Air-stable (phenylbuta-1,3-diynyl)palladium(II) complexes: Highly active initiators for living polymerization of isocyanides

Article abstract of DOI:10.1021/ja5004747

A family of air-stable (phenylbuta-1,3-diynyl)palladium(II) complexes were designed and prepared in a facile synthetic procedure. Their structures were characterized by ¹H and ¹³C NMR, MS, and X-ray analysis. These Pd complexes were

Full text of DOI:10.1021/ja5004747

Article
pubs.acs.org/JACS
Air-Stable (Phenylbuta-1,3-diynyl)palladium(II) Complexes: Highly
Active Initiators for Living Polymerization of Isocyanides
Ya-Xin Xue,†,§ Yuan-Yuan Zhu,†,§ Long-Mei Gao,† Xiao-Yue He,† Na Liu,† Wu-Yi Zhang,† Jun Yin,†
Yunsheng Ding,† Hongping Zhou,‡ and Zong-Quan Wu*,†
†Department of Polymer Science and Engineering, School of Chemical Engineering, Hefei University of Technology and Anhui Key
Laboratory of Advanced Functional Materials and Devices, Hefei 230009, China
‡College of Chemistry and Chemical Engineering, Key Laboratory of Functional Inorganic Materials Chemistry of Anhui Province,
Anhui University, Hefei 230039, China
S
* Supporting Information
ABSTRACT: A family of air-stable (phenylbuta-1,3-diynyl)-
palladium(II) complexes were designed and prepared in a facile
1
synthetic procedure. Their structures were characterized by H
and 13C NMR, MS, and X-ray analysis. These Pd complexes
were revealed to efficiently initiate the polymerization of phenyl
isocyanides in a living/controlled chain growth manner, which
led to the formation of poly(phenyl isocyanide)s with
controlled molecular weights and narrow molecular weight distributions. 13C NMR analysis indicated the isolated poly(phenyl
isocyanide) was of high stereoregularity. The Pd unit at the end of the polymer chain could undergo further copolymerization
with phenyl isocyanide monomers to give block copolymers. It was also found that incorporation of an electron-donating group
on the phenyl group of the Pd complex could improve the catalytic activities. Furthermore, these Pd complexes were tolerant to
most organic solvents and applicable to a wide range of isocyanide monomers including alkyl and phenyl isocyanides and even
phenyl isocyanide with bulky substituents at the ortho position and diisocyanide monomers. Therefore, this polymerization
system is versatile in the preparation of well-defined polyisocyanides with controlled sequence. Bi- and trifunctional Pd
complexes with two and three Pd units incorporated onto the same phenyl ring were designed and synthesized. They were also
able to initiate the living polymerization of phenyl isocyanide to afford telechelic linear and star-shaped polyisocyanides with
controlled molecular weights and narrow molecular weight distributions.
systems they cannot control the sequence and molecular weights
of the resulting polyisocyanides.9
INTRODUCTION
■
The helix is the central structural motif in biological macro-
molecules, such as DNA,1 proteins,2 and polysaccharides,3 and is
also the ubiquitous object in nature from microscopic to
macroscopic points of view. Inspired by sophisticated biological
helices and related unique functions, chemists have been
challenged to develop artificial helical polymers4 and oligomers
(foldamer)5 not only to mimic biological helices and functions
but also for their potential applications as materials.6 Among the
synthetic helical polymers, polyisocyanides have been the focus
of intense efforts in recent decades due to their unique rigid
helical structures.4f,g,j,q,s,7 They have a distinctive π-conjugated
system with CN double bonds at the carbons that connect the
monomer units, which twist along the polymer backbone to
create a helical conformation. In addition to their interesting rigid
helical structure, polyisocyanides have exhibited wide applica-
tions in enantiomeric separation, asymmetric catalysis, and chiral
recognition as well as in material sciences.8 Well-defined
polyisocyanides are commonly prepared by the polymerization
of appropriate isocyanide monomers with transition metal
complexes as catalyst or initiator.4s,7d Although several transition
metal complexes have been discovered to promote the
polymerization of isocyanides, as nonliving polymerization
Among the transition metal complexes, organonickel com-
plexes have been widely used for isocyanide polymerizations. For
example, Notle et al. have reported that some Ni(II) salts are
efficient catalysts for the polymerization of alkyl and aryl
isocyanides through the so-called “merry-go-round” mecha-
nism,4g,10 while Deming et al. revealed that π-allylnickel
complexes are effective catalysts for living polymerization of
alkyl isocyanides in noncoordinating solvents.11 Very recently,
Asaoka et al. disclosed a fast, living/controlled polymerization of
isocyanide by using aromatic isocyanide as monomer and
aromatic nucleophile adduct of tetra(tert-butylisocyano)nickel-
(II) complex as initiator.12 We found that poly(3-hexylthio-
phene) with a nickel complex residing at the chain end can
initiate the block copolymerization of phenyl isocyanide,13 alkyl
isocyanide,14 and 4,5-dimethyl-1,2-diisocyanobenzene mono-
mers15 in controlled chain extension fashions. Suginome and co-
workers demonstrated that arylnickel complexes are effective
initiators for the living polymerization of both phenyl isocyanide
and 1,2-diisocyanobenzenes.16 In contrast to the widely used
Received: January 16, 2014
Published: March 7, 2014
© 2014 American Chemical Society
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Ni(II) complexes, the utilization of other transition metal
complexes as catalysts or initiators for isocyanide polymerization
are relatively limited. Onitsuka et al. have found that arylrhodium
complexes can promote the polymerization of aryl isocyanides in
a living/controlled manner. Nevertheless, a large amount of
phosphine ligands were needed to stabilize the living chain end,
which made product purification difficult.17
Scheme 1. Synthesis of (Phenylbuta-1,3-diynyl)palladium(II)
Complexes 1a−e
Palladium complexes have been widely used in organic
synthesis and polymer preparation,18 and for promoting the
polymerization of 1,2-diisocyanobenzenes.19 However, examples
of the polymerization of isocyanide and its derivatives with a Pd
complex as catalyst or initiator are limited. The challenge in
designing Pd catalysts for this purpose may be their great
tendency in forming a trimer intermediate involving intra-
molecular N−Pd coordination to terminate the polymerization
progress.9,16,20 Two decades ago, Onitsuka et al. reported that a
dinuclear Pd−Pt μ-ethynediyl complex can catalyze the living
polymerization of aryl isocyanide.21 Although isocyanides were
demonstrated to exclusively insert into the carbon−Pd bond, Pt
played an important role since no polymerization occurred with a
similar (phenylethynyl)palladium(II) complex.21c In our pre-
vious studies, we found that such a Pd−Pt μ-ethynediyl complex
was sensitive to air and hard to synthesize.22 Moreover, it is
ineffective for the polymerization of bulky phenyl and alkyl
isocyanides. Therefore, the development of a novel air-stable,
facile accessible Pd complex for efficient living polymerization of
different isocyanides is highly desirable.
We herein describe the synthesis of a new kind of (phenylbuta-
1,3-diynyl)palladium complexes from the reaction of phenyl-
buta-1,3-diynyl with trans-bis(triphenylphosphine)palladium(II)
dichloride in high yields. In contrast to their analogues
(phenylethynyl)- and ethynylpalladium complexes, which are
inactive for the polymerization of isocyanides,21b these new
complexes can efficiently initiate the living polymerization of
phenyl isocyanide to afford poly(phenyl isocyanide) with
controlled molecular weight (Mn) and narrow molecular weight
distribution (Mw/Mn). They can tolerate most organic solvents
and are applicable to a wide range of isocyanide monomers,
including phenyl and alkyl isocyanides and bulky phenyl
isocyanides as well as diisocyanobenzene monomers. Further-
more, we further prepared bi- and trifunctional Pd complexes by
attaching two or three Pd atoms to a straight or triangular ligand.
We demonstrate that these multifunctional complexes can also
initiate the living polymerization of phenyl isocyanide to yield
well-defined telechelic linear and star-shaped polyisocyanides
with controlled Mn and narrow Mw/Mn.
Compound 1c was further reacted with 7c in dichloromethane in
the presence of copper chloride at room temperature to produce
complex 1e in 75% yield. All complexes 1a−e were soluble in
common organic solvents, such as chloroform, n-hexane, toluene,
and THF. Their structures were completely characterized by 1H
and 13C NMR, MS, FT-IR, and elemental microanalysis. It is
noteworthy that, the precursors 7a−d are unstable, their
colorless chloroform solutions easily turn to brown and
eventually to dark within one week. Straight-phase thin layer
chromatography analysis indicated new spots were formed on
the plate (Figure S1, see Supporting Information [SI]). In sharp
contrast, 1a−e are very stable; their 1H NMR spectra in CDCl3
showed no changes after the samples were deposited for two
weeks at ambient temperature.
Single crystals of 1a, 1b, 1c, and 1e suitable for X-ray analysis
were obtained by slow evaporation of their solutions in the
mixture of ethyl acetate and petroleum ether at room
temperature, and their crystal structures are shown in Figure 1.
It can be found that for all the complexes, the benzene ring is
perpendicular to the Pd coordination plane, probably to
minimize the steric hindrance between the phenyl ring and the
ethyl groups on the phosphine ligands. For 1a−c, the linear
phenyldiynyl moiety is not straight, but a little bent. In contrast,
1e adopts a straight linear conformation.
The catalysis of 1a−e for the polymerization of decyl 4-
isocyanobenzoate 213a was then investigated. The activity of 1a
was first examined by treating it with 100 equiv of 2 in THF at
room temperature ([2]0 = 0.2 M, [2]0/[1a]0 = 100) (Scheme 2).
No polymeric product was isolated after even 20 h, and
monomer 2 was recovered. At 55 °C, the polymerization could
take place, the originally colorless solution turned to brown-
yellow, and the viscosity of the solution also increased
considerably. After 10 h, methanol was added, and the formed
precipitate was isolated by centrifugation to give poly-a2100 (the
footnote indicates the initial feed ratio of monomer to initiator)
as a yellow solid in almost quantitative yield. The Mn and Mw/Mn
values of poly-a2100 were determined to be 2.9 × 104 and 1.11,
respectively, by size exclusion chromatography (SEC) with
polystyrene (PSt) standards (Scheme 2). The FT-IR spectrum of
poly-a2100 showed an absorption band at 1599 cm−1 due to
ν(CN), which is characteristic for polyisocyanides (Figure S2,
SI),17b but no absorption of the monomer around 2260 cm−1. 1H,
13C, and 31P NMR and UV−vis spectra of the polymer also
confirmed its structure (Figures S3−S6, SI). It is worthy to note
that the polymerization could be carried out in an open system to
give the same polymer.
RESULTS AND DISCUSSION
■
(Phenylbuta-1,3-diynyl)palladium complexes 1a−e with various
substituents on the para position of the benzene ring was
prepared in the same procedure as outlined in Scheme 1. The
synthesis of 1a was briefly described here. First, 1-ethynyl-4-
methylbenzene was brominated by NBS with AgNO3 as catalyst
in acetone at room temperature to afford 5a in almost
quantitative yield. The isolated 5a was then reacted with
ethynyltrimethylsilane (TMSA) by Sonogashira coupling
reaction to afford 6a, and removal of the TMS group with
potassium carbonate gave 7a in 83% yield over two steps.
Complex 1a was then obtained in 65% yield by treating 7a with
trans-dichlorobis(triethylphosphine)palladium(II) in dichloro-
methane in the presence of diethylamine as base and copper
chloride as catalyst at room temperature.23 By using similar
procedures, 1b−d could be prepared and isolated in high yields.
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Figure 1. Crystal structures of Pd complex 1a (a), 1b (b), 1c (c), and 1e (d). The H atoms were omitted for clarity.
isocyanide)s prepared with nickel complexes as catalyst have low
stereoregularity of the imino groups.25
Scheme 2. Polymerization of Phenyl Isocyanide 2 with Pd
Complex 1a in THF at 55 °C and Block Copolymerization of 2
with Poly-a2m As Macroinitiator, and the SEC
The narrow molecular weight distribution of poly-a2100
suggested that the polymerization of 2 initiated by 1a might
proceed in a living or controlled chain growth fashion. To test
this, 1a-initiated polymerizations of 2 in different initial feed
ratios (m) were performed under the same conditions ([2]0 = 0.2
M, [2]0/[1a]0 = m, THF, 55 °C). SEC chromatograms of the
prepared polymers were shown in Figure 2a. All the polymers
exhibited symmetric and single model SEC traces. The Mn of the
produced poly-a2m was found to increase linearly and in
proportion to the initial feed ratio of 2 to 1a up to 180 (Figure
2b). All the samples showed narrow molecular weight
distribution (Mw/Mn < 1.20) which became narrower with the
increase of the polymerization degree. The absolute Mn value was
Chromatograms of Poly-a250, poly-a2100 and the Resulting
Block Copolymer Poly(a2100-b-2100
)
1
determined by H NMR from the integral ratio of the methyl
signal of the Pd complex on the polymer chain end to the OCH2
signal on the polymer repeat units. The Mn (1H NMR) values of
poly-a2m were consistent with the Mn (SEC) and in good
agreement with the ideal Mn values calculated from the initial
feed ratio of [2]0/[1a]0, suggesting the polymerization did
proceed in a living/controlled chain growth manner. This result
also revealed the initiation efficiency of 1a was extremely high
and almost quantitative. Due to the living nature of the
polymerization, a range of poly-a2m’s with different molecular
weights and narrow molecular weight distributions could be
prepared using this new initiator by simply varying the initial feed
ratio of monomer and initiator. The polymerization results are
shown in Table 1.
To further confirm the living nature of the generated
poly(phenyl isocyanide) and also to examine the stability of
the living chain end of the Pd moiety of the synthetic
polyisocyanide, block copolymerization was carried out for the
isolated poly-a2100 (Mn = 2.9 × 104, Mw/Mn = 1.11) and 2 in
THF at 55 °C ([2]0 = 0.2 M, [2]0/[poly-2100]0 = 100/1). SEC
analysis of the resulting product demonstrated that block
copolymerization did take place because the SEC chromatogram
of the product shifted to the higher-molecular weight region
(Scheme 2). The molecular weight of the obtained copolymer
increased by 1 time and also kept narrow distribution (Mn= 5.8 ×
104, Mw/Mn = 1.07). This result supported the fact that the
isolated poly-a2100 was still stable to keep the active living chain
end group −Pd(PEt3)2Cl to initiate the copolymerization.
We then carried out the polymerizations in various organic
solvents to investigate the influence of the solvent. The results are
Stereoregularity is one of the most important parameters for
natural and synthetic polymers and has significant influence on
the optical activity for helical polyisocyanides. It is thus worth
examining the stereoregularity of the polymer prepared through
this new method. The stereoregularity of poly-a2100 was
evaluated through a calculation of half-bandwidth of the main
chain imino carbon signal on 13C NMR spectroscopy in CDCl3 at
room temperature.17b,24 A relatively sharp resonance corre-
sponding to the imino carbons of the polymer backbone was
displayed at δ 162.56 ppm, and the half-bandwidth was estimated
to be 30 Hz (Figure S4, SI), which suggested that poly-a2100 had a
high stereoregularity of the imino groups. In contrast, poly(aryl
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Figure 2. (a) SEC chromatograms of poly-a2m prepared from 2 with 1a as initiator in THF at 55 °C with different initial feed ratios. (b) Plot of Mn and
Mw/Mn values of poly-a2m as a function of the initial feed ratios of 2 to 1a. Mn and Mw/Mn were determined by SEC with PSt standard (eluent = THF,
temperature = 40 °C).
a
Table 1. Polymerization Results of Phenyl Isocyanide 2 with Pd Complex 1a as Initiator at 55 °C
b
c
c
d
run
[2]0/[1a]0
solvent
polymer
Mn
Mw/Mn
yield, %
1
2
3
4
5
6
7
8
9
50
100
200
50
THF
poly-a250
1.5 × 104
2.9× 104
5.8 × 104
1.4 × 104
2.6 × 104
1.5 × 104
2.7 × 104
1.5 × 104
9.5 × 103
1.19
1.11
1.07
1.29
1.32
1.21
1.11
1.36
1.38
96
96
91
94
96
94
90
90
68
THF
poly-a2100
THF
poly-a2200
toluene
toluene
CHCl3
CHCl3
DMF
poly-a250(t)
poly-a2100(t)
poly-a250(c)
poly-a2100(c)
poly-a250(d)
poly-a250(a)
100
50
100
50
50
Acetone
a
b
c
The poly-a2m was synthesized according to Scheme 2. The initial feed ratio. The Mn and Mw/Mn were determined by SEC and reported as
d
equivalent to standard PSt. Isolated yield.
summarized in Table 1. It can be seen that the polymerization
proceeded smoothly at 55 °C in most of the solvents of varying
polarity, including toluene, CHCl3, acetone, THF, and DMF
(runs 4−9 in Table 1, Figures S7−S9, SI). In n-hexane and
methanol, the polymerization also took place, but the resulting
polymers were not soluble, and thus only short oligomers or
polymers were obtained as precipitates. The polymers formed in
acetone and DMF exhibited a little bit larger Mw/Mn probably
due to the poor solubility of the Pd complex and the polymers,
and poly-a2m with higher Mn could not be obtained in these
solvents.
To get deep insight into the new polymerization system, the
relationship between the conversion of monomer 2 and Mn of the
obtained poly-a2m was also investigated. The polymerization
([2]0 = 0.2 M, [2]0/[1a]0 = 90) was performed in THF in the
presence of PSt (Mn = 2630, Mw/Mn = 1.06) as internal standard
to facilitate the calculation of the monomer conversion.22a The
process was followed by SEC analyses of aliquots of the reaction
solution at appropriate time intervals to estimate the Mn and Mw/
Mn of the generated polyisocyanide as well as the monomer
conversion. The results are shown in Figure 3. It can be found
that the polymerization was relatively fast and >80% of the
monomer was consumed within 5.5 h. Both conversion-Mn and
conversion-Mw/Mn relationships are plotted in Figure 4a. The
Mn values of the isolated polymers are linearly correlated with the
conversion of 2 and kept a narrow distribution, further
confirming the living nature of the polymerization. Kinetic
studies revealed that the polymerization obeyed the first-order
rate law. The rate constant was estimated to be ∼7.58 × 10−5 s−1
according to the kinetic plot (Figure 4b), which is comparable to
that of the analogous initiator Pd−Pt μ-ethynediyl complex in
the polymerization of phenyl isocyanides.21c
Figure 3. Time-dependent SEC chromatograms for 1a-initiated
polymerization samples of 2 in THF at 55 °C with PSt (Mn = 2630,
Mw/Mn = 1.06) as internal standard ([2]0 = 0.2 M, [2]0/[1a]0 = 90; SEC
condition: eluent = THF; temperature = 40 °C).
Pd complexes 1b−d (Scheme 1) were further synthesized to
investigate their activity for polymerization of 2. The reactions
were also followed by SEC as used for 1a-initiated reaction. It was
found that these complexes also exhibited high activity to lead to
the formation of polyisocyanide in high isolated yield and with
designed Mn and narrow Mw/Mn. Time-dependent SEC
chromatograms of the polymerizations with 1b, 1c, and 1d
(Figures S10−15 in SI) indicated the polymerizations all
proceeded in a living chain growth fashion. Kinetic studies
revealed the polymerizations also obeyed the first-order rate law,
and the reaction rate constants for 1b, 1c, and 1d were estimated
to be 1.61 × 10−4, 6.51 × 10−5, and 4.78 × 10−5 s−1, respectively
(Figure 4b). The sequence of the polymerization rate is 1b > 1a >
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Figure 4. (a) Plot of Mn and Mw/Mn values as a function of 1a-initiated conversion of 2 in THF at 55 °C ([2]0 = 0.2 M, [2]0/[1a]0 = 90). (b) First-order
kinetics plots for polymerizations of 2 initiated by 1a (red ●), 1b (black ■), 1c (blue ▲), and 1d (magenta ▼).
1c > 1d, demonstrating the incorporation of an electron-
donating group to the benzene ring of the complex improved the
polymerization activity. However, the difference in rate constants
was not prominent, and decreased with the increase of the
polymerization degree because the substituent on the benzene
ring is far away from the living chain end. Even at higher
temperature and with longer time, 1e did not initiate the
polymerization of 2. This result suggested that the chloride
counterion of complexes 1a−d might play an important role in
the polymerization and is indispensable for this series of Pd
complexes. The polymerization of 2 with 1a−d may take place
through multiple and successive insertion of isocyanides into a
Pd−C bond following the mechanism proposed for the
polymerization of the dinuclear Pd−Pt μ-ethynediyl complex.21b
The insertion was believed to proceed by nucleophilic
substitution of the chloride with an isocyanide monomer to
form a cationic intermediate in which both isocyanide and
phenylbuta-1,3-diynyl groups are bonded to the Pd atom, and
simultaneously the phenylbuta-1,3-diynyl group migrates to the
isocyanide. In 1a−d, the phenylbuta-1,3-diynyl group may
promote the nucleophilic attack and facilitate its migration from
Pd to isocyanide. The two phenylbuta-1,3-diynyl groups on 1e
hampered the nucleophilic substitution, and thus it could not
initiate the polymerization. The electron-donating group on the
phenyl ring should promote the migration. Therefore, 1b
exhibited the highest reaction rate.
The above results clearly demonstrate 1a−d were effective in
initiating the polymerization of phenyl isocyanide in a living/
controlled chain growth fashion. Because the conformational
stability of helical poly(aryl isocyanide)s should be affected by
the steric factor of the pendants, helical poly(aryl isocyanide)s
with a substituent at the ortho position of its benzene ring are
attractive targets to prepare. We thus further investigated the
ability of 1a−d for initiating the polymerization of 2-(2-
(trimethylsilyl)ethynyl)benzenisocyanide 3a, which bears a
bulky (trimethylsilyl)ethynyl group at the ortho position of the
benzene ring. Due to steric hindrance, 3a could only be
polymerized with aryl rhodium complexes as an initiator in the
presence of an excess of phosphine ligands as additives.17b
Because 1b was the most active among the new Pd complexes,
polymerization of 3a was investigated in THF at 55 °C with 1b as
initiator (Scheme 3). However, no polymeric product was
isolated after 20 h. When the reaction was run in toluene at 90 °C
([3a]0/[1b]0 = 50, [3a]0 = 0.2 M), polymerization took place to
lead to the formation of yellow-brown poly-b3a50 with Mn = 5.0
× 103 and Mw/Mn = 1.18 as determined by SEC (Figure 5a).
The polymerization of 1-isocyanohexadecane 3b could also be
initiated in toluene by 1b at 90 °C ([3c]0/[1b]0 = 50, [3c]0 = 0.2
Scheme 3. Polymerization of Isocyanides 3a−f and
Diisocyanide 4 Initiated by 1b
M). The Mn (3.8 × 103) and Mw/Mn (1.08) of the resulting
product poly-b3b50 were determined by SEC. The yield of poly-
b3b50 was very low, and polymers of higher Mn could not be
obtained due to poor solubility of the product in toluene. The
polymerization of isocyanides 3c−f could also be initiated by 1b.
The structures of the resulting polymers were confirmed by 1H
NMR, FT-IR, and SEC (Figure 5a) and the results are
summarized in Table 2. The polymerization of enantiomers 3e
and 3f gave optically active poly-b3e100 and poly-b3f100, which
respectively exhibited positive and negative Cotton effects at 364
nm in the circular dichroism (CD) spectra (Figure 5b). The
Δε364 of poly-b3e100 and poly-b3f100 were estimated to be −11.8
and +12.8, respectively. These results indicated that a single-
handed helical polyisocyanide was formed through the induction
of the chiral pendants. To extend the reaction scope, we further
prepared diisocyanobenzene 4,5-dimethyl-1,2-diisocyanoben-
zene 4,14 and found that, although 1b did not initiate its
polymerization in THF at 55 °C, the polymerization could take
place in toluene at 90 °C to give poly-b450 with Mn = 5.4 × 103
and Mw/Mn = 1.19 as estimated by SEC (Table 2).
Telechelic polymers represent a class of chain-end function-
alized polymers. In contrast to monofunctional polymers which
are widely applied for surface functionalization26 or drug
immobilization,27 telechelic polymers offer an additional func-
tional group on the other chain end and thus can combine
different molecules such as biomacromolecules with, for
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Figure 5. (a) SEC chromatograms of the polymerization of 3a−e and 4 prepared by using 1b as initiator (SEC conditions: eluent = THF; temperature =
40 °C). (b) CD and UV−vis spectra of poly-b3e100 and poly-b3f100 measured in CHCl3 at 25 °C (c = 0.10 mg/mL).
a
Table 2. Polymerization Results of Isocyanide 3a−f and Diisocyanide 4 with 1b as Initiator.
b
c
c
d
run
monomer
[M]0/[1b]0
solvent
T, °C
polymer
Mn
Mw/Mn
yield, %
1
3a
3a
3b
3c
3c
3d
3e
3e
3f
3f
4
50
100
50
toluene
toluene
toluene
THF
90
90
90
55
55
90
55
55
55
55
90
90
poly-b3a50
poly-b3a100
poly-b3b50
poly-b3c50
poly-b3c100
poly-b3d50
poly-b3e50
poly-b3e100
poly-b3f50
poly-b3f100
poly-b450
5.0 × 103
8.3 × 103
3.8 × 103
1.1 × 104
1.7 × 104
3.6 × 103
1.0 × 104
1.9 × 104
1.1 × 104
2.2 × 104
5.4 × 103
1.18
1.08
1.04
1.22
1.14
1.05
1.29
1.22
1.16
1.21
1.19
1.20
70
83
30
90
89
59
95
93
97
95
75
58
2
3
4
50
5
100
50
THF
6
toluene
THF
7
50
8
100
50
THF
9
THF
10
11
12
100
50
THF
toluene
4
100
toluene
poly-b4100
7.4 × 103
a
b
c
The polymers were synthesized according to Scheme 3. The initial feed ratio of monomer to initiator. The Mn and Mw/Mn were determined by
d
SEC and reported as equivalent to standard PSt. Isolated yield.
Scheme 4. Synthesis of Bi- and Trifunctional Pd Complexes 1f and 1g and Their Initiation for the Polymerization of 2
example, dyes or inorganic tracers to produce intriguing
properties or functions.28,29 We envisioned that introducing
more than one Pd unit to the benzene ring might produce a
multifunctional initiator for preparing telechelic polyisocyanides.
We thus further synthesized Pd complexes 1f and 1g by attaching
two and three Pd(PEt3)2Cl moieties to 1,4-diethynylbenzene or
1,3,5-triethynylbenzene (Scheme 4). The polymerizations of 2
initiated by 1f and 1g were then investigated in THF at 55 °C.
Both complexes displayed high activity (Figures S16 and S17 and
Table S1, SI). The reaction in the initial feed ratio of 60 gave
poly-f260 with Mn = 1.5 × 104 and Mw/Mn = 1.19 (Figure 6a).
The narrow Mw/Mn also supported a living/controlled polymer-
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Figure 6. (a) Representative SEC chromatograms of the polymerization of 2 initiated by 1f and 1g in THF at 55 °C (SEC conditions: eluent = THF;
temperature = 40 °C). (b) 31P NMR spectra of 1f and 1g and the resulting poly-f230 and poly-g245 in CDCl3 at room temperature.
ization mechanism, and the Mn of the resulting polymers could
be controlled by varying the initial feed ratio (Figures S18 and
S19, SI). With 1g as initiator, living polymerization also took
place because linear dependence of the Mn values of the resulting
star polymers with the initial feed ratio was also observed
(Figures S20 and S21, SI).
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, spectral data of palladium complexes,
isocyanide monomers and polymers, SEC chromatograms of
polyisocyanides, and details of X-ray analysis. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
The structures of these two- and three-armed polymers were
1
1
characterized by H and 31P NMR, SEC, and FT-IR. The H
NMR spectra of the telechelic polymers were similar to that of
poly-a2100 (Figures S3, S61, and S62, SI). The formation of such
AUTHOR INFORMATION
Corresponding Author
zqwu@hfut.edu.cn
■
multiarmed polymers were also tracked and confirmed with 31
P
Author Contributions
NMR spectroscopy.30 For comparison, 31P NMR spectra of 1a
and the resulting poly-a250 were further recorded (Figure S6, SI).
The spectrum of 1a displayed a single peak at δ 18.8 ppm. This
peak disappeared after the polymerization of 2 took place, which
was accompanied with a new doublet at 14.4 ppm, corresponding
to the formation of poly-a250. Di- and trifunctional initiators 1f
and 1g exhibited a single peak around δ 17.9 ppm in 31P NMR
spectra. The peak disappeared after polymerization, while a new
doublet at δ 14.4 ppm was observed, again supporting the
formation of desired polymers (Figure 6b). All these results
suggested that the Pd units in these multifunctional initiators
functioned as reactive points for the polymerization to afford the
well-defined multiarmed telechelic polymers.
§Y.-X.X. and Y.-Y.Z. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is partially supported by the National Natural
Scientific Foundation of China (21104015, 21172050,
21302035, 21371043, 51303044, and 21304027), Thousand
Young Talents Program of China and Fundamental Research
Funds for the Central Universities of China (2011HGRJ0005,
2012HGZY0012 and 2013HGCH0013). 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.
CONCLUSION
■
In summary, we have designed and synthesized a series of air-
stable (phenylbuta-1,3-diynyl)palladium(II) complexes and
demonstrated their robust ability for initiating the polymer-
ization of phenyl isocyanides in living and controlled chain
growth manner. The poly(phenyl isocyanide)s prepared using
the new initiators possess controlled Mn and narrow Mw/Mn and
high stereoregularity, which is important for maintaining the
helical sense of the polymer main chain. The obtained polymers
can keep the activity of the living chain end of Pd(PEt3)2Cl
complex to further undergo block copolymerization with
isocyanide monomers. The new Pd complexes can promote
the living polymerization of a broad range of isocyanides and
diisocyanides in a number of solvents, which leads to the
formation of a variety of polymers. Furthermore, the bi- and tri-
Pd catalysts can be used to prepare multifunctional poly-
isocyanides with the same arm length. We believe that the
present systematic studies will provide not only a useful living
polymerization method for facile preparation of a broad range of
polyisocyanides with controlled Mn and narrow Mw/Mn but also
a clue for designing novel catalysts and initiators.
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