Synthesis and Characterization of Dinuclear Salan Rare-Earth Metal

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

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Synthesis and Characterization of Dinuclear Salan Rare-Earth Metal Complexes and Their Application in the Homo- and Copolymerization of Cyclic Esters Hao Ouyang,† Dan Yuan,† Kun Nie,*,†,‡ Yong Zhang,† Yingming Yao,*,†,§ and Dongmei Cui§

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†

Key Laboratory of Organic Synthesis of Jiangsu Province and the State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Taishan University, Taian 271021, China § State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China S Supporting Information *

ABSTRACT: Four rare-earth-metal aryloxo complexes stabilized by a tetradentate Salan ligand were prepared, and their catalytic properties for the (co)polymerization of lactides and ε-caprolactone were elucidated. The proton-exchange reactions of (C5H5)3Ln(THF) with the Salan ligand N,N′(CH2Ph)2-N,N′-[CH2(2-OH-C6H2-Me2-3,5)]2 (LH2) in a 1:1 molar ratio, and subsequently with 1 equiv of pmethylphenol, gave the rare-earth-metal aryloxides [LLn(OC6H4-4-CH3)(THF)n]2 [n = 0 and Ln = Y (1), Sm (2), and Nd (3); n = 1 and Ln = La (4)] in good isolated yields. These complexes were fully characterized by elemental analysis, IR, and NMR spectroscopy (for complexes 1 and 4). Solid-state structures of complexes 1−4 were confirmed by single-crystal X-ray diffraction analysis. Complexes 1−4 have dinuclear solid-state structures, with a Ln2O2 core bridging the Salan ligands. The coordination geometry around each of the metals is a slightly distorted octahedron in complexes 1−3, whereas it is a capped trigonal prism in complex 4. It was found that complexes 1−4 can initiate efficiently the homopolymerization of L-lactide (L-LA) and rac-lactide (rac-LA) at 30 °C in tetrahydrofuran. The increasing activity of these complexes is in agreement with increasing ionic radii. A kinetic study revealed that seven-coordinated lanthanum complex 4 is more active for rac-LA polymerization compared with L-LA. A further study revealed that complex 4 was also an efficient initiator for the random copolymerization of L-LA and ε-caprolactone with the simultaneous addition of these two monomers, and the Tg values of the copolymers obtained increase linearly from −30.2 to +38.3 °C with an increase of the percentage of LA units. A mechanism study revealed that transesterification plays a crucial role in the formation of a random copolymer.

■

INTRODUCTION

Although numerous organometallic complexes were reported to be efficient catalysts/initiators for the homopolymerization of LA and ε-CL,11 most of them cannot catalyze the random copolymerization of these two monomers. The main reason is that the polymerization rates of LA and ε-CL are quite different. Generally, the homopolymerization of ε-CL is faster than that of LA under the similar reaction conditions, whereas the polymerization rates are reversed in the copolymerization of these two monomers.4 Thus, the onepot copolymerizations of LA and ε-CL generally produce tapered, gradient, or blocky copolymers, instead of the expected random copolymers.5 Until now, it is still a big

Polylactide (PLA) and polycaprolactone (PCL) as biodegradable polymer materials are widely applied in tissue engineering, packaging, pharmaceuticals, and medical fields in recent decades.1 However, the physical properties of PLA and PCL are quite different. For instance, PLA has a short half-time (several weeks) in vivo and it is hardly permeable for most drugs, whereas PCL has a long half-time (almost 1 year) in vivo and it is easily permeable for drugs.2 Because the physical properties of these two homopolymers are complementary, their random copolymers are expected to have significant advantages in comparison with their corresponding homopolymers.3 Therefore, the preparation of random copolymers of εcaprolactone (ε-CL) and lactide (LA) has received considerable attention in recent years.4−10 © XXXX American Chemical Society

Received: April 16, 2018

A

DOI: 10.1021/acs.inorgchem.8b01046 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of Salan Rare-Rarth-Metal Aryloxides 1−4

challenge to achieve random copolymers from ε-CL and LA catalyzed by an organometallic complex. To obtain random copolymers from LA and ε-CL, a popular strategy is to reduce the reactivity gap of ε-CL and LA by altering the structures of the metal-based initiators/catalysts. For example, a lot of catalytic systems derived from aluminum,6 zinc,7 magnesium,8 and group 4 metal9 complexes have been developed for the random copolymerization of LA and ε-CL via this strategy. However, adjustment of the reactivity ratios of LA and ε-CL via catalyst/initiator is still difficult, and successful cases are relatively rare. On the other hand, transesterification is another potential, but less studied method to synthesize random copolymers from ε-CL/LA.9 Transesterification is a common side reaction during LA or εCL homopolymerization catalyzed by organometallic complexes, but this reaction can be applied to the preparation of random copolymers of LA and ε-CL. It has been reported that Sn(Oct)2 can catalyze the copolymerization of LA and ε-CL under melting conditions to give random copolymers.10 Schafer and co-workers reported that pyridonate- and amidatetitanium alkoxides can randomly copolymerize ε-CL and LA, which was proposed via the transesterification reaction.9b,f Rare-earth-metal complexes have been developed as initiators for the homopolymerization of ε-CL and LA.12 However, many attempts to prepare a ε-CL/LA copolymer initiated by rare-earth-metal complexes failed. Some organorare-earth-metal complexes were tried to catalyze the one-pot copolymerization of ε-CL/LA, but only homo-PLA was obtained.13 A block copolymer can be achieved only when the homopolymerization of ε-CL is completed, and then LA was introduced; otherwise, block copolymerization will be suppressed.13,14 Density functional theory (DFT) calculations revealed that there is a mismatch between the activities of the PCL and PLA active chains and provided explanations for the incapacity of rare-earth-metal complexes in ε-CL/LA copolymerization.15 To date, only two examples of rare-earth-metal initiators for ε-CL/LA random copolymerization have been reported. The first one involving a LnCl3−propylene oxide binary system was found by Shen et al. about 20 years ago and was suggested to undergo transesterification.16 Very recently, Visseaux and co-workers reported that mixed allyl−rare-earthmetal borohydride complexes can initiate the random copolymerization of ε-CL and LA, although the copolymerization mechanism is not involved yet.17 Now, some rare-earth-metal aryloxides stabilized by a tetradentate Salan ligand have been prepared, and their catalytic behaviors for the homo- and copolymerization of cyclic esters have been studied. These complexes are found to

be efficient initiators for the ring-opening polymerization (ROP) of L-lactide (L-LA) and rac-lactide (rac-LA), and the corresponding lanthanum complex is active for the copolymerization of L-LA and ε-CL, affording random ε-CL/LA copolymers with the simultaneous addition of these two monomers. We herein report these results.

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RESULTS AND DISCUSSION

Synthesis and Characterization of Rare-Earth-Metal Aryloxides. We previously reported that the readily available (C5H5)3Ln(THF) (THF = tetrahydrofuran) are useful starting materials for rare-earth-metal aryloxides or alkoxides synthesis.18 Therefore, a similar method was adopted to prepare the rare-earth-metal aryloxides stabilized by the Salan ligand. The reactions of (C5H5)3Ln(THF) with N,N′-(CH2Ph)2N,N′-[CH2(2-OH-C6H2-Me2-3,5)]2 (LH2), without separation and subsequently with 1 equiv of p-methylphenol, gave the desired rare-earth-metal aryloxo complexes [LLn(OC6H4-4CH3)(THF)n]2 [n = 0 and Ln = Y (1), Sm (2), and Nd (3); n = 1 and Ln = La (4)] in high isolated yields (Scheme 1). These complexes were well characterized by elemental analysis, IR, and NMR spectroscopy (for complexes 1 and 4). In the 1H NMR spectrum of complex 1, the CH2 protons of ArCH2N groups from the skeleton of the Salan ligand resonate as eight doublets at 5.22, 4.96, 4.54, 4.44, 4.24, 3.34, 2.94, and 2.86 ppm, which is different from two sharp peaks at 3.60 and 3.50 ppm for the free ligand LH2 (see the Supporting Information). This difference may arise from the fact that complex 1 dimerizes through bridging O atoms of Salan ligands (Scheme 1), which leads to four CH2 groups of ArCH2N from one Salan ligand in different chemical environments. In addition, two protons of each methylene group (ArCH2N) resonate as two AB doublets because the conformation (axial and equatorial) of the H atoms in the ArCH2N group is fixed because of coordination of the O and N atoms from the Salan ligand to the metal center.19 Complexes 1−4 are unstable when exposed to air or moisture, while their crystals and solution are rather stable under argon. All complexes are soluble in toluene and slightly soluble in hexane. The solubility of these complexes in toluene decreases with an increase of the ionic radii of rare-earth metals. To gain solid-state structure information, single-crystal X-ray diffraction analyses were conducted for complexes 1−4. Selected bond parameters are tabulated in Table S2. Crystals of complex 1 were obtained from a toluene solution at room temperature, and the crystals of the other complexes were obtained from a mixed THF/toluene solution. X-ray structure determination showed that complexes 1−3 are isomorphous, whereas there is an additional THF molecule coordinated to B


Article

Inorganic Chemistry lanthanum in complex 4. Thus, only the ORTEP diagrams of complexes 2 and 4 are shown in Figures 1 and 2, respectively.

Complexes 1−4 have symmetric dinuclear structures, and the coordination environments around the two rare-earth metals in each of these complexes are identical. These complexes contain a Ln2O2 skeleton, bridged by the O atoms of the OAr groups from the Salan ligands, instead of the O atoms from the 4-methylphenoxo groups. This phenomenon is quite different from the other Salan rare-earth-metal complexes and indium complexes reported.20 Apparently, this dimerizing nature in complexes 1−4 originates from the insufficient bulkiness of the o-methyl substituents of the Salan ligands because a similar situation was observed in rare-earthmetal ketoiminate complexes.21 In complexes 1−3, each of the metal ions is six-coordinated by two O atoms and two N atoms from one Salan ligand, one O atom from another Salan ligand, and one O atom from the 4-methylphenoxo group to form a distorted octahedron, in which O1, O2, O3, and N1 occupy the equatorial positions and O2A and N2 occupy the axial positions. In complex 4, each metal ion is seven-coordinated by four O atoms and two N atoms, as described in complexes 1−3, and another O atom from a THF molecule. The coordination geometry around lanthanum is a slightly distorted capped trigonal prism, in which the N1 atom occupies the capping position. The average Ln−O(Salan) bond distances in complexes 1− 4 are 2.187(18), 2.280(3), 2.325(2), and 2.403(2) Å, and the average Ln−N(Salan) bond distances are 2.555(2), 2.623(4), 2.667(3), and 2.825(3) Å, respectively. The increasing trends in these bond lengths are consistent with the increasing ionic radii (Y < Sm < Nd < La). These bond distances are comparable with those observed in the Salan rare-earth-metal complexes provided the difference in the ionic radii is considered.20a The Ln−O(Ar) distances of 2.0913(19), 2.147(4), 2.171(2), and 2.261(2) Å in complexes 1−4, respectively, which are similar to those in rare-earth-metal aryloxo derivatives.18a Homopolymerization of rac-LA and L-LA by Complexes 1−4. The catalytic properties of complexes 1−4 for the ROP of rac-LA and L-LA were tested at 30 °C in THF, and the preliminary results are listed in Table 1. Ionic radii of the rare-earth metals have an obvious influence on the activity of these complexes for LA polymerization. For instance, the lanthanum complex can polymerize 200 equiv of rac-LA in 12 h to give 95% yield, whereas the yttrium, samarium, and neodymium complexes give apparently low yields under the same reaction conditions (Table 1, entries 1−4). The activity decreasing order of La > Nd > Sm > Y is consistent with the

Figure 1. ORTEP diagram of complex 2 with an atom numbering scheme. Thermal ellipsoids are shown at 10% probability, and H atoms are omitted for clarity. Complexes 1 and 3 are isomorphous with complex 2.

Figure 2. ORTEP diagram of complex 4 with an atom numbering scheme. Thermal ellipsoids are shown at 10% probability, and H atoms are omitted for clarity.

Table 1. Polymerization of LAs Initiated by Complexes 1−4a entry

monomer

catalyst

time (h)

yieldb (%)

1 2 3 4 5 6 7 8

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

1 2 3 4 1 2 3 4

12 12 12 12 12 12 12 12

21 43 60 95 18 61 89 92

Mcc × 10−3

Mnd × 10−3

D̵ d

Pre

17.0 27.4

10.8 42.9

1.31 1.49

0.73 0.75

25.6 26.5

32.5 30.5

1.16 1.14

a

General polymerization conditions: THF as the solvent, [LA]0:[I]0 = 400:2 (I stands for the initiator, with two initiators per dinuclear rare-earthmetal complex), [LA]0 = 1 mol L−1, and 30 °C. bYield: weight of polymer obtained/weight of monomer used. cMc = 144[LA]0/[I]0(polymer yield) (%). dMeasured by GPC calibrated with standard polystyrene samples and corrected by a factor of 0.58 for PLA. ePr is the probability of racemic linkages between monomer units, which is obtained from the methine region of the homonuclear decoupled 1H NMR spectrum. C


Article

Inorganic Chemistry

relationship. A linear plot of kapp versus [I] was also observed (Figure 4), suggesting a first-order dependence of the rate on

order of their ionic radii. The same observation has been reported in other rare-earth-metal initiator systems for LA polymerization.18a,22 The activities of these rare-earth-metal complexes for rac-LA polymerization were obviously lower than those of Salen rare-earth-metal aryloxides,23 which may be attributed to the dimeric structures of complexes 1−4. Both complexes 3 and 4 led to polymers of moderate heteroselectivity, and the Pr values (Pr = probabilities of racemic enchainment) are 0.73 and 0.75, respectively (Figures S9 and S10). It seems that ionic radii of rare-earth metals do not significantly influence the stereoselectivity for rac-LA polymerization. Compared with the catalytic activity of complex 4 (Table 1, entry 4), the Salalen Sm alkoxo complex22a and chiral Salen In complexes24 exhibit higher activity. For example, the Salalen samarium complex polymerized 800 equiv of rac-LA in 2 h to give 95% yield. However, the Salan Al methyl complex32a and similar Salan Zn complexes25 show lower catalytic activity. For example, the Salan Al complex polymerized 100 equiv of rac-LA in 24 h to give 75% yield. In the study of the rac-LA stereoselectivity, the Salalen Sm alkoxo complex polymerized rac-LA with moderate heteroselectivity (Pr = 0.73), and the chiral Salen In complexes (Pm = 0.77) and Salan Al complexes (Pm = 0.79) showed isotactic selectivity for rac-LA polymerization, but predominately atactic PLA was formed, initiated by Salan Zn complexes. In Table 1, the molecular weights of the polymers (Mn) are found to be larger than the theoretical values (Mc), suggesting that the number of actual initiating groups may be between 1 and 2 on average per dinuclear complex. To understand the catalytic differences of these Salan rareearth-metal aryloxides for rac-LA and L-LA polymerization, the kinetics of polymerization initiated by complex 4 were studied. The reaction order of rac-LA was determined by conducting polymerization with different molar ratios of monomer to initiator at 30 °C in THF, and the plot of ln([rac-LA]0/[racLA]t) versus polymerization time is shown in Figure 3. It is conclusive that the polymerization follows first-order kinetics for the rac-LA concentration, as corroborated by a good linear

Figure 4. Plot of kapp versus [I] for rac-LA using complex 4 as the catalyst in THF at 30 °C (linear fit, R2 = 0.9929).

the initiator concentration. Thus, the kinetic equation of racLA polymerization initiated by complex 4 is −d[rac-LA]/dt = kp[rac-LA][I]. To gain more insight into rac-LA polymerization, the kinetics at different temperatures were also studied, and the results are shown in Figure 5. The propagation rate constants

Figure 5. Kinetic plots of rac-LA polymerization using complex 4 as the catalyst at different temperatures. Polymerization conditions: THF as the solvent, [rac-LA]0 = 1 M, [rac-LA]0/[I]0 = 400:2. (a) 30 °C; ■, kapp = (1.14 ± 0.02) × 10−4 s−1 (linear fit, R2 = 0.9996); (b) 35 °C; ◇, kapp = (1.87 ± 0.04) × 10−4 s−1 (linear fit, R2 = 0.9933); (c) 40 °C; ▲, kapp = (3.99 ± 0.05) × 10−4 s−1 (linear fit, R2 = 0.9995); (d) 50 °C; □, kapp = (8.99 ± 0.53) × 10−4 s−1 (linear fit, R2 = 0.9941).

(kp) were determined according to the equation kapp = kp[initiator]0, which are determined as 0.0228 ± 0.0004 (30 °C), 0.0374 ± 0.0008 (35 °C), 0.0798 ± 0.0010 (40 °C), and 0.1798 ± 0.0106 (50 °C) M−1 s−1. Apparently, the polymerization rate increased with increasing reaction temperature, and kp at 50 °C was found to be about 8 times that at 30 °C. The temperature dependence of kapp is shown with an Arrhenius plot in Figure 6. The value of the apparent activation energy

Figure 3. Kinetic plots of the rac-LA using complex 4 as the catalyst in THF at 30 °C: [rac-LA]0 = 1 M; ■, [rac-LA]0/[I]0 = 400:2, kapp = (1.14 ± 0.02) × 10−4 s−1 (linear fit, R2 = 0.9996); ▲, [rac-LA]0/[I]0 = 600:2, kapp = (0.55 ± 0.02) × 10−4 s−1 (linear fit, R2 = 0.9899); □, [rac-LA]0/[I]0 = 800:2, kapp = (0.38 ± 0.01) × 10−4 s−1 (linear fit, R2 = 0.9972); ◆, [rac-LA]0/[I]0 = 1000:2, kapp = (0.19 ± 0.01) × 10−4 s−1 (linear fit, R2 = 0.9967). D


Article

Inorganic Chemistry

polymerization (85.8 ± 6.9 kJ mol−1). In contrast, the polymerization of rac-LA has a kapp (1.14 × 10−4 s−1) approximately 2 times that (0.57 × 10−4 s−1) for polymerization of L-LA ([LA]0/[I]0 = 400:2, Figures 3 and S1). The value of Pr = 0.75 for the polymerization of rac-LA with complex 4 at 30 °C suggests a relative rate difference of three processes, which means kR/SS > kR/RR (or kS/RR > kS/SS). This is consistent with the observation that the polymerization of L-LA proceeds slower than that of rac-LA. Similar phenomena were reported in the literature.27 These results revealed that the ROP of L-LA catalyzed by complex 4 is more difficult in comparison with that of rac-LA. Copolymerization of L-LA and ε-CL. The catalytic behaviors of these Salan metal complexes for the one-pot copolymerization of L-LA and ε-CL were also evaluated, and the results are summarized in Table 2. Conversions of L-LA were over 93% in all cases after the polymerization was conducted in 12 h in the presence of 0.25 mol % initiator (entries 1−4). At the same time, ε-CL polymerization also occurred, and the conversions of ε-CL were 32% using the yttrium complex (1) as the catalyst and over 80% using the samarium (2), neodymium (3), and lanthanum (4) complexes as the catalyst, indicating that the ionic radii of the central metals obviously affect the copolymerization. A similar influence of the ionic radii of rare-earth metals on the rac-LA polymerization rate has been reported.28 1H NMR analysis revealed the presence of LA and CL linkages in the copolymer. The signals for methylene groups (−COO−CH2− and −CH2−CO−) attributed to CL−CL homosequences were observed at about 4.0 and 2.3 ppm (Figures S11−S16), and the signals in the downfield region (4.1 and 2.4 ppm) can be attributed to the methylene protons of CL−LA heterosequences, which are consistent with those reported in the literature.9a According to the 1H NMR spectra, the copolymer obtained by complex 1 has about 25% CL and 75% LA units, whereas the copolymers initiated by complexes 2−4 have about 50% CL and 50% LA units, which was consistent with the initial feed ratio. This evidence revealed that copolymerization of ε-CL and L-LA occurred successfully in this system. This result is quite different from that of systems initiated by homoleptic rare-earth-metal aryloxides,14a a dimeric neodymium borohydride complex bearing aminebridged bis(phenolate) ligand,13 and a divalent samarocene complex,14b in which only PLA was obtained when ε-CL and LLA were added simultaneously. The successful copolymerization of ε-CL and L-LA in our system demonstrated that the

Figure 6. Plot of ln kapp versus 1/T for polymerization of rac-LA with complex 4 as the catalyst (linear fit, R2 = 0.9870). Eapp = (85.8 ± 6.9) kJ mol−1.

(Eapp) in this system is determined to be (85.8 ± 6.9) kJ mol−1, which is higher than those of rac-LA polymerization catalyzed by an aluminum Schiff base26 and a lanthanum complex.23 The kinetics of L-LA polymerization catalyzed by complex 4 were also studied under different molar ratios of monomer to initiator ([L-LA]0/[I]0 = 150, 200, 250, and 300). A first-order dependence on the monomer and initiator concentrations was observed (Figures S5 and S6). Thus, the kinetic equation of LLA polymerization initiated by complex 4 is −d[L-LA]/dt = kp[L-LA][I]. The influence of the reaction temperature on the ROP of LLA was also evaluated using complex 4 as the catalyst. It can be seen that the polymerization rate increased with increasing reaction temperature (Figure S7), and the propagation rate constants (kp) of L-LA polymerization are 0.0114 ± 0.0002 (30 °C), 0.0228 ± 0.0010 (35 °C), 0.0658 ± 0.0030 (40 °C), and 0.2160 ± 0.0136 (50 °C) M−1 s−1. The value of kp at 50 °C was found to be 20 times that at 30 °C. This difference is apparently larger than that of rac-LA polymerization (vide supra). These results indicated that the influence of the reaction temperature on L-LA polymerization is more significant than that on rac-LA polymerization. It can be seen that there is a good linear relationship between ln kapp and 1/T (Figure S8). From the slope of the plot, the apparent activation energy Eapp of L-LA polymerization is 122.0 ± 9.0 kJ mol−1, which is obviously larger than that of rac-LA

Table 2. One-Pot Copolymerization of L-LA and ε-CL by Complexes 1−4a entry

catalyst

1 2 3 4 5g 6h 7i

1 2 3 4 4 4 4

CCL, CLAb (%) 32, 84, 85, 84, 84, 90, 91,

96 93 94 97 91 86 83

CL/LAc (%) 25:75 48:52 48:52 46:54 48:52 51:49 53:47

LLA, LCLd 8.1, 4.3, 4.1, 2.9, 2.8, 2.8, 1.8,

2.3 4.8 4.6 3.0 3.3 3.0 1.9

CL−LA/CL−CLe (%)

Mcf × 10−3

Mn × 10−3

D̵

45:55 21:79 24:76 36:64 38:62 40:60 51:49

34.9 45.9 46.5 47.1 45.4 45.3 44.7

63.6 99.6 128.8 69.9 96.2 100.6 34.4

1.51 1.75 1.72 1.70 1.77 1.77 1.80

General polymerization conditions: toluene as the solvent, [ε-CL]0:[L-LA]0:[I]0 = 400:400:2, [ε-CL]0 = [L-LA]0 = 1 mol L−1, and 90 °C for 12 h. Monomer conversion was measured according to 1H NMR spectroscopy. cCL/LA mole ratio in the copolymer. dThe average chain length was measured according to 13C NMR spectroscopy. eMolar ratio of two linkages in the copolymer. fMc = 114[ε-CL]0/[I]0CCL (%) + 144[L-LA]0/ [I]0CLA (%). g24 h. h110 °C. i130 °C. a

b

E


Article

Inorganic Chemistry

copolymer (Table 2, entry 5). However, increasing the polymerization temperature has an obvious influence on the copolymerization. When the reaction temperature increased to 130 °C, the conversion of ε-CL was obviously increased, and an increased amount of LA−CL heterodiads was also found (Table 2, entry 7). Especially, the average sequence lengths of the resultant copolymer became LCL = 1.9 and LLA = 1.8, which is near that of the ideal random LA−CL copolymer. These results demonstrated that complex 4 can catalyze a highly random copolymerization of ε-CL and L-LA at 130 °C. In order to further know the catalytic behavior of complex 4, the one-pot copolymerization of ε-CL and L-LA with different monomer feed ratios was tested, and the results are listed in Table 3. All of the copolymerization proceeded smoothly, giving conversions of ε-CL and L-LA over 70%, and the molar ratios of the LA and CL units in the resultant copolymers were consistent with the monomer feed ratios. Furthermore, 1H NMR analysis revealed that there are CL−LA heterodiads in the copolymer, and the percentages of the CL−LA linkages increased with an increase of the ratio of LA (Table 3, entries 1−5). Meanwhile, the values of randomness (R) of the resultant copolymers range from 0.75 to 1.07. These results suggested that the random copolymers were formed. Thus, starting from different monomer feed ratios, random L-LA/εCL copolymers of various compositions formed straightforwardly. The thermal properties of these copolymers were determined by differential scanning calorimetry (DSC). It was found that no Tg was detected when the content of PLA in the copolymer was low (entries 1 and 2). The values of Tg rose from −30.2 °C (Table 3, entry 3) to +38.3 °C (Table 3, entry 9) with an increase of the molar ratio of the LA units in the copolymers. In the meantime, there is a linear relationship between the Tg values and the molar percentage of the PLA linkages (Figure 8). These results indicated that the thermal properties of the copolymers depend strongly on the compositions of the copolymers. Generally, to form an ideal random copolymer of ε-CL and LA, the reactivity ratios of these two monomers should be equal6b or the transesterification should play a key role in redistributing the copolymer chain structure.9b,17 To understand the random copolymerization mechanism in this system, monitoring experiments of the chain microstructure with the polymerization time were carried out, and the results are shown in Figures 9 and 10, respectively. The results in Figure 9 demonstrated that L-LA polymerized rapidly in the first 3 min,

ligand frameworks around rare-earth metals significantly affect their catalytic property. The chain microstructures of the copolymers were determined by quantitative 13C NMR spectroscopy. According to literature reports, the carbonyl signals of the copolymers range from 169 to 174 ppm (Figure 7), which can be assigned

Figure 7. 13C NMR spectra of carbonyl groups of the copolymers obtained by complexes 1−4.

accurately to different possible monomer linkages,6b,29 and the corresponding average lengths of the lactidyl (LLA) and caproyl (LCL) units can be calculated according to the equations suggested by Kasperczyk and Bero.30 The ionic radii of the rare-earth metals are found to have a profound effect on the copolymer chain microstructure. In the copolymer obtained with the yttrium complex 1, the value of LLA is 8.1, whereas the value of LCL is only 2.3 (Table 2, entry 1), which deviates from a random structure (ideally LCL = LLA = 2).6b With an increase in the ionic radius of the initiator, an obviously shorter lactidyl (LLA) sequence length was observed. When the lanthanum complex 4 was used as the initiator, the average sequence lengths of the resulting copolymer are LCL = 3.0 and LLA = 2.9 (Table 2, entry 4), suggesting a practically random structure. A further study revealed that lengthening of the polymerization time has a slight influence on the chain microstructure of the

Table 3. Copolymerization of L-LA and ε-CL with Different Feed Ratios by Complex 4a entry

[ε-CL]0:[L-LA]0:[I]0

1 2 3 4 5 6 7 8 9

720:80:2 640:160:2 560:240:2 480:320:2 400:400:2 320:480:2 240:560:2 160:640:2 80:720:2

CCL, CLAb (%) 87, 87, 82, 92, 91, 84, 75, 78, 86,

78 77 90 84 83 81 82 83 70

CL/LAc (%)

LLA, LCLd

Re

CL−LA/CL−CLf (%)

Mcg × 10−3

Mn × 10−3

D̵

Tg (°C)

91:9 82:18 68:32 62:38 53:47 41:59 28:72 19:81 12:88

0.8, 7.7 0.9, 4.0 1.1, 2.3 1.3, 2.0 1.8, 1.9 2.3, 1.4 3.3, 1.2 5.0, 1.1 11.5, 1.0

0.75 0.80 0.88 0.85 0.82 0.96 0.99 1.02 1.07

14:86 24:76 44:56 48:52 51:49 70:30 79:21 83:17 85:15

40.2 40.6 41.7 44.5 44.7 43.3 43.3 45.4 40.2

93.9 76.3 52.8 54.3 34.4 33.9 32.5 34.7 30.6

1.63 1.63 1.58 1.62 1.80 1.80 1.85 1.76 1.80

−30.2 −23.9 −12.7 3.4 21.6 31.4 38.3

General polymerization conditions: toluene as the solvent, ([L-LA]0 + [ε-CL]0):[I]0 = 800:2, [L-LA]0 + [ε-CL]0 = 2 mol L−1, and 130 °C for 12 h. Monomer conversion was measured according to the 1H NMR spectrum. cMole ratio of CL and LA in the copolymer. dThe average chain length was measured according to the 13C NMR spectrum. eDegree of randomness, R = lRLL/leLL (lRLL = (k + 1)/2k, where k = [Cap]/[L]). f(CL−LA linkages)/(CL−CL linkages) in the copolymer. gMc = 114[ε-CL]0/[I]0CCL (%) + 144[L-LA]0/[I]0CLA (%). a

b

F


Article

Inorganic Chemistry

rule out the possibility of equal reactivity of these two monomers and suggest that transesterification is a predominate role in random copolymer formation in this system. Thus, the proposed copolymerization mechanism is shown in Scheme 2. At the first step of copolymerization, the rare-earth-metal complex initiated LA polymerization to form long PLA chains. Then ε-CL inserted a Ln−PLA linkage to form an active caproyl center. Subsequently, an inter- or intramolecular transesterification process occurred, leading to cleavage of the lactidyl groups along with the formation of LCL linkages. The intramolecular transesterification may lead to cyclic polymer formation, which may undergo a ring opening to form a chain polymer. Finally, a random copolymer was formed by a continuous insertion/transesterification reaction. To prove the above mechanism, we conducted the experiment below, in which PLA was mixed with complex 4, followed by the addition of ε-CL. After 12 h of reaction at 130 °C, the resulting polymer was obtained as a random copolymer in 83% yield. This result strongly supports a transesterification-driven random copolymerization.

Figure 8. Relationship between the Tg values of the copolymers and the composition of the L-LA units in the copolymers at 130 °C.

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CONCLUSION In summary, four dinuclear Salan rare-earth-metal aryloxides were prepared and characterized via X-ray structure determination. Their catalytic properties for homo- and copolymerization of cyclic esters were elucidated. A comparative study on the polymerization kinetics of L-LA and rac-LA was carried out in detail. It was found that the polymerization is first-orderdependent on both the monomer and initiator concentrations, respectively, and the apparent activation energy Eapp of L-LA is obviously higher than that of rac-LA. Furthermore, these rareearth-metal complexes are efficient catalysts for the random copolymerization of ε-CL and L-LA through the simultaneous addition of both monomers, and the ionic radii of rare-earth metals have a profound effect on their catalytic properties. In the presence of the lanthanum complex 4, perfect random copolymers of L-LA and ε-CL with different compositions can be prepared conveniently by adjusting the monomer feed ratios. Thermal analysis revealed that the thermal behaviors of the resultant copolymers obviously depend on the compositions of the copolymers. A further copolymerization mechanism study revealed that transesterification plays a crucial role in the monomer linkage redistribution and the random-chain microstructure formation in this system. This is one of the rare examples of well-defined rare-earth-metal complexes for the one-pot random copolymerization of L-LA and ε-CL. A further study on the synthesis of rare-earth-metal-based initiators and their catalytic property for the random copolymerization of cyclic esters is ongoing in our laboratory.

Figure 9. Consumption of ε-CL and L-LA with the polymerization time catalyzed by complex 4.

whereas ε-CL decreased rather slowly. On the other hand, it can be seen that only homo-PLA is formed within the first 3 min from Figure 10 because only the carbonyl signal of PLA at 169.7 ppm was observed in the 13C NMR spectrum. When the polymerization time was prolonged to 10 min, the carbonyl signal of PCL appeared. When polymerization time was further prolonged, the carbonyl signal of PLA got weaker, and the carbonyl signals of different monomer linkages got stronger. Furthermore, a strong signal at about 171 ppm, a characteristic signal for the CLC (C for caproyl unit, L for lactic ester) sequence, was observed in the 13C NMR spectra. Obviously, the CLC linkages must come from transesterification.6c,7a In addition, a control experiment was carried out. Two reaction vessels containing complex 4/ε-CL and complex 4/L-LA reacted for 1 h at 130 °C, and then the solution in the two vessels was mixed. The mixture was reacted at 130 °C for another 12 h. Analysis of the resulting polymer with 1H NMR spectroscopy showed the presence of two homopolymers, which is further corroborated by gel permeation chromatography (GPC) spectra (Figures S18 and S19). These findings

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EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under a purified argon atmosphere using standard Schlenk techniques. Solvents were degassed and distilled from sodium benzophenone ketyl under argon prior to use. (C5H5)3Ln(THF)31 and the Salan ligand LH232 were prepared according to the literature. pMethylphenol was dried over 4 Å molecular sieves for 1 week and distilled before use. L-LA and rac-LA were recrystallized twice from dry toluene and then sublimed under vacuum at 50 °C. ε-CL was distilled over CaH2 under reduced pressure prior to use. Carbon, hydrogen, and nitrogen analyses were measured by direct combustion with a Carlo-Erba EA-1110 instrument. 1H and 13C NMR spectra were recorded with a Unity Varian-400 spectrometer. The IR spectra were recorded with a Nicolet-550 Fourier transform infrared G


Article

Inorganic Chemistry

Figure 10. 13C NMR spectra of carbonyl groups of copolymers obtained at different polymerization times.

Scheme 2. Proposed Random Copolymerization Mechanism

Synthesis of [LY(OC6H4-4-CH3)]2•3(C7H8) (1). To a THF solution (10 mL) of (C5H5)3Y(THF) (0.71 g, 2.00 mmol) was added a THF solution (8 mL) of LH2 (1.02 g, 2.00 mmol). The reaction mixture was stirred for 1 h at room temperature, and then p-methylphenol (0.21 mL, 2.00 mmol) was added by syringe. The mixture was stirred for 24 h at 50 °C, and a white powder formed slowly. The solution was concentrated to about 7 mL, the white powder was collected by centrifugation, and then toluene (30 mL) was added to dissolve the residue. Colorless crystals were obtained from a toluene solution at room temperature in several days (0.91 g, 65%). Anal. Calcd for C103H114N4O6Y2: C, 73.56; H, 6.83; N, 3.33. Found: C, 73.63; H, 6.91; N, 3.24. 1H NMR (400 MHz, C6D6): δ 7.13−6.33 (51H, ArH, ligand and toluene), 5.22 (d, 2H, CH2Ph), 4.96 (d, 2H, CH2Ph), 4.54 (d, 2H, CH2Ph), 4.44 (d, 2H, CH2Ph), 4.24 (d, 2H, CH2Ph), 3.34 (d, 2H, CH2Ph), 3.10 (6H, CH3Ar), 2.94 (d, 2H, CH2Ph), 2.86 (d, 2H, CH2Ph), 2.81−2.64 (m, 8H, NCH2CH2N), 2.32 (6H, CH3Ar), 2.26

spectrometer as KBr pellets. The molecular weight (Mn) and molecular weight distribution (D̵ ) were determined against a polystyrene standard by GPC with an HLC-8320 GPC instrument, and THF (HPLC-grade) was used as an eluent at a flow rate of 1.0 mL min−1 at 40 °C. Samples were prepared by dissolving a maximum weight of 5 mg of polymer in 5 mL of THF. Microstructures of copolymers were measured by 1H and 13C NMR spectroscopy in CDCl3. Glass transition temperatures (Tg) of the copolymers were measured by differential scanning calorimetry (DSC) using a 2010 DSC V 4.4E instrument in a nitrogen flow with a heating and cooling rate of 10 °C min−1 in the range of −80 to +200 °C. For DSC measurements, the samples were first heated to 200 °C at a rate of 10 °C min−1 and cooled to −80 °C. The samples were then heated to 200 °C at a heating rate of 10 °C min−1 to determine their glass transition temperature (Tg). H


Article

Inorganic Chemistry (6H, CH3Ar), 2.16 (9H, CH3, toluene), 2.12 (12H, CH3Ar). 13C NMR (100 MHz, C6D6): δ 155.3, 138.2, 133.1, 132.7, 132.6, 131.5, 131.3, 131.2, 131.1, 129.9, 129.7, 129.4, 128.9, 126.0, 124.4, 123.4, 120.5 (ArC), 58.6, 57.8 (ArCH2N), 32.3 (NCH2CH2N), 23.4, 21.8, 20.9, 20.8, 20.7, 20.3, 17.9, 14.7 (PhCH3). IR (KBr, cm−1): 2915(w), 2857(w), 1605(m), 1503(s), 1479(s), 1359(w), 1291(s), 1272(s), 1237(s), 1165(m), 1028(m), 863(s), 818(s), 727(s), 702(s). Synthesis of [LSm(OC6H4-4-CH3)]2 (2). The synthesis of complex 2 was carried out in the same way as that described for complex 1, but (C5H5)3Sm(THF) (0.84 g, 2.00 mmol) was used instead of (C5H5)3Y(THF). A yellow powder was collected by centrifugation, and then toluene (20 mL) and THF (4 mL) were added to dissolve the residue. Yellow crystals were obtained at room temperature in several days (1.11 g, 73%). Anal. Calcd for C82H90N4O6Sm2: C, 64.44; H, 5.94; N, 3.67. Found: C, 65.12; H, 6.06; N, 3.23. IR (KBr, cm−1): 2914(w), 2855(w), 1653(m), 1504(s), 1476(s), 1283(s), 1268(s), 1239(s), 1163(m), 1028(m), 986(m), 865(s), 814(s), 768(s), 705(s), 678(s). Synthesis of [LNd(OC6H4-4-CH3)]2 (3). The synthesis of complex 3 was carried out in the same way as that described for complex 1, but (C5H5)3Nd(THF) (0.82 g, 2.00 mmol) was used instead of (C5H5)3Y(THF). A blue powder was collected by centrifugation, and then toluene (25 mL) and THF (3 mL) were added to dissolve the residue. Blue crystals were obtained at room temperature in several days (1.06 g, 70%). Anal. Calcd for C82H90N4O6Nd2: C, 64.96; H, 5.98; N, 3.70. Found: C, 65.53; H, 6.17; N, 3.12. IR (KBr, cm−1): 2913(w), 2856(w), 1604(m), 1502(s), 1476(s), 1444(m), 1281(s), 1268(s), 1236(s), 1162(m), 1028(w), 1003(w), 859(s), 813(s), 785(s), 727(s), 677(s). Synthesis of [LLa(OC6H4-4-CH3)]2(THF)2 (4). The synthesis of complex 4 was carried out in the same way as that described for complex 1, but (C5H5)3La(THF) (0.81 g, 2.00 mmol) was used instead of (C5H5)3Y(THF). A white powder was collected by centrifugation, and then toluene (20 mL) and THF (10 mL) were added to dissolve the residue. Colorless crystals were obtained at room temperature in several days (1.32 g, 80%). Anal. Calcd for C90H106N4O8La2: C, 65.53; H, 6.48; N, 3.40. Found: C, 65.67; H, 6.40; N, 2.94. 1H NMR (400 MHz, CDCl3): δ 7.63−7.30 (m, 28H, ArH), 7.21−7.06 (m, 8H, ArH), 7.05−6.35 (m, 16H, ArCH2N), 3.93 (m, 8H, α-CH2 THF), 3.20−2.55 (m, 8H, NCH2CH2N), 2.52 (s, 6H, ArCH3), 2.47−2.10 (m, 24H, ArCH3), 1.91 (m, 8H, β-CH2 THF). 13 C NMR (100 MHz, CDCl3): δ 137.9, 132.8, 132.4, 131.9, 131.6, 131.4, 130.2, 129.7, 129.2, 128.4, 128.2, 128.1, 127.9, 125.5, 119.7 (ArC), 77.4 (ArCH2N), 68.7 (α-CH2 THF), 57.6 (NCH2CH2N), 25.6 (β-CH2 THF), 21.6 (PhCH3), 20.7, 20.6 17.5 (PhCH3). IR (KBr, cm−1): 3587(w), 2915(w), 1605(m), 1569(m), 1500(s), 1475(s), 1456(m), 1436(m), 1314(m), 1269(s), 1227(m), 859(s), 822(s), 810(s), 761(s), 729(s), 706(s). General Procedure for Homopolymerization of LA. A 15 mL Schlenk flask, equipped with a magnetic stirring bar, was charged with the desired amount of LA and THF. The mixture was stirred vigorously at 30 °C for the desired time, during which time an increase in the viscosity was observed. The reaction mixture was quenched by the addition of ethanol and then poured into ethanol (20 mL) to precipitate the polymer, which was dried under vacuum and weighed. General Procedure for Copolymerization of L-LA and ε-CL. A 15 mL Schlenk flask was charged with L-LA (576 mg), ε-CL (0.43 mL), toluene (3.6 mL), and complex 4 (15 mg). Polymerization was conducted at 90 °C for 12 h. The polymerization was quenched by adding ethanol after cooling to room temperature, and the mixture was poured into ethanol (30 mL) to precipitate the polymer. The resultant polymer was washed with a large amount of ethanol, then dried under vacuum, and weighed. X-ray Crystallographic Structure Determination. Suitable single crystals of complexes 1−4 were sealed in a thin-walled glass capillary for determination of the single-crystal structures. The intensity data were collected on an Agilent Xcalibur diffractometer equipped with Mo Kα radiation (graphite-monochromated, λ = 0.71073 Å). The diffracted intensities were corrected for Lorentz/

polarization effects and empirical absorption corrections. Details of the intensity data collection and crystal data are given in Table S1. The structures were solved by direct methods and refined by fullmatrix least-squares procedures based on |F|.2 The H atoms in these complexes were generated geometrically, assigned the appropriate isotropic thermal parameters, and allowed to ride on their parent C atoms. All of the H atoms were held stationary and included in the structure factor calculation in the final stage of full-matrix leastsquares refinement. The structures were solved and refined using SHELEXL-97 programs.

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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.8b01046. Synthesis and characterization of proligand LH2, NMR spectra, tables of crystallographic data and selected bond parameters of complexes 1−4, kinetic data for L-LA polymerization, 1 H NMR spectra of ε-CL/ L -LA copolymers, and a GPC spectrum (PDF) Accession Codes

CCDC 1570041−1570044 contain 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 Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: (86)512-65882806. Fax: (86)512-65880305. ORCID

Yingming Yao: 0000-0001-9841-3169 Dongmei Cui: 0000-0001-8372-5987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Grants 21402138 and 21674070), the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry (CIAC), PAPD, and the project of scientific and technologic infrastructure of Suzhou (Project SZS201708) for financial support.

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REFERENCES

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