New catalysts for the alternating copolymerization of 4-tert-butylstyrene/CO

Article abstract of DOI:10.1016/S0022-328X(00)00670-7

New cationic palladium(II) complexes, containing C_S-nitrogen chelate ligands with pyrazol moieties, catalyze the copolymerization of styrene with CO under mild conditions, to produce a syndiotactic copolymer. Molecular weight and polydispersity for the obtained polymers are among the best reported for bisnitrogen planar ligands.

Full text of DOI:10.1016/S0022-328X(00)00670-7

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 619 (2001) 287–292
New catalysts for the alternating copolymerization of
4-tert-butylstyrene/CO
Amaia Bastero a, Aurora Ruiz a, J. Antonio Reina b, Carmen Claver a,*,
Ana M. Guerrero c, Felix A. Jalo´n c, Blanca R. Manzano c
a Departament de Qu´ımica F´ısica i Inorga`nica, Uni6ersitat Ro6ira i Virgili, Pl. Imperial Ta`rraco I, 43005 Tarragona, Spain
b Departament de Qu´ımica Anal´ıtica i Orga`nica, Uni6ersitat Ro6ira i Virgili, Pl. Imperial Ta`rraco I, 43005 Tarragona, Spain
c Departamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica, Facultad de Qu´ımicas, UCLM, A6da. Camilo Jose´ Cela, 10,
13071 Ciudad Real, Spain
Received 4 July 2000; accepted 15 September 2000
Abstract
New cationic palladium(II) complexes, containing CS-nitrogen chelate ligands with pyrazol moieties, catalyze the copolymeriza-
tion of styrene with CO under mild conditions, to produce a syndiotactic copolymer. Molecular weight and polydispersity for the
obtained polymers are among the best reported for bisnitrogen planar ligands. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Catalysis; Copolymerization; Styrene; Carbon monoxide; Palladium complexes; Nitrogen ligands
1. Introduction
Chiral ligands have been successfully used for the
copolymerization of carbon monoxide with propene
[16,17] and styrene [18–21]. With styrene, however,
only oligomers with low molecular weight are obtained
with phosphine ligands. This has been attributed to a
favoured b-hydride-elimination [1].
The first systems studied with styrene were Pd(II)
catalyst based on planar 2,2%-bypiridine or 1,10-
phenantroline to give syndiotactic poly(styrene-alt-CO)
[22–24] (Scheme 1). This high stereochemical control
(\90%) has been attributed to a chain-end control due
to the interaction of the growing chain with the incom-
ing styrene unit, which inserts exclusively in the 2,1-
fashion [3,25].
The copolymerization of carbon monoxide and alke-
nes using palladium catalyst is providing much interest
[1–3]. Ethylene/carbon monoxide copolymerization has
been widely studied and results have been best with
diphosphine ligands. The high activity of these systems
has raised industrial interest in the process [4–9]. Sev-
eral groups are reporting on the exploration of new
systems [10–15].
C2-symmetric chiral N–N ligands have also been
successfully explored to yield isotactic copolymers
[18,19,25,26], although in some cases the chain-end
control is more efficient than the enantiosite control
caused by the ligand and this results in the formation of
syndiotactic polymers [25].
C1-symmetric chiral P–N ligands have been used to
yield isotactic polymers due to the site-selective coordi-
nation of the olefin in the chiral environment generated
by the ligand [21,25]. For these systems, using P–N
ligands, high CO pressures and moderate temperatures
are required.
Scheme 1.
* Corresponding author. Tel.: +34-977-559574; fax: +34-977-
559563.
E-mail address: claver@quimica.urv.es (C. Claver).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00670-7

288
A. Bastero et al. / Journal of Organometallic Chemistry 619 (2001) 287–292
lieve that this stereochemistry cannot be explained in
terms of electronic effects. The sterical hindrance
caused by the hydrogen H6 in compounds 4 and 5 may
be larger than the one caused by the H3% of the pyrazole
fragment, which could force the methyl to be cis to the
pyrazole in both molecules. This agrees with the stereo-
chemistry observed in 6, where the steric demand of the
methyl group (Me)3 of the pyrazole ring is greater than
the bulkiness of H6, thus placing the methyl group trans
to the pyrazole moiety.
Spectroscopic data suggest that these configurations
are in agreement with the stereochemistry of the related
[PdClMe(N–N)], although in the case of [Pd-
ClMe(pz*pm)] there is also a minor isomer with the
methyl ligand trans to the pyrimidine ring [27].
Scheme 2. (N-N%)-Ligand systems.
Since planar N-N ligands are efficient and since there
is an interest in exploring new types of ligands, we
report on the synthesis of new well-defined cationic Pd
(II) catalyst using the unsymmetrical chelate ligands
2-(1-pyrazolyl) pyridine (pzpy, 1), 2-(1-pyrazolyl)
pyrimidine (pzpm, 2) and 2-(1-(3,5-dimethyl)pyrazolyl)
pyrimidine (pz*pm, 3), which yield syndiotactic alter-
nating copolymers of 4-tert-butyl styrene and CO at
mild pressure and temperature (Scheme 2).
In the case of 6, it is also noteworthy the fluxional
behaviour found for the pz*pm ligand (3). An NOE
experiment carried out at low temperature (243 K)
reflects the expected NOE interaction between the
methyl group and H6. However, at room temperature
an NOE effect has been found not only with this
proton but also with H4 being the observed percent of
2.4% for H6 and 0.6% for H4. This implies a process of
Pd–N bond rupture, internal rotation of the ligand
around the pz*–pm bond and reformation of the Pd–
N bond. The two separated resonances for H4 and H6
at room temperature indicate that the process is in a
slow exchange regime at this temperature. This Pd–N
bond rupture is also observed in related Pd(II) com-
plexes with this type of ligands [28].
2. Results and discussion
2.1. Synthesis and characterization of the palladium
catalyst precursors [Pd(Me)(NCMe)(N–N%)]BAr% (4–6)
4
The cationic complexes [Pd(Me)(NCMe)(N–N%)]-
BAr4% were obtained by adding a previously prepared
solution of the neutral complexes [PdCl(Me)(N–N%)]
[27] to an equimolar solution of NaBAr% in MeCN.
4
NOE difference NMR experiments showed that
cationic precursors 4 and 5 had the methyl group trans
to pyridine and pyrimidine ring, respectively, while 6
had the methyl group trans to the pyrazole moiety
(Scheme 3). Because of the large trans influence of the
methyl group, we should expect it to be coordinated
trans to the less basic ring (pyrazole in compound 4 and
pyrimidine in compounds 5 and 6). We therefore be-
2.2. Copolymerization experiments
Whereas C2-symmetric bipyridyl-or phenantroline-
type ligands are a well-known class of C2-symmetric
Scheme 3. Stereochemistry around palladium of compounds 4, 5 and 6.

A. Bastero et al. / Journal of Organometallic Chemistry 619 (2001) 287–292
289
Table 1
Copolymerization of 4-tert-butylstyrene and CO with 4, 5 and 6 at 1 bar CO and r.t. a
Entry
Catalyst precursor
pCO (bar)
t (h)
% Conversion b
MN (MW/MN)
Productivity (g (g Pd h)−1
)
1
2
3
4
4
5
6
4
5
4
4
5
1
1
1
1
1
1
5
5
24
24
24
48
48
24
24
24
38.45
55
21 440 (1.1)
36 353 (1.3)
15 720 (1.5)
72 360 (1.1)
70 680 (1.2)
30 380 (1.4)
15 570 (1.1)
7155 (1.2)
8.83
11.75
5.88
7.75
7.05
19.86
3.42
2.5
25.78
67.49
68.88
43.15
14.59
10.73
5
6 c
7
8
a Reaction conditions: solvent: 5 ml chlorobenzene, alkene/catalyst=310.
b Styrene conversion calculated from the isolated polymer weight.
c Alkene/catalyst=620.
planar ligands for the styrene/CO copolymerization
reaction, few systems have been reported in which a
nonsymmetrical N-N’ planar ligand leads to similar
activities and molecular weights [29]. The new com-
plexes 4, 5 and 6 were explored as catalysts in this
reaction at mild conditions (room temperature and 1
bar CO) (Table 1).
All the systems we studied yield prevailing syndiotac-
tic copolymers, which is indicative of a stereocontrol
from the growing polymer chain (chain-control). Anal-
ysis of the decoupled 13C spectra indicated a substantial
degree of stereoregularity (ca. 85% of syndiotactic di-
ads) by integration of the signals of the methylene
carbon atoms. The greater resonance at 43.2 ppm was
assigned to the syndiotactic uu-triad by comparison
with the spectrum of the epimerized polymer and with
literature values [18,21] (Fig. 2).
The catalyst precursor 5 seemed the most active after
24 h, with a productivity of 11.75 g (g Pd h)−1
,
although the cationic system 4 behaved in a similar way
(entries 1 and 2). Increasing the reaction time from 24
to 48 h led, as expected, to higher conversions and
molecular weights although the difference in activity
between the two catalysts (4 and 5) narrowed, probably
due to the combined effects of the increasing difficulty
of the monomers to access the catalytic site and decom-
position of catalyst 5 to palladium metal (entries 4 and
5). The molecular weights (20000–70000) and the poly-
dispersities (1.1–1.3) are in the order of the best ones
reported for bisnitrogen systems using styrene deriva-
tives [24,26]. The activity and molecular weight ob-
tained with catalyst precursor 6 were lower than those
obtained with precursors 4 and 5 (entries 1–3). This
lower productivity may be attributed to the sterically
more demanding ligand pz*pm (3), as previously re-
ported for other systems [25]. When the substrate/cata-
lyst ratio is 620 for precursor 4, productivity and
molecular weight increased (entries 1 and 6).
3. Conclusions
This study shows that new cationic palladium(II)
compounds containing the bisnitrogen ligands pzpy (1),
pzpm (2) and pz*pm (3) are active as catalyst precur-
sors in the alternating CO/4-tert-butylstyrene copoly-
merization. The CS-symmetry of the ligands did not
seem to affect stereocontrol, which was determined by
the chain end. These systems behave similar to the
previously studied C2-symmetric bipyridyl and
phenantroline ligands that yield readily syndiotactic
copolymers.
We also studied the effect of CO pressure. When the
CO pressure was increased to 5 bar, conversion and
molecular weight decreased, although polydispersities
remained stable (entries 7 and 8).
To compare the activity of the most active catalyst
precursors 4 and 5, we varied reaction times from 7 to
72 h and plotted styrene conversion against reaction
time (Fig. 1). With catalyst 5 the initial reaction rate
was higher but prolonged reaction time with 4, how-
ever, resulted in similar conversions, as mention above.
Fig. 1. Compared activity of catalysts 4 and 5, containing ligands
pzpy and pzpm, respectively.

290
A. Bastero et al. / Journal of Organometallic Chemistry 619 (2001) 287–292
4.2. Synthesis of catalyst precursors
4.2.1. [Pd(Me)(NCMe)(pzpy)](BAr%) (4)
4
A previously prepared solution of [PdCl(Me)(pzpy)]
[27] (0.05 g, 0.17 mmol) in 3 ml of CH2Cl2 was added
to a solution of NaBAr% (Ar%=3,5-(CF3)2C6H3) (0.147
4
g, 0.17 mmol) in the minimum amount of MeCN. The
light orange solution formed was stirred for about an
hour, filtrated through Kieselghur and evaporated to
dryness. The white-brownish compound was crys-
tallised from CH2Cl2 / hexane. Yield: 75%. Anal.
Found: C, 44.7; H, 2.1; N, 4.7. Calc. for
1
C43H25BF24N4Pd: C, 44.1; H, 2.1; N, 4.8%. H-NMR
(400 MHz, CDCl3, room temperature (r.t.): l 8.15
3
4
5
(ddd, J=5.2 Hz, J=1.6 Hz, J=0.8 Hz, 1H, H6),
3
3
8.08 (d, J=3.2 Hz, 1H, H5’), 7.9 (ddd, J=8.4 Hz,
4
3
3J=7.6 Hz, J=1.6 Hz, 1H, H4), 7.8 (d, J=2.0 Hz,
1H, H3’), 7.71 (s, 8H, Hb), 7.52 (s, 4H, Hd), 7.45 (td,
3J=8.4 Hz, 4J=0.8 Hz, 5J=0.8 Hz, 1H, H3), 7.23
(ddd, 3J=7.6 Hz, 3J=5.2 Hz, 4J=0.8 Hz, 1H, H5), 6.7
(dd, 3J=3.2 Hz, 3J=2 Hz, 1H, H4’), 2.38 (s, 3H,
CH3CN), 1.21 (s, 3H, Pd–CH3). 13C-NMR (100.5
MHz, CDCl3, r.t.): l 161.9 (q, JC–B=198.3 Hz, Ca),
147.5 (s, C6), 142.4 (s, C3% or C4), 142.3 (s, C4 or C3%),
135 (s, Cb), 129.5 (s, C5%), 129.1 (m, Ce), 126 (s, Cc),
124.4 (s, C5), 123.3 (s, CH3CN), 117.7 (s, Cd), 111.2 (s,
C4% or C3), 111.1 (s, C3 or C4%), 3.3 (s, CH3CN), −0.8
(s, Pd–CH3).
Fig. 2. Methylene carbon atom-region of the copolymers obtained
with precursors 4, 5 and 6. The reference spectrum is epimerized poly
(4-tert-butylstyrene-alt-CO).
4. Experimental
4.1. General procedure
4.2.2. [Pd(Me)(NCMe)(pzpm)](BAr%) (5)
4
All reactions were carried out in a nitrogen atmo-
sphere at room temperature using standard Schlenk
techniques. Solvents were distilled and deoxygenated
Compound 5 was obtained from [PdCl(Me)(pzpm)]
[27] as a white-brownish solid in a similar way to
compound 4, although it does not analyse as a pure
prior to use unless otherwise stated. The salt NaBAr%
1
4
solid. Yield: 72%. H-NMR (300 MHz, CDCl3, r.t.): l
(Ar%=3,5-(CF3)2-C6H3) was prepared according to re-
ported methods [30]. Ligands 1 and 2 were prepared
according to published methods [28]. Ligand 3 was
prepared in a similar way.
3
4
8.74 (dd, J=4.8 Hz, J=2.4 Hz, 1H, H4), 8.57 (dd,
3J=3.2 Hz, 4J=0.8 Hz, 1H, H5%), 8.33 (dd, 3J=5.2 Hz,
4J=2.4 Hz, 1H, H6), 7.84 (dd, 3J=2.4 Hz, 4J=0.8 Hz,
1H, H3%), 7.71 (s, 1H, Hb), 7.52 (s, 1H, Hd), 7.14 (dd,
3J=5.2 Hz, 3J=4.8 Hz, 1H, H5), 6.76 (dd, 3J=3.2 Hz,
3J=2.4 Hz, 1H, H4%), 2.32 (s, 3H, CH3CN), 1.3 (s, 3H,
Pd–CH3). 13C-NMR (100.5 MHz, CDCl3, r.t.): l 161.9
(q, JC–B=198.3 Hz, Ca), 161.6 (s, C6), 156.3 (s, C4),
143.8 (s, C3%), 135 (s, Cb), 132.3 (s, C5%), 129.2 (m, Ce),
126 (s, Cc), 123.3 (s, CH3CN), 120.8 (s, C5), 117.7 (s,
Cd), 111.2 (s, C4%), 3.2 (s, CH3CN), −0.1 (s, Pd–CH3).
1H- and 13C-NMR spectra were recorded on a Varian
1
Gemini spectrometer with a H resonance frequency of
300 MHz and on a Varian Mercury VX spectrometer
1
with a H resonance frequency of 400 MHz. Chemical
1
shifts were reported relative to tetramethylsilane for H
and 13C. Some assignments in NMR spectra were deter-
1
mined by H-13C-COSY, DEPT and NOE experiments.
IR spectra (range 4000–400 cm−1) were recorded on a
Midac Grams/386 spectrophotometer in KBr pellets.
Elemental analyses were carried out on a Carlo Erba
Microanalyser EA 1108. The molecular weight of the
copolymers and molecular weight distributions were
determined by gel permeation chromatography (GPC-
MALLS) measurements made in THF on a Waters 510
gel-permeation chromatography device using a three-se-
rial column system (SHODEX K80M and PLGEL
MIXED-D and MIXED-E linear columns) with a Wy-
att mini-DAWN Light Scattering and a SHIMADZU
RID-6A refractive index detector.
4.2.3. [Pd(Me)(NCMe)(pz*pm)](BAr%) (6)
4
Compound
6
was synthesised from [Pd-
Cl(Me)(pz*pm)] [27] following a similar procedure to
compound 4 and isolated as a light grey solid. Yield:
82.1%. Anal. Found: C, 44.6; H, 2.3; N, 5.6. Calc. for
1
C44H28BF24N5Pd: C, 44.1; H, 2.3; N, 5.8%. H-NMR
3
4
(400MHz, CDCl3, r.t.): l 8.8 (dd, J=4.8 Hz, J=2.2
3
4
Hz, 1H, H4), 8.48 (dd, J=6 Hz, J=2.2 Hz, 1H, H6),
3
7.71 (s, 8H, Hb), 7.53 (s, 4H, Hd), 7.29 (dd, J=6 Hz,
3J=4.8 Hz, 1H, H5), 6.19 (s, 1H, H4%), 2.7 (s, 3H, (Me)3
or (Me)5), 2.35 (s, 3H, CH3CN), 2.23 (s, 3H, (Me)5 or

A. Bastero et al. / Journal of Organometallic Chemistry 619 (2001) 287–292
291
(Me)3), 1 (s, 3H, Pd–CH3). 13C-NMR (75.4 MHz,
Acknowledgements
CDCl3, r.t.): l 161.9 (q, JC–B=198.2 Hz, Ca), 160.6 (s,
C6), 156 (s, C4), 135 (s, Cb), 129.1 (m, Ce) 126.5 (s, Cc),
122.9 (s, CH3CN), 119.3 (s, C5), 117.7 (s, Cd), 113.5 (s,
C4%), 15.4 (s, (Me)3 or (Me)5), 13.3 (s, (Me)5 or (Me)3),
5.5 (s, CH3CN), 3.4 (s, Pd–CH3).
We thank the Spanish Ministerio de Educacio´n y
Cultura for financial support (DGES PB-97-0407-C05-
01, DGES PB-98-0315 and 2FD97-1565) and for award-
ing a research grant (to A.B.). We also thank Ramo´n
Guerrero (Servei de Recursos Cientifics, Universitat
4.3. Copolymerization reactions — general procedure
1
Rovira i Virgili) for carrying out the NOE and H-13C-
COSY experiments.
The 4-tert-butylstyrene was passed through a small
column of Al2O3 prior to use. Chlorobenzene was used
as purchased from Aldrich.
In a typical procedure the cationic precursor 4, 5 or
6 (0.0125 mmol) was dissolved in 5 ml of chlorobenzene
in a previously flushed Schlenk and the N2 atmosphere
replaced with CO. 4-tert-Butylstyrene (0.7 ml, 3.875
mmol) was then introduced and the reaction was allowed
to take place at r.t. and 1 bar of CO. The experiments
under 5 bar of CO pressure were carried out in a 100 ml
stainless steel Berghoff autoclave. The reaction mixture
was introduced into the autoclave by suction and the
pressure level was kept constant by continuous feeding
from a gas reservoir. Reaction times varied from 7 to 96
h. Work-up included filtration of the reaction mixture
through Kieselghur and precipitation of the polymeric
material by adding the reaction solution dropwise into
100 ml of rapidly stirring methanol. The off-white
powder was collected by filtration, washed with methanol
and dried in a vacuum oven at 70°C overnight. Percent-
age conversions were calculated from the weight of the
isolated polymeric material.
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