Soldier-Sergeant-Soldier triblock copolymers: Revealing the folded structure of single-chain polymeric nanoparticles

Article abstract of DOI:10.1039/c4cc02789b

Soldiers-Sergeant-Soldiers experiments performed on single-chain polymeric nanoparticles (SCPNs) with an ABA-type triblock architecture carrying chiral and achiral benzene-1,3,5-tricarboxamides (BTAs) in different blocks reveal that the BTAs form segregated, multiple stacks in a single SCPN. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02789b

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‘‘Soldier–Sergeant–Soldier’’ triblock copolymers:
revealing the folded structure of single-chain
polymeric nanoparticles†
Cite this: Chem. Commun., 2014,
50, 7990
Received 15th April 2014,
Accepted 3rd June 2014
Nobuhiko Hosono, Anja R. A. Palmans* and E. W. Meijer*
DOI: 10.1039/c4cc02789b
www.rsc.org/chemcomm
‘‘Soldiers–Sergeant–Soldiers’’ experiments performed on single-chain
polymeric nanoparticles (SCPNs) with an ABA-type triblock architec-
ture carrying chiral and achiral benzene-1,3,5-tricarboxamides (BTAs)
in different blocks reveal that the BTAs form segregated, multiple
stacks in a single SCPN.
Single-chain polymeric nanoparticles (SCPNs), which are synthetic
polymers capable of folding into nanoparticles by covalent/
supramolecular interactions, attract considerable interest to
mimic biomaterials.1–3 We have recently developed SCPNs
consisting of a synthetic polymer chain with complementary
hydrogen-bonding side groups that can induce a large chain
collapse through intramolecular self-assembly providing
dynamic crosslinking.4,5
Our investigations on SCPNs predominantly focused on systems
with ureidopyrimidinone (UPy)4 and/or benzene-1,3,5-tricarboxy-
amide (BTA)5 as the supramolecular crosslinking units. BTA is
a well-explored discotic molecule that forms a helical aggregate,
stabilized by threefold, lateral hydrogen bonding.6 The folding
behaviour of BTA-based SCPNs has been investigated extensively
by our group for facile construction of nanometer-sized objects
(Fig. 1a), with the ultimate aim to create artificial enzymatic
systems.5b–d Since a better understanding of the folding process
is crucial for tailoring the structure and functions of SCPNs, a
quantitative estimation of the extent of folding was attempted by
Fig. 1 Schematic representations and chemical structures of BTA-based
SCPNs. (a) Pn[B*B*B*] and (b) Pn[BB*B] (n = 1, 2, and 3).
means of model analysis of temperature-dependent circular dichro- ellipsoidal shapes with an unexpectedly high aspect ratio.5f
ism (CD) spectra.5d However, gathering direct experimental evidence Increasing the degree of polymerization of the SCPN resulted in
on the internal folding structure still remains challenging since we an increase of the aspect ratio while the cross-sectional radius
are neither able to quantify the length of BTA stacks formed in remained constant. As a result, we concluded that the BTA moieties
SCPNs nor to distinguish between single and multi-domains present self-assemble into separate, individual stacks providing multiple,
in the folded core. Using a combination of scattering methods, folded domains alongside the backbone polymer.5h
we recently realized that water-soluble BTA-based SCPNs adopt
In SCPNs designed for organic solvents, where the hydrophobic
effect is absent, it remained unclear how BTA aggregation occurs
and how it affects the global conformation of the polymers.5a
Here we report intramolecular ‘‘Soldiers–Sergeant–Soldiers’’ (S–S–S)
experiments using SCPNs with triblock architectures as a novel
tool to elucidate the folding behavior of SCPNs in organic media.
To investigate the extent of supramolecular self-assembly, the
Institute for Complex Molecular Systems (ICMS), Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
E-mail: a.palmans@tue.nl, e.w.meijer@tue.nl; Fax: +31 402-451-036
† Electronic supplementary information (ESI) available: Synthesis and character-
izations of the polymers. See DOI: 10.1039/c4cc02789b
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S–S–S experiment is a powerful tool that has been applied to a
variety of self-assembling motifs including BTAs. In ‘‘free’’ BTA
systems (not bound on a polymer chain), it is known that only
5 mol% of Sergeant BTA, which possesses chiral side chains at
the periphery, induces full chiral amplification of the achiral
BTA stacks.6a This indicates that one chiral BTA imposes
its preferred handedness on twenty achiral BTAs on average.
Thus, in a SCPN the S–S–S experiment provides experimental
evidence for the possibility of the chiral sergeant transferring
its helical preference to the achiral soldiers.
Fig. 2 Envisioned folding structures of Pn[BB*B] S–S–S triblock copolymers
with chiral BTA (red) and achiral BTA (green). SCPN with (a) a segregated and
(b) an intercalated core, and (c) separated multiple cores.
The chemical structures of S–S–S folding triblock copolymers
are depicted in Fig. 1b. The ABA-type triblock copolymers were
designed to be so-called ‘‘Soldier–Sergeant–Soldier’’ architec-
tures. Sergeants of chiral BTA (B*) and Soldiers of achiral BTA
(B) are separately incorporated into the middle (B) and both
end (A) blocks, respectively. The subscripts B and * denote BTA
and the presence of a chiral center in the peripheral side
chains, respectively. We synthesized the S–S–S triblock copoly-
mers with three different molecular weights ranging from 32
to 120 kg molÀ1, denoted as Pn[BB*B] (n = 1, 2, and 3). The
triblock architectures were first realized by two-step atom-
transfer radical polymerization (ATRP) in the presence of the
Cu(I)Br/N-butyl-2-pyridylmethanimine catalyst with isobornyl
methacrylate and 10 mol% of post-functionalizable comonomers
(propargyl methacrylate for the middle block, and hydroxyethyl
methacrylate for both end blocks). The backbone triblock copolymers
were functionalized through two consecutive Cu(I)-catalyzed azide-
alkyne cycloaddition (CuAAC) for B* and ester conjugation reactions
for B.†5e Sergeant fractions, fB*, are around 37% for all S–S–S
copolymers. For comparison, triblock copolymers with all chiral
BTAs, denoted as Pn[B*B*B*] (n = 1, 2, and 3), were prepared in an
identical manner starting with the identical backbone polymers,
but now fB* is 100% (Fig. 1a). The characterization data for all
triblock copolymers are given in Table 1.
cores (Fig. 2a and b). In the latter case, the most likely option is
an internal structure that consists of multiple cores, in which
communication between B* and B is restricted in the individual
stacks resulting in no or poor chiral amplification (Fig. 2c).
Temperature-dependent CD measurements using 1,2-dichloro-
ethane (DCE) as a solvent were applied to Pn[BB*B] and Pn[B*B*B*]
at a fixed BTA concentration of cBTA = 50 mM (Fig. 3). The polymer
solutions were heated up to 80 1C, then slowly cooled down to 20 1C
at the rate of 1 1C minÀ1 to ensure that BTA self-assembly is under
thermodynamic control. The CD signal intensity was monitored
at 225 nm. Above 60 1C, all of the polymers were CD-silent,
indicating that the polymers were molecularly dissolved
and adopted unfolded conformations (Fig. 3a, c and e). Upon
cooling, a negative Cotton effect was observed for all of the
polymers, indicative of the preference for left-handed helical
A number of possibilities can be distinguished with respect to
how the chiral amplification behavior takes place in the single
polymer chain. If full chiral amplification occurs, we expect to
observe an identical CD signal intensity between Pn[BB*B] and
Pn[B*B*B*] at a fixed BTA concentration. If not, a smaller CD
signal of Pn[BB*B] compared to that of Pn[B*B*B*] indicates that
the formation of only a single, long BTA stack in the SCPN is no
longer realistic, neither with segregated nor with intercalated
Table 1 Data of triblock copolymers
nBTA in each blocka
[A–B–A]
Total nBTA
chain
/
Mn/
fB* (%) kg molÀ1 (Ð)c
b
Polymer
P1[BB*B]
P2[BB*B]
P3[BB*B]
P1[B*B*B*] [3.2B*–3.8B*–3.2B*
P2[B*B*B*] [5.7B*–6.3B*–5.7B*
[3.2B–3.8B*–3.2B]
[5.7B–6.3B*–5.7B]
[14.0B–16.6B*–14.0B] 44.6
10.2
17.7
37
36
37
100
100
100
31.0 (1.20)
53.0 (1.42)
120.8 (1.56)
32.5 (1.19)
54.8 (1.43)
119.8 (1.70)
]
]
10.2
17.7
44.6
P3[B*B*B*] [14.0B*–16.6B*–14.0B*
]
a
Number of chiral (B*) and achiral BTA (B) groups determined by
1H NMR with reference to the corresponding numbers determined
previously (Fig. S1–S3, ESI).5e b Sergeant fraction: nB*/(nB + nB*). Deter-
c
Fig. 3 CD cooling curves (a, c and e) and spectra at 20 1C (b, d and f) for
Pn[BB*B] (green) and Pn[B*B*B*] (red) (n = 1,2, and 3; cBTA = 50 mM in DCE;
cooling rate: 1 1C minÀ1). (a, b) P1 (c, d) P2, and (e, f) P3 series of polymers.
mined by SEC using THF as the eluent, calibrated using polystyrene
standards (Fig. S4, ESI). Ð = Mw/Mn.
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incorporation numbers of B, numbered as P3[–B*–] ( fB* 100%),
P3[BB*B]a ( fB* 72%), P3[BB*B]b ( fB* 66%), P3[BB*B]c ( fB* 62%),
and P3[BB*B]d ( fB* 45%), where – denotes the absence of any B
unit in the end blocks.† The incorporation number of B* is 16.6
for all polymers, and of B is 6.4, 8.6, 10.4, and 20.2, respectively.
The CD intensity at 225 nm measured at 20 1C after cooling from
80 1C in DCE (cBTA = 50 mM) was plotted for all above polymers as
a function of fB* (Fig. 4a). CD spectra and cooling curves of all
these polymers are given in Fig. S5.† The plot showed a peculiar
non-linear dependence on fB*. In particular, P3[BB*B] (orange
marker) with the highest incorporation density of B but the
lowest fB* of 37% showed a stronger signal and exhibited the
most pronounced double-peak CD spectrum in this polymer
series.† This fact could suggest that the inter-block communica-
tion starts taking place at a higher incorporation density of B.
Taking all observations into account, we propose a folded
structure of BTA-based SCPNs that consists of multiple stacked
BTA aggregates (Fig. 4b). As a direct result, the organosoluble
copolymers are likely to adopt an ellipsoidal, elongated shape,
similar to their watersoluble counterparts.5f,h The CD cooling
Fig. 4 (a) Variable B experiment on the P3 series of S–S–S polymers. (cBTA
50 mM in DCE). P3[–B*–] (red), P3[BB*B]a (blue), P3[BB*B]b (light blue),
=
P3[BB*B]c (green), P3[BB*B]d (yellow), and P3[BB*B] (orange). An imaginary curves of P1–P3 are almost identical, indicating that the
zero-CD point was plotted using a black marker. (b) The envisioned folding
length of the polymers does not affect the melting curves. This
structure of BTA-based S–S–S SCPNs.
observation implies a lack of strong cooperativity in the folding
of these polymers.5h
In conclusion, we have demonstrated ‘‘Sergeant-and-Soldiers’’
columnar aggregates inside the polymer, which is in accor- experiments in the BTA-based SCPNs with a ‘‘Soldier–Sergeant–
dance with our previous reports.6 The temperature at which the Soldier’’ triblock architecture. The results revealed that BTA units
CD signal starts to appear, Te (elongation temperature), was incorporated into the polymer chain tend to form segregated,
60 1C for all polymers, Pn[B*B*B*] and Pn[BB*B]. This confirms multiple stacks in the SCPNs with some mixing of the BTA units
that the same incorporation density of BTAs (10 mol%), either of the different blocks.
B or B*, was achieved in both polymers, since Te is dependent
N.H. is thankful to the Japan Society for the Promotion of
Science (JSPS) for a Young Scientist Fellowship. This work was
on the local concentration of BTA units.5a
Evaluation of the CD cooling curves shows that the CD financially supported by the European Research Council (ERC),
signal intensities of all S–S–S polymers Pn[BB*B] never reached the Dutch Science Foundation (NWO) and the Dutch Ministry of
the ones of Pn[B*B*B*], indicating that the chiral amplification Education, Culture and Science (Gravity program 024.001.035).
is not perfectly operative within the triblock architectures, ICMS animation studio is acknowledged for the artwork.
despite the fact that the local Sergeant fraction, fB*, is high
(36–37%) and well above the required 5% for free BTAs. The
magnitude of the CD effects at room temperature, on the other
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