Transition metal acetylide catalysts for polymerization of alkynes 1. Effect of ligands on catalytic activity of nickel complexes

Article abstract of DOI:10.1016/S1381-1169(00)00559-8

The catalytic activity of a series of nickel acetylides and chlorides containing different phosphine and alkynyl ligands: NiL₂(C≡CR)₂; NiL₂(C≡CR)Cl and NiL₂Cl₂ (L₂ = (PPh₃)₂, (PBu₃)₂ and Ph₂PCH₂CH₂PPh₂; R = p-C₆H₄C≡CH, C₆H₅, H, CH₂OH and CH₂OCOCH₃), towards p-diethynylbenzene polymerization is compared. The catalytic property of these acetylides is related to chlorine, phosphines and alkynyl ligands bonded to the metal atoms. The polarity, steric hindrance and electron-donating ability of ligands are main factors influencing the catalytic activity of these complexes. Nickel acetylides were found to be novel efficient initiating systems for the alkyne polymerization. The polymerization proceeds under mild conditions to give soluble π-conjugated polymers with weight average molecular weights as high as 4.8 × 10⁴ in yields as high as 95%.

Full text of DOI:10.1016/S1381-1169(00)00559-8

Journal of Molecular Catalysis A: Chemical 169 (2001) 27–31
Transition metal acetylide catalysts for polymerization of alkynes
1. Effect of ligands on catalytic activity of nickel complexes
Xiaowei Zhana,b,∗, Mujie Yanga
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a
b
Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, PR China
Received 21 August 2000; accepted 16 November 2000
Abstract
The catalytic activity of a series of nickel acetylides and chlorides containing different phosphine and alkynyl ligands:
≡
≡
≡
NiL2(C CR)2; NiL2(C CR)Cl and NiL2Cl2 (L2 = (PPh3)2, (PBu3)2 and Ph2PCH2CH2PPh2; R = p-C6H4C CH, C6H5,
H, CH2OH and CH2OCOCH3), towards p-diethynylbenzene polymerization is compared. The catalytic property of these
acetylides is related to chlorine, phosphines and alkynyl ligands bonded to the metal atoms. The polarity, steric hindrance and
electron-donating ability of ligands are main factors influencing the catalytic activity of these complexes. Nickel acetylides
were found to be novel efficient initiating systems for the alkyne polymerization. The polymerization proceeds under mild
conditions to give soluble ␲-conjugated polymers with weight average molecular weights as high as 4.8 × 104 in yields as
high as 95%. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Transition metal acetylide; Catalyst; Alkyne; Polymerization
1. Introduction
procedures such as thermal, high pressure and sheer,
␥- and UV-irradiation, electroinitiated and catalytic
polymerizations, almost led to insoluble and unstable
products or resulted in partially soluble oligomers
with low molecular weights in low yields [11–14].
The research and development on organometallic
catalysts for synthesis of polymers is an important
and attractive field in polymer science and indus-
try. In recent years, transition metal acetylides were
found to be effective catalysts for alkynes polymer-
ization [15–20]. To our knowledge, however, there
have been no reports on effect of transition metals
and ligands on catalytic activity of transition metal
acetylides towards alkynes polymerization. We have
accomplished polymerization of propargyl alcohol,
p-diethynylbenzene (p-DEB) and their derivatives
with transition metal acetylide catalysts to give sol-
uble polymers in high yields [21–25]. We found that
Polyacetylenes possessing alternating olefinic
bonds along the main chain are of intense interest
because of their unique physical properties such as
photoconductivity, optical nonlinear susceptibility,
magnetic susceptibility, chirality and liquid crys-
tallinity [1–5]. Poly(p-diethynylbenzene) (PDEB) is
one member of the polyacetylenes family. Recently,
we found that soluble PDEB shows conductivity,
photoluminescence, electroluminescence, third-order
nonlinear optical property and humidity sensitivity
[6–10]. Research devoted to methods of synthesiz-
ing PDEB has been reported. However, the reported
∗ Corresponding author. Tel.: +86-571-7952444;
fax: +86-571-7952444.
E-mail address: yangmj@cmsce.zju.edu.cn (M. Yang).
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(00)00559-8

28
X. Zhan, M. Yang / Journal of Molecular Catalysis A: Chemical 169 (2001) 27–31
the metal–carbon ␴-bond plays an important role in
the polymerization of alkynes initiated by these transi-
tion metal acetylides. However, the knowledge about
properties of the metal–carbon ␴-bond seems still in-
sufficient. Only a few discussions on the infrared and
electronic spectra of transition metal acetylides can
be found [26–28].
In this work, we discussed the effect of ligands on
the infrared and electronic spectra and catalytic activ-
ity of nickel acetylides towards the polymerization of
alkynes.
and degassed at room temperature for 30 min, and
p-DEB (2.1 mmol, 265 mg) was added. The reaction
mixture was then stirred for 72 h at room tempera-
ture. The solid was filtered off and the solvent was
evaporated to give an orange oil. The oily residue
was dissolved in 5 ml benzene and n-hexane (20 ml)
was added. The orange crystals were filtered and re-
crystallized from benzene/n-hexane to yield orange
crystals (163 mg, 23%): mp (decomposition), 170◦C.
UV–VIS λmax (THF, nm): 355; 331; 308; 262. FT-IR
(nujol mull, cm−1): 3280 (m); 2090 (s); 1600 (m);
1305 (m); 1190 (s); 1175 (s); 1120(s); 1100 (m);
830 (s); 695 (s); 530 (m); 510 (m). Anal. Calcd. for
NiC46H34P2: C, 78.10; H, 4.84. Found: C, 77.55; H,
4.90%.
2. Experiments
2.1. Materials
2.3.2. Bis(triphenylphosphine)(p-ethynylphenyl-
ethynyl)chloronickel
Only analytical grade quality chemicals were
used. Diethylamine was dried over CaH2 and dis-
tilled under N2 atmosphere prior to use. Other sol-
vents were dried with activated alumina. p-DEB
[29], Ni(PPh3)2Cl2 [30], Ni(Ph2PCH2CH2PPh2)Cl2
A suspension of Ni(PPh3)2Cl2 (1 mmol, 654 mg)
in dry HNEt2 (50 ml) was stirred and degassed at
room temperature for 30 min, and p-DEB (1.0 mmol,
126 mg) was added. The reaction mixture was then
stirred for 5 h at room temperature. The solid is fil-
tered off and the solvent was evaporated to give an
orange oil. The oily residue was dissolved in 5 ml
CHCl3 and n-hexane (20 ml) was added. The or-
ange crystals were filtered off and recrystallized from
CHCl3/n-hexane to yield orange crystals (186 mg,
25%): mp (decomposition), 135◦C. UV–VIS λmax
(THF, nm): 368; 304; 258. FT-IR (nujol mull, cm−1):
3280 (w); 2089 (s); 1600 (m); 1435 (s); 1305 (w);
1100 (s); 1025 (w); 1000 (w); 840 (s); 740 (s);
695 (s); 628 (w); 550 (w); 522 (s); 510 (s); 497
(m); 460 (w); 420 (w); 390 (w). Anal. Calcd. for
NiC46H35P2Cl: C, 74.27; H, 4.74. Found: C, 74.54; H,
4.83%.
≡
≡
[31], Ni(PPh3)2(C CC6H4C CH) 2 [32], Ni(PBu3)2
≡
≡
≡
(C CC6H4C CH)2 [32], Ni(PPh3)2(C CPh)2 [32],
≡
≡
Ni(PBu3)2(C CPh)2 [33], Ni(PBu3)2(C CH)2 [33],
Ni(PBu3)2(C CCH2OH)2 [33], Ni(PBu3)2(C CCH2
OCOCH3)2 [23] and Ni(PPh3)2(C CCH2OCOCH3)2
[23] were prepared by literature methods or modify-
ing reported procedures.
≡
≡
≡
2.2. Measurements
Melting points were determined on a Yanaco
MP-500 melting point apparatus. IR spectra were
taken on a Nicolet 5-DX FT-IR with nujol mulls.
UV–VIS spectra were recorded on a Beckman DU-50
spectrophotometer. Elemental analyses were carried
out by a Carlo Erba Model 1106 elemental analyzer.
The weight-average molecular weight (Mw) and poly-
dispersity index (Mw/Mn) of polymers were measured
on a PL-GPC Model 210 chromatograph at 35◦C using
THF as eluent and standard polystyrene as reference.
2.4. Polymerization
A typical polymerization procedure is as follows:
378 mg (3 mmol) of sublimed p-DEB and 25 mg
(0.03 mmol) of [bis(triphenylphosphine)bis(p-ethy-
nylphenyleneethynyl)]nickel complex were dis-
solved in 1,4-dioxane (1.25 ml) and toluene (1.25 ml)
mixed solvents. Under purified nitrogen atmo-
sphere, the polymerization proceeded at 25◦C
for 3 h. The resulting polymer was precipitated
by the addition of methanol, filtered from the
2.3. Catalyst synthesis
2.3.1. [1,2-Bis(diphenylphosphino)ethane]bis(p-ethy-
nylphenyleneethynyl)nickel
A
suspension of Ni(Ph2PCH2CH2PPh2)Cl2
(1 mmol, 528 mg) in dry HNEt2 (50 ml) was stirred

X. Zhan, M. Yang / Journal of Molecular Catalysis A: Chemical 169 (2001) 27–31
29
Table 2
solution, washed with CH3OH, and dried un-
der vacuum at room temperature for 24 h to give
PDEB. Other polymerizations were carried out
analogously.
Comparison of catalytic activity of Ni(PPh3)2Cl2, Ni(PPh3)
a
≡
≡
≡
≡
2(C CC6H4C CH)Cl and Ni(PPh3)2(C CC6H4C CH)2
Complex
Time Yield Mw
Mw/Mn
(h)
(%)
× 10−4
≡
≡
Ni(PPh3)2(C CC6H4C CH)2 3.5 81.2
Ni(PPh3)2(C CC6H4C CH)Cl
Ni(PPh3)2Cl2
1.7
2.1
–
2.3
1.7
–
≡
≡
5
48
71.2
0
3. Results and discussion
a Conditions: solvent, DMSO; catalyst concentration, [cat]0
3.1. Effect of phosphine ligands on the metal–carbon
stretching frequency and the lowest energy band of
nickel acetylides
= 0.01 mol l−1
;
monomer concentration, [M]0 = 1.0 mol l−1
;
30◦C.
3.2. Comparison of the catalytic activity of nickel
chloride and acetylide
Table 1 lists the influence of phosphine ligands
on the lowest energy band and the metal–carbon
≡
stretching frequency (ν(M–C)) of NiL2(C CC6H4
The results of p-DEB polymerization with nickel
chloride and acetylide catalysts in DMSO are given
in Table 2. Nickel dialkynyl complex shows a higher
catalytic activity than nickel monoalkynyl complex,
and nickel monoalkynyl complex much higher than
≡
C CH)2. The influence of phosphine ligands (L) such
as PPh3, PBu3 and Ph2PCH2CH2PPh2 on ν(M–C) is
negligible. The intensity of absorption bands is of the
order of four in log ε scale, attributed to intramolecu-
lar charge transfer transition [28,33]. The complexes
almost show three intense absorption bands, and the
lowest energy band is empirically assigned to the
transition between the molecular orbitals involved
in metal–carbon bonds [28,33]. We only discuss
the lowest energy band because it is related to ac-
tivity of the metal–carbon and the complexes. In
contrast to infrared spectra, phosphine ligands ex-
ert an appreciable effect on the wavelength of the
lowest energy band (λmax). The lowest band shows
a blue shift and λmax follows the following order:
L = PPh3 > Ph2PCH2CH2PPh2 > PBu3. The en-
ergy of the lowest band of the complexes increases
with increasing the electron-donating ability of phos-
phines. The basicity and the ␴-related donating ability
of phosphines increase the density of electron cloud
of the metal–carbon bond, and somewhat exert effect
on metal–alkynyl charge transfer transitions and the
lowest energy band.
≡
≡
nickel chloride: Ni(PPh3)2(C CC6H4C CH)2
>
≡
≡
Ni(PPh3)2(C CC6H4C CH)Cl ꢀ Ni(PPh3)2Cl2.
In fact, the chloride shows no activity for p-DEB
polymerization under tested conditions. It is evident
that the chlorine ligand decreases the catalytic activity
of the nickel complexes.
3.3. Effect of phosphine ligands on the catalytic
activity of the acetylides
The phosphine ligands bonded to the metal atom
play an important role on the catalytic properties of
these complexes. Table 3 gives yields and molecu-
lar weights of PDEB obtained with nickel acetylides
containing different phosphine ligands. The p-DEB
polymerization can proceed at 30◦C in the presence of
≡
≡
Ni(PPh3)2(C CC6H4C CH)2 to afford PDEB with
Table 3
Effect of phosphine ligands on catalytic activity of
a
≡
≡
NiL2(C CC6H4C CH)2
Table 1
Effect of phosphine ligands on the metal–carbon stretching vibra-
L2
Tempera- Time Yield Mw
Mw/Mn
tion and the lowest energy band of NiL2(C CC6H4C CH)2
ture (◦C) (h)
(%)
× 10−4
≡
≡
ν(M–C) (cm−1
)
λmax (nm)
(PPh3)2
(PBu3)2
Ph2PCH2CH2PPh2 60
30
60
3.5 81.2 1.7
2.3
3.1
2.6
L2
9
95.2 2.2
33.3 1.9
(PPh3)2
(PBu3)2
Ph2PCH2CH2PPh2
539
542
540
371
353
355
61
a Conditions: [cat]0 = 0.01 mol l−1
;
[M]0 = 1.0 mol l−1
;
DMSO.

30
X. Zhan, M. Yang / Journal of Molecular Catalysis A: Chemical 169 (2001) 27–31
Table 5
a high molecular weight in a high yield. The p-DEB
Effect of alkynyl ligands on catalytic activity of Ni(PBu3)2
(C CR)2
≡
≡
polymerization with Ni(PBu3)2(C CC6H4C CH)2
a
≡
≡
≡
and Ni(Ph2PCH2CH2PPh2)(C CC6H4C CH)2 cat-
Time (h) Yield (%) Mw × 10−4 Mw/Mn
alysts can undergo only at 60◦C. The p-DEB poly-
merization with Ni(Ph2PCH2CH2PPh2)(C CC6H4-
C CH)2 catalyst produced a polymer in a low
yield (33%). Thus, the activity of nickel acetylides
with different phosphines follows the sequence:
R
≡
≡
p-C6H4C CH 48
27.8
47.1
76.7
78.0
51.6
2.5
3.3
4.8
4.4
2.8
3.0
3.4
3.6
3.1
2.6
≡
C6H5
H
48
33
33
CH2OH
CH2OCOCH3 33
≡
≡
≡
Ni(PPh3)2(C CC6H4C CH)2
>
Ni(PBu3)2(C
a Conditions: [cat]0 = 0.01 mol l−1
1,4-dioxane/toluene = 1; 60◦C.
;
[M]0 = 1.0 mol l−1
;
≡
≡
CC6H4C CH)2 > Ni(Ph2PCH2CH2PPh2)(C CC6-
H4C CH)2. The nickel complex with tributylphos-
≡
phine ligand exhibits a lower activity than that with
triphenylphosphine ligand, probably owing to the
greater basicity and the ␴-related donating ability of
PBu3 which increases the density of electron cloud
of nickel–carbon bond and decreases the reaction
activity of this bond. The poor catalytic activity of
p-DEB polymerization in 1,4-dioxane/toluene mixed
solvents at 25◦C. Nickel acetylides with PPh3 ligand
are efficient initiators for p-DEB polymerization, and
the polymerization can occur at room temperature to
give satisfactory results. Nickel acetylides containing
≡
≡
≡
≡
Ni(Ph2PCH2CH2PPh2)(C CC6H4C CH)2 is related
to the great steric hindrance of 1,2-bis(diphenylpho-
sphino)ethane bridging ligand. The influence of phos-
phine ligands on catalytic activity of nickel complexes
is similar to the effect of phosphine ligands on the
lowest energy band of these complexes. The energy of
the lowest energy band increases, and the catalytic ac-
tivity of the complexes decreases with increasing the
␴-related electron-donating ability of the phosphine
ligands.
non-polar alkynyl ligands, i.e. p-C CC6H4C CH and
≡
C CC6H5, show a little higher activity than that with
≡
polar ligand, i.e. C CCH2OCOCH3, probably due
to slight instability of the latter complex. We failed
in our attempt to synthesize Ni(PPh3)2(C CH)2 and
Ni(PPh3)2(C CCH2OH)2 because of their instability.
≡
≡
Effect of polarity of alkynyl ligands on catalytic
activity of nickel acetylides with PBu3 ligand is
different from that with PPh3 ligand as shown in
Table 5. The complexes containing polar alkynyl
ligands show higher catalytic activity than the cor-
responding complexes containing non-polar ligands.
The acetylides containing small-bulk alkynyl ligands
show higher catalytic activity than the corresponding
complexes containing large-bulk ligands. The nickel
acetylides with PBu3 ligand show much more sta-
ble and inert in solution than that with PPh3 ligand
owing to the great basicity and the electron-donating
ability of PBu3. The extremely good stability and
inertness of initiators are unfavorable to catalysis.
The nickel acetylides containing polar alkynyl lig-
ands possess medium stability and fair activity, but
that with non-polar alkynyl ligands are too stable
to be favorable catalysts for p-DEB polymerization.
On the other hand, the steric hindrance of PBu3
ligand is greater than that of PPh3 ligand resulting
from the long chain of n-butyl group. The steric re-
pulsion of PBu3 prevents the coordination of triple
bonds of the monomer with metal atoms of the cat-
alysts and the ␲-complex formation. The great-bulk
alkynyl ligands are harmful to the ␲-complex
3.4. Effect of alkynyl ligands on the catalytic
activity of the acetylides
The alkynyl ligands exhibit a great influence on the
catalytic activity of nickel acetylides towards p-DEB
polymerization. The effect of alkynyl ligands is dif-
ferent with varying phosphine ligands. Table 4 com-
pares the catalytic activity of nickel acetylides con-
taining non-polar and polar alkynyl ligands towards
Table 4
Effect of alkynyl ligands on catalytic activity of Ni(PPh3)2
a
≡
(C CR)2
R
Time (h) Yield (%) Mw × 10−4 Mw/Mn
≡
p-C6H4C CH
C6H5
3
3
74.3
71.5
57.7
1.4
1.7
1.3
2.4
2.3
2.0
CH2OCOCH3 10
a Conditions: [cat]0 = 0.01 mol l−1
1,4-dioxane/toluene = 1; 25◦C.
;
[M]0 = 1.0 mol l−1
;

X. Zhan, M. Yang / Journal of Molecular Catalysis A: Chemical 169 (2001) 27–31
31
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necessary to catalytic polymerization. Therefore,
the p-DEB polymerization in the presence of nickel
acetylides with PBu3 and great-bulk alkynyl ligands
gives PDEB in lower yields.
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Nickel acetylides are novel efficient initiating sys-
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catalytic activity of transition metal acetylides. The
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