Bis(pyrazole)- and bis(pyrazolyl)-palladium complexes as phenylacetylene polymerization catalysts

Article abstract of DOI:10.1016/j.poly.2007.11.033

Two types of pyrazole-based palladium complexes were used to catalyze the polymerization of phenylacetylene. Catalysts with electron-withdrawing linkers, [{1,3-(3,5-R₂pzCO)₂C₆H₄}Pd₂Cl₂(μ-Cl

Full text of DOI:10.1016/j.poly.2007.11.033

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 1017–1023
www.elsevier.com/locate/poly
Bis(pyrazole)- and bis(pyrazolyl)-palladium complexes
as phenylacetylene polymerization catalysts
Kelin Li a, M. Sarah Mohlala a, Tebogo V. Segapelo b, Poslet M. Shumbula a,
Ilia A. Guzei c, James Darkwa b,*
a Department of Chemistry, University of The Western Cape, Private Bag X17, Bellville 7535, South Africa
b Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, P.O. Box 524, Johannesburg 2006, South Africa
c Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA
Received 29 March 2006; accepted 28 November 2007
Available online 3 January 2008
Abstract
Two types of pyrazole-based palladium complexes were used to catalyze the polymerization of phenylacetylene. Catalysts with
electron-withdrawing linkers, [{1,3-(3,5-R2pzCO)2C6H4}Pd2Cl2(l-Cl)2] (R = tBu (1), Ph (2), Me (3), [{2,6-(3,5-R2pzCO)2C5H3N)}-
PdCl2] (R = tBu (4), Me (5)), show high conversion; whilst those with simple pyrazole ligands, [(3,5-R2pz)2PdCl2] (R = H (6), Me
t
(7), Bu (8)), [(3,5-tBu2pz)2PdCl(Me)] (9), have much lower conversions. Conversion greatly improved when 9 was used to catalyze
the co-polymerization of sulfur dioxide and phenylacetylene. Both types of catalysts produce predominantly trans–cisoidal
polyphenylacetylene.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Pyrazole; Pyrazolyl; Palladium complexes; Catalysts; Phenylacetylene; Polymerization
1. Introduction
ate)(H2O)] [3i,6]. The palladium-based catalysts are
mainly palladium(II) phosphine complexes or simple salts
like [Pd(OAc)2] [3k]. Even though nitrogen donor palla-
dium complexes have been used extensively to catalyze
olefin polymerization reactions [7], to date there have
been no well-defined palladium(II) nitrogen donor ligand
complexes that are known to catalyze the polymerization
of phenylacetylene. Attempts by LaPointe and Brookhart
to use cationic palladium(II) methyl or vinyl complexes to
polymerize alkynes led to insertion products and no poly-
mers [8]. The only palladium nitrogen donor ligand com-
plexes that catalyze the polymerization of phenylacetylene
are [Pd(NN0O)Cl] (NN0O = 2-acetylpyridinebenzohydraz-
one, or 2-formylpyridinebenzohydrazone) [9], although
catalysts activities are moderate (32–52% conversions).
This paper reports our investigation of how cationic pyr-
azole and pyrazolyl palladium(II) complexes catalyze the
polymerization of phenylacetylene to relatively high-
molecular weight polyphenylacetylene.
Masuda and co-workers first polymerized phenylacety-
lene in 1974 with WCl6 and MoCl5 catalysts to give high-
molecular weight polymers [1]. Since then various Groups
5 and 6 [2] and late [3] transition metal catalysts have
been used to polymerize phenylacetylene and other substi-
tuted acetylenes. Most of the late transition metal cata-
lysts are rhodium- and palladium-based compounds
[3a,3b,3c,3d, 3e], but by far the most efficient late transi-
tion metal phenylacetylene polymerization catalysts are
from the precursor complexes [(PPh3)2Rh(CCPh)(nbd)]
[4] [(AAEMA)Rh(cod)] (AAEMA = 2-(acetoacetoxy)eth-
ylmethacrylate) [5], [(diene)RhCl]2 and [(diene)Rh(tosyl-
*
Corresponding author. Tel.: +27 11 559 2838; fax: +27 11 559 2810.
E-mail address: jdarkwa@uj.ac.za (J. Darkwa).
0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.11.033

1018
K. Li et al. / Polyhedron 27 (2008) 1017–1023
2. Experimental
2.3. Polymerization
2.1. Materials and instrumentation
In a typical experiment a solution of silver triflate
(0.078 g, 0.28 mmol) in a 15 mL mixture of CH2Cl2 and
MeCN (7:1) was added to complex 1 (0.10 g, 0.14 mmol)
in 25 mL of CH2Cl2 and MeCN (7:1). A white precipitate
of AgCl formed immediately and the mixture was stirred
for at most 5 min and filtered to give a yellow solution of
the active catalyst. To the solution of the active catalyst
was added 50 equivalents of phenylacetylene (0.78 mL,
7.0 mmol). The resultant solution changed to dark red
immediately. After stirring for 2 min the solution was evap-
orated to give a dark brown residue, which was dissolved in
10 mL of CH2Cl2 followed by the addition of 30 mL of
All reactions were performed under dry deoxygenated
nitrogen atmosphere using standard Schlenk techniques.
Dichloromethane was dried and distilled over P2O5 and
acetonitrile was dried over P2O5 and stored over molecular
sieves. All the catalyst precursors, [{1,3-(3,5-R2pzCO)2C6-
H4}Pd2Cl2(l-Cl)2] (R = tBu (1) [10], Me (3) [10], [{2,6-
(3,5-R2pzCO)2C5H3N)}PdCl2] (R = tBu (4), Me (5)) [11],
[(3,5-R2pz)2PdCl2] (R = H (6), Me (7), tBu (8)) [12],
[(3,5-tBu2pz)2PdCl(Me)] (9) [12], were prepared by the liter-
ature methods, except 2, which was prepared as described
below. Phenylacetylene (98%) was obtained from Merck
and used as received. NMR spectra were recorded on a
Gemini 2000 instrument at 200 MHz for 1H and
50.3 MHz for 13C. Polymer molecular weights were deter-
mined by gel permeation chromatography on a Waters
600E instrument with a Waters 410 differential refractom-
methanol to precipitate
a yellowish-orange powder.
Yield = (0.58 g, 90%). The product was characterized by
a combination of 1H, 13C{1H} NMR spectroscopy and
GPC analysis.
2.4. X-ray crystallography
eter detector (THF, at 30 °C, rate = 1.0 mL minꢀ1
)
and PL-MIXED-CTM columns, using polystyrene as
standards.
The crystal evaluation and data collection were per-
formed on a Bruker CCD-1000 diffractometer with Mo
˚
Ka (k = 0.71073 A) radiation and the diffractometer to
2.2. Synthesis
crystal distance of 4.9 cm. The initial cell constants were
obtained from three series of x scans at different starting
angles. The reflections were successfully indexed by an
automated indexing routine built in the SMART program
[13]. The absorption correction was based on fitting a func-
tion to the empirical transmission surface as sampled by
multiple equivalent measurements. A successful solution
by the direct methods provided most non-hydrogen atoms
from the E-map. The remaining non-hydrogen atoms were
located in an alternating series of least-squares cycles and
difference Fourier maps. All non-hydrogen atoms were
refined with anisotropic displacement coefficients. All
hydrogen atoms were included in the structure factor calcu-
lation at idealized positions and were allowed to ride on the
neighboring atoms with relative isotropic displacement
coefficients.
2.2.1. 1,3-Bis(3,5-diphenylpyrazolyl-1-carbonyl)benzene
(L1)
To a solution of isophthaloyl dichloride (0.50 g;
2.46 mmol) in dry toluene (50 mL) was added diphenylpy-
razole (1.08 g; 4.92 mmol). The colourless solution turned
yellow upon addition of Et3N (8 mL). The solution was
vigorously stirred under nitrogen at room temperature
for 24 h. After the specified reaction time, the mixture
was filtered and the filtrate evaporated to dryness at
reduced pressure to give a yellow solid, which was purified
by column chromatography using silica gel and
CH2Cl2:Et2O (8:1) as eluent. Yield = 0.64 g, 46%. Anal.
Calc. for C38H26N4O2: C, 79.98; H, 4.59; N, 9.82. Found:
1
C, 79.96; H, 4.57; N, 9.65%. H NMR (CDCl3): d 8.95
(s, 1H, Ph); 8.42 (d, 2H, Ph, JHH = 10 Hz); 7.69 (t, 1H,
2
Ph); 7.46 (m, 20H, Ph on pz); 6.90 (s, 2H, 4-pz).
3. Result and discussion
2.2.2. Di-l-chloro-dichloro{1,3-bis(3,5-diphenylpyrazolyl-1-
carbonyl)benzene}dipalladium(II) (2)
3.1. Synthesis of bis(diphenylpyrazolyl-1-carbonyl)benezene
ligand and palladium(II) complex (2)
To L1 (0.33 g, 0.58 mmol) and [PdCl2(NCMe)2] (0.15 g,
0.58 mmol) was added dry dichloromethane (70 mL). The
resulting dark red solution was stirred at room temperature
under nitrogen for 18 h, turning yellow with formation of
small amount of precipitate. The reaction mixture was fil-
tered and the filtrate was evaporated to dryness to give
an orange solid. Yield = 0.40 g, 63%. Anal. Calc. for
C40H30Cl8N4O2Pd2: C, 47.06; H, 2.91; N, 5.78. Found: C,
The synthesis of both ligand and complex is similar to
the other substituted pyrazolyl compounds and their pal-
ladium complexes we reported recently [10]. Scheme 1
1
shows how L1 and 2 were prepared. The H NMR spec-
trum of the ligand is typical of other 1,3-benzenedicar-
bonyl pyrazolyl ligands. The C2, C4 and C5 protons
on the benzene appear as a triplet (7.69 ppm), a doublet
(8.42 ppm) and doublet of a doublet (8.56 ppm) respec-
tively. The 1H NMR spectrum of 2 suggests that the
ligand binds to the Pd in a similar fashion as reported
for 1,3-benzenedicarbonyl pyrazolyl ligands [10] to form
1
47.79; H, 2.46; N, 5.78%. H NMR (CDCl3): 9.08 (d, 2H,
2
2
Ph, JHH = 4 Hz); 9.05 (d, 2H, Ph, JHH = 4 Hz); 8.38
(m, 5H, Ph on pz); 8.31 (t, 1H, Ph); 7.62 (s, 6H, Ph on
pz); 7.49 (s, 10H, Ph on pz); 6.79 (s, 2H, 4-pz).

K. Li et al. / Polyhedron 27 (2008) 1017–1023
1019
Ph
O
O
Ph
O
Cl
N
Ph
Ph
Cl
N
N
Ph
Ph
N
Pd
Pd
PdCl2(NCMe)2
2
+
Cl
Cl
N
N
H
N
Ph
Cl
Cl
N
Ph
N
N
O
O
O
Ph
Ph
L1
2
Scheme 1.
a dinuclear complex. The structures of the ligand and the
dinuclear complex were confirmed by X-ray crystallo-
graphy.
3.2. Descriptions of the structure of L1 and 2
Single crystals of L1 and 2 were obtained from recrystal-
lization of the respective compounds from CH2Cl2 and
ether at ꢀ15 °C. The crystal quality of 2 was poor and
the crystals collapsed once it was dry, but X-ray diffraction
analysis could be carried out on the crystals if solvent of
crystallization was present; although the quality of data
from these crystals was not so good. The refinement was
therefore only good to establish the connectivity of atoms,
and as such we have not discussed any bond parameters.
The structure of 2 was found to be isomorphous with the
solid-state structures of 1 and 3 [10]. The molecular struc-
tures of L1 and 2 are shown in Figs. 1 and 2 respectively.
Crystallographic information is tabulated in Table 1 while
selected bond lengths and angles for L1 are listed as part of
the captions for Fig. 1.
Fig. 2. Molecular structure of 2. Due to the poor quality of the X-ray data
only an ortep drawing is shown to indicate the connectivity of atoms.
Table 1
Crystal data and structure refinement for L1 and 2
L1
2
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Space group
C
570.63
173(2)
0.71073
P21/n
38H26N4O2
C40H30Cl8N4O2Pd2
1095.08
100(2)
0.71073
P21/n
˚
Crystal system
monoclinic
9.6723(16)
28.979(5)
10.1433(17)
92.980(3)
2839.3(8)
4
monoclinic
12.819(6)
16.503(8)
19.401(9)
94.703(8)
4091(3)
˚
a (A)
˚
b (A)
˚
c (A)
b (°)
˚
V (A)
Z
4
Dc (Mg mꢀ3
)
1.335
0.084
1.778
1.444
l(Mo K) (mmꢀ1
)
Crystal size
Absorption correction/mm
Tmax/Tmin
0.44 ꢁ 0.34 ꢁ 0.17
multiscan
0.9859/0.9640
0.10 ꢁ 0.10 ꢁ 0.05
0.9313/0.8691
The solid state structure of 1,3-(3,5-R2pzCO)2C6H4 and
t
1,2-(3,5-R2pzCO)2C6H4 (R = Me, Bu) possess either a C2
Fig. 1. Molecular structure of 1,3-bis(3,5-diphenylpyrazolyl-1-
or Cs symmetry [10]. These observation can be extended
to 1,3-(3,5-Ph2pzC(O))2C6H4. The C2 symmetry is attained
when the pyrazolyl substituents on the benzene ring reside
on the opposite sides of its plane. The Cs symmetry is
˚
carbonyl)benzene. Selected bond lengths (A) and angles (°): N(1)–N(2)
1.374(18), N(2)–C(16) 1.431(19), O(1)–C(16) 1.206(19), C(16)–C(17)
1.485(2). (N91)–N(2)–C(16) 117.31(12), O(1)–C(16)–N(2) 120.03(14),
O(1)–C(16)–C(17) 124.22(14).

1020
K. Li et al. / Polyhedron 27 (2008) 1017–1023
observed when both substituents are on the same side of
the ring with the mirror plane perpendicular to the plane
of the ring. Based on the above assessment 1,3-(3,5-
Ph2pzCO)2C6H4 has a Cs symmetry. Molecular modeling
revealed that the two symmetry conformations are energet-
ically equivalent and are close in energy to a number of cal-
culated energetic minima corresponding to various Ci
arrangements. It has also been established that the most
energetically favourable conformations are those in which
the carbonyl is coplanar with the pyrazole ring.
Compound 1,3-(3,5-Ph2pzC(O))2C6H4 cannot achieve
planarity due to either close proximity of the pyrazolyl sub-
stituents in the 2-position to the ortho hydrogen of the phe-
nyl ring, or due to unfavorable interactions between the
nitrogen lone pair and the ortho hydrogen. The C@O group
can, however, be coplanar with either ring since both pyr-
azolyl and phenyl groups possess a delocalized p-system,
with the preferred ring being the pyrazole. The C–N bond
ated in situ by reacting each palladium complex with silver
triflate in a mixture of CH2Cl2 and MeCN. The presence of
MeCN is necessary to stabilize the active catalyst, yet the
coordination of MeCN has to be weak enough to allow
the monomer access to the active centre of the catalysts
in order to initiate the polymerization.
The stereochemistry of the polyphenylacetylene pro-
duced was determined by 1H NMR spectroscopy. Polymers
from solvents mixtures were mainly cis–transoidal polyphe-
nylacetylene, which showed a sharp singlet at 5.85 ppm for
the vinylic protons in the polymer and broad singlets at
6.66 ppm (ortho) 6.97 ppm (meta and para) protons of
the phenyl groups in the polymer. In MeCN and THF
1
the H NMR spectra of the polymers produced were very
broad and centred at 7.25 ppm typical of trans–cisoidal
polyphenylacetylene.
Polymerization data, showing the effect of catalysts, sol-
vents and ratio of PA:Pd on polymerization, are listed in
Tables 2 and 3. All the catalysts showed moderate to high
activity and a general trend where substituents on the pyr-
azolyl unit or pyrazole determined the molecular weight of
polyphenylacetylene formed, was observed. Other factors
that affected polymer molecular weights are reaction time
and nature of solvent. Polymerization with catalyst 1
resulted in very high conversion, with very little variation
in polymers molecular weight when monomer to catalyst
ratio was kept at 50:1 and CH2Cl2:MeCN ratio of 7:1
(Table 2 (entries 1–4)). Increase in MeCN content of the
mixture of solvents, as well as monomer to catalyst ratio
(Table 2, entries 5–8) resulted in lower conversion and
lower polymer molecular weights. The results suggest that
with increased concentration of MeCN, the monomer com-
petes with MeCN for the catalyst. The steric bulk of the
catalyst also has profound effect on polymer molecular
weight. With bulky substituents on the pyrazolyl ligands
(i.e. tertiary butyl and phenyl) and at similar polymeriza-
tion conditions, high-molecular weight polyphenylactylene
were formed (Table 2, entries 4 and 9), but the less bulky
methyl substituent only produced oligomers (Table 2, entry
10). Steric bulk of catalyst is a well established property
that promotes chain propagation in olefin polymerization
reactions; a phenomenon that underlies the success of a-
diimine late transition metal complexes as catalysts for
the polymerization of olefins [7a].
in the O@C–N–N linkage is shorter than the generic Csp2
–
Nsp2 single bond due to electronic resonance. The C–N
bond is the shortest when the torsion angle O–C–N–N is
180° and longest when the angle is 90°. The average C–N
bond distances in 1,3-(3,5-Ph2pzCO)2C6H4 averaged
˚
1.4271 A. This value falls within the range for similar com-
pounds reported by Darkwa et al. [10].
3.3. Polymerization of phenylacetylene
The polymerization of phenylacetylene (PA) was exam-
ined with three types of pyrazolyl palladium complexes as
catalysts (Scheme 2). These catalysts differ by the nature
of the ligand systems, all designed to vary the electron den-
sity on the metal centre and the active catalysts were gener-
R
O
R'
R
Cl
N
N
R'
Pd
Pd
O
N
Cl
Cl
Cl
N
N
N
O
Cl
Pd
N
R
R'
N
N
Cl
O
R'
R
We also observed that when polymerization reactions
were run in neat solvents (MeCN and THF) with the
active catalyst generated as described, the polymerization
was fast but polymer molecular weights were lower than
in the solvent mixtures. Both MeCN and THF are donor
solvent and would compete with phenylacetylene for the
catalytic centre. It therefore comes as no surprise that
reactions in these solvents produced lower molecular
weight polymers.
Time dependent polymerization experiments with cata-
lyst 1 showed little variation in monomer conversion,
(Table 2, entries 1–4). However, there was no increase in
activity with time (Fig. 2) as expected if the catalyst is still
(R' = tBu (4), Me(5))
t
(R = Bu (1), Ph (2), Me (3))
R
tBu
N
H
N
H
N
N
Cl
R
tBu
Me
Pd
H
Pd
H
Cl
Cl
N
N
N
N
R
tBu
R
tBu
(R = H (6), Me (7), tBu (8))
(9)
Scheme 2.

K. Li et al. / Polyhedron 27 (2008) 1017–1023
1021
Table 2
Polymerization of phenylacetylene catalyzed by bis(pyrazolyl)Pd(II)a
Entry
Catalyst
Solventb
Time (min)
% Con
Mw
Mw/Mn
1
2
3
4
5
6
7c
8d
9
10
11
12
13
14
15
16
17
18
a
1
1
1
1
1
1
1
1
2
3
2
2
3
3
4
4
4
5
7:1
7:1
7:1
7:1
5:3
1:3
7:1
7:1
2
30
60
90
85
87
92
86
76
58
60
100
45
93
100
43
50
96
82
87
73
11890
11609
12919
12114
11521
8546
8955
6444
10237
1203
5257
1610
1581
3949
10119
10831
3798
2.36
2.25
2.20
2.12
2.08
1.80
2.99
2.20
2.29
1.68
1.65
1.71
2.82
3.27
1.68
1.44
3.23
2.32
120
1440
1440
1440
1440
120
120
180
180
120
120
240
240
240
240
7:1
7:1
MeCN
THF
MeCN
THF
7:1
5:1
3:1
7:1
639
All reactions were run in 40 mL solvent.
Solvent is a mixture of CH2Cl2 and MeCN; [Pd] = 3.5 ꢁ 10ꢀ3 M; PA:Pd = 50:1.
b
c
PA:Pd = 100:1.
d
PA:Pd = 200:1; temperature = 25 °C.
Table 3
Polymerization of phenylacetylene catalyzed by bis(pyrazole)Pd(II) complexesa
Entry
Catalyst
Solventb
Time (min)
% Con
Mw
Mw/Mn
1
2
3
4
5
6
7
8
6
7
8
9
9
9
9
9
9
9
9
9
9
9
5:3
5:3
5:3
5:3
5:3
5:3
5:3
5:3
5:3
5:3
5:3
5:3
5:3
5:3
15
15
15
15
2
5
30
60
120
1440
15
30
60
35
31
38
47
38
41
51
54
58
99
74
80
90
97
13475
16822
15407
23237
26541
27084
22343
24212
26000
6458
2.67
2.56
3.06
2.16
2.52
2.34
2.48
2.09
2.19
1.81
2.84
2.84
2.82
2.66
9
10
11c
12c
13c
14c
a
21794
20481
19452
15372
120
All reactions were run in 40 mL solvent.
Solvent is a mixture of CH2Cl2 and MeCN; [Pd] = 3.5 ꢁ 10ꢀ3 M; PA:Pd = 50:1.
b
c
Reactions run with SO2; temperature = 25 °C.
active. This is typical of catalyst deactivation, which sug-
gests decomposition of catalyst. Indeed a change in ligand
design by using a pyridine linker produced catalysts 4 and
5; both of which are substantially more stable, albeit less
active. Reactions with 4 and 5 took much longer at similar
conditions to reach % conversions found for catalysts 1 and
2; but the steric effect on polymer molecular weight (Table
2, entries 15–18) remained similar to molecular weights
observed for catalysts 1–3. It, therefore, appears that upon
removal of chloride ligands to give the active catalysts, the
uncoordinated pyrazolyl units in 4 and 5 enters the coordi-
nation sphere of the palladium and block the coordination
site the monomer needs to occupy in order to be
polymerized.
In order to establish the full effects of both the carbonyl
and pyridine linkers in catalysts 1–5, simple pyrazole palla-
dium complexes (6–9) were investigated as catalysts for
phenylacetylene polymerization.
A crucial difference
between catalysts 6–9 and 1–5 is the higher amount of
MeCN required to stabilize the active catalysts (i.e.
CH2Cl2:MeCN = 5:3). Furthermore, % conversion of
monomer was generally low with catalysts 6–9, clearly
showing that the catalysts centres in 6–9 are less electro-
philic. Time dependent experiments with 9 showed a grad-
ual increase in % conversion (Table 3, entries 5–9); but
never achieved the high conversions observed with 1–5.
We took advantage of the slow kinetics exhibited by cat-
alyst 9 the co-polymerize phenylacetylene and another

1022
K. Li et al. / Polyhedron 27 (2008) 1017–1023
monomer. We recently reported the insertion of SO2 into
the Pd–Me bond in 9 [14]. By saturating CH2Cl2:MeCN
solutions of the active catalyst generated from 9 with
SO2, we could perform time dependent experiments involv-
ing the co-polymerization of SO2 and phenylacetylene.
Monomer conversions improved dramatically (Table 3,
entries 11–14), but polymer molecular weights decreased
with time. Liaw and Lay have co-polymerized CO and
phenylacetylene with an in situ generated catalyst from
2,20-bipyridine and [Pd(MeCN)4][BF4]2 [15]. The polysulf-
one formed with catalyst 9 has characteristic v(SO) peaks
at 1259 cmꢀ1 in addition to v(C@C) of the acetylenic func-
tion catalysts as the slow conversion gives the co-monomer
sufficient time to insert to the Pd–C bond, in this case sulfur
dioxide, thus enabling chain growth that involves the inser-
tion of sulfur dioxide to produce polyphenylacetylene
sulfone.
5. Supplementary material
CCDC 278053 and 275054 contain the supplementary
crystallographic data for L1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033, or e-mail: deposit@
ccdc.cam.ac.uk.
tionality at 1550 cmꢀ1
.
All the catalysts showed moderate to high activity.
The ability of cationic pyrazole palladium catalysts used
here to catalyze phenylacetylene polymerization is not
surprising. We have observed the formation of linear
high density polyethylene catalyzed by bis(pyra-
zole)nickel(II) [16] and palladium(II) [11] as well as by
pyrazolyl palladium(II) [10,12]. This observation confirms
the high electrophilicity of these nickel and palladium
catalysts, that allows rapid insertion of monomer, gives
rise to a much higher rate of chain propagation over
chain termination; thus avoiding branching that is com-
mon to late transition metal olefin polymerization cata-
lysts. We believe it is this high electrophilicity of the
cationic species formed by 1–9 that make them active
catalysts for phenylacetylene polymerization, when other
cationic palladium nitrogen donor compounds [8,7] can-
not polymerize acetylenes.
In general complexes 1, 2 and 4 were the most active,
and catalysts with bulky substituents gave high-molecular
weight polyphenylacetylene than catalysts that have less
bulky substituents. Catalysts with less bulky substituents
formed phenylacetylene oligomers. Compared with known
Pd(II) catalysts, the (N^N)Pd(II) catalysts described here
are more active, although they produce lower-molecular
weight polymers than some of the palladium catalysts
reported in the literature. For instance while Pd(II) halide
and acetate salts are known to polymerize phenylacetylene
to low-molecular weight polymers, the use of Pd(II) acetate
is highly exothermic and at times even explosive [3k]. It is
also worth noting that the slow conversions by catalyst 9
allowed it to be used as co-polymerization catalyst for
SO2 and phenylacetylene.
Acknowledgement
This work was funded by a Grant from the National Re-
search Foundation, South Africa.
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