Efficient synthesis of hydroxylated polyethylene via copolymerization of ethylene with 5-norbornene-2-methanol using bis(β-enaminoketonato)titanium catalysts

Article abstract of DOI:10.1021/om200526n

The copolymerizations of ethylene with 5-norbornene-2-methanol (NBM) using bis(β-enaminoketonato)titanium complexes [RNC(CH₃)CHCO(CF ₃)]₂TiCl₂ [1a: R = Ph, 1b: R = C ₆H₄Me(p), 1c: R = C

Full text of DOI:10.1021/om200526n

ARTICLE
pubs.acs.org/Organometallics
Efficient Synthesis of Hydroxylated Polyethylene via
Copolymerization of Ethylene with 5-Norbornene-2-methanol using
Bis(β-enaminoketonato)titanium Catalysts
Miao Hong,†,‡ Yong-Xia Wang,† Hong-Liang Mu,†,‡ and Yue-Sheng Li*,†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, China
‡Graduate School of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
S Supporting Information
b
ABSTRACT: The copolymerizations of ethylene with 5-norbor-
nene-2-methanol (NBM) using bis(β-enaminoketonato)titanium
complexes [RNC(CH3)CHCO(CF3)]2TiCl2 [1a: R = Ph, 1b:
R = C6H4Me(p), 1c: R = C6H4tBu(p), 1d: R = C6H4F(p), 1e: R =
C6H4Me(o)] and [R2NC(CF3)CHCO(R1)]2TiCl2 [2a: R1 = Ph,
R2 = Ph; 2b: R1 = Ph, R2 = C6H4Me(p); 2c: R1 = Ph, R2 =
C6H4tBu(p); 2d: R1 = Ph, R2 = C6H4F(p); 2e: R1 = C6H4Me(p),
R2 = Ph] have been investigated. By optimizing the catalyst
structures and polymerization conditions, outstanding catalytic
activity and efficient polar comonomer incorporation have been
achieved. Catalysts 1b,c and 2b,c with electron-donating groups
(Me and tBu) display improved tolerance toward polar group,
exhibiting higher catalytic activities. Among these catalysts, 1b
produced the copolymer with the highest NBM incorporation. Using Et2AlCl as the pretreated reagent, the high NBM
incorporation up to 22.0 mol % with a catalytic activity of 700 kg/molTi h has been achieved by catalyst 1b. DFT calculations
3
have been utilized to explain the high catalytic activity and efficient comonomer incorporation in the present catalyst system. The
alternating ethyleneꢀNBM sequence can be detected at high NBM incorporation, as demonstrated by 13C NMR (DEPT) spectra.
’ INTRODUCTION
examples have been achieved in titanium-catalyzed copolymer-
ization of ethylene with a polar monomer.7,8 Fujita and co-
workers have demonstrated that bis(phenoxyimine)titanium
complexes (Ti-FI) are potent catalysts for ethylene/5-hexene-
1-yl-acetate copolymerization, and this result opened the door to
polar monomer copolymerization with titanium catalysts.7 Re-
cently, a class of titanium complexes bearing tridentate ligands
([OꢀNSR]TiCl3) developed by Tang and his colleagues has
been proved to be efficient catalysts for copolymerization of
ethylene with ω-alkenol or ω-alkenoic acid with comonomer
incorporation up to 11.2 mol %.8 However, the synthesis of
functional polyethylene that has the efficient polar comonomer
incorporated with alternating sequence has not been reported yet
in titanium-catalyzed copolymerization. Additionally, the polar
comonomers used in titanium-catalyzed copolymerizations were
limited to polar R-olefins. Compared with polar R-olefins, polar
norbornene derivatives such as 5-norbornene-2-methanol9 are
more convenient for easy synthesis via DielsꢀAlder reaction.10
Moreover, the copolymerization of ethylene with a polar nor-
bornene derivative would provide a functional cyclic olefin
copolymer with improved performance.6e,9 To date, few studies
Since the discovery of ZieglerꢀNatta catalysts, it has been a
long scientific interest and technologically important subject to
investigate the routes to incorporate functional (polar) groups
into a polyolefin chain, because the polyolefins with functional
groups possess improved performance such as excellent dye-
ability, adhesion, and wettability.1
Conventionally, functional polyethylenes are produced by free
radical copolymerizations of ethylene and polar comonomers at
high temperature and extremely high pressure.2 In recent years,
much attention has been paid to the copolymerization of
ethylene with polar comonomers initiated by transition metal
catalysts, because it provides an easy and low-cost access to
functional polyethylene. Significant progress has been made in
developing late transition metal catalysts that can copolymerize
ethylene with a polar comonomer under mild conditions.3ꢀ5
Conversely, early transition metal catalysts suitable for the
copolymerization of ethylene with polar comonomers are scarce
due to their high oxophilicity, although these catalysts have
offered remarkable opportunities for the synthesis of novel
olefin-based materials.6ꢀ9 Especially, the titanium catalysts,
which are extremely oxophilic and deactivated easily in the
presence of polar monomers, are normally regarded as unpro-
mising for such a copolymerization. To date, only two successful
Received: June 18, 2011
Published: August 18, 2011
r
2011 American Chemical Society
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Organometallics
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Scheme 1. Synthetic Route for Hydroxylated Polyethylene
and Structures of Catalysts Used
% = {[4(I3.6 ppm + I3.4 ppm)]/[2I0.7ꢀ2.3 ppm ꢀ 5(I3.6 ppm + I3.4 ppm)]} ꢁ
100%, where I3.6 ppm and I3.4 ppm are the peak areas of protons at 3.6 and
3.4 ppm, and I0.7ꢀ2.3 ppm is the total peak area of protons at 0.7ꢀ2.3 ppm.
NBM was purchased from Aldrich, dried over molecular sieves for
3 days, and vacuum-distilled before use. Anhydrous toluene was purified
by a solvent purification system purchased from MBraun. Modified
methylaluminoxane (MMAO, 7% aluminum in heptane solution) and
diethylchloroaluminum (Et2AlCl) were purchased from Akzo Nobel
Chemical Inc. and Albemarle Corporation, respectively, and used with-
out further purification.
Synthesis of Ligands. p-MeC6H4NHC(CH3)CHCOCF3. To a stir-
red solution of 1,1,1-trifluoro-2,4-pentanedione (3.14 g, 20.0 mmol) in
dried toluene (20 mL) were added 4-methylaniline (2.68 g, 25 mmol)
and p-toluenesulfonic acid (ca. 20 mg) as a catalyst at room temperature.
The mixture was heated and refluxed, and the water formed was
removed azotropically using a DeanꢀStark apparatus for 7 h. During
the stirring period, a red solution was formed. Evaporation of toluene
gave a deep red oil. The crude product was purified by column
chromatography on silica gel with petroleum etherꢀethyl acetate
(200:1) as an eluent, affording product as colorless crystals in 85%
yield. Ligands [RNHC(CF3)CHCOPh] were prepared via a procedure
similar to that for [RNHC(CH3)CHCO(CF3)] except with refluxing
for 24 h. 1H NMR (CDCl3): δ 12.52 (s, 1H, N-H), 7.21ꢀ7.18 (d, J = 9
Hz, 2H, Ph-H), 7.04ꢀ7.01 (d, J = 9 Hz, 2H, Ph-H), 5.50 (s, 1H, dCH),
2.36 (s, 3H, p-CH3), 2.07 (s, 3H, CH3). Anal. Calcd for C12H12F3NO:
C, 59.26; H, 4.94; N, 5.76. Found: C, 59.00; H, 4.83; N, 5.67.
p-tBuC6H4NHC(CH3)CHCOCF3. Yield: 90%, colorless crystal. 1H
NMR (CDCl3): δ 12.55 (s, 1H, N-H), 7.42ꢀ7.39 (d, J = 9 Hz, 2H,
Ph-H), 7.09ꢀ7.06 (d, J = 9 Hz, 2H, Ph-H), 5.51 (s, 1H, dCH), 2.10 (s,
3H, CH3), 1.31 (s, 9H, tBu). Anal. Calcd for C15H18F3NO: C, 63.16; H,
6.32; N, 4.91. Found: C, 63.28; H, 6.33; N, 4.90.
have been reported on the use of a polar norbornene derivative as
the comonomer in titanium-catalyzed copolymerization.
Recently, we demonstrated that the bis(β-enaminoketona-
to)titanium catalysts [PhNC(CF3)CHCO(Ph)]2TiCl2 (1a) and
[PhNC(CH3)CHCO(CF3)]2TiCl2 (1b) (Scheme 1) are very
effective for the copolymerization of ethylene with norbornene
or norbornene derivatives such as dicyclopentadiene.11 How-
ever, these catalysts are extremely oxophilic and easily deacti-
vated in the presence of a polar monomer. In order to improve
the tolerance of the catalyst toward polar groups, we modified the
electronic and steric properties of the titanium active site by
introducing different substituents at different positions of the
ligands. The trifluoromethyl group on the framework has been
retained in the modification because it plays a crucial role in
determining catalytic activity in the bis(β-enaminoketona-
to)titanium system.11a Thus, a series of catalysts 1bꢀe and
2bꢀe (Scheme 1) bearing a trifluoromethyl group on the
framework were designed, and a detailed, systematic investiga-
tion regarding the copolymerization of ethylene with 5-norbor-
nene-2-methanol (NBM) was carried out in this study.
p-FC6H4NHC(CH3)CHCOCF3. Yield: 86%, colorless crystal. 1H NMR
(CDCl3): δ 12.48 (s, 1H, N-H), 7.26ꢀ7.09 (m, 4H, Ph-H), 5.55 (s, 1H,
CH), 2.12 (s, 3H, ꢀCH3). Anal. Calcd for C11H9F4NO: C, 54.35; H,
3.67; N, 5.67. Found: C, 54.21; H, 3.72; N, 5.64.
o-MeC6H4NHC(CH3)CHCOCF3. Yield: 56%, yellow oil. 1H NMR
(CDCl3): δ 12.43 (s, 1H, N-H), 7.30ꢀ7.21 (m, 3H, Ph-H),
7.10ꢀ7.07 (m, 1H, Ph-H), 5.54 (s, 1H, dCH), 2.27 (s, 3H, o-CH3),
1.98 (s, 3H, CH3). Anal. Calcd for C12H12F3NO: C, 59.26; H, 4.94; N,
5.76. Found: C, 59.17; H, 4.89; N, 5.50.
p-MeC6H4NHC(CF3)CHCOPh. Yield: 58%, yellow oil. 1H NMR
(CDCl3): δ 12.27 (s, 1H, N-H), 7.95ꢀ7.93 (m, 2H, Ph-H),
7.56ꢀ7.43 (m, 3H, Ph-H), 7.17ꢀ7.11 (m, 4H, Ph-H), 6.40 (s, 1H,
dCH), 2.35 (s, 3H, CH3). Anal. Calcd for C17H14F3NO: C, 66.88; H,
4.62; N, 4.59. Found: C, 66.71; H, 4.58; N, 4.69.
’ EXPERIMENTAL SECTION
General Procedure and Materials. All work involving air- and/
or moisture-sensitive compounds was carried out in an MBraun glove-
box or under an argon atmosphere by using standard Schlenk techni-
ques. The molecular weights (MWs) and polydispersity indices (PDIs)
of the polymer samples were determined at 150 °C by a PL-GPC 220-
type high-temperature gel permeation chromatography. 1,2,4-Trichlor-
obenzene was employed as the solvent at a flow rate of 1.0 mL/min.
The calibration was made by polystyrene standard Easi-Cal PS-1
(PL Ltd.). The differential scanning calorimetry (DSC) measurements
were performed on a Perkin-Elmer Pyris 1 DSC instrument under
N2 atmosphere. The samples were heated at a rate of 20 °C/min and
cooled at a rate of 20 °C/min. The 1H NMR spectra of the ligands and
catalysts were recorded on a Varian Unity-300 MHz spectrometer. The
13C NMR spectra of the catalysts were recorded on a Varian Unity-
1
p-tBuC6H4NHC(CF3)CHCOPh. Yield: 65%, yellow crystal. H NMR
(CDCl3): δ 12.30 (s, 1H, N-H), 7.95ꢀ7.92 (m, 2H, Ph-H), 7.53ꢀ7.44
(m, 3H, Ph-H), 7.37ꢀ7.35 (d, J = 6 Hz, 2H, Ph-H), 7.17ꢀ7.15 (d, J = 6
Hz, 2H, Ph-H), 6.39 (s, 1H, dCH), 1.31 (s, 9H, tBu). Anal. Calcd for
C20H20F3NO: C, 69.16; H, 5.76; N, 4.03. Found: C, 69.71; H, 5.58;
N, 4.19.
p-FC6H4NHC(CF3)CHCOPh. Yield: 72%, orange crystal. 1H NMR
(CDCl3): δ 12.17 (s, 1H, N-H), 7.95ꢀ6.80 (m, 9H, Ph-H) 6.38 (s,
1H, dCH). Anal. Calcd for C17H14F3NO2: C, 62.13; H, 3.56; N, 4.53.
Found: C, 63.25; H, 3.33; N, 4.30.
PhNHC(CF3)CHCO(p-MeC6H4). Yield: 68%, orange oil. 1H NMR
(CDCl3): δ 12.30 (s, 1H, N-H), 7.61 (m, 4H, Ar-H), 7.35ꢀ7.22 (m,
5H, Ph-H), 6.41 (s, 1H, dCH), 2.41 (s, 3H, CH3). Anal. Calcd for
C17H14F3NO: C, 66.82; H, 4.58; N, 4.58. Found: C, 66.02; H, 4.80;
N, 4.67.
1
600 MHz spectrometer. The H and 13C NMR spectra of the poly-
mers were recorded on a Varian Unity-400 MHz spectrometer. All
deuterated NMR solvents were stored over molecular sieves under a
nitrogen atmosphere, and all chemical shifts are given in ppm and
referenced to Me4Si. The NBM content of copolymer was determined
by 1H NMR spectra and calculated according to the formula NBM mol
Synthesis of Titanium Complexes 1bꢀe and 2bꢀe. [p-
MeC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1b). To a stirred solution of com-
pound p-MeC6H4NHC(CH3)CHCOCF3 (2 mmol) in dried diethyl
ether (20 mL) at ꢀ78 °C was added dropwise a 2.5 M n-butyllithium
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hexane solution (0.84 mL) over 5 min. The mixture was allowed to warm
to room temperature and stirred for 4 h. Then the mixture was added
dropwise to TiCl4 (1 mmol) in dried diethyl ether (10 mL) at ꢀ78 °C
with stirring over 30 min. The mixture was allowed to warm to room
temperature and stirred overnight. The evaporation of the solvent under
vacuum yielded a crude product. To the crude product was added 30 mL
of dried dichloromethane, and the mixture was stirred for 10 min and
then filtered. The filtrate was evaporated under vacuum to afford a solid
residue. Dried dichloromethane and dried n-hexane were added to the
solid residue, and the mixture was stirred for 10 min. The filtration of the
mixture gave complex 1b as a red crystal in 70% yield. 1H NMR
(CDCl3): δ 7.24ꢀ7.21 (d, J = 9 Hz, 2H, Ph-H), 7.09ꢀ7.03 (m, 4H,
Ph-H), 6.45ꢀ6.42 (d, J = 9 Hz, 2H, Ph-H), 5.78 (s, 2H, dCH), 2.32 (s,
6H, p-CH3), 1.85 (s, 6H, CH3). 13C NMR (CDCl3): δ 172.40, 146.50,
136.61, 134.38, 130.54, 130.03, 128.86, 125.10, 120.21, 106.32, 90.85,
24.88, 20.98, 20.80, 20.22. Anal. Calcd for C24H22Cl2F6N2O2Ti: C,
47.77; H, 3.65; N, 4.64. Found: C, 46.62; H, 3.64; N, 4.70. Other
complexes were prepared via a similar procedure.
[p-tBuC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1c). Yield: 85%, red crystal.
1H NMR (CDCl3): δ 7.45ꢀ7.42 (d, J = 9 Hz, 2H, Ph-H), 7.26ꢀ7.23 (d,
J = 9 Hz, 2H, Ph-H), 7.12ꢀ7.09 (d, J = 9 Hz, 2H, Ph-H), 6.48ꢀ6.45 (d,
J = 9 Hz, 2H, Ph-H), 5.75 (s, 2H, dCH), 1.85 (s, 6H, CH3), 1.29 (s,
18H, tBu). 13C NMR (CDCl3): δ 172.27, 156.46, 156.22, 149.88,
146.39, 126.95, 126.37, 125.74, 125.24, 124.43, 120.00, 106.49, 34.59,
31.21, 25.09. Anal. Calcd for C30H34Cl2F6N2O2Ti: C, 52.41; H, 4.95; N,
4.08. Found: C, 52.52; H, 4.74; N, 4.10.
2.02 (s, 6H, CH3). 13C NMR (CDCl3): δ 174.50, 156.64, 148.60,
143.50, 138.34, 136.43, 129.75, 129.40, 128.00, 127.36, 126.34, 120.66,
99.32, 20.67. Anal. Calcd for C34H26Cl2F6N2O2Ti: C, 60.09; H, 3.83; N,
4.12. Found: C, 60.00; H, 3.92; N, 4.32.
Crystallographic Studies. Single crystals of complexes 1bꢀd
suitable for X-ray analyses were grown from a saturated CH2Cl2ꢀhex-
ane solution. The intensity data were collected with the ω scan mode
(186 K) on a Bruker Smart APEX diffractometer with a CCD detector
using Mo KR radiation (λ = 0.71073 Å). Lorentz and polarization factors
were made for the intensity data, and absorption corrections were
performed using the SADABS program. The crystal structures were
solved using the SHELXTL program and refined using full matrix least
squares. The positions of hydrogen atoms were calculated theoretically
and included in the final cycles of refinement in a riding model along
with attached carbons.
DFT Calculations. All density functional theory (DFT) calcula-
tions were performed by using the Amsterdam Density Functional
(ADF) program package. The structures and energies are obtained
based on the local density approximation augmented with Becke’s
nonlocal exchange correction and Perdew’s nonlocal correlation correc-
tion. A triple STO basis set was employed for Ti, while all other atoms
were described by a double-ζ plus polarization STO basis. The frozen
core approximation was employed for the 1s electrons of the C, N, O,
and F atoms, as well as the 1sꢀ2p electrons of the Ti atom. Finally, first-
order scalar relativistic corrections were added to the total energy of the
system.
[p-FC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1d). Yield: 68%, red crystal. 1H
NMR (CDCl3): δ 7.26ꢀ6.98 (m, 16H, Ph-H), 5.30 (s, 2H, CH), 2.07 (s,
6H, -CH3). 13C NMR (CDCl3): δ 176.90, 168.10, 162.42, 160.76,
132.81, 127.26, 116.39, 90.91, 20.15. Anal. Calcd for C22H16Cl2F8N2O2-
Ti: C, 43.24; H, 2.64; N, 4.58. Found: C, 43.41; H, 2.60; N, 4.54.
[o-MeC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1e). Yield: 62%, red crystal.
1H NMR (CDCl3): δ 7.23ꢀ6.89 (m, 16H, Ph-H), 6.27ꢀ6.09 (m,
General Procedure for Pretreating Comonomer with
Et2AlCl. To a solution of comonomer (0.04 mol) in toluene (20 mL)
was added carefully Et2AlCl (1.2 equiv for NBM) at 0 °C. The resulting
mixture was allowed to warm to room temperature. Then the desired
amount of toluene was added until the total volume of the solution was
40 mL, and the obtained solution was stored under nitrogen for
further use.
Typical Copolymerization Procedure. Copolymerizations
were carried out under atmospheric pressure in toluene in a 150 mL
glass reactor equipped with a mechanical stirrer. The total volume of the
copolymerization solution was 50 mL. The reactor was charged with a
prescribed volume of toluene and the described comonomer under
argon atmosphere, and then the ethylene gas feed was started, followed
by the addition of MMAO to the reactor. After equilibration at the
desired polymerization temperature for 5 min, the polymerization was
initiated by the toluene solution of the catalyst. After a desired period of
time, the reactor was vented. The resultant copolymers were precipi-
tated from hydrochloric acidꢀethanol (2%V), filtered, washed three
times with ethanol and acetone, then immersed in ethanol for 12 h to
remove the unreacted comonomer, and then dried in vacuo at 40 °C to a
constant weight.
2H, dCH), 2.27ꢀ2.17 (m, 6H, o-CH3), 1.89ꢀ1.84 (m, 6H, CH3). 13
C
NMR (CDCl3): δ 173.77, 149.93, 146.10, 135.85, 133.92, 131.30,
131.18, 130.97, 130.75, 130.64, 128.10, 127.33, 127.19, 126.82,
126.39, 126.31, 125.13, 107.49, 23.94, 18.55, 18.22, 17.86. Anal. Calcd
for C24H22Cl2F6N2O2Ti: C, 47.77; H, 3.65; N, 4.64. Found: C, 46.90;
H, 3.72; N, 4.61.
[p-MePhNHC(CF3)CHCO(Ph)]2TiCl2 (2b). Yield: 70%, brown crystal.
1H NMR (CDCl3): δ 7.53ꢀ7.50 (d, J = 9 Hz, 4H, Ph-H), 7.34ꢀ7.31 (d,
J = 9 Hz, 2H, Ph-H), 7.16ꢀ7.13 (d, J = 9 Hz, 4H, Ph-H), 7.06ꢀ6.93 (m,
4H, Ph-H), 6.80ꢀ6.77 (d, J = 9 Hz, 4H, Ph-H), 6.47 (s, 2H, dCH), 2.40
(s, 6H, CH3). 13C NMR (CDCl3): δ 175.08, 157.10, 145.51, 136.49,
132.89, 132.62, 128.83, 128.45, 128.07, 127.96, 126.35, 121.60, 99.20,
20.50. Anal. Calcd for C34H26Cl2F6N2O2Ti: C, 56.14; H, 3.60; N, 3.85.
Found: C, 56.32; H, 3.54; N, 3.80.
[p-tBuC6H4NC(CF3)CHCO(Ph)]2TiCl2 (2c). Yield: 90%, black crystal.
1H NMR(CDCl3):δ7.70ꢀ7.67 (d, J = 9 Hz, 4H, Ph-H), 7.49ꢀ7.45(t, J =
6 Hz, 2H, Ph-H), 7.38ꢀ7.32 (t, J = 9 Hz, 4H, Ph-H), 7.27ꢀ7.24 (d, J = 9
Hz, 2H, Ph-H), 7.05ꢀ6.99 (t, J = 9 Hz, 4H, Ph-H), 6.72ꢀ6.70 (d, J = 6
Hz, 2H, Ph-H), 6.52 (s, 1H, dCH), 0.92 (s, 9H, tBu). 13C NMR
(CDCl3): δ 174.82, 157.24, 157.06, 149.30, 145.08, 132.95, 132.53,
128.65, 128.05, 125.99, 124.99, 124.43, 121.51, 98.77, 34.06, 30.82. Anal.
Calcd for C40H38Cl2F6N2O2Ti: C, 59.19; H, 4.69; N, 3.45. Found: C,
59.22; H, 4.58; N, 3.40.
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Bis(β-enaminoketonato)-
titanium Complexes. The desired titanium complexes were
obtained in good yields (62ꢀ90%) by the reaction of TiCl4 with
2 equiv of the relevant lithium salts of β-enaminoketonates in
dried diethyl ether. These complexes were fully identified by 1H
NMR spectra and elemental analyses. The crystals of complexes
1bꢀd suitable for crystallographic study were grown from the
CH2Cl2ꢀhexane mixture solution, and their molecular struc-
tures were further confirmed by X-ray crystallographic analyses.
The crystallographic data, collection parameters, and refinement
parameters for the X-ray diffraction analyses of complexes 1bꢀd
are summarized in Table S1 (see Supporting Information). As
shown in Figure 1, complexes 1bꢀd adopt a distorted octahedral
geometry around the titanium center with the trans oxygen atoms
[p-FC6H4NC(CF3)CHCO(Ph)]2TiCl2 (2d). Yield: 76%, black crystal. 1H
NMR (CDCl3): δ 7.70ꢀ6.54 (m, 18H, Ph-H), 6.40 (s, 2H, dCH). 13
C
NMR (CDCl3): δ 177.00, 168.30, 162.48, 160.72, 132.95, 132.81,
132.53, 128.65, 127.26, 124.43, 116.31, 92.91. Anal. Calcd for
C32H20Cl2F8N2O2Ti: C, 52.25; H, 2.72; N, 3.81. Found: C, 51.75; H,
2.92; N, 3.62.
[PhNC(CF3)CHCO(p-MeC6H4)]2TiCl2 (2e). Yield: 71%, black crystal.
1H NMR (CDCl3): δ 7.49ꢀ6.86 (m, 18H, Ph-H), 6.58 (s, 2H, dCH),
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Figure 1. Molecular structures of complexes 1aꢀd with thermal ellipsoids at the 30% probability level (H atoms are omitted for clarity).
Table 1. Selected Bond Distances (Å) and Bond Angles (deg) for Complexes 1aꢀd
1a
1b
1c(1)
1c(2)
1d
Bond Distances (Å)
TiꢀCl(1)
TiꢀCl(2)
Ti1ꢀO(1)
Ti1ꢀO(2)
Ti1ꢀN(1)
Ti1ꢀN(2)
2.2777(9)
2.2777(9)
1.900 (2)
1.900 (2)
2.198(2)
2.198(2)
2.3035(15)
2.2697(15)
1.893(3)
1.881(3)
2.209(4)
2.174(4)
2.282(2)
2.276(2)
1.873(4)
1.878(4)
2.180(5)
2.192(5)
2.2780(19)
2.267(2)
1.872(4)
1.883(4)
2.176(5)
2.204(5)
2.2804(9)
2.2804(9)
1.889(2)
1.889(2)
2.198(2)
2.198(2)
Bond Angles (deg)
O(1)ꢀTiꢀO(2)
N(1)ꢀTiꢀN(2)
Cl(1)ꢀTiꢀCl(2)
O(1)ꢀTiꢀN(1)
O(2)ꢀTiꢀN(1)
O(1)ꢀTiꢀN(2)
O(1)ꢀTiꢀCl(1)
O(2)ꢀTiꢀCl(2)
N(1)ꢀTiꢀCl(1)
N(2)ꢀTiꢀCl(1)
170.82(13)
84.00(13)
98.18(5)
83.22(9)
89.95(9)
89.95(9)
93.64(7)
93.27(7)
89.06(7)
171.77(7)
172.43(13)
82.03(13)
102.71(5)
83.09(13)
90.24(13)
91.52(13)
92.46(10)
94.48(11)
87.19(10)
167.98(11)
171.08(17)
83.41(18)
100.07(8)
83.55(17)
89.23(18)
90.55(18)
93.06(14)
91.92(14)
171.58(15)
88.94(14)
172.61(18)
82.54(17)
101.92(7)
82.73(19)
91.07(18)
90.94(18)
92.61(14)
91.20(13)
169.00(13)
87.60(13)
173.50 (13)
82.22 (12)
100.42 (5)
82.92(8)
92.17(9)
82.92 (9)
90.22(7)
93.95 (7)
89.08(6)
168.677(6)
and cis chloride atoms as well as cis nitrogen atoms similar to
nonsubstituted complex 1a.12 Selected bond distances and angles
for complexes 1aꢀd are summarized in Table 1.12 Complex 1c
crystallizes two independent molecules in the asymmetric unit
cell with slightly different bond lengths and angles, and the
average values of bond lengths and angles are used for compar-
ison. Although the TiꢀO, TiꢀN, and TiꢀCl bond distances for
complexes 1aꢀd are very similar, the bond angles (e.g., O-
(1)ꢀTiꢀO(2), N(1)ꢀTiꢀN(2), and Cl(1)ꢀTiꢀCl(2)) for
these complexes are significantly different. Especially, there is
an evident discrepancy in the Cl(1)ꢀTiꢀCl(2) angle which
would provide the vacant coordination site at the active center in
the presence of MMAO (1a: 98.18°, 1b: 102.71°, 1c: 101.00, 1d:
100.42). The wider Cl(1)ꢀTiꢀCl(2) angle of complex 1b
indicates that this complex may have a sterically more open
active site and is expected to produce copolymer with higher
comonomer incorporation.
Ethylene/NBM Copolymerization. Upon treatment with
MMAO, catalysts 1bꢀd and 2bꢀe exhibit high catalytic activities
toward ethylene polymerization (except catalyst 1e, discussed
later), affording linear polyethylenes with high molecular weights
and unimodal molecular weight distributions (Table S2, see
Supporting Information). The copolymerization behavior of
these catalysts (1bꢀe and 2bꢀe) took our attention and was
explored in depth. For comparison, the copolymerization pro-
moted by nonsubstituted catalysts 1a and 2a was also investi-
gated. Representative results are summarized in Table 2.
Overall, for the catalysts bearing the same substituents at
the para-position of the N-aryl moiety of the ligands, catalysts
1aꢀd, with a methyl on the framework, exhibited higher
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Table 2. Ethylene/NBM Copolymerization by Bis(β-enaminoketonato)titanium Catalystsa
d
entry
cat.
comon (mmol)
Al/Ti
yield (mg)
activityb
incorpc (mol %)
Mwd (kg/mol)
Mw/Mn
Tme (°C)
1
1a
1b
1b
1b
1c
1c
1c
1d
2a
2b
2b
2b
2c
2c
2c
2d
2e
4.5
2.0
3.0
4.5
2.0
3.0
4.5
4.5
4.5
2.0
3.0
4.5
2.0
3.0
4.5
4.5
4.5
2500
1500
2000
2500
1500
2000
2500
2500
2500
1500
2000
2500
1500
2000
2500
2500
2500
70
390
213
160
450
267
200
40
53
300
160
120
337
200
150
30
5.5
2.8
5.0
10
22.8
89.0
53.4
36.5
95.1
51.9
31.7
7.61
18.7
84.5
50.4
28.0
102.0
60.0
36.2
1.9
2.6
2.5
1.9
1.7
2.2
1.9
2.1
1.9
1.7
2.1
1.8
1.6
1.9
1.7
81.4
102.7
90.0
2
3
4
5
1.6
3.4
6.0
6.8
5.2
1.8
4.6
9.2
1.5
3.1
5.6
6.5
1.2
120.6
93.0
87.0
93.2
97.4
105.0
96.0
6
7
8
9
55
41
10
11
12
13
14
15
16
17
320
173
107
360
200
133
25
240
130
80
270
150
100
15
120.0
91.2
89.0
47
35
32.5
1.8
110.0
a Conditions: catalyst, 8 μmol; cocatalyst, MMAO; ethylene, 1 atm; Vtotal = 50 mL; T = 25 °C; time =10 min. b Catalytic activity: kg polymer/molTi h.
3
c Comonomer incorporation (mol %) established by 1H NMR spectra. d Weight-average molecular weights and polydispersity indices determined by
GPC at 150 °C in 1,2,4-C6Cl3H3 vs narrow polystyrene standards. e Melting temperatures were measured by DSC.
To gain more insight into the ligand effect on catalytic activity
in polar monomer copolymerization, DFT calculations were
used to estimate the energy difference (ΔE = EE ꢀ EO) between
ethylene-coordinated (EE) and oxygen-coordinated (EO) cationic
complexes, which is an indication of functional group tolerance
(Figure 2).7 The calculated structures of ethylene-coordinated
and oxygen-coordinated species are provided in Figures S1 and
S2 (see Supporting Information). The oxygen-coordinated ca-
tionic methyl complexes are more stable than ethylene-coordi-
nated cationic methyl complexes. The ΔE values for catalysts
1aꢀd are 43.47, 36.16, 31.77, and 64.37 kJ/mol, respectively.
Figure 2. Energy difference (ΔE) between ethylene-coordinated and
oxygen-coordinated cationic complexes (NB-OCH3 was used in DFT
The lower ΔE value of a catalyst suggests that it has a less
oxophilic titanium center and will display better tolerance toward
a functional group. Accordingly, among these catalysts, 1b,c, with
electron-donating groups (tBu and Me), have much lower
oxophilic titanium centers, while 1d, with an electron-withdraw-
ing group (F), has the highest oxophilic titanium center. Con-
sistent with the order of experimental catalytic activity, the order
of functional group tolerance for catalysts 1aꢀd predicted
by DFT calculations is 1c > 1b > 1a > 1d. These calculation
results provide a reasonable demonstration that a less oxophilic
Ti center, generated by a more electron-donating ligand, exhibits
better tolerance toward functional groups, giving rise to an
enhanced catalytic activity.
The comonomer incorporation is strongly influenced by not
only the catalyst structures but also the reaction conditions.
Overall, relative to nonsubstituted catalysts 1a and 2a, catalysts
with substituents at the para-position of the N-aryl moiety
(1bꢀd and 2bꢀd) produced the copolymer with higher NBM
incorporation. Among these catalysts with substituents at the
para-position of the N-aryl moiety (1bꢀd and 2bꢀd), the NBM
incorporation increases in the sequence 1c ≈ 2c (tBu) < 1d ≈ 2d
(F) < 1b ≈ 2b (Me). Remarkably, catalysts 1b and 2b, with
methyl groups, produce the copolymers with significantly en-
hanced NBM incorporations, which are nearly twice as high
as those produced by nonsubstituted catalysts (Table 2, entry 1
calculations for simplicity).
activity than those (2aꢀd) with a phenyl group on the frame-
work (1a vs 2a, 1b vs 2b, 1c vs 2c, 1d vs 2d). Compared with
nonsubstituted catalysts 1a and 2a, 1b,c and 2b,c, bearing
electron-donating groups (Me and tBu) at the para-position of
the N-aryl moiety, displayed much higher activities, while
the introduction of electron-withdrawing groups (F, 1d and
2d) led to lower activities. Additionally, the activities of
catalysts 1c and 2c, with tert-butyl groups, are higher than
those of catalysts 1b and 2b, with methyl groups (Table 2, entry
3 vs 6 and entry 11 vs 14). Since the introduction of an electron-
donating group can enhance the catalytic activity, we thus tried
to synthesize the catalyst with a methyl group adjacent to the
imine moiety (1e). However, only a trace amount of poly-
ethylene was obtained, which may be caused by the steric effect of
ortho-methyl groups hindering ethylene from accessing the titanium
center (Table S2, see Supporting Information). Moreover, complex
2e, with a methyl group at the para-position of the phenyl group
on the framework, was also synthesized and utilized in ethylene/
NBM copolymerization. However, lower activity and NBM incor-
poration were observed compared with that of catalyst 2a (Table 2,
entry 9 vs 17).
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Figure 3. Calculated structures of cationic methyl complexes derived from complexes 1aꢀd (H atoms are omitted for clarity).
Table 3. Ethylene/Pretreated NBM Copolymerization by Bis(β-enaminoketonato)titanium Catalystsa
e
entry
cat.
comonb (mmol)
yield (mg)
activityc
incorpd (mol %)
Mwe (kg/mol)
Mw/Mn
Tmf (°C)
1
2
3
4
5
6
1b
1b
1b
1c
1c
1c
4.5
10
507
320
187
667
523
360
1900
1200
700
7.0
15
78.0
60.0
51.8
93.4
72.0
68.0
1.9
1.9
1.8
1.6
2.3
2.0
80.0
18
22
4.5
10
2500
1960
1350
5.3
12
86.5
18
16
a Conditions: catalyst, 8 μmol; cocatalyst, MMAO; ethylene, 1 atm; Vtotal = 50 mL; T = 25 °C; time = 2 min. b Comonomer was pretreated by 1.2 equiv of
Et2AlCl. c Catalytic activity: kg polymer/molTi h. d Comonomer incorporation (mol %) established by 1H NMR spectra. e Weight-average molecular
3
f
weights and polydispersity indices determined by GPC at 150 °C in 1,2,4-C6Cl3H3 vs narrow polystyrene standards. Melting temperatures were
measured by DSC.
vs 4 and entry 9 vs 12). As the NBM feed rises gradually from 2.0
to 4.5 mmol, the NBM incorporation for catalyst 1b increases
significantly from 2.8 to 10.0 mol % (Table 2, entries 2ꢀ4).
Generally, high comonomer incorporation is associated with the
sterically open nature of the active site and high electrophilicity of
the metal center achieved by introducing the electron-with-
drawing groups.13 However, in this work, the catalysts 1b and
2b, with electron-donating groups (Me), produced the copoly-
mer with even higher NBM incorporation than those of the
catalysts 1d and 2d, with electron-withdrawing groups (F), which
is probably caused by the more open active site of the catalysts 1b
and 2b.
In order to obtain the structures of real active species after the
initiation reaction, DFT calculations were carried out. As the
active species derived from a group 4 transition metal complex is
now widely accepted to be a cationic alkyl complex,13,14 the
cationic methyl species (for simple calculation) derived from
complexes 1aꢀd were calculated. The resultant structures of
active species are shown in Figure 3. As observed, the alkene
approaches the titanium center on the side of either the
NꢀTiꢀC or OꢀTiꢀC angle. In fact, the coordination reactions
for both ethylene and NBM all occur in the space between the
NꢀTiꢀC angles (Figures S1 and S3, see Supporting In-
formation). It can be clearly explained by more open environment
(120.59ꢀ124.95°) of the NꢀTiꢀC angle relative to the
OꢀTiꢀC angle (1a: 85.33°, 1b: 83.96°, 1c: 85.41°, 1d:
86.04°), which is propitious for the alkene to access the titanium
center with less steric congestion. Accordingly, the cationic methyl
species with a wider NꢀTiꢀC angle has more open coordination
space. The NꢀTiꢀC angles of active species derived from 1aꢀd
are 120.59°, 124.95°, 121.37°, and 121.35°, respectively, indicating
the coordination space has a sequence of 1b > 1d ≈ 1c > 1a.
The highest comonomer incorporation of catalyst 1b can be
clearly explained by its widest coordination space. Catalyst 1d
displays higher NBM incorporation than 1c though both catalysts
have similar coordination space, which may be due to the higher
electrophilicity of 1d originating from the electron-withdrawing
groups.
In our work, the catalysts with improved functional group
tolerance (1b,c) exhibited high activity even without the pre-
treatment procedure (120ꢀ337 kg polymer/molTi h, Table 2,
3
entries 2ꢀ7). In order to further increase the catalytic activity,
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Figure 5. 1H NMR spectrum of ethylene/NBM copolymer (NBM mol
% = 16.0 mol %, Table 3, entry 6) in o-C6D4Cl2 at 125 °C.
Figures S4ꢀ6, Supporting Information). With increasing como-
nomer incorporation, Tm values of copolymers decrease. More-
over, as expected for a polymer generated from a chemically
homogeneous catalyst, all the copolymers possess narrow mo-
lecular weight distributions (Mw/Mn = 1.6ꢀ2.6; typical GPC
curves are provided in Figure S7, Supporting Information). The
molecular weights of copolymers vary with catalyst structures
and reaction conditions. Overall, the electron-donating substi-
tuted catalysts producecopolymerswith higher molecular weights
than those nonsubstituted and electron-withdrawing substituted
catalysts. Copolymers obtained by pretreatment procedure have
higher molecular weights than those obtained without pretreat-
ment procedure.
Microstructure of Ethylene/NBM Copolymer. The micro-
structure of ethylene/NBM copolymer is established by 1H and
13C NMR (DEPT) spectra. The typical 13C NMR (DEPT)
spectra for ethylene/NBM copolymers are presented in Figure 4.
The copolymer with low NBM incorporation (7.0 mol %) shows
a similar 13C NMR spectrum (Figure 4a) to that previously
reported.9d The analysis of spectra indicated that the bis(β-
enaminoketonato)titanium catalysts promote addition-type po-
lymerization of NBM, which contains both exo and endo en-
chainment. The peaks at 67.4 and 64.8 ppm are assigned to C8a
(exo) and C8b (endo) in the CH2OH substituent, respectively.
Those peaks in the region of 29.60ꢀ49.00 ppm are ascribed to
the carbons in the norbornenyl moiety and successive ethylene
units. Additionally, the 13C NMR spectra for the copolymer with
low NBM incorporation also reveal that NBM is randomly
incorporated into the polymer with the isolated NBM unit
present in the polyethylene sequences, while the continuous
NBMꢀNBM sequence and alternating ethyleneꢀNBM se-
quence are not detectable. The negligible NBM-NBM sequence
can be clearly explained by steric congestion of titanium active
species associated with the successive insertion of NBM units,
which is consistent with the fact that these catalysts exhibit no
catalytic activity in an attempted homopolymerization of NBM.
With NBM incorporation increasing from 7.0 mol % to 16.0
mol %, the 13C NMR spectrum (Figure 4b) is different from the
previous report.9d The new peaks at 56.35 [2a0 (exo)], 51.75 [2b0
(endo)], 48.96 [3b0 (endo)], 47.52 [3a0 (exo)], 46.27 [1b0
(endo)], 44.92 [4a0 (exo)], 43.20 [4b0 (endo)], 42.48 [1a0
(exo)], 40.80 [5a0 (exo)], 40.45 [5b0 (endo)], 39.46 [7a0 (exo)],
37.15 [7b0 (endo)], and 32.29 ppm [6a0 (exo)] can be tracked,
which are attributed to the alternating ethyleneꢀNBM sequence.
These peaks are assigned on the basis of the 13C NMR spectra
with low NBM incorporation, the molar ratio of exo and endo in
Figure 4. Comparison of the 13C NMR (DEPT) spectra of ethylene/
NBM copolymers in o-C6D4Cl2 at 125 °C: (a) NBM mol % = 7.0 mol %
(Table 3, entry 1); (b) NBM mol % = 16 mol % (Table 3, entry 6); (c)
NBM mol % = 22.0 mol % (Table 3, entry 3); (d) 13C NMR (DEPT)
spectra (NBM mol % = 16 mol %). The detailed 13C NMR spectra in the
range 38ꢀ50 ppm are provided at the bottom.
the copolymerizations of ethylene with NBM using Et2AlCl as
the pretreated reagent were also carried out by catalysts 1b,c,
and the results are summarized in Table 3. The influence of
catalyst structure on catalytic activity and comonomer incor-
poration in ethylene/Et2AlCl-pretreated NBM copolymeriza-
tions was similar to those in nonpretreated copolymerizations. The
highest catalytic activity, up to 2500 kg/molTi h, has been achieved
3
by catalyst 1c with NBM incorporation of 5.3 mol % (Table 3, entry
4). Increasing the NMB feed resulted in higher comonomer
incorporation, and the highest incorporation, up to 22.0 mol %,
has been achieved by catalyst 1b with an activity of 700 kg/
molTi h (Table 3, entry 3). As far as we know, the present
3
catalyst system is the first example of successful titanium-
catalyzed ethylene/polar comonomer copolymerization with
efficient polar comonomer incorporation.
All the copolymers obtained by nonpretreated or pretreated
copolymerizations display single melting temperatures (Tm)
in the measurement of DSC, indicating the component of co-
polymer is homogeneous (typical DSC curves are provided in
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Organometallics
ARTICLE
the copolymer, and a comparison of 13C NMR and 13C NMR
(DEPT) spectra. When NBM incorporation increases to 22.0
mol % (Figure 4c), the intensities of these peaks corresponding
to the alternating ethyleneꢀNBM sequence are further en-
hanced. As far as we are aware, the present catalyst system based
on 1b,c represents the first example of successful titanium-
catalyzed polar monomer copolymerization that has efficient
polar comonomer incorporation with alternating sequence.
A typical 1H NMR spectrum of E/NBM copolymer is
provided in Figure 5. The peaks at 3.6 and 3.4 ppm are assigned
to the protons of methylene in CH2OH corresponding to endo
and exo enchainments, respectively. 1H and 13C NMR spectra all
indicate both endo and exo isomers are incorporated into the
polymer chain and the intensity ratio is almost 1:1. Regarding
NBM comonomer used in our work composed of exo and endo
isomers with the molar ratio of 0.3:1, the present catalytic system
is considered to prefer copolymerizing the exo isomer to the endo
isomer.
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’ CONCLUSIONS
A series of hydroxylated polyethylenes with high hydroxyl
group contents have been efficiently synthesized via copolymer-
izations of ethylene with 5-norbornene-2-methanol using newly
designed bis(β-enaminoketonato)titanium complexes. The catalysts
with electron-donating groups (Me and tBu, 1b,c and 2b,c),
possessing less oxophilic Ti centers as confirmed by DFT
calculations, have improved tolerance toward the functional
groups, exhibiting higher catalytic activities. Among these cata-
lysts, 1b produced the copolymer with the highest NBM
incorporation on account of its widest coordination space as
illuminated by DFT calculations. Using Et2AlCl as the pretreated
reagent, the high NBM incorporation up to 22.0 mol % with a
catalytic activity of 700 kg/molTi h has been achieved by catalyst
3
1b. 13C NMR (DEPT) spectra revealed that the alternating
ethyleneꢀNBM sequence can be detected at high NBM incor-
poration. As far as we know, the present catalyst system is the first
example of successful titanium-catalyzed ethylene/polar como-
nomer copolymerization that has efficient polar comonomer
incorporation with alternating sequence.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data, collec-
b
tion parameters, and refinement parameters for the X-ray
1
diffraction analyses of complexes 1bꢀd, selective H NMR
spectra for complexes, ethylene polymerization, calculated struc-
tures for ethylene-coordinated, oxygen-coordinated, and NBM-
coordinated species, and typical DSC and GPC curves for
copolymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: +86-431-85262039. E-mail: ysli@ciac.jl.cn.
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X. K.; Wang, Z.; Xie, Z. W.; Tang, Y. Angew. Chem., Int. Ed. 2009, 48,
8099–8102.
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’ ACKNOWLEDGMENT
The authors are grateful for a subsidy provided by the National
Natural Science Foundation of China (Nos. 20734002 and
20923003).
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