Selective Chain-End Functionalization of Polar Polyethylenes: Orthogonal Reactivity of Carbene and Polar Vinyl Monomers in Their Copolymerization with Ethylene

Article abstract of DOI:10.1021/jacs.8b10335

Pd(II)-catalyzed copolymerization of ethylene, carbene species, and polar vinyl monomers is successfully developed for the first time to obtain chain-end-functionalized polar polyethylenes, with selective distribution of the polar vinyl monomers within the main chain and carbene at the chain-end. Notably, >99% chain-end-capping with the carbene species can be achieved under the optimized conditions. Mechanistic investigations suggest that the carbene precursor reacts with the generated alkyl palladium complex to terminate the growing chain-end and regenerate the catalyst.

Full text of DOI:10.1021/jacs.8b10335

Communication
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Selective Chain-End Functionalization of Polar Polyethylenes:
Orthogonal Reactivity of Carbene and Polar Vinyl Monomers in
Their Copolymerization with Ethylene
Xiaoming Wang and Kyoko Nozaki*
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
*
S Supporting Information
2
chain. Thus, chain-end functionalization via chain transfer
method (2)) is more preferable as a one-pot in situ approach
ABSTRACT: Pd(II)-catalyzed copolymerization of ethyl-
ene, carbene species, and polar vinyl monomers is
successfully developed for the first time to obtain chain-
end-functionalized polar polyethylenes, with selective
distribution of the polar vinyl monomers within the
main chain and carbene at the chain-end. Notably, >99%
chain-end-capping with the carbene species can be
achieved under the optimized conditions. Mechanistic
investigations suggest that the carbene precursor reacts
with the generated alkyl palladium complex to terminate
the growing chain-end and regenerate the catalyst.
(
3
and produces a large number of polymers per catalyst. Early
transition-metal catalysts such as group 3 and 4 metal
complexes have been widely used for this chain-transfer
chain-end functionalization: examples include σ-bond meta-
thesis with the H−X bond or C−X bond of an amine (H−
NR ), borane (H−BR ), alane (R−AlR ), thiophene (H−
2
2
2
3
C H S), etc. Postmodification of preformed unsaturated
4
3
chain-ends (method (3)) presents several disadvantages
because quantitative chain-end unsaturation is required, and
the chemical transformations are often hampered by the low
concentration of the chain-end double bonds as well as the
poor solubility of PEs.
hain-end-functionalized polyethylenes (Cef-PEs) play a
Despite the significant progress in the synthesis of Cef-PEs,
few examples can be applied to one-pot in situ chain-end
functionalization of polar polyethylenes, which are statistical
copolymers of ethylene and polar vinyl monomers. The
reasons are as follows: (1) early transition-metal catalysts
cannot be used, as they are poisoned easily by polar functional
C
paramount role in the construction of complex macro-
1
molecular architectures (Scheme 1a). Three general methods
Scheme 1. Copolymerization of Ethylene, Carbene, and
Vinyl Monomers
3
groups; and (2) by using group 10 metal catalysts, the
ethylene/polar vinyl monomer copolymerization takes place,
but the alkylmetal species thus formed often undergoes β-
hydride elimination, affording copolymers bearing a terminal
5
vinyl group and/or in-chain CC bonds. The only functional
group introduced successfully thus far is a furyl group in the
copolymerization of ethylene and carbic anhydride, using
6
vinylfuran as a chain-end functionalization reagent.
In recent years, intensive efforts have been devoted to the
development of late-transition-metal-catalyzed copolymeriza-
tion of ethylene with polar monomers, which enables the
incorporation of polar functional groups into polyethylene
5
chains (Scheme 1b). Particularly, palladium/phosphine−
sulfonate complexes have been proven to be tolerant toward
a large scope of polar monomers to produce highly linear
7
copolyethylenes. The unsymmetric ligand structure bearing a
strong σ-donor and a weak σ-donor is proposed to play an
indispensable role, suppressing chain-transfer reactions by β-
hydride elimination. However, the polar comonomers utilized
can be used to synthesize Cef-PEs: (1) controlled end-capping
of living PE chains after living coordination polymerization of
ethylene; (2) chain-end functionalization via chain-transfer
5
2
to date are limited to vinyl compounds and/or carbon
8
monoxide. In this study, we envisioned that functionalized
reactions during coordination−insertion polymerization of
3
carbenes would be a novel class of comonomers for the
ethylene; and (3) postmodification of the unsaturated
4
chain-ends of preformed polyethylenes. From the viewpoint
of efficiency and productivity, method (1) inherently imposes a
Received: September 25, 2018
restriction that one catalyst molecule forms only one polymer
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b10335
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
synthesis of such functionalized polymers. It is well known that
In addition, no functionalized products from 1-dodecene 2
were detected in the reaction of 1-dodecene 2 and 1 in the
presence of catalyst A (Scheme 2b). Only ∼7% of 2 was
isomerized to the internal olefins. Thus, this suggests that the
dE moiety is incorporated by carbene insertion to an
alkylpalladium bond and subsequent β-hydride elimination,
but not via the reaction of the carbene with 1-alkene as a
carbenes can undergo migratory insertion into transition-
9
metal−carbon bonds. In addition, diazoacetates (N 
2
CHCO R) can be polymerized under transition-metal
2
catalysis, with the release of N gas, to access functional
2
10
11
polymethylenes, as pioneered by Ihara and de Bruin
Scheme 1c). These papers support our idea of using carbene
(
14
comonomers. To the best of our knowledge, however, there is
only one reported example of the successful copolymerization
of ethylene with a carbene precursor, in the presence of a
transient intermediate.
Motivated by the above-mentioned results, we investigated
the copolymerization of ethylene and carbene precursor 1 with
palladium complexes A−E in toluene at 80 °C (Table 1,
entries 1−5). All these reactions afforded dE-incorporated
copolymers, thereby demonstrating that the palladium-
catalyzed ethylene polymerization is compatible with 1. The
NMR analysis of these copolymers indicated that the majority
of the incorporated dE is present as an unsaturated chain-end
(UdE), and the minority of dE is present as a saturated chain-
1
2
rhodium catalyst. The copolymerization afforded a mixture
of carbene homopolymers and carbene/ethylene random
copolymers with only 2−11 mol % ethylene incorporation in
the whole mixture, and the copolymers had many branches.
We hypothesized whether a stable carbene precursor such as
dimethyl diazomalonate 1 could be used as a comonomer for
selective chain-end functionalization. Our mechanistic hypoth-
esis begins with olefin polymerization by well-known
palladium/phosphine−sulfonate catalysts (Scheme 1). Con-
tinuous insertion of olefins (either ethylene or polar vinyl
monomers) results in the formation of an alkyl palladium
complex I. Three pathways from intermediate I can be
considered: (i) chain propagation by subsequent olefin
insertion, (ii) chain transfer via β-hydride elimination, and
1
end (SdE). For example, the H NMR spectrum of the
copolymer corresponding to entry 3 (Figure 1a) clearly
showed the characteristic resonances of the UdE chain-ends
at 7.04 (t) (C), 3.86 (s) (A/B), 3.83 (s) (B/A), and 2.38 (m)
(D) ppm, as well as those of SdE at 3.78 (s) (E) and 3.40 (t)
(F) ppm. In the olefinic region, a vinyl group (a, b) and a 1-
propenyl group (−CHCHCH ) (d) were also detected.
3
(
iii) reaction with 1 to form Pd−carbene complex II. Once II
Notably, the number of saturated chain-ends (sum of SdE and
ethyl group) was equal to the number of unsaturated chain-
ends (sum of UdE, −CHCH , and −CHCHCH ), as
is generated, key intermediate III may be formed by the
migratory insertion of the carbene into the Pd−alkyl bond. We
anticipated that owing to the steric hindrance of the ester-
substituted carbon atom connected to the palladium atom,
chain propagation from these carbene insertion intermediates
might be suppressed. Accordingly, direct chain transfer from
III would be favorable to afford chain-end-functionalized
copolymers and generate the Pd−H species to reinitiate the
olefin polymerization. The formation of a conjugate double
bond in the malonic ester unit may also be a driving force.
To investigate the feasibility of our hypothesis, we first
examined the homopolymerization of 1 using catalyst A at 80
2
3
1
3
determined by quantitative C NMR analyses (see Supporting
Information). These results provide important insights into the
polymerization mechanism. The polymerization is initiated by
ethylene insertion either into the Pd−CH bond of complexes
3
A−E or into a Pd−H bond generated by β-hydride
elimination. Carbene insertion to a Pd−H bond is likely to
be responsible for the formation of SdE structures. Notably,
there is no notable incorporation of 1 into the main chains,
suggesting that intermediate III, formed by carbene insertion
into the Pd−alkyl species, does not undergo further chain
propagation. This might be caused by the larger steric
hindrance of III than the intermediate formed from the
carbene insertion to the Pd−H species. The polymerization is
possibly terminated by β-hydride elimination either from
carbene-inserted palladium species III to form UdE or from
alkylpalladium species I to form a terminal vinyl group (partial
migration of the C−C double bond occurs).
°
C. The results indicated no significant formation of
homopolymethylene. Next, stepwise copolymerization of
ethylene and 1 was performed (Scheme 2a). Complex A was
Scheme 2. Preliminary Mechanistic Investigations
As revealed by entry 1 in Table 1, the incorporated dE is
entirely located at the unsaturated chain-end of the copolymer,
rather than the saturated chain-ends (<1%), suggesting that the
carbene precursor is less reactive than ethylene for insertion
into the Pd−CH (and Pd−H) bonds in the initiation step of
3
the polymerization. The majority of the unsaturated chain-end
was dE based, UdE (90.0%). As shown in entries 2 and 4 of
Table 1, complexes B and D also produced low-molecular-
weight copolymers with UdE as the major structure of the
unsaturated chain-end, while complex C resulted in a decrease
in UdE (entry 3). In the presence of complex E, the reaction
afforded a copolymer with an especially high ratio of UdE
(>99%), thus implying exclusive chain-end functionalization
(entry 5). For the saturated chain-end, a relatively higher ratio
of SdE (38.3%) was observed. Increasing the amount of the
carbene precursor and decreasing the ethylene pressure
improved the ratio of the chain-end functionalization with
dE: UdE was improved to 93.4 and 99.0% when using catalyst
reacted with 1.0 atm of ethylene at room temperature, which
resulted in the formation of a palladium complex bearing a
1
13
long alkyl chain, as detected by H NMR analysis. After
excess ethylene was removed, 1 was added to the mixture. The
reaction afforded a PE (M = 1900; M /M = 1.8) with diester
w
w
n
(
dE)-based end groups as the major chain-end (UdE, 74.0%)
and alkenyl groups as the minor chain-end (26.0%). The
incorporated dE was located entirely at the unsaturated chain-
end, and no sequential incorporation of carbene was detected.
A (entries 6 and 7), although M decreased in both cases.
n
B
DOI: 10.1021/jacs.8b10335
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Journal of the American Chemical Society
Communication
a
Table 1. Copolymerization of Ethylene and Dimethyl Diazomalonate Using Palladium Complexes
cat.
“carbene”
(mmol)
yield
b
activity
(kg/mol·h)
M
M /
n
i.r.
(%)
saturated chain-end,
unsaturated chain-end,
no.
chains/Pd
n
w
M
c
c
d
e
f
entry (μmol)
(g)
(g/mol)
SdE (%)
UdE (%)
1
2
3
4
5
6
7
8
9
A (4.0)
B (4.0)
C (4.0)
D (4.0)
E (4.0)
A (4.0)
A (4.0)
E (10.0)
E (10.0)
2.0
2.0
2.0
2.0
2.0
4.0
2.0
1.0
1.0
0.877
1.049
1.500
0.552
0.038
0.448
0.242
0.545
0.583
18.2
17.5
31.2
11.5
0.8
9.3
5.0
4.5
4.8
2,300
3,700
6,400
2,800
4,700
1,500
1,100
7,900
15,400
2.5
2.3
4.2
2.6
2.6
2.3
2.0
3.0
2.5
0.93
0.42
0.10
0.80
0.64
1.24
1.45
0.29
0.11
<1
1.0
13.0
1.1
38.3
<1
<1
34.0
10.3
90.0
71.4
17.6
88.8
>99
93.4
99.0
>99
>99
95
71
58
49
2
75
55
7
g
h
4
a
A mixture of the catalyst (4.0 or 10.0 μmol) and dimethyl diazomalonate in toluene (10.0 mL) was stirred under an ethylene atmosphere (3.0
b
c
MPa) in a 50 mL autoclave at 80 °C. Isolated yields after precipitation with methanol. Molecular weights determined by size-exclusion
chromatography using polystyrene standards and corrected by universal calibration. Incorporation ratio of dE in mol %. The saturated chain-ends
containing dE in mol %. The unsaturated chain-ends containing dE in mol %. Ethylene atmosphere (1.0 MPa). 50 °C.
d
e
f
g
h
methylenemalonate 3 and used it for copolymerization with
ethylene (Scheme 2c). However, the reaction only afforded the
polyethylene without significant incorporation of 3. The steric
hindrance of 3 may have prevented the insertion of the
15
carbon−carbon double bond into the Pd−alkyl bond. The
difference in reactivities between 1 and 3 in the copolymeriza-
tion with olefins highlights the significance of the unique
reactivity of carbenes toward transition metals. Two other
carbene precursors, ethyl diazoacetate and α-benzoyl sulfur
ylide, were also tested in the copolymerization with ethylene.
However, the reactions only afforded PE without significant
incorporation of the polar monomers (for details, see the
Supporting Information).
A variety of typical polar vinyl monomers were incorporated
successfully with orthogonal reactivity of the dimethyl
diazomalonate 1 in the terpolymerization with ethylene using
catalyst E. Under pressure-reactor conditions, the exposure of
catalyst E to 3.0 MPa of ethylene, 1.0 mmol of 1, and 2.0 mL
of allylic acetate (AAc) at 80 °C resulted in the formation of a
novel heterofunctionalized PE (Table 2, entry 1). Notably,
high selectivity of the AAc in the main chain (>99%) and dE-
1
Figure 1. H NMR spectrum of the copolymers obtained in entries 3
a) and 7 (b) of Table 1 and entry 3 of Table 2 (c).
(
When using catalyst E, reducing the amount of 1 improved the
activity and M appreciably, without significantly changing the
selectivity (entry 8 vs entry 5). Decreasing the temperature
from 80 °C to 50 °C resulted in a significant improvement of
M (up to 15 400) while maintaining the excellent selectivity
for UdE (>99%, entry 9).
Figure 1b shows a representative H NMR chart for the
product with selective chain-end functionalization (Table 1,
entry 7): the characteristic resonances for UdE are detected as
the exclusive structure of the unsaturated chain-end.
Quantitative C NMR analyses of the obtained polyethylenes
from entries 7 and 8 (Table 1) revealed that the reactions
produced predominantly linear polymers containing less than
one branch per 1000 carbon atoms in their polyethylene
backbone. It is noteworthy that the present catalytic system
produces more than one chain per Pd catalyst (chain transfer,
1
based unsaturated chain-end (>99%) was achieved. The H
n
NMR spectrum of the obtained copolymer (Figure 1c) clearly
showed the orthogonal distribution of the AAc within the main
chain and carbene at the unsaturated chain-end. Increasing the
amount of AAc significantly improved the incorporated ratio of
AAc, with little change in the AAc distribution (Table 2, entry
n
1
2
). The yield and M of the copolymer were clearly improved
n
when the reaction temperature was reduced to 50 °C (Table 2,
entry 3). Furthermore, allylic chloride (AC), methyl acrylate
(MA), and 3-butenylacetate (BAc) were applied in this
terpolymerization. In general, the vinyl monomers and 1
exhibited excellent orthogonal selectivities, in that the vinyl
monomers were incorporated selectively in the main chain,
while the dE was incorporated preferentially at the unsaturated
chain-end. It is noteworthy that very high ratios of unsaturated
chain-ends with dE (90.1% and >99%) were also obtained in
the presence of MA and BAc, respectively, as successful
examples of unprecedented chain-end-functionalized polar
polyethylenes (Table 2, entries 6 and 8).
1
3
2
−95 chains/Pd), highlighting the higher efficiency compared
to that of end-capping in living coordination polymerization.
Expecting the formation of intermediate III in the
mechanism outlined in Scheme 1, we synthesized dimethyl
C
DOI: 10.1021/jacs.8b10335
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Journal of the American Chemical Society
Communication
a
Table 2. Orthogonal Copolymerization of Ethylene, Carbene, and Vinyl Monomers Using a Palladium Complex
d
i.r. (%)
unsaturated chain-ends
b
c
c
e
f
g
h
entry monomer (X mL) yield (g)
M (g/mol) M /M
n
FG
dE
IFG/FG (%) saturated chain-ends, SdE (%) UdE (%)
UFG (%)
n
w
1
2
3
4
5
6
7
8
AAc (2.0)
AAc (5.0)
AAc (2.0)
AC (2.0)
AC (2.0)
MA (2.0)
MA (5.0)
BAc (5.0)
0.078
0.056
0.230
0.103
0.084
0.449
0.450
0.203
14 000
9400
2.1
0.36
1.49
0.16
0.57
0.62
0.65
1.50
1.10
0.21
0.26
0.06
0.10
0.16
0.23
0.24
0.27
>99
97.2
>99
>99
>99
97.9
96.0
>99
7.1
2.5
6.0
0.8
>99
80.3
87.7
66.0
70.6
90.1
76.7
>99
<1
13.6
<1
<1
<1
6.3
23.3
<1
3.0
2.4
2.0
2.3
3.3
3.0
2.7
i
i
23 500
18 000
12 000
10 100
8900
1.4
17.8
16.4
11.3
9800
a
A mixture of the catalyst (10.0 μmol), monomer (X mL), and dimethyl diazomalonate (1.0 mmol) in toluene (10.0 − X mL) was stirred under an
b
c
ethylene atmosphere (3.0 MPa) in a 50 mL autoclave at 80 °C. Isolated yields after precipitation with methanol. Molecular weights determined
d
by size-exclusion chromatography using polystyrene standards and corrected by universal calibration. Incorporation ratio of FG and dE in mol %.
e
f
g
Vinyl monomers in the main chain/all the incorporated vinyl monomers. The saturated chain-ends containing dE in mol %. The unsaturated
h
i
chain-ends containing dE in mol %. The vinyl monomers at the unsaturated chain-ends. 50 °C.
In conclusion, highly efficient chain-end functionalization of
polar polyethylenes was achieved successfully by the
palladium-catalyzed copolymerization of ethylene, a carbene
precursor, and vinyl monomers. A series of attractive
heterofunctionalized PEs with selective distribution of the
polar vinyl monomers within the main chain and carbene at
the chain-end were obtained. The application of carbene into
olefin copolymerization in this work would trigger the
development of novel polyolefins that are inaccessible via
traditional polymerization methods and stimulate future work
for the development of novel carbene species and other types
of polar monomers.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b10335.
Pathways for In Situ Polyolefin Functionalization with Heteroatoms:
Catalytic Chain Transfer. Angew. Chem., Int. Ed. 2008, 47, 2006.
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(PDF)
AUTHOR INFORMATION
Corresponding Author
nozaki@chembio.t.u-tokyo.ac.jp
■
*
ORCID
Xiaoming Wang: 0000-0001-7981-3757
Kyoko Nozaki: 0000-0002-0321-5299
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JST CREST Grant No.
JPMJCR1323, Japan and JSPS KAKENHI JP18H05259. We
are grateful to Dr. Shingo Ito (NTU) and Dr. Megan Hill
(
UTokyo) for the fruitful discussions and careful proofreading.
(
4) For selected examples of functionalization of unsaturated chain-
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