Synthesis and lithographic patterning of polycarbosilanes with pendant cobalt carbonyl clusters

Article abstract of DOI:10.1021/ma048368u

The synthesis of highly metallized (25 wt% Co) polycarbosilane (PCS) by ring opening polymerization of an acetylide substituted 1-silacyclobutane followed by clusterization with [Co₂(CO)₈] was discussed. The acetylide-substituted sil

Full text of DOI:10.1021/ma048368u

Volume 38, Number 6
March 22, 2005
© Copyright 2005 by the American Chemical Society
Communications to the Editor
carbosilane, demonstrate a convenient method for mo-
Synthesis and Lithographic Patterning of
Polycarbosilanes with Pendant Cobalt
Carbonyl Clusters
lecular weight control, and illustrate the application of
this material as a direct-write resist for electron beam
lithography (EBL) to create metal-rich microscopic
features with potential preceramic and catalytic ap-
plications.
Sharonna Greenberg, Scott B. Clendenning,
Kun Liu, and Ian Manners*
To prepare cobalt-clusterized polycarbosilanes, we
attempted a similar strategy to that used to prepare
polyferrocenylsilane analogues (Co-PFS),17,18 which
function as processable precursors to magnetic ceramics
containing Fe/Co alloy nanoparticles. This involved
clusterization of pendent acetylide groups on a polymer
precursor.19,20
Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, Ontario M5S 3H6, Canada
Ste´phane Aouba and Harry E. Ruda
Centre for Advanced Nanotechnology, University of Toronto,
Toronto, Ontario M5S 3E4, Canada
Received August 8, 2004
The acetylide-substituted silacyclobutane 2 was pre-
pared by the reaction of Li[CtCPh] with the com-
mercially available 1-chloro-1-methylsilacyclobutane (1),
which resulted in selective substitution of the Cl atom
for a phenylacetylene group (Scheme 1). A very slight
excess of the silane was used to avoid anionic initiation
of ring-opening polymerization by the lithium acetylide.
Upon workup, the desired product (2) was obtained in
good yield (64%) as a slightly yellow, highly viscous oil.
The 1H, 13C{1H}, and 29Si{1H} NMR spectra of 2 were
consistent with the assigned structure. For example, the
13C{1H} NMR spectrum showed resonances at 108.3 and
93.2 ppm, attributed to the acetylenic carbon atoms
bound to the phenyl group and the silicon atom,
respectively. This assignment was made in accord with
the analogous acetylide-substituted [1]silaferroceno-
phane.21 The methylene carbon atoms on the silacyclo-
butyl ring were detected at δ ) 19.2 and 16.1 ppm (cf.
δ ) 21.1 and 16.4 ppm for 1). A single resonance was
observed in the 29Si NMR spectrum at a chemical shift
of δ ) -6.8 ppm, shifted upfield compared to 1 (δ ) 33.2
ppm).
Revised Manuscript Received December 16, 2004
Polycarbosilanes (PCS)1-12 represent structural hy-
brids between polysilanes and polyolefins which have
found applications as lithographic resists,2 liquid crys-
tals,3,4 and pyrolytic precursors to silicon carbide-based
ceramic fibers and monoliths.1,5 Ring-opening polymer-
ization (ROP) provides the most convenient route to
these materials: poly(silylenemethylene)s [RR′SiCH2]n,
poly(silylenepropylene)s [RR′SiCH2CH2CH2]n and other
related polycarbosilanes can be synthesized by thermal
or transition-metal-catalyzed ROP of 1,3-disilacyclo-
butanes or 1-silacyclobutanes, respectively.6,9,10,12b An-
ionic ROP routes can also be used for 1-silacyclobutanes
to afford moderate molecular weight polymers with
narrow polydispersities.7,8,11,12a The introduction of metal
atoms into polymers is of interest as a means of creating
functional macromolecular and supramolecular materi-
als which combine processability with interesting redox,
optical, electronic, preceramic, and catalytic proper-
ties.13-16 However, relatively few examples of metal-
functionalized linear polycarbosilanes have been re-
ported.1 In this communication we report the synthesis
and characterization of a novel cobalt-clusterized poly-
Treatment of 1-silacyclobutanes with a transition-
metal catalyst (e.g., Pt or Rh complexes) has been shown
to afford high molecular weight polycarbosilanes.1,22
Ring-opening polymerization of 2 in the presence of
Karstedt’s catalyst (Pt-divinyltetramethyldisiloxane
complex in xylenes) occurred after 6 h at 60 °C in
* To whom correspondence should be addressed: e-mail
imanners@chem.utoronto.ca; Ph (416)-978-6157; Fax (416)-978-
6157.
10.1021/ma048368u CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/17/2005

2024 Communications to the Editor
Macromolecules, Vol. 38, No. 6, 2005
Scheme 1. Synthesis of the Acetylide-Substituted
Silacyclobutane 2 and Its ROP To Afford the
Polycarbosilane (PCS) 3 Which Yields Co-PCS 4
Following Clusterization with Dicobalt Octacarbonyl
ppm for 2). The molecular weight of 3, as measured by
GPC, was determined to be in the range 3.9 × 104 <
Mn < 6.8 × 104 over several trials. The polydispersities
(PDI) and yields for these polymers were in the ranges
of 1.8-2.0 and ca. 50-60%, respectively.
To provide a means of conveniently controlling the
molecular weight of 3, we adapted a method first used
to control the chain length of low molecular weight
oligocarbosilanes (Mn < 4800)9 and, in our group, high
molecular weight polyferrocenylsilanes23,24 (5000 < Mn
< 50000) which involves the addition of Si-H functional
silanes such as Et3SiH to the metal-catalyzed polym-
erization mixture. It is believed that the Si-H bond
competes with the strained monomer for oxidative
addition at the catalytic metal center and subsequent
reductive eliminations yield end-functionalized materi-
als (e.g., with Et3Si and Si-H end groups).23,25 The
addition of Et3SiH to the ROP of 2 in the presence of
Karstedt’s catalyst (toluene, 6 h, 60 °C) indeed permit-
ted excellent molecular weight control over the range
of 5.0 × 102 < Mn < 5.8 × 104, as determined by GPC
(Figure 1). The isolated yields of end-capped PCS 3 were
lower (ca. 40%) for ratios of 2:Et3SiH ) 2:1 and 10:1
but were fairly consistent around ca. 60% for larger
ratios of 2:Et3SiH.
Apart from the main chain Si atoms, four 29Si NMR
resonances were observed for end-capped 3 at δ ) -18.9,
-6.2, 1.7, and 6.5 ppm, which were assigned to the
silicon atoms in the Si-H, Si-OH, Si-OMe, and
Et3Si-C end groups, respectively. Presumably, the Si-
OMe end group resulted from the methanolysis of the
initial Si-H end group upon precipitation of the polymer
into methanol. The presence Si-OH group could be
attributed to hydrolysis of the initial Si-H end group
toluene to afford PCS 3, as an off-white gummy material
following precipitation into methanol (Scheme 1).
The 1H, 13C{1H}, and 29Si{1H} spectra of 3 were
consistent with the assigned structure. Only two reso-
nances corresponding to the methylene protons were
observed at δ ) ca. 1.95 and 1.0 ppm, instead of four
resonances observed for 2, supporting a linear chain
structure. These peaks were shifted upfield compared
to 2, which may be attributed to the steric or electronic
effects of ring opening. In the 13C{1H} NMR spectrum,
resonances assigned to the acetylenic carbon atoms were
observed at δ ) 107.6 and 93.7 ppm, which were very
similar to the corresponding monomer 2. The methylene
carbon atoms bound to silicon exhibited a significant
downfield shift compared to 2, with a resonance at δ )
19.9 ppm (cf. 16.1 ppm for 2). The 29Si{1H} NMR
spectrum displayed a peak at δ ) -14.8 ppm (cf. -6.8
1
and/or the Si-OMe group. In the H NMR spectrum,
the methyl protons of the Et3Si-C end group resonated
at δ ) 1.0 ppm, but the methylene protons of the
Et3Si-C end group could not be observed, presumably
because they overlap with the relatively broad peaks of
the polymer backbone. Two weak singlets at δ ) 3.00
and 3.02 ppm were ascribed to the two possible dia-
stereotopic environments for the Si-OMe end groups.
To clusterize PCS 3, the polymer was reacted with
excess dicobalt octacarbonyl [Co2(CO)8]. This resulted
in the generation of polymer 4, by selective clusteriza-
Figure 1. Plot of Mn vs mole ratio of 2:Et3SiH, demonstrating molecular weight control for the synthesis of PCS 3.

Macromolecules, Vol. 38, No. 6, 2005
Communications to the Editor 2025
tion of the triple bond with a dicobalt hexcarbonyl
moiety (Scheme 1). A powdery, brown, air- and moisture-
stable polymer was obtained after precipitation from
THF into methanol in 67% yield.
The Co-PCS 4 was characterized by 1H, 13C{1H}, and
29Si NMR spectroscopy. The resonances for 4 were
shifted downfield compared to those for 3. The 1H NMR
and 13C{1H} NMR spectra of 4 showed broadened peaks;
however, the observed resonances were consistent with
1
the assigned structure. In the H NMR spectrum, the
broad peaks tended to overlap with regions at which
resonances for 3 are located, and as a result, it was
difficult to estimate the degree of clusterization of 4.
Nonetheless, the region from 1.9 to 2.0 ppm, where some
of the methylene protons of 3 are expected, did not
display any peaks, suggesting that clusterization with
dicobalt hexacarbonyl was quantitative. The resonances
at δ ) 1.55 and 1.05 ppm were assigned to the protons
of the methylene groups. In the 13C{1H} NMR spectrum,
the carbonyl groups were detected at δ ) 200.9 ppm,
while the acetylenic carbon atoms were observed at δ
) 107.6 and 78.9 ppm. Peaks corresponding to the
methylene carbon atoms were located at δ ) 21.8 and
19.9 ppm. Solid-state 29Si NMR spectroscopy revealed
a broad singlet for 4 at δ ) 1.3 ppm.
Three stretches corresponding to terminal carbonyl
groups were found in the IR spectrum of 4, located at
2086, 2049, and 2023 cm-1. In addition, the CtC stretch
at 2157 cm-1 present in the PCS precursor 3 was
notably absent in the IR spectrum of 4, suggesting that
clusterization was quantitative. We assume that clus-
terization of 3 proceeds without significant chain cleav-
age,26 and molecular weight analysis of 4 by GPC was
not attempted due to potential adsorption to the column
material. The Tg of polymer 4 was observed at 48 °C by
differential scanning calorimetry (DSC). This thermal
transition was fully reversible, indicating that the
transition did not correspond to the endothermic loss
of labile carbonyl ligands. This value is higher than for
the unclusterized polymer 3 (Tg ) -9 °C) as expected
for a material with bulkier substituents and therefore
lower skeletal flexibility. Thermogravimetric analysis
of 4 under an atmosphere of N2 to 1000 °C resulted in
the formation of a black ceramic material in 45%
yield.
Co-PCS (4) is an attractive lithographic resist for the
formation of patterned, functional, metal-containing
ceramics due to its excellent film-forming ability and
high concentration (25 wt %) of a transition metal
exhibiting bulk ferromagnetism. Direct-write patterning
of Co-PFS using EBL to give features rich in Fe and
Co was recently reported,27 and Co-PCS (4) should
allow access to highly metallized cobalt-rich features.
Co-PCS 4 was indeed found to operate as a negative
tone resist for EBL. This behavior may be the result of
chain cross-linking induced by homolytic cleavage of
σ-bonds and radical dimerization caused by exposure
to the electron beam.27 Regions of the film that were
exposed to the electron beam were firmly attached to
the substrate, while the unexposed material could be
washed away. Using a thin film (175 nm) of Co-PCS 4
as a resist, arrays of bars ca. 0.5 µm × 4.0 µm were
fabricated using a dose of 5.0 mC cm-2 followed by
development with THF. An optical micrograph of a large
array of bars is shown in Figure 2a. Sectional analysis
of an atomic force micrograph of the bars (Figure 2b)
revealed an average height of 175 ( 3 nm.
Figure 2. Microbars of Co-PCS (4) patterned by electron
beam lithography visualized by (A) optical microscopy and (B)
tapping mode AFM with sectional analysis.
In summary, we have synthesized a highly metallized
(25 wt % Co) polycarbosilane by ROP of an acetylide-
substituted 1-silacyclobutane, followed by clusterization
with [Co2(CO)8]. Furthermore, a convenient method for
molecular weight control using a silane capping reagent
has been thoroughly demonstrated, and this process is
likely to be transferable to other monomers, allowing a
general route for molecular weight control of high
molecular weight polycarbosilanes which function as a
desirable alternative to the painstaking anionic polym-
erization method. A preliminary study of materials
science applications of Co-PCS 4 has revealed its
potential use as a resist for EBL in the direct writing
of cobalt-rich patterned films. These offer access to
ordered arrays of magnetic ceramics containing cobalt
nanoparticles on pyrolysis or plasma treatment28 which
are expected to be harder magnets compared to those
obtained from iron or a mixture of iron and cobalt. These

2026 Communications to the Editor
Macromolecules, Vol. 38, No. 6, 2005
(13) Manners, I. Synthetic Metal-Containing Polymers; Wiley-
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nanostructures may find applications in spintronics as
an isolating magnetic layer in a nanogranular in-gap
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(19) For the use of alkyne clusterization to prepare organosilicon
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Acknowledgment. I.M. and H.E.R. thank NSERC
for an AGENO grant. I.M. thanks the Canadian govern-
ment for a Canadian Research Chair. S.B.C. acknowl-
edges NSERC for a PDF. H.E.R and S.A. thank NSERC,
MMO, CITO, ORDCF, and CIPI. We thank Dr. Tim
Burrow and Dr. Sara Bourke for assistance with the
HMBC pulse sequence, Dr. Andrew J. Baer for as-
sistance with solid-state NMR spectroscopy, and Patrick
Yang for assistance with AFM.
Supporting Information Available: Complete descrip-
tion of materials and equipment as well as experimental
details for the synthesis and characterization of 2-4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(20) For a recent report of block copolymers with pendent Co
clusters attached to one block using this strategy, see:
Mˆıinea, L. A.; Sessions, L. B.; Ericson, K.; Glueck, D. S.;
Grubbs, R. B. Macromolecules 2004, 37, 8967.
(21) Berenbaum, A.; Lough, A. J.; Manners, I. Organometallics
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MA048368U