Synthesis of Polyethylene with In-Chain α,β-Unsaturated Ketone and Isolated Ketone Units: Pd-Catalyzed Ring-Opening Copolymerization of Cyclopropenone with Ethylene

Article abstract of DOI:10.1002/anie.201906990

Although various functionalized units can be incorporated into polyolefins by transition metal catalyzed coordination copolymerizations of nonfunctionalized olefins with polar functional monomers, the incorporated functional units are largely limited to a C1 unit from either CO or C2 units from vinyl monomers. Reported here is the Pd-catalyzed copolymerization of ethylene with cyclopropenone, leading to incorporation of C3 units with functional groups, α,β-unsaturated ketones, in the chain. Coordination-insertion of the carbonyl group and ring opening of the strained three-membered ring are proposed as the key steps in the mechanism. Under different reaction conditions an isolated ketone structure was afforded as the major carbonyl unit, and could be generated by the copolymerization of ethylene with CO formed in situ from cyclopropenone.

Full text of DOI:10.1002/anie.201906990

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Accepted Article
Title: Synthesis of Polyethylene with in-chain α,β-Unsaturated Ketone
and Isolated Ketone Units by Pd-catalyzed Ring-opening
Copolymerization of Cyclopropenones with Ethylene
Authors: Xiaoming Wang, Falk Seidel, and Kyoko Nozaki
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906990
Angew. Chem. 10.1002/ange.201906990
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10.1002/anie.201906990
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COMMUNICATION
Synthesis of Polyethylene with in-chain α,β-Unsaturated Ketone
and Isolated Ketone Units by Pd-catalyzed Ring-opening
Copolymerization of Cyclopropenones with Ethylene
Xiaoming Wang, Falk William Seidel, and Kyoko Nozaki*
Abstract: Although various functionalized units can be incorporated
into polyolefins via transition-metal catalyzed coordination
copolymerizations of non-functionalized olefins with polar functional
monomers, the incorporated functional units are largely limited to a
C1 unit from CO or C2 units from vinyl monomers. Here we report
the Pd-catalyzed copolymerization of ethylene with cyclopropenone,
leading to incorporation of C3 units with functional groups in chain,
that are α,β-unsaturated ketones. Coordination-insertion of the
carbonyl group and ring opening of the strained three-membered
ring is proposed as the key steps in the mechanism. Under a
different condition an isolated ketone structure was afforded as the
major carbonyl unit, which could be generated by the
copolymerization of ethylene with in-situ formed CO from
cyclopropenone.
ketone unit. As another reaction pathway, decomposition is
known for cyclopropenone to generate alkynes and CO under
high temperature pyrolysis, UV irradiation or in the presence of
catalysts.[7]
The
ring-opening
coupling
reactions
of
cyclopropenones with olefins were also reported to form five-
membered cycloaddition products via a strain-driven oxidative
addition of the C–C bond to a transition metal as the key step.[8,9]
Although the chemistry of cyclopropenones has been
extensively investigated in synthetic methodology, their
application as polar monomers in olefin polymerization remains
rather unexplored.[10] Herein we report the first application of a
C3 polar monomer to the olefin copolymerization through
palladium-catalyzed
ring-opening
copolymerization
of
cyclopropenones with ethylene. The copolymers have highly
linear structures with novel in-chain α,β-unsaturated ketone units.
In addition, the reaction could be switched to afford an in-chain
isolated ketone incorporated polyethylene under a different
condition, incorporating the in-situ generated CO.[7] Both of the
α,β-unsaturated ketone and isolated ketone units constitute
Rapidly growing demand of functionalized polyolefins in various
areas has accentuated the need for the development of new
polymerization chemistry for the ready introduction of
functionalized groups into polyolefins.[1] In recent years late-
transition-metal catalyzed polymerization has become an
attractive strategy for the synthesis of functionalized polyolefins
via a direct copolymerization of non-functionalized olefins with
interesting
anchoring
sites
for
introducing
further
functionalities.[11] The photo-degradability of the products
suggested that the α,β-unsaturated ketone units in the
copolymer can be partially cleaved.
polar
monomers
(Scheme
1a).[2]
Particularly
palladium/phosphine−sulfonate complexes Pd[P-O] have been
proven to be tolerant toward a large scope of polar monomers to
produce highly linear functional polyethylenes.[3] However the
polar co-monomers utilized to date are mainly limited to vinyl
compounds and/or carbon monoxide.[4] We became interested in
the development of alternative pathways based on new
incorporation mechanisms to introduce functional groups into
polyolefins, using monomers other than polar vinyl compounds.
Currently, cyclopropenones are receiving significant attention
as an important activated coupling partner in organic synthesis
(Scheme 1b).[5] Cyclopropenones show a zwitterionic resonance
structure in which a negative charge is located on the oxygen
and a positive charge on the three-membered ring, that is
stabilized
by
a
2-electron
aromaticity.
Accordingly
cyclopropenones bear an extremely electrophilic carbonyl group
which has been used in insertion reactions with nucleophilic
organo-metal species.[6] The strained ring can be opened by the
subsequent β-carbon elimination to form an α,β-unsaturated
Scheme 1. Transition metal catalyzed copolymerization of ethylene with
cyclopropenone.
[*] Dr. X. Wang, F. W. Seidel, Prof. K. Nozaki
Department of Chemistry and Biotechnology, Graduate School of
Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: nozaki@chembio.t.u-tokyo.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201906990
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Table 1. Copolymerization of ethylene and cyclopropenone using palladium complexes[a]
COMMUNICATION
activity
(kg/mol.h)
4.4
11.8
8.1
16.9
7.5
12.0
2.3
3.4
entry
cat.
T (oC)
yield (g)[b]
Mn (g/mol)[c]
Mw/Mn[c]
i.r. of α (%)[d]
i.r. of β (%)[d]
α/β[e]
Tm[f]
1
2
3
4
5
6
7[g]
8[h]
A
B
C
D
D
D
D
D
80
80
80
80
60
120
80
80
0.526
1.420
0.976
2.035
0.896
1.446
0.275
0.408
5,400
7,600
7,600
25,300
50,600
13,800
5,500
3,800
2.6
2.1
2.3
2.5
1.8
2.4
1.6
1.7
1.93
0.69
0.85
1.01
0.41
0.85
3.35
4.19
1.04
0.20
0.35
0.17
0.04
0.93
1.39
1.53
1.8
3.4
2.4
5.9
10.2
0.9
2.4
2.7
110.7
119.8
85.1 / 96.6
[a] A mixture of the catalyst (0.01 mmol) and cyclopropenone 1a (1.0 mmol) in toluene (10.0 mL) was stirred under ethylene atmosphere (3.0 MPa) in a 50
mL autoclave. [b] Isolated yields after precipitation with methanol. [c] Molecular weights determined by size-exclusion chromatography using polystyrene
standards and corrected by universal calibration. [d] Incorporation ratio in mol% determined by 1H-NMR analysis. [e] The ratio was determined by 1H-NMR
analysis. [f] Melting temperatures determined by differential scanning calorimetry (DSC) analysis. [g] 1.0 MPa ethylene. [h] 5.0 mmol cyclopropenone 1a.
Our mechanistic hypothesis begins with the olefin polymerization
by Pd[P-O] (Scheme 1c). A Pd catalyst may initiate the
polymerization of ethylene to generate a long alkyl palladium
complex I. The Pd−C bond might undergo migratory insertion
Copolymerization of ethylene with 1a catalyzed by D generated
the highest activity, Mn and the ratio of α/β in the initial survey,
albeit with the reduction in the incorporation ratio (Table 1, entry
4 vs 1-3).
into the carbonyl group of the cyclopropenone, and
a
subsequent β-carbon elimination of intermediate II gives a ring-
opened alkenyl intermediate III. Further chain propagation from
III with ethylene is proposed to be favorable to afford a novel
α,β-unsaturated
ketone
incorporated
polyethylene.Other
possibilities are discussed in the Supporting Information..
Copolymerization of ethylene with commercially available
diphenylcyclopropenone (1a) was successfully accomplished
with Pd[P-O] catalysts A-D (entries 1-4, Table 1). The polymers
were purified by precipitation with MeOH to give them in an
essentially pure form as solids. NMR analysis of these four
obtained polymers clearly supported that a substantial amount of
1a was successfully incorporated into the polymer chain in two
different forms (named α and β). Unit α was assessed to be the
proposed α,β-unsaturated ketone and unit β was evaluated to be
an isolated ketone structure which might be generated by the
copolymerization of ethylene with in-situ formed CO from
cyclopropenone. For example, the 1H NMR spectrum of the
copolymer corresponding to entry 4 (Figure 1a) showed the
characteristic resonances of the α,β-unsaturated ketone unit (α)
at 2.58 (t) (B), 2.47 (t) (F), and 7.17-6.99 (aromatic range) ppm.
Quantitative 13C NMR analyses also showed the resonance of
ketone at 206 ppm (E), total of ten peaks for the aromatic
carbons and alkenyl carbons at 144-126 ppm, and carbons at α-
and β-positions of C=O or C=C (F, B, A, G) (Figure 1b). The
signals for unit β were found at 2.41 (t) (I) ppm in 1H-NMR
spectrum and 210 ppm (H) in 13C-NMR spectrum. It should be
noted that the two phenyl groups are cis in unit α, suggesting
that intermediate III does not undergo isomerization in the
catalysis. No obvious incorporation of the polar monomer is
seen at either chain ends. Quantitative 13C NMR analysis of the
obtained polymers from entries 1-4 (Table 1) revealed that the
reactions produced predominantly linear polymers containing
less than one branch per 1000 carbon atoms in their
polyethylene backbone. The results of the copolymerization
including activity, Mn and the ratio of α/β, are highly related to
the catalysts used. The complexes bearing alkylphosphine
ligands exhibited higher catalytic activity than arylphosphine
ligand (entries 2-4 vs 1). Catalysts B and C afforded similar
copolymers with higher Mn and activities than catalyst A.
Figure 1. Assignment of characteristic NMR resonances. (a) 1H NMR
spectrum of the copolymer in Table 1, entry 4. (b) Quantitative 13C NMR
spectrum of the copolymer in Table 1, entry 4. (c) 1H NMR spectrum of the
copolymer after the UV irradiation (d) 1H NMR spectrum of the copolymer in
the terpolymerization with AAc (allyl acetate) at 80 oC with catalyst D. (e) 1H
NMR spectrum of the copolymer of ethylene with 1a at 120 oC with catalyst C.
(f) 1H NMR spectrum of the polyketone from the copolymerization of ethylene
with dimesitylcyclopropenone (1e).
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10.1002/anie.201906990
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COMMUNICATION
electron-donating groups (p-methoxyphenyl and ethyl) can
preferably stabilize the cyclopropenone’s zwitterionic resonance
Detailed investigations of the reaction conditions were further
carried out using catalyst D. Mn of the copolymer was increased
structure, leading to
a diminished reactivity and stronger
Table 2. Copolymerization of ethylene and cyclopropenone using Pd catalyst D[a]
activity
(kg/mol.h)
3.6
entry
cyclopropenone (R)
yield (g)[b]
Mn (g/mol)[c]
Mw/Mn[c]
i.r. of α (%)[d]
i.r. of β (%)[d]
α/β[e]
1
2
1b (R = p-methoxyphenyl)
1c (R = p-Bromophenyl)
0.436
1.467
4,000
11,200
2.4
2.4
0.88
0.34
<0.10
0.10
-
12.2
3.4
3[f]
1d (R = Ethyl)
0.479
4.0
5,200
3.3
0.34
<0.10
-
[a] A mixture of the catalyst D (10.0 μmol) and cyclopropenone (1.0 mmol) in toluene (10.0 mL) was stirred under ethylene atmosphere (3.0 MPa) in a 50 mL
autoclave. [b] Isolated yields after precipitation with methanol. [c] Molecular weights determined by size-exclusion chromatography using polystyrene
standards and corrected by universal calibration. [d] Incorporation ratio in mol% determined by 1H-NMR analysis. [e] The ratio was determined by 1H-NMR
analysis. [f] 0.5 mmol 1d was used.
coordination of the polarized ketone to the catalyst.
to 50,600 by lowering the reaction temperature to 60 °C with
lower incorporation ratio and a higher ratio of α/β (Table 1, entry
5). Increasing the polymerization temperature from 80 °C to
120 °C resulted in a significant drop in Mn and activity but
evidently higher incorporation ratio of unit β (Table 1, entry 6).
Increasing the ratio of [comonomer, 1a] vs P[ethylene] by
decreasing the pressure of ethylene (entry 7) and/or raising the
amount of 1a (entry 8) led to an increase in the α and β
incorporation ratio with a decrease in catalytic activity, molecular
weight and the ratio of α/β.
As shown above, an isolated ketone unit (β) is generated in a
linear polyethylene chain as a minor structure. Actually, the
production of non-alternating/isolated CO and ethylene
copolymers is one of the main aims in polyketone research in
order to obtain new materials since non-alternating polyketones
exhibit lower melting points as compared to perfectly alternating
materials.[4] Since 2002, it was reported that the use of Pd[P-O]
catalysts enables successive insertion of several ethylene units
into the growing polyketone chain for the synthesis of non-
perfectly alternating copolymerization of CO and ethylene.[14,15]
Although low content of CO was used in these cases, the
obtained polyketones still contain significant alternating units
which show strong inter-chain dipolar interactions. On the
contrary, CO can be slowly generated in situ by the
decarbonylation of cyclopropenone in this study. In order to
obtain a non-alternating polyketone, the copolymerization of 1.0
Notably, the differential scanning calorimetry (DSC) analysis
of the copolymers obtained from entry 1 and 4 revealed
endothermic peaks at Tm = 110.7 °C and 119.8 °C, respectively.
The copolymer from entry 8 showed two endothermic peaks at
Tm = 85.1 °C and 96.6 °C. All of them are significantly lower
than highly linear PEs in the observed MW range.[12] The photo-
degradability of the copolymer obtained from entry 4 was further
tested under UV light. The Mn of the recovered polymer was
o
MPa ethylene with 1.0 mmol 1a at 120 C was carried out using
1
catalyst C. The reaction led to the formation of a copolymer with
units β as the dominant polar structures (Figure 1d, i.r. 1.80%, 9
ketone groups per 1000 CH2 units, Mn 4,100). Unit α is at trace
amount by 13C-NMR analysis. Besides these, copolymerization
of ethylene with in situ formed diphenylacetylene was also
detected (Figure 1d, for details, see SI). In order to decrease the
decreased to 15,600 and the PDI was turned to be 4.0. The H
NMR analysis also showed that the signals B and F due to the
enone moiety decreased (For details, see SI). These results
suggested that the α,β-unsaturated ketone units of the
copolymer were partially decomposed by the irradiation.[13]
Terpolymerization of ethylene with 1a and a polar vinyl
monomer (allyl acetate; AAc) was further tested using catalyst D
to investigate the relative activities of these two polar monomers.
Figure 1c clearly showed that both polar monomers were
successfully incorporated in the main chain of the obtained
copolymer, suggesting that two different incorporation
mechanisms are compatible together. Compared with AAc,
much lower [1a] (0.10 M 1a vs. 1.85 M AAc) in the present
catalysis led to higher 1a incorporation than AAc (1.24% vs.
0.60%), highlighting the high activity of 1a towards the
copolymerization
of
ethylene
with
this
alkyne,
dimesitylcyclopropenone (1e) bearing bulky mesityl groups was
synthesized and tested. The copolymer obtained showed
isolated ketone units are dominant (Figure 1e, i.r. 1.29%, 6
ketone groups per 1000 CH2 units, Mn 3,500), without a distinct
alternating polyketone structure. Since the properties of
polyketone are highly related to the microstructure, this finding
offers opportunity for the studies of the structure-property
relationships in polykeone research area.[16]
In conclusion, the copolymerization of ethylene and C3 polar
monomer (cyclopropenone) using palladium (II) phosphine-
sulfonate catalysts was successfully achieved. Cyclopropenone
was incroporated into the main chain by the proposed insertion-
ring opening mechanism, generating a novel α,β-unsaturated
copolymerization with ethylene.
To further investigate the scope and versatility of the
copolymerization, cyclopropenone derivatives 1b-d were
examined under the standard conditions. As shown in Table 2,
all the co-monomers smoothly underwent the copolymerization
with relative lower incorporation ratio than 1a. Namely, the
reactions using diarylcyclopropenone with electron-donating
groups (1b) and diethylcyclopropenone (1d) gave similar results
regarding the activities and Mn, which were much lower than the
reaction using comonomer 1c and/or 1a. It is proposed that
ketone
structure,
which
was
photodegradable.
The
terpolymerization of ethylene, cyclopropenone and allyl acetate
suggested that both of these two polar comonomers can be
incorporated into the main chain by their respective mechanisms.
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G. Wu, M. Liu, W. Gao, J. Ding, X. Huang, H. Wu, Org. Chem. Front.
2018, 5, 1651.
In addition, benefiting from the slow in situ release of CO from
cyclopropenone, polyethylene bearing isolated ketone units in
chain was also obtained as the major component in this
copolymerization, which may contribute to the studies of their
structure-property relationships.
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Acknowledgements
This work was supported by JST CREST Grant Number
JPMJCR1323, Japan. F.W. Seidel was supported by a MEXT
scholarship provided by the Ministry of Education, Culture,
Sports, Science and Technology.
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Keywords: Copolymerization • C3 polar monomer •
Cyclopropenones • Pd catalyzed • Polyethylene
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Text for Table of Contents
Author(s), Corresponding Author(s)*
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Title
((Insert TOC Graphic here))
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Xiaoming Wang, Falk William Seidel,
and Kyoko Nozaki*
Page No. – Page No.
Synthesis of Polyethylene with in-
chain α,β-Unsaturated Ketone and
Isolated Ketone Units by Pd-catalyzed
Ring-opening Copolymerization of
Cyclopropenones with Ethylene
Text for Table of Contents
A C3-polar monomer unit was successfully incorporated into a polyethylene chain for the first time by the Pd catalyzed copolymerization of
cyclopropenones with ethylene. A polyethylene containing α,β-unsaturated ketone units in chain was obtained possibly via the coordination-
insertion of the carbonyl group and ring opening of the strained three-membered ring of cyclopropenone. Under a different condition an
isolated ketone structure was afforded as the major carbonyl unit, which could be generated by the copolymerization of ethylene with in-situ
formed CO from cyclopropenone.
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