Hyperbranched polycarbosilanes of homogeneous architecture: Regioselective hydrosilylation of AB2 monomers and consecutive functionalization

Article abstract of DOI:10.1021/ma902425y

A novel AB₂ monomer, bis(4-(but-3-enyl)phenyl)methylsilane, la, and its analogue AB structure lb were synthesized and polymerized with Karstedt's, Speier's, and a Pt-NHC catalyst to obtain hyperbranched and linear polycarbosilanes. Only the Pt-

Full text of DOI:10.1021/ma902425y

934 Macromolecules 2010, 43, 934–938
DOI: 10.1021/ma902425y
Hyperbranched Polycarbosilanes of Homogeneous Architecture:
Regioselective Hydrosilylation of AB2 Monomers and Consecutive
Functionalization
†
Ulrike Will, Draganco Veljanovski, Peter Harter, and Bernhard Rieger*
‡
‡
,†
€
†Institut fu€r Siliciumchemie, WACKER-Lehrstuhl fu€r Makromolekulare Chemie, Technische Universita€t
Mu€nchen, Lichtenbergstrasse 4, 85747 Garching bei Mu€nchen, Germany, and ‡Institut fu€r Siliciumchemie,
€
€
Lehrstuhl fu€r Anorganische Chemie, Technische Universitat Munchen, Lichtenbergstrasse 4,
85747 Garching bei Mu€nchen, Germany
Received September 15, 2009
ABSTRACT: A novel AB2 monomer, bis(4-(but-3-enyl)phenyl)methylsilane, 1a, and its analogue
AB structure 1b were synthesized and polymerized with Karstedt’s, Speier’s, and a Pt-NHC catalyst to
obtain hyperbranched and linear polycarbosilanes. Only the Pt-NHC complex afforded a regioselective
hydrosilylation reaction. As a consequence, this led to a uniform polymer microstructure and at the same
time to significantly increased molecular weights. Functionalization of the remaining double bonds in the
obtained polymer was achieved by two exemplary routes. Hydrosilylation with a functional silane yielded
chloromethylsilyl capped polymers, hydroboration and subsequent oxidation led to hydroxy-terminated
polymers.
Introduction
Experimental Section
All reactions were performed under an atmosphere of argon
using standard Schlenk techniques. All chemicals were purchased
from Sigma-Aldrich or Acros Organics and used as received. Dry
solvents were obtained by filtering with a solvent purification
system (MB SPS-800, MBraun). Flash chromatography was
performed with silica gel 60 (Fluka) with a particle size of
0.040-0.063 mm. 1H, 13C and 29Si NMR spectra were recorded
on a Bruker ARX 300 spectrometer at ambient temperature. 29Si
NMR spectra of the polymers were recorded on a Bruker Avance
500 spectrometer. The spectra were referenced to the residual
solvent signals of the deuterated solvents (Deutero); chemical
shifts are reported in δ (ppm). GPC was performed with a
Polymer Laboratories GPC 50 Plus chromatograph running with
HPLC-grade THF at a flow rate of 1 mL/min. Polystyrene
standards were used for calibration. GC-MS was measured on
a Varian CP-3800 with a quadrupol 1200 L mass spectrometer.
Elemental analysis was performed at the microanalytic labora-
tory, institute of inorganic chemistry, TU Munich.
Hydrosilylation describes the addition of hydrosilanes to
multiple bonds. Since the discovery of hexachloroplatinic acid
as a very efficient catalyst by Speier in 1957,1 hydrosilylation has
become one of the most important reactions for preparing
organosilicon compounds in both industry and academia.
As a quantitative, high yielding reaction hydrosilylation is
also used in the synthesis of dendrimers and hyperbranched
polymers, one of the fastest growing areas of interest in
polymer science in the last few decades.2 Although dendrimers
with their perfectly branched structure and symmetry are of
natural beauty, they have the disadvantage of a very elaborate
and purification intensive synthesis. By contrast, hyper-
branched polymers offer similar properties with the benefit
of a simple one pot polymerization strategy. The utilization of
hydrosilylation in the synthesis of hyperbranched polycarbo-
silanes was first reported by Muzafarov et al. in 1993, who
polymerized, among others, methyldiallylsilane and methyldi-
vinylsilane.3 Apart from aliphatic polyalkenylsilanes,4 hyper-
branched polycarbosilanes with aromatic5 and thiophene6
moieties are also known.7
1-Bromo-4-(but-3-enyl)benzene. The compound was synthe-
sized according to the known literature procedure.14 The crude
product was purified by column chromatography (pentane,
Rf = 0,43) to yield the product as a colorless liquid (83%). 1H
NMR (CDCl3, 300.13 MHz): δ = 7.40 (d, 3JHH = 8.40 Hz, 2H,
In the synthesis of hyperbranched polycarbosilanes mostly
platinum complexes, especially Speier’s or the more active
Karstedt’s catalyst, are used. Unfortunately, applying these
catalysts also leads to numerous side reactions like R-addition
Ar-H), 7.06 (d, 3JHH = 8.40 Hz, 2H, Ar-H), 5.82 (ddt, 3JHH
=
6.57, 10.20, 16.84 Hz, 1H, H2CdCH-), 5.02 (m, 2H, H2C =
CH-), 2.67 (t, 3JHH = 8.40 Hz, 2H, Ar-CH2-), 2.35 (m, 2H,
Ar-CH2-CH2-).
yielding the Markovnikov product, dehydrogenative silyla-
8
ꢀ
tion and isomerization. To overcome these problems, Marko
AB2 Monomer Bis(4-(but-3-enyl)phenyl)methylsilane (1a). In a
three-necked round-bottom flask equipped with dropping fun-
nel, reflux condenser, and argon inlet was dispersed 0.49 g of
magnesium (0.02 mol) in 10 mL of dry THF. A solution of 4.30 g
of 1-bromo-4-(but-3-enyl)benzene (0.02 mol) in 20 mL of dry
THF was slowly added to the stirred magnesium dispersion.
After completion of the addition, the grayish solution was
heated to reflux for 1 h and afterward cooled to room tempera-
ture. Then 1.17 g of dichloromethylsilane (0.01 mol) was added
dropwise; meanwhile, a white solid precipitated. The reaction
et al. developed several N-heterocyclic carbene platinum(0)
complexes which showed high efficiency and selectivity
in hydrosilylation experiments.9-13 We report here on the
regioselective synthesis of a novel family of hyperbranched
polycarbosilanes with good control of microstructure and
polymer architecture via hydrosilylation polymerization
using a Pt-NHC complex.
*Corresponding author. E-mail: rieger@tum.de.
pubs.acs.org/Macromolecules
Published on Web 11/18/2009
r 2009 American Chemical Society

Article
Macromolecules, Vol. 43, No. 2, 2010 935
Scheme 1. Synthesis and Polymerization of AB and AB2 Monomer
mixture was again heated to reflux for 2 h. After the mixture was
cooled to room temperature, 10 mL of a saturated NH4Cl
solution was carefully added. The white precipitate was dis-
solved by adding 10 mL of water. The organic phase was
separated, washed with water and a saturated NaCl solution,
dried over MgSO4, and concentrated in vacuo. The crude
product was purified by column chromatography (pentane,
pentane/ethyl acetate 98:2, Rf (pentane) = 0.11) to yield 2.03
g (65%) of the product as a colorless liquid. 1H NMR (CDCl3,
300.13 MHz): δ = 7.49 (d, 3JHH = 7.60 Hz, 4H, Ar-H), 7.21 (d,
of chloromethyldimethylsilane was added to the solution.
Karstedt’s catalyst was added, and the mixture was stirred at
60 °C for 12 h. After evaporation of the solvent, the residue
was dissolved in 1 mL of chloroform and precipitated from
cold methanol. The polymer was filtered off and dried in
vacuo to yield 90 mg of a brownish solid.
For the hydroboration, 0.50 g (double bond content:
1.6 mmol) of the polymer was dissolved in 15 mL of dry THF,
the mixture was cooled to -10 °C, and 6.5 mL of a 9-BBN
solution (3.2 mmol, 0.5 M solution in THF) was added slowly.
The mixture was stirred at -10 °C for 3 h and at room
temperature overnight. For oxidation, the solution was cooled
to -10 °C again and 4 mL of a 6 N solution of NaOH followed
by 5 mL of a 30% aqueous solution of H2O2 was added. After
being stirred for 1 h at -10 °C, the mixture was allowed to warm
to room temperature and stirred at 50 °C for another hour. The
mixture was cooled to room temperature and the organic phase
was separated, washed with a saturated solution of NaCl three
times, dried over MgSO4 and the solvent was removed in vacuo.
The residue was dissolved in 2 mL of chloroform again and
precipitated from cold methanol. The polymer was filtered off
and dried in vacuo to give 0.41 g of a colorless sticky solid.
3
3JHH = 7.60 Hz, 4H, Ar-H), 5.87 (ddt, JHH = 6.54, 10.18,
16.92 Hz, 2H, H2CdCH-), 5.02 (m, 4H, H2CdCH-), 4.91 (q,
3
3JHH = 3.78 Hz, 1H, Si-H), 2.72 (t, JHH = 8.40 Hz, 4H,
3
Ar-CH2-), 2.38 (m, 4H, Ar-CH2-CH2-), 0.60 (d, JHH
=
3.84 Hz, 3H, Si-CH3). 13C NMR (75.47 MHz): δ = 143.3,
138.0, 134.9, 132.4, 128.1, 114.9, 35.4, 35.3, -4.9. 29Si NMR
(59.63 MHz): δ = -17.9. Anal. Calcd for C21H26Si: C, 82.29;
H, 8.55; Si, 9.16. Found: C, 82.40; H, 9.02; Si, 10.02. GC-MS
(M - Hþ): calcd, 305.17; found, 305.18.
AB Monomer 4-(But-3-enyl)phenyldimethylsilane (1b). The
AB monomer was synthesized analogously to the AB2 monomer
by reaction of 0.36 g of magnesium (0.015 mol), 3.15 g of
1-bromo-4-(but-3-enyl)benzene (0.015 mol), and 1.41 g of
chlorodimethylsilane (0.015 mol). The crude product was puri-
fied by column chromatography (pentane, Rf = 0.38) to yield
2.12 g (75%) of the product as a colorless liquid. 1H NMR
Results and Discussion
One of the possibilities to synthesize hyperbranched polymers
is the polyaddition of ABx monomers (x g 2). For hyperbranched
polycarbosilanes the AB2 monomers require, for example, one
Si-H group and two double bonds. We synthesized bis(4-(but-
3-enyl)phenyl)methylsilane (1a) as a novel AB2 monomer and
(4-(but-3-enyl)phenyl)dimethylsilane (1b) as its AB monomer
analogue by two consecutive Grignard reactions each starting
from 4-bromobenzyl bromide (see Scheme 1). The monomers
were purified by flash chromatography and obtained in good
yields. We chose this monomer structure with an aryl spacer
between silicon and double bond because simpler spacers like
alkyl spacers lead to problems. Medium length alkyl spacers
result in cyclization instead of polymerization and longer spacers
are not as easily accessible.
3
(CDCl3, 300.13 MHz): δ = 7.48 (d, JHH = 8.01 Hz, 2H,
=
Ar-H), 7.21 (d, 3JHH = 8.10 Hz, 2H, Ar-H), 5.88 (ddt, 3JHH
6.54, 10.19, 16.83 Hz, 1H, H2CdCH-), 5.03 (m, 2H,
3
H2CdCH-), 4.43 (m, 1H, Si-H), 2.72 (t, JHH = 8.98 Hz,
3
2H, Ar-CH2-), 2.39 (dt, JHH = 7.05, 14.16 Hz, 2H,
3
Ar-CH2-CH2-), 0.34 (d, JHH = 3.60 Hz, 6H, Si-CH3).
13C NMR (75.47 MHz): δ = 143.0, 138.0, 134.3, 134.1, 128.0,
114.9, 35.3, -3.7. 29Si NMR: (CDCl3, 59.63 MHz): δ (ppm) =
-17.2. Anal. Calcd for C12H18Si: C, 75.71; H, 9.53; Si, 14.75.
Found: C, 75.28; H, 9.35; Si, 13.42. GC-MS (M - Hþ): calcd,
189.11; found, 189.19
General Polymerization Procedure. In a Schlenk flask, 0.25 g
of the monomer was dissolved in 3 mL of dry solvent and the
catalyst was added. The reaction mixture was stirred at 60 °C for
24 h (48 h when using the Pt-NHC catalyst). After the mixture
had cooled to room temperature, the solvent was removed
in vacuum. Reactions using Karstedt’s catalyst (substrate to
catalyst ratio = 10000/1) or the Pt-NHC complex (1000/1)
were conducted in dry toluene, reactions with Speier’s catalyst
(100/1) were performed in dry isopropanol. The polymers were
obtained as sticky, colorless to brownish liquids to solids,
depending on the degree of polymerization and the catalyst
used. For NMR assignment of the polymer signals, see the
Supporting Information.
For polymerization of the monomers three different catalysts
were employed. Apart from the well-known Speier (2) and Karstedt
(3) systems, one of the N-heterocyclic carbene platinum(0) com-
ꢀ
plexes developed by Marko et al. was also utilized for hydrosilyla-
tion (see Figure 1). We decided to use (N,N’-bis(2,4,6-trimethyl-
phenyl)imidazol-2-ylidene)platinum(divinyltetramethyldisiloxane)
(4) (Pt-NHC) because of its reported high selectivity and reacti-
vity. The complex was synthesized according to the known
literature procedure.10
All polymerization reactions with Karstedt’s catalyst and the
Pt-NHC complex were conducted in toluene at 60 °C, the
reactions using the Speier system were performed in isopropanol
at the same temperature. Table 1 summarizes the polymerization
Functionalization of polymers. For the hydrosilylation,
0.10 g (double bond content: 0.33 mmol) of the polymer
was dissolved in 5 mL of dry toluene and 0.35 g (3.3 mmol)

936 Macromolecules, Vol. 43, No. 2, 2010
Will et al.
Figure 1. Hydrosilylation catalysts used for polymerization of AB2
monomers.
Table 1. Summary of Polymerization Results with Different Catalysts
entry monomer catalysta % isomerizationb Mnc (g/mol) PDIc
P1
P2
P3
P4
P5
AB2
AB2
AB2
AB
2
3
4
3
4
10
29
1400
3900
18 600
4800
1.30
6.38
2.41
2.08
1.77
d
-
6
d
AB
-
9500
a Reactions with Speier’s and Karstedt’s catalyst were stirred at 60 °C
for 24 h, reactions with Pt-NHC catalyst for 48 h. b Determined by 1H
NMR spectroscopy. c Determined by GPC in THF versus polystyrene
standards. d Only trace amounts of isomerized double bonds were
detectable.
Figure 3. 29Si NMR of (a) partially isomerized polymer P2 (Karstedt’s
catalyst) and (b) polymer P3 without isomerization (Pt-NHC)
(500 MHz, CDCl3, 300 K).
bond is unreactive in hydrosilylation under the given conditions
this reaction limits the number of reactive double bonds available
for polymer growth.7
ꢀ
The Pt-NHC complexes developed byMarko et al. are known
to be less active but more selective than Karstedt’s catalyst with
hardly anyisomerization observed. Therefore, 4 was also tested in
the hydrosilylation of the AB2 monomer 1a. The resulting
polymer P3 showed only trace amounts of isomerized double
1
bonds in the H NMR spectrum (Figure 2c) and the molecular
weight was significantly higher (see Table 1). At the same
time a narrow molecular weight distribution was found, which
also corresponds to the homogeneous polymer architecture.
The uniformity of polymer P3 is also clarified by its 29Si NMR
spectrum (Figure 3b), which mainly shows one signal at
-7.8 ppm. Figure 3a displays the spectrum of the partially
isomerized polymer P2 with additional signals at -0.7, -2.6,
and -11.7 ppm, most likely deriving from the inhomogenous
polymer microstructure.
The trends observed for the hyperbranched polymers were also
detected for the linear polymers which result from hydrosilylation
of the AB monomer 1b. Compared to Karstedt’s catalyst the
molecular weight of the polymer is significantly increased and
again no internal double bonds could be detected when using the
Pt-NHC complex. The lower degree of isomerization observed
for the linear polymer in comparison to the hyperbranched
polymer clearly results from the higher reaction rate of the
hydrosilylation compared to isomerization. During the polym-
erization of the AB2 monomer only 50% of the double bonds can
be consumed, the others remain unreacted in the polymer. By
comparison, in the polymerization of the AB monomer, all
double bonds will eventually be consumed during the reaction.
All remaining double bonds can be isomerized by the catalyst
during the whole polymerization process which should lead to
higher degrees of isomerization for the hyperbranched polymer
because of the higher double bond content.
Figure 2. 1H NMR spectra of (a) AB2 monomer 1a, (b) partially
isomerized polymer P2 (Karstedt’s catalyst) and (c) polymer P3 without
isomerization (Pt-NHC) (300 MHz, CDCl3, 300 K, solvent and H2O
signals are marked with an asterisk).
results and trends obtained for the different catalysts with
selected examples P1 - P5.
When using Speier’s or Karstedt’s catalyst, polymers with
moderate molecular weights and high PDI were obtained. How-
ever, considerable amounts of internal double bonds occurred
from a side reaction during polymerization. Figure 2a shows the
1H NMR spectrum of AB2 monomer 1a with the signals for the
terminal double bond at 5.9 and 5.0 ppm and the signal for the
Si-H group at 4.9 ppm. The spectrum of the hyperbranched
polymer P2 obtained with Karstedt’s catalyst (Figure 2b) clearly
shows two new signals at 5.6 and 3.3 ppm which can be assigned
to the double bond protons and the adjacent methylene group of
an isomerized internal double bond. This isomerization is a well-
known side reaction in hydrosilylation.15 As the internal double
The improvement of molecular weight and PDI with the
Pt-NHC catalyst can also clearly be seen from Figure 4 which
shows the GPC traces of polymer P3 (solid line) and P2 (dashed
line). Not only does P3 have the higher molecular weight but also
it has a much narrower weight distribution, resulting in a more
uniform polymer. In our opinion, this improvement of molecular

Article
Macromolecules, Vol. 43, No. 2, 2010 937
Another possibility of functionalization, already applied to
carbosilane dendrimers,16 is a hydroboration of the double bonds
with 9-BBN and subsequent oxidation with H2O2 leading to a
hydroxyl-group-containing hyperbranched polymer. Figure 5b
shows that all double bonds have successfully been converted to
hydroxy groups. The addition of the borane proceeds in an anti-
Markovnikov fashion and the functionalized polymer could
easily be separated from impurities like 1,5-cyclooctanediol by
precipitation from methanol.
The simple application of the two presented derivatization
methods and the homogeneity of the resulting polymers show
that these hyperbranched polymers could be used as an alter-
native to dendrimers which are much more tedious to prepare.
In particular, the hydrosilylation approach for functionaliza-
tion is very promising as any functional silane with one Si-H
group can be utilized. Apart from chloromethyldimethylsilane
used in the presented work, a lot of other silanes such as,
e.g., ethoxydimethylsilane are imaginable. Functionalization
and polymerization rely on the same reaction, the hydro-
silylation, so the same catalyst can be used in both. The
derivatization could be a follow up-reaction to the polymeri-
zation without the need of a work-up in between. First
experiments in our group show that a one pot functionali-
zation without preceding isolation of the polymer is possible.
This is a straightforward approach toward diversely functio-
nalized dendritic polymers.
Figure 4. GPC traces of partially isomerized polymer P2 (Karstedt’s
catalyst, dashed line) and polymer P3 without isomerization (Pt-NHC,
solid line).
Conclusion
Our novel AB and AB2 monomers were polymerized with
Karstedt’s, Speier’s, and a Pt-NHC catalyst to obtain linear and
hyperbranched polycarbosilanes. 2 and 3 only yielded polymers
with moderate molecular weights and high PDI due to the double
bond isomerization. Additionally such polymers are of nonuni-
form polymer microstructure. However, application of the
Pt-NHC complex 4 gave polymers with higher molecular
weights and narrower weight distributions. We believe this
improvement to be caused by the regioselectivity of 4, noticeable
in the absence of any isomerized, unreactive double bonds,
always observed with Karstedt’s and Speier’s catalyst. The
regioselectivity greatly increases the uniformity of the hyper-
branched polymers synthesized with the Pt-NHC catalyst.
The polymers were quantitatively functionalized in an additional
hydrosilylation step or by hydroboration of the remaining double
bonds. This complete derivatization is a consequence of the
uniformity of the polymer, as only the terminal double bonds
can easily be addressed for functionalization. The Pt-NHC
catalyst thus not only yields polymers with higher mole-
cular weights but also gives the opportunity for quantitative
functionalization. The homogeneity of the polymers and the
straightforward introduction of functional groups make these
hyperbranched polycarbosilanes an easily accessible mimic of
dendrimers which would require a tedious multistep synthesis.
Figure 5. 1H NMR spectra of functionalized hyperbranched polymers.
(a) after hydrosilylation and (b) after hydroboration and oxidation (300
MHz, CDCl3, 300 K).
weight arises principally for the following two reasons. The
insertion of the olefin into the Pt-H bond at the active center
of 4 occurs regioselectively, which leads to no isomerization. As
all internal double bonds are unavailable for further polymer
growth, isomerization can be regarded as a termination reaction
of the hydrosilylation polymerization. A selective catalyst sup-
pressing isomerization should therefore lead to higher molecular
weights. Second the lower activity of the Pt-NHC complex
compared to Karstedt’s catalyst results in a slower but more
uniform growth of the polymer and therefore in smaller PDIs.
In an additional step the obtained polymer was functionalized
in two different ways. Because of its homogeneous microstructure
the polymer obtained with the Pt-NHC catalyst is suited
particularly well for functionalization because all remaining
double bonds are equally reactive. The polymer gained, e.g., with
Karstedt’s catalyst on the other hand partly consists of iso-
merized internal double bonds that are less reactive and thus
harder to functionalize. As a first method for introducing func-
tional groups to the polymer we performed an additional hydro-
silylation step. A functional silane, chloromethyldimethylsilane,
was hydrosilylated to the remaining double bonds of the polymer.
Acknowledgment. The authors thank WACKER Chemie
AG for financial support and especially Dr. Haller and Mr. Hahn
for measuring 500 MHz 29Si NMR spectra.
Supporting Information Available: Figures showing NMR
spectra of monomers and polymers. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
References and Notes
The success of this functionalization can be seen from the H
NMR spectrum of the polymer (Figure 5a). The signals of the
double bond at 5.9 and 5.0 ppm have completely disappeared and
new signals originating from the functionalization emerged
(signals a-f in Figure 5a).
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