Synthesis of high molecular weight polyethylene using iminopyridyl nickel catalysts

Article abstract of DOI:10.1039/c6cc00457a

A series of iminopyridyl Ni(ii) catalysts containing both the dibenzhydryl and the naphthyl moieties can polymerize ethylene with high activity and high thermal stability, generating polyethylene with a molecular weight of up to one million. In α-olefin polymerization, semicrystalline polymers with high melting temperatures are generated.

Full text of DOI:10.1039/c6cc00457a

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Synthesis of high molecular weight polyethylene
using iminopyridyl nickel catalysts†
Cite this:DOI: 10.1039/c6cc00457a
Shengyu Dai, Xuelin Sui and Changle Chen*
Received 17th January 2016,
Accepted 14th March 2016
DOI: 10.1039/c6cc00457a
www.rsc.org/chemcomm
A series of iminopyridyl Ni(II) catalysts containing both the dibenzhydryl
and the naphthyl moieties can polymerize ethylene with high activity
and high thermal stability, generating polyethylene with a molecular
weight of up to one million. In a-olefin polymerization, semicrystalline
polymers with high melting temperatures are generated.
Since the Nobel Prize winning discovery of Ziegler and Natta
catalysts in the 1950s, tremendous efforts have been dedicated
to the development of olefin polymerization catalysts. Originally,
the metal nickel found its way into this field as a ‘‘poison’’. The
famous ‘‘nickel effect’’ was discovered by Ziegler in 1952, who
found that ethylene was converted exclusively to 1-butene using
aluminium alkyls in the presence of nickel salts.1 The scenario
has changed dramatically after decades of research. Recently,
numerous Ni(II) based catalysts have been developed that are
capable of generating high molecular weight polyolefin materials,
including some SHOP-type catalysts (Scheme 1, I; SHOP =
Shell Higher Olefin Process),2 a-diimine based Ni(II) catalysts
(Scheme 1, II),3,4 neutral salicylaldimine Ni(II) catalysts
(Scheme 1, III),5 and some phosphine–sulfonate Ni(II) catalysts
(Scheme 1, IV).6
Scheme 1 Previously reported nickel based ethylene polymerization cata-
lysts (I–IV) and iminopyridyl complexes (V–XI; R = Me, iPr; X = Cl, Br; these
complexes are usually in dimeric forms).
Some interesting results have been reported for iminopyridyl
based Ni(II) catalysts.7 A series of dimeric iminopyridyl Ni(II)
catalysts (Scheme 1, V) have been shown to be active in ethylene
polymerization, generating polyethylene with Mn of less than for these catalysts (Mn o 1000) compared with the original
3500 at temperatures higher than 20 1C (Mn o 20000 at 0 1C).8 iminopyridyl Ni(II) catalysts (Scheme 1, V). By increasing the
By utilizing a sterically bulky dibenzhydryl group, Sun et al. steric bulkiness on the other side of the iminopyridyl ligand,
showed that the resulting Ni(II) catalysts (Scheme 1, VI) were only ethylene oligomerization Ni(II) catalysts could be obtained
highly active in ethylene polymerization with activities of up to (Scheme 1, VII and VIII).10 The Ni(II) catalysts with a fused
107 g (PE) molÀ1 (Ni) hÀ1 9
. However, the increase in steric iminopyridyl structure (Scheme 1, IX) led to the formation of
bulkiness led to the reduction in polyethylene molecular weight polyethylene with a similar molecular weight to those generated
by the original iminopyridyl Ni(II) catalysts (Scheme 1, V).11
Key Laboratory of Soft Matter Chemistry, Chinese Academy of Sciences,
Department of Polymer Science and Engineering, University of Science and
Technology of China, Hefei, 230026, China. E-mail: change@ustc.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
graphical spectra, and kinetics data. CCDC 1446577 contains supplementary
crystallographic data for 3. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c6cc00457a
Similar to the cases in VII and VIII, the installation of sub-
stituents on the other side of the iminopyridyl ligand led to
ethylene oligomerization using Ni(II) catalysts (Scheme 1, X). Sun
et al. also studied a series of naphthylamine based iminopyridyl
ligands (Scheme 1, XI). These Ni(II) catalysts were highly active in
ethylene polymerization, affording very low molecular weight
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Fig. 1 Molecular structure of L3. Hydrogen atoms have been omitted for
clarity. Atoms are drawn at a 30% probability level. Selected bond lengths
(Å) and angles (deg): C1–C2 = 1.467(3), N1–C1 = 1.261(2), N1–C7 =
1.420(2); C2–C1–N1 = 123.59(18), C1–N1–C7 = 117.41(15).
Scheme 2 Synthesis of ligands L1–L3 and Ni(II) complexes 1–3.
polyethylene (Mn o 500).12 Two conclusions can be drawn from
these studies: (i) iminopyridyl Ni(II) catalysts are capable of only
oligomerizing ethylene or generating low molecular weight
polyethylene; (ii) the polyethylene molecular weight is not sensi-
tive to ligand sterics in this system.
Multiple attempts to determine the molecular structure of the
Ni(II) complexes failed due to their instability. A dimeric struc-
ture may be present in the solid state similar to those previously
reported iminopyridyl Ni(II) catalysts shown in Scheme 1.
Upon activation with methylaluminoxane (MAO) or Et2AlCl,
complexes 1–3 become highly active in ethylene polymerization
(Table 1). Complex 1 was used to screen the influence of
polymerization conditions. When the temperature was increased
from 20 1C to 50 1C and 80 1C, both the activity and the
polyethylene molecular weight decreased dramatically (Table 1,
entries 1–3). The branching density increased. The melting
temperature of the resulting polyethylene also significantly
decreased. Some exciting results were obtained when the poly-
merization was carried out at 5 1C (Table 1, entries 4 and 5). By
decreasing the temperature from 50 1C to 20 1C, Mn of poly-
ethylene increased from 5.2 Â 104 to 14.1 Â 104. By decreasing
the temperature from 20 1C to 5 1C, Mn increased from 14.1 Â
104 to 44.6 Â 104. The difference between Mn’s at 5 1C and 20 1C
seems to be large. However, this actually agrees very well with
previous reports. In ref. 8a, Mn of the polyethylene generated
by catalyst V with the iPr substituent (Scheme 1) increased from
Based on previous studies, sterically bulky ligands are
usually required to afford Ni(II) catalysts capable of generating
high molecular weight polymers. For example, Long et al.
showed that some sterically bulky a-diimine Ni(II) complexes
bearing a dibenzhydryl moiety (Scheme 1, XII) were highly
active in ethylene polymerization, generating polyethylene with
high molecular weight (Mn 4 105).13 Also, our group showed
that the dibenzhydryl moiety could lead to highly stable and
active a-diimine Pd(II) catalysts, generating semicrystalline
polyethylene and ethylene-methyl acrylate copolymers with
high molecular weight.14 Recently, Brookhart, Coates, Daugulis
et al. studied the olefin polymerization properties of some
arylnaphthylamine derived a-diimine and salicylaldimine
Ni(II) catalysts (Scheme 1, XIII).15 Inspired by these studies,
we herein report the synthesis and olefin polymerization studies
of a series of iminopyridyl Ni(II) catalysts bearing both the
dibenzhydryl and the arylnaphthyl moieties. These Ni(II) com-
plexes are highly active in ethylene polymerization when
activated with aluminium cocatalysts. Most surprisingly, these
catalysts could generate polyethylene with a molecular weight
on the order of millions, about two orders of magnitude higher
than all of the previously reported iminopyridyl Ni(II) catalysts.
This is a very good demonstration of the power of the ligand
structures to influence the polymerization processes.
8-(4-R-phenyl)-naphthalen-1-amine (R = H, Me, Ph) was
easily synthesized using the literature procedure (Scheme 2).15a,16
Subsequently, the reaction with two equiv. of Ph2CHOH led to
the formation of anilines in 93–96% yields. The condensation
reaction with picolinaldehyde afforded the desired imino-
pyridine ligands L1–L3 in 76–87% yields. All of these organic
compounds were characterized by 1H, 13C NMR spectroscopy
and mass spectrometry. The Ni(II) complexes (1–3) were generated
in 92–96% yields by the reaction of the ligands with 1 equiv. of
(DME)NiBr2 (DME = ethylene glycol dimethyl ether) and were
characterized by mass spectrometry and elemental analysis.
Table 1 Ethylene polymerization mediated by Ni(II) complexes 1–3a
Ent. Cat. Cocat. T (1C) Yield (g) Act.b Mn (Â10À4) PDIc Bd Tm (1C)
c
e
1
2
3
1
1
1
1
1
1
1
2
3
1
1
2
2
3
3
MAO
MAO
MAO
MAO
MAO
20 0.76
50 0.35
80 0.21
7.6 14.1
1.77 49 74.0
1.86 66 58.4
2.00 71 44.8
2.00 25 105.2
1.83 26 112.3
1.38 24 111.3
1.43 25 115.5
1.47 58 71.4
2.10 52 83.6
3.5
2.1
5.2
4.8
4
5
5
1.42
3.32
14.2 44.6
8.30 101.2
22.1 72.5
16.0 142.5
6.1 13.5
5.6 21.1
5 f
6
MAO À20 2.21
7 f
8
MAO À20 6.41
MAO
MAO
20 0.61
20 0.56
9
10
11
12
13
14
15
Et2AlCl 20 4.30
Et2AlCl 80 2.29
Et2AlCl 20 2.63
Et2AlCl 80 1.68
Et2AlCl 20 3.23
Et2AlCl 80 2.52
43.0
22.9
26.3
16.8
32.3
25.2
8.3
2.8
8.1
2.1
9.9
2.4
1.78 — g 67.8
h
2.10 — g
—
2.01 — g 70.1
h
2.13 — g
—
1.96 — g 69.8
h
1.97 — g
—
a
Conditions: 2 mmol pre-catalyst, 1000 eq. cocatalyst, 5 mL CHCl3,
The molecular structure of L3 was determined by X-ray 45 mL toluene, 8 atm, 30 min. Activity (Act.) = 105 g (mol Ni h)À1
.
b
c
Molecular weight was determined by GPC using polystyrene standards.
diffraction analysis (Fig. 1). Based on the structural analysis,
the phenyl substituent on the naphthyl moiety would be able to
d
e
B = branches per 1000 carbon atoms, determined by 1H NMR analysis.
f
Determined by differential scanning calorimetry (DSC). Polymeriza-
g
h
block the axial position of the Ni center in the metal complexes. tion for 2 h. Not determined. Less than 25 1C.
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Table 2 Polymerization of a-olefins by Ni(II) complexes 1–3a
3.5 Â 103 to 20 Â 103 when the polymerization temperature was
decreased from 20 1C to 0 1C. In ref. 8b, Mn of the polyethylene
generated by catalyst V with the Me substituent (Scheme 1)
increased from 2.5 Â 103 to 14 Â 103 when the polymerization
temperature was decreased from 20 1C to 0 1C. At 5 1C and 2 h of
polymerization time, Mn could reach 101.2 Â 104. Meanwhile,
the branching density was dramatically decreased to ca. 25/1000C,
along with the dramatically increased polymer melting tempera-
ture. When the temperature was further decreased to À20 1C,
the activity further increased, along with increased polyethylene
molecular weight (Mn = 142.5 Â 104) and melting temperatures
(Table 1, entries 6 and 7).
d
Yield
Mn
f
Ent. Cat. Cocat. a-Olefinb (mg) Act.c (Â10À4) PDId Be Tm (1C)
1
2
3
4
1
1
1
1
2
3
1
3
MAO
C6
291
9.7 2.1
1.61 69 44.9
1.78 64 49.7
1.73 46 70
1.61 32 89
2.01 75 39.3
1.76 70 56.5
1.52 29 97.9
1.49 26 105.5
Et2AlCl C6
Et2AlCl C8
Et2AlCl C10
Et2AlCl C6
Et2AlCl C6
Et2AlCl C10
Et2AlCl C10
587 19.6 1.7
372 12.4 1.4
430 14.3 2.1
171
291
64
5
5.7 3.6
9.7
6
1
7g
8g
0.26 1.4
0.19 1.3
45
a
Conditions: 10 mmol pre-catalyst, 200 eq. cocatalyst, 1.0 M a-olefin.
b
2 mL CHCl3, total volume 20 mL, 3 h. C6 = 1-hexene, C8 = 1-octene,
C10 = 1-decene. Activity (Act.) = 103 g (mol Ni h)À1
.
Molecular weight
c
d
e
When the activator was changed from MAO to Et2AlCl, the
was determined by GPC using polystyrene standards. B = branches per
catalytic activity increased by 5 to 10 times (Table 1, entries 8–15). 1000 carbon atoms, determined by 1H NMR analysis. Determined by
f
g
DSC. 0.1 M C10 for 24 h.
However, the polyethylene molecular weight and the melting
temperature decreased. The catalyst system is more stable with
Et2AlCl as the activator compared with MAO, maintaining very 2,o-enchainment could occur from 1,2-monomer insertion
high activity even at 80 1C. For catalyst 1/Et2AlCl at 80 1C, the followed by chain walking.4 Recently, Coates et al. showed that
polyethylene yield increased from 2.29 g to 3.32 g and 3.50 g semicrystalline polymers (Tm 4 100 1C) could be obtained in a
(Table S1, ESI†), when the time was increased from 30 min to 1 h sandwich type a-diimine Ni(II) catalyzed a-olefin polymeriza-
and 2 h. This suggested that significant decomposition of the tion.15b This was attributed to the combination of regioselective
catalyst occurred after 30 min. It should be noted that the 2,1-monomer insertion and precision chain walking events. A
sterically bulky a-diimine Ni(II) complexes bearing a dibenzhydryl similar phenomenon was reported by Wu et al. in an amine-
moiety (Scheme 1, XII) reported by Long et al. were only shown to imine Ni(II) system.18 Herein, we report another example of
be stable on the time scale of 30 min.13
such a kind of precision a-olefin polymerization using imino-
The microstructure of the polyethylene was analysed by 13C pyridyl Ni(II) catalysts.
NMR (Table S2, ESI†).17 For the polyethylene obtained using
Compared with cocatalyst MAO, Et2AlCl led to a catalyst
1/MAO at 80 1C (Table 1, entry 3), 13C NMR analysis (Fig. 2) system with much higher activity, slightly lower molecular
indicates a branching density of 77/1000C (agrees well with that weight, lower branching density and higher melting point
determined by 1H NMR) including methyl (60.2%), ethyl (6.4%), in complex 1 initiated 1-hexene polymerization (Table 2,
propyl (2.9%), butyl (6.3%), sec-butyl (6.4%) and long chain entries 1 and 2). Therefore, the Et2AlCl cocatalyst was used
(17.8%) branches. For the polyethylene generated at lower for the a-olefin polymerization studies. When 1-octene and
temperatures (Table 1, entries 1 and 2 at 20 and 50 1C), fewer 1-decene were used, a similar polymer molecular weight,
percentages of long chain branches and more percentages of lower branching density and higher melting temperatures
methyl branches were observed.
were observed (Table 2, entries 3 and 4). In terms of polymer-
Because of the highly unsymmetric feature of the ligand ization selectivity and correspondingly the polymer melting
structure, the performance of complexes 1–3 in a-olefin poly- points, complex 3 showed better properties than complexes
merization was also investigated (Table 2). 1,2- or 2,1-insertion 1 and 2 (Table 2, entries 5 and 6). At lower concentration of
could occur during a-olefin polymerization. For chain walking 1-decene, much lower polymer branching density and much
olefin polymerization catalysts, 1,o-enchainment could occur higher polymer melting points were observed (Table 2,
from 2,1-monomer insertion followed by chain walking, and entries 7 and 8).
In summary, we described the synthesis and characterization
of some iminopyridyl Ni(II) catalysts containing both the dibenz-
hydryl and the naphthyl moieties. In ethylene polymerization,
these complexes showed activities of above 106 g (mol Ni h)À1
.
With Et2AlCl as the cocatalyst, very high activity could be
maintained up to 80 1C. Above 20 1C, polyethylene with a
molecular weight (Mn) of up to 2 Â 105 could be generated.
Moreover, a polymer with a molecular weight of up to 1 Â 106
could be generated when the polymerization was carried out at
5 1C. This is ca. two orders of magnitude higher than all of the
previously reported iminopyridyl Ni(II) catalysts. In a-olefin
polymerization, significant chain straightening was observed
from the combination of 2,1-monomer insertion and precision
chain walking. As a result, polymers with very high melting
points (Tm up to 105.5 1C) were generated.
Fig. 2 13C NMR of the polyethylene generated by 1/MAO at 80 1C
(Table 1, entry 3).
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7 C. Bianchini, G. Giambastiani, L. Luconi and A. Meli, Coord. Chem.
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This work was supported by the National Natural Science
Foundation of China (NSFC, 21374108, 51522306), the Fundamen-
tal Research Funds for the Central Universities (WK3450000001),
and the Recruitment Program of Global Experts. We thank
Dr S. M. Zhou (HFNL, USTC) for the determination of the
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