Preparation of Polymer Supported Phosphine Ligands by Metal Catalyzed Living Radical Copolymerization and Their Application to Hydroformylation Catalysis

Article abstract of DOI:10.1002/cctc.201200446

A series of well-defined polystyrene-supported tertiary phosphine ligands were prepared by copper-catalyzed atom transfer radical polymerization (ATRP), involving direct copolymerization of styrene and 4-diphenylphosphinostyrene (or 4-styryldiphenylphosphine, SDPP). Copolymerization of the two monomers at different molar ratios showed a decreasing level of control as the SDPP molar fraction (f_SDPP) increased. A satisfactory level of control was achieved for f_SDPP≤0.25 such that there was a constant concentration of growing living chains , and linear M_n growth with conversion and low dispersity. Copper-free polymers with different chain lengths were prepared and tested as polymeric ligands in the rhodium-catalyzed hydroformylation of 1-octene. The polymeric ligands yielded higher linear/branched selectivity and lower activity relative to PPh₃ at the same P/Rh ratio. The selectivity increased slightly as a function of the polymer chain length.

Full text of DOI:10.1002/cctc.201200446

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DOI: 10.1002/cctc.201200446
Preparation of Polymer Supported Phosphine Ligands by
Metal Catalyzed Living Radical Copolymerization and Their
Application to Hydroformylation Catalysis
[
a, b, c]
[a, c]
[b, c]
[b, c]
Andrꢀs F. Cardozo,
Henri Delmas,
Eric Manoury,
Carine Julcour,
Jean-FranÅois Blanco,
[
b, c]
[a, c]
[a, c, d]
Florence Gayet,
and Rinaldo Poli*
A series of well-defined polystyrene-supported tertiary phos-
phine ligands were prepared by copper-catalyzed atom trans-
fer radical polymerization (ATRP), involving direct copolymeri-
zation of styrene and 4-diphenylphosphinostyrene (or 4-styryl-
diphenylphosphine, SDPP). Copolymerization of the two mono-
mers at different molar ratios showed a decreasing level of
control as the SDPP molar fraction (fSDPP) increased. A satisfac-
tory level of control was achieved for fSDPP ꢀ0.25 such that
there was a constant concentration of growing “living chains”,
and linear Mn growth with conversion and low dispersity.
Copper-free polymers with different chain lengths were pre-
pared and tested as polymeric ligands in the rhodium-cata-
lyzed hydroformylation of 1-octene. The polymeric ligands
yielded higher linear/branched selectivity and lower activity
relative to PPh3 at the same P/Rh ratio. The selectivity in-
creased slightly as a function of the polymer chain length.
Introduction
[
7–8]
The ability to construct functionalized polymers with precise
architecture, topology, composition, molecular weight, mole
fraction and location of specific functional groups leads to an
essentially limitless number of advanced materials with specific
munity.
For instance, polymer-supported rhodium com-
[9]
plexes were first described by Manassen and then developed
[10–11]
[12]
[13]
ˇ
Capka and Pittman as hydro-
in the 1970s by Grubbs,
genation, hydrosilylation and hydroformylation catalysts. Since
then, many polymer-supported phosphine ligands have been
[
1]
functions. In this area, giant steps have been made in recent
times by the development of controlled radical polymerization
techniques that combine the precision in macromolecular syn-
thesis of living and/or controlled polymerization and the typi-
cal advantages of radical polymerization, namely the large
choice of polymerizable monomers and the tolerance to
[7,14–16]
reported in the literature.
Many strategies have been
used to access polymer-supported phosphine ligands, mostly
leading to materials with anchored triphenylphosphine. These
can be divided into two classes, the first involving chemical
modification of a pre-existing phosphine-free polymer and the
second involving the introduction of phosphine-containing
monomers during the macromolecular synthesis. An example
of the first strategy is treatment of bromo functionalized poly-
styrene resins with butyllithium and subsequent chlorodiphe-
[
2–6]
impurities.
Among the functionalities that can be attached to a polymer
backbone, phosphines have, so far, received little attention
with respect to the controlled radical polymerization approach.
However, phosphine functionalized polymers have been pre-
pared and studied for quite some time by the catalysis com-
[17]
nylphosphine addition. Merrifield resins, containing chloro-
or bromomethyl anchoring functional groups can be treated
with lithium or potassium diphenylphosphide to obtain sup-
[14,18]
ported benzyldiphenylphosphine ligands.
By using the
[a] Dr. A. F. Cardozo, Dr. E. Manoury, Dr. F. Gayet, Prof. R. Poli
second strategy, 4-diphenylphosphinostyrene (or 4-styryldiphe-
nylphosphine, SDPP) can be polymerized or copolymerized to-
gether with styrene by anionic or free radical methods. Free
radical SDPP/styrene copolymerization in the absence or pres-
ence of additional crosslinking comonomers (e.g., p-divinylben-
zene or 1,4-bis(4-vinylphenoxy)-butane) gives access to soluble
linear or insoluble crosslinked materials that have, however,
broad molecular weight distributions owing to the uncontrol-
CNRS, LCC (Laboratoire de Chimie de Coordination)
2
05 route de Narbonne
BP 44099, F-31077 Toulouse Cedex 4 (France)
Fax: (+33)561553003
E-mail: rinaldo.poli@lcc-toulouse.fr
[
b] Dr. A. F. Cardozo, Dr. C. Julcour, Dr. J.-F. Blanco, Prof. H. Delmas
CNRS, LGC (Laboratoire de Gꢀnie Chimique)
4
Allꢀe Emile Monso, BP 84234, 31030 Toulouse Cedex 4 (France)
[
c] Dr. A. F. Cardozo, Dr. E. Manoury, Dr. C. Julcour, Dr. J.-F. Blanco,
Prof. H. Delmas, Dr. F. Gayet, Prof. R. Poli
Universitꢀ de Toulouse, UPS, INPT
[19–20]
led nature of the polymerization.
Anionic living polymeri-
zation of SDPP provides homopolymers with narrow MW distri-
bution, which are extended subsequently into diblock copoly-
F-31077 Toulouse Cedex 4 (France)
[
d] Prof. R. Poli
[21]
mer structures by polymerization of styrene. An alternative
Institut Universitaire de France
method is the radical polymerization of SDPP, initiated by radi-
cal centers generated by g-radiation of a polypropylene sup-
port, resulting in the build-up of short surface-grafted SDPP
1
03, bd Saint-Michel, 75005 Paris (France)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200446.
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[
22]
homopolymer chains. More sophisticated systems that con-
mer and polymer. A potential problem under ATRP conditions
was an attack of the alkyl halide chain end (a benzylic halide
for styrene polymerization) by the tertiary phosphine function
of the monomer or polymer, inducing formation of quaternary
phosphonium salts (see Scheme 1). A second potential prob-
lem is dehydrobromination of the halogenated chain end,
leading to an unsaturated chain end and HBr. Although, to the
[
23]
[24]
tain bulky phosphines
and phosphites
have also been
made, mainly through the polymer chemical functionalization
strategy.
Generally, the above methods yield materials with positions
and relative amount of phosphine groups that cannot be fully
controlled. Only one precedent for the incorporation of a phos-
phine functionalized monomer
into a copolymer by controlled
radical polymerization is known.
This involves, however, the si-
multaneous incorporation of the
phosphine functionalized mono-
mer SDPP, a halogen-terminated
polyacrylate macroinitiator, and
a crosslinker (ethylene glycol di-
methyl
dimethacrylate),
by
a
ruthenium-catalyzed atom
transfer approach to yield core
crosslinked and phosphine-func-
tionalized core-shell copoly-
[
25–26]
mers.
This report describes
the synthesis of simpler linear
triphenylphosphine containing
polystyrene chains by controlled
SDPP/styrene radical copolymeri-
zation. In particular, we aim to
Scheme 1. Atom transfer process and potential nucleophilic attack by phosphine for the dormant chain end of
poly(styrene-co-SDPP) made by ATRP. k =propagation rate constant.
p
establish if an equally good level of control in terms of M and
best of our knowledge, this phenomenon has never been re-
ported for the ATRP of styrene under the same conditions, it
may be favored in this case by the presence of the phosphine
groups. These reactions are highly undesirable because they
lead to consumption of the halogen chain ends and formation
of dead chains and networks. It is therefore necessary to probe
the possible presence and extent of these reactions under the
polymerization conditions.
n
molecular weight distribution (as measured by the dispersity
index, ꢁ=M /M ) is obtained for different levels of SDPP incor-
w
n
poration, or how these parameters would depend on it. In ad-
dition, we investigate if the controlled polymer microstructure
has an effect on the ligand performance in a benchmark cata-
lytic application: the Rh-catalyzed hydroformylation of linear a-
olefins (represented in this study by 1-octene).
Previous work has shown that the halogenated chain end of
polystyrene and polyacrylates prepared by ATRP may indeed
be transformed into phosphonium salts upon addition of
phosphines, with the reaction accelerating upon an increase in
temperature and the solvent polarity (i.e., THF<DMSO<aceto-
Results and Discussion
Polymer synthesis and characterization
[28]
The controlled incorporation of SDPP in a copolymer structure
by atom transfer radical polymerization (ATRP) has already
been reported with use of complex RuCl (PPh ) as the atom
nitrile). We therefore decided to test for the presence of this
reaction in toluene. The halide chain end was emulated by 1-
phenylethylbromide and the reactive phosphine function by
2
3 3
transfer catalyst. However, as the functionalized monomer can
PPh . To model the polymerization conditions as closely as pos-
3
bind the ruthenium catalyst just as well as PPh , the polymer
sible, a large PPh /alkyl bromide ratio (12.5) was used. The
3
3
obtained contained the Ru catalyst coordinated by the poly-
mer-anchored phosphine functional groups and entrapped
within the polymer star core. Although a procedure to com-
pletely remove the metal from the polymer exists, we wished
to simplify the procedure and used instead the CuBr/tris(2-(di-
same copper catalyst system as that in the ATRP process was
also added to the reaction mixture and the mixture was
1
heated to 1008C for several hours with continuous H and
31
P NMR monitoring. After up to 26 h, which covered the time
range for the polymerization process, no signals other than
those of the reactants were observable. In particular, the PPh3
singlet at d=ꢁ4.89 remained the only observable resonance
methylamino)ethyl)amine (Me TREN) system as an ATRP cata-
6
lyst. This catalytic system yielded a fast and well-controlled
polymerization of styrene, resulting in polymers with target
31
in the P NMR spectrum, comparable to a 20% conversion
[
27]
[28]
molecular weights and low dispersities. Moreover, the che-
after 24 h if using the same reagents at 808C in acetonitrile.
lating power of the tetradentate Me TREN ensured efficient se-
Therefore, the nucleophilic substitution side-reaction should
not take place under these conditions and toluene was there-
fore selected as the ATRP solvent. The possible presence of de-
6
questering of the copper ion, inhibiting any copper coordina-
tion by the phosphine functional groups present in the mono-
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1
hydrobromination was probed by H NMR spectroscopy on the
prepared polymer samples (see above).
Copolymers made from comonomer mixtures with SDPP
molar fractions (fSPDD) of 0, 0.15, 0.25, 0.5, and 1 were prepared
by ATRP with CuBr/Me TREN as the catalyst to study the incor-
6
poration behavior of the phosphine monomer. Ethyl bromoiso-
butyrate (EBiB) was used as the initiator owing to its faster acti-
vation relative to the oligomeric secondary radical species gen-
erated after styrene or SDPP addition, which ensured fast ini-
[
29]
tiation.
A small amount (10%) of CuBr2 deactivator was
added to the reagent mixture to prevent a slower deactivation
rate relative to the propagation rate at the beginning of the
reaction. The other conditions were inspired by the literature
report on styrene homopolymerization (see the Experimental
[
27]
Section). The SDPP and styrene reactivity ratios (r) for radical
copolymerization were determined under free radical polymeri-
[
19]
zation conditions (rstyrene =0.52, rSDPP =1.43),
indicating that
SDPP should be incorporated into the macromolecular chain
slightly faster than styrene. These parameters should not
Figure 1. Evolution of the SDPP conversion for the copolymerization with
3
1
f
3
SDPP =0.25, by P NMR monitoring in CDCl .
[
30–31]
change if transposed to an ATRP process,
which suggests
that the SDPP/styrene copolymerization should yield a slight
compositional gradient copolymer with a higher inclusion of
SDPP at the beginning of the polymerization and lower at the
end, relative to the ratio in the comonomer initial feed. The re-
sults obtained are shown in Table 1.
counted for no more than 1% of the sample or recovered
polymer in each case. No other signals were detected on the
31
P NMR spectra.
The SDPP monomer conversion could be determined by de-
31
convolution of the P NMR signal. The advantage of using
31
P NMR was the simplicity of the spectrum and that the two
[
a]
Table 1. Synthesis of poly(SDPP-co-styrene)-Br copolymers.
resonances of interest were well separated from those of po-
tential impurities (notably traces of phosphine oxide). The
SDPP homopolymerization (fSDPP =1) was fast and uncontrolled,
reaching 86% conversion in only 10 h (see Figure S1 in the
Supporting Information). Styrene, however, yielded a very low
dispersity homopolymer in a well-controlled fashion, as docu-
[
b]
[d]
f
SDPP EBiB /styrene/
t
Styrene
SDPP
M
n,th
M
n,SEC
ꢁ
[
c]
SDPP/Cu /L
conversion conversion
h] [%] [%]
[
0
0
0
0
1
1:50:0:0.5:0.5
.15 1:85:15:0.5:0.5
.25 1:37.5:12.5:0.5:0.5 24 82
.5 1:25:25:0.5:0.5
1:0:50:1:1
20 81
32 73
–
4420 6800 1.04
10120 18300 1.37
6710 12570 1.29
9130 22220 1.51
13890 19680 1.36
80
92
94
97
[27]
mented in the literature
Figure S2).
(see the Supporting Information,
24 83
24 –
For the copolymerization experiments, as the concentration
of the two monomers could be monitored independently, the
individual comonomer inclusion rate into the polymers during
the reaction could be determined, confirming that SDPP is in-
corporated at a faster rate than styrene. All the experiments
were stopped after approximately 80% global monomer con-
version. For the copolymerizations with fSDPP =0.15 and 0.25,
the comonomer consumption followed a pseudo first order
rate law (Figure 2). Bending of the first order plot beyond 70%
conversion could be related to loss of a fraction of active chain
ends, a phenomenon that is well documented for the ATRP of
[
a] Conditions: t=1008C in toluene. [b] [EBiB]=0.044m for fSDPP =0, 0.25,
and 0.5; 0.014m for fSDPP =1 and 0.02m for fSDPP =0.15. [c] CuBr/CuBr
Me TREN=0.9:0.1:1. [d] Mn,th =theoretical number-average molecular
weight, calculated from the monomer conversion data.
2
/
6
After purification, all polymers were white, indicating the re-
moval of the Cu salts. This demonstrated that the copper ion
had greater affinity for the chelating Me TREN ligand than for
6
the polymer-supported phosphine. The possibility that copper
remained trapped as a less colored phosphine complex in the
I
[32]
reduced Cu state in the polymer was checked by a UV-visible
styrenic monomers at high conversions. In principle, a devia-
study (see the Supporting Information); the fraction of residual
copper remaining in the polymer was estimated to be approxi-
mately 1.5% of the amount used for the polymer synthesis.
This result differed to the RuCl (PPh ) -catalyzed ATRP leading
tion from first order behavior could also be caused by the reac-
tivity ratios and the resulting comonomer ratio drift with con-
centration. This effect, however, should be too small to be
clearly visible in this plot and should ultimately reflect into
a small deviation of the phosphine comonomer distribution
along the polymer chain (small concentration gradient). The
copolymerization with fSDPP =0.5 was faster, in addition it was
less controlled, as judged from the dispersity of the recovered
polymer (Table 1). The polymers recovered from the fSDPP =0.25
and 0.15 copolymerizations show properties in agreement
2
3 3
to SDPP-containing polymer nanogels, in which the catalyst re-
[
25]
mained trapped within the polymer. The monomer units in-
corporated in the polymer backbone exhibit a different
3
1
P NMR resonance (dꢂꢁ5.8, broader) from that of the free
monomer (dꢁ5.39, sharper), see Figure 1. In a few cases, traces
of oxide were found in the final product, nevertheless, they ac-
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sponding to initiator efficiencies
in the 41–65% range. These ini-
tiator efficiencies are typical for
this initiator and are generally
attributed to side reactions that
may occur for the primary radi-
cal if this was generated too rap-
idly relative to the rate of addi-
[33–34]
tion to monomer.
1
The H NMR analysis of the
isolated polymers (fSDPP =0, 0.15,
and 0.25) provided useful infor-
mation on the chain ends. The
Figure 2. Semi-logarithmic kinetic plots for the copolymerizations of SDPP (ꢂ) and styrene (*) at 1008C in toluene
with a) fSDPP =0.25 and b) 0.15. Conditions are given in Table 1. M=monomer concentration
region between d=3 and
5
clearly showed the chain end
functional groups (the initiator ꢁ
OCH CH signal at dꢂ3.6 and
2
3
with a controlled radical polymerization process. The polymer
molecular weight evolutions with conversion are close to
linear, not significantly perturbed by small drift of comonomer
ratio during polymerization (see Figure 3), and the dispersities
the terminating ꢁCHPhBr signal at dꢂ4.5, see Figure S5). The
approximate 2:1 ratio of these two resonances allowed the es-
timation that at least 90% of the halogen functionalities were
preserved. Furthermore, the spectra revealed minor sharp reso-
nance peaks attributable to vinyl protons in the region be-
tween d=5 and 6. Extensive efforts were made to remove all
traces of residual styrene monomer from the isolated polymer,
but it was not possible to exclude that these resonances owed
simply to such an impurity, especially given the narrow line-
width relative to the polymer resonances. However, they could
also be attributed to the polymer unsaturated chain ends gen-
erated by the dehydrobromination processes (Scheme 1). In
1
that case, the H NMR resonance integration combined with
knowledge of the average degree of polymerization accounted
for no more than 10% of such unsaturated chain ends.
Further investigations were performed on the copolymers at
fSDPP =0.25 and 0.15. Adequate quantities of five different co-
polymers were prepared by quenching the polymerization re-
action at different times. The characterization data of the puri-
fied polymers are summarized in Table 2. As before, the poly-
merizations occurred in a controlled manner. The molecular
weight distributions were uniform as shown by the analysis of
selected size-exclusion chromatography (SEC) traces (see
Figure 4).
Figure 3. Evolution of the molecular weight and dispersity as a function of
total monomer conversion for the copolymerization with fSDPP =0.25 (*) and
0
.15 (*).
are rather low. The corresponding data for the fSDPP =0.5 co-
polymerization (see the Supporting Information, Figure S3)
showed a much less-controlled
behavior.
As the composition was
changing constantly, the refrac-
tive index of the polymer solu-
tions was also in continuous
evolution. Therefore, a multian-
gle light-scattering (MALLS) de-
tector was used to determine
the molecular weight of the co-
polymer. The polymers Mn are
greater than those expected on
the basis of the monomer/initia-
tor ratio and conversion, corre-
Table 2. Synthesis of different length poly(SDPP-stat-styrene)-Br copolymers CP1–5 at fSDPP =0.25 and 0.15.
[
a]
[b]
[c]
Entry
f
SDPP
t
Styrene
SDPP
R
0
-(SDPP
x
-stat-S
y
)-Br
M
n,SEC
ꢁ
dn/dc P content
initial feed
conversion conversion
ꢁ1
[
h]
[%]
[%]
x
y
[mmolg ]
CP1 0.25
CP2 0.25
CP3 0.25
CP4 0.15
CP5 0.15
2.25 26
31
56
86
57
80
9
12
16
15
26
22
40
57
59
103
4900 1.19 0.181 1.77
7520 1.38 0.210 1.54
10270 1.33 0.200 1.48
10490 1.38 0.212 1.40
18300 1.37 0.204 1.41
6
18
42
70
11.5 30
32 73
[a] R
0
=primary radical generated by the EBiB initiator. [b] Values found by using the MALLS detector. [c] Quan-
3
1
tified from P NMR analysis (refers only to the non oxidized phosphine). For reference, the ideal polymer with
ꢁ1
2
5% incorporation of SDPP has a P content of 1.64 mmolg .
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[
a]
Table 3. Homogeneous hydroformylation of 1-octene.
4
[c]
Run
Ligand P/Rh
t
Residual
l/b
ratio
Initial rateꢂ10
[
b]
[b]
1-octene
ꢁ
1
ꢁ1
[
h]
[%]
[moll s ]
1
2
3
4
5
6
7
8
9
PPh
PPh
PPh
CP1
CP2
CP3
CP3
CP3
CP4
CP5
3
3
3
8
8
8
8
8
8
4
15
8
8
0.33
0.33 10
0.33
2
2
2
2
2
2
7
2.7
2.8
2.5
3.8
4.7
4.9
3.8
5.0
4.4
3.9
28.5
25.0
29.1
7.9
4.2
4.8
8.7
4.0
5.9
7.5
[
[
d]
e]
9
10
17
16
12
16
15
14
10
2
Figure 4. Size-exclusion chromatograms [refractive index detector, gel per-
meation chromatography (GPC)] for the three isolated poly(SDPP-stat-sty-
rene) polymers CP1 (g), CP2 (b), and CP3 (c), obtained with
ꢁ3
[
a] [Rh(acac)(CO)
2
]=2.0ꢂ10 m; [1-octene]=1.0m; T=908C; syngas pres-
sure=20 bar (CO/H
2
=1). [b] From the GC data. [c] Derived from the
slope at the beginning of the reaction of the number of moles produced
vs. time according to the syngas consumption. [d] In the presence of
f
SDPP =0.25.
ꢁ3
PhCHBrCH
styrene (M
3
(2.0ꢂ10 m). [e] In the presence of bromine-terminated poly-
=3820 gmol ; 2.0ꢂ10 m of Br functional groups).
ꢁ
1
ꢁ3
n
Application in 1-octene hydroformylation
The copolymers obtained by the above method were used as
ligands for the Rh-catalyzed hydroformylation of octene under
homogenous conditions. The desired reaction products are 1-
nonanal (linear isomer) and 2-methyl-1-octanal (branched
isomer), with internal octene isomers derived from catalyzed
isomerization as the main by-products, see Scheme 2. Other
end group perturbed the hydroformylation process, two addi-
tional experiments were performed using PPh in the presence
3
of 1-phenylethylbromide (run 2) or bromine-terminated poly-
styrene (run 3), in amounts chosen to emulate the PPh /Br
3
ratio of run 4. The conversion, initial rate and selectivity were
very close to those found from
the standard PPh3 system
(
run 1), suggesting the absence
of interference of the benzylic
bromide groups in hydroformy-
lation under these conditions.
Runs 4–6 were performed
with the three polymers ob-
Scheme 2. Possible products from the catalyzed hydroformylation of 1-octene.
tained using fSDPP =0.25 (CP1–3)
under the same conditions and
notably with the same P/Rh
possible by-products derive from hydrogenation (octane), hy-
droformylation products of the isomerized olefins, and prod-
ucts of further aldehyde hydrogenation (alcohols).
ratio. Two additional experiments (runs 7 and 8) were per-
formed with CP3 at different P/Rh ratios. Finally, runs 9 and 10
show the results obtained with the two polymers prepared
using fSDPP =0.15. The reaction chemoselectivity was excellent
as, in analyses performed on a gas chromatograph equipped
with a flame ionization detector, aldehydes were the only reac-
tion products detected: except for those resulting from
a minor amount of isomerization, 1-nonanal (linear) and 2-
methyl octanal (branched product) were the two major prod-
ucts. Neither other aldehyde isomers, nor octane or aldehyde
hydrogenation products (alcohols) were detected. Two octene
isomers were detected by GC–MS but in all cases they ac-
counted for less than 1% of the converted 1-octene. For the
The key reaction parameters (solvent, temperature, catalyst
concentration and P/Rh ratio) had already been optimized for
the homogeneous hydroformylation of 1-octene catalyzed by
[
35–37]
PPh and rhodium.
Our choice of conditions for the cata-
3
lytic tests was inspired by these previous investigations. Tolu-
ene was chosen as the solvent because it was a good solvent
for the phosphine-functionalized polymers described above.
The active catalytic species is formed in situ by reaction of Rh-
(
acac)(CO) and the phosphine ligand in the presence of
2
[
38–39]
H .
The results are summarized in Table 3.
2
[36]
The polymers were used directly as isolated from the ATRP
synthesis, therefore they still contained the benzylic bromine
same system, Uenveren and Schomꢃcker observed a greater
degree of isomerization in the initial stages of the reaction if
1
atom as the chain end, as revealed by H NMR spectroscopy
the PPh ligand was used. A rather interesting observation is
3
(Figure S5). The reaction progress and the material balance
that the linear-to-branched (l/b) selectivity increases on switch-
were monitored by gas chromatography; the total amount of
aldehydes produced was consistent with the syngas consump-
tion. To verify whether the presence of the residual halogen
ing from PPh to the polymer-supported ligands. Comparison
3
of runs 4–6 showed that the selectivity also increases slightly
with the polymer chain length at fSDPP =0.25. At similar chain
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length, the l/b ratios obtained with the polymers containing
lower phosphorus content (CP4 and CP5) were slightly smaller.
It is known that the l/b ratio increases with the P/Rh ratio until
fects. Two adjacent phosphines are separated by 3–4 styrene
units on average for the CP1, CP2 and CP3 systems, or 5–6
units for CP4 and CP5. These arrangements may have a direct
influence on the spatial conformation of the substituents on
the rhodium complex. The PꢁRhꢁP bite angle is critical in
terms of controlling the relative stability of equatorial–equato-
rial (eq-eq) and equatorial–axial (eq-ax) bis-phosphine isomers
in the pentagonal bipyramidal geometry with strong repercus-
sion on the l/b selectivity, larger bite angles favoring the more
[
35]
reaching a plateau;
comparison of runs 6–8 showed that
a P/Rh ratio of 8 was sufficient to yield the maximum selectivi-
ty for the polymer catalytic system.
The initial hydroformylation rate in the presence of the poly-
mer-supported phosphine ligands was always lower than with
PPh . This rate could have correlated with the l/b ratio, as it de-
3
[50–51]
creased with increasing selectivity. In accordance with observa-
selective eq–eq isomer.
Hence, the greater selectivity may
[
35]
tions with PPh3,
increasing the P/Rh ratio from 4 to 15
be induced by the imposition of a slightly greater (on average)
bite angle by the constraints of the chain folds. For the poly-
mers obtained from the fSDPP =0.25 comonomer feed, the ob-
served increase of the l/b ratio with chain length may result
from an increase of structural rigidity. However, the lower l/
b ratio and relative insensitivity of this ratio as a function of
chain length for the polymers resulting from the fSDPP =0.15 co-
monomer feed could result from the longer average spacing
between the phosphine ligands. Therefore, the greater flexibili-
ty of the polymer chain backbone relaxes the structure to-
wards thermodynamically more favored situations of lower
bite angles. This situation is summarized in the schematic
drawing of Figure 5.
(runs 6–8) diminished the reaction rate by a factor of 2.
An improvement in selectivity and the slowdown of the re-
action rate for polymer-immobilized phosphine ligands had al-
ready been reported for the hydroformylation of substrates
[
40–41]
[22]
[42]
such as 1-pentene,
1-hexene, 1-octene, and 1-dode-
[
43]
cene. For instance, Hartley et al. reported a l/b ratio of 6 for
the hydroformylation of 1-hexene at 858C using a phosphinat-
[
22]
ed polypropylene, twice the value obtained for the mono-
[
44–45]
meric catalyst.
The decrease in the reaction rate was ra-
tionalized in terms of a reduced accessibility of the catalytic
sites for the olefin; the immobilization and confinement of the
complex into the polymer coils limited the diffusion rate of the
catalytic complex and the olefin access inside the polymer.
However, our observation that the rate did not have a strong
dependence on the polymer size or phosphine content
showed that the olefin diffusion into the polymer folded
chains was not a kinetically relevant process or that the differ-
ence between the chain lengths was too small to detect this
effect. The Rh atoms may have also played the role of intra-
chain or inter-chain crosslinkers, leading to diffusional limita-
tion. Owing to the low polymer concentrations used in our cat-
alytic experiments and the good solvent properties, the macro-
molecular chains were likely to be relatively unfolded and iso-
lated from each other. Hence, intra-chain crosslinking should
have dominated. Furthermore, for entropic reasons, coordina-
tion of the Rh atom by adjacent phosphine donors in the poly-
mer chain should have been preferred. Consequently, the mac-
romolecule should have remained flexible and the catalytic
centers continued to be relatively accessible. The relatively
small number of SDPP units per chain (see Table 2) and the
high P/Rh ratio suggested that the chains only contain 1–3 Rh
atoms on average (ꢂ1 for CP1, 2 for CP3 and CP4, and 3 for
CP5). This low degree of crosslinking relative to the chain
length, especially in a good solvent, may not have been suffi-
cient to influence the activity.
Figure 5. Bidentate coordination to the catalytically active pentagonal bipyr-
amidal Rh center in the HRh(CO)(polymerꢁPPh
3 2
) intermediate. &: Open co-
ordination site).
Conclusions
We describe the first generation of triphenylphosphine ligands
anchored on linear polystyrene chains by controlled radical
polymerization. This synthesis method enables easy tailoring of
polymer chain length, with a narrow molecular weight distribu-
tion, and phosphine incorporation. Application of these poly-
mer-supported phosphines in Rh-catalyzed 1-octene hydrofor-
mylation provides comparable results to those obtained with
Following the extensive mechanistic studies of this catalyzed
reaction, the catalyst structure for the PPh system is known to
3
be of type RhX(CO) (PPh ) or RhX(CO)(PPh ) (X=H, alkyl or
2
3 n
3 n
acyl as the molecule moves around the catalytic cycle), con-
taining between one and three phosphine ligands and yielding
PPh -containing polymers obtained by other less-controlled
3
methods in terms of activity and selectivity but allows, for the
first time, an analysis of these parameters as a function of
chain length and degree of phosphine incorporation. This
study reveals a slight dependence of the hydroformylation
linear/branched selectivity, for comparable phosphine incorpo-
ration, on the polymer chain length for chains containing ap-
proximately 25% of functionalized monomers. In perspective,
[
46]
different catalytic performance.
The lower activity of the
polymer-supported catalyst may be related to the chelating
nature of the macromolecular ligand, disfavoring the more
active and less regioselective configurations with a lower
[
35,47–49]
number of coordinated phosphine ligands.
Their higher
selectivity may also result from other polymer structuring ef-
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this synthetic method allows for better control of the polymer
architecture and, thereby, a finer tailoring of the ligand perfor-
mance. The method also takes advantage of the residual bro-
minated chain end to create elaborate and more complex
polymer architectures for more specific catalytic applications.
Studies in this direction are currently ongoing in our
laboratories.
polystyrene (0.184). Gas chromatographic analyses were performed
on a Trace GC 2000 Thermo Fisher chromatograph equipped with
a flame ionization detector. The separations were performed with
a capillary CP-WAX 52 CB (25 mꢂ0.25 mm, 0.2 mm film thickness)
column using helium as carrier gas. To follow the styrene conver-
sion during the polymerizations, dodecane was used as an internal
standard. For the hydroformylation reactions, samples withdrawn
for kinetic monitoring were diluted into diethyl ether and anisole
was added as an internal standard. Prior to the analytical tests,
standard solutions were prepared in the range of investigated con-
centrations of reactant (1-octene) and product (n-nonanal), and cal-
ibration curves were plotted for quantification. It was assumed
that the response factor is the same for isomers as for the linear
compounds. The identification of the compounds was confirmed
by GC–MS analysis.
Experimental Section
Materials
Unless otherwise stated, all operations were performed under an
argon atmosphere. Styrene (99%, Aldrich) was distilled under re-
duced pressure from CaH (90–95%, Aldrich) and stored under
2
argon at ꢁ208C before use. Toluene (99.7%, Aldrich) was dried
and distilled under argon. CuBr (99.999%, Aldrich), CuBr (99.0%,
Synthetic protocols
2
Fluka), triphenylphosphine (99%, Aldrich), Rh(CO) (acac) (99% Alfa
Aesar), hexane (>96.5%, Aldrich), anisole (99.0%, Fluka), octane (>
2
All the polymerizations were performed in glass flasks equipped
with a three way stopcock and a magnetic stir bar. Solvents were
flashed by an argon flow and introduced using argon-purged sy-
ringes fitted with stainless steel needles. For the homopolymeriza-
tion of styrene, the polymer purification was performed in air.
9
9.0%, Aldrich), 1-octene (>99%, ACROS), nonanal (>97%, Alfa
Aesar), dodecane (99%, Aldrich), EBiB (98%, Aldrich), 1-phenyl eth-
ylbromide (97%, Aldrich), ethyl diphenyl phosphine oxide (98%,
Lancaster) and SDPP (97%, Aldrich) were used without further pu-
rification. Me TREN was synthesized according to a literature proto-
6
[
27]
col. Carbon monoxide and hydrogen were supplied by Linde gas
S.A. Syngas was prepared by mixing equivalent molar amounts of
Styrene homopolymerization
H and CO in a reservoir vessel.
2
In a typical experiment, CuBr (55.3 mg, 0.38 mmol), CuBr (9.4 mg,
2
0
.04 mmol), toluene (13 mL), Me TREN (110 mL, 0.42 mmol), styrene
6
(5.0 mL, 42 mmol) and dodecane (1 mL) were sequentially intro-
Instrumentation
duced in a 50 mL round-bottomed Schlenk flask at room tempera-
ture. EBiB (128 mL, 0.84 mmol) was then introduced with a microsyr-
inge and the mixture was rapidly cooled-down by immersion into
a liquid nitrogen-filled Dewar flask. The mixture was degassed by
three freeze–pump–thaw cycles. The flask was then immersed in
an oil bath thermostated at 1008C to start the reaction. At timed
intervals, aliquots of the reaction solution were withdrawn with
argon-purged syringes, followed by filtration through a small neu-
tral-alumina column to remove the catalyst. One part of the color-
less samples was analyzed by gas chromatography to determine
the conversion and the remaining part was used for the SEC analy-
sis. After 20 h, the Schlenk flask was rapidly cooled down and the
mixture was diluted with toluene and then filtered through a neu-
tral-alumina column. The resulting solution was concentrated by
evaporation under reduced pressure and the polymer was precipi-
tated by addition of cold hexane, filtered off, and dried under
vacuum until total residual solvent evaporation (ca. 3 d) to yield
The conversion during the SDPP polymerization was monitored
and the phosphorus content in the purified polymers was mea-
3
1
1
sured by P{ H} NMR with a Bruker Avance 300 spectrometer
equipped with a 5 mm TXO probe head. Chemical shifts are report-
1
ed in ppm relative to tetramethylsilane ( H), referenced to the
chemical shift of residual solvents resonances, or from external
3
1
8
5% H PO ( P). All experiments were performed in CDCl at
3 4 3
2
98.0 K. Long relaxation delays (t=40 s) were used for the quanti-
tative P analyses to ensure full magnetization recovery, and the rel-
ative peak area was calculated by signal deconvolution and fitting
procedures. Absolute amounts were determined relative to known
added amounts of ethyl diphenylphosphine oxide as an internal
standard. The phosphine content (in molP atoms
mined directly from its relative intensities in the P{ H} NMR spec-
tra [Eq. (1)]:
ꢁ
1
g
) was deter-
polymer
31
1
ꢁ1
a white powder. The yield was 3.6 g (82%). Mn,SEC =6800 gmol
ꢁ=1.04.
;
mref
Ipol
ꢃ mpol
Pcontent
¼
ꢃ I
ð1Þ
Mwref
ref
in which Mwref is the molecular weight of ethyl diphenylphosphine
oxide, mref and mpol are the weights of ethyl diphenylphosphine
oxide and polymer, respectively, Iref and Ipol are the intensities from
the spectra. SEC analyses were performed on a PL gel (5 mm parti-
cles, 50ꢂ7.5 mm) guard column and a PL-gel 5 mm mixed-D (300ꢂ
Stability test of PPh under ATRP conditions: phosphonium
salt formation
3
CuBr (26.3 mg, 0.18 mmol), CuBr2 (4.5 mg, 0.02 mmol) and PPh3
(1.311 g, 5 mmol) were charged on a round-bottom flask with
a magnetic stir bar. The flask was degassed and filled with argon
7
.5 mm) column (Polymer Laboratories), in filtered THF as eluent.
Molecular weights were measured by a MALLS detector (mini-
DAWN TriStar, Wyatt Technology Corporation) coupled with a re-
fractive index detector (RI2000, Sopares), set at 358C. Molecular
weight values for the copolymers and the SDPP homopolymer
were determined using the MALLS detector. The total mass recov-
ery protocol was employed, which in turn yielded the correspond-
ing dn/dc values. Notably, a literature value of dn/dc was used for
by three vacuum-argon cycles. Toluene (5.0 mL) and Me TREN
6
(55 mL, 0.2 mmol) were added in turn using argon-purged syringes.
1-phenylethyl bromide (55 mL, 0.4 mmol) was then introduced with
a microsyringe and the mixture was rapidly cooled-down by im-
mersion into a liquid nitrogen-filled Dewar flask. The mixture was
degassed by three freeze–pump–thaw cycles to remove oxygen
traces. The flask was then immersed in an oil bath thermostated at
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1
008C for 26 h. At timed intervals, aliquots of the reaction solution
speed to 1200 rpm and data acquisition commenced. The stirring
speed was checked to ensure it was high enough for the reaction
to occur in the chemical regime, as no improvement was found
upon increasing it to 1500 rpm. The initial hydroformylation rate
was obtained from the pressure variations in the reservoir that was
used to feed the reactor, which was maintained at a constant pres-
sure. The selectivity toward linear aldehyde and the conversion
were monitored by gas chromatographic analyses of additional
were withdrawn by using argon-purged syringes, followed by filtra-
tion under argon through a small neutral alumina column to
remove the metal complexes, and finally analyzed by P NMR
spectroscopy.
31
SDPPꢁstyrene copolymerization
liquid samples (after 20, 40 and 60 min for PPh and 30, 60, 90 and
3
The same procedure was used to obtain an SDPP homopolymer or
an SDPP/styrene statistical copolymer. Only the reaction performed
with fSDPP =0.25 will be described in detail: in a 50 mL round-
bottom Schlenk flask were sequentially introduced CuBr (59.3 mg,
1
20 min for the polymeric ligands) during the reaction. The
amounts of polymeric ligand used, calculated on the basis of their
phosphorus content, were: CP1: 920 mg; CP2: 1061 mg; CP3: 549;
1
098 and 2060 mg for the P/Rh ratios of 4, 8, and 15 respectively;
0
.41 mmol), CuBr2 (10.1 mg, 0.05 mmol), SDPP (3.344 g,
CP4: 1165 mg and CP5: 1160 mg. Two additional runs were per-
11.25 mmol), toluene (15 mL), Me TREN (120 mL, 0.45 mmol), sty-
6
formed in which PPh (432 mg, 1.63 mmol) was used as the ligand
3
rene (4.0 mL, 33.75 mmol) and dodecane (1 mL) at room tempera-
in the presence of 1-phenylethyl bromide (28 mL, 0.2 mmol) or bro-
ture. Then EBiB (140 mL, 0.9 mmol) was introduced with a microsyr-
inge and the mixture was rapidly cooled by immersion into
a liquid-nitrogen-filled Dewar flask. The solution was then de-
gassed by three freeze–pump–thaw cycles and the flask was finally
immersed in an oil bath thermostated at 1008C. At timed intervals,
aliquots of the reaction solution were withdrawn with argon-
purged syringes, followed by filtration under argon through
a small neutral-alumina column to remove the catalyst. One part of
the colorless samples was analyzed by gas chromatography to de-
termine the styrene conversion and the remaining part was used
for the SEC and P NMR analyses. After 24 h, the Schlenk flask was
rapidly cooled down and the mixture was diluted with toluene and
filtered through a neutral alumina column. The polymer was pre-
cipitated by addition of cold hexane (0.5 L), filtered off, and dried
under vacuum until total residual solvent evaporation (ca. 3 d) to
ꢁ
1
mide-end polystyrene (Mn,SEC =3820 gmol ; 771.5 mg, 0.2 mmol).
The procedure was the same as described above.
Acknowledgements
We are grateful to the CNRS (Centre National de la Recherche Sci-
entifique), the ANR (Agence Nationale de la Recherche, Grant No.
ANR-11-BS07-025-01), and the IUF (Institut Universitaire de
France) for financial support. We also thank the PRES University
of Toulouse for a Ph.D. fellowship to A. F. Cardozo.
3
1
Keywords: hydroformylation
·
phosphane
ligands
·
yield
1
a white powder. The yield was 4.2 g (60%). Mn,SEC =
polymerization · radicals · rhodium
ꢁ
1
2570 gmol ; ꢁ=1.29. The same reaction was repeated under
the same conditions and quenched at different times to produce
copolymer chains with different lengths: CP1: reaction time=2 h
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Received: July 5, 2012
Published online on January 30, 2013
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2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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