One-pot synthesis of brush copolymers bearing stereoregular helical polyisocyanides as side chains through tandem catalysis

Article abstract of DOI:10.1021/ma502283f

An air-stable phenylethynyl Pd(II) complex containing a polymerizable norbornene unit was designed and synthesized. Such a Pd(II) complex can initiate the living/controlled polymerization of phenyl isocyanide, giving stereoregular poly(phenyl isocyanide)s

Full text of DOI:10.1021/ma502283f

Article
pubs.acs.org/Macromolecules
One-Pot Synthesis of Brush Copolymers Bearing Stereoregular
Helical Polyisocyanides as Side Chains through Tandem Catalysis
Zhi-Qiang Jiang, Ya-Xin Xue, Jia-Li Chen, Zhi-Peng Yu, Na Liu, Jun Yin, Yuan-Yuan Zhu,
and Zong-Quan Wu*
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology
and Anhui Key Laboratory of Advanced Functional Materials and Devices, Hefei 230009, China
S
* Supporting Information
ABSTRACT: An air-stable phenylethynyl Pd(II) complex
containing a polymerizable norbornene unit was designed and
synthesized. Such a Pd(II) complex can initiate the living/
controlled polymerization of phenyl isocyanide, giving stereo-
regular poly(phenyl isocyanide)s in high yields with controlled
molecular weights and narrow molecular weight distributions.
The norbornene unit on the Pd(II) complex can undergo ring-
opening metathesis polymerization (ROMP) with Grubbs’
second-generation catalyst, affording polynorbornene bearing
Pd(II) complex pendants under a living/controlled manner.
Interestingly, the Pd(II) complex pendants on the isolated
polynorbornene are active enough to initiate the living/controlled polymerization of phenyl isocyanides, yielding well-defined
brush-like copolymers with polynorbornene backbone and helical poly(phenyl isocyanide) as side chains. 31P NMR analyses
indicate almost all the Pd(II) units on the polynorbornene participated in the polymerization, and the grafting density of the
brush copolymer is high. Further studies revealed the brush copolymer can be readily achieved in one-pot via tandem catalysis.
By using this method, a range of brush copolymers with different structures and tunable compositions were facilely prepared in
high yields with controlled molecular weights and narrow molecular weight distributions. The synthesized brush copolymers
were revealed to form worm-like cylindrical morphologies and helical rod architectures in film state by atomic force microscope
observations.
defined conjugated block copolymers with controlled molecular
weights and tunable compositions.10 However, polyisocyanides
prepared with Ni(II) complexes usually possess low stereo-
regularity, which is important to the optical activity of a helical
polymer.11 Besides the nickel complexes, other metal
complexes for living/controlled polymerization of isocyanide
are very limited. The aryl−rhodium complex and μ-ethynediyl
Pt−Pd binuclear complex have been reported to promote the
living/controlled polymerization of aryl isocyanide.12 By using
the μ-ethynediyl Pt−Pd complex as catalyst, we developed a
facile synthetic method for preparation of single-handed helical
poly(phenyl isocyanide)s.13 Nevertheless, their applications in
preparation of brush copolymers containing polyisocyanide
segment have been difficult because of their narrow tolerance to
initiator and terminator structures and polymerization con-
ditions. Very recently, we reported a class of air-stable
(phenylbuta-1,3-diynyl)- and phenylethynylpalladium(II) com-
plexes which were unexpectedly found to promote the living/
controlled polymerization of isocyanides, leading to the
formation of well-defined polyisocyanides with controlled
INTRODUCTION
■
The helix is one of the most important secondary structures of
biological macromolecules and is closely related to the
characteristic functions.1 Inspired by sophisticated biological
helices and related unique functions, chemists have been
challenged to develop artificial helical polymers not only for
mimic biological helices and functions but also for their
applications in the field of material science.2 Although some
investigations have been carried out in recent decades, the
number of artificial polymers with stable helical conformation is
still limited. Examples of such artificial helical polymers include
sterically restricted poly(methacrylate ester)s3 and poly(aryl
vinyl)s,4 polyisocyanides,5 polyisocyanate,6 polyguanidines,7
and polyacetylenes.8 Among them, polyisocyanides have
attracted considerable research interests in recent years owing
to their unique fascinating rigid helical structures and wide
applications in many fields.9 Well-defined polyisocyanides can
be obtained through the polymerization of appropriate
isocyanide monomers with transition metal complexes as
catalysts or initiators. The nickel(II) complex is by far the
most used catalyst for the isocyanide polymerization; even the
Ni(II) species reside on a polymer chain end.5,9 We found that
the Ni(II)-terminated regioregular poly(3-hexylthiophene) can
initiate the living polymerization of isocyanide, affording well-
Received: November 11, 2014
Revised: December 11, 2014
Published: December 22, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis of Pd(II) Complex 1 and Brush Copolymer Poly(1m-g-2n)
molecular weight (Mn), narrow molecular weight distribution
(Mw/Mn), and high stereoregularity.14 Utilization of these
synthetic methods, block and star-shaped copolymers contain-
ing well-defined stereoregular polyioscyanide segments have
been successfully developed. We envision that this polymer-
ization method may be further applied to the preparation of
brush copolymers bearing well-defined polyisocyanide as side
chains through modification on the structure of the Pd(II)
complex.
In this contribution, we designed and synthesized a new class
of air-stable phenylethynyl Pd(II) complexes (1, Scheme 1)
that are connected with a polymerizable norbornene unit. Such
Pd(II) complex can initiate the living/controlled polymer-
ization of phenyl isocyanides, leading to the formation of well-
defined, stereoregular poly(phenyl isocyanide)s in controlled
Mns and narrow Mw/Mns. In addition, the norbornene unit on
the Pd(II) complex can undergo ROMP with Grubbs’ second-
generation catalyst (G2), affording well-defined polynorbor-
nene containing multiple Pd(II) complexes. Combination of
the ROMP and the polymerization of phenyl isocyanide, well-
defined brush copolymers with polynorbornene backbone
bearing helical poly(phenyl isocyanide)s as side chains with
tunable compositions and high grafting density were readily
achieved in a single-pot via tandem catalysis. The synthesized
brush copolymer was founded to form worm-like cylindrical
morphologies and helical rod architectures by atomic force
microscope observations.
Brush copolymer is a class of one-dimensional macro-
molecules containing a high density of side chains connected to
a linear backbone.15 Because of their crowding arrangement,
those side chains are stretched away from the backbone to form
a brush-like or a worm-like cylindrical conformation.16 Thus,
brush copolymers with controlled structures are expected to
provide relatively precise shape and size control in each
dimension. They can afford unique nanoscale morphologies
that are unavailable by self-assembly of block copolymers.17
Because of the unique interesting helical structure and
functions, brush copolymers containing helical polymers as
side chains have attracted considerable research attentions in
the past few years.18 Chen et al. developed a series of brush
copolymers with polynorbornene backbone bearing helical
poly(γ-benzyl-L-glutamate) branches through integrated ring-
opening metathesis polymerization (ROMP) and the polymer-
ization of amino acid N-carboxyanhydrides.19 Deming and co-
workers reported the preparation of cylindrical polypeptide
brushes via the tandem catalysis approach.20 Maeda and co-
workers synthesized chiral polymer brushes with helical
polyacetylene backbone and helical polyisocyanates as side
chains.21 However, to the best of our knowledge, well-defined
brush copolymers bearing rigid helical polyisocyanides as side
chains have never been reported to date, probably due to the
limited synthetic method.
RESULTS AND DISCUSSION
■
Synthesis of Pd(II) Complex. The phenylethynyl Pd(II)
complex 1 containing a polymerizable norbornene unit was
prepared in a synthetic procedure similar to that we reported
previously and was briefly described here.14 As depicted in
Scheme 1, cis-5-norbornene-endo-2,3-dicarboxylic anhydride
reacted with 4-bromoaniline in toluene at 110 °C to give 3 in
72% yield. The isolated 3 was then reacted with trimethylsi-
lylacetylene (TMSA) by Sonogashira coupling reaction to
afford 4. Removal of the TMS group with potassium carbonate
gave 5 in 81% yield over the two steps. The norbornene
incorporated phenylethynyl Pd(II) complex 1 was obtained in
60% yield by treating 5 with trans-dichlorobis-
(triethylphosphine)palladium(II) in dichloromethane in the
presence of diethylamine as base and copper chloride as catalyst
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at room temperature. The Pd(II) complex 1 is air-stable and
shows good solubility in most common organic solvents such as
n-hexane, chloroform, THF, toluene, and ethyl acetate. The
structures of 1 and the related intermediates were characterized
by 1H and 13C NMR, FT-IR, and mass spectroscopies as well as
microanalysis (Figures S1−S7, Supporting Information). As
1
shown in Figure 1a, H NMR analysis of 1 showed a series of
Figure 2. 31P NMR spectra of the Pd(II) complex 1, poly-2n, poly-1m,
and poly(1m-g-2n) measured in CDCl3 at room temperature.
THF with [2]0 = 0.2 M and [2]0/[1]0 = 55. The
polymerization failed at room temperature because no
polymeric product could be isolated even though the
polymerization time was elongated to 20 h, and the monomer
was recovered. However, the polymerization succeeded at 55
°C as confirmed by size exclusion chromatography (SEC)
analysis. As shown in Figure 3a, the recorded SEC chromato-
gram of the isolated poly-255 (the footnote indicates the initial
feed ratio of monomer to catalyst) showed a monomodal
elution peak at the high molecular weight region. The Mn and
Mw/Mn of poly-255 were estimated to be 1.7 × 104 and 1.19,
respectively, based on SEC analysis with polystyrene standard.
The FT-IR spectrum of poly-255 in KBr showed an absorption
band at 1600 cm−1 due to the υ(CN), which is characteristic for
poly(phenyl isocyanide)s (Figure S8, Supporting Information).
1
The H NMR spectrum of the isolated poly-255 is depicted in
Figure 1b. The resonances of phenyl and −CO2CH2− protons
coming from the isocyanide repeating units appeared at 7.33,
5.76 and 3.45−4.50 ppm, respectively. All the resonances are
relatively broad due the rigid helical conformation of the main
chain which restricted motion of the backbone. 13C NMR of
the resulting poly-255 measured in CDCl3 at room temperature
also confirmed the structure of the isolated polymer. Moreover,
a relatively sharp resonance corresponding to the imino carbons
of the polymer backbone was displayed at δ 162.5 ppm, and the
half-bandwidth was estimated to be 29 Hz (Figure S9,
Supporting Information), which suggested that the isolated
polyisocyanide had a high stereoregularity of the imino groups
of backbone. To further confirm the polymerization of phenyl
isocyanide with 1 as initiator proceeded in a living/controlled
chain-growth manner, a series of polymerizations of 2 with 1 as
catalyst were then carried out with different initial feed ratios of
monomer to catalyst. The SEC chromatograms of the isolated
poly-2ns are displayed in Figure 3a. It was found that all the
polymers exhibited single modal elution peaks. The Mn and
Mw/Mn values of the afforded poly-2ns estimated from SEC
analyses were plotted against the initial feed ratios of monomer
to catalyst and are summarized in Figure 3b. All the isolated
1
Figure 1. H NMR spectra (600 MHz, CDCl3) of 1 (a), poly-2n (b),
poly-1m (c), and poly(1m-g-2n) (d) at 25 °C.
characteristic resonances, including those of phenyl protons (a
at 7.28 and b at 6.99 ppm), norbornene alkene protons (c at
6.24 ppm), and the protons of PCH2CH3 on the Pd unit (f, at
1.93 ppm). The integration area ratios of a:c:f are 2:2:12, which
were in good agreement with the proton number ratios of the
proposed structure. The 31P NMR spectrum of the Pd(II)
complex shows a singlet at 17.80 ppm (Figure 2), which is
almost the same as that of the reported (phenylbuta-1,3-
diynyl)palladium(II) and phenylethynyl Pd(II) complexes,
confirming the chemical structure of 1.
Polymerization of Isocyanide. The polymerization
behavior of Pd(II) complex 1 with phenyl isocyanide was
investigated by treating 1 with decyl 4-isocyanobenzoate (2) in
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Figure 3. (a) Size exclusion chromatograms of poly-2ns prepared from phenyl isocyanide 2 with Pd(II) complex 1 as initiator in THF at 55 °C with
different initial feed ratios of 2 to 1. (b) Plot of Mn and Mw/Mn values of the isolated poly-2ns as a function of the initial feed ratios of 2 to 1. Mn and
Mw/Mn values were determined by SEC with polystyrene standard (SEC conditions: eluent = THF, temperature = 40 °C).
polymers exhibited narrow Mw/Mn. A linear correlation
between the Mn and the initial feed ratios was observed,
suggesting the polymerization of phenyl isocyanide 2 with
Pd(II) complex 1 as initiator did proceed in a living/controlled
chain-growth manner. These results demonstrated that
incorporation of a polymerizable norbornene unit on the
phenylethynyl Pd(II) complex has no negative influence on the
living/controlled polymerization of phenyl isocyanide. The
double bonds of the norbornene did not interfere with the
isocyanide polymerization, and no side reaction took place
during the polymerization. Thus, a range of poly-2ns with
different Mn and narrow Mw/Mn were readily prepared by just
varying the initial feed ratio of monomer to catalyst (Table 1).
ROMP of Pd(II) Complex 1. The ROMP behavior of the
norbornene unit of Pd(II) complex 1 was examined by treating
1 with Grubbs’ catalyst G2 in THF at room temperature with
[1]0 = 0.2 and [1]0/[G2]0 = 20 (Scheme 1). The polymer-
ization was relatively fast, and 1 was almost completely
1
consumed within 1 h. SEC and H NMR analyses of the
isolated poly-120 suggest the successful transformation of 1 into
expected polynorbornene bearing Pd(II) complexes pendants.
As shown in Figure 4a, SEC chromatogram of the isolated
material exhibited a monomodal elution peak at high molecular
weight region. Based on SEC calculations, poly-120 was found
to have a Mn of 1.2 × 104 Da and Mw/Mn of 1.37. The 1H NMR
spectrum of the isolated poly-120 is shown in Figure 1c, which
revealed that 1 was nearly completely consumed (>99%) as the
essential absence of the sharp resonances of alkene protons at
6.24 ppm. In addition, a series of characteristic resonances
assignable to the proposed polynorbornene structure were
observed, including those of the phenyl protons (a at 7.28 ppm
and b at 7.01 ppm), alkene protons of the polynorbornene-
based main chain (c, at 5.50−6.95 ppm), and PCH2CH3
protons (CH2 at 1.95 ppm and CH3 at 1.20 ppm). The
integration area ratio of a:b:c is 2:2:2, which agreed very well
with the proton number ratio of the proposed structure,
indicating almost quantitatively one Pd(II) functionality per
repeating unit of poly-120. As depicted in Figure 2, the 31P
NMR spectrum of poly-1m showed a sharp resonance appearing
at 17.8 ppmthe same as that of monomer 1. No other peaks
could be detected on the 31P NMR spectrum, suggesting no
side reaction occurred on the Pd(II) unit during the
Table 1. Selected Results for the Polymerization of 2 with 1
a
as Initiator in THF at 55 °C
b
c
c
d
run
[2]0/[1]0
polymer
Mn
Mw/Mn
yield (%)
1
2
3
4
5
30
45
55
75
85
poly-230
poly-245
poly-255
poly-275
poly-285
9.0 × 103
1.4 × 104
1.7 × 104
2.2 × 104
2.4 × 103
1.19
1.18
1.19
1.16
1.20
90
94
92
92
89
a
b
The polymers were synthesized according to Scheme 1. The initial
feed ratio of 2 to 1. The Mn and Mw/Mn data were determined by
SEC and reported as equivalent to standard polystyrene. Isolated
c
d
yield.
Figure 4. (a) Size exclusion chromatograms of poly-1ms prepared from Pd(II) complex 1 with G2 as catalyst in THF at room temperature with
different initial feed ratios of 1 to G2. (b) Plots of Mn and Mw/Mn values of the isolated poly-1ms as a function of the initial feed ratios of 1 to G2. Mn
and Mw/Mn were determined by SEC with polystyrene standard (SEC conditions: eluent = THF, temperature = 40 °C).
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polymerization of norbornene. Further studies revealed that the
ROMP of 1 in THF at room temperature also proceeds in a
living/controlled fashion. A range of poly-1ms were synthesized
by the ROMP method with different initial feed ratios of 1 to
G2. All the isolated polymers showed single modal elution
peaks on SEC chromatograms and exhibited narrow Mw/Mns.
The Mns of the synthesized poly-1ms showed a linear
relationship with the initial feed ratios of monomer to catalyst
(Figure 4b). Thus, a series of poly-1ms with different Mn and
narrow Mw/Mn can be facile prepared in high yields under
controlled manner (Table 2). These results demonstrated that
The elution peak of recorded SEC did not shift to higher
molecular weight region as compared to the corresponding
macromonomer, and poly-255 was recovered. Probably, the
steric hindrance of the rigid helical poly(phenyl isocyanide)s
prevents the ROMP of the norbornene terminus.
The brush copolymer was then attempt to be prepared
through the polymerization of phenyl isocyanide with the
isolated poly-1m bearing Pd(II) complexes pendants as multiple
macroinitiators. First, a test reaction of small amount of 2 with
1
poly-120 was investigated in THF at 55 °C. The H NMR
spectrum of the isolated polymer is shown in Figure 1d. In
addition to the resonances come from the polynorbornene
backbone, broad signals ascribe to the phenyl isocyanide units
were clearly observed, including phenyl peaks located at 7.45
and 5.80 ppm and −CO2CH2− resonance at 3.61−4.50 ppm.
This result indicates the Pd(II) units on the poly-1m are active
enough to reacted with phenyl isocyanide. Then, the
polymerization of 2 with poly-110 as macroinitiator was
performed to achieve brush copolymers with long side chains.
The recorded SEC chromatogram of the obtained materials
(poly(110-g-210)) is shown in Figure 5. As expected, the brush
polymer exhibited a monomodal elution peak on SEC
chromatogram and was shifted to higher molecular weight
region as compared with the respective polyfuntional initiator
poly-110. No elution peaks corresponding to macroinitiator
poly-110 and monomer 2 could be observed on the SEC
chromatogram, suggesting the phenyl isocyanide monomer was
completely participated in the polymerization. The Mn of the
isolated poly(110-g-210) was estimated to be 3.1 × 104 Da based
on SEC analysis, larger than that of the poly-110 precursor (Mn
= 7.4 × 103 Da, Mw/Mn = 1.25). Although the Mn was
considerably increased, the Mw/Mn was kept narrow which was
estimated to be 1.28 by SEC analysis. The FT-IR spectrum of
the brush copolymer showed a characteristic absorption band
of imino backbone at 1600 cm−1 (Figure S10, Supporting
Table 2. Selected Results for ROMP of 1 with G2 as Catalyst
a
in THF at 25 °C
b
c
c
d
run
[1]0/[G2]0
polymer
Mn
Mw/Mn
yield (%)
1
2
3
4
5
10
15
20
25
30
poly-110
poly-115
poly-120
poly-125
poly-130
7.4 × 103
8.6 × 103
1.2 × 104
1.5 × 104
1.7 × 104
1.39
1.36
1.37
1.30
1.32
87
92
93
89
85
a
b
The polymers were synthesized according to Scheme 1. The initial
c
feed ratio of monomer 1 to catalyst G2. The Mn and Mw/Mn were
determined by SEC and reported as equivalent to standard
d
polystyrene. Isolated yield.
the G2 catalyst and the polymerization condition have no
negative influence on the Pd(II) complex units. That is, the
Pd(II) complex is robust enough to tolerate the polymerization
condition and the structure of the Pd(II) complexes was
maintained during the ROMP.
Synthesis of Brush Copolymers. The experiments
mentioned above have demonstrated that the polymerizable
norbornene incorporated phenylethynyl Pd(II) complex 1 is
applicable to both living/controlled polymerization of phenyl
isocyanide and ROMP by using G2 as catalyst. Therefore, brush
copolymer with polynorbornene as backbone and helical
polyisocyanide as side chains may be readily prepared through
the combination of ROMP and the living polymerization of
phenyl isocyanides. To verify, we initially attempt to prepare
such brush copolymer using “grafting through” strategy by
ROMP of the norbornene-terminated poly-255 as macro-
monomer with G2 as catalyst in THF at room temper-
aturethe same condition as the polymerization of Pd(II)
complex 1. However, SEC analysis indicated the polymerization
was failed because no molecular weight increase was observed.
1
Information). The H NMR spectrum of a brush copolymer
with long side chains is similar to that of the poly-2n
homopolymer. Signals assignable to the polynorbornene
backbone protons are difficult to observe, probably due to
the high grafting density (Figure S11, Supporting Information).
The 13C NMR spectrum is also similar to that of the poly-2n
homopolymer and showed a sharp resonance of the imino
carbons of the backbone of side chains at 162.5 ppm with the
half-bandwidth of 30 Hz, suggesting the poly(phenyl
isocyanide) side chains of the brush copolymer also possess
Figure 5. (a) SEC chromatograms of poly-110 and the respective brush copolymer poly(110-g-210). (b) SEC chromatograms of poly-120 and the
respective brush copolymer poly(120-g-210) and the side-chain-extended poly(120-g-220) prepared from the polymerization of 2 with poly(120-g-210)
as macroinitiator in THF at 55 °C.
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a
Table 3. Selected Polymerization Results for Brush Copolymer Poly(1m-g-2n)
b
c
c
d
b
c
c
d
run
polymer
Mn (Da)
Mw/Mn
yield (%)
[2]0/[1]0
brush copolymer
Mn (Da)
Mw/Mn
yield (%)
1
2
3
4
5
poly-110
poly-115
poly-120
poly-125
poly-115
poly-115
poly-110
poly-110
poly-130
7.4 × 103
9.4 × 103
1.2 × 104
1.5 × 104
9.4 × 103
9.1 × 103
7.6 × 103
7.6 × 103
1.8 × 104
1.25
1.38
1.37
1.40
1.30
1.31
1.30
1.30
1.38
92
89
87
89
85
10
10
10
20
5
poly(110-g-2a10)
poly(115-g-2a10)
poly(120-g-2a10)
poly(125-g-2a20)
poly(115-g-2b5)
3.1 × 104
4.0 × 104
5.9 × 104
1.7 × 105
3.5 × 104
5.1 × 104
3.3 × 104
5.4 × 104
5.3 × 105
1.28
1.27
1.25
1.25
1.23
1.19
1.29
1.31
1.20
84
78
75
76
84
80
70
76
e
6
10
10
20
50
poly(115-g-2b10)
poly(110-g-2c10)
poly(110-g-2c20)
poly(130-g-2c50)
e
7
e
8
e
9
80
a
b
c
The brush copolymers were prepared according to Scheme 1. The footnotes indicate the initial feed ratios of the monomer to initiator. Mn and
d
e
Mw/Mn data were estimated by SEC with polystyrene standard. Isolated yield. Prepared in one-pot via tandem catalysis.
high stereoregularity. The 31P NMR spectrum of the isolated
brush copolymer is shown in Figure 2, which shows multiple
peaks at 14.7 ppm; no peaks could be detected at 17.8 ppm,
corresponding to the unreacted Pd complex in the current
experimental conditions. These results undoubtedly demon-
strated that most of the Pd(II) complexes pendants on the
poly-1m take part in the phenyl isocyanide polymerization.
Thus, the grafting density of the afforded brush polymer is high.
By using this synthetic approach, a range of brush copolymers
of polybornene backbone bearing well-defined, stereoregular
helical poly(phenyl isocyanide)s as side chains were facilely
prepared in high yields (Table 3). It is worthy to note that the
Pd(II) complexes reside at the side chain end of the isolated
brush copolymers are still active and can undergo further
copolymerization with phenyl isocyanide to give a brush
copolymer with extended poly(phenyl isocyanide) side chains.
For example, adding monomer 2 to a THF solution of
poly(120-g-210) at 55 °C ([2]0/[Pd]0 = 10), SEC analysis of the
resulting polymer poly(120-g-220) indicated the chain extension
polymerization occurred. As shown in Figure 5b, the isolated
side-chain-extended poly(120-g-220) showed a single modal
elution peak and was shifted to the higher molecular weight
region as compared with the poly(120-g-210) precursor and also
kept a narrow molecular weight distribution (Mw/Mn = 1.28).
The synthesis of the brush copolymer of poly(1m-g-2n)s was
then investigated to be accomplished in one-pot via tandem
catalysis. To verify, Pd(II) complex 1 was added to the mixture
solution of G2 and monomer 2 in THF, and the mixture was
first stirred at room temperature for the ROMP of 1 with G2.
After 1 was completely consumed, the Mn ceased to increase as
indicated by SEC analyses of the aliquots removed from the
polymerization mixture; the polymerization solution was then
heated up to 55 °C to allow the polymerization of phenyl
isocyanide. In the aforementioned experiments, we have
demonstrated that, catalyzed by G2, ROMP of 1 was a very
fast process and could reach near complete conversion within 1
h at room temperature, whereas polymerization of 2 with 1
required higher temperature (55 °C) and longer time (usually
several hours) to obtain significant polymerization rate and
monomer conversion. Therefore, with much faster polymer-
ization rate for ROMP of 1 than that for the polymerization of
2 under the same conditions, essentially ROMP and isocyanide
insertions should occurred sequentially. SEC analysis of the
isolated materials indicated the one-pot polymerizations were
succeeded. All the isolated brush copolymers exhibited single
modal elution peaks on the SEC chromatograms (Figures S15−
S18, Supporting Information). The structures of the one-pot
synthesized brush copolymers were also confirmed by 1H,
NMR, and FT-IR spectra, which were the same to that
prepared via two steps. These results demonstrated the well-
defined brush copolymers with polynorbornene backbone
bearing rigid helical poly(phenyl isocyanide)s as side chains
can be readily achieved in one-pot via tandem catalysis (Table
3). Moreover, the molecular weights and the compositions of
the brush copolymer are easily controlled through the variation
on the ratios of monomer to catalyst.
The morphology of the synthesized brush copolymers in the
film state was investigated using atomic force microscopy
(AFM). The thin film for AFM observation was prepared by
spin-casted a solution of poly(1m-g-2n) in appropriate solvent
onto silicon wafers. After the sample was annealed under the
vapor of the corresponding solvent and slowly dried in air, it
was subjected to AFM observation. The tapping-mode phase
and height images of the thin film spin-casted from brush
copolymer poly(130-g-250) in diluted toluene are shown in
Figures 6a and 6b. As expected, worm-like cylindrical structures
with ca. 160 nm in length and ca. 80 nm in width were clearly
Figure 6. AFM phase (a) and height (b) images of thin film spin-
casted from a solution of brush copolymer poly(130-g-250) in toluene
(c = 0.10 mg/mL) and AFM phase (c) and height (d) images of the
thin film spin-casted from a solution of poly(130-g-250) in THF at
room temperature (c = 0.10 mg/mL).
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(triphenylphosphine)palladium(II), trans-dichlorobis-
(triethylphosphine)palladium(II), and Grubbs second-generation Ru
catalyst were purchased from Aladdin and Sigma-Aldrich Co. Ltd. and
were used as received without further purification. The phenyl
isocyanide 2 was prepared according to the literatures and the
observed. The average height of the worm-like supramolecular
structures was estimated to be ca. 12.4 nm. Interestingly, the
worm-like structures were further self-assembled into a well-
defined ladder-like supramolecular architecture on the silicon
wafer. However, the morphology of the self-assembly
architecture was dependent on the solvent used. When the
thin film was spin-casted from a diluted THF solution, rigid-rod
helical structures with distinct handedness were clearly
observed (Figures 6c and 6d). The dimensions of the
assembled helical rods were estimated to have several
micrometers in lengths, ca. 150 nm in width, and ca. 9.0 nm
in height. For comparison, the morphology of the linear poly-
130 precursor was examined by AFM under the identical
conditions. However, in sharp contrast to the brush copolymers
poly(130-g-250), no well-defined morphology was observed by
AFM on the thin film spin-casted from corresponding poly-130
(Figure S19, Supporting Information). Probably, the distinct
morphology of the synthesized brush copolymers in film state
was ascribed to the rigid-rod structure of the polynorbornene
backbone and steric congestion offered by the densely
populated and rigid stereoregular helical poly(phenyl iso-
cyanide) side chains.
10
1
structure was confirmed by H NMR.
Synthesis of 3. This compound was synthesized according to the
literature with slight modification.22 Anhydrous toluene (15 mL) was
added to the mixture of exo-norbornene anhydride (0.50 g, 3.05
mmol) and 4-bromoaniline (0.35 mL, 3.05 mmol) under a dry
nitrogen atmosphere. After the reaction mixture was stirred at 110 °C
for 16 h, the solvent was removed by evaporation under reduced
pressure. The crude product was purified by column chromatography
with ethyl acetate and petrol ether (v/v = 5/1) as eluent to yield 3 as a
white solid (0.70 g, 72%). 1H NMR (600 MHz, CDCl3): δ 7.55 (d, J =
9.0 Hz, 2H, aromatic), 7.03 (d, J = 9.0 Hz, 2H, aromatic), 6.25 (s, 2H,
CH of double bond), 3.51 (s, 2H, CH), 3.44−3.43 (m, 2H, CH), 1.79
(d, J = 8.4 Hz, 1H, CH2), 1.61 (d, J = 8.4 Hz, 1H, CH2).
Synthesis of 4. A mixture of 3 (1.85 g, 5.84 mmol), CuI (32 mg,
0.17 mmol), and Pd(PPh3)2Cl2 (0.12 g, 0.17 mmol) in a 100 mL
three-neck flask was degassed and refilled with N2. After this procedure
was repeated three times, triethylamine (30 mL) and trimethylsilyla-
cetylene (0.90 mL, 6.43 mmol) were added via a syringe. The reaction
solution was stirred at 55 °C for 6 h, and then the solvent was
evaporated to dryness under reduced pressure. The residue was further
purified by column chromatography with ethyl acetate and petrol ether
(v/v = 4/1) as eluent to afford 4 as white solid (1.63 g, 83%); mp
108.2−109.5. 1H NMR (400 MHz, CDCl3): δ 7.49 (d, J = 8.4 Hz, 2H,
aromatic), 7.09 (d, J = 8.4 Hz, 2H, aromatic), 6.25 (s, 2H, CH of
double bond), 3.51−3.49 (m, 2H, CH), 3.43−3.41 (m, 2H, CH), 1.79
(d, J = 8.8 Hz, 1H, CH2), 1.61 (d, J = 8.8 Hz, 1H, CH2), 0.24 (s, 9H,
TMS). 13C NMR (100 MHz, CDCl3, 25 °C): δ 176.53, 134.66,
132.52, 132.28, 131.74, 128.23, 126.40, 123.49, 104.13, 95.47, 52.31,
45.84, 45.56. FT-IR (KBr, cm−1): 2160 (νCC), 1710 (vCO). HRMS
m/z calcd for C20H22NO2Si [M + H]+: 336.1342; Found:
C20H22NO2Si, 336.1419. Anal. Calcd (%) for C20H21NO2Si: C,
71.60; H, 6.31; N, 4.18; Found (%): C, 71.31; H, 6.60; N, 4.00.
Synthesis of 5. To a solution of 4 (0.89 g, 2.65 mmol) in
dichloromethane (20 mL) and methanol (20 mL) was added K2CO3
(0.70 g, 5.3 mmol). After the mixture was stirred at room temperature
for 1 h, the solvent was evaporated to dryness under reduced pressure.
The afforded residue was extracted with ether and washed successively
with H2O (10 mL × 3) and brine (10 mL × 3). The organic layer was
dried over Na2SO4. After filtration and evaporation, the resulting crude
product was purified by column chromatography with petrol ether as
eluent to afford 5 as a white solid (0.68 g, 97%). The isolated 5 was
directly used in next step without further characterization due to
instability.
CONCLUSION
■
In summary, we have designed and synthesized a simple
phenylethynyl Pd(II) complex containing a polymerizable
norbornene unit. Such a Pd(II) complex was found to promote
the living/controlled polymerization of phenyl isocyanide,
affording well-defined helical poly(phenyl isocyanide)s with
controlled Mn, narrow Mw/Mn, and high stereoregularity. The
norbornene unit on the Pd(II) complex can undergoROMP by
using Grubbs’ second-generation catalyst in living/controlled
manner. Combination of these two polymerizations, brush
copolymers with polynorbornene backbone bearing well-
defined, stereoregular helical poly(phenyl isocyanide) side
chains can be readily achieved in high yields with controlled
molecular weights and tunable compositions. The synthesis of
these brush copolymers can be readily accomplished in one-pot
via tandem catalysis. Worm-like morphologies and helical rod
architectures formed from the brush copolymer were clearly
observed by AFM. Given the modifications on the structure of
Pd(II) complex and isocyanide monomers, we believe this
novel polymerization technique will allow easily access to
numerous hybrid materials with novel functions and broad
applications.
Synthesis of 1. To a solution of 5 (110 mg, 0.42 mmol) in
diethylamine (15 mL) and dichloromethane (15 mL) was added trans-
dichlorobis(triethylphosphine)palladium (178 mg, 0.43 mmol) and
copper(I) chloride (3 mg, 0.03 mmol). The mixture solution was
stirred at room temperature for 3 h. After the solvent was removed by
evaporation under reduced pressure, the residue was purified by
column chromatography with ethyl acetate and petrol ether (v/v = 2/
1) as eluent. The afforded product was recrystallized from petrol ether
and methanol to give 1 as a white solid (160 mg, 60%); mp 112.5−
EXPERIMENTAL SECTION
■
1
Instruments. The H, 13C, and 31P NMR spectra were recorded
using a Bruker 600 or 400 MHz (H) spectrometer. Melting points
were obtained with a Mel-Temp apparatus and are uncorrected. SEC
was performed on Waters 1515 pump and a Waters 2414 differential
refractive index (RI) detector (set at 40 °C) using a series of linear
Styragel columns (HR1, HR2, and HR4) with THF as eluent (flow
rate is 0.3 mL/min). Mn and Mw/Mn data are reported relative to
polystyrene standards. FT-IR spectra were recorded on PerkinElmer
Spectrum BX FT-IR system using KBr pellets at room temperature.
AFM images were acquired in tapping mode with a Digital
Instruments Dimension 3100 scanning probe microscope using
standard silicon cantilevers with a nominal spring constant of 50 N/
m and resonance frequency of ∼300 kHz.
1
113.6 °C. H NMR (400 MHz, CDCl3): δ 7.28 (d, J = 8.8 Hz, 2H,
aromatic), 6.99 (d, J = 8.8 Hz, 2H, aromatic), 6.24 (s, 2H, CH of
double bond), 3.51−3.50 (m, 2H, CH of norbornene), 3.42−3.41 (m,
2H, CH of norbornene), 1.99−1.91 (m, 12H, PCH2CH3), 1.80−1.77
(m, 1H, CH2 of norbornene), 1.62−1.60 (m, 1H, CH2 of
norbornene), 1.23−1.15 (m, 18H, PCH2CH3). 13C NMR (150
MHz, CDCl3, 25 °C): δ 177.01, 134.70, 131.33, 128.98, 128.42,
126.35, 106.22, 97.60, 52.36, 45.90, 45.62, 15.46, 8.44. 31P NMR
(121.5 MHz, CDCl3, 25 °C): δ 17.80. FT-IR (KBr, cm−1): 2970
(νC−H), 2930 (νC−H), 2880 (νC−H), 2120 (νCC), 1720 (vCO).
HRMS m/z calcd for C29H43ClNO2P2Pd [M + H]+: 640.1414. Found:
C29H43ClNO2P2Pd, 640.1922. Anal. Calcd (%) for C29H42ClNO2P2Pd:
C, 54.38; H, 6.61; N, 2.19. Found (%): C, 54.09; H, 6.90; N, 2.00.
Materials. All solvents were obtained from Sinopharm. Co. Ltd.
and were purified by the standard procedures before use. 4-
Bromoaniline, exo-norbornene anhydride, trimethylsilylacetylene,
copper(I) iodide, copper(I) chloride, trans-dichlorobis-
87
dx.doi.org/10.1021/ma502283f | Macromolecules 2015, 48, 81−89

Macromolecules
Article
Typical Procedure for Polymerization of 2 with 1 as Initiator
(Poly-255). A 10 mL oven-dried flask was charged with monomer 2
(50 mg, 0.17 mmol), THF (0.77 mL), and a stir bar. To this stirring
solution was added a solution of 1 in THF (0.035 M, 0.09 mL) via a
microsyringe at room temperature. The concentrations of monomer 2
and catalyst 1 were 0.2 and 0.0036 M, respectively ([2]0/[1]0 = 55).
The reaction flask was then immersed into an oil bath at 55 °C and
stirred for 12 h. After cooled to room temperature, the polymerization
solution was precipitated into a large amount of methanol, collected by
centrifugation, and dried in vacuum at room temperature overnight to
afford poly-255 as a yellow solid (45 mg, 89% yield). SEC: Mn = 1.7 ×
ASSOCIATED CONTENT
* Supporting Information
Additional spectral data and SEC chromatograms. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail zqwu@hfut.edu.cn (Z.-Q.W.).
Author Contributions
■
Z.-Q.J. and Y.-X.X. contributed equally.
1
104, Mw/Mn = 1.19. H NMR (400 MHz, CDCl3, 25 °C): δ 7.33 (br,
Notes
2H, aromatic), 5.76 (br, 2H, aromatic), 4.50−3.45 (br, 2H, CO2CH2),
1.63−0.83 (br, 19H, CH2 and CH3). 13C NMR (150 MHz, CDCl3, 25
°C): δ 164.95, 162.54, 150.34, 129.63, 127.22, 117.03, 64.87, 31.87,
29.72, 29.64, 29.59, 29.42, 29.30, 28.60, 25.07, 22.62, 14.03. 31P NMR
(121.5 MHz, CDCl3, 25 °C): δ 14.70. FT-IR (KBr, cm−1): 2965
(νC−H), 2928 (νC−H), 2851 (νC−H), 1720 (νCO), 1600 (νCN).
Typical Procedure for ROMP of 1 with G2 as Catalyst (Poly-
120). A solution of G2 in THF (0.0039 M, 0.1 mL) was added to a
degassed solution of Pd(II) complex 1 (50 mg, 0.08 mmol) in THF
(0.30 mL) via a microsyringe. The initial concentrations of 1 and
catalyst G2 were 0.2 and 0.01 M, respectively ([1]0/[G2]0 = 20). After
the reaction mixture was stirred at room temperature for 1 h, thin-layer
chromatography (TLC) analysis indicated 1 was completely
consumed. The polymerization was then quenched by addition of
ethyl vinyl ether (1.0 mL) and poured into a large amount methanol.
The precipitated solid was collected by centrifugation, washed with
methanol, and dried under vacuum to afford poly-120 (44 mg, 88%
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by National Natural Scientific
Foundation of China (21104015, 21172050, 21371043,
51303044, and 21304027). Z.W. thanks the Thousand Young
Talents Program for Financial Support. J.Y. expresses his thanks
for Specialized Research Fund for the Doctoral Program of
Higher Education (20130111120013) and Research Founda-
tion for Returned Overseas Chinese Scholars of the Ministry of
Education of China.
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