Synthesis, structural characterization, and olefin polymerization behavior of vanadium(III) complexes bearing bidentate phenoxy-phosphine ligands

Article abstract of DOI:10.1002/pola.26292

Vanadium(III) complexes bearing phenoxy-phosphine ligands (2a-g) (2-R ₁-4-R₂-6-PPh₂-C₆H ₂O)VCl₂(THF)₂ (2a: R₁ = R ₂ = H; 2b: R₁ = F, R₂ = H; 2c: R₁ = Ph, R₂ = H; 2d: R₁ = tBu, R₂ = H; 2e: R ₁ = R₂ = Me; 2f: R₁ = R₂ = tBu; 2g: R₁ = R₂ = CMe₂Ph) were prepared from VCl ₃(THF)₃ by treating with 1.0 equiv of the ligand in tetrahydrofuran (THF) in the presence of excess triethylamine (TEA). The reaction of VCl₃(THF)₃ with 2.0 equiv of the ligand in THF in the presence of excess TEA afforded vanadium(III) complexes bearing two phenoxy-phosphine ligands (3c-f). These complexes were characterized by FTIR and mass spectrum as well as elemental analyses. Structures of 2f and 3c were further confirmed by X-ray crystallographic analyses. Complexes 2a-g and 3c-f were employed as the catalysts for ethylene polymerization under various reaction conditions. On activation with Et₂AlCl, these complexes exhibited high catalytic activities (up to 41.3 kg PE/mmol _V·h·bar) even at high temperature (70°C), and produced high molecular weight polymer with unimodal molecular weight distributions, indicating the polymerization took place in a single-site nature. Complexes 3c-f displayed better thermal stability than the corresponding complexes 2a-g under similar conditions. In addition, copolymerizations of ethylene and 1-hexene with precatalysts 2a-g were also explored in the presence of Et₂AlCl. Catalytic activity, comonomer incorporation, and properties of the resultant polymers can be controlled over a wide range by tuning catalyst structures and reaction parameters. Copyright

Full text of DOI:10.1002/pola.26292

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Synthesis, Structural Characterization, and Olefin Polymerization
Behavior of Vanadium(III) Complexes Bearing Bidentate
Phenoxy-Phosphine Ligands
Sen-Wang Zhang,1,2 Ling-Pan Lu,1,2 Bai-Xiang Li,1 Yue-Sheng Li1
1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
2Graduate School of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
Correspondence to: Y.-S. Li (E-mail: ysli@ciac.jl.cn)
Received 12 June 2012; accepted 26 July 2012; published online
DOI: 10.1002/pola.26292
ABSTRACT: Vanadium(III) complexes bearing phenoxy-phos-
phine ligands (2a–g) (2-R1-4-R2-6-PPh2-C6H2O)VCl2(THF)2 (2a: R1
¼ R2 ¼ H; 2b: R1 ¼ F, R2 ¼ H; 2c: R1 ¼ Ph, R2 ¼ H; 2d: R1 ¼ tBu, R2
¼ H; 2e: R1 ¼ R2 ¼ Me; 2f: R1 ¼ R2 ¼ tBu; 2g: R1 ¼ R2 ¼ CMe2Ph)
were prepared from VCl3(THF)3 by treating with 1.0 equiv of the
ligand in tetrahydrofuran (THF) in the presence of excess trie-
thylamine (TEA). The reaction of VCl3(THF)3 with 2.0 equiv of the
ligand in THF in the presence of excess TEA afforded vanadiu-
m(III) complexes bearing two phenoxy-phosphine ligands (3c–f).
These complexes were characterized by FTIR and mass spec-
trum as well as elemental analyses. Structures of 2f and 3c were
further confirmed by X-ray crystallographic analyses. Com-
plexes 2a–g and 3c–f were employed as the catalysts for ethyl-
ene polymerization under various reaction conditions. On
activation with Et2AlCl, these complexes exhibited high catalytic
activities (up to 41.3 kg PE/mmolVꢀhꢀbar) even at high tempera-
ture (70ꢁC), and produced high molecular weight polymer with
unimodal molecular weight distributions, indicating the poly-
merization took place in a single-site nature. Complexes 3c–f
displayed better thermal stability than the corresponding com-
plexes 2a–g under similar conditions. In addition, copolymeriza-
tions of ethylene and 1-hexene with precatalysts 2a–g were also
explored in the presence of Et2AlCl. Catalytic activity, comono-
mer incorporation, and properties of the resultant polymers can
be controlled over a wide range by tuning catalyst structures
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and reaction parameters. 2012 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: catalysts; phenoxy-phosphine ligand; polyolefins;
Ziegler-Natta polymerization
oxidation state of vanadium systems.16 Designing and
synthesizing ancillary ligands to stabilize active vanadium
species is another powerful approach to keep vanadium in
high-oxidation and subsequently prolong the catalyst life-
time.17–36 For example, Lorber et al. found the amine
bis(phenolate) ligand can stabilize vanadium complexes that
exhibited high activity for ethylene polymerization.17 Nomura
obtained many different (arylimino) vanadium complexes
containing aryloxo and alkoxo ligands, which possessed
excellent efficiency toward olefin (co)polymerization.18,19
Aryloxide-based vanadyl complexes, reported by Redshaw’s
group, exhibited excellent performance in olefin polymeriza-
tion at an elevated temperature.20 However, complexes with
high thermal stability in olefin polymerizations were scarce.
INTRODUCTION A significant number of advances in olefin
polymerization catalysis have been reported in the past
decade, and the development of new generation ‘‘nonmetallo-
cene’’ catalysts has attracted great interest recently.1–5
Among the transition metals, vanadium catalysts exhibited
promising characteristics, especially for the syntheses of high
molecular weight polyethylene,6–10 syndiotactic polypropyl-
ene,11,12 and poly(ethylene-co-propylene) and poly(ethylene-
co-propylene-co-diene) elastomers.13–15 Nonetheless, the
reports on vanadium catalysts are limited owing to low
activity, which is ascribed to their deactivation during the
polymerization, especially at high polymerization tempera-
ture, due to reduction of catalytically active vanadium
species to low valent, less active, or inactive species. This
problem could be overcome by reactivation of inactive vana-
dium(II) center to active vanadium(III) species by addition
of Cl3CCOOEt or chlorinated hydrocarbons (namely ‘‘rejuve-
nators’’ or ‘‘promoters’’ in the literature), which has proven
to be effective reagents for maintaining the higher (active)
In Group 4 metal complexes, the phenoxide donors tend to
be matched with relatively hard first-row nitrogen and oxy-
gen-based donors.37–40 Bearing hard nitrogen and oxygen-
based donors, salicylaldiminato ligands have been used in
Additional Supporting Information may be found in the online version of this article.
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2012 Wiley Periodicals, Inc.
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calibration was made by polystyrene standard EasiCal PS-1
(PL Ltd).
Ethyl trichloroacetate (ETA) were purchased from Aldrich,
dried over calcium hydride at a room temperature, and then
distilled. The n-butyllithium solution in hexane and
VCl3(THF)3 was purchased from Aldrich. Et2AlCl, AlMe3,
AlEt3, and AliBu3 were purchased from Albemarle Corpora-
tion. Modified methylaluminoxane (MMAO, 7% aluminum in
heptane solution) was purchased from Akzo Nobel Chemical.
Commercial ethylene was directly used for polymerization
without further purification. The other reagents and solvents
were commercially available. Various phenoxy-phosphine
ligands containing different substituent on R1 and R2 posi-
tions, 2-R1-4-R2-6-PPh2-C6H2OH (2a: R1 ¼ H, R2 ¼ H; 2b: R1
¼ F, R2 ¼ H; 2c: R1 ¼ Ph, R2 ¼ H; 2d: R1 ¼ tBu, R2 ¼ H;
CHART 1 Previously reported salicylaldiminato vanadium-(iii)
complexes.
transition metal organometallics and have already been
shown to afford highly active olefin polymerization catalysts
for Groups 4, 6,41 and 10 metal systems.42–44 Previously, our
group had synthesized a series of mono and bis(salicylaldi-
minato) vanadium complexes, which showed high catalytic
activity toward ethylene polymerization (Chart 1).45,46 How-
ever, the deactivation or chain transfer to aluminum took
place, and broad molecular weight distribution polymers
were obtained at an elevated polymerization temperature by
the mono salicylaldiminato vanadium(III) complexes. In addi-
tion, there is growing evidence that softer second-row
donors may offer beneficial stabilization of the highly reac-
tive metal center.47–52 The combination of hard donors with
soft phosphine donors in early transition metal polymeriza-
tion catalysis increased the configurational stability of the
catalyst, which was found by Gibson and Tang.53–63 This
work was initially aimed at introducing softer phosphine
donors to the ligands, which were used to synthesize mono
and bis(phenoxy-phosphine) vanadium complexes. These
catalysts displayed some different characteristics from
salicylaldiminato vanadium complexes for ethylene
(co)polymerization.
2e: R1 ¼ Me, R2 ¼ Me; 2f: R1 ¼ tBu, R2 ¼ tBu; 2g: R1
¼
CMe2Ph, and R2 ¼ CMe2Ph) were prepared according to lit-
erature procedures.55,64,65
Synthesis and Characterization of Vanadium Complexes
(2-PPh2-C6H2O)VCl2(THF)2 (2a)
To a stirred solution of VCl3(THF)3 (0.75 g; 2.0 mmol) in
dried tetrahydrofuran (THF) (20 mL) was added slowly a so-
lution of 2-PPh2-C6H2OH (0.56 g; 2.0 mmol) in THF (20 mL).
The red reaction mixture was stirred for 10 min, and Et3N
(0.3 mL, 216 mg, and 2.1 mmol) was added. After stirring
for 4 h at room temperature the solution was concentrated
to about 10 mL, and then filtered to remove NH4Cl. Crystalli-
zation by diffusion of n-hexane (20 mL) into the clear
solution yielded red-black crystals of 2a (0.65 g; 60%). Com-
pounds 2b–g were prepared analogously. IR (KBr pellets): m
3365, 3042, 2963, 1828, 1635, 1584, 1539, 1462, 1422,
1411, 1402, 1356, 1322, 1265, 1232, 1154, 1120, 1103,
1078, 1024, 976, 926, 872, 852, 757, 752, 728, 702, 658,
635, 563, 522, 493, and 434 cmꢂ1. EI-MS (70 eV): m/z ¼
542 [Mþ]. Anal. Calcd for C26H30ClO3PV: C, 57.47; H, 5.57.
Found: C, 57.56; H, 5.51.
EXPERIMENTAL
General Procedure and Materials
All manipulation of air- and/or moisture-sensitive com-
pounds was carried out under a dry argon atmosphere
using standard Schlenk techniques or under a dry argon
atmosphere in an MBraun glovebox unless otherwise noted.
All solvents were purified from an MBraun solvent purifica-
tion system. The NMR data of the ligands were obtained on
a Bruker 300 MHz spectrometer at an ambient temperature,
with CDCl3 as a solvent. The IR spectra were recorded on a
Bio-Rad FTS-135 spectrophotometer. Elemental analyses
were recorded on an elemental Vario EL spectrometer. Mass
spectra were obtained using electron impact (EI-MS) and
LDI-1700 (Linear Scientific). The NMR data of PE and ethyl-
ene/1-hexene copolymers were obtained on a Bruker 400
MHz spectrometer at 125ꢁC with o-C6D4Cl2 as a solvent.
DSC measurements were performed on a Perkin–Elmer
Pyris 1 differential scanning calorimeter at a rate of 10ꢁC/
min. The weight-average molecular weight (Mw) and the
polydispersity index (PDI) of polymer samples were deter-
mined at 150ꢁC by a PL-GPC 220 type high-temperature
chromatograph equipped with three Plgel 10 mm Mixed-B
LS type columns. 1,2,4-Trichlorobenzene (TCB) was
employed as the solvent at a flow rate of 1.0 mL/min. The
(2-F-6-PPh2-C6H2O)VCl2(THF)2 (2b)
Yield: 69%. IR (KBr pellets): m 3414, 3059, 2967, 2953,
1971, 1903, 1822, 1601, 1575, 1553, 1466, 1441, 1382,
1310, 1262, 1237, 1193, 1123, 1068, 1027, 998, 896, 857,
780, 740, 727, 705, 693, 661, 618, 602, 561, 488, and
449 cmꢂ1. EI-MS (70 eV): m/z ¼ 560 [Mþ]. Anal. Calcd
for C26H29Cl2FO3PV: C, 55.63; H, 5.21. Found: C, 55.46; H,
5.27.
(2-Ph-6-PPh2-C6H2O)VCl2(THF)2 (2c)
Yield: 51%. IR (KBr pellets): m 3390, 3057, 2974, 2901,
1828, 1600, 1579, 1588, 1483, 1448, 1436, 1401, 1342,
1351, 1294, 1236, 1206, 1187, 1159, 1123, 1096, 1073,
1024, 999, 919, 875, 852. 803, 759, 752, 728, 702, 667, 635,
577, 563, 522, 491, and 467 cmꢂ1. EI-MS (70 eV): m/z ¼
618 [Mþ]. Anal. Calcd for C32H34Cl2O3PV: C, 62.05; H, 5.53.
Found: C, 62.11; H, 5.48.
(2-tBu-6-PPh2-C6H2O)VCl2(THF)2 (2d)
Yield: 48%. IR (KBr pellets): m 3401, 3029, 2954, 2934,
1968, 1857, 1835, 1634, 1566, 1534, 1465, 1427, 1362,
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1306, 1251, 1233, 1165, 1123, 1068, 1034, 992, 886, 868,
773, 740, 727, 705, 692, 654, 625, 602, 561, 476, and 455
cmꢂ1. EI-MS (70 eV): m/z ¼ 598 [Mþ]. Anal. Calcd for
EI-MS (70 eV): m/z
¼
768 [Mþ]. Anal. Calcd for
C44H44ClO3P2V: C, 68.71; H, 5.77. Found: C, 68.75; H, 5.69.
(2-tBu-4-tBu-6-PPh2-C6H2O)2VCl(THF) (3f)
C
30H38Cl2O3PV: C, 60.11; H, 6.39. Found: C, 60.15; H, 6.34.
Yield: 57%. IR (KBr pellets): m 3411, 3021, 2866, 1862,
1641, 1537, 1548, 1426, 1411, 1408, 1379, 1354, 1311,
1267, 1225, 1143, 1108, 1066, 1031, 1021, 972, 866, 835,
(2-Me-4-Me-6-PPh2-C6H2O)VCl2(THF)2 (2e)
Yield: 45%. IR (KBr pellets): m 3254, 3015, 2876, 1815, 1633,
1576, 1526, 1438, 1421, 1401, 1378, 1356, 1324, 1263, 1232,
1151, 1119, 1087, 1032, 1024, 975, 958, 872, 864, 852, 773,
735, 716, 702, 648, 633, 620, 578, 534, 515, 476, and 422
cmꢂ1. EI-MS (70 eV): m/z ¼ 570 [Mþ]. Anal. Calcd for
812, 773, 724, 702, 664, 623, 572, 538, 448, and 427 cmꢂ1
.
EI-MS (70 eV): m/z
¼
936 [Mþ]. Anal. Calcd for
C56H68ClO3P2V: C, 71.75; H, 7.31. Found: C, 71.68; H, 7.25.
Ethylene Polymerization
C28H34Cl2O3PV: C, 58.86; H, 6.00. Found: C, 58.75; H, 6.06.
Polymerization was carried out under atmospheric pressure
in toluene in a 150 mL glass reactor equipped with a mechan-
ical stirrer. Toluene (50 mL) was introduced into the nitrogen-
purged reactor and stirred vigorously (600 rpm). The toluene
was kept at a prescribed polymerization temperature, and
then ethylene gas feed was started. After 15 min, a solution of
Et2AlCl in toluene and a solution of ETA in toluene were
added and stirred for 5 min. Then a toluene solution of the va-
nadium complexes was added into the reactor with vigorous
stirring (900 rpm) to initiate polymerization. After a pre-
scribed time, acidic alcohol (10 mL) was added to terminate
the polymerization reaction, and the ethylene gas feed was
stopped. The resulting mixture was added to acidic alcohol.
The solid polyethylene was isolated by filtration, washed with
alcohol, and dried at 60ꢁC for 24 h in a vacuum oven.
(2-tBu-4-tBu-6-PPh2-C6H2O)VCl2(THF)2 (2f)
Yield: 56%. IR (KBr pellets): m 3321, 3017, 2952, 2923,
1965, 1832, 1807, 1634, 1563, 1525, 1467, 1423, 1361,
1325, 1278, 1233, 1165, 1123, 1068, 1034, 992, 886, 868,
763, 745, 737, 715, 682, 674, 621, 602, 563, 466, and 427
cmꢂ1. EI-MS (70 eV): m/z ¼ 654 [Mþ]. Anal. Calcd for
C34H46Cl2O3PV: C, 62.29; H, 7.07. Found: C, 62.25; H, 7.01.
(2-CMe2Ph-4-CMe2Ph-6-PPh2-C6H2O)VCl2(THF)2 (2g)
Yield: 53%. IR (KBr pellets): m 3327, 3015, 2866, 1804,
1653, 1545, 1521, 1439, 1421, 1408, 1372, 1345, 1321,
1275, 1237, 1154, 1118, 1067, 1031, 1022, 977, 876, 865,
843, 772, 733, 725, 706, 639, 631, 575, 538, 520, 473, and
449 cmꢂ1. EI-MS (70 eV): m/z ¼ 778 [Mþ]. Anal. Calcd for
C34H46Cl2O3PV: C, 67.78; H, 6.46. Found: C, 67.74; H, 6.42.
Ethylene/1-Hexene Copolymerization
(2-Ph-6-PPh2-C6H2O)2VCl(THF) (3c)
Copolymerization was carried out in toluene in a 150 mL
glass reactor equipped with a mechanical stirrer. The reactor
was charged with 20 mL of toluene and the prescribed
amount of 1-hexene. Next, the ethylene gas feed was started
followed by equilibration at the desired polymerization tem-
perature. After 15 min, a solution of Et2AlCl in toluene and a
solution of ETA in toluene were added and stirred for 5 min.
Subsequently, a toluene solution of vanadium complexes was
added into the reactor with vigorous stirring (900 rpm) to
initiate polymerization. After a prescribed time, ethanol (10
mL) was added to terminate the polymerization reaction,
and the ethylene gas feed was stopped. The resulting mix-
ture was added to acidic ethanol. The polymer was isolated
by filtration, washed with ethanol, and dried at 60ꢁC for 12
h in a vacuum oven.
To a stirred solution of VCl3(THF)3 (0.36 g; 1.0 mmol) in
dried THF (20 mL) was added slowly a solution of 2-Ph-6-
PPh2-C6H2OH (0.71 g; 2.0 mmol) in THF (20 mL). The red
reaction mixture was stirred for 20 min, and Et3N (0.3 mL,
216 mg, and 2.1 mmol) was added. After stirring overnight
at room temperature the solution was concentrated to about
10 mL and then the mixture was filtered to remove NH4Cl.
Crystallization by diffusion of n-hexane (20 mL) into the
clear solution and chilling the solution (ꢂ40ꢁC) yielded
0.535 g of 3c (62%) as red-brown crystals. Compounds 3d–f
were prepared analogously. Yield: 51%. IR (KBr pellets): m
3364, 3017, 2835, 1823, 1644, 1537, 1548, 1426, 1411,
1408, 1379, 1354, 1311, 1267, 1243, 1152, 1121, 1068,
1031, 1021, 973, 872, 861, 823, 771, 735, 716, 706, 633,
624, 572, 538, 523, 448, and 427 cmꢂ1. EI-MS (70 eV): m/z
¼ 864 [Mþ]. Anal. Calcd for C52H44ClO3P2V: C, 72.18; H, 5.13.
Found: C, 72.11; H, 5.18.
Crystallographic Studies
Crystals for X-ray analysis were obtained as described in the
preparations. The crystallographic data, collection parame-
ters, and refinement parameters are listed in Supporting
Information Table S1. The crystals were manipulated in a
glovebox. The intensity data were collected with the x scan
mode (186 K) on a Bruker Smart APEX diffractometer with
CCD detector using Mo Ka radiation (k ¼ 0.71073Å). Lor-
entz, polarization factors were made for the intensity data,
and absorption corrections were performed using the
SADABS program. The crystal structures were solved using
the SHELXTL program and refined using full matrix least
squares. The positions of hydrogen atoms were calculated
theoretically and included in the final cycles of refinement in
a riding model along with attached carbons.
(2-tBu-6-PPh2-C6H2O)2VCl(THF) (3d)
Yield: 51%. IR (KBr pellets): m 3421, 3015, 2943, 2921,
1962, 1837, 1806, 1634, 1545, 1517, 1456, 1422, 1374,
1321, 1274, 1231, 1175, 1112, 1065, 1032, 998, 874, 856,
737, 711, 705, 647, 635, 621, 544, 466, and 438 cmꢂ1. EI-
MS (70 eV): m/z ¼ 824 [Mþ]. Anal. Calcd for C48H52ClO3P2V:
C, 69.86; H, 6.35. Found: C, 69.81; H, 6.28.
(2-Me-4-Me-6-PPh2-C6H2O)2VCl(THF) (3e)
Yield: 44%. IR (KBr pellets): m 3321, 3016, 2942, 2927,
1953, 1832, 1807, 1632, 1544, 1512, 1453, 1427, 1371,
1321, 1269, 1231, 1174, 1132, 1078, 1037, 998, 874, 856,
823, 726, 717, 708, 663, 635, 628, 565, 447, and 433 cmꢂ1
.
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SCHEME 1 General synthetic route of the vanadium com-
plexes used in this study.
FIGURE 1 Molecular structure of complex 2f. Thermal ellip-
soids are drawn at the 30% probability level, and H atoms are
omitted for clarity.
RESULTS AND DISCUSSION
Synthesis and Characterization of Phenoxy-phosphine
Vanadium(III) Complexes
catalysts. However, IR spectra of mono and relevant bis(phe-
noxy-phosphine) vanadium were similar.
A general synthetic route for new vanadium complexes used
in this study is shown in Scheme 1. The phenoxy-phosphine
ligands 1a–g were prepared via the known procedure.55,64,65
The reaction of VCl3(THF)3 with 1.0 equiv of 2-R1-4-R2-6-
PPh2-C6H2OH (1a–g) in THF in the presence of excess triethyl-
amine (TEA) afforded vanadium(III) complexes 2a–g in mod-
erate yields (2a, 60%; 2b, 69%; 2c, 51%; 2d, 48%; 2e, 45%;
2f, 56%; 2g, 53%). These reactions took place along with evo-
lution of hydrochloride, and the pure samples as dark red or
brown crystallized solids were isolated from the chilled con-
centrated mixture of THF and hexane solution. These com-
plexes were identified by FTIR and mass spectrum as well as
elemental analyses (the sample was dried at room tempera-
ture for 5 h in a vacuum pump). The resonances are broad-
ened to such an extent that they become effectively unobserv-
able in the 1H (or 13C) NMR spectra for the complexes in
CDCl3 or C6D6, indicating that they are paramagnetic species.
Crystals suitable for crystallographic analyses were grown
from the chilled concentrated THF-hexane mixture solution.
The ratio of THF-hexane was adjusted in the range of 1/1 to
1/3, according to the solubility of the complexes. A dark red
block microcrystal of 2f and 3c suitable for X-ray crystallo-
graphic analysis was grown. The crystallographic data to-
gether with the collection and refinement parameters are
summarized in Supporting Information Table S1. Molecular
structures and the selected bond lengths and bond angles
for 2f and 3c are shown in Figures 1 and 2.
Also shown in Scheme 1, reaction of VCl3(THF)3 with 2.0
equiv of phenoxy-phosphine ligands 1c-f in the presence of
excess TEA in THF afforded bis(phenoxy-phosphine) com-
plexes 3c–f in moderate yields (3c, 62%; 3d, 51%; 3e, 44%;
3f, 57%). The reaction products were identified by mass
spectra, FTIR, and elemental analysis. 1H NMR spectra indi-
cated that these complexes are also paramagnetic species,
which is similar to the case of complexes 2a–g.
The representative IR spectrums of 1c and 2c were studied
and the results were present in Supporting Information
Figure S1. It clearly indicated the disappearance of the band
related to the OAH stretching in free ligand at about 3350
cmꢂ1. Instead, stretching frequencies of furan at about 3390,
3057, and 2974 cmꢂ1 were observed. In addition, the strong
band of m(P-Ph) at around 1183 cmꢂ1 shifts to lower fre-
quency around 1123 cmꢂ1. The proposed structure is in line
with the elemental analysis. Obvious difference in IR spec-
trums was observed for other ligands and the corresponding
FIGURE 2 Molecular structure of complex 3c. Thermal ellip-
soids are drawn at the 30% probability level, and H atoms are
omitted for clarity.
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As shown in Figure 1, complex 2f has a six-coordinate dis-
torted octahedral geometry around the V metal center. The
two chlorine atoms are situated in the cis position, and the
two THF molecules are also in cis position to each other, as
seen in the bond angles for O(1)-V-O(2) (83.96ꢁ) and Cl(2)-
V-Cl(3) (95.25ꢁ). However, in our previous work,45 complex
[C6H5N¼¼CH(OC6H2tBu2-2,4)] VCl2(THF)2, bearing the same
substituents in the aryloxy group, possesses a different con-
figuration, in which two chlorine atoms are situated in the
trans position, while the two THF molecules are in cis posi-
tion to each other. This may be due to steric hindrance of
two benzene rings on the phosphorus atom.
vating agent to reactivate the low-valent, less active, or inac-
tive species to active vanadium(III) species. The typical
results of the polymerization are collected in Table 1. The
data in Table 1 (Entries 1–4 and 6) indicate that Et2AlCl is
the most effective cocatalyst among the five alkylaluminums
under the similar conditions, while a much lower activity
was obtained using MMAO or AlMe3 or AlEt3 as a cocatalyst,
and even no polymer was formed using AliBu3 as a cocata-
lyst, due to the strong reducibility. We found that AlEt2Cl
displayed at least order of magnitude higher activities
compared with (MMAO), AlMe3, AlEt3, and AliBu3. Then we
choose AlEt2Cl as cocatalyst for the following experiments.
The catalytic activities of 2e and 3e first rapidly enhanced
and then gradually declined with the increase of Al/V molar
ratio, and the maximal value was obtained when the Al/V
ratio equals 4000 (Fig. 3). However, the molecular weights of
the polymer obtained gradually decreased with the increase
of Et2AlCl concentration, which indicates that chain transfer
to aluminum took place during the polymerization. All the
polymers prepared were linear polyethylene confirmed by
13C NMR spectra, which is similar to that b-enaminoketonato
Selected bond lengths (Å) and bond angles (deg): V-O(3),
1.897(4); V-O(1), 2.109(4); V-O(2), 2.162(4); V-Cl(2),
2.2968(17); V-Cl(1), 2.3345(17); V-P, 2.4943(17); O(3)-V-
O(1), 92.20(15); O(3)-V-O(2), 83.69(15); O(1)-V-O(2),
83.96(15); O(3)-V-Cl(2), 95.11(12); O(1)-V-Cl(2), 91.69(12);
O(2)-V-Cl(2), 175.43(12); O(3)-V-Cl(1), 166.17(13); O(1)-V-
Cl(1), 96.63(12); O(2)-V-Cl(1), 86.67(11); Cl(2)-V-Cl(1),
95.25(6); O(3)-V-P, 78.33(11); O(1)-V-P, 170.41(12); O(2)-V-P,
93.37(11); Cl(2)-V-P, 90.70(6); Cl(1)-V-P, 92.40(6).
or
salicylaldiminato
vanadium
catalysts
reported
previously.45,68
Compared with the structure of II and [p-CF3C6
H4N¼¼CH(C6H4O)]2VCl(THF) reported previously,52 complex
3c displays different geometry, in which the equatorial posi-
tions are occupied by two phosphorus atoms of the phe-
noxy-phosphine ligand and two oxygen atoms, one from the
THF molecule and the other from ligand (Fig. 2). Chlorine
atom and oxygen atom from the ligand are coordinated on
the axial position, which may also be attributed to the steric
effect of diphenylphosphine substitute. The bond length V-P
2.5544Å in complex 3c is slightly longer than 2f (2.4943Å)
and some reported vanadium complexes (2.367–2.500Å),66,67
indicating that the interaction between V and P is not so
strong, probably because two bulky coordinated ligands
repulsed each other. The crowded environment of the two
crystal structures may have a great influence on polymeriza-
tion results. Other bond lengths, such as V-O, V-Cl in 2f and
3c are somewhat analogous.
The 13C and 1H NMR spectra revealed that the signals
belonging to chain-end double bonds were not detected for
low molecular weight copolymers, indicating that chain
transfer to aluminum was the dominant chain-transfer path-
way under these reaction conditions.
The reaction temperature considerably influenced polymer-
ization behaviors, as shown in Figure 4. It is noteworthy that
the catalytic activities of complexes 2a–g displays the highest
activities at 50ꢁC, and then slightly decreased at 70ꢁC. How-
ever, the molecular weights of the polymer obtained sharply
decreased with the increase of reaction temperature, indicat-
ing that high temperature accelerates chain transfer reaction.
In addition, substituents in the ligands greatly influence cata-
lytic activities and the molecular weights of the polymers
obtained. Complex 2a, without any substituents on the
ligand, displays relative low catalytic activity. Complex 2b,
bearing F-substituted phenoxy-phosphine ligand, also shows
only low to mild catalytic activities, indicating that electron-
withdrawing effects may be helpless for catalytic perform-
ance. Comparing 2c–g, however, we found catalytic activities
was improved from 13.4 (2c), 23.0 (2d) to 41.3 (2f) (kg PE/
mmolVꢀhꢀbar) with increasing of the steric hindrance, imply-
ing that introducing bulky substituents on aryloxy moiety of
the phenoxy-phosphine ligand can efficiently improve cata-
lytic activity. Although the steric hindrance of 2e, bearing Me
substituents, was not very large, it displayed higher activities
(27.8) than 2c and 2d, indicating that para-methyl on ary-
loxy moiety of the phenoxy-phosphine ligand directly influ-
ences vanadium catalyst’s performance. Complex 2g bearing
bulky CMe2Ph on the aryloxy moiety, seeming to be an
exceptant, exhibits relative low catalytic activity. Previous
investigation showed that mono salicylaldiminato vanadium I
produced the PE with broad molecular weight distributions
when reaction temperature was 70ꢁC, although it displayed
Selected bond lengths (Å) and bond angles (deg): V-O(2),
1.8951(13); V-O(1), 1.9357(13); V-O(3), 2.1222(13); V-Cl(1),
2.3134(6); V-P(2), 2.5510(6); V-P(1), 2.5544(6); O(2)-V-O(1),
96.15(6); O(2)-V-O(3), 84.78(6); O(1)-V-O(3), 88.83(5); O(2)-
V-Cl(1), 101.71(4); O(1)-V-Cl(1), 162.12(4); O(3)-V-Cl(1),
93.55(4); O(2)-V-P(2), 172.16(4); O(1)-V-P(2), 76.14(4);
O(3)-V-P(2), 96.33(4); Cl(1)-V-P(2), 85.98(2); O(2)-V-P(1),
79.17(4); O(1)-V-P(1), 87.70(4); O(3)-V-P(1), 163.11(4);
Cl(1)-V-P(1), 94.81(2); P(2)-V-P(1), 98.865(19).
Ethylene Polymerization
Co-catalysts, especially alkylaluminums, play an irreplaceable
role in the olefin polymerization promoted by vanadium cat-
alysts, and the performance of catalysts depended very much
on the nature of the cocatalyst. In this article, we investi-
gated the performance of complex catalyzing ethylene poly-
merization with the aid of various kinds of alkylaluminums,
such as modified methylaluminoxane (MMAO), AlMe3, AlEt3,
AliBu3, and Et2AlCl. Cl3CCOOEt (ETA) was used as a reacti-
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TABLE 1 Typical Results of Ethylene Polymerization by Complexes 2a–g and 3c–fa
Activityb
Mw (ꢃ104)
Mw/Mn
c
c
Entry
Complex
Temp (ꢁC)
Cocatalyst
Time (min)
Polymer (g)
1
2f
2f
2f
2f
2f
2f
2f
2a
2b
2c
2d
2e
2g
3c
3d
3e
3f
I
50
50
50
50
25
50
70
50
50
50
50
50
50
50
50
50
50
25
50
70
25
50
70
MMAO
AliBu3
10
10
10
10
5
0.03
Trace
0.01
0.14
1.48
1.72
1.23
0.27
0.31
0.56
0.96
1.16
0.58
0.64
0.59
0.63
0.94
0.93
0.88
0.53
0.35
0.43
0.41
0.36
–
2
3
AlEt3
0.12
1.68
35.5
41.3
29.5
6.48
7.44
13.4
23.0
27.8
13.9
15.4
14.2
15.1
22.6
22.3
21.1
12.7
8.4
4
AlMe3
5
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
12.3
4.2
2.3
6.3
5.7
5.2
4.2
6.1
5.7
6.8
5.5
8.2
7.6
13.8
5.7
3.8
17.1
6.1
1.5
2.4
2.1
3.1
2.4
1.9
2.4
3.0
2.7
1.8
2.7
2.2
2.6
2.0
2.7
3.8
12.0
1.9
2.2
2.6
6
5
7
5
8
5
9
5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
a
5
5
5
5
5
5
5
5
5
I
5
I
5
II
5
II
5
10.3
9.84
II
5
Reaction conditions: 1 bar of ethylene pressure, 0.5 lmol of vanadium complex, Cl3CCO2Et/V (molar ratio) ¼ 300, Al/V (molar ratio) ¼ 4000, toluene
50 mL.
b
kg/mmolVꢀhꢀbar.
c
Determined by GPC in 1,2,4-trichlorobenzene versus polystyrene standard.
high catalytic activity. In contrast, 2d, 2e, and 2f not only
showed much higher activities than I, but also produced
polymers with relatively high molecular weights with unimo-
dal molecular weight distributions, confirmed by GPC at
70ꢁC, which attribute to benefits of introduction of softer
second-row donor atoms and proper steric hindrance.
FIGURE 3 Plots of catalytic activity of vanadium complex and
weight-average molecular weight of the polymers obtained vs.
Al/V molar ratio. Reaction conditions: 0.5 lmol vanadium com-
plex, Cl3CCO2Et 0.15 mmol, ethylene 1 atm, toluene 50 mL,
and at 50ꢁC polymerization for 5 min.
FIGURE 4 Catalytic activities of complexes 2a–g toward ethyl-
ene polymerization at different temperatures. Reaction condi-
tions: 0.5 lmol of vanadium complex, Cl3CCO2Et 0.15 mmol,
Et2AlCl 2 mmol, ethylene 1 bar, toluene 50 mL, and polymeriza-
tion for 5 min.
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tion, which observed from 13C NMR spectra. The data of
entries 1–6 and 14 in Table 2 indicate that catalytic activities
was improved in the order of increasing steric bulk on ary-
loxy moiety of the phenoxy-phosphine ligand (2a < 2c <
2d, and 2e < 2f), implying that introducing bulky substitu-
ents can efficiently improve catalytic activity. However, con-
tinuing to increase steric bulk of the ligand, compared with
catalyst 2f, 2g display slightly lower activities for the copoly-
merization, which indicated that introduction of too bulky
substituents was baleful to chain growth. Complex 2b, bear-
ing F-substituted phenoxy-phosphine ligand, also show only
low to mild catalytic activities, indicating that electron-with-
drawing effects may be harmful for catalytic performance.
The data of entries 1–6 and 14 in Table 2 also indicated that
the 1-hexene incorporation increases in the order of decreas-
ing steric bulk on aryloxy moiety of the phenoxy-phosphine
ligand (2a/2b > 2c > 2d and 2e > 2f > 2g), which showed
distinct regularity character.
FIGURE 5 Catalytic activities of complexes 3c–f toward ethyl-
ene polymerization at different temperatures. Reaction condi-
tions: 0.5 lmol of vanadium complex, Cl3CCO2Et 0.15 mmol,
Et2AlCl 2 mmol, ethylene 1 bar, toluene 50 mL, and polymeriza-
tion for 5 min.
The effect of reaction conditions on the copolymerization of
ethylene/1-hexene was also investigated using catalyst 2f.
An enhancement in polymerization temperature from 25 to
50ꢁC led to an increase in the catalytic activity (entry 6, 4.56
kg/mmolVꢀhꢀbar; entry 7, 5.10 kg/mmolVꢀhꢀbar) and 1-hex-
ene incorporation (from 2.54% to 3.45%), but a decrease in
the molecular weight of the resultant copolymer, while the
molecular weight distribution of the copolymers almost
remained constant. The increase of initial hexene concentra-
tion led to an increase in the 1-hexene incorporation (entries
6 and 8–11), while catalytic activity, the melting tempera-
ture, and the Mw of the resultant polymers gradually
decreased. A dependence of catalytic activity on Et2AlCl con-
centration is observed. As shown in entries 6, 12, and 13
(Table 2), a maximal activity of 4.56 kg/mmolVꢀhꢀbar when
the Al/V (molar ratio) equals 4000, whereas the molecular
weight of the resultant polymers decreases with the increase
of Al/V (molar ratio) from 1000 to 4000, indicating chain
transfer reaction to Al occurred. Also the 1-hexene incorpo-
ration of the copolymer slightly increased with the increase
of Et2AlCl concentration. In addition, the molecular weight
distributions remained constant, indicating a single-site cata-
lytic behavior.
Although bis(salicylaldiminato) vanadium complexes are
effective catalyst precursors for ethylene polymerization,
they show much lower catalytic activities than mono salicy-
laldiminato complexes. In contrast, bis(phenoxy-phosphine)
vanadium complexes 3c–f show only slightly lower catalytic
activities than monophenoxy-phosphine vanadium catalysts,
as shown in Figure 5. Interestingly, all bis(phenoxy-phos-
phine) vanadium complexes displayed comparable activities
under the same conditions. This is perhaps because the
structures of bis(phenoxy-phosphine) vanadium complexes
themselves already had the crowded environment, so that
steric effect of the phenoxy moiety of the ligands was not so
obvious as 2c–f. An increase in reaction temperature from
50 to 70ꢁC results in little decreasing catalytic activity for
most of the complexes, while slightly enhancing catalytic ac-
tivity for 3e, suggesting the potential high thermal stability
of these catalysts. This hypothesis could be confirmed in
Supporting Information Figure S2. The catalytic activities of
the phenoxy-phosphine complexes such as 2e sharply decline
with prolonged reaction time, while those of the bis(phe-
noxy-phosphine) complexes such as 3e only slightly
decrease. Moreover, all the bis(phenoxy-phosphine) vana-
dium complexes show much higher activities than II under
the same condition, which may also attribute to benefits of
introduction of softer second-row donor atoms.
The microstructures of ethylene/1-hexene copolymers are
established by 13C NMR in o-C6D4Cl2 at 125ꢁC, with the
assignment of the microstructure following previous work
reported by Hsieh and Randall.69,70 As shown in Figure 6,
the resultant copolymer only possessed isolated 1-hexene
inserted unit ([EHE] assigned as Tdd) among the repeated
ethylene insertions when 1-hexene incorporation of the
copolymer was not high (entries 6 and 9). However, when
1-hexene incorporation reached 11.4 mol% (entry 11), the
resultant copolymer possessed isolated 1-hexene inserted
unit ([EHE] assigned as Tdd) among the repeated ethylene
insertions, and the alternating sequence ([HEH] assigned as
Ethylene/Hexene Copolymerization by Complexes 2a–g
The copolymerizations of ethylene/1-hexene were also
studied, and the representative results are summarized in
Table 2. In the presence of cocatalyst Et2AlCl and promoter
Cl3CCOOEt, catalysts 2a–g show high activities towards eth-
ylene/1-hexene copolymerization and produce copolymers
with high molecular weight and unimodal molecular weight
distributions. A quantitative 13C NMR technique was used
for measuring 1-hexene incorporation and microstructure of
the copolymers. Chain termination mechanism in ethylene/
hexene copolymerization coincides with ethylene polymeriza-
S
bb) was also present with a low extent. In addition, the
resonances ascribed to two 1-hexene repeating unit ([EHH]
Tbd) were seen in the spectrum, and even tiny peaks due to
block-type sequence of 1-hexene repeating units were
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TABLE 2 Copolymerization of Ethylene and 1-Hexene by 2a–ga
d
Temp.
(ꢁC)
1-Hexene
(mol/L)
Polymer
(g)
Hexene
Incorpc
Mw
Activityb
(ꢃ103)
Mw/Mn
Tm (ꢁC)e
d
Entry
Complex
1
2a
2b
2c
2d
2e
2f
25
25
25
25
25
25
50
25
25
25
25
25
25
25
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.4
0.8
1.2
0.2
0.2
0.2
0.14
0.20
0.33
0.43
0.39
0.76
0.85
0.89
0.53
0.36
0.25
0.53
0.68
0.62
0.84
1.20
1.98
2.58
2.34
4.56
5.10
5.34
3.18
2.16
1.50
3.18
4.08
3.72
3.75
3.44
3.11
2.92
2.96
2.54
3.45
1.70
5.68
7.91
11.4
2.66
2.60
2.33
41.1
38.5
47.2
52.1
49.4
55.2
28.2
66.7
35.4
22.3
12.4
80.2
61.3
61.3
2.2
2.4
2.6
1.8
2.1
1.9
1.7
2.4
2.6
2.1
2.5
2.3
2.0
2.2
105.3
106.2
109.3
110.1
110.5
111.2
106.3
115.6
101.3
91.9
2
3
4
5
6
7
2f
8
2f
9
2f
10
11
12f
13g
14
a
2f
2f
78.2
2f
110.3
110.5
112.1
2f
2g
d
e
f
Conditions: toluene þ comonomer ¼ 30 mL, ethylene 1 atm, catalyst
1.0 lmol, cocatalyst Et2AlCl 4.0 mmol, ETA/V ¼ 300 (molar ratio), 10
min.
GPC data in 1,2,4-trichlorobenzene versus polystyrene standard.
Determined by DSC.
Al/V (molar ratio) ¼ 1000.
b
g
Activity in kg of polymer/mmolVꢀhꢀbar.
Al/V (molar ratio) ¼ 2000.
c
Comonomer content (mol %) estimated by 13C NMR spectra.
observed (assigned as Saa). The behavior is different from
the resultant polymers obtained by the corresponding
tion sequences of 1-hexene were also found in the spectra.
The possible monomer sequences in the copolymer were
summarized in Scheme 2.
mono(b-enaminoketonato)
vanadium(III)
catalysts
or
VCl3(THF)3,68 which only contained isolated hexene units
even with high hexene incorporation (16.0 mol%). Moreover,
as shown in Figure 6, the signals at 28.0 ppm (Sbc), 32.5
ppm (Sab), and 38.9 ppm (Tcd) caused by the inverse inser-
The triad distributions data and reactivity ratios re and rh
for each catalyst were calculated at 50ꢁC according to 13C
NMR using the method of Hsieh and Randall, as shown in
Table 3. The steric hindrance and electron effect of the
FIGURE 6 13C NMR of ethylene/1-hexene copolymer using catalyst 2f: (a) 2.54 mol % (entry 6); (b) 5.68 mol % (entry 9); (c) 11.4
mol % (entry 11) in Table 2.
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SCHEME 2 Possible monomer sequences in poly(ethylene-co-hexene)s.
ligands exerted obvious influence on the value of re and rh,
which manifested as the capability of incorporation of 1-hex-
ene. Compared with complex 2a (rh ¼ 0.16; re ¼ 31.4), com-
plex 2b showed lower tendency to incorporate 1-hexene (rh
¼ 0.08; re ¼ 32.9), indicating that electron-withdrawing
effects may be unfavorable for 1-hexene insertion. The
change in re from 31.4 (for 2a) to 46.0 (for 2e) is in turn
more remarkable than rh (from 0.01 to 0.17). We could also
find that 2c (rh ¼ 0.17; re ¼ 42.1) showed a little higher
tendency to incorporate 1-hexene than 2d (rh ¼ 0.14; re ¼
44.3), which is broadly consistent with that at 25ꢁC. Com-
pared with complex 2e (rh ¼ 0.11; re ¼ 46.0), complex 2f
showed much lower tendency to incorporate 1-hexene (rh ¼
0.01; re ¼ 45.9), indicating that steric hindrance effect of the
ligands may also be unfavorable for 1-hexene insertion.
Finally, the value of 1-hexene incorporation of 2g (0.030) is
close to 2f (0.031), which is a stark contrast with the results
at 25ꢁC, indicating that increasing temperature is beneficial
for chain growth and 1-hexene insertion. These results indi-
cate that the conclusions from reactivity ratios and 1-hexene
incorporation calculated from 13C NMR are unanimous. That
is steric effect can hinder the insertion of 1-hexene, but high
temperature can weaken this effect.
At last, we have tried to polymerize hexene alone. However,
we have not found the white precipitation appearing, which
maybe attributed to no polymer generation or getting a
small amount of low molecular weight oligomer. For details,
go to Supporting Information.
CONCLUSIONS
In summary, a series of vanadium(III) complexes bearing
phenoxy-phosphine ligands have been synthesized, character-
ized and investigated as efficient catalysts for olefin polymer-
ization. Both the ligand structures and the reaction parame-
ters such as the cocatalyst type, the Al/V molar ratio and
polymerization reaction temperature play an important role
in ethylene polymerization. The steric hindrance effect can
improve the catalytic performance of the mono phenoxy-
phosphine vanadium complexes, and bis(phenoxy-phosphine)
vanadium complexes exhibited higher thermal stability than
mono complexes at high temperature due to the steric hin-
drance effect. The resultant polymers possessed high molec-
ular weights with unimodal distributions, strongly suggesting
that these polymerizations proceeded with a single catalyti-
cally active species. In addition, these complexes also exhib-
ited high catalytic activity for ethylene/1-hexene
TABLE 3 Triad Distributions and Reactivity Ratios for Ethylene/1-Hexene Copolymerization
Cat.
[H]feed
[H]copolymer
[EHE]
[EHH]
[HHH]
[HEH]
[HEE]
[EEE]
re
rh
2a
2b
2c
2d
2e
2f
0.677
0.677
0.677
0.677
0.677
0.677
0.677
0.053
0.046
0.042
0.038
0.036
0.031
0.030
0.033
0.037
0.024
0.025
0.025
0.030
0.024
0.020
0.010
0.019
0.013
0.011
0.009
0.007
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.086
0.083
0.074
0.063
0.061
0.061
0.054
0.861
0.870
0.883
0.899
0.903
0.900
0.885
31.4
32.9
42.1
44.3
46.0
45.9
52.4
0.16
0.08
0.17
0.14
0.11
0.01
0.08
2g
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23 Chan, M.; Cole, J. M.; Gibson, V. C.; Howard, J. Chem. Com-
mun. 1997, 2345–2346.
copolymerization. Changing the substituents in ligands signif-
icantly influenced the activity for ethylene polymerization
and molecular weight of resultant polymers as well as the
selectivity of 1-hexene. To further investigate the copolymer-
ization system, the reactivity ratios of comonomers were
determined using 13C NMR methods of Hsieh and Randall.
The steric hindrance and electronic effect of the ligands exert
obvious influence on the value of re and rh, which manifested
as the copolymerization capability of catalysts. We believe
that the results through this study would introduce impor-
tant information for designing efficient transition metal cata-
lysts for olefin polymerization.
24 Tomov, A. K.; Gibson, V. C.; Zaher, D.; Elsegood, M.; Dale,
S. H. Chem. Commun. 2004, 1956–1957.
25 Wu, J. Q.; Mu, J. S.; Zhang, S. W.; Li, Y. S. J. Polym. Sci.
Part. A: Polym. Chem. 2010, 48, 1122–1132.
26 Cuomo, C.; Milione, S.; Grassi, A. J. Polym. Sci. Part A:
Polym. Chem. 2006, 44, 3279–3289.
27 Milione, S.; Cavallo, G.; Tedesco, C.; Grassi, A. Dalton
Trans. 2002, 8, 1839–1846.
28 Tang, L. M.; Wu, J. Q.; Duan, Y. Q.; Pan, L.; Li, Y. G.; Li, Y.
S. J. Polym. Sci. Part A: Polym. Chem. 2008, 46, 2038–2048.
29 Hagen, H.; Bezemer, C.; Boersma, J.; Kooijman, H.; Lutz, M.;
Spek, A. L.; van Koten, G. Inorg. Chem. 2000, 39, 3970–3977.
ACKNOWLEDGMENTS
30 Janas, Z.; Wisniewska, D.; Jerzykiewicz, L. B.; Sobota, P.;
Drabent, K.; Szczegot, K. Dalton Trans. 2007, 20, 2065–2069.
The authors are grateful for subsidy provided by the National
Natural Science Foundation of China (No. 20923003).
31 Bialek, M.; Czaja, K. J. Polym. Sci. Part A: Polym. Chem.
2008, 46, 6940–6949.
32 Bialek, M.; Liboska, O. J. Polym. Sci. Part A: Polym. Chem.
2010, 48, 471–478.
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