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Micelle Formation from Amphiphilic УCylindrical BrushФЧCoil Block Copolymers Prepared by Metallocene Catalysis.

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Communications
Rod–Coil Copolymers
Micelle Formation from Amphiphilic “Cylindrical
Brush”—Coil Block Copolymers Prepared by
Metallocene Catalysis**
Michael W. Neiser, Sandra Muth, Ute Kolb,
J. Robin Harris, Jun Okuda,* and Manfred Schmidt*
Although the synthetic access to rod–coil block copolymers
has been known for a long time,[1] the recent discovery of their
self-organization into complex supramolecular assemblies has
initiated a number of publications on the synthesis and
properties of novel rod–coil block copolymers.[2]
So far, most of the block copolymers prepared exhibit
rather short rigid blocks consisting of either liquid-crystalline
segments or helices. Herein we present the first report of
micelle formation from novel giant rod–coil amphiphiles,
which consist of a stiff cylindrical brush connected to a linear
poly(methacrylic acid) chain.
[*] Dr. M. W. Neiser, S. Muth, Dr. U. Kolb, Prof. Dr. M. Schmidt
Institut fr Physikalische Chemie
Johannes Gutenberg Universit't
Jakob-Welder-Weg 11, 55128 Mainz (Germany)
Fax: (+ 49) 6131-392-2970
E-mail: mschmidt@mail.uni-mainz.de
Prof. Dr. J. Okuda
Institut fr Anorganische Chemie
RWTH Aachen
Prof.-Pirlet-Strasse 1, 52056 Aachen (Germany)
Fax: (+ 49) 241-809-2644
E-mail: jun.okuda@ac.rwth-aachen.de
J. R. Harris
Institut fr Zoologie
Johannes Gutenberg-Universit't
55099 Mainz (Germany)
[**] We thank the Fonds der Chemischen Industrie and the Deutsche
Forschungsgemeinschaft (SFB 625) for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3192
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200353259
Angew. Chem. Int. Ed. 2004, 43, 3192 –3195
Angewandte
Chemie
Regular comblike polymers with an extremely high
grafting density of the side chains adopt the shape of
cylindrical brushes, if the main chain is much longer than
the side chains. The observed stiffening of the normally
flexible main chain originates from the steric repulsion of the
densely grafted side chains. Cylindrical brushes may be
prepared by the polymerization of macromonomers,[3] by
the polymerization of the side chains from macroinitiators,[4]
or by grafting side chains onto functionalized linear chains.[5]
Each of these procedures has specific advantages and
disadvantages as described elsewhere.[6] Regularly branched
side chains (“dendrons”) have been also successfully utilized
for the preparation of cylindrical macromolecules.[7]
So far, the polymerization of macromonomers to much
higher degrees of polymerization for the main chain than the
side chain could only be achieved by free-radical polymerization of highly concentrated macromonomer solutions.[3] All
other attempts to achieve this goal by other methods,
particularly by living or controlled polymerizations such as
anionic,[8] radical,[9] ring opening,[10] or metallocene-catalyzed[11] polymerization failed in so far that the degree of
polymerization of the main chain hardly exceeded that of the
side chain. The resulting structures could be described as
spherical or star like.
Recently, we reported the synthesis of extremely highmolar-mass polymacromonomers by metallocene-catalyzed
polymerization.[6] The polydispersities of the samples were
relatively broad (typically 1.8 M̄w/M̄n 2; M̄w and M̄n are
the mass-average and number-average molar masses, respectively) and the degree of polymerization was not controllable.
Accordingly, we wondered whether living chain ends persisted during the course of the reaction. The synthesis of
cylindrical-brush–coil block copolymers would prove that the
metallocene-initiated polymerization of macromonomers is at
least partly controlled and proceeds by a living-polymerization mechanism.
The synthesis is shown in Scheme 1. Briefly, a concentrated solution of methacryloyl end-functionalized polystyrene macromonomer 1 (PS-MM; DPn = 18.3, DPw/DPn = 1.05;
DPw and DPn are the mass-average and number-average
degrees of polymerization; Scheme 2) was polymerized by the
Scheme 1. Synthesis of a cylindrical brush–coil block copolymer
3-OCs+.
Angew. Chem. Int. Ed. 2004, 43, 3192 –3195
Scheme 2. Chemical structure of the macromonomer 1 and of the
samarocene catalyst 2.
organosamarium(iii) catalyst 2 in THF for 2 h as described
previously.[6] After a small aliquot (500 mL) had been taken
from the reaction mixture, tert-butyl methacrylate (tBuMA)
was added. After two more hours the polymerization was
terminated by ethanol and the polymer 3-OtBu was precipitated. The sample was investigated by NMR spectroscopy,
gel-permeation chromatography (GPC), light scattering,
atomic force microscopy (AFM), and transmission electron
microscopy (TEM). Details of the synthesis and characterization procedures are given in the Supporting Information.
The AFM image of the copolymer 3-OtBu is shown in
Figure 1. Bright spots can be seen at the end of the cylindrical
brush chains, which most probably originate from the
collapsed p-tBuMA coil block (arrow 1). Furthermore,
homopolymer brushes can be seen (arrow 2), which clearly
did not initiate a linear coil block. Additional evidence for the
formation of block copolymers is given by NMR spectroscopy, which shows an increase in the ratio of the aliphatic to
aromatic protons from 0.8:1 to 0.87:1. From these numbers
the block-length ratio is calculated: DPn(PS-MM):DPn(ptBuMA) = 2.0. The degree of polymerization DPw of the
Figure 1. AFM picture of the cylindrical brush–coil copolymer 3-OtBu.
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3193
Communications
cylindrical brush block is estimated to be about 890 (see
below and Supporting Information). If an unreacted macromonomer fraction of 20 % and DPw/DPn 2.1 are taken into
account, the minimum degree of polymerization of the
tBuMA block DPmin
n (tBuMA) is estimated to be about 265
(see Supporting Information for details). DPmin
n (tBuMA)
represents a lower limit because it is assumed that all PSbrush chains are living and able to initiate a p-tBuMA block.
Unfortunately, there is no simple way to determine the
fraction of cylindrical brush homopolymer in the reaction
mixture, because the total molar-mass fraction of p-tBuMA in
the copolymer amounts to 4 % only. Static light scattering on
the copolymer brush in toluene yields M̄w = 2.0 D 106 g mol1
and a gyration radius hR2gi1=2
z = 53 nm, which represent mostly
the characteristics of the cylindrical brush block, because the
mass fraction and contrast (an increment in the refractive
index) of p-tBuMA in toluene are negligible. This value
compares well to that estimated by GPC and to the results
obtained from light scattering on a similar homopolymer
brush (see Supporting Information).
To unambiguously demonstrate that a block copolymer
was synthesized, the p-tBuMA block was hydrolyzed to the
polymethacrylic acid 3-OH by HCl in dioxane and neutralized
with CsOH to give 3-OCs+ (Scheme 1). The resulting
amphiphilic block copolymer 3-OCs+ in THF forms giant
spherical micelles as shown in Figure 2. The increased
Figure 2. TEM picture of a giant cylindrical brush–coil block copolymer
3-OCs+.
contrast at the core of the micelle can be ascribed to an
increased concentration of cesium counterions. Additional
pictures are presented in the Supporting Information, which
show that the size distribution of the micelles is unusually
broad. The micelle formation for such samples is quite
complex and has not been investigated in detail. In particular,
the formation of “twin” micelles and aging of the micellar
solutions over weeks leading to an enhanced contrast of the
3194
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
core provide insight into the peculiarities of the structures
presented.
We have reported herein on the synthesis of a novel high
molar-mass amphilic block copolymer, which consists of a
coiled and highly soluble stiff cylindrical brush block. The
block copolymers from core–shell micelles have a hydrophilic
flexible core and a hydrophobic shell comprising the stiff
block. In analogy to the known low-molar-mass rod–coil
block copolymers, an interesting variety of micellar structures
may be expected as a function of the composition of the block
copolymers, which will be investigated in the future.
Received: November 5, 2003
Revised: March 9, 2004 [Z53259]
.
Keywords: block copolymers · metallocene catalysts · micelles ·
rod-coil polymers
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