Preparation of Pd Colloids in Block Copolymer Micelles
J. Am. Chem. Soc., Vol. 119, No. 42, 1997 10117
Table 1. Colloid-Analytical Data of the
(OAc)2 was obtained from Degussa and used as received. Tri-o-
tolylphosphine (TOP) was prepared following a procedure described
elsewhere.25
Polystyrene-b-poly-4-vinylpyridine Block Copolymers Useda
Mn/g
Mw/g
mol-1
mol-1
D ) Mw/Mn
f4VP
dh/nm; σ
Preparation of Colloidal Palladium Catalysts. A 250-mL three-
necked flask equipped with a gas inlet, a bubbler, and a septum was
evacuated several times and then filled with argon. In an argon stream,
0.5 g of the desired block copolymer and 100 mL of toluene (previously
stored over sodium wire) were charged into the flask. After complete
dissolution of the polymer, palladium acetate (0.25 mol of Pd(OAc)2
per mol of 4-VP) was added to the solution. After being stirred over
night (solubilization), the solution was degassed and superhydride (2
mol of superhydride per mol of Pd(OAc)2 as 1 M LiB(C2H5)3H in THF)
was added through a septum to the yellow solution. The resulting dark
brown solution was stored under argon.
polymer
PS-3.3
PS-5.2
PS-7.1
PS-11.1
17500
28014
21200
18980
20150
36736
21760
19850
1.15
0.27
0.54
0.49
0.28
27.2; 0.121
64.0; 0.112
57.3; 0.096
34.6; 0.278
1.311
1.027
1.05
a Mn and Mw are number and weight averaged molecular weights,
respectively; D denotes the polydispersity index. These quantities were
determined by DMF-GPC. f4VP is the relative amount of 4-VP (“core”)
in each micelle. The hydrodynamic diameter of the micelles in toluene,
dh, and the Gaussian width of its distribution, σ, as determined by
dynamic light scattering are given in the last column.
Heck-Coupling Reactions. Heck-coupling reactions of haloarenes
(generally 20 mmol) and olefins in toluene (typically 45 mL) with tri-
n-butylamine (NBu3) as base and different Pd catalyst systems and
tetradecane as internal standard were carried out in lab-autoclaves under
inert gas conditions. Other reaction parameters were varied as listed
in Tables 2 and 3. Workup was accomplished by pouring samples of
the reaction mixture into 2 N HCl, followed by extraction with toluene,
drying over Na2SO4, and GC analysis.
metal colloids formation. The resulting hybrids between block
copolymer micelles and metal colloids can be regarded as
molecular entities which inherited the profitable solution
behavior of polymers and the catalytic activity of metal colloids.
In some publications,16,19,20 it was shown for various noble
metals that the colloid size depends on the micelle size and the
molar ratio of metal to 4-vinylpyridine, but is mainly controlled
by the type of reducing agent applied. In all cases, the stability
of the formed colloids turned out to be very high, which is
directly related to the stabilization of the final metal particles
by a shell of block copolymer. Application of Pd, Pt, and Au
monometallic and bimetallic nanoparticles as catalysts in the
hydrogenation reaction of cyclohexene, 1,3-cyclooctadiene, and
1,3-cyclohexadiene was also examined.19 Selected colloidal
catalysts showed about the same reactivity as commercial,
optimized systems, but possessed a much higher stability and
selectivity.
In the present paper, we want to focus on a more demanding
synthesis, namely the Heck reaction of aryl halides with alkenes,
which represents an important synthetic method in organic as
well as in polymer chemistry.21,22 In most cases the Heck
reaction is restricted to bromo- or iodoarenes. The usage of
chloroarenes is still a challenging demand due to economic
considerations, but is realized just for some systems with an
otherwise high reactivity.23,24 In polymer chemistry the Heck
reaction is also used for the synthesis of conjugated polymers
for electronic and optical applications, where traces of residual
palladium may ruin the electrooptical properties. For this
application, small catalyst amounts relative to the educts and a
good removability after reaction are advantageous. For both
purposes, employment of block copolymer stabilized colloidal
catalysts seems to be promising.
Results and Discussion
Preparation of Metal Colloids. Solutions of palladium
colloidssprepared as shown abovesare stable for months and
can be precipitated and redissolved without changing the
colloidal properties. This is already a significant advantage
compared to low molecular weight catalysts which require rapid
consumption within days after their synthesis.
It was shown in preceding publications that the number and
size of the metal particles in the micelle core is strongly affected
by the reducing agent used. Reduction of the solubilized Pd-
(OAc)2 by superhydride (LiB(C2H5)3H) leads to a fast nucleation
and growth of metal colloids in the micelle cores resulting in a
so-called raspberry morphology. Here, a large number of small
noble metal colloids or clusters (ca. 10-1000) are formed in
each micelle core.16,19 This morphology offers a very high
colloid surface area, is X-ray amorphous due to the small
crystallites, and turned out to be the most appropriate morphol-
ogy for hydrogenation and disproportionation reactions. For a
better illustration and understanding of the following discussion,
Figure 1 depicts electron micrographs of three arrays of dried
block copolymer micelles with Pd colloids reduced by different
reducing agents.
All three different degrees of dispersity were used as catalysts
in the Heck reaction, and it turned out that the reduction with
“fresh” superhydride leads to the most active catalyst systems,
related to the finest distribution of Pd inside the micelle core.
In this publication we will mainly discuss the Pd colloids
reduced by “fresh” superhydride.
Pd-Catalyzed Coupling Reactions of Haloarenes and
Olefins. To evaluate the catalytic activity of block copolymer
stabilized Pd colloids, Heck reactions were performed under
different reaction conditions. In some cases, we also tested the
reproducability of the procedure of catalyst production, which
turned out to be satisfactory (e.g. see Table 2, entry 6 and Table
3 entries 7, 10, and 11). Reexamination of the produced colloids
by dynamic light scattering and TEM showed the same particle
size and dispersion, i.e. the procedure is general and allows for
direct repetition. This is important in the whole area of colloidal
catalysts. In a first set of experiments with the system (PS-
3.3, Pd) as a reference block copolymer catalyst, significantly
different reactivities were found for different educts, as listed
in Table 2. High reaction temperatures were necessary with
Experimental Section
The anionic synthesis of PS-b-P4VP block copolymers was carried
out by the procedure described elsewhere.17,18 For the preparation of
metal colloids, four different PS-b-P4VP block copolymers were chosen.
Their molecular characteristics are summarized in Table 1.
Na2PdCl4, superhydride (LiB(C2H5)3H), haloarenes, olefins, tetrade-
cane, and the different solvents were obtained from Aldrich and used
as received (except tetradecane, which was distilled prior to use). Pd-
(18) Fo¨rster, S.; Zisenis, M.; Wenz, E.; Antonietti, M. J. Chem. Phys.
1996, 104, 9956.
(19) Serigina, M.; Bronstein, L.; Platonova, O. A.; Chernyshov, D. M.;
Valetsky, P. M.; Wenz, E.; Hartmann, J.; Antonietti, M. Chem. Mater. 1997,
9 (4), 923-931.
(20) Spatz, J. P.; Mo¨ssmer, S.; Mo¨ller, M. Chem. Eur. J. 1996, 2, 1552.
(21) Heck, R. F. Org. React. 1982, 27, 345.
(22) Greiner, A.; Heitz, W. The Polymeric Materials Encyclopedia;
Salomone, J. C., Ed.; CRC Press: Bocca Raton, 1996; Vol. 7, p 4865.
(23) Ben-David, Y.; Portnoy, M.; Gozin, M.; Milstein, D. Organome-
tallics 1992, 11, 1995.
(25) Sander, R.; Klingelho¨fer, S.; Hopmeier, M.; Go¨bel, E. O.; Heitz,
W.; Greiner, A. In preparation.
(24) Portnoy, M.; Milstein, D. Organometallics 1993, 12, 1655, 1665.