Efficient Preparation of Cyclic Poly(methyl
acrylate)-block-poly(styrene) by Combination of
Atom Transfer Radical Polymerization and Click
Cyclization
Dawanne M. Eugene and Scott M. Grayson*
Department of Chemistry, Tulane UniVersity,
New Orleans, Louisiana 70118
ReceiVed April 28, 2008
ReVised Manuscript ReceiVed June 7, 2008
Introduction. Diblock copolymers demonstrate a unique
ability to self-assemble into complex nanoscale morphologies
resulting from the phase segregation of contrasting blocks.
Extensive research has been devoted to studying how linear
diblocks segregate in bulk to yield a diverse range of nanoscale
symmetries,
1,2
including lamellae, hexagonally packed cylinders,
bicontinuous gyroids, and body-centered-cubic arrays of spheres.
Diblock copolymers also demonstrate the ability to self-assemble
in solution to yield micellar aggregates which can be used to
encapsulate guests and can be stabilized via cross-links to afford
more robust nanoscale carriers.
3
With the development of
synthetic techniques to prepare more architecturally complex
block copolymers including star block polymers,
4
ABC triblocks,
5–7
ABCD tetrablocks,
8,9
and miktoarm stars,
10–12
the relationship
between the block copolymer architecture and the resultant self-
assembly is being explored. In the case of triblock copolymers
the addition of the third block leads to a vastly increased set of
complex morphologies.
13
Cyclic block copolymers belong to a
unique polymer topological that is expected to exhibit charac-
teristic changes in both solution and bulk properties.
14,15
However, exploration of these materials has been very lim-
ited,
16,17
largely because of the technical difficulties in preparing
and purifying well-defined cyclic block copolymers. Recently,
cyclic block copolymers have been prepared using the 60Co
γ-ray-induced polymerization from cyclic dithioester initiators,
though this technique has not seen widespread use.
18
The most
common synthetic route has been the end-group coupling of
telechelic ABA triblock copolymers prepared using anionic
polymerization, including poly(dimethylsiloxane)-block-poly-
(styrene)-block-poly(dimethylsiloxane),
16,19,20
poly(2-vinylpy-
ridine)-block-poly(styrene)-block-poly(2-vinylpyridine),
16
poly-
(styrene)-block-poly(butadiene)-block-poly(styrene),
17,21
and poly-
(styrene)-block-poly(isoprene)-block-poly(styrene),
22
which were
cyclized with difunctional coupling agents such as dichlorodim-
ethylsilane or di(bromomethyl)benzene. While anionic polym-
erization techniques can yield very narrow polydispersity (PDI)
linear precursors, the nonspecific coupling chemistry between
two bifunctional reagents results in significant quantities of linear
oligomers from multiple intermolecular coupling events, in
addition to unreacted linear precursors. The anionic preparation
of block copolymers with complementary amine and carboxylic
acid end groups
23
provides a more specific functional group
coupling via their dicyclohexylcarbodiimide (DCC) coupling
to form an amide linkage, but the low efficiency of the DCC
coupling requires extensive purification. Among the reported
anionic techniques for preparing cyclic diblocks, the isolation
of reasonably pure cyclics requires tedious and repetitive
fractionation to remove acyclic impurities. For physical studies
of block copolymer self-assembly as well as exploration of these
materials for biomedical applications, narrow polydispersity and
high cyclic purity are critical for providing meaningful and
reproducible data. Therefore, a more efficient route toward well-
defined cyclic block copolymers with diverse side chain
functional groups would be invaluable.
Herein, an alternative cyclization approach is investigated that
promises to provide access to a diverse range of high-purity
cyclic block copolymers. Laurent and Grayson
24
recently
demonstrated that the functional group tolerance, facile end-
group modification, and well-controlled polymerizations offered
by atom transfer radical polymerization (ATRP)
25
can be paired
with the Huisgen 1,3-dipolar cycloaddition “click” coupling
26–28
to yield high-purity cyclic polystyrene without excess quantities
of solvent or time-consuming fractionation methods to purify
the product. Because of the functional group tolerance of both
ATRP and the click reaction, this procedure can provide an
efficient route to prepare a broad range of homopolymers. In
addition, because ATRP has demonstrated high efficiency in
reinitiating macroinitiators,
29,30
this procedure should be equally
well-suited to preparing diverse cyclic block copolymers. The
application of this technique toward the preparation of well-
defined cyclic block copolymers of methyl acrylate and styrene
(Scheme 1) is reported below.
Results and Discussion. Using a previously reported alkyne
initiator,
31
1, methyl acrylate (MA) was polymerized in bulk
using Cu(I)Br catalyst with N,N,N′,N′′,N′′-pentamethyldiethyl-
enetriamine (PMDETA) ligand at 50 °C after a series of freeze-
pump-thaw cycles to remove oxygen from the reaction
environment. The resulting trimethylsilyl (TMS)-alkyne-
terminated poly(methyl acrylate) (TMS-CtC-PMA-Br) ho-
mopolymers 2a,cwere purified by washing their methylene
chloride solution with water, passing the organic layer through
Scheme 1
.
Polymerization and Cyclization of Poly(methyl
acrylate (MA))-b-poly(styrene (S)): (a) n≈26, m≈38; (b) n≈
26, m≈56; (c) n≈30, m≈23
a
a
The degree of polymerization for PMA and PS blocks was
calculated from the average of the number-average molecular weight
(Mn) values measured by gel permeation chromatography (GPC) and
matrix-assisted laser desorption time-of-flight mass spectrometry
(MALDI-TOF MS).
5082 Macromolecules 2008,41, 5082-5084
10.1021/ma800962z CCC: $40.75 2008 American Chemical Society
Published on Web 06/25/2008
