Highly active ortho-phenyl substituted α-diimine Nickel(II) catalysts for "chain walking polymerization" of ethylene: Synthesis of the nanosized dendritic polyethylene

Article abstract of DOI:10.1016/j.molcata.2013.01.016

Three new α-diimine Ni(II) complexes {bis[N,N′-(4-fluoro-2,6- diphenylphenyl)imino]acenaphthene}dibromonickel 4a, {bis[N,N′-(4-chloro-2, 6-diphenylphenyl)imino]acenaphthene}dibromonickel 4b, and {bis[N,N′-(4- methyl-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4c, were synthesized and characterized. The crystal structure of the complex 4a was determined by X-ray crystallography. Complex 4a has pseudo-tetrahedral geometry about the nickel center, showing C_2v molecular symmetry. These complexes, activated by diethylaluminum chloride (DEAC) were tested in the polymerization of ethylene under mild conditions. Complex 4a bearing 2,6-diphenyl and strong electron-withdrawing 4-fluorine groups, activated by diethylaluminum chloride (DEAC) shows highly catalytic activity for the polymerization of ethylene [4.95 × 10⁶ g PE/(mol Ni h bar)] and produced dendritic polyethylene (153.3 branches/1000 C). The dendritic polyethylene particle size obtained by 4a/DEAC can be controlled in the 1-20 nm under 0.2 bar ethylene pressure, and could be expected to become a nano-targeted drug carrier after modified with water-soluble oligo(ethylene glycol) (OEG).

Full text of DOI:10.1016/j.molcata.2013.01.016

Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Highly active ortho-phenyl substituted ␣-diimine Nickel(II) catalysts
for “chain walking polymerization” of ethylene:
Synthesis of the nanosized dendritic polyethylene
Jianchao Yuan∗, Fuzhou Wang, Bingnian Yuan, Zong Jia, Fengying Song, Jing Li
Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry & Chemical
Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e i n f o
a b s t r a c t
Three new ␣-diimine Ni(II) complexes {bis[N,Nꢀ-(4-fluoro-2,6-diphenylphenyl)imino]acenaphthene}
dibromonickel 4a, {bis[N,Nꢀ-(4-chloro-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4b, and
{bis[N,Nꢀ-(4-methyl-2,6-diphenylphenyl)imino]acenaphthene}dibromonickel 4c, were synthesized and
characterized. The crystal structure of the complex 4a was determined by X-ray crystallography. Com-
plex 4a has pseudo-tetrahedral geometry about the nickel center, showing C2v molecular symmetry.
These complexes, activated by diethylaluminum chloride (DEAC) were tested in the polymerization of
ethylene under mild conditions. Complex 4a bearing 2,6-diphenyl and strong electron-withdrawing
4-fluorine groups, activated by diethylaluminum chloride (DEAC) shows highly catalytic activity for
the polymerization of ethylene [4.95 × 106 g PE/(mol Ni h bar)] and produced dendritic polyethylene
(153.3 branches/1000 C). The dendritic polyethylene particle size obtained by 4a/DEAC can be controlled
in the 1–20 nm under 0.2 bar ethylene pressure, and could be expected to become a nano-targeted drug
carrier after modified with water-soluble oligo(ethylene glycol) (OEG).
Article history:
Received 26 July 2012
Received in revised form 15 January 2013
Accepted 20 January 2013
Available online 28 January 2013
Keywords:
Ni(II) complexes
␣-Diimine ligand
Crystal structure
Chain walking polymerization
Dendritic polyethylene
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
play a very important role in tumor targeted anti-cancer drugs.
The key part of this method is to be the synthesis of highly active
Late transition metal olefin polymerization catalysis has
attracted increasing attention due to their high functional group
tolerance and their ability to produce branched or dendritic poly-
mer [1–24]. Nanosized dendrimers are artificial macromolecules
with tree-like structures and synthesized from branched monomer
units in a stepwise manner [25]. Due to their surface functional
groups and highly branched architectures, nanosized dendrimers
have an enormous capacity for solubilization of hydrophobic drugs
and can be modified or conjugated with various interesting guest
molecules [25,26]. Based on the “Enhanced Permeability and Reten-
tion (EPR) effect” [27–30], the nanosized dendrimers have shown
great promise in the development of anticancer drug delivery
systems [31]. However, it is very difficult to prepare dendrimers
through stepwise synthesis. Recently, Guan and his coworker
reported a simple method, chain walking polymerization (CWP)
followed by atom transfer radical polymerization (ATRP), to effi-
ciently synthesize water-soluble core–shell [core: polyethylene,
PE; shell: oligo(ethylene glycol), OEG] dendritic nanoparticles with
tunable sizes and reactive surface functionalities [32], which could
late transition metal catalysts, which could generate dendritic
polyethylene and control the polyethylene particle diameter in the
nanometer range. The dendritic polyethylene could be expected to
become a nano-targeted drug carrier after modified with water-
soluble oligo(ethylene glycol) (OEG).
In this work, we report the synthesis and characterization of
three new ␣-diimine Ni(II) complexes of the type [NiBr2(Ar-BIAN)]
(Ar-BIAN = bis(arylimino)acenaphthene) bearing two bulky phenyl
groups in the ortho-aryl position and different groups (strong
electron-withdrawing group F, electron-withdrawing group Cl and
electron-donating group methyl) in the para-aryl position of the
arylimino group, in order to study the influence of different steric
effects and electron densities at the metal center on the cata-
lyst activity and, in particular, on the microstructure and size of
polyethylene, and find new ␣-diimine Ni(II) complexes, which
could produce nanosized dendritic polyethylene.
2. Experimental
2.1. General procedures and materials
∗
All manipulations involving air and/or moisture-sensitive
compounds were carried out with standard Schlenk techniques
Corresponding author. Tel.: +86 931 7971539; fax: +86 931 7971261.
E-mail address: jianchaoyuan@nwnu.edu.cn (J. Yuan).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.01.016

J. Yuan et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
133
under nitrogen. Methylene chloride and o-dichlorobenzene were
pre-dried with 4 A molecular sieves and distilled from CaH2 under
2.4. Synthesis of 4-fluoro-2,6-diphenylbenzenamine 2a
Pd(OAc)2 (0.01 g, 0.04 mmol), 2,6-dibromo-4-
˚
dry nitrogen. Toluene, diethyl ether, and 1,2-dimethoxyethane
(DME) were distilled from sodium/benzophenone under N2
atmosphere. Anhydrous NiBr2 (99%), phenylboronic acid,
Pd(OAc)2 and diethylaluminum chloride (DEAC, 0.9 M solution
in toluene) were obtained from Acros. Acenaphthoquinone
fluorobenzenamine (0.54 g, 2 mmol), K2CO3 (0.55 g, 4 mmol)
and phenylboronic acid (0.54 g, 4.4 mmol) were placed in a 100 ml
flask and allowed to stir at 25 ◦C for 24 h, in the presence of
10 ml PEG-400. The mixture was extracted three times with 10 ml
diethyl ether. The combined organic phase was dried over MgSO4,
filtered, and the solvent was removed. The residue was purified
by chromatography on silica gel with petroleum ether/ethyl ester
(v/v = 20:1) to give 4-fluoro-2,6-diphenylbenzenamine (0.35 g,
66% yield). 1H NMR (400 MHz, CDCl3): ␦ 3.66 (s, 2H, NH2), 6.85
(d, 2H, benzenamine), 7.34 (t, 2H, phenyl ring), 7.43–7.49 (m, 8H,
phenyl ring). 13C NMR (400 MHz, CDCl3): ␦ 115.81 (benzenamine
carbon near F), 127.63 (benzenamine carbon connected with
phenyl), 128.83 (phenyl carbon), 128.92 (phenyl carbon near
benzenamine), 129.12 (phenyl carbon), 136.90 (phenyl carbon
connected with benzenamine), 138.80 (benzenamine carbon
connected with NH2), 154.63 (benzenamine carbon connected
with F).
(98%),
2,6-dibromo-4-fluorobenzenamine
(98%)
(1a),
4-
methylbenzenamine (98%), and 4-chlorobenzenamine (98%),
were purchased from Alfa Aesar, and used without further purifi-
cation. [NiBr2(DME)] was synthesized according to the literature
[33].
NMR spectra were recorded at 400 MHz on a Varian Mer-
cury Plus-400 instrument, using TMS as internal standard. FTIR
spectra were recorded on
a Digilab Merlin FTS 3000 FTIR
spectrophotometer on KBr pellets. The molecular weights and
molecular weight distributions (Mw/Mn) of the polymers were
determined by gel permeation chromatography/size-exclusion
chromatography (GPC/SEC) via a Waters Alliance GPCV2000 chro-
matograph, using 1,2,4-trichlorobenzene as eluent, at a flow
rate of 1.0 ml/min and operated at 140 ◦C. Effective hydro-
dynamic diameters of the dendritic polyethylenes were measured
by particle size analyzer at 20 ◦C using a dynamic light scat-
tering photometer (Nano ZS ZEN3600, Malvern Instruments
Ltd., United Kingdom) equipped with laser at a wavelength of
633 nm.
2.5. Synthesis of 4-chloro-2,6-diphenylbenzenamine 2b
Pd(OAc)2
(0.01 g,
0.04 mmol),
2,6-dibromo-4-
chlorobenzenamine (0.57 g, 2 mmol), K2CO3 (0.55 g, 4 mmol)
and phenylboronic acid (0.54 g, 4.4 mmol) were placed in a 100 ml
flask and allowed to stir at 25 ◦C for 24 h, in the presence of
10 ml PEG-400. The mixture was extracted three times with 10 ml
diethyl ether. The combined organic phase was dried over MgSO4,
filtered, and the solvent was removed. The residue was purified
by chromatography on silica gel with petroleum ether/ethyl ester
(v/v = 20:1) to give 4-chloro-2,6-diphenylbenzenamine (0.34 g,
60% yield). 1H NMR (400 MHz, CDCl3): ı 3.71 (s, 2H, NH2), 7.00
(s, 2H, benzenamine), 7.28 (t, 2H, phenyl ring), 7.33–7.40 (m, 8H,
phenyl ring). 13C NMR (400 MHz, CDCl3): ı 124.82 (benzenamine
carbon connected with Cl), 127.47 (benzenamine carbon con-
nected with phenyl), 128.38 (benzenamine carbon near Cl), 128.98
(phenyl carbon), 129.58 (phenyl carbon near benzenamine),
130.46 (phenyl carbon), 138.46 (phenyl carbon connected with
benzenamine), 139.74 (benzenamine carbon connected with
NH2).
2.2. Synthesis of 2,6-dibromo-4-chlorobenzenamine 1b
Acetic acid (2 ml) was added to a stirred solution of 4-
chlorobenzenamine (0.64 g, 5 mmol) in CH2Cl2. The solution was
stirred for 30 min. After the solution cooled to 5 ◦C with ice bath, Br2
(2.00 g, 12.5 mmol) in 5 ml CH2Cl2 was slowly added to the stirred
solution. The mixture was stirred for 5 h and neutralized by 10%
saturated aqueous sodium hydroxide solution. The mixture was
extracted three times with 50 ml petroleum ether. The combined
organic phase was dried over MgSO4, filtered, and the solvent was
removed. The residue was purified by chromatography on silica gel
with petroleum ether/ethyl ester (v/v = 15:1) to give 2,6-dibromo-
4-chlorobenzenamine (0.94 g, 66% yield). 1H NMR (400 MHz,
CDCl3): ␦ 4.55 (s, 2H, NH2), 7.38 (s, 2H, C6H2Br2ClNH2). 13C NMR
(400 MHz, CDCl3): ␦ 108.43 (benzenamine carbon connected with
Br), 130.48 (benzenamine carbon connected with Cl), 131.46 (ben-
zenamine carbon), 141.25 (benzenamine carbon connected with
NH2).
2.6. Synthesis of 4-methyl-2,6-diphenylbenzenamine 2c
Pd(OAc)2
(0.01 g,
0.04 mmol),
2,6-dibromo-4-
methylbenzenamine (0.53 g, 2 mmol), K2CO3 (0.55 g, 4 mmol)
and phenylboronic acid (0.51 g, 4.2 mmol) were placed in a 100 ml
flask and allowed to stir at 25 ◦C for 24 h, in the presence of
10 ml PEG-400. The mixture was extracted three times with 10 ml
diethyl ether. The combined organic phase was dried over MgSO4,
filtered, and the solvent was removed. The residue was purified
by chromatography on silica gel with petroleum ether/ethyl
ester (v/v = 20:1) to give 4-methyl-2,6-diphenylbenzenamine
(0.31 g, 58% yield). 1H NMR (400 MHz, CDCl3): ı 2.21 (s, 3H,
CH3 of benzenamine ring), 3.59 (s, 2H, NH2), 6.86 (s, 2H, ben-
zenamine ring), 7.25 (t, 2H, phenyl ring), 7.34 (t, 4H, phenyl
ring), 7.41 (d, 4H, phenyl ring near benzenamine). 13C NMR
(400 MHz, CDCl3): ı 20.41 (CH3), 127.15 (benzenamine carbon
connected with phenyl), 127.88 (phenyl carbon), 128.69 (phenyl
carbon near benzenamine), 129.27 (phenyl carbon), 129.42 (ben-
zenamine carbon connected with methyl), 130.23 (benzenamine
carbon near methyl), 138.13 (phenyl carbon connected with
benzenamine), 139.74 (benzenamine carbon connected with
NH2).
2.3. Synthesis of 2,6-dibromo-4-methylbenzenamine 1c
Acetic acid (2 ml) was added to a stirred solution of 4-
methylbenzenamine (0.54 g, 5 mmol) in CH2Cl2. The solution was
stirred for 30 min. After the solution cooled to 5 ◦C with ice bath,
Br2 (2.00 g, 12.5 mmol) in 5 ml CH2Cl2 was slowly added to the
stirred solution. The mixture was stirred for 5 h and neutralized
by 10% saturated aqueous sodium hydroxide solution. The mix-
ture was extracted three times with 50 ml petroleum ether. The
combined organic phase was dried over MgSO4, filtered, and the
solvent was removed. The residue was purified by chromatog-
raphy on silica gel with petroleum ether/ethyl ester (v/v = 15:1)
to give 2,6-dibromo-4-methylbenzenamine (0.97 g, 73% yield).
1H NMR (400 MHz, CDCl3): ␦ 2.20 (s, 3H, CH3), 4.38 (s, 2H,
NH2), 7.19 (s, 2H, C6H2Br2CH3NH2). 13C NMR (400 MHz, CDCl3):
␦ 19.75 ( CH3), 108.69 (benzenamine carbon connected with Br),
129.31 (benzenamine carbon connected with CH3), 132.13 (ben-
zenamine carbon), 139.48 (benzenamine carbon connected with
NH2).

134
J. Yuan et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
2.7. Synthesis of
near CH3), 127.85 (phenyl carbon near benzenamine), 128.68
(phenyl carbon), 129.68 (benzenamine carbon connected with
phenyl), 130.41 (phenyl carbon connected with benzenamine),
130.71 (naphthyl ring), 131.69 (naphthyl ring), 134.85 (naphthyl
ring), 137.93 (naphthyl ring), 139.21 (benzenamine carbon con-
nected with CH3), 139.74 (benzenamine carbon connected with N),
160.28 (acenaphthoquinone ring carbon connected with N). Anal.
Calcd. for C50H36N2: C, 90.33; H, 5.46; N, 4.21. Found: C, 90.31; H,
5.49; N, 4.23.
bis[N,Nꢀ-(4-fluoro-2,6-diphenylphenyl)imino]acenaphthene 3a
4-Methylbenzenesulfonic acid (20 mg, 0.1 mmol) was added to
a stirred solution of acenaphthenequinone (0.18 g, 1.00 mmol) and
4-fluoro-2,6-diphenylbenzenamine (0.58 g, 2.2 mmol) in benzene
(20 ml). The mixture was refluxed for 24 h, and then the solvent
was removed. The residue was purified by chromatography on sil-
ica gel with petroleum ether/ethyl ester (v/v = 20:1) to give ligand
3a (0.45 g, 67% yield). 1H NMR (400 MHz, CDCl3): ı 6.76 (t, 2H,
naphthyl ring), 6.93 (t, 4H, phenyl ring), 7.04 (d, 2H, naphthyl ring),
7.10 (t, 8H, phenyl ring), 7.27 (t, 8H, phenyl ring), 7.29 (d, 4H, ben-
zenamine ring), 7.66 (d, 2H, naphthyl ring). 13C NMR (400 MHz,
CDCl3): ı 116.81 (naphthyl ring carbon), 122.53 (naphthyl ring car-
bon), 127.01 (benzenamine carbon near F), 127.11 (phenyl carbon),
128.10 (phenyl carbon near benzenamine), 129.01 (phenyl car-
bon), 129.69 (benzenamine carbon connected with phenyl), 130.37
(phenyl carbon connected with benzenamine), 133.27 (naphthyl
ring carbon), 138.78 (naphthyl ring), 140.05 (naphthyl ring), 142.99
(naphthyl ring), 158.51 (benzenamine carbon connected with
N), 161.13 (acenaphthoquinone ring carbon connected with N),
161.77 (benzenamine carbon connected with F). Anal. Calc. for
2.10. Synthesis of {bis[N,Nꢀ-(4-fluoro-2,6-
diphenylphenyl)imino]acenaphthene}dibromonicke 4a
NiBr2(DME) (0.16 g, 0.5 mmol), ligand 3a (0.33 g, 0.5 mmol) and
dichloromethane (40 ml) were mixed in a Schlenk flask and stirred
at room temperature for 24 h. The resulting suspension was fil-
tered. The solvent was removed under vacuum and the residue was
washed with diethyl ether (3 × 15 ml), and then dried under vac-
uum at room temperature to give complex 4a 0.37 g (82% yield).
Anal. Calcd. for C48H30Br2F2N2Ni: C, 64.68; H, 3.39; N, 3.14. Found:
C, 64.64; H, 3.43; N, 3.17. FT-IR (KBr) 1,645 cm−1 (C N). Single
crystals of complex 4a suitable for X-ray analysis were obtained
at −30 ◦C by dissolving the nickel complex in CH2Cl2, following by
slow layering of the resulting solution with n-hexane.
C48H30F2N2: C, 85.69; H, 4.49; N, 4.16. Found: C, 85.66; H, 4.52;
N, 4.18.
2.8. Synthesis of
2.11. Synthesis of {bis[N,Nꢀ-(4-chloro-2,6-
bis[N,Nꢀ-(4-chloro-2,6-diphenylphenyl)imino]acenaphthene 3b
diphenylphenyl)imino]acenaphthene}dibromonicke 4b
4-Methylbenzenesulfonic acid (20 mg, 0.1 mmol) was added to
a stirred solution of acenaphthenequinone (0.18 g, 1.00 mmol) and
4-chloro-2,6-diphenylbenzenamine (0.62 g, 2.2 mmol) in benzene
(20 ml). The mixture was refluxed for 24 h, and then the solvent
was removed. The residue was purified by chromatography on sil-
ica gel with petroleum ether/ethyl ester (v/v = 20:1) to give ligand
3b (0.46 g, 65% yield). 1H NMR (400 MHz, CDCl3): ı 6.83 (t, 2H, naph-
thyl ring), 6.92 (t, 8H, phenyl ring), 7.05 (t, 4H, phenyl ring), 7.26
(d, 8H, phenyl ring), 7.30 (d, 2H, naphthyl ring), 7.35 (s, 4H, ben-
zenamine ring), 7.70 (d, 2H, naphthyl ring). 13C NMR (400 MHz,
CDCl3): ı 122.66 (naphthyl ring carbon), 127.10 (naphthyl ring car-
bon), 127.40 (phenyl carbon), 127.70 (benzenamine carbon near
Cl), 128.00 (phenyl carbon near benzenamine), 129.06 (phenyl car-
bon), 129.66 (benzenamine carbon connected with phenyl), 130.18
(phenyl carbon connected with benzenamine), 130.64 (naphthyl
ring), 131.76 (naphthyl ring), 132.82 (benzenamine carbon con-
nected with Cl), 137.78 (naphthyl ring), 138.39 (naphthyl ring),
139.36 (benzenamine carbon connected with N), 160.36 (ace-
naphthoquinone ring carbon connected with N). Anal. Calcd. for
NiBr2(DME) (0.16 g, 0.5 mmol), ligand 3b (0.34 g, 0.5 mmol) and
dichloromethane (40 ml) were mixed in a Schlenk flask and stirred
at room temperature for 24 h. The resulting suspension was fil-
tered. The solvent was removed under vacuum and the residue
was washed with diethyl ether (3 × 16 ml), and then dried under
vacuum at room temperature to give complex 4b (0.39 g, 85%
yield). Anal. Calcd. for C48H30N2Cl2NiBr2: C, 62.38; H, 3.27; N,
3.03. Found: C, 62.35; H, 3.29; N, 3.06. FT-IR (KBr) 1647 cm−1
(C N).
2.12. Synthesis of {bis[N,Nꢀ-(4-methyl-2,6-
diphenylphenyl)imino]acenaphthene}dibromonicke 4c
NiBr2(DME) (0.16 g, 0.5 mmol), ligand 3c (0.33 g, 0.5 mmol) and
dichloromethane (40 ml) were mixed in a Schlenk flask and stirred
at room temperature for 24 h. The resulting suspension was fil-
tered. The solvent was removed under vacuum and the residue was
washed with diethyl ether (3 × 15 ml), and then dried under vac-
uum at room temperature to give complex 4c 0.41 g (93% yield).
Anal. Calcd. for C50H36Br2N2Ni: C, 67.98; H, 4.11; N, 3.17. Found: C,
68.02; H, 4.09; N, 3.14. FT-IR (KBr) 1649 cm−1 (C N).
C48H30Cl2N2: C, 81.70; H, 4.29; N, 3.97. Found: C, 81.73; H, 4.26; N,
3.95.
2.9. Synthesis of
bis[N,Nꢀ-(4-methyl-2,6-diphenylphenyl)imino]acenaphthene 3c
2.13. X-ray structure determinations
4-Methylbenzenesulfonic acid (20 mg, 0.1 mmol) was added to
a stirred solution of acenaphthenequinone (0.18 g, 1.00 mmol) and
4-meyhyl-2,6-diphenylbenzenamine (0.57 g, 2.2 mmol) in benzene
(20 ml). The mixture was refluxed for 24 h, and then the solvent
was removed. The residue was purified by chromatography on sil-
ica gel with petroleum ether/ethyl ester (v/v = 20:1) to give ligand
3c (0.41 g, 62% yield). 1H NMR (400 MHz, CDCl3): ı 2.37 (s, 6H, CH3
of benzenamine ring), 6.84 (t, 8H, phenyl ring), 6.94 (t, 4H, phenyl
ring), 7.07 (s, 4H, benzenamine ring), 7.21 (d, 8H, phenyl ring),
7.42 (t, 2H, naphthyl ring), 7.36 (d, 2H, naphthyl ring), 7.55 (d, 2H,
naphthyl ring). 13C NMR (400 MHz, CDCl3): ı 20.32 ( CH3 of ben-
zenamine ring), 122.65 (naphthyl ring carbon), 126.42 (naphthyl
ring carbon), 126.72 (phenyl carbon), 127.25 (benzenamine carbon
Single crystals of complex 4a suitable for X-ray analysis were
obtained at −30 ◦C by dissolving the nickel complex in CH2Cl2, fol-
lowing by slow layering of the resulting solution with n-hexane.
Data collections were performed at 296(2) K on a Bruker SMART
APEX diffractometer with a CCD area detector, using graphite
˚
monochromated MoK˛ radiation (ꢀ = 0.71073 A). The determina-
tion of crystal class and unit cell parameters was carried out by
the SMART program package. The raw frame data were processed
using SAINT and SADABS to yield the reflection data file. The struc-
tures were solved by using the SHELXTL program. Refinement was
performed on F2 anisotropically for all non-hydrogen atoms by the
full-matrix least-squares method. The hydrogen atoms were placed
at the calculated positions and were included in the structure

J. Yuan et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
135
Table 1
The precipitated polymer was filtered, washed with methanol and
dried under vacuum at 60 ◦C to a constant weight.
Summary of crystallographic data for complex 4a.
Empirical formula
Formula mass
Temperature (K)
Wavelength (Å)
Crystal size (mm3)
Crystal system
Space group
a (Å)
C48H30Br2F2N2Ni
891.27
296 K
3. Results and discussion
0.71073
0.29 × 0.27 × 0.26
Monoclinic
C2/c
3.1. Synthesis and characterization of ligands 3a–c and
complexes 4a–c
˚
16.390 (8) A
After protection of the amino group by acetic acid, 4-
R-benzenamines were brominated to form 2,6-dibromo-4-R-
benzenamines 1a–c (1a: R = F; 1b: R = Cl; 1c: R = CH3) (Scheme 1).
The Suzuki coupling reaction of 2,6-dibromo-4-R-benzenamines
and phenylboronic acid catalyzed by Pd(OAc)2, in PEG-400,
led to the corresponding amine precursors 2,6-diphenyl-4-R-
benzenamines 2a–c in 58–66% yield. The ␣-diimine ligands 3a–c
were finally obtained by acid catalyzed condensation of the 2,6-
diphenyl-4-R-benzenamines and acenaphthoquinone, and purified
by chromatography on silica gel with petroleum ether/ethyl ester.
The ligands (3a–c) were characterized by 1H NMR, 13C NMR and
elemental analysis. Compared with other ␣-diimine ligands, the
synthesis of ligands (3a–c) was difficult due to two bulky ortho-
phenyl groups of the anilines (2a–c). At the beginning, ligands
(3a–c) were synthesized in methanol at 40–50 ◦C using formic acid
as catalyst, but failed. Ligands (3a–c) were successfully synthesized
at high temperature in benzene using 4-methylbenzenesulfonic
acid as catalyst (yield, 62–67%).
˚
b (Å)
c (Å)
V (Å3)
16.470 (8) A
˚
3
18.445 (9) A
˚
4530 (4) A
Z
4
Density (calcd.) (mg/cm3)
1.307
2.23
1792
2.4–20.5
Absorption coefficient (mm−1
)
F(000)
Theta range for data collection (◦)
Limiting indices
−19 ≤ h ≤ 17 0 ≤ k ≤ 190 ≤ l ≤ 21
Reflections collected
Independent reflections
Rint
3919
250
0.0000
99.6%
Completeness to ꢁ = 25.20◦
Final R indices [I > 2ꢂ(I)]
R indices (all data)
R1 = 0.0396, wR2 = 0.1104
R1 = 0.0824, wR2 = 0.1285
Full-matrix least-squares on F2
1.015
0.5637 and 0.5945
0.37 and −0.32
Refinement method
Goodness-of-fit on F2
Max. and min. transmission
Largest diff. peak and hole (e A
−3
˚
)
calculation without further refinement of the parameters. Crys-
tal data, data collection, and refinement parameters are listed in
Table 1.
The reaction of equimolar amounts of [NiBr2(DME)] and the
␣-diimine ligands 3a–c in CH2Cl2 led to the displacement of 1,2-
dimethoxyethane and afforded the catalyst precursors 4a–c as
moderately air-stable microcrystalline solids in high yields. The
result of elemental analysis of complex 4a is consistent with the
theoretical value of the molecular structure obtained by X-ray crys-
tallography (see below).
2.14. Procedure for the polymerization of ethylene
The polymerization of ethylene was carried out in a flame
dried 250 ml crown capped pressure bottle sealed with neoprene
septa. After drying the polymerization bottle under N2 atmosphere,
50 ml of dry toluene was added to the polymerization bottle. The
resulting solvent was then saturated with a prescribed ethylene
pressure. The co-catalyst (DEAC) was then added in Al/Ni molar
ratios in the range of 200–1000 to the polymerization bottle via
a syringe. At this time, the solutions were thermostated to the
desired temperature and allowed to equilibrate for 15 min. Subse-
quently, an o-dichlorobenzene solution of Ni catalyst was added to
the polymerization reactor. The polymerization, conducted under a
dynamic pressure of ethylene (0.2 bar), was terminated by quench-
ing the reaction mixtures with 100 ml of a 2% HCl–MeOH solution.
Suitable crystals of complex 4a for X-ray diffraction were
obtained at −30 ◦C by double layering a CH2Cl2 solution of the
complex with n-hexane. The molecular structure complex 4a
was determined and the corresponding diagram is shown in Fig.
1, while selected bond distances and angles are summarized in
Table 2. The structure of complex 4a has pseudo-tetrahedral geom-
etry about the nickel center, showing C2v molecular symmetry.
The two imino C N bonds have typical double bond character with
˚
C
N bond lengths of 1.284 A. Both aryl rings bonded to the iminic
nitrogens of the ␣-diimine lie nearly perpendicular to the plane
formed by the nickel and coordinated nitrogen atoms. The phenyl
rings in the 2,6-position of the benzenamine fragments in 4a point
Scheme 1. Syntheses of ␣-diimine ligands 3a–c and their corresponding ␣-diimine nickel(II) dibromide complexes 4a–c.

136
J. Yuan et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
Fig. 1. ORTEP drawing of complex 4a with 30% probability displacement ellipsoid.
toward each other above and below the plane, thus shielding the
apical positions of the Ni(II) center. Its structure is similar to those
reported in the literature for other similar [NiBr2(␣-diimine)]
compounds characterized by X-ray diffraction, {bis[N,Nꢀ-4-bromo-
2,6-dimethylphenyl)imino]acenaphthene}dibromonickel [34] and
{bis[N,Nꢀ-(2, 4, 6-trimethylphenyl)imino]acenaphthene}dibromo-
polymerization of ethylene, under the same reaction conditions.
The results of the polymerization experiments are shown in Table 3.
Noteworthy is the fact that blank experiments carried out with
DEAC alone, under similar conditions, showed its inability to poly-
merize ethylene on its own.
Three ␣-diimine Ni(II)/DEAC catalytic systems are highly
active for ethylene polymerization due to the bulky ortho-phenyl
substituents on the aryl rings of ␣-diimine Ni(II) complexes
[the highest activities: 4a: 4.95 × 106 g PE/(mol Ni h bar); 4b:
2.89 × 106 g PE/(mol Ni h bar); 4c: 4.00 × 106 g PE/(mol Ni h bar)]
(Table 3).
The performances of the nickel precatalysts are also affected
by 4-position substituents on the benzenamine rings of ␣-diimine
Ni(II) complexes (Table 3). To our surprise, ␣-diimine nickel
complex (4a) bearing strong electron-withdrawing 4-fluorine
groups [the highest activity, 4.95 × 106 g PE/(mol Ni h bar) was
obtained at 600 of Al/Ni ratio at 0 ◦C, Run 3], activated by DEAC,
shows higher activities for ethylene polymerization than those
observed for 4b and 4c in same conditions (normally, a com-
plex bearing 4-position electron-withdrawing substituents on the
benzenamine rings shows lower activity than that of a complex
bearing 4-position electron-donating substituents [8]). Complex
4b bearing electron-withdrawing 4-chlorine groups [the high-
est activity, 2.59 × 106 g PE/(mol Ni h bar) was obtained at 600 of
Al/Ni ratio at 20 ◦C, Run 11] exhibits an activity close to com-
plex 4c bearing electron-donating 4-methyl groups [the highest
activity, 4.00 × 106 g PE/(mol Ni h bar) was obtained at 600 of Al/Ni
ratio at 20 ◦C, Run 16]. This difference, observed between com-
plex 4a and the others, may be partially due to the presence of
the very strong electron-withdrawing F group in the 4-position of
the 2,6-diphenylbenzenamine rings. In fact, the activity of these
catalysts depends not only on the coordination step (␲-complex
formed between the metal atom of the catalyst and the double
bond of the monomer), but also on the activation energy of the
migratory insertion step. The barriers to migratory insertions as
measured by low-temperature NMR spectroscopy in the nickel
systems lie in the range of 13–14 kcal/mol with systems bearing
˚
nickel [35]. In fact, the Ni N bond distances in complex 4a (2.032 A)
are similar to those determined for these compounds (2.026 and
˚
˚
2.021 A, respectively), as well as the Ni Br bond distances (2.3336 A
˚
for complex 4a vs. 2.3229 and 2.323 A, respectively). However, the
N
Ni Br angles (96.97◦ for complex 4a) are very different to those
determined for those compounds (113.32 and 114.4◦, respectively)
due to the great steric hindrance of the four ortho benzene rings of
complex 4a around Ni(II) center.
3.2. Polymerization of ethylene with nickel complexes 4a–c
The three ␣-diimine nickel (II) complexes 4a, 4b, and 4c,
activated by DEAC, were tested as catalyst precursors for the
Table 2
Selected bond lengths (Å) and angles (◦) for complex 4a.
Bond lengths
(Å)
Bond angles
(◦)
81.34 (17)
Ni1 N1i
Ni1 N1
Ni1 Br1
Ni1 Br1i
C1 N1
2.032 (3)
2.032 (3)
2.3336 (9)
2.3337 (9)
1.435 (4)
1.355 (5)
1.377 (6)
1.360 (8)
1.385 (8)
1.378 (7)
1.284 (4)
1.461 (5)
1.485 (7)
1.376 (5)
1.417 (5)
N1i Ni1 N1
N1i Ni1 Br1
N1 Ni1 Br1
N1i Ni1 Br1i
N1 Ni1 Br1i
Br1 Ni1 Br1i
C19 N1 Ni1
C1 N1 Ni1
C2 C1 N1
C6 C1 N1
C3 C4 F1
F1 C4 C5
N1 C19 C20
N1 C19 C19i
C19 N1 C1
96.97 (9)
132.88 (9)
132.88 (9)
96.97 (9)
116.19 (5)
112.4 (2)
122.2 (2)
117.3 (3)
120.2 (3)
118.1 (4)
119.0 (4)
135.4 (4)
116.9 (2)
124.5 (3)
C4 F1
C14 C15
C15 C16
C16 C17
C17 C18
C19 N1
C19 C20
C19 C19i
C20 C25
C20 C21

J. Yuan et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
137
Table 3
Polymerization of ethylene with complexes 4a–c/DEAC.a
T (◦C)
t (min)
Yield (g)
Activityb
TOFc (h bar)−1
Mn (kg/mol)d
Mw/Mn
Branchese/1000 C
Df (nm)
d
Run
Complex
[Al]/[Ni]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4c
4c
4c
4c
4c
200
400
600
800
1000
600
600
600
600
600
600
600
600
600
600
600
600
600
0
0
0
0
0
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
1.75
1.78
2.22
1.98
1.77
1.51
0.88
0.70
0.86
1.01
1.25
1.05
0.54
0.41
1.72
1.81
1.59
1.25
3.91
3.97
4.95
4.42
3.94
3.37
2.05
1.56
1.98
2.43
2.89
2.43
1.23
0.91
2.84
4.00
3.52
2.81
1.40
1.42
1.77
1.58
1.41
1.20
0.74
0.57
0.71
0.87
1.03
0.87
0.44
0.33
1.01
1.43
1.16
1.00
115.3
109.8
102.3
16.2
16.0
14.4
11.3
10.7
17.1
19.1
20.4
11.4
9.7
1.68
1.68
1.73
1.78
1.82
1.85
1.87
1.95
1.74
1.79
1.87
1.99
1.75
1.83
1.88
1.94
1.97
2.08
–
–
–
–
–
–
–
–
–
–
20
40
60
0
10
20
40
60
0
10
20
40
60
93.2
125.7
153.3
–
13.5 5.1
11.2 4.9
9.75 1.2
–
–
–
86.3
114.2
122.2
–
16.8 3.2
15.1 3.5
11.3 2.4
–
13.1
14.6
12.3
10.4
6.5
–
–
78.4
96.1
109.9
19.2 4.1
14.2 3.2
12.6 2.1
a
Polymerization conditions: n(4a) = 2.24 ␮mol, n(4b) = 2.17 ␮mol, n(4c) = 2.26 ␮mol; ethylene relative pressure = 0.2 bar, ethylene absolute pressure = 1.2 bar;
t = polymerization time; solvent = toluene (50 ml); T = polymerization temperature.
b
Activity in 106 g PE/(mol Ni h bar).
c
Turnover frequency in 105 mol ethylene/(mol Ni h bar).
d
Determined by GPC.
e
Estimated by 1H NMR [39]. Branches/1000C = (1/3)ICH /(([ICH − (1/3)ICH )]/2) + ((1/3)ICH3 )) × 1000
3
2
3
f
Estimated by particle size analyzer.
the bulkiest alkyl ligands (the most electron-donor character of
the Ni atom) exhibiting the lowest insertion barriers (the high-
est insertion rate). Increased steric bulk of the diimine ligand aryl
substituents (electron-donor character of the Ni atom) leads to an
increase in the ground-state energy of the resting-state species rel-
ative to the migratory insertion transition state in nickel species
(Scheme 2). Consequently, lower migratory insertion barriers (high
insertion rate) were expected with bulkier diimine substituents
(electron-donor character of the Ni atom) [20]. Thus, the increase
of the electron-donor character of the metal atom is expected to
increase the insertion rate while the increase of its electrophilicity
is expected to increase the stability of resting state. Therefore, both
factors must be balanced when predicting the effect of an elec-
tron donor or acceptor substituent on the catalyst activity. As a
result, the following activity trend can be summarized for our sub-
stituted precatalysts under low ethylene pressure (0.2 bar), in the
range 0–60 ◦C: 4a > 4b ∼ 4c.
depend on reaction parameters such as the reaction temperature,
ethylene pressure and ligand structure [20]. Generally, low eth-
ylene pressure and high polymerization favor the Chain-Walking,
and afford high branched polyethylenes [20]. However, the effect
of ligand structure on polyethylene branching is much more com-
plicated.
Previous studies by Guan and coworkers demonstrated that in
ethylene polymerization the PE branching topology could be sim-
ply controlled by the pressure of ethylene, with linear PE being
formed at high pressure and dendritic PE being obtained at very
low pressure [36–38]. So, in this study, the low ethylene pressure
(0.2 bar) was employed. Also, in order to keep the high productivity
of polyethylene at low ethylene pressure, three new catalysts con-
taining two bulky ortho-phenyl substituents on benzenamine rings
were synthesized.
Our previous studies show that the more electron-rich catalyst
affords a more linear polymer. Conversely, the more electron-
deficient catalyst affords a more dendritic polymer [8]. So,
complexes 4a and 4b containing electron-withdrawing para-F/Cl
substituents on benzenamine rings were synthesized to prepared
nanosized dendritic polyethylene.
As shown in Table 3 and Fig. 2, the catalyst systems 4a/DEAC,
containing a strong electron-withdrawing F groups, generated
polyethylene with high degrees of branching. The total branching
degrees of the polymer samples prepared with 4a/DEAC (Runs 6–8,
branching degree: 93.2, 125.7 and 153.3 branches/1000 C at 20, 40
and 60 ◦C, respectively; diameter: 13.5, 11.2 and 9.75 nm at 20, 40
and 60 ◦C, respectively) are much higher than those observed for
4b/DEAC (Runs 11–13, branching degree: 86.3, 114.2 and 122.2
branches/1000 C at 20, 40 and 60 ◦C, respectively; diameter: 16.8,
15.1 and 11.3 nm at 20, 40 and 60 ◦C, respectively), and 4c/DEAC
At 0 ◦C, the activity of complex 4a increases slightly with the
increase of [Al]/[Ni] ratio (Runs 1–3), the maximum around [Al]/[Ni]
ratio 600 (Run 3), and then the activity decreases slightly with the
increase of [Al]/[Ni] ratio (Runs 3–5). For a ratio [Al]/[Ni] 600, an
increase in the polymerization temperature in the range 0–60 ◦C
decreases slightly the activity of the precatalyst 4a (Runs 3, 6–8).
However, precatalysts 4b and 4c show the highest activities around
20 ◦C (Runs 9–13 for complex 4b; Runs 14–18 for complex 4c).
3.3. Control of the polyethylene topology and size by ˛-diimine
ligand structures and polymerization conditions
The type and amount of branches formed in the polymeriza-
tion of ethylene promoted by typical ␣-diimine nickel precatalysts
Scheme 2. Mechanism of ethylene insertion.

138
J. Yuan et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
4. Conclusions
Three new ␣-diimine ligands 3a–c, and their Ni(II) complexes
4a–c have been prepared and characterized. Ligands 3a–3c were
modified in an attempt to change steric effects and the electronic
density of the metal center and eventually to improve the activity
in the polymerization of ethylene and control the microstructure
and size of the polyethylenes obtained. The results obtained show
that complex 4a, activated by DEAC produces highly active cata-
lyst system for the polymerization of ethylene. Complex 4a bearing
strongly electron-withdrawing 4-fluorine groups produced nano-
sized dendritic polyethylene at 60 ◦C. A more electron-deficient
ligand may better stabilize the transition state for monomer chain
walking than for insertion, which should afford a more dendritic
polyethylene. The dendritic polyethylene particle size obtained by
4a/DEAC can be controlled in the 1–20 nm under 0.2 bar ethylene
pressure, and could be expected to become a nano-targeted drug
carrier after modified with water-soluble oligo(ethylene glycol)
(OEG).
Acknowledgements
We thank the National Natural Science Foundation of China
(20964003), Chinese Ministry of Education Returned Overseas Stu-
dents Scientific Research Foundation (2009-1341) and Chinese
Ministry of Human Resources and Social Security Overseas Stu-
dents Science and Technology Activities Merit-based Foundation
(2009-416) for funding. We also thank Key Laboratory of Eco-
Environment-Related Polymer Materials of Ministry of Education,
Key Laboratory of Polymer Materials of Gansu Province (Northwest
Normal University), for financial support.
Fig. 2. 1H NMR (CDCl3/o-dichlorobenzene, v/v = 1:3) spectrum of the nanosized
Appendix A. Supplementary data
dendritic polyethylene catalyzed by 4a/DEAC at 60 ◦C (Table 3, Run 8).
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
2013.01.016.
systems (Runs 16–18, branching degree: 78.4, 96.1 and 109.9
branches/1000 C at 20, 40 and 60 ◦C, respectively; diameter: 19.2,
14.2 and 12.6 nm at 20, 40 and 60 ◦C, respectively). Also the total
branching degrees of the polymer samples prepared with 4a/DEAC
was higher than those observed for similar precatalyst/DEAC
systems such as {bis[N,Nꢀ-(4-tert-butyl-diphenylsilyl-2,6-diiso-
propylphenyl)imino]acenaphthene}dibromonickel (45 branches/
1000 C, at 20 ◦C) [20] or precatalyst/MAO systems such
as {bis[N,Nꢀ-(2,6-diisopropylphenyl)imino]-1,2-dimethylethane}
dibromonickel (30, 67, 80 and 90 branches/1000 C, at 25,
50, 65 and 80 ◦C, respectively) [20], and {bis[N,Nꢀ-(2,6-
References
[1] L.K. Johnson, C.M. Killian, M. Brookhart, J. Am. Chem. Soc. 117 (1995)
6414–6415.
[2] Z. Zhang, Z. Ye, Chem. Commun. 48 (2012) 7940–7942.
[3] G. Sun, J. Hentschel, Z. Guan, ACS Macro. Lett. 1 (2012) 585–588.
[4] C.S. Popeney, M.C. Lukowiak, C. Böttcher, B. Schade, P. Welker, D. Mangoldt, G.
Gunkel, Z. Guan, R. Haag, ACS Macro. Lett. 1 (2012) 564–567.
[5] H. Gao, H. Hu, F. Zhu, Q. Wu, Chem. Commun. 48 (2012) 3312–3314.
[6] X. Shi, Y. Zhao, H. Gao, L. Zhang, F. Zhu, Q. Wu, Macromol. Rapid Commun. 33
(2012) 374–379.
[7] C.M. Killian, D.J. Tempel, L.K. Johnson, M. Brookhart, J. Am. Chem. Soc. 118
(1996) 11664–11665.
[8] J.C. Yuan, T.J. Mei, P.T. Gomes, M.M. Marques, X.H. Wang, Y.F. Liu, C.P. Miao, X.L.
Xie, J. Organomet. Chem. 696 (2011) 3251–3256.
diisopropylphenyl)imino]acenaphthene}dibromonickel
(65
branches/1000 C, at 25 ◦C) [20], although the reaction condi-
tions are not exactly the same as in the present work. The total
branching degree of the polymer sample obtained with 4c bearing
electron-donating methyl groups is the lowest among our three
complexes.
Our results (Table 3) indicate that the more electron defi-
cient the catalyst, the more dendritic the polymer formed. A
more electron-deficient ligand may better stabilize the transi-
tion state for monomer chain walking than for insertion, which
should afford a more dendritic polymer. These results provide
[9] J.C. Yuan, L.C. Silva, P.T. Gomes, J.M. Campos, M.R. Riberio, P.S. Valerga, J.C.W.
Chien, M.M. Marques, Polymer 46 (2005) 2122–2132.
[10] J. Pelesˇka, Z. Hosˇtálek, D. Hasalíková, J. Merna, Polymer 52 (2011) 275–281.
[11] M.M. Wegner, A.K. Ott, B. Rieger, Macromolecules 43 (2010) 3624–3633.
[12] Y. Choi, J.B.P. Soares, Polymer 51 (2010) 2271–2276.
[13] D.H. Camachoa, Z. Guan, Chem. Commun. 46 (2010) 7879–7893.
[14] Z. Guan, C.S. Popeney, Top. Organomet. Chem. 26 (2009) 179–220.
[15] D. Takeuchi, Y. Fukuda, S. Park, K. Osakada, Macromolecules 42 (2009)
5909–5912.
[16] L. Li, M. Jeon, S.Y. Kim, J. Mol. Catal. A: Chem. 303 (2009) 110–116.
[17] C. Popeney, Z. Guan, Organometallics 24 (2005) 1145–1155.
[18] F.S. Liu, H.B. Hu, Y. Xu, L.H. Guo, S.B. Zai, K.M. Song, H.Y. Gao, L. Zhang, F.M. Zhu,
Q. Wu, Macromolecules 42 (2009) 7789–7796.
[19] A.E. Cherian, J.W. Rose, E.B. Lobkovsky, G.W. Coates, J. Am. Chem. Soc. 127 (2005)
13770–13771.
[20] S.D. Ittel, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000) 1169–1203.
[21] L.K. Johnson, S. Mecking, M. Brookhart, J. Am. Chem. Soc. 118 (1996) 267–268.
[22] S. Mecking, L.K. Johnson, L. Wang, M. Brookhart, J. Am. Chem. Soc. 120 (1998)
888–899.
a
strategy to control the branching topology by controlling
the electronic effect of the catalysts and polymerization condi-
tions.
The dendritic polyethylene particle size obtained by 4a/DEAC
can be controlled in the 1–20 nm under 0.2 bar ethylene pres-
sure, and could be expected to become a nano-targeted drug
carrier after modified with water-soluble oligo(ethylene glycol)
(OEG).
[23] A.C. Gottfried, M. Brookhart, Macromolecules 36 (2003) 3085–3100.

J. Yuan et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 132–139
139
[24] H.S. Schrekker, V. Kotov, P. Preishuber-Pflugl, P. White, M. Brookhart, Macro-
molecules 39 (2006) 6341–6354.
[25] W.J. Yang, Y.Y. Cheng, T.W. Xu, X.Y. Wang, L.P. Wen, Eur. J. Med. Chem. 44 (2009)
862–868.
[26] S. Svenson, D.A. Tomalia, Adv. Drug Deliv. Rev. 57 (2005) 2106–2129.
[27] Y. Matsumura, H. Maeda, Cancer Res. 46 (1986) 6387–6392.
[28] H. Maeda, T. Sawa, T. Konno, J. Control. Release 74 (2001) 47–61.
[29] H. Maeda, G.Y. Bharate, J. Daruwalla, Eur. J. Pharm. Biopharm. 71 (2009)
409–419.
[30] F. Danhier, O.R. Feron, V. Préat, J. Control. Release 148 (2010) 135–146.
[31] E.R. Gillies, J.M.J. Fr’echet, Drug Discov. Today 10 (2005) 35–43.
[32] G. Chen, D. Huynh, P.L. Felgner, Z. Guan, J. Am. Chem. Soc. 128 (2006)
4298–4302.
[33] L.G.L. Ward, J.R. Pipal, Inorg. Synth. 13 (1972) 162–163.
[34] C.L. Song, L.M. Tang, Y.G. Li, X.F. Li, J. Chen, Y.S. Li, J. Polym. Sci. Pol. Chem. 44
(2006) 1964–1974.
[35] R.J. Maldanis, J.S. Wood, A. Chandrasekaran, M.D. Rausch, J.C.W. Chien, J.
Organomet. Chem. 645 (2002) 158–167.
[36] Z. Guan, P.M. Cotts, E.F. McCord, S.J. McLain, Science 283 (1999)
2059–2062.
[37] Z. Guan, Chem. Eur. J. 8 (2002) 3086–3092.
[38] P.M. Cotts, Z. Guan, E.F. McCord, S.J. McLain, Macromolecules 2000 (33) (2000)
6945–6952.
[39] D. Meinhard, M. Wegner, G. Kipiani, A. Hearley, P. Reuter, S. Fischer, O. Marti,
B. Rieger, J. Am. Chem. Soc. 129 (2007) 9182–9191.