Efficient copolymerization of ethylene with norbornene or its derIVatIVes using half-metallocene zirconium(IV) catalysts

Article abstract of DOI:10.1039/c6ra11501b

A series of half-metallocene zirconium complexes CpZr(thf)Cl₂[O-2,4-^tBu-6-(P-PhR)C₆H₂] (Cp = C₅H₅, 2a: R = Me, 2b: R = ^tBu, 2c: R = Ph, 2d: R = 4-F-Ph) have been synthesized by r

Full text of DOI:10.1039/c6ra11501b

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Efficient copolymerization of ethylene with
norbornene or its derivatives using half-
Cite this: RSC Adv., 2016, 6, 59590
metallocene zirconium(IV) catalysts†
*
Yulian Li, Jixing Yang, Bin Wang and Yuesheng Li
A series of half-metallocene zirconium complexes CpZr(thf)Cl2[O-2,4-tBu-6-(P-PhR)C6H2] (Cp ¼ C5H5, 2a:
R ¼ Me, 2b: R ¼ tBu, 2c: R ¼ Ph, 2d: R ¼ 4-F-Ph) have been synthesized by reacting CpZrCl3 with the
corresponding ligands in THF. All the complexes were characterized by (1H, 13C and 31P) NMR
spectroscopy as well as elemental analysis. X-ray structural analysis for 2a and 2d revealed a six-
coordinate distorted octahedral geometry around the zirconium center in the solid state. With the
activation of a suitable cocatalyst, half-metallocene zirconium complexes 2a–d were employed to
catalyze ethylene polymerization and ethylene copolymerization with cycloolefins including norbornene
(NBE), 5-ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB). All the complexes exhibited
high efficiency toward the copolymerizations. High comonomer incorporations up to 62% (NBE), 76%
(ENB) and 40.6% (VNB), respectively, with high catalytic activities above 103 kgpolymer per molZr per h
were achieved using catalyst 2c. Steric hindrance and electronic effects of the complexes or the
comonomers greatly influence copolymerization behavior. Thus, catalytic activity and copolymer chain
structure, including comonomer incorporation and the molecular weight, can be easily tuned over
a wide range by changing catalyst and reaction conditions.
Received 4th May 2016
Accepted 15th June 2016
DOI: 10.1039/c6ra11501b
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special comonomers. A practicable alternative is to choose
comonomers bearing additional functional groups that do not
1. Introduction
interfere with the copolymerization, which allows for post-
functionalization through simple transformations.7,8 5-
Ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB)
satisfactorily meet these requirements.7,9–19
Cyclic olen copolymers (COCs) are good candidates for engi-
neering plastics and optical apparatus, thanks to their excellent
transparency, and high thermal and chemical stability relative
to polyethylene or ethylene/a-olen copolymer. These desirable
properties have led to the emergence of commercially available
copolymers such as TOPAS and APEL.1,2 The existence of ring
strain in the structure of norbornene (NBE) and its derivatives
makes them facile polymerizable, thus NBE has become one
of the most active participants in ethylene/cyclo-olen
copolymerization.3
In spite of commercial success, an issue that should be
properly addressed in the application of COCs is their poor
surface properties, resulting from the nonpolar nature of the
material. As the simplest method of functionalization, copoly-
merization of olen with polar monomer using early transition
metal catalysts is rarely reported because of the highly oxophilic
nature of polar monomer.4,5 Although this can be realized by
using certain late transition metal catalysts,6 the polymerization
oen suffers from either low efficiency or the high cost of
Catalysts play a crucial role in the production of ethylene/
ENB or ethylene/VNB copolymers, and most of catalysts re-
ported are ansa-metallocenes.20 For example, when Ph2-
C(Flu)(Cp)ZrCl2 (Cp ¼ C5H5, Flu ¼ uorenyl) was used in
ethylene/VNB copolymerization, the catalytic activity could be
up to 3520 kg per molZr per h with VNB incorporation of 32
mol%.12 When used in ethylene/ENB copolymerization, the
catalytic activity of rac-Et(Ind)2ZrCl2 (Et ¼ C2H4, Ind ¼ indenyl)
reached 1645 kg per molZr per h and ENB incorporation was
38.4 mol%. In addition, some constrained geometry catalysts
(CGCs) were also efficient for ethylene/VNB or ethylene/ENB
copolymerization.15,21 For example, Mu and his coworkers re-
ported a CGC-type titanium catalyst, which showed activity as
high as 1320 kg per molTi per h with ENB incorporation of 50.5
mol%.15 Recently, half-metallocene, possessing the merits of
easy synthesis and attractively catalytic performance, had
received more and more attention.22–37 The half-metallocene
scandium complexes reported by Hou and his colleagues
showed excellent catalytic activity for ethylene/norbornene
(NBE) and ethylene/dicyclopentadiene (DCPD) copolymeriza-
tion, with comonomer incorporation of 44 and 45 mol%,
Tianjin Key Lab Composite & Functional Materials, School of Materials Science and
Engineering, Tianjin University, Tianjin 300072, China. E-mail: ysli@tju.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1475551 and
1475552. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6ra11501b
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molecular weights (MWs) and the polydispersities of the poly-
ꢀ
mer samples were determined at 150 C by a PL-GPC 220 type
high-temperature chromatograph equipped with three PL gel 10
mm Mixed-B LS type columns. 1,2,4-Trichlorobenzene (TCB) was
employed as the solvent at a ow rate of 1.0 mL minꢁ1. The
calibration was made by the polystyrene standard Easi Cal PS-1
(PL Ltd.).
CpZrCl3 and trityltetrakis(pentauorophenyl)borate ([Ph3C]
[B(C6F5)4]) were purchased from Aldrich and used without
further purication. Triisobutylaluminium (AliBu3) and modi-
ed methylaluminoxane (MMAO, 7% aluminium heptane
solution) were purchased from Akzo Nobel Chemical Inc. Dried-
MAO was prepared by removing toluene and Al(CH3)3 in MAO.
Commercial ethylene was directly used for polymerization
without further purication. ENB and VNB were stirred over
CaH2 for 24 h and distilled under N2 before use. The NBE was
stirred with sodium (Na) for 8 h and distilled under N2 before
use. The other reagents and solvents were commercially
available.
Scheme 1 Synthesis of complexes 2a–d.
respectively.38,39 Nomura and his coworkers reported that half-
titanocenes containing pyrazolato ligands showed unique
characteristics not only for ethylene and syndio-specic styrene
polymerization, but also for the copolymerization of ethylene
with 1-hexene, styrene or NBE.23
2.2 Typical copolymerization procedure
Copolymerizations were carried out under atmospheric pres-
sure in toluene in a 150 mL glass reactor equipped with
a mechanical stirrer. The total volume of the solution was 30
mL. The reactor was saturated with ethylene rstly, then
charged with comonomer, prescribed volume of toluene and
AliBu3 sequentially. Aer equilibration at the desired polymer-
ization temperature for 5 min, toluene solution of the catalyst
was added, and the polymerization was initiated by the addition
of [Ph3C][B(C6F5)4] to the reactor. Aer a desired period of time,
the reactor was vented. The resultant copolymers were precipi-
tated from hydrochloric acid/ethanol (volume ratio: 1/50),
ltered, washed three times with ethanol, then dried in
Recently, we reported a series of phosphine-phenolate based
half-titanocenes, which showed high catalytic activities for
ethylene/NBE
copolymerization.40,41
Furthermore,
half-
zirconocenes supported by ligands with the similar structures
showed higher catalytic activities for ethylene polymeriza-
tion.42,43 These results prompted us to explore catalytic behavior
of the half-zirconocenes for ethylene/ENB and ethylene/VNB
copolymerization. Thus, on the basis of the reported half-
zirconocenes (2c: R ¼ Ph),42 we synthesized a series of novel
phosphine-phenolate-based half-zirconocenes (CpZr(thf)Cl2[O-
2,4-tBu-6-(PPhR)C6H2], 2a: R ¼ CH3, 2b: R ¼ tBu, 2d: R ¼ 4-F-Ph,
Scheme 1) and employed them catalyzing copolymerization of
ethylene with VNB or ENB. By tuning the substitute on P atom,
the inuence of steric hindrance and electronic effect on
copolymerization behavior was examined. As a result, high
catalytic activities up to 103 kg per molZr per h and high cyclo-
olen incorporations were achieved.
ꢀ
vacuum at 60 C to a constant weight.
2.3 Density functional theory calculations
Density functional theory (DFT) calculations were used for
optimizing active species of catalysts and nding out the
differences between these four catalysts in coordination space.
Geometry optimizations were performed using the local density
approximation augmented with Beckes nonlocal exchange
correction and Perdews nonlocal correction.44,45 A triple SOT
basis set was used for Zr atom, whereas all other atoms were
described by a double-z plus polarization SOT basis. The 1s
electrons of the C, P and O atoms, as well as the 1s–2p electrons
of Zr atom, were treated as frozen core. Finally, rst-order scalar
relativistic corrections were added to the total energy of the
system.
2. Experimental section
2.1 General procedure and materials
All manipulation of air- and/or moisture-sensitive compounds
was carried out under nitrogen atmosphere by using standard
Schlenk techniques or in an Mbraun glove box. All solvents were
puried from an Mbraun SPS system. All H, 13C and 31P NMR
1
spectra of ligands and complexes were recorded on a Bruker-400
MHz spectrometer at ambient temperature, with CDCl3 or d6-
2.4 Synthesis of complex 2a, 2b and 2d
1
˚
DMSO as the solvent (dried by MS 4 A). Moreover, the H and The ligand 1a–1d was synthesized according to the literature
13C NMR spectra were taken TMS (trimethylchlorosilane) as an method,46 and directly used for next procedures. Then the
internal reference, however, H3PO4 was employed as an external remaining steps to get the catalysts 2a–b and 2d were similar to
reference for 31P NMR spectra. 1H and 13C NMR spectra of that of 2c.42
polymer samples were obtained on the same Bruker-400 MHz
spectrometer at 135 ꢀC, with o-C6D4Cl2 as a solvent. The according to the literature procedures40,41,47–49 and RPPhCl was
2,4-tBu2-6-(PPhCH3)-C6H2OH (1a). The synthesis of 1a was
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t
prepared via the reaction between CH3MgBr and PPhCl2. Yield: 4H, –OCH2), 1.80 (s, 4H, –CH2), 1.4 (d, J ¼ 21.2 Hz, 18H, Bu),
75%, 1H NMR (400 MHz, CDCl3, 298 K): d 7.40–7.28 (m, 5H, Ar- 1.24 (dd, J ¼ 20.7 Hz, J ¼ 11.7 Hz, 9H, tBu). 13C (101 MHz, CDCl3,
H), 7.10 (dd, J ¼ 5.4, 2.4 Hz, 1H, Ar-H), 6.91 (d, J ¼ 8.3 Hz, 1H, Ar- 298 K), d 167.16, 166.90, 142 (d, J ¼ 3.8 Hz), 137.53 (d, J ¼ 4.9
t
H), 1.65 (d, J ¼ 1.8 Hz, 3H, CH3), 1.46 (s, 9H, Bu), 1.23 (s, 9H, Hz), 132.69, 129.52, 128.62, 127.44, 118.39, 118.10, 116.50,
tBu). 13C NMR (101 MHz, CDCl3, 298 K): d 154.92 (d, J ¼ 19.8 77.24, 35.64 (d, J ¼ 8.5 Hz), 35.18, 34.47, 31.61, 29.53, 25.28. 31
P
Hz), 141.25 (d, J ¼ 1.6 Hz), 137.76, 134.07, 130.06 (d, J ¼ 16.2 (162 MHz, CDCl3, 298 K), d 19.98, 8.26. Anal. calc. for C35H53-
Hz), 127.39 (d, J ¼ 6.1 Hz), 127.03, 125.59, 125.10, 120.58, 34.06, Cl2O2PZr: C, 60.15; H, 7.64. Found: C, 60.10; H, 7.60.
33.46, 30.44 (d, J ¼ 11.0 Hz), 28.59, 10.19 (d, J ¼ 9.0 Hz). 31P
NMR (202 MHz, CDCl3, 298 K), d ꢁ54.76.
CpZr(thf)Cl2[C6H2O-2-tBu-6(PPh-4-F-Ph)] (2d). The synthesis
of 2d was similar to that of 2b, except that 2,4-tBu2-6-(PPh-4-F-
2,4-tBu2-6-(PPhtBu)-C6H2OH (1b). The synthesis of 1b was Ph)-C6H2OH was used in place of 2,4-tBu2-6-(PPhtBu)C6H2OH.
t
similar to that of 1a, while BuMgBr was used in place of CH3- Yield: 0.40 g (59.0%). 1H NMR (400 MHz, CDCl3, 298 K): d 7.66
MgBr. Yield: 60%. 1H NMR (400 MHz, CDCl3, 298 K): d 7.66–7.52 (m, 4H, Ar-H), 7.46 (d, J ¼ 2.4 Hz, 1H, Ar-H), 7.41 (s, 3H, Ar-H),
t
(m, 3H, Ar-H), 7.41–7.29 (m, 5H, Ar-H), 1.47–1.36 (s, 9H, Bu), 7.18 (dd, J ¼ 6.2, 2.1 Hz, 1H, Ar-H), 7.06 (d, J ¼ 11.9 Hz, 2H, Ar-
t
t
1.32–1.24 (s, 9H, Bu), 1.23–1.13 (d, J ¼ 13.4 Hz, 9H, Bu). 13C H), 6.53 (m, 5H, Cp), 3.72 (s, 4H, –OCH2), 1.63 (m, 4H, –CH2),
NMR (101 MHz, CDCl3, 298 K): d 157.33 (d, J ¼ 20.9 Hz), 141.01, 1.46 (s, 9H, tBu), 1.27 (s, 9H, tBu). 13C (101 MHz, CDCl3, 298 K),
134.99 (d, J ¼ 25.9 Hz), 134.22 (d, J ¼ 19 Hz), 133.68 (d, J ¼ 17.0 d 167.42, 167.12, 164.86, 162.35, 142.58, 137.49, 132.47, 129.81,
Hz), 129.22, 128.81, 128.22 (d, J ¼ 20.6 Hz), 127.78 (d, J ¼ 7.2 129.23, 128.18 (d, J ¼ 31.3 Hz), 118.33, 117.96, 116.70, 116.17,
Hz), 125.94, 118.45, 35.13, 34.36, 31.49 (m), 29.58, 28.64 (d, J ¼ 73.48, 35.20 (d, J ¼ 1.6 Hz), 34.51, 31.52, 29.67, 24.94. 31P (202
13.5 Hz), 27.46 (d, J ¼ 13.3 Hz). 31P NMR (162 MHz, CDCl3, 298 MHz, CDCl3, 298 K), d 3.14, ꢁ3.82. Anal. calc. for C37H48Cl2-
K), d ꢁ19.62.
FO2PZr: C, 60.31; H, 6.57. Found: C, 60.27; H, 6.50.
2,4-tBu2-6-(PPh4-F-Ph)-C6H2OH (1d). The synthesis of 1d was
similar to that of 1b, while 4-F-PhMgBr was used in place of
tBuMgBr. Yield: 70%. 1H NMR (400 MHz, DMSO, 298 K): d 8.18
(d, J ¼ 3.1 Hz, 1H, Ar-H), 7.38 (d, J ¼ 4.1 Hz, 3H, Ar-H), 7.20 (m,
3. Results and discussion
3.1 Synthesis and characterization
7H, Ar-H), 6.47 (dd, J ¼ 4.4, 2.4 Hz, 1H, –OH), 1.36 (s, 9H, tBu), Complex 2c was synthesized and characterized in our previous
t
1.04 (s, 9H, Bu). 13C NMR (101 MHz, CDCl3, 298 K): d 164.76, report, but it was not used to catalyze ethylene/cycloolens
162.28, 155.91 (d, J ¼ 19.7 Hz), 142.47 (d, J ¼ 2.4 Hz), 135.47, copolymerization. Similar to the synthesis of complex 2c,
133.20 (d, J ¼ 18.3 Hz), 130.94 (d, J ¼ 3.4 Hz), 128.94 (d, J ¼ 4.2 novel phosphine-phenolate-based half-zirconocenes 2a–b and
Hz), 128.68 (d, J ¼ 7.2 Hz), 126.39, 119.98, 115.85 (d, J ¼ 21.1, 8.0 2d have been prepared in moderate yields (50.0–66.7%) by
Hz), 35.13 (d, J ¼ 1.5 Hz), 34.44, 31.46, 29.69. 31P NMR (162 treating CpZrCl3 with phosphine-phenolate sodium salt (1.0
MHz, DMSO, 298 K), d ꢁ21.38 (d, J ¼ 4.0 Hz).
equiv.), which was prepared from the corresponding ligands
CpZr(thf)Cl2[C6H2O-2-tBu-6(PPhCH3)] (2a). The synthesis of with NaH, as shown in Scheme 1. Pure samples were collected
2a was according to the literature procedures.42 To the solution from the chilled concentrated mixture of THF and hexane
of CpZrCl3 (0.262 g, 1.0 mmol) in dried tetrahydrofuran (THF, solution placed in a freezer (ꢁ30 ꢀC). These complexes were
20 mL) a solution of 2,4-tBu2-6-(PPhCH3)-C6H2ONa in THF (20 identied by 1H, 13C and 31P NMR spectra as well as elemental
mL), which was obtained by treating 2,4-tBu2-6-(PPhCH3)- analyses. The H NMR spectra showed no complexity and the
1
C6H2OH (0.328 g, 1.0 mmol) with NaH (0.048 g, 2.0 mmol) in integration of complexes conrms a 1 : 1 ratio of Cp groups to
THF (20 mL) for 6 h at room temperature, was added slowly. The phosphine-phenolate ligands. The resonances ascribed to the
reaction mixture was stirred overnight and then the solvent was protons in THF were observed at around d 3.72 and 1.63 ppm,
removed under reduced pressure. The resultant mixture was which were up-eld from those in the corresponding free THF
dissolved in CH2Cl2 and then the mixture was ltered to remove in CDCl3, indicating that the THF molecular coordinated to the
NaCl. Recrystallization by diffusion of n-hexane (20 mL) into the zirconium center. The chemical shi values of these four
clear solution yielded a white solid of 2a. Yield: 0.60 g (66.7%). complexes in the 31P NMR spectra were different, because of
1H NMR (400 MHz, CDCl3, 298 K): d 7.45 (m, 6H, Ar-H), 7.16 (s, different substitutes on phosphorus (P) atom. The steric
1H, Ar-H), 6.50 (m, 5H, Cp), 3.57 (m, 4H, –OCH2), 1.89 (d, J ¼ 7.6 hindrance and electron effect of these ligands are quite
Hz, 3H, –CH3), 1.73 (m, 4H, –CH2), 1.46 (s, 9H, –tBu), 1.29 (m, different. Their steric hindrance was decreased in the order: 2b
9H, –tBu). 13C NMR (101 MHz, CDCl3, 298 K), d 166.93, 142.98, > 2c ꢂ 2d > 2a.
137.08 (d, J ¼ 5.9 Hz), 131.09 (d, J ¼ 9.2 Hz), 128.64 (d, J ¼ 53.0
Block microcrystals of 2a and 2d suitable for X-ray crystal-
Hz), 128.30, 127.58 (d, J ¼ 36.5 Hz), 127.10, 119.26, 116.49, lographic analyses were grown from the chilled concentrated
73.65, 35.13 (d, J ¼ 1.5 Hz), 34.57, 31.64, 29.53, 24.97. 31P (202 THF–hexane solution mixtures. The crystallographic data
MHz, CDCl3, 298 K), d ꢁ3.64, ꢁ8.43. Anal. calc. for C32H47Cl2- together with the collection and renement parameters are
O2PZr: C, 58.52; H, 7.21. Found: C, 58.48; H, 7.16.
summarized in Table S1.† Selected bond distances and angles
CpZr(thf)Cl2[C6H2O-2,4-tBu-6(PPhtBu)] (2b). The synthesis of for 2a and 2d are listed in Table 1. Structures for 2a and 2d are
2b was similar to that of 2a, except that 2,4-tBu2-6-(PPhtBu)- shown in Fig. 1 and S1 (see ESI†).
C6H2OH was used in place of 2,4-tBu2-6-(PPhCH3)-C6H2-OH.
When the centroid of the cyclopentadienyl ring is considered
Yield: 0.31 g (50.0%). 1H NMR (400 MHz, CDCl3, 298 K): d 7.91 to be a single coordination site, all complexes adopt a six-
(s, 2H, Ar-H), 7.81–7.33 (m, 6H, Ar-H), 6.26 (s, 5H, Cp), 3.72 (s, coordinate, distorted octahedral geometry around the
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Table 1 Selected bond distances (A˚) and angles (ꢀ) for complexes 2a
and 2d
and the P atom are coordinated at axial positions (Fig. 1 and
S1†). Moreover, in complexes 2a and 2d, P(1) is trans to the Cp
ring, while O(1) and O(2) are cis to the Cp ring. The Zr–P bond
distance in 2d is longer than that in 2a, because of the electron
withdrawing property of F, resulting in weaker interaction
between Zr and P atom. Similarly, the Zr–O(1 or 2) distances in
2d are both shorter than those in 2a to compensate the de-
ciency of electron donation property of phosphorus moiety. The
bond angle of C(15)–P(1)–C(21) in 2d is larger than that in 2a
because of the repellence between the two benzene rings con-
nected on P atom in 2d.
2a
2d
˚
Bond distances in A
Zr–O(1)
Zr–O(2)
Zr–P(1)
Zr–Cl(1)
2.033(2)
2.309(2)
2.7599(9)
2.4929(9)
2.4806(9)
1.805(3)
1.828(3)
1.823(4)
1.363(4)
2.020(3)
2.292(3)
2.8353(11)
2.4878(12)
2.4817(12)
1.806(4)
1.834(4)
1.844(4)
1.352(3)
Zr–Cl(2)
P(1)–C(6/1)
P(1)–C(15)
P(1)–C(21)
O(1)–C(1/6)
3.2 Ethylene polymerization
Bond angles in ꢀ
O(1)–Zr–P(1)
Ethylene polymerizations by CpZr(thf)Cl2[O-2,4-tBu-6-(P-PhR)
C6H2] (2a–d) in the presence of MMAO were rstly examined to
explore the effect of the steric hindrance and electron effect
near the catalytic active centre on the polymerization behavior,
and the typical results are depicted in Table 2. In all, complexes
2a–d showed moderate catalytic activity towards ethylene poly-
merization, yielding polymers with low molecular weight (MW)
(Table 2). Catalyst 2a and 2b with electron donating group on P
atom showed higher activity at 75 ꢀC, while the catalytic activity
70.70(6)
91.00(3)
135.62(19)
98.55(11)
148.24(7)
87.72(6)
78.10(3)
76.94(3)
84.69(5)
153.31(6)
82.43(8)
76.39(6)
114.2(2)
101.89(16)
69.74(8)
91.26(4)
137.6(2)
97.19(13)
148.79(8)
90.43(9)
80.43(3)
76.33(4)
81.98(8)
153.60(8)
82.78(11)
77.39(7)
114.3(3)
104.15(18)
Cl(1)–Zr–Cl(2)
Zr–O(1)–C(1/6)
Zr–P(1)–C(6/1)
O(1)–Zr–Cl(1)
O(1)–Zr–Cl(2)
P(1)–Zr–Cl(1)
P(1)–Zr–Cl(2)
O(2)–Zr–Cl(1)
O(2)–Zr–Cl(2)
O(2)–Zr–O(1)
O(2)–Zr–P(1)
ꢀ
of 2c and 2d were much higher at 50 C. Therefore, it can be
concluded that increasing the electron donating ability of P
atom can improve the stability of active species at high
temperature. However, at 75 ꢀC, the MWs of polymers produced
by 2c and 2d with electron withdrawing groups are higher than
those obtained by 2a and 2b (run 2, 7, 10 and 13 in Table 2).
Catalyst 2a and 2b may have more stable active species,
affording more chance for aluminum alkyls or b-H to compete
with monomers. All catalysts show the lowest activities at 100
ꢀC, suggesting the active species under this condition are not
stable. The inuence of electronic and steric properties of these
four catalysts on catalytic activity is irregular. At 50 ꢀC, the
activity is decreased in the order: 2c (1600 kg per molZr per h) >
2b (560 kg per molZr per h) > 2d (380 kg per molZr per h) > 2a
(182 kg per molZr per h). Perhaps, these catalysts are heavily
inuenced by the amount of alkyl-aluminum and their optimal
conditions are different.
P(1)–C(6/1)–C(1/6)
C(15)–P(1)–C(21)
The kind of cocatalyst also exhibits a profound effect on
polymerization catalysis behavior. For example, when the
polymerization was carried out by 2a in the presence of dry MAO
(d-MAO), the MWs of the resultant polymers increased greatly
(run 2 and 3). This result suggests that chain transfer to
aluminum alkyls is crucial among different chain-transfer
pathways for phosphine-phenolate-based half-zirconocenes
here. The molecular weight of the polymer was largely deter-
mined by the chain transfer reaction. The components of these
three kinds of cocatalysts are different. MMAO contains both
the free Al(CH3)3 and AliBu3. The alkylation of catalyst and
chain transfer to aluminum alkyls are very easy. However, d-
MAO contains little of free Al(CH3)3. Thus, alkylation and
Fig. 1 Molecular structure of 2a with thermal ellipsoids at 30% prob-
ability level. Hydrogen atoms are omitted for clarity.
zirconium center, similar to the reported catalyst 2c.42 For 2a chain transfer reactions are both difficult to occur. The condi-
and 2d, the equatorial positions are occupied by the oxygen tion to AliBu3/[Ph3C][B(C6F5)4] is compromised. The chain
atom of the chelating phosphine-phenolate ligand, the oxygen transfer to aluminum alkyls was relatively difficult owning to
atom of THF and two chlorine atoms. The cyclopentadienyl ring larger steric hindrance relative to Al(CH3)3. Therefore, polymers
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Table 2 Typical results of ethylene polymerization by catalysts 2a–da
Activity (kg
per molZr per h)
Run
Catalyst
Temp. (ꢀC)
Yield (g)
Mwd (ꢃ100)
PDId
1
2a
2a
2a
2a
2a
2b
2b
2b
2c
2c
2c
2d
2d
2d
50
75
75
75
100
50
75
100
50
75
100
50
0.091
0.37
0.15
0.20
0.046
0.28
0.38
0.046
0.80
0.14
0.043
0.19
182
740
300
400
92
560
760
92
1600
280
86
380
190
—
890
890
107 700
5400
1050
930
680
690
960
1720
1390
920
1110
—
1.27
1.35
1.93
2.50
1.40
1.32
1.14
1.15
1.28
2.07
1.48
1.28
1.65
—
2
3b
4c
5
6
7
8
9
10
11
12
13
14
75
100
0.095
Trace
a
b
Conditions: catalyst 3 mmol, MMAO 3 mmol, ethylene 1 atm, Vtotal ¼ 30 mL, reaction for 10 min. d-MAO (prepared by removing toluene and
AlMe3 from ordinary MAO) 3 mmol. AliBu3 1 mmol, [Ph3C][B(C6F5)4] 6 mmol. Determined by GPC at 150 ꢀC in 1,2,4-C6Cl3H3 vs. narrow PS
c
d
standards.
with highest molecular weight were obtained when using d- mol%) > 2c (42.0 mol%) > 2a (38.0 mol%) > 2b (32.1 mol%).
MAO as cocatalyst.
Notably, when the comonomer in feed is increased to 1.0 M, the
1
The H NMR spectra of ethylene polymers were collected to NBE incorporation for catalyst 2b only enhanced 7.2%, which is
investigate characteristics of the oligomers (Fig. S2 in ESI†). It obviously different from the other catalysts. The lowest incor-
turns out that the characteristic peaks of terminal methyl can be poration capability of 2b is also ascribed to the bulk substituent
observed clearly for polymers (runs 2 and 4). Moreover, the on P atom. In homopolymerization, the polymerization behav-
chemical shis corresponding to double bonds can only be seen iors of 2b are comparative to those of 2a and 2d. Thus, the steric
with appropriate augmented. Therefore, chain transfer to hindrance was not a main factor in determining the catalytic
aluminum alkyls and b-H elimination can both exist in the activity of catalyst in ethylene homopolymerization. Therefore,
polymerization system contained aluminum alkyls. Chain introducing cyclic comonomers with easier coordination and
transfer to aluminum alkyls is the main way in this polymeri- incorporation capabilities into the polymerization system can
zation system. When AliBu3/[Ph3C][B(C6F5)4] is used as the adjust polymerization.41 From the viewpoint of electronic effect,
cocatalyst, ethylene polymerization by 2a also produces poly- we can assume that the more electron-withdrawing property of
mers with higher MWs compared with MMAO (run 2 and 4 in the substitute on P atom, the copolymers with higher NBE
Table 2). When d-MAO or AliBu3/[Ph3C][B(C6F5)4] as the cocat- incorporation will be produced. All these copolymers exhibit
alysts, the catalytic activities reduced half. Taking all these into high MW, and their variation tendency is in accordance with
consideration, we choose AliBu3/[Ph3C][B(C6F5)4] as the cocat- that of the incorporation (run 1–4, Table 3). Some DFT calcu-
alyst, because the catalytic activity and MWs of copolymers are lations about the differences of 2a–d in coordination space and
both satisfying.
electronic properties can be seen in the ESI.† The DFT calcu-
lations show that the coordination space is decreased in the
order: 2a ꢂ 2c ꢂ 2d [ 2b, while as to electron withdrawing
capabilities: 2d > 2c > 2a > 2b. These DFT calculations are in
accordance with our experimental results.
3.3 Ethylene/NBE copolymerization
Ethylene copolymerizations with NBE catalyzed by 2a–d/AliBu3/
[Ph3C][B(C6F5)4] were carried out with different NBE concen-
trations in feed and at different reaction temperature. Typical
results are shown in Table 3, and all of them produced relatively
high MW copolymers (Mw ¼ 35.0–170.3 kg per molZr per h) with
relative narrow and unimodal molecular weight distributions
(MWDs, PDI ¼ 1.26–2.43). When conducted at 75 ꢀC, catalyst 2a
with a methyl group on P atom displayed the highest activity.
The catalytic activity decreases in the order: 2a (4020 kg per
molZr per h) > 2d (3900 kg per molZr per h) > 2c (3700 kg per
molZr per h) > 2b (2180 kg per molZr per h). Therefore, this result
indicated that increasing the steric bulk of phosphorus moiety
could reduce the catalytic activity evidently. The NBE incorpo-
rations for the four catalysts are decreased in the order: 2d (46.9
Since catalysts 2b and 2c are different in steric hindrance
and electronic effect, these two typical catalysts are used to
explore the effect of the reaction parameters like reaction
temperature, Al/Zr molar ratio, and concentration of como-
nomer in the feed on the copolymerization behaviors. As the
ꢀ
ꢀ
reaction temperature increases from 50 C to 100 C, the cata-
lytic activity of both catalysts increased rstly and then
decreased, while NBE incorporations are always increased.
Thus, to this kind of catalysts, it is relatively difficult to proceed
ꢀ
polymerization at 50 C in this ethylene/NBE copolymerization
system. Because the processes about removing alkyls by Lewis
acid and the formation of four-membered-ring transition state
are both endothermic.41 The higher temperature would
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decrease the solubility of ethylene in toluene, resulting in
higher comonomers incorporation. Therefore, the effect of
raising temperature on comonomer incorporation is similar to
that of increasing comonomer concentration in feed. For both
catalysts, the catalytic activity, comonomer incorporation and
the MW of the copolymers were all enhanced with increasing
concentration of comonomer in feed (compared run 2 with 6 or
3 with 10, Table 3). Hence, the MWs of copolymers can be both
inuenced by temperature and comonomer incorporation.
Higher temperature can result in lower MWs because of the
increased chain transfer reaction. However, higher comonomer
incorporation will retard the chain transfer reaction, resulting
higher molecular weight. For catalyst 2b with electron-donating
group at P atom, the increase in reaction temperature leads to
a decrease in MWs of the resultant copolymers. However, the
ꢀ
MW of copolymer produced by 2c at 75 C is higher than that
produced at 50 ꢀC in the same concentration of NBE (run 3 and
9, Table 3), thanks to higher NBE incorporation at 75 ꢀC.
Moreover, 2c shows both high activity reached 7140 kg per molZr
per h and high comonomer incorporation up to 62 mol% at 75
ꢀC (run 10, Table 3). The increase in Al/Zr ratio for catalyst 2b
causes a slightly increase in comonomer incorporation as well
as catalytic activity and a large extent decrease in the MW (run 2
and 7 in Table 3). Thus, chain-transfer to aluminum alkyls
cannot be neglected for 2b in ethylene/NBE copolymerization.
The effect of the amount aluminum alkyls on copolymerization
is very complicated, all kinds of parameters can be inuenced
with different degree, which is similar to the case reported
previously.50,51
Fig. 2 13C NMR spectra of ethylene/NBE copolymers produced by
different catalysts (a, run 6, 39.3 mol%; b, run 11, 57.1 mol% in Table 3).
copolymer with NBE incorporation of 57.1% shows new peaks
assigned to NBE diads and triads (Fig. 2b), which is similar to
the case reported previously.56,60,61
Compared with ethylene homopolymerization, catalytic
activity and MW of the resultant polymers in copolymerization
can be both substantially increased because of the introduction
of comonomers. We assume that the cycloolens are easier to
coordinate and be incorporated in comparison with ethylene,
similar to our previous reported literature.41 The more bulky
size of cycloolens can inhibit chain transfer effectively, which
results in producing the higher MW copolymers.
The typical 13C NMR spectra of the copolymers (Fig. 2 and
S3†) show that the microstructure formed by catalyst 2b
possesses isolated and alternated NBE units without NBE
continue sequences when the incorporation is below 50 mol%
(Fig. 2a). With the increasing of NBE incorporation, more
alternative NBE units will be traced (Fig. S3†). The NBE incor-
poration of copolymers catalyzed by 2c can reach to higher than
50 mol%. Compared with Fig. 2a, the 13C NMR spectrum of the
3.4 Ethylene/ENB copolymerization
Copolymerizations of ethylene with ENB catalyzed by complexes
2a–d were subsequently explored under the similar conditions.
All the catalytic systems employed are able to copolymerize
Table 3 Ethylene/NBE copolymerization by 2a–da
NBE
Yield
(g)
Activity (kg
per molZr per h)
Run
Catalyst
Temp. (ꢀC)
(mol Lꢁ1
)
Incorp.c (mol%)
Mw (ꢃ103)
PDId
d
1
2
3
4
2a
2b
2c
2d
2b
2b
2b
2b
2c
2c
2c
75
75
75
75
50
75
75
100
50
75
0.5
0.5
0.5
0.5
0.5
1.0
0.5
0.5
0.5
1.0
0.5
38.0
32.1
42.0
46.9
28.1
39.3
37.4
39.4
37.2
62.0
57.1
2.01
1.09
1.85
1.95
0.64
1.57
1.34
0.74
1.09
3.57
1.43
4020
2180
3700
3900
1280
3140
2680
1480
2180
7140
2860
70.0
55.3
118.0
170.3
57.5
60.4
35.0
38.2
1.60
1.58
1.95
1.96
1.77
1.26
1.59
1.58
2.20
1.69
2.43
5
6
7b
8
9
10
11
107.0
166.0
74.9
100
a
b
Conditions: catalyst 3 mmol, AliBu3 1 mmol, [Ph3C][B(C6F5)4] 6 mmol, ethylene 1 atm, Vtotal ¼ 30 mL, reaction for 10 min. AliBu3 1.5 mmol.
c
Established by 13C NMR spectra at 135 ꢀC with o-C6D4Cl2 as a solvent (run 1, 3, 4 and 9–11 in Table 4), while (run 2, 5–8) established by 13C
d
NMR spectra at 25 ꢀC with CDCl3 as a solvent, referred to the literatures.52–59 Determined by GPC at 150 ꢀC in 1,2,4-C6Cl3H3 vs. narrow PS
standards.
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ethylene with ENB and produce copolymers with high MWs and not play a crucial role in chain transfer for catalyst 2b. It
unimodal MWDs. The typical polymerization results are conrms the conclusion again that aluminum alkyls do play
summarized in Table 4.
important and complicated role for these catalysts in ethylene/
The catalyst 2b with the bulky tert-butyl group on the P atom NBE-derivates copolymerization, similar to the situation previ-
exhibited lower activity compared with the other catalysts under ously reported.50,51
the same conditions (run 1–4, Table 4). Moreover, ethylene/ENB
It is also noteworthy that the ethylene/ENB copolymers
copolymers produced by 2b also contain lower ENB content and afforded by catalyst 2b can be dissolved in THF and CHCl3 at
have lower MWs than those of other copolymers obtained by 2a room temperature, while those produced by catalyst 2c can not.
and 2c–d. Similar to the ethylene/NBE copolymerization, the This may be determined by the MW and microstructure of
catalysts 2c–d with electron-withdrawing group on the P atom copolymers, which is the same as E/NBE copolymers. From the
1
displayed higher comonomer incorporation capacities than 2a– typical H NMR spectra of ENB and ethylene/ENB copolymer,
b with electron donating group on the P atom. Catalysts 2b and only encyclo double bond participates in addition polymeriza-
2c are also chosen to explore the effect of polymerization tion, which conguration is similar with that of comonomer
parameters like reaction temperature, Al/Zr ratio, and como- (Fig. 3). Moreover, cross-linking can be ignored because of the
nomer concentration in the feed on copolymerization behav- narrow and unimodal molecular weight distributions of E/ENB
iors. It is interesting that the catalytic activities of catalyst 2b at
50 ꢀC and 100 ꢀC are the same and are almost half of that at 75
ꢀC. However, the copolymer produced at 100 ꢀC show higher
ENB incorporation and lower MW compared with copolymer
ꢀ
obtained at 50 C. The reason about the inuence of tempera-
ture on catalytic activity and comonomers incorporation is the
same as E/NBE copolymerization.
For catalyst 2c with electron withdrawing substitute, the
catalytic activity decreases with the increase of polymerization
temperature, on account of unstable active species at high
temperature. However, it is worth noting that the molecular
weight produced at 100 ꢀC is higher than that at 50 ꢀC, thanks to
higher comonomer incorporation at 100 ꢀC. For both catalysts,
increasing the ENB concentration can largely improve catalytic
activity, ENB incorporation and MW. If more AliBu3 is used
initially, MWs of copolymers produced by catalyst 2c decrease
substantially without too much change in catalytic activity and
ENB incorporation (run 3 and 10). However, the increase of Al/
Zr ratio causes a decrease in catalytic activity and an increase in
the incorporation and MW for 2c (run 2 and 6). Therefore, chain
Fig. 3 1H NMR spectra of ENB in CDCl3 and poly(E-co-ENB) sample in
o-C6D4Cl2 at 135 ꢀC (run 12 in Table 4).
transfer to aluminum alkyls is an important way of chain
transfer in the copolymerization by 2c, but aluminum alkyls do
Table 4 Ethylene/ENB copolymerization by 2a–da
ENB
Yield
(g)
Activity (kg
per molZr per h)
Run
Catalyst
Temp. (ꢀC)
(mol Lꢁ1
)
Incorp.c (mol%)
Mw (ꢃ103)
PDId
d
1
2
3
4
2a
2b
2c
2d
2b
2b
2b
2b
2c
2c
2c
2c
75
75
75
75
50
75
75
100
50
75
0.5
0.5
0.5
0.5
0.5
0.5
1.0
0.5
0.5
0.5
1.0
0.5
2.50
1.40
2.45
2.45
0.74
0.55
2.27
0.74
2.80
2.40
2.41
2.20
5000
2800
4900
4900
1480
1100
4540
1480
5600
4800
8033
4400
32.1
28.8
40.4
39.4
23.6
32.3
37.5
34.7
30.3
35.8
76.6
58.0
124.8
50.1
180.0
192.4
95.3
59.7
96.0
41.7
96.0
1.77
1.54
1.40
1.69
1.64
1.64
1.55
1.58
1.62
1.73
1.91
1.86
5
6b
7
8
9
10b
11e
12
17.4
347.0
134.0
75
100
a
b
Conditions: catalyst 3 mmol, AliBu3 1 mmol, [Ph3C][B(C6F5)4] 6 mmol, ethylene 1 atm, Vtotal ¼ 30 mL, reaction for 10 min. AliBu3 1.5 mmol.
c
Comonomer incorporation (mol%) established by 1H NMR spectra at 135 ꢀC with o-C6D4Cl2 as a solvent (run 1, 3, 4 and 9–12 in Table 5),
while that of the others (run 2, 5–8) established by 1H NMR spectra at 25 ꢀC with CDCl3 as a solvent, referred to the literatures.12,13,17
d
e
Determined by GPC at 150 ꢀC in 1,2,4-C6Cl3H3 vs. narrow polystyrene standards. Reaction for 6 min.
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Table 5 Ethylene/VNB copolymerization by 2a–da
VNB
Yield
(g)
Activity (kg
per molZr per h)
Run
Catalyst
Temp. (ꢀC)
(mol Lꢁ1
)
Incorp.c (mol%)
Mw (ꢃ103)
PDId
d
1
2
3
4
2a
2b
2c
2d
2b
2b
2b
2b
2c
2c
2c
2c
75
75
75
75
50
75
75
100
50
75
0.5
0.5
0.5
0.5
0.5
1.0
0.5
0.5
0.5
1.0
0.5
0.5
2.30
0.63
2.24
2.08
0.69
0.28
0.41
0.19
2.50
3.03
2.10
0.54
4600
1260
4480
4160
1380
560
820
380
5000
6060
4200
108
28.2
18.6
34.7
37.8
13.4
21.2
18.0
24.3
30.3
40.8
23.6
40.6
40.0
37.4
43.0
46.7
26.7
88.4
16.9
18.0
48.9
155.0
72.0
48.6
1.75
1.77
2.00
1.65
1.65
1.69
1.56
1.60
2.16
1.79
1.92
1.63
5
6
7b
8
9
10
11b
12
75
100
a
b
Conditions: catalyst 3 mmol, AliBu3 1 mmol, [Ph3C][B(C6F5)4] 6 mmol, ethylene 1 atm, Vtotal ¼ 30 mL, reaction for 10 min. AliBu3 1.5 mmol.
c
Established by 1H NMR spectra at 25 ꢀC with CDCl3 as a solvent except for run 10, which was established by 1H NMR spectra at 135 ꢀC with
d
o-C6D4Cl2 as a solvent, referred to the literatures.12,13,17 Determined by GPC at 150 ꢀC in C6Cl3H3 vs. narrow polystyrene standards.
copolymers (PDI
conditions.
< 2.0) and good solubility in certain
3.5 Ethylene/VNB copolymerization
Ethylene undergoes facile copolymerization with VNB in
toluene under different conditions. The typical results of
ethylene/VNB copolymerization catalyzed by 2a–d/Al(iBu3)/
[Ph3C][B(C6F5)4] systems under different conditions are
summarized in Table 5. Complex 2a with methyl substitute on P
atom shows the highest catalytic activity (run 1, Table 5).
However, introducing the tert-butyl group on P atom (2b) seri-
ously decreased the activity under the same conditions (run 2).
Similarly, catalysts with more electron-withdrawing group on P
moiety exhibited higher VNB incorporation capacities.
Therefore, we chose catalyst 2b and 2c to examine the
inuence of different conditions on copolymerization. At
elevated temperatures, the catalytic activities of 2b and 2c will
Fig. 4 1H NMR spectra of VNB in CDCl3 and a sample of ethylene/VNB
copolymer in CDCl3 (run 8, 24.3% in Table 5).
ꢀ
decrease, especially when the reaction temperature is 100 C.
However, higher VNB incorporations are achieved because of
lower solubility of ethylene in toluene at higher temperature.
Notably, the MW of copolymer produced by 2b at 75 ꢀC is
higher than that obtained at 50 ꢀC (run 2 andꢀ9 in Table 5) and
the MW of copolymer afforded by 2c at 100 C is higher than
that obtained at 75 ꢀC (run 3 and 12, Table 5). This result is the
same as ethylene/VNB copolymers. For both catalysts, the
increase in VNB concentration in feed from 0.5 M to 1.0 M can
cause an increase in catalytic activity, MW and incorporation
(run 2 vs. 6 and 3 vs. 10). Nonetheless, a slightly degree of
increment in VNB incorporation is found, compared with
ethylene/ENB and ethylene/NBE copolymerization. Mean-
while, monomer incorporation and the MW of the E/VNB
copolymers were both much lower than those of E/ENB or E/
NBE copolymers. Similar results were found in our previous
reports.11 It may be ascribed to the different exocyclic double
bond in VNB. Perhaps, the vinyl double bond in VNB can
interact with the active species remotely, resulting in smaller
incorporation space and easier chain transfer reaction. These
results strongly indicated that the minor difference in
structures of catalysts and comonomers can both affect the
polymerization behaviors.
The increase in Al/Zr ratio for catalyst 2b gave rise to
a decrease both in catalytic activity and MW of the copolymer
(run 2 and 7, Table 5). Thus, chain transfer to aluminum alkyls
was serious in ethylene/VNB copolymerization for catalyst 2b.
As for 2c, more AliBu3 in feed will result in higher MW and lower
comonomer incorporation with unobviously change in the
catalytic activity (run 3 and 11, Table 5). The 1H NMR spectra of
VNB and typical E/VNB copolymers are shown in the Fig. 4.
Obviously, only encyclo double bond can be region-selectively
incorporated into the polymer chains. All the copolymers
produced by catalysts 2b and 2c can be dissolved in THF and
CHCl3 at room temperature. All the aforementioned results
suggest that cross-linking can be ignored and this phosphine-
phenolate-based half-zirconocenes showed high regiose-
lectivity to VNB.
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11 J. Y. Liu, S. R. Liu, L. Pan and Y. S. Li, Adv. Synth. Catal., 2009,
351, 1505–1511.
4. Conclusions
12 H. Lasarov and T. T. Pakkanen, Macromol. Chem. Phys., 2000,
201, 1780–1786.
13 S. Marathe and S. Sivaram, Macromolecules, 1994, 27, 1083–
1086.
The phosphine-phenolate-based half-zirconocenes 2a–d with
different substitutes on phosphorus atom show moderate
activities in ethylene polymerization, affording polymers with
low molecular weights. However, they are good candidates for
ethylene/ENB, ethylene/VNB and ethylene/NBE copolymeriza-
tion. The titanium analogues would be poisoned partly by the
extra vinyl and ethylidene double bond in VNB and ENB
respectively, resulting in moderate activity about 5 ꢃ 105 kg per
molTi per h. Steric hindrance of the ligands in catalysts plays
a more important role in catalytic activity, and the catalytic
activity decreases in the order: 2a (methyl) > 2c (phenyl) ꢂ 2d (4-
F-phenyl) [ 2b (tert-butyl). Electron effect of the substitute on
the phosphorus atom play a decisive role in comonomer
incorporation capability, thus the catalysts bearing more elec-
tron withdrawing ligands showed higher comonomer incorpo-
ration ability and produce copolymers with higher molecular
weights. The three kinds of comonomers NBE, ENB and VNB
possess different steric hindrance and reactivity, resulting in
different copolymerization behaviors under same conditions.
The obtained copolymers with pendent double bonds are
amenable to post-functionalization through epoxidation reac-
tions or thio–ene click reaction to introduce polar groups into
the polyolens. As far as we know, this phosphine-phenolate-
based half-zirconocenes are the rare examples among half-
metallocenes which can catalyze ethylene/cyclic-olen copoly-
merization with high efficiency and region-selectivity.
14 Y. Y. Long, Y. X. Wang, J. Y. Liu, X. F. Li and Y. S. Li, J. Polym.
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