Fast and Regioselective Polymerization of para-Alkoxystyrene by Palladium Catalysts for Precision Production of High-Molecular-Weight Polystyrene Derivatives

Article abstract of DOI:10.1021/acs.macromol.9b02274

Polymerization of polar vinyl monomers by a coordination-insertion approach is a topic of fundamental importance to the field of polymer synthesis. Herein, we initially report the coordination-insertion polymerization of para-alkoxystyrene (pAOS) monomers by the dibenzobarrelene-based α-diimine palladium catalysts. The unprecedented polymerization characteristics including rapid initiation, fast chain growth, controlled chain transfer, and high-molecular-weight polymer with a narrow distribution (M_w > 1000 kg/mol, PDI < 1.32) reflected previously unrecognized aspects of palladium-catalyzed pAOS polymerization. Chain-end analysis and characterization of palladium intermediates showed that the pAOS monomer was rapidly inserted into the primary palladium species in a full 1,2-regioselectivity, and the chain transfer took place by monomer-assisted β-H abstraction at high monomer concentrations. The resultant polymers showed improved mechanical properties and thermal stabilities and were also attractive candidates of hydrophilic styrenic resin.

Full text of DOI:10.1021/acs.macromol.9b02274

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Fast and Regioselective Polymerization of para-Alkoxystyrene by
Palladium Catalysts for Precision Production of High-Molecular-
Weight Polystyrene Derivatives
Guangfu Liao, Zefan Xiao, Xiaolin Chen, Cheng Du, Liu Zhong, Chi Shing Cheung,
and Haiyang Gao*
School of Materials Science and Engineering, PCFM Lab, GD HPPC Lab, Sun Yat-sen University, Guangzhou 510275, China
S
* Supporting Information
ABSTRACT: Polymerization of polar vinyl monomers by a coordination−
insertion approach is a topic of fundamental importance to the field of polymer
synthesis. Herein, we initially report the coordination−insertion polymerization
of para-alkoxystyrene (pAOS) monomers by the dibenzobarrelene-based α-
diimine palladium catalysts. The unprecedented polymerization characteristics
including rapid initiation, fast chain growth, controlled chain transfer, and high-
molecular-weight polymer with a narrow distribution (Mw > 1000 kg/mol, PDI <
1.32) reflected previously unrecognized aspects of palladium-catalyzed pAOS
polymerization. Chain-end analysis and characterization of palladium inter-
mediates showed that the pAOS monomer was rapidly inserted into the primary
palladium species in a full 1,2-regioselectivity, and the chain transfer took place
by monomer-assisted β-H abstraction at high monomer concentrations. The
resultant polymers showed improved mechanical properties and thermal
stabilities and were also attractive candidates of hydrophilic styrenic resin.
polar monomers (<10 mol %), and no consecutive insertions
of the vinyl monomer into the polymer chain are observed
except for consecutive insertions of acrylate using phosphine-
sulfonato palladium catalysts till now.24,25 Consecutive units of
polar vinyl monomers in Ni/Pd-catalyzed homo- and
copolymerization of polar vinyl monomers with ethylene is
currently inclined to ionic or radical polymerization.26−29 An
alternative strategy of heteroatom-assisted olefin polymer-
ization has been recently developed, and a new family of
heteroatom-functionalized polyolefins is synthesized by rare-
earth metal catalysts.30−35 Despite these achievements, CIP of
polar vinyl monomers has been far from successful.
para-Methoxystyrene (pMOS) is an electron-rich monomer
because of a strong electron-donating effect of the methoxy
group. pMOS is readily polymerized by cationic or radical
method,36−38 and is also prone to coordinate to the metal
center by a π-binding manner in metal-mediated catalysis.
Although pMOS has been polymerized with titanium and
palladium catalysts, the obtained homopolymers have low
molecular weight (<30 kg/mol), and polymerization mecha-
nisms are not clearly demonstrated.39−41 CIP of pMOS is only
achieved by rare-earth metal catalytic systems.32,33,42,43
INTRODUCTION
■
Coordination−insertion polymerization (CIP) is a powerful
method to synthesize polymers with precisely designed
macromolecular architecture, especially chain composition,
molecular weight and distribution, and stereochemistry.1,2 In
the modern “Plastic Age”, CIP of ethylene and propylene is
employed for the production of more than 100 million tons of
polyolefins annually.3 However, CIP of polar vinyl monomers
has remained elusive due to the deleterious side reactions that
arise from heteroatom interactions with metal centers.4−6 In
contrast to ethylene and α-olefins, polar vinyl monomers are
predominately polymerized via radical and ionic methods.
Therefore, polymerization of polar vinyl monomers by a
coordination−insertion approach is a topic of fundamental
importance to the field of polymer synthesis.7,8
CIP of polar vinyl monomers using early metal catalysts is
very difficult because of their high oxophilicity unless
appropriate strategies, such as steric shielding or electronic
protecting of the functional groups of the polar monomers.4,9
Late transition metal Ni/Pd catalysts are less oxophilic by
comparison to early transition metal catalysts, and therefore
can directly catalyze copolymerization of olefins and polar vinyl
monomers.10−23 Although late transition metal catalysts,
especially palladium-based catalysts, are compatible with a
broad scope of polar vinyl monomers, CIP of polar vinyl
monomers normally suffers from the heavily reduced activity
and a low molecular weight of the copolymer produced. The
obtained copolymers usually contain low incorporation of
In this paper, we initially report the coordination−insertion
polymerization of polar para-alkoxystyrene (pAOS) monomers
Received: October 30, 2019
Revised: December 11, 2019
© XXXX American Chemical Society
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Figure 1. Molecular structures of dibenzobarrelene-based α-diimine palladium complexes.
(CH2CH(Ph-4-OR), R = Me, Et) using dibenzobarrelene-
based α-diimine palladium catalysts. The bulky dibenzobarre-
lene backbone of α-diimine palladium catalysts markedly
accelerated the chain growth rate and improved the 1,2-
regioselectivity of pAOS polymerization. At high monomer
concentrations, chain transfer reaction occurred exclusively by
monomer-assisted β-H abstraction, and narrowly dispersed
poly(para-alkoxystyrene)s (PAOS) with high molecular
weights up to 1000 kg/mol were precisely produced after
only a few minutes.
ization of styrenic monomers through a coordination−
insertion mechanism. Styrene derivatives were previously
used as chain quenching agents to quench ethylene polymer-
ization by α-diimine palladium catalysts.46,47 For our
dibenzobarrelene-based α-diimine palladium systems, styrene
(St) polymerization also did not produce polymers (entry 1 in
Table 1) because of the rapid benzyl transfer reaction (Scheme
S4 and Figure S36). Strikingly, dibenzobarrelene-based
palladium catalysts 1−6 were highly active for pMOS
polymerization with high monomer conversions.
The influences of the catalyst structure on pMOS polymer-
ization were first evaluated. Under the same conditions, a
comparison of pMOS polymerization using catalysts 1−3
clearly demonstrated the steric effects of o-aryl substituents.
With increasing steric bulk of o-aryl substituents, polymer-
ization activity and monomer conversion increased slightly.
However, the polymer molecular weight was independent of
the steric bulk of o-aryl substituents, and the least bulky
palladium catalyst 1 without a substituent produced a high-
molecular-weight poly(para-methoxystyrene) (PMOS) with a
molecular weight of ∼1000 kg/mol. This observation was
different from the previously reported steric effect of o-aryl
substituents on ethylene polymerization using α-diimine nickel
and palladium catalysts.11 Although the least bulky palladium
catalyst 1 produced narrowly dispersed polymers, increasing
the steric bulk of o-aryl substituents led to a decreased
polydispersity index (PDI) of polymers. This observation
indicated that bulky o-aryl substituents more effectively
suppressed the chain transfer. The electronic effects of
meta-/para-aryl substituents were also illuminated by pMOS
polymerizations using palladium catalysts 1 and 4−6 (entries 5
and 17−19 in Table 1). The introduction of electron-donating
methoxy groups on meta-/para-positions of aniline hardly
affected pMOS polymerization, and catalysts 1 and 4−6 almost
showed the same activity and produced the same polymers. As
a comparison, a classic methyl-based α-diimine palladium
catalyst 7 was also used to catalyze pMOS polymerization
(entry 20 in Table 1).11 Under the same conditions, methyl-
based α-diimine palladium catalyst 7 produced traces of
polymers, indicating that the dibenzobarrelene backbone was a
strategic substituent for pMOS polymerization.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Palladium Com-
plexes. A series of dibenzobarrelene-based α-diimine
palladium complexes (Figure 1) was prepared by the
complexation of dibenzobarrelene-based α-diimine ligands
L1−L6 with different substituents and (COD)PdMeCl
(COD: 1,5-cyclooctadiene) (Scheme S2 in Supporting
Information).45 Palladium complexes were fully confirmed
and characterized by 1H and 13C NMR spectroscopies (Figures
S5−S28). Single crystal structures of palladium complexes 2−6
were previously reported,19,44,45 and a single crystal of
palladium complex 1 suitable for X-ray diffraction analysis
was also obtained herein. Palladium complex 1 displays a
distorted square planar coordination around the palladium
center, and the rigid dibenzobarrelene backbone effectively
shields the back space of the palladium center (Figure 2).
Polymerization of para-Alkoxystyrene. After treatment
with sodium tetrakis(3,5-bis-trifluoromethyl)phenyl)borate
(NaBArF), chloromethylpalladium complexes 1−6 were
activated for olefin polymerization.21 It was reported that α-
diimine palladium catalysts were usually inactive for polymer-
Note that pMOS polymerizations using all 1−6 catalysts
afforded narrowly dispersed polymers (PDI ≈ 1.2), suggesting
living/controlled characteristics. Therefore, pMOS polymer-
Figure 2. Crystal structure of palladium complex 1. The hydrogen
atoms and 0.5 CH2Cl2 molecules are omitted for clarity.
B
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a
Table 1. Polymerization Results of pAOS Catalyzed by Palladium Catalysts
b
c
c
entry
catalyst
monomer
time (min)
yield (g)
conv. (%)
act.
Mn (kg/mol)
PDI
1
2
1−3
1
St
1440
5
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pMOS
pEOS
pEOS
pEOS
pEOS
0.1819
0.3432
0.5126
0.8014
0.2082
0.3912
0.5839
0.8943
0.4788
0.6378
0.8162
0.9896
0.3265
0.7149
0.9868
0.8072
0.7800
0.7992
traces
18.02
31.82
50.80
79.43
20.63
38.77
57.87
88.63
47.45
63.21
80.89
98.07
32.36
70.85
98.06
80.00
77.30
79.21
437
412
410
321
500
469
467
358
1149
766
653
396
131
143
132
323
312
320
1011
987
962
853
933
923
902
909
943
917
908
889
749
791
735
935
858
877
1.15
1.16
1.16
1.21
1.11
1.12
1.14
1.17
1.08
1.09
1.13
1.15
1.17
1.19
1.19
1.22
1.24
1.26
3
4
5
6
7
8
9
10
11
12
13
14
15
1
1
1
2
2
2
2
3
3
3
3
3
3
3
4
5
10
15
30
5
10
15
30
5
10
15
30
30
60
90
30
30
30
30
5
d
d
d
16
17
18
19
20
21
22
23
24
6
7
e
3
3
3
3
0.3924
0.6019
0.8085
0.9767
39.64
60.80
81.67
98.66
942
722
647
391
900
886
881
824
1.09
1.10
1.11
1.17
10
15
30
a
b
Reaction conditions: 5 μmol of Pd, 1.2 equiv of NaBArF, 10 mL of CH2Cl2, 1 mL of monomer, 30 °C. Activity, in the unit of kg PAOS/(mol·Pd·
c
d
e
h). Determined by GPC using THF as the eluent. Polymerization proceeded under air with RH of 45%. 7 is a conventional methyl-based α-
diimine palladium complex, and traces of polymer was produced.
a
Table 2. pAOS Polymerizations by Sequential Monomer Addition Using Catalyst 3
b
c
c
entry
M/t1 (min)
M/t2 (min)
yield (g)
conv. (%)
act.
Mn (kg/mol)
PDI
1
2
3
4
5
6
7
pMOS/30
pMOS/30
pMOS/30
pMOS/30
pMOS/30
pMOS/30
pMOS/30
0.4894
0.7504
0.8676
0.9629
0.7335
0.8887
0.9572
97.01
74.37
85.99
95.43
73.72
88.91
95.77
195
180
174
144
176
177
164
668
684
716
711
705
751
720
1.29
1.33
1.29
1.30
1.32
1.27
1.29
pMOS/10
pMOS/20
pMOS/30
pEOS/10
pEOS/20
pEOS/30
a
b
Reaction conditions: 5 μmol of palladium catalyst 3, 1.2 equiv of NaBArF, 10 mL of CH2Cl2, 30 °C, 0.5 mL of monomer for each stage. Activity
c
in kg PAOS/(mol·Pd·h). Molecular weight and PDI were determined by GPC using tetrahydrofuran as the eluent.
izations at different times were performed to gain deep insight
into polymerization characteristics. As shown in Table 1,
catalysts 1−3 showed the same trend within the nearly full
conversion of monomers. Polymer molecular weight reached
∼1000 kg/mol at a short time of 5 min, indicating a very fast
chain growth rate. With prolonging polymerization time from 5
to 30 min, the monomer conversion increased consistently,
whereas the Mn of polymer slightly decreased. This finding
strongly indicated that dibenzobarrelene-based α-diimine
palladium catalysts were not living systems for pMOS
polymerization. During polymerization, chain termination
typically occurred to produce a high-molecular-weight polymer
chain and new active species that was able to reinitiate pMOS
polymerization.
dispersed PMOS (entries 14−16 in Table 1). However, the
monomers were fully consumed after 90 min, indicating that
the moisture and the oxygen in air only reduced the chain
growth rate, and did not poison the palladium species and
quench the polymerization. Palladium catalyst 3 was also
highly active for polymerization of para-ethoxystyrene (pEOS)
and produced narrowly dispersed poly(para-ethoxystyrene)
(PEOS) with high molecular weight. As observed, the pEOS
polymerization behavior was the same as that of pMOS
(entries 21−24 in Table 1).
The polymerization of pAOS using catalyst 3 was further
studied by sequential monomer addition. As shown in Table 2,
polymerization of half an amount of pMOS (0.5 mL) was first
performed for 30 min to ensure full conversion of the pMOS
monomer and resulted in the formation of the PMOS with Mn
of 668 kg/mol and PDI of 1.29. Additional half an amount of
pMOS monomer was sequentially added into the system, and
the polymerization was reinitiated. After 10 min, the molecular
weight of the final PMOS was slightly increased to 684 kg/mol,
Catalyst 3 was in fact able to catalyze pMOS polymerizations
under mild reaction conditions. pMOS polymerizations
smoothly proceeded under an atmosphere of air with relative
humidity (RH) of 45% instead of nitrogen. Under air, catalyst
3 showed reduced activity and still produced narrowly
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1
Figure 3. H and 13C NMR spectroscopy of low-molecular-weight PMOS.
and the PDI remained narrow (PDI = 1.33). When pEOS was
used as an additional monomer instead of pMOS, the same
trend was observed. Despite narrow distribution, the polymeric
products were a blend of two homopolymers rather than a
block copolymer because of almost no increase in polymer
molecular weight and a tiny discrepancy of the molecular
weight between two homopolymers (Figure S35).
Generally, the para-alkoxystyrene (pAOS) polymerization
using dibenzobarrelene-based α-diimine palladium catalysts
showed unique characteristics including rapid initiation, fast
chain growth, controlled chain transfer, nearly quantitative
conversion of monomer and high activity, and high molecular
weight. The active species was long-lived and showed good
tolerance toward polar groups and high atomic economics
because the polymerization of newly added pAOS monomers
could be reinitiated to produce new polymer chains.
Mechanistic Study. Currently, a crucial polymerization
mechanism involving the nature of the active species and the
chain transfer manner for pAOS polymerization remains
unanswered. Typically, pAOS could be polymerized by a
cationic method because of the strong electron-donating effect
of the alkoxy group.48−50 However, the cationic polymerization
of pAOS usually produced well-defined polymers (PDI < 1.2)
with lower than 100 kg/mol molecular weight under harsh
conditions. The control pMOS polymerization using NaBArF
as a cationic initiator produced a trace of polymer with a
relatively low molecular weight (Mn = 61 kg/mol, PDI = 2.45)
under adopted conditions (Table S6 in SI). Therefore, the
influence of NaBArF should be excluded in pMOS polymer-
izations. Besides, pMOS polymerizations exposed to air
produced high-molecular-weight polymers with a narrow
distribution in our palladium system. No palladium black
(Pd(0)) was observed during the pMOS polymerization, but it
was previously reported that palladium black was closely
related to the cationic polymerization of vinyl ethers with a
classic methyl-based α-diimine palladium catalyst.28 Further-
more, the copolymerization of pMOS with ethylene using
catalyst 3 produced the copolymer with low ethylene
incorporation of 5.4 mol % because of the slow ethylene
insertion rate. The sequential addition copolymerization
produced a blend of two homopolymers because of fast
chain growth and controlled chain transfer of pMOS
polymerization (Table S4 in SI). These copolymerization
results of pMOS with ethylene strongly indicated a
coordination−insertion mechanism. In combination with the
catalyst structure-polymerization activity relationship, it was
deduced that palladium-catalyzed polymerization of pAOS
should obey a coordination−insertion mechanism, and the
cationic mechanism was ruled out.
Chain-End Analysis. The polymer chain-end analysis can
provide more information on the chain initiation and
termination and further elucidates the polymerization mech-
anism. The low-molecular-weight PMOS (Mn = 5200, Figure
S68) was prepared at a very low monomer concentration for
NMR analysis. The polymer exhibited weak signals Hh at 5.2−
1
5.7 ppm in H NMR spectroscopy (Figure 3A), which was
indicative of the vinyl end group (CH2CH(4-MeO-Ph)−)
produced by β-H elimination or monomer-assisted β-H
abstraction. 13C NMR spectrum of the polymer also proved
the existence of the vinyl end group (Figure 3B) on the basis of
the assignment of the weak peak Ch at 110.1 ppm. The peaks
Cl and Cm at 134.6 and 126.5 ppm were safely assigned to
ortho- and meta-phenyl protons of the chain end, not to the
doubt bond of the vinylene end group (CH(4-MeO-Ph)
CH−) according to previous assignments.51,52 No signals were
observed at 5.8−6.0 ppm in 1H NMR spectroscopy, suggesting
no existence of the vinylene end group (CH(4-MeO-Ph)
CH−) in the polymer chain end.53 The major 4-MeO-benzyl
terminal (CH2(4-MeO-Ph)−CH2−) was also confirmed by
the peak at 32.74 ppm in 13C NMR spectroscopy. The minor
methyl terminal (CH3CH(4-MeO-Ph)−CH2−) generated at
the chain initiation step was also confirmed by the weak peak
1
at 0.84 ppm in H NMR spectroscopy.
The polymer chain structure of the low-molecular-weight
PMOS was further demonstrated by MALDI-TOF mass
analysis. As shown in Figure 4, two series of polymers with a
molecular weight difference of 14 were observed, which were
separately attributed to the vinyl-end polymer I with a 4-MeO-
benzyl terminal and polymer II with a methyl terminal. This
result was well consistent with NMR analysis.
The appearance of the vinyl end group (CH2CH(4-MeO-
Ph)−) proved that the pMOS polymerization did not obey a
cationic mechanism, which typically produced a vinylene end
group because of the 2,1-insertion of the monomer.48−50
Therefore, the vinyl end group (CH2CH(4-MeO-Ph)−)
was originated from the chain transfer by β-H elimination or/
and monomer-assisted β-H abstraction in the aprotic CH2Cl2
solvent.54,55 A direct way to discriminate between the two
mechanism is through the study of the effects of monomer
concentrations [M] on the degree of polymerization (DP).56
D
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was prepared by treatment of chloromethylpalladium complex
3 with NaBArF and 10 equiv pMOS in CH2Cl2 (Scheme
S5).59 pMOS-treated palladium complexes were obtained in
high yield after the removal of polymeric product. The
1
obtained palladium products showed two sets of peaks in H
NMR spectroscopy in a ratio of ∼10/1 (Figure 6). H-H COSY
Figure 4. MALDI-TOF mass spectrum of low-molecular-weight
PMOS.
Chain transfer constants have been determined using
variations of the Mayo equation from theoretical consid-
erations of polymerization kinetics,57,58 a form of it is given
below on the basis of the omission of the chain transfer to the
cocatalyst and solvent:
1
Figure 6. H NMR spectrum of pMOS-treated palladium mixture
(Pd-MOS1 and Pd-MOS2).
ktrM
kp
ktH
kp[M]
1
DP
analysis (Figure S37) further confirmed that the isolated
palladium products were a mixture of two π-benzyl palladium
complexes Pd-MOS1 [(L3)Pd(η3 -CH(CH2 CH3 )-
C6H5]+BArF− and Pd-MOS2 [(L3)Pd(η3-CH(CH3)-
C6H5]+BArF− but not diastereomers (anti- and syn-isomers)
of the π-benzyl palladium complex on the basis of previous
assignment.60 No syn-isomer was observed by NMR spectros-
copy because of unfavorable interactions of the anti-methyl
group and the bulky dibenzobarrelene-based α-diimine ligand.
These interactions were also observed for anti-isomer of
palladium on the basis of great shift of the proton of the
methylene toward the high field (−0.21 ppm) because of
extremely bulky ligand.60
=
+
where DP is the number-average degree of polymerization, kp
is the propagation rate coefficient, ktrM is the rate coefficient of
monomer-assisted β-H abstraction, ktH is the rate coefficient of
β-H abstraction, and [M] is the monomer concentration.
A series of polymerization experiments at different monomer
concentrations were performed (Table S2), and polymer-
izations were quenched at short times to assure low monomer
conversion (<20%) and to reduce the influence of monomer
concentration change. The effect of [M] on the DP of the
resultant polymer is shown in Figure 5. Two linearities and a
A single crystal of the major palladium product Pd-MOS1
suitable for X-ray analysis was obtained and depicted in Figure
7. The single-crystal structure confirmed that Pd-MOS1 was
Figure 5. Plot of 1/DP versus 1/[M] for pMOS polymerizations
catalyzed by palladium catalyst 3 with low monomer conversions.
Figure 7. Crystal structure of η3-π-benzyl palladium complex Pd-
MOS1.
critical point corresponding to critical [M] of ca. 0.59 mol/L
were clearly observed. DP is independent of [M] above critical
[M], strongly indicating the occurrence of monomer-assisted
β-H abstraction in pMOS polymerization. Below critical [M],
chain transfer took place by β-H elimination and monomer-
assisted β-H abstraction. Generally, the two transfer constants
(ktrM/kp and ktH/kp) were small (10−4 order of magnitude);
therefore, high-molecular-weight polymers were formed.
Palladium Intermediate Analysis. To gain a deep insight
into the palladium-active species for coordination−insertion
polymerization of pMOS, pMOS-chelated palladium complex
an η3-π-benzyl complex with anti-configuration. Palladium
complex Pd-MOS1 was formed by the insertion of pMOS into
a Pd−Me bond in a 2,1-mode and followed methyl migration
to the methylene carbon (Scheme 1). Palladium complex Pd-
MOS2 was produced by the benzyl transfer reaction of Pd-
MOS1 with excess pMOS.59 The isolated palladium mixture
was stable and inactive for pMOS polymerization, which
indicated that the π-benzyl palladium complexes were inert
E
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Scheme 1. Proposed Mechanistic Pathways for Palladium-Catalyzed pMOS Polymerization
Figure 8. 13C NMR of high-molecular-weight PMOS (A) and PEOS (B) (entries13 and 24 in Table 1).
species for pMOS polymerization. The true active species was
not isolated and afforded polymer because of the fast chain
growth rate.
monomers is only observed for [N,N] bidentate palladium
catalysts with 1,10-phenanthroline, 2,2′-bipyridyl, bisoxazoline,
and conventional α-diimine ligands.59,62 To the best of our
knowledge, this is the first report on 1,2-regioselective Pd-
catalyzed homopolymerization of styrenic monomers in sharp
contrast to the predominant 2,1-selectivity by radical and ionic
polymerization.
Building on the above experimental results, the pMOS
polymerization mechanism catalyzed by dibenzobarrelene-
based α-diimine palladium catalysts is proposed (Scheme 1).
In the presence of NaBArF, the chloromethylpalladium
complex is activated to form a methylpalladium complex
with a vacant orbit. The pMOS monomer majorly inserted a
Pd−Me bond in 2,1-fashion, anti-η3-π-benzyl palladium
complex Pd-MOS1 is obtained by the methyl migration to
the methylene carbon. Palladium complex Pd-MOS2 is further
produced by the benzyl transfer reaction of Pd-MOS1 with an
excess pMOS monomer. η3-π-Benzyl palladium complexes are
stable and inert for pMOS polymerization. When the 1,2-
insertion of pMOS into a Pd−Me bond occurs, the palladium
In combination with the above chain-end analysis, the
methyl terminal was originated from the pMOS monomer
insertion into the Pd−Me in a 1,2-fashion at the initiation step
and the 4-MeO-benzyl terminal (CH2(4-MeO-Ph)−CH−)
came from the monomer-assisted β-H abstraction in 1,2-
regioselectivity. 13C NMR spectrum of low-molecular-weight
polymer also showed regioregular head-to-tail enchainment
and no detectable head-to-head and tail-to-tail enchainments
(Figure 3B). These results showed that the pMOS monomer
inserted into the Pd−R bond in a 1,2-mode with full
regioselectivity. It is reported that an electron-rich ethyl vinyl
ether is majorly inserted into the Pd−R bond with a 52% 1,2-
selectivity in the copolymerization of ethylene and ethyl vinyl
ether by a phosphine-sulfonato palladium catalyst.61 1,2-
Selective insertion of styrenic monomers has been rarely
observed for phosphine-ligated Pd-catalyzed copolymerizations
of CO and styrene, while the 2,1-insertion of styrenic
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Figure 9. DSC curves (A) and WAXD patterns (B) of PMOS and PEOS (entries13 and 24 in Table 1).
Figure 10. Comparison of mechanical, thermal, and hydrophilic properties for PS, CPMOS, and PMOS samples.
species for pMOS polymerization is produced. The chain is
rapidly propagated by 1,2-insertion of pMOS. The 1,2-
regioselectivity can be reasonably explained by the enantiose-
lectivity of the α-olefin in a coordination−insertion mecha-
nism. It is widely acceptable that steric factors instead of
electronic factors determined the enantioface of α-olefin,
thereby the bulky and rigid dibenzobarrelene backbone leads
to high 1,2-regioselectivity (Scheme S6).53,62−66 This is well
consistent with the experimental observation that the
dibenzobarrelene-based palladium catalyst is much more active
than the methyl-based analogue for pMOS polymerization.
The chain transfer takes place by monomer-assisted β-H
abstraction to produce Pd−CHCH2(Ph-4-OMe) species with
the Pd···phenyl interaction, which can reinitiate pMOS
polymerization by 1,2-insertion. In the whole process, the
chain growth rate is much faster than the chain transfer rate,
thereby producing a high-molecular-weight polymer with a
narrow distribution.
Polymer Characterization and Properties. All high-
molecular-weight poly(para-alkyloxystyrene)s produced by
dibenzobarrelene-based palladium catalysts showed low stereo-
regularity. 13C NMR analysis confirmed that the obtained
PMOS and PEOS were atactic polymers, which was a result of
poor stereoselectivity of Ni/Pd catalysts (Figure 8). In the
carbon atom C1 region, three peaks at 138.3, 137.7, and137.4
ppm were assigned to isotactic triad (mm), heterotactic triad
(mr), and syndiotactic triad (rr), respectively, indicating that
the obtained PMOS (rr = 52%) and PEOS (rr = 62%) was a
syndio-rich polymer. DSC curves of the poly(para-
alkyloxystyrene)s exhibited a single glass transition temper-
ature (Tg = 110 °C for PMOS and 90 °C for PEOS) without a
melting point (Figure 9A). The wide-angle X-ray diffraction
(WAXD) analysis of the PAOS showed a broad peak (2θ =
18.6° for PAOS and 23.1° for PEOS), indicating that PMOS
and PEOS were amorphous (Figure 9B).
High molecular weight was expected to endow polymeric
materials with improved properties. Commercial polystyrene
(PS) purchased from SINOPEC (Mn = 110 kg/mol, PDI =
2.47, Figure S71) and PMOS prepared by a cationic
polymerization using BF3·(OEt)2 as an initiator (Mn = 130
kg/mol, PDI = 2.81, Figure S72) (CPMOS) were chosen for a
comparison. The stress−strain curves of three samples showed
plastic deformation behavior of typical thermoplastics. The
mechanical properties of high-molecular-weight PMOS,
including tensile strength and breaking elongation, are
somewhat improved by a factor of ∼15% (Figure 10). High
molecular weight also enhanced the thermal stability of
polymeric materials. The thermogravimetric analysis (TGA)
of three samples showed that PMOS catalyzed by palladium
catalysts had the highest decomposition onset temperature
(temperature at 5% weight loss, Td,5%) up to 392 °C. The water
surface contact angle (WSCA) was tested to evaluate the
hydrophilic properties of these three samples. The PMOS
showed a contact angle of 61.6 0.6°, substantially lower than
that observed for commercial PS (98.6 0.5°), CPMOS (80.9
0.3°, Mn =130 kg/mol) produced by a cationic method, and
syndiotactic PMOS produced by yttrium catalysts (83.7°, Mn =
55 kg/mol).42 Therefore, the PMOS should display superior
adhesion to polar surfaces. The enhanced hydrophilic proper-
ties might be mainly ascribed to the high-density polar
methoxy group existing in the long polymer chains.67−69
Collectively, high-molecular-weight PMOS prepared by our
palladium systems showed improved mechanical properties
and thermal stability, and the polarity of methoxy groups
substantially enhanced the adhesion of materials to polar
surfaces relative to hydrophobic PS.
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direct methods, while further refinement with full-matrix least-squares
on F2 was obtained with the SHELXTL program package. All
nonhydrogen atoms were refined anisotropically. Hydrogen atoms
were introduced in calculated positions with the displacement factors
of the host carbon atoms.
CONCLUSIONS
■
In summary, we have reported fast and regioselective
coordination−insertion polymerization of polar para-alkoxys-
tyrene (CH2CH(Ph-4-OR), R = Me, Et) by dibenzobarre-
lene-based α-diimine palladium catalysts. The bulky dibenzo-
barrelene backbone markedly accelerated the chain growth
rate, and narrowly dispersed polymers with high molecular
weight (Mw > 1000 kg/mol, PDI < 1.3) were produced after
only 30 min with nearly quantitative monomer conversions.
Chain-end analysis of the polymer confirmed that palladium-
catalyzed pAOS polymerization obeyed a coordination−
insertion mechanism rather than a cationic method. The
mechanistic study demonstrated that the primary palladium
species was highly active and catalyzed pAOS polymerization
in a full 1,2-regioselectivity. The chain transfer occurred by the
monomer-assisted β-H abstraction at high monomer concen-
trations to produce a high-molecular-weight polymer with a
narrow distribution. The resulting atactic PMOS showed
improved mechanical properties, thermal stability, and hydro-
philicity, and had a high potential for applications as high-
performance styrenic resins.
Synthesis of para-Alkoxystyrene. para-Methoxystyrene
(pMOS) and para-ethoxystyrene (pEOS) were synthesized by Wittig
reactions according to the related literature (Scheme S1).42,70
A
typical synthetic route of pMOS was shown as follows. In a dry 250
mL three-necked round-bottom flask, the methyltriphenylphospho-
nium bromide (10.6004 g, 29.7 mmol), potassium tert-butoxide
(3.0485 g, 27.2 mmol), and dry THF (100 mL) were added with
stirring at 0 °C under a nitrogen atmosphere. After stirring for 30 min,
para-methoxybenzaldehyde (24.7 mmol, 3 mL), which dissolved in 20
mL dry THF, was added dropwise into the flask. The reaction was
stirred for 24 h, and then 10 mL deionized water was added to the
flask to terminate the reaction. The mixture was extracted with
petroleum ether 6 times (6 × 20 mL), and the organic layer was
collected and dried with anhydrous Na2SO4 overnight. The solution
was filtered, evaporated to remove the solvents, and then was purified
by column chromatography using pure petroleum ether as the solvent.
The solution was evaporated to remove the solvents and finally
distilled under reduced pressure over CaH2 to obtain a colorless liquid
(yield: 80%). The pEOS were also synthesized by a similar method.
1
pMOS, H NMR (400 MHz, CD3OD), δ (ppm): 7.37−7.32 (m,
EXPERIMENTAL SECTION
2H, Ar-H), 6.89−6.83 (m, 2H, Ar-H), 6.70−6.60 (q, 1H, CH), 5.64−
5.57 (dd, 1H, CH2), 5.15−5.09 (dd, 1H, CH2), 3.80 (s, 3H, OCH3).
13C NMR (100 MHz, CDCl3), δ (ppm): 159.28 (Ar-C), 136.14
(CH), 130.30 (Ar-C), 127.27 (Ar-C), 113.79 (Ar-C), 111.37 (CH2),
55.04 (OCH3).
■
General Procedures. All manipulations involving air- and
moisture-sensitive compounds were carried out under an atmosphere
of dried and purified nitrogen using standard vacuum-line, Schlenk, or
glovebox techniques.
1
pEOS, H NMR (400 MHz, CD3OD), δ (ppm): 7.36−7.31 (m,
Materials. Dichloromethane was distilled from P2O5, tetrahy-
drofuran was distilled from CaH2, and toluene and hexane were from
Na/K alloy under nitrogen. Anthracene, vinylene carbonate, and
trifluoroacetic acid were purchased from Energy Chemical. 2,6-
Diisopropylaniline was purchased from Aldrich Chemical and was
distilled under reduced pressure before use. ZnCl2 was purchased
from TCI and stored in an oven at 110 °C overnight before use.
Methyltriphenylphosphonium bromide, BF3·(OEt)2, potassium tert-
butoxide, para-methoxybenzaldehyde, para-ethoxybenzaldehyde,
ortho-methoxybenzaldehyde, aniline, 2,6-dimethylaniline, 2,6-diiso-
propylaniline, and potassium oxalate were also purchased from Energy
Chemical. All α-diimine palladium complexes were prepared by the
reaction of Pd(COD)MeCl with the corresponding α-diimine ligand
according to the previously reported method.45 The ligands and
2H, Ar-H), 6.88−6.82 (m, 2H, Ar-H), 6.70−6.60 (q, 1H, CH), 5.63−
5.57 (dd, 1H, CH2), 5.13−5.08 (dd, 1H, CH2), 4.07−4.00 (q, 2H,
OCH2CH3), 1.44−1.38 (t, 3H, OCH2CH3). 13C NMR (100 MHz,
CDCl3), δ (ppm): 158.64 (Ar-C), 136.17 (CH), 130.10 (Ar-C),
127.08 (Ar-C), 114.20 (CH2), 111.12 (Ar-C), 63.09 (OCH2CH3),
14.69 (OCH2CH3).
Polymerization Procedure. A detailed polymerization procedure
of pAOS was described as follows. A round-bottom Schlenk flask with
a stirring bar was heated for 1 h at 150 °C under vacuum and then
cooled to room temperature. Next, 1 mL of pMOS and 9 mL of
CH2Cl2 were injected into the round-bottom Schlenk flask at 30 °C.
Then, 1 mL of palladium catalyst solution consisting of palladium
complexes (5 μmol) and NaBArF (6 μmol) in CH2Cl2 was then
injected into the round-bottom Schlenk flask at 30 °C. The reaction
mixture was continuously stirred at a polymerization temperature of
30 °C. After a certain polymerization time, the reaction was quenched
with 100 mL mixture of 10% HCl solution of methanol. The resulting
precipitated polymers were collected and treated by filtration, washed
with methanol three times, and dried in a vacuum at 40 °C to a
constant weight.
palladium complexes were fully characterized by H and 13C NMR
(Figures S5−S28 in the Supporting Information).
1
Measurements. Elemental analyses were performed on a Vario
EL microanalyzer. NMR spectra of all samples were carried out on
Bruker 400 or 600 MHz instruments in CDCl3 using TMS as a
reference. MALDI-TOF spectroscopy was performed on a Bruker
ultrafleXtreme. GPC analyses of the molecular weights and molecular
weight distributions (PDI = Mw/Mn) of the polymers were performed
on an Agilent technologies 1260 infinity equipped with a differential
refractive-index detector. Tetrahydrofuran (THF) was used as the
eluent at a flow rate of 1.0 mL/min. DSC analyses were conducted
with a Perkin-Elmer DSC-4000 system. The DSC curves were
recorded as second heating curves from 30 to 200 °C at a heating rate
of 10 °C/min and a cooling rate of 10 °C/min. Thermogravimetric
analysis (TGA) was carried out with a Perkin-Elmer TGA-7 thermal
gravimetric analyzer at a heating rate of 10 °C/min from 30 to 800 °C
under a nitrogen flow. The mechanical properties were determined by
stress/strain measure under uniaxial tension (5 mm/min) using
CMT4104. The water surface contact angle (WSCA) test was
Synthesis of Low-Molecular-Weight PMOS. Following the
polymerization procedure, 0.01 mL of pMOS monomer was used for
polymerization. The low-molecular-weight PMOS was obtained, and
its molecular weight was analyzed by GPC analysis (Mn = 5200, PDI
= 2.27).
Synthesis of CPMOS by Cationic Polymerization. A detailed
synthesis procedure of CPMOS by cationic polymerization38 was
described as follows. A round-bottom Schlenk flask with a stirring bar
was heated for 1 h at 150 °C under vacuum and then cooled to room
temperature. Next, 1 mL of pMOS and 10 mL of toluene were
injected into the round-bottom Schlenk flask at 0 °C. Then, 0.1 mL of
BF3·(OEt)2 was injected into the round-bottom Schlenk flask at 0 °C
for 0.5 h. The reaction was initiated and continuously stirred at 0 °C.
After 0.5 h, the reaction was quenched with 100 mL mixture of 10%
HCl solution of methanol. The resulting precipitated polymers were
collected and treated by filtration, washed with methanol several
times, and dried in a vacuum at 40 °C to a constant weight. The
conducted on a goniometer (Kruss, DSA100S).
̈
Crystal Structure Determination. The crystals of palladium
complexes were mounted on a glass fiber and transferred to a Bruker
CCD platform diffractometer. Data obtained with the ω−2θ scan
mode were collected on a Bruker SMART 1000 CCD diffractometer
with graphite-monochromated Cu Kα radiation (λ = 1.54184 Å) or
Mo Kα radiation (λ = 0.71073 Å). The structures were solved using
molecular weight and distribution were analyzed by GPC (Mn
=
130 631, PDI = 2.81).
H
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Preparation Process of Polymer Films. A detailed procedure of
polymer films was described as follows. The polymer (10 mg) was
dissolved in 10 mL of CH2CL2, and then it was subjected to
ultrasound for 1 h. Subsequently, the homogeneous polymer solution
was cast onto a level glass plate for 12 h, and polymer films were
successfully prepared.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.macromol.9b02274.
NMR spectra of ligands and palladium complexes,
crystallographic data of palladium complexes, and
NMR, DSC, GPC, and XRD of polymers (PDF)
Crystallographic data (cif) (cif)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gaohy@mail.sysu.edu.cn. Fax: +86-20-84114033.
Tel.: +86-20-84113250.
(15) Liu, J.; Chen, D.; Wu, H.; Xiao, Z.; Gao, H.; Zhu, F.; Wu, Q.
Polymerization of α-Olefins Using a Camphyl α-Diimine Nickel
Catalyst at Elevated Temperature. Macromolecules 2014, 47, 3325−
3331.
ORCID
Guangfu Liao: 0000-0003-1299-8106
Haiyang Gao: 0000-0002-7865-3787
(16) Liu, F.-S.; Hu, H.-B.; Xu, Y.; Guo, L.-H.; Zai, S.-B.; Song, K.-
M.; Gao, H.-Y.; Zhang, L.; Zhu, F.-M.; Wu, Q. Thermostable α-
Diimine Nickel(II) Catalyst for Ethylene Polymerization: Effects of
the Substituted Backbone Structure on Catalytic Properties and
Branching Structure of Polyethylene. Macromolecules 2009, 42, 7789−
7796.
(17) Zhong, L.; Li, G.; Liang, G.; Gao, H.; Wu, Q. Enhancing
Thermal Stability and Living Fashion in α-Diimine−Nickel-Catalyzed
(Co)polymerization of Ethylene and Polar Monomer by Increasing
the Steric Bulk of Ligand Backbone. Macromolecules 2017, 50, 2675−
2682.
(18) Hu, H.; Chen, D.; Gao, H.; Zhong, L.; Wu, Q. Amine−Imine
Palladium Catalysts for Living Polymerization of Ethylene and
Copolymerization of Ethylene with Methyl Acrylate: Incorporation
of Acrylate Units into the Main Chain and Branch End. Polym. Chem.
2016, 7, 529−537.
(19) Zhong, S.; Tan, Y.; Zhong, L.; Gao, J.; Liao, H.; Jiang, L.; Gao,
H.; Wu, Q. Precision Synthesis of Ethylene and Polar Monomer
Copolymers by Palladium-Catalyzed Living Coordination Copoly-
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(20) Guo, L.; Gao, H.; Guan, Q.; Hu, H.; Deng, J.; Liu, J.; Liu, F.;
Wu, Q. Substituent Effects of the Backbone in α-Diimine Palladium
Catalysts on Homo- and Copolymerization of Ethylene with Methyl
Acrylate. Organometallics 2012, 31, 6054−6062.
(21) Du, C.; Zhong, L.; Gao, J.; Zhong, S.; Liao, H.; Gao, H.; Wu,
Q. Living (Co)Polymerization of Ethylene and Bio-Based Furfuryl
Acrylate Using Dibenzobarrelene Derived α-Diimine Palladium
Catalysts. Polym. Chem. 2019, 10, 2029−2038.
(22) Zhong, L.; Du, C.; Liao, G.; Liao, H.; Zheng, H.; Wu, Q.; Gao,
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by grants from the National Natural
Science Foundation of China (Project 51873234 and
21674130).
DEDICATION
■
Dedicated to Professor Qing Wu on the occasion of his 65th
birthday.
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