Polymerization of ε-Caprolactone Using Bis (phenoxy)-amine

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Polymerization of ε‑Caprolactone Using Bis(phenoxy)-amine Aluminum Complex: Deactivation by Lactide Phongnarin Chumsaeng,† Setsiri Haesuwannakij,‡ Sareeya Bureekaew,§ Vuthichai Ervithayasuporn,† Supawadee Namuangruk,∥ and Khamphee Phomphrai*,‡ †

Department of Chemistry, Faculty of Science, Mahidol University, Ratchathewi, Bangkok 10400, Thailand Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), Wangchan, Rayong 21210, Thailand § Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), Wangchan, Rayong 21210, Thailand ∥ National Nanotechnology Center, National Science and Technology Development Agency, Klong Luang, Pathumthani 12120, Thailand

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‡

S Supporting Information *

ABSTRACT: Polymerizations of biodegradable lactide and lactones have been the subjects of intense research during the past decade. They can be polymerized/copolymerized effectively by several catalyst systems. With bis(phenolate)-amine aluminum complex, we have shown for the first time that lactide monomer can deactivate the aluminum complex during the ongoing polymerization of ε-caprolactone to a complete stop. After hours of dormant state, the aluminum complex can be reactivated again by heating at 100 °C without the addition of any external chemicals still giving polymer with narrow dispersity. Studies using NMR, in situ FTIR, and single-crystal X-ray crystallography indicated that the coordination of the carbonyl group in lactyl unit was responsible for the unusual behavior of lactide. In addition, the unusual methyl-migration from methyl lactate ligand to the amine side chain of the aluminum complex was observed through intermolecular nucleophilic-attack mechanism.

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INTRODUCTION Ring-opening polymerization (ROP) of cyclic esters is one of the most promising methodologies for the preparation of polyesters, especially from ε-caprolactone (CL) and lactide (LA).1,2 They are biodegradable and biocompatible polymers that provide eco-friendly alternative to replace the nondegradable polymers and have found uses in biomedical and pharmaceutical applications (e.g., artificial skins, dissolvable sutures, drug delivery).3,4 In order to synthesize the polyesters via ROP, metal alkoxide complexes were extensively used as initiators.1,3,5−10 The catalyst design has a crucial impact on the outcome of the polymerization to control molecular weight, stereochemistry, and dispersity of the resulting polymer.6,7 In the past decades, a wide range of bis(phenolate) ligands have been developed due to the ease of ligand synthesis and modification.11 The ligands feature N2O2 bowl-shaped structure that can stabilize the metals in its adjustable pocket. Previously, bis(phenolate) ligands have been developed as an alternative to nonmetallocene catalysts for α-olefin polymerization.12−20 Pioneer work reported by Kol and Okuda shown that metal group IV bearing bis(phenolate) complexes act as excellent catalysts for α-olefin polymerization.11,21−28 This © XXXX American Chemical Society

ligand framework was also expanded to titanium and zirconium metals in the ring-opening polymerization (ROP) of lactide.29−31 The polymerization activity depended on the metal, coordination number, and the phenolate substituents. A more open site on metal (larger metal, less coordination number, and less bulky group) leads to higher activity. However, the activity of titanium complexes was rather low and the PDI values of PLAs derived from titanium and zirconium complexes were both broad at high conversion.29,30 Aluminum is also one of the most studied active metals having good catalytic reactivity toward ROP of lactide and cyclic lactone resulting in good stereocontrol and narrow dispersity.32−34 Hillmyer and Tolman,35 Chen,36 and Gibson37 studied aluminum complexes containing similar bis(phenolate)-amine ligand templates. The catalysts exhibited living character and well-controlled polymerization with narrow dispersity for both CL and LA. Notably, the polymerization rates of LA and CL are very different where the reactivity of LA is greatly slower than that of CL. Recently, Liang38 reported that titanium tridentate bis(phenolate) Received: May 18, 2018

A

DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry complexes bearing an N-alkyl substituent acted as single-site, living catalysts in ROP of CL. In contrast, they were found to be ineffective for the ROP of rac-LA or L-LA under various conditions (e.g., toluene, 25 or 80 °C, 24 h; melt at 150 °C, 24 h). We also revisited the chemistry of this bis(phenolate)amine ligand template with variation of substituents on amine side arm (Scheme 1).39 The bis(phenoxy)-amine aluminum

entropy of ROP, both ROP of LA and CL monomers are not significantly different, ΔG° ≈ −11 kJ mol−1 at 300 K,40 although the interactions between the catalysts and monomers would also determine the polymerization rates.41 Moreover, if the diethylamine group on the ligand was modified to a pyridine group, the aluminum catalyst was more active toward LA but less active toward CL. This result was very puzzling and required further study that may shed light to a better understanding of the ring-opening event. We hypothesize that the chelation of the ester group during the resting state of the polymerization may play a crucial role and, once thoroughly understood, would lead to a better design of more efficient catalysts. Herein, the different resting states of the polymerization of LA and CL catalyzed by bis(phenoxy)-amine aluminum complex were studied and observed experimentally. We aimed to clarify the effect of carbonyl chelation in the ringopening process to the catalytic performance. To demonstrate their striking structural differences, we showed for the first time that LA can be added to deactivate the aluminum complex during the ongoing polymerization of CL. Subsequently, the deactivated complex can be reactivated by heat treatment even after hours of dormant state.

Scheme 1. (a) Polymerization of Cyclic Esters Catalyzed by Aluminum Complex 1 and (b) Ring-Opened Products of LA (I) and CL (II) (from Ref 39)

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RESULT AND DISCUSSION The ROP of cyclic esters consists of three common processes: initiation, propagation/monomer insertion, and chain-transfer/ termination. The monomer insertion is undoubtedly among the most important events in ROP. If we consider the structures of CL and LA, both monomers contain a similar ester group. However, they are drastically different in the intermediate structures after the ring-opening process. The ester group of LA in the ring-opened product readily forms a five-membered metallacyclic ring with the metal center, while the ester group of CL does not coordinate due to its longer distance between metal and carbonyl group (Scheme 1b). We

complex 1 (LAlOiPr, where L is a bis(phenoxy)-amine ligand) exhibited unusual behavior for ROP of CL and LA (Scheme 1a). The catalyst exhibited high activity for the polymerization of CL ([CL]0/[catalyst] = 300), giving complete consumption of monomer in only 2.5 min at 70 °C. On the other hand, the reactivity toward polymerization of LA was totally shut down under the same condition.39 If we consider the thermodynamic driving force, ΔH° relates to ring-strain and ΔS° relates to the

Figure 1. 1H NMR spectra (C6D6, 30 °C) of (a) 1 + CL; (b) after subsequent addition of LA left for 10 min; and (c) followed by subsequent addition of CL left for 2 h. B

DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(Me)C(O)OCH(Me)C(O)OiPr. This product afforded two νCO frequencies at 1749 and 1674 cm−1 apparently as a result of the noncoordinating CO and coordinating CO, respectively. The coordinating νCO frequency was confirmed by comparison with νCO in LAl(ML), where there is only one CO bond, appeared lower at 1674 cm−1 due to electron donation to the metal center upon coordination. On the other hand, the reaction of complex 1 and CL afforded only one νCO frequency at 1723 cm−1 (Figure 2b) that is similar to those of free PCL and CL (at 1738 and 1726 cm−1, respectively), confirming that the carbonyl group of ringopened CL does not coordinate to the metal center. In accordance with the result from FTIR technique, the 13 C{1H} NMR spectrum of complex 1 + LA revealed two signals of the carbonyl groups at δ = 189.77 and 167.79 ppm (see Supporting Information (SI), Table S1 and Figure S3). These data also strongly confirmed the coordination of CO in the ring-opened product in solution for LA. The resonance for coordinated carbonyl group revealed the significantly downfield shift from 169.44 ppm (carbonyl of PLA)42 to 189.77 ppm. NMR spectrum of LAl(ML) was also examined to compare the chemical shift of the coordinated carbonyl group (Figure S7). It showed almost the same chemical shift at 190.33 ppm, confirming the identity of the coordinated carbonyl group as shown in Scheme 1b. Complex 1 was used as a catalyst for the polymerization of CL at higher monomer:catalyst ratio. Three sets of polymerizations using [CL]:[1] molar ratios of 100:1, 200:1, and 300:1 in benzene were studied at room temperature (Figure 3). CL

proposed that the chelation of the carbonyl group to the metal complex can play an important role in ROP process if the interaction is strong enough. To confirm this hypothesis, complex 1 was reacted with LA and CL to compare the ringopened products. Lactyl-containing analogue, LAlOCH(Me)C(O)OMe (LAl(ML)), was synthesized separately from the reaction of LAlMe and methyl (s)-lactate (ML) for comparison to mimic the coordination of carbonyl group to Al atom. The insertion step was investigated by a sequential addition of monomers to complex 1 (Figure 1) using a low monomer:catalyst mole ratio so that the chain end can be observed by NMR. First, only 1 equiv of CL was reacted with 1 to mimic poly(ε-caprolactone) (PCL) chain attached to the aluminum complex. The insertion completed immediately (Figure 1a). Another equivalent of LA was then subsequently added (Figure 1b). 1H NMR spectra revealed that only one LA molecule readily inserted to Al−OCL bond at room temperature. The O−CH2 signal of the CL unit attached to Al metal (4.48 ppm, position a of Figure 1a) disappeared and shifted to high field (4.02 ppm, position a of Figure 1b) along with the occurrence of O−CH(Me) of lactide at δ = 4.60 ppm (position h). The single insertion of LA was confirmed by MALDI-TOF mass spectra of the obtained oligomeric PCL having a mass pattern of 114.07n + 227.1 assignable to H(LA)(CL)nOiPr + Na+ (Supporting Information (SI), Figure S12). To demonstrate if LA can inhibit further insertion of CL, another equivalent of CL was subsequently added (Figure 1c). Surprisingly, the insertion of CL to the Al−OLA bond did not take place and the CL monomer remained intact even after the reaction was left for 2 h at room temperature. This was confirmed by MALDI-TOF mass spectra where the same mass pattern of 114.07n + 227.1 was still observed (SI, Figure S12). In addition, the characteristic NMR signals of the catalyst before and after the addition of LA or CL were still identical. This observation suggests that LA does not destruct the ligandAl framework and the deactivation of the aluminum complex after LA addition is not a result of catalyst decomposition. The chelation of carbonyl group in solution was investigated using in situ FTIR spectroscopy (Figure 2). The νCO frequencies of free LA and polylactide (PLA) appear at 1771 and 1760 cm−1, respectively. The reaction of complex 1 with 1 equiv of LA (1 + LA) gave the inserted product LAlOCH-

Figure 3. Polymerizations of CL (100:1, 200:1, and 300:1) and DVL (100:1) using 1 at room temperature with subsequent addition of 5 equiv of LA.

was consumed rapidly by 1 giving conversions around 40% after 4, 6, and 9 min for the 100:1, 200:1, and 300:1 molar ratio, respectively. At these time intervals, 5 equiv of LA was immediately added into the reaction mixtures. Upon addition of LA, the catalyst was deactivated and the reactions decelerated dramatically and halted within a few minutes. We found that the catalyst remained deactivated even after 12 h. Other lactones such as β-butyrolactone, γ-butyrolactone, and δ-valerolactone (DVL), were also tested in place of CL. Only DVL can be rapidly polymerized by 1 and the catalyst was deactivated similarly after the addition of 5 equiv of LA (Figure 3). We have shown in Figures 1 and 3 that the addition of lactide monomer can deactivate the aluminum complex. The strong coordination of the carbonyl group of lactyl unit

Figure 2. In situ FTIR spectra in chloroform showing ester ketonic vibration’s region of (a) LA and (b) CL series. C

DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX

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(position a), PCL having Mn = 8050 g/mol43 and Đ = 1.07 was obtained. The molecular weight of PCL remained rather unchanged even after 5 h of catalyst deactivation (position b, Mn = 8260 g/mol,43 Đ = 1.09). After thermal reactivation at 100 °C, the molecular weight of PCL increased to Mn = 11550 g/mol43 and still remained monomodal distribution with narrow dispersity (Đ = 1.20). This result revealed that, once deactivated by LA, the aluminum complex was dormant for hours and was reactivated back on after heating. Interestingly, CL insertion continued from the previously idle active site. However, the slower rate of polymerization after heating at 100 °C (kobs = 5.8 × 10−3 s−1) was observed compared to the initial faster polymerization rate (kobs = 7.4 × 10−2 s−1) (SI, Figure S15). The slower polymerization rate and a slight broadening of dispersity were the result of a residual LA monomer in the reaction. At the beginning of thermal reactivation of the catalyst, there is still a small amount of LA left (position b in Figure 4a). We attempted to understand how LA was incorporated into the PCL chain during this heating period. Microstructure of the polymer after thermal activation was therefore investigated. At this point (Figure 4a, position b), approximately 5 mol % of LA and 40 mol % of CL monomer remained in the solution. Thus, we set up a separate polymerization to mimic the polymerization of LA and CL at the same monomer ratio (1/LA/CL = 1:5:40) to study the incorporation of residual LA into the propagating chain and to avoid the interference of the previous PCL segment. After the addition of CL into the mixture of LA and the catalyst, the microstructure of the polymer was monitored every 3 h, and the result of mimicked ROP was shown in Table 1. 1H NMR analysis revealed that, in term of completion percentage, CL was consumed faster than LA. Notably, the completion percentage of LA and CL was observed in parallel pattern along the polymerization time as shown in Figure 5, and the mole ratio of the incorporated LA in the polymer is almost constant at different polymerization time. To understand the incorporation behavior of LA in the PCL main chain, GPC and 13C NMR analyses were examined. GPC traces (see SI, Figure S18) showed monomodal distribution with increasing molecular weight at longer time. The resulting polymers have low dispersity values of ≤1.30 with molecular weight in agreement with the expected values. The carbonyl regions of time-resolved 13C NMR spectra were reported in Figure 6. Triad sequence signal were assigned based on the reported literature.44 Characteristic triad sequences of single lactidyl units (LLCapCap, CapLLCap, and CapCapLL) were apparently observed throughout the polymerization. In contrast, the double/triple lactidyl units in triad sequences (LLLLCap and LLLLLL) were very minor. The average length of lactidyl (LLA) units calculated from the 13 C NMR intregation45 remained closed to 1 unit (Table 1). These values revealed the structure of the polymer obtained after thermal reactivation of the catalyst to be short PCL blocks separated by, on average, only one LA unit distributed over the polymer chain. Thus, the overall polymer structure can be described as a long PCL chain (before LA addition) followed by shorter PCL chains that were separated by only one LA unit (after thermal reactivation). We have shown that the chelation of the ester group dramatically affects the activity of the aluminum catalyst. Although the chelating product was evidenced in solution, we have attempted to characterize the chelating product crystallographically. Therefore, the lactyl-containing analogue,

prevents the next monomer to coordinate and, as a result, deactivates the catalyst. The structures of the species resulted from adding CL or LA to complex 1 are depicted in Scheme 2. Scheme 2. Proposed Structures of Complex 1 after Reacted with CL (left) and LA (right)

To resume the catalytic reactivity, the strong coordination of the carbonyl group to Al center must be alleviated. Thus, polar and nonpolar solvents, such as pyridine, THF, dichloromethane, and toluene, were tested but still cannot reactivate the complex. Alternatively, heat was investigated as a mean to reactivate the aluminum complex to resume the polymerization without the addition of any external chemicals. To test this hypothesis, a polymerization of CL catalyzed by 1 ([CL]:[1] = 100:1) was carried out at room temperature. A few minutes after the addition of 5 equiv of lactide, the aluminum catalyst was deactivated completely giving a conversion at 60% (Figure 4a, position a). The polymerization remained off even after 5 h

Figure 4. (a) Polymerization deactivated by LA and reactivated by heat ([CL]0 = 0.5 M in C6D6, [CL]0/[1] = 100:1 and (b) GPC traces of the PCL produced at positions a, b, and c.

(position b). The reaction was subsequently heated at 100 °C. As anticipated, the polymerization resumed and continued to 92% conversion (position c) proving that heat can be used to reactivate the aluminum catalyst. The molecular weights of the polymers taken at different reaction time were examined by GPC as shown in Figure 4b. After the addition of lactide D

DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. ROP of CL in the Present of LAa conversion (%)

composition (%)

time (h)

LA

CL

LA

CL

Mn(calc)b (kDa)

Mnc (kDa)

Đc

LLAd

LCLe

3 6 9 12 15

38 53 64 70 75

66 85 91 95 96

6.0 6.7 7.1 7.5 7.9

94.0 93.3 92.9 92.5 92.1

7.3 9.5 10.3 10.8 10.9

6.5 8.3 8.7 9.2 9.6

1.30 1.29 1.30 1.28 1.26

1.06 1.08 1.08 1.08 1.09

18.28 16.03 15.18 15.21 14.42

Polymerizations was prepared in toluene at 100 °C, [CL]0 = 0.54 M, [LA]0/[CL]0 = 5:40, [LA + CL]/[I] = 100:1. bCalculated Mn = [conversion(LA) × M(LA)/I × MW(LA)] + [conversion (CL) × M(CL)/I × MW(CL)]. cMeasured by GPC in THF calibrated with polystyrene standards and corrected using correction factor 0.45. dExperimental average length of lactidyl unit as determined by 13C NMR analysis. e Experimental average length of caproyl unit as determined by 13C NMR analysis. a

Figure 5. Relationship of % conversion (blue circles, LA; red triangle, CL) and % composition of LA (gray ×, LA) at different polymerization time.

Figure 7. X-ray crystal structure of complex 2 with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Al1−O1 = 1.757(2), Al1−O2 = 1.770(2), Al1−N1 = 2.153(2), Al1−O3 = 1.781(2), Al1−O4 = 1.880(2), O1−Al1−O2 = 116.8(1), O1−Al1− O4 = 93.29(9), O4−Al1−N1 = 173.4(1).

position with the axial O4−Al1−N1 angle of 173.4(1)° close to the ideal linear angle (180°). The O−C−O angle in the carboxylate group (125.3(3)°) is rather bigger than the related bond angle in a free acid (123.29(8)°)47 as a result of the coordination to the metal. The two C−O bond lengths of the carboxylate group are rather similar (1.230(4) and 1.295(4) Å) indicating delocalization of the negative charge on carboxylate group. Moreover, the negative charge also affects the Al− O(carbonyl) bond distance providing shorter Al(1)−O(4) distance (1.880(2) Å) compared to the coordinating neutral ester groups to aluminum observed in other aluminum complexes (2.018−2.165 Å).48−50 The Al(1)−O(4) distance is slightly longer than Al(1)−O(3) distance (1.781(2) Å) due to the resonance of a negative charge on carboxylate group. The migration of methyl to amine group results in a positive charge on nitrogen atom causing a repulsion between the ammonium side chain and Al center. The methyl migration from methyl lactate is very unusual considering that methyl lactate has been used to probe the resting state of the catalyst species without prior evidence of structural decomposition through Oester-Me bond.51,52 To the best of our knowledge, this is the first time that the methyl migration from methyl lactate ever happened in complexation

Figure 6. Carbonyl range of 13C NMR spectra (150 MHz, CDCl3) of resulting polymer from Table 1 at different time.

LAlOCH(Me)C(O)OMe (LAl(ML)) was recrystallized in the mixed solvent between hexane and chloroform or dichloromethane. Surprisingly, a clean unexpected methyl-migration product (complex 2) was observed within a week with the change of 1H NMR signal of the OMe group from 4.09 to 3.05 ppm (SI, Figures S6 and S8, respectively). The crystal structure of complex 2 was obtained as shown in Figure 7. The structural geometry of aluminum core of complex 2 can be viewed as a distorted trigonal bipyramid with τ value46 of 0.76. The structure obviously reveals the coordination of carbonyl group giving a five-membered metallacyclic ring. The unexpected methyl migration from methyl lactate to amine side chain was observed generating the carboxylate and ammonium ions. The chelating carboxylate locates on axial E

DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The transition states of the first (TS1) and second (TS2) nucleophilic substitution were shown in Figure 8b and c, respectively. The angles of O−CMe-N in both TS1 and TS2 indicate good linearities, 173.4° and 171.8°, respectively, which were commonly found in nucleophilic substitution reaction. Then, the Berry pseudorotation occurred to obtain complex 2 with the activation barrier of 6.7 kcal mol−1 (Scheme 3 and Figure 10). The geometry of TS3 is intermediate with the geometry index (τ) of 0.64. In comparison between the formation of INT2 and complex 2, the formation of complex 2 is more thermodynamically favorable than that of INT2, due to smaller ΔG, as shown in Figure 10. We also confirmed this methyl migration mechanism experimentally by using a bulkier isopropyl (s)-lactate analogue (SI, Figure S10) in place of methyl (s)-lactate. As expected, there is no observed isopropyl migration after leaving the compound over a week indicating that the bulkier isopropyl group prevented the nucleophilic substitution that would have led to the alkyl migration. Complex 2 was also studied for the ROP of CL. Unfortunately, it could not initiate the polymerization even when the reaction was heated up to 100 °C for 12 h (SI, Figure S14). Even though the five-coordinated Al center was suitable for the coordination of monomer at the metal center, the negatively charged carboxylate group decreased Lewis acidity of Al metal, which may limit the coordination of the monomer.

with a metal. Nevertheless, examples of the alkylation of tertiary amines by other esters, which appeared unintentionally and in an intramolecular context, have been investigated elsewhere.53−56 The unexpected migration product thus requires further investigation. A plausible mechanism of the methyl migration was proposed via a nucleophilic substitution. Nitrogen atom of amine acts as a nucleophile attacking the electron deficient methyl carbon atom. The O−CMe bond then dissociated followed by a rearrangement process. To gain a better understanding of methyl migration process, computational studies were used to gain insight information on unknown species and mechanism. First, the structure of LAl(ML) was optimized and shown in Figure 8a. The

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CONCLUSION We have demonstrated the importance of the structural differences of cyclic esters. The polymerizations of LA and lactones have been studied extensively in literature and yet still fascinate us by their structural differences that give rise to different activities and chemistry. The majority of the catalysts can polymerize or copolymerize these monomers with ease. However, this is the first time that LA monomer was shown to be a reagent that induced catalyst deactivation during the ongoing polymerization of lactones. This unusual behavior comes from the coordination of carbonyl group in lactyl unit giving a stable five-membered metallacycle. The aluminum catalyst was shown to be living even after hours of dormant state and can be reactivated again by heat without the addition of any external chemicals. The resulting polymer can be described as a long PCL chain followed by short blocks of PCL that were separated by single-LA units. In addition, the unusual methyl-migration from methyl lactate ligand to the amine side chain was observed through intermolecular nucleophilic-attack mechanism. The bis(phenoxy)-amine aluminum complex is the first example to show that LA can be used to temporarily halt the ongoing polymerization of lactones that may find uses in active polymer chain or active catalyst counting. It can be used to expand the synthesis of polymer or copolymer having end-functionalization of lactide for biomedical applications.

Figure 8. Gas-phase optimized geometry of (a) LAl(ML) complex, (b) TS1, (c) TS2, and (d) TS3. Hydrogen atoms of ligand are omitted for clarity.

geometry of optimized LAl(ML) is distorted square-based pyramidal with the geometry index (τ) of 0.35.46 The Mulliken charge of Al atom is 1.06 which indicates slightly Lewis acidic properties of LAl(ML). The enthalpies of the whole possible methyl migration process were performed in Figure 9. Starting from LAl(ML), the nucleophilic substitution between nitrogen side arm as a nucleophile and methyl ester of methyl lactate analogue as an electrophile was calculated. The intermolecular reaction is energetically more favorable as compared to its intramolecular analogue with the energy of 25.4 and 33.1 kcal mol−1, respectively.

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EXPERIMENTAL DETAILS

General Information. All reactions were carried out under dry nitrogen atmosphere using standard Schlenk and drybox techniques. All solvents were dried using MB-SPS solvent purification system. N,N-Bis[methyl(2-hydroxy-3-tert-butyl-5-methylphenyl)]-N′,N′-diethylethylenediamine (LH),57 LAlOiPr (1),39 1 + LA,39 LAlMe,39 and isopropyl (s)-lactate58 was synthesized according to the literature. All reagents such as aluminum isopropoxide, trimethyl aluminum, and methyl (s)-lactate (ML), were purchased from commercial suppliers and used as received. L-lactide was sublimed three times and recrystallized in dried toluene before use. ε-Caprolactone was dried over CaH2 and distilled prior to use and stored at −30 °C under N2.

Figure 9. Comparison of enthalpy profile of intermolecular and intramolecular nucleophilic substitution. F

DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 3. Plausible Mechanism for Intermolecular Methyl Migration via Nucleophilic Substitution Process, whereas TS = Transition State and INT = Intermediate

1.0 mL min−1 at 35 °C. Molecular weights and molecular weight distributions were calibrated with polystyrene standards ranging from 1200 to 4200000 amu. Elemental analyses were performed on a TruSpec Micro CHNS. Synthesis and Characterization. LAl-OCH(CH3)C(O)OCH3. The mixture of LAlMe (10 mg, 0.020 mmol) and methyl (s)-lactate (2.05 mg, 0.020 mmol) in 0.5 mL of CDCl3 were loaded in a J. Young NMR tube. The mixture was shaken and NMR spectra was taken after 1 h at room temperature. 1H NMR (600 MHz, CDCl3, 30 °C): δ 6.98 (s, 2H, ArH), 6.65 (s, 2H, ArH), 4.50 (q, J = 6.9 Hz, 1H, AlOCH(CH3)), 4.09 (s, 3H, COOCH3), 3.88 (dd, J = 13.5, 5.1 Hz, 2H, ArCH2N), 3.71 (br, 2H, ArCH2N), 2.90 (t, J = 7.4 Hz, 1H, NCH2CH2N), 2.61 (t, J = 7.0 Hz, 2H, NCH2CH2N), 2.42 (q, J = 7.1 Hz, 4H, N(CH2CH3)2), 2.22 (s, 6H, ArCH3), 1.36 (s, 18H, C(CH3)3), 1.34 (d, J = 7.1 Hz, 3H, AlOCH(CH3)), 0.94 (t, J = 7.1 Hz, 6H, N(CH2CH3)2). 13C{1H} NMR (150 MHz, CDCl3): δ 190.33 (CHCOO), 156.16, 156.08 (ipso-C), 138.19, 138.16, 127.63, 127.26, 127.21, 125.06, 125.01, 121.93 (Ar), 68.35 (AlOCH(CH3)), 55.69 (OCH3), 50.92, 47.59 (N(CH2CH3)2), 45.50 (N(CH2)2N), 34.71, 34.69 (C(CH3)3), 29.39, 29.38 (C(CH3)3), 21.90 (AlOCH(CH3)), 20.90, 20.88 (ArCH3), 11.95 (N(CH2CH3)2). (L(CH3))+Al-OCH(CH3)COO−, 2. LAl-OCH(CH3)C(O)OCH3 was transformed to complex 2 cleanly after recrystallization. LAlOCH(CH3)C(O)OCH3 (0.180 g, 0.302 mmol) was dissolved in a mixed solvent of hexane and chloroform or dichloromethane (1:1) and placed at −30 °C. Colorless crystals were obtained after a week. The crystals were separated by decanting the residue and washing with hexane yielding 0.126 g (70%). 1H NMR (600 MHz, CDCl3, 30 °C): δ 6.98 (dd, J = 8.3, 2.2 Hz, 2H, ArH), 6.63 (dd, J = 10.0, 2.2 Hz, 2H, ArH), 4.12 (q, J = 7.0 Hz, 1H, AlOCH(CH3)), 4.10−2.81 (br, 8H, ArCH2N, N(CH 2)2N), 3.58, 3.48, 3.38, 3.31 (m, 4H, N(CH2CH3)2), 3.05 (s, 3H, NCH3(CH2CH3)2), 2.21 (s, 3H, ArCH3), 2.19 (s, 3H, ArCH3), 1.37 (s, 9H, C(CH3)3), 1.36 (s, 9H, C(CH3)3, 1.28 (d, J = 6.7 Hz, 3H, AlOCH(CH3)), 1.21 (t, J = 15.2 Hz, 3H, NCH 3 (CH 2 CH 3 ) 2 ), 1.18 (t, J = 15.2 Hz, 3H, NCH3(CH2CH3)2). 13C{1H} NMR (150 MHz, CDCl3): δ 185.11 (CHCOO), 156.84, 156.68 (ipso-C), 138.16, 127.46, 127.43, 124.78, 124.59 (Ar), 70.29 (OCH(CH3)), 57.78 (NCH3(CH2CH3)2), 54.63 (ArCH2N), 52.62 (N(CH2)2N), 47.76 (NCH3(CH2CH3)2), 34.83 (C(CH3)3), 29.74, 29.71 (C(CH3)3), 23.63 (AlOCH(CH3)), 20.87, 20.85 (ArCH3), 7.93, 7.89 (NCH3(CH2CH3)2). Anal. Calcd for C34H53AlN2O5: C, 68.43; H, 8.95; N, 4.69. Found: C, 68.84; H, 8.82; N, 4.51. LAl-[O(CH2)5CO]nOCH(CH3)2. The mixture of complex 1 (10 mg, 0.018 mmol) and ε-CL (6.19 mg, 0.054 mmol) in 0.5 mL of C6D6 were loaded in a J. Young NMR tube. The mixture was shaken and

Figure 10. Enthalpy and free energy profile of methyl migration process. The enthalpy and free energy of LAl(ML) is set to 0.0 kcal mol−1, where TS = transition state and INT = intermediate. Deuterated solvents were dried over molecular sieve and stored under N2. 1 H and 13C{1H} NMR spectra were recorded on a Bruker AscendTM 600 and referenced to protio impurities of commercial CDCl3 (residual internal CHCl3, δ = 7.26 ppm) or C6D6 (residual internal C6D5H, δ = 7.16 ppm) as internal standards. IR spectra were obtained with a ReactIR 15, equipped with a 6.3 mm AgX DiComp probe. The X-ray crystallographic data were collected at 100 K on a Bruker D8 venture using Photon II detector and IμS 3.0 Microfocus source, Mo Kα radiation (λ = 0.71073 Å). Data collection was carried out using Bruker software suite APEX3. Data integration was performed with the SAINT software, and intensity data were corrected based on the intensities symmetry-related reflections measured at different angular setting (SADABS). The space group was determined with the XPREP software. The crystal structure was solved by direct method using intrinsic phasing (SHELXT program) and refined by full-matrix least-squares against F2 using the program SHELXL. Mass spectra were acquired using a Bruker Autoflex Speed MALDI-TOF mass spectrometer. Gel permeation chromatography (GPC) analyses were carried out on a Malvern GPCmax instrument equipped with refractive index detector and three 300 mm × 8.0 mm ID columns packed with porous styrene divinylbenzene copolymer. The GPC columns were eluted using tetrahydrofuran with flow rate of G

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Inorganic Chemistry NMR spectra was taken after 1 h at room temperature. 1H NMR (600 MHz, C6D6, 30 °C): δ 7.28 (d, J = 2.2 Hz, 2H, Ar), 6.57 (s, 2H, Ar), 5.03 (m, 1H, OCH(CH3)2), 4.49 (t, J = 6.3 Hz, 2H, AlOCH2CH2CH2CH2CH2CO), 4.02−3.96 (m, 6H, OCH2CH2CH2CH2CH2CO), 3.25 (br, 4H, ArCH2N), 2.74 (br, 4H, N(CH2)2N), 2.36 (t, 2H, 7.2 Hz, AlOCH2CH2CH2CH2CH2CO), 2.32 (s, 6H, ArCH3), 2.15−2.04 (m, 6H, OCH2CH2CH2CH2CH2CO), 2.04−1.83 (m, 8H, AlOCH2CH2CH2CH2CH2CO, AlOCH2CH2CH2CH2CH2CO, N(CH2CH3)2), 1.73 (s, 18H, C(CH3)3), 1.71 (m, 2H, AlOCH2CH2CH2CH2CH2CO), 1.55−1.45 (m, 6H, OCH2CH2CH2CH2CH2CO), 1.45−1.35 (m, 6H, OCH2CH2CH2CH2CH2CO), 1.23−1.12 (m, 6H, OCH2CH2CH2CH2CH2CO), 1.11−1.03 (m, 6H, OCH(CH3)2), 0.74 (m, 6H, N(CH2CH3)2). 13C{1H} NMR (150 MHz, CDCl3): δ 172.77 (CH2COO), 156.47 (ipso-C), 139.05, 128.15, 127.98, 125.06, 122.01 (Ar), 67.23 (OCH(CH3)2), 64.11 (OCH2CH2CH2CH2CH2CO), 63.33 (AlOCH2CH2CH2CH2CH2CO), 58.87 (ArCH2N), 49.21 (N(CH2CH 3)2, 40.77 (N(CH2)2N), 35.80 (AlOCH2CH2CH2CH2CH2CO), 35.26 (C(CH3)3), 34.93 (AlOCH2CH2CH2CH2CH2CO), 34.21 (OCH2CH2CH2CH2CH2CO), 30.10 (C(CH3)3), 28.78 (OCH2CH2CH2CH2CH2CO), 26.94 (AlOCH2CH2CH2CH2CH2CO), 25.94 (AlOCH2CH2CH2CH2CH2CO), 25.85 (OCH2CH2CH2CH2CH2CO), 24.93 (OCH2CH2CH2CH2CH2CO), 21.90 (OCH(CH3)2), 21.09 (ArCH3), 9.22 (N(CH2CH3)2). LAl-OCH(CH3)COOCH(CH3)2. LAlMe (10 mg, 0.020 mmol) was reacted with isopropyl (s)-lactate (2.64 mg, 0.020 mmol) in 0.5 mL of CDCl3 and loaded in a J. Young NMR tube. The mixture was shaken and NMR spectra was taken after 1 h. 1H NMR (600 MHz, CDCl3, 30 °C): δ 6.99 (s, 2H, ArH), 6.66 (s, 2H, ArH), 5.35 (m, 1H, OCH(CH3)2), 4.46 (q, J = 6.9 Hz, 1H, AlOCH(CH3)), 3.89 (dd, J = 13.4, 4.6 Hz, 2H, ArCH2N), 3.72 (br, 2H, ArCH2N), 2.92 (t, J = 7.3 Hz, 2H, NCH2CH2N), 2.62 (t, J = 7.6 Hz, 2H, NCH2CH2N), 2.43 (q, J = 7.1 Hz, 4H, N(CH2CH3)2), 2.23 (s, 6H, ArCH3), 1.47 (d, J = 6.3 Hz, 6H, OCH(CH3)2), 1.38 (s, 18H, C(CH3)3), 1.34 (d, J = 6.9 Hz, 3H, AlOCH(CH3)), 0.95 (t, J = 7.1 Hz, 6H, N(CH2CH3)2). 13 C{1H} NMR (150 MHz, CDCl3): δ 189.26 (CHCOO), 156.27, 156.17 (ipso-C), 138.12, 138.07, 127.64, 127.23, 127.16, 124.98, 124.92, 122.03 (Ar), 74.73 (OCH(CH3)2), 68.52 (AlOCH(CH3)), 50.97, 47.59 (N(CH2CH3)2, 45.52 (N(CH2)2N), 34.74 (C(CH3)3), 29.39 (C(CH3)3), 21.95 (AlOCH(CH3)), 21.77, 21.73 (OCH(CH3)2), 20.91, 20.89 (ArCH3), 11.96 (N(CH2CH3)2). Polymerization of ε-Caprolactone. NMR-Scale Polymerization. Polymerization of ε-CL was carried out in a screw-cap NMR tube. The following representative polymerization is for 100:1 mol ratio of CL/1. The amount of catalyst was adjusted accordingly for 200:1 and 300:1 while keeping the monomer concentration constant. Polymerizations of other cyclic lactones, β-butyrolactone, γ-butyrolactone, and δ-valerolactone, were also carried out similarly. A solution of ε-CL (61.9 mg, 0.542 mmol) in 1.00 mL of C6D6 was loaded into a screw-cap NMR tube. A solution of complex 1 (5.42 μmol, 90.3 mM, 60 μL) in C6D6 was added by micro syringe into the screw-cap NMR tube, and the mixture was monitored by NMR technique every 1 min until about 50% conversion. At this point, 60 μL of L-LA solution (27.1 μmol, 0.452 M) was immediately added by micro syringe into the reaction tube. The reaction was continuously monitored by NMR technique for over 1 h. The reaction tube was then heated in oil bath at 100 °C, and the data was collected at desired time until over 90% conversion. Large-Scale Polymerization. A solution of ε-CL (0.619 g, 5.42 mmol) in 10.00 mL of toluene was prepared in 100 mL Schlenk flask. A solution of complex 1 (54.2 μmol, 90.3 mM) in 0.600 mL toluene was added to the polymerization flask by syringe. The mixture was stirred and sampled at the desired time. At about 50% conversion, 0.78 mL of L-LA solution (271 μmol, 347 mM) was immediately added by syringe to the reaction flask. A small aliquot was taken for analysis. The reaction was then stirred at room temperature for 5 h. A small aliquot was taken again for analysis. The Schlenk flask was subsequently immersed in oil bath heated at 100 °C until more than 90% conversion was observed. Each polymer samples were quenched with 10% acetic acid in methanol and dried under vacuum. The

conversion and molecular weight were determined by NMR spectroscopy and GPC technique, respectively. Polymerization of CL in the Present of LA. Stirred solution of LLA (57.2 mg, 0.40 mmol) in 5 mL of toluene in 100 mL Schlenk flask was added a solution of complex 1 (20 mg, 36 μmol) in 1 mL of toluene. ε-CL (0.368 g, 3.22 mmol) in 1 mL of toluene was then added to the mixture. The Schlenk flask was immersed in oil bath heated at 100 °C. The mixture was sampled at the desired time. Each polymer samples were quenched with 10% acetic acid in methanol and dried under vacuum. Conversion was determined by 1H NMR spectroscopy. The polymer was then purified by precipitation in methanol. Composition and molecular weight was determined by NMR spectroscopy and GPC technique, respectively. DFT Calculations. All calculations were carried out with Gaussian 09 program.59 The mechanistic intermediate have been calculated by using DFT with the B3LYP60−62 functional and 6-31G(d,p)63 basis set. Transition states were located using standard Berny transitionstate optimization method.64 All transition states were located at minima or saddle points with only one imaginary frequency, whereas none of the negative frequencies were observed on the reactants and products. The energy profiles were given in enthalpy at 298 K. The single point energies and solvent effects in dichloromethane were computed using the polarizable continuum model (PCM) model.65−67

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01364. Experimental details, 1H NMR and 13C NMR spectra of all compounds, polymerization studies, MALDI-TOF spectra, and DFT calculation details (PDF). Accession Codes

CCDC 1551834 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Khamphee Phomphrai: 0000-0002-3132-680X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from The Thailand Research Fund, Mahidol University, and Vidyasirimedhi Institute of Science and Technology (RSA5680029). We gratefully acknowledge financial support from the Royal Golden Jubilee PhD Program (PHD/0159/2554), The Thailand Research Fund. Financial support from Frontier Research Center, Vidyasirimedhi Institute of Science and Technology is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01364 Inorg. Chem. XXXX, XXX, XXX−XXX