Ethylene/polar monomer copolymerization behavior of bis(phenoxyimine)ti complexes: Formation of polar monomer copolymers

Full text of DOI:10.1021/ja8060479

Published on Web 12/09/2008
Ethylene/Polar Monomer Copolymerization Behavior of Bis(phenoxy–imine)Ti
Complexes: Formation of Polar Monomer Copolymers
Hiroshi Terao, Seiichi Ishii, Makoto Mitani, Hidetsugu Tanaka, and Terunori Fujita*
Research Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba, 299-0265, Japan
Received August 14, 2008; E-mail: Terunori.Fujita@mitsui-chem.co.jp
For some years now, academic and industrial scientists have
been hard at work trying to discover molecular catalysts that
allow for the copolymerization of olefinic hydrocarbon and polar
monomers.1 This is because polyolefins having polar functional
groups are believed to possess not only modified surface
properties such as dyeability, adhesion, and wettability, but also
unique bulk properties.
solid state (distance between the Cl and its nearest C in the tert-
Bu or Ph group, complex 2; 3.97 Å, complex 3; 3.76 Å).6
For the copolymerization of olefinic hydrocarbon and polar
monomers, late transition metal catalysts have been at the
forefront of catalyst development.2,3 Conversely, although early
transition metal catalysts represented by metallocene catalysts
have offered remarkable opportunities for the synthesis of novel
olefin-based materials,4 they are normally regarded as unpromis-
ing for such a copolymerization. This is because early transition
metal catalysts are typically oxophilic and thus are deactivated
by the presence of polar monomers in a polymerization medium.
Therefore, these early transition metal catalysts have limited
applications and require reaction modifications such as masking
the functionality as an innocuous species and/or precomplexation
of functional groups by stoichiometric amounts of Lewis acidic
species.1a,5
We have found that bis(phenoxy–imine) group 4 transition metal
complexes (now known as FI catalysts) with appropriate cocatalysts
exhibit unique and versatile polymerization catalysis, including
highly isospecific and syndiospecific polymerizations of both
propylene and styrene.6,7 As part of our investigations into FI
catalysis, we studied ethylene/polar monomer copolymerization
using FI catalysts, resulting in the discovery of Ti-FI catalysts
capable of promoting the copolymerization of ethylene and 5-hex-
ene-1-yl-acetate. We therefore describe ethylene/5-hexene-1-yl-
acetate copolymerization behavior of a series of Ti-FI catalysts in
this Communication.
Figure 2. Molecular structure of complex 3.
Common FI and metallocene catalysts; namely, complexes 1 and
2, Cp2MCl2 (M ) Zr, Ti) and Me2Si(Me4Cp)(NtBu)TiCl2 (CGC)
were investigated for their potential as ethylene/5-hexene-1-yl-
acetate copolymerization catalysts using dried MAO (DMAO) as
a cocatalyst. The results demonstrated that complex 1, Cp2MCl2
(M ) Zr, Ti) and CGC displayed practically no reactivity under
the given conditions, generating neither polymeric nor oligomeric
materials probably because of the deactivation caused by the
coordination of the carbonyl moiety of the polar monomer. To our
surprise, however, bis(phenoxy–imine)Ti complex 2 afforded a high
molecular weight product that contains 0.13 mol % of 5-hexene-
1-yl-acetate (Table 1, entry 1), which may suggest the considerable
potential of Ti-FI catalysts for polar monomer copolymerization.
To obtain further information about the catalytic properties of
Ti-FI catalysts for this copolymerization, we studied complexes
3-8, which displayed higher ethylene polymerization activities than
complex 2 (see Supporting Information). Copolymerization results
are compiled in Table 1.
Complexes 3-7 produced copolymers with enhanced polar
monomer contents (0.50-0.90 mol %). These results show that
the phenyl group ortho to the phenoxy-O induces higher polar
monomer incorporation than the tert-Bu group. While this appears
contradictory to the X-ray structures, density functional theory
(DFT) calculations indicate that the phenyl group provides a
sterically more open active site than the tert-Bu group due to its
rotation to evade the steric congestion (Figure 3). Increasing the
amount of charged 5-hexene-1-yl-acetate resulted in the higher
incorporation of this monomer (entries 8-11, ca. 2 mol %).
Significantly, complexes 3 and 4 produced copolymers even in the
presence of an excess amount of 5-hexene-1-yl-acetate to DMAO
(entries 13, 14).
Figure 1. FI catalysts employed in this study.
Shown in Figure 1 are bis(phenoxy–imine) Zr and Ti
complexes (Zr- and Ti-FI catalysts) 1-8 utilized in this study,
which were synthesized either according to published procedures
or in a similar manner.8 An X-ray diffraction study has revealed
that although complex 3 (R3 ) Ph) adopts the same configuration
(trans-O, cis-N, and cis-Cl arrangement) as complex 2 (R3 )
tert-Bu), it possesses a sterically more encumbered environment
around the Cl-bound sites (potential polymerization sites) in the
The copolymers formed with complexes 3-7 possess narrow
molecular weight distributions (Mw/Mn, 1.6-2.4), as expected for
a polymer generated from a chemically homogeneous catalyst. GPC-
IR together with 13C NMR analysis of the copolymer obtained with
complex 3 (entry 8) showed a narrow comonomer distribution and
the randomly distributed nature of the comonomer (Figure 4). These
results represent the first examples of group 4 metal catalysts that
9
17636 J. AM. CHEM. SOC. 2008, 130, 17636–17637
10.1021/ja8060479 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
exhibit high performance for the copolymerization of ethylene and
a polar monomer with ester functionality.
For the purposes of having a quantitative discussion on the ligand
effect on polar monomer copolymerization, we estimated the energy
difference (∆E) between ethylene-coordinated and carbonyl-co-
ordinated cationic complexes (an indication of functional group
tolerance). The carbonyl-coordinated cationic methyl complexes are
more stable, and the ∆E values obtained with complexes 2-8 are
47.9 (2), 55.7 (3), 55.4 (4), 45.2 (5), 37.4 (6), 49.5 (7), and 60.5
kJ/mol (8), which are much smaller ∆E values than the ∆E values
for the metallocene catalysts [104.6 (Cp2TiCl2), 108.8 (Cp2ZrCl2),
104.9 kJ/mol (CGC)]. These results indicate that Ti-FI catalysts
display much higher functional group tolerance than the metallocene
catalysts though both are group 4 metal catalysts, probably because
of the coordination of [O-, N] ligands (already “poisoned”). The
basic trend observed is that the introduction of an electron-donating
group to the phenyl on the imine-N leads to smaller ∆E values,
and Vice Versa. These calculation results are essentially consistent
with the experimental data described so far, suggesting the
possibility of a rational catalyst design using theoretical methods.
Further research on copolymerizations using other polar monomers
as well as theoretical studies is underway.
Figure 3. Calculated structures of 5-hexene-1-yl-acetate-coordinated cat-
ionic methyl complexes derived from complexes 2 and 3.
Table 1. Results of Ethylene/5-Hexene-1-yl-acetate
Copolymerization with Complexes 2-8a
comonomer
contentd
(mol %)
e
comonomerb
(mmol)
Mw
(×10-3
entry complex
yield (g) activityc
)
Mw/Mne
1
2
3
4
5
6
7
8
2
3
4
5
6
7
8
3
4
5
6
7
3
4
1.00
1.00
1.00
1.00
1.00
1.00
1.00
2.00
2.00
2.00
2.00
2.00
5.25
5.25
0.287
1.12
1.14
1.72
1.17
0.593
0.093
0.227
0.235
0.202
0.185
0.058
0.035
0.051
86
337
341
515
353
178
28
68
71
61
56
0.13
0.81
0.90
0.74
0.66
0.50
497
269
273
387
252
190
2.1
2.2
2.2
2.4
2.2
2.4
In summary, we have demonstrated that bis(phenoxy–imine) Ti
complexes (Ti-FI catalysts) are potent catalysts for ethylene/5-
hexene-1-yl-acetate copolymerization. The steric and electronic
nature of the substitution groups of the FI ligands has a significant
effect on copolymerization behavior, which can reasonably be
explained by DFT calculations. The results introduced herein may
open the door to polar monomer copolymerization with early
transition metal catalysts.
f
f
f
–
–
–
1.97
1.90
1.82
1.98
59
69
67
55
1.8
1.7
2.2
1.8
9
10
11
12
13
14
f
f
f
17
11
15
–
–
–
2.45
3.20
20
23
1.8
1.6
Supporting Information Available: Synthesis of 4-8, crystal-
lographic information file (CIF) for 3, polymerization procedures and
results, GPC-IR data, DFT calculations. This material is available free
of charge via the Internet at http://pubs.acs.org.
a Conditions: toluene, 250 mL; complex, 20 µmol; DMAO, 5.00
mmol as Al; 25 °C, 10 min, ethylene, 0.1 MPa. b Charged 5-hexene-
1-yl-acetate. c As kg of polymer/(mol of cat. h). d Determined by 1H
NMR. e Determined by GPC using PS calibration. f Not determined.
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