Self-Complementary ABC Triblock Copolymers via Ring-Opening Metathesis Polymerization

Article abstract of DOI:10.1021/ma034683p

The synthesis of self-complementary triblock copolymers using controlled ring-opening metathesis polymerization was presented. It was shown that polymers possess ordered sequences of complementary molecular recognition blocks and could be synthesized efficiently. It was found that changing the triblock copolymer sequence resulted in materials with different self-assembly properties.

Full text of DOI:10.1021/ma034683p

Macromolecules 2003, 36, 7899-7902
7899
Self-Com p lem en ta r y ABC Tr iblock
Cop olym er s via Rin g-Op en in g Meta th esis
P olym er iza tion
shown in Scheme 1. 2-Acetylamino-6-aminopyridine was
prepared via the reaction of 2,6-diaminopyridine and
acetyl chloride and was then reacted with 6-bromo-
hexanoyl chloride to give 5. Monomer 4 was obtained
in good yield and high purity from the reaction of 5 with
monomer 2 and K2CO3 in DMF.
1
1
Ha ssa n S. Ba zzi, J ea n Bou ffa r d , a n d
Ha n a d i F . Sleim a n *
We have previously shown that the ROMP reaction
of 2 is living and that well-defined polymers and
copolymers can be readily generated. The ROMP reac-
Department of Chemistry, McGill University,
8
01 Sherbrooke Street West, Montreal,
5
Quebec, Canada H3A 2K6
tion of monomer 4 was examined in dichloromethane
Received May 22, 2003
Revised Manuscript Received September 3, 2003
7
at room temperature using Cl2(PCy3)2RudCHPh, 1
(
6,7
1
Scheme 1). Fast initiation was detected, and H NMR
One of the unique features of DNA is its ability to
associate with its complementary partner into a well-
defined secondary structure. This property, which is
showed that the monomer consumption was 95% after
30 min. The reaction was quenched with ethyl vinyl
ether and precipitated in hexanes. Gel permeation
chromatography (GPC) analysis of polymer 6 showed a
monomodal and narrow molecular weight distribution
(polydispersity index, PDI ) 1.11; see Table 1 of the
Supporting Information), indicative of a living polym-
1
unrivalled by synthetic macromolecules, is derived from
the placement of a well-defined sequence of hydrogen-
1
bonding units along a regioregular polymer backbone.
While DNA has been used as a highly specific building
2
12
1
block by a number of laboratories in materials science,
erization. Polymer 6 has a 67:33 trans:cis ratio ( H
6
,7
the challenges associated with its synthesis and stability
have impeded more general use of this molecule. The
construction of synthetic polymers which can mimic the
specificity of this biopolymer, but can be readily accessed
in large quantities, is thus an especially attractive goal.
One approach toward DNA-inspired synthetic poly-
mers is through the use of living polymerization to
generate block copolymers containing a well-defined
sequence of molecular recognition blocks. While poly-
mers containing molecular recognition units have been
previously reported, these were typically generated
using nonliving methods or by statistical attachment
of these units to a polymer backbone.3,4 We have
recently initiated a research program to examine the
synthesis of polymers containing nucleic acid bases or
their analogues by living ring-opening metathesis po-
NMR) and is soluble in common organic solvents but
shows some turbidity in non-hydrogen-bonding solvents,
such as dichloromethane, likely due to the self-associa-
tion of the diacetamidopyridine units in the polymer
1
0
13
backbone. The UV-vis spectrum of polymer 6 showed
absorption peaks similar to those of monomer 4, with a
slight bathochromic shift (6 nm).
Having established that monomers 2 and 4 undergo
well-behaved and controlled ROMP, we examined the
generation of an ABC-triblock copolymer (Scheme 2) by
the sequential addition of monomers 2 (block A), 3 (block
B), and 4 (block C). Blocks A and C are complementary
to each other through a three-point hydrogen-bonding
interaction between their diacetamidopyridine and di-
1
0
carboximide units (Scheme 2), and block B is relatively
hydrophobic, due to the alkyl chains in the repeat units.
The synthesis of the triblock copolymer started with
the addition of the Grubbs catalyst 1 to a solution of 2
5
lymerization (ROMP). The ROMP reaction is a highly
efficient method to generate polymers and copolymers
in a living fashion.6 We here report the synthesis of
the first ABC triblock copolymers containing specific
sequences of complementary molecular recognition blocks.
Importantly, different sequences of these copolymers
can be readily constructed by sequential polymerization
of the three monomers in different order, and these
polymers are nearly monodisperse. Finally, we report
the first example of a dramatic change in polymer
morphology which is brought about merely by changing
the sequence of the molecular recognition blocks in this
triblock copolymer.
,7
1
(10 equiv) in THF (5 mL). H NMR showed that
monomer consumption was complete after 5 min. Half
of the polymerization solution was taken out, quenched
with ethyl vinyl ether, precipitated into hexanes, and
dried (polymer 7). To the remaining half, monomer 3
(50 equiv in 2.5 mL of THF) was added.9 1H NMR
analysis showed complete conversion of 3 in 20 min.
Again, half of the solution was syringed out, quenched,
and precipitated into hexanes (diblock copolymer 8).
Finally, monomer 4 (10 equiv in 2.5 mL of CH Cl ) was
2
2
added to the polymerization solution. Monomer conver-
1
The ease of synthesis of the building blocks for our
polymers was of great importance in our approach.
Therefore, monomers 2-4 were used. exo-7-Oxabicyclo-
sion was complete in 30 min ( H NMR), after which the
reaction mixture was quenched and precipitated into
hexanes and the resulting ABC triblock copolymer 9 was
collected and dried.
[
2.2.1]hept-5-ene-2,3-dicarboximide, 28 (thymine ana-
logue, Chart 1), was readily synthesized by the Diels-
Alder reaction of furan and maleimide. The second,
hydrophobic monomer exo-N-butyl-7-oxabicyclo[2.2.1]-
hept-5-ene-2,3-dicarboximide, 3, was obtained by react-
ing 2 with 1-bromobutane and K2CO3 in DMF. Our third
monomer 4 contains an acetylated diaminopyridine
The synthesis and investigation of the rich morpho-
logical behavior of ABC triblock copolymers have been
the subject of increasing recent interest. However,
14
9
because of monomer crossover problems, there are very
few examples in the literature of the synthesis of
triblock copolymers containing different block se-
quences, merely by changing the order of addition of
(
DAP), which is complementary to the dicarboximide
moiety in 2, through a strong, three-point hydrogen-
14
monomers. In addition, there are very few previously
bonding interation.10 Monomer 4 was synthesized as
reported examples of the use of ROMP to access ABC
1
5
triblock copolymers. Because of the highly controlled
*
Corresponding author: e-mail hanadi.sleiman@mcgill.ca.
nature of the ROMP reaction of 2 and 4, we were thus
1
0.1021/ma034683p CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/17/2003

7
900 Communications to the Editor
Macromolecules, Vol. 36, No. 21, 2003
Ch a r t 1
Sch em e 1
Sch em e 2
intrigued by the possibility of synthesizing different
sequences of these triblock copolymers. Thus, we carried
out the sequential addition of monomers 3, 2, and 4 to
the Grubbs catalyst 1, using a similar procedure to the
above synthesis of triblock 9. Again, conversion of all
three monomers was quantitative ( H NMR), and BAC
triblock copolymer 12 was obtained in high yield (Scheme
polydispersity indexes (PDI ) 1.08-1.14).16 Figure 1
shows well-resolved GPC traces for both the ABC
triblock copolymer 9 and the BAC triblock copolymer
12. The absence of traces at higher retention time (lower
molecular weight) indicates that there was essentially
no chain termination during the sequential addition of
the monomers to the polymerization reaction. A slight
variation was detected in the observed molecular weights
1
2
).
Gel permeation chromatography data of polymers
-12 are shown in Figure 1. All GPC traces displayed
3
of triblock copolymers 9 and 12 (21.1 and 22.7 × 10 ),
6
although they have the same calculated molecular
weights. The different block sequence can account for
monomodal molecular weight distributions with low

Macromolecules, Vol. 36, No. 21, 2003
Communications to the Editor 7901
F igu r e 1. Gel permeation chromatography traces of the
triblock copolymers 9 and 12 and their precursors.
F igu r e 2. Transmission electron micrographs of copolymer
9
3
from a CHCl solution
this variation, thus likely resulting in different hydro-
dynamic volumes for the ABC and BAC polymers. The
compositions of the triblock copolymers were 10:50:10
for 9 (poly(2):poly(3):poly(4), respectively) and 50:10:10
for 12 (poly(3):poly(2):poly(4), respectively) as confirmed
by both NMR and GPC data. In addition, copolymers 9
and 12 showed UV/vis spectra similar to that of the
diaminopyridine monomer 4, with a slight bathochromic
shift similar to that of polymer 6.
of the same building blocks in different sequences. The
complementary dicarboximide and diacetamidopyridine
units in these copolymers can selectively associate via
1
8
a hydrogen-bond-mediated interaction (Chart 1). Ex-
amination of the self-assembly properties of 9 and 12
revealed dramatic differences. Dynamic light scattering
studies (DLS) of a solution of ABC copolymer 9 in
CHCl3, taken at multiple angles, showed the presence
of a bimodal distribution of small (20-40 nm) and large
(200-700 nm) aggregates. A solution of 9 in CHCl3 (1%
w/v) was evaporated on a carbon-coated copper grid and
examined by transmission electron microscopy (TEM).
Large aggregates of small spherical micelles (approxi-
mately 20 nm) were observed, in accordance with the
DLS results (Figure 2). On the other hand, no ag-
gregates were detected when solutions of BAC copoly-
mer 12 in CHCl3 were examined by DLS and TEM.
Thus, mere modification of the sequence of the blocks
in copolymers 9 and 12 results in a dramatic change in
polymer morphology. While we are examining the
mechanism of this self-assembly in more detail, this
preliminary study shows the potential of the ROMP
reaction with monomers such as 2, 3, and 4 to yield
novel triblock copolymers with deliberate variation of
sequence and self-assembly properties.
In summary, we have shown the synthesis of the first
self-complementary triblock copolymers by controlled
ring-opening metathesis polymerization. These copoly-
mers possess ordered sequences of complementary mo-
lecular recognition blocks and can be synthesized effi-
ciently with deliberate variation of this sequence.
Changing the triblock copolymer sequence results in
materials with dramatically different self-assembly
properties. These novel systems introduce a new pa-
Finally, we attempted the synthesis of a triblock
copolymer ACB, containing a third sequence of these
blocks by the sequential addition of monomers 2, 4, and
3
to catalyst 1 (Scheme 2). However, after the complete
consumption of monomer 2 and addition of monomer 4,
the solution became turbid, and the conversion of
monomer 4 stopped at 70%. Thus, although homopoly-
mers poly(2) and poly(4) are soluble in THF, their
solubility behavior changes dramatically when they are
covalently linked in copolymer 13. This is likely due to
the H-bond complementarity of the two molecular
recognition units in the adjacent blocks of 13. Despite
the incomplete monomer conversion in the second block,
1
9
1
monomer 3 was added to the solution. Interestingly, H
NMR showed that the polymerization had resumed, and
monomer 3 was fully consumed in 30 min. ACB triblock
1
7
copolymer 14 was thus also obtained, albeit less
efficiently than polymers 9 and 12. Because of its limited
solubility in THF, GPC data could not be obtained.
1
However, H NMR of 14 after its isolation showed the
presence of the three monomeric units (2:4:3) in a 10:
8
:50 ratio.
The synthesis of different sequences of the triblock
copolymers allowed us to address the question of the
effect of molecular recognition block sequence on self-
assembly properties. Copolymers 9 and 12 are composed

7
902 Communications to the Editor
Macromolecules, Vol. 36, No. 21, 2003
rameter to the study of the self-assembly of ABC triblock
copolymers, namely molecular recognition and comple-
mentarity between the different blocks. Further studies
on the interplay of molecular recognition and mi-
crophase separation in the self-assembly of these tri-
block copolymers are underway.
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(12) We performed a preliminary investigation of the living
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1
by H NMR (monomer:initiator 10:1). A linear dependence
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13) See Supporting Information.
Refer en ces a n d Notes
(
(
14) Cochran, E. W.; Morse, D. C.; Bates, F. S. Macromolecules
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MA034683P