The Effect of SiMe3 and SiEt3 Para Substituents for High Activity and Introduction of a Hydroxy Group in Ethylene Copolymerization Catalyzed by Phenoxide-Modified Half-Titanocenes

Article abstract of DOI:10.1002/anie.202010559

Remarkable effects of SiMe₃ and SiEt₃ para-substituents in the phenoxide-modified half-titanocenes, Cp*TiCl₂(O-2,6-ⁱPr₂-4-R-C₆H₂) [R=SiMe₃ (6), SiEt₃ (7)],

Full text of DOI:10.1002/anie.202010559

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Accepted Article
Title: Effect of SiMe3, SiEt3 Para-Substituents for Exhibiting
High Activity, Introduction of Hydroxy Group in Ethylene
Copolymerization Catalyzed by Phenoxide-Modified Half-
Titanocenes
Authors: Suphitchaya Kitphaitun, Qing Yan, and Kotohiro Nomura
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10.1002/anie.202010559
Angewandte Chemie International Edition
COMMUNICATION
Effect of SiMe3, SiEt3 Para-Substituents for Exhibiting High
Activity, Introduction of Hydroxy Group in Ethylene
Copolymerization Catalyzed by Phenoxide-Modified Half-
Titanocenes
Suphitchaya Kitphaitun,[a] Qing Yan,[a] and Kotohiro Nomura*[a]
[a]
S. Kitphaitun, Q. Yan, and Prof. Dr. K. Nomura*
Department of Chemistry, Graduate School of Science
Tokyo Metropolitan University
1-1 Minami Osawa, Hachioji, Tokyo 192-0376, Japan
E-mail: ktnomura@tmu.ac.jp
Supporting information for this article is given via a link at the end of the document.
Abstract: Remarkable effects of SiMe3, SiEt3 para-substituent in the
phenoxide-modified half-titanocenes, Cp*TiCl2(O-2,6-iPr2-4-R-C6H2)
[R = SiMe3 (6), SiEt3 (7)], toward the catalytic activities in ethylene
copolymerizations with 2-methyl-1-pentene, 1-decene, 1-dodecene
and with 9-decen-1-ol (DC-OH) have been demonstrated. The
activities by 6, 7 at 50 ºC showed higher than those conducted at 25
ºC in all cases in the presence of MAO cocatalyst. Efficient synthesis
of high molecular weight (HMW) ethylene copolymers incorporating
DC-OH (or 5-hexen-1-ol, HX-OH) has been attained in the
copolymerization by 7, which showed better DC-OH (HX-OH)
incorporation at 50 ºC to afford the HMW copolymers, poly(ethylene-
co-DC-OH)s, with high activities (activity 1.21–3.81×105 kg-
polymer/mol-Ti·h, Mn = 6.55-10.0×104, DC-OH 2.3-3.6 mol %).
Modified half-titanocenes have been focused in this study,
because the catalysts enable synthesis of ethylene copolymers
with disubstituted α-olefins,[12] branched α-olefins,[13] and with
cyclic olefins[4] by adopting the ligand modification.[4] A series of
the dichloride complexes containing various para-substituents on
the 2,6-diisopropylphenoxy ligand, Cp*TiCl2(O-2,6-iPr2-4-R-C6H2)
[R = Ph (3), CHPh2 (4), CPh3 (5), SiMe3 (6), SiEt3 (7)],[11] were
prepared by treating Cp*TiCl3 with the corresponding phenol in
Et2O in the presence of NEt3 [Scheme 1, details are shown in the
Supporting Information (SI)].[14] The prepared complexes were
identified by NMR spectra and elemental analysis; structures of
complexes 5-7 were determined by X-ray crystallography (Figure
1).[14] It turned out that the Ti‒O‒C bond angles in 6, 7 [172.5(3),
174.62(19)º for 6,7, respectively][14] are close to those by
complexes 1,[15] 2[16a] [173.0(3), 174.0(3)º for 1,2, respectively].[14-
17] It has been considered that a large Ti‒O‒C bond angle affects
the high activity due to increased stabilization of the active
species by O→Ti π-donation.[4c,15,16a] In contrast, the angle in 5 is
rather small [163.96(10)º]. The Ti‒O bond distances in 5-7
[1.7919(11), 1.787(4), 1.7965(19) Å, respectively] are rather long
compared to 1, 2 [1.772(3), 1.777(4) Å, respectively],[15,16]
whereas no apparent differences were observed in the other bond
distances and angles (Table S7-2, SI).[14]
Design of efficient molecular catalysts for precise olefin
polymerization has been a long-term interest in the field of
catalysis, organometallic chemistry, and of polymer chemistry.[1,2]
In particular, development of the molecular catalysts, which
enable synthesis of new copolymers (containing sterically
encumbered monomers or cyclic olefins that are not incorporated
by ordinary catalysts,[3,4] or by incorporation of polar
functionalities[5-10]), have been considered as the fascinating
goals. Efficient synthesis of high molecular weight ethylene
(propylene) copolymers containing hydroxy group has been the
attractive target in early transition metal catalysis,[7-9] because the
polymerization generally proceeds with low catalytic activities
affording (rather) low molecular weight polymers due to a strong
interaction of oxygen with the centered metal,[7-9] even though the
OH group in alken-1-ol was protected with trialkylsilyl group[7c,d] or
AliBu3 etc. in advance.[7-9] The direct ethylene copolymerization
using nickel, palladium catalysts generally afforded copolymers
with certain branching.[2,5,6] We herein communicate the efficient
synthesis of high molecular weight ethylene copolymers
containing hydroxy group by adopting Cp*TiCl2(O-2,6-iPr2-4-
SiEt3-C6H2) (7)‒MAO catalyst, through remarkable effects of
SiMe3, SiEt3 groups in the aryloxo para-substituent toward the
Scheme 1. Synthesis of Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = Ph (3), CHPh2 (4),
CPh3 (5), SiMe3 (6), SiEt3 (7)].
Ethylene polymerizations by 1-7 were conducted in toluene in
the presence of MAO [Ti 1.5×10-8 mmol (0.015 μmol), ethylene 4
atm, 25 ºC, 10 min, MAO 3.0 mmol, Table S2-1, SI].[18] It turned
out that the activity increased in the order: 5 (R = CPh3, activity
activity,
comonomer
incorporation
in
the
ethylene
copolymerizations with long chain α-olefins [1-dodecene (DD), 1-
decene (DC)], 2-methyl-1-pentene (2M1P), and with 9-decen-1-ol
(DC-OH).[11]
1
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10.1002/anie.202010559
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COMMUNICATION
whereas the activities were slightly decreased upon increasing
2M1P concentration charged (Table S3-2, SI).[18]
Scheme 2. Ethylene polymerization and the copolymerizations with 1-decene
(DC), 1-dodecene (DD), 2-methyl-1-pentene (2M1P) by Cp*TiCl2(O-2,6-iPr2-4-
R-C6H2)‒MAO catalysts.
Table 2 summarizes results in ethylene copolymerization with
DC in the presence of 9-decen-1-ol (DC-OH) by 1, 6, 7‒MAO
catalysts (Scheme 3), and the results conducted by the linked
half-titanocene, [Me2Si(C5Me4)(NtBu)]TiCl2 (8, constrained
geometry type),[1e,13] are also placed for comparison.[18] The
hydroxy group in DC-OH was pretreated with AliBu3 for the
protection according to the previous reports.[8,9,14]
Figure 1. Structures of Cp*TiCl2(O-2,6-iPr2-4-RC6H2) [R = CPh3 (5, top left),
SiMe3 (6, top right), SiEt3 (7, bottom). Thermal ellipsoids are drawn at 30%
probability level; H atoms are omitted for clarity.[14]
36800 kg-PE/mol-Ti·h) < 2 (tBu, 45600), 4 (CHPh2, 46000), 1 (H,
47400) <
3 (Ph, 50700) < 6 (SiMe3, 54300), 7 (SiEt3,
56800).[18]The activities at 25 ºC by 6 and 7 further increased upon
increasing the Al/Ti molar ratios [Ti 5.0×10-9 mmol (0.005 μmol),
activity = 131000, 157000 kg-PE/mol-Ti·h, for 6, 7 respectively,
Table S2-1], whereas the activity by 1 decreased under these
conditions. It was also revealed that the activities by 7 increased
at 60 ºC, whereas decrease in the activity was observed by 1.[18]
These results thus suggest that the SiMe3 (6) and the SiEt3 (7)
analogues exhibited better catalyst performances in the ethylene
polymerization at high temperature.[18] The resultant polymers by
7 possessed high molecular weights with unimodal molecular
weight distributions (Table S2-1).[18]
Selected results in the ethylene copolymerizations with 1-
decene (DC), 1-dodecene (DD), and with 2-methyl-1-pentene
(2M1P) are summarized in Table 1 (Scheme 2).[18] It was revealed
that complexes 6 and 7 showed the higher catalytic activities than
the others (runs 7,9 vs runs 1,3-6) in the ethylene/DD
copolymerization at 25 ºC. Note that the activities by 6 and 7
increased at 50 ºC, whereas decrease in the activity was
observed by 1; the similar trend was observed in the ethylene/DC
copolymerization (runs 12-17). The activities in the
copolymerization with DD by 3-5 were lower than that by 1,
whereas improvements in the activities by 2-4 were seen at 50 ºC
(Table S3-1, SI).[18] The resultant polymers were high molecular
weight copolymers with unimodal molecular weight distributions;
no apparent differences in the Mn values and the DD or DC
contents were seen in the copolymers prepared by 1 and 6, 7.[18,19]
As reported previously,[12] 1 afforded poly(ethylene-co-2M1P)s
with uniform compositions, whereas reported examples for
synthesis of ethylene copolymers containing disubstituted 1-
alkenes still have been limited.[3,4,12] It was revealed that the SiMe3
(6) and SiEt3 (7) analogues showed the higher activities than 1,
although the activity by 1 slightly increased at 50 ºC. Magnitude
in the increases in the activities by 6, 7 at 50 ºC were high
compared to 1, whereas no apparent differences in the Mn values
and the 2M1P contents (estimated by DSC thermograms) were
observed.[18,19] The activity by 7 further increased at 80 ºC,
Scheme 3. Ethylene copolymerization with 1-decene (DC) and 9-decen-1-ol
(DC-OH) [1-hexene (HX) and 5-hexen-1-ol (HX-OH)], protected with AliBu3 in
situ, by Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) [R = H (1), SiMe3 (6), SiEt3 (7)],
[Me2Si(C5Me4)(NtBu)]TiCl2 (8)‒MAO catalysts.
It should be noted that both 6 and 7 showed higher catalytic
activities than 1, and the activity increased at 50 ºC (runs 24-27,
29,30). The SiMe3 analogue (6) showed the notable increase in
the activity at 50 ºC (run 27), and the further increase at 80 ºC
was observed by 7 (Table S4-1, SI).[18] In contrast, the activity by
1 and 8 decreased at 50 ºC (runs 25, 40). Noteworthy, the activity
by 7 at 50 ºC increased upon increasing the DC-OH concentration
(run 31), whereas decreases in the activities were observed by 6
and 8 under the same conditions (runs 28,41); the polymer yield
by 8 became negligible. Importantly, apparent decrease in the Mn
value was not observed with increasing the DC-OH content (2.5
mol%, run 31). The increase in the activity by 7 was also observed
in the ethylene/1-hexene copolymerization upon co-presence of
5-hexen-1-ol (HX-OH) at 50 ºC (runs 35,36); the resultant
polymers also possessed high molecular weights irrespective of
the HX-OH content.[19]
2
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10.1002/anie.202010559
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Table 1. Ethylene Copolymerization with 1-Decene (DC), 1-Dodecene, and with 2-Methyl-1-pentene (2M1P) by Cp*TiCl2(O-2,6-iPr2-4-RC6H2) [R = H (1), tBu (2), Ph
(3), CHPh2 (4), CPh3 (5), SiMe3 (6), SiEt3 (7)]‒MAO Catalysts[a]
catalyst
comonomer
(M)[b]
ethylene
/ atm
temp.
/ ºC
yield
/ mg
activity
cont.[d]
/ mol%
[c]
run
Mn[c]×10-4
Mw/Mn
(μmol)
/ kg-polymer/mol-Ti·h
1
2
1 (0.0025)
1 (0.0025)
2 (0.001)
3 (0.005)
4 (0.005)
5 (0.005)
6 (0.001)
6 (0.001)
7 (0.001)
7 (0.001)
Cp2ZrCl2 (0.01)[e]
1 (0.005)
1 (0.005)
6 (0.0025)
6 (0.001)
7 (0.0025)
7 (0.001)
1 (0.05)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DD (0.75)
DC (0.88)
DC (0.88)
DC (0.88)
DC (0.88)
DC (0.88)
DC (0.88)
2M1P (1.35)
2M1P (1.35)
2M1P (1.35)
2M1P (1.35)
2M1P (1.35)
2M1P (1.35)
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
4
4
4
4
4
25
50
25
25
25
25
25
50
25
50
25
25
50
25
50
25
50
25
50
25
50
25
50
161
151
73.9
99.8
176
108
91.1
188
104
161
301
173
121
191
144
168
81.1
41.7
68.0
164
183
75.6
168
644000
604000
739000
200000
352000
216000
911000
1880000
1040000
1610000
1810000
346000
242000
764000
1440000
672000
811000
5000
18.9
16.5
18.4
16.7
16.7
16.9
15.0
15.3
16.7
16.2
56.2
19.7
16.0
18.4
16.2
15.4
15.0
10.7
7.21
6.69
5.29
5.91
6.19
1.79
1.85
1.60
1.59
1.57
1.52
1.62
1.54
1.62
1.60
1.99
1.61
1.73
1.67
1.74
1.55
1.57
1.64
1.54
1.62
1.60
1.55
1.58
17.6
3
18.3
4
5
6
7
8
16.3
15.3
16.7
2.0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
21.4
20.1
21.3
21.4
3.9
1 (0.025)
6 (0.05)
16300
19700
6 (0.025)
7 (0.05)
43900
3.3
2.9
9070
7 (0.025)
40300
[a] Conditions: toluene and 1-decene (DC) 1-dodecene (DD) or 2-methyl-1-pentene (2M1P) 5.0 mL total 30.0 mL, MAO 2.0 mmol for 6 min (DC, DD) or 3.0 mmol
10 min (2M1P). [b] Initial monomer concentration. [c] GPC data in o-dichlorobenzene vs polystyrene standards. [d] Comonomer content estimated by 13C NMR
spectra.[19] [e] Cited from reference 13.
Note that the ethylene copolymerization with DC-OH by 7‒MAO
catalyst proceeded with high activity (381000 kg-polymer/mol-
Ti·h) affording high molecular weight copolymer with unimodal
molecular weight distribution (Mn = 65500, Mw/Mn = 1.83), uniform
composition [confirmed by DSC thermograms as observed sole
melting temperature (Tm) at 94.1 ºC (run 32, Scheme 4, Figure
S6-2, SI)].[19] The activity was higher than that in the co-presence
of DC (run 31) without change in the DC-OH content and the Mn
value. Moreover, the high activity was preserved upon increasing
the DC-OH concentration; surprisingly, the Mn value did not
change with increasing the DC-OH content (run 33). The similar
trend was observed in the copolymerization with HX-OH,
affording high molecular weight copolymers with uniform
compositions (runs 37,38, Figures S6-3, SI).[19]
Scheme 4. Ethylene copolymerization with 9-decen-1-ol (DC-OH) by
Cp*TiCl2(O-2,6-iPr2-4-SiEt3-C6H2) (7)‒MAO catalyst.
3
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Table 2. Copolymerization of ethylene (E) with 1-Decene (DC) and 9-Decen-1-ol (DC-OH) by Cp*TiCl2(O-2,6-iPr2-4-RC6H2) [R = H (1), SiMe3 (6), SiEt3 (7)],
[Me2Si(C5Me4)(NtBu)]TiCl2 (8)‒MAO Catalysts[a]
[e]
Mn[c]×10-4
Mw/Mn
DC-OH[d]
/ mol%
DC[d]
Tm
[c]
catalyst
E
DC
DC-OH
(M)[b]
temp
/ ºC
yield
/ mg
activity
/ kg-polymer/mol-Ti·h
run
(μmol)
/ atm
(M)[b]
/ mol%
/ ºC
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
1 (0.025)
1 (0.01)
6 (0.025)
6 (0.01)
6 (0.01)
7 (0.025)
7 (0.01)
7 (0.01)
7 (0.01)
7 (0.01)
7 (0.01)
7 (0.01)
7 (0.01)
7 (0.01)
7 (0.01)
8 (0.025)
8 (0.01)
8 (0.01)
6
6
6
6
6
6
6
6
6
6
8
6
6
6
8
6
6
6
0.79
0.79
0.79
0.79
0.70
0.79
0.79
0.70
---
0.093
0.093
0.093
0.093
0.19
25
50
25
50
50
25
50
50
50
50
50
50
50
50
50
25
50
50
154
53.0
266
305
89.0
189
79.9
122
635
212
223
39.4
101
120
185
61.0
11.1
5.0
37000
31800
63800
183000
53300
45400
47900
73200
381000
127000
134000
23600
60600
72000
111000
14600
6660
13.8
7.29
1.83
1.81
0.8
1.2
15.2
17.8
14.6
15.0
13.7
15.2
16.7
14.1
--
13.0 (14.1)[f] 1.72 (1.81)[f]
1.2
7.58
7.10
11.7
6.82
6.70
6.55
7.16
10.0
5.32
6.80
10.1
10.5
8.97
--
1.68
1.67
1.81
1.70
1.69
1.83
1.84
1.99
1.51
1.69
2.01
2.08
1.79
--
1.5
1.8
0.093
0.093
0.19
1.3
1.5
2.5
0.19
2.6
94.1
93.2
97.6
---
0.28
3.6
--
---
0.28
2.3
--
1.13[g]
1.00[g]
---
0.14[g]
0.28[g]
0.28[g]
0.28[g]
0.093
0.093
0.19
0.3[h]
0.9[h]
2.4[h]
0.7[h]
0.9
16.0[h]
15.2[h]
--
110
115
---
--
0.79
0.79
0.70
14.0
3060
--
--
[a] Conditions: toluene and 1-decene (DC) 4.0 (0.70M) or 4.5 mL (0.79 M) and 9-decen-1-ol (DC-OH) 0.5 (0.093 M) or 1.0 mL (0.19 M) total 30.0 mL, ethylene 6
atm, MAO 3.0 mmol, AliBu3 3.0 (DC-OH 0.5 mL) or 6.0 mmol (DC-OH 1.0 mL), 10 min. [b] Initial monomer concentration. [c] GPC data in o-dichlorobenzene vs
polystyrene standards. [d] DC-OH, DC content (mol%) estimated by 1H NMR spectra.[19] [e] By DSC thermograms.[19] [f] GPC data in THF. [g] 1-Hexene and/or
5-hexen-1-ol were used. [h] 1-Hexene and/or 5-hexen-1-ol contents estimated by 1H NMR spectra.[19]
As demonstrated previously in poly(1-octene-co-7-octen-1-
ol)s,[20] the hydroxy group in the resultant polymer in
poly{ethylene-co-DC-co-(DC-OH)} was treated with AlEt3 and
subsequent addition of ε-caprolactone (CL) at 80 ºC gave the
amphiphilic graft copolymers, poly{ethylene-co-DC-co-(DC-OH)-
gr-poly(CL)}, via Al-alkoxide initiated ring-opening polymerization
(ROP, Scheme 5, detailed procedures are shown in Table S4-3,
SI).[14,18,19] The Mn values increased over time course consistent
with unimodal molecular weight distributions due to a living nature
in this ROP;[20] the resultant amphiphilic polymers were identified
by NMR spectra. Precise grafting by the living ROP has thus
been demonstrated.
Scheme 5. Synthesis of amphiphilic block/graft copolymers by post-
modification from the OH group (run 26) by Al-alkoxide initiated ring-opening
We have demonstrated efficient ethylene copolymerizations
with 2-methyl-1-pentene, 1-dodecene especially by phenoxide-
modified half-titanocenes containing SiMe3 (6) or SiEt3 (7) group
in the phenoxy para-substituent. In particular, these catalysts (6,
7) exhibited high activities for ethylene copolymerization with 1-
decene (DC) and 9-decen-1-ol (DC-OH), affording high molecular
weight copolymers containing hydroxy group with unimodal
molecular weight distributions. The activities by 6, 7 at 50 ºC
showed higher than those conducted at 25 ºC. In particular, 7
polymerization (ROP) of ε-caprolactone (CL). Both Mn and Mw/Mn data are GPC
data in THF versus polystyrene standards (Table S4-3, SI).
showed better DC-OH (HX-OH) incorporation at 50 ºC, and the
copolymerizations with DC-OH (HX-OH) afforded high molecular
weight copolymers with high catalytic activities. As far as we know,
this is the first successful demonstration of the efficient synthesis
of high molecular weight ethylene copolymers containing hydroxy
4
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P. T. Gomes, A. R. Dias, P. Foster, M. D. Rausch, J. C. W. Chien, J.
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Adv. 2015, 5, 98929–98933.
group by (at least) group 4 transition metal catalysts. We
speculate that an introduction of SiMe3, SiEt3 group could
contribute to a stabilization of the active species at this
moment.[11] Further studies concerning copolymerization with
various monomers containing polar functionalities (including the
scope and limitation) will be introduced in the near future.
[8]
[9]
a) J. Imuta, N. Kashiwa, J. Am. Chem. Soc. 2002, 124, 1176–1177; b) N.
Kashiwa, T. Matsugi, S. Kojoh, H. Kaneko, N. Kawahara, S. Matsuo, T.
Nobori, J. Imuta, J. Polym. Sci.: Part A: Polym. Chem. 2003, 41, 3657–
3666; c) Y. Inoue, T. Matsugi, N. Kashiwa, K. Matyjaszewski,
Macromolecules 2004, 37, 3651–3658.
Acknowledgements
a) H. Hagihara, M. Murata, T. Uozumi, Macromol. Rapid Commun. 2001,
22, 353–357; b) H. Hagihara, K. Tsuchihara, K. Takeuchi, M. Murata, H.
Ozaki, T. Shiono, J. Polym. Sci.: Part A: Polym. Chem. 2004, 42, 52–58;
c) H. Hagihara, K. Tsuchihara, J. Sugiyama, K. Takeuchi, T. Shiono,
Macromolecules 2004, 37, 5145-5148.
This project was partly supported by Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(JSPS, Grant No. 18H01982). The authors express their heartfelt
thanks to Mr. K. Preradović (University of Belgrade, Serbia) for
experimental assistances through IAESTE program, and to Tosoh
Finechem Co. for donating MAO.
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Keywords: titanium catalyst • ligand effect • half-titanocenes •
ethylene copolymerization • polyolefin containing polar group
[11] These data were partly presented by S.K. at Asian Polyolefin Workshop
2019 (APO2019), Hiroshima, Japan, December, 2019; Pure and Applied
Chemistry International Conference 2020 (PACCON 2020), Bangkok,
Thailand, February, 2020. It was assumed that an introduction of SiMe3,
SiEt3 group might contribute to a stabilization of the active species
leading to the higher activity (by PM3 calculation, unpublished results).[4c]
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[18] Additional polymerization data are shown in the Supporting Information.
In the ethylene polymerization , the activity by 7 at 80 ºC under the
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5
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Remarkable effect of SiMe3, SiEt3 phenoxy para-substituent in Cp*TiCl2(O-2,6-iPr2-4-R-C6H2) toward the activity in the ethylene
copolymerizations with 2-methyl-1-pentene, 1-dodecene and with 9-decen-1-ol have been demonstrated; efficient synthesis of high
molecular weight ethylene copolymers containing 9-decen-1-ol (5-hexen-1-ol) has been attained by Cp*TiCl2(O-2,6-iPr2-4-SiEt3-
C6H2)‒MAO catalyst.
6
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