of the data can be obtained, free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK.

■

Research and Development Institute (KURDI) is also acknowledged for equipment support. We also gratefully acknowledge Prof. Philippe A. Bopp for his useful discussions.

■

(1) (a) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000, 12, 1841−1846. (b) Chiellini, E.; Solaro, R. Adv. Mater. 1996, 8, 305−313. (c) Amass, W.; Amass, A.; Tighe, B. Polym. Int. 1998, 47, 89−144. (d) Ragauskas, A. J. Science 2006, 311, 484−489. (2) (a) Albertsson, A.-C.; Varma, I. K. Biomacromolecules 2003, 4, 1466−1486. (b) Gupta, A. P.; Kumar, V. Eur. Polym. J. 2007, 43, 4053−4074. (c) Mainil-Varlet, P.; Rahn, B.; Gogolewski, S. Biomaterials 1997, 18, 257−266. (d) Langer, R.; Vacanti, J. Science 1993, 260, 920−926. (e) Auras, R.; Harte, B.; Selke, S. Macromol. Biosci. 2004, 4, 835−864. (3) (a) Li, G.; Lamberti, M.; Pappalardo, D.; Pellecchia, C. Macromolecules 2012, 45, 8614−8620. (b) Bakewell, C.; Platel, R. H.; Cary, S. K.; Hubbard, S. M.; Roaf, J. M.; Levine, A. C.; White, A. J. P.; Long, N. J.; Haaf, M.; Williams, C. K. Organometallics 2012, 31, 4729−4736. (c) Lamberti, M.; D’Auria, I.; Mazzeo, M.; Milione, S.; Bertolasi, V.; Pappalardo, D. Organometallics 2012, 31, 5551−5560. (d) Darensbourg, D. J.; Karroonnirun, O.; Wilson, S. J. Inorg. Chem. 2011, 50, 6775−6787. (e) Normand, M.; Dorcet, V.; Kirillov, E.; Carpentier, J.-F. Organometallics 2013, 32, 1694−1709. (f) Maudoux, N.; Roisnel, T.; Dorcet, V.; Carpentier, J.-F.; Sarazin, Y. Chem. - Eur. J. 2014, 20, 6131−6147. (g) Tabthong, S.; Nanok, T.; Kongsaeree, P.; Prabpai, S.; Hormnirun, P. Dalton Trans. 2014, 43, 1348−1359. (4) (a) D’Auria, I.; Lamberti, M.; Mazzeo, M.; Milione, S.; Roviello, G.; Pellecchia, C. Chem. - Eur. J. 2012, 18, 2349−2360. (b) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229− 3238. (c) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V. G., Jr.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125, 11350−11359. (d) Drouin, F.; Oguadinma, P. O.; Whitehorne, T. J. J.; Prud’homme, R. E.; Schaper, F. Organometallics 2010, 29, 2139−2147. (e) Wang, H.; Yang, Y.; Ma, H. Macromolecules 2014, 47, 7750−7764. (5) (a) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.; Williams, D. J. Chem. Commun. 2001, 283−284. (b) Aubrecht, K. B.; Hillmyer, M. A.; Tolman, W. B. Macromolecules 2002, 35, 644−650. (6) (a) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072−4073. (b) Cao, T.-P.-A.; Buchard, A.; Le Goff, X. F.; Auffrant, A.; Williams, C. K. Inorg. Chem. 2012, 51, 2157−2169. (c) Yang, S.; Du, Z.; Zhang, Y.; Shen, Q. Chem. Commun. 2012, 48, 9780−9782. (d) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2011, 50, 7718−7728. (e) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Organometallics 2009, 28, 1469−1475. (f) Buffet, J.C.; Kapelski, A.; Okuda, J. Macromolecules 2010, 43, 10201−10203. (g) Arnold, P. L.; Buffet, J.-C.; Blaudeck, R.; Sujecki, S.; Wilson, C. Chem. - Eur. J. 2009, 15, 8241−8250. (h) Bakewell, C.; White, A. J. P.; Long, N. J.; Williams, C. K. Inorg. Chem. 2015, 54, 2204−2212. (7) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 11583−11584. (b) Wu, J.; Chen, Y.-Z.; Hung, W.-C.; Lin, C.-C. Organometallics 2008, 27, 4970−4978. (c) Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C.; Iyer, S. S.; Pacold, M.; Phomphrai, K. J. Am. Chem. Soc. 2000, 122, 11845−11854. (d) Tang, H.-Y.; Chen, H.-Y.; Huang, J.-H.; Lin, C.-C. Macromolecules 2007, 40, 8855−8860. (8) (a) Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg. Chem. 2004, 43, 6717−6725. (b) Darensbourg, D. J.; Choi, W.; Karroonnirun, O.; Bhuvanesh, N. Macromolecules 2008, 41, 3493− 3502. (c) Hill, M. S.; Hitchcock, P. B. Chem. Commun. 2003, 1758− 1759. (d) Cushion, M. G.; Mountford, P. Chem. Commun. 2011, 47, 2276−2278. (9) (a) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem. 2006, 118, 7982−7985. (b) Sinenkov, M.; Kirillov, E.; Roisnel, T.; Fukin, G.; Trifonov, A.; Carpentier, J.-F. Organometallics 2011, 30, 5509−5523. (c) Buchard, A.; Platel, R. H.; Auffrant, A.; Le Goff, X. F.; Le Floch, P.;

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01381. Synthetic procedure of L2AlMe (2)−L7AlMe (7); crystallographic data for compound 3 in CIF format; tables of crystallographic data; plots of PLA Mn and molecular weight distribution as a function of monomer conversion for a rac-LA polymerization using 1−6/ PhCH2OH; semilogarithmic plots of L-lactide conversion versus time using 1−7/PhCH 2 OH; homonuclear decoupled 1H NMR spectra of PLA produced by 1,2,4,5,6/PhCH2OH; selected bond lengths and angles of the optimized geometries of complexes 1−7; calculated free energy profile for the ring-opening insertion of the first (S,S)-LA monomer by 1,2,4−7/ PhCH2OH; optimized structures of complexes 1−7; selected geometrical parameters for TS1 of complexes 1−7; selected geometrical parameters for TS2 of complexes 1−7; ring-opening insertion mechanism for the second (S,S)-LA monomer by 3; close-up of the TS1′and TS2′ structures for the (SS,SS) assembly mode of insertion by 3; free energy profiles for the second lactide insertion in different assembly modes mediated by 1,2,4,5,6; selected geometrical parameters for TS1′ structures of complexes 1−7; selected geometrical parameters for TS2′ structures of complexes 1−7 (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Thailand Research Fund (TRF) (Grant DIG5180026). S.T. thanks the Graduate School Kasetsart University of his scholarship for supporting the publication in an international journal. P.K. and S.P. thank Mahidol University and the Center for Innovation in Chemistry (PERCH-CIC) for financial support. Kasetsart University N

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Williams, C. K. Organometallics 2010, 29, 2892−2900. (d) Stevels, W. M.; Ankoné, M. J. K.; Dijkstra, P. J.; Feijen, J. Macromolecules 1996, 29, 3332−3333. (10) For a reviews on lactide polymerization, see: (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem. Soc., Dalton Trans. 2001, 2215−2224. (b) Dechy-Cabaret, O.; Martin- Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147−6176. (c) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Polym. Rev. 2008, 48, 11−63. (d) Dove, A. P. Chem. Commun. 2008, 6446−6470. (e) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165−173. (f) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Dalton Trans. 2009, 4832−4846. (g) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev. 2010, 39, 486−494. (h) Wu, J.; Yu, T.-L; Chen, C.-T; Lin, C.-C. Coord. Chem. Rev. 2006, 250, 602−626. (i) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Pure Appl. Chem. 2007, 79, 2013−2030. (j) Dijkstra, P. J.; Du, H.; Feijen, J. Polym. Chem. 2011, 2, 520−527. (k) Guillaume, S. M.; Kirillov, E.; Sarazin, Y.; Carpentier, J.-F. Chem. Eur. J. 2015, 21, 7988−8003. (11) Spassky, N.; Wisniewski, M.; Pluta, C.; Le Borgne, A. Macromol. Chem. Phys. 1996, 197, 2627−2637. (12) (a) Ovitt, T. M.; Coates, G. W. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4686−4692. (b) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316−1326. (c) Radano, C. P.; Baker, G. L.; Smith, M. R. J. Am. Chem. Soc. 2000, 122, 1552−1553. (d) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Angew. Chem., Int. Ed. 2002, 41, 4510−4513. (e) Zhong, Z.; Dijkstra, P. J.; Feijen, J. J. Am. Chem. Soc. 2003, 125, 11291−11298. (13) (a) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. J. Am. Chem. Soc. 2002, 124, 5938−5939. (b) Tang, Z.; Chen, X.; Pang, X.; Yang, Y.; Zhang, X.; Jing, X. Biomacromolecules 2004, 5, 965−970. (c) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072−4073. (d) Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem. - Eur. J. 2007, 13, 4433−4451. (e) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; Pugh, R. I.; White, A. J. P. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15343−15348. (f) Cross, E. D.; Allan, L. E. N.; Decken, A.; Shaver, M. P. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1137−1146. (g) Agatemor, C.; Arnold, A. E.; Cross, E. D.; Decken, A.; Shaver, M. P. J. Organomet. Chem. 2013, 745−746, 335−340. (h) Chen, H.-L.; Dutta, S.; Huang, P.-Y.; Lin, C.-C. Organometallics 2012, 31, 2016− 2025. (i) Du, H.; Pang, X.; Yu, H.; Zhuang, X.; Chen, X.; Cui, D.; Wang, X.; Jing, X. Macromolecules 2007, 40, 1904−1913. (j) Chisholm, M. H.; Gallucci, J. C.; Quisenberry, K. T.; Zhou, Z. Inorg. Chem. 2008, 47, 2613−2624. (14) Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2004, 126, 2688−2689. (15) Press, K.; Goldberg, I.; Kol, M. Angew. Chem., Int. Ed. 2015, DOI: 10.1002/anie.201503111. (16) Du, H.; Velders, A. H.; Dijkstra, P. J.; Sun, J.; Zhong, Z.; Chen, X.; Feijen, J. Chem. - Eur. J. 2009, 15, 9836−9845. (17) Sumrit, P.; Hormnirun, P. Macromol. Chem. Phys. 2013, 214, 1845−1851. (18) (a) Whitelaw, E. L.; Lorain, G.; Mahon, M. F.; Jones, M. D. Dalton Trans. 2011, 40, 11469−11473. (b) dos Santos Vieira, I.; Whitelaw, E. L.; Jones, M. D.; Herres-Pawlis, S. Chem. - Eur. J. 2013, 19, 4712−4716. (c) Hancock, S. L.; Mahon, M. F.; Jones, M. D. Dalton Trans. 2013, 42, 9279−9285. (19) Pilone, A.; Press, K.; Goldberg, I.; Kol, M.; Mazzeo, M.; Lamberti, M. J. Am. Chem. Soc. 2014, 136, 2940−2943. (20) (a) Du, H.; Velders, A. H.; Dijkstra, P. J.; Zhong, Z.; Chen, X.; Feijen, J. Macromolecules 2009, 42, 1058−1066. (b) Schwarz, A. D.; Chu, Z.; Mountford, P. Organometallics 2010, 29, 1246−1260. (c) Zpeitia, H. F. S.; Cortes-Llamas, S. A.; Vengoechea-Gómez, F. A.; Rufino-Felipe, E.; Crespo-Velasco, N. T.; Muñoz-Hernández, M.-Á . Main Group Chem. 2011, 10, 127−140. (21) For selected references of DFT studies, see: (a) Marshall, E. L.; Gibson, V. C.; Rzepa, H. S. J. Am. Chem. Soc. 2005, 127, 6048−6051. (b) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; Rzepa, H. S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 2006, 128, 9834−9843. (c) Fang, J.; Yu, I.; Mehrkhodavandi, P.; Maron, L. Organometallics 2013, 32, 6950−6956. (d) Barros, N.; Mountford, P.; Guillaume, S.

M.; Maron, L. Chem. - Eur. J. 2008, 14, 5507−5518. (e) Miranda, M. O.; DePorre, Y.; Vazquez-Lima, H.; Johnson, M. A.; Marell, D. J.; Cramer, C. J.; Tolman, W. B. Inorg. Chem. 2013, 52, 13692−13701. (f) Della Monica, F.; Luciano, E.; Roviello, G.; Grassi, A.; Milione, S.; Capacchione, C. Macromolecules 2014, 47, 2830−2841. (g) Börner, J.; dos Santos Vieira, I.; Pawlis, A.; Döring, A.; Kuckling, D.; HerresPawlis, S. Chem. - Eur. J. 2011, 17, 4507−4512. (22) Weber, J. H. Inorg. Chem. 1967, 6, 258−262. (23) Atwood, D. A.; Harvey, M. J. Chem. Rev. 2001, 101, 37−52. (24) Benn, R.; Rufińska, A. Angew. Chem., Int. Ed. Engl. 1986, 25, 861−881. (25) The τ value ranges from 0 (perfectly square pyrimidal) to 1 (perfectly trigonal bipyradal). (a) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356. (b) Munoz-Hernandez, M.-A.; Keizer, T. S.; Wei, P.; Parkin, S.; Atwood, D. A. Inorg. Chem. 2001, 40, 6782−6787. (26) (a) Barakat, I.; Dubois, P.; Jérôme, R.; Teyssié, P. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 505−514. (b) Baran, J.; Duda, A.; Kowalski, A.; Szymanski, R.; Penczek, S. Macromol. Rapid Commun. 1997, 18, 325−333. (c) Kowalski, A.; Duda, A.; Penczek, S. Macromolecules 1998, 31, 2114−2122. (27) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2008, 47, 6840−6849. (28) Ishii, R.; Nomura, N.; Kondo, T. Polym. J. 2004, 36, 261−264. (29) Tang, Z.; Chen, X.; Yang, Y.; Pang, X.; Sun, J.; Zhang, X.; Jing, X. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 5974−5982. (30) Pang, X.; Du, H.; Chen, X.; Wang, X.; Jing, X. Chem. - Eur. J. 2008, 14, 3126−3136. (31) Yu, I.; Acosta-Ramírez, A.; Mehrkhodavandi, P. J. Am. Chem. Soc. 2012, 134, 12758−12773. (32) Pilone, A.; De Maio, N.; Press, K.; Venditto, V.; Pappalardo, D.; Mazzeo, M.; Pellecchia, C.; Kol, M.; Lamberti, M. Dalton Trans. 2015, 44, 2157−2165. (33) (a) 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. (b) Thakur, K. A. M.; Kean, R. T.; Hall, E. S.; Kolstad, J. J.; Munson, E. J. Macromolecules 1998, 31, 1487−1494. (c) Thakur, K. A. M.; Kean, R. T.; Zell, M. T.; Padden, B. E.; Munson, E. J. Chem. Commun. 1998, 1913−1914. (d) Chisholm, M. H.; Iyer, S. S.; McCollum, D. G.; Pagel, M.; Werner-Zwanziger, U. Macromolecules 1999, 32, 963−973. (e) Zell, M. T.; Padden, B. E.; Paterick, A. J.; Thakur, K. A. M.; Kean, R. T.; Hillmyer, M. A.; Munson, E. J. Macromolecules 2002, 35, 7700−7707. (f) Coudane, J.; Ustariz-Peyret, C.; Schwach, G.; Vert, M. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 1651−1658. (34) (a) Zhao, U.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 1−17. (b) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (c) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157−167. (35) Hehre, W. J.; Ditchfie, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (36) Honrado, M.; Otero, A.; Fernández-Baeza, J.; Sánchez-Barba, L. F.; Garcés, A.; Lara-Sánchez, A.; Rodríguez, A. M. Organometallics 2014, 33, 1859−1866. (37) Gottlieb, H. E.; Kotlyar, B.; Nudelman, A. J. Org. Chem. 1997, 62, 7512−7515. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson,G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; O

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc.: Wallingford, CT, 2009. (39) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119. (40) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

P

DOI: 10.1021/acs.macromol.5b01381 Macromolecules XXXX, XXX, XXX−XXX