Longstanding living polymerization of ethylene: Substituent effect on bridging carbon of 2-pyridinemethanamine nickel catalysts

Article abstract of DOI:10.1039/c000176g

Bulky 2-pyridinemethanamine nickel complex activated by MAO was used as a catalyst to conduct longstanding living ethylene polymerization under atmospheric pressure to produce branched polyethylene.

Full text of DOI:10.1039/c000176g

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Longstanding living polymerization of ethylene: substituent effect
on bridging carbon of 2-pyridinemethanamine nickel catalysts w
Shaobo Zai,ab Fengshou Liu,a Haiyang Gao,*a Cong Li,a Guiyu Zhou,a Shan Cheng,a
Lihua Guo,a Ling Zhang,a Fangming Zhua and Qing Wu*ab
Received 6th January 2010, Accepted 14th April 2010
First published as an Advance Article on the web 12th May 2010
DOI: 10.1039/c000176g
Bulky 2-pyridinemethanamine nickel complex activated by
MAO was used as a catalyst to conduct longstanding living
ethylene polymerization under atmospheric pressure to produce
branched polyethylene.
short-chain oligomers were simultaneously produced, which
can be attributed to different catalytic species yielded from
isomerization equilibrium by the rotation of the amino-aryl
bond.9 Based on a strategy of suppressing the rotation of the
amino-aryl bond by increasing the steric hindrance of the
bridge joining the pyridine and aniline moieties, 2-pyridine-
methanamine nickel complexes 1 and 2 (Scheme 1) with
substituents of different sizes on the bridging carbon were
synthesized. Herein, we report the substituent effect on the
bridging carbon of 2-pyridinemethanamine nickel catalysts for
long-lifetime living polymerization of ethylene under atmospheric
pressure.
Living alkene polymerizations have been paid special attention
in the last ten years because these reactions allow for the
synthesis of polyolefins with precise architectures, such as mono-
disperse polymers, block copolymers and end-functionalized
materials, and improved physical properties.1 Many early
transition metal catalysts have been reported to catalyze living
ethylene polymerization to produce linear polyethylene.2
However, most of the living polymerizations last only a very
short time. As compared to early transition metal catalysts,
there are much fewer late transition metal catalysts that have
been reported to catalyze living ethylene polymerization.
A crucial reason is that b-H transfer can rapidly happen for
late transition metal catalysts,3 and C–H activation through a
free rotation C–NAr bond was also proposed as one potential
deactivation pathway.4 One noteworthy example is the cationic
a-diimine palladium catalyst that can produce branched
polyethylene with living behavior.5 Although several catalysts
based on nickel can produce polyethylenes with narrow
polydispersities within short polymerization time, the living
properties are not adequately explored.6 An a-keto-b-diimine
nickel catalyst for living ethylene polymerization within 60 min
was recently reported.7 Despite all the work mentioned, long
lifetime living polymerization of ethylene by late-transition
metals remains a challenge.
Ligands were prepared by a reduction reaction of imine
with trimethylaluminium or Grignard reagent, and nickel
complexes 1 and 2 were obtained in high yield by the addition
of ligands to a stirring suspension of (DME)NiBr2 in CH2Cl2
at room temperature.10 Single crystals of complexes 1 and 2ꢀ
H2O suitable for X-ray diffraction analysis were obtained
from dichloromethane solutions layered with hexane.z
Because of a reaction with moisture during crystallization,
the single crystal obtained from complex 2 contains a H2O
molecule coordinated to the nickel metal. Repeated efforts to
obtain a single crystal of complex 2 without H2O were not
successful. As shown in Fig. 1, complex 1 exists as a bimolecular
structure bridging by Br atoms, while complex 2ꢀH2O is
of a monomolecular structure. The aniline moieties of both
complexes are roughly perpendicular to the five-membered
planes (N1–C5–C6–N2–Ni1), and the isopropyl substituents
on the aryls are positioned at the axial directions of the nickel
centers. In the case of complex 2ꢀH2O, the 2,4,6-trimethyl-
phenyl plane (C7–C15) is also perpendicular to the five-
membered plane (87.31), and leans toward the aniline moiety
with an angle of 56.671. It is obvious that the 2,4,6-trimethyl-
phenyl group of complex 2 introduces much bulkier steric
hindrance to the rotation of the aniline moiety than the
methyl of complex 1, which should make the isopropyl sub-
stituents on the aryl group blocking the axial sites of the
metal more effective at preventing b-H elimination and chain
transfer.4,11
On the other hand, pyridylaniline ligands have been
developed for Zr, Ti, Hf, and Pd based catalysts for olefin
polymerizations due to their rich variation and potential for
the control in olefin catalysis.8 Recently, a new type of nickel
catalysts bearing 2-pyridinemethanamine ligands was also
explored for ethylene polymerization by our group. Without
a substituent on the bridging methane of the complexes,
branched polymers with a high molecular weight as well as
a DSAPM Lab, Institute of Polymer Science, School of Chemistry and
Chemical Engineering, Sun Yat-Sen (Zhongshan) 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 (Zhongshan) University,
Guangzhou, 510275, China
w Electronic supplementary information (ESI) available: Synthesis
and characterization of the ligands, complexes, and polymers. CCDC
754729 & 754730. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c000176g
Scheme 1 Complexes 1 and 2.
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Table 1 Results of ethylene polymerization initiated by 1, 2, 2ꢀH2Oa
Tp/1C Yield/g Act.b Mn [ꢃ104] PDI Tg/1C br
c
d
Entry Cat.
1
2
3
4
5
6
7
8
9
10
1
1
2
2
2
2
2
2
2
ꢂ20 0.270
1.35 1.08
0.96 0.13
0.97 0.94.
2.12 1.72
2.19 1.75
3.13 1.71
1.29 0.81
2.16 1.88
0.25 0.37
2.05 1.78
2.21 ꢂ37.3 82
1.98 ꢂ59.2 93
1.19 ꢂ29.3 57
1.19 ꢂ35.2 66
1.27 ꢂ42.0 67
1.49 ꢂ46.0 68
1.56 ꢂ56.8 75
1.25 ꢂ31.2 80
1.75 ꢂ45.2 78
1.19 ꢂ36.2 68
0
0.192
ꢂ20 0.194
ꢂ10 0.424
0
20
40
0.438
0.626
0.258
ꢂ10 0.431
ꢂ10 0.050
2ꢀH2O ꢂ10 0.410
Fig. 1 Molecular structures of complex 1 (a) and complex 2ꢀH2O (b).
Both depicted with 30% thermal ellipsoids. Hydrogen atoms and the
uncoordinated CH2Cl2 molecules have been omitted for clarity.
a
Polymerization conditions:
2
ꢃ
10ꢂ5 mol nickel complexes,
800 equiv. activator (MAO: entries 1–7 and 10; AlEt2Cl: entry 8;
AlMe3: entry 9), 1.2 atm ethylene pressure, 40 mL toluene, 1 h.
b
c
Activity, 104 g PE/[molꢀNiꢀh]. Mn [g molꢂ1] measured by GPC
Complexes 1 and 2 were used as catalyst precursors for
ethylene polymerization under 1.2 atm ethylene pressure
(Table 1). As compared with complex 1, complex 2 activated
by methylaluminoxane (MAO) at 0 1C shows a more than
1-fold increase in activity, and produces a polymer with much
higher Mn (increase by one order of magnitude) and a much
narrower PDI value (1.27 vs. 1.98). At ꢂ10 and ꢂ20 1C,
complex 2 produces PEs with a narrow PDI value of 1.19,
suggesting living polymerizations. However, even at ꢂ20 1C,
complex 1 produces a relatively wide molecular weight
distribution PE. These results provide a demonstration that
2,4,6-trimethylphenyl on the bridge backbone plays a dominant
role in resisting the rotation of the CAr–N bond to eliminate
chain transfer, while methyl substituent on the bridging
carbon is not bulky enough to resist the rotation effectively.
With an elevated temperature, complex 2 exhibited the highest
activity for ethylene polymerization at 20 1C, and an obvious
decrease in molecular weight at 40 1C. AlEt2Cl can also serve
as an activator with the same activity and a slightly boarder
PDI value (1.25), while using AlEt3 as an activator leads to a
board PDI value (1.75) and a dramatic decrease of activity.
The PEs obtained by these catalytic systems are branched
polymers mainly containing methyl branches as revealed by
13C NMR spectra. The branch densities are around 60–90
branches/1000 C, indicating a ‘‘chain walking’’ process of the
catalysts.12 As shown in Table 1 (entry 4 and 10), there is little
difference between the polymerizations catalyzed separately by
complex 2 and 2ꢀH2O under the same conditions, suggesting
that the coordinated water in 2ꢀH2O can be removed by
excessive MAO.
d
vs. linear polyethylene standards. Branches per 1000 C atoms
1
determined by H NMR spectroscopy.
Fig. 2 (a) Plot of Mn (’) and Mw/Mn (.) as a function of
polymerization time at ꢂ10 1C under 1.2 atm of ethylene; catalyst
2/MAO. (b) GPC traces.
Table 2 Living ethylene polymerizations catalyzed by 2/MAOa
b
c
Entry Time/h Yield/g Mn [ꢃ104] Mn,t [ꢃ104] PDI brd
1
2
3
4
5
6
0.5
1
2
3
4
0.170
0.424
0.747
1.042
1.542
2.143
0.88
1.72
3.21
4.66
6.21
8.66
0.85
2.12
3.74
5.21
7.71
10.72
1.22 69
1.19 66
1.17 66
1.15 63
1.17 68
1.21 67
6
a
Polymerization conditions: 2 ꢃ 10ꢂ5 mol nickel complex, 800 equiv.
b
MAO, ꢂ10 1C, 1.2 atm ethylene pressure, 40 mL toluene. Mn
[g molꢂ1] measured by GPC vs. linear polyethylene standards. The
c
d
theoretical number-average molecular weight. Branches per 1000 C
1
atoms determined by H NMR spectroscopy.
Living polymerization of ethylene using catalyst 2/MAO
was carried out in toluene at ꢂ10 1C under 1.2 atm of ethylene
and quenched with ethanol after desired time. The results are
summarized in Table 2, and a plot of Mn and Mw/Mn as a
function of polymerization time and the GPC traces are shown
in Fig. 2. The Mn value increases linearly with the polymer-
ization time, and the Mw/Mn value can be maintained in
1.15–1.21 for 6 h. Although the Mw/Mn values are a little
higher than that produced by an a-diimine palladium catalyst,5
the stable linear increase of the Mn value indicates a long-
lifetime living polymerization. To the best of our knowledge,
this is the first report of a nickel complex to catalyze long-
standing living ethylene polymerization. Longstanding living
ethylene polymerization catalyzed by other transition metals
has been achieved only in rare instances.5a
As shown in Table 2, the number-average molecular weights
of the polymers obtained at different polymerization time are
basically consistent with the theoretical number-average
molecular weights (Mn,t) which is calculated gravimetrically
on the basis of moles of catalyst employed and the weight of
polymer produced, suggesting that all Ni metals have been
successfully activated and each Ni metal center efficiently
initiated the growth of one polymer chain.13 A little lower
values of Mn than the corresponding Mn,t can be attributed to
the measurement of Mn by GPC with linear polyethylenes as
standards. Because of the differences in the hydrodynamic
volumes of these branched PEs and linear PEs standards, the
Mn values determined for these polymers are smaller than the
real number-average molecular weights.14
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a = 13.4346(13), b = 13.4926(13), c = 17.6159(17) A, U = 3075.7(5) A3,
T = 173(2) K, space group P21/c, Z = 4, 15490. Reflections measured,
6637 unique (Rint = 0.0358) which were used in all calculations. The final
wR(F2) was 0.1064 (all data).
1 (a) G. J. Domski, J. M. Rose, G. W. Coates, A. D. Bolig and
M. Brookhart, Prog. Polym. Sci., 2007, 32, 30–92;
(b) G. W. Coates, P. D. Hustad and S. Reinartz, Angew. Chem.,
Int. Ed., 2002, 41, 2236–2257.
2 (a) S. Bambirra, D. van Leusen, A. Meetsma, B. Hessen and
J. H. Teuben, Chem. Commun., 2003, 522–523; (b) K. Mashima,
S. Fujikawa, H. Urata, E. Tanaka and A. Nakamura, J. Chem.
Soc., Chem. Commun., 1994, 1623–1624; (c) J. Tian, P. D. Hustad
and G. W. Coates, J. Am. Chem. Soc., 2001, 123, 5134–5135;
(d) S. Reinartz, A. F. Mason, E. B. Lobkovsky and G. W. Coates,
Organometallics, 2003, 22, 2542–2544; (e) R. Furuyama,
M. Mitani, J. Mohri, R. Mori, H. Tanaka and T. Fujita,
Macromolecules, 2005, 38, 1546–1552; (f) M. Mitani, J. Mohri,
Y. Yoshida, J. Saito, S. Ishii, K. Tsuru, S. Matsui, R. Furuyama,
T. Nakano, H. Tanaka, S. Kojoh, T. Matsugi, N. Kashiwa and
T. Fujita, J. Am. Chem. Soc., 2002, 124, 3327–3336; (g) X. Li,
K. Dai, W. Ye, L. Pan and Y. Li, Organometallics, 2004, 23,
1223–1230; (h) S. Yu and S. Mecking, J. Am. Chem. Soc., 2008,
130, 13204–13205.
3 D. P. Gates, S. A. Svejda, E. Onate, C. M. Killian, L. K. Johnson,
P. S. White and M. Brookhart, Macromolecules, 2000, 33,
2320–2334.
4 D. J. Tempel, L. K. Johnson, R. L. Huff, P. S. White and
M. Brookhart, J. Am. Chem. Soc., 2000, 122, 6686–6700.
5 (a) A. C. Gottfried and M. Brookhart, Macromolecules, 2001, 34,
1140–1142; (b) A. C. Gottfried and M. Brookhart, Macromolecules,
2003, 36, 3085–3100.
Fig. 3 (a) Plot of Mn as a function of time for a postpolymerization at
ꢂ10 1C under 1.2 atm of ethylene; catalyst 2/MAO. (b) GPC traces of
PE and PE-b-PH.
A postpolymerization method was used to further test the
stability of ‘‘living spots’’ in the absence of reacting monomer,
which affects the application of the catalyst system to the
preparation of block copolymers.2f After the polymerization
was conducted for 1 h, ethylene was vented and the system was
charged with N2. Ten hours later, the system was recharged
with ethylene to continue the polymerization. As shown in
Fig. 3a, the polymerization system still remains living, and the
Mn value continues to increase linearly with the polymerization
time after recharging ethylene monomer and the PDI values
are around 1.2. Therefore, the longstanding living polymerization
system should have a high potential for synthesis of ethylene-
based block copolymers.
6 (a) M. Schmid, R. Eberhardt, M. Klinga, M. Leskela and
B. Rieger, Organometallics, 2001, 20, 2321–2330; (b) F. A. Hicks,
J. C. Jenkins and M. Brookhart, Organometallics, 2003, 22,
3533–3545; (c) S. J. Diamanti, P. Ghosh, F. Shimizu and
G. C. Bazan, Macromolecules, 2003, 36, 9731–9735.
7 (a) J. D. Azoulay, R. S. Rojas, A. V. Serrano, H. Ohtaki,
G. B. Galland, G. Wu and G. C. Bazan, Angew. Chem., Int. Ed.,
2009, 48, 1089–1092; (b) J. D. Azoulay, Y. Schneider,
G. B. Gallandb and G. C. Bazan, Chem. Commun., 2009,
6177–6179.
A further attempt at the synthesis of diblock copolymer
polyethylene-b-poly(1-hexene) was made by catalyst 2/MAO
at ꢂ10 1C. After polymerizing ethylene for 0.5 h, the ethylene
monomer was removed, and then 5 mL 1-hexene was added
and stirred for 5 h. As shown in GPC elution curves (Fig. 3b),
the peak of polyethylene obtained at 0.5 h appears at a longer
retention time (Mn = 0.88 ꢃ 104, Mw/Mn = 1.22) and another
peak of the final polyethylene-b-poly(1-hexene) copolymer
shifts to the shorter retention time region (Mn = 2.14 ꢃ
104, Mw/Mn = 1.18), indicating a successful preparation of
an A–B diblock polymer under these conditions.
8 (a) G. J. Domski, E. B. Lobkovsky and G. W. Coates, Macro-
molecules, 2007, 40, 3510–3513; (b) T. R. Boussie, G. M. Diamond,
C. Goh, K. A. Hall, A. M. LaPointe, M. K. Leclerc, V. Murphy, J.
A. W. Shoemaker, H. Turner, R. Rosen, J. C. Stevens, F. Alfano,
V. Busico, R. Cipullo and G. Talarico, Angew. Chem., Int. Ed.,
2006, 45, 3278–3283; (c) Y. Lin, K. Yu, S. Huang, Y. Liu,
Y. Wang, S. Liu and J. Chen, Dalton Trans., 2009, 41,
9058–9067; (d) L. Annunziata, D. Pappalardo, C. Tedesco and
In conclusion, we have successfully developed a bulky
2-pyridinemethanamine nickel catalyst for longstanding living
ethylene polymerization to yield branched polyethylene.
Sufficiently bulky substitution on the bridging carbon of
the complex plays a crucial role in resisting the rotation of
the CAr–N bond to prevent chain transfer reaction during the
polymerization process. A diblock copolymer polyethylene-
b-poly(1-hexene) can also be synthesized by catalyst 2/MAO.
Further reports will address the influences of varied steric and
electronic substituents and polymerizations with different
monomers.
C.
(e) L. Annunziata, D. Pappalardo, C. Tedesco and C. Pellecchia,
Macromolecules, 2009, 42, 5572–5578; (f) F. Guerin,
Pellecchia,
Organometallics,
2009,
28,
688–697;
´
D. H. McConville and J. J. Vittal, Organometallics, 1997, 16,
1491–1496.
9 Z. Huang, K. Song, F. Liu, J. Long, H. Hu, H. Gao and Q. Wu,
J. Polym. Sci., Part A: Polym. Chem., 2008, 46, 1618–1628.
10 The synthesis and detailed characterization of the ligands and
complexes can be seen in the supporting information.
11 (a) T. Schleis, T. P. Spaniol, J. Okuda, J. Heinemann and
R. Mulhaupt, J. Organomet. Chem., 1998, 569, 159–167;
(b) L. K. Johnson, C. M. Killian and M. Brookhart, J. Am. Chem.
Soc., 1995, 117, 6414–6415; (c) D. H. Camacho, E. V. Salo,
J. W. Ziller and Z. Guan, Angew. Chem., Int. Ed., 2004, 43,
1821–1825; (d) F. Liu, H. Hu, Y. Xu, L. Guo, S. Zai, K. Song,
H. Gao, L. Zhang, F. Zhu and Q. Wu, Macromolecules, 2009, 42,
7789–7796.
Supports by NSFC (Projects 20734004 and 20974125),
NSFG (Project 8251027501000018) and the Ministry of
Education of China (Foundation for PhD Training) are
gratefully acknowledged.
Notes and references
12 S. D. Ittel, L. K. Johnson and M. Brookhart, Chem. Rev., 2000,
100, 1169–1203.
z Crystal data for complex 1. C39H54Br4Cl2N4Ni2, M = 1086.82,
monoclinic, a = 18.4583(17), b = 15.3616(14), c = 15.5084(14) A,
U = 4394.3(7) A3, T = 173(2) K, space group P21/c, Z = 4, 29190.
Reflections measured, 9553 unique (Rint = 0.0443) which were used in
all calculations. The final wR(F2) was 0.0769 (all data). Crystal data
for complex 2ꢀH2O. C28H38Br2Cl2N2NiO, M = 708.03, monoclinic,
13 K. Zhang, Z. Ye and R. Subramanian, Macromolecules, 2008, 41,
640–649.
14 D. Papanagopoulos, J. Polym. Sci., Part B: Polym. Phys., 2005, 43,
2388–2391.
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This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4321–4323 | 4323