Group 4 metal complexes containing the salalen ligands: Synthesis, structural characterization and studies on the ROP of cyclic esters

Article abstract of DOI:10.1016/j.jorganchem.2018.03.035

The synthesis of homoleptic salalen ligand supported group 4 complexes and their applications in the ring-opening polymerization (ROP) of ε-caprolactone (?-CL) and lactide (LA) are described. All these complexes of type M(L)₂ (LH = (E)-N-(2-(3,

Full text of DOI:10.1016/j.jorganchem.2018.03.035

Accepted Manuscript
Group 4 metal complexes containing the salalen ligands: Synthesis, structural
characterization and studies on the ROP of cyclic esters
Debashis Chakraborty, Bijja Rajashekhar, Mrinmay Mandal, Venkatachalam
Ramkumar
PII:
S0022-328X(18)30204-3
10.1016/j.jorganchem.2018.03.035
JOM 20385
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 11 December 2017
Revised Date: 19 March 2018
Accepted Date: 21 March 2018
Please cite this article as: D. Chakraborty, B. Rajashekhar, M. Mandal, V. Ramkumar, Group 4 metal
complexes containing the salalen ligands: Synthesis, structural characterization and studies on the ROP
of cyclic esters, Journal of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.03.035.
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ACCEPTED MANUSCRIPT
Group 4 metal complexes containing the salalen ligands: Synthesis,
structural characterization and studies on the ROP of cyclic esters
a
b
Debashis Chakraborty a,*, Bijja Rajashekhar , Mrinmay Mandal , Venkatachalam
Ramkumar a
aDepartment of Chemistry, Indian Institute of Technology Madras, Chennai-600 036, Tamil
Nadu, India.
bDepartment of Chemistry, Indian Institute of Technology Patna, Bihta-801103, Bihar, India.
Correspondence to: D. Chakraborty (E-mail: dchakraborty@iitm.ac.in), Fax: +044-
22574202; Tel: +044-22574223.
ABSTRACT
The synthesis of homoleptic salalen ligand supported group 4 complexes and their
applications in the ring-opening polymerization (ROP) of ε-caprolactone (ɛ-CL) and lactide
(LA) are described. All these complexes of type M(L)2 (LH = (E)-N-(2-(3,5-dibromo-2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide: M = Ti (1), M = Zr (4), M
=
Hf
(7);
LH
=
(E)-N-(2-(2-hydroxybenzylideneamino)phenyl)-4-
methylbenzenesulfonamide: M = Ti (2), M = Zr (5), M = Hf (8); LH = (E)-N-(2-(3,5-di-t-
butyl-2-hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide: M = Ti (3), M =
Zr (6), M = Hf (9)) are characterized by various spectroscopic techniques. The single crystal
X-ray diffraction analysis confirmed that the metal centers in complexes 6 and 8 adopt
distorted dodecahedral geometry. The self-assembly of the complexes in solid state through
the molecular interactions demonstrated that the supramolecular network forms molecular
channels which act as a host lattice for the assembly of toluene solvent molecules. Complexes
(1–9) are effective catalysts for the ROP of ɛ-CL and LA and produced high molecular
weight (Mn) polycaprolactone (PCL) and poly (lactic acid) (PLA) respectively with narrow
molecular weight distributions (MWDs) in the range of 1.17–1.28. PLA produced from the
polymerization of rac-LA is atactic. However, better control over the polymerization process
was achieved in the presence of toluene as solvent at 80 °C. All these polymerizations follow
first-order kinetics with respect to monomer concentration. MALDI-TOF and NMR spectral
analysis suggested that the polymerization proceeds through the coordination-insertion
mechanism.
Keywords: Homoleptic, salalen ligand, ROP, polyesters, coordination-insertion mechanism.
1

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1. Introduction
In the last three decades aliphatic polyesters are accepted as biorenewable,
biocompatible and biodegradable polymers which possess versatile physical properties [1,2].
These polymers have wide range of applications in the biomedical, pharmaceutical and food
packaging industry [3-6]. Ring-opening polymerization (ROP) of cyclic esters and the ring-
opening copolymerization (ROCOP) of epoxides and cyclic anhydrides provide a promising
route to synthesize well defined polymers [7-14]. Extensive research efforts have been made
to develop efficient catalysts for the ROP of cyclic esters [15-24]. Various new catalyst
systems have been developed for ROP and considered as effective catalyst systems for the
synthesis of polymers with great control over the polymer microstructure, molecular weight
(Mn) and molecular weight distributions (MWDs) [25-29]. In these catalyst systems, the
ligand moiety is one of the crucial factors which determine the activity and the
stereoselectivity of the system [1,24,30].
In the past, salen and salan supported metal complexes were widely used as catalysts
in various organic transformations and polymerization reactions [31-37]. The majority of
work has been focused on symmetrical salen ligands. Surprisingly, only a few reports are
available on the asymmetrical derivatives [36]. In fact, it is believed that the structural
asymmetry allows better control over the catalytic properties of the metal complexes [38].
Salalen ligands could be considered as a potential alternative for the asymmetric salen
derivatives, which are easily synthesized by the condensation of salicylaldehyde with
different amines [39,40]. Salalen ligands are the reduced analogues of salen and due to their
reduced structures and nitrogen donor of the salalen ligands, it provide quite different
electronic and steric environment from the salen ligands [41].
In the last decade, increasing attention has been paid to the salalen ligands due to their
high flexibility and ease of tuning the structures of the resulting metal complexes [42-50].
Recent reports on group 4 complexes showed excellent polymerization activity for the
formation of high Mn polymer, narrow MWDs and stereoregularity in the polymer chain [51-
60]. Recently, Kol et al., have reported salalen ligands supported titanium complex which
shows extraordinarily high selectivity towards propylene polymerization and produced
isotactic polypropylene with [mmmm] > 99 % [61]. In addition, Kol et al., [62] Jones et al.,
[39,63] Darensbourg et al., [64] Yao et al. [38] and Lamberti et al. [65,66] have divulged
various salalen ligands supported metal complexes for the ROP of LA. Chiral salalen
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supported Al complexes are found to be the most effective catalyst in the polymerization of
LA [65]. In 2010, Mountford et al., reported a first detailed study of any sulfonamide-
supported group 4 catalyst system. All these complexes were effective catalysts for the
controlled ROP of ɛ-CL and rac-LA [67,68]. Pedrido et al., have reported half-salen ligands
supported Ni(II), Cu(II), dinuclear Cd(II) and higher nuclearity Zn(II) complexes which
exhibit great structural variety due to the asymmetry by incorporating a dansyl pendant in the
salen skeleton [69]. This report inspired us to synthesize the sulfonamide supported salalen
ligands as alternatives to conventional salalen ligand systems and our continued interest in
the development of group 4 catalysts for the ROP. Herein we report the synthesis and
characterization of homoleptic unsymmetrical salalen ligand supported group 4 complexes
and their catalytic activity in the ROP of ɛ-CL and LA. To the best of our knowledge this is
the first report of homoleptic sulfonamide-supported salalen group 4 catalysts for the ROP.
Scheme 1. Synthetic routes for the preparation of 1–9.
2. Experimental
2.1. Materials and methods
All reactions were performed under a dry argon atmosphere employing standard
Schlenk techniques or in a glove box levels maintaining the oxygen and moisture at below 1
ppm. The reactions for the synthesis of complexes (1−9) were done in toluene which was
3

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rigorously dried over sodium benzophenone and distilled fresh prior to use. CDCl3 used for
NMR studies was dried by stirring over CaH2, distilled and stored in the glove box. Bruker
1
13
Avance 400/500 NMR instruments were employed to record H and C NMR with the
chemical shifts referenced to residual solvent resonances and are reported as parts per million
relative to SiMe4. ESI-MS spectra were recorded using a Waters Q-Tof micro mass
spectrometer and MALDI-TOF spectra were procured using a Bruker Daltonics instrument in
dihydroxy benzoic acid matrix. Elemental analyses were performed with a Perkin Elmer
Series 11 analyzer. Ti(OiPr)4, Zr(OiPr)4·iPrOH and Hf(OtBu)4 were purchased from Sigma-
Aldrich and used without further purification. The rac-lactide (rac-LA), L-lactide (L-LA) and
ε-caprolactone (ε-CL) were purchased from Sigma-Aldrich. The rac-LA and L-LA was
purified by sublimation repeatedly and stored in a glove box. The ε-CL was purified by
drying over calcium hydride overnight and distilled fresh prior to use. Solvents used for this
study were purchased from Ranchem India. Mn and MWDs of the polymers were determined
by 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 mm×300 mm) were serially connected in a series.
Measurements were done in THF at 27 °C and the flow rate was maintained at 1 mL/min for
all the cases. Mn and MWDs (Mw/Mn) of the polymers were measured relative to polystyrene
standards. Mn was corrected according to Mark-Houwink corrections [70]. The ligands
employed in this study were synthesized by the literature reported methods [69].
2.1.1. Synthesis of complexes 1-9
Complex 1
In an argon filled glove box, to a stirred solution of (E)-N-(2-(3,5-dibromo-2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.18 g, 0.36 mmol) in 10
mL of toluene was added a solution of Ti(OiPr)4 (0.05 g, 0.18 mmol) in 5 mL toluene at room
temperature. The colour of the resulting solution immediately turned orange. The reaction
mixture was stirred for additional 24 h followed by the removal of solvent under vacuum.
1
The resulting orange solid was crystallized from toluene. Yield = 72 %. Mp = 154 °C. H
NMR (400 MHz, CDCl3): δ = 2.39 (s, Ar-CH3, 6H), 6.76-6.88 (m, Ar-H, 2H), 6.90-6.94 (m,
Ar-H, 2H), 7.09-7.20 (m, Ar-H, 4H), 7.20-7.22 (d, J = 8 Hz, Ar-H, 2H), 7.24-7.26 (m, Ar-H,
4H), 7.30 (s, Ar-H, 4H), 7.44 (s, Ar-H, 2H), 7.56 (s, Ar-H, 2H), 7.91 (s, Ar-H, 2H), 8.44 (s,
CH=N, 2H). 13C NMR (100 MHz, CDCl3): δ = 21.39 (Ar-CH3), 115.72 (Ar-Br), 120.93 (Ar-
Br), 123.36 (Ar-C), 125.39 (Ar-C), 127.99 (Ar-C), 128.31(Ar-C), 128.39 (Ar-C), 129.02 (Ar-
4

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C), 129.12 (Ar-C), 130.88 (Ar-C), 137.43 (Ar-C), 140.22 (Ar-C), 141.14 (Ar-S), 143.93 (Ar-
N), 159.14 (Ar-O), 161.18 (CH=N). Anal. Calc. for C40H28Br4N4O6S2Ti: C 43.98, H 2.58, N
5.13. Found: C 44.08, H 2.69, N 5.18. ESI m/z calculated for [M]+ calculated 1092.29 found
1092.78.
Complex 2
In an argon filled glove box, to a stirred solution of (E)-N-(2-(2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.13 g, 0.36 mmol) in 10
mL of toluene was added a solution of Ti(OiPr)4 (0.05 g, 0.18 mmol) in 5 mL toluene at room
temperature. The colour of the resulting solution immediately turned light orange. The
reaction mixture was stirred for additional 24 h followed by the removal of solvent under
vacuum. The resulting light orange solid was crystallized from toluene. Yield = 74 %. Mp =
160 °C. 1H NMR (400 MHz, CDCl3): δ = 2.27 (s, Ar-CH3, 6H), 6.80 (s, Ar-H, 4H), 6.93-6.96
(m, Ar-H, 4H), 7.05 (s, Ar-H, 4H), 7.19-7.26 (m, Ar-H, 6H), 7.55-7.57 (m, Ar-H, 2H), 8.11
(s, Ar-H, 4H), 8.36 (s, CH=N, 2H). 13C NMR (100 MHz, CDCl3): δ = 21.50 (Ar-CH3),
116.05 (Ar-C), 116.51 (Ar-C), 119.10 (Ar-C), 119.72 (Ar-C), 121.03 (Ar-C), 122.04 (Ar-C),
122.36 (Ar-C), 128.25 (Ar-C), 128.81 (Ar-C), 129.00 (Ar-C), 129.06 (Ar-C), 129.49 (Ar-C),
129.82 (Ar-C), 134.97 (Ar-C), 135.26 (Ar-C), 135.77 (Ar-S), 142.92 (Ar-N), 159.57 (Ar-O),
160.98 (CH=N). Anal. Calc. for C40H32N4O6S2Ti: C 61.85, H 4.15, N 7.21. Found: C 62.10,
H 4.21, N 7.39. ESI m/z calculated for [M]+ calculated 776.70 found 776.70.
Complex 3
In an argon filled glove box, to a stirred solution of (E)-N-(2-(3,5-di-t-butyl-2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.17 g, 0.36 mmol) in 10
mL of toluene was added a solution of Ti(OiPr)4 (0.05 g, 0.18 mmol) in 5 mL toluene at room
temperature. The colour of the resulting solution immediately turned bright orange. The
reaction mixture was stirred for additional 24 h followed by the removal of solvent under
vacuum. The resulting yellow solid was crystallized from toluene. Yield = 72 %. Mp = 172
1
°C. H NMR (400 MHz, CDCl3): δ = 1.22 (s, C(CH3)3, 18H), 1.39 (s, C(CH3)3, 18H), 2.26
(Ar-CH3, 6H), 6.73-6.77 (m, Ar-H, 2H), 6.84-6.90 (m, Ar-H, 2H), 6.92-7.08 (m , Ar-H, 4H),
7.16-7.17 (m, Ar-H, 2H), 7.17-7.19 (m, Ar-H, 2H), 7.24-7.28 (m, Ar-H, 2H), 7.42-7.55 (m,
13
Ar-H, 4H), 8.42 (s, CH=N, 2H). C NMR (100 MHz, CDCl3): δ = 21.33 (Ar-CH3), 29.87
(C(CH3)3), 31.60 (C(CH3)3), 35.09 (C(CH3)3), 115.66 (Ar-C), 120.87 (Ar-C), 123.30 (Ar-C),
5

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125.32 (Ar-C), 127.92 (Ar-C), 128.25 (Ar-C), 128.33 (Ar-C), 128.96 (Ar-C), 129.06 (Ar-C),
130.82 (Ar-C), 137.37 (Ar-C), 140.16 (Ar-C), 141.07 (Ar-S), 143.87 (Ar-N), 159.08 (Ar-O),
161.12 (CH=N). Anal. Calc. for C56H64N4O6S2Ti: C 67.18, H 6.44, N 5.60. Found: C 67.43,
H 6.69, N 5.90. ESI m/z calculated for [M]+ calculated 1001.13 found 1001.03.
Complex 4
In an argon filled glove box, to a stirred solution of (E)-N-(2-(3,5-dibromo-2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.14 g, 0.26 mmol) in 10
mL of toluene was added a solution of Zr(OiPr)4·iPrOH (0.05 g, 0.13 mmol) in 5 mL toluene
at room temperature. The colour of the resulting solution immediately turned pale yellow.
The reaction mixture was stirred for additional 24 h followed by the removal of solvent under
vacuum. The resulting yellow solid was crystallized from toluene. Yield = 65 %. Mp = 180
1
°C. H NMR (400 MHz, CDCl3): δ = 2.45 (s, Ar-CH3, 6H), 6.78-6.80 (m, Ar-H, 2H), 6.89-
6.93 (m, Ar-H, 2H), 6.99-7.02 (m, Ar-H, 2H), 7.19-7.22 (m, Ar-H, 4H), 7.26-7.31 (m, Ar-H,
2H), 7.33-7.48 (m, Ar-H, 2H), 7.50-7.55 (m, Ar-H, 4H), 7.57-7.62 (m, Ar-H, 2H), 8.09 (s,
13
Ar-H, 2H), 8.87 (s, CH=N, 2H). C NMR (100 MHz, CDCl3): δ = 21.58 (Ar-CH3), 116.18
(Ar-Br), 121.18 (Ar-Br), 121.88 (Ar-C), 127.34 (Ar-C), 128.27 (Ar-C), 128.41(Ar-C), 128.54
(Ar-C), 129.08 (Ar-C), 129.40 (Ar-C), 129.57 (Ar-C), 131.90 (Ar-C), 138.47 (Ar-C), 139.45
(Ar-C), 140.39 (Ar-C), 140.50 (Ar-C), 141.45 (Ar-S), 142.71 (Ar-N), 159.83 (Ar-O), 161.67
(CH=N). Anal. Calc. for C40H28Br4N4O6S2Ti: C 43.98, H 2.58, N 5.13. Found: C 44.28, H
2.79, N 5.38. ESI m/z calculated for [M]+ calculated 1135.64 found 1135.87.
Complex 5
In an argon filled glove box, to a stirred solution of (E)-N-(2-(2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.09 g, 0.26 mmol) in 10
mL of toluene was added a solution of Zr(OiPr)4·iPrOH (0.05 g, 0.13 mmol) in 5 mL toluene
at room temperature. The colour of the resulting solution immediately turned yellowish
orange. The reaction mixture was stirred for additional 24 h followed by the removal of
solvent under vacuum. The resulting yellowish orange solid was crystallized from toluene.
Yield = 70 %. Mp = 170 °C. 1H NMR (400 MHz, CDCl3): δ = 2.27 (s, Ar-CH3, 3H), 2.34 (s,
Ar-CH3, 3H), 6.68-6.74 (m, Ar-H, 4H), 6.88-6.95 (m, Ar-H, 4H), 7.05-7.08 (m, Ar-H, 4H),
7.15-7.24 (m, Ar-H, 6H), 7.32-7.34 (m, Ar-H, 2H), 8.16 (s, Ar-H, 4H), 8.67 (s, CH=N, 1H),
8.75 (s, CH=N, 1H). 13C NMR (100 MHz, CDCl3): δ = 21.44 (Ar-CH3), 116.45 (Ar-C),
6

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119.03 (Ar-C), 120.96 (Ar-C), 121.25 (Ar-C), 121.97 (Ar-C), 122.30 (Ar-C), 128.18 (Ar-C),
128.74 (Ar-C), 128.93 (Ar-C), 128.99 (Ar-C), 129.42 (Ar-C), 129.75 (Ar-C), 135.70 (Ar-S),
142.85 (Ar-N), 159.50 (Ar-O), 160.91 (CH=N). Anal. Calc. for C40H28Br4N4O6S2Zr: C 42.30,
H 2.49, N 4.93. Found: C 42.60, H 2.67, N 5.15. ESI m/z calculated for [M]+ calculated
818.08 found 818.45.
Complex 6
In an argon filled glove box, to a stirred solution of (E)-N-(2-(3,5-di-t-butyl-2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.12 g, 0.26 mmol) in 10
mL of toluene was added a solution of Zr(OiPr)4·iPrOH (0.05 g, 0.13 mmol) in 5 mL toluene
at room temperature. The colour of the resulting solution immediately turned yellow. The
reaction mixture was stirred for additional 24 h followed by the removal of solvent under
vacuum. The resulting yellow solid was crystallized from toluene. Yield = 70 %. Mp = 188
°C. 1H NMR (400 MHz, CDCl3): δ = 1.36 (s, C(CH3)3, 18H), 1.52 (s, C(CH3)3, 18H), 2.36 (s,
Ar-CH3, 3H), 2.42 (s, Ar-CH3, 3H), 6.75-6.86 (m, Ar-H, 2H), 6.88-6.90 (m, Ar-H, 4H), 6.96-
7.01 (m , Ar-H, 2H), 7.14-7.26 (m, Ar-H, 6H), 7.28-7.44 (m, Ar-H, 4H), 7.46-7.59 (m, Ar-H,
13
2H), 7.60 (s, Ar-H, 1H), 8.84 (s, CH=N, 1H), 8.92 (s, CH=N, 1H). C NMR (100 MHz,
CDCl3): δ = 21.48 (Ar-CH3), 29.80 (C(CH3)3), 31.34 (C(CH3)3), 35.24 (C(CH3)3), 115.39
(Ar-C), 116.08 (Ar-C), 118.53 (Ar-C), 121.08 (Ar-C), 127.24 (Ar-C), 127.77 (Ar-C), 128.17
(Ar-C), 128.31 (Ar-C), 128.44 (Ar-C), 128.98 (Ar-C), 129.30 (Ar-C), 129.47 (Ar-C), 131.80
(Ar-C), 138.37 (Ar-C), 139.35 (Ar-C), 140.40 (Ar-C), 141.35 (Ar-S), 142.61 (Ar-N), 159.73
(Ar-O), 161.57 (CH=N). Anal. Calc. for C56H64N4O6S2Zr: C 64.40, H 6.18, N 5.36. Found: C
64.60, H 6.48, N 5.66. ESI m/z calculated for [M]+ calculated 1044.48 found 1044.24.
Complex 7
In an argon filled glove box, to a stirred solution of (E)-N-(2-(3,5-dibromo-2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.11 g, 0.22 mmol) in 10
mL of toluene was added a solution of Hf(OtBu)4 (0.05 g, 0.11 mmol) in 5 mL toluene at
room temperature. The colour of the resulting solution immediately turned orange. The
reaction mixture was stirred for additional 24 h followed by the removal of solvent under
vacuum. The resulting orange solid was crystallized from toluene. Yield = 72 %. Mp = 172
1
°C. H NMR (400 MHz, CDCl3): δ = 2.41 (s, Ar-CH3, 6H), 6.85-6.87 (d, J = 8 Hz, Ar-H,
2H), 7.11-7.13 (m, Ar-H, 2H), 7.16-7.18 (m, Ar-H, 4H), 7.23-7.26 (m, Ar-H, 6H), 7.29-7.31
7

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13
(d, J = 8 Hz, Ar-H, 2H), 7.66 (s, Ar-H, 2H), 7.95 (m, Ar-H, 2H), 8.32 (s, CH=N, 2H). C
NMR (100 MHz, CDCl3): δ = 21.17 (Ar-CH3), 115.50 (Ar-Br), 120.71 (Ar-Br), 123.14 (Ar-
C), 125.17 (Ar-C), 127.77 (Ar-C), 128.10 (Ar-C), 128.17 (Ar-C), 128.80 (Ar-C), 128.90 (Ar-
C), 130.66 (Ar-C), 137.22 (Ar-C), 137.74 (Ar-C), 140.00 (Ar-C), 140.92 (Ar-S), 143.71 (Ar-
N), 158.92 (Ar-O), 160.97 (CH=N). Anal. Calc. for C40H28Br4HfN4O6S2: C 39.29, H 2.31, N
4.58. Found: C 39.68, H 2.68, N 4.86. ESI m/z calculated for [M+H]+ calculated 1223.92
found 1223.71.
Complex 8
In an argon filled glove box, to a stirred solution of (E)-N-(2-(2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.08 g, 0.22 mmol) in 10
mL of toluene was added a solution of Hf(OtBu)4 (0.05 g, 0.11 mmol) in 5 mL toluene at
room temperature. The colour of the resulting solution immediately turned yellowish orange.
The reaction mixture was stirred for additional 24 h followed by the removal of solvent under
vacuum. The resulting yellowish orange solid was crystallized from toluene. Yield = 71 %.
Mp = 185 °C. 1H NMR (400 MHz, CDCl3): δ = 2.29 (s, Ar-CH3, 3H), 2.36 (s, Ar-CH3, 3H),
6.70 (s, Ar-H, 3H), 6.93-7.00 (m, Ar-H, 4H), 7.07-7.10 (m, Ar-H, 5H), 7.17-7.26 (m, Ar-H,
4H), 7.52 (s, Ar-H, 2H), 8.03 (s, Ar-H, 2H), 8.16 (s, Ar-H, 2H), 8.67 (s, CH=N, 1H), 8.75 (s,
13
CH=N, 1H). C NMR (100 MHz, CDCl3): δ = 21.46 (Ar-CH3), 116.01 (Ar-C), 116.48 (Ar-
C), 119.06 (Ar-C), 119.68 (Ar-C), 120.99 (Ar-C), 122.00 (Ar-C), 122.32 (Ar-C), 128.21 (Ar-
C), 128.77 (Ar-C), 128.96 (Ar-C), 129.46 (Ar-C), 134.93 (Ar-C), 135.22 (Ar-S), 135.73 (Ar-
S), 142.88 (Ar-N), 159.54 (Ar-O), 160.94 (CH=N). Anal. Calc. for C40H32HfN4O6S2: C 52.95,
H 3.55, N 6.17. Found: C 53.15, H 3.85, N 6.47. ESI m/z calculated for [M]+ calculated
907.32 found 907.50.
Complex 9
In an argon filled glove box, to a stirred solution of (E)-N-(2-(3,5-di-t-butyl-2-
hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide (0.10 g, 0.22 mmol) in 10
mL of toluene was added a solution of Hf(OtBu)4 (0.05 g, 0.11 mmol) in 5 mL toluene at
room temperature. The colour of the resulting solution immediately turned yellow. The
reaction mixture was stirred for additional 24 h followed by the removal of solvent under
vacuum. The resulting yellow solid was crystallized from toluene. Yield = 70 %. Mp = 179
1
°C. H NMR (400 MHz, CDCl3): δ = 1.35 (s, C(CH3)3, 18H), 1.62 (s, C(CH3)3, 18H), 2.27
8

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(Ar-CH3, 6H), 6.87-6.89 (d, J = 8 Hz, Ar-H, 4H), 7.16-7.19 (m, Ar-H, 6H), 7.24-7.26 (m, Ar-
13
H, 2H), 7.28-7.32 (m, Ar-H, 6H), 7.68 (s, Ar-H, 2H), 8.33 (s, CH=N, 2H). C NMR (100
MHz, CDCl3): δ = 21.25 (Ar-CH3), 29.79 (C(CH3)3), 31.53 (C(CH3)3), 35.02 (C(CH3)3),
115.58 (Ar-C), 120.79 (Ar-C), 123.22 (Ar-C), 125.25 (Ar-C), 127.85 (Ar-C), 128.18 (Ar-C),
128.25 (Ar-C), 128.88 (Ar-C), 128.98 (Ar-C), 130.74 (Ar-C), 137.30 (Ar-C), 137.82 (Ar-C),
140.08 (Ar-S), 141.00 (Ar-S), 143.79 (Ar-N), 159.00 (Ar-O), 161.05 (CH=N). Anal. Calc. for
C56H64HfN4O6S2: C 59.43, H 5.70, N 4.95. Found: C 59.93, H 5.99, N 5.25. ESI m/z
calculated for [M]+ calculated 1131.75 found 1131.31.
2.1.2. X-ray structure determination of compounds 6 and 8
The crystallographic data for complexes 6 and 8 were collected on Bruker AXS
(Kappa Apex 2) CCD diffractometer equipped with graphite monochromated MoKα (λ =
0.7107 Å) radiation source. The data were collected with 100 % completeness for
θ up to 25°.
ω
and scans were employed to collect the data. The frame width for was set to 0.5° for
φ
ω
data collection. The frames were integrated and data were reduced for Lorentz and
polarization corrections using SAINT-NT (Bruker, 2008). The multi-scan absorption
correction was applied to the data set. All structures were solved using SIR-92 and refined
using SHELXL-97 program [71]. Among the several compounds synthesized in this study,
suitable crystals for X-ray diffraction studies could be obtained from complexes 6 and 8.
Single crystals were grown in a glove box at −25 °C from dilute toluene solutions of the
respective compounds over a period of ten days. The location of all the hydrogen atoms could
be found in the difference Fourier map. The hydrogen atoms attached to carbon atoms were
fixed at chemically meaningful positions and were allowed to ride with the parent atom
during refinement. The molecular structures of complexes 6 and 8 are depicted in Fig. 1 and
Fig. 2 respectively. These data were deposited with CCDC with the following numbers:
CCDC 1063215 and 1063214 respectively.
2.1.3. General procedure for the bulk polymerization of rac-LA, L-LA and ε-CL
The procedure for the polymerization of respective monomers using 1‒9 in 200: 1
stoichiometric ratio is as follows. For L-LA or rac-LA polymerization, 17.34 µmol of
catalysts and 0.50 g L-LA or rac-LA (3.47 mmol) and for ɛ-CL polymerization, 23.66 µmol
of catalysts and 0.54 g (0.50 mL, 4.73 mmol) of ɛ-CL were used during the polymerization.
The polymerizations were carried out in an inert atmosphere. The flask was heated to 80 °C
9

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in case of ɛ-CL polymerization and 140 °C in case of lactide polymerization. The progress of
1
polymerization was monitored by recording the H NMR spectra of the crude mixture
periodically. The polymerization was quenched by gradually cooling the contents to ambient
temperature. The contents were dissolved in minimum amount of CH2Cl2 and poured into
cold methanol in case of rac-LA/L-LA and cold heptane in case of ɛ-CL. The formed
polymer was collected by filtration. The filtered product was dried in vacuum until a constant
weight was achieved.
2.1.4. Kinetics of rac-LA polymerization
To determine the kinetics for the polymerization of rac-LA, we carried out the
polymerization reactions in a sealed tube at a temperature of 140 °C under argon atmosphere.
The polymerizations were performed at 200: 1 (monomer: catalyst) ratios. The time of
polymerization was divided into six equal intervals for each monomer and a set of six
polymerization reactions was performed at each time interval. The polymerizations were
conducted by charging 17.34 µmol of catalyst and 0.50 g (3.47 mmol) of rac-LA under an
argon atmosphere. Next, the contents were taken out separately and analyzed. For rac-LA, 1H
NMR was recorded to determine the % conversion of the monomer into the corresponding
polymer by comparing the methine proton of the unreacted monomer and polymer. Using
these data, we created a linear plot by plotting ln ([rac-LA]0/[ rac-LA]t) vs. time. Apparent
rate constants (kapp) were obtained from the slopes of the best fit lines. Next, we ran the GPC
to get the number average molecular weight (Mn) and MWDs after quenching the aliquots
removed after the same time intervals as above. From this we plotted Mn vs. % conversion vs.
MWDs.
3. Result and discussion
3.1. Synthesis and characterization of complexes (1–9)
The synthesis of salalen ligand precursor involves two steps. In the first step,
benzene-1,2-diamine was treated with 4-methylbenzene-1-sulfonyl chloride to obtain N-(2-
aminophenyl)-4-methylbenzene-sulfonamide. Next, the resultant product was reacted with 2-
hydroxybenzaldehyde derivative to afford an unsymmetrical salalen ligand and purified by
the crystallization from toluene. All the unsymmetrical salalen ligands were azeotropically
distilled in toluene and dried under high vaccum for several hours prior to their use in the
10

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metal complexation reactions. The salalen ligands were treated with group 4 metal alkoxides
in the presence of toluene maintaining a 2: 1 (ligand: metal precursors) stiochiometric ratio to
afford the complexes (1–9) in good yields (Scheme 1). All these complexes were
recrystallized from toluene and thoroughly characterized by various spectroscopic techniques
and the compositions were confirmed by elemental analysis. The complex structure was
1
determined by single crystal X-ray diffraction analysis. In H NMR spectra, all the expected
1
signals are present in correct integration ratio. H NMR analysis results revealed that the
signals corresponding to the –OH and sulphonamide –NH protons (in the free ligand which
appeared ~11 and ~10 ppm, respectively) were absent in 1–9, which confirmed the
coordination between the metal center and ligand. In addition, the imine proton signals are
slightly downfield shifted which shows that the imine nitrogen is coordinated to the metal
centres and a significant downfield shift was observed in case of benzenesulfonamide protons
which is attributed to the coordination with the metal centers. The conclusions drawn from
1
the H NMR analysis were fully supported by the 13C NMR results. ESI-MS results
confirmed monomeric structure of these complexes.
3.2. Single crystal X-ray diffraction studies
From the single crystal X-ray diffraction studies, it is clear that complexes 6 and 8
(Figs. 1 and 2 respectively) are monomeric in the solid state and the corresponding metal
atom occupies distorted dodecahedral geometry. X-ray crystal structure revealed that two
ligands are perpendicular to each other with respect to the plane containing the coordinating
centres of the each ligand. As a result of which five coordinate centres fall in about same
plane making rest of the coordinate centres to occupy two above the plane and one below the
plane, which is clear representation of dodecahedral geometry. Single crystal X-ray
diffraction studies of complex 6 revealed that the Zr(1)–N(3) bond distance is shorter than the
Zr(1)–N(4) as it is a covalent bond. Bond distance of S(2)–O(5) is longer than the S(2)–O(6)
bond distance which implies the coordination of the O(5) atom to the Zr metal centre. Bond
angles N(1)–Zr(1)–N(2), N(3)–Zr(1)–N(4) and O(3)–Zr(1)–O(5) deviates from 60°, 72° and
90° which are standard bond angles constitutes a regular dodecahedral geometry respectively.
From the above observations it is clear that the metal centre in the complex 6 adopts distorted
dodecahedral geometry. Selected bond lengths and angles are listed in Table S1 & Table S2
respectively (Supplementary material). Similar observations were also noticed from the
11

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single crystal X-ray diffraction studies of complex 8. The crystal data for the compounds 6
and 8 is presented in Table S3 (Supplementary material).
Fig. 1. Molecular structure of 6; thermal ellipsoids were drawn at 30 % probability level. Selected bond lengths
[Å] and angles [°]: Zr(1)–O(1) 2.007(16), Zr(1)–O(2) 2.003(16), Zr(1)–O(5) 2.504(16), Zr(1)–N(1) 2.214(2)
Zr(1)–N(2) 2.400(2), Zr(1)–N(4) 2.423(2), O(1)–Zr(1)–O(2) 91.44(7), O(3)–Zr(1)–O(5) 99.99(6), N(1)–Zr(1)–
O(5) 79.09(7), O(5)–Zr(1)–N(3) 58.37(6), N(1)–Zr(1)–N(4) 125.75(7), O(1)–Zr(1)–N(1) 144.28(7).
Fig. 2. Molecular structure of 8; thermal ellipsoids were drawn at 30 % probability level. Selected bond lengths
[Å] and angles [°]: Hf(1)–O(1) 2.004 (19), Hf(1)–O(2) 2.399 (18), Hf(1)–O(5) 2.454(2), Hf(1)–N(1) 2.186(2),
Hf(1)–N(2) 2.380(2), O(1)–Hf(1)–O(2) 84.76(8), O(4)–Hf(1)–O(5) 88.46(8), N(1)–Hf(1)–O(5) 59.30(8), O(4)–
Hf(1)–N(3) 146.26(8), O(2)–Hf(1)–N(4) 140.4(10), N(1)–Hf(1)–N(4) 129.53(8), 129.53(8) 68.54(8).
The supramolecular network in the complexes 6 and 8 is stabilized by strong C-H…O
hydrogen bonds and π…π interactions. The details of the hydrogen bond interactions are
given in Table S4 and Table S5 respectively (Supplementary material). In the complex 6,
inversion related molecules are linked through a C-H…O hydrogen bond C5-H5…O5i to
form a supramolecular ring, whose centroid lies on the crystallographic inversion centre at (0,
0, ½). Inversion related supramolecular dimers are interlinked through C56-H56…O6ii
12

ACCEPTED MANUSCRIPT
hydrogen bond around the crystallographic inversion center at (1/2, 0, 1/2), which extends
infinitely along the [1 0 0] direction forming a one dimensional supramolecular tape as shown
in Fig. S28 (Supplementary material). C36-H36A…O4iii connects the adjacent molecular tape
constructing a two dimensional sheet parallel to the (0 1 0) plane. Parallel stacking of (0 1 0)
sheets along the b-axis are stabilized by π…π interaction between the inversion related
carbon atoms C24 at (x, y, z) and C24 at (1-x, 1-y, 1-z), with an interaction distance of
C24…C24i = 3.318Å, to build a three dimensional network in the crystalline solid.
Interestingly, the supramolecular network forms molecular channels which act as a host
lattice for the assembly of toluene solvent molecules as shown in Fig. S29 (Supplementary
material). The nearby toluene molecules get trapped in the supramolecular channels are
arranged one another with respect to the crystallographic centre of inversion at (1/2, 0 0),
(1/2, 1/2, 0) and so on as shown in Fig. S30 (Supplementary material). In complex 8, the
centrosymmetrically related complex molecules are linked through two C-H…O hydrogen
bonds C2-H2…O6i and C35-H35…O6i forming a ring dimer with respect to the
crystallographic inversion centre at (½, ½, ½). In the complex super-molecule, the oxygen
atom O6 acts as a bifurcated acceptor for the carbon atoms C2 and C35. Similar adjacent
dimers are interlinked through C33-H33…O5iii hydrogen bond thus generating an infinite one
dimensional supramolecular tape extend along [0 1 0] direction, as shown in Fig. S31
(Supplementary material). The hydrogen bond C11-H11…O3ii connects the [0 1 0]
supramolecular tape to construct a two dimensional supramolecular sheer parallel to the (1 0
0) plane and is illustrated in Fig. S32 (Supplementary material). In addition, a π…π
interaction is observed between the de-localized carbon atoms C7 and C9 with an interaction
distance of C7(Sp2)…C9(Sp2) = 3.328Å. Parallely stacked (1 0 0) sheets are further form
three dimensional network stabilized by π…π interaction existing between the de-localized
carbon atoms C18 and C20 with interaction distance of C18(Sp2)…C20(Sp2) = 3.313Å.
3.3. Polymerization studies
Complexes 1–9 were tested as catalysts towards the ROP of ɛ-CL, L-LA and rac-LA.
It was found that these complexes are potential catalysts for the ROP of ɛ-CL, L-LA and rac-
LA. The representative polymerization data is summarized in Table 1. All the
polymerizations were performed under solvent free conditions at 140 °C and 80 °C for LA
and ɛ-CL respectively. The polymerization results revealed that the complexes 1–9 produced
high Mn polymers with narrow MWDs in the range of 1.15–1.27. The narrow MWDs range
13

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confirms the controlled nature of the polymerization process, although the experimental Mn of
the resulting polymers was higher than the theoretical Mn (Table 1). The progress of the
1
polymerization reactions was monitored by H NMR spectroscopy and a linear relationship
between Mn and monomer conversions with almost unchanged MWDs indicated that the
polymerizations proceed in well controlled fashion (Fig. 3). In case of complexes 1–9, rac-
LA polymerization yielded high Mn PLA and the tacticity of the resulting PLA was
determined by the inspection of the methine region in their homonuclear decoupled 1H NMR
spectrum.
The microstructural analysis revealed that the catalysts 1–9 were found to produce
atactic PLA from rac-LA (Fig. 4). In fact, the influence of ionic radii of the metal center on
the catalysts activity and selectivity is not so obvious. Moreover, the substitution on the
phenolate ring does not affect much on the activity and selectivity of the catalysts (Table 1,
Entry 1 vs. 2 vs. 3). The polymerization rates were high for ɛ-CL as compared to LA (Table 1,
Entries 1–18 vs. 19–27).
Fig. 3. Plot of Mn vs. MWDs vs. % conversion for rac-LA polymerization at 140 °C using 1, 4 and 7.
Fig. 4. Methine region of homonuclear decoupled 1H NMR spectrum of PLA obtained from rac-LA
polymerization initiated by 1 in 200: 1 respectively (CDCl3, 500 MHz).
14

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Table 1
Polymerization data for rac−LA, L−LA and ɛ-CL with 1−9 in the ratio [M]0/[C]0 = 200 at 140 °C for LA and 80
°C for ɛ-CL.
Entry Catalysts Monomer Timea [min] Yieldb [%] Mn, obsc [kg/mol] Mn, theod [kg/mol] Mw/Mn
1
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
L-LA
ɛ-CL
28
29
32
34
37
38
40
43
46
22
24
27
30
33
35
38
40
43
10
11
14
16
19
21
24
26
27
98
97
97
96
96
96
95
96
95
99
98
98
97
97
96
96
95
95
99
99
98
98
97
97
97
96
96
52.37
52.21
51.64
45.59
47.04
46.34
41.85
42.72
40.94
57.42
55.15
54.23
52.12
51.33
50.02
48.92
46.38
45.24
39.81
38.08
36.65
34.81
33.03
32.12
31.66
30.28
29.90
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
23.35
23.19
23.31
29.35
29.19
29.31
29.35
29.19
29.31
1.17
1.18
1.18
1.19
1.20
1.22
1.25
1.25
1.27
1.15
1.15
1.17
1.18
1.18
1.20
1.22
1.23
1.25
1.17
1.18
1.18
1.19
1.20
1.20
1.21
1.22
1.24
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
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
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
aTime of polymerization measured by quenching the polymerization reaction at 100 % conversion.
bIsolated yield at 100 % conversion.
cMeasured by gel permeation chromatography at 27 °C in THF relative to polystyrene standards with Mark-
Houwink corrections.
dMn (theoretical) at 100% conversion = [M]0/[C]0 × mol wt (monomer) + Mend groups
.
The Mn of the resulting PLAs appear to be notably higher than the theoretical Mn
calculated from the initial catalyst loading and conversion, assuming a single chain for each
15

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catalyst (Table 1). This increase in Mn can be accounted for slow initiation step vs.
propagation, plausible due to the large initiating group. In fact, when the polymerizations
were carried out in the presence of benzyl alcohol (BnOH, 1 equiv), the polymerization
proceeds faster and Mn of resulted PLA exhibited good agreement with the theoretical Mn
(Table S6, Supplementary material). The supremacy of polymerization process using BnOH
compare to polymerization process without BnOH is explained by the appearance of lower
transesterification product during the polymerization reaction using BnOH (discussed later).
To investigate the influence of the solvent over the polymerization process, all
polymerizations were performed in toluene and the results revealed that the polymerizations
proceed in more controlled fashion. The Mn of resulted polymers has a close agreement with
the theoretical Mn with narrow MWDs (1.11–1.18) (Table 2).
Table 2
Polymerization data for rac−LA, L−LA and ɛ-CL with 1−9 in the ratio [M]0/[C]0 = 200 at 80 °C in toluene.
Entry Catalysts Monomer Timea [min] Yieldb [%] Mn, obsc [kg/mol] Mn, theod [kg/mol] Mw/Mn
1
2
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
rac-LA
L-LA
90
98
95
95
94
93
92
93
91
90
90
96
96
95
94
95
95
94
93
92
95
94
94
30.41
30.11
29.87
29.78
29.62
29.27
28.87
28.54
28.11
30.64
30.06
29.84
29.36
29.03
28.55
28.12
28.01
27.88
24.27
24.03
23.93
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
29.35
29.19
29.31
1.13
1.14
1.14
1.15
1.16
1.17
1.17
1.16
1.18
1.11
1.11
1.12
1.14
1.14
1.15
1.16
1.16
1.17
1.12
1.13
1.13
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
3
103
120
128
140
160
169
182
82
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
L-LA
89
L-LA
99
L-LA
114
123
134
150
158
170
40
L-LA
L-LA
L-LA
L-LA
L-LA
ɛ-CL
ɛ-CL
45
ɛ-CL
52
16

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22
23
24
25
26
27
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
ɛ-CL
60
67
92
92
93
92
91
92
23.31
22.83
22.15
22.04
21.67
21.22
29.35
29.19
29.31
29.35
29.19
29.31
1.14
1.14
1.16
1.15
1.16
1.17
4
5
6
7
8
9
78
87
96
105
aTime of polymerization measured by quenching the polymerization reaction at 100 % conversion.
bIsolated yield at 100 % conversion.
cMeasured by gel permeation chromatography at 27 °C in THF relative to polystyrene standards with Mark-
Houwink corrections.
dMn (theoretical) at 100 % conversion = [M]0/[C]0 × mol wt (monomer) + Mend groups
.
In order to confirm the controlled nature of the polymerization process, different
monomer to catalyst ratios have been used and the polymerization results are summarized in
Table S7 (Supplementary material). Further, rac-LA was used as the representative monomer
in combination with complexes 1, 4 and 7 and the polymerizations were carried out under
solvent free conditions at 140 °C. However, despite the slow initiation, the Mn increases with
the progress of the reaction, and the MWDs are almost unaltered (Fig. 5), indicative of
controlled behavior of the polymerization.
Fig. 5. Plot of Mn and [M]0/[C]0 vs. MWDs for rac-LA polymerization at 140 °C using 1, 4 and 7.
3.4. Kinetics of polymerization
To determine the rates of polymerization, complexes 1, 4 and 7 were used as
representative catalysts for the kinetic studies of rac-LA polymerization. The monomer
conversion was increased linearly with time and the conversion was almost constant after a
certain point. A conversion plot of rac-LA vs. time resulted in a sigmodial curve (Fig. S33,
Supplementary material). The kinetic studies revealed that the polymerizations show a first-
order dependence on rac-LA concentration. No induction period was observed during the
17

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polymerization. The plot of [rac-LA]0/[ rac-LA]t vs. time is linear (Fig. 6). The values of the
apparent rate constant (kapp) for rac-LA polymerizations initiated by 1, 4 and 7 were
evaluated from the slope of these lines and found to be 2.10 × 10-3 sec-1, 1.63 × 10-3 sec-1 and
1.26 × 10-3 sec-1 respectively.
Fig. 6. Semi-logarithmic plots of rac-LA conversion in time initiated by 1, 4 and 7: [rac-LA]0/[C]0 = 200 at 140
°C.
3.5. Mechanism of polymerization
To have an insight into the polymerization mechanism, low molecular weight
oligomer of rac-LA was synthesized by stirring rac-LA with 2 in 10: 1 molar ratio under
solvent free conditions at 140 °C. The resultant low molecular weight oligomer was
1
thoroughly examined by MALDI-TOF and 1H NMR techniques. H NMR and MALDI-TOF
analysis results revealed that the salalen ligand is present in the polymer chain as one of the
end group (Fig. 7 and Fig. 8 respectively). All the characteristic peaks were appeared in the
1H NMR spectrum of the oligomer. The signals at 5.15 ppm and 1.52 ppm correspond to the
methine and methyl protons of the oligomers chain respectively. The imine proton appears as
a singlet at ∼8.2 ppm. The aromatic protons resonate in 6.8-7.9 region and methyl protons of
the ligand appeared at 2.3 ppm (Fig. 7). MALDI-TOF analysis also suggested that the ligand
moiety is one of the end terminal groups. The expected peaks appeared at regular interval
corresponds to the sequential growth of the oligomer chain containing ligand moiety as one
of the end group. Moreover, the low intensity peaks present in the spectrum are assigned as
transesterification products (Fig. 8). MALDI-TOF and NMR analysis of the oligomer
revealed that the polymerization proceeds through coordination-insertion mechanism. The
proposed mechanism has shown in Scheme 2. In addition, the MALDI-TOF analysis of the
low molecular weight oligomer of rac-LA synthesized by stirring rac-LA and 2 using BnOH
in 10: 1: 1 molar ratio indicated the formation of very less transesterification products which
18

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attributed to the predominance of synthesis of polymers using BnOH (Fig. S34,
Supplementary material).
1
Fig. 7. H NMR spectrum of the product obtained from a reaction between rac-LA and 2 in 10: 1 ratio at 140
°C.
m
m a s s
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
2 1
7 9 2 . 1 0 1
8 6 3 . 8 7 5
9 3 6 . 0 0 8
1 0 0 8 . 0 1
1 0 7 9 . 8 5
1 1 5 2 . 9 5
1 2 2 4 . 7 6
1 2 9 6 . 3 3
1 3 6 7 . 9 9
1 4 3 9 . 5 6
1 5 1 1 . 6 3
O
O
.
Na
H
O
O
O
O
N
O
n
m
n
3
4
5
6
7
8
9
mass
NH
S
749.25
821.28
893.31
965.34
1037.37
1109.40
1181.43
O
O
10 1253.46
11 1325.49
12 1397.52
13 1469.55
14 1541.58
Fig. 8. MALDI-TOF of the crude product obtained from a reaction between rac-LA and 2 in 10: 1 ratio at 140
°C.
19

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Hf(RO)2
O
O
O
Hf(OR)
RO
O
O
RO
O
Hf(OR)
O
O
O
O
O
O
O
RO
O
O
O
Hf(OR)
O
O
rac-LA
O
O
RO
O
H
O
n
O
O
N
R =
NH
O
S O
Scheme 2. Proposed mechanistic pathway for the ROP of LA.
4. Conclusions
In summary, we have synthesized a series of homoleptic unsymmetrical salalen ligand
supported group 4 complexes (1–9) and fully characterized by the NMR and mass
spectroscopy. The compositions and purity were confirmed by elemental analysis. X-ray
analysis results (6 and 8) confirmed the monomeric structure of the complexes. The self-
assembly of the complex molecules in solid state through molecular interactions is explained.
These complexes are good catalysts for the ROP of ɛ-CL and LA. The disagreement between
observed Mn of the resulting PLAs and theoretical Mn can be attributed to slow initiation vs.
propagation due to the presence of large initiating group. However, despite the slow initiation
polymerizations are well controlled as it evident from the Mn increases with the progress of
the reaction. In addition, the polymerizations were well-controlled in toluene and exhibited
good agreement with the theoretical Mn. MALDI-TOF and 1H NMR analysis of the oligomer
revealed that the polymerization proceeds through coordination-insertion mechanism. The
microstructure analysis revealed that the resultant PLA was atactic in nature. In addition, the
nature of metal center and substituents on the ligand backbone of the catalyst has no
significant effect on the activity and selectivity of the catalyst systems. The kinetic results
confirmed the polymerization follows first-order kinetics with respect to monomer
concentration.
20

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Acknowledgements
BR thanks the University Grants Commission New Delhi for research fellowship.
Supplementary material
Supplementary material is available in the online version of this article.
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Homoleptic salalen ligand supported group 4 complexes were synthesized.
All the complexes adopted distorted dodecahedral geometry.
Effective catalysts for the ROP of ɛ-CL and LA.
Atactic polymer was formed.
Polymerizations proceeded through the coordination-insertion mechanism.