2-[1-(2,6-Dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl] pyridyliron(II) dichlorides: Synthesis, characterization and ethylene polymerization behavior

Article abstract of DOI:10.1016/j.polymer.2012.02.050

A series of 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1- (arylimino)ethyl]pyridine ligands (L1-L5) as well as the ligand 2,6-bis[1-(2,6-dibenzhydryl-4-chloro-phenylimino)ethyl]pyridine (L6) were synthesized and reacted with FeCl₂·4H₂O to afford the iron(II) dichloride complexes [LFeCl₂] (Fe1-Fe6). All new compounds were fully characterized by elemental and spectroscopic analysis, and the molecular structures of the complexes Fe1, Fe2 and Fe4 were determined by single-crystal X-ray diffraction, which revealed a pseudo-square-pyramidal geometry at iron. Upon activation with either MAO or MMAO, all iron pre-catalysts exhibited very high activity in ethylene polymerization with good thermal stability. To the best of our knowledge, the current system showed the highest activity amongst iron bis(imino)pyridine pre-catalysts reported to-date. The polymerization parameters were explored to determine the optimum conditions for catalytic activity, which were typically found to be 2500 eq. Al to Fe at 60 °C in the presence of MMAO, and 80 °C in the presence of MAO. The resultant polyethylene possessed a narrow molecular polydispersity index (PDI) consistent with the formation of single-site active species.

Full text of DOI:10.1016/j.polymer.2012.02.050

Polymer 53 (2012) 1870e1880
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Polymer
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2-[1-(2,6-Dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]
pyridyliron(II) dichlorides: Synthesis, characterization and ethylene
polymerization behavior
,
Xiaoping Cao a, Fan He a b, Weizhen Zhao b, Zhengguo Cai c, Xiang Hao b, Takeshi Shiono c,
**
,
a,b,
*
Carl Redshaw d , Wen-Hua Sun
a State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
b Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
c Department of Applied Chemistry, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
d School of Chemistry, University of East Anglia, Norwich NR4 7TJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine ligands
(L1eL5) as well as the ligand 2,6-bis[1-(2,6-dibenzhydryl-4-chloro-phenylimino)ethyl]pyridine (L6)
were synthesized and reacted with FeCl2$4H2O to afford the iron(II) dichloride complexes [LFeCl2] (Fe1
eFe6). All new compounds were fully characterized by elemental and spectroscopic analysis, and the
molecular structures of the complexes Fe1, Fe2 and Fe4 were determined by single-crystal X-ray
diffraction, which revealed a pseudo-square-pyramidal geometry at iron. Upon activation with either
MAO or MMAO, all iron pre-catalysts exhibited very high activity in ethylene polymerization with good
thermal stability. To the best of our knowledge, the current system showed the highest activity amongst
iron bis(imino)pyridine pre-catalysts reported to-date. The polymerization parameters were explored to
determine the optimum conditions for catalytic activity, which were typically found to be 2500 eq. Al to
Fe at 60 ꢀC in the presence of MMAO, and 80 ꢀC in the presence of MAO. The resultant polyethylene
possessed a narrow molecular polydispersity index (PDI) consistent with the formation of single-site
active species.
Received 8 December 2011
Received in revised form
17 February 2012
Accepted 27 February 2012
Available online 3 March 2012
Keywords:
Ethylene polymerization
Linear polyethylene
Iron pre-catalysts
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
phenanthrolines [28], 2-(benzoxazolyl)-1,10-phenanthrolines [29],
2,8-bis(1-aryliminoethyl)quinolines [30], N-((pyridin-2-yl)methy-
The discovery of bis(imino)pyridyliron(II) chloride pre-catalysts
in ethylene polymerization was a relatively recent milestone within
polyolefin science [1,2]. Subsequently, extensive research has
focused on the derivatization of such pre-catalysts via the variation
of the substituents on the framework of the parent bis(imino)
pyridine ligand set [3e18]. Moreover, there have been a number of
investigations into new, but related iron pre-catalysts which
employ sp2-nitrogen-donating tridentate ligand sets such as 6-
benzimidazolyl-2-iminopyridines [19e21], 6-benzoxazolyl-2-
iminopyridines [22,23], 2-quinoxalinyl-6-iminopyridines [24], 2-
imino-1,10-phenanthrolines [25e27], 2-(2-benzimidazolyl)-1,10-
lene)-8-amino-quinolines [31], 8-(1H-benzimidazol-2-yl)quino-
lines [32], and a few reports involving bidentate ligands such as 8-
benzimidazolylquinoline [33,34] and 8-iminoquinaldine ligands
[35]. Although high catalytic activities have been achieved for most
of these iron pre-catalyst systems, as reported in a number of recent
review articles [36e42], the critical problems of catalyst deactiva-
tion together with the formation of low molecular weight products
at elevated reaction temperature, have not been overcome. Using
knowledge accumulated for the numerous models of iron pre-
catalysts, it should be possible to address these problematic
issues to enable such iron-based catalysts to be more suited for
industrial applications.
Ethylene polymerization is a highly exothermic reaction, and the
industrial process prefers the polymerization to be conducted at
temperatures between 60 and 90 ꢀC. Given this, the catalytic
system should remain highly active at such high reaction temper-
atures. Furthermore, it is desirable for the catalytic system to
produce polyethylene waxes with narrow PDIs, which are both very
* Corresponding author. Key Laboratory of Engineering Plastics, Beijing National
Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China. Tel.: þ86 10 62557955; fax: þ86 10 62618239.
** Corresponding author.
E-mail addresses: Carl.Redshaw@uea.ac.uk (C. Redshaw), whsun@iccas.ac.cn
(W.-H. Sun).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.02.050

X. Cao et al. / Polymer 53 (2012) 1870e1880
1871
useful and often of greater commercial value than the common
polyethylenes. Having these required catalytic features in mind,
there are only a limited number of papers relating to iron pre-
catalysts ligated by 2,8-bis(1-aryliminoethyl)quinolines [30], 8-
benzimidazolylquinolines [33,34] and unsymmetrical bis(imino)
pyridines bearing extremely bulky substituents [43e46]. However,
the extensive use of bis(imino)pyridines possessing bulky substit-
uents has not been widely explored, basically due to the rather
limited number of commercially available bulky anilines.
Encouraged by the success of the 2-[1-(2,6-dibenzhydryl-4-
methylphenylimino) ethyl]-6-[1-(arylimino)ethyl]pyridyliron(II)
pre-catalyst system [43], which, for this type of pre-catalyst, per-
formed with the highest observed activity in ethylene polymeri-
zation reported at that time (we note however that such high
activities are usually achieved in the presence of large amounts of
alkylaluminium co-catalyst), iron pre-catalysts bearing 2-[1-(2,6-
dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]
pyridines were synthesized and revealed higher catalytic activities
for ethylene polymerization [45]. Noting the enhanced catalytic
activities of iron pre-catalysts bearing ligands with electron-
withdrawing groups, 2,6-dibenzhydryl-4-chloroaniline was
prepared and was subsequently used to form a new family of 2-[1-
(2,6-dibenzhydryl-4-chloro-phenylimino)ethyl]-6-[1-(arylimino)
ethyl]pyridine ligands. The title iron chloride complexes were
readily formed and were investigated for their catalytic behavior
toward ethylene polymerization. To the best of our knowledge, the
iron pre-catalysts possess the highest catalytic activities for
ethylene polymerization reported to-date for bis(imino)pyridine
containing pre-catalysts. Moreover, they afforded polyethylene
with the narrowest PDIs in the family of unsymmetrical bis(imino)
pyridines bearing extremely bulky substituents, indicative of
a single-site catalytic system. Herein, the synthesis, characteriza-
tion, and catalytic behavior of the 2-[1-(2,6-dibenzhydryl-4-
chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridyliron(II)
complexes are reported and discussed in detail.
a catalytic amount of p-toluenesulfonic acid in toluene (125 mL)
was refluxed for 4 h. After removing the solvent in vacuo, the
crude product was purified by silica gel column chromatography
(30:1 (v/v) petroleum ether/ethyl acetate) to afford 3.15 g (52%) of
2-acetyl-6-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]pyri-
dine as a yellow powder and 2.09 g of L6 (20%) as a white power.
Data for 2-acetyl-6-[1-(2,6-dibenzhydryl-4-chlorophenylimino)
ethyl]pyridine: Mp: 156e158 ꢀC. FT-IR (KBr, cmꢁ1): 3027, 2920,
2170, 1700, 1650 (
n
C]N), 1494, 1446, 1360, 1240, 1118, 1076, 1029,
8.14 (d,
819, 761, 738, 700. 1H NMR (400 MHz, CDCl3, TMS):
d
J ¼ 7.8 Hz, 1H, Py-Hm), 8.10 (d, J ¼ 7.6 Hz, 1H, Py-Hm), 7.87 (t,
J ¼ 7.8 Hz, 1H, Py-Hp), 7.29e7.13 (m, 12H, aryl-H), 7.00 (t, J ¼ 8.0 Hz,
8H, aryl-H), 6.87 (s, 2H, aryl-H), 5.24 (s, 2H, CHPh2), 2.66 (s, 3H,
O]CCH3), 1.06 (s, 3H, N]CCH3). 13C NMR (100 MHz, CDCl3, TMS):
d
200.0, 170.0, 155.1, 152.4, 146.9, 142.7, 141.7, 137.3, 134.5, 129.8,
129.4, 128,8, 128.6, 128.4, 128.1, 127.0, 126.6, 126.5, 124.7, 122.6,
52.3, 25.7, 17.0. Anal. Calcd for C41H33ClN2O (605.17): C, 80.99; H,
5.68; N, 4.48. Found: C, 81.37; H, 5.50; N, 4.63. Data for L6: Mp:
270e272 ꢀC. FT-IR (KBr, cmꢁ1): 3058, 3025, 1974, 1638 (
n
C]N),
1493, 1446, 1369, 1242, 766, 743, 698. 1H NMR (400 MHz, CDCl3,
TMS):
d
8.10 (d, J ¼ 7.8 Hz, 2H, Py-Hm), 7.80 (t, J ¼ 7.8 Hz, 1H, Py-
Hp), 7.30e7.09 (m, 24H, aryl-H), 7.04e6.99 (m, 16H, aryl-H), 6.88 (s,
4H, aryl-H), 5.28 (s, 4H, CHPh2), 0.91 (s, 6H, N]CCH3). 13C NMR
(100 MHz, CDCl3, TMS):
d170.7, 154.6, 147.0, 142.9, 141.8, 136.7,
134.5, 129.8, 129.4, 128.6, 128.3, 128.2, 128.1, 126.5, 122.3, 52.1, 17.0.
Anal. Calcd for C73H57Cl2N3 (1047.16): C, 83.31, H, 5.69; N, 3.88.
Found: C, 83.73; H, 5.49; N, 4.01.
2.2.2. Synthesis of 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)
ethyl]-6-[1-(arylimino)ethyl]pyridine (L1eL5)
2.2.2.1. 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-
(2,6-dimethylphenylimino)ethyl]pyridine (L1). A mixture of 2-
acetyl-6-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl] pyri-
dine (3.02 g, 5.0 mmol), 2,6-dimethylaniline (0.91 g, 7.5 mmol) and
a catalytic amount of p-toluenesulfonic acid in toluene (50 mL) was
refluxed for 8 h. The solution was evaporated in vacuo and the
residual solid was purified by silica gel column chromatography
(30:1 (v/v) petroleum ether/ethyl acetate) to afford 1.93 g (55%) of
L1 as a yellow powder. Mp: 222e223 ꢀC. FT-IR (KBr, cmꢁ1): 3057,
2. Experimental
2.1. General considerations
3023, 2916, 2361, 2161, 2034, 1639 (
n
C]N), 1494, 1448, 1365, 1242,
All manipulations of air and moisture-sensitive compounds were
performed under a nitrogen atmosphere using standard Schlenk
techniques. Toluene was refluxed over sodium-benzophenone and
distilled under nitrogen prior to use. Methylaluminoxane (MAO,
a 1.46 M solution in toluene) and modified methylaluminoxane
(MMAO, 1.93 M in heptane, 3A) were purchased from Akzo Nobel
Corp. Other reagents were purchased from Acros Chemicals or local
suppliers. 1H and 13C NMR spectra of compounds and PE samples
were recorded on Bruker DMX 400 MHz instrument at ambient
temperature using TMS as an internal standard. IR spectra were
recorded on a PerkineElmer System 2000 FT-IR spectrometer.
Elemental analysis was carried out using a Flash EA 1112 micro
analyzer. Molecular weights and polydispersity index of the poly-
ethylenes were determined by a PL-GPC220 at 150 ꢀC, with 1,2,4-
trichlorobenzene as the solvent. DSC trace and melting points of
polyethylenes were obtained from the second scanning run on
PerkineElmer DSC-7 at a heating rate of 10 ꢀC/min.
1120, 1076, 1032, 762, 699. 1H NMR (400 MHz, CDCl3, TMS):
d
8.42
(d, J ¼ 7.7 Hz, 1H, Py-Hm), 8.02 (d, J ¼ 7.7 Hz, 1H, Py-Hm), 7.84 (t,
J ¼ 7.8 Hz, 1H, Py-Hp), 7.31e7.11 (m, 12H, aryl-H), 7.08 (d, J ¼ 7.5 Hz,
2H, aryl-H), 7.02 (t, J ¼ 7.2 Hz, 8H, aryl-H), 6.95 (t, J ¼ 7.5 Hz, 1H,
aryl-H), 6.86 (s, 2H, aryl-H), 5.27 (s, 2H, CHPh2), 2.11 (s, 3H, N]
CCH3), 2.06 (s, 6H, aryl-o-CH3), 1.27 (s, 3H, N]CCH3). 13C NMR
(100 MHz, CDCl3, TMS):
d 170.4, 167.2, 155.0, 154,6, 148.7, 147.0,
142.7, 141.7, 136.7, 134.4, 129.8, 129.4, 128.5, 128.2, 128.0, 127.9,
126.4, 126.3, 125.4, 123.0, 122.3, 122.2, 52.1, 18.0, 17.0, 16.4. Anal.
Calcd for C49H42ClN3 (708.33): C, 82.75; H, 6.10; N, 5.77. Found: C,
83.09; H, 5.98; N, 5.93.
2.2.2.2. 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-
(2,6-diethylphenylimino)ethyl]pyridine (L2). A procedure similar to
that for L1 was used, but using 3.02 g (5.0 mmol) of 2-acetyl-6-[1-
(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]pyridine and 1.12 g
(7.5 mmol) of 2,6-diethylaniline, to afford 1.75 g (48%) of L2 as
a yellow powder. Mp: 202e203 ꢀC. FT-IR (KBr, cmꢁ1): 3024, 2963,
2.2. Synthesis and characterization
2868, 2170, 2033, 1974, 1639(
n
C]N), 1494, 1448, 1366, 1233, 1120,
1074, 764, 699. 1H NMR (400 MHz, CDCl3, TMS):
d
8.41 (d,
2.2.1. Synthesis of 2-acetyl-6-[1-(2,6-dibenzhydryl-4-
chlorophenylimino)ethyl]pyridine and 2,6-bis[1-(2,6-dibenzhydryl-
4-chlorophenylimino)ethyl]pyridine (L6)
A solution of 2,6-dibenzhydryl-4-chlorophenylamine (4.60 g,
10.0 mmol), 2,6-diacetylpyridine (1.63 g, 10.0 mmol) and
J ¼ 7.8 Hz, 1H, Py-Hm), 8.03 (d, J ¼ 7.8 Hz, 1H, Py-Hm), 7.84 (t,
J ¼ 7.8 Hz, 1H, Py-Hp), 7.30e7.15 (m, 12H, aryl-H), 7.13 (d, J ¼ 7.5 Hz,
2H, aryl-H), 7.03 (m, 9H, aryl-H); 6.87 (s, 2H, aryl-H); 5.28 (s, 2H,
CHPh2); 2.40 (m, 4H, CH2CH3); 2.12 (s, 3H, N]CCH3), 1.16 (m, 9H,
CH3). 13C NMR (100 MHz, CDCl3, TMS):
d 170.6, 167.1, 155.2, 154.8,

1872
X. Cao et al. / Polymer 53 (2012) 1870e1880
147.9, 147.1, 142.8, 141.9, 136.8, 134.5, 131.3, 130.0, 129.5, 128.6,
128.4, 128.2, 128.1, 126.6, 126.1, 123.5, 122.4, 122.3, 52.2, 24.7, 17.2,
16.9, 13.9. Anal. Calcd for C51H46ClN3 (736.38): C, 82.97, H, 6.62; N,
5.68. Found: C, 83.18; H, 6.30; N, 5.71.
room temperature for 8 h, which was then filtered and washed
with diethyl ether (3 ꢂ 5 mL). The pure complex was obtained as
a blue powder after drying under vacuo.
Complex Fe1 was prepared in 88% yield. FT-IR (KBr, cmꢁ1): 3059,
3025, 2968, 2168,1977, 1582 (nC]N), 1495, 1431, 1364, 1267, 1214,
2.2.2.3. 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-
1077, 1031, 813, 767, 743, 699. Anal. Calcd for C49H42Cl3FeN3
(835.08): C, 70.02; H, 5.45; N, 4.86. Found: C, 70.48; H, 5.07; N, 5.03.
Complex Fe2 was prepared in 92% yield. FT-IR (KBr, cmꢁ1): 3061,
3026, 2966, 2915, 2165, 1977, 1580 (nC¼N), 1495, 1434, 1366, 1268,
1208, 1076, 1031, 810, 769, 743, 700. Anal. Calcd for C51H46Cl3FeN3
(863.14): C, 70.61; H, 5.86; N, 4.69. Found: C, 70.97; H, 5.37; N, 4.87.
Complex Fe3 was prepared in 87% yield. FT-IR (KBr, cmꢁ1): 3060,
(2,6-diisopropylphenylimino)ethyl]pyridine
(L3). A
procedure
similar to that for L1 was used, but using 3.02 g (5.0 mmol) of 2-
acetyl-6-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]pyri-
dine and 1.33 g (7.5 mmol) of 2,6-diisopropylaniline, to afford 1.72 g
(45%) of L3 as a yellow powder. Mp: 179e180 ꢀC. FT-IR (KBr, cmꢁ1):
3062, 3027, 2958, 2167, 1636 (
nC]N), 1495, 1432, 1369, 1235, 1126,
823, 767, 699. 1H NMR (400 MHz, CDCl3, TMS):
d
8.39 (d, J ¼ 7.8 Hz,
3027, 2959, 2913, 2165, 2077, 1579 (nC]N), 1496, 1434, 1370, 1268,
1H, Py-Hm), 8.02 (d, J ¼ 7.8 Hz, 1H, Py-Hm), 7.84 (t, J ¼ 7.7 Hz, 1H, Py-
Hp), 7.30e7.08 (m, 15H, aryl-H), 7.02 (t, 8H, J ¼ 8.0 Hz, aryl-H), 6.87
(s, 2H, aryl-H), 5.28 (s, 2H, CHPh2), 2.77 (m, 2H, CHMe2), 2.13 (s, 3H,
1207, 1028, 808, 770, 751, 702. Anal. Calcd for C53H50Cl3FeN3
(891.19): C, 71.08; H, 5.93; N, 4.53. Found: C, 71.43; H, 5.66; N, 4.72.
Complex Fe4 was prepared in 84% yield. FT-IR (KBr, cmꢁ1): 3065,
N]CCH3), 1.18 (d, J ¼ 6.7 Hz, 12H, CH3), 1.14 (s, 3H, N]CCH3). 13
C
3032, 2970, 2918, 2168, 1980, 1581 (nC]N), 1495, 1430, 1363, 1267,
NMR (100 MHz, CDCl3, TMS):
d
170.6, 167.1, 155.1, 154.8, 147.1, 146.5,
1221,1077,1030, 767, 700. Anal. Calcd for C50H44Cl3FeN3 (849.11): C,
70.42; H, 5.57; N, 4.77. Found: C, 70.73; H, 5.22; N, 4.95. Complex
Fe5 was prepared in 86% yield. FT-IR (KBr, cmꢁ1): 3062, 3030, 2960,
142.8, 141.8, 136.8, 135.9, 134.5, 129.9, 129.4, 128.5, 128.3, 128.1,
128.1, 126.5, 126.5, 123.75, 123.11, 122.4, 122.3, 52.2, 28.4, 23.4, 23.0,
17.2, 17.1. Anal. Calcd for C53H50ClN3 (764.44): C, 83.02, H, 7.03; N,
5.35. Found: C, 83.27; H, 6.59; N, 5.50.
2915, 2165, 1978, 1581 (nC]N), 1496, 1445, 1373, 1267, 1217, 1033,
870, 807, 767, 704. Anal. Calcd for C52H48Cl3FeN3 (877.16): C, 70.83;
H, 5.80; N, 4.65. Found: C, 71.20; H, 5.52; N, 4.79. Complex Fe6 was
prepared in 70% yield. FT-IR (KBr, cmꢁ1): 3026, 2168, 1978, 1575
2.2.2.4. 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-
(2,4,6-trimethylphenylimino)ethyl]pyridine
(L4). A
procedure
(nC]N), 1494, 1433, 1269, 1203, 1075, 1030, 808, 766, 700. Anal.
similar to that for L1 was used, but using 3.02 g (5.0 mmol) of 2-
acetyl-6-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]pyri-
dine and 1.01 g (7.5 mmol) of 2,4,6-trimethylaniline, to afford
1.51 g (45%) of L4 as a yellow powder. Mp: 228e229 ꢀC. FT-IR (KBr,
Calcd for C73H57Cl4FeN3 (1173.91): C, 74.22; H, 5.23; N, 3.33. Found:
C, 74.69; H, 4.89; N, 3.58.
2.3. X-ray crystallographic studies
cmꢁ1): 3060, 3025, 2914, 1943, 1643 (
n
C]N), 1493, 1431, 1365,
1216, 1122, 740, 698. 1H NMR (400 MHz, CDCl3, TMS):
d
8.41 (d,
Single-crystal X-ray diffraction studies for Fe1, Fe2 and Fe4
were carried out on a Rigaku Saturn724 þ CCD diffractometer
J ¼ 7.8 Hz, 1H, Py-Hm), 8.02 (d, J ¼ 7.8 Hz, 1H, Py-Hm), 7.82 (t,
J ¼ 7.8 Hz, 1H, Py-Hp), 7.33e7.12 (m, 12H, aryl-H), 7.02 (t, 8H,
J ¼ 7.6 Hz, aryl-H), 6.90 (s, 2H, aryl-H), 6.86 (s, 2H, aryl-H), 5.27 (s,
2H, CHPh2), 2.30 (s, 3H, aryl-p-CH3), 2.10 (s, 3H, N]CCH3), 2.02 (s,
6H, aryl-o-CH3), 1.11 (s, 3H, N]CCH3). 13C NMR (100 MHz, CDCl3,
with graphite-monochromated Mo K
a
radiation (
l
¼ 0.71073 Å) at
173 (2) K. Cell parameters were obtained by global refinement of
the positions of all collected reflections. Intensities were corrected
for Lorentz and polarization effects and empirical absorption. The
structures were solved by direct methods and refined by full-
matrix least-squares on F2. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were placed in
calculated positions. Structure solution and refinement were
performed by using the SHELXL-97 package [47]. Crystal data and
processing parameters for Fe1, Fe2 and Fe4 are summarized in
Table 1.
TMS):
d 170.5, 167.4, 155.1, 154.6, 147.0, 146.2, 141.7, 136.7, 134.4,
132.2, 129.8, 129.4, 128.6, 128.5, 128.2, 128.0, 126.4, 126.3, 125.2,
122.2, 122.1, 52.1, 20.7, 17.9, 17.0, 16.3. Anal. Calcd for C50H44ClN3
(722.36): C, 82.86, H, 6.44; N, 5.63. Found: C, 83.14; H, 6.14; N,
5.82.
2.2.2.5. 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-
(2,6-diethyl-4-methylphenylimino)ethyl]pyridine (L5). A procedure
similar to that for L1 was used, but using 3.02 g (5.0 mmol) of 2-
acetyl-6-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]pyri-
dine and 1.22 g (7.5 mmol) of 2,6-diethyl-4-methylaniline, to
afford 1.77 g (47%) of L5 as a yellow powder. Mp: 211e212 ꢀC. FT-
2.4. General procedure for ethylene polymerization
2.4.1. Ethylene polymerization at ambient pressure
The pre-catalyst was dissolved in toluene using standard
Schlenk techniques, and the reaction solution was stirred with
a magnetic stir bar under ethylene atmosphere (1 atm) with
a steam bath for controlling the desired temperature. Finally, the
require amount of co-catalyst (MMAO) was added by a syringe.
After the reaction was carried out for the required period, the
reaction solution was quenched with acidified ethanol solution
containing 10% hydrochloric acid. The precipitated polymer was
collected by filtration, washed with ethanol and water, and dried in
a vacuum at 60 ꢀC until of constant weight.
IR (KBr, cmꢁ1): 3060, 3026, 2964, 2170, 1943, 1639 (
1447, 1364, 1211, 1121, 860, 740, 698. 1H NMR (400 MHz, CDCl3,
TMS):
nC]N), 1494,
d
8.39 (d, J ¼ 7.8 Hz, 1H, Py-Hm), 8.02 (d, J ¼ 7.8 Hz, 1H, Py-
Hm), 7.82 (t, J ¼ 7.8 Hz, 1H, Py-Hp), 7.30e7.12 (m, 12H, aryl-H), 7.02
(t, 8H, J ¼ 8.1 Hz, aryl-H), 6.94 (s, 2H, aryl-H), 6.86 (s, 2H, aryl-H),
5.27 (s, 2H, CHPh2), 2.35 (m, 4H, CH2CH3), 2.34 (s, 3H, aryl-p-CH3),
2.11 (s, 3H, N]CCH3), 1.14 (m, 9H, CH3). 13C NMR (100 MHz, CDCl3,
TMS):
d 170.6, 167.1, 155.2, 154.8, 147.9, 147.1, 142.8, 141.9, 136.8,
134.5, 131.3, 129.9, 129.5, 128.6, 128.4, 128.2, 128.1, 126.6, 123.5,
122.4, 122.3, 52.2, 24.7, 17.2, 16.9, 13.9. Anal. Calcd for C52H48ClN3
(750.41): C, 82.80, H, 6.78; N, 5.46. Found: C, 83.23; H, 6.45;
N, 5.60.
2.4.2. Ethylene polymerization at 10 atm pressure
A 250 mL stainless steel autoclave, equipped with a mechanical
stirrer and a temperature controller, was employed for the reac-
tion. Firstly, 50 mL toluene (freshly distilled) was injected into the
autoclave which was full of ethylene. Then 30 mL toluene solution
2.2.3. Synthesis of iron complexes (Fe1eFe6)
To the corresponding ligand, 1.0 equivalents of FeCl2$4H2O and
freshly distilled ethanol were added in a Schlenk tube. A blue
precipitate was formed while this reaction mixture was stirred at
of the complex (1.5
mmol), the require amount of co-catalyst (MAO,
MMAO) and 20 mL toluene were added by syringe successively

X. Cao et al. / Polymer 53 (2012) 1870e1880
1873
Table 1
Crystal data and structure refinement for Fe1, Fe2 and Fe4.
Fe1
Fe2
Fe4
Empirical formula
Fw
T (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C49H42Cl3FeN3
835.06
173(2)
0.71073
Monoclinic
Cc
24.024(5)
14.396(3)
15.970(3)
90
125.06(3)
90
4520.8(16)
4
1.227
C51H46Cl3FeN3
863.11
173(2)
0.71073
Monoclinic
Cc
23.941(5)
14.652(3)
15.929(3)
90
123.57(3)
90
4655.7(16)
4
1.231
C50H45Cl3FeN3
850.09
173(2)
0.71073
Monoclinic
Cc
24.125(5)
14.632(3)
16.093(3)
90
126.62(3)
90
4559.8(16)
4
1.238
0.543
b (Å)
c (Å)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
V (Å3)
Z
D calcd. (g cmꢁ3
)
m
(mmꢁ1
)
0.546
0.533
F (000)
Cryst size (mm)
range (ꢀ)
Limiting indices
1736
1800
1772
0.29 ꢂ 0.13 ꢂ 0.08
1.75e25.32
ꢁ20 ꢃ h ꢃ 28
ꢁ16 ꢃ k ꢃ 17
ꢁ19 ꢃ l ꢃ 16
11672
0.24 ꢂ 0.20 ꢂ 0.12
1.72e25.37
ꢁ27 ꢃ h ꢃ 28
ꢁ17 ꢃ k ꢃ 17
ꢁ19 ꢃ l ꢃ 9
8704
0.36 ꢂ 0.12 ꢂ 0.11
1. 74e25.33
ꢁ28 ꢃ h ꢃ 28
ꢁ16 ꢃ k ꢃ 16
ꢁ19 ꢃ l ꢃ 19
12700
q
No. of rflns collected
No. unique rflns [R (int)]
6206 (0.0497)
99.8
None
5093 (0.0406)
99.1
None
7019 (0.0523)
98.6
None
Completeness to
Abs corr.
q (%)
data/restraints/params
6206/2/505
1.070
5093/21/533
1.101
7019/2/514
1.030
Goodness of fit on F2
Final R indices [I > 2
R indices (all data)
s
(I)]
R1 ¼ 0.0747
wR2 ¼ 0. 1957
R1 ¼ 0.0801
wR2 ¼ 0. 2020
0.643 and ꢁ0.455
R1 ¼ 0. 0548
wR2 ¼ 0.1459
R1 ¼ 0. 0582
wR2 ¼ 0. 1495
0.442 and ꢁ0.435
R1 ¼ 0.0756
wR2 ¼ 0.1904
R1 ¼ 0. 0839
wR2 ¼ 0. 1983
0. 566 and ꢁ0. 502
Largest diff peak and hole (e Åꢁ3
)
after the autoclave was heated to the required reaction tempera-
ture. The reaction mixture was intensively stirred for the desired
time under 10 atm pressure of ethylene through the entire
experiment. The reaction was terminated and analyzed using the
same method as above for ethylene polymerization at ambient
pressure.
3. Result and discussion
3.1. Synthesis and characterization of the ligands and complexes
Procedures for the preparation of the ligands and complexes are
shown in Scheme 1. The ligand 2,6-bis[1-(2,6-dibenzhydryl-4-
DBCPh-NH2
TsOH
+
N
N
N
O
O
N
O
N
N
DBCPh
DBCPh
PhDBC
L6
Ph NH2 Ph
NH2
R1
R1
Ph
Ph
.
FeCl2 4H2O
TsOH
R2
Cl
2,6-dibenzhydryl-4-chloro
benzenamine(DBCPh-NH2)
.
FeCl2 4H2O
R1
N
R1
N
N
N
N
N
DBCPh
R2
L3 L4 L5
Fen Fe1 Fe2 Fe3 Fe4 Fe5
DBCPh
Fe
Cl
Cl
R1
R2
R1
Fe1 - Fe6
L1 - L5
Ln
L1 L2
L6
Fe6
R1
R2
CHPh2
Cl
Me Et
i-Pr Me Et
Me Me
H
H
H
Scheme 1. Synthesis of the ligands and iron complexes.

1874
X. Cao et al. / Polymer 53 (2012) 1870e1880
Fig. 3. Molecular structure of the complex Fe4; thermal ellipsoids are shown at 30%
probability. Hydrogen atoms have been omitted for clarity.
Fig. 1. Molecular structure of the complex Fe1; thermal ellipsoids are shown at 30%
probability. Hydrogen atoms have been omitted for clarity.
spectroscopy and by elemental analysis. According to the FT-IR
spectra, when compared with the corresponding free ligands
(1636e1643 cmꢁ1), the stretching vibrations for C]N in these
complexes (1575e1582 cmꢁ1) shifted to lower wave-numbers and
the peak intensities were decreased, consistent with effective
coordination between the imino-nitrogen and the cationic metal.
The molecular structures of complexes Fe1, Fe2 and Fe4 were
confirmed by single-crystal X-ray diffraction studies.
chlorophenylimino)ethyl]pyridine (L6) was isolated as
a by-
product in the preparation of 2-acetyl-6-[1-(2,6-dibenzhydryl-4-
chlorophenylimino)ethyl]pyridine, the latter being conveniently
synthesized by the condensation of 2,6-diacetylpyridine with one
equivalent of 2,6-dibenzhydryl-4-chlorophenylamine in refluxing
toluene using a catalytic amount of p-toluenesulfonic acid. Further
condensation of this product with the corresponding aniline
produced the 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-
6-[1-(arylimino)ethyl]pyridine ligands (L1eL5) in good yields. All
ligands were fully characterized by elemental analysis, 1H and 13C
NMR and FT-IR spectroscopy.
3.2. X-ray crystallographic studies
Single crystals of Fe1, Fe2 and Fe4 were grown by diffusing an n-
hexane layer into dichloromethane solutions of the respective
complexes. The molecular structures are shown in Figs. 1e3,
Reactions of ligands (L1eL6) with iron(II) dichloride afforded
the iron complexes (Fe1eFe6), which were characterized by FT-IR
Table 2
Selected bond lengths (Å) and angles (ꢀ) for complexes Fe1, Fe2 and Fe4.
Fe1
Fe2
Fe4
Bond lengths (Å)
Fe1eN1
2.274 (6)
2.223 (4)
2.283 (6)
Fe1eN2
Fe1eN3
Fe1eCl1
Fe1eCl2
N1eC6
N1eFE1 0
N2eFE1
N2eC5
2.109 (5)
2.216 (6)
2.325 (2)
2.266 (2)
1.290 (9)
1.421 (9)
1.402 (9)
1.306 (9)
1.278 (9)
1.405 (9)
2.088 (5)
2.193 (5)
2.3264 (17)
2.2676 (19)
1.294 (7)
1.439 (7)
1.347 (7)
1.359 (7)
1.278 (8)
1.450 (7)
2.091 (6)
2.209 (6)
2.331 (2)
2.276 (2)
1.257 (9)
1.430 (8)
1.362 (9)
1.335 (8)
1.300 (10)
1.419 (9)
N3eC8
N3eC42
Bond angles (ꢀ)
N2eFe1 eN3
N2eFe1 eN1
N3eFe1 eN1
N2eFe1 eCl2
N3eFe1 eCl2
N1eFe1 eCl2
N2eFe1 eCl1
N3eFe1 eCl1
N1eFe1 eCl1
Cl2eFe1 eCl1
73.4 (2)
72.4 (2)
141.4 (2)
154.61 (17)
108.29 (16)
95.29 (16)
93.56 (16)
94.76 (15)
104.82 (15)
111.27 (8)
73.23 (19)
72.15 (18)
139.48 (18)
155.48 (14)
107.54 (14)
95.72 (13)
92.93 (13)
96.21 (14)
105.90 (12)
111.10 (7)
73.3 (2)
72.0 (2)
140.4 (2)
155.68 (18)
109.67 (17)
94.78 (15)
92.97 (18)
93.72 (16)
106.60 (15)
110.59 (8)
Fig. 2. Molecular structure of the complex Fe2; thermal ellipsoids are shown at 30%
probability. Hydrogen atoms have been omitted for clarity.

X. Cao et al. / Polymer 53 (2012) 1870e1880
1875
Table 3
Ethylene polymerization with Fe1eFe6/MMAO.a
Activityb
Adjusted activityc
Mnd/104 g molꢁ1
Mwd/104 g molꢁ1
Mw/Mn
d
e
Tm /ꢀC
Run
Cat.
P/atm
T/ꢀC
T/min
Al/Fe
Yield/g
1
2
3
4
5
6
7
8
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe1
Fe2
Fe3
Fe5
Fe6
1
1
1
1
5
0
20
40
60
60
20
20
20
20
40
50
60
70
80
100
60
60
60
60
60
60
60
60
60
60
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
5
10
15
45
60
30
30
30
30
30
2500
2500
2500
2500
2500
1500
2000
2500
3000
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
1.09
0.89
0.86
0.53
7.73
2.08
2.62
2.81
1.45
1.19
1.15
0.71
2.06
0.28
0.35
0.38
0.35
0.47
1.60
2.46
2.12
1.35
e
7.22
8.18
10.5
8.28
24.0
1.90
2.40
2.58
2.42
4.30
16.6
28.7
27.7
19.6
e
0.29
0.21
0.11
0.13
0.48
5.8
1.1
2.1
0.65
1.1
1.0
7.0
0.76
0.34
0.23
3.0
120
38
9.5
1.1
7.3
4.4
4.2
3.3
1.4
e
24
129.7
124.7
119.9
106.4
127.0
134.7
133.7
131.2
122.4
132.5
129.5
128.7
128.7
125.9
e
3.6
3.1
1.8
6.3
21
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
35
4.5
1.7
6.6
4.4
5.1
3.6
1.9
e
9
2.64
3.54
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
11.98
18.47
15.90
10.11
trace
6.90
11.20
14.42
22.13
24.69
17.35
7.28
0.83
0.91
0.75
e
5.52
4.48
3.85
1.96
1.65
2.31
0.97
0.50
1.12
e
64.4
52.3
44.9
22.9
19.2
26.9
11.3
5.83
13.1
e
0.47
0.46
0.76
0.77
0.89
0.56
0.49
0.45
0.48
e
0.66
1.8
2.1
7.8
10
1.7
1.4
1.2
1.7
e
1.4
3.8
2.7
10
126.4
126.9
129.2
130.2
130.5
126.4
126.2
126.9
126.7
e
11
3.1
2.9
2.6
3.5
e
3.72
8.40
trace
a
General conditions:1.5 mmol of Fe; 50 mL toluene for 1 atm ethylene, 100 mL toluene for 5 atm and 10 atm ethylene.
b
c
106 g molꢁ1(Fe)$hꢁ1 atmꢁ1
106 g molꢁ1(Fe)$hꢁ1 Cꢁ1
.
.
ethylene
d
e
Determined by GPC.
Determined by DSC.
respectively, and selected bond lengths and angles are shown in
Table 2. These complexes (Fe1, Fe2 and Fe4) are penta-coordinate
with pseudo-square-pyramidal geometry at the metal, with Cl1
occupying the apical position and N1, N2, N3 and Cl2 forming the
square plane. The metal center is pressed out of the chelated plane
due to the associated sterics: the iron atom lies 0.484 Å out of the
chelated plane (N1eN2eN3) in Fe1, 0.551 Å in Fe2 and 0.517 Å in
Fe4. It is noticeable that the plane of the 2,6-dibenzhydryl-4-
chlorophenyl rings is oriented essentially orthogonal to the plane
of the bis(imino)pyridine ligand backbone (ranging between 79ꢀ
and 84ꢀ), whereas the angle between the plane of the other aryl
ring and the backbone is smaller (ranging between 60ꢀ and 70ꢀ).
Therefore the assumption that the two benzhydryl ortho-substit-
uents on the aryl ring can constrain the free rotation of the aryl-
nitrogen bond and thus shield the active site, especially at
elevated temperatures during the ethylene polymerization, is put
forward as a reason for the high observed catalytic activity in the
current iron pre-catalysts.
catalytic conditions found for Fe4/MMAO. The results, tabulated in
Table 2, showed the effects of these parameters on the activities of
the pre-catalysts, the molecular weights and the PDIs of the
resultant polyethylenes.
Under an ambient pressure of ethylene and with the molar ratio
of Al/Fe at 2500, the steady decrease in the catalytic activity of Fe4
from 0 to 60 ꢀC (Runs 1e4 in Table 3) was the result of the change of
the ethylene solubility at elevated reaction temperature. In order to
eliminate the influence of ethylene solubility in toluene at different
temperatures and various ethylene pressures on the catalytic
activity, adjusted activities were introduced, which were obtained
3.3. Catalytic behavior toward ethylene polymerization
In order to determine the best co-catalyst to use herein, various
alkylaluminum reagents were employed in polymerization runs,
and it was founded that pre-catalysts treated with modified
methylaluminoxane (MMAO) or methylaluminoxane (MAO)
produced polyethylene with the highest observed catalytic
activities.
3.3.1. Ethylene polymerization with Fe1eFe6/MMAO
The catalytic system of Fe4/MMAO was investigated for the
optimum catalytic conditions by variation of the molar ratio of Al/
Fe, the ethylene pressure, the reaction temperature and the reac-
tion time. Pre-catalysts with different aryl substituents (Fe1eFe6)
were then employed in polymerization runs using the optimum
Fig. 4. GPC curves of PEs obtained by Fe4/MMAO under ambient pressure with various
temperatures (Runs 1e4 in Table 3).

1876
X. Cao et al. / Polymer 53 (2012) 1870e1880
6
5
4
3
2
1
0
0
10
20
30
40
50
60
Reaction Time (min)
Fig. 7. The effect of the reaction time on the catalytic activity for the Fe4/MMAO
system (see Runs 12, 16e20 in Table 3).
Fig. 5. GPC curves of PEs obtained by Fe4/MMAO under various ethylene pressures
(Runs 4, 5, 12 in Table 3).
by the calculation such as the linearly decreasing solubility of
ethylene about 30% in toluene from 20 ꢀC to 60 ꢀC under 10 atm
[48]. Therefore, the best adjusted activity under ambient pressure
of ethylene was observed for the catalytic system Fe4/MMAO at
40 ꢀC (Run 3 in Table 3). Meanwhile, the GPC curves of the resultant
polyethylenes in Fig. 4, which are bimodal at 0 ꢀC and unimodal at
higher reaction temperature with a narrow PDI, suggested the
dominance of enhanced chain transfer to aluminum [49] at
elevated temperature.
possible that the higher pressure of ethylene enhanced the rate of
chain propagation over the rate of chain transfer. Additionally,
precipitated polymeric products could be contributing to the
observed bimodal distribution, in particular the peak associated
with the higher molecular weights.
Further studies concerning the effect of reaction temperature
and ethylene pressure on the active species, and on the pre-cata-
lysts’ thermo-stability and catalytic activity were carried out under
10 atm pressure of ethylene. When the Al/Fe molar ratios were
raised from 1500 to 3000, the catalytic activities for the Fe4/MMAO
system at 20 ꢀC initially increased and then decreased (Runs 6e9 in
Table 3) with an optimum ratio of 2500 (Run 8 in Table 3). As shown
in Fig. 6, the variation of the GPC curves from multi-modal to
unimodal suggested that the dominant termination process was
chain transfer to aluminum, or simply that as a result of poly-
ethylene precipitation, the equilibrium between the iron and
aluminum species responsible for chain transfer was disturbed;
similar observations were noted lanthanocene-based systems
[50,51].
Ethylene polymerizations with the catalytic system Fe4/MMAO
were conducted under different pressures of ethylene (Runs 4, 5, 12
in Table 3). The adjusted catalytic activities increased
ꢁ1
significantly from 8.28 ꢂ 106 g molꢁ1(Fe) hꢁ1
C
to 2.87 ꢂ
ethylene
ꢁ1 ꢁ1
107 g molꢁ1(Fe) h
C
on variation of the ethylene pressure,
ethylene
attributable to the higher monomer concentration around the active
iron centers at higher pressure. As shown in Table 3 and Fig. 5, the
higher the ethylene pressure employed, the greater the molecular
weights and PDIs observed for the polyethylene products. It is
When the reaction temperature was elevated from 20 to 100 ꢀC
(Runs 8, 10e15 in Table 3; 10 atm ethylene, Al/Fe molar ratio
2500:1), the catalytic activity for Fe4 increased to
2.46 ꢂ 106 g molꢁ1(Fe) hꢁ1 atmꢁ1 at 60 ꢀC (Run 12 in Table 3) and
then decreased. Although Fe4 gradually deactivated on increasing
the temperature from 60 to 100 ꢀC, the associated lower molecular
weights and narrower PDI values were also attributed to the
dominance of chain transfer to aluminum at elevated temperature.
The catalytic system Fe4/MMAO under 10 atm pressure of
ethylene and the Al/Fe molar ratio 2500 at 60 ꢀC was quenched over
different reaction periods (Runs 12, 16e20 in Table 3) in order to
further understand the lifetime of the active species and the effect
of the reaction time on the polymerization behavior. As the reaction
time increased from 5 to 60 min, the catalytic activity dropped
from 5.52 ꢂ 106 g molꢁ1(Fe) hꢁ1 atmꢁ1 to 1.65 ꢂ 106 g molꢁ1
(Fe) hꢁ1 atmꢁ1, indicative of little or no induction time during the
catalytic process. The effect of reaction time on the catalytic activity
is shown in Fig. 7.
Additionally, the PDIs of the resultant polyethylenes (Runs 12,
16e20 in Table 3) varied over different reaction periods, principally
relying on the competition between chain propagation and chain
transfer. Once the polymeric chains on the active species achieve
the requisite length and are ready for the termination stage, this
Fig. 6. GPC curves of PEs obtained by Fe4/MMAO with various Al/Fe molar ratios (Runs
6e9 in Table 3).

X. Cao et al. / Polymer 53 (2012) 1870e1880
1877
Table 4
Ethylene polymerization with Fe1eFe6/MAO.a
Activityb
Adjusted activityc
Mnd/104 g molꢁ1
Mwd/104 g molꢁ1
Mw/Mn
d
e
Tm /ꢀC
Run
Cat.
T/ꢀC
Al/Fe
Yield/g
1
2
3
4
5
6
7
8
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe4
Fe1
Fe2
Fe3
Fe5
Fe6
20
20
20
20
40
60
70
80
90
100
80
80
80
80
80
1500
2000
2500
3000
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
2500
1.43
1.92
2.55
2.17
5.05
5.65
9.60
10.69
10.20
1.97
9.85
8.40
2.94
10.30
trace
0.19
0.26
0.34
0.29
0.67
0.75
1.28
1.43
1.36
0.26
1.31
1.12
0.32
1.37
e
1.31
1.76
2.34
1.99
6.13
8.78
16.7
20.8
21.9
4.65
19.0
16.3
4.82
19.9
1.2
96
26
23
67
25
41
2.4
2.3
2.2
0.65
2.0
20
80
40
38
20
29
3.4
2.4
2.8
3.1
2.5
1.7
2.1
2.8
3.1
e
133.2
133.5
130.7
129.7
130.0
129.5
130.7
130.2
131.0
124.4
130.5
130.0
128.0
130.5
e
0.66
0.60
0.34
0.87
1.2
1.0
0.83
0.70
0.26
1.2
0.95
0.41
0.68
e
9
10
11
12
13
14
15
1.1
2.1
a
General conditions:1.5 mmol of Fe 10 atm ethylene; 100 mL toluene; 30 min.
b
c
106 g molꢁ1(Fe)$hꢁ1 atmꢁ1
.
106 g molꢁ1(Fe)$hꢁ1 Cꢁ1
.
ethylene
d
e
Determined by GPC.
Determined by DSC.
can be achieved by either chain transfer to aluminum [49e51] or
via -H transfer to the ethylene monomer [3,52e54]. On extending
the polymerization time, the available amount of MMAO decreased,
and chain propagation was more favorable. Such a combination of
events resulted in polyethylene products with higher molecular
weights and broader molecular distributions.
In the following, all of the pre-catalysts were employed for
polymerization under the optimum catalytic conditions found for
Fe4/MMAO (10 atm ethylene, Al/Fe ¼ 2500, 60 ꢀC). All systems
exhibited high activities and produced polyethylenes with narrow
PDIs. The catalytic activities of the pre-catalysts bearing different
substituent’s varied in the order Fe1 [2,6-di(Me)] > Fe2 [2,6-
di(Et)] > Fe3 [2,6-di(i-Pr)] > Fe6 [2,6-di(Benzhydryl)-4-Cl], Fe4
[2,4,6-tri(Me)] > Fe1 [2,6-di(Me)] and Fe5 [2,6-di(Et)-4-Me] > Fe2
[2,6-di(Et)] (Runs 12, 21e25 in Table 3 and Runs 8,11e15 in Table 4),
which suggested that a reduction in the steric bulk at the ortho-aryl
position and/or replacement of the para-aryl proton with a methyl
group can increase the activity [1e6]. The highest catalytic activity
of these pre-catalysts was obtained by the system Fe4/MMAO,
namely 2.46 ꢂ 106 g molꢁ1(Fe) hꢁ1 atmꢁ1. Such catalytic activity,
and that of the analogous cobalt pre-catalysts [46], is higher than
those previously reported by pre-catalysts based on these metals.
Thus, given iron pre-catalysts generally show better activities than
their cobalt analogs, the current iron pre-catalyst exhibits the
highest catalytic activity (as well as good thermal stability) among
all iron-based bis(imino)pyridine pre-catalysts reported to-date.
b
3.3.2. Ethylene polymerization with Fe1eFe6/MAO
The catalytic system comprising Fe4/MAO was initially
employed to determine the optimum catalytic conditions by
varying the molar ratios of Al/Fe (Runs 1e4 in Table 4) and the
reaction temperature (Runs 5e10 in Table 4). At 10 atm ethylene
Fig. 8. 1H NMR spectrum of polyethylene prepared with catalyst Fe4/MMAO (Run 4 in Table 3).

1878
X. Cao et al. / Polymer 53 (2012) 1870e1880
Fig. 9. 13C NMR spectrum of polyethylene prepared with catalyst Fe4/MMAO (Run 4 in Table 3).
pressure, the optimum molar ratio of Al/Fe was 2500 (Run 3 in
Table 4), whilst the optimal reaction temperature was 80 ꢀC (Run 8
in Table 4). Hence, Fe4/MAO exhibited better thermal stability than
observed for Fe4/MMAO, and produced polyethylene products
having narrow molecular weight distributions above 60 ꢀC (Run 6
in Table 4).
All of the pre-catalysts were employed for ethylene polymeri-
zation under the optimum conditions found using the Fe4/MAO
system. The order of catalytic activities was similar to that found in
the presence of MMAO, namely Fe1 > Fe2 > Fe3 > Fe6, Fe4 > Fe1
and Fe5 > Fe2. Although the activity was lower, the thermal
stability of the Fe1eFe5/MAO catalytic systems was better than
those of the Fe1eFe5/MMAO catalytic systems, and the PDIs of the
resultant polyethylene products were narrower (1.7e3.1).
In addition, the formation of low molecular weight polyethylene
consequence of the chain transfer to alkylaluminum
is
a
[2,4,55e57], which is consistent with the NMR observations on the
PE samples. On comparison with the literature [58,59], the analysis
of the polyethylene by 1H and 13C NMR spectroscopy clearly
showed the absence of signals for olefinic protons and carbons, and
thus the products were highly linear polyethylenes, saturated with
i-propyl end groups when using MMAO (Run 4 in Table 3) as the co-
catalyst (Figs. 8 and 9) and with methyl end groups when using
MAO (Run 10 in Table 4) as the co-catalyst (Figs. 10 and 11). Under
the optimum catalytic conditions of Fe1eFe5/MMAO system and
Fe1eFe5/MAO system, chain transfer to aluminum was dominant
in the catalytic process; the resultant polyethylenes were of low
molecular weight and narrow PDI.
Fig. 10. 1H NMR spectrum of polyethylene prepared with catalyst Fe4/MAO (Run 10 in Table 4).

X. Cao et al. / Polymer 53 (2012) 1870e1880
1879
Fig. 11. 13C NMR spectrum of polyethylene prepared with catalyst Fe4/MAO (Run 10 in Table 4).
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4. Conclusion
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[16] Bianchini C, Giambastiani G, Rios IG, Meli A, Oberhauser W, Sorace L, et al.
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The series of iron(II) complexes (Fe1eFe6) bearing 2-[1-(2,6-
dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]
pyridine ligands (L1eL5) or the ligand 2,6-bis[1-(2,6-dibenzhydryl-
4-chloro-phenylimino)ethyl]pyridine (L6) were synthesized and
fully characterized. On treatment with MMAO or MAO as the co-
catalyst, the pre-catalysts (Fe1eFe5) exhibited the highest
activity observed for bis(imino)pyridine-based iron systems, with
good thermal stability, producing highly linear polyethylene of low
molecular weights and with narrow PDIs (single-site active
species). Pre-catalysts with such features have the potential to be
applied in the industrial production of polyethylene waxes.
Acknowledgment
[20] Chen Y, Hao P, Zuo W, Gao K, Sun WH. J Organomet Chem 2008;693(10):
1829e40.
[21] Xiao L, Gao R, Zhang M, Li Y, Cao X, Sun WH. Organometallics 2009;28(7):
2225e33.
[22] Gao R, Zhang M, Liang T, Wang F, Sun WH. Organometallics 2008;27(21):
5641e8.
This work is supported by MOST 863 program No.
2009AA034601. CR thanks the EPSRC for an Overseas Travel Grant.
[23] Gao R, Li Y, Wang F, Sun WH, Bochmann M. Eur J Inorg Chem 2009;2009(27):
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692(21):4506e18.
[25] Sun WH, Jie S, Zhang S, Zhang W, Song Y, Ma H, et al. Organometallics 2008;
25(3):666e77.
Appendix. Supplementary data
Supplementary data associated with this article can be found, in
the online version at doi:10.1016/j.polymer.2012.02.050.
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483e91.
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