Synthesis and solution self-assembly of side-chain cobaltocenium-containing block copolymers

Article abstract of DOI:10.1021/ja1037726

The synthesis of side-chain cobaltocenium-containing block copolymers and their self-assembly in solution was studied. Highly pure monocarboxycobaltocenium was prepared and subsequently attached to side chains of poly(tert-butyl acrylate)-block-poly(2-hyd

Full text of DOI:10.1021/ja1037726

Published on Web 06/14/2010
Synthesis and Solution Self-Assembly of Side-Chain
Cobaltocenium-Containing Block Copolymers
Lixia Ren, Christopher G. Hardy, and Chuanbing Tang*
Department of Chemistry and Biochemistry and Nanocenter, UniVersity of South Carolina,
631 Sumter Street, Columbia, South Carolina 29208
Received May 10, 2010; E-mail: tang.c@chem.sc.edu
Among metallopolymers,1 metallocene-containing polymers have
Scheme 1. Synthesis of Monocarboxycobaltocenium and
Side-Chain Cobaltocenium-Containing Block Copolymers
attracted significant attention since they have great potential in
catalytic, optical, magnetic, and biological applications due to the
unique geometries and physicochemical properties of metallocenes.2
Compared to widely studied ferrocene and ferrocene polymers,1,2
cobaltocene has received far less attention, partly because of the
greater difficulty in preparing substituted derivatives.3 Cobaltocene
(19-e) has one more valence electron than ferrocene (18-e). The
ionization potential of cobaltocene is 5.56 eV,4 only slightly above
that of alkali metals, so cobaltocene can lose an electron readily to
form the cobaltocenium cation (18-e),5 isoelectronic with ferrocene.
Given the ease oxidation of cobaltocene and the great inertness of
cobaltocenium salts, it is extremely difficult to prepare substituted
derivatives from cobaltocene or cobaltocenium. Although a few
main-chain cobaltocenium polymers were synthesized via conden-
sation or ring-opening polymerization,6 there have been almost no
reports on the synthesis of side-chain cobaltocenium polymers.7
Due to their cationic nature, side-chain cobaltocenium polymers
exhibit interesting solubility behaviors. For example, we found that
poly(tert-butyl acrylate)-block-poly(2-hydroxyethyl acrylate) (PtBA-
b-PHEA-Br). Well-defined block copolymers PtBA-b-PHEA-Br
with polydispersity indexes < 1.2 were prepared by atom-transfer
radical polymerization (ATRP). A bromine-terminated poly(tert-
butyl acrylate) (PtBA-Br) macroinitiator was synthesized and then
chain-extended with 2-hydroxyethyl acrylate. With the same starting
PtBA-Br, different block lengths of PHEA or relative ratios of
PHEA and PtBA-Br were achieved by controlling the conversion
of HEA monomers during polymerization. In this study, we focus
on one well-defined block copolymer, PtBA80-b-PHEA38. The
esterification reaction of PtBA-b-PHEA-Br with cobaltocenium acyl
chloride 6 was performed with the aid of triethylamine in dimeth-
-
cobaltocenium acrylate polymers with PF6 as anions are soluble
in water and acetone but not soluble in less polar solvents such as
chloroform. Thus, the cationic features and unique solubility of
cobaltocenium-containing block copolymers may present interesting
solution self-assembly, which could lead to useful catalytic,
magnetic, and redox properties.
As the first step toward developing this class of organometallic
polymers, herein we report novel side-chain cobaltocenium-
containing block copolymers and their solution self-assembly,
starting with unambiguous evidence on the synthesis of highly pure
monocarboxycobaltocenium. As shown in Scheme 1, our synthetic
strategy is based on the preparation of a block copolymer substrate
followed by attachment of cobaltocenium to one of the blocks.
Synthesis of pure (∼100%) monocarboxycobaltocenium is a vital
step toward side-chain cobaltocenium polymers, as trace amounts
of coexisting 1,1′-disubstitued cobaltocenium could result in cross-
linked polymers. We modified a procedure reported by Sheats and
co-workers3a with exhaustive extraction and recrystallization to
remove any remaining 1,1′-dicarboxycobaltocenium. Nearly 100%
pure monocarboxycobaltocenium 4 with an overall yield of
14-20% was obtained. The structures and purity of compound 4
1
ylformamide. Figure 1 shows the H NMR spectra of PtBA-b-
PHEA-Br and resulting PtBA-b-poly(2-acryloyloxyethyl cobalto-
ceniumcarboxylate) (PtBA-b-PAECo-Br). Chemical shifts at 3.6 and
4.2 ppm corresponded to the ethyl protons of PtBA-b-PHEA-Br.
1
were confirmed by H and 13C NMR and MS spectra (Figures
S1-S5, Supporting Information (SI)), with three different charac-
teristic chemical shifts of cyclopentadienyl (Cp) protons in the 1H
1
NMR spectrum (Figure S3), while H NMR of dicarboxycobalto-
cenium 5 showed a different and characteristic A2B2 pattern of
Cp protons (two peaks, unresolved; Figure S3). The monocarboxy-
cobaltocenium 4 was readily converted into the relatively stable
cobaltocenium acyl chloride 6 under reflux of thionyl chloride.
The cobaltocenium moiety was then attached to side chains of
block copolymers through reaction between the acyl chloride 6 and
Figure 1. 1H NMR spectra of PtBA-b-PHEA-Br and PtBA-b-PAECo-Br
block copolymers in acetone-d6.
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10.1021/ja1037726  2010 American Chemical Society

C O M M U N I C A T I O N S
After esterification, characteristic chemical shifts at 5.8-6.4 ppm
corresponded to protons from Cp of the cobaltocenium units.
Moreover, ethyl protons from cobaltocenium-linked units shifted
to lower field around 4.4 ppm, indicating the successful attachment
of cobaltocenium to the side chains of block copolymers. However,
due to possible steric hindrance of bulky cobaltocenium units, the
esterification yield was about 70%, as a small peak at 3.6 ppm was
observed in the PtBA-b-PAECo-Br block copolymers, correspond-
ing to ethyl protons from unreacted PHEA block. Thermogravi-
metric analysis (Figure S10, SI) indicated that these block
copolymers have three stages of weight loss: 200-280, 300-500,
and 850 °C and above, respectively corresponding to decomposition
of tert-butyl group, complete degradation of polymer skeleton
derived from PtBA-b-PHEA-Br precursors, and decomposition of
Cp rings from cobaltocenium. A plateau between 500 and 850 °C
was observed and assigned to cobaltocenium salts, indicating high
thermal stability of cobaltocenium structures.
We then explored self-assembly behaviors of cobaltocenium-
containing block copolymers in solution. The micellization of PtBA-
b-PAECo-Br was first carried out by dissolving the block copoly-
mers in acetone and then adding water to induce micelle formation.
PAECo blocks are soluble in both acetone and water, while PtBA
is soluble only in acetone. Figure 2a shows the PtBA-b-PAECo-Br
block copolymer micellar aggregates in acetone and water mixture.
Transmission electron microscopy (TEM) images show that these
micellar aggregates exhibited vesicle morphology. These vesicles
are not uniform, with diameter ranging from 50 to 300 nm. In
unstained TEM images, we were only able to see the electron-rich
cobaltocenium-containing PAECo domains. Thus, we believe that
the dark rings are formed by the PAECo blocks. Size analyses from
dynamic light scattering (DLS) experiments (Figure 2b) indicated
that the hydrodynamic radius of vesicles falls into two major
populations, 50 ( 30 and 240 ( 120 nm, consistent with TEM
results.
indicating that the center is the cavity of the nanotubes while the
shell contains electron-rich PAECo domains. Direct comparison
of the same areas of samples between bright-field and dark-field
modes further indicated the formation of nanotubular structures
(Figure S14).
Figure 3. TEM micrographs of PtBA-b-PAECo-Br self-assembled ag-
gregates in the mixture of acetone and chloroform. Samples were viewed
(a) under bright-field mode and (b) under dark-field mode. The inset in
panel a is a proposed illustration of the self-assembled nanotubes. Scale
bar: 1000 nm.
In conclusion, we have synthesized highly pure monosubstituted
carboxycobaltocenium and subsequently prepared side-chain co-
baltocenium-containing block copolymers. These block copolymers
exhibited self-assembled vesicle and nanotube structures depending
on the solvent system used. Currently, effects of different anions
and hydrophobic blocks on the micellar morphology are under
investigation.
Acknowledgment. We acknowledge the University of South
Carolina for providing start-up funds. We thank Dr. John E. Sheats
for helpful discussion.
Supporting Information Available: Experimental details and
complete characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
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