Titanium complexes bearing amine bis(phenolate) ligands: Synthesis, structure and catalysis in ring-opening polymerization of lactide

Article abstract of DOI:10.1002/aoc.3688

Monomeric bis(isopropoxy) titanium complexes LTi(OⁱPr)₂ (L?=?─OC₆H₂–4-R¹–6-R²–2-CH₂N[(CH₂)₂N(R³)₂]CH₂–4-R⁴–6-R⁵-C₆H₂O─, R¹?=?R²?=?^tBu, R³?=?Et, R⁴?=?R⁵?=?Cl, (L¹)Ti(OⁱPr)₂; R¹?=?R²?=?Me, R³?=?Et, R⁴?=?R⁵?=?Me, (L²)Ti(OⁱPr)₂; R¹?=?R²?=?^tBu, R³?=?Et, R⁴?=?OMe, R⁵?=?^tBu, (L³)Ti(OⁱPr)₂; R¹?=?R⁴?=?OMe, R³?=?Et, R²?=?R⁵?=?^tBu, (L⁴)Ti(OⁱPr)₂; R¹?=?R²?=?^tBu, R³?=?Me, R⁴?=?OMe, R⁵?=?^tBu, (L⁵)Ti(OⁱPr)₂) supported by amine bis(phenolate) ligands were synthesized and characterized using NMR spectroscopy and elemental analysis. The solid-state structure of (L³)Ti(OⁱPr)₂ was determined using single-crystal X-ray diffraction. (L^{1 –5})Ti(OⁱPr)₂ were all found to initiate the ring-opening polymerization of l-lactide and rac-lactide in a controlled manner at 110–160°C. As shown by kinetic studies, (L¹)Ti(OⁱPr)₂ polymerized l-lactide faster than did (L^{2 –5})Ti(OⁱPr)₂. In addition, good number-average molecular weight and narrow polydispersity index (1.00–1.71) of polymers were also obtained. The microstructure of the polymers and a possible mechanism of coordination–insertion of polymerization were evidenced by MALDI-TOF and ¹H NMR spectra of the polylactides.

Full text of DOI:10.1002/aoc.3688

Received: 6 July 2016
DOI 10.1002/aoc.3688
Revised: 20 October 2016
Accepted: 23 October 2016
F U L L PA P E R
Titanium complexes bearing amine bis(phenolate) ligands:
Synthesis, structure and catalysis in ring‐opening polymerization
of lactide
Baolong Wang | Jianing Zhang | Huanqin Zhao | He Huang | Junpei Zheng | Liying Wang |
Junmin Sun | Yongfeng Zhang | Zhenzhu Cao
School of Chemical Engineering, Inner Mongolia
Monomeric bis(isopropoxy) titanium complexes LTi(OiPr)2 (L = ─OC6H2–4‐R1–6‐
University of Technology, Institute of Coal
t
R2–2‐CH2N[(CH2)2N(R3)2]CH2–4‐R4–6‐R5‐C6H2O─, R1 = R2 = Bu, R3 = Et,
Conversion and Cyclic Economy, Hohhot, 010051,
R4 = R5 = Cl, (L1)Ti(OiPr)2; R1 = R2 = Me, R3 = Et, R4 = R5 = Me, (L2)Ti(OiPr)2;
People's Republic of China
t
t
R1 = R2 = Bu, R3 = Et, R4 = OMe, R5 = Bu, (L3)Ti(OiPr)2; R1 = R4 = OMe,
Correspondence
Liying Wang, School of Chemical Engineering,
Inner Mongolia University of Technology, Institute
of Coal Conversion and Cyclic Economy, Hohhot,
010051, People's Republic of China.
Email: wangliying7704@163.com
R3 = Et, R2 = R5 = Bu, (L4)Ti(OiPr)2; R1 = R2 = Bu, R3 = Me, R4 = OMe,
t
t
R5 = Bu, (L5)Ti(OiPr)2) supported by amine bis(phenolate) ligands were synthe-
t
sized and characterized using NMR spectroscopy and elemental analysis. The
solid‐state structure of (L3)Ti(OiPr)2 was determined using single‐crystal X‐ray dif-
fraction. (L1–5)Ti(OiPr)2 were all found to initiate the ring‐opening polymerization
of L‐lactide and rac‐lactide in a controlled manner at 110–160°C. As shown by
kinetic studies, (L1)Ti(OiPr)2 polymerized L‐lactide faster than did (L2–5)Ti(OiPr)2.
In addition, good number‐average molecular weight and narrow polydispersity index
(1.00–1.71) of polymers were also obtained. The microstructure of the polymers and
a possible mechanism of coordination–insertion of polymerization were evidenced
by MALDI‐TOF and 1H NMR spectra of the polylactides.
Funding information
National Science Foundation for Young Scientists
of China, Grant/Award Number: 51562029 and
21302083; Teaching Reform Project of Inner
Mongolia University of Technology, Grant/Award
Number: 2014217 and 2014212; Major Program
of Natural Science Foundation of Inner Mongolia
University of Technology, China, Grant/Award
Number: X201503
KEYWORDS
amine bis(phenolate) ligands, ring‐opening polymerization of lactide, titanium
complexes
1
|
INTRODUCTION
polymerizations of lactide have been carried out using com-
plexes of rare earth metals,[7,8] aluminium,[9,10] calcium,[11,12]
zinc[13,14] and magnesium.[15,16] With the rapid development
of biodegradable biocompatible PLA from natural renewable
resources,[17,18] titanium complexes have recently gained
much attention for ROP of lactides because they are non‐toxic,
inexpensive, easily prepared and less air‐sensitive.[19–24]
Symmetric amine bis(phenolate) titanium complexes
were been synthesized and observed to initiate the ROP of
ε‐caprolactone at an initial monomer‐to‐catalyst ratio of
100, but ROP of lactides was not discussed.[25] Jones and
co‐workers reported titanium complexes bearing amine
triphenolate ligands, which initiated the ROP of rac‐lactide
at an initial monomer‐to‐catalyst ratio of 300, and conver-
sions of monomer to PLA of up to 95% could be reached
within 2 h in the melt, yielding atactic PLA with PDIs in
Because of its environmentally friendly biodegradable char-
acteristics, poly(lactic acid) (PLA), being an alternative to
synthetic petrochemical‐based polymers, has been attracting
widespread attention in research and industry over the past
two decades. PLA can be synthesized from inexpensive and
renewable resources such as corn, sugar beet and starch,
and forms lactic acid by hydrolytic degradation, which can
be metabolized in the environment.[1–3]
Controlled ring‐opening polymerization (ROP) using a
metal complex as catalyst is a convenient method for the syn-
thesis of poly(cyclic ester)s such as poly(propylene carbon-
ate)[4–6] and PLA, owing to the advantages of achieving
well‐controlled number‐average molecular weights (Mn) and
narrow polydispersity index (PDI).[2] Stereocontrolled
Appl Organometal Chem 2016; e3688;
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1 of 9
DOI: 10.1002/aoc.3688

2 of 9
WANG ET AL.
the range 1.09–2.07 under melt conditions.[26,27] To the best
of our knowledge, non‐symmetric amine bis(phenolate) tita-
nium complexes for ROP of lactide have not been reported
to date. With the aim of designing and structurally defining
initiators of non‐toxic metals which are capable of exerting
control over the polymerization process, based on previous
studies,[28–32] we synthesized a series of monomeric titanium
complexes bearing dianionic amine bis(phenolate) ligands
and investigated their behavior towards ROP of lactide under
melt conditions. Furthermore, a high conversion of monomer
to PLA of up to 89% could be reached when a large molar
ratio of [rac‐LA]0/[Ti]0 = 2570:1 was adopted in the melt.
by rotary evaporation gave a viscous oil, to which was added
a solution of paraformaldehyde (0.5700 g, 0.0190 mol) and
2,4‐dichlorophenol (2.7752 g, 0.0170 mol) in methanol
(20 ml) at 80°C for 12 h with magnetic stirring. The mixture
was cooled and kept at −18°C to afford colorless crystals in
1
40% yield. H NMR (CDCl3, 500 MHz, δ, ppm): 7.23 (d,
1H, J = 2 Hz, ArH), 7.20 (d, 1H, J = 2 Hz, ArH), 6.91 (d,
1H, J = 2 Hz, ArH), 6.84 (d, 1H, J = 1.5 Hz, ArH), 3.59
(s, 2H, Ar─CH2─N), 3.57 (s, 2H, Ar─CH2─N), 2.66 (m,
8H, N─CH2─CH2─N, N─(CH2CH3)2), 1.38 (s, 9H,
Ar─C(CH3)3), 1.27 (s, 9H, Ar─C(CH3)3), 1.13 (t, 6H,
J = 7 Hz, N─(CH2CH3)2). 13C NMR (CDCl3, 125 MHz, δ,
ppm): 153.13, 152.27, 144.64, 136.02, 129.18, 128.62,
125.09, 124.42, 123.46, 122.84, 122.62, 120.71 (all Ar─C),
2
| EXPERIMENTAL
54.34
(N─CH2─Ar),
49.70
(CH2CH2),
34.51
(N─(CH2CH3)2), 31.67 (C(CH3)3), 29.59 (C(CH3)3). Anal.
Calcd for C28H42N2O2Cl2 (%): C, 66.01; H, 8.25; N, 5.50.
Found (%): C, 66.39; H, 8.15; N, 5.62.
2.1 | Materials and methods
All reactions with air‐ and/or water‐sensitive compounds
were carried out under dry nitrogen atmosphere using
standard Schlenk‐line or glovebox techniques. Toluene
and n‐hexane were refluxed over sodium benzophenone ketyl
prior to use. Chloroform‐d and other reagents were carefully
dried and stored in a glovebox. Lactide (Aldrich) was recrys-
tallized with dry toluene and then sublimed twice under
vacuum at 80°C. L2H and (L2)Ti(OiPr)2 were prepared fol-
lowing literature procedures.[33] All other chemicals were pur-
chased from Aladdin (China) and used after appropriate
purification. Glassware and vials used in the polymerization
were dried in an oven at 120°C overnight and exposed to
vacuum–argon cycle three times.
L3H. The procedure was the same as that for L1H, except
that 3‐tert‐butyl‐4‐hydroxyanisole (3.0000 g, 0.0171 mol)
was used to afford ligand L3H as colorless crystals in 34%
1
yield. H NMR (CDCl3, 500 MHz, δ, ppm): 7.19 (s, 1H,
ArH), 6.86 (s, 1H, ArH), 6.86 (s, 1H, ArH), 6.78 (d, 1H,
J = 2.5 Hz, ArH), 6.46 (d, 1H, J = 2.5 Hz, ArH), 3.73 (s,
3H, Ar─OCH3), 3.57 (d, 4H, J = 17 Hz, Ar─CH2─N),
2.63 (m, 8H, N─CH2─CH2─N, N─(CH2CH3)2), 1.39 (s,
9H, Ar─C(CH3)3), 1.37 (s, 9H, Ar─C(CH3)3), 1.27 (s, 9H,
Ar─C(CH3)3), 1.09 (t, 6H, J = 7 Hz, N─(CH2CH3)2). 13C
NMR (CDCl3, 125 MHz, δ, ppm): 112.5, 113.1, 121.3,
122.7, 123.4, 124.8, 136.0, 138.1, 140.1, 149.7, 151.4,
153.2
(all
Ar─C),
77.2
(N─CH2─CH3),
77.0
NMR spectra were recorded with Agilent 500 MHz DD2
(N─CH2─CH3), 56.5 (Ar─CH2─N), 55.7 (N─CH2─Ar),
49.8 (CH2CH2), 48.8 (CH2CH2), 45.6 (N─CH2─CH3), 35.0
(C(CH3)3), 34.9 (C(CH3)3), 34.1 (C(CH3)3), 29.6
(C(CH3)3), 29.4 (O─CH3). Anal. Calcd for C33H54N2O3
(%): C, 75.29; H, 10.27; N, 5.32. Found (%): C, 75.66; H,
10.34; N, 5.45.
spectrometers at 25°C unless otherwise stated. Chemical
1
shifts for H NMR and 13C NMR spectra were referenced
internally using the residual solvent resonances and reported
relative to tetramethylsilane. Elemental analyses were per-
formed with an EA‐1106 instrument. Spectroscopic analyses
of polymers were performed in CDCl3. Gel permeation chro-
matography (GPC) analyses were carried out with a Waters
instrument (M515 pump, Optilab Rex injector) in tetrahydro-
furan (THF) at 25°C, at a flow rate of 1 ml min−1. Calibration
standards were commercially available narrowly distributed
linear polystyrene samples that covered a broad range of
molar masses (103–2 × 106 g mol−1).
L4H. The procedure was the same as that for L2H, except
that 3‐tert‐butyl‐4‐hydroxyanisole (3.6000 g, 0.0200 mol)
was used to afford ligand L4H as colorless crystals in 40%
yield. 1H NMR (CDCl3, 500 MHz, δ, ppm): 6.79 (d,
J = 3.0 Hz, 2H ,ArH), 6.46 (d, J = 3.0 Hz, 2H, ArH), 3.73
(s, 6H, CH3O─Ar), 3.55 (s, 4H, Ar─CH2─N), 2.68–2.55
(m, 8H, NCH2CH2N, N(CH2CH3)2), 1.37 (s, 18H,
C(CH3)3), 1.08 (t, J = 7.1 Hz, 6H, N(CH2CH3)2). 13C
NMR (CDCl3, 125 MHz, δ, ppm): 151.36, 149.64, 138.24,
122.62, 113.12, 112.48 (all Ar─C), 56.31 (CH3O─Ar),
55.66 (Ar─CH2─N), 49.56 (CH2CH2), 48.79 (CH2CH2),
45.11 (CH2CH3), 35.03 (C(CH3)3), 29.37 (C(CH3)3), 9.54
(CH2CH3). Anal. Calcd for C30H48N2O4 (%): C, 71.90; H,
9.59; N, 5.59. Found (%): C, 72.11; H, 9.62; N, 5.44.
L5H. The procedure was the same as that for L1H, except
that N,N‐dimethylethylenediamine (1.5000 g, 0.0171 mol)
and 3‐tert‐butyl‐4‐hydroxyanisole (3.0000 g, 0.0171 mol)
were used to afford ligand L5H as colorless crystals in 40%
2.2 | Synthesis
L1H. N,N‐Diethylethylenediamine (1.9815 g, 0.0170 mol)
and a solution of 3,5‐bis(1,1‐dimethylethyl)‐2‐hydroxyben-
zaldehyde (3.9906 g, 0.0170 mol) in methanol (20 ml) were
heated to reflux for 9 h. After cooling to room temperature,
sodium borohydride (1.3000 g, 0.0342 mol) was subse-
quently added to the yellow solution at 0°C (ice water bath)
and the mixture was stirred for 12 h. The reaction mixture
was extracted with methylene dichloride. The organic phase
was dried over anhydrous MgSO4. Removal of the solvent

WANG ET AL.
3 of 9
1
yield. H NMR (CDCl3, 500 MHz, δ, ppm): 7.19 (d, 1H,
Ar─C(CH3)3), 1.31 (d, 6H, J = 6 Hz, O─CH(CH3)2), 1.28
(s, 9H, Ar─C(CH3)3), 0.97 (t, 6H, J = 7 Hz, N─(CH2CH3)2),
0.85 (d, 3H, J = 6 Hz, O─CH(CH3)2), 0.82 (d, 3H, J = 6 Hz,
O─CH(CH3)2). 13C NMR (CDCl3, 125 MHz, δ, ppm): 160.8,
156.8, 150.6, 139.1, 137.3, 134.8, 124.6, 123.9, 123.6, 123.3,
112.9, 111.3 (all Ar─C), 78.2 (N─CH2CH3), 78.0
(N─CH2CH3), 77.3 (C(CH3)3), 77.0 (C(CH3)3), 76.8
(C(CH3)3), 55.8 (Ar─CH2─N), 47.6 (N─CH2─Ar), 35.2
(C(CH3)3), 35.1 (C(CH3)3), 34.1 (C(CH3)3), 31.8
(CH3O─Ar), 30.2 (O─C(CH3)2), 30.0 (O─C(CH3)2), 26.5
(CH2CH2), 26.4 (CH2CH2), 25.9 (O─C(CH3)2), 25.8
(O─C(CH3)2), 9.8 (N─CH2CH3). Anal. Calcd for
C39H66N2O5Ti (%): C, 67.83; H, 9.57; N, 4.06. Found (%):
C, 68.14; H, 9.69; N, 4.16.
J = 2 Hz, ArH), 6.87 (d, 1H, J = 2 Hz, ArH), 6.79 (d, 1H,
J = 3 Hz, ArH), 6.47 (d, 1H, J = 3 Hz, ArH), 3.73 (s, 3H,
Ar─OCH3), 3.59 (d, 4H, J = 18 Hz, Ar─CH2─N), 2.56 (m,
4H, N─CH2─CH2─N), 2.28 (s, 6H, N─(CH3)2), 1.38 (s,
9H, Ar─C(CH3)3), 1.37 (s, 9H, Ar─C(CH3)3), 1.27 (s, 9H,
Ar─C(CH3)3). 13C NMR (CDCl3, 125 MHz, δ, ppm):
153.26, 151.44, 149.65, 140.15, 138.37, 136.11, 124.80,
123.38, 123.05, 121.52, 113.18, 112.51 (all Ar─C), 77.00
(Ar─CH2─N), 56.00 (N─CH2─Ar), 49.00 (CH2CH2), 44.7
(CH2CH2), 34.99 (N─(CH3)2), 34.05 (C(CH3)3), 31.07
(C(CH3)3), 29.44 (C(CH3)3). Anal. Calcd for C31H50N2O3
(%): C, 74.70; H, 10.04; N, 5.62. Found (%): C, 74.36; H,
9.86; N, 5.42.
[(L1)Ti(OiPr)2]. The ligand L1H (1.0000 g, 0.0019 mol)
was added slowly to a solution of Ti(OiPr)4 (0.3850 g,
0.0010 mol) in toluene (20 ml) in a Schlenk flask. The solu-
tion was stirred for 4 h at room temperature. The yellow
solids were afforded by solvent evaporation under vacuum,
which were then recrystallized from toluene–pentane mixture
and kept at −40°C to give yellow crystals in 76% yield (two
[(L4)Ti(OiPr)2]. Following a procedure similar to that
described for [(L1)Ti(OiPr)2], L4H (0.1883 g, 0.0004 mol)
was treated with Ti(OiPr)4 (0.1137 g, 0.0004 mol) in toluene
(20 ml) at room temperature to afford yellow solids. Recrys-
tallization from toluene–pentane mixture afforded yellow
1
crystals in 60% yield. H NMR (CDCl3, 500 MHz, δ, ppm):
6.79 (s, 2H, ArH), 6.47 (s, 2H, ArH), 5.00 (m, 1H
J = 6.0 Hz, OCH(CH3)2), 4.65 (m, 1H, OCH(CH3)2), 4.07
(d, 2H, J = 14.0 Hz, Ar─CH2─N), 3.75 (s, 6H, Ar
—O─CH3), 3.49 (d, 2H, J = 13.0 Hz, Ar─CH2─N),
2.83–2.42 (m, 8H, NCH2CH2N, N(CH2CH3)2), 1.47 (s,
18H, ArC(CH3)3), 1.32 (d, 3H, J = 5.7 Hz, O─CH(CH3)2),
1.25 (d, 3H, J = 5.6 Hz, O─CH(CH3)2), 1.20 (d, 3H,
J = 5.9 Hz, O─CH(CH3)2), 0.97 (t, 6H, J = 7.0 Hz,
Ar─O─CH3), 0.88 (d, 3H, J = 4.8 Hz, O─CH(CH3)2). 13C
NMR (CDCl3, 125 MHz, δ, ppm): 137.9, 129.0, 128.2,
125.3, 112.8, 111.7 (all Ar─C), 64.8(CH3O─Ar) 55.7
(Ar─CH2─N), 47.0 (CH2CH3), 34.9 (O─C(CH3)2), 29.4
(C(CH3)3),25.3 (C(CH3)3), 21.5(C(CH3)3), 1.0 (CH2CH3).
Anal. Calcd for C36H60N2O6Ti (%): C, 64.99; H, 9.03; N,
4.21. Found (%): C, 65.43; H, 9.34; N, 4.47.
1
crops). H NMR (CDCl3, 500 MHz, δ, ppm): 7.26 (s, 2H,
ArH), 6.90 (s, 1H, ArH), 6.88 (s, 1H, ArH), 5.00 (s, 1H,
OCH(CH3)2), 4.55 (s, 1H, OCH(CH3)2), 4.39–3.99 (m, 2H,
Ar─CH2─N), 3.40 (dd, J = 46.3, 12.4 Hz, 2H, Ar─CH2─N),
2.86–2.71 (m, 4H, N(CH2CH3)2), 2.48 (s, 2H, NCH2CH2),
2.30 (s, 2H, NCH2CH2), 1.48 (s, 9H, Ar─C(CH3)3), 1.39
(s, 6H, O─CH(CH3)2), 1.28 (s, 9H, Ar─C(CH3)3), 1.06 (s,
6H, O─CH(CH3)2), 0.90 (s, 3H, N(CH2CH3)2), 0.78 (s, 3H,
N(CH2CH3)2). 13C NMR (CDCl3, 125 MHz, δ, ppm):
156.90, 138.59, 133.55, 127.89, 126.30, 125.47, 123.15,
122.99, 122.37, 121.69 (all Ar─C), 77.3 (C(CH3)3,
O─C(CH3)2), 77.0 (C(CH3)3), 63.42 (N─CH2CH3), 62.27
(Ar─CH2─N), 52.87 (Ar─CH2─N), 51.21 (N(CH2CH3)2),
34.11 (ArC(CH3)3), 33.11 (ArC(CH3)3), 30.71 (ArC(CH3)3),
29.57 (ArC(CH3)3), 25.04 (O‐C(CH3)2), 24.58 (O‐C(CH3)2).
Melting point: 153.1–156.7°C. Anal. Calcd for
C34H54N2O4Cl2Ti (%): C, 60.62; H, 8.02; N, 4.16. Found
(%): C, 61.03; H, 8.25; N, 4.47.
[(L5)Ti(OiPr)2]. Following a procedure similar to that
described for [(L1)Ti(OiPr)2], L5H (1.0000 g, 0.0019 mol)
was treated with Ti(OiPr)4 (0.5399 g, 0.0019 mol) in toluene
(20 ml) at room temperature to afford yellow solids. Recrys-
tallization from toluene–pentane mixture afforded yellow
[(L3)Ti(OiPr)2]. Following a procedure similar to that
described for [(L1)Ti(OiPr)2], L3H (0.5000 g, 0.0009 mol)
was treated with Ti(OiPr)4 (0.2558 g, 0.0009 mol) in toluene
(20 ml) at room temperature to afford yellow solids. Recrys-
tallization from toluene–pentane mixture afforded yellow
crystals in 71% yield (two crops). 1H NMR (CDCl3,
500 MHz, δ, ppm): 7.22 (d, 1H, J = 2 Hz, ArH), 6.88 (d,
1H, J = 2 Hz, ArH ), 6.81 (d, 1H, J = 2 Hz, ArH), 6.48 (d,
1H, J = 2 Hz, ArH), 5.00 (m, 1H, OCH(CH3)2), 4.59 (m,
1H, OCH(CH3)2), 4.18 (d, 1H, J = 14 Hz, Ar‐CH2─N),
3.86 (d, 1H, J = 13 Hz, Ar─CH2─N), 3.76 (s, 3H,
Ar─OCH3), 3.63 (d, 1H, J = 14 Hz, Ar─CH2─N), 3.53 (d,
1
crystals in 70% yield. H NMR (CDCl3, 500 MHz, δ, ppm):
7.24 (d, 1H, J = 2.5 Hz, ArH), 6.88 (d, 1H, J = 2 Hz,
ArH), 6.85 (d, 1H, J = 3 Hz, ArH), 6.49 (d, 1H, J = 3 Hz,
ArH), 5.06 (m, 1H, OCH(CH3)2), 4.59 (m, 1H, OCH(CH3)2),
4.35 (d, 1H, J = 13.5 Hz, Ar─CH2─N), 4.08 (d, 1H,
J = 13 Hz, Ar─CH2─N), 3.76 (s, 3H, Ar─OCH3), 3.48 (d,
1H, J = 13 Hz, Ar─CH2─N), 3.32 (d, 1H, J = 13.5 Hz,
Ar─CH2─N), 2.79 (m, 1H, CH2─N─(CH3)2), 2.61 (m, 1H,
CH2─N─(CH3)2), 2.35 (s, 6H, N─(CH3)2), 2.25 (m, 1H,
CH2─N─CH2), 1.50 (s, 9H, Ar─C(CH3)3), 1.49 (s, 9H,
Ar─C(CH3)3), 1.35 (d, 6H, J = 6 Hz, O─CH(CH3)2), 1.29
(s, 9H, Ar─C(CH3)3), 1.21 (d, 1H, CH2─N─CH2), 0.84 (d,
3H, J = 6 Hz, O─CH(CH3)2), 0.79 (d, 3H, J = 6 Hz,
O─CH(CH3)2). 13C NMR (CDCl3, 125 MHz, δ, ppm):
161.2, 157.1, 150.1, 138.4, 137.1, 134.6, 124.6, 123.9,
1H,
J
=
13 Hz, Ar─CH2─N), 2.84 (m, 1H,
CH2CH2─N─(CH2CH3)2), 2.76 (m, 4H, N─(CH2CH3)2),
2.62 (m, 2H, CH2CH2─N─CH2), 2.53 (m, 1H, CH2CH2—
N─(CH2CH3)2), 1.48 (s, 9H, Ar─C(CH3)3), 1.46 (s, 9H,

4 of 9
WANG ET AL.
123.7, 123.6, 113.1, 111.6 (all Ar─C), 77.3 (C(CH3)3), 77.2
(N─CH3), 77.1 (N─CH3), 77.0 (C(CH3)3), 76.8 (C(CH3)3),
64.7 (Ar─CH2─N), 64.5 (N─CH2─Ar), 58.4 (C(CH3)3),
55.8 (C(CH3)3), 52.6 (C(CH3)3), 50.0 (CH3O─Ar), 35.3
(O─C(CH3)2), 35.2 (O─C(CH3)2), 30.6 (CH2CH2), 30.4
(CH2CH2), 26.0 (O─C(CH3)2), 25.9 (O─C(CH3)2). Anal.
Calcd for C37H62N2O5Ti (%): C, 67.07; H, 9.37; N, 4.23.
Found (%): C, 67.58; H, 9.58; N, 4.45.
TABLE 1 Crystal data and structure refinement for (L3)Ti(OiPr)2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C46H74N2O5Ti C7H8
782.97
273(2)
Triclinic
P‐1
10.0053(18)
13.060(2)
18.457(3)
91.370(6)
103.493(5)
98.561(6)
2314.7(7)
2
b (Å)
c (Å)
2.3 | X‐ray structure determination of [(L3)Ti(OiPr)2]
α (°)
β (°)
Suitable crystals of [(L3)Ti(OiPr)2] for X‐ray analysis were
grow from saturated solution in toluene, n‐hexane solution
or toluene–pentane mixture, at −40°C. Diffraction data were
collected using a Bruker D8 VENTURE diffractometer with
graphite‐monochromated Mo Kα (λ = 0.71073 Å) radiation.
All data were collected at 20°C using the ω‐scan techniques.
All structures were solved by direct methods and refined
using Fourier techniques. An absorption correction based
on SADABS was applied.[34] All non‐hydrogen atoms were
refined by full‐matrix least‐squares on F2 using the
SHELXTL program package.[35] Hydrogen atoms were
located and refined by the geometry method. The cell refine-
ment, data collection and reduction were done using Bruker
SAINT.[36] The structure solution and refinement were per-
formed using SHELXS‐97[37] and SHELXL‐97,[38] respec-
tively. Further crystal data and details of measurements are
summarized in Tables 1 and 2. Molecular structures were
generated using the ORTEP program.[39]
γ (°)
Volume (Å3)
Z
Densitycalc (mg m−3
)
1.123
Absorption coefficient (mm−1
)
0.228
F(000)
852
Crystal size (mm)
0.2 × 0.2 × 0.2
2.82 to 25.90
θ range for data collection (°)
Limiting indices
−12 ≤ h ≤ 12, −15 ≤ k ≤ 15, −22 ≤ l ≤ 22
62 942/8907 [R(int) = 0.0987]
0.7 and 0.5
Reflections collected/unique
Max. and min. transmission
Data / restraints / parameters
Goodness‐of‐fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
8907 / 204 / 504
1.053
R1 = 0.1075, wR2 = 0.3033
R1 = 0.1383, wR2 = 0.3282
1.766 and −0.575 e
Largest diff. peak and hole (Å−3
)
TABLE 2 Selected bond lengths (Å) and angles (°) for (L3)Ti(OiPr)2
2.4 | General polymerization procedure
Ti(1)–O(2)
Ti(1)–O(3)
Ti(1)–N(1)
1.777(4)
1.783(3)
2.326(4)
Ti(1)–O(4)
Ti(1)–O(1)
Ti(1)–N(2)
1.938(3)
1.940(3)
2.453(4)
Bulk polymerizations of L‐lactide were carried out by heating
the monomer and the catalyst in a closed glass vessel for a
period of time by which the melt became viscous. The
catalyst dissolved in 0.1 ml of dry toluene and a lactide‐to‐
catalyst molar ratio of 100:1–2570:1 were employed for all
polymerizations. The polymerization runs were terminated
by the addition of 1 ml of methanol. The reaction mixture
was dissolved in dichloromethane, followed by removal of
the volatiles under reduced pressure. The resulting polymer
was washed with excess methanol to remove any unreacted
monomer, filtered and dried under vacuum for 12 h at 60°C.
O(2)–Ti(1)–O(3)
O(2)–Ti(1)–O(4)
O(3)–Ti(1)–O(4)
O(2)–Ti(1)–O(1)
O(3)–Ti(1)–O(1)
O(4)–Ti(1)–O(1)
O(2)–Ti(1)–N(1)
N(1)–Ti(1)–N(2)
103.72(18)
94.17(17)
99.03(16)
94.85(17)
98.47(16)
157.80(15)
89.73(16)
76.39(14)
O(3)–Ti(1)–N(1)
O(4)–Ti(1)–N(1)
O(1)–Ti(1)–N(1)
O(2)–Ti(1)–N(2)
O(3)–Ti(1)–N(2)
O(4)–Ti(1)–N(2)
O(1)–Ti(1)–N(2)
166.54(16)
79.08(14)
80.71(14)
166.11(17)
90.15(16)
82.61(15)
83.79(14)
The stoichiometric structures of (L1–5)Ti(OiPr)2 were fur-
ther confirmed on the basis of 1H NMR and 13C NMR spec-
troscopies as well as elemental analysis. In the H NMR
3
| RESULTS AND DISCUSSION
1
3.1 | Synthesis and characterization of Ti complexes
spectrum of complex (L3)Ti(OiPr)2, the two protons of each
methylene in Ar─CH2─N units are magnetically non‐equiva-
lent and give rise to two doublets, as compared to the singlet
for the free ligand. Similar phenomena are also displayed in
the proton spectra of the other titanium complexes. It is worth
noting that for these titanium complexes, the resonances of
methylene in the bridging unit (NCH2CH2N) exhibit unex-
pected coupling modes. For instance, in the 1H NMR
The tetradentate bis(phenolate) ligands (L1–5H) were synthe-
sized by the reactions outlined in Scheme 1. Titanium com-
plexes (L1–5)Ti(OiPr)2 were obtained via the reaction of
ligands L1–5H and Ti(OiPr)4 in 1:1 molar ratio in toluene at
room temperature. Analytically pure samples of (L1–5
)
Ti(OiPr)2 were isolated as yellow solids in 65–70% yields
after work‐up (Scheme 1).

WANG ET AL.
5 of 9
SCHEME 1 Synthetic routes for ligands and complexes
spectrum of (L3)Ti(OiPr)2, the two protons of each methylene
in (NCH2CH2N) units are magnetically non‐equivalent and
give rise to a multiple signal as compared to the singlet in
the free ligand (supporting information). Similar phenomena
are also displayed in the proton spectra of the other titanium
complexes. All these indicate the participation of all nitrogen
donors in the coordination with the titanium center.
complexes (1.781(1) and 1.800(1) Å).[25] Because of the ste-
ric repulsion between the titanium center and the ethyl groups
of CH2CH2N(CH2CH3)2 bridge, the bond length of Ti─N2
(2.453(4) Å) is slightly longer than that of Ti─N1 (2.326(4)
Å). The angle O4─Ti─O1 = 157.80(15)° in (L3)Ti(OiPr)2
is significantly more open than that of corresponding angles
of symmetric amine bis(phenolate) titanium complexes
(121.15(5)°)[25] for the coordination between titanium core
and N2 atom of the CH2CH2N(CH2CH3)2 bridge.
Single crystals of (L3)Ti(OiPr)2 suitable for X‐ray diffrac-
tion studies were obtained by slightly cooling the saturated
toluene–pentane mixture. An ORTEP drawing of the molec-
ular structure of (L3)Ti(OiPr)2 is given in Figure 1. Crystal
data and structure refinement are summarized in Table 1
and selected bond lengths and angles are listed in Table 2.
As depicted in Figure 1, (L3)Ti(OiPr)2 has a monomeric
structure in the solid state, in which the titanium atom is
six‐coordinate by four heteroatom donors of the ligand L3H
and two isopropoxy groups adopting a distorted octahedral
geometry around the metal center. The bond lengths of tita-
nium core and oxygen atoms (Ti─O2 and Ti─O3) in (L3)
Ti(OiPr)2 are 1.777(4) and 1.783(3) Å, respectively, which
are close to those in symmetric amine bis(phenolate) titanium
3.2 | Polymerization of L‐lactide and rac‐lactide
As can be seen from the data compiled in Table 3, the com-
plexes (L1–5)Ti(OiPr)2 are efficient initiators for the ROP of
L‐lactide and rac‐lactide in the melt. The polymers pro-
duced have relatively narrow molecular weight distributions
(Mw/Mn = 1.00–1.56). As depicted in Figure 2, the linear
relationship of a plot of Mn versus monomer conversion
([LA]0/[(L3)Ti(OiPr)2]0 = 100:1, at 140°C) indicates that
the polymerization proceeds in a living manner. The exper-
imental values of Mn are very close to their theoretical
values. However, the molecular weights of polymer samples
are considerably lower than the calculated values. Particu-
larly, when [LA]0/[Ti]0 > 250, the Mn values deviate much
from the theoretical values. For instance, the titanium com-
plex (L3)Ti(OiPr)2 exhibits a markedly higher activity, com-
paring with titanium complexes bearing similar amine
phenolate ligands (300 equiv. of rac‐lactide was
adopted)[25–27] in the ROP of rac‐lactide. Even for
1000–2570 equiv. of rac‐lactide in the melt, the polymeriza-
tion initiated by (L3)Ti(OiPr)2 still proceeds but produces
PLA with Mn of only 1.00 × 104–5.19 × 104 g mol−1 (runs
27, 28, 29). Most likely, on the one hand, in the MALDI‐
TOFmassspectrumofanoligomerobtainedbynon‐symmetric
complexes (L1)Ti(OiPr)2 and (L2)Ti(OiPr)2 (supporting infor-
mation, Figures S1 and S2), a series of signals end‐capped
with OiPr group and a hydroxyl group are dominant; mean-
while, the spectrum also reveals two kinds of signals end‐
capped with OiPr group unit. So there are possibly two grow-
ing polymer chains due to two OiPr groups on each metal
FIGURE
1
ORTEP diagram of molecular structure of (L3)Ti(OiPr)2.
Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms
are omitted for clarity

6 of 9
WANG ET AL.
TABLE 3 Ring‐opening polymerization of lactide initiated by (L1–5)Ti(OiPr)2
b
c
c
d
Run
Cat.
Monomer
[LA]0/[Ti]0
T/(oC)
T/min
Conv.a (%)
(104)Mn,caled
(104)Mn
Mw/Mn
Pr
1.
(L1)Ti(OiPr)2
L‐LA
100:1
100:1
100:1
100:1
100:1
200:1
250:1
300:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
200:1
250:1
300:1
800:1
1000:1
2000:1
2570:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
100:1
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
110
110
110
110
140
140
140
140
140
140
140
160
140
140
140
140
140
140
140
140
140
15
60
11
54
68
92
95
94
90
87
20
39
82
86
95
25
37
74
87
34
58
90
96
94
68
90
90
26
50
25
89
74
85
89
93
95
46
57
66
87
0.16
0.77
0.98
1.33
1.37
2.70
3.26
3.75
0.29
0.56
1.17
1.24
1.37
0.36
0.53
1.06
1.25
0.49
0.84
1.30
1.38
1.36
1.97
3.25
3.89
3.05
7.25
7.10
32.92
1.07
1.23
1.28
1.34
1.37
0.67
0.82
0.94
1.26
‐
‐
‐
‐
‐
‐
2.
L‐LA
0.76
0.84
1.34
1.34
2.34
1.41
1.55
‐
1.05
1.14
1.24
1.28
1.50
1.24
1.23
‐
3.
L‐LA
80
4.
L‐LA
2×60
15
5.
rac‐LA
rac‐LA
rac‐LA
rac‐LA
L‐LA
0.48
6.
20
0.46
7.
20
0.47
8.
20
0.46
9.
(L2)Ti(OiPr)2
30
‐
10
L‐LA
60
0.64
1.07
1.24
1.34
0.60
0.61
1.09
1.21
0.59
0.83
1.03
1.38
1.34
1.10
2.84
1.83
‐
1.00
1.18
1.19
1.36
1.00
1.00
1.01
1.21
1.00
1.14
1.17
1.31
1.26
1.34
1.56
1.25
‐
‐
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.
L‐LA
2.5×60
3×60
4×60
45
‐
L‐LA
‐
L‐LA
‐
(L3)Ti(OiPr)2
L‐LA
‐
L‐LA
60
‐
L‐LA
2×60
3×60
30
‐
L‐LA
‐
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
rac‐LA
L‐LA
0.58
40
0.53
60
0.54
80
0.51
30
0.48
60
0.53
60
0.50
60
0.51
60
‐
12×60
12×60
27×60
3×60
4×60
5×60
6×60
7×60
4×60
5×60
8×60
13×60
1.02
1.00
2.83
0.91
0.97
0.92
1.13
1.27
0.66
0.78
0.84
0.93
1.18
1.15
1.71
1.14
1.20
1.15
1.27
1.27
1.01
1.05
1.13
1.14
0.60
0.65
0.51
(L4)Ti(OiPr)2
‐
‐
‐
‐
‐
‐
‐
‐
‐
L‐LA
L‐LA
L‐LA
L‐LA
(L5)Ti(OiPr)2
L‐LA
L‐LA
L‐LA
L‐LA
aDetermined by 1H NMR spectroscopy.
bMn,calcd = ([LA]0/[Ti]0) × 144.13 × conv.
cDetermined by GPC analysis with polystyrene standards in THF.
dProbability of a racemic linkage between two repeat units calculated from homonuclear decoupled 1H NMR spectra.
core which is however similar to the results obtained for
bis(anilinephenolate) titanium complexes.[40]
S3) of the PLAs show that the polymer chains are end‐capped
by isopropyl ester and hydroxyl groups. All these features
indicated a possible coordination–insertion mechanism in
the process of polymerization (Figure 4). A six‐coordinate
intermediate is formed by ketonic oxygen of lactide and tita-
nium core of complex. Then monomer inserts into the
Ti─OiPr bond to trigger the ROP. The acyl group is formed
at the end cap of polymer, and a new Ti─OiPr bond is formed
at another end cap. The monomer continues to insert the
On the other hand, the increased amount of impurities in
large molar ratio of lactide monomer act as chain transfer
reagents during the polymerization process and lead to a
decrease of the molecular weight of resultant PLAs. In addi-
tion, the MALDI‐TOF mass spectrum of an oligomer
obtained by symmetric complex (L4)Ti(OiPr)2 (Figure 3)
1
and the H NMR spectra (supporting information, Figure

WANG ET AL.
7 of 9
non‐symmetric structure of complex (supporting informa-
tion, Figures S1 and S2). At the end of the polymerization,
two kinds of polymer chains with OiPr groups are obtained.
In addition, there are also a series of cyclic polylactides
derived from the intrachain transesterification and the inter-
molecular transesterification[41] (supporting information,
Figures S1 and S2).
The structure of the ancillary ligands also has a signif-
icant influence on the catalytic activity of the correspond-
ing titanium complexes. To investigate the exact influence
of ligand substituents on the polymerization rate of
lactide, preliminary kinetic studies of L‐lactide and rac‐
lactide polymerization initiated by (L1–5)Ti(OiPr)2 were
carried out at 140°C. The semi‐logarithmic plots for these
polymerizations are shown in Figure 5. In each case, the
kinetics of a first‐order dependence on the monomer
concentration is observed. A clear decreasing tendency of
the apparent propagation rate is observed in the order of
(L1)Ti(OiPr)2 (kobs = 2.08 × 10−2 min−1) > (L2)Ti(OiPr)2
(kobs = 1.28 × 10−2 min−1) > (L3)Ti(OiPr)2 (kobs = 1.18 ×
FIGURE 2 Mn and Mw/Mn versus conversion plots for ROP of rac‐lactide
initiated by (L3)Ti(OiPr)2 ([LA]0/[Ti]0 = 100:1, at 140°C, in the melt)
10−2 min−1) > (L4)Ti(OiPr)2 (kobs = 7.26 × 10−3 min−1
)
> (L5)Ti(OiPr)2 (kobs = 2.56 × 10−3 min−1). It is found
FIGURE 3 MALDI‐TOF mass spectrum of the polymerization of L‐lactide
using symmetric complex (L4)Ti(OiPr)2 as initiator (doped with ammonium
citrate) ([LA]0/[Ti]0 = 100:1, at 140°C)
Ti─OiPr and to result in chain propagation. Meanwhile,
another Ti─OiPr group also initiates the ROP leading to
another polymer chain. There is a difference between the
initiation rates of two Ti─OiPr groups because of
FIGURE 5 Semi‐logarithmic plots of L‐lactide conversion versus time
initiated by (L1–5)Ti(OiPr)2 ([LA]0/[Ti]0 = 100:1, in the melt, at 140°C)
FIGURE 4 Proposed mechanism for lactide polymerization initiated by (L1–5)Ti(OiPr)2

8 of 9
WANG ET AL.
that the presence of substituents, particularly at ortho posi-
tion of the phenoxide unit of the ligand, plays an important
role in determining the polymerization rate. In general, the
introduction of electron‐withdrawing substituents, such as
chloro substituents, leads to an increase of the polymeriza-
tion rate. Decreasing the electron density (by introduction
of electron‐withdrawing group) makes the titanium an excel-
lent electrophilic metal center by making the lone pair of
electrons on nitrogen less available to the titanium metal
center. This changes titanium into a strong Lewis acid metal
center and its nucleophilic group (─OiPr) into a weak nucle-
ophile. Thus, this weak nucleophile will promote polymer
chain growth.[5] Complex (L1)Ti(OiPr)2 with 6‐chloro sub-
demanding tert‐butyl group shows certain preference for
heterotactic dyad enchainment during lactide polymerization
and Pr can be improved to 0.65 (run 28). As illustrated in
Figure S4 (supporting information), for (L3)Ti(OiPr)2, the
apparent rate constant of rac‐lactide polymerization is
higher than that of L‐lactide of the homochiral monomers
on a polymerization scale. This indicates that heterotactic‐
enriched PLA produced by these complexes is a result of
chain‐end control.[47,48]
4
| CONCLUSIONS
stituents exhibits higher catalytic activity than (L2–5
)
Several amine bis(phenolate) ligands were synthesized and
their titanium metal complexes were studied. Structural
analyses reveal that all the complexes possess a six‐coordi-
nate titanium center with four heteroatom donors of the
tetradentate ligand and two isopropoxy groups and adopt
a distorted octahedral geometry. The titanium complexes
(L1–5)Ti(OiPr)2 acted as initiators for the ROP of L‐lactide
and rac‐lactide in the melt. All of these titanium com-
plexes exhibit good catalytic activity. Meanwhile, the
molecular weight of the polymer increased linearly with
conversion of monomer, indicating a controlled polymeri-
zation. The feed molar ratio of lactide and titanium com-
plex reached 2570:1 for the polymerization in the melt at
160°C. However, the molecular weights of polymer sam-
ples are considerably lower than theoretical values when
[LA]0/[Ti]0 > 250. The activity in the polymerization of
L‐lactide and rac‐lactide using (L1–5)Ti(OiPr)2 as initia-
tors/catalysts depends upon the ligand substitution pattern.
For example, the introduction of electron‐withdrawing sub-
stituents leads to an increase of the polymerization rate.
Meanwhile, the observed activity increases with a decrease
in the steric bulkiness of the ortho substituent on the
phenoxide unit. The introduction of less steric chloro sub-
stituents at the ortho and para positions of phenoxide
units increases the polymerization rate. However, reduction
of one carbon in the N(CH2CH3)2 backbone to obtain
N(CH3)2 backbone lowers markedly the polymerization
rate. The preparation of related complexes of group 4
metals and studies of their activity in the polymerization
process are underway.
Ti(OiPr)2 with electron‐donating substituents for the poly-
merization of L‐lactide and rac‐lactide. It is easy to under-
stand that electron‐withdrawing substituents would
enhance greatly the electrophilicity of the Ti center and
increase the activity of (L1)Ti(OiPr)2. Simultaneously, (L2)
Ti(OiPr)2 with 6‐methyl substitution has the higher activity
for ROP than both (L3)Ti(OiPr)2 with one 6‐methoxy sub-
stituent and (L4)Ti(OiPr)2 with two 6‐methoxy substituents.
The reason for this is that the electron‐donating capability
of methyl substitution is weaker than that of methoxy sub-
stitution, and the more electron‐donating substitutions are
disadvantageous to the catalytic activity. Certainly, the
effect of the less‐hindered chloro substituents is also hard
to ignore, that is, the presence of bulky groups in the
coordination sphere of the central metal tends to block the
coordination–insertion of incoming monomer and hence is
disadvantageous to the catalytic activity. The less‐hindered
chloro substituents would make the titanium atom more
accessible for the incoming monomer. A similar phenome-
non was observed by Wang and Ma in the polymerization
of rac‐lactide with zinc complex supported by tridentate
aminophenoxide ligands, which involved chloro substituents
at the ortho and para positions of phenoxide units.[42,43]
Nevertheless, there was an opposite trend for L‐lactide
polymerization in the melt at 140°C with zirconium
complexes supported by ONSO‐tetratridentate ligands.[44]
Bearing the same substituents on the phenoxy ring, (L3)
Ti(OiPr)2 with N(CH2CH3)2 backbone is far more active
than (L5)Ti(OiPr)2 with N(CH3)2 backbone (Table 3, runs
15 versus 35). Such marked influences of one‐carbon‐
elongation in the backbone on the catalytic activity were also
observed for other metal complexes, such as salen–Al
complexes as well as tridentate zinc complexes.[43,45,46] It
is a likely reason for higher activity of (L3)Ti(OiPr)2
towards lactide polymerization than (L5)Ti(OiPr)2 that the
more hindered two ethyl groups are advantageous for break-
ing the Ti─O bond between isopropoxy group and metal
center.
ACKNOWLEDGMENTS
This work was financially supported by the National
Science Foundation for Young Scientists of China (No.
21302083 and 51562029); Teaching Reform Project of
Inner Mongolia University of Technology (No. 2014212
and 2014217); and the Major Program of Natural Science
Foundation of Inner Mongolia University of Technology,
China (No. X201503). The authors thank the Institute of
Coal Conversion and Cyclic Economy for their support of
these experiments.
As can be seen from the data compiled in Table 3,
PLAs with slightly heterotactic‐enriched microstructure are
obtained using (L1–5)Ti(OiPr)2. (L3)Ti(OiPr)2 with sterically

WANG ET AL.
9 of 9
REFERENCES
[28] B. J. Jeffery, E. L. Whitelaw, D. Garcia‐Vivo, J. A. Stewart, M. F. Mahon, M.
G. Davidson, M. D. Jones, Chem. Commun. 2011, 47, 12328.
[1] S. K. Roymuhury, D. Chakraborty, V. Ramkumar, Dalton Trans. 2015, 44,
10352.
[29] R. Zhao, T. Liu, L. Wang, H. Ma, Dalton Trans. 2014, 43, 12663.
[2] S. K. Roymuhury, D. Chakraborty, V. Ramkumar, New J. Chem 2015, 39,
[30] C.‐K. Su, H.‐J. Chuang, C.‐Y. Li, C.‐Y. Yu, B.‐T. Ko, J.‐D. Chen, M.‐J.
Chen, Organometallics 2014, 33, 7091.
5218.
[3] D. Chakraborty, D. Mandal, V. Ramkumar, V. Subramanian, J. Vijaya
[31] J.‐C. Buffet, A. N. Martin, M. Kol, J. Okuda, Polym. Chem. 2011, 2, 2378.
Sundar, Polymer 2015, 56, 157.
[32] X. Wang, A. Thevenon, J. L. Brosmer, I. Yu, S. I. Khan, P. Mehrkhodavandi,
[4] S. Narang, E. Macova, D. Berek, S. N. Upadhyay, R. Mehta, J. Appl. Polym.
Sci. 2016, 123, 43099.
P. L. Diaconescu, J. Am. Chem. Soc. 2014, 136, 11264.
[33] D. Peri, C. M. Manna, M. Shavit, E. Y. Tshuva, Eur. J. Inorg. Chem. 2011,
2011, 4896.
[5] S. Narang, D. Berek, S. N. Upadhyay, R. Mehta, J. Polym. Res. 2016, 23, 96.
[6] P. Singla, R. Mehta, S. N. Upadhyay, Appl. Clay Sci. 2014, 95, 67.
[34] SADABS, BrukerNonius area detector scaling, absorption correction‐
V2.05, Bruker AXS Inc., Madison, W1, 1996.
[7] Z. Zhang, X. Xu, W. Li, Y. Yao, Y. Zhang, Q. Shen, Y. Luo, Inorg. Chem.
2009, 48, 5715.
[35] G. M. Sheldrick, SHELXTL 5.10 for Windows NT, Structure determination
software programs, Bruker Analytical X‐ray Systems Inc., Madison, W1,
1997.
[8] H. Ma, T. P. Spaniol, J. Okuda, Inorg. Chem. 2008, 47, 3328.
[9] H. Du, A. H. Velders, P. J. Dijkstra, Z. Zhong, X. Chen, J. Feijen, Macromol-
ecules 2009, 42, 1058.
[36] SAINT, Version 6.02, Bruker AXS Inc., Madison, W1, 1999.
[37] G. M. Sheldrick, SHELXS‐97, Program for the solution of crystal structures,
University of Göttingen, Germany, 1990.
[10] M. H. Chisholm, J. C. Gallucci, K. T. Quisenberry, Z. Zhou, Inorg. Chem.
2008, 47, 2613.
[38] G. M. Sheldrick, SHELXL‐97, Program for the refinement of crystal struc-
tures, University of Göttingen, Germany, 1997.
[11] D. J. Darensbourg, W. Choi, O. Karroonnirun, N. Bhuvanesh, Macromole-
cules 2008, 41, 3493.
[39] L. J. Farrugia, J. Appl. Crystallogr. 1997, 30, 565; ORTEP‐III for Windows-
[12] M. H. Chisholm, J. Gallucci, K. Phomphrai, Chem. Commun. 2003, 1, 48.
[13] H.‐Y. Chen, H.‐Y. Tang, C.‐C. Lin, Macromolecules 2006, 39, 3745.
Version 2.0, University of Glasgow, 2008.
[40] A. Stopper, I. Goldberg, M. Kol, Inorg. Chem. Commun. 2011, 14, 715.
[41] H. Ma, J. Okuda, Macromolecules 2005, 38, 2665.
[14] C. K. Williams, L. E. Breyfogle, S. K. Choi, W. Nam, J. Victor, G. Young,
M. A. Hillmyer, W. B. Tolman, J. Am. Chem. Soc. 2003, 125, 11350.
[42] M. Mandal, D. Chakraborty, V. Ramkumar, RSC Adv. 2015, 5, 28536.
[43] L. Wang, H. Ma, Dalton Trans. 2010, 39, 7897.
[15] L. Wang, H. Ma, Macromolecules 2010, 43, 6535.
[16] M. Y. Shen, Y. L. Peng, W. C. Hung, C. C. Lin, Dalton Trans. 2009, 44, 9906.
[17] R. Platel, L. Hodgson, C. Williams, Polym. Rev. 2008, 48, 11.
[44] A. Stopper, J. Okuda, M. Kol, Macromolecules 2012, 45, 698.
[45] S. Song, X. Zhang, H. Ma, Y. Yang, Dalton Trans. 2012, 41, 3266.
[18] O. Dechy‐Cabaret, B. Martin‐Vaca, D. Bourissou, Chem. Rev. 2004, 104,
6147.
[46] H. Du, X. Pang, H. Yu, X. Zhuang, X. Chen, D. Cui, X. Wang, X. Jing,
Macromolecules 2007, 40, 1904.
[19] F. Della Monica, E. Luciano, G. Roviello, A. Grassi, S. Milione, C.
Capacchione, Macromolecules 2014, 47, 2830.
[47] H. Wang, Y. Yang, H. Ma, Macromolecules 2014, 47, 7750.
[20] F. Della Monica, E. Luciano, A. Buonerba, A. Grassi, S. Milione, C.
[48] B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt, E. B. Lobkovsky,
Capacchione, RSC Adv. 2014, 4, 51262.
G. W. Coates, J. Am. Chem. Soc. 2011, 123, 3229.
[21] S. Pappuru, E. R. Chokkapu, D. Chakraborty, V. Ramkumar, Dalton Trans.
2013, 42, 16412.
SUPPORTING INFORMATION
[22] C. J. Chuck, M. G. Davidson, G. Gobius du Sart, P. K. Ivanova‐Mitseva,
G. I. Kociok‐Kohn, L. B. Manton, Inorg. Chem. 2013, 52, 10804.
Additional Supporting Information may be found online in
the supporting information tab for this article.
[23] L. Azor, C. Bailly, L. Brelot, M. Henry, P. Mobian, S. Dagorne, Inorg.
Chem. 2012, 51, 10876.
[24] A. J. Chmura, D. M. Cousins, M. G. Davidson, M. D. Jones, M. D. Lunn,
How to cite this article: Wang B, Zhang J, Zhao H,
Huang H, Zheng J, Wang L, Sun J, Zhang Y, Cao Z.
Titanium complexes bearing amine bis(phenolate)
ligands: Synthesis, structure and catalysis in ring‐
opening polymerization of lactide, Appl Organometal
Chem. 2016;e3688. doi: 10.1002/aoc.3688
M. F. Mahon, Dalton Trans. 2008, 11, 1437.
[25] A. J. Chmura, M. G. Davidson, M. D. Jones, M. D. Lunn, M. F. Mahon,
Dalton Trans. 2006, 7, 887.
[26] E. L. Whitelaw, M. D. Jones, M. F. Mahon, G. Kociok‐Kohn, Dalton Trans.
2009, 41, 9020.
[27] A. J. Chmura, M. G. Davidson, C. J. Frankis, M. D. Jones, M. D. Lunn,
Chem. Commun. 2008, 11, 1293.