Direct Synthesis of Functionalized High-Molecular-Weight Polyethylene by Copolymerization of Ethylene with Polar Monomers

Article abstract of DOI:10.1002/anie.201607152

The introduction of even a small amount of polar functional groups into polyolefins could excise great control over important material properties. As the most direct and economic strategy, the transition-metal-catalyzed copolymerization of olefins with polar, functionalized monomers represents one of the biggest challenges in this field. The presence of polar monomers usually dramatically reduces the catalytic activity and copolymer molecular weight (to the level of thousands or even hundreds Da), rendering the copolymerization process and the copolymer materials far from ideal for industrial applications. In this contribution, we demonstrate that these obstacles can be addressed through rational catalyst design. Copolymers with highly linear microstructures, high melting temperatures, and very high molecular weights (close to or above 1 000 000 Da) were generated. The direct synthesis of polar functionalized high-molecular-weight polyethylene was thus achieved.

Full text of DOI:10.1002/anie.201607152

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607152
German Edition: DOI: 10.1002/ange.201607152
Copolymerization
Direct Synthesis of Functionalized High-Molecular-Weight Polyethy-
lene by Copolymerization of Ethylene with Polar Monomers
Shengyu Dai and Changle Chen*
[
4]
Abstract: The introduction of even a small amount of polar
functional groups into polyolefins could excise great control
over important material properties. As the most direct and
economic strategy, the transition-metal-catalyzed copolymer-
ization of olefins with polar, functionalized monomers repre-
sents one of the biggest challenges in this field. The presence of
polar monomers usually dramatically reduces the catalytic
activity and copolymer molecular weight (to the level of
thousands or even hundreds Da), rendering the copolymeriza-
tion process and the copolymer materials far from ideal for
industrial applications. In this contribution, we demonstrate
that these obstacles can be addressed through rational catalyst
design. Copolymers with highly linear microstructures, high
melting temperatures, and very high molecular weights (close
to or above 1000000 Da) were generated. The direct synthesis
of polar functionalized high-molecular-weight polyethylene
was thus achieved.
sation (ADMET) followed by hydrogenation were devel-
oped to prepare functionalized linear polyolefins. However,
special monomers are required, making these methods not
economically viable for large-scale applications. The radical
copolymerization of olefins with polar vinyl monomers is
[
5]
widely practiced in industry. However, harsh conditions
(high temperatures and pressures) are usually required,
affording highly branched copolymers with poor mechanical
[5,6]
properties.
The above-mentioned and some other multi-
step strategies generally suffer from low cost efficiency, high
energy consumption, and, in some cases, poor control over the
polymer microstructure. In contrast, transition-metal-medi-
ated catalytic copolymerization holds great potential to
overcome these limitations and achieve better control over
the polar/nonpolar ratios and the copolymer microstructures.
Despite many years of research, the direct coordination
copolymerization of olefins with unprotected polar mono-
mers remains to be a great challenge. This was also recognized
as one of the last “holy grails” in this field. The development
of novel catalyst systems that can tolerate polar functional
groups is the key. A milestone discovery was made by
Brookhart and co-workers in the 1990s (Scheme 1, cata-
[
7]
S
ince the Nobel Prize winning discovery made by Ziegler
and Natta, transition-metal-catalyzed olefin polymerization
has enjoyed great success in both industry and academia.
Currently, polyolefin materials constitute almost half of the
[
1]
II
3
00 million tons of the worldwide plastics production.
lyst A), who demonstrated the a-diimine–Pd catalyzed
Among the many strategies to modify the properties of
polyolefins, the incorporation of functional groups has
copolymerization of ethylene (E) with methyl acrylate
[
2]
attracted considerable attention. The introduction of even
a small amount of polar functionalities can dramatically
improve important properties, such as the surface (e.g.,
paintability, printability), adhesion, toughness, dyeability,
[2]
compatibility, and barrier properties, and many others.
Several techniques are available to prepare functionalized
polyolefin materials. The post-polymerization functionaliza-
tion method is widely used because a huge number of non-
functionalized polyolefins can be used as starting materials,
and no new copolymerization technology is required. How-
ever, harsh conditions are usually required owing to the
chemical inertness of polyolefins, which could also adversely
impact their properties. Ring opening metathesis polymeri-
Scheme 1. Representative transition-metal catalysts that can catalyze
the copolymerization of olefins with polar monomers.
[
3]
zation (ROMP) and acyclic diene metathesis polyconden-
[
*] S. Dai, Prof. C. Chen
Key Laboratory of Soft Matter Chemistry
Chinese Academy of Sciences
Department of Polymer Science and Engineering
University of Science and Technology of China
Hefei, 230026 (China)
E-mail: changle@ustc.edu.cn
Homepage: http://staff.ustc.edu.cn/~changle
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201607152.
[
8]
(MA). Subsequently, various functional copolymers with
[
9]
ester and ether functional moieties were prepared. Grubbs
and co-workers showed that salicylaldimine–Ni catalysts can
efficiently copolymerize ethylene with polar-substituted nor-
bornene (Scheme 1, catalyst B).
bearing phosphine-sulfonate ligands or derivatives thereof
have emerged as a powerful class of catalysts for olefin–polar
monomer copolymerization (Scheme 1, catalysts C). Some
II
[
10]
II
Recently, Pd catalysts
[
11]
disadvantages are shared by this limited number of successful
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examples, such as greatly reduced activities and copolymer
molecular weights.
One of the most distinguishing features of the Brookhart-
II
type a-diimine–Pd catalyst (Scheme 1, A) is its chain-
walking capability, which can migrate the metal center
along the polymer chain by b-H elimination and reinsertion
with opposite regiochemistry. However, this also represents
one of the biggest limitations and a long-standing challenge
for this class of catalysts, as highly branched (ca. 100 branches
per 1000 C atoms) and totally amorphous polymer and
[9]
copolymer materials are generated. Recently, we studied
the ethylene polymerization and copolymerization properties
II
of a series of a-diimine–Pd complexes with dibenzhydryl
[
12]
moieties (Scheme 1, D).
Very high molecular weight
5
polyethylene (Mn up to 5.4 ꢀ 10 ) and E–MA copolymers
M up to 1.9 ꢀ 10 ) could be obtained. Most interestingly, the
4
(
n
branching density could be reduced to about 25 per 1000
C atoms, leading to semicrystalline products. Inspired by
these promising results, new palladium complexes with
further increased ligand steric bulk were designed and
prepared by replacing the phenyl groups with naphthalene
or benzothiophene units.
Figure 1. Molecular structure of 1. Hydrogen atoms omitted for clarity.
Ellipsoids set at 30% probability. Selected bond lengths [ꢀ] and angles
[8]: Pd1–C52 2.0998(11), Pd1–Cl1 2.192(4), Pd1–N1 2.069(4), Pd1–N1’
2.069(4); N1-Pd1-N1’ 76.1(3), N1’-Pd1-C52 97.1(3), C52-Pd1-Cl1 86.4-
(3), N1-Pd1-Cl1 101.83(18).
The a-diimine ligands L1 and L2 were prepared on
multigram scale from the reaction of 2,3-butanedione with the
corresponding anilines. The palladium complexes 1 and 2
were generated from the reaction of the ligands with
[
a]
Table 1: Ethylene polymerization with complexes 1 and 2.
[
b]
[b]
Mn[c]
[d]
Tm[e]
Entry Cat.
T
Yield
Act.
PDI
B
À14
[8C] [g]
[ꢁ10
]
[8C]
(
COD)PdMeCl (COD = 1,5-cyclooctadiene; Scheme 2).
1
2
3
4
5
6
7
8
9
0
1
2
3
4
1
1
1
1
1
1
2
2
2
2
2
2
1
2
0
20
40
60
80
100
0
20
40
60 10.24
80
100
30 18.24
30 19.52
0.38
1.63
4.23
5.56
3.64
1.45
0.44
5.12
5.50
1.52
6.52
16.92
22.24
14.56
5.80
3.38
1.07 18
1.06 18
1.09 18
1.05 18
1.25 19
1.49 20
118.2
110.1
109.2
109.5
109.4
108.5
127.2
121.1
122.5
119.0
118.3
116.5
110.4
120.1
16.35
53.16
51.77
33.34
8.12
11.52
78.17
90.72
42.05
32.35
12.36
1.76
1.36
1.21
6
9
20.48
22.00
40.96
23.92
12.80
1.26 13
1.23 20
1.23 21
1.53 23
1.25 18
1.26 10
1
1
1
1
1
Scheme 2. Synthesis of the palladium complexes 1 and 2.
5.98
3.20
[
[
f]
f]
7.60 393
8.13 802
1
These palladium complexes were characterized by H and
[
a] Conditions: precatalyst (10 mmol), NaBAF (1.2 equiv), CHCl (5 mL,
for 1) or CH
1
3
3
C NMR spectroscopy, mass spectrometry, and elemental
2 2
Cl (5 mL, for 2), toluene (45 mL), 8 atm ethylene, 15 min.
analysis. The molecular structure of complex 1 is shown in
Figure 1. The Pd center adopts a distorted square-planar
geometry, and the effective blockage of the axial positions of
the Pd center can be observed from the structure.
These palladium complexes are highly active in ethylene
polymerization when activated using 1.2 equiv tetrakis(3,5-
bis(trifluoromethyl)phenyl)borate (NaBAF). For both com-
plexes, the catalytic activities reached their maximum at 608C
in the temperature range of 08C to 1008C (Table 1, entries 1–
[
b] The yields and activities are averages of at least two runs. Activity
5
À1
À1 À1
(Act.)=10 gmol Pd
h . [c] Determined by GPC in trichlorobenzene
at 1508C. [d] B=number of branches per 1000 C atoms, determined by
1
H NMR spectroscopy. [e] Determined by differential scanning calorim-
etry (DSC). [f] Precatalyst (2 mmol), NaBAF (1.2 equiv), CHCl (5 mL, for
1
3
) or CH Cl (5 mL, for 2), toluene (145 mL), 8 atm ethylene, 12 h.
2 2
totally amorphous polyethylene. The branching densities of
the resulting polyethylene materials were dramatically lower
1
2). High activities were maintained even at 1008C whereas
1
3
the classic Brookhart catalyst A (Scheme 1) completely lost
its activity at 608C. It should be noted that significant loss of
catalytic activity was observed after 20–30 min at 808C for
catalysts 1 and 2 (see the Supporting Information, Figure S3).
Further improvements in thermal stability are required for
this class of catalysts to become commercially viable.
(as low as 6/1000 C) for catalysts 1 and 2. The C NMR
spectrum of a polyethylene sample prepared according to
Table 1, entry 7 (Figure S21) showed that the polymer is
essentially linear, and very similar to commercial polyethy-
lene produced using early-transition-metal catalysts. The
branching density of the polyethylene generated by catalyst
1 is independent of the polymerization temperature, and it is
similar to that obtained with catalyst A and our previously
As mentioned above, the classic Brookhart catalyst A
(
Scheme 1) generated highly branched (ca. 100/1000 C),
2
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reported catalyst D (Scheme 1). Interestingly, the branching
density was influenced by the polymerization temperature for
catalyst 2. When the polymerization was carried out for 12 h
at 308C, polyethylene with an extremely high molecular
weight was generated (Table 1, entries 13 and 14; M = 3.9 ꢀ
n
6
6
1
0 for 1 and M = 8.0 ꢀ 10 for 2). With catalysts 1 and 2, high-
molecular-weight (M up to 4.4 ꢀ 10 ), relatively linear, semi-
n
5
n
crystalline (Tm close to 1208C) E–MA copolymers were
generated (Table 2, entries 1–4; Table S1).
Scheme 3. Comparison of the ethylene–polar monomer copolymeriza-
Brookhart catalyst A represents a major breakthrough in
the field of coordination copolymerization of ethylene with
tion properties of classic Brookhart catalyst A and catalysts 1 and 2.
in the presence of these polar mon-
omers, although no comonomer
[
a]
Table 2: Ethylene/polar monomer copolymerization.
incorporation
was
observed
[
b]
[c]
[d]
[d]
[e]
[f]
Entry
Cat.
Monomer
[M]
molL
Yield
[g]
Act.
XM
[%]
Mn
PDI
B
Tm
[8C]
(Table 2, entries 5–7). The ligand
steric bulk and the high stability of
À1
[b]
4
[
]
[ꢁ10 ]
[
[
[
[
g]
g]
h]
h]
1
2
3
4
5
6
7
8
9
0
1
2
1
2
2
2
2
2
2
2
2.4
2.4
1.0
1.0
0.5
0.5
0.5
0.5
0.5
2.0
1.49
3.64
0.92
0.85
3.50
4.97
0.38
0.22
2.04
0.12
1.24
3.03
0.77
0.71
35.0
49.7
3.80
2.20
20.4
0.61
<0.1
0.71
0.88
0
0
0
0.88
0.56
2.04
44.20
282
5.54
1.68
1.57
1.89
2.35
3.04
1.89
2.06
2.56
1.76
2.84
18
10
18
9
12
13
8
16
15
15
120.0 catalyst 2 might be responsible for
123.8
117.0
119.0
121.3
121.5
125.6
125.0
this phenomenon.
When a spacer was put between
the double bond and the polar
functional group (the second group
of polar monomers shown in
Scheme 3), no polymer was gener-
ated for classic Brookhart catalyst
7.21
15.53
61.86
3.26
7.54
39.65
4.86
122.6
A as expected. In combination with
the fast chain-walking feature of
catalyst A, fast b-halide (for the
monomers CH =CH(CH ) Cl and
1
1
1
1
1
1.20
5.20
9.90
5.90
124.5
112.9
121.4
110.9
125.2
1
2
3
4
1
2
1
2
0.5
0.5
0.5
0.5
0.52
0.99
0.59
1.10
0.68
0.91
0.65
0.99
9.57
57.14
14.10
34.18
2.62
2.11
2.18
1.88
22
19
22
13
2
2 9
CH =CH(CH ) Br) or b-OR elimi-
2
2 9
11.0
nation (for CH =CHCH OMe) or
2
2
[
2
(
a] Conditions: precatalyst (10 mmol), NaBAF (1.2 equiv), total volume of toluene and polar monomer: the formation of stable p-benzyl
0 mL, 1 h, 8 atm ethylene. [b] The yields and activities are averages of at least two runs. Activity
intermediates (for CH =CHCH Ph)
2
2
4
À1
À1 À1
1
Act.)=10 gmol Pd
h
. [c] X =Comonomer incorporation (mol%), determined by H NMR
M
or stable chelate complexes (for
CH =CHCH OAc)
efficiently
spectroscopy . [d] Determined by GPC in trichlorobenzene at 1508C. [e] B=number of branches per
1
added to the total branches. [f] Determined by DSC. [g] CHCl (5 mL, for 1) or CH Cl (5 mL, for 2), total
volume of toluene and polar monomer: 95 mL, 8 atm ethylene, 12 h. [h] CHCl (5 mL, for 1) or CH Cl
1
2
2
000 C atoms, determined by H NMR spectroscopy. The branches ending with functional groups were
[13]
quenched the catalytic activity.
In contrast, high activities (up to
3
2
2
3
2
2
5
À1 À1
(
5 mL, for 2), total volume of toluene and polar monomer: 45 mL, 1 atm, 12 h.
2 ꢀ 10 gmol h ), high copolymer
molecular weights (M up to 5.7 ꢀ
n
5
1
0 ), and good polar monomer
polar monomers. However, only amorphous copolymers
could be generated, and the polar monomer scope is very
limited. Many polar monomers (e.g., vinyl halides, vinyl ether,
acrylonitrile, vinyl acetate, or styrene) completely shut down
the polymerization for various reasons, such as b-halide
elimination, formation of stable chelates, or cationic poly-
incorporation (up to 2.0%) were observed using catalysts
1
13
1 and 2 (Table 2, entries 8–14). The H and C NMR spectra
of these copolymers are shown in the Supporting Information.
It should be noted that all of these polar monomers were
analytically pure and used without any purification. This also
demonstrates the robustness of catalyst 2.
[13]
merization. When a long spacer is put between the double
bond and the polar functional groups (the second group of
polar monomers shown in Scheme 3), it becomes easier to
incorporate the polar monomers in ethylene polymerization.
This strategy has proven to be effective for some early-
Clearly, the capability of catalysts 1 and 2 to significantly
slow down the chain-walking process enables these copoly-
merization reactions. This greatly broadens the polar mono-
II
mer scope for a-diimine–Pd catalyst systems, leading to the
formation of a variety of semicrystalline copolymers. Early-
transition-metal catalysts are widely used in industry for the
production of semicrystalline polyolefin materials. However,
they are easily poisoned by polar functional groups. The
classic Brookhart catalyst A can copolymerize olefins with
[
14]
transition-metal and nickel catalysts. However, this strategy
should not work for Brookhart catalyst A because of the very
fast chain-walking property, which will quickly bring the
functional groups close to the metal center.
[13]
[9]
As in previous reports, the first group of polar mono-
mers shown in Scheme 3 (vinyl ether, vinyl acetate, and
acrylonitrile) completely shut down the ethylene polymeri-
zation process for classic Brookhart catalyst A. In contrast,
significant polymerization activity was observed for catalyst 2
some polar monomers, affording amorphous copolymers.
II
The recently described phosphine-sulfonate Pd catalysts can
generate semicrystalline copolymers, while low copolymer
[
11]
molecular weights were usually achieved. Although special
polar monomers are required, the current catalyst system
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holds great potential for future investigations because of its
unique properties for the generation of various high-molec-
ular-weight, semicrystalline, polar functionalized polyolefin
materials.
pressure of 1 atm, much higher comonomer incorporation
(1.41–6.67%) was realized (Table 3, entries 12–16). This
system is highly robust as high activities and molecular
weights were still observed even with a carboxylic acid
containing polar monomer (Table 3, entries 10 and 11).
Because of the livingness of this system, it is also possible to
prepare block copolymers with blocks of polyethylene and
blocks of polar functionalized polyolefins with molecular
weights of > 1000000 Da (Table S2).
Ultrahigh molecular weight polyethylene (UHMWPE) is
referred to as a specialty class of polyethylene with M values
n
6
7
in the range of 10 –10 . UHMWPE is of great importance and
has wide applications ranging from artificial joints to bullet-
proof vests. This material is predominantly prepared using
early-transition-metal catalysts, which are easily poisoned by
polar monomers.
[15]
As discussed in the introduction, even a small amount of
incorporated polar monomer can dramatically alter the
copolymer properties. The surface properties of the copoly-
mers were evaluated by determining the water contact angle
(WCA) of a copolymer film generated using a solvent process
(see the Supporting Information). The WCA for pure
polyethylene is 1108 (Figure 2, sample from Table 1,
entry 2). The WCAs were determined to be 92.38 and 85.78
when the polar monomer incorporation ratio was 0.35%
(Table 3, entry 6) and 2.48% (Table 3, entry 15). For the acid-
functionalized copolymer (Table 3, entry 11), a WCA of 74.18
was measured. The WCAs of these copolymers are even
Very few late-transition-metal catalysts can afford
[
16]
UHMWPE. Owing to the poisoning of such metal centers
by polar functional groups, the presence of polar monomers
will dramatically reduce the catalytic activity and copolymer
molecular weight (to the level of thousands or even hundreds
Da). This represents one of the biggest challenges to over-
come in this field. With our system, however, very high
activities and particularly high copolymer molecular weights
(
even on the level of millions Da) could be achieved.
When some specific polar monomers were used, it was
[
17]
possible to achieve the direct synthesis of polar functionalized
UHMWPE with this system (Table 3). For entries 1–3, 5, 6,
and 8, copolymers with molecular weights close to or above
lower than that of polystyrene (928), which is a paintable
polymer.
In conclusion, we have demonstrated that the installation
of a naphthalene or benzothiophene substituent in the
a-diimine ligand gave palladium catalysts that perform
extremely well in ethylene polymerization and copolymeriza-
tion. In ethylene homopolymerization, these catalysts showed
high activity and great thermal stability, generating polyeth-
ylene materials with extremely high
1
000000 Da could be generated. Meanwhile, significant
amounts (0.12–1.21%) of the polar monomers were incorpo-
rated. Low branching densities (10–20/1000 C) and high
melting temperatures (up to 123.78C) were also observed.
When the polymerization was carried out at an ethylene
[
a]
molecular weights, low branching
densities, and high melting temper-
atures. In ethylene–methyl acrylate
copolymerization, high activities
and high molecular weights were
Table 3: Ethylene/polar monomer copolymerization.
[
b]
[c]
[d]
4
[d]
[e]
[f]
Entry
Cat.
Monomer
[M]
molL
Yield
[g]
Act.
XM
Mn
[10 ]
PDI
B
Tm
À1
[b]
[
]
[%]
[8C]
1
2
3
4
5
6
7
8
9
1
1
1
1
2
2
2
2
2
2
2
1
1
2
2
2
0.2
0.4
0.6
1.2
0.6
1.2
2.4
1.2
2.4
1.2
2.4
0.2
0.6
0.2
0.6
1.2
10.94
4.32
2.71
0.90
31.30
17.61
3.97
3.20
0.88
25.10
17.36
0.07
0.03
0.44
0.16
0.06
9.12
3.60
2.26
0.75
0.06
0.12
0.21
0.32
0.12
0.35
0.73
1.21
2.21
1.61
1.78
3.68
6.67
1.41
2.48
4.76
242
152
1.37
1.48
1.52
1.65
1.21
1.36
1.24
1.85
1.89
1.89
2.17
1.61
1.78
2.59
2.08
1.82
22
20
20
22
10
11
11
14
14
12
15
25
38
15
16
24
105.5
105.8 also achieved.
98.7
26.2
284
105
32.9
105.0
105.2
119.2
118.9
115.7
123.7
119.4
118.7
When a spacer was put between
the double bond and the polar
functional groups (the second
group of polar monomers shown in
Scheme 3), classic Brookhart cata-
lyst A was completely deactivated
owing to its fast chain-walking
properties. In contrast, catalysts
26.1
14.7
3.31
2.67
0.74
20.92
14.47
0.06
0.03
0.37
0.13
0.05
93.3
32.3
61.9
39.6
1.6
1
1
1
1
1
1
1
0
1
2
3
4
5
6
1
and 2 enabled the efficient copoly-
119.6 merization of ethylene with these
106.7 polar monomers with high activity,
[
[
[
[
[
g]
g]
g]
g]
g]
high copolymer molecular weights,
and good comonomer incorpora-
tion. In some cases, copolymers
with molecular weights close to or
1.1
95.7
4.4
122.8
2.1
121.0
1.7
118.6
even above 1000000 Da were gen-
[
a] Conditions: Precatalyst (10 mmol), NaBAF (1.2 equiv), CHCl (5 mL, for 1) or CH Cl (5 mL, for 2),
erated. Furthermore, it was demon-
strated that the introduction of
polar functional groups can dramat-
ically improve the surface proper-
ties of these polymeric materials.
This is a clear demonstration of the
power of ligand design for the
3
2
2
total volume of toluene and polar monomer: 95 mL, 12 h, 8 atm ethylene. [b] The yields and activities are
averages of at least two runs. Activity (Act.)=10 gmol Pd
(
[
ending with functional groups were added to the total branches. [f] Determined by DSC. [g] CHCl3
(
ethylene, 12 h.
4
À1
À1 À1
h . [c] X =Comonomer incorporation
M
1
mol%), determined by H NMR spectroscopy. [d] Determined by GPC in trichlorobenzene at 1508C.
1
e] B=number of branches per 1000 C atoms, determined by H NMR spectroscopy. The branches
5 mL, for 1) or CH Cl (5 mL, for 2), total volume of toluene and polar monomer: 45 mL, 1 atm
2 2
4
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Keywords: a-diimine ligands · copolymerization ·
palladium catalysts · polar monomers · polymerization
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Published online: && &&, &&&&
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5
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Copolymerization
S. Dai, C. Chen*
&&&& — &&&&
Direct Synthesis of Functionalized High-
Molecular-Weight Polyethylene by
Copolymerization of Ethylene with Polar
Monomers
The chain-walking capability of the a-
diimine–palladium catalyst developed by
Brookhart was significantly reduced
through ligand modifications. A new
slow-chain-walking a-diimine–palladium
catalyst enables the copolymerization of
ethylene with a variety of polar mono-
mers, affording semicrystalline copoly-
mers with molecular weights close to or
above 1000000 Da.
6
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These are not the final page numbers!