Palladium/IzQO-Catalyzed coordination-insertion copolymerization of ethylene and 1,1-disubstituted ethylenes bearing a polar functional group

Article abstract of DOI:10.1021/jacs.7b12593

Coordination-insertion copolymerization of ethylene with 1,1-disubstituted ethylenes bearing a polar functional group, such as methyl methacrylate (MMA), is a long-standing challenge in catalytic polymerization. The major obstacle for this process is the huge difference in reactivity of ethylene versus 1,1-disubstituted ethylenes toward both coordination and insertion. Herein we report the copolymerization of ethylene and 1,1-disubstituted ethylenes by using an imidazo[1,5-a]quinolin-9-olate-1-ylidene-supported palladium catalyst. Various types of 1,1-disubstituted ethylenes were successfully incorporated into the polyethylene chain. In-depth characterization of the obtained copolymers and mechanistic inferences drawn from stoichiometric reactions of alkylpalladium complexes with methyl methacrylate and ethylene indicate that the copolymerization proceeds by the same coordination-insertion mechanism that has been postulated for ethylene.

Full text of DOI:10.1021/jacs.7b12593

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Article
Palladium/IzQO-Catalyzed Coordination–Insertion Copolymerization of
Ethylene and 1,1-Disubstituted Ethylenes Bearing a Polar Functional Group
Hina Yasuda, Ryo Nakano, Shingo Ito, and Kyoko Nozaki
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12593 • Publication Date (Web): 08 Jan 2018
Downloaded from http://pubs.acs.org on January 8, 2018
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Journal of the American Chemical Society
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Palladium/IzQO-Catalyzed Coordination–Insertion Copolymeriza-
tion of Ethylene and 1,1-Disubstituted Ethylenes Bearing a Polar
Functional Group
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Hina Yasuda, Ryo Nakano, Shingo Ito, and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hon-
go, Bunkyo-ku, Tokyo 113-8656, Japan
Supporting Information Placeholder
ABSTRACT: Coordination–insertion copolymerization of ethylene with 1,1-disubstituted ethylenes bearing a polar functional
group, such as methyl methacrylate (MMA), is a long-standing challenge in catalytic polymerization. The major obstacle for this
process is the huge difference in reactivity of ethylene versus 1,1-disubstituted ethylenes towards both coordination and inser-
tion. Herein we report the copolymerization of ethylene and 1,1-disubstituted ethylenes by using an imidazo[1,5-a]quinolin-9-
olate-1-ylidene (IzQO)-supported palladium catalyst. Various types of 1,1-disubstituted ethylenes were successfully incorporated
into the polyethylene chain. In-depth characterization of the obtained copolymers and mechanistic inferences drawn from stoi-
chiometric reactions of alkylpalladium complexes with methyl methacrylate and ethylene indicate that the copolymerization
proceeds by the same coordination–insertion mechanism that has been postulated for ethylene.
insertion steps: having higher steric demands than that of
INTRODUCTION
ethylene, the 1,1-disubstituted ethylenes generally show
much lower reactivity towards coordination/insertion of a
monomer and subsequent propagation. For example, Meck-
ing and coworkers’ attempts to copolymerize ethylene and
MMA using palladium/phosphine–sulfonate complexes only
resulted in exclusive formation of linear polyethylene even in
the presence of substantial amounts of MMA.7 Gibson et al.
successfully incorporated MMA into the polyethylene chain
using nickel/phosphine–enolate catalysts; however, 2,1-
insertion of MMA resulted in chain termination, indicated by
the formation of enolate-terminated polyethylene.8 Sen and
colleagues reported that the activation enthalpy of MMA
insertion into cationic palladium/α-diimine complexes is 4
kcal/mol higher than that of methyl acrylate (MA) insertion,
explaining the inability of MMA to insert in α-diimine-
catalyzed copolymerization reactions.9
Coordination–insertion copolymerization of ethylene and
polar monomers has attracted much attention as a powerful
method for the synthesis of functionalized polyethylenes.
The presence of polar functional groups alters their physical
and surface properties such as paintability and dyeability.1 To
date, group 10 metal catalysts bearing various ligands includ-
ing α-diimine,2 phosphine–sulfonate,3 bisphosphine monox-
ide, 4 and imidazo[1,5-a]quinolin-9-olate-1-ylidene (IzQO) 5
have successfully copolymerized ethylene and polar mono-
mers such as acrylates, vinyl acetate, and allyl monomers
(Scheme 1a).1d Copolymers of ethylene with 1,1-disubstituted
ethylenes are expected to exhibit superior material properties
and higher stability when compared to those with monosub-
stituted ethylenes.6 Thus, 1,1-disubstituted ethylenes such as
methyl methacrylate (MMA) are highly lucrative comono-
mers. To this end, the challenge of coordination–insertion
copolymerization of ethylene and 1,1-disubstituted ethylenes
bearing a polar functional group has been the subject of on-
going investigation (Scheme 1b).
Another concern in these transformations is the elucida-
tion of the mechanism of copolymerization. Although there
are some reports on the “coordination–insertion” copolymer-
ization of ethylene and polar 1,1-disubstituted ethylenes,10,11,12
Scheme 1. Copolymerization of Ethylene and (a) Po-
lar Vinyl Monomers and (b) 1,1-Disubstituted Eth-
ylene Bearing a Polar Functional Group.
drawing conclusions inclined towards
a coordination–
insertion mechanism might be erroneous, since most polar
1,1-disubstituted ethylenes are also susceptible to ionic or
radical polymerization. For instance, Li and colleagues re-
ported the copolymerization of ethylene and MMA by [(β-
ketoiminato)NiPh(PPh3)]/MMAO systems and suggested
that coordination–insertion mechanism was operational
(Scheme 2a).11b By contrast, Mecking and coworkers have
shown that a mixture of homopolymers, polyethylene and
polyMMA (Scheme 2b), were formed when similar neutral
nickel catalysts were used, indicating that the coordination–
insertion polymerization of ethylene and the radical
polymerization of MMA proceeded simultaneously and inde-
pendently.13,14 Furthermore, it was reported that similar [(sa-
licylaldiminato)NiPh(PPh3)] complexes produced multiblock
The major difficulty in copolymerization of ethylene with
1,1-disubstituted ethylenes is related to the coordination–
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Page 2 of 10
copolymers by combining a coordination–insertion mecha-
ethylene as depicted in Scheme 2a. By investigating stoichi-
1
2
3
4
nism for ethylene and a radical mechanism for MMA
(Scheme 2c).15 From the above, mechanistic conclusions of
repetitive reports on ethylene/MMA copolymerization are
often inconclusive and contradictory.
ometric reactions of palladium/IzQO complexes with MMA
followed by reactions with ethylene, we conclusively demon-
strate that the copolymerization proceeds via the coordina-
tion–insertion mechanism.
An inconclusive picture of polymerization process also ex-
ists on the other ethylene/1,1-disubstituted ethylene copoly-
merization. Recently, it was reported that palladi-
um/phosphine–sulfonate complexes produced copolymers of
ethylene and 1,1-disubstituted ethylenes bearing two polar
functional groups, such as ethyl 2-cyanoacrylate and trifluo-
romethyl acrylic acid, which are claimed statistical copoly-
mers as shown in Scheme 2a.12 However, our comparison of
spectroscopic data of their copolymers with those of some
model compounds revealed inconsistencies (vide infra). Thus,
we felt that the implied coordination–insertion mechanism
needed further investigation.
5
6
7
8
Scheme 3. Copolymerization of Ethylene and 1,1-
Disubstituted Ethylenes Bearing a Polar Functional
Group by Palladium/IzQO Complex 1.
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Scheme 2. Polymer Structures Postulated for Eth-
ylene/1,1-Disubstituted Ethylene Copolymers.
RESULTS AND DISCUSSION
Copolymerization of Ethylene and 1,1-Disubstituted Ethyl-
enes Bearing a Polar Functional Group. The copolymerization
of ethylene and 1,1-disubstituted ethylenes 2 was investigated
by using palladium/IzQO complex 1 (Table 1). Copolymeriza-
tion of ethylene (2.0 MPa) and MMA (2a; 25 vol % in tolu-
ene) with 1 (10 ꢀmol) was performed in the presence of BHT
(200 mg) as a radical inhibitor to afford an ethylene/MMA
copolymer with an MMA incorporation of 0.46 mol % (entry
1). When the concentration of MMA was increased to 50
vol % (entry 2) and 100 vol % (entry 3), the incorporation
ratio was increased to 0.62 mol % and 0.85 mol %, respec-
tively, although the polymerization activity and polymer
molecular weight were decreased. Reducing the ethylene
pressure from 2.0 to 1.0 MPa significantly changed the incor-
poration ratio of MMA up to 2.5 mol % (entry 4). The palla-
dium/IzQO catalysts were also employed for the copolymeri-
zation of ethylene with other 1,1-disubstituted ethylenes
bearing a polar functional group (entries 5–8 of Table 1). Me-
thallyl monomers (CH2=CMeCH2X, where X = OR, NR2, hal-
ogen, etc.), industrially produced from isobutene,16 are a po-
tential class of candidates. Methallyl phenyl ether 2b was
successfully incorporated to form the ethylene/2b copoly-
mers with ca. 3–5 times higher activity than those of MMA
We chose the palladium complexes bearing an IzQO lig-
and (1) to perform coordination–insertion copolymerization
of ethylene and 1,1-disubstituted ethylenes bearing a polar
functional group, since previous reports from our group
demonstrated that the same catalysts could efficiently incor-
porate polar monomers under even low concentration.5a This
choice was also warranted by its high durability under high
temperature conditions, favoring incorporation of 1,1-
disubstituted ethylenes. Here, we report the copolymeriza-
tion of ethylene with various 1,1-disubstituted ethylenes bear-
ing a polar functional group (2), including MMA, using the
palladium/IzQO catalyst (Scheme 3). In-depth NMR analyses
of the obtained copolymers and comparison with model
compounds revealed that these 1,1-disubstituted ethylenes
were successfully incorporated into the main chain of poly-
Table 1. Copolymerization of Ethylene and 1,1-Disubstituted Ethylenes 2a–c by Palladium/IzQO Complex 1a
c
e
entry
comonomer
ethylene
(MPa)
monomer
(mL)
solvent yieldb
activity
(g⋅mmol–1⋅h–1)
Mn
(103)
Mw/ incorp.d
Tpm
(°C)
c
(mL)
(mg)
Mn
(mol %)
1f
2f
3f
4 f
5
6
7
8
2a (FG = CO2Me)
2a
2.0
2.0
2.0
1.0
2.0
2.0
2.0
2.0
2.0
4.0
8.0
8.0
4.0
8.0
4.0
8.0
6.0
4.0
0
0
4.0
0
130
101
0.87
0.67
0.33
0.16
2.3
1.5
2.1
0.85
8.7
8.6
2.2
0.8
18
5.1
27
18
1.8
1.7
2.1
2.9
1.9
2.3
1.7
1.9
0.46
0.62
0.85
2.5
0.30
0.41
0.11
104.1
106.6
108.6
br
106.7
109.5
107.0
102.8
2a
2a
49.1
23.7
343
227
315
2b (FG = CH2OPh)
2b
2c (FG = CH2CH2OAc)
2c
4.0
0
127
0.23
a A mixture of catalyst 1 (10 µmol), ethylene, and monomer 2a–c in toluene was stirred in a 50-mL autoclave with glass tube
for 15 h at 120 °C. b Isolated yields after precipitation with methanol. c Molecular weights determined by SEC using polysty-
rene standards and corrected by universal calibration. Molar incorporation ratios of polar monomers determined by H
d
1
e
f
NMR analysis. Peak melting temperatures determined by DSC analysis. Reaction was performed in the presence of BHT
(200 mg).
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(entries 5 and 6). The 2b incorporation ratio could be in-
creased up to 0.41 mol % by conducting the copolymerization
a,c h,F
1
2
3
4
5
6
7
8
(a)
at a higher concentration of 2b (entry 6). This catalytic sys-
tem was also applied to the copolymerization of homome-
thallyl monomers having a structure of CH2=CMeCH2CH2X,
which can be obtained from isobutene.17 3-Methylbut-3-en-1-
yl acetate 2c (X = OAc), a volatile fragrant constituent of
various fruits,18 produced ethylene/2c copolymers with activi-
ties comparable to those of 2b (entries 7 and 8). Similar to
the copolymerizations with methacrylic and methallyl mon-
omers, the incorporation ratio of 2c was increased to 0.23
mol % when the concentration was increased, albeit at the
expense of the polymerization activity and polymer molecu-
lar weight.
b
C
A
i
chain
end
E
p,E
m
j
9
4
3
2
1
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(ppm)
a
(b)
f
Characterization of Ethylene/1,1-Disubstituted Ethylene Co-
polymers. The structures of the obtained ethylene/MMA
copolymers were determined via in-depth NMR analyses
e
d
b
c
1
including H, 13C, DEPT, COSY, HSQC, and HMBC experi-
g
i
h
1
F
E
ments. In the H NMR spectrum (Figure 1a), the proton sig-
B
A
C
D
m
j
p
nals of methoxy group A and methyl group C were observed
as a singlet at 3.697 and 1.186 ppm, respectively, while the
signals of methylene group E were observed as multiplets
around 1.67 and 1.45 ppm due to their diastereotopic charac-
ter. The minor proton signals (ca. 2% of incorporated MMA
units) of methoxy groups observed at 3.71 ppm were also
observed and assigned as chain-end group through the com-
parison with several model compounds (vide infra).19,20 In the
quantitative 13C NMR spectrum (Figure 1b), the signal of the
quaternary carbon at the α position of the carbonyl group
(D) and the signals of methylene carbons at the β and γ posi-
tions (E and F, respectively) were observed in the ratio of 1.0 :
2.0 : 2.0.19 The characterization was also supported by
DEPT135 analysis, indicating that A and C are primary or
tertiary carbons, E and F are secondary carbons, and B and D
are quaternary carbons (Figure S22). Furthermore, the struc-
tures of the obtained copolymers were confirmed by two-
dimensional NMR analyses. The HSQC spectrum (Figure
S24) showed the cross signals between proton and carbon
signals for A, C, and E, and the HMBC spectrum (Figure S26)
revealed cross signals of methylene proton E (1.67 ppm) with
carbons B (177.5 ppm), D (45.9 ppm), and F (24.3 ppm). The-
se results strongly indicated that the obtained polymers were
without doubt statistical copolymers of ethylene and MMA,
and that MMA was mainly incorporated into the main chain
of polyethylenes as illustrated in Scheme 2a.19 It is worth
noting that no units in which multiple MMA are successively
inserted into the main chain were observed and that only
minor amounts of MMA units incorporated into the chain
ends were observed in the NMR spectra. As for the eth-
ylene/2b copolymers and ethylene/2c copolymers, the struc-
tures were determined in the same manner to confirm in-
chain incorporation of these comonomers into polyethylene
chain (Figures S33–S49 and S50–S66, respectively).19
180 175
50
40
(ppm)
30
20
10
Figure 1. (a) H NMR and (b) Quantitative 13C NMR spectra
of the ethylene/MMA copolymer obtained in entry 3 of Table
1 (1,1,2,2-tetrachloroethane-d2, 120 °C).
1
All the characteristic signals were in accord with each other
with great accuracy, again suggesting that a single unit of
MMA was incorporated into the main chain of polyethylene
during the chain propagation step (Scheme 2a). In addition,
infrared (IR) spectra of the ethylene/MMA copolymer, model
compound I, and commercial poly(methyl methacrylate)
(PMMA) (Figures S70–S72) revealed that the carbonyl ab-
sorption of the copolymer was observed at 1734 cm–1, which
was almost identical to that of model compound I at 1732 cm–
1, but was different from that of PMMA (1722 cm–1). The com-
parison of NMR spectra with model compounds II–VI pro-
vided important information on the chain-end structures
(Table 2). For the ease of comparison, the chemical shifts of
1
the methoxy group are focused. In the H NMR spectrum of
the ethylene/MMA copolymer, two types of signals corre-
1
sponding to chain ends were observed in the H NMR spec-
trum (3.706 and 3.714 ppm) as well as the aforementioned in-
chain signal at 3.697 ppm. These two signals are consistent
with those of model compounds II, III, and IV and no signal
corresponding to model compounds V and VI was observed.
Comparison of Ethylene/Methyl Methacrylate Copolymers
and Model Compounds. In order to support the assignments,
1
we compared the H and 13C NMR spectra of the obtained
13
Quantitative C NMR spectra also revealed the methoxy sig-
ethylene/MMA copolymers and those of several model com-
pounds shown in Figure 2, which were reasonably chosen
from the mechanistic consideration (See Scheme S1 for de
nals of the copolymer corresponds to model compounds II,
III, and IV. Thus, chain-end of the ethylene/MMA copolymer
is confirmed as the structure corresponding to model com-
pounds II, III, and IV.19
1
tail). H and 13C NMR spectra of the copolymer obtained in
this study and model compound I are shown in Figure 3.
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Figure 2. List of model compounds.
(c)
i
E
E
F
F
D
A
C
1
Table 2. H and 13C NMR Chemical Shifts (ppm) of the
Methoxy Group of the Ethylene/MMA Copolymer Ob-
tained in Entry 3 of Table 1 and Model Compounds II–VI.
other
CH2
(d)
CH2CH3
CH2CH3
C
D
E/MMA
A
II
III
IV
V
VI
copolymer
3.697 (major)
3.706, 3.714
(minor)
1H
3.708 3.705 3.717 3.783 3.802
51.1 50.8 50.9 55.6 51.5
50
40
30
(ppm)
20
10
13C
50.8
1
Figure 3. H NMR spectra of (a) the ethylene/MMA copoly-
mer obtained in entry 3 of Table 1 and (b) model compound I
(1,1,2,2-tetrachloroethane-d2, 120 °C). Quantitative 13C NMR
spectra of (c) the ethylene/MMA copolymer obtained in en-
try 3 of Table 1 and (d) model compound I (1,1,2,2-
tetrachloroethane-d2, 120 °C).
Table 3. 13C NMR Chemical Shifts (ppm) of Ethylene/MMA Copolymers and PMMA.a
condi-
sample
A
B
C
D
E
F
tionb
[A]
E/MMA copolymer
(entry 3, Table 1)
50.8
50.6
177.5
177.0
21.3
21.5
45.9
46.1
39.1
39.5
24.2
24.7
[B]
19.4–19.1 (m),
17.4 (br)
19.9–19.6 (m),
17.9 (br)
18.9 (br), 16.6
(br)
[A]
[B]
54.6–53.8 (m) 177.4–176.4 (m)
54.6–54.3 (m) 177.3–176.3 (m)
54.6–54.1 (m) 178.3–177.1 (m)
51.3 45.3–44.9 (m)
51.1 45.6–45.3 (m)
51.9 45.0–44.7 (m)
---
---
---
commercial PMMA (TCI)
[C]
[B]
[B]
E/MMA copolymer in ref. 11b
E/MMA copolymer in ref. 11c
54.9
54.6
177.5
177
18.6
51.5
51.3
45.9
45.9
27.6
not as-
signed
not as-
signed
18.3
E/MMA copolymer in ref. 10
[C]
[C]
54c
55c
179–177 (m)
180
not assigned
21.1, 18.8, 16.6
52c
52c
46–44 (m)c
45.5, 44.8,
44.6
E/MMA copolymer in ref. 11a
29.8
a
b
m = multiple signals, br = broad signal. [A]: 1,1,2,2-tetrachloroethane-d2, 120 °C; [B]: 1,2-dichlorobenzene-d4, 120 °C; [C]:
chloroform-d, ambient temperature. c Values read off the NMR spectra shown in the paper.
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Comparison with Commercial PMMA and Ethylene/Methyl
Methacrylate Copolymers in Literature. As stated in the in-
1
troductory part, there are some reports on the late transi-
tion-metal-catalyzed copolymerization of ethylene and
MMA.10,11 Nonetheless, a comparison of our copolymers with
those reported in literature10,11 as well as to commercial
PMMA validated the originality of our copolymers. 13C NMR
chemical shifts of our ethylene/MMA copolymer, commercial
PMMA, and reported ethylene/MMA copolymers10,11 are
summarized in Table 3. Our copolymer exhibited strikingly
different 13C NMR signals from those of the other
(co)polymers. In particular, the chemical environment of
methylene carbon E should be significantly different between
–ethylene–MMA–ethylene– and –(MMA)n– units. The chem-
ical shifts of carbon E in the reported copolymers were ob-
served at 44–46 ppm, which significantly differ from that of
our copolymer (39.1 ppm) and rather resembled those of
commercial PMMA (45–46 ppm). Similar trends were ob-
served in the chemical shifts of methoxy carbon A and quer-
tanary carbon D. This strongly implies that all of the previ-
ously reported ethylene/MMA copolymers in references10,11
have successive MMA units in the main chain as shown in
Schemes 2b and/or 2c. These poly-MMA units would be
formed by the intermediacy of radical-related processes, con-
sidering that coordination–insertion processes typically af-
ford copolymers with random and statistical distribution of
each comonomers.
2
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Figure 4. 1H NMR spectra of (a) ethyl 2-cyano-2-
hexylotanoate, (b) poly(ethyl 2-cyanoacrylate), and (c)
ethylene/ethyl 2-cyanoacrylate copolymer in reference 12a
(1,1,2,2-tetrachloroethane-d2, 130 °C). NMR spectrum (c)
was adapted with permission from reference 12a. Copy-
right 2015 American Chemical Society.
Comparison of Reported Ethylene/1,1-Disubstituted Ethylene
Copolymers and Model Compounds. It was reported in 2015
that the copolymerization of ethylene with difunctional 1,1-
disubstituted ethylenes was mediated by palladi-
um/phosphine–sulfonate catalysts.12 However, our compari-
son with model compounds resulted in a conclusion contra-
dictive to the authors’ claim that the obtained copolymers
have a sequence of –ethylene–(1,1-disubstituted ethylene)–
1
ethylene–. For example, Figures 4 and 5 show H and 13C
NMR spectra of (a) ethyl 2-cyano-2-hexylotanoate as a model
compound for in-chain incorporation of ethyl 2-
cyanoacrylate, (b) poly(ethyl 2-cyanoacrylate),21 and (c) the
reported ethylene/ethyl 2-cyanoacrylate copolymer.12a The
comparison showed that spectrum (c) rather resembled (b)
rather than (a), suggesting successive ethyl 2-cyanoacrylate
units are present in the copolymer, which is different from
the polymer structures the authors proposed to have formed.
We also compared the NMR spectra for ethylene/2-
acetamidoacrylic acid, ethylene/methyl 2-acetamidoacrylate,
and ethylene/2-bromoarylic acid copolymers and obtain es-
sentially the same results.19 From the above consideration,
we strongly believe that the previously reported ethylene/1,1-
disubstituted ethylene copolymers in references 12 did not
have the proposed sequence of –ethylene–(1,1-disubstituted
ethylene)–ethylene– (Scheme 2a), but contained blocky mi-
crostructures generated by successive 1,1-disubstituted eth-
ylene incorporation presumably via ionic/radical mecha-
nisms (Schemes 2b and/or 2c).
Figure 5. 13C NMR spectra of (a) ethyl 2-cyano-2-
hexylotanoate, (b) poly(ethyl 2-cyanoacrylate), and (c) eth-
ylene/ethyl 2-cyanoacrylate copolymer in ref. 12a (1,1,2,2-
tetrachloroethane-d2, 130 °C). Note: * = solvent impurities; #
= monomer residue. NMR spectrum (c) was adapted with
permission from reference 12a. Copyright 2015 American
Chemical Society.
Stoichiometric Methyl Methacrylate Insertion. In order to
elucidate the polymerization mechanism, a stoichiometric
reaction of alkylpalladium complex 3 with MMA was per-
formed (Scheme 4). The reaction of methylpalladium com-
plex 3 with MMA in the presence of silver carbonate for 18 h
at 75 °C led to the formation of MMA-inserted complex 4 in
93% NMR yield. This result indicates that MMA indeed un-
derwent coordination and insertion into the palladium–
methyl bond and that the same process occurs in other al-
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kylpalladium species formed during the polymerization pro-
formation of INT-2, which bears the polymer chain with the
1
2
3
4
5
6
7
8
cess. It should be noted here that no 2,1-insertion complex
was detected by NMR spectroscopy, suggesting that MMA
inserts into the palladium–alkyl bond of palladium/IzQO
complexes exclusively in a 1,2-fashion.5a Recrystallization
furnished complex 4 in 45% yield as single crystals appropri-
ate for X-ray crystallographic analysis.19 The molecular struc-
ture, shown in Figure 6, clearly indicates that 1,2-insertion of
MMA into the palladium–methyl group of complex 3 oc-
curred to form a five-membered-ring chelation. When com-
pared to the reported MMA-inserted cationic complex bear-
ing an α-diimine ligand,9 the observed angle for C1–Pd1–O1 is
similar (81.21(13)° versus 81.41(19)°), while the observed bond
length of Pd1–O1 is longer (2.124(3) Å versus 2.041(4) Å). The-
se results suggest that the strong trans influence of the car-
bene ligand weakens the Pd1–O1 bond in neutral complex 4.
methoxycarbonyl terminus. Polymer 5 is formed via chain
termination of INT-2, and Polymer 6, i.e. regular polyeth-
ylene, is obtained by polymerization initiated by palladium–
hydride species, which was generated by β-hydride elimina-
tion during the polymerization process. From a MALDI-TOF-
MS analysis of the supernatant of the reaction mixture, sig-
nals assignable to the palladium species formed by several
consecutive insertions of ethylene into starting complex 4 [4
+ (ethylene)n] (e.g. INT-1 and INT-2) were observed (Figure
S75). These palladium species provide further convincing
evidence for the insertion of ethylene into complex 4.
9
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It is worth noting that the condition required for the initi-
ation of ethylene polymerization from 4 (2.0 MPa, 100 °C) is
quite reasonable compared to the actual copolymerization
conditions (1.0–2.0 MPa, 120 °C). Moreover, the mass balance
of the methoxycarbonyl unit based on 4 was as high as 95%;
the recovery of 4 was 86% while 9% of the methoxycarbonyl
unit (65% from the 14% conversion of 4) was found at the
chain end of the obtained polyethylene. These results suggest
that ethylene coordinated to the palladium center of com-
plex 4 via de-chelation and subsequently inserted into the
palladium–alkyl bond to successfully start chain propagation.
Hence, the stoichiometric reactions of complex 3 with MMA
followed by ethylene strongly support the coordination–
insertion mechanism in the palladium/IzQO-catalyzed co-
polymerization of ethylene and MMA.
Scheme 4. Synthesis of MMA-Inserted Complex 4 by
the Reaction of Methylpalladium Complex 3 with
MMA.
Na+
N
O
MMA
N
O
Ag2CO3
N
Me
Cl
N
Pd
Pd
THF
75 ºC, 18 h
O
OMe
3
4
93%(NMR)
Scheme 5. Homopolymerization of Ethylene Using
Complex 4.
N
N
H
CO2Me
+
Pd
R
R
n
n
toluene
100 °C
1 h
O
O
OMe
OMe
5
6
4
ethylene
coordination
O
O
C
Pd
O
C
O
OMe
Pd
n
INT-2
insertion
INT-1
Figure 6. X-ray structure of complex 4 with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (°): Pd1–C1 = 2.026(4), Pd1–C7 =
1.949(4), Pd1–O1 = 2.124(3), Pd1–O3 = 2.056(3), C7–Pd1–O3 =
90.44(14), C1–Pd1–O1 = 81.21(13).
Discussion on the Advantages of the Palladium/IzQO System
towards Ethylene/MMA Copolymerization. Finally, we discuss
the advantages of our palladium/IzQO systems over the oth-
er systems. In the previous reports,7,9 the trials of the copol-
ymerization of ethylene and MMA using palladium/α-
diimine complexes9 and palladium/phosphine–sulfonate
complexes7 did not afford the corresponding copolymers,
rather, only polyethylenes were obtained. This was attributed
to the large differences of activation barriers between eth-
ylene insertion and MMA insertion, which were experimen-
tally determined to be 4 kcal/mol in palladium/α-diimine
systems9 and 5 kcal/mol in palladium/phosphine–sulfonate
systems.7 On the other hand, our previous study revealed
that the palladium/IzQO system can incorporate both elec-
tron-rich and -deficient polar monomers even under consid-
erably low concentration, compared to the first two sys-
tems.5a Such insensitivity of the palladium/IzQO system to-
wards steric hindrance on alkenes can be further supported
by its ability to polymerize 1-butene, which could not be pol-
Reactivity of MMA-Inserted Complex 4. The potential of
MMA-inserted complex 4 to further incorporate ethylene
was confirmed experimentally, as shown in Scheme 5. Upon
heating complex 4 under 2.0 MPa ethylene pressure at 100 °C
for a period of 1 h, a mixture of two kinds of polyethylene, 5
and 6, were obtained. Polymer 5 features a methoxycarbonyl
(–CH2CMe2CO2Me) terminus while polymer 6 features a hy-
drogen terminus. The structures of polymer 5 were unam-
1
biguously characterized by NMR analyses; the H NMR sig-
nals of the methoxycarbonyl group in polymer 5 (3.706 ppm)
were in good accordance with those of model compound II
(3.708 ppm).19
A mechanism for the formation of polymers 5 and 6 is
proposed in Scheme 5. Ethylene coordination and insertion
into complex 4 followed by chain propagation lead to the
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ymerized by palladium/phosphine–sulfonate system. There-
EXPERIMENTAL SECTION
1
2
3
4
5
6
7
8
fore, it could be postulated that the reactivity gap between
ethylene and MMA arising from their steric difference is less
emphasized in the case of palladium/IzQO system.
A General Procedure of the Copolymerization of Methyl
Methacrylate with Ethylene (Table 1, entries 1–4). A 50-mL
stainless steel autoclave was dried in an oven at 120 °C, and
then cooled inside a glovebox under argon atmosphere.
Complex 1 (5.7 mg, 10 µmol), BHT (200 mg), MMA (x mL; x =
2, 4, 8), and toluene (8−x mL) were added into the autoclave
(NOTE: In order to avoid the formation of PMMA, MMA
should be added directly to the bottom of the autoclave, so
as not to touch the inner sidewall of the autoclave). After the
autoclave was charged with ethylene (1.0–2.0 MPa), the reac-
tion mixture was stirred in an isothermal heating block at
120 °C for 15 hours. After cooling to room temperature and
venting redundant ethylene, the reaction was quenched by
addition of methanol (ca. 20 mL). The formed precipitates
were filtered, washed with methanol (NOTE: The product
should be carefully washed because insufficient washing re-
sults in the formation of PMMA during the subsequent dry-
ing process), and dried under high vacuum for >3 hours at
100 °C to afford ethylene/MMA copolymers. The molecular
weights and molecular weight distributions were determined
by size exclusion chromatography. The incorporation ratio of
Another important feature of the palladium/IzQO system
is selective 1,2-insertion of substituted ethylenes.5a As for
MMA, the above stoichiometric study revealed almost quan-
titative formation of the 1,2-inserted product (Scheme 4). As
shown in Scheme 6, after 1,2-insertion of MMA, β-hydrogen
elimination cannot occur because of the absence of β-
hydrogen in the resulting alkylpalladium species. In contrast,
2,1-insertion of MMA affords a species bearing multiple β-
hydrogens and stabilized tertiary alkyl chain-end hindering
the following propagation, both of which lead to undesirable
chain termination. Indeed, the previous report on the stoi-
chiometric reaction of a palladium/phosphine–sulfonate
complex with MMA proceeded in both 1,2- and 2,1-insertion
fashion and the 2,1-insertion product was subject to facile β-
hydrogen elimination.7
9
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Based on the discussion above, we conclude that our suc-
cessful coordination–insertion copolymerization of ethylene
and MMA by the palladium/IzQO system stems from the
insensitivity towards steric hindrance and the selective 1,2-
insertion mode.
1
polar monomers was determined by quantitative H NMR
analyses. The melting temperature was determined by DSC
analysis.
Scheme 6. Insertion of Methyl Methacrylate into a
Palladium–Alkyl Bond during the Polymerization
Process.
A General Procedure of the Copolymerization of 1,1-
Disubstituted Ethylenes with Ethylene (Table 1, entries 5–8).
A 50-mL stainless steel autoclave was dried in an oven at
120 °C, and then cooled inside a glovebox under argon at-
mosphere. Complex 1 (5.7 mg, 10 µmol), 1,1-disubstituted
ethylene (x mL; x = 2, 4, 8), and toluene (8−x mL) were added
into the autoclave. After the autoclave was charged with eth-
ylene (2.0 MPa), the reaction mixture was stirred in an iso-
thermal heating block at 120 °C for 15 hours. After cooling to
room temperature and venting redundant ethylene, the reac-
tion was quenched by addition of methanol (ca. 20 mL). The
formed precipitates were filtered, washed with methanol,
and dried under high vacuum for >3 hours at 100 °C to afford
ethylene/1,1-disubstituted ethylene copolymers. The molecu-
lar weights and molecular weight distributions were deter-
mined by size exclusion chromatography. The incorporation
CONCLUSION
In summary, palladium/imidazo[1,5-a]quinolin-9-olate-1-
ylidene (Palladium/IzQO) catalysts were successfully applied
to copolymerize ethylene with a series of polar functional
group bearing 1,1-disubstituted ethylenes. For the first time,
we obtained statistical copolymers of ethylene and 1,1-
disubstituted ethylene uniquely featuring nonsequential in-
corporation of the polar units into the main chain. The struc-
tures of the obtained copolymers were rigorously confirmed
via thorough NMR spectroscopic analyses. In sharp contrast
to earlier reports, our copolymers, containing sequences of –
ethylene–(1,1-disubstituted ethylene)–ethylene–, are devoid
of undesirable blocky microstructures. Isolation of an MMA-
inserted complex obtained via a stoichiometric reaction of
alkylpalladium complex with MMA supported the hypothesis
that MMA can coordinate to the complex and then insert.
Subsequent initiation of ethylene from the MMA-inserted
complex showed that the chain propagation can indeed oc-
cur after the insertion of MMA. These mechanistic inferences,
together with comparison our copolymers with model com-
pounds and copolymers reported earlier, suggest that our
study indeed offers the first report on the coordination–
insertion copolymerization of ethylene with 1,1-disubstituted
ethylene.
1
ratio of polar monomers was determined by quantitative H
NMR analyses. The melting temperature was determined by
DSC analysis.
Preparation of Complex 3: Sodium {(SP-4-3)-(chlorido)[2-(2,6-
diisopropylphenyl)imidazo[1,5-a]quinolin-9-olato-κO-1-
ylidene-κC1](methyl)palladate}: To a mixture of 2-(2,6-
diisopropylphenyl)imidazo[1,5-a]quinolinium-9-olate
(51.7
mg, 0.15 mmol) and sodium bis(trimethylsilyl)amide (30.3
mg, 0.17 mmol) in a 15-mL vial was added THF (7.5 mL) at
room temperature and the mixture was stirred for 15 minutes
at room temperature. To the resulting solution was added
PdMeCl(cod) (39.8 mg, 0.15 mmol), and the mixture was
stirred for 1 hour. The resulting mixture was passed through
a plug of Celite with THF, and concentrated under reduced
pressure to ca. 0.2 mL. The obtained solution was poured
into hexane (ca. 10 mL) to form precipitates. After filtration
and extraction with dichloromethane, the resulting solution
was dried in vacuo. Then, complex 3 was obtained as a yellow
1
solid (69.3 mg, 0.13 mmol, 88% yield). H NMR (500 MHz,
CDCl3) δ 7.42 (t, J = 7.8 Hz, 1H), 7.22 (d, J = 7.9 Hz, 1H), 7.10–
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6.95 (m, 6H), 6.77 (dd, J = 7.6, 1.5 Hz, 1H), 2.64 (sept, J = 6.7
1
2
3
4
5
6
Hz, 2H), 1.48 (d, J = 6.7 Hz, 6H), 1.10 (d, J = 6.7 Hz, 6H), 0.33
(s, 3H); 13C NMR (126 MHz, CDCl3) δ 154.4, 149.8, 142.1 (2C),
133.9, 126.7, 126.5, 123.9, 123.7, 122.7, 122.3, 120.5 (2C), 119.3,
112.6, 112.0, 110.2, 25.1 (2C), 22.2 (2C), 20.4 (2C), −11.3; HRMS-
ESI (m/z) calcd for C24H26ClN2NaOPd ([M−Na+]−) 499.0774,
found 499.0766.
(1) (a) Functional Polymers: Modern Synthetic Methods and Novel
Structures; Patil, A. O., Schulz, D. N., Novak, B. M., Eds. ACS Sympo-
sium Series 704; American Chemical Society: Washington, DC, 1998.
(b) Boffa, L. S.; Novak, B. M. Chem. Rev. 2000, 100, 1479–1493. (c)
Chung, T. C. Functionalization of Polyolefins; Academic Press: Lon-
don, 2002. (d) Nakamura, A.; Ito, S.; Nozaki, K. Chem. Rev. 2009, 109,
5215–5244. (e) Dai, S.; Zhou, S.; Zhang, W.; Chen, C. Macromolecules
2016, 49, 8855−8862.
(2) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem.
Soc. 1996, 118, 267–268. (b) Mecking, S.; Johnson, L. K.; Wang, L.;
Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888–899. For reviews, see:
(c) Camacho, D. H.; Guan, Z. Chem. Commun. 2010, 46, 7879–7893.
(d) Franssen, N. M. G.; Reek, J. N. H.; de Bruin, B. Chem. Soc. Rev.
2013, 42, 5809–5832.
(3) (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R.
I. Chem. Commun. 2002, 744. For a review, see: (b) Nakamura, A.;
Anselment, T. M. J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking,
S.; Rieger, B.; Sen, A.; van Leeuwen, P. W. N. M.; Nozaki, K. Acc.
Chem. Res. 2013, 46, 1438–1449.
(4) (a) Carrow, B. P.; Nozaki, K. J. Am. Chem. Soc. 2012, 134, 8802–
8805. (b) Mitsushige, Y.; Carrow, B. P.; Ito, S.; Nozaki, K. Chem. Sci.
2016, 7, 737–744.
(5) (a) Nakano, R.; Nozaki, K. J. Am. Chem. Soc. 2015, 137, 10934–
10937. See also: (b) Tao, W.; Nakano, R.; Ito, S.; Nozaki, K. Angew.
Chem. Int. Ed. 2016, 55, 2835–2839.
(6) (a) Park. J. B.; Lakes. R. S. In Biomaterials: An Introduction, 3rd
ed.; Springer: New York, 2007; p. 185. (b) Brydson, J. A. In Plastics
Materials, 6th ed.; Butterworth-Heinemann: Oxford, 2013; p. 408.
(7) Rünzi, T.; Guironnet, D.; Göttker-Schnetmann, I.; Mecking, S.
J. Am. Chem. Soc. 2010, 132, 16623–16630.
(8) Gibson, V. C.; Tomov, A. Chem. Commun. 2001, 1964–1965.
(9) Borkar, S.; Yennawar, H.; Sen, A. Organometallics 2007, 26,
4711–4714.
(10) For copper systems, see: Galletti, A. M. R.; Carlini, C.;
Giaiacopi, S.; Martinelli, M.; Sbrana, G. J. Polym. Sci., A: Polym. Chem.
2007, 45, 1134–1142.
7
8
9
Preparation of Complex 4: (SP-4-3)-[2-(2,6-diisopropyl-
phenyl)imidazo[1,5-a]quinolin-9-olato-κO-1-ylidene-κC1][3-
methoxy-2,2-dimethyl-3-oxo-κO-prop-1-yl-κC1]palladium: To
a solution of complex 3 (29.2 mg, 55.8 µmol) in THF (4.0 mL)
in a 20-mL Schlenk tube were added MMA (297.1 µL, 2.79
mmol) and silver carbonate (15.4 mg, 55.8 µmol) at room
temperature. In the dark, the mixture was stirred for 18 h at
75 °C. The yield of complex 4 was determined to be 93% by
NMR analysis using 1,2,4,5-tetrabromobenzene as an internal
standard (Figure S5). The resulting mixture was passed
through a plug of Celite, and evaporated to dryness. The re-
sulting solid was dissolved in diethyl ether, and filtered to
remove some precipitates. Then, slow evaporation of the
solvent afforded the crystal of complex 4 as pale yellow rec-
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23
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27
28
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32
33
34
35
36
37
38
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
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59
60
1
tangular cylinders (14.2 mg, 45% yield). H NMR (500 MHz,
CDCl3) δ 7.44 (t, J = 7.8 Hz, 1H), 7.22–7.15 (m, 4H), 7.11 (d, J =
9.5 Hz, 1H), 7.02 (s, 1H), 6.96 (d, J = 9.5 Hz, 1H), 6.69 (dd, J =
7.2, 1.7 Hz, 1H), 3.89 (s, 3H), 2.63 (sept, J = 6.7 Hz, 2H), 1.35 (d,
J = 6.7 Hz, 6H), 1.07 (d, J = 6.7 Hz, 6H), 0.98 (s, 6H), 0.48 (s,
2H); 13C NMR (126 MHz, CDCl3) δ 189.8, 153.9, 151.1, 142.3,
133.9, 126.5, 126.2, 124.5, 123.6, 123.5, 121.7, 120.4 (2C), 118.3,
112.6, 109.6, 108.8, 51.6, 45.3, 27.1, 25.4 (2C), 25.0 (2C), 22.3
(2C), 20.0 (2C), 17.9; Anal. calcd for C29H34N2O3Pd C, 61.65; H,
6.07; N, 4.96 found C, 61.57; H, 6.25; N, 4.86.
ASSOCIATED CONTENT
Supporting Information
(11) For nickel systems, see: (a) Carlini, C.; Martinelli, M.; Galletti,
A. M. R.; Sbrana, G. Macromol. Chem. Phys. 2002, 203, 1606–1613. (b)
Li, X.; Li, Y.; Li, Y.; Chen, Y.; Hu, N. Organometallics 2005, 24, 2502–
2510. (c) Carlini, C.; de Luise, V.; Martinelli, M.; Galletti, A. M. R.;
Sbrana, G. J. Polym. Sci., A: Polym. Chem. 2006, 44, 620–633.
(12) (a) Gaikwad, S. R.; Deshmukh, S. S.; Gonnade, R. G.; Rajamo-
hanan, P. R.; Chikkali, S. H. ACS Macro Lett. 2015, 4, 933–937. (b)
Gaikwad, S. R.; Deshmukh, S. S.; Koshti, V. S.; Poddar, S.; Gonnade,
R. G.; Rajamohanan, P. R.; Chikkali, S. H. Macromolecules 2017, 50,
5748–5758.
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org. Experimental procedures,
characterization of palladium compounds and polymers, and
crystallographic data for 4 (PDF)
AUTHOR INFORMATION
Corresponding Author
nozaki@chembio.t.u-tokyo.ac.jp
NOTES
(13) Ölscher, F.; Göttker-Schnetmann, I.; Monteil, V.; Mecking, S.
J. Am. Chem. Soc. 2015, 137, 14819–14828.
(14) Similar [N–O]Ni complexes were reported to promote homo-
polymerization of MMA involving a radical-related process. See: He,
X.; Wu, Q. Appl. Organomet. Chem. 2006, 20, 264−271.
(15) (a) Leblanc, A.; Grau, E.; Broyer, J.-P.; Boisson, C.; Spitz, R.;
Monteil, V. Macromolecules 2011, 44, 3293−301. (b) Leblanc, A.; Broy-
er, J.-P.; Boisson, C.; Spitz, R.; Monteil, V. Pure Appl. Chem. 2012, 84,
2113−20.
(16) (a) Krähling, L.; Krey, J.; Jakobson, G.; Grolig, J.; Miksche, L.
In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH:
Weinheim, 2014. (b) In Encyclopedia of chemical technology, 4th ed.;
Kroschwitz, J. I.; Howe-Grant, M. Eds. Wiley: New York, 1992; Vol. 4,
p 728. (c) In The Merck Index: An Encyclopedia of Chemicals, Drugs,
and Biologicals, 15th ed.; O'neil, M. J.; Heckelman, P. E. Eds. Royal
Society of Chemistry: Cambridge, 2013; p 381.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by JST CREST Grant Number
JPMJCR1323, Japan. A part of this work was conducted at the
Research Hub for Advanced Nano Characterization, The
University of Tokyo, supported by MEXT, Japan. We are
grateful to Dr. Yusuke Ota and Dr. Yusuke Mitsushige for
fruitful discussion and Dr. Shrinwantu Pal for proofreading.
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industrially synthesized from isobutene. See: Hoelderich, F. H.;
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(21) The homopolymer was obtained by the reaction performed
under the 1/5 scale of the same condition in reference 12a in the ab-
sence of the palladium catalyst.
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(19) See Supporting Information for details.
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