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Dalton Transactions Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: D. Chakraborty, S. Pappuru, E. R. Chokkapu and R. V, Dalton Trans., 2013, DOI: 10.1039/C3DT52065J.

Volume 39 | Number 3 | 2010

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Dalton Transactions An international journal of inorganic chemistry www.rsc.org/dalton

Volume 39 | Number 3 | 21 January 2010 | Pages 657–964

Dalton Transactions

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Group (IV) complexes containing the benzotriazole phenoxide ligand as catalysts for the ring-opening polymerization of lactides, epoxides and as precatalysts for the polymerization of ethylene Sreenath Pappuru, Eswara Rao Chokkapu, Debashis Chakraborty* and Venkatachalam Ramkumar 5

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A series of Ti(IV), Zr(IV) and Hf(IV) benzotriazole phenoxide (BTP) complexes were synthesized and characterized by various spectroscopic techniques, elemental analysis and X-ray crystallography. The monosubstituted Zr(IV) BTP complexes [(µ-L)Zr(OiPr)3]2 1−3[ L = C1BTP-H (1), TClBTP-H (2), pentBTPH (3)] and tetrasubstituted Zr(IV), Hf(IV) complexes ZrL4 4−6 [L = C1BTP-H (4), TClBTP-H (5), pentBTPH (6)] and HfL4 7−9 [L = C1BTP-H (7), TClBTP-H (8), pentBTP-H (9)] were prepared by the reaction of Zr(OiPr)4·(iPrOH) and Hf(OtBu)4 in toluene with the respective ligands in different stoichiometric proportions. The reaction between BTP and TiCl4, ZrCl4 and HfCl4 in 2:1 stoichiometric reaction resulted in the formation of disubstituted group IV chloride complexes L2MCl2 10−12 [L = C1BTP-H, M = Ti, Zr and Hf].The molecular structures of complexes 1, 4, 7, 10, 11, and 12 were determined by single-crystal X– ray studies. The X−ray structure of 1 reveals a dimeric Zr(IV) complex containing a Zr2O2 core bridging through the oxygen atoms of the phenoxide groups. Each Zr atom is distorted from an octahedral symmetry. These complexes were found to be active towards the ring-opening polymerization (ROP) of L−Lactide (L−LA) and rac–Lactide (rac–LA). Complex 1 produced highly heterotactic poly(lactic acid) (PLA) from rac–LA under melt conditions with narrow molecular weight distributions (MWDs) and well controlled number average molecular weights (Mn). Additionally epoxide polymerizations using rac– cyclohexene oxide (CHO), rac–propylene oxide (PO), and rac–styrene oxide (SO) were also carried out with these complexes. The yield and molecular weight of the polymer was found to increase with the extension of reaction time. Compounds 1−12 were activated by methylaluminoxane (MAO) and show good activity for ethylene polymerization and produced high molecular weight polyethylene.

Introduction:

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Poly(lactic acid) (PLA) is an important biodegradable material.1,2 The industry and academic community are synthesizing well characterized biodegradable polymers, such as poly (ε– caprolactone) (PCL) and PLA as well as their copolymers by ring opening polymerization (ROP)3-8 that are normally mediated by metal catalysts.9 These biodegradable polymers have numerous medical applications in both surgical and pharmacological processes over the last three decades such as tissue and bone repairing engineering and drug delivery by drug-loaded biodegradable devices.10-12 Among several methods available for ROP, the coordination-insertion polymerization has been extensively used because of its capability of producing high molecular weight (Mn) polymers with narrow molecular weight distributions (MWDs).9 Before the last two decades scientists were using commercially available metal alkoxides as catalysts for ROP.13,14 But to control the molecular weight (Mn) and MWDs, different metal complexes have been explored with This journal is © The Royal Society of Chemistry [year]

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single active site containing ancillary ligands for showing a potent catalytic activity. Significant controlled properties such as controlled Mn and MWDs were achieved.15-17 Such single-site catalysts may be denoted by the general formula LnMR, where M is the active metal center, R is an initiating group, generally an alkoxide, and Ln are ancillary ligands which are not directly involved in the polymerization but minimize the side reactions during the polymerization. Complexes of aluminum,15 zinc,15d,16 magnesium,17,16d iron,18 indium,19 tin (II)20 and the rare earth metals21 as well as group 4 metals22,23 have been investigated as catalysts for ROP. Polyethers are an important class of materials with great commercial value are commonly used to manufacture products such as foams, sealants, surfactants, elastomers, and biomedical components.24 The ROP of rac–cyclohexene oxide (CHO), rac– propylene oxide (PO), and rac–styrene oxide (SO) has been previously reported with zinc,25 aluminum,26 cobalt27 or titanium28 catalysts. There are very few reports with group IV complexes towards the homopolymerization of epoxides.28 We explored the potent catalytic activity of these complexes of group [journal], [year], [vol], 00–00 | 1

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4 metals for the solvent free ROP of epoxides. In addition to PLA and polyethers, group IV chloride complexes of the BTP ligand were synthesized and have been utilized for ethylene polymerization. Ever since the discovery of Ziegler–Natta catalysis, metallocene and non-metallocene compounds, have been known to be good catalysts for ethylene polymerization.29 Non-metallocene group 4 metal complexes containing various types of ligand precursors were found to be active for olefin polymerization30 and most of the nonmetallocene group 4 precatalysts were dihalide (or dialkyl or monohalide monoalkyl) complexes. Many examples of ML2X2 compounds containing monodentate or bidentate anionic ligands have been reported.31 In this contribution, we have reported new dichloro complexes of group 4 metals containing the BTP ligands and their catalytic activity for ethylene homopolymerization. We have recently reported a family of group 4 catalysts towards the synthesis of biodegradable polymers.32 In our continued search for well-defined catalysts for ROP, we have synthesized BTP containing group 4 metal complexes. The synthesis and characterization of complexes with BTP containing Al,33 Mg,34 Pd,35 and Zn34a,36 were reported. The BTP derivatives of Ti(IV) from the ligands and Ti(OiPr)4 was reported recently.37 These complexes were used as catalyst for the polymerization of L–LA. Herein, we report the stereoselective ROP of rac–LA under solvent free conditions using Ti(IV), Zr(IV) and Hf(IV) catalysts supported by BTP ligands (Fig. 1) which offer good catalytic activity for ROP. Recent results have highlighted the potential of BTP ligands in ROP and have motivated us to explore the potential utility of Ti(IV), Zr(IV) and Hf(IV) complexes as catalysts for ROP of LA, epoxides and as precatalysts for the polymerization of ethylene.

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Results and discussion 35

The BTP ligands used for our studies are depicted in Figure 1. 95

X-ray diffraction studies of complexes 1, 4, 7, 10, 11 and 12

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(i)

(ii)

(iii)

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Fig. 1 Ligand precursors used in this study: (i) C1BTP-H; (ii) TCl BTP-H; (iii) PentBTP-H

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resulted in the formation of the monosubstituted dimeric zirconium analogue (2, [(µ-TClBTP-H) Zr(OiPr)3]2 and 3, [(µpent BTP-H) Zr(OiPr)3]2) in good yield. Attempts to synthesize the disubstituted product remained futile as a complicated mixture of several products were seen in the 1H NMR of the crude reaction mixture. Similarly, complexes of Hf analogous to 1–3 could not be prepared as a 1:1 stoichiometry reaction between ligand and Hf(OtBu)4 resulted in a complicated reaction mixture. However a 4:1 stochiometric reaction between the various ligands (Fig. 1) and Zr and Hf alkoxides resulted in the formation of tetrasubstituted Zr, and Hf complexes [Zr(C1BTP)4, 4; Zr(TClBTP)4, 5; Zr(pentBTP)4, 6; and Hf(C1BTP)4,7; Hf(TClBTP)4, 8; Hf(pentBTP)4, 9] respectively in good yield. A 2 : 1 stoichiometry reaction between C1BTP-H ligand (Fig. 1) with group 4 metal chlorides in toluene resulted in the synthesis of disubstituted compounds [Ti(C1BTP)2Cl2, 10; Zr(C1BTP)2Cl2, 11 and Hf(ClBTP)2Cl2, 12]. The formation of these complexes 1-12 were clearly explained by the disappearance in the 1H NMR spectra of the O−H signal of the ligands (∼ 11.7 ppm).37 All the compounds were fully characterized by spectroscopic studies and elemental analysis. In case of complexes 1–3, the 1H NMR spectrum exhibited two chemically inequivalent methine protons present in the OiPr groups bound to the Zr. These appear much downfield shifted at 4.72 ppm and 4.51 ppm for 1, 4.46 ppm and 4.34 ppm for 2, 4.41 ppm and 4.09 ppm for 3. The 13C NMR of complexes 1–3 shows the presence of two different signals corresponding to the methine carbons of the OiPr groups attached to the Zr. It can be explained as the two OiPr groups of the complex have different spatial arrangement as compared to the other four OiPr groups. The NMR spectra of the eight coordinated monomeric complexes 4−9 displayed one set of BTP signals, indicating that these complexes retains their symmetry in solution. Again, the six-coordinate monomeric complexes 10−12 displayed one set of BTP signals. The overall conclusions drawn from 13C NMR studies are in agreement with the conclusions drawn from the 1H NMR spectra of 1–12. The ESI-MS spectra of 1–12 clearly explains that 1–3 are dimeric and 4–12 are monomeric in the solid state. The purity of 1–12 was assured through correct elemental analysis.

The synthetic routes for 1–12 are depicted in Scheme 1. One equivalent of the 2-(2H-benzotriazol-2-yl)-4-methylphenol (C1BTP-H) was treated with one equivalent of Zr(OiPr)4·(iPrOH) in toluene resulting in the formation of the monosubstituted dimeric complex [(µ-C1BTP)Zr(OiPr)3]2 (1) in good yield (83 %). Following the same procedure, treatment of sterically bulky 2,4di-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)phenol (TClBTP-H) and 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol (pentBTPH) with Zr(OiPr)4·(iPrOH) in toluene in 1:1 stoichiometric ratio 2 | Journal Name, [year], [vol], 00–00

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The complexes 1, 4, 7, 10, 11, and 12 were crystallized from saturated toluene solution over a period of 8 days and subjected to X-ray diffraction studies. The molecular structure of complexes 1, 4, 7, 10, 11, and 12 along with selected bond lengths and angles are shown in Figures 2–5 and ESI Figs. S37, S38 respectively. The crystal data is depicted in ESI Table I. The molecular structures of 1–3 explains a dimeric Zr(IV) complex containing a Zr2O2 core bridging through the oxygen atoms of the phenoxide groups. Each Zr atom is bonded to two bridging phenolic oxygen atoms, a N atom of the bidentate BTP ligands, and three O atoms from the OiPr groups. Compounds 4–9 demonstrate the homoleptic and monomeric features with a eightcoordinated zirconium center supported by four N,O-bidentate BTP ligands, forming four six-membered chelating rings. The complexes 10−12 are monomeric in the solid state containing a single metal center supported by two O atoms, two N atoms of This journal is © The Royal Society of Chemistry [year]


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Scheme 1 Synthetic routes for complexes 1−12 50

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the bidentate BTP ligands and two Cl atoms. The structures of complexes, 1–3 are dimeric and each Zr adopts a distorted octahedral environment.37 In complexes 4 and 7, the coordination polyhedron of the central metal is a slightly distorted square antiprism. Similar eight-vertex coordination compounds have been reported to adopt square antiprismatic configurations.38 The metal centre of complexes 10−12 adopts an octahedral environment. In complex 1, the distances between the Zr atom and atoms O(1), O(1A), O(2), O(3) and O(4) are 2.2222(17), 2.2429(18), 1.944(2), 1.9412(19) and 1.926(2) Å, respectively. These distances match well with the literature.37 In the case of complexes 4 and 7, bond lengths of the Zr–O and Hf–O moieties fall within 2.027−2.058 Å and correspond to the literature for four coordinated complexes with phenolate ligands (Zr(IV), This journal is © The Royal Society of Chemistry [year]

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2.027 − 2.047Å, Hf(IV), 2.034 − 2.074 Å).38,39 The mean value of the chelate bond angles O–Zr–Oav and O–Hf–Oav are 95.4° and 140.25° respectively. The Zr–containing bond lengths of Zr−O(1) = 2.042(2) Å, Zr−O(2) = 2.045(2) Å, Zr−O(3) = 2.058(2) Å, and Zr–O(4) = 2.034(2) Å. The Hf–containing bond lengths of Hf–O(1) = 2.048(2) Å, Hf–O(2) = 2.042(2) Å, Hf–O(3) = 2.0272(19) Å, and Hf–O(4) = 2.030(2) Å. These distances and angles match well with the literature.40 The crystal structure of complex 7 is similar to that of 4 with the bond lengths and bond angles nearly matching. In complex 10, the Ti–containing bond lengths of Ti–O(1A) = 1.8382(13) Å, Ti–O(2A) = 2.2849 (13) Å, Ti–O(1B) = 1.8377(13) Å, and Ti–O(2B) = 1.8368 (13) Å, Ti– Cl(1A) = 2.2843(6) Å, Ti–Cl(2A) = 2.2849(6) Å, Ti–Cl(1B) = 2.3014(5) Å, and Ti–Cl(2B) = 2.2739(6) Å. The crystal structures Journal Name, [year], [vol], 00–00 | 3



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Fig. 2 Molecular structure of 1; thermal ellipsoids were drawn at 30 % probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (Ǻ) and angles (◦): Zr(1)–O(1) 2.2 222(17), Zr(1)–O(1)i 2.2429(18), Zr(1)–O(2) 1.944(2), Zr(1)– O(3) 1.9412(17), Zr(1)–O(4) 1.926(2), Zr(1)–N(1) 2.468(2), O(1) –Zr(1)–N(1) 84.02.05(8), O(1)i–Zr(1)–N(1) 81.26(7), O(1)– Zr(1)–O(4) 140.07(9), O(1)–Zr(1)–O(3) 95.75(9).

Fig. 3 Molecular structure of 4; thermal ellipsoids were drawn at 30 % probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (Ǻ) and angles (◦): Zr(1)–O(1) 2.042(2), Zr(1)–O(2) 2.045(2), Zr(1)–O(3) 2.058(2), Zr(1)–O(4) 2.034(2), Zr(1)–N(3) 2.469(3), Zr(1)–N(6) 2.470(3), Zr(1)–N(9) 2.448(3), Zr(1)–N(12) 2.478(3), O(1)–Zr(1)–O(2) 97.21(9), O(1)–Zr(1)–N(3) 72.35(9).

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Fig. 4 Molecular structure of 7; thermal ellipsoids were drawn at 30 % probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (Ǻ) and angles (◦): Hf(1)–O(1) 2.048(2), Hf(1)–O(2) 2.042(2), Hf(1)–O(3) 2.0272(19), Hf(1)– O(4) 2.030(2), Hf(1)–N(11) 2.434(2), Hf(1)–N(14) 2.452(3), Hf (1)–N(17) 2.467(2), Hf(1)–N(20) 2.455(3), O(2)–Hf(1)–O(1) 140.40(8), O(3)–Hf(1)–N(11) 71.89(8). of complexes 11 and 12 are similar to 10, the bond lengths and bond angles being nearly identical.41 4 | Journal Name, [year], [vol], 00–00

Fig. 5 Molecular structure of 10; thermal ellipsoids were drawn at 30 % probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (Ǻ) and angles (◦): Ti(1)–O(1), 1.8382(13), Ti(1)–O(2) 1.8474(13), Ti(1)–N(1) 2.2176(16), Ti(1) –N(4) 2.2425, Ti(1)–Cl(1) 2.2843(6), Ti(1)–Cl(2) 2.2849(6), O(1) –Ti(1)–N(1) 79.30(6), O(2)–Ti(1)–N(4) 85.44(6). (Two molecules were found in one unit cell) Ring-opening polymerization of rac–LA and L–LA

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The performance of these complexes towards the polymerization of rac–LA and L–LA was explored. These complexes have shown good catalytic activity for the solvent free polymerization of both the monomers at 140 °C as well as polymerizations done in the presence of benzyl alcohol (BnOH) at 140 °C. The results This journal is © The Royal Society of Chemistry [year]



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DOI: 10.1039/C3DT52065J

Entry

Cat

[L-LA]o/ [Cat]o/[BnOH]ₒ

timea (min)

Yield (%)

Mn (GPC)b (kg/mol)

Mn (theor)c (kg/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

1 1 1 1 1 2 2 2 3 3 4 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 11 12 12

100/1/0 100/1/5 200/1/0 400/1/0 800/1/0 100/1/0 100/1/5 200/1/0 200/1/0 100/1/5 100/1/0 100/1/5 200/1/0 200/1/0 100/1/5 200/1/0 100/1/5 200/1/0 100/1/5 200/1/0 100/1/5 200/1/0 100/1/5 200/1/0 100/1/5 100/1/0 100/1/5 200/1/0 200/1/0 100/1/5

20 15 30 55 95 15 10 20 35 18 30 25 45 40 20 50 30 65 36 60 33 70 40 45 18 30 15 40 65 45

99 99 98 99 98 99 99 99 99 99 99 98 98 98 98 98 98 99 99 98 97 99 98 99 98 99 99 98 98 97

14.36 3.36 28.78 58.15 115.10 14.70 2.94 27.91 26.85 2.84 14.55 3.40 23.11 22.78 2.79 22.47 2.52 21.30 3.31 20.81 2.49 19.58 2.45 24.60 3.14 13.56 3.22 25.60 20.16 2.35

14.47 2.99 28.89 57.71 115.36 14.47 2.99 28.89 28.89 2.99 14.64 2.99 29.05 29.19 2.99 29.18 2.99 25.11 2.99 29.19 2.99 29.18 2.99 29.05 2.99 14.64 2.99 25.11 25.11 2.99

Mn (NMR)d (kg/mol) 14.14 3.12 27.64 57.61 115.45 14..41 3.01 28.14 26.78 2.91 14.50 2.97 24.79 23.54 2.65 21.63 2.68 20.13 2.90 21.57 2.65 20.52 2.71 25.28 2.98 14.40 2.99 24.84 20.51 2.51

Mw/Mn

1.10 1.12 1.13 1.15 1.12 1.13 1.12 1.16 1.26 1.19 1.19 1.17 1.25 1.28 1.21 1.24 1.25 1.20 1.19 1.28 1.25 1.22 1.24 1.25 1.20 1.18 1.19 1.26 1.22 1.25

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Time of polymerization measured by quenching the polymerization reaction when all monomer was found consumed. Measured by GPC at 27 °C in THF relative to polystyrene standards with Mark-Houwink corrections for Mn. c Mn(theoretical) at 100 % conversion = [M]o/[C]o × mol wt (monomer) + molecular weight of end group (without BnOH); cMn(theoretical) at 100 % conversion (in presence of BnOH) = [M]o/[BnOH]o × mol wt (monomer) + mol wt (BnOH). dObtained from 1H NMR analysis. b

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are depicted in Tables 1 and 2 respectively. High conversions of monomer to polymer were usually obtained within an hour and PLAs with well controlled number average molecular weights (Mn) and relatively narrow MWDs (M w / M n = 1.07 – 1.28) were obtained. Experimental results showed that 1 offer a fantastic combination of high heterotactic stereocontrol for rac–LA polymerization (Pr ≥ 0.80) as described by homonuclear decoupled 1H NMR (see ESI, Figs. S39, 40). From Tables 1 and 2 it was found that the experimental Mn of the produced polymers catalyzed by 1−12 are close to the theoretical molecular weight (Mn(theo)). Due to the presence of isopropoxide groups, 1–3 are taking less time for ROP, whereas in monomeric eight coordinated compounds (4–6 and 7–9), 4–6 exhibits good activity due to the presence of more acidic Zr metal centre in the system. Introduction of a electron withdrawing chloro substituent on the benzotriazole (2, 5 and 8) resulted in higher activities for ROP. This journal is © The Royal Society of Chemistry [year]

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We have performed polymerization of rac˗LA and L˗LA in the presence of benzyl alcohol (BnOH) by using catalysts 1–12 (Table 1 and 2). In all the cases, the polymerization results reflect a close proximity between Mn observed and Mn expected with areasonable control in the MWDs. From Figure 6 it is found that for 1 as a catalyst there exists a linear relationship between the number average molecular weight (Mn) and the monomer to initiator concentration ratio([M]ₒ/[Zr1]ₒ), and the MWDs of PLAs produced remains controlled (1.07−1.15), which suggest that these polymerizations proceed in a controlled fashion. Again a linear relationship of molecular weight (Mn) versus % of conversion could be obtained using complex 1 and 4 (see ESI, Fig. S41) and the resultant PLA have relatively narrow MWDs, suggesting a well-controlled polymerization process. Journal Name, [year], [vol], 00–00 | 5



Table 1 Polymerization data for L–LA catalyzed by complexes 1 – 12

Dalton Transactions


DOI: 10.1039/C3DT52065J


entry

[rac-LA]o/

time a

Yield

Mn(GPC)b

Mn(calcd)c

Mn(NMR)d

[Cat]o/[BnOH]o

(min)

(%)

(kg/mol)

(kg/mol)

(kg/mol)

Cat

Mw/Mn

Pr e

1

1

100/1

25

99

14.21

14.47

14.39

1.07

0.80

1

1

100/1/5

20

99

3.11

2.99

2.97

1.12

0.80

2

0.85

1

200/1

33

98

28.52

28.89

29.10

1.15

3

1

400/1

65

98

58.24

57.71

57.35

1.12

4

1

800/1

110

99

115.62

115.36

115.22

1.20

5

2

100/1

18

99

14.20

14.47

14.56

1.15

0.70

6

2

100/1/5

15

99

3.05

2.99

3.10

1.16

0.75

7

2

200/1

30

98

28.21

28.89

28.95

1.17

0.75

8

3

200/1

40

98

27.95

28.89

28.62

1.24

9

3

100/1/5

23

99

2.96

2.99

2.78

1.19

10

4

100/1

40

99

13.10

14.64

14.60

1.21

11

4

100/1/5

35

99

2.85

2.99

2.89

1.20

12

4

200/1

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98

24.61

29.05

24.93

1.23

13

5

200/1

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98

24.52

29.19

23.35

1.26

14

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100/1/5

30

98

2.65

2.99

2.54

1.25

15

6

200/1

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98

23.12

29.18

22.50

1.36

16

6

100/1/5

38

99

2.53

2.99

2.56

1.21

17

7

200/1

65

99

21.85

25.11

20.85

1.24

18

7

100/1/5

45

99

2.47

2.99

2.35

1.20

19

8

200/1

60

98

21.78

29.19

22.45

1.31

20

8

100/1/5

40

98

2.45

2.99

2.61

1.28

21

9

200/1

75

98

20.99

29.18

20.10

1.35

22

9

100/1/5

40

98

2.41

2.99

2.58

1.30

23

10

100/1

42

98

12.59

14.64

13.11

1.25

24

10

200/1

43

98

25.56

25.11

23.38

1.28

25

10

100/1/5

30

98

2.95

2.99

2.98

1.21

26

11

100/1

40

99

13.85

14.64

13.97

1.19

27

11

100/1/5

25

98

3.10

2.99

3.20

1.23

28

11

200/1

40

98

26.36

25.11

22.50

1.25

29

12

200/1

55

98

21.58

25.11

22.06

1.25

30

12

100/1/5

40

98

2.50

2.99

2.61

1.22

a

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10

Time of polymerization measured by quenching the polymerization reaction when all monomer was found consumed. Measured by GPC at 27 °C in THF relative to polystyrene standards with Mark-Houwink corrections for Mn. cMn(theoretical) at 100 % conversion = [M]o/[C]o × mol wt (monomer) + molecular weight of end group (without BnOH); cMn(theoretical) at 100 % conversion (in presence of BnOH) = [M]o/[BnOH]o × mol wt (monomer) + mol wt (BnOH). dObtained from 1H NMR analysis. eThe probability for heterotactic enchainment calculated from homonuclear decoupled 1H NMR spectrum. b

Polymerization mechanism 15

To understand the polymerization characteristics and final 6 | Journal Name, [year], [vol], 00–00

composition of the polymer produced, it was decided to investigate the polymerization of rac–LA more extensively. Low molecular weight oligomer of rac–LA was synthesized by stirring This journal is © The Royal Society of Chemistry [year]


Table 2 Polymerization data for rac–LA catalyzed by complexes 1 – 12

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Scheme 2 Mechanism of ring-opening polymerization initiated by complexes 1–3 1.6 30

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L-LA rac-LA

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L-LA rac-LA

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Mw/Mn

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Kinetic studies

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0.8 100 200 300 400 500 600 700 800 900

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Fig. 6 Plot of Mn and MWD vs. [M]ₒ/[Zr1]ₒ for L-LA and racLA polymerization at 140 °C using 1.

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rac-LA with 1 or 4 as prototypes for compounds having OiPr groups and without in 20:1 molar ratio under solvent free conditions at 140 °C. The low molecular weight oligomer produced was quenched with cold methanol and the oligomer recovered was subjected to 1H NMR spectroscopy and MALDITOF (Figs. 7, 8 and ESI, Figs. S42, S43). End group analysis of the isolated oligomer, from the MALDI-TOF spectra confirms that this polymerization is initiated by the isopropoxide groups in case of 1−3. In the case of 4−12, the BTP ligand initiates the polymerization. Besides, there were peaks corresponding to intermolecular transesterification as well as dioxane type intermediate products were observed. These can’t be differentiated in the 1H NMR spectrum (see ESI, Fig. S44). In the case of 1−3 the MALDI-TOF spectrum contains all the peaks as acetonitrile adduct resulting from the oligomers but in the case of 4−12 the correspond peaks appearing as proton adduct. Based on MALDI-TOF and 1H NMR spectroscopic evidences, we have proposed that the polymerization reaction proceeds through the coordination-insertion mechanism42 as depicted in Schemes 2 and 3 (moved to ESI as Scheme I) respectively. This mechanism involves first the coordination of the monomer to the electrophilic metal center through the exocyclic carbonyl oxygen. Formation of this adduct activates the carbonyl group of the monomer This journal is © The Royal Society of Chemistry [year]

towards the nucleophilic attack of the active function (X= isopropoxy or BTP). This step is followed by the cleavage of the acyl-oxygen bond, i.e., a spontaneous ring-opening. As a result the polymerization reaction initiates, chain propagation proceeds and eventually leads to the building up of (BTP)(iPr)M– {O···C(O)}X species. When all of the monomer is being consumed, addition of MeOH in the reaction medium deactivates those (BTP)(iPr)M–{O···C(O)}X species to give eventually H– [polymer]–X chains.

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In the next portion of our work, we have performed the kinetic studies for the polymerizaton of rac–LA using 2, 5, 8 and 11 with a ratio of 200 : 1 at 140 °C. The results are described in Fig. 9. The plots suggest that there is a first order with respect to rac–LA up to a conversion of 98 % (0.5 h), as evident from a linear relationship of ln([rac-LA]o/[ rac-LA]t) versus time (min). There is no induction period. A plot of % conversion of rac–LA against time (see ESI, Fig S45) produces a sigmodial curve. From the slope of the plots (Fig. 9), the first order rate constants (kapp) for rac–LA polymerizations initiated by 2, 5, 8 and 11 were found to be 5.1×10-2, 4.5×10-2, 4.1×10-2 and 3.9×10-2 min-1 respectively. From the rate contansts of polymerization, it can be concluded that the rate is fastest for 2 followed by 5 and then 8 and 11. This is justified by the time taken for the polymerization. Density functional theory (DFT) calculations In order to gain some insight on the structure and reactivity of these complexes towards ROP, theoretical calculations were carried out at the DFT level using the hybrid functional B3LYP with the Gaussian 0944 suite of programs with LANL2DZ basis set.45 The calculations agree satisfactorily with the experimental results. MPA (Mulliken Population Analysis) method was carried out on all the complexes with the basis set LANL2DZ.45 Mulliken net charges are –0.873, –0.863, –0.861 and –0.858 respectively for the O-atoms of OiPr in 1, is significantly more electron rich than the oxygen atoms (–0.834) in the OPh ligands of 1 i.e., the isopropoxide is the nucleophilic center (Fig. 10). Hence, the isopropoxide group initiates the ROP via coordination insertion mechanism. The OPh ligand is just a spectator. In the case of 4 and 7 phenolic oxygen is more electron Journal Name, [year], [vol], 00–00 | 7


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DOI: 10.1039/C3DT52065J

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Fig. 7 1H NMR spectrum (500 MHz, CDCl3) of the crude product obtained from a reaction between rac–LA and 1 in 20:1 ratio at 140 o C. O

O H

O

O

O O 40

n 4 5 6 7 8 9 10 11 12

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mass 675.999 819.529 963.355 1107.334 1251.386 1395.464 1539.541 1683.599 1827.631

CH3CN

O

H

O

.

O O

n

m 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

CH3CN

m

mass 675.999 747.715 819.529 891.416 963.355 1035.331 1107.334 1179.354 1251.386 1323.424 1395.464 1467.504 1539.541 1611.572 1683.599 1755.619 1827.631 1899.636

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Fig. 8 MALDI-TOF spectrum of the crude product obtained from a reaction between rac–LA and 1 in 20:1 ratio at 140 °C. 70

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ln([M]o/[M]t)

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For all solid state structures HOMO (Highest occupied molecular orbitals) and LUMO (Lowest un occupied molecular orbitals) energy gaps are calculated at the B3LYP/LANL2DZ level (Fig. 11). HOMO and LUMO energy gap explains the eventual charge transfer interactions taking place within the molecule. The low energy difference between HOMO and LUMO for all the complexes results in high reactivity towards ROP. For all the complexes, MPA method showed the presence of maximum electron density on the oxygen atom of the isopropoxide or phenoxide moietities. Hence, all the complexes are highly reactive. Ring opening homopolymerization of epoxides

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Time (min) Fig. 9 Semilogarithmic plots of rac–LA, conversion in time initiated by 2, 5, 8 and 11: [rac-LA]o/[Cat]o = 200 at 140 °C.

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rich than the nitrogen atoms in the triazole ring due to more resonance stabilization energy of triazole ring in the BTP ligand. The calculated atomic charges (MPA) are –0.775, –0.778, –0.769 and –0.767 for the O-atoms of OPh and − 0.309, −0.301, and −0.274 for the N-atoms in 4 respectively. Compared to 4 complex 7 is less reactive towards ROP. From MPA method it is clear that the oxygen atoms in the OPh ligands of 7 are less electron rich (∼0.59) than the oxygen atoms in the OPh ligands of 4. In the case of 10−12 the O-atoms of OPh ligand has more electron rich (∼0.77) than the chlorine atoms (∼0.275). From the MPA method, 11 (∼0.783) is more reactive than the 12 (∼0.752) followed by 10 (∼0.589) respectively towards the ROP. Mulliken net charges of metal complexes are given in ESI, Fig. S46 and Table II. The mechanism of polymerization of LA is expected to consist of a succession of insertions of carbonyl groups, C=O, into metal–O bond containing the isopropoxide in the case of 1–3 and OPh ligand in the case of 4–12. The polymer obtained has isopropoxide or ligand as end group, indicating carbonyl insertion into a metal–O bond in the first step of the path.

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The reactivity of 1−12 towards the ROP of rac–PO, rac–CHO and rac–SO was explored next. Herein, we report results of our initial screening of 1−12 for the homopolymerization of epoxides (Table 3). Yield and molecular weight of polymer increases with the extension of reaction time. All the polymerization reactions of PO, CHO and SO were performed in a high pressure autoclave under an argon atmosphere at 60 °C, 100 °C and 130 °C respectively and two different monomer to catalyst ratios namely 1000:1 and 10000:1 were considered. For all polymer samples 1H NMR (see ESI, Fig. S47–49) was recorded and conversion was calculated. Polymerization of PO yielded oily polypropylene oxide (PPO) polymer. The 13C NMR spectrum of this polymer (see ESI, Fig. S50) shows numerous resonances in the regions of 76.2 and 65.2ppm assignable to methine and methylene carbons, respectively.43 The 13C NMR spectrum of the oily polymer revealed that the PPO component was atactic. MALDI-TOF analysis of the PCHO polymer catalyzed by using 1 indicates that the polymerization is initiated by the BTP ligand (see ESI, Fig. S51). In the case of 1−3, the polycyclohexene oxide (PCHO) did not contain any proportions of the corresponding product initialized by the isopropoxide fragment. From the experimental results, the reactivity of these complexes towards epoxide homopolymerization is similar like ROP of LA. The complexes 1, 2 and 3 as catalyst with the extension of the reaction time, % conversion of CHO to the respective polymers increases and the MWDs of polycyclohexenoxides (PCHOs) produced remains almost invariant ( see ESI, Fig. S52). Ethylene polymerization by 1–12

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Fig. 10 Mulliken partial charges of complex 1. This journal is © The Royal Society of Chemistry [year]

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The catalytic activity of complexes 1−12 activated by MAO towards the polymerization of ethylene was studied in detail and the results are summarized in Table 4 (for full detail results see ESI, Table III). These complexes exhibit good catalytic activity for ethylene polymerization. The catalytic activity of these complexes is dependent on the [MAO]/[C] molar ratio. The maximal catalytic activities were obtained at [MAO]/[C] = 1000. Similar behaviour has been observed with complexes 2 and 3. An increase in the [MAO]/[C] ratio induces a reduction in the molecular weight of the polymers (see entries 1–9, Table III in ESI). A higher MAO concentration induced lower activities, achieving a plateau at a ratio of ~1400 (see ESI, Fig. S53), due to the acceleration of the chain transfer reactions to alkylaluminum. Journal Name, [year], [vol], 00–00 | 9


2 Y = 0.09722 + 0.05094X 11 Y = 0.1053 + 0.04541X 5 Y = 0.06526 + 0.04112X 8 Y = 0.10584 + 0.03989X

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Fig. 11 Frontier molecular orbital diagrams of complexes 1, 4, 7, 10, 11 and 12. Conclusions

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A family of Ti(IV), Zr(IV) and Hf(IV) complexes, with the BTP ligand were synthesized and evaluated for the ROP of L-LA, racLA, epoxides and as precatalysts for ethylene polymerization. All the complexes have shown good reactivities toward the polymerization of the various monomers. Modifying the benzotriazole phenoxide system with different substituents at ortho and para–position do not affect the catalytic activity in a significant manner. Experimental results showed that, due to the presence of isopropoxides 1–3 have higher catalytic activity towards ROP of L–LA, rac–LA, rac–epoxides and polymerization of ethylene than 4–9 and their activities increases due to the presence of chloro substituent and decrease due to the presence of sterically hindering substituents. The complexes 10– 12 are relatively slower in their reactivity towards the polymerizations in comparison to 1–3. General experimental section

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All reactions involving air and moisture sensitive compounds were manipulated using either standard Schlenk techniques or glove box techniques under a dry argon atmosphere. Toluene was dried by refluxing for at least 24 h over sodium/benzophenone and distilled fresh before use. The CDCl3 for NMR studies was purchased from Aldrich and purified by distilling over calcium hydride and stored in a glove box. All 1H and 13C NMR spectra were recorded on a Bruker Avance 400 MHz or 500 MHz spectrometers with chemical shifts given in parts per million (ppm). ESI-MS spectra of the compounds were performed using Waters Q-Tof micro mass spectrometer. Elemental analyses were performed using Perkin Elmer Series 11 analyzer. MALDI-TOF results were performed on a Bruker Daltonics instrument in dihydroxy benzoic acid matrix. Commercial reagents, namely, Zr(O-iPr)4·iPrOH, Hf(O-tBu)4, TiCl4, ZrCl4, HfCl4 and MAO were purchased from Aldrich and used without further purification. The monomers rac–LA, L–LA, rac–CHO, rac–PO and rac–SO 10 | Journal Name, [year], [vol], 00–00

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were purchased from Aldrich and Acros and sublimed twice prior to use. The ligands, 2-(2H-benzotriazol-2-yl)-4-methyl phenol, 2,4-di-tert-butyl-6-(5-chloro-2H-benzotriazol-2-yl)phenol, and 2(2H-benzotriazol-2-yl)-4,6-di-tert-pentyl phenol were purchased from Aldrich and used in a glove box without further purification. Preparation of complexes 1 12

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Complex 1. To a stirred solution of 2-(2H-benzotriazol-2-yl)-4methylphenol (0.05 g, 0.22 mmol) in 10 mL of dry toluene was added a solution of Zr(OiPr)4·(iPrOH) (0.086 g, 0.22 mmol) in 5 mL of dry toluene at –24 °C. The colour of the solution immediately changed to yellow. The reaction mixture was warmed up to room temperature and stirred for 24 h. The solvent was removed under vacuum. The resulting solid was crystallized from concentrated toluene solution to yield yellow crystals. (Yield 0.19 g, 85 %). Mp: 162 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 7.99 (s, 2H, Ar-H), 7.75−7.73 (m, 4H, Ar-H), 7.17−7.15 (m, 6H, Ar-H), 6.99 (d, J = 8 Hz, 2H, Ar-H), 4.72 (sept, J = 6 Hz, 4H, O–CH−(CH3)2), 4.51 (m, 2H, O– CH−(CH3)2), 2.34 (s, 6H, Ar-CH3), 1.26 (d, J = 6 Hz, 12H, CH(CH3)2), 1.01 (d, J = 6.5 Hz, 24H, CH(CH3)2). 13C NMR (500 MHz, CDCl3, ppm): δ = 155.3 (Ar–O), 142.9 (Ar=N), 140.0 (Ar=N), 131.5 (Ar–CH3), 129.1(Ar–C), 128.3 (Ar–C), 127.9 (Ar– C), 127.5 (Ar–C), 125.4 (Ar–C), 122.1 (Ar–C), 120.9 (Ar–C), 117.9 (Ar–N), 80.0 (O–CH−(CH3)2), 76.3 (O–CH−(CH3)2), 29.8 (O– CH−(CH3)2), 26.6 (O–CH−(CH3)2), 20.6 (Ar-CH3). ESI m/z calculated for [M+H]+. C44H62N6O8Zr2: 982.37 found 983.89. Anal. Calc. for C44H62N6O8Zr2: C, 63.61; H, 8.28; N, 7.67. Found: C, 63.53; H, 8.34; N, 7.53. Complex 2. 2,4-di-tert-butyl-6-(5-chloro-2Hbenzotriazol-2-yl phenol (0.05 g, 0.13 mmol) and Zr(OiPr)4·(iPrOH) (0.052 g, 0.13 mmol) were reacted and the reaction was carried out in an identical manner as described for 1. (Yield 0.13 g, 80 %). Mp: This journal is © The Royal Society of Chemistry [year]



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Time (h)

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Mn(calc)c (kg/mol)

Mn(GPC)d (kg/mol)

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1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28 29. 30. 31. 32. 33. 34. 35. 36. 37. 38 39

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

CHO CHO CHO CHO CHO CHO CHO CHO CHO CHO CHO CHO CHO CHO CHO PO PO PO PO PO PO PO PO PO PO PO PO SO SO SO SO SO SO SO SO SO SO SO SO

1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:10000 1:10000 1:10000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000 1:1000

12 12 12 12 12 12 12 14 15 12 12 14 48 45 45 12 12 12 15 16 15 15 18 18 14 14 16 12 12 12 15 17 18 17 18 18 14 12 18

95 98 96 85 83 81 80 80 80 88 90 81 79 82 80 80 87 83 79 76 75 76 80 73 80 81 80 80 85 81 78 75 75 76 73 70 81 81 71

79.2 81.6 80.0 70.8 69.1 67.5 66.6 57.1 53.3 73.3 75.0 57.8 164.5 182.2 177.7 66.6 72.5 69.1 52.6 47.5 50.0 50.6 44.4 40.5 57.1 57.8 50.0 66.6 70.8 67.5 52.0 44.1 41.6 44.7 40.5 38.8 62.8 67.5 39.4

98.32 98.46 98.45 98.32 98.46 98.45 98.32 98.46 98.45 98.32 98.32 98.32 981.22 981.36 981.35 58.22 58.36 58.35 58.22 58.36 58.35 58.22 58.36 58.35 58.22 58.22 58.22 120.32 120.46 120.45 120.32 120.46 120.45 120.32 120.46 120.45 120.32 120.32 120.32

92.11 99.14 96.30 92.12 88.36 86.52 84.35 81.52 80.60 90.32 94.91 80.35 980.35 984.50 981.40 55.20 56.60 55.81 50.54 50.15 49.01 48.31 48.21 46.67 52.20 53.30 47.25 117.30 121.50 120.26 112.41 110.63 107.25 105.58 103.54 95.12 114.97 115.31 100.21

1.12 1.18 1.16 1.24 1.29 1.28 1.22 1.19 1.27 1.19 1.22 1.24 1.15 1.20 1.12 1.19 1.21 1.15 1.22 1.22 1.29 1.29 1.24 1.20 1.20 1.24 1.19 1.21 1.24 1.18 1.27 1.24 1.27 1.25 1.29 1.29 1.17 1.29 1.21

Polymerization run with 2 mL of epoxide for 12 h unless otherwise indicated. aBased on crude polymer weight and 1H NMR analysis. b TOFs calculated as (mol of epoxide consumed)/(mol of catalyst × reaction time). cMn(theoretical) = [M]o/[C]o × mol wt (monomer) + molecular weight of end group. dMeasured by GPC at 27 °C in THF relative to plyostyrene standards. SO represents styrene oxide. 171 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 8.00 (s, 2H, ArH), 7.82 (s, 2H, Ar-H), 7.71 (d, J = 9.5 Hz, 2H, Ar-H), 7.55 (s, 2H, Ar-H), 7.18 (d, J = 7 Hz, 2H, Ar-H), 4.46 (sept, J = 6 Hz, 4H, O–CH–(CH3)2), 4.34 (sept, J = 6 Hz, 2H, O–CH−(CH3)2), 1.62 (s, 18H, C(CH3)3), 1.40 (s, 18H, C(CH3)3), 0.79 (d, J = 4.5 Hz, 12H, O–CH−(CH3)2), 0.69 (d, J = 4 Hz, 24H, O–CH−(CH3)2). 13 C NMR (500 MHz, CDCl3, ppm): δ = 152.5 (Ar–O), 143.3 (Ar=N), 141.3 (Ar=N), 140.38 (Ar–C(CH3)3), 138.7(Ar– C(CH3)3), 133.3 (Ar–Cl), 129.1 (Ar-C), 128.3 (Ar-C), 126.2 (ArC), 125.4 (Ar-C), 118.4 (Ar–N), 116.7 (Ar–N), 72.5 (O– CH−(CH3)2), 71.8 (O–CH−(CH3)2), 36.0 (C(CH3)3), 34.7 This journal is © The Royal Society of Chemistry [year]

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(C(CH3)3), 31.7 (C(CH3)3), 30.1(C(CH3)3), 26.3 (CH(CH3)2), 26.2 (CH(CH3)2). ESI m/z calculated for [M–iPrOH + H]+. C55H81Cl2N6O7Zr2: 1187.41 found 1188.01 Anal. Calc. for C58H88Cl2N6O8Zr2: C, 55.44; H, 6.85; N, 7.05. Found: C, 55.52; H, 6.73; N, 7.13.

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Complex 3. 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol (0. 05 g, 0.14 mmol) and Zr(OiPr)4·(iPrOH) (0.055 g, 0.14 mmol) were reacted and the reaction was carried out in an identical manner as described for 1. (Yield 0.14 g, 79 %). Mp: 178 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 7.98 (s, 2H, Ar-H), 7.72 (s, Journal Name, [year], [vol], 00–00 | 11



Table 3 Ring-opening polymerization of epoxides catalyzed by complexes 1−12

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DOI: 10.1039/C3DT52065J

Table 4 Data for the polymerization of ethylene catalyzed by complexes 1−12 with MAO

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4H, Ar-H), 7.41 (s, 2H, Ar-H), 7.15 (s, 4H, Ar-H), 4.41 (sept, J = 6 Hz, 2H, O–CH−(CH3)2), 4.08 (sept, J = 6 Hz, 4H, O– CH−(CH3)2), 2.40-2.36 (m, 2H, −C(CH3)2CH2CH3), 2.35-1.89 (m, 2H, −C(CH3)2CH2CH3), 1.88-1.67 (m, 4H, −C(CH3)2CH2CH3), 1.39 (s, 24H, −C(CH3)2CH2CH3), 0.79 -0.73 (m, 36H, O–CH−(CH3)2), 0.67 (t, J = 1.5 Hz, 12H, −C(CH3)2CH2CH3). 13C NMR (500 MHz, CDCl3, ppm): δ = 152.7 (Ar–O), 142.8 (Ar=N), 138.3 (Ar=N), 129.3 (Ar– C(CH3)2CH2CH3), 127.8 (Ar–C), 126.6 (Ar–C), 119.3 (Ar–C), 117.6 (Ar–C), 81.7 (O–CH−(CH3)2), 72.3 (O–CH−(CH3)2), 39.6 (−C(CH3)2CH2CH3), 37.8 (−C(CH3)2CH2 CH3), 37.2 (–C(CH3)2) CH2CH3), 32.9 (−C(CH3)2CH2CH3), 29.0 (−C(CH3)2CH2CH3), 28.6 (−C(CH3)2CH2CH3), 28.5 (−C(CH3)2CH2CH3), 28.2 (– C(CH3)2CH2CH3), 26.3 (O–CH−(CH3)2), 26.2 (– C(CH3)2CH2CH3), 25.5 (O–CH−(CH3)2), 9.9 (−C(CH3)2CH2CH3) , 9.2 (−C(CH3)2CH2CH3). ESI m/z calculated for [M–iPrOH+H]+. C59H91N6O7Zr2: 1175.52 found 1176.02 Anal. Calc. for C62H98N6O8Zr2: C, 60.24; H, 7.78; N, 7.13. Found: C, 60.42; H, 7.73; N, 7.21. Complex 4. To a stirred solution of 2-(2H-benzotriazol-2-yl)-4methylphenol (0.05 g, 0.22 mmol) in 20 mL of dry toluene was added a solution of Zr(OiPr)4·(iPrOH) (0.344 g, 0.88 mmol) in 5 mL of dry toluene at –24 °C. The colour of the solution immediately changed to yellow. The reaction mixture was stirred at 70 °C for 36 h, during which a yellow precipitate formed. The resulting precipitate was collected by filtration over a pad of celite and the resulting filtrate was then dried in vacuum to give a yellow solid. The resulting solid was crystallized from the concentrated toluene solution to yield yellow crystals. (Yield 0.18 g, 81 %). Mp: 188 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 7.77-7.75 (m, 8H, Ar-H), 7.60 (s, 4H, Ar-H), 7.11-6.99 (m, 12H, Ar-H), 6.98 (d, J = 3.5 Hz, 4H Ar-H), 2.25 (s, 12H, Ar-CH3). 13C NMR (500 MHz, CDCl3, ppm): δ 152.3 (Ar–O), 142.9 (Ar=N), 138.0 (Ar=N), 130.8 (Ar–CH3), 129.1 (Ar–C), 128.3 (Ar–C), 127.0 (Ar–C), 125.4 (Ar–C), 121.8 (Ar–C), 118.6 (Ar–N), 117.8 (Ar–C), 21.6 (Ar-CH3). ESI m/z calculated for [M]+. C52H40N12O4Zr: 986.21 found 986.05. Anal. Calc. for 12 | Journal Name, [year], [vol], 00–00

C52H40N12O4Zr: C, 63.20; H, 4.08; N, 17.01. Found: C, 63.12; H, 4.03; N, 17.13. 50

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Complex 5. 2,4-di-tert-butyl-6-(5-chloro-2Hbenzotriazol-2-yl) phenol (0.05 g, 0.13 mmol) and Zr(OiPr)4·(iPrOH) (0.201 g, 0.52 mmol) were reacted and the reaction was carried out in an identical manner as described for 4. (Yield 0.17 g, 84 %). Mp: 179 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 8.26 (s, 4H, ArH), 8.05 (d, J = 3 Hz, 4H, Ar-H), 7.74 (s, 4H, Ar-H), 7.57 (s, 4H, Ar-H), 7.18 (d, J = 3.5 Hz, 4H, Ar-H), 1.64 (s, 36H, C(CH3)3), 1.51 (s, 36H, C(CH3)3). 13C NMR (500 MHz, CDCl3, ppm): δ = 146.8 (Ar–O), 142.0 (Ar=N), 141.3 (Ar=N), 140.3 (Ar– C(CH3)3), 138.9 (Ar– C(CH3)3), 133.4 (Ar–Cl), 129.1 (Ar–C), 126.2 (Ar–C), 125.6 (Ar–C), 118.9 (Ar–C), 116.7 (Ar–C), 116.3 (Ar–N), 35.8 (C(CH3)3), 34.7 (C(CH3)3), 31.6 (C(CH3)3), 30.1 (C(CH3)3). ESI m/z calculated for [M–C21H27ClN3O]+. C60H69Cl3N9O3Zr: 1158.41 found 1158.87. Anal. Calc. for C80H92Cl4N12O4Zr: C, 62.03; H, 5.99; N, 10.85. Found: C, 62.12; H, 5.90; N, 10.41.

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Complex 6. 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol (0. 05 g, 0.14 mmol) and Zr(OiPr)4·(iPrOH) (0.217 g, 0.56 mmol) were reacted and the reaction was carried out in an identical manner as described for 4. (Yield 0.16 g, 76 %). Mp: 192 oC. 1H NMR (500 MHz, CDCl3, ppm): δ 8.23 = (s, 4H, Ar-H), 7.99-7.91 (m, 8H, Ar-H), 7.70 (s, 4H, Ar-H), 7.49-7.44 (m, 8H, Ar-H), 2.41-2.32 (m, 8H, –C(CH3)2CH2CH3), 2.03-1.92 (m, 8H, – C(CH3)2CH2CH3), 1.75-1.67 (m, 16H, –C(CH3)2CH2CH3), 1.46 (s, 48H,−C(CH3)2 CH2CH3), 0.77 (t, 24H, –C(CH3)2CH2CH3). 13C NMR (500 MHz, CDCl3, ppm): δ =152.7 (Ar–O), 142.8 (Ar=N), 138.0 (Ar=N), 129.3 (Ar– C(CH3)2CH2CH3), 128.3 (Ar– C(CH3)2CH2CH3), 127.8 (Ar–N), 127.5 (Ar–C), 119.3 (Ar–C), 117.7 (Ar–C), 39.6 (–C(CH3)2CH2CH3), 37.2 (–C(CH3)2CH2CH3) , 37.1 (–C(CH3)2CH2CH3), 32.9 (–C(CH3)2CH2 CH3), 29.0 (– C(CH3)2CH2CH3), 28.7 (–C(CH3)2CH2CH3), 28.5 (–C(CH3)2 CH2 CH3), 28.2 (–C(CH3)2CH2CH3), 9.9 (–C(CH3)2CH2CH3), 9.7 (– C(CH3)2CH2CH3). ESI m/z calculated for [M–C22H28N3O]+. C66H84N9O3Zr1: 1140.61 found 1140.33. Anal. Calc. for C88H112N12O4Zr1: C, 69.37; H, 7.41; N, 11.03. Found: C, 69.54; This journal is © The Royal Society of Chemistry [year]



a

entry Cat. Yield Activity Mn (kg/mol) Mw/Mn 1 1 1.8 7.0 140.1 2.8 2 2 1.5 7.5 118.0 2.5 3 3 1.2 6.0 111.5 2.5 4 4 0.8 3.2 87.4 2.6 5 5 5.5 3.3 84.8 2.4 6 6 5.3 3.1 76.8 3.1 7 7 0.7 3.0 90.4 2.9 8 8 4.8 3.0 82.6 2.7 9 9 4.5 2.8 71.3 2.9 10 10 2.4 5.4 102.1 3.7 11 11 2.3 5.6 107.0 3.7 12 12 1.5 4.1 80.6 4.8 All experiments were performed in toluene at MAO:catalyst ratio = 1000, unless otherwise indicated. Ethylene pressure = 8 atm, 80 °C for 30 min, catalyst = 50 mg, solvent = 45 mL. a g of PE obtained after 30 min. bA = Activity in (g PE/mol cat × h) ×104. c Determined by GPC in 1,2,4-C6Cl3H3 vs narrow polystyrene standards.

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Complex 7. To a stirred solution of 2-(2H-benzotriazol-2-yl)-4methylphenol (0.05 g, 0.22 mmol) in 20 mL of dry toluene was added a solution of Hf(OtBu)4 (0.414 g, 0.88 mmol) in 5 mL of dry toluene at –24 °C. The colour of the solution immediately changed to yellow. The reaction mixture was stirred at 70 °C for 24 h, during which a yellow precipitate formed. The resulting precipitate was collected by filtration over a pad of celite and the resulting filtrate was then dried in vacuum to give a yellow solid. The resulting solid was crystallized from the concentrated toluene solution to yield yellow crystals. (Yield 0.18 g, 75 %). Mp: 184 o C. 1H NMR (500 MHz, CDCl3, ppm): δ = 7.82-7.80 (m, 8H, ArH), 7.68 (s, 4H, Ar-H), 7.21-7.19 (m, 12H, Ar-H), 7.17 (d, J = 5 Hz, 4H, Ar-H), 2.36 (s, 12H, Ar-CH3). 13C NMR (500 MHz, CDCl3, ppm): δ 152.5 (Ar–O), 142.8 (Ar=N), 138.0 (Ar=N), 131.0 (Ar–CH3), 129.1 (Ar–C), 128.3 (Ar–C), 127.0 (Ar–C), 125.4 (Ar–C), 122.4(Ar–C), 121.9(Ar–C), 118.5 (Ar–N), 117.8 (Ar–C), 21.6 (Ar-CH3). ESI m/z calculated for [M–C13H10N3O]+. C39H30HfN9O3: 852.21 found 852.14. Anal. Calc. for C52H40HfN12O4: C, 55.03; H, 3.55; N, 14.81. Found: C, 55.21; H, 3.59; N, 14.87. Complex 8. 2,4-di-tert-butyl-6-(5-chloro-2Hbenzotriazol-2-yl) phenol (0.05 g, 0.13 mmol) and Hf(OtBu)4 (0.244 g, 0.52 mmol) were reacted and the reaction was carried out in an identical manner as described for 7. (Yield 0.17 g, 79 %). Mp: 176 oC. 1H NMR (500 MHz, CDCl3, ppm): δ 8.26 (s, 4H, Ar-H), 8.05 (d, J = 2.5 Hz, 4H, Ar-H), 7.74 (s, 4H, Ar-H), 7.57 (s, 4H, Ar-H), 7.18 (d, J = 3.5 Hz, 4H, Ar-H), 1.64 (s, 36H, C(CH3)3), 1.51 (s, 36H, C(CH3)3). 13C NMR (500 MHz, CDCl3, ppm): δ = 146.8 (Ar–O), 143.1 (Ar=N), 142.0 (Ar=N), 140.4 (Ar– C(CH3)3), 138.9 (Ar– C(CH3)3), 129.2 (Ar–Cl), 129.1 (Ar–C), 128.3 (Ar–C), 126.4 (Ar–C), 125.6 (Ar–C), 118.9 (Ar–C), 116.7 (Ar–N), 36.1(– C(CH3)3), 35.8 (–C(CH3)3), 30.4(–C(CH3)3), 29.7 (C(CH3)3). ESI m/z calculated for [M-C21H27ClN3O]+. C60H69Cl3N9O3Hf: 1248.41 found 1248.54. Anal. Calc. for C80H92Cl4N12O4Hf: C, 57.69; H, 5.57; N, 10.09. Found: C, 57.65; H, 5.49; N, 10.17. Complex 9. 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol (0. 05 g, 0.14 mmol) and Hf(OtBu)4 (0.263 g, 0.56 mmol) were reacted and the reaction was carried out in an identical manner as described for 7. (Yield 0.16 g, 71 %). Mp: 219 oC.1H NMR (500 MHz, CDCl3, ppm): δ = 8.24 (s, 4H, Ar-H), 7.77-7.70 (m, 8H, Ar-H), 7.29 (s, 4H, Ar-H), 7.18-713 (m, 8H, Ar-H), 2.60-2.51 (m, 4H, −C(CH3)2CH2CH3), 2.04-1.98 (m, 8H, −C(CH3)2CH2CH3), 1.83-1.75 (m, 4H,–C(CH3)2CH2CH3), 1.37 (s, 48H,−C(CH3)2 CH2CH3), 0.75 (t, J = 5 Hz, 24H, –C(CH3)2CH2CH3). 13C NMR (500 MHz, CDCl3, ppm): δ = 152.8 (Ar–O), 142.8 (Ar=N), 139.9 (Ar=N), 129.7 (Ar–C(CH3CH2CH3), 129.1 (Ar–C(CH3)2CH2CH3) , 127.5 (Ar–C), 127.3 (Ar–C), 119.4 (Ar–C), 117.7 (Ar–C), 39.7 (−C(CH3)2CH2CH3), 37.8 (−C(CH3)2CH2CH3), 37.1 (– C(CH3)2CH2CH3), 32.3 (−C(CH3)2CH2CH3), 29.1 (−C(CH3)2CH2 CH3), 28.7 (−C(CH3)2CH2CH3), 28.3 (−C(CH3)2CH2CH3), 27.9 (–C(CH3)2CH2CH3), 9.9 (−C(CH3)2CH2CH3), 9.7 (–C(CH3)2CH2 CH3). ESI m/z calculated for [M–C22H28N3O]+. C66H84HfN9O3: 1230.61 found 1230.70. Anal. Calc. for C88H112HfN12O4: C, 64.45; H, 6.88; N, 10.25. Found: C, 64.41; H, 6.79; N, 10.34. This journal is © The Royal Society of Chemistry [year]

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Complex 10. To a suspension of NaH (0.005 g, 0.22 mmol) in 10 mL of THF was added a solution of 2-(2H-benzotriazol-2-yl)-4methylphenol (0.05 g, 0.22 mmol) in 15 mL of THF at −24 °C. The reaction mixture was warmed to room temperature and stirred for 5 h . The solvent was removed under vacuum and 20 ml of toluene was added to the solid .The solution was cooled to 24 oC and added dropwise to a solution of TiCl4 (0.1ml, 0.11 mmol) in 15 mL of toluene. The reaction mixture was allowed to warm to room temperature and stirred for 12 h. The resulting solid was filtered off and washed with toluene to remove NaCl. The filtrate was then dried in vacuum to give a red solid which was crystallized from concentrated toluene solution to yield red crystals. (Yield 0.1 g, 77 %). Mp: 196 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 8.42 (s, 2H, Ar-H), 8.15 (d, J = 6.5 Hz, 4H, ArH), 7.65 (d, J = 6.5 Hz, 4H, Ar-H), 7.31 (d, J = 8.5 Hz, 2H, ArH), 7.07 (d, J = 8 Hz, 2H, Ar-H), 2.35 (s, 6H, Ar-CH3). 13C NMR (500 MHz, CDCl3, ppm): δ = 153.3 (Ar–O), 142.1 (Ar=N), 138.0 (Ar=N), 131.5 (Ar-CH3), 129.2 (Ar–C), 128.3 (Ar–C), 125.4 (Ar– C), 121.8 (Ar–C), 119.0 (Ar–C), 116.8 (Ar–N), 21.6 (Ar-CH3). ESI m/z calculated for M+. C26H20Cl2N6O2Ti: 566.11 found 566.40. Anal. Calc. for C26H20Cl2N6O2Ti: C, 55.05; H, 3.55; N, 14.82. Found: C, 55.13; H, 3.61; N, 14.75. Complex 11. NaH (0.005 g, 0.22 mmol), 2-(2H-benzotriazol-2yl)-4-methylphenol (0.05 g, 0.22 mmol) and ZrCl4 (0.026 g, 0.11 mmol) were reacted and the reaction was carried out in an identical manner as described for 10. (Yield 0.11 g, 84 %). Mp: 186 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 8.54 (s, 2H, ArH), 7.94 (m, 4H, Ar-H), 7.59 (m, 4H, Ar-H), 7.06-6.99 (m, 4H, Ar-H), 2.23 (s, 6H, Ar-CH3). 13C NMR (500 MHz, CDCl3, ppm): δ = 152.7 (Ar–O), 148.8 (Ar=N), 138.0 (Ar=N), 131.9(Ar–CH3), 129.1 (Ar–C), 128.0 (Ar–C), 125.4 (Ar–C), 121.2 (Ar–C), 119.3 (Ar–C), 118.1 (Ar–N), 21.6 (Ar–CH3). ESI m/z calculated for M+. C26H20Cl2N6O2Zr: 608.01 found 608.49. Anal. Calc. for C26H20Cl2N6O2Zr : C, 51.14; H, 3.30; N, 13.76. Found: C, 51.21; H, 3.37; N, 13.65. Complex 12. NaH (0.005 g, 0.22 mmol), 2-(2H-benzotriazol-2yl)-4-methylphenol (0.05 g, 0.22 mmol) and HfCl4 (0.035 g, 0.11 mmol) were reacted and the reaction was carried out in an identical manner as described for 10. (Yield 0.12 g, 79 %). Mp: 217 oC. 1H NMR (500 MHz, CDCl3, ppm): δ = 8.54 (s, 2H, ArH), 7.89-7.83 (m, 4H, Ar-H), 7.47-7.39 (m, 4H, Ar-H), 7.18-7.15 (m, 6H, Ar-H), 7.04-7.01 (m, 2H, Ar-H), 2.35 (s, 6H, Ar-CH3). 13 C NMR (500 MHz, CDCl3, ppm): δ 154.7 (Ar–O), 148.9 (Ar=N), 135.4 (Ar=N), 132.0 (Ar–CH3), 129.2 (Ar–C), 128.3 (Ar–C), 125.4 (Ar–C), 121.6 (Ar–C), 119.4 (Ar–N), 118.2 (ArC), 21.6 (Ar-CH3). ESI m/z calculated for [M–Cl]+. C26H20ClN6O2Hf: 663.12 found 663.45. Anal. Calc. for C26H20Cl2N6O2Hf : C, 47.14; H, 3.04; N, 12.69. Found: C, 47.21; H, 3.12; N, 12.75. General procedure for the polymerization of rac-LA and LLA

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H, 7.56; N, 11.18.

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General procedure for the hompolymerization of epoxides 20

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A 100 mL high pressure autoclave equipped with a mechanical stirrer was dried under vacuum at 100 °C for 4 h and then transferred to a drybox to cool to ambient temperature. Here, 30 mg of the desired catalyst was added into a metal vessel in the autoclave. 1.8–3.7 g epoxide monomers (1000 mol or 10000 mol) were added under nitrogen via an injection port. The autoclave was then heated to the required temperature for the desired time. The autoclave was cooled to yield a large polymer mass, which was dissolved in minimum dichloromethane and added to acidic methanol under stirring. The polymer was collected by filtration and then dried in vacuo to a constant mass. Molecular weight determination and MWDs were determined by using GPC measurements at 27 °C with Waters 510 pump and Waters 410 differential refractometer as the detector. Three columns namely WATERS STRYGEL-HR5, STRYGEL-HR4 and STRYGELHR3 each of dimensions (7.8 × 300 mm) were connected in series. Measurements were done in THF at 27 °C. Number average molecular weights (Mn) and MWDs (Mw/Mn) of polymers were measured relative to polystyrene standards.

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All the Density functional theory calculations were performed at B3LYP/LANL2DZ level of theory as implemented in the GAUSSIAN 09 (Rev C.01) package44 of quantum chemical programs. Molecular coordinates for the DFT calculations were extracted from single crystal XRD data. MPA (Mulliken Population Analysis) were performed using MPA 3.1 program as implemented in the Gaussian 09W package at the B3LYP/LANL2DZ level45 in order to understand various secondorder interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem.

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Suitable single crystals of complexes 1, 4, 7, 10, 11, and 12 were obtained by crystallization from saturated solution of toluene at room temperature for X-ray structural determinations. Single crystals were mounted on Bruker AXS (Kappa Apex 2) CCD diffractometer equipped with graphite monochromated Mo (Kα) (λ = 0.7107 Å) radiation source. A full sphere of data was collected with 100 % completeness for θ up to 25°. ω and φ scans was employed to collect the data. The frame width for ω was set to 0.5 for data collection. The frames were integrated and data were reduced for Lorentz and polarization corrections using SAINT-NT. The multi-scan absorption correction was applied to the data set. All structures were solved using SIR-92 and refined using SHELXL-97. The non-hydrogen atoms were refined with anisotropic displacement parameter. All the hydrogen atoms could be located in the difference Fourier map. The hydrogen atoms bonded to carbon atoms were fixed at chemically meaningful positions and were allowed to ride with the parent atom during refinement. Computational details

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A 100 mL flask was equipped with an ethylene inlet, a magnetic stirrer, and a vacuum line. The flask was charged with 45 mL of freshly distilled toluene, along with the required amount of MAO and the flask was placed in a bath at the required polymerization temperature for the desired time. The polymerization reaction was started by adding a toluene solution of 50 mg in 5 mL of 1– 12 with a syringe. The polymerization was quenched with the acidic methanol after 30 min. The polymer was collected by filtration and dried till constant weight was achieved. Molecular weights (Mn and Mw) and the MWDs (Mw/Mn) of polyethylene samples were determined by GPC instrument with Waters 510 pump and Waters 2414 differential refractometer as the detector. The used column namely WATERS STRYGEL-HR4 of dimensions (4.6 × 300 mm) was connected during the experiment. Measurements were done in trichloro benzene (TCB). Number average molecular weights (Mn) and molecular

weight distributions (MWDs) of poly olefins were measured relative to polystyrene standards.

This work was supported by the Department of Science and Technology, New Delhi, India. S. P. thanks the University Grants Commission, New Delhi for research fellowship.

Notes and references a

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Department of Chemistry, Indian Institute of Technology Madras, Chennai-600 036, Tamil Nadu, India. E-mail: [email protected]; Fax: +914422574202; Tel:+914422574223 † Electronic Supplementary Information (ESI) available: Crystallographic data for the structural analysis of complexes 1, 4, 7, 10, 11 and 12 have been deposited at the Cambridge Crystallographic Data Center (CCDC). CCDC reference numbers are 924655–924660. See DOI: 10.1039/b000000x/ 1 C. K. Williams and M. A. Hillmyer, Polym. Rev., 2008, 48, 1–10. 2 R. E. Drumright, P. R. Gruber and D. E. Henton, Adv. Mater., 2000, 12, 1841–1846. 3 R. H. Platel, L. M. Hodgson and C. K. Williams, Polym. Rev., 2008, 48, 11–63. 4 C. Jérôme and P. Lecomte, Adv. Drug Delivery Rev., 2008, 60, 1056–1076.



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glass vessel to 140 °C for a period of time by which the melt had become viscous. Here, 173.4 µmol of desired catalyst and 5 g LLA or rac-LA were introduced into a dry reaction vessel equipped with a magnetic bar. The reaction mixture was dissolved in dichloromethane and the polymer was precipitated in cold methanol filtered and dried to a constant weight. The conversion yield of L-LA and rac-LA were analyzed by 1H NMR spectroscopic studies. Data concerning molecular weights (Mn) and the MWDs (Mw/Mn) of the polymer samples obtained by the ROP of LA were determined by using a GPC instrument with Waters 510 pump and Waters 410 differential refractometer as the detector. Three columns namely WATERS STRYGEL-HR5, STRYGEL-HR4 and STRYGEL-HR3 each of dimensions (7.8 × 300 mm) were connected in series. Measurements were done in THF at 27 °C. Number average molecular weights (Mn) and MWDs (Mw/Mn) of polymers were measured relative to polystyrene standards.

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