Article pubs.acs.org/Macromolecules
Highly Isoselective and Active Zinc Catalysts for rac-Lactide Polymerization: Effect of Pendant Groups of Aminophenolate Ligands Chao Kan, Jianwen Hu, Yang Huang, Haobing Wang,* and Haiyan Ma* Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China S Supporting Information *
ABSTRACT: Highly isoselective and active ring-opening polymerization of rac-lactide to obtain stereocomplexed polylactide is a long-standing challenge. In this contribution, zinc catalysts with good isoselectivities and high activities (Pm = 0.87, TOF = 3312 h−1 at 25 °C; Pm = 0.92, TOF = 117 h−1 at −20 °C, toluene) for rac-lactide polymerization are reported. These catalysts are coordinated by a well-designed chiral oxazolinyl or achiral benzoxazolyl aminophenolate ligand. Both types of zinc complexes afforded multiblock isotactic PLAs, proving to be formed via a chain-end control mechanism regardless of the existence of a chiral group or not. Preliminary studies suggested that the dihedral angle between the phenoxy and oxazolinyl/benzoxazolyl planes in these complexes reflects the steric bulkiness of the ligand and has a close relation with the isoselectivity of the complex, which might serve as a guide in obtaining new isoselective catalysts of similar types. Furthermore, unprecedented combination of excellent activity (TOF up to 44 000 h−1) and sufficient isoselectivity (Pm = 0.80) could be achieved under solvent-free immortal conditions with catalyst loadings as low as 0.005 mol % (vs monomer), which shows the potential of industrial application.
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INTRODUCTION Poly(lactic acid) (PLA) is one of the most important synthetic biodegradable polymers and alternatives to those petrochemical-based polymers.1−3 The environmental advantages of PLA, such as derived from renewable resources and degradable to metabolites, impel its applications in packaging, agricultural, and biomedical fields.1−4 The stereochemistry of PLA dramatically affects its physical, mechanical, and degradation properties.5−9 For instance, isotactic stereocomplex PLAs (Scheme 1) derived from rac-lactide (rac-LA) possess enhanced
light of these benefits, isoselective ring-opening polymerization (ROP) of rac-LA capable of obtaining stereocomplexed PLA is currently attracting considerable interest. The most isoselective catalytic systems are based on Al− Salen complexes and their derivatives.12−27 Although the isoselectivities (Pm) of some complexes exceed 0.95, these initiators suffer from low activities (taking days to reach completion) even at elevated temperatures (70−110 °C) as well as the need for high initiator loadings (typically ∼1 mol %). Seeking to address this problem, initiators based on other metals, such as Zn,28−46 Mg,29,30,32,34,39,47−52 Ca,53−55 In,56−60 K,61−64 group IV metals,65−73 and rare-earth metals,74−86 have been extensively studied, and some of them have displayed excellent stereocontrol in the catalytic ROP of rac-LA. Nevertheless, only a few discrete isoselective catalysts with moderate to high activities have emerged recently.41,55,60,63,86 Thus, exploring new initiators which integrate excellent isoselectivity with high activity as well as high productivity toward the ROP of rac-LA is still a challenge. Isotactic PLAs derived from rac-LA mainly possess tapered diblock (Scheme 1A) or multiblock (Scheme 1B) microstructures.7−9 The former can be achieved via an enantiomorphic-site control mechanism (SCM) by using chiral cata-
Scheme 1. Microstructures of Isotactic Poly(lactic acid)
physical and mechanical properties in comparison with homochiral PLLA and PDLA, such as higher melting temperatures (170−220 °C) and better crystallinity.7−9 The elevated Tm is due to an abnormal phenomenon in which sequences/blocks of the -RRRR- unit of PLA cocrystallize with those of the -SSSS- unit, giving rise to a double-helical stereocomplex microstructure with improved properties.10,11 In © XXXX American Chemical Society
Received: July 4, 2017 Revised: September 11, 2017
A
DOI: 10.1021/acs.macromol.7b01420 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules lysts.12,18 The multiblock isotactic PLAs can be obtained through either a chain-end control mechanism (CEM)15,19,22,23 or a polymeryl exchange process between chiral active centers.14,16 Recently, isoselective polymerization was also achieved by the co-occurrence of SCM and CEM.26,39 For instance, we reported that isotactic multiblock PLA (Pm = 0.84) contributed dominantly by CEM with slight misinsertion defects (e.g., -RRRRSSRRRR-) governed by SCM was obtained by the zinc complex bearing a chiral pyrrolidinyl aminophenolate ligand (I, Scheme 2).38,39 Therein, the occurrence of
minor enantiomorphic-site control was potentially attributed to the strong chiral induction effect around the zinc center (multiple stereogenic centers RNRZnSCSN).38,39 The resultant PLA showed a Tm of 165 °C, lower than those of homochiral PLLA (Tm = 170 °C) and PLA with identical isotacticity (Pm = 0.84, Tm = 178 °C) obtained from rac-LA via a pure CEM.84 To surmount this obstacle, we decided to weaken the chiral induction effect by changing a chiral amino N center in the ligand framework to an achiral imino N atom. We envisioned that the coordination of an achiral imino N atom instead of the chiral pyrilidinyl N atom might suppress the minor SCM to produce PLA with higher isotacticity and less stereodefect. Herein we report the new oxazolinyl/benzoxazolyl aminophenolate zinc complexes 1−9 (Scheme 2), which could realize the highly isoselective and active ROP of rac-LA via CEM. To the best of our knowledge, some of them also serve as the most isoselective and meanwhile highly active metal initiators to date.
Scheme 2. Synthesis of 1−9 and the Structure of Ia
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RESULTS AND DISCUSSION Synthesis and Structures of Zinc Complexes. The oxazolinyl- and benzoxazolyl-based aminophenol proligands L1−9H with a trityl group on the ortho-position of the phenol ring were synthesized from substituted chloromethyloxazolines, primary amines, and 2-bromomethyl-4-methyl-6-(triphenylmethyl)phenol, according to modifications of the published procedures (Scheme S1, Supporting Information). L1−9H reacted smoothly with 1 equiv of Zn[N(SiMe3)2]2, affording the corresponding zinc silylamido complexes 1−9 in yields of 26−59% (Scheme 2). In addition to the inherent carbon chirality in L1−6H, the zinc center and the skeleton N atom become new stereogenic centers upon complexation; thus, two pairs of diastereomers may be conceived for complexes 1−6.38 However, complexes 1−4 and 6 are diastereopure according to their 1H and 13C NMR spectra, with the other isomers being suppressed. Having a benzyl group on the oxazolinyl ring, a pair of diastereomers with a ratio of 7:3 were observed for complex 5. The solid state structures of complexes 1, 3, 4, and 6 were further determined by X-ray diffraction studies, indicating the absolute configurations of (RC4RNRZn)-1, 3 and (SC4SNSZn)-4, 6 respectively (Figure 1; also see Figures S1−S4 in the Supporting Information). Clearly, the carbon chirality on the fourth position of the oxazolinyl ring induces the same configurations at both the zinc center and the skeleton N atom. Complexes 7−9 were obtained from proligands L7−9H as
a
For clarity, only the descriptors of stereogenic N and Zn centers are noted.
Figure 1. Typical X-ray molecular structures of 3 (left) and 6 (right). Thermal ellipsoids represent the 50% probability surfaces. Hydrogen atoms are omitted for clarity. B
DOI: 10.1021/acs.macromol.7b01420 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. ROP of rac-LA Using Complexes 1−9a run
cat.
feed ratio
T (min)
convb (%)
TOFc (h−1)
Mn,cacldd (kg/mol)
Mne (kg/mol)
PDIe
P mf
1 2g 3 4 5g 6 7g 8g,h 9g,i 10 11g 12 13j 14g 15g,i 16g,k 17g 18g 19 20g 21 22g 23 24g 25g,i
1 1 2 2 2 3 3 3 3 4 4 5 5 5 5 5 6 3/6 7 7 8 8 9 9 9
200:1:1 200:1:1 200:1:1 500:1:1 500:1:1 500:1:1 500:1:1 200:1:1 200:1:1 500:1:1 500:1:1 500:1:1 1500:1:1 500:1:1 200:1:1 200:1:1 500:1:1 500:1:1 200:1:1 200:1:1 500:1:1 500:1:1 200:1:1 200:1:1 200:1:1
600 900 6 23 34 25 34 20 150 14 18 14 25 16 90 720 32 27 540 600 28 28 19 23 300
93 93 94 93 96 92 93 95 96 93 87 94 92 91 88 91 93 93 96 96 90 90 93 95 86
19 12 1880 1213 847 1140 820 570 77 1993 1450 2014 3312 1706 117 15 872 1033 21 19 964 964 587 496 34
26.8 26.8 27.1 67.0 69.1 66.2 67.0 27.4 27.6 67.0 62.6 67.7 198 65.6 25.3 26.2 67.0 67.0 27.6 27.6 64.8 64.8 26.8 27.4 24.8
33.7 26.7 41.1 87.7 66.3 78.2 70.9 32.1 32.7 86.9 82.1 71.8 190 73.8 20.9 20.5 75.1 68.3 33.9 28.1 89.5 65.0 28.6 33.1 34.5
1.43 1.23 1.10 1.08 1.29 1.16 1.13 1.07 1.06 1.44 1.30 1.32 1.37 1.22 1.06 1.07 1.09 1.12 1.32 1.41 1.09 1.28 1.45 1.15 1.03
0.69 0.69 0.81 0.78 0.80 0.83 0.85 0.89 0.91 0.61 0.65 0.86 0.87 0.88 0.92 0.93 0.84 0.84 0.82 0.82 0.84 0.82 0.87 0.87 0.92
[LA]0 = 1.0 M, in toluene, feed ratio = [LA]0/[Zn]0/[iPrOH]0, 25 °C. bDetermined by 1H NMR spectroscopy. cTurnover frequency (TOF) = mol of product (polylactides)/mol of catalyst per hour. dMn,calcd = [LA]0/[iPrOH]0 × conv % × 0.14413 kg/mol. eDetermined by GPC. fThe probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR spectroscopy. gIn THF. hAt 0 °C. iAt −20 °C. j[LA]0 = 1.5 M. kAt −40 °C.
a
Further decreasing the reaction temperature to −20 °C, the polymerization could still be accomplished in 150 min and achieved an even higher isoselectivity of Pm = 0.91. By comparing the structural features of complexes 2 and 3, we speculated that a sterically hindered oxazolinyl ring might be beneficial for the enhancement of isoselectivity. In this regard, complex 4 with 4,5-bis(phenyl)-substituted oxazolinyl was evaluated. To our surprise, this complex displayed a rather poor isoselectivity in both solvents, although accompanied with increased activities (runs 10 and 11). Probably, the 5-phenyl of the oxazolinyl is located far away, hardly influencing the steric hindrance around the metal center. When the substituent on the 4-position of the oxazolinyl ring was changed from phenyl to benzyl, complex 5 gained the highest isoselectivity among these complexes toward rac-LA polymerization (Pm = 0.86− 0.88, runs 12−14), meanwhile exhibiting a significantly higher activity than complex 3 with similar isoselectivity. For instance, polymerization of 1500 equiv of monomer could be accomplished in 25 min by complex 5 (TOF = 3312 h−1, run 13). The isoselectivity of complex 5 notably increased to Pm = 0.92 at −20 °C and to Pm = 0.93 at −40 °C (runs 15 and 16; Figure S10), while moderate activities were still maintained. The isoselectivity achieved here is also the highest value recorded for zinc-based complexes.32−44 Zinc complexes 7−9, bearing achiral benzoxazolyl derivative ligands prepared from cheap raw material instead of the highcost chiral reagents, were also accessed for the ROP of rac-LA. In comparison with complex 1 having a tert-butyl group on the skeleton N atom, analogous complex 7 showed similar activities but enabled a significant increase in isoselectivity with the Pm
a pair of racemic mixtures. Noteworthily, 1.1 equiv of Zn[N(SiMe3)2]2 was needed for the synthesis of 8 to suppress the bisligated complex (L8)2Zn, possibly due to the small steric hindrance of n-butyl and benzoxazolyl. The solid structures of complexes 7−9 determined by X-ray diffraction are depicted in Figures S5−S7; both racemates are found in the centrosymmetric crystal structures. Isoselective Polymerization of rac-LA Using Zinc Complexes. Complexes 1−9 could effectively initiate the ROP of rac-LA at ambient temperature either alone or in the presence of 2-propanol in a controlled manner, and the polymerization results are summarized in Table 1 (also see Tables S3 and S4). All complexes showed slightly lower activities in THF than in toluene, but the coordinative solvent had a negligible influence on their stereoselectivity. Contrasted with these polymerization parameters, the substituents on the ligand framework showed remarkable influence on the activity and stereoselectivity of these zinc complexes. In the presence of 2-propanol, complex 1 with a tert-butyl group on the skeleton N atom displayed a moderate activity in toluene, and isotactic-enriched PLA was obtained. In stark contrast, simply by varying the substituent on the skeleton N atom to a n-butyl, the activity of complex 2 was nearly 100-fold higher than that of complex 1; meanwhile, the Pm value of the resultant PLA was greatly improved to 0.81 (Table 1, run 1 vs run 3). Complex 3, having a sterically more hindered oxazolinyl ring, exhibited similar activity but slightly increased isoselectivity when compared to complex 2 (Pm = 0.83−0.85, runs 6 and 7). The Pm value was improved to 0.89 at 0 °C in THF, while high monomer conversion of 95% was still reached in 20 min. C
DOI: 10.1021/acs.macromol.7b01420 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 2. Semilogarithmic plots of lactides conversion versus time mediated by complexes 3 (left) and 7 (right). For 3: D-LA (black squares, kapp = (8.88 ± 0.29) × 10−2 min−1, R2 = 0.997); L-LA (red circles, kapp = (8.84 ± 0.66) × 10−2 min−1, R2 = 0.994); rac-LA (green triangles, kapp = (4.30 ± 0.62) × 10−2 min−1, R2 = 0.996). Conditions: T = 25 °C; [LA]0/[iPrOH]0/[Zn]0 = 500:1:1; [LA]0 = 0.5 M, THF as solvent. For 7: D-LA (black squares, kapp = (1.22 ± 0.02) × 10−2 min−1, R2 = 0.999); L-LA (red circles, kapp = (1.23 ± 0.04) × 10−2 min−1, R2 = 0.998); rac-LA (green triangles, kapp = (0.80 ± 0.01) × 10−2 min−1, R2 = 0.996). Conditions: T = 25 °C; [LA]0/[iPrOH]0/[Zn]0 = 100:1:1; [LA]0 = 0.5 M, THF as solvent.
Pm = 0.90 at −15 °C. Such an overall comparison does indicate the leading performance of 5 and 9 toward the isoselective polymerization of rac-LA. Besides, the molecular weights of PLAs produced by these zinc complexes are more controllable than previous isoselective zinc complexes bearing chiral pyrrolidinyl aminopholate38,39,43 or achiral heteroscorpionate ligands.40 PLAs with experimental molecular weights (Mn, GPC) close to theoretical values, and relatively narrow molecular weight distributions are obtained thoroughly (PDI = 1.03−1.45, more generally
