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Well-defined Phosphino-phenolate Neutral Nickel(II) Catalysts for Efficient
Yan-Ping Zhang,1,2 Wei-Wei Li,1,2 Bai-Xiang Li,1 Hong-Liang Mu,*,1 Yue-Sheng Li*,1,3
1
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China 2
University of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
3
School of Material Science and Engineering, Tianjin University, Tianjin 300072, China
*To whom correspondence should be addressed: [email protected], [email protected]
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(Co)polymerization of Norbornene and Ethylene
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Abstract. Phosphino-phenolate neutral nickel catalysts 1-3/B(C6F5)3, without the help of any organoaluminum compound, were found to be efficient catalytic systems for norbornene polymerization and it’s copolymerization with norbornene derivatives. The amount of B(C6F5)3
high molecular weight polymers were obtained (> 106 g/mol). Efficient incorporation of polar monomers NBC, NBA, NBM were also achieved in a controllable fashion, giving high molecular weight copolymers. Catalysts 1-3 were highly active for ethylene polymerization as single component catalysts, with an activity up to 107 g/molNi·h, and catalyst 3 was more readily initiated under lower temperature. Catalysts 1-3 were also efficient to incorporate norbornene (up to 30%) into polyethylene backbone. Bisligated phosphino-phenolate nickel complex 4 and salicylaldimine complex 5 were also studied for comparison, which further verified the unique performance of catalysts 1-3. Preliminary NMR analyses were conducted to explore the norbornene polymerization mechanism. Key words: Ziegler-Natta polymerization, ethylene polymerization, addition polymerization, neutral nickel catalysts
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required to achieve high efficiency (3 equiv) was markedly lowered relative to previous reports, and
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INTRODUCTION It is well known that norbornene and its derivatives can be polymerized via three different routes,
polymerization (addition polymerization).1-5 These vinyl-type polymers exhibit excellent properties such as high glass-transition temperatures (Tg) and decomposition temperatures (Td), low dielectric constant, high chemical resistance, high refractive index, and low birefringence.1, 6-9 Early transition metal catalysts (especially zirconocene type catalysts) were reported to produce stereoregular crystalline polynorbornene (PNB) with high melting temperature (Tm) (close to Td), leading to poor processibility and poor solubility in common organic solvent.10-12 Compared with the PNB produced by early transition metal catalysts, those yielded by late metal catalysts showed improved solubility, although the Tg values were still very high. On the other hand, the excellent tolerance of late metal catalysts toward polar functional groups also attracted the attention from both academic and industrial community.13-14 In this respect, nickel catalysts became a promising candidate after experiencing the resurgence due to the work of Brookhart et al.15-17 and Grubbs et al.18-19 In combination with MAO or MMAO, nickel halides bearing various [N,N],[N,P] or [N,N,X] (X = N, O, P, S) ligands8, 20-32 were reported to be active for norbornene polymerization. Neutral nickel
catalysts
supported
salicylaldiminato ligands9,
35-38
by
fluorinated
β-diketiminato,33-34
imino-pyrrolyto5
and
also showed high activies (106 to 107 g PNB/molNi·h) for
norbornene polymerization when activated by MAO or MMAO. While Pd(II)-systems have been applied to copolymerize norbornene and its derivatives since 1990s,39-44 similar applications using nickel(II) complexes were not abundant. Wu and He’s group both applied bisligand Ni(II) complexes to achieve this purpose.45-49 In the presence of AlMe3/B(C6F5)3, complex (Cp)Ni(Cl)(PPh3) copolymerized norbornene with methyl 5-norbornene-2-carboxylate (NBC) to give high yields of copolymers with variable contents of comonomer units (17.4–60.7 mol%).50 Some groups also worked on the copolymerization of NB with 1-alkenes used nickel(II) complexes.24,
49, 51-52
α-Iminocarboxamidato neutral nickel catalysts reported by Bazan et al.
showed quasi-living characteristics with the help of Ni(COD)2 for both ethylene polymerization and the copolymerization of ethylene with 5-norbornen-2-yl acetate (NBA), which was taken
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ring-opening metathesis polymerization (ROMP), cationic or radical polymerization, and vinyl
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advantage of for the manufacture of several kinds of block copolymers.53-57 Nevertheless, most of these aforementioned metal catalysts, both early and late metal ones, required the activation of large amount of aluminium compounds, the residue of which proved to be
aluminum compounds were thus highly desirable. Thanks to its unique property, lewis acidic B(C6F5)3 has been widely used as an activator in ethylene polymerization,58-65 and thus also attractive in norbornene polymerizations. Janiak’s group reported that nickel(II) complexes bearing bidentate phosphane ligands showed activity of 103-104 g PNB/molNi·h when activated with B(C6F5)3/TEA, which was similar to that of the MAO promoted systems66 and it was possible to activate some of these complexes with B(C6F5)3 alone.67 Jang’s group also investigated the effect of B(C6F5)3 on the norbornene polymerization, but the Ni(II) complexes combined with B(C6F5)3 alone did not result in any activity regardless of the structures of the complexes.51, 68 Recently, neutral nickel catalysts based on β-ketiminato ligands were reported to be active for norbornene polymerization in the presence of B(C6F5)3 as a singular cocatalyst, however, the amount of B(C6F5)3 required was large.4, 69 As an ongoing research in our group, we are interested in the property of [P,O] chelate neutral nickel catalysts for the (co)polymerization of norbornene. While SHOP type catalysts have been rigorously studied for ethylene oligomerization or polymerization as in situ catalysts,51,
70-71
well-defined catalysts based on phosphino-phenolate ligands were rarely reported,72-75 especially their
activity
towards
norbornene
and
its
derivatives.
In
this
report,
well-defined
phosphino-phenolate Ni(Ⅱ)-Me pyridine complexes 1-3 (Scheme 1) were examined in combination with low dosage of B(C6F5)3 in norbornene (co)polymerization, yielding high molecular weight PNB
or
functionalized
PNB.
The
ethylene
polymerization
and
ethylene/norbornene
copolymerization were also carried out using these catalysts. Bisligated [P,O] complex 4 and salicylaldimine Ni(II)-Me pyridine complex 5 were studied here for comparison.
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detrimental to the resultant polymer materials, catalytic systems that do not need the help of
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R1
t
Me
Py
Bu
O Ni P R3 R2
O P
Ph
1: R1 = tBu, R2 = R3 = Ph 2: R1 = C6F5, R2= R3 = Ph 3: R1 = tBu, R2 = tBu, R3 = Ph
Me Ni
P O
Ni Ph
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Ph
Ph
t
O t
N
Bu
Bu
4
5
Scheme 1. Nickel catalysts used in this report for norbornene (co)polymerizations
RESULT AND DISCUSSION Synthesis and Characterization of the Complexes. The phosphine-phenolate ligands and complexes 1, 3 were synthesized according to literature method.76 We synthesized complexes 2, 4 and 5 by a route shown in Scheme 2. Bisligated nickel complex was routinely prepared by the ready reaction of free ligand with dihalide nickel complexes, such as (DME)NiBr2.77 However, it was interesting to note that the reaction of nickel source (temda)Ni(CH3)2 with free ligand 2-diphenylphosphanyl-6-tert-butyl-phenol followed by the addition of pyridine afforded high yield of bisligated complex 4 rather than the expected nickel-methyl pyridine complex 1. The yield of this reaction was significantly influenced by the reaction conditions, such as the overall reaction time, the adding order of the reactants, and reaction temperature. Py OH C6F5
PPh2
OH t
Bu
PPh2
Py2Ni(CH3)2
Ni
O C6F5
Me
PPh2
2
Ph2 P
1). (temda)Ni(CH3)2
Ni
2). Py
O
4
2
t
Bu
t
OH N Ar t
Bu
Bu
Py2Ni(CH3)2 O Ar = 2,6-iPrC6H3
Py 5
Ni N Ar
Me
Scheme 2. Synthesis of Nickel Complexes 2, 4 and 5.
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Py
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Single crystals suitable for X-ray diffraction analysis were grown from concentrated toluene/hexane solution of the complexes (Table S1). Similar to the salicylaldimine neutral nickel catalysts reported previously78-79 and in this work (complex 5), the phosphino-phenolate nickel complexes exhibited a
phosphanyl moiety. The nickel-pyridine bond length in complex 1 was slightly higher than that in complex 2 (Ni1-N1, 1.951(3) Å vs. 1.945(3) Å). Unfortunately, multiple attempts of long time crystallization led to the deactivation of complex 3, giving hydroxy nickel complex 3-OH or bisligated complex 3-Bis. This may result from the reaction of Ni-methyl with trace amount of water in the solvent, indicating a less stable nature of complex 3 relative to complexes 1, 2 and 5.
Figure 1. ORTEP plot of 1 (left) and 2 (right). Ellipsoids are shown with 30% probability. Hydrogen atoms are omitted for clarity.
Figure 1. ORTEP plot of bisligated complex 4. Ellipsoids are shown with 30% probability. Hydrogen atoms are omitted for clarity.
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square planar geometry around the metal center, with the pyridine ligand positioned trans to the
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Figure 3. ORTEP plot of complex 5. Ellipsoids are shown with 30% probability. Hydrogen atoms are omitted for clarity.
Figure 4. ORTEP plot of 3-OH (left) and 3-Bis (right). Ellipsoids are shown with 30% probability. Hydrogen atoms (except the Ni-OH hydrogen) are omitted for clarity.
Homopolymerization of NB In the presence of B(C6F5)3, phosphine-phenolate Ni(Ⅱ)-Me pyridine complexes 1-3 were first investigated for NB polymerization in toluene under mild conditions (Table 1). In contrast to previous reports, these catalysts showed higher activity at lower B/Ni molar ratio (B/Ni = 3). These catalysts displayed very high activity, with catalyst 1 converted 89% of the monomers in 3 min. Compared to the electron-withdrawing complex 2 (90.5% conversion), the electron-rich analogue 1 showed a little higher monomer conversion (98.6%) and catalytic activity (4.59 × 106 g PNB/molNi⋅h)
in 5 min trials (entry 1 vs. entry 2). To replace a phenyl group in complex 1 by
tert-butyl group (complex 3) resulted in a decrease in activity (entry 4 vs. entry 1). The bisligand complex 4 bearing the same ligand as complex 1 showed a much lower catalytic efficiency (32.2% conversion, entry 5), which may originate from the less ready formation of active species due to the more stable bisligated structure. With that said, complex 4 was still far more active than [N,O] or
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other bisligated complexes in former reports.47, 69 Under the same conditions, salicylaldimine nickel complex 5 was also investigated, and only 50.9% monomer conversion was observed (entry 7), which was much lower than phosphino-phenolate complexes 1-3, although elongating the reaction
(entries 8 and 9). All these produced homopolynorbornenes by [P,O] chelate complexes 1-4 displayed high molecular weight (Mw ~1220 kg/mol), narrow PDI (1.7-1.8) and high Tg values (336-347 oC). These Tg values were all determined by modulated differential scanning calorimetry (MDSC) rather than conventional DSC considering the presence of serious enthalpy relaxation. The produced homopolymers are soluble at room temperature in common organic solvent such as chloroform, 1,2,4-trichlorobenzene, indicating a low stereoregularity. The results of FT-IR and 1H NMR analyses supported a typical vinylic polymerization mechanism. Tabel 1. Selected results of NB polymerization with neutral nickel complexes 1-5a Entry
Complex
Polymer (g)
Conversion (%)
Activityb
Mwc (kg/mol)
Mw/Mnc
1 0.765 98.6 4.59 1201 1 e 2 0.695 89.0 6.95 1251 1 3 0.707 90.5 4.24 1220 2 4 0.682 87.3 4.09 1162 3 5 0.252 32.2 1.51 854 4 f 0.455 58.3 2.73 1097 6 4 7 0.395 50.9 2.37 1475 5 8g 0.482 62.1 2.89 1395 5 9h 0.585 75.4 3.51 1415 5 a Raction conditions: 20 mL of toluene, under 60 oC, 2 µmol of catalyst, C[Norbornene]
Tgd (oC)
1.8 340 1.7 347 1.8 338 1.8 339 1.6 337 1.7 336 1.6 329 1.6 344 1.6 354 is 0.416 mol/L, 3
equiv of B(C6F5)3, reaction for 5 minutes. b In the unit of 106 g polymer/molNi⋅h. c Determined by GPC versus PS standards. d Determined by MDSC. e Reaction for 3 min. f B/Ni = 5. g Reaction for 10 min. hB/Ni = 6.
Complex 1 was selected as a model catalyst to examine the effect of reaction conditions. B/Ni ratio was found to be the most important factor that affected the catalytic efficiency (Table 2). With the B/Ni ratio increased from 1 to 3, the norbornene conversion in 5 min dramatically improved from 18.6% to 98.6%. Even when the molar ratio of B/Ni is 1, the activity can be still up to 0.88 × 106 g PNB/molNi⋅h, albeit no polymer was obtained by single component complex 1. It should be noted that large amount of B(C6F5)3 was needed to achieve high activity for the previously reported nickel catalysts4, 69. Despite the great influence on the activity of the catalysts by the amount of B(C6F5)3, the MWs of the resultant PNBs were virtually identical, indicating the same active species generated in these cases, which was in contrast with that reported by Chen et al.4
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time or elavating the B(C6F5)3/Ni ratio would improve the monomer conversion to some extent
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Table 2. Influence of the molar ratios of B/Ni on norbornene polymerization by complex 1a B/Ni
Polymer (g)
Conversion (%)
Activityb
Mwc (kg/mol)
Mw/Mnc
Tgd (oC)
1 1 0.146 18.6 0.88 1248 1.6 345 2 2 0.524 66.8 3.14 1294 1.8 339 3 3 0.765 98.6 4.59 1201 1.8 340 4 5 0.773 99.0 4.64 1269 1.8 343 a Raction conditions: 20 mL of toluene, under 60 oC, reaction for 5 min, 8.3 mmol of NB, 2 µmol of catalyst. b
in the unit of 106 g polymer/molNi⋅h. c Determined by MDSC. d Determined by GPC versus PS standards.
Variation of the reaction volume means changes in the concentration of NB, catalyst and B(C6F5)3, affecting the monomer conversion, MWs and catalytic activities (Table 3). As expected, a high conversion (98.6%) was observed when the total reaction volume was 20 ml, which monotonically decreased with the increasing amount of toluene. Table 3. Influence of the reaction volume on polymerization by complex 1a Entry
Volume (mL)
Polymer (g)
Conversion (%)
Activityb
Mwc (kg/mol)
Mw/Mnc
1 20 0.765 98.6 4.59 1201 1.8 2 30 0.663 85.0 3.98 1104 1.7 3 40 0.637 81.7 3.82 1048 1.6 a Raction conditions: polymerization temperature is 60 oC, reaction for 5 minutes, 8.3 mmol of NB, 2 µmol of catalyst, 3 equiv of B(C6F5)3. standards.
b
in the unit of 106 g polymer/molNi⋅h.
c
Determined by GPC versus PS
With the elevating of polymerization temperatures, the monomer conversion first increased from 20 to 40 oC, and reached its maximum value at 40 oC , then decreased from 60 to 80 oC, which may caused by the decomposition of the active species under higher temperatures. The Mw of these polymers decreased from 1541 to 1005 kg/mol with the temperature elevated to 80 oC (Figure 5), indicating a more prominent chain transfer reactions under these conditions.
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Entry
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1600
110
1500 100
80
1200
70
1100 1000
60 20
30
40
50
60
70
80
Temperature ( C) o
Figure 5. Plot of monomer conversion (■) and polymer Mw(○) vs polymerization temperature. 2 µmol of complex 1, 0.781 g of NB feed, B/Ni = 3, Vtol is 20 ml, polymerization time is 5 min.
Copolymerization of NB with Its Polar Derivatives Polar norbornene derivatives methyl 5-norbornene-2-carboxylate (NBC), 5-norbornene-2-yl acetate (NBA) and 5-norbornene-2-methanol (NBM) were used as comonomers for the copolymerization catalyzed by Ni(Ⅱ)-Me pyridine complexes 1-3 (Table 4-6). Salicylaldimine nickel complex 5 was also studied for comparison. Similar to NB homopolymerization, B(C6F5)3/1-3 ratios in this work were again lower than that in previous reports.4,
69
For the NB/NBC copolymerization, good
monomer conversions (87.0%) and high catalytic activities (1.02 × 106 g polymer/molNi⋅h, entry 4) was achieved by complex 2 when the NBC:NB ratio in feed was 1:9. Complex 2 bearing a -C6F5 group in the ortho-phenoxy position showed higher activity than the analogous complex 1 under the same reaction conditions (see entries 1-6), indicating that electron-withdrawing groups may promote the copolymerization process. Complex 3 bearing a tert-butyl group at the phosphorus atom showed the lowest activity among catalysts 1-3, suggesting a less favorable effect of electron-donating groups. In case of [N,O] chelate complex 5, the monomer conversion was significantly lower than the aforementioned complexes 1-3, with similar (slightly lower) NBC incorporations (entries 8-10). With the increasing of NBC/NB ratio in feed, the catalytic activities of these complexes decreased. For example, the activity of complex 2 decreased from 1.02 to 0.4 × 106 g polymer/molNi⋅h with the content of NBC in feed changing from 10 to 50 mol% (entry 4 vs. entry 6). In fact, the homopolymerization of NBC was also achieved by complex 2, in which the monomer conversion was only 2.1% and the activity sharply decreased to 0.02 × 106 g
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Conversion (%)
1300
Mw (kg/mol)
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1400 90
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polymer/molNi⋅h (entry 7).
Entry
Cat.
NBC (mol%)
Polymer (g)
Yield (%)
Activityb
Incorp. (mol%)
Tgc (oC)
Mwd (kg/mol)
Mw/Mnd
1
1
10
0.816
83.4
0.98
9.7
324
417
1.8
2
1
30
0.462
42.1
0.55
24.5
313
262
1.8
3
1
50
0.200
16.3
0.24
34.3
299
156
1.7
4
2
10
0.851
87.0
1.02
9.6
335
361
1.8
5
2
30
0.578
52.6
0.69
24.6
297
242
1.7
6
2
50
0.334
27.2
0.40
37.6
294
185
1.6
7
2
100
0.016
2.1
0.02
100
261
--
--
8
3
10
0.788
80.6
0.95
9.1
327
409
1.8
9
3
30
0.333
30.4
0.40
22.1
299
253
1.7
10
3
50
0.162
13.2
0.20
36.9
290
165
1.8
a
0.78 11 5 10 0.653 66.7 8.4 327 583 1.9 Copolymerization conditions: 20 mL of toluene, under 60 oC, reaction for 10 min, 5 µmol of catalyst, B/Ni =
4, C(NB + NBC) = 0.50 mol/L. bin the unit of 106 g polymer/molNi⋅h. cDetermined by MDSC. dDetermined by GPC versus PS standards.
The MW of the copolymers obtained by complexes 1-3 /B(C6F5)3 systems are over 105 kg/mol and decreased with the increasing content of NBC in the copolymer. Moreover, the PDIs of the obtained polymers are all relatively narrow (1.6 to 1.8) and appear as a single modal in the GPC spectrums (Figure S2), which indicated that the copolymerization occurs at single active sites and the products are true copolymers rather than blends of the homopolymers. A clear absorption signals of carbonyl group (C=O) at 1738 cm-1 in FTIR spectrum signified the incorporation of NBC comonomers (Figure 7). That no absorptions observed in the vibration bands of carbon-carbon double bonds indicated a vinyl-type NB/NBC copolymerization. The 1H NMR data of the copolymers containing different contents of NBC were collected in deuterated chloroform (CDCl3) at room temperature (Figure 6). The absence of any trace amount of multiple signals in 5.0-6.0 ppm also confirmed a viny type copolymerization. The incorporation of NBC in copolymer was calculated through the signal intensity of -COOCH3 protons at 3.66 ppm. With the molar ratio of NBC in the feed increasing from 10 to 50 mol%, the NBC incorporation raised up to 37.6% (entries 4-6, Table 4). By controlling the monomer conversion (
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This article can be cited before page numbers have been issued, to do this please use: Y. Zhang, W. Li, B. Li, H. Mu and Y. Li, Dalton Trans., 2015, DOI: 10.1039/C5DT00074B.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Well-defined Phosphino-phenolate Neutral Nickel(II) Catalysts for Efficient
Yan-Ping Zhang,1,2 Wei-Wei Li,1,2 Bai-Xiang Li,1 Hong-Liang Mu,*,1 Yue-Sheng Li*,1,3
1
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China 2
University of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
3
School of Material Science and Engineering, Tianjin University, Tianjin 300072, China
*To whom correspondence should be addressed: [email protected], [email protected]
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(Co)polymerization of Norbornene and Ethylene
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Abstract. Phosphino-phenolate neutral nickel catalysts 1-3/B(C6F5)3, without the help of any organoaluminum compound, were found to be efficient catalytic systems for norbornene polymerization and it’s copolymerization with norbornene derivatives. The amount of B(C6F5)3
high molecular weight polymers were obtained (> 106 g/mol). Efficient incorporation of polar monomers NBC, NBA, NBM were also achieved in a controllable fashion, giving high molecular weight copolymers. Catalysts 1-3 were highly active for ethylene polymerization as single component catalysts, with an activity up to 107 g/molNi·h, and catalyst 3 was more readily initiated under lower temperature. Catalysts 1-3 were also efficient to incorporate norbornene (up to 30%) into polyethylene backbone. Bisligated phosphino-phenolate nickel complex 4 and salicylaldimine complex 5 were also studied for comparison, which further verified the unique performance of catalysts 1-3. Preliminary NMR analyses were conducted to explore the norbornene polymerization mechanism. Key words: Ziegler-Natta polymerization, ethylene polymerization, addition polymerization, neutral nickel catalysts
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required to achieve high efficiency (3 equiv) was markedly lowered relative to previous reports, and
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INTRODUCTION It is well known that norbornene and its derivatives can be polymerized via three different routes,
polymerization (addition polymerization).1-5 These vinyl-type polymers exhibit excellent properties such as high glass-transition temperatures (Tg) and decomposition temperatures (Td), low dielectric constant, high chemical resistance, high refractive index, and low birefringence.1, 6-9 Early transition metal catalysts (especially zirconocene type catalysts) were reported to produce stereoregular crystalline polynorbornene (PNB) with high melting temperature (Tm) (close to Td), leading to poor processibility and poor solubility in common organic solvent.10-12 Compared with the PNB produced by early transition metal catalysts, those yielded by late metal catalysts showed improved solubility, although the Tg values were still very high. On the other hand, the excellent tolerance of late metal catalysts toward polar functional groups also attracted the attention from both academic and industrial community.13-14 In this respect, nickel catalysts became a promising candidate after experiencing the resurgence due to the work of Brookhart et al.15-17 and Grubbs et al.18-19 In combination with MAO or MMAO, nickel halides bearing various [N,N],[N,P] or [N,N,X] (X = N, O, P, S) ligands8, 20-32 were reported to be active for norbornene polymerization. Neutral nickel
catalysts
supported
salicylaldiminato ligands9,
35-38
by
fluorinated
β-diketiminato,33-34
imino-pyrrolyto5
and
also showed high activies (106 to 107 g PNB/molNi·h) for
norbornene polymerization when activated by MAO or MMAO. While Pd(II)-systems have been applied to copolymerize norbornene and its derivatives since 1990s,39-44 similar applications using nickel(II) complexes were not abundant. Wu and He’s group both applied bisligand Ni(II) complexes to achieve this purpose.45-49 In the presence of AlMe3/B(C6F5)3, complex (Cp)Ni(Cl)(PPh3) copolymerized norbornene with methyl 5-norbornene-2-carboxylate (NBC) to give high yields of copolymers with variable contents of comonomer units (17.4–60.7 mol%).50 Some groups also worked on the copolymerization of NB with 1-alkenes used nickel(II) complexes.24,
49, 51-52
α-Iminocarboxamidato neutral nickel catalysts reported by Bazan et al.
showed quasi-living characteristics with the help of Ni(COD)2 for both ethylene polymerization and the copolymerization of ethylene with 5-norbornen-2-yl acetate (NBA), which was taken
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ring-opening metathesis polymerization (ROMP), cationic or radical polymerization, and vinyl
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advantage of for the manufacture of several kinds of block copolymers.53-57 Nevertheless, most of these aforementioned metal catalysts, both early and late metal ones, required the activation of large amount of aluminium compounds, the residue of which proved to be
aluminum compounds were thus highly desirable. Thanks to its unique property, lewis acidic B(C6F5)3 has been widely used as an activator in ethylene polymerization,58-65 and thus also attractive in norbornene polymerizations. Janiak’s group reported that nickel(II) complexes bearing bidentate phosphane ligands showed activity of 103-104 g PNB/molNi·h when activated with B(C6F5)3/TEA, which was similar to that of the MAO promoted systems66 and it was possible to activate some of these complexes with B(C6F5)3 alone.67 Jang’s group also investigated the effect of B(C6F5)3 on the norbornene polymerization, but the Ni(II) complexes combined with B(C6F5)3 alone did not result in any activity regardless of the structures of the complexes.51, 68 Recently, neutral nickel catalysts based on β-ketiminato ligands were reported to be active for norbornene polymerization in the presence of B(C6F5)3 as a singular cocatalyst, however, the amount of B(C6F5)3 required was large.4, 69 As an ongoing research in our group, we are interested in the property of [P,O] chelate neutral nickel catalysts for the (co)polymerization of norbornene. While SHOP type catalysts have been rigorously studied for ethylene oligomerization or polymerization as in situ catalysts,51,
70-71
well-defined catalysts based on phosphino-phenolate ligands were rarely reported,72-75 especially their
activity
towards
norbornene
and
its
derivatives.
In
this
report,
well-defined
phosphino-phenolate Ni(Ⅱ)-Me pyridine complexes 1-3 (Scheme 1) were examined in combination with low dosage of B(C6F5)3 in norbornene (co)polymerization, yielding high molecular weight PNB
or
functionalized
PNB.
The
ethylene
polymerization
and
ethylene/norbornene
copolymerization were also carried out using these catalysts. Bisligated [P,O] complex 4 and salicylaldimine Ni(II)-Me pyridine complex 5 were studied here for comparison.
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detrimental to the resultant polymer materials, catalytic systems that do not need the help of
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R1
t
Me
Py
Bu
O Ni P R3 R2
O P
Ph
1: R1 = tBu, R2 = R3 = Ph 2: R1 = C6F5, R2= R3 = Ph 3: R1 = tBu, R2 = tBu, R3 = Ph
Me Ni
P O
Ni Ph
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Ph
Ph
t
O t
N
Bu
Bu
4
5
Scheme 1. Nickel catalysts used in this report for norbornene (co)polymerizations
RESULT AND DISCUSSION Synthesis and Characterization of the Complexes. The phosphine-phenolate ligands and complexes 1, 3 were synthesized according to literature method.76 We synthesized complexes 2, 4 and 5 by a route shown in Scheme 2. Bisligated nickel complex was routinely prepared by the ready reaction of free ligand with dihalide nickel complexes, such as (DME)NiBr2.77 However, it was interesting to note that the reaction of nickel source (temda)Ni(CH3)2 with free ligand 2-diphenylphosphanyl-6-tert-butyl-phenol followed by the addition of pyridine afforded high yield of bisligated complex 4 rather than the expected nickel-methyl pyridine complex 1. The yield of this reaction was significantly influenced by the reaction conditions, such as the overall reaction time, the adding order of the reactants, and reaction temperature. Py OH C6F5
PPh2
OH t
Bu
PPh2
Py2Ni(CH3)2
Ni
O C6F5
Me
PPh2
2
Ph2 P
1). (temda)Ni(CH3)2
Ni
2). Py
O
4
2
t
Bu
t
OH N Ar t
Bu
Bu
Py2Ni(CH3)2 O Ar = 2,6-iPrC6H3
Py 5
Ni N Ar
Me
Scheme 2. Synthesis of Nickel Complexes 2, 4 and 5.
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Py
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Single crystals suitable for X-ray diffraction analysis were grown from concentrated toluene/hexane solution of the complexes (Table S1). Similar to the salicylaldimine neutral nickel catalysts reported previously78-79 and in this work (complex 5), the phosphino-phenolate nickel complexes exhibited a
phosphanyl moiety. The nickel-pyridine bond length in complex 1 was slightly higher than that in complex 2 (Ni1-N1, 1.951(3) Å vs. 1.945(3) Å). Unfortunately, multiple attempts of long time crystallization led to the deactivation of complex 3, giving hydroxy nickel complex 3-OH or bisligated complex 3-Bis. This may result from the reaction of Ni-methyl with trace amount of water in the solvent, indicating a less stable nature of complex 3 relative to complexes 1, 2 and 5.
Figure 1. ORTEP plot of 1 (left) and 2 (right). Ellipsoids are shown with 30% probability. Hydrogen atoms are omitted for clarity.
Figure 1. ORTEP plot of bisligated complex 4. Ellipsoids are shown with 30% probability. Hydrogen atoms are omitted for clarity.
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square planar geometry around the metal center, with the pyridine ligand positioned trans to the
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Figure 3. ORTEP plot of complex 5. Ellipsoids are shown with 30% probability. Hydrogen atoms are omitted for clarity.
Figure 4. ORTEP plot of 3-OH (left) and 3-Bis (right). Ellipsoids are shown with 30% probability. Hydrogen atoms (except the Ni-OH hydrogen) are omitted for clarity.
Homopolymerization of NB In the presence of B(C6F5)3, phosphine-phenolate Ni(Ⅱ)-Me pyridine complexes 1-3 were first investigated for NB polymerization in toluene under mild conditions (Table 1). In contrast to previous reports, these catalysts showed higher activity at lower B/Ni molar ratio (B/Ni = 3). These catalysts displayed very high activity, with catalyst 1 converted 89% of the monomers in 3 min. Compared to the electron-withdrawing complex 2 (90.5% conversion), the electron-rich analogue 1 showed a little higher monomer conversion (98.6%) and catalytic activity (4.59 × 106 g PNB/molNi⋅h)
in 5 min trials (entry 1 vs. entry 2). To replace a phenyl group in complex 1 by
tert-butyl group (complex 3) resulted in a decrease in activity (entry 4 vs. entry 1). The bisligand complex 4 bearing the same ligand as complex 1 showed a much lower catalytic efficiency (32.2% conversion, entry 5), which may originate from the less ready formation of active species due to the more stable bisligated structure. With that said, complex 4 was still far more active than [N,O] or
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other bisligated complexes in former reports.47, 69 Under the same conditions, salicylaldimine nickel complex 5 was also investigated, and only 50.9% monomer conversion was observed (entry 7), which was much lower than phosphino-phenolate complexes 1-3, although elongating the reaction
(entries 8 and 9). All these produced homopolynorbornenes by [P,O] chelate complexes 1-4 displayed high molecular weight (Mw ~1220 kg/mol), narrow PDI (1.7-1.8) and high Tg values (336-347 oC). These Tg values were all determined by modulated differential scanning calorimetry (MDSC) rather than conventional DSC considering the presence of serious enthalpy relaxation. The produced homopolymers are soluble at room temperature in common organic solvent such as chloroform, 1,2,4-trichlorobenzene, indicating a low stereoregularity. The results of FT-IR and 1H NMR analyses supported a typical vinylic polymerization mechanism. Tabel 1. Selected results of NB polymerization with neutral nickel complexes 1-5a Entry
Complex
Polymer (g)
Conversion (%)
Activityb
Mwc (kg/mol)
Mw/Mnc
1 0.765 98.6 4.59 1201 1 e 2 0.695 89.0 6.95 1251 1 3 0.707 90.5 4.24 1220 2 4 0.682 87.3 4.09 1162 3 5 0.252 32.2 1.51 854 4 f 0.455 58.3 2.73 1097 6 4 7 0.395 50.9 2.37 1475 5 8g 0.482 62.1 2.89 1395 5 9h 0.585 75.4 3.51 1415 5 a Raction conditions: 20 mL of toluene, under 60 oC, 2 µmol of catalyst, C[Norbornene]
Tgd (oC)
1.8 340 1.7 347 1.8 338 1.8 339 1.6 337 1.7 336 1.6 329 1.6 344 1.6 354 is 0.416 mol/L, 3
equiv of B(C6F5)3, reaction for 5 minutes. b In the unit of 106 g polymer/molNi⋅h. c Determined by GPC versus PS standards. d Determined by MDSC. e Reaction for 3 min. f B/Ni = 5. g Reaction for 10 min. hB/Ni = 6.
Complex 1 was selected as a model catalyst to examine the effect of reaction conditions. B/Ni ratio was found to be the most important factor that affected the catalytic efficiency (Table 2). With the B/Ni ratio increased from 1 to 3, the norbornene conversion in 5 min dramatically improved from 18.6% to 98.6%. Even when the molar ratio of B/Ni is 1, the activity can be still up to 0.88 × 106 g PNB/molNi⋅h, albeit no polymer was obtained by single component complex 1. It should be noted that large amount of B(C6F5)3 was needed to achieve high activity for the previously reported nickel catalysts4, 69. Despite the great influence on the activity of the catalysts by the amount of B(C6F5)3, the MWs of the resultant PNBs were virtually identical, indicating the same active species generated in these cases, which was in contrast with that reported by Chen et al.4
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time or elavating the B(C6F5)3/Ni ratio would improve the monomer conversion to some extent
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Table 2. Influence of the molar ratios of B/Ni on norbornene polymerization by complex 1a B/Ni
Polymer (g)
Conversion (%)
Activityb
Mwc (kg/mol)
Mw/Mnc
Tgd (oC)
1 1 0.146 18.6 0.88 1248 1.6 345 2 2 0.524 66.8 3.14 1294 1.8 339 3 3 0.765 98.6 4.59 1201 1.8 340 4 5 0.773 99.0 4.64 1269 1.8 343 a Raction conditions: 20 mL of toluene, under 60 oC, reaction for 5 min, 8.3 mmol of NB, 2 µmol of catalyst. b
in the unit of 106 g polymer/molNi⋅h. c Determined by MDSC. d Determined by GPC versus PS standards.
Variation of the reaction volume means changes in the concentration of NB, catalyst and B(C6F5)3, affecting the monomer conversion, MWs and catalytic activities (Table 3). As expected, a high conversion (98.6%) was observed when the total reaction volume was 20 ml, which monotonically decreased with the increasing amount of toluene. Table 3. Influence of the reaction volume on polymerization by complex 1a Entry
Volume (mL)
Polymer (g)
Conversion (%)
Activityb
Mwc (kg/mol)
Mw/Mnc
1 20 0.765 98.6 4.59 1201 1.8 2 30 0.663 85.0 3.98 1104 1.7 3 40 0.637 81.7 3.82 1048 1.6 a Raction conditions: polymerization temperature is 60 oC, reaction for 5 minutes, 8.3 mmol of NB, 2 µmol of catalyst, 3 equiv of B(C6F5)3. standards.
b
in the unit of 106 g polymer/molNi⋅h.
c
Determined by GPC versus PS
With the elevating of polymerization temperatures, the monomer conversion first increased from 20 to 40 oC, and reached its maximum value at 40 oC , then decreased from 60 to 80 oC, which may caused by the decomposition of the active species under higher temperatures. The Mw of these polymers decreased from 1541 to 1005 kg/mol with the temperature elevated to 80 oC (Figure 5), indicating a more prominent chain transfer reactions under these conditions.
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Entry
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1600
110
1500 100
80
1200
70
1100 1000
60 20
30
40
50
60
70
80
Temperature ( C) o
Figure 5. Plot of monomer conversion (■) and polymer Mw(○) vs polymerization temperature. 2 µmol of complex 1, 0.781 g of NB feed, B/Ni = 3, Vtol is 20 ml, polymerization time is 5 min.
Copolymerization of NB with Its Polar Derivatives Polar norbornene derivatives methyl 5-norbornene-2-carboxylate (NBC), 5-norbornene-2-yl acetate (NBA) and 5-norbornene-2-methanol (NBM) were used as comonomers for the copolymerization catalyzed by Ni(Ⅱ)-Me pyridine complexes 1-3 (Table 4-6). Salicylaldimine nickel complex 5 was also studied for comparison. Similar to NB homopolymerization, B(C6F5)3/1-3 ratios in this work were again lower than that in previous reports.4,
69
For the NB/NBC copolymerization, good
monomer conversions (87.0%) and high catalytic activities (1.02 × 106 g polymer/molNi⋅h, entry 4) was achieved by complex 2 when the NBC:NB ratio in feed was 1:9. Complex 2 bearing a -C6F5 group in the ortho-phenoxy position showed higher activity than the analogous complex 1 under the same reaction conditions (see entries 1-6), indicating that electron-withdrawing groups may promote the copolymerization process. Complex 3 bearing a tert-butyl group at the phosphorus atom showed the lowest activity among catalysts 1-3, suggesting a less favorable effect of electron-donating groups. In case of [N,O] chelate complex 5, the monomer conversion was significantly lower than the aforementioned complexes 1-3, with similar (slightly lower) NBC incorporations (entries 8-10). With the increasing of NBC/NB ratio in feed, the catalytic activities of these complexes decreased. For example, the activity of complex 2 decreased from 1.02 to 0.4 × 106 g polymer/molNi⋅h with the content of NBC in feed changing from 10 to 50 mol% (entry 4 vs. entry 6). In fact, the homopolymerization of NBC was also achieved by complex 2, in which the monomer conversion was only 2.1% and the activity sharply decreased to 0.02 × 106 g
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Conversion (%)
1300
Mw (kg/mol)
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1400 90
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polymer/molNi⋅h (entry 7).
Entry
Cat.
NBC (mol%)
Polymer (g)
Yield (%)
Activityb
Incorp. (mol%)
Tgc (oC)
Mwd (kg/mol)
Mw/Mnd
1
1
10
0.816
83.4
0.98
9.7
324
417
1.8
2
1
30
0.462
42.1
0.55
24.5
313
262
1.8
3
1
50
0.200
16.3
0.24
34.3
299
156
1.7
4
2
10
0.851
87.0
1.02
9.6
335
361
1.8
5
2
30
0.578
52.6
0.69
24.6
297
242
1.7
6
2
50
0.334
27.2
0.40
37.6
294
185
1.6
7
2
100
0.016
2.1
0.02
100
261
--
--
8
3
10
0.788
80.6
0.95
9.1
327
409
1.8
9
3
30
0.333
30.4
0.40
22.1
299
253
1.7
10
3
50
0.162
13.2
0.20
36.9
290
165
1.8
a
0.78 11 5 10 0.653 66.7 8.4 327 583 1.9 Copolymerization conditions: 20 mL of toluene, under 60 oC, reaction for 10 min, 5 µmol of catalyst, B/Ni =
4, C(NB + NBC) = 0.50 mol/L. bin the unit of 106 g polymer/molNi⋅h. cDetermined by MDSC. dDetermined by GPC versus PS standards.
The MW of the copolymers obtained by complexes 1-3 /B(C6F5)3 systems are over 105 kg/mol and decreased with the increasing content of NBC in the copolymer. Moreover, the PDIs of the obtained polymers are all relatively narrow (1.6 to 1.8) and appear as a single modal in the GPC spectrums (Figure S2), which indicated that the copolymerization occurs at single active sites and the products are true copolymers rather than blends of the homopolymers. A clear absorption signals of carbonyl group (C=O) at 1738 cm-1 in FTIR spectrum signified the incorporation of NBC comonomers (Figure 7). That no absorptions observed in the vibration bands of carbon-carbon double bonds indicated a vinyl-type NB/NBC copolymerization. The 1H NMR data of the copolymers containing different contents of NBC were collected in deuterated chloroform (CDCl3) at room temperature (Figure 6). The absence of any trace amount of multiple signals in 5.0-6.0 ppm also confirmed a viny type copolymerization. The incorporation of NBC in copolymer was calculated through the signal intensity of -COOCH3 protons at 3.66 ppm. With the molar ratio of NBC in the feed increasing from 10 to 50 mol%, the NBC incorporation raised up to 37.6% (entries 4-6, Table 4). By controlling the monomer conversion (
