Imino phenoxide complexes of group 4 metals: Synthesis, structural characterization and polymerization studies

Article abstract of DOI:10.1039/c3nj40916c

A complete library of new alkoxides containing group 4 metals and the imino phenol ligand were synthesized in high yields and purity. These compounds showed catalytic activity towards the polymerization of l-lactide (l-LA), rac-lactide (rac-LA), ε-caprolactone (CL), δ-valerolactone (VL), rac-β-butyrolactone (rac-BL), ethylene and propylene. The zirconium catalysts were found to yield better polymerization results in comparison to the titanium and hafnium analogues. The number average molecular weight (M _n) values of the polymers are very high for LA and CL with controlled molecular weight distributions (MWDs). Analyses of MALDI-TOF and ¹H NMR spectra of low M_n oligomers reveal that the ligand initiates the ring-opening polymerization (ROP). For ethylene and propylene polymerization, we have achieved moderate to good activity using MAO as a co-catalyst.

Full text of DOI:10.1039/c3nj40916c

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Imino phenoxide complexes of group 4 metals:
synthesis, structural characterization and
polymerization studies†
Cite this: NewJ.Chem., 2013,
37, 949
Tanmoy Kumar Saha, Mrinmay Mandal, Debashis Chakraborty* and
Venkatachalam Ramkumar
A complete library of new alkoxides containing group 4 metals and the imino phenol ligand were
synthesized in high yields and purity. These compounds showed catalytic activity towards the
polymerization of L-lactide (L-LA), rac-lactide (rac-LA), e-caprolactone (CL), d-valerolactone (VL), rac-b-
butyrolactone (rac-BL), ethylene and propylene. The zirconium catalysts were found to yield better
polymerization results in comparison to the titanium and hafnium analogues. The number average
molecular weight (Mn) values of the polymers are very high for LA and CL with controlled molecular
weight distributions (MWDs). Analyses of MALDI-TOF and 1H NMR spectra of low Mn oligomers reveal
that the ligand initiates the ring-opening polymerization (ROP). For ethylene and propylene
polymerization, we have achieved moderate to good activity using MAO as a co-catalyst.
Received (in Victoria, Australia)
11th October 2012,
Accepted 8th January 2013
DOI: 10.1039/c3nj40916c
www.rsc.org/njc
a hot area of current interest16–19 since it is a very convenient
and easy method to synthesize these polymers. Out of the
1 Introduction
In the modern era, there has been a continuous decrease in research
based on commodity polymers owing to the rapid consumption of
non-renewable petroleum resources. Non-biodegradability of these
polymers is also an important issue that has posed a key threat
to the ultimate fate of such polymers.1,2 Currently, scientists are
more focused on studying biodegradable polymers due to
growing environmental awareness of the harmful effects of
non-biodegradable materials.1,3–6 This has initiated an active
impetus for extensive research on polymers that can be derived
from completely renewable natural resources like corn and
sugar beet.7–10 These polymers are not only biodegradable,
but also bioassimilable as their hydrolysis in physiological
media furnishes non-toxic side products.10 At the same time,
they are completely environmentally benign. Besides, they have
tremendous impact on the biomedical and pharmaceutical
industry in vital applications like drug delivery, fabrication of
biodegradable sutures, medical implants, ligating clips, bone
pins and scaffolds for tissue engineering.3,11–15 Synthesis of
these biodegradable polymers by ROP of cyclic esters and LA is
numerous methods available, the coordination-insertion
mechanism20–28 is a popular choice for ROP as it has the ability
to produce high molecular weight polymers (Mn) with narrow
MWDs. In recent times, we have reported several group 4 metal
catalysts containing a bis(imino)phenoxide ligand backbone
for the ROP of cyclic esters and lactides (LA)29,30 as group 4
metal catalysts31,32 are very efficient and popular for performing
such polymerization. We observed excellent activity of these
complexes towards the ROP of cyclic esters and LA, but the
major drawback was that the observed Mn was far higher than
the theoretical value. Inspired by these results, we wanted to
explore the effect of the imino phenoxide ligand complexes of
group 4 metals upon the polymerization of cyclic esters and LA.
In 2008, Davidson’s group reported group 4 metal catalysts
comprising of a chiral imino phenoxide type ligand that has a
benzyl spacer arm.32c Our goal was to see the electronic effect of
the imino phenoxide ligand without any spacer.
The invention of high density polyethylene and methyl-
aluminoxane (MAO) as a potent activator of sandwich type
metallocene complexes showed a new path in the area of olefin
polymerization.33 An important reason for the synthesis of
molecular catalysts stems from the provision of opportunities
related to the issues concerning polymerization mechanism and
stereospecificity. Although single site homogeneous catalysts
have gained appreciable attention for olefin polymerization,
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036,
Tamilnadu, India. E-mail: dchakraborty@iitm.ac.in; Fax: +91 44 2257 4202;
Tel: +91 44 2257 4223
† Electronic supplementary information (ESI) available: Includes NMR spectra
and experimental details. CCDC 891894–891896. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3nj40916c
c
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scientists are interested in catalysts based on non-metallocene with Waters 510 pump and Waters 2414 differential refracto-
systems. The non-metallocene complexes discovered in recent times meter as the detector. The column used, namely WATERS
have enabled the synthesis of hyperbranched poly(ethylene)s, STRYGEL-HR4 of dimensions (4.6 Â 300 mm), was connected
copolymers containing ethylene and polar monomers, mono- during the experiment. Measurements were done in trichloro-
disperse poly(higher-a-olefin)s and higher a-olefin based block benzene. Number average molecular weights (Mn) and mole-
copolymers that are otherwise not possible to produce using cular weight distributions (MWDs) of the poly olefins were
group 4 metallocene catalysts and constrained geometry ascertained relative to polystyrene standards.
catalyst systems.34
2.2 Materials
Among the several highly active catalytic systems for
olefin polymerization, the chloro complexes of group 4 metals Methylaluminoxane (MAO), Ti(O–iPr)4, Zr(O–iPr)4(HO–iPr) and
containing an imino phenoxide ligand developed by Fujita Hf(O–tBu)4 were purchased from Sigma-Aldrich and used with-
and later by Coates deserve considerable attention.35 These out further purification. rac-LA, L-LA, CL, VL and rac-BL were
compounds are popularly called ‘‘FI’’ catalysts. There has been purchased from Sigma-Aldrich. rac-LA and L-LA were sublimed
active impetus in developing high performance group 4 metal- under an argon atmosphere and stored in a glove box. Ligands
containing FI catalysts by both academic and industrial L1, L2 and L3 were prepared according to literature reported
research groups. Our compounds have a good deal of structural procedures.36–38 CL, VL and rac-BL were dried over calcium
resemblance to these FI catalysts and this prompted us to hydride overnight and distilled fresh prior to use. Highly pure
investigate their catalytic activity towards the polymerization ethylene and propylene gas were purchased from Indogas,
of ethylene and propylene. We have tested these compounds Bangalore, India.
towards the polymerization of ethylene and propylene as
they would contribute towards the synthesis of new ‘‘FI’’ non-
2.3 Syntheses and characterization
metallocene compounds.
Compound 1. In an argon filled glove box, a solution of
Ti(O–iPr)4 (0.050 g, 0.176 mmol) in 5 mL of dry toluene and
ligand L1 (0.138 g, 0.351 mmol) in 10 mL of dry toluene were
cooled for 6 h at À25 1C. After cooling, these solutions were
mixed and stirred for 24 h at ambient temperature. Subse-
2 Experimental
2.1 General information and instrumentation
All the reactions were performed under a dry argon atmosphere quently, the mixture was evaporated to dryness to afford a
using standard Schlenk techniques or glove box techniques greenish-yellow residue. The residue was crystallized from dry
with the rigorous exclusion of moisture and air. Toluene was toluene. Yield = 0.141 g (84%). Mp: 163 1C. 1H NMR (400 MHz,
dried by heating under refluxing conditions for 6 h over sodium CDCl3): d = 1.20–1.28 (m, O–CH(CH3)2, 12 H), 1.29–1.50
and benzophenone and distilled fresh prior to use. CDCl3 used (m, CH(CH3)2, 24 H), 1.53 (s, C(CH3)3, 18 H), 1.57 (s, C(CH3)3,
for NMR spectral measurements was dried over calcium 18 H), 2.98–3.05 (m, CH(CH3)2, 4 H), 4.49–4.54 (m, O–CH(CH3)2,
hydride for 48 h, distilled and stored in a glove box. 1H and 2 H), 7.18–7.54 (m, Ar–H, 10 H), 8.33 (s, CHQN, 2 H). 13C NMR
13C NMR spectra were recorded with a Bruker Avance 400 or (100 MHz, CDCl3): d = 22.59 (Ar–CH3), 23.77 (O–CH(CH3)2),
1
Bruker Ascend 500 instrument. Chemical shifts for H and 13
C
25.94 (CH(CH3)2), 26.25 (CH(CH3)2), 26.70 (CH(CH3)2), 27.83
NMR spectra were referenced to residual solvent resonances (CH(CH3)2), 31.56 (C(CH3)3), 31.77 (C(CH3)3), 34.35 (C(CH3)3),
and are reported as parts per million relative to SiMe4. ESI-MS 35.13 (C(CH3)3), 76.83 (O–CH(CH3)2), 123.11 (Ar–CH3), 123.30
spectra of the samples were recorded using Waters Q-Tof micro (Ar–C), 123.39 (Ar–C), 125.36 (Ar–C), 125.51 (Ar–C), 126.80
mass spectrometer. MALDI-TOF measurements were performed (Ar–C), 128.25 (Ar–C), 129.18 (Ar–CH3), 137.29 (Ar–C), 139.01
on a Bruker Daltonics or Bruker Ultraflextreme instrument in a (Ar–CH3), 140.60 (Ar–C), 146.53 (Ar–C), 158.59 (Ar–O), 167.66
dihydroxybenzoic acid matrix. Elemental analyses were performed (CHQN). ESI† m/z calculated for [M + H]+. C60H90N2O4Ti:
with a Perkin Elmer Series 11 analyzer.
950.638 found 951.906. Anal. calc. for C60H90N2O4Ti: C 75.76,
Molecular weights (Mn) and the polydispersity indices H 9.54, N 2.94. Found: C 75.61, H 9.41, N 2.88.
(MWDs) of the polymer samples produced by the ROP of
Compound 2. In an argon filled glove box, a solution of
various cyclic ester monomers and LA were determined by Ti(O–iPr)4 (0.050 g, 0.176 mmol) in 5 mL of dry toluene and L2
using a GPC instrument with Waters 510 pump and Waters ligand (0.101 g, 0.351 mmol) in 10 mL of dry toluene were
410 differential refractometer as the detector. Three columns, mixed and heated at 100 1C for 24 h. After 24 h, the mixture was
WATERS STRYGEL-HR5, STRYGEL-HR4 and STRYGEL-HR3 evaporated to dryness to afford a yellow residue. The residue
each of dimensions (7.8 Â 300 mm) were serially connected was purified from dry toluene. Yield = 0.103 g (83%). Mp: 151 1C.
one after another. Measurements were done in THF at 27 1C for 1H NMR (400 MHz, CDCl3): d = 1.22–1.35 (m, O–CH(CH3)2,
all the cases. Measurement of number average molecular 12 H), 1.47 (s, C(CH3)3, 18 H), 1.49 (s, C(CH3)3, 18 H), 1.57
weights (Mn), weight average molecular weights (Mw) and poly- (s, C(CH3)3, 18 H), 4.49–4.53 (m, O–CH(CH3)2, 2 H), 7.11
dispersity (Mw/Mn) (MWDs) of the polymers were performed (s, Ar–H, 2 H), 7.37 (s, Ar–H, 2 H), 8.36 (s, CHQN, 2 H). 13C
relative to polystyrene standards. Molecular weights (Mn and Mw) NMR (100 MHz, CDCl3): d = 25.94 (O–CH(CH3)2), 29.62
and the polydispersity indices (Mw/Mn) of polyethylene and (C(CH3)3), 29.87 (C(CH3)3), 31.69 (C(CH3)3), 34.28 (C(CH3)3),
poly(propylene) samples were obtained by using a GPC instrument 35.18 (C(CH3)3), 56.89 (N–C(CH3)3), 76.35 (O–CH(CH3)2),
c
950 New J. Chem., 2013, 37, 949--960
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118.07 (Ar–C), 126.01 (Ar–C), 126.66 (Ar–C), 136.93 (Ar–C), 8.35 (s, CHQN, 2 H). 13C NMR (100 MHz, CDCl3): d = 26.05
139.75 (Ar–C), 158.88 (Ar–O), 160.82 (CHQN). ESI† m/z calcu- (O–CH(CH3)2), 26.82 (C(CH3)3), 29.86 (C(CH3)3), 31.68 (C(CH3)3),
lated for [M]+. C44H74N2O4Ti: 742.936 found 742.252. Anal. calc. 34.27 (C(CH3)3), 35.18 (C(CH3)3), 56.89 (N–C(CH3)3), 74.95
for C44H74N2O4Ti: C 71.13, H 10.04, N 3.77. Found: C 71.22, H (O–CH(CH3)2), 118.09 (Ar–C), 126.00 (Ar–C), 126.67 (Ar–C),
10.15, N 3.69.
137.03 (Ar–C), 139.75 (Ar–C), 158.89 (Ar–O), 160.81 (CHQN).
Compound 3. In an argon filled glove box, a solution of ESI† m/z calculated for [M]+. C44H74N2O4Zr: 742.936 found
Ti(O–iPr)4 (0.050 g, 0.176 mmol) in 5 mL of dry toluene and 742.252. Anal. calc. for C44H74N2O4Zr: C 67.21, H 9.49, N 3.56.
ligand L3 (0.154 g, 0.351 mmol) in 10 mL of dry toluene were Found: C 67.10, H 9.59, N 3.67.
cooled for 6 h at À25 1C. After cooling, these solutions were
Compound 6. In an argon filled glove box, a solution of
mixed and stirred for 24 h at ambient temperature. Sub- Zr(O–iPr)4 (HO–iPr) (0.050 g, 0.129 mmol) in 5 mL of dry
sequently, the mixture was evaporated to dryness to afford a toluene and ligand L3 (0.113 g, 0.257 mmol) in 10 mL of dry
yellow residue. The residue was crystallized from dry toluene. toluene were cooled for 6 h at À25 1C. After cooling, those
Yield = 0.163 g (88%). Mp: 168 1C. 1H NMR (400 MHz, CDCl3): d solutions were mixed and stirred for 24 h at ambient tempera-
= 0.98–1.14 (m, O–CH(CH3)2, 12 H), 1.15–1.22 (m, CH(CH3)2, ture. Subsequently, the mixture was evaporated to dryness to
24 H), 3.26–3.40 (m, CH(CH3)2, 4 H), 4.56–4.61 (m, O–CH(CH3)2, afford a yellow residue. The residue was crystallized from dry
2 H), 7.05–7.74 (m, Ar–H, 10 H), 8.03 (s, CHQN, 2 H). 13C NMR toluene. Yield = 0.130 g (92%). Mp: 168 1C. 1H NMR (400 MHz,
(100 MHz, CDCl3): d = 21.60 (Ar–CH3), 24.66 (O–CH(CH3)2), CDCl3): d = 0.99–1.27 (m, O–CH(CH3)2, 12 H), 1.27–1.35
25.13 (CH(CH3)2), 25.99 (CH(CH3)2), 27.77 (CH(CH3)2), 27.99 (m, CH(CH3)2, 24 H), 2.95–3.01 (m, CH(CH3)2, 4 H), 3.52–3.60
(CH(CH3)2), 78.62 (O–CH(CH3)2), 123.28 (Ar–C), 123.75 (Ar–C), (m, O–CH(CH3)2, 2 H), 7.17–7.74 (m, Ar–H, 10 H), 8.03
125.43 (Ar–C), 126.05 (Ar–C), 126.38 (Ar–C), 128.36 (Ar–C), (s, CHQN, 2 H). 13C NMR (100 MHz, CDCl3): d = 21.60 (Ar–CH3),
129.16 (Ar–CH3), 134.43 (Ar–CH3), 139.56 (Ar–C), 140.41 23.42 (O–CH(CH3)2), 25.37 (CH(CH3)2), 26.60 (CH(CH3)2),
(Ar–C), 150.51 (Ar–C), 161.83 (Ar–O), 165.97 (CHQN). ESI† m/z 27.58 (CH(CH3)2), 28.01 (CH(CH3)2), 70.69 (O–CH(CH3)2), 122.16
calculated for [M]+. C44H54Br4N2O4Ti: 1042.394 found 1042.182. (Ar–CH3), 123.59 (Ar–C), 123.77 (Ar–C), 124.44 (Ar–C), 125.44
Anal. calc. for C44H54Br4N2O4Ti: C 50.70, H 5.22, N 2.69. Found: (Ar–C), 127.06 (Ar–C), 128.37 (Ar–C), 129.18 (Ar–CH3), 136.29
C 50.77, H 5.15, N 2.78.
(Ar–C), 140.85(Ar–C), 141.60 (Ar–C), 161.16 (Ar–O), 171.72 (CHQN).
Compound 4. In an argon filled glove box, a solution of ESI† m/z calculated for [M]+. C44H54Br4N2O4Zr: 1085.751 found
Zr(O–iPr)4 (HO–iPr) (0.050 g, 0.129 mmol) in 5 mL of dry 1085.374. Anal. calc. For C44H54Br4N2O4Zr: C 48.67, H 5.01, N 2.58.
toluene and L1 ligand (0.102 g, 0.257 mmol) in 10 mL of dry Found: C 48.59, H 5.10, N 2.51.
toluene were heated at 100 1C for 24 h. After 24 h, the mixture
Compound 7. In an argon filled glove box, a solution of
was evaporated to dryness to afford yellow residue. Hf(O–tBu)4 (0.050 g, 0.106 mmol) in 5 mL of dry toluene and
a
The residue was purified from dry toluene. Yield = 0.102 g ligand L1 (0.041 g, 0.106 mmol) in 10 mL of dry toluene were
1
(80%). Mp: 165 1C. H NMR (400 MHz, CDCl3): d = 1.20–1.29 cooled for 6 h at À25 1C. After cooling, these solutions were mixed
(m, O–CH(CH3)2, 12 H), 1.31–1.41 (m, CH(CH3)2, 24 H), 1.52 and stirred for 24 h at ambient temperature. Subsequently, the
(s, C(CH3)3, 18 H), 1.53 (s, C(CH3)3, 18 H), 2.99–3.06 mixture was evaporated to dryness to afford a greenish-yellow
(m, CH(CH3)2,
4 H), 4.34–4.41 (m, O–CH(CH3)2, 2 H), residue. The residue was crystallized from dry toluene. Yield =
7.18–7.54 (m, Ar–H, 10 H), 8.32 (s, CHQN, 2 H). 13C NMR 0.074 g (88%). Mp: 168 1C. H NMR (400 MHz, CDCl3): d = 1.14
(100 MHz, CDCl3): d = 22.59 (Ar–CH3), 23.77 (O–CH(CH3)2), (s, O–C(CH3)3, 27 H), 1.20–1.37 (m, CH(CH3)2, 12 H), 1.52
26.07 (CH(CH3)2), 26.67 (CH(CH3)2), 27.00 (CH(CH3)2), 28.17 (s, C(CH3)3, 9 H), 1.57 (s, C(CH3)3, 9 H), 2.37 (Ar–CH3), 3.03–3.09
(CH(CH3)2), 31.62 (C(CH3)3), 31.81 (C(CH3)3), 34.35 (C(CH3)3), (m, CH(CH3)2, 2 H), 6.94 (s, Ar–H, 1 H), 7.18–7.29 (m, Ar–H, 3 H),
35.34 (C(CH3)3), 71.65 (O–CH(CH3)2), 123.30 (Ar–C), 125.36 7.55 (s, Ar–H, 1 H), 8.15 (s, CHQN, 1 H). 13C NMR (100 MHz,
(Ar–C), 125.45 (Ar–C), 126.80 (Ar–C), 128.25 (Ar–C), 128.37 CDCl3): d = 22.60 (Ar–CH3), 23.09 (O–C(CH3)3), 25.31(CH(CH3)2),
(Ar–C), 129.18 (Ar–CH3), 137.29 (Ar–C), 139.01 (Ar–CH3), 28.37 (CH(CH3)2), 31.63 (C(CH3)3), 31.75 (C(CH3)3), 34.15
140.60 (Ar–C), 146.25 (Ar–C), 158.59 (Ar–O), 167.66 (CHQN). (C(CH3)3), 35.60 (C(CH3)3), 75.47 (O–C(CH3)3), 123.76 (Ar–C),
ESI† m/z calculated for [M + H]+. C60H90N2O4Zr: 992.595 found 125.46 (Ar–C), 125.92 (Ar–C), 128.38 (Ar–C), 128.59 (Ar–C), 129.19
993.267. Anal. calc. for C60H90N2O4Zr: C 72.46, H 9.12, N 2.82. (Ar–CH3), 131.23(Ar–C), 139.17 (Ar–CH3), 140.07(Ar–C), 140.60
1
Found: C 72.34, H 9.23, N 2.89.
(Ar–C), 143.55 (Ar–C), 154.89 (Ar–O), 164.46 (CHQN). ESI† m/z
Compound 5. In an argon filled glove box, a solution of calculated for [M]+. C39H65NO4Hf: 790.428 found 790.300. Anal.
Zr(O–iPr)4 (HO–iPr) (0.050 g, 0.129 mmol) in 5 mL of dry calc. For C39H65NO4Hf: C 59.26, H 8.29, N 1.77. Found: C 59.37, H
toluene and L2 ligand (0.075 g, 0.257 mmol) in 10 mL of dry 8.23, N 1.85.
toluene were mixed and heated at 100 1C for 24 h. After 24 h,
Compound 8. In an argon filled glove box, a solution of
the mixture was evaporated to dryness to afford a yellow Hf(O–tBu)4 (0.050 g, 0.106 mmol) in 5 mL of dry toluene and L2
residue. The residue was purified from dry toluene. Yield = ligand (0.031 g, 0.106 mmol) in 10 mL of dry toluene were
0.083 g (82%). Mp: 157 1C. 1H NMR (400 MHz, CDCl3): d = 1.18– mixed and heated at 100 1C for 24 h. After 24 h, the mixture was
1.36 (m, O–CH(CH3)2, 12 H), 1.40 (s, C(CH3)3, 18 H), 1.45 evaporated to dryness to afford a yellow residue. The residue
(s, C(CH3)3, 18 H), 1.48 (s, C(CH3)3, 18 H), 4.24–4.51 was purified from dry toluene. Yield = 0.057 g (79%). Mp: 160 1C.
(m, O–CH(CH3)2, 2 H), 7.10 (s, Ar–H, 2 H), 7.36 (s, Ar–H, 10 H), 1H NMR (400 MHz, CDCl3): d = 1.39 (s, O–C(CH3)3, 27 H),
c
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1.51 (s, C(CH3)3, 9 H), 1.59 (s, C(CH3)3, 9 H), 1.66 (s, C(CH3)3, 5 g L-LA or rac-LA (34.7 mmol) (867.2 mmol of benzyl alcohol
9 H), 7.15 (s, Ar–H, 1 H), 7.42 (s, Ar–H, 1 H), 8.48 (s, CHQN, 1 H). was added wherever benzyl alcohol was used during LA poly-
13C NMR (100 MHz, CDCl3): d = 29.85 (O–C(CH3)3), 31.18 merization) and for CL polymerization, 236.6 mmol of catalyst
(C(CH3)3), 31.67 (C(CH3)3), 33.56 (C(CH3)3), 34.28 (C(CH3)3), and 5.4 g CL (5 mL, 47.3 mmol) (1183 mmol of benzyl alcohol
35.16 (C(CH3)3), 60.85 (N–C(CH3)3), 75.30 (O–C(CH3)3), 122.59 was added wherever benzyl alcohol was used during CL poly-
(Ar–CH3), 125.99 (Ar–C), 126.65 (Ar–C), 129.03 (Ar–CH3), 129.13 merization) in 20 mL of toluene. The reaction mixture was
(Ar–CH3), 139.06 (Ar–C), 139.71 (Ar–C), 158.91 (Ar–O), 160.79 charged into a 100 mL stainless steel autoclave reactor with
(CHQN). ESI† m/z calculated for [M + Na]+. C31H57NO4Hf: mechanical stirring under an inert atmosphere. The autoclave
686.279 found 709.235. Anal. calc. for C31H57NO4Hf: C 54.25, was heated up to 80 1C and stirred for 12 h. The contents were
H 8.37, N 2.04. Found: C 54.36, H 8.25, N 2.10.
quenched by pouring the material into cold heptane in the case
Compound 9. In an argon filled glove box, a solution of of CL and cold methanol in the case of L-LA and rac-LA after
Hf(O–tBu)4 (0.050 g, 0.129 mmol) in 5 mL of dry toluene and cooling it to ambient temperature. The polymer was collected
ligand L3 (0.046 g, 0.106 mmol) in 10 mL of dry toluene were by filtration. The filtered material was dried in vacuum con-
cooled for 6 h at À25 1C. After cooling, these solutions were mixed tinuously to attain a constant weight.
and stirred for 24 h at ambient temperature. Subsequently, the
2.5 General procedure for ethylene and propylene
polymerization
mixture was evaporated to dryness to afford a yellow residue. The
residue was crystallized from dry toluene. Yield = 0.080 g (90%).
Mp: 172 1C. 1H NMR (400 MHz, CDCl3): d = 1.49–1.79 The polymerizations were performed in a 100 mL stainless steel
(m, CH(CH3)2, 12 H), 1.80 (m, (O–C(CH3)3), 27 H), 3.63–3.69 autoclave reactor with mechanical stirring under an argon atmo-
(m, CH(CH3)2, 2 H), 7.72–7.88 (m, Ar–H, 10 H), 8.48 (s, CHQN, sphere. The container was charged with an argon atmosphere
1 H). 13C NMR (100 MHz, CDCl3): d = 23.00 (O–C(CH3)3), with 50 mg of catalyst, 45 mL of toluene along with the required
28.02 (CH(CH3)2), 28.40 (CH(CH3)2), 32.48 (CH(CH3)2), 75.68 amount of MAO. Consequently, the autoclave was heated up to
(O–C(CH3)3), 122.67 (Ar–C), 123.65 (Ar–C), 123.84 (Ar–C), 126.52 80 1C and gas was continuously bubbled through the proper
(Ar–C), 129.19 (Ar–CH3), 135.23 (Ar–C), 137.45 (Ar–C), 140.14 channel. The gas feed was passed for 30 min at a pressure of 8
(Ar–C), 140.56 (Ar–C), 151.52 (Ar–C), 159.48 (Ar–O), 170.63 atm and subsequently the polymerization was quenched with
(CHQN). ESI† m/z calculated for [M]+. C31H47Br2NO4Hf: 836.007 acidic methanol. The polymer produced was collected by filtra-
found 836.389. Anal. Calc. For C31H47Br2NO4Hf: C 44.54, H 5.67, tion and dried until a constant weight was achieved.
N 1.68. Found: C 44.48, H 5.73, N 1.63.
2.6 X-ray structure determination of compounds 3, 6 and 7
2.4 General procedure for the bulk polymerization of rac-LA,
L-LA, CL, VL and rac-BL
Amongst all the compounds synthesized in this study, suitable
crystals for X-ray diffraction studies were obtained from 3, 6 and 7.
These polymerizations were performed under bulk solvent-free Single crystals were grown in a glove box at À25 1C from diluted
condition. For L-LA and rac-LA polymerization, 173.4 mmol of toluene and acetonitrile (few drops) solutions of the respective
catalyst and 5 g L-LA or rac-LA (34.7 mmol) (867.2 mmol of benzyl compounds over a period of 10 days. X-ray data was collected with
alcohol was added wherever benzyl alcohol was used during LA a Bruker AXS (Kappa Apex 2) CCD diffractometer equipped with a
polymerization) and for CL polymerization, 236.6 mmol of catalyst graphite monochromated Mo (Ka) (l = 0.7107 Å) radiation source.
and 5.4 g CL (5 mL, 47.3 mmol) (1183 mmol of benzyl alcohol was The data were collected with 100% completeness for y up to 251. o
added wherever benzyl alcohol was used during CL polymerization) and f scans were employed to collect the data. The frame width for
were used during the polymerization. The contents were taken into o was fixed to 0.51 for data collection. The frames were subjected
a 100 mL stainless steel autoclave reactor with mechanical stirring to integration and data were reduced for Lorentz and polarization
under an argon atmosphere. The steel autoclave was heated to corrections using SAINT-NT. The multi-scan absorption correction
110 1C in the case of CL polymerization and 130 1C in the case of was applied to the data set. All structures were solved using SIR-92
LA. The contents were rapidly stirred. The progress of polymeriza- and the refinement was done using SHELXL-97.39 Location of all
tion was monitored by periodically recording the 1H NMR spectra the hydrogen atoms could be found in the difference Fourier map.
of the reaction mixture. The polymerization was quenched by The hydrogen atoms attached to carbon atoms were fixed at
gradually cooling the autoclave to ambient temperature in almost chemically meaningful positions and were allowed to ride with
30 min and pouring the contents into cold heptane in the case of the parent atom during refinement. Compound 7 has four mole-
CL and cold methanol in the case of L-LA and rac-LA. The polymer cules in the asymmetric unit, which gave rise to the doubt that this
formed was collected by filtration. The filtered product was dried in might be due to the doubling of any axis due to pseudo-transla-
a vacuum until a constant weight was achieved. For VL and rac-BL, tions. PLATON was used to confirm it’s non-existence.
the same procedure and workup were followed.
3 Results and discussion
2.5 General procedure for solution polymerization of rac-LA,
L-LA and CL
3.1 Synthesis and spectral characterization of compounds
The solution polymerization was done in dry toluene. These All the complexes 1–9 were synthesized by reacting various
polymerizations were done by taking 173.4 mmol of catalyst and imino phenol ligands, L1–L3 with group 4 metal alkoxides in
c
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Scheme 1 Imino phenoxide complexes of group 4 metals.
toluene in 1 : 1 and 2 : 1 stoichiometric ratios (Scheme 1). The state and the corresponding metal atom is present in a
products formed were completely purified by crystallization distorted octahedral geometry. The crystal data is shown in
from toluene at À25 1C and isolated in high yield and purity Table 1 for 3, 6 and 7. Compound 3 has a crystallographically
after removing the solvent under reduced pressure. For L1 and imposed twofold symmetry with the Ti1 atom on the twofold
L3 the reactions went to completion at room temperature. axis. Moreover, the two coordinated N centers are trans to each
However, in the case of L2, the reaction was not complete at other and the alkoxide groups are cis to each other. All bond
room temperature as understood from spectroscopic analysis distances are identical in the case of Ti and the difference is
of the crude reaction mixture. The reaction mixture was heated marginal for Zr. On the other hand, for 7, an acetonitrile
up to 100 1C for completion. We have tried the reactions in a molecule has coordinated to the metal center in order to make
1 : 1 stoichiometric ratio for all the cases, but we observed that it octahedral. The Hf–O (alkoxide) bond lengths are almost the
the unreacted metal alkoxides remained along with the same and shorter than the bond distance of the Hf–O (phenolic)
expected product involving Ti and Zr, as per the conclusion moiety. Both Hf–N bond distances are also almost identical.
drawn from 1H NMR of the crude. Whereas we have not
From Fig. 1 and 2 it is clear that the trans-N, cis-O and cis-
observed any unreacted metal alcoholate signal in the OiPr isomer is predominant in the solid state. This isomer is
1H NMR spectrum for Hf. However, an unreacted phenolic predominant because the steric repulsion between the isopro-
–OH signal was found in all reactions with Hf in 1 : 2 stoichio- pyl groups contained as part of the ligand is minimum. This is
metric ratios even after heating at 100 1C. On the other hand, different to the observations gathered from analogous halide
the unreacted phenolic –OH signal is completely absent in case containing derivatives. Contributions from the research group
of Ti or Zr. All these derivatives were completely characterized of Fujita clearly indicate the possibility of the presence of six
using various spectroscopic techniques and their purity was different octahedral isomers. This was ascertained on the basis
assured by the proper elemental analysis results. Electrospray of the appearance of different signals for the imine moieties in
ionization mass spectrometry (ESI-MS) spectra reveal that these the 1H NMR at different temperatures.35 In order to under-
compounds have a mononuclear structure.
stand, we performed variable temperature 1H NMR and 13C NMR
studies employing compounds 1 and 4, respectively (see ESI,†
Fig. S28–S31). To our surprise we did not find any significant
3.2 Single crystal X-ray diffraction studies
From single crystal X-ray diffraction studies, it is clear that changes in the NMR results at different temperatures, implying
compounds 3, 6 and 7 (Fig. 1–3) are monomeric in the solid that this isomer is predominant in solution.
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Fig. 3 Molecular structure of 7; thermal ellipsoids were drawn at 30% prob-
ability level. Selected bond lengths (Å) and angles (1): Hf(04)–O(4c), 1.923(3);
Hf(04)–O(2c), 1.948(3); Hf(04)–O(3c), 1.952(4); Hf(04)–O(1c), 2.068(3); Hf(04)–
N(1c), 2.450(4); Hf(04)–N(2c), 2.430(4); O(4c)–Hf(4)–O(2c), 99.75(16); O(4c)–
Hf(4)–O(3c), 102.25(16); O(3c)–Hf(4)–O(2c), 100.95(17); O(4c)–Hf(4)–O(1c),
95.23(14); O(4c)–Hf(4)–N(2c), 81.90(16); N(2c)–Hf(4)–N(1c), 81.11(13). One of
the four independent molecules in the asymmetric unit is depicted.
Fig. 1 Molecular structure of 3; thermal ellipsoids were drawn at 30% probability
level. Selected bond lengths (Å) and angles (1): Ti(1)–O(1), 2.000(3); Ti(1)–O(3),
1.790(3); Ti(1)–N(1), 2.242(3); Ti(1)–O(1)_1, 2.000(3); Ti(1)–O(3)_1, 1.790(3);
Ti(1)–N(1)_1, 2.242(3); O(3)–Ti(1)–O(1), 168.32(12); O(3)–Ti(1)–O(3)_1, 99.66(18);
O(3)_1–Ti(1)–O(1), 90.18(12); O(3)–Ti(1)–O(1)_1, 90.18(12); N(1)_1–Ti(1)–O(1)_1,
83.33(11); N(1)–Ti(1)–N(1)_1, 169.28(18). The superscript i in the atom labels
indicates that these atoms are at an equivalent position (2 À x, y, 1/2 À z).
temperature resulted in the formation of low molecular weight
oligomers. For cyclic ester polymerizations, the temperature
was set at 80 1C and for LA, the polymerizations were done at
130 1C. The polymerization results are summarized in Table 2.
From Table 2, it can be easily inferred that the Mn value is
highest for zirconium catalysts followed by the titanium
catalyst and finally the hafnium catalyst. In addition, all Mn
values are much higher than the theoretical value, indicating
slow initiation during the polymerization with MWDs between
1.11–1.39. We observed very high Mn in the cases of CL and LA.
The earlier report of imino phenoxide complexes of Ti and Zr had
shown good reactivity with greater degree of stereoselectivity.32c In
these cases, solution polymerization of rac-LA produced moderate
Mn with well controlled MWDs (1.02–1.08). However, MWDs were
much broader in bulk polymerization of rac-LA. Screening all
these results we can say that our results are better than these
reports and literature reports based on group 4 metals.31c,d,m,r,32h
Again, we have extended our study by doing solution poly-
merization for rac-LA, L-LA and CL in toluene using these
catalysts (see ESI†). It is clear that there is a close proximity
between observed Mn and theoretical Mn value and MWDs are
extremely narrow (1.02–1.09) for the polymers concerned. In
these solution polymerizations, we were able to control Mn and
MWDs in an efficient manner.
Fig. 2 Molecular structure of 6; thermal ellipsoids were drawn at 30% prob-
ability level. Selected bond lengths (Å) and angles (1): Zr(01)–O(1), 2.107(3);
Zr(01)–O(2), 2.099(3); Zr(01)–O(3), 1.925(3); Zr(01)–O(4), 1.918(3); Zr(01)–N(1),
2.378(4); Zr(01)–N(2), 2.378(4); O(4)–Zr(01)–O(3), 99.41(13); O(4)–Zr(01)–O(2),
92.65(13); O(2)–Zr(01)–O(3), 161.56(12); O(2)–Zr(01)–N(1), 89.65(12); O(1)–
Zr(01)–N(1), 77.17(12); N(2)–Zr(01)–N(1), 165.95(12).
3.3 Polymerization characteristics and activities
We wanted to test the potential of 1–9 as catalysts towards the
To see the effect of a co-initiator like benzyl alcohol, we have
ROP of L-LA, rac-LA, e-CL, d-VL and rac-BL. Our prime intention performed polymerizations of rac-LA, L-LA and CL using 1–9 as
was to see the effect of one imine group in the ligand upon the catalysts in the presence of benzyl alcohol (BnOH) (Table 3).
polymerization. Here, complexes 1–9 have shown high catalytic Here, we have found that Mn values of the respective polymers
activity towards the polymerization of these monomers. All the are higher compared to the theoretical value expected. This
polymerizations were done under solvent free conditions first. actually reveals that we were not able to control the Mn when
In this regard, we have performed all the polymerization in a the polymerizations were done in the presence of benzyl
200 : 1 monomer to catalyst ratio. Polymerization at ambient alcohol. MWDs (1.10–1.17) are controlled in a much better
c
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Table 1 Crystal data for the structures 3, 6 and 7
Compound
3
6
7
Empirical formula
Formula weight
Crystal system
Space group
Temp/K
Wavelength (Å)
a (Å)
b (Å)
c (Å)
a (1)
b (1)
C48H60Br4N4O4Ti
1124.54
Monoclinic
C2/c
173(2)
0.71073
24.4152(13)
13.1350(6)
18.3821(10)
90
116.306(3)
90
5284.5(5)
4
1.413
C51H62Br4N2O4Zr
1177.89
Monoclinic
P2(1)/n
173(2)
0.71073
21.9206(11)
10.1128(5)
23.1925(12)
90
96.119(2)
90
5112.0(4)
4
1.530
C41H68N2O4Hf
831.46
Triclinic
P1
173(2)
0.71073
16.4326(9)
21.7830(11)
25.0841(13)
88.239(2)
85.703(2)
79.019(2)
8788.5(8)
8
g (1)
V (Å3)
Z
Dcalc (g cmÀ3
)
1.257
Reflns collected
No. of indep reflns
GOF
22480
4878
1.052
28555
9436
1.007
118536
42436
1.074
R1 = 0.0474, wR2 = 0.1171
R1 = 0.0741, wR2 = 0.1360
Final R indices (I 4 2s(I))a
R1 = 0.0458, wR2 = 0.1233
R1 = 0.0804, wR2 = 0.1408
P
R1 = 0.0453, wR2 = 0.1042
R1 = 0.0864, wR2 = 0.1211
R indices (all data)a
P
P
P
a
2
R1
=
|F0| À |Fc|/ |F0|, wR2 = [ (F0 À Fc2)2/ w(F02)2]1/2
.
fashion than without co-initiator in these polymerizations. done at 130 1C. A plot of % conversion of rac-LA against time
However, in the presence of toluene, the same polymerization (see ESI,† Fig. S42) produces a sigmoidal curve. This disclosed
results depict a close correlation between observed Mn and that the polymer formation was at a very high rate initially and
theoretical Mn with narrow MWDs (1.10–1.14) (see ESI†). It was after some time conversion rate was almost constant. From the
independently verified that BnOH reacts with 4 at the respective kinetic experiment results, it was ascertained that there is a
temperatures of the various polymerizations to yield the benzyl- first-order dependence of the rate of polymerization on rac-LA
oxide compound, which is the true catalyst (see ESI,† Fig. S45 concentration without any induction period. The plot of
and S46).
ln([rac-LA]o/[rac-LA]t) vs. time was found to be linear (Fig. 5).
We were very curious to study the variation of Mn and MWDs The values of the apparent rate constant (kapp) for rac-LA
with increasing [rac-LA]/[C] ratio using 3, 6 and 9. In these polymerization catalyzed by 3, 6 and 9 were evaluated from
cases, we have observed that with increase of [rac-LA]/[C] ratio, the slope of these straight lines and were found to be 33.82 Â
Mn increased continuously and appreciably. A plot of molecular 10À2 minÀ1, 24.14 Â 10À2 minÀ1 and 18.28 Â 10À2 minÀ1
,
weight (Mn) vs. [M]o/[C]o ratio depicts that Mn is rising steeply respectively. From the rate contants of polymerization, it can be
with the variation of ratios. The MWDs (Mw/Mn) remain almost concluded that the rate is fastest for Ti followed by Zr and then
invariant with increasing ratios (Fig. 4).
Hf. This is justified by the time taken for the polymerization.
Again, a plot of Mn against % conversion of monomer (see
ESI†) shows a sharp increment of Mn with increasing conver- 3.5 Polymerization mechanism
sion of monomer.
Low molecular weight oligomers of rac-LA were synthesized by
Another interesting observation is that the PHB sample from
entry 30 (Table 2) showed a very strong contribution from the
r-centered diad (d = 169.32 ppm) indicating syndiotacticity. The
methylene region showed one prominent signal (d = 40.80 ppm)
corresponding to the rr triad. On the other hand, the polymer-
ization of rac-LA leads to the high Mn polymer with predomi-
nantly heterotactic enchainment and not much difference in
heterotacticity was observed when the polymerizations were
done in the absence or presence of benzyl alcohol.
stirring rac-LA with 6 in 10 : 1 molar ratio under solvent free
conditions at 130 1C. The product was extracted with heptane.
After removal of heptane, the residue was examined thoroughly
1
using MALDI-TOF and H NMR techniques. The predominant
product comprised of the ligand fragment as one of the end
terminal groups in the oligomer chain, as determined through
the analysis of MALDI-TOF and 1H NMR spectra. The oligomer
did not contain any proportions of the corresponding product
initiated by the isopropoxide fragment. The same observations
were seen using the Hf catalyst 7. In addition to the above,
peaks corresponding to intramolecular transesterification pro-
ducts were seen (see ESI,† Fig. S35–S38). When the same
reaction was done in the presence of benzyl alcohol in
10 : 1 : 2 using 4, the resultant polymer had the OBn moiety as
It must be noted that the structures of the Hf compounds (7–9)
is different from the others. Hence, it is not reasonable to compare
the polymerization activity of these with the others.
3.4 Kinetics of polymerization
In the next part of our study, we have performed the kinetics of one of the end groups. This was ascertained using MALDI-TOF
1
rac-LA polymerization using 3, 6 and 9. The kinetic studies for and H NMR spectra. Besides there were peaks corresponding
the polymerization of rac-LA in ratio [rac-LA]o/[C]o = 200 were to the ligand initiated oligomer along with products of
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Table 2 Polymerization data for L-LA, rac-LA, CL, VL and rac-BL using 1–9 in
Table 3 Polymerization data for rac-LA, L-LA and CL using 1–9 in the presence of
benzyl alcohol in 200 : 1 : 5 ratio at 80 1C for CL and 130 1C for LA
200 : 1 ratio
T/ Yield Timea/ Mn
/
Mw/
Entry Initiatora Monomer Time (min) Monbs b/kg molÀ1 Mw/Mn Prc
b
Entry Initiator Monomer 1C (%)
min
kg molÀ1 Mn Prc
1
2
3
4
5
6
7
8
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
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
CL
CL
CL
CL
CL
CL
CL
CL
CL
10
15
9
8
9.5
7
8.5
11
12
5
7.5
6.5
8
8.5
5.5
12
13
11.5
4
8
6
14.56
13.20
16.34
17.45
18.98
19.56
17.88
15.31
17.22
15.55
14.85
12.11
17.76
15.97
15.35
14.20
12.19
15.49
10.32
12.78
12.36
14.54
13.25
15.21
13.65
12.40
11.51
1.13
1.14
1.12
1.16
1.11
1.15
1.13
1.14
1.11
1.10
1.12
1.11
1.14
1.12
1.13
1.14
1.13
1.14
1.11
1.13
1.14
1.12
1.14
1.15
1.17
1.15
1.16
0.69
0.70
0.71
0.73
0.72
0.70
0.68
0.69
0.70
1
2
3
4
5
6
7
8
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
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
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
CL
CL
CL
CL
CL
CL
CL
CL
CL
130 98
130 99
130 99
130 98
130 98
130 97
130 99
130 99
130 99
130 99
130 98
130 98
130 96
130 96
130 97
130 98
130 97
130 97
80 97
80 99
80 99
80 98
80 99
80 98
80 99
80 94
80 99
80 96
80 98
80 97
80 97
80 98
80 95
80 98
80 98
80 96
80 95
80 97
80 96
80 96
80 97
80 96
80 98
80 97
80 98
12
17
14
13
18.5
18
15
21
29
11
15
12.5
15
17
15
22
28
28
8
13
10
10
16.5
12.5
16
19
17.5
6
38.55
40.55
41.62
88.22
77.54
82.52
42.22
40.96
41.44
42.81
38.33
40.22
92.61
76.42
81.62
40.26
38.55
38.55
68.27
64.11
67.77
76.82
70.81
74.98
47.62
44.29
45.26
56.72
53.99
55.34
60.44
55.81
57.22
43.32
39.67
42.52
33.01
29.01
31.02
35.92
31.39
33.12
30.21
27.71
29.65
1.13 0.70
1.16 0.69
1.11 0.71
1.12 0.72
1.14 0.68
1.12 0.73
1.18 0.68
1.22 0.70
1.18 0.69
1.12
1.14
1.11
1.11
1.15
1.13
1.20
1.21
1.21
1.33
1.29
1.31
1.36
1.34
1.34
1.39
1.37
1.35
1.35
1.28
1.26
1.31
1.30
1.23
1.37
1.32
1.31
1.29
1.32
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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
40
41
42
43
44
45
5
9
4.5
10
12
11
a
Time of polymerization measured by quenching the polymerization
VL
VL
VL
VL
VL
VL
VL
VL
b
reaction when all monomer was found consumed. Measured by GPC
11
at 27 1C in THF relative to polystyrene standards with Mark–Houwink
corrections for Mn for LA polymerization; Mn (theoretical) at 100%
9.5
8.5
12
11
10
14
12.5
14
20.5
18
17
conversion = [M]o/[C]o  mol wt (monomer) + 108.14; Mn (theoretical)LA
=
c
5.87 kg molÀ1 and Mn (theoretical)CL = 4.67 kg molÀ1
homonuclear decoupled 1H NMR spectrum.
. Calculated from
VL
rac-BL
rac-BL
rac-BL
rac-BL
rac-BL
rac-BL
rac-BL
rac-BL
rac-BL
1.36
1.24
1.27
1.36
1.29
1.21
1.24
24
21
22
27
24
a
Time of polymerization measured by quenching the polymerization
b
reaction when all monomer was found consumed. Measured by GPC
at 27 1C in THF relative to polystyrene standards with Mark–Houwink
c
corrections for Mn for LA polymerization. Calculated from homo-
nuclear decoupled 1H NMR spectrum.
intramolecular transesterification. This was understood from
the MALDI-TOF spectrum. In an independent study, it was seen
that compound 4 reacts with BnOH to produce the corres-
ponding OBn product. This initiates the polymerization (see
ESI,† Fig. S39 and S40).
Fig. 4 Plot of Mn and Mw/Mn vs. [M]o/[C]o for rac-LA polymerization at 130 1C
using 3, 6 and 9.
Table 4, it is clear that molecular weight is highest for Zr
catalysts and lowest for Hf catalysts. Ti catalysts show inter-
3.6 Ethylene polymerization
After achieving good results with cyclic esters and LA, we mediate activity. The activity followed the order Zr 4 Ti 4 Hf.
investigated the polymerization of ethylene. Interestingly, here In all the cases, we have achieved good polymer yield. In most
we found that compounds 1–9, in the presence of MAO, are cases our activities are lower than Fujita’s catalysts.35a
potent catalysts towards the polymerization of ethylene. These
These polymerizations were carried out in different solvents
polymerizations were performed at 80 1C in toluene. From to investigate the effect on activity of these catalysts.
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Fig. 5 Semi-logarithmic plots of rac-LA conversion in time initiated by 3, 6 and
9: [rac-LA]o/[C]o = 200 at 130 1C.
Fig. 7 Plot of activity vs [MAO]/[C] ratio for 1,
4 and 7 for ethylene
polymerization.
Table 4 Polymerization data for ethylene using 1–9 along with MAO
Entry Catalysta Ab Yieldc (g) Mw (kg molÀ1) Mn (kg molÀ1) Mw/Mn
Table 5 Polymerization data for propylene using 1–9 along with MAO
Yieldc Mw
Mn
Mw/
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
5.65 1.54
5.31 1.79
5.42 1.30
5.85 1.55
5.43 1.83
6.07 1.40
3.80 1.20
4.09 1.49
3.71 1.11
109.72
100.13
110.67
117.88
110.59
111.81
109.66
94.82
61.30
55.02
59.50
64.42
59.14
60.44
56.82
48.38
51.50
1.79
1.82
1.86
1.83
1.87
1.85
1.93
1.96
1.91
Entry Catalysta Activityb (g) (kg molÀ1) (kg molÀ1) Mn % mmmm
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
4.66
3.41
4.58
4.96
4.24
4.95
3.98
3.21
3.74
1.23 66.78
1.15 62.43
1.10 61.56
1.25 71.38
1.43 61.57
1.14 61.21
1.26 56.72
1.17 54.79
1.12 52.56
40.23
38.78
39.21
42.24
37.54
39.49
34.80
35.12
34.35
1.65 14.2
1.61 13.1
1.57 16.9
1.69 20.1
1.64 18.4
1.55 20.3
1.63 12.9
1.56 11.7
1.53 13.6
98.36
a
All experiments were performed in toluene at MAO : catalyst ratio =
1000, ethylene pressure = 8 atm, 80 1C for 30 min, catalyst = 50 mg,
solvent = 45 mL. A = Activity in (g PE per mol cat  h)  104. g of PE
b
c
a
All experiments were performed in toluene at MAO : catalyst ratio =
obtained after 30 min.
1000, propylene pressure = 8 atm, 80 1C for 30 min, catalyst = 50 mg,
solvent = 45 mL. Activity in (g PP per mol cat  h)  104. g of PP
b
c
From Fig. 6, we can conclude with ease that toluene is the best
solvent for doing these polymerizations since the activity of 4 is
highest in toluene.
Again, when we varied the [MAO]/[C] ratio from 500 to 4000
for 1, 4 and 7, we observed that a [MAO]/[C] ratio of 1000 is
optimum for performing these polymerizations, as other ratios
led to reduction of the activity value of these catalysts (Fig. 7).
obtained after 30 min.
propylene. Noticeably, here we have seen that compounds 1–9
are quite active catalysts towards the polymerization of propy-
lene using MAO as a co-catalyst. These polymerizations were
done at 80 1C in toluene. Table 5 depicts that Mn value is
highest for Zr cases and lowest for Hf systems. Here, polymer
yield was found to be moderate.
3.7 Propylene polymerization
Table 6 denotes the pentad distribution for selected samples
of polymer obtained using 4 along with different ratios of MAO.
Here, we have noticed that the percentage of mmmm pentad
distributions is reduced with increasing ratio of MAO. The
polymerization is not stereoselective and the polymer produced
is atactic.
Our next goal was to investigate the effectiveness of these
compounds as catalysts towards the polymerization of
Further, these polymerizations were performed in different
solvents to find out the effect on activity of these compounds.
We have inferred that toluene is the preferred solvent for
carrying out these polymerizations as the activity of 4 is much
greater in toluene in comparison with other solvents (see ESI,†
Fig. S32).
We wanted to see the effect on activity upon the variation of
[MAO]/[C] ratio from 500 to 4000 for 1, 4 and 7. A plot of activity
vs [MAO]/[C] ratio clearly reveals that the 1000 ratio is the best
option in order to perform propylene polymerization. (see ESI,†
Fig. S33). Once again it may be noted that the activity of 7–9
Fig. 6 Activity of 4 in different solvent in ethylene polymerization.
c
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Table 6 Pentad distributions (in %) for selected polymers obtained in propylene polymerization catalyzed by complex 4
[MAO] /[4] ratio
[mmmm]
[mmmr]
[rmmr]
[mmrr]
[mmrm] + [rrmr]
[mrmr]
[rrrr]
[rrrm]
[mrrm]
500
1000
2000
21.2
20.1
18.7
14.2
15.3
13.1
6.9
7.4
8.2
11.9
14.6
15.8
12.3
13.7
9.8
9.1
6.7
9.3
7.7
9.5
8.3
9.3
8.2
5.9
5.6
4.9
10.1
4 G. W. Coates, J. Chem. Soc., Dalton Trans., 2002, 467–475.
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1–22.
6 R. E. Drumright, P. R. Gruber and D. E. Henton, Adv. Mater.,
2000, 12, 1841–1846.
may not be compared with the rest or Fujita’s work since their
structures are different.
The polymerization proceeds through the activation of the
catalyst using MAO as a result of abstraction of an alkoxide
moiety and proceeds through a cationic intermediate, which is
the true active species. Such abstraction of an alkoxide group by
MAO is well documented in the literature.40
7 G. W. Coates and M. A. Hillmyer, Macromolecules, 2009, 42,
7987–7989.
8 T. Biela, A. Kowalski, J. Libiszowski, A. Duda and S. Penczek,
Macromol. Symp., 2006, 47–55.
9 C. K. Williams and M. A. Hillmyer, Polym. Rev., 2008, 48,
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10 Y. Ikada and H. Tsuji, Macromol. Rapid Commun., 2000, 21,
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4 Conclusion
We have synthesized a complete library of new alkoxide com-
plexes containing group 4 metals and the imino phenol ligand.
These compounds have catalytic activity towards the polymer-
ization of different cyclic esters, LA, ethylene and propylene.
The zirconium catalysts were found to yield better polymeriza-
tion results in comparison to the titanium and hafnium analo-
gues. The presence of different substituents either on the aryl
ring or on different anilines present in the various ligands does
not affect the polymerizations in a significant manner. Good
control over MWDs and Mn were achieved when these poly-
merizations were performed in toluene. Using MAO as a co-
catalyst, we achieved good results in the ethylene and propylene
polymerizations.
The ligand moiety is responsible for initiating the ROP
instead of the isopropoxide groups, as understood from the
17 R. H. Platel, L. M. Hodgson and C. K. Williams, Polym. Rev.,
2008, 48, 11–63.
1
MALDI-TOF and H NMR spectra analysis. In the presence of
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20 C. M. Silvernail, L. J. Yao, L. M. R. Hill, M. A. Hillmyer and
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BnOH, the polymers contain the OBn moiety as one of the end
groups. The formed poly(3-hydroxy butyrate) is a syndiotactic
polymer as it clearly gave the corresponding 13C NMR signals.
Homonuclear decoupled spectra of the poly(lactic acid) formed
showed that the polymer was heterotactically enriched. Tacti-
city is not influenced in an appreciable manner with increasing
steric bulk of the complexes.
Acknowledgements
TKS thanks The Council of Scientific and Industrial Research
Delhi for a fellowship. This work was supported by the Depart-
ment of Science and Technology, New Delhi.
24 H. R. Kricheldorf, H. Hachmann-Thiessen and G. Schwartz,
Macromolecules, 2004, 34, 6340–6345.
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