A thermally robust amine-imine nickel catalyst precursor for living polymerization of ethylene above room temperature

Article abstract of DOI:10.1039/c2cc17154f

A bulky amine-imine nickel complex containing two 2,6-diisopropyl substituents after activation with MMAO or Et₂AlCl can polymerize ethylene in a living fashion over a period of 120 minutes at room temperature or above. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17154f

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COMMUNICATION
A thermally robust amine–imine nickel catalyst precursor for living
polymerization of ethylene above room temperaturew
b
Haiyang Gao,z*a Haibin Hu,z Fangming Zhua and Qing Wu*ab
Received 17th November 2011, Accepted 7th February 2012
DOI: 10.1039/c2cc17154f
A bulky amine–imine nickel complex containing two 2,6-diisopropyl
substituents after activation with MMAO or Et2AlCl can polymerize
ethylene in a living fashion over a period of 120 minutes at room
temperature or above.
at 5 1C7 and a-keto-b-diimine nickel catalyst that is capable of
producing semicrystalline PE under living conditions at ꢀ10 1C.8
An approach of increasing axially steric hindrance of a-diimine
nickel and palladium complexes sheds light on this issue, which not
only can slow down the growing chain transfer process (associative
displacement or chain transfer to bound monomer),5c,9 but
also can suppress deactivation of metal species resulting from
a free rotation C–NAr bond.4b,c,10
Living catalytic olefin polymerization is unsurpassed in terms of
microstructure control of the polymer, which allows controlled
synthesis of polyolefins with precise architectures, such as mono-
disperse polymers, block copolymers and end-functionalized
materials.1 Currently, living coordination polymerization of
olefins is a long-standing scientific challenge because of the
processes of chain termination by b-H elimination and polymeryl
transfer to the cocatalyst or monomer.2,3 Besides, short lifetime and
rapid deactivation of living metal species at elevated temperature
also limit precise tuning of polymer structures. Lowering reaction
temperature and reducing/eliminating the use of alkylaluminium
cocatalysts have thereby been devised to stabilize the living
metal center and decrease the rate of chain termination/
transfer such that living systems can be formed.2 However,
lowering reaction temperature generally leads to low catalytic
activity and limits molecular weight attainable because of poor
dissolution of polyolefins in reaction media.4 Design of novel
thermostable transition metal catalysts for living polymerization
of ethylene thus remains a persistent challenge.
Recently, an amine–pyridine nickel catalyst precursor with
2,4,6-trimethylphenyl on the bridge carbon has been reported
to be capable of producing PEs with narrow molecular weight
distributions in a living fashion below ꢀ10 1C by our group.11
In view of less axially steric effect of the pyridine moiety in
amine–pyridine nickel and the alkyl substituents on the amine
moiety in aminoaldimine nickel,12 we herein design new
amine–imine nickel complexes containing two N-aryl substituents
for long-lifetime living polymerization of ethylene above room
temperature.
Amine–imine ligands were prepared by a reduction reaction
of a-diimine with trimethylaluminium (TMA) in high yields,13
and nickel complexes 1, 2, and 3 with different substituents
(Scheme 1) were obtained by addition of the ligands to a
stirring suspension of (DME)NiBr2 in CH2Cl2 (cf. ESIw).
Single crystals of 1 (Fig. 1), 2, and 3 (ESIw) suitable for
X-ray diffraction analysis were obtained by slow diffusion of
hexane into nickel complexes solution in CH2Cl2. The molecular
connectivity of 1 is consistent with a neutral N,N-bound amine–
imine ligand. The imine moiety of 1 is roughly perpendicular to
Late transition metal catalysts, such as a-diimine nickel and
palladium, can produce branched polyethylenes (PEs) with
relatively good dissolution,5 but there are rare examples of
living polymerization of ethylene with late transition metal
catalysts in the current literature. Despite some successful
applications of a-diimine nickel catalysts in living polymerizations
of a-olefins such as propylene, hexene, and 4-methyl-1-pentene,6 to
date no living characteristics for ethylene polymerization have been
observed.4b Two noteworthy examples are a-diimine palladium
catalyst that can produce highly branched PE in a living manner
Scheme 1 Structures of amine–imine nickel complexes 1–3 and 4.
a DSAPM Lab, Institute of Polymer Science, School of Chemistry and
Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275,
China. E-mail: gaohy@mail.sysu.edu.cn, ceswuq@mail.sysu.edu.cn;
Fax: +86-20-84114033; Tel: +86-20-84114033
b PCFM Lab, OFCM Institute, Sun Yat-Sen University, Guangzhou
510275, China
w Electronic supplementary information (ESI) available: Experimental
procedures, detailed synthesis and characterization of compounds and
polymers. CCDC 867170–867172. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2cc17154f
z Both authors contributed equally to this work.
Fig. 1 Crystal structure of amine–imine nickel complex 1.
c
3312 Chem. Commun., 2012, 48, 3312–3314
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Table 1 Ethylene polymerization results with nickel precursors 1–4a
in a wide range of ethylene pressures. The Et2AlCl compound can
also replace MMAO as the activator for living polymerization of
ethylene. The 1/Et2AlCl system is less active than the 1/MMAO
system, and produces lower molecular weight polymers with
narrower distributions (entries 11 and 12). Additionally, 1 can
be effectively activated with only a small amount of MMAO or
Et2AlCl (Al/Ni = 50) and exhibits good activity for ethylene
polymerization. There is no substantial change in catalytic activity,
molecular weight, and polydispersity of the obtained polymers
when the Al/Ni ratio varies from 50 to 500 (entries 1, 5–7 for
MMAO, Table S2 in ESIw for Et2AlCl), strongly indicating no
occurrence of chain transfer to the aluminium cocatalyst.
Entry Cat. Al/Ni Tp/1C Yield/g Act.b Mn PDIc Brd Tge/1C
c
1
2
1
2
3
1
1
1
1
1
1
1
1
1
1
1
4
200
200
200
200
50
100
500
200
200
200
200
200
200
200
200
20
20
20
20
20
20
20
0
35
50
20
35
50
75
20
0.582
0.849
1.085
0.440
0.502
0.544
0.524
0.411
0.482
0.392
0.188
0.373
0.358
0.124
0.385
58.2 107 1.02 153 ꢀ53
84.9
109
92 1.13 213 ꢀ64
h
3
3 1.59 175 —
4f
5
880 339 1.08 119 ꢀ41
50.2
54.4 109 1.02 151 ꢀ52
52.4 114 1.02 155 ꢀ53
97 1.03 147 ꢀ51
6
7
8
9
10
11
12
13
14
15g
41.1
94 1.05 123 ꢀ49
48.2 104 1.08 164 ꢀ59
39.2
18.8
37.3
35.8
12.4
70 1.24 175 ꢀ61
28 1.03 171 ꢀ53
50 1.04 180 ꢀ55
48 1.08 185 ꢀ56
22 1.65 194 ꢀ64
Generally, late metal catalysts are highly sensitive to
polymerization temperature. High temperature (above room
temperature) for Ni and Pd catalysts usually leads to a low
molecular weight product due to chain transfer, and causes a
decay of polymerization activity because of deactivation of
metal species.4,5 The polymerization results using 1/MMAO
over the temperature range from 0 to 50 1C (entries 1, 8–10)
show that the catalytic activity and molecular weight of the PE
reach the maximum values at 20 1C and then slightly decrease
with elevated temperature. When the stronger Lewis-acid
Et2AlCl is used instead of MMAO (entries 11–14), the highest
catalytic activity can be achieved at 35 1C and the obtained
polymer still has very narrow molecular weight distribution
(PDI = 1.04) (entry 12). Higher temperatures lead to increased
PDIs and decreased activities. A long-standing polymerization
of ethylene at elevated temperatures was performed to further
probe the stability of the active site. Fig. 2 illustrates the time
dependence of the rate of ethylene consumption over a period
of 120 minutes at 35 1C for the 1/Et2AlCl system, and clearly
shows a constant rate curve and rapid initiation. This proves
that the amine–imine nickel catalyst is highly stable at 35 1C
and no catalyst deactivation occurs over a 120 minute period
under the adopted conditions. It is distinct from a-diimine
nickel catalysts, for which a rapid decay of activity above room
temperature could be usually observed.5,10a Good thermal
stability of amine–imine nickel 1 should exhibit a great potential
for precise synthesis of monodisperse PE with different branch
topology structures and corresponding block copolymers.
The living feature of ethylene polymerizations with 1 was
further investigated at 20 1C using MMAO as the activator
and at 35 1C using Et2AlCl instead of MMAO, respectively.
Fig. 3b shows symmetric GPC traces of the polymers obtained
at different polymerization times, which shift to the higher
molecular weight region with the increasing polymerization
time. Plots of absolute number-average molecular weight (Mn)
and Mw/Mn (PDI) determined by GPC using a light scattering
770 168 1.73 136 ꢀ50
a
Polymerization conditions: 10 mmol of nickel, 60 min, 3 psig, 28 mL
toluene and 2 mL CH2Cl2, activators: MMAO for entries 1–10, 15;
Et2AlCl for entries 11–14. Activity: kg PE (mol Ni)ꢀ1 hꢀ1
(in units of 103 g molꢀ1) and PDI were determined by gel permeation
b
c
. Mn
chromatography (GPC) in 1,2,4-trichlorobenzene at 150 1C using a light
scattering detector. Branching density, branches per 1000 carbon atoms
d
e
f
determined by 13C NMR spectroscopy. Determined by DSC. 2 mmol
g
of nickel, 15 min, 300 psig, 28 mL toluene and 2 mL CH2Cl2. 2 mmol of
a-diimine nickel, 15 min, 3 psig, 30 mL toluene. Not determined.
h
the slightly distorted five-membered coordination plane (dihedral
angle of 78.51), while the amine moiety of 1 is almost completely
perpendicular (87.61). Besides, one of the isopropyl groups on the
amine moiety is closer to nickel (C(10)–Ni 3.6491 A) than those
on the imine moiety (C(24)–Ni 4.2417 A) due to the distorted
tetrahedral configuration of nitrogen on the amine, which causes
a more crowded space on one side of the coordination plane.12
Two methyl substituents on the backbone carbon ((Me)2CNH)
are oriented toward the axial space, showing steric effect on
the metal center (Fig. 1b). Compared with the ‘‘classic’’
a-diimine nickel analogue,5 the amine–imine nickel complex
exhibits more effective steric blocks at the axial sites, anticipated
to suppress chain transfer from the axial direction.
A series of ethylene polymerizations were carried out using
catalyst precursors 1, 2, and 3 under various conditions (Table 1).
With 1 after activation with MMAO, ethylene polymerization can
produce extremely narrow-dispersed polyethylene (PDI = 1.02)
in a living fashion with moderate activity under atmospheric
pressure at 20 1C (entry 1). Reducing the steric hindrance of
the amine–imine ligand by substituting o-methyl groups for
o-isopropyl groups results in an increased polymerization
activity, but a reduced molecular weight polymer with a slightly
broadened polydispersity (precursor 2). Lack of substituents on the
N-aryl moiety leads to a remarkable drop in molecular weight of
polyethylene and an obvious increase in the PDI value
(precursor 3). Compared with the previously reported amine–
pyridine nickel catalyst,11 the amine–imine nickel catalyst can
more actively polymerize ethylene to produce higher molecular
weight PE with narrow polydispersity at higher temperature.
Increasing ethylene pressure from atmospheric pressure to
300 psig (entry 4) for the 1/MMAO system leads to an
increased activity and affords higher molecular weight PE
with a PDI value of 1.08. Only a little broader polydispersity
of PE is observed under high pressure, suggesting that living
polymerization of ethylene catalyzed by 1/MMAO can be achieved
Fig. 2 Plots of polymerization rate vs. reaction time in ethylene
polymerization using 1/Et2AlCl at 35 1C (10 mmol of nickel).
c
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Chem. Commun., 2012, 48, 3312–3314 3313

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In summary, we successfully developed conveniently accessible
novel amine–imine nickel precursors that can catalyze ethylene
polymerization to produce highly branched polymers in a living
fashion. For complex 1 containing 2,6-diisopropyl substituents on
amine and imine moieties, a stationary polymerization kinetics
proves no occurrence of catalyst deactivation within 120 minutes at
35 1C, and narrow-dispersed polyethylene (PDI o 1.10) with high
molecular weight can be produced at room temperature or above
in the presence of MMAO or Et2AlCl. Living polymerization of
ethylene using a thermally robust catalyst provides a viable access
to precise synthesis of monodisperse PE with various branch
topology structures by changing reaction temperature, corres-
ponding block copolymers, and functionalized PE. Further
optimization of variations in amine–imine frameworks will
enable improvements in polymerization control.
Fig. 3 (a) Plots of Mn (’,m,E) and Mw/Mn (PDI) (&,n,B) as a function
of polymerization time using 1/MMAO at 20 1C or using 1/Et2AlCl at
35 and 50 1C (polymerization conditions: 3 psig, 10 mmol Ni, 200 equiv.
activator). (b) GPC traces at different times using a light scattering detector.
detector (ESIw) as a function of polymerization time (Fig. 3a) also
illustrate that Mn grows linearly with polymerization time, and
Mw/Mn values are below 1.10 within 2 hours, proving living
polymerizations with long lifetime using 1/MMAO at 20 1C and
1/Et2AlCl at 35 1C. PE with an Mn of 2 ꢁ 105 g molꢀ1 is formed
after 2 hours using 1/MMAO, and its molecular weight is still
precisely controlled (PDI = 1.04) (ESIw). Polyethylene-block-
polyhexene copolymers (PE-b-PH) with narrow PDI of B1.05
are also successfully synthesized under the adopted living
polymerization conditions by subsequent copolymerization
(ethylene: 0.25 h, hexene: 6 h) (ESIw), further supporting that
amine–imine nickel 1 species is long-standing and shows living
nature for olefin polymerization. To the best of our knowledge,
this is the first report on living polymerization of ethylene above
room temperature with a late transition metal catalyst. At the
higher temperature (50 1C), Mn grows linearly with time in the
early stages of polymerization (B30 min), but then the slope
begins to decrease and PDI also becomes broad.
The financial supports from NSFC (Projects 20974125,
20734004 and 21174164), and the Fundamental Research
Funds for the Central Universities (Project 10lgpy10) are
gratefully acknowledged.
Notes and references
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It is informative at this point to compare different behaviors
of ethylene polymerization using amine–imine nickel 1 and a
classic a-diimine nickel 4 (Scheme 1), which have seemingly
similar metal–ligand frameworks. The catalytic system 4/MMAO
has been known to polymerize ethylene in a nonliving fashion4,5
and herein produces a PE with a PDI value of 1.73 under the
same conditions (entry 15). This molecular weight distribution is
much broader than that of PE produced by amine–imine nickel 1
(entry 1), although the higher catalytic activity is achieved by 4.
The living catalytic ethylene polymerization behavior of
amine–imine nickel 1 not only arises from the effective axially
steric block, but also may be attributed to the weak Lewis base of
amine with N-aryl (ArNH), which can result in retardation of
chain transfer to the ethylene monomer and the alkylaluminium
cocatalyst.5c,d
Like the PEs produced by a-diimine nickel,5 the PEs produced
by these amine–imine nickel catalysts are also highly branched
products with methyl branches predominating as revealed by
13C NMR spectra (cf. ESIw).14 Various branches originate from a
chain walking process involving a b-agostic nickel complex.9,15
The PEs obtained under living polymerization conditions are
branched, suggesting that chain termination through b-H
elimination of the b-agostic nickel complex cannot occur. This
may be explained as a quite low barrier to chain isomerization
and an extremely unstable hydride–olefin nickel complex from
thermodynamic aspect.9
13 (a) R. E. Murray, WO 2003051935, 2003; (b) M. Bhadbhade, G. K. B.
Clentsmith and L. D. Field, Organometallics, 2010, 29, 6509.
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Macromolecules, 1999, 32, 1620; (b) F. A. Kunrath, E. F. Monta,
O. L. Casagrande, R. S. Mauler and R. F. de Souza, Macromol.
Chem. Phys., 2002, 203, 2407.
15 V. M. Mohring and G. Fink, Angew. Chem., Int. Ed., 1985, 24, 1001.
¨
c
3314 Chem. Commun., 2012, 48, 3312–3314
This journal is The Royal Society of Chemistry 2012