Efficient Catalysts for Ring-Opening Polymerization ... - ACS Publications

Dec 20, 2012 - Experimental results demonstrated that complex 1 is an active catalyst for the ring-opening polymerization of ε-caprolactone (ε-CL) a...
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Facile Synthesis of Well-Defined Titanium Alkoxides Based on Benzotriazole Phenoxide Ligands: Efficient Catalysts for RingOpening Polymerization of Cyclic Esters Chen-Yu Li, Chia-Jung Yu, and Bao-Tsan Ko* Department of Chemistry, Chung Yuan Christian University, Chung-Li 320, Taiwan S Supporting Information *

ABSTRACT: New titanium alkoxide complexes incorporated by mono- or bisBTP ligands (BTP = N,O-bidentate benzotriazole phenoxide) were synthesized and structurally characterized. The reaction of 2-(2H-benzotriazol-2-yl)-4-(2,4,4trimethylpentan-2-yl)phenol (C8BTP-H) with Ti(OiPr)4 (1.0 mol equiv) in hexane produced the monoadduct complex [(μ-C8BTP)Ti(OiPr)3]2 (1), but treatment of Ti(OiPr)4 with 2.0 mol equiv of C8BTP-H at 50 °C gave the bis-adduct titanium complex [( C8 BTP) 2 Ti(O i Pr) 2 ] (2). The monomeric titanium analogue ([(t‑BuBTP)2Ti(OiPr)2] (3), [(TMClBTP)2Ti(OiPr)2] (4)) was prepared by employing sterically bulky 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol (t‑BuBTP-H) or 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol (TMClBTP-H) as the ligand according to the same synthetic route with a ligand to metal precursor ratio of 2:1. X-ray diffraction of single crystals indicates that 1 displays a dimeric Ti(IV) complex containing a Ti2O2 core bridging through one of the aryloxy oxygen atoms and the geometry around each Ti atom is distorted from an octahedral environment. Catalysis for lactone polymerizations of BTP-containing Ti complexes was investigated. Experimental results demonstrated that complex 1 is an active catalyst for the ring-opening polymerization of ε-caprolactone (ε-CL) and L-lactide (L-LA) with “living” and “immortal” characters. Kinetic studies of ε-CL polymerization initiated by 1 were investigated using in situ 1H NMR spectroscopy; the polymerization exhibited first-order dependence on both complex 1 and ε-CL concentrations.

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INTRODUCTION Metal-catalyzed ring-opening polymerization (ROP) of lactones is one of the most promising processes for preparations of environmentally friendly and biomedical polyesters, and it provides an alternative in the synthesis of well-characterized biodegradable polymers, such as poly(ε-caprolactone) (PCL) and poly(lactide) (PLA) as well as their copolymers. However, undesired intermolecular transesterification and intramolecular cyclization reactions may take place when some commercially available metal alkoxides are employed as initiators for the ROP of cyclic esters.1 To diminish such side reactions, a variety of groups have successfully developed metal complexes with a socalled single active site supported by diverse ancillary ligands to achieve efficient catalytic activities with controlled properties.2 These metal alkoxides are mostly derived from various metal complexes, including aluminum, lanthanide, magnesium, tin(II), yttrium, and zinc as well as titanium complexes. Among them, titanium alkoxide based initiator systems can be easily obtained via a one-step synthetic route from titanium tetraalkoxide precursors and seem to be suitable initiators for ROP due to their high Lewis acidity and low toxicity.3−7 For instance, the titanium isopropoxide complexes incorporating amine-phenolate ligands were synthesized by a direct reaction between the ligand precursors and Ti(OiPr)4, and these complexes were demonstrated to be active initiators for lactide polymerizations.6a © XXXX American Chemical Society

In order to design well-defined catalysts for ROP and CO2/ epoxide coupling, we have successfully developed catalytic systems of metal benzotriazole phenoxide (BTP) complexes, where BTP ligands include unmodified and amino- and imineBTP ligands.8 The syntheses and structural characterizations of BTP-containing Al,9 Mg,10 Pd,11 and Zn10a,12 complexes were reported recently, and the zinc complexes catalyzed the polymerization of ε-caprolactone (ε-CL) with excellent catalytic activity in a controlled manner.12b Despite the good efficiency of the aforementioned BTP-bearing complexes for the ROP of cyclic esters, it is necessary to add an extra alcohol initiator in a proper catalyst to alcohol ratio to give wellcontrolled polymers. As a result, more useful well-defined catalytic systems for lactone polymerizations could be denoted as [LnMt(OR)m]y (L = ligand; Mt = metal; OR = alkoxy; n, m, y = integer). For this reason, our current interest focuses on developing group 4 metal complexes supported by both BTP ligands and ancillary alkoxy groups. Moreover, the Lewis acidity of BTP derivatives with various metal systems might lead to dramatic differences in polymerization performances. Herein, we describe the synthesis, structure, and ROP catalysis of novel titanium complexes based on various BTP ligands (Figure 1). Received: October 13, 2012

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Figure 1. Ligand precursors used in this study: (left) (middle) t‑BuBTP-H; (right) TMClBTP-H.

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of −OiPr groups and the corresponding methylene signal (δ 1.68 ppm) from the −C8H17 alkyl group of the BTP ligand with the integration ratio 3:2. The NMR spectra of bis-adduct complexes 2−4 displayed one set of BTP and OiPr signals, indicating that each complex forms a C2-symmetry structure in solution. Attempts to isolate the monoadduct Ti complexes modified with these sterically bulky BTP ligands were unsuccessful. All of these compounds were isolated as yellow crystalline solids and fully characterized by spectroscopic studies as well as microanalyses. Their molecular structures were further confirmed by X-ray single-crystal analysis. Crystal Structure Studies of Complexes 1−4. Single crystals of complexes 1−4 suitable for X-ray structural determinations were grown from their saturated hexane or toluene solutions. Oak Ridge thermal ellipsoid plot (ORTEP) drawings illustrating selected bond distances and angles of the molecular structures of 1−4 are shown in Figures 2−4. The solid-state structure of 1 is a dimer, including a Ti2O2 core bridging through one of the phenoxy oxygen atoms of the C8 BTP− ligands. The geometry around each Ti is distorted from an octahedral environment, with the metal center hexacoordinated by one N atom and two bridging O atoms of two BTP ligands and by three O atoms from three isopropoxide groups. The distances between the Ti atom and atoms O(1), O(1A), O(2), O(3), and O(4) are 2.1145(12), 2.1038(12), 1.7947(13), 1.8190(13), and 1.8129(13) Å, respectively. These distances are all within the normal range previously reported for titanium catecholates.4c The six-membered rings consisting of a Ti center chelated with N,O-bidentate ligands for 1 are nonplanar, displaying twisted conformations. Compound 2 reveals a monomeric Ti(IV) complex that contains two N,O-bidentate C8 BTP− ligands. The Ti atom is hexacoordinated by two O atoms and two N atoms from the bidentate BTP ligands and two O atoms from the −OiPr groups, displaying a distortedoctahedral geometry. The equatorial plane, which is defined by Ti, O(1), N(1), O(2), and O(3), is almost coplanar with a mean deviation of 0.0936 Å; the angle O(4)−Ti−N(4) formed by the axial bonds is 173.43(8)°. The Ti−containing bond lengths of Ti−O(1) = 1.9004(16) Å, Ti−O(2) = 1.9141(16) Å,

C8

BTP-H;

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes of titanium alkoxides (1−4) supported by benzotriazole phenoxide ligands are shown in Scheme 1. The reaction of Ti(OiPr)4 with 1.0 mol equiv of 2-(2H-benzotriazol-2-yl)-4-(2,4,4trimethylpentan-2-yl)phenol (C8BTP-H) in hexane afforded the monoadduct dimeric complex [(μ-C8BTP)Ti(OiPr)3]2 (1) in high yield (77%). However, the bis-adduct titanium complex [(C8BTP)2Ti(OiPr)2] (2) was synthesized in 72% yield through the reaction of C8BTP-H (2.0 equiv) with Ti(OiPr)4 in hexane at 50 °C. Alternatively, complex 1 reacted further with 2.0 mol equiv of C8BTP-H in hexane to afford monomeric titanium complex 2 in good yield. Similarly, treatment of sterically bulky 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol (t‑BuBTP-H) or 2-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)-4-methylphenol (TMClBTP-H) with Ti(OiPr)4 according to the same synthetic route with a ligand to Ti precursor ratio of 2:1 gave the monomeric titanium analogue (3, [(t‑BuBTP)2Ti(OiPr)2]; 4, [(TMClBTP)2Ti(OiPr)2]) in high yield. The formations of expected titanium complexes 1−4 were demonstrated by the disappearance in the 1H NMR studies of the O−H signal of the C8 BTP-H, t‑BuBTP-H, or TMClBTP-H group (∼11.6 ppm) and by the downfield shift of resonances for the methine protons of isopropoxide groups bound to the titanium atom. For instance, the 1H NMR spectrum of complex 1 exhibited signals at 4.47 and 4.72 ppm for two chemically inequivalent methine protons Scheme 1. Synthetic Routes for Complexes 1−4

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Figure 2. ORTEP drawing of complex 1 with probability ellipsoids drawn at the 50% level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ti−O(1) = 2.1145(12), Ti−O(1A) = 2.1038(12), Ti−O(2) = 1.7947(13), Ti−O(3) = 1.8190(13), Ti−O(4) = 1.8129(13), Ti−N(1A) = 2.3284(16); O(1)−Ti−O(2) = 161.79(5), O(1)−Ti−O(3) = 91.24(6), O(1)−Ti−O(4) = 92.27(5), O(2)−Ti−O(3) = 102.37(6), O(2)−Ti−O(4) = 96.93(6), O(3)−Ti−O(4) = 100.76(6), O(1)−Ti−O(1A) = 70.70(5), O(2)−Ti−O(1A) = 92.57(6), O(3)−Ti− O(1A) = 155.37(6), O(4)−Ti−O(1A) = 96.68(6), O(1)−Ti−N(1A) = 83.38(5), O(2)−Ti−N(1A) = 85.35(6), O(3)−Ti−N(1A) = 86.78(6), O(4)−Ti−N(1A) = 171.41(6), O(1A)−Ti−N(1A) = 74.91(5).

Figure 3. ORTEP drawing of complex 2 with probability ellipsoids drawn at the 50% level. All hydrogen atoms are omitted for clarity.

1.7992(17) Å, which is ∼0.1 Å shorter than the average Ti− O(phenoxy) bond distance (1.9073(16) Å), indicating better covalent bonding between the titanium atom and alkoxy oxygen atom. It is worthy of note that complex 2 adopts a cis geometrical configuration with the adjacent oxygen atoms from

Ti−O(3) = 1.8062(17) Å, Ti−O(4) = 1.7921(17) Å, Ti−N(1) = 2.295(2) Å, and Ti−N(4) = 2.300(2) Å are all comparable with those observed for the monomeric titanium complexes bearing the phenoxyamine derivatives.4b,5a,7f The average bond distance between the Ti atom and O(isopropoxide) is C

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Figure 4. ORTEP drawings of (a) complex 3 and (b) complex 4 with probability ellipsoids drawn at the 50% level. All hydrogen atoms are omitted for clarity.

two −OiPr groups in the distorted coordination octahedron. The crystal structures of complexes 3 and 4 are isostructural with 2, and their molecular structures also exhibit a monomeric titanium complex, six-coordinated by t‑BuBTP− or TMClBTP− ligands and ancillary isopropoxide groups, in which the Ticontaining bond distances and angles are similar to those found for complex 2, as compared in Table 1.

decreasingly sterically hindering substituents at the 4- and 6positions of the BTP ligand. Accordingly, polymerizations initiated by 1 and 2 were systematically examined for their “living” character under optimal conditions (Table 2, entries 1, 10−14 and 3, 6−9). Basically, the actual molecular weight Mn of the produced polymers catalyzed by both initiators is close to the calculated molecular weight with the ratio [ε-CL]0/[Ti]0 in a range of 75−450 or 50−400. Furthermore, a linear relationship (Figure 5) between the number-averaged molecular weight (Mn) and the ratio of monomer to initiator ([εCL]0/[Ti]0) exists by using 1 as an initiator, and the PDIs of PCLs remain narrow (1.12−1.26), implying a “living” character of the polymerization. This was further verified by a polymerization resumption experiment (Table 2, entry 17). Another portion of the ε-CL monomer ([ε-CL]0/[Ti]0 = 150) was added after the polymerization of the first addition ([εCL]0/[Ti]0 = 150) had gone to completion. It was found that the produced narrow molecular weight distribution polymer with Mn(GPC) = 22400 is similar to the molecular weight from the addition of [ε-CL]0/[Ti]0 = 300 (Table 2, entry 17 vs entry 13). As illustrated in Figure 6, the 1H NMR spectrum displays that the PCL chain is capped with one isopropyl ester and one hydroxyl chain end with an integration ratio close to 6:2:1 among Hg, Hc, and Ha. To realize the capability of synthesizing PCL by using only small amounts of catalyst (“immortal” character), ε-CL polymerizations initiated by 1 using excess equivalent ratios of isopropyl alcohol (IPA, up to 9 equiv) as the chain transfer agent (entries 15 and 16) were carried out. For instance, 9-fold IPA can be added to [ε-CL]0/[Ti]0 = 300 for 6 h, producing a narrow-PDI polymer with an Mn value that is similar to the molecular weight from the addition of [ε-CL]0/ [Ti]0 = 75 (Table 2, entry 16 vs 10). It is worth noting that the catalytic performance of our Ti initiator 1 for ROP of ε-CL is superior to that of the previously published titanium isopropoxide complexes with amine bis(phenolate) ligands.4b On the basis of the good catalytic efficiency of ε-CL polymerizations catalyzed by titanium alkoxide 1, we also explored the catalytic activity in the ROP of lactide (LA). Representative results of the polymerizations of LA using 1 as an initiator under dry N2 are given in Table 3. Experimental results indicated that optimal conditions were [Ti]0 = 0.005 M

Table 1. Selected Bond Lengths (Å) and Angles (deg) for Complexes 2−4 2

3

4

Ti−O(1) Ti−O(2) Ti−O(3) Ti−O(4) Ti−N(1) Ti−N(4)

1.9004(16) 1.9141(16) 1.8062(17) 1.7921(17) 2.295(2) 2.300(2)

1.9201(14) 1.9277(14) 1.7692(14) 1.7807(14) 2.2926(17) 2.2946(16)

1.9188(17) 1.9155(17) 1.7678(18) 1.7717(18) 2.3050(2) 2.3130(2)

O(1)−Ti−O(2) O(3)−Ti−N(1) O(4)−Ti−N(4)

153.99(7) 168.65(8) 173.43(8)

157.36(6) 168.25(6) 167.70(7)

156.25(8) 168.14(8) 169.10(8)

Ring-Opening Polymerization of Cyclic Esters. Welldefined complexes [LnMt(OR)m] (L = ligand, OR = alkoxy, n = 1, 2, and m = 1−3), where Mt = Ti, Zr, Hf, have been demonstrated to be effective initiators for lactone polymerizations with good catalytic activities in a controlled manner.4−7 ROP of ε-caprolactone (ε-CL) initiated by Ti complexes 1−4 was first accomplished to evaluate their catalytic performances, and representative results are summarized in Table 2. Optimum conditions were found to be [Ti]0 = 0.01 M in toluene (10 mL) at 30 or 80 °C. Experimental results showed that complexes 1 and 2 are active initiators to give poly(ε-CL)s with narrow molecular weight distributions (PDI < 1.30), as depicted in Table 2, entries 1−5. Monoadduct complex 1 exhibits efficient activity within 6 h at 30 °C, whereas the bis-adduct complex 2 can achieve similar conversion in 10 h; however this requires a higher reaction temperature (80 °C). It is obvious that the catalytic performance of these complexes has the order 1 > 2 > 4 > 3, and their activities seem to increase along with the D

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Table 2. Ring-Opening Polymerization of ε-Caprolactone (ε-CL) Catalyzed by Complexes 1−4a entry

cat.

temp (°C)

[ε-CL]0/[Ti]0/[IPA]0

time (h)

conversn (%)b

Mn(calcd)c

Mn(obsd)d

Mn(NMR)e

PDIf

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17h

1 2 2 3 4 2 2 2 2 1 1 1 1 1 1 1 1

30 30 80 80 80 80 80 80 80 30 30 30 30 30 30 30 30

400/1/0 400/1/0 400/1/0 400/1/0 400/1/0 50/1/0 100/1/0 150/1/0 200/1/0 75/1/0 150/1/0 225/1/0 300/1/0 450/1/0 300/1/3 300/1/9 150 (150)/1/0

6 6 10 24 24 7 7 7 7 6 6 6 6 6 6 6 6 (6)

99 trace 92 66 76 99 99 99 99 99 99 99 99 99 99 99 99 (99)

15100 g 21100 15100 17400 2900 5700 8500 11400 2900 5700 8500 11400 17000 5700 2900 11400

29000 (16200) g 39000 (21800) 39000 (21800) 37600 (21000) 7000 (3900) 11000 (6100) 16500 (9200) 22000 (12000) 5000 (2800) 10400 (5800) 16500 (9200) 22000 (12300) 32300 (18000) 10200 (5700) 5200 (2900) 22400 (12500)

15800 g 21800 15600 18000 3800 5800 8300 11200 2900 5800 8300 11700 17700 5700 2900 12000

1.26 g 1.16 1.60 1.49 1.14 1.24 1.20 1.23 1.12 1.19 1.20 1.26 1.22 1.17 1.08 1.28

a

Conditions: [Ti]0 = 0.01 M, 10 mL of toluene. bObtained from 1H NMR determinations. cCalculated from the molecular weight of lactone (114.14 g/mol) times [ε-CL]0/n[Ti]0 (n = 3 for complex 1; n = 2 for complexes 2−4) times conversion yield plus the molecular weight of IPA (60.1 g/mol). d Obtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC times 0.56.14 e Obtained from 1H NMR analysis. fObtained from GPC analysis. gNot available. hPrepolymerization of ε-CL with initiator for 6 h followed by the addition of ε-CL and stirred for another 6 h.

Figure 5. Polymerization of ε-CL initiated by titanium complex 1 in toluene at 30 °C for 6 h. The relationship between the Mn (■) or the PDI value (□) of the polymer and the initial molar ratio [ε-CL]0/[Ti]0 is shown.

in toluene (10 mL) at 80 °C after several trials of L-LA polymerizations with the effects of solvent, concentration, temperature, and reaction time (Table 3, entries 1−4). To investigate the “living” fashion, different monomer to initiator ratios ([L-LA]0/[Ti]0 = 75−300) were systematically studied. It can be seen from Table 3, entries 4−7, that >90% L-LA conversion was achieved in 10 h and the actual molecular weight Mn of all produced polymers is similar to the theoretical value under optimum conditions. Figure 7 displays a linear relationship between the observed Mn and ([L-LA]0 − [L-LA])/ [Ti]0, and the PDIs of poly(L-LA)s initiated by 1 are narrow (1.13−1.27), which suggest that polymerizations proceed in a living fashion. End group analysis from the 1H NMR spectrum (Figure S1, Supporting Information) shows that the PLLA chain is also capped by one isopropyl ester and one hydroxyl

chain end, implying that back-biting reactions leading to the formation of macrocycles do not occur. Complex 1 catalyzes the ROP of L-LA in not only a “living” fashion but also with an “immortal” character (entries 8 and 9). It was further demonstrated that extra IPA can be added in a 9-fold amount to yield a narrow-PDI polymer with an Mn value only ∼1/4 of that with the addition of [L-LA]0/[Ti]0 = 300/1 (Table 3, entry 9 vs 4). To understand the microstructure of PLA catalyzed by BTP-modified Ti complexes, polymerization of rac-LA and the stereoselectivity catalyzed by Ti complex 1 were investigated. It was found that the catalytic activity is little affected when racLA is used as the monomer (Table 3, entry 10), and a virtually atactic polymer (Pm = 0.63) is obtained (Figure S2, Supporting Information). E

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Figure 6. 1H NMR spectra of PCL-25 (Table 2, entry 10) in CDCl3.

Table 3. Ring-Opening Polymerization of Lactide (LA) Catalyzed by Complex 1 entry

solvent

temp (°C)

[L-LA]0/[Ti]0/[IPA]0

time (h)

conversn (%)d

Mn(calcd)e

Mn(obsd)f

Mn(NMR)g

PDIh

a

toluene CH2Cl2 toluene toluene toluene toluene toluene toluene toluene toluene

80 30 90 80 80 80 80 80 80 80

300/1/0 300/1/0 300/1/0 300/1/0 225/1/0 150/1/0 75/1/0 300/1/3 300/1/9 300/1/0

8 48 8 10 10 10 10 10 10 10

92 trace 94 93 91 91 92 92 93 91

13300 i 13600 13500 9900 6600 3400 6700 3400 13200

30800 (18000) i 27400 (15600) 23400 (13600) 16900 (9800) 11000 (6400) 5300 (3100) 11600 (6700) 6300 (3700) 21400 (12400)

13500 i 13400 13400 10800 6600 3200 6800 3300 12600

1.35 i 1.44 1.13 1.26 1.26 1.27 1.24 1.21 1.19

1 2b 3b 4b 5b 6b 7b 8b 9b 10c a

Conditions: L-LA as the monomer, [Ti]0 = 0.01 M. bConditions: L-LA as the monomer, [Ti]0 = 0.005 M. cConditions: rac-LA as the monomer, [Ti]0 = 0.005 M. dObtained from 1H NMR determinations. eCalculated from the molecular weight of lactide (144.13 g/mol) times [LA]0/3[Ti]0 times the conversion yield plus the molecular weight of IPA (60.1 g/mol). fObtained from GPC analysis and calibrated by polystyrene standard. Values in parentheses are the values obtained from GPC times 0.58.15 gObtained from 1H NMR analysis. hObtained from GPC analysis. iNot determined.

order with regard to monomer and initiator concentrations for ROP of ε-CL by mono-BTP-modified titanium alkoxides, complex 1 was used to systematically investigate the kinetic behavior with various concentrations of 1 (5, 7, 8, and 9 mM, respectively) and a constant concentration of [ε-CL] (1.25 M) at 30 °C. As shown in Figure 8, plots of ln([ε-CL]0/[ε-CL]t) versus time at different initiator concentrations are linear during the propagation period (>80 min), suggesting that polymerization proceeds with a first-order dependence on monomer concentration. The rate law of polymerization can therefore be expressed as −d[ε-CL]/dt = kobs[ε-CL]1, where kobs= kp[1]x and kp is the propagation rate constant. To further determine the order in titanium complex 1 and kp, the linear relationship between ln kp versus ln [1] is plotted as demonstrated in Figure 9. From the fitted regression line of ln kobs = 1.211 + 1.147 ln [1], the slope of the regression line is ca. 1.0 (x = 1.147), indicating that the reaction is first order in initiator 1. The y intercept of the regression line (1.211) equals ln kp and allows us to obtain the polymerization rate constant, kp = 3.36 M−1 min−1. On the basis of these kinetic studies, it was concluded that the polymerization of ε-CL is first order in both complex 1 and monomer concentrations, and the overall rate equation is −d[ε-CL]/dt = kp[1]1[ε-CL]1. It is worth noting that the rate law −d[ε-CL]/dt = kp[1]1[ε-CL]1 is consistent with typical

Figure 7. Polymerization of L-LA initiated by titanium complex 1 in toluene at 80 °C for 10 h. The relationship between the Mn (■) or the PDI value (□) of polymer and the molar ratio ([L-LA]0 − [L-LA])/ [Ti]0 is shown.

Kinetic Studies of ε-Caprolactone Polymerization Initiated by 1. In order to thoroughly understand the reaction F

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out under a dry nitrogen atmosphere. Solvents and reagents were dried by refluxing for at least 24 h over sodium/benzophenone (hexane, toluene, tetrahydrofuran (THF)), or over phosphorus pentoxide (CH2Cl2, isopropyl alcohol (IPA)). Deuterated solvents and εcaprolactone (ε-CL) were dried over 4 Å molecular sieves. L-Lactide and rac-lactide were recrystallized from a toluene solution and sublimed twice prior to use. Ti(OiPr)4 (97%, Aldrich) was purified by vacuum distillation prior to use. The ligands 2-(2H-benzotriazol-2-yl)4-(2,4,4-trimethylpentan-2-yl)phenol (C8BTP-H), 2-(2H-benzotriazol2-yl)-4,6-di-tert-butylphenol (t‑BuBTP-H), and 2-tert-butyl-6-(5-chloro2H-benzotriazol-2-yl)-4-methylphenol (TMClBTP-H) were purchased from Aldrich and used without further purification. 1H NMR and 13C NMR spectra were recorded on a Bruker Aveance (300 and 400 MHz) spectrometer with chemical shifts given in parts per million from the peak of internal TMS. Microanalyses were performed using a Heraeus CHN-O-RAPID instrument. Gel permeation chromatography (GPC) measurements were performed on a Jasco PU-2080 plus system equipped with a RI-2031 detector using THF (HPLC grade) as an eluent. The chromatographic column was Phenomenex Phenogel 5 μ 103 Å and the calibration curve used to calculate Mn(GPC) was constructed from 10 polystyrene standards, the molecular weights of which range from 1580 to 288000. The GPC results were calculated using the Scientific Information Service Corporation (SISC) chromatography data solution 3.1 edition. Synthesis of [(μ-C8BTP)Ti(OiPr)3]2 (1). To an ice-cold solution (0 °C) of C8BTP-H (1.29 g, 4.0 mmol) in hexane (30 mL) was slowly added Ti(OiPr)4 (1.18 mL, 4.0 mmol). The mixture was stirred at room temperature for 18 h, during which time the formation of a yellow precipitate was observed. The resulting precipitate was collected by filtration, and the resulting filtrate was then dried in vacuo to give yellow solids. The resulting solid was recrystallized from the saturated hexane solution to yield yellow crystals. Yield: 1.68 g (77%). Anal. Calcd for C58H90N6O8Ti2: N, 7.67; C, 63.61; H, 8.28. Found: N, 7.73; C, 63.62; H, 8.43. 1H NMR (CDCl3, ppm): δ 8.04 (s, 2H, Ar-H), 7.81−7.78 (m, 4H, Ar-H), 7.20−7.16 (m, 6H, Ar-H), 6.95 (d, J = 9.0 Hz, 2H, Ar-H), 4.72 (sept, J = 6.0 Hz, 2H, CH(CH3)2), 4.47 (sept, J = 6.0 Hz, 4H, CH(CH3)2), 1.68 (s, 4H, −C(CH3)2CH2C(CH3)3), 1.34 (s, 12H, −C(CH3)2CH2C(CH3)3), 1.24 (d, J = 6.0 Hz, 24H, CH(CH3)2), 1.05 (d, J = 6.0 Hz, 12H, CH(CH3)2), 0.63 (s, 18H, −C(CH3)2CH2C(CH3)3). 13C NMR (CDCl3, ppm): δ 154.4, 142.7, 140.3, 128.5, 127.8, 127.2, 120.5, 119.0, 118.0 (Ar), 79.6, 76.2 (CH(CH 3 ) 2 ), 56.6 (−C(CH 3 ) 2 CH 2 C(CH 3 ) 3 ), 38.1 (−C(CH3)2CH2C(CH3)3), 32.2 (−C(CH3)2CH2C(CH3)3), 31.8 (−C(CH3)2CH2C(CH3)3), 31.7 (−C(CH3)2CH2C(CH3)3), 26.5, 25.2 (CH(CH3)2). Synthesis of [(C8BTP)2Ti(OiPr)2] (2). To an ice-cold solution (0 °C) of C8BTP-H (1.29 g, 4.0 mmol) in hexane (30 mL) was slowly added Ti(OiPr)4 (0.59 mL, 2.0 mmol). The mixture was stirred at 50 °C for 16 h, during which time the formation of a yellow precipitate was observed. The resulting precipitate was collected by filtration, and the resulting filtrate was then dried in vacuo to give yellow solids. The resulting solid was recrystallized from the saturated hexane solution to yield yellow crystals. Yield: 1.17 g (72%). Anal. Calcd for C46H62N6O4Ti: N, 10.36; C, 68.13; H, 7.71. Found: N, 10.34; C, 68.08; H, 7.49. 1H NMR (CDCl3, ppm): δ 8.07 (s, 2H, Ar-H), 7.82− 7.79 (m, 4H, Ar-H), 7.22−7.16 (m, 6H, Ar-H), 6.97 (d, J = 9.0 Hz, 2H, Ar-H), 4.74 (sept, J = 6.0 Hz, 2H, CH(CH3)2), 1.69 (s, 4H, −C(CH3)2CH2C(CH3)3), 1.36 (s, 12H, −C(CH3)2CH2C(CH3)3), 1.06 (d, J = 6.0 Hz, 12H, CH(CH3)2), 0.64 (s, 18H, −C(CH3)2CH2C(CH3)3). 13C NMR (CDCl3, ppm): δ 154.4, 142.7, 140.3, 128.5, 127.8, 127.2, 120.5, 119.0, 118.0 (Ar), 79.6 (CH(CH 3 ) 2 ), 56.6 (−C(CH3)2CH2C(CH3)3), 38.1 (−C(CH3)2CH2C(CH3)3), 32.3 (−C(CH3)2CH2C(CH3)3), 31.9 (−C(CH3)2CH2C(CH3)3), 31.8 (−C(CH3)2CH2C(CH3)3), 25.3 (CH(CH3)2). Synthesis of [(t‑BuBTP)2Ti(OiPr)2] (3). To an ice-cold solution (0 °C) of t‑BuBTP-H (1.29 g, 4.0 mmol) in hexane (30 mL) was slowly added Ti(OiPr)4 (0.59 mL, 2.0 mmol). The mixture was stirred at 50

Figure 8. Kinetic plots of ln([ε-CL]0/[ε-CL]t) versus time for εcaprolactone polymerizations in d-toluene at 30 °C with different concentrations of complex 1 as an initiator ([ε-CL]0 = 1.25 M; [1] = 5, 7, 8, 9 mM). The inset shows linear fitting for the first-order region during the propagation period.

Figure 9. Linear plot of ln kobs versus ln [1] showing the first-order dependence on catalyst concentration (x = 1.147). All polymerizations were performed in d-toluene at 30 °C with [ε-CL]0 = 1.25 M.

lactone polymerizations that are first order in monomer and first order in catalyst and is in good agreement with the previously reported catalytic systems.13

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CONCLUSION Four titanium alkoxides based on N,O-bidentate benzotriazole phenoxide ligands have been synthesized and fully characterized by spectroscopic studies as well as X-ray single-crystal structure analysis. The molecular structures of these complexes differ in the solid state: monoadduct complex 1 is a dimeric species containing a Ti2O2 core bridging through one of the BTP oxygen atoms, whereas the bis-adduct complexes 2−4 are monomeric Ti(IV) complexes with two −OiPr groups. Polymerizations of ε-CL and L-LA initiated by Ti complex 1 display efficient activities with good molecular weight control and produce polymers with narrow molecular weight distributions (PDI < 1.30). Kinetic studies of ε-CL polymerizations initiated by 1 are first order in both complex 1 and εCL concentrations with the polymerization rate constant kp = 3.36 M−1 min−1. G

dx.doi.org/10.1021/om300962k | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

°C for 18 h, during which time the formation of a yellow precipitate was observed. The resulting precipitate was collected by filtration, and the resulting filtrate was then dried in vacuo to give yellow solids. The resulting solid was recrystallized from the saturated toluene solution to yield yellow crystals. Yield: 1.29 g (80%). Anal. Calcd for C46H62N6O4Ti: N, 10.36; C, 68.13; H, 7.71. Found: N, 10.34; C, 68.08; H, 7.49. 1H NMR (CDCl3, ppm): δ 8.15 (s, 2H, Ar-H), 7.61− 7.58 (m, 4H, Ar-H), 7.52 (s, 2H, Ar-H), 7.03−7.00 (m, 4H, Ar-H), 4.36 (sept, J = 6.0 Hz, 2H, CH(CH3)2), 1.56 (s, 18H, C(CH3)3), 1.44 (s, 18H, C(CH3)3), 0.70 (t, 12H, CH(CH3)2). 13C NMR (CDCl3, ppm): δ 155.6, 142.8, 140.1, 139.2, 129.3, 127.0, 125.1, 117.7, 117.4 (Ar), 79.6 (CH(CH3)2), 35.8, 34.6 (C(CH3)3), 31.7, 30.1 (C(CH3)3), 25.0, 24.8 (CH(CH3)2). Synthesis of [(TMClBTP)2Ti(OiPr)2] (4). To an ice cold solution (0 °C) of TMClBTP-H (1.26 g, 4.0 mmol) in hexane (30 mL) was slowly added Ti(OiPr)4 (0.59 mL, 2.0 mmol). The mixture was stirred at 50 °C for 20 h, during which time the formation of a yellow precipitate was observed. The resulting precipitate was collected by filtration, and the resulting filtrate was then dried in vacuo to give yellow solids. The resulting solid was recrystallized from the saturated toluene solution to yield yellow crystals. Yield: 1.39 g (88%). Anal. Calcd for C40H48Cl2N6O4Ti: N, 10.56; C, 60.38; H, 6.08. Found: N, 10.28; C, 60.09; H, 6.13. 1H NMR (CDCl3, ppm): 1H NMR (CDCl3, ppm): δ 8.01 (s, 2H, Ar-H), 7.68 (s, 2H, Ar-H), 7.55 (d, J = 9.0 Hz, 2H, Ar-H), 7.28 (s, 2H, Ar-H), 7.00 (d, J = 9.0 Hz, 2H, Ar-H), 4.37 (sept, J = 6.0 Hz, 2H, CH(CH3)2), 2.43 (s, 6H, Ar-CH3), 1.53 (s, 18H, C(CH3)3), 0.74 (t, 12H, CH(CH3)2). 13C NMR (CDCl3, ppm): δ 155.1, 143.3, 141.1, 140.1, 133.0, 129.3, 129.2, 128.8, 126.9, 120.8, 118.6, 116.8 (Ar), 79.9 (CH(CH3)2), 35.5 (C(CH3)3), 29.9 (C(CH3)3), 25.2, 24.9 (CH(CH3)2), 21.1 (Ar-CH3). Polymerization of ε-CL Catalyzed by Titanium Benzotriazole Phenoxide Complexes. A typical polymerization procedure is exemplified by the synthesis of PCL-100 using 1 as an initiator at 30 °C. Polymerizations were carried out under a dry nitrogen atmosphere. A mixture of [(μ-C8BTP)Ti(OiPr)3]2 (1; 0.054 g, 0.05 mmol) and ε-CL (3.3 mL, 30 mmol) in toluene (10 mL) was stirred at 30 °C for 6 h. The conversion yield (99%) of PCL-100 was analyzed by 1H NMR spectroscopic studies. After the reaction was quenched by the addition of excess water (0.5 mL), the polymer was precipitated into hexane (100 mL). The final polymer was then redissolved in THF (20 mL) and purified upon precipitation again in MeOH (150 mL), collected, and dried under vacuum. Yield: 2.97 g (90%). Polymerization of L-lactide Catalyzed by Titanium Benzotriazole Phenoxide Complex 1. A typical polymerization procedure is exemplified by the synthesis of PLLA-100 using 1 as an initiator at 80 °C. Polymerizations were carried out under a dry nitrogen atmosphere. A mixture of [(μ-C8BTP)Ti(OiPr)3]2 (1; 0.027 g, 0.025 mmol) and L-LA (2.16 g, 15 mmol) in toluene (10 mL) was stirred at 80 °C for 10 h. The conversion yield (93%) of PLLA-100 was analyzed by 1H NMR spectroscopic studies. After the reaction was quenched by the addition of excess water (0.5 mL), the polymer was precipitated into hexane (50 mL). The final polymer was then redissolved in THF (10 mL) and purified upon precipitation again in MeOH (70 mL), collected, and dried under vacuum. Yield: 1.90 g (88%). X-ray Crystallographic Studies. Suitable crystals of complexes 1−4 were mounted onto glass fibers using perfluoropolyether oil and cooled rapidly under a stream of cold nitrogen gas to collect diffraction data at 100 K using a Bruker APEX2 diffractometer. Intensity data were collected in 1350 frames with increasing w (width of 0.5° per frame). The absorption correction was based on the symmetryequivalent reflections using the SADABS program.16 The space group determination was based on a check of the Laue symmetry and systematic absence and was confirmed by the structure solution. The structures were solved with direct methods using the SHELXTL package.16 All non-H atoms were located from successive Fourier maps, and hydrogen atoms were treated as a riding model on their parent C atoms. Anisotropic thermal parameters were used for all nonH atoms, and fixed isotropic parameters were used for H atoms. Drawings of the molecules were created using Oak Ridge thermal

ellipsoid plots (ORTEP).17 Crystallographic data of complexes 1−4 are summarized in Table S1 (Supporting Information).

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ASSOCIATED CONTENT

* Supporting Information S

Figures giving additional NMR spectra and CIF files giving crystallographic data for 1−4. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC files 886656−886659 also contain the supplementary crystallographic data for complexes 1−4.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 886-3-2653327. Fax: 886-32653399. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Council of Taiwan (NSC101-2113-M-033-008-MY3 and NSC100-2632-M-033-001-MY3).

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REFERENCES

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