A thermal stable α-diimine palladium catalyst for copolymerization of ethylene with functionalized olefins

Article abstract of DOI:10.1016/j.molcata.2014.03.008

Copolymerization of polar monomers and olefins catalyzed by transition metal under mild conditions is a challenge. α-Diimine palladium complexes have great potential to catalyze the copolymerization of alkylacrylates and olefins. But they are prone to deactivation under mild conditions which limits their industrial application. A novel unsymmetrical α-diimine palladium complex with bulkier α-diimine ligand was provided in this paper. They were active in ethylene homopolymerization at 80 °C. At 50 °C they could produce ethylene/methylacrylate copolymer with moderate methylacrylate incorporation. The probable reason for the high thermal stability of these complexes was proposed.

Full text of DOI:10.1016/j.molcata.2014.03.008

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Journal of Molecular Catalysis A: Chemical
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A thermal stable ␣-diimine palladium catalyst for copolymerization of
ethylene with functionalized olefins
Huijie Pan1, Liang Zhu1, Jiewei Li, Dandan Zang, Zhisheng Fu∗, Zhiqiang Fan
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou
310027, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Copolymerization of polar monomers and olefins catalyzed by transition metal under mild conditions
is a challenge. ␣-Diimine palladium complexes have great potential to catalyze the copolymerization
of alkylacrylates and olefins. But they are prone to deactivation under mild conditions which limits
their industrial application. A novel unsymmetrical ␣-diimine palladium complex with bulkier ␣-diimine
ligand was provided in this paper. They were active in ethylene homopolymerization at 80 ◦C. At 50 ◦C
they could produce ethylene/methylacrylate copolymer with moderate methylacrylate incorporation.
The probable reason for the high thermal stability of these complexes was proposed.
Received 7 December 2013
Received in revised form 8 March 2014
Accepted 15 March 2014
Available online xxx
Keywords:
Alpha-diimine
Palladium
© 2014 Elsevier B.V. All rights reserved.
Ethylene
Methylacrylate
Copolymerization
1. Introduction
ligand to form palladacyclic intermediates (Scheme 1) [22]. But
those ␣-diimine palladium complexes bearing 2,6-diphenyl ani-
Compared with polyolefins, polar polyolefin copolymers show
better properties, such as mar resistance, toughness and printing
properties. They are commercially manufactured by free radical
polymerization under very harsh conditions with branched struc-
tures and broad molecular weight distributions [1,2]. It is difficult
to control the copolymer composition [3]. Coordination polymer-
ization catalyzed by transition metal would permit the synthesis
of polar polyolefin copolymers under mild conditions, reduce pro-
duction costs and allow better control of their composition and
microstructure. Therefore, for many years an intense research
effort to overcome the difficulties in controlling the copolymeri-
zation of polar monomers and olefins has been sustained [2,4–7].
In 1995 Brookhart found that ␣-diimine palladium complexes
could catalyze the copolymerization of ethylene with alkylacry-
lates [8,9]. It encouraged researchers to develop novel ␣-diimine
palladium catalysts that could copolymerize polar monomers and
olefins [10–21]. However, ␣-diimine palladium catalysts are prone
to deactivation under mild conditions in comparison to Ziegler-
Natta catalysts. Brookhart et al. demonstrated that the deactivation
took place by C H activation with alkyl groups on the ␣-diimine
line moieties (complex 1, as shown in Fig. 1) whose aniline-phenyl
groups without alkyl could not rotate freely around their N Carom
bond decomposed within minutes during the polymerization reac-
tion [23]. The other ␣-diimine palladium complexes bearing bulky
ligands, such as cyclophane-based Pd(II) ␣-diimine palladium
catalysts [24] and camphyl ␣-diimine palladium catalyst [25], gen-
erally catalyze the copolymerization of ethylene with alkylacrylates
below 35 ◦C with low activity. However, in terms of prospective
commercialization, such catalysts would typically need to operate
at temperatures of between 80 and 100 ◦C [26].
Since the ␣-diimine nickel complexes bearing 2,6-diphenyl
aniline moieties (the analogue of complex 1) exhibited unprece-
dentedly high activity at ambient temperature [23], C H activation
might be not the only reason for the deactivation of the correspond-
ing ␣-diimine palladium complexes. Because the ␣-diimine ligand
in complex 1was a large and rigid molecule, the push and squeeze
of the ligand around the metal center might be the other reason.
The vibration and fluctuation of the methyl groups on the back-
bone of the ligand in complex 1 will repel the substituents bonded
to the imine groups. And finally the repulsion will be conducted
to the metal center. Nickel has a smaller atomic radius (0.1243 nm)
than palladium (0.1376 nm). Therefore, nickel complexes are stable
while palladium complexes are unstable.
∗
Corresponding author. Tel.: +86 571 87953754.
E-mail address: fuzs@zju.edu.cn (Z. Fu).
In this paper, two methyl groups on the backbone and one 2,6-
diphenyl aniline moiety were replaced by a naphthyl group and a
1
These authors contributed equally.
http://dx.doi.org/10.1016/j.molcata.2014.03.008
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Scheme 1. Possible intermediates for palladacycle formation via ␣-diimine C H activation [22].
2,6-diisopropyl aniline, respectively, to afford a novel unsymmetric
␣-diimine palladium complex2 (Fig. 1). The ethylene homopoly-
merization and ethylene/methylacrylate (MA) copolymerization
catalyzed by complex 2 were investigated.
2.2. Synthesis of 2,6-dibromoaniline
2. Experimental
To a mixture of 50.0 g sulfanilamide, 850 mL water and 200 mL
48% HBr solution, 90.0 g sodium perborate was added at 85 ◦C. The
mixture was stirred at 80–85 ◦C for 30 min and then was cooled
to room temperature and filtered. The residue was washed to be
neutral by water. The solid was desulfonated by reacting with
70% sulfuric acid solution for 3 h under reflux. Crude product was
obtained by steam distillation and further purified by recrystalliza-
tion in ethanol. 1H NMR (500 MHz, CDCl3, ı in ppm): 7.35–7.40 (d,
2H, Ar-Hm), 6.5 (t, 1H, Ar-Hp).
All manipulations of air- and/or moisture-sensitive compounds
were performed under a nitrogen atmosphere using standard
Schlenk techniques.
2.1. Materials
Toluene, diethyl ether and hexane were purified by reflux-
ing and distillation over sodium, and kept under a dry nitrogen
atmosphere. Methylene chloride was purified by refluxing and dis-
tillation over CaH2, and kept under a dry nitrogen atmosphere.
Methylene chloride-d2 was dried by distillation over P2O5, vacuum-
transferred and stored in the dry box. Methyl acrylate (MA) was
2.3. Synthesis of
2-[(2,6-diisopropylphenyl)imino]acenaphthylen-1-one
˚
distilled to remove radical inhibitor, stored over 4 A molecular
sieves and purged with nitrogen prior to use. Phenylboronic acid,
p-tert-butyl phenylboronic acid and p-methoxyphenylboronicacid
were purchased from Frontier Scientific and used as received. 2,6-
Diisopropyl aniline (Acros) and acenaphthequinone (Alfa Aesar)
were used as received. The synthesis and characterization of
(COD)PdMeCl [1] has been previously reported.
2,6-Diisopropyl aniline (1.45 g, 8.22 mmol) dissolved in ethanol
(50 mL) was added slowly to a mixture of acenaphthequinone(2.0 g,
11.0 mmol) with ethanol (65 mL) at 60 ◦C. The catalytic amount
Fig. 1. ␣-Diimine palladium complexes bearing 2,6-diphenyl aniline moieties.
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of formic acid (1 mL, 26.5 mmol) was added. The solution was
heated to under reflux for 20 h. Unreacted acenaphthequinone
was removed by filtration at room temperature. The filtrate
was further purified by column chromatography (silica gel, ethyl
acetate:hexane = 1:16) to give an orange red solid (1.68 g, yield
60%).1H NMR (500 MHz, CDCl3, ı in ppm): 8.20 (d, 1H, An-Ho), 8.17
(d, 1H, An-Hp), 7.98 (d, 1H, An-Ho), 7.41 (t, 1H, An-Hm), 7.26 (m, 3H,
Ar-H), 6.64 (d, 1H, An-Hp), 2.84 (sept, 2H, CH(CH3)2), 1.18 (d, 6H,
CH(CH3)2), 0.91 (d, 6H, CH(CH3)2).
8H, Ar-H), 7.18–7.29 (m, 8H, Ar-H), 6.90 (d, 1H, An-Hm), 6.39 (d,
1H, An-Hp), 2.84 (sept, 2H, CH(CH3)2), 1.23 (s, 18H, CH3), 1.22 (d,
6H, CH(CH3)2), 0.78 (d, 6H, CH(CH3)2). Anal. Calcd. for C50H52N2: C,
88.19; H, 7.70; N, 4.11. Found: C, 87.82; H, 7.78; N, 4.21.
2.7. Synthesis of Ligand 2b
Scheme 2 showed the synthesis route of ␣-diimine palladium
catalysts. Triethylene-diamine (0.267 g, 2.4 mmol) was added to
a solution of 2,6-di(4-methoxy)phenylaniline (0.286 g, 0.8 mmol)
in chlorobenzene (9 mL). To this solution, a solution of TiCl4
2.4. Synthesis of 2,6-di(4-methoxy)phenylaniline
4-methoxy phenylboronic acid (2.72 g, 18 mmol) dissolved in
ethanol (12 mL) and 2 M Na2CO3 (24 mL, 48 mmol) were added to a
solution of 2,6-dibromoaniline (1.56 g, 6 mmol) in toluene (60 mL).
To this solution, Pd(PPh3)4 (0.84 g, 0.67 mmol) was added. The
solution was heated to reflux for 72 h. After cooling to room tem-
perature, the mixture was filtered and an excess of aqueous HCl was
slowly added to the filtrate until no more white solid precipitated.
The residue was suspended in diethyl ether after filtration, and then
an excess of 2 M Na2CO3 was slowly added until the solid was totally
dissolved. The organic layer was dried over Na2SO4 and then the
solvent was removed to give light yellow solid (1.10 g, yield 60.3%).
1H NMR (500MHz, CDCl3, ı in ppm): 7.45 (d, 4H, Ar-Hm), 7.10 (d,
2H, Ar-Hm), 7.00 (d, 4H, Ar-Hm), 6.85–6.90 (t, 1H, Ar-Hp), 3.85–3.88
(s, 6H, OCH3).
(0.2 mL, 1.8 mmol) in chlorobenzene (2 mL) was slowly added
at 90 ◦C. Then 2-[(2,6-diisopropylphenyl)imino]acenaphthylen-1-
one (0.204 g, 0.4 mmol) dissolved in chlorobenzene (3.5 mL) was
added at once. The mixture was stirred at 150 ◦C for 48 h. The sol-
vent was removed in vacuum and the residue was dissolved in
CH2Cl2. After filtration, the solution was concentrated and purified
by chromatography (silica gel, CH2Cl2/Hexane/EtOAc = 40/40/1) to
give a yellow solid (0.16 g, yield 63.6%). 1H NMR (500 MHz, CDCl3,
ı in ppm): 7.85 (d, 1H, An-Ho), 7.75 (d, 1H, Ar-Ho), 7.74–7.75 (m,
8H, Ar-H), 7.20–7.30 (d, 4H, Ar-Hm), 6.87 (d, 1H, An-Hm), 6.72 (d,
4H, Ar-Hm), 6.39 (d, 1H, An-Hp), 3.72 (s, 6H, OCH3), 2.84 (sept, 2H,
CH(CH3)2), 1.22 (d, 6H, CH(CH3)2), 0.78 (d, 6H, CH(CH3)2). Anal.
Calcd. for C44H40N2O2: C, 84.04; H, 6.41; N, 4.46. Found: C, 83.11;
H, 6.59; N, 4.39.
2.5. Synthesis of 2,6-di(4-tert-butyl)phenylaniline
4-tert-Butylphenylboronic acid (4.81 g, 27.0 mmol) dissolved in
ethanol (18 mL) and 2 M Na2CO3 (36 mL, 72 mmol) were added
to a solution of 2,6-dibromoaniline (2.258 g, 9.0 mmol) in toluene
(90 mL). To this solution, Pd(PPh3)4 (1.26 g, 1.0 mmol) was added.
The mixture was heated to reflux for 72 h. After cooling to room
temperature, the mixture was filtered and an excess of aqueous
HCl was slowly added to the filtrate until no more white solid pre-
cipitated. The residue was suspended in diethyl ether after filtration
and then an excess of 2 M Na2CO3 was slowly added until the solid
was totally dissolved. The organic layer was dried over Na2SO4 and
then the solvent was evaporated to give white solid (1.80 g, yield
56%). 1H NMR (500 MHz, CDCl3, ı in ppm): 7.47–7.52 (m, 8H, Ar-
Hm), 7.15 (d, 2H, Ar-Ho), 6.89 (t, 1H, Ar-Hp), 3.95 (s,2H, NH2), 1.40
(s, 18H, CH3).
2.8. Synthesis of complex 2a
A solution of ligand 2a (0.30 g, 0.44 mmol) in CH2Cl2 (10 mL) was
added to a solution of (COD)PdMeCl (0.11 g, 0.42 mmol) in CH2Cl2
(10 mL). The mixture was stirred at 25 ◦C for 24 h. The solvent
was removed in vacuum and the residue was washed with diethyl
ether (3 × 10 mL). The residue was evaporated in vacuum to give an
orange powder (0.32 g, yield 91.1%). Anal. Calcd. for C51H55N2ClPd:
C, 73.11; H, 6.62; N, 3.34. Found: C, 71.62; H, 6.77; N, 3.19.
Single crystals of complex 2a suitable for X-ray diffraction
analysis were obtained by laying hexane on the dichloromethane
solution at −10 ◦C. With graphite monochromated Mo K␣ radiation
˚
(ꢀ = 0.71073 A) at 173(2) K, cell parameters were obtained by global
2.6. Synthesis of Ligand 2a
refinement of the positions of all collected reflections. Intensities
were corrected for Lorentz and polarization effects and empiri-
cal absorption. The structures were solved by direct methods and
refined by full-matrix least squares on F2. All hydrogen atoms were
placed in calculated positions. Structure solution and refinement
were performed by using the SHELXL-97 package [27].
Triethylene-diamine (1.33 g, 12 mmol) was added to a solu-
tion of 2,6-di(4-tert-butyl)phenylaniline (1.43 g, 4.0 mmol) in
chlorobenzene (20 mL). To this solution, a solution of TiCl4 (0.8 mL,
7.3 mmol) in chlorobenzene (2 mL) (Scheme 2) was slowly added
at 90 ◦C. Then 2-[(2,6-diisopropylphenyl)imino]acenaphthylen-1-
one (0.68 g, 2.0 mmol) dissolved in chlorobenzene (6.0 mL) was
added at once. The mixture was stirred at 150 ◦C for 48 h. The sol-
vent was removed in vacuum and the residue was dissolved in
CH2Cl2. After filtration, the solution was concentrated and purified
by chromatography (silica gel, CH2Cl2/Hexane/EtOAc = 40/40/1) to
give a yellow solid (0.64 g, yield 47.1%).1H NMR (500 MHz, CDCl3,
ı in ppm): 7.82 (d, 1H, An-Ho), 7.78 (d, 1H, Ar-Ho), 7.38–7.52 (m,
2.9. Synthesis of complex 2b
A solution of ligand 2b (0.30 g, 0.48 mmol) in CH2Cl2 (10 mL) was
added to a solution of (COD)PdMeCl (0.12 g, 0.45 mmol) in CH2Cl2
(10 mL). The mixture was stirred at 25 ◦C for 24 h. The solvent was
removed in vacuum and the residue was washed with diethyl ether
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Scheme 2. The synthesis route of ␣-diimine palladium catalysts.
Table 1
(3 × 10 mL). The residue was evaporated in vacuum to give a red
powder (0.32 g, yield 89.4%). Anal. Calcd. for C45H43N2O2ClPd: C,
68.68; H, 5.51; N, 3.56. Found: C, 68.54; H, 6.11; N, 3.42.
Selected bond lengths (Å) and angles (degree) for complex 2a.
Bond lengths (Å)
Bond Angles (◦)
Pd(1) N(1)
Pd(1) N(2)
Pd(1) Cl(1)
Pd(1) C(51)
N(1) C(13)
N(1) C(9)
2.069
2.195
2.266
2.097
1.291
1.436
1.276
1.434
Cl(1) Pd(1) C(51)
Pd(1) N(1) C(9)
Pd(1) N(2) C(25)
N(1) Pd(1) N(2)
Pd(1) N(1) C(13)
Pd(1) N(2) C(14)
N(1) C(13) C(14)
N(2) C(14) C(13)
84.41
128.00
132.08
78.70
110.63
114.24
118.07
118.10
2.10. General procedure for ethylene homo-polymerization and
ethylene/methyl acrylate co-polymerization
A 50 mL Schlenk flask was heated by heat-gun (300–350 ◦C)
under vacuum for more than 5 min, purged with ethylene three
times, and immersed into oil-bath at the designated temperature.
To this flask, 9 mL of dry toluene was added by syringe, and 0.1 MPa
of ethylene was introduced. As to ethylene/methyl acrylate copo-
lymerization, a designated amount of methyl acrylate was added by
syringe at this moment. A 10 mL Schlenk flask was heated by heat-
gun (300–350 ◦C) under vacuum for more than 5 min then thrice
purged with N2 and cooled to room temperature. The flask was
transferred into dry box and charged with 10 ␮mol of complex 2a
or 2b and10 ␮mol of [Li(Et2O)2.8][B(C6F5)4]. To this flask, 1 mL of
dry toluene was added. The catalyst solution was then transferred
into the 50 mL Schlenk flask by cannula. The polymerization was
carried out for a designated time. The resultant polymer solution
was filtered through Celite. The filtrate was dried by rotary evapo-
ration. Finally, the resultant polymer was dried in vacuum at 60 ◦C
overnight.
N(2) C(14)
N(2) C(25)
A single crystal of palladium complex 2a suitable for X-ray
diffraction analysis was obtained by slow diffusion of hexane into
methylene chloridesolution of 2a. The molecular structure of 2a
was analyzed by single-crystal X-ray diffraction. The molecular
structure of 2a is shown in Fig. 2, and the selected bond lengths and
angles are tabulated in Table 1. The crystal, intensity collection, and
refinement data for complex 2a were presented in Table 2. Fig. 2
shows the perspective of the molecular structure along the square
plane of the complex. As shown in Fig. 1, each side of the two N-aryl
rings lies roughly perpendicular (85.5◦ and 95.2◦) to the molecular
Table 2
Crystallographic Data for Complex2a.
2.11. Characterization
Parameters
Data
Empirical formula
Formula Wt
Temp. (K)
a(Å)
b(Å)
c(Å)
˛(deg)
ˇ(deg)
ꢁ(deg)
V(Å3)
C51H55N2PdCl
837.9
140
11.7297(3)
19.8691(4)
19.8742(4)
90.00
95.700(2)
90.00
1H NMR spectra of ligands and catalysts were recorded on a
Bruker Avance DMX500. 1H NMR spectra of polymers were con-
ducted at room temperature on a Bruker Avance DMX400. All NMR
chemical shifts are reported as ı in parts per million (ppm). 1H NMR
spectra are reported relative to residual solvent. Elemental analysis
(EA) was conducted on a Thermo Electron SPA EA1112. Molec-
ular weights of polymers were determined using a Waters 1515
Gel Permeation Chromatographer with a Waters 2414 differential
refractometer in THF.
4608.9(2)
4
1.330
Z
Dt(Mg/m3)
Abs coeff (mm−1
)
0.613
F(0 0 0)
Crystal size (mm)
ꢂ range (deg)
1920
3. Results and discussion
0.29 × 0.39 × 0.45
3.24–25.35
3.1. Synthesis and characterization of ligands and complexes
Index ranges
h: −14 to 12
k: −23 to 18
l: −23 to 17
As shown in Scheme 3, the condensation reaction of ace-
naphthene with one equivalent of 2,6-diisopropyl aniline led
to C1. The bromination and then hydrolysis reactions of sul-
fanilamide generated 2,6-dibromoaniline. The Suzuki reactions of
2,6-dibromoaniline with various phenylboronic acids led to ter-
phenyl anilines (C2). The condensation reaction of C1 with C2 led
to ␣-diimine ligands 2a and 2b. Ligand 2a or 2b reacted with
[(COD)Pd(CH3)Cl] (COD = ␩2,␩2-cycloocta-1,5-diene) to form pal-
ladium complexes 2a or 2b in good yields.
Completeness
0.998
Multi-scan
1.060
Absorption correction
Goodness-of-fit on F2
Final R indices
R1 = 0.0572
wR2 = 0.1657
R indices (all data)
R1 = 0.0719
wR2 = 0.1527
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Scheme 3. The total synthesis route of complex 2.
plane. This structure is critical to block the axial sites and generate
high molecular weight polymers.
It indicated that the lifetime of complex 2a for polymerization reac-
tion was longer than 2 h. Namely, unlike complex 1, complex 2a was
stable and did not decompose within minutes during the polymer-
ization reaction. Compared with Brookhart’s research results [8],
the catalytic activity of complex 2a was lower than that of the clas-
sical (␣-diimine) palladium catalyst probably due to the relatively
higher steric hindrance of the ligand.
As shown in Table 3, raising the polymerization temper-
ature to 50 ◦C resulted in slight increase in catalytic activity
(5.8 × 103 gPE/molPd•h, Table 3, entry 3). Further raising the
temperature to 80 ◦C did not significantly affect the catalytic
3.2. Ethylene homopolymerizations catalyzed by complex 2
Since complex
1 decomposed within minutes during the
polymerization reaction, firstly the stability of complex 2 was
tested. The ethylene homopolymerization with 10 ␮mol of com-
plex 2a at 0.1 MPa, 25 ◦C for 2 h yielded 0.1224 g of polyethylene
(Table 1, entry 1). As the polymerization time was prolonged to
6.5 h, 0.3244 g of polyethylene was obtained (Table 1, entry 2).
Fig. 2. Molecular structure of depicted with 30% thermal ellipsoids and with hydrogen atoms omitted.
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Table 3
Ethylene homopolymerization catalyzed by complex 2a.
Entry
Condition
Yield (g)
Activityb
Branchesc
Mw ×104
Mw/Mn
1
2
3
4
5
2a, 25 ◦C, 2 h
0.1224
0.3244
0.3771
0.3642
0.3901
6.1
5.0
5.8
5.6
6.0
-
-
-
-
2a, 25 ◦C, 6.5 h
2a, 50 ◦C, 6.5 h
2a, 80 ◦C, 6.5 h
2b, 50 ◦C, 6.5 h
5.64
3.41
1.48
4.10
1.46
1.61
2.12
1.42
107
128
104
a
Polymerization condition: 10 ␮mol complex 2, 10 ␮mol cocatalyst (Li(Et2O)2.8B(C6F5)4), 10 mL toluene, 0.1 MPa ethylene.
×103gPE/molPd•h.
b
c
CH3/1000 C, number of methyl group per 1000 carbons determined by 1H NMR.
activity (5.6 × 103 gPE/molPd•h, Table 3, entry 4). As temperature
increased, the branches of the resultant polyethylene increased
obviously, at the same time, the molecular weights of the resultant
polyethylene dropped significantly and the molecular weight dis-
tributions of polyethylene broadened to some extent. Complex 2b
was also stable at 50 ◦C and produced polyethylene with catalytic
activity of 6.0 × 103 gPE/molPd•h (Table 3, entry 5).
with each other. According to Rieger’s results [23], the observed
Pd bond lengths for complex 1b were 2.004(4) Å and 2.028(4)
N
Å for N1 Pd and N2 Pd, respectively. The C1 C2 bond length was
1.463(7) Å. The C1 N1 bond length was 1.275(6) Å. The C2 N2
bond length was 1.280(6) Å. The N1 Pd N2 angle was 79.73(15)◦.
According to these data and the data in Table 1, the schematic dia-
grams of the five-membered ␣-diimine ring of complex 1b and
complex 2a could be deduced and shown in Fig. 3. Each edge of the
␣-diimine ring of complex 2a is longer than the corresponding edge
of complex 1b. The framework of complex 2a is larger than that
of complex 1b. Furthermore, the distance between Pd and C1 C2
3.3. Ethylene/methyl acrylate copolymerizations catalyzed by
complex 2
˚
bond in complex 2a (2.778 A) is longer than that in complex 1b
Since complex 2 was stable and could efficiently catalyze
ethylene polymerization at the temperature higher than 35 ◦C,
ethylene/methyl acrylate copolymerizations catalyzed by complex
2 were further investigated.
˚
(2.690 A). Thus, the Pd atom in complex 2a experiences less repul-
sion force from the ␣-diimine ligand. We propose that this could
be the probable reason for the good thermal stability of complex
2a in the polymerization reaction.
As shown in Table 4, complex 2 was also stable and pro-
duced ethylene/methylacrylate (MA) copolymer with moderate
MA incorporation at 25 ◦C. As expected, the fraction of MA incor-
poration was directly proportional to its concentration in the
reaction solution (entries 1ꢀ–3ꢀ and entries 5ꢀ–7ꢀ). Interestingly, as
the copolymerization temperature increased from 25 to 50 ◦C, both
polymerization activity and MA incorporation increased (entries
3ꢀ, 4ꢀ and entries 7ꢀ, 8ꢀ). Especially, complex 2b afforded copol-
ymer of 4.03 mol% MA at 50 ◦C, which was almost twice that at
25 ◦C. At the same time, the catalytic activity of complex 2b at
50 ◦C was also twice that at 25 ◦C. During the copolymerization,
a six-membered chelate complex is formed after insertion of MA
(as shown in Scheme 4). Further chain growth requires coordi-
nation and insertion of ethylene. This is the turnover-limiting
step. Therefore, the chelate complex is the catalyst resting state
[9]. Probably, raising the polymerization temperature resulted in
the activation of the six-membered chelate complex. Namely,
the higher the polymerization temperature, the more readily the
six-membered chelate complex could open. Thus, raising the poly-
merization temperature resulted in an increase in ethylene and MA
turnovers.
3.4. The probable reason for thermal stability of complex 2
For deactivation in ␣-diimine metal catalysts, a possible rea-
son has been proposed that the metal center reacts with the C
H
bonds of the ortho-aryl substituents on aniline moiety (C H activa-
tion) to give a six-membered metallacycle [22]. However, complex
1 did not possess ortho-aryl alkyl substituents. Using complex 1 to
catalyze ethylene polymerization at room temperature in CH2Cl2,
it decomposed quickly during the polymerizationreaction [23]. On
the contrary, complex 2 who possessed ortho-isopropyl could cat-
alyze ethylene polymerization at 80 ◦C with moderate activity. The
experimental results in this paper suggest that probably C H bond
activation is not the only route for deactivation of ␣-diimine palla-
dium catalysts.
To reveal the origin of the high thermal stability for complex 2,
the crystal structure of complex 1b and complex 2a was compared
Fig. 3. Schematic diagrams of the five-membered ␣-diimine ring: (a) complex 1b;
(b) complex 2a.
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MOLCAA-9042; No. of Pages7
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H. Pan et al. / Journal of Molecular Catalysis A: Chemical xxx (2014) xxx–xxx
7
Table 4
Ethylene/methylacrylate copolymerization catalyzed by complex 2a.
b
Entry
Condition
[MA]0
MA%c
Acti.d
Mw1e/PDI1
Mw2e/PDI2
1ꢀ
2ꢀ
3ꢀ
4ꢀ
5ꢀ
6ꢀ
7ꢀ
8ꢀ
2a, 25 ◦
2a, 25 ◦
2a, 25 ◦
2a, 50 ◦
2b, 25 ◦
2b, 25 ◦
2b, 25 ◦
2b, 50 ◦
C
C
C
C
C
C
C
C
58
0.84
1.67
2.01
2.45
1.44
1.93
2.11
4.03
907
583
151
154
678
319
123
246
-
1.4/1.5
0.8/1.2
0.7/1.2
0.4/1.2
-
-
-
-
116
174
174
58
116
174
174
3.3/1.3
2.8/1.3
3.9/1.6
5.8/1.2
5.5/1.2
4.8/1.2
4.1/1.2
a
Polymerization condition: 10 ␮mol complex 2, 10 ␮mol cocatalyst (Li(Et2O)2.8B(C6F5)4), 10 mL toluene, 0.1 MPa ethylene.
mmol/L.
Molar percentage.
b
c
d
e
g/molPd•h.
×104. For some samples (entry 2ꢀ–4ꢀ) there were two peaks on the GPC curve.
Scheme 4. The mechanistic pathway of ethylene/methyl acrylate copolymerization catalyzed by ␣-diimine palladium catalyst [9].
4. Conclusions
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(1996) 11664–11665.
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888–899.
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428–447.
A thermal stable ␣-diimine palladium complex for ethylene
homopolymerization and ethylene/methylacrylate copolymeriza-
tion was prepared. The unique structure of the unsymmetrical
␣-diimine ligand, especially the rigid naphthyl group on the back-
bone, significantly reduces the push and squeeze of the ligand
around Pd atom. Presumably, this suppresses the repulsion force
from the ␣-diimine ligand, thus enhancing the thermal stability of
the ␣-diimine palladium complex. Further mechanistic studies and
structural modification of the thermal stable ␣-diimine palladium
complexes are currently under way.
[12] Z. Guan, P.M. Cotts, E.F. McCord, S.J. McLain, Science 283 (1999) 2059–2062.
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6945–6952.
[14] Z. Guan, Chem. Eur. J. 8 (2002) 3086–3092.
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Acknowledgment
[19] T.M.J. Anselment, S.I. Vagin, B. Rieger, Dalton Trans. (2008) 4537–4548.
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122 (2000) 6686–6700.
Support by Specialized Research Fund for the Doctoral Program
of High Education (grant no. 20100101110136) and the Major State
Basic Research Programs (Grant No. 2011CB606001) are gratefully
acknowledged.
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(2001) 2321–2330.
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Please cite this article in press as: H. Pan, et al., J. Mol. Catal. A: Chem. (2014), http://dx.doi.org/10.1016/j.molcata.2014.03.008