Supramolecular Multiblock Copolymers Featuring Complex Secondary Structures

Article abstract of DOI:10.1021/jacs.7b06201

This contribution introduces main-chain supramolecular ABC and ABB′A block copolymers sustained by orthogonal metal coordination and hydrogen bonding between telechelic polymers that feature distinct secondary structure motifs. Controlled polymerization t

Full text of DOI:10.1021/jacs.7b06201

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Article
pubs.acs.org/JACS
Supramolecular Multiblock Copolymers Featuring Complex
Secondary Structures
Elizabeth Elacqua, Kylie B. Manning, Diane S. Lye, Scott K. Pomarico, Federica Morgia,
and Marcus Weck*
Department of Chemistry and Molecular Design Institute, New York University, New York, New York 10003, United States
S
* Supporting Information
ABSTRACT: This contribution introduces main-chain supra-
molecular ABC and ABB′A block copolymers sustained by
orthogonal metal coordination and hydrogen bonding between
telechelic polymers that feature distinct secondary structure
motifs. Controlled polymerization techniques in combination
with supramolecular assembly are used to engineer hetero-
telechelic π-sheets that undergo high-fidelity association with
both helical and coil-forming synthetic polymers. Our design
features multiple advances to achieve our targeted structures,
in particular, those emulating sheet-like structural aspects using
poly(p-phenylenevinylene)s (PPVs). To engineer hetero-
telechelic PPVs in a sheet-like design, we engineer an iterative
one-pot cross metathesis−ring-opening metathesis polymer-
ization (CM-ROMP) strategy that affords functionalized Grubbs-II initiators that subsequently polymerize a paracyclophane-
diene. Supramolecular assembly of two heterotelechelic PPVs is used to realize a parallel π-sheet, wherein further orthogonal
assembly with helical motifs is possible. We also construct an antiparallel π-sheet, wherein terminal PPV blocks are adjacent to a
flexible coil-like poly(norbornene) (PNB). The PNB is designed, through supramolecular chain collapse, to expose benzene and
perfluorobenzene motifs that promote a hairpin turn via charge-transfer-aided folding. We demonstrate that targeted helix−(π-
sheet)−helix and helix−(π-sheet)−coil assemblies occur without compromising intrinsic helicity, while both parallel and
antiparallel β-sheet-like structures are realized. Our main-chain orthogonal assembly approach allows the engineering of
multiblock copolymer scaffolds featuring diverse secondary structures via the directional assembly of telechelic building blocks.
The targeted assemblies, a mix of sequence-defined helix−sheet−coil and helix−sheet−helix architectures, are Nature-inspired
synthetic mimics that expose α/β and α+β protein classes via de novo design and cooperative assembly strategies.
collapse,5,16,19,20 which, although elegant in approach, is limited
in the complexity of polymer backbones that can be integrated
within the target scaffold. In this contribution, we exploit the
INTRODUCTION
■
The design of complex architecturally well-defined frameworks
reminiscent of Nature necessitates a delicate synergy between
controlled polymerization of structurally simple yet complex
covalent and noncovalent chemistries. Structures of biomole-
monomers to engineer polymers containing secondary
cules (e.g., enzymes and proteins) are orchestrated through an
structures such as helices, sheets, or random coils4,21 that are
array of cooperative and orthogonal noncovalently driven
capable of further assembly into localized tertiary structures.
processes that, as the result of hierarchical self-assembly, imbue
Our strategy takes these elements and, through high-fidelity
long-range order across multiple domains. The engineering of
well-defined structures reminiscent of Naturein a sequence-
orthogonal molecular recognition processes, engineers three-
specific and monodisperse manner1−3is currently beyond the
dimensional (3D) architectures from well-defined secondary-
structure-containing building blocks.
realization of synthetic chemists. Strategies that achieve well-
defined polymers capable of sustaining an orthogonal and
Natural designs feature a vast array of structural motifs,
arranged within discrete patterns that form tertiary structures
directional self-assembly, as well as rendering compartmental-
upon self-assembly into compact, localized, and quasi-
ized domains of individual helices, coils, and sheets within
synthetic frameworks, however, are substantive.4−18 Controlled
independent chains. Key structural facets include compartmen-
talized domains, intrastrand interactions, and binding pock-
polymerization routes allow for the fabrication of well-defined
polymers with a high degree of control over both molecular
weight and monomer placement, thus featuring the potential to
ets.5,20 Proteins and enzymes feature a large combination of α-
helices and β-sheets, along with lesser-organized flexible regions
sustain precision and sequence-specific materials.
To date, well-established routes to fabricate bio-inspired
Received: June 14, 2017
polymers are limited and rely mainly on single-chain polymer
© XXXX American Chemical Society
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Scheme 1. Schematic Representation of the Target Supramolecular Block Copolymers Comprising π-Sheets, Helices, and Coils
via a Plug-and-Play Strategy Utilizing Orthogonal Metal Coordination and Hydrogen Bonding
comprised of random coils, in addition to supersecondary
structures (e.g., β-hairpins and β-barrels). Bio-inspired synthetic
polymers have achieved the construction of β-turn mimics22 as
well as compartmentalized single-strand structures5,20 that
feature small-molecule self-assembly into a dynamic helical
pattern using side-chain-functionalized polymeric systems.11,23
Strategies that exploit the planarity of rigid backbones to drive
the formation of secondary structures into foldamers are also
prominent.24−30 Other approaches feature supramolecular
motifs that fold via high-fidelity molecular recognition
processes between polymer chains and/or pendant units,
affording helical31−35 and sheet-like oligomers36,37 and
polymers predicated upon single supramolecular interactions.
For example, poly(acrylate)s featuring helical benzene-1,3,5-
tricarboxamide and either self-associating 2-ureido-4[1H]-
pyrimidinone moieties13 or complementary cyanuric acid and
Hamilton wedge (HW) side chains can be assembled in an
orthogonal manner.23 We have recently introduced well-
defined secondary structures in main-chain supramolecular
block copolymers4 and observed that the secondary structures
remain intact and localized upon assembly both in solution and
in the solid state. While these studies yield synthetic foldamers,
multiple directional and orthogonal interactions must be
integrated to fully probe and comprehend the folding behavior
of more complex synthetic models.
Hierarchical designs analogous to protein structural classes
(e.g., α+β, and α/β proteins) require precisely sequencing
multiple structural motifs in a proximal and well-defined
manner.38 Analogous hierarchical architectures are not likely to
be realized in synthetic systems using solely pendant-chain
interactions to drive assembly. Such structures, however, can be
envisioned via de novo designs that feature secondary structure
elements intrinsic to the backbone of each building block. Our
strategy is to engineer a library of telechelic synthetic building
blocks that emulate natural self-assembled materials. We
hypothesize that such a library of well-defined synthetic
polymers will lead to unique biomimetic tertiary structures
through directed self-assembly. We disclose orthogonal supra-
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molecular assemblies that feature three or more well-defined
secondary structures that mirror proteinic α-helices, β-sheets,
and random coils via delicate manipulation of both covalent
and noncovalent bonds. The building blocks are obtained using
a combination of ring-opening metathesis polymerization
(ROMP), reversible addition−fragmentation chain-transfer
(RAFT) polymerization, anionic polymerization, and/or
atom-transfer radical polymerization (ATRP), affording helical-,
sheet-, and coil-based synthetic polymers.
A fundamental prerequisite for engineering tertiary structure
mimics lies in sequencing the structural elements of proteins
together with precision.38 Our strategy uses main-chain
supramolecular polymerization of secondary-structure-featuring
polymer chains to engineer sequence-specific tertiary motifs.
We achieve synthetic designs that feature α/β and α+β-like
structures via de novo supramolecular synthesis of our telechelic
building blocks (Scheme 1). Combinations of helical and sheet-
like domains comprise prominent binding sites within enzyme
folds; thus, investigation of synthetic α/β and α+β-like systems
that not only can be achieved in a straightforward manner but,
also feature unique intrinsic properties will provide for a
comprehensive understanding of how to achieve complexity
within synthetic designs while maintaining simple synthetic
building blocks.
(iii) spectroscopic methodologies that can be used to explicitly
investigate conjugated polymer folding motifs have recently
emerged.45,50,51
We envision that the construction of a sheet-like building
block featuring PPV π-strands would allow us to monitor
folding modes through fluorescence spectroscopy, as PPVs that
engage in persistent face-to-face π−π stacking would be
expected to exhibit fluorescence quenching relative to the
unassembled parent polymers. We utilize orthogonal assembly
approaches and ensuing cooperative effects to arrange hetero-
telechelic π-strands in both parallel and antiparallel sheet-like
arrangements (Scheme 2). Parallel strands were realized via
Scheme 2. Noncovalent and Covalent Strategies toward
(Left) Parallel and (Right) Antiparallel π-Stacked Sheets via
Successive CM-ROMP and Self-Assembly
RESEARCH DESIGN
■
The selection criteria for our building blocks include three
design requirements: (i) the propensity to form secondary
structures; (ii) control over dispersity and molecular weights;
and (iii) ease of functional end-group installment. We have
reported telechelic helix, sheet, and coil-like building blocks
using ROMP, RAFT, ATRP, and anionic polymerization, all of
which had a single functional end-group either through the use
of a functional initiator or terminating agent.4 Instrumental to
the design of more complex main-chain supramolecular
architectures featuring three or more unique and well-defined
secondary structure building blocks is the capacity to facilitate
multiple orthogonal assembly steps. While side-chain function-
alization routinely leads to pendant interactions via monomer
choice, only two well-defined dual-gated motifs have been
achieved to date.39 Judicious selection of monomers in concert
with initiator and terminator design can afford scaffolds
featuring both multiple functional building blocks and
compartmentalized domains by design. Multiple living polymer-
ization methods are reported that install molecular recognition
units with high fidelity.4,8,23,40 We select ROMP for the
preparation of the heterotelechelic building blocks, owing to
the ability to utilize functional initiators and terminators in
succession while avoiding post-polymerization functionalization
routes.
Our design for the heterotelechelic building block is through
the ROMP of a disubstituted paracyclophanediene (pCpd) in
the presence of a functional initiator, and subsequent
quenching with a functional chain terminator (CT). Essential
to this design is the development of a new functional initiator
based upon Grubbs II; we realize this through an iterative cross-
metathesis−ring-opening metathesis polymerization (CM-
ROMP) strategy that installs an α-chain end upon the PPV.
We choose PPV scaffolds for multiple reasons: (i) conjugated
polymer frameworks serve as hosts for a variety of applications,
ranging from electronics to sensing;41−43 (ii) homopolymers
and block copolymers featuring PPVs are well-studied in the
context of optical properties and molecular structures;44−49 and
metallosupramolecular assembly of complementary hetero-
telechelic PPVs. Antiparallel strands are achieved using
orthogonal supramolecular motifs. We rationalize that a
covalent rod−coil−rod copolymer, wherein the coil includes
a β-hairpin-like turn could promote π−π stacking of antiparallel
strands by design. The β-turn unit is engineered through
supramolecular charge-transfer complexation of phenyl and
perfluorophenyl units spatially separated within the coil block.
The ensuing folded block copolymers are then assembled
with complementary polymers using recognition events that are
orthogonal to the charge-transfer event. Complementary
polymers were rationally chosen that feature a high density of
pendant aromatic units that engage with the π-strands upon
assembly. We hypothesize that successful assemblies, thus,
would favor disruption of prominent PPV−PPV intrachain
interactions while enforcing a compartmentalized structure that
features either parallel or antiparallel π-sheets as a key
component.
Our two helical blocks are based upon poly(N-1-
(naphthalene-1-yl)ethyl)methacrylamide52 and poly(aryl iso-
cyanide) (PIC).53,54 We synthesize monotelechelic PMAc
through RAFT polymerization in the presence of a functional
chain-transfer agent (CTA), while telechelic PIC is prepared via
anionic polymerization with an alkynyl-palladium-function-
alized initiator.4,21 PMAc helices present a relatively more
dynamic backbone, wherein polymer tacticity and sterics dictate
helicity, as opposed to static PIC helices that demonstrate a
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high inversion barrier and feature a rigid backbone.55,56 Helical
PMAc can be influenced by its environment, while the overall
structure is dependent on intrinsic hydrogen bonding51 akin to
common biomolecular helices. Both static and dynamic helical
domains are prominent in Nature;57 thus, we envision that their
combined incorporation into bio-inspired synthetic designs will
engender investigations into the interplay of both helices,
including how they assemble cooperatively. Additionally,
interconversions between dynamic and static helices are well-
documented, owing to the responsive nature of dynamic
helices, and can be triggered by both soft confinement58 and
conformational constraints,59 as well as solvent and steric
effects.60,61
For our random coil building block, we select poly(styrene)
(PS). PS features the ability to install pendant side chains with
relative ease, which can impact folding properties in more
complex structures. Moreover, akin to sections of proteins
lacking a well-defined structure that feature weak amino-based
interactions between side chains, PS only organizes via weak
intermolecular π−π interactions.
We have reported on monotelechelic helical polymer systems
based upon PIC and PMAc,21,62,63 respectively that achieve
high-fidelity directed assembly (Ka ≅ 103−104 M−1) and retain
their individual helicity during the assembly event. In addition,
we have identified multiple homotelechelic coil-forming blocks
including PS that can serve as a random coil mimic.
Furthermore, we have demonstrated that PPVs can be
functionalized to achieve main-chain supramolecular polymers
with high-fidelity molecular recognition processes, affording
supramolecular AA and AB diblock copolymers.4,64
PMAc and Pyr-PIC, respectively (Figures S-15 and S-16). SCS-
PdII-functionalized PS (Pin-PS) is available via ATRP in the
presence of a 1,3-di(methylthiophenyl) phenyl ether moiety
that undergoes post-polymerization palladation to install the
SCS-PdII motif. Pin-PS is characterized through GPC, wherein
a Đ of 1.21 is observed (Figure S-17).
Synthesis of HW-Functionalized GII and Subsequent
Heterotelechelic Sheets via CM-ROMP. Taking advantage
of ROMP to engineer heterotelechelic polymers with functional
initiators and CTs, we synthesized α-HW-functionalized PPVs
containing ω-Pin or Pyr moieties. The design necessitated the
successful incorporation of the HW motif onto a GII initiator
scaffold. Whereas the synthesis and isolation of functional
Grubbs I-based initiators has been reported,68 along with dye-
functionalized Hoveyda Grubbs derivatives,69 GII-based func-
tional initiators have not been realized to date for engineering
telechelic polymers. Owing to the poor air stability of GII-based
complexes in solution, we designed an iterative one-pot CM-
ROMP strategy to achieve heterotelechelic PPVs. The CM of a
HW-functionalized styrene in dichloroethane results in the
formation of the desired HW-GII in one hour (Figure 1). The
With these building blocks, we target synthetic tertiary
architectures through supramolecularly directed sequencing of
secondary structure elements. Bio-inspired sequences of helix−
sheet−coil and helix−sheet−helix are pursued. Helix−sheet−
coil assemblies comprising both unimeric π-strand PPVs and
antiparallel π-sheets are targeted owing to the prevalence of all
three structural elements within biomacromolecules; we view
these assemblies as models to study synergistic effects
stemming from the integration of three synthetic conforma-
tions into a single compact structure. Helix−sheet−helix
assemblies are more interesting structures. Sequences of α-
helices and β-sheets are dominant in key folding classes, such as
α+β and α/β domains.65 A mixture of solely α-helices and β-
sheets highlights the former class, while the latter is
distinguished by parallel β-sheets that are surrounded with
peripheral α-helices, with β-α-β motifs being prominent
supersecondary structures.66 Furthermore, α/β-barrel domains
are common binding domains featured within enzyme folding
pockets comprised of antiparallel sheet motifs.67 We engineer
helix−sheet−helix assemblies that, de facto, present both
designed parallel and antiparallel β-sheet-like interior blocks.
Figure 1. Functionalized HW-GII and its 1H NMR spectrum (CDCl3,
Av600).
GII carbene signal at δ = 19.15 decreases in intensity with the
concomitant appearance of a new species at δ = 18.7 ppm,
consistent with the formation of the HW-GII carbene (1H
NMR spectrum, Figure 1). The formation of HW-GII is also
evidenced using both 13C NMR spectroscopy and MALDI-ToF
(Figures S-8−S-10). The CM reaction proceeds in 98%
conversion, and is used without purification to initiate the
ROMP of pCpd. The polymerizations are monitored for
completion via 1H NMR spectroscopy, wherein the propagating
carbene is always observed with an integration of 1H relative to
the monomer feed ratio and/or polymer conversion. After the
polymerization reaches completion, an excess of Pin- or Pyr-
substituted vinyl ether is added to afford the targeted
heterotelechelic polymers. The ensuing polymers, HW-PPV-
Pin and HW-PPV-Pyr are characterized by 1H NMR
spectroscopy, wherein resonances consistent with the successful
orchestration of the CM-ROMP strategy are distinguished, as
both end groups within the polymer backbone are observed. In
particular, HW-amido protons are observed at 8.6 ppm as well
as Pin-related resonances at 4.6 ppm. GPC was used to
determine the Đ of HW-PPV-Pin and HW-PPV-Pyr (1.14 and
RESULTS AND DISCUSSION
■
Synthesis of Monotelechelic Helical and Coil Building
Blocks. Monotelechelic helical and coil building blocks are
realized through controlled polymerization routes. Barbiturate-
functionalized PMAc (Ba-PMAc) is synthesized via RAFT
polymerization in the presence of a Ba-conjugated CTA, while
pyridine-functionalized PIC (Pyr-PIC) is synthesized via
anionic polymerization in the presence of a pyridine-function-
alized palladium initiator. Both building blocks are evaluated
through gel-permeation chromatography (GPC), wherein
dispersity (Đ) values of 1.16 and 1.22 are observed for Ba-
1
1.18, respectively, see Figures S-11 and S-12), while H NMR
spectroscopic analysis was used to yield more reliable molecular
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weights through comparative integration of end group and
backbone signals (Table 1).
Supramolecular ABC Triblock Copolymers via Or-
thogonal Metal Coordination and Hydrogen Bonding.
Heterotelechelic HW-PPV-Pin and HW-PPV-Pyr can provide
access to a variety of block copolymers via orthogonal assembly.
The individual assembly behavior of HW-PPV-Pin and HW-
PPV-Pyr were evaluated with two complementary secondary
structure blocks. The assemblies were evaluated in a stepwise
manner to allow for the characterization of the intermediate
assembled products.
Table 1. Characterization Data for Telechelic Polymers
c
polymer
Mn (SEC)
Mw (SEC)
Đ
Mn (NMR )
a
a
a
a
b
b
b
HW-PPV-Pin
HW-PPV-Pyr
HW-1T-Pin
HW-1T-Pyr
Pyr-PIC
33 000
33 000
99 000
39 000
6 100
39 000
37 000
120 000
49 000
7 400
1.18
1.14
1.21
1.28
1.22
1.16
1.21
16 000
18 400
23 700
17 800
−
First, we investigated the formation of a helix−(π-sheet)−
coil ABC triblock copolymer using HW-PPV-Pyr as the
internal heterotelechelic block. We targeted the metallosupra-
molecular assembly of HW-PPV-Pyr with Pin-PS,79 followed
by subsequent installation of helical Ba-PMAc via hydrogen
bonding. The successful formation of HW-PPV-Pin_Pyr-PS
Ba-PMAc
6 900
7 400
−
14 400
Pin-PS
14 000
17 000
a
b
c
Recorded in CHCl3, SEC. Recorded in THF, SEC. Recorded in
CDCl3, NMR.
1
was evidenced using H NMR spectroscopy and GPC (Figure
1
2). The H NMR spectrum displays an upfield shift of the
Dialing in Complexity: Synthesis of Heterotelechelic
PPV Rod−Coil−Rod Block Copolymers Featuring Flexi-
ble Interior Segments via CM-ROMP. We sought to design
a covalent block copolymer that aids in the organization of
adjacent PPV blocks while featuring an antiparallel π-stacked
sheet. There are reports detailing the use of flexible linkages to
tether PPV backbones so as to attain collapsed order of
chromophore interactions.70 We envision a noncovalent
analogue could enforce chromophore ordering via the collapse
of a coil-like interior segment. Specifically, we hypothesize that
the synthesis of a PPV-PNB-PPV scaffold, wherein the PNB
features pendant phenyl groups adjacent to perfluorophenyl
motifs71 that are able to undergo charge-transfer-assisted chain
collapse within favorable solvent systems (e.g., chloroform),10
will lead to an interior flexible segment reminiscent of a β-
hairpin turn. The use of perfluoroarene/arene interactions has
been exploited as a supramolecular synthon72,73 with the
aptitude to preorganize both molecular and polymeric systems,
affording molecular co-crystals,74 as well as liquid crystals75 and
collapsed β-hairpin mimics.10
1
Figure 2. (Left) Partial H NMR spectroscopic overlay of (top to
bottom) HW-PPV-Pyr, HW-PPV-Pyr_Pin-PS, and Ba-PMAc_HW-
PPV-Pyr_Pin-PS (CDCl3). (Right) SEC traces (CHCl3) of HW-
PPV-Pyr (red) and HW-PPV-Pyr_Pin-PS (black) and schematic of
supramolecular ABC triblock copolymer.
We utilize our CM-ROMP strategy to generate HW-
functionalized PPVs. Upon completion of the ROMP, an exo-
norbornene phenyl ester (Ph) is added to the polymerization
solution and followed by a structurally analogous perfluor-
ophenyl ester (PFP). We target incorporation of a PNB block
such that the PNB:PPV ratio is less than 1.5.76 Smaller flexible
units are demonstrated to induce PPV chromophore align-
ment.77 The ensuing PPV−PNBPh-PNBPFP block copolymer is
then treated with a fresh equivalent of pCpd and polymerized
to completion. Polymers are subsequently quenched with vinyl
ethers containing either Pin or Pyr motifs, affording the desired
HW-1T-Pin and HW-1T-Pyr (wherein 1T denotes the PPV−
PNBPh-PNBPFP-PPV backbone has one designed flexible turn)
heterotelechelic block copolymers. The final polymers were
characteristic Pin-related resonance at 7.8 ppm to 7.7 ppm, on
par with prior reports of Pin-Pyr coordination in PPV-based
systems.4 Resonances related to the α-HW terminus are still
present at 8.6 ppm, owing to their orthogonality to the metal-
coordination event. Further evidence of successful assembly
was confirmed via GPC, wherein a shift to a higher molecular
weight species relative to HW-PPV-Pyr is observed, (Mn = 32
800 to Mn = 114 000) coupled with a low Đ of 1.09.
Subsequent assembly with helical Ba-PMAc was characterized
by a downfield shift of the HW amide resonance at 8.6 ppm to
9.4 ppm, along with the appearance of the Ba hydrogen-
bonding signal at 12.2 ppm (Figure 2).
We investigate the capacity of HW-PPV-Pin to juxtapose
two telechelic secondary structures, allowing for the fabrication
of a helix−(π-sheet)−helix wherein both persistent dynamic
and static helical blocks are incorporated. We first assemble
HW-PPV-Pin with Pyr-PIC and evaluate the metallosupra-
molecular assembly using 1H NMR spectroscopy. Upon
coordination, we observe the disappearance of the α-Pyr
resonance at 8.8 ppm. Addition of the Ba-PMAc, results in the
shifting of the protons related to the HW amide from 8.6 to 8.9
ppm along with the concomitant appearance of a sharp
1
characterized by H NMR spectroscopy, wherein resonances
consistent with the HW initiator end group, as well as both the
PPV and PNB backbones were present. GPC was used to
determine Đ of the Pin- and Pyr-terminated polymers (1.25
1
and 1.21, respectively, see Figures S-13 and S-14), while H
NMR spectroscopic analysis was required to estimate more
reliable molecular weights (Table 1) and confirm block ratios.
For instance, HW-1T-Pin features block ratios of 30:15:15:30.
Following this iterative ROMP strategy, we are able to generate
additional “turns” within the polymeric backbone, leading to
essentially A(BCA)n covalent block copolymers of variable
molecular weights.78
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resonance related to the Ba hydrogen-bonding N−H protons at
12.7 ppm (Figure S-28).
ordered spectroscopy (DOSY) is used to further evaluate
ABB′A tetrablock copolymer formation, as the assembled
structures should exhibit different diffusion coefficients in
comparison to the parent HW-PPV-Pin_Pyr-PPV-HW. The
metallosupramolecular PPV has a diffusion coefficient of 3.89 ×
10−10 m2 s1. Upon block copolymer formation with two
equivalents of Ba-PMAc, the diffusion coefficient decreases to
Construction of a Parallel π-Stacked β-Sheet-like
Supramolecular Block Copolymer. Having evaluated the
assembly potential of HW-PPV-Pyr and HW-PPV-Pin, we
looked to utilize their complementary end-groups to engineer a
synthetic system that would conceptually mimic parallel β-
sheets. Given that the preliminary PPV-related assemblies are
successful, we envisioned that the two polymers could
additionally self-assemble into HW-PPV-Pin_Pyr-PPV-HW,
wherein the short saturated linkage between the blocks allows
for the flexibility to manifest a hairpin-like turn. Prior studies
using PPV systems have demonstrated that folded sheet-like
structures can be engineered via interruption of the conjugated
backbone with short saturated linkages70 akin to those present
leading up to the supramolecular junction. The self-assembly
would, thus, allow for the further incorporation of comple-
mentary telechelic secondary structures, affording an ABB′A
tetrablock copolymer via orthogonal assembly. Whereas
oligoamide antiparallel β-sheet-like frameworks have been
achieved via π−π stacking,36,37 reports realizing parallel
synthetic β-sheet-like architectures remain largely dormant.
We view this as a platform to design symmetrical hierarchical
structures, as the initiating end of the polymer species can be
changed in a facile manner via CM reactions to engineer a
versatile ABB′A building block.
2.36 × 10−10 m2 s1
.
Characterization of Tri- and Tetrablock Architectures.
We characterized the supramolecular block copolymers by CD
and fluorescence spectroscopies, along with wide-angle X-ray
scattering (WAXS). CD spectroscopy is used to confirm the
helical features of Ba-PMAc and Pyr-PIC upon assembly
within the three different interior PPV blocks. Both helical
domains retain the helical character in the ABC triblock
copolymers, displaying wavelength-specific CD indicative of
structures with helicity in enantiomeric excess (Figure 4).80 For
The successful assembly of the PPV building blocks is
characterized by 1H NMR spectroscopy and GPC. Upon metal
coordination, we observe that the HW-related downfield
Figure 4. CD spectra corresponding to ABC triblock copolymers:
(left) red, Ba-PMAc; black, Ba-PMAc_HW-PPV-Pyr_Pin-PS; (right)
purple, Ba-PMAc; red, Pyr-PIC; black, Ba-PMAc_HW-PPV-Pin_-
Pyr-PIC). Spectra were obtained in dichloroethane.
1
resonances are intact in the H NMR spectrum, while signals
related to the Pin and Pyr functionalities remain hidden within
the backbone (Figure 3). A clean shift to higher molecular
the helix−sheet−coil assembly, only the helix block exhibits a
CD trace. In contrast, the helix−(π-sheet)−helix system
displays two different helices, each varying in relative magnitude
of their Cotton effects. In both supramolecular block
copolymers, traces of a negative CD signal at 230 nm, along
with positive CD at 236 and 283 nm are characteristic of
contributions from Ba-PMAc, with the largest signal being the
result of the naphthyl π−π* transitions. For the helix−(π-
sheet)−helix assembly, the signature CD signals at 249 and 364
nm relate to the PIC backbone n−π* and π−π* transitions; in
addition, those of Ba-PMAc are still maintained within the
supramolecular block copolymer. The helicity of Ba-PMAc
within the ABB′A block copolymer is also investigated, wherein
upon assembly with the parallel π-sheet, a profile similar to that
of the unassembled helical polymer is observed (Figure S-34).
The CD profiles collectively depict that the helical structures of
Ba-PMAc and Pyr-PIC are retained within the supramolecular
multiblock copolymers.
We next evaluated the π-sheet PPV solid-state assemblies
using WAXS. Both heterotelechelic polymers, being structurally
similar to dioctyloxy PPV (DO-PPV), exhibit analogous
signature peaks corresponding to the interbackbone separation
of alkyloxy chains, interbackbone monomer repeat units, and
interbackbone face-to-face π−π stacking (d = 14.1−14.3 Å, 6.7
Å, and 4.5 Å, respectively)48 that leads to 2D stacks sustained
via π−π stacking and alkyl−alkyl interdigitation between
neighboring stacks in an orthogonal direction. To achieve
compartmentalized π-sheet structures upon assembly, breaking
up the PPV long-range order is essential. Unlike DO-PPV, the
parent polymers display the more prominent alkyl−alkyl
Figure 3. (Left) Partial 1H NMR spectroscopic overlay of (bottom to
top) HW-PPV-Pyr, HW-PPV-Pin, HW-PPV-Pin_Pyr-PPV-HW, and
ABB′A assembly (CDCl3). (Right) SEC traces (CHCl3) of HW-PPV-
Pin (red), HW-PPV-Pyr (black), and HW-PPV-Pin_Pyr-PPV-HW
(blue) and schematic of the supramolecular ABB′A tetrablock
copolymer.
weight (Mn = 96 000 to Mn = 107 000) is observed by GPC
proving the chain extension via self-assembly (Figure 3).
Further assembly with the parallel-stacked π-sheet HW-PPV-
Pin_Pyr-PPV-HW was conducted with complementary Ba-
PMAc, whereupon the HW-related amide resonances at 8.6
ppm shift downfield to 9.3 ppm, along with concomitant
appearance of the Ba N−H protons at 12.7 ppm. Diffusion-
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spacing peak at a shorter distance, owing to the heightened
ability of the chains to tilt48 with the lessened density of side-
chains. DO-PPV scaffolds demonstrate a transverse span of 18.4
Å.48 Whereas the majority of the homopolymer side-chains can
remain tilted, both polymers feature a broadened shoulder
between 19 and 22 Å, suggestive of some extended side chains
being present (Figure 5).
helix−(parallel π-sheet)−helix, we note that (i) the ABB′A
assembly maintains a molecular structure featuring both alkyl−
alkyl and face-to-face π−π interactions; (ii) upon assembly with
the hydrogen-bonding helix, the degree of side-chain
interactions is lessened; and (iii) the relative intensity of both
signals, although broadened, suggests equal contributions of the
two interactions in determining the long-range ordered
structure. Prior to the integration of Ba-PMAc, the PPV
backbones likely maximize both side-chain and backbone
interactions to afford largely stacked 2D assemblies. The fully
assembled ABB′A tetrablock copolymer appears to support
equal contributions of interbackbone alkyl−alkyl and π−π
stacking, suggestive of a parallel π-sheet being maintained in a
compartmentalized manner, with the helical units suppressing
further PPV−PPV interactions.
The homopolymers and assemblies are further evaluated
using fluorescence spectroscopy. Although the complementary
helices and coils do not exhibit fluorescence, the emission
properties of PPV homopolymers81 and covalent block
copolymers, as well as their aggregation-influenced properties
are well documented.46,82,83 The heterotelechelic PPV homo-
polymers exhibit emission maxima at 520 nm with a broad
shoulder at 560 nm (Figure S-33). Both ABC triblock
copolymers exhibit mainly relative changes in intensity
compared to their parent PPV polymers. Upon sequencing
HW-PPV-Pyr_Pin-PS and Ba-PMAc_HW-PPV-Pyr_Pin-PS,
we observe a decrease in intensity upon diblock formation.
Triblock formation, however, partially restores some of the
fluorescence behavior (Figure S-33). In contrast, with HW-
PPV-Pin_Pyr-PIC and Ba-PMAc_HW-PPV-Pin_Pyr-PIC, we
observe that the fluorescence intensity maintains a steady
decrease (ca. 50% overall) suggestive of achieving more face-to-
face π-stacks upon sequencing that would allow for quenching.
Both systems exhibit a slight blue shift (2 nm) in the emission
maximum upon hydrogen bonding assembly.
Figure 5. WAXS diffractograms corresponding to ABC triblock
copolymers obtained: (left) green, HW-PPV-Pyr; red, Ba-PMAc;
blue, HW-PPV-Pyr_Pin-PS; black, Ba-PMAc_HW-PPV-Pyr-Pin-PS;
(right) green, HW-PPV-Pin; red, Pyr-PIC; blue, HW-PPV-Pin_Pyr-
PIC; black, Ba-PMAc_HW-PPV-Pin-Pyr-PIC.
Following metal coordination with Pin-PS or Pyr-PIC, the
(π-sheet)−coil and (π-sheet)−helix diblocks feature less
intense signals in the low 2θ region, indicative of alkyl−alkyl
interdigitation between stacks being lessened. Both diblock
copolymers still feature a significant degree of π−π stacking,
with HW-PPV-Pin_Pyr-PIC also featuring a broad signal
centered at d = 21.5 Å that represents the hexagonal packing of
PIC and correlates to the polymer stem, with overlap from the
aforementioned shoulder of the parent PPV. Upon hydrogen
bonding to Ba-PMAc, both ABC triblock copolymers feature
signals related to π−π stacking (d = 4.2−4.8 Å). In both, the
lessened intensity of the peaks related to the interbackbone
alkyl−alkyl spacing signifies that the long-range order leading to
2D stacks is partially broken upon assembly; thus, the
assembled polymers feature solely 1D π-stacked sheets.
In the ABB′A mutliblock copolymer, the metallosupra-
molecular assembly exhibits roughly identical signature peaks
relative to the parent PPVs. The alkoxy side-chain separation
remains the most prominent signal (d = 12.98 Å; Figure 6),
with the concomitant broadening of the low 2θ shoulder
corresponding to the presence of extended side chains (d =
19.28 Å). Upon hydrogen bonding to Ba-PMAc to generate the
We then investigate the fluorescence of the assembled PPV
diblock and ABB′A block copolymer (Figure 7). After
metallosupramolecular self-assembly, we observe fluorescence
that closely matches the parent polymers. Upon hydrogen
bonding with Ba-PMAc, the ABB′A polymer exhibited sharply
contrasting optical behavior, most notably a hypsochromic shift
in emission, as well a sufficient decrease in fluorescence
Figure 7. Excitation (left) and emission (right) spectra related to the
ABB′A parallel π-sheet supramolecular assembly (blue = HW-PPV-
Pyr; red = HW-PPV-Pin; purple = HW-PPV-Pin_Pyr-PPV-HW; and
black = ABB′A). Polymers are excited at 450 nm, and emission is fixed
at 520 nm for each excitation scan in CHCl3 (note: excitation spectra
of HW-PPV-Pin and HW-PPV-Pyr are identical; we plot only one for
clarity).
Figure 6. WAXS diffractograms corresponding to ABB′A multiblock
copolymer: (left) metallosupramolecular assembly and (right) both
assemblies (red, HW-PPV-Pin; black, HW-PPV-Pyr; blue, HW-PPV-
Pin_Pyr-PPV-HW; and purple, ABB′A multiblock copolymer).
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intensity (66% less intense), relative to the parent homopol-
ymers and assembled diblock. Specifically, the emission
maximum shifts to 510 nm, with a concomitant shift in the
excitation maximum from 450 to 420 nm (Figure 7). The
combined results suggest that the assembly with helical Ba-
PMAc leads to compartmentalization and isolation of the PPV
polymer chains (i.e., breaking of the 2D long-range order
normally propagated by alkyl−alkyl interactions), with
heightened face-to-face interactions leading to fluorescence
quenching behavior akin to H-aggregates.84 PPV systems are
known to exhibit a blue-shifted emission as a result of strong
interchain interactions and π−π stacking within solution, and
suppression of intrachain interactions.46
synthesizing the (antiparallel π-sheet)-coil metallosupra-
molecular block copolymer, we incorporate the helical block
using hydrogen bonding between HW and Ba.
We evaluate the Ka using H NMR spectroscopic titration,
wherein the HW amido N−H protons are monitored upon
addition of Ba-PMAc (Figure 9). Upon achieving a 1:1
1
Engineering of Supramolecular Block Copolymers
Featuring Antiparallel π-Sheets. We pursued the con-
struction of a helix−(antiparallel π-sheet)−coil ABC supra-
molecular triblock copolymer via orthogonal and stepwise
assembly. HW-1T-Pyr is assembled with Pin-PS using metal
coordination, with the assembled material characterized both by
1H NMR spectroscopy and GPC (Figure 8). Upon
Figure 9. Plot of (left) chemical shifts (in CDCl3) related to
hydrogen-bonding N−H amido protons upon addition of Ba-PMAc to
HW-1T-Pyr_Pin PS and (right) specific viscosity (ηsp) versus
concentration for related block copolymer assemblies in CHCl3
(red, HW-2T-Pyr; black, HW-2T-Pyr_Pin-PS; and blue, Ba-
PMAc_HW-2T-Pyr_Pin-PS).
assembly, the amido protons shift downfield from 8.5 to 9.2
ppm, along with concomitant appearance of the Ba imido
protons at 12.5 ppm. The association constant of the
supramolecular assembly was calculated to be (3.41 0.6) ×
104 M−1, which correlates well with our previous assemblies,4
and others featuring HW-based hydrogen-bonding junc-
tions.63,67,85 The overall helix−(antiparallel π-sheet)−coil
sequence is further evaluated by measuring the specific viscosity
(ηsp) upon supramolecular association relative to the parent
heterotelechelic PPV-based polymer. The viscosity studies are
conducted using an Ubbelohde viscometer at 25 °C in CHCl3.
Relative to the heterotelechelic PPV-based polymer, the ηsp
increases upon addition of the Pin-PS block, and further
increases with the installation of the hydrogen-bonding Ba-
PMAc, which correlates with the increase in the estimated
molecular weight of the assembled block copolymers (Figure
9).
The final supramolecular assembly target is a helix−
(antiparallel π-sheet)−helix, wherein the terminal blocks
comprise chemically distinct helical polymers. Pyr-PIC is
coordinated to HW-1T-Pin, followed by the hydrogen bonding
of the α-HW terminus to Ba-PMAc. The assembly is evaluated
by 1H NMR spectroscopy. HW amido protons exhibit a
downfield shift from 8.5 ppm to 8.9 ppm, along with the
emergence of Ba imido protons at 12.7 ppm upon assembly
with both helical polymers (Figure S-32). Further NMR
spectroscopic characterization of the supramolecular helix−
(antiparallel π-sheet)−helix assembly was provided by DOSY
experiments. Relative to HW-1T-Pin (diffusion coefficient =
8.43 × 10−10 m2 s1), the diffusion coefficients of the metallo-
supramolecular diblock and ABC triblock are clearly decreasing
with the increase in molecular weight (7.19 × 10−10 and 4.84 ×
10−10 m2 s1, respectively). The successful helix−(antiparalllel
π-sheet)−helix main-chain sequencing is evidenced by
viscometry measurements. Akin to the triblock copolymer
obtained using HW-2T-Pyr, the ηsp relative to the parent 2T
polymer increases upon addition of Pyr-PIC, with a further
increase upon addition of the hydrogen-bonding Ba-PMAc
(Figure 10).
Figure 8. (Top) 1H NMR spectra overlay (CDCl3) of (bottom to top)
HW-1T-Pyr, HW-1T-Pyr_Pin-PS, and Ba-PMAc_HW-1T-Pyr_Pin-
PS. (Bottom) SEC traces (CHCl3) highlighting a shift in molecular
weight upon metal coordination (red, HW-1T-Pyr; black, HW-1T-
Pyr_Pin-PS) and schematic depicting two orthogonal interactions and
Ba-PMAc_HW-1T-Pyr_Pin-PS.
metallosupramolecular assembly, the HW amide proton
resonance remains centered at approximately 8.5 ppm, while
the Pin and Pyr resonances remain hidden within the aromatic
backbone signals. GPC is used to confirm the assembly,
wherein the trace obtained shows a distinct shift to a higher
molecular weight (Mn = 38 600 to Mn = 131 000; Đ = 1.16)
upon sequencing the antiparallel sheet and coil. After
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diminished intensity with a concomitant 2 nm shift in emission
maximum (Figure 11). The emission stemming from the PPV
Figure 10. (Left) Specific viscosity (ηsp) versus concentration for
related helix−(antiparallel π-sheet)−helix block copolymer assemblies
in CHCl3 relative to HW-2T-Pin. (Right) CD spectral overlay of Pyr-
PIC (red), Ba-PMAc (purple), and helix−(antiparallel π-sheet)−helix
(black) measured in dichloroethane.
Characterization of ABC Triblock Copolymers Featur-
ing Antiparallel π-Sheets. The effect of supramolecular
assembly on Ba-PMAc was evaluated using CD. Within the
helix−(antiparallel π-sheet)−coil assembly, the only contribu-
tions would stem from the helicity of Ba-PMAc. The
supramolecular triblock copolymer exhibits a negative CD
signal at 230 nm, along with positive CD at 236 and 283 nm
(Figure S-36). The bisignate profile observed correlates well
with the telechelic unassembled Ba-PMAc, suggestive of
retaining helical structure. The helical features of Ba-PMAc
and Pyr-PIC upon stepwise assembly with the antiparallel π-
sheet are also confirmed. We observe that the helix−
(antiparallel π-sheet)−helix exhibits characteristic CD signals
consistent with both helices. Specifically, traces of a negative
CD signal at 230 nm, along with positive CD at 236 and 283
nm from the PMAc block naphthyl π−π* transitions, with
additional signals at 249 and 364 nm related to the PIC
backbone n-π* and π−π* transitions are observed (Figure 10).
At the corresponding wavelengths, signal intensities in the fully
formed triblock copolymer result from the summation of
absorbances contributed from each polymer species. The CD
profiles collectively depict that the helix−(antiparallel π-
sheet)−helix triblock copolymers retain the helical structures
of Ba-PMAc and Pyr-PIC.
Relative to DO-PPV, the WAXS of HW-1T-Pyr and HW-
1T-Pin still feature strong Bragg diffraction peaks correspond-
ing to the interbackbone alkyloxy chain separation, monomer
repeat units, and face-to-face π−π stacking. Additionally, the
high 2θ peak is broadened owing to the perfluoroarene/arene
interactions of the PNB interior block exhibiting Bragg
diffraction at 2θ = 17.5° (Figures S-37 and S-38). Upon
metallosupramolecular diblock formation, the most prominent
feature corresponds to the face-to-face π−π stacking of the PPV
backbone as well as the perfluoroarene/arene interactions (d =
4.56 Å). In HW-1T-Pyr_Pin-PS, a broadened signal related to
the loosely packed phenyl stacks within the coil-like Pin-PS
backbone is still distinguishable. Upon hydrogen bonding with
Ba-PMAc, the molecular structure remains mostly unaffected,
although the intensity of the HW-1T-Pyr and HW-1T-Pin
signature peaks are about equal (i.e., the relative influence of
π−π stacking is decreased), suggestive of the presence of
unimers that do not interact with neighboring assemblies,
unlike the parent polymer.
Figure 11. Excitation (left) and emission (right) profiles related to
HW-1T-Pyr supramolecular assembly (red, HW-1T-Pyr; black, HW-
1T-Pyr_Pin-PS; blue, Ba-PMAc_HW-1T-Pyr_Pin-PS; and dashed
purple, HW-PPV-Pyr). Polymers are excited at 450 nm, and emission
is fixed at 520 nm for each excitation scan in CHCl3.
backbone does not undergo a substantial hypsochromic shift,
despite the presence of “defects” within the π-conjugated
framework, as the interior PNB units break up electronic
communication. The difference observed in intensity is
attributed to the ability of HW-1T-Pyr to enforce π−π stacking
of the PPV backbones with a heightened degree of face-to-face
stacking that suppresses fluorescence intensity.
Upon sequential block copolymer formation with Pin-PS
and Ba-PMAc, we observe the relative emission intensity
decreases (ca. 65% relative to parent HW-1T-Pyr), suggestive
of more H-aggregate-like face-to-face stacking upon assembly.
Additionally, both assemblies support a hypsochromic shift in
excitation (450 to 440 nm), likely owing to the disruption of
interactions that normally dominate the PPV aggregated
state.46,48 For the helix−(antiparallel π-sheet)−helix upon
both metal coordination and hydrogen bonding, we observe
an overall decrease in fluorescence intensity (Figure S-39).
These collective results suggest maintenance of the antiparallel
π-sheets in the wake of assembly, while the complementary
polymers successfully compete with the intrachain associations
common in PPV-based scaffolds.
CONCLUSION
■
Herein, we report a versatile method to access architecturally
well-defined Nature-inspired multiblock copolymers through
designed supramolecular sequencing of telechelic helical, π-
sheet, and coil-forming polymers. Essential to our design is the
one-pot iterative CM-ROMP strategy that first engineers a
functional GII initiator, followed by heterotelechelic π-sheet
PPVs. The heterotelechelic PPVs exploit orthogonal self-
assembly or further living polymerization to achieve non-
covalently driven parallel π-sheets or combined covalently and
noncovalently driven antiparallel π-sheets, respectively. Both
parallel and antiparallel π-sheets, which bear structural
similarity, to proteinic β-sheets, are intentionally utilized as
the interior assembly block to probe assembly. Complementary
polymers were rationally chosen that feature a high density of
aromatic units that engage with the π-strands upon assembly.
Successful assemblies, thus, disrupt prominent intrachain PPV−
PPV interactions while enforcing a compartmentalized
Fluorescence spectroscopy is used to gain insights into the
assembly behavior of HW-1T-Pyr and HW-1T-Pin. We first
compare the emission spectrum of HW-1T-Pyr to HW-PPV-
Pyr. At the same concentration, HW-1T-Pyr displays a
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structure that can be facilely monitored using fluorescence
spectroscopy.
AUTHOR INFORMATION
■
Corresponding Author
*marcus.weck@nyu.edu
ORCID
Elizabeth Elacqua: 0000-0002-1239-9560
Marcus Weck: 0000-0002-6486-4268
Notes
Three unique structures are combined through hydrogen
bonding and metal coordination, and retained within the final
assemblies. We investigate PPV-based helix−sheet−coil and
helix−sheet−helix assemblies as simple building systems that
feature homology to α/β and α+β protein classes in wake of
constituting a fully synthetic design. Supramolecular sequencing
occurs in an orthogonal manner, while disrupting the intrachain
PPV−PPV interactions and maintaining the helicity of both Ba-
PMAc and Pyr-PIC. We demonstrate the realization of a
parallel-stacked π-sheet based upon an ABB′A network,
wherein the addition of helical termini aids in establishing a
compartmentalized structure. The compartmentalized nature of
the folded interior sheet was evidenced both in the solid state
through WAXS and in solution via fluorescence spectroscopy,
wherein hypsochromic shifts in excitation and emission, along
with concomitant fluorescence quenching supported the lack of
intrachain interactions and PPV aggregation, as well as
enhanced degrees of face-to-face π−π stacking.
We engineered a heterotelechelic PPV-based rod−coil−rod
block copolymer featuring internal nonconjugated blocks that
interact using perfluoroarene/arene stacking to aid in the
establishment of an antiparallel π-sheet. We observe that HW-
1T-Pin and HW-1T-Pyr feature heightened degrees of face-to-
face stacking as evidenced by both WAXS and fluorescence
spectroscopy, likely owing to the ability of the PNB segments
to be flexible enough to induce a folded structure, but rigid
enough to prevent unwanted aggregation from occurring. Upon
assembly with complementary helical and coil polymers, a
decrease in the fluorescence intensity was observed, indicative
of pronounced face-to-face π−π stacking. All assemblies,
despite the complex nature of the building blocks, maintain
prominent secondary structures.
The introduced system provides a versatile platform to
combine synthetic polymers in a well-defined and precise
manner via a supramolecular sequencing strategy that supports
the formation of tertiary structures. Our methodology presents
an approach to utilize supramolecular interactions in concert to
not only afford main-chain synthetic foldamers but also
promote further assembly into more complex and compart-
mentalized systems. While much work is dedicated to the
engineering of bio-inspired synthetic folding systems that
feature individual helical and sheet-like segments, few non-
amide systems dial in design complexity, while still maintaining
simplistic routes to analyze assembled structures. We view this
strategy as a rational platform to further develop directionally
assembled polymer systems on demand while increasing the
complexity of synthetic systems that feature structural
homology to biomacromolecular architectures.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was primarily supported by the National Science
Foundation (NSF) under award number CHE-1506890. We
thank the MRSEC Program of the NSF (DMR-1420073) for
partial financial support of this research. The MALDI-ToF mass
spectrometer was acquired through the NSF (CHE-0958457).
We also acknowledge the NSF CRIF Program (CHE-0840277)
for the purchase of a Bruker GADDS microdiffractometer and
the National Institutes of Health for the purchase of the Avance
III-600 CPTCI-cryoprobe head (S10 grant, OD016343).
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DOI: 10.1021/jacs.7b06201
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX