Highly Robust Yttrium Bis(phenolate) Ether Catalysts for Excellent

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Highly Robust Yttrium Bis(phenolate) Ether Catalysts for Excellent Isoselective Ring-Opening Polymerization of Racemic Lactide Tie-Qi Xu,*,† Guan-Wen Yang,†,‡ Chuang Liu,† and Xiao-Bing Lu† †

College of Chemistry, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, P. R. China MOE Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China

‡

S Supporting Information *

ABSTRACT: The highly isoselective ring-opening polymerization (ROP) of racemic lactide is regarded as a valuable strategy for the synthesis of stereocomplex polylactide. In the present contribution, we report the novel yttrium bis(phenolate) ether complexes for catalyzing ROP of racemic lactide in high activity (TOF) and excellent isotactic selectivity (Pi) (Pi = 0.90, TOF = 120 h−1 at −15 °C; Pi = 0.84, TOF = 2280 h−1 at 25 °C). Kinetic and computational studies provide a fundamental understanding of the stereocontrol mechanism governing polymerization reactions mediated by yttrium catalysts.

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INTRODUCTION Polylactide (PLA), a biodegradable and biocompatible material derived from biorenewable feedstocks, is considered to be an excellent replacement for conventional petroleum-based materials.1−8 By changing isomers of lactide (LA) monomer, the selectivity of initiators, and the polymerization conditions, various PLAs with different microstructures can be synthesized. The stereocomplex PLA of them is especially important and useful because it shows higher Tm and better thermal stability than other PLAs. For instance, isotactic poly(L-lactide) exhibits a Tm of 180 °C, while stereocomplex PLA shows a Tm of up to 230 °C.9 As an efficient way to produce a stereocomplex PLA, metal complexes catalyzing isoselective ring-opening polymerization (ROP) of racemic lactide (rac-LA) have attracted considerable interest. Aluminum complexes bearing Schiff base ligands are by far the most successful systems in generating stereocomplex PLA from rac-LA. Many Al complexes and their derivatives showed high isoselectivities (Pi) of greater than 0.9.10−34 However, Al catalysts often suffer from low reactivity, typically requiring a long reaction time (days) for acceptable polymerization yield. Therefore, intensive efforts have been devoted to the discovery of initiators combining both high rates and excellent stereocontrol for rac-LA polymerization.35−55 However, only a few of them can afford isotactic enriched PLAs from rac-LA with good activity. The pioneering work is attributed to the discovery of the homochiral yttrium phosphine oxide alkoxide complexes with high activity and good isoselectivity (Pi = 0.81 at −18 °C, TOF = 1200 h−1) for the ROP of rac-LA to afford stereocomplex PLAs.56,57 Soon thereafter, a few zinc,58−61 indium,62,63 and zirconium64,65 based isoselective catalysts © XXXX American Chemical Society

bearing chiral ligands were reported. Although these chiral ligands supported initiators polymerize rac-LA in an isoselective manner, they also exhibit two disadvantageous features: (i) catalysts selectively initiate one enantiomer to polymerize rapidly but the other to do so slowly; (ii) the expensive chiral ligands enhance polymerization cost. The breakthrough came from the successful preparation of the rare earth complexes with achiral phosphasalen ligand providing distinguished isoselectivity and good activity for the ROP of rac-LA. They showed isoselectivity Pi = 0.84 and activity TOF = 26 h−1 at −15 °C (Pi = 0.84, TOF = 61 h−1 at 25 °C for lutetium) (Scheme 1).66,67 Rare earth initiators have the added advantage of being easy to prepare and relatively redox-inactive. A number of rare earth complexes have been reported to produce heterotactic PLAs.68−73 However, rare earth complexes, as potential ideal candidates combining both high isoselectivity and high catalytic activity for rac-lactide polymerization, still Scheme 1. Rare Earth Phosphasalen Complexes Reported by Williams

Received: November 11, 2016 Revised: December 22, 2016

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activity (runs 1 and 2 in Table 2). The coordinating solvent (THF) significantly decreased the catalytic activity (run 4 in Table 2). The complex 3 showed much lower activity compared to complexes 1, 2, and 4 because of its instability in CH2Cl2. The PLA tacticity was obtained by comparison of the tetrad resonanes for the methane CHMeC proton on the polymer backbone observed in the homonuclear-decoupled 1H NMR spectra (Figure 3).76 The Pi values can be calculated using Bernoullian statistics based on five tetrads, and their average value was used as the Pi (Table 2).77 The microstructural analysis of PLAs formed from rac-LA with complexes 1−4 revealed that the structure of the ligand also has a significant influence on the stereospecificity of the growing polymer chain (Table 2). The tert-butyl-substituted complexes 1 and 2 were shown to be effective catalysts for highly isoselective polymerization of rac-LA. A maximum Pi value of 0.90 was observed for complex 1 (run 7 in Table 1). This is the highest selectivity obtained to date for high activity catalysis system. The isoselectivity of complexes 1−4 was improved as the size of the bis(phenolate) ligand increased. The tert-butylsubstituted complexes 1 and 2 afford highly isotactic PLA with Pi = 0.84 and 0.83 at 25 °C, respectively (runs 1 and 9 in Table 2). It should be noted that catalysts capable of yielding such Pi values under ambient conditions remain rare. In contrast, complexes 3 and 4 only produced low isotactic PLA (runs 10 and 11 in Table 2). To exploit their promising properties, the polymerization was conducted at a lower temperature (−15 °C), and the high catalytic activity (TOF = 120 h−1) and even higher Pi value (0.90) were observed. Changing solvent polarity did not noticeably affect the isoselectivity of PLA (runs 1 and 2 in Table 2). More interestingly, this system showed a tolerance of change in the polymerization temperature. At 70 °C, the complex 1 still shows a high isoselectivity with Pi = 0.77 (run 3 in Table 2). However, the coordinating solvent significantly affected the tacticity of the resulting PLA. Thus, polymerization in THF lost stereoselectivity and yielded an essentially atactic polymer (run 4, Table 2), presumably because of competing solvent coordination to the metal center with monomer coordination. Furthermore, the polymerization rates remained high, and the polymerization control was good. The variation of the bridge from C4 to C3 led to only a slight improvement for Pi value (from 0.83 to 0.84). No epimerization of lactide was observed, as L-LA was polymerized to a purely isotactic PLA (run 8, Table 2). To understand the unusual isoselectivity exhibited by complex 1, we analyzed the relative intensities of the stereoerror tetrad signals in the PLA produced by them (Figure 1). The close intensity ratio of sii:iis:isi to 1:1:1 as well as the basically ignorable sis signal suggests that the polymermain chain is essentially stereoblock (e.g., -RRRRRRSSSS-), and the stereocontrol occurs by a chain end control mechanism (CEM).19 Moreover, the PLAs obtained at different polymerization stages maintain a constant isotacticity (Pi = 0.82−0.84) without optical activity ([a]D20 ∼ 0), suggesting that both enantiomers are transferred equally into the polymer chains (Table S1, Supporting Information); a stereoblock polymer was formed. Analysis of the polymer by differential scanning calorimetry (DSC) showed a maximum Tm value of 186 °C, further confirming the formation of a stereocomplex polymer (Figure 2).78,79 The chain ends of the oligomeric PLA obtained by employing complex 1 at a low monomer-toinitiator ratio are capped by a CH2SiMe3 end group according 1 H NMR (Figure S20). In addition, investigation by MALDI-

need to be explored. Thus, exploring faster and higher isoselective initiators without the need for expensive chiral ligands is still an obviously promising but challenging subject. Recently, a new family of metal complexes with bis(phenolate) ether [OOOO]-type ligands has been reported to control the stereoselectivity during the coordination polymerization of olefin or polar monomer. Waymouth et al. reported the group 4 bis(phenolate) ether complexes produced highly isotactic polypropylenes.74 Earlier this year, we reported the first effective yttrium bis(phenolate) ether catalysts for highly isotactic 2-vinylpyridine polymerization.75 Herein, we report new yttrium bis(phenolate) ether complexes for rac-LA polymerization, showing both excellent isoselectivity and high activity (Table 1). Table 1. Coordination Polymerization Systems Combining Both High Isoselectivity and Excellent Activity Using Nonchiral Ligands Supported Initiators no.

catalyst or initiator

Tp (°C)

Pi (%)

TOF (h−1)

ref

1 2 3 4

yttrium phosphasalen lutetium phosphasalen lutetium phosphasalen yttrium bis(phenolate) ether

−15 25 −16 25 −15

84 84 89 84 90

26 61 5 2280 120

66 66 12b this work

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RESULTS AND DISCUSSION Synthesis and Structure of Yttrium Complexes 1−4. As was reported for yttrium complexes 3 and 4,75 the derivatives 1 and 2 were readily synthesized by using the alkane elimination reaction of [Y(CH2SiMe3)3(THF)2] with the corresponding bridged sterically bulkier bis(phenolate) ether in n-hexane in 54−65% yield (Scheme 2). Complexes 1 and 2 were characterized by 1H and 13C NMR spectroscopy, 1 H−1H COSY spectroscopy, and elemental analysis. Scheme 2. Yttrium Bis(phenolate) Ether Complexes 1−4

Stereoselective Polymerization of rac-LA. Some experimental results of rac-LA polymerization with complexes 1− 4 as catalysts are summarized in Table 2. Complexes 1−4 were all highly active for ROP of rac-LA at ambient temperature in CH2Cl2, and the ligands’ structures significantly influenced the catalytic activity (Table 2). Complex 1 gave the highest catalytic activity, with TOF = 2280 h−1 (run 1 in Table 2). The catalysts were also found to be efficient in low initiator loading (0.1 mol %). Complex 1 showed nearly complete conversion of 1000 equiv of rac-LA in ∼1.5 h (run 5 in Table 2). Changing solvent polarity from polar dichloromethane (CH2Cl2, ε = 8.93) to relatively nonpolar toluene (ε = 2.38) decreased polymerization B

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Macromolecules Table 2. Results of rac-LA Polymerization Using Complexes 1−4a

no.

cat.

solvent

time (min)

convh (%)

TOFi (h−1)

Mn × 10−4 (g mol−1)

Mn × 10−4 j (g mol−1)

Mw/Mnj

Pik

Tm (°C)

1 2b 3b,c 4 5d 6e 7f 8g 9 10 11

1 1 1 1 1 1 1 1 2 3 4

CH2Cl2 toluene toluene THF CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

5 10 5 60 90 60 90 5 5 60 5

95 92 94 51 90 92 90 94 90 70 90

2280 1104 2256 102 120 184 120 2256 2160 140 2160

2.74 2.65 2.74 1.47 13.0 2.65 2.59 2.71 2.59 2.02 2.59

2.72 2.57 2.45 1.10 18.4 2.70 2.41 2.65 3.45 3.21 4.12

1.34 1.76 1.80 1.34 1.72 1.37 1.27 1.29 1.26 1.62 1.61

0.84 0.82 0.77 0.37 0.83 0.85 0.90 1.00 0.83 0.41 0.64

175 171 165 174 175 186 176

CH2Cl2, [Y]:[rac-LA] = 1:200, [Y] = 5 mM, 25 °C. bToluene. c70 °C. d[Y]:[rac-LA] = 1:1000. e0 °C. f−15 °C. gL-Lactide was used. hDetermined by integration of the methyne region of the 1H NMR spectrum. iTurnover frequency (TOF) = mol of product (polylactides)/mol of catalyst per hour. jDetermined by GPC. kDetermined by analysis of all of the tetrad signals in the methyne region of the homonuclear-decoupled 1H NMR spectrum. a

Figure 1. Methine region of the homonuclear decoupled 1H NMR spectrum of isotactic PLA produced by complex 1 ([rac-LA]0:[Y]0= 200:1, [Y]0 = 5 mM, in CH2Cl2 at −15 °C). Figure 3. Plots of the first-order kinetics of ln([rac-LA]0/[rac-LA]t) vs time (min) for the polymerization of rac-LA by complex 1 activated in CH2Cl2 at 25 °C. Conditions: [rac-LA]0 = 0.24 M; [Y]0 = 0.24 (▲), 0.50 (▼), 1.05 (■), and 2.51 mM (●).

Figure 2. DSC thermograms of PLA with Pi = 0.90 (a) and Pi = 0.84 (b) (runs 7 and 1 in Table 1).

TOF mass spectrometry of a PLA sample also revealed the presence of a CH2SiMe3 end group as well as transesterification indicated by the cluster of peaks separated by 72 mass units (Figure S22). Kinetics of LA Polymerization. Kinetic experiments employed [rac-LA]0/[Y]0 ratios ranging from 100 to 1000, clearly showing first-order dependence on [rac-LA] for all the ratios (Figure 3). Furthermore, a double-logarithm plot (Figure 4) of the apparent rate constants (kapp), obtained from the slopes of the best fit lines to the plots of ln([rac-LA]0/[rac-

Figure 4. Plot of ln(kapp) vs ln[Y] for rac-LA polymerization by complex 1 in CH2Cl2 at 25 °C.

LA]t) vs time as a function of ln[Y]0, was fit to a straight line (R2 = 0.999) with a slope of 1.058. Thus, the kinetic order with respect to [Y], given by the slope of 1, reveals that the C

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value was calculated to be 1.15 × 10−3 s−1, which is approximately 7 times smaller than the observed k(L−L)app (7.35 × 10−3 s−1). This kinetic study produces a stereoselectivity (Pi = 0.86) that is consistent with the analysis of the poly(rac-LA) obtained (Pi = 0.84, run 3, Table 1).12,80 Stereocontrol Mechanism in rac-LA Polymerization. To understand the stereocontrol mechanism governing these polymerization reactions, we performed computational studies of these reactions using a B3LYP density functional (DFT) procedure.81 The postulated mechanism of the yttriumcatalyzed LA polymerization is shown in Figure 7. In particular, special attention has been paid to the propagation process since it determines the stereoselectivity. We have analyzed the isotactic selectivity of the process by studying the addition of two successive LA units at Y center of complex 2. First, the geometry of complex 2 with first insertion (S,S)-LA1 (I) was optimized. This is followed by coordination of a second monomer unit, (S,S)-LA2, in which THF molecule is displaced, forming complex INT1 and resulting in the LA2 O(carbonyl)− Y bond (2.532 Å). In the third step, intramolecular nucleophilic attack by the Y−alkoxide onto the coordinated carbonyl group of LA2 (C−O: 1.868 Å), forming a tetrahedral transition state (TS1). The complex then rearranges via two tetrahedral intermediates (INT2, INT3), resulting in close approach (2.212 Å) of the LA2 O acyl atom to the Y atom center. The new propagating Y−alkoxide bond forms in TS2 (Y−O: 2.309 Å), with concerted dissociation of the LA2 O(carbonyl)−Y bond (2.259 Å) and cleavage of the original LA2 C−O acyl bond (1.807 Å) to achieve ring-opening process. Finally, the system evolves to the final THF adduct of a five-membered metallacyclic product (P) by dissociation of the polymer chain from the coordination sphere of the yttrium atom. For the stereoselectivity of polymerization process, we computed eight assembly modes, considering LA1 and LA2 may both be either (R,R) or (S,S), and the approach (si or re faces) of LA2 may occur on either face of the LA1 lactite− chelate. Accordingly, we have calculated eight TS1 (Figures S23 and S24) and their associated TS2 (Figure 8 and Figure S25) geometries. In all eight cases, all si faces are lower in energy (Table 3) and therefore represent the approach of LA2 to LA1 lactite−chelate. For all four si face cases, TS2 is higher in energy (Table 3) and consequently determines the reaction rate for this polymerization process. The explanation for isotactic polymer formation is therefore included within the four competing TS2 geometries shown in Figure 8. Calculation results reveal that for an R,R growing chain addition of another R,R LA monomer molecule on the si face of the chain is clearly favored. Competition is with addition of an S,S LA molecule on the si face of growing chain, which is disfavored by 1.0 kcal/mol due to steric repulsion between the Me groups of the chain and the monomer. Likewise, for an S,S chain, addition of another S,S LA molecule on the si face of the growing chain is favored over addition of an R,R LA molecule on the si face by 2.4 kcal/mol. In a word, calculations demonstrate that for the catalyst 2 the R,R chain obviously favors addition of another R,R LA molecule on the si face of the chain, and the S,S chain also favors addition of another S,S LA molecule on the si face of the chain. As the calculation showed R,R over S,S selectivity for an R,R chain, and S,S over R,R selectivity for an S,S chain, the resulting PLA should display a high isotactic selectivity. In summary, we have successfully developed a highly active yttrium catalyst stabilized by achiral bis(phenolate) ether ligand

propagation is also first-order in catalyst concentration, indicating that the rac-LA polymerization follows the monometallic, intramolecular coordination−insertion mechanism. To understand the origin of the high isotacic selectivity, two kinetic experiments were attempted: (1) complex 1 was treated with equivalent (R)-tert-butyl lactate to give intermediate with R chain-end, which showed difference induction period (T0) for L-LA and D-LA polymerization in dilute solution ([Y] = 0.4 mM, [L-LA] = [D-LA] = 0.05 M). The comparison between DLA (T0 = 10 s) and L-LA (T0 = 120 s) (Figure 5) revealed that

Figure 5. First-order kinetic plots for ROP of lactides (L-LA (■); DLA(●)) promoted by intermediate produced by complex 1 and (R)tert-butyl lactate. [Y] = 4.0 × 10−4 M; [LA]/[Y] = 125; T = 25 °C; k(L−L)app = 11.1 × 10−3 s−1 ; k(D−D)app = 10.5 × 10−3 s−1. The induction periods for D-LA and L-LA polymerization are 10 and 120 s, respectively.

the chain-end chirality powerfully differentiates an enantiomer with the same chiral sense from one that has the opposite chiral sense, and the monomer with the same chiral sense preferentially enters the reaction site for the propagation reaction. After onset of polymerization, the rate of polymerization (kapp values) of each enantiomer was approximate (k(L−L)app = 11.1 × 10−3 s−1; k(D−D)app = 10.5 × 10−3 s−1) (Figure 5); (2) the first-order rate constant for the polymerization of rac-lactide (k(rac)app = 4.25 × 10−3 s−1) is smaller than that observed under identical conditions with L-lactide (k(L−L)app = 7.35 × 10−3 s−1) (Figure 6). By kinetic rate equations of rac-LA and L-LA polymerizations,3,18 the k(D−L)app

Figure 6. First-order kinetic plots for ROP of lactides (L-LA; rac-LA) promoted by complex 1. [Y] = 1.0 × 10−2 M; [LA]/[Y] = 250; T = 25 °C; k(L−L)app = 7.35 × 10−3 s−1 ; k(rac)app = 4.25 × 10−3 s−1. D

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Figure 7. Calculated free energy values (kcal mol−1) for the reaction coordinate corresponding to the SS chain/SS monomer/si face assembly mode.

Table 3. Calculated Activation Free Energies (ΔG298) for TS1 and TS2 Assembly assembly mode

ΔG TS1 (kcal/mol)

ΔG TS2 (kcal/mol)

RR-chain/RR-monomer/si face RR-chain/RR-monomer/re face RR-chain/SS-monomer/si face RR-chain/SS-monomer/re face SS-chain/SS-monomer/si face SS-chain/SS-monomer/re face SS-chain/RR-monomer/si face SS-chain/RR-monomer/re face

12.6 11.0 11.1 11.7 12.2 16.4 17.5 11.1

13.3 20.1 14.3 16.2 15.8 14.7 18.2 21.3

formation of an isotactic polymer originates chiefly from interactions between the methyl groups on the chiral C atom of the LA ring of both the monomer and the last inserted LA unit of the chain and the auxiliary ligand.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02439. Complete experimental details and additional characterization data including Figures S1−S25 and Table S1 (PDF)

Figure 8. TS2 geometries for the competitive LA (RR or SS configuration) addition on an RR chain and an SS chain. The approach of second LA occur on si face of the polymer chain. The energies (kcal/mol) are relative to the geometry of complex 2 with first insertion LA (I) (H atoms omitted: gray = C; red = O; blue = Y).

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to induce high isoselective polymerization (Pi = 0.90) of rac-LA to give stereoblock isotactic PLA. Kinetic studies revealed that chain-end chirality powerfully differentiates an enantiomer with the same chiral sense from one that has the opposite chiral sense, and the monomer with the same chiral sense preferentially enters the reaction site for the propagation reaction. Computational studies have led to a detailed mechanistic understanding on stereocontrol, in which the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.-Q.X.). ORCID

Tie-Qi Xu: 0000-0003-1777-630X Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21274015 and 21574016), the Program for Liaoning Excellent Talents in University (LJQ2015025), and the Chang Jiang Scholars Program (No. T2011056) from Ministry of education, People’s Republic of China.

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DOI: 10.1021/acs.macromol.6b02439 Macromolecules XXXX, XXX, XXX−XXX