Controlled folding of a novel electroactive polyolefin via multiple sequential orthogonal intra-chain interactions

Article abstract of DOI:10.1039/c2cc37157j

We report here the synthesis of a novel electroactive polyolefin and demonstrate through a combination of SEC-MALS and DLS how this type of functional polymer can be folded in a controlled fashion from an expanded coil to a tightly wound globule using sequentially activated noncovalent and covalent intra-chain interactions.

Full text of DOI:10.1039/c2cc37157j

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Controlled folding of a novel electroactive polyolefin
via multiple sequential orthogonal intra-chain
interactions†‡
Cite this: Chem. Commun., 2013,
49, 4178
Received 1st October 2012,
Accepted 3rd December 2012
Danming Chao,ab Xiaoteng Jia,b Bryan Tuten,a Ce Wangb and Erik B. Berda*a
DOI: 10.1039/c2cc37157j
www.rsc.org/chemcomm
We report here the synthesis of a novel electroactive polyolefin and variety of intra-chain interactions operating synergistically to
demonstrate through a combination of SEC-MALS and DLS how this fold chains in a controlled manner are conspicuously absent.
type of functional polymer can be folded in a controlled fashion Likewise, reports on the folding of polymer chains that contain
from an expanded coil to a tightly wound globule using sequen- some intrinsic functionality are limited. In light of this, we
tially activated noncovalent and covalent intra-chain interactions.
report here the use of multiple, orthogonal intra-chain inter-
actions triggered sequentially to fold an electroactive polymer.
We chose a simple architecture for our study, poly(oxano-
As advances continue in the area of well-defined functional
polymer synthesis, a paradigm shift is occurring: focus on the rbornene anhydride-co-cyclooctadiene) synthesized using ROMP
fabrication of devices and nanostructures based on the manipula- chemistry, based on previous success in our lab15 creating folded
tion of single polymer chains is intensifying.1 Controlled single- structures based on similar scaffolds. Here, the anhydride groups
chain folding in particular is receiving increased attention.2,3 along the backbone provide a reactive handle that can be easily
When compared to the elegant biomacromolecules from which decorated with a variety of nucleophiles, simultaneously inducing
synthetic folded structures draw their inspiration, this technique folding and installing functionality. Scheme 1 outlines the
remains in its infancy. However, the growing volume of literature
on this topic has delineated many of the benchmarks and
requirements for inducing an individual macromolecule to fold
by means of intra-chain chemical or physical cross-links. The
scope of this emergent area is eclectic; fabrication of nano-
particles for biomedical uses,4–6 fluorescent nanoparticles,7
synthetic enzyme mimics,8 polymer supported dynamic combi-
natorial libraries,9 sensors,10 and processable supramolecular
networks11,12 highlights the range of possible applications.
Other important studies tackle some of the most pressing
fundamental challenges in attempting to mimic Nature’s design,
such as reversible folding via orthogonal supramolecular13,14 and
dynamic covalent cross-links,15 the importance of primary struc-
ture and monomer sequences,16,17 and the roles temperature and
solvent effects play in the folding process.18 While the array of
chemistries, both covalent and noncovalent, which have been
investigated as handles to effect polymer folding is impressive,
it is by no means exhaustive. Further, reports that utilize a
a Department of Chemistry and Materials Science Program,
University of New Hampshire, Durham, NH 03824, USA.
E-mail: Erik.Berda@unh.edu; Fax: +1 603-862-4278; Tel: +1 603-862-1762
b Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun,
130012, P. R. China
† This article is part of the ChemComm ‘Emerging Investigators 2013’ themed issue.
‡ Electronic supplementary information (ESI) available: Full experimental details
and supporting figures. See DOI: 10.1039/c2cc37157j
Scheme 1 Controlled single-chain folding via sequential intra-chain interactions.
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route we adopted to produce folded electroactive chains. First,
an aniline tetramer (AT) was introduced. This both installs the
electroactive properties and begins the sequential folding pro-
cess via supramolecular interactions between aniline tetramer
pendant groups. Next, the remaining anhydride groups were
reacted with p-aminoaniline. This serves to further fold the
chain by pulling in the remainder of the oxanorbornene seg-
ments. Finally, the polycyclooctadiene segments are folded
around the initial structure by connecting the olefins installed
within the backbone during the ROMP polymerization using
thiol–ene click chemistry. The result is a tightly folded structure
with a hydrodynamic volume that is 70% smaller than that of
the original coil.
The aniline tetramer was selected as the electroactive
component based on our familiarity with its synthesis and
properties.19–22 The cyclic voltammogram (Fig. 1A) of the ani-
line tetramer functionalized polymer shows two reversible
redox processes: two pairs of redox current peaks at 335 mV/
160 mV and 500 mV/410 mV. These were, respectively, assigned
to the transition from a reduced leucoemeraldine base (LEB) to
the half-oxidized emeraldine base (EB), and then from EB to
the fully oxidized pernigraniline base (PNB). The polymer
also exhibited interesting spectral changes when subjected to
oxidative or reductive species in solution (see ESI,‡ Fig. S1).
Spectroelectrochemical studies were performed on a polymer
coated electrochromic electrode in 0.5 mol LÀ1 H2SO4
coupled at applied potentials of À0.2, 0, 0.2, 0.4, 0.6, and
0.8 V (vs. Ag/AgCl). The polymer film shows different UV-Vis
absorbance at various applied potentials (Fig. 1B). An absorp-
tion peak appeared at about 430 nm, which is related to the
polarons of the oligoaniline segment. With an increase in
applied potential to 0.4 and 0.6 V, the peak assigned to
polaronic and bipolaronic species appeared and centered at
Fig. 1 Representative cyclic voltammogram, scan rate of 50 mV sÀ1 in 1.0 M
H2SO4 (A) and spectroelectrochemical data (B) for the aniline tetramer
functionalized polymers.
800 nm. Further oxidation beyond 0.6 V to 0.8 V results in a size that is directly related to the amount of AT present (see
decrease in the absorbance at 700 nm, but this may be ascribed ESI‡ for more information on the SEC-MALS characterization).
to the decomposition of the oligoaniline segment and/or the This result was corroborated by DLS, which showed a decrease
damage to the polymer/ITO electrochromic device after in hydrodynamic radius (Rh) as the amount of AT in the
extended operation at low pH. The colour of the polymer thin polymer increased (see ESI‡). Efforts are currently underway
film changed drastically from a transmissive grey (at 0 V), to to evaluate the feasibility of using the electroresponsive nature
yellow-green (at 0.2 V), to blue (at 0.4 V), and finally to blue- of these pendants to modulate the conformation of the
black (at 0.6 V) (Fig. 1).
folded chain.
In our folding studies, we first accessed the ability of the
Each type of covalent interaction was then explored sepa-
aniline tetramer pendants to induce conformational changes in rately to examine its interplay with the aniline tetramer seg-
the polymer via p–p and hydrogen bonding interactions ments. In each case discussed here we started with a sample in
between these side chains. This was accomplished by decorating which half of the original anhydride segments were substituted
the parent ROMP copolymer with varying amounts of AT with AT. First, we reacted the remaining anhydride units along
pendant groups. The resultant change in hydrodynamic size the polymer backbone with varying amounts of a diamine
was then characterized using dynamic light scattering (DLS) (p-aminoaniline), a method with which we have had previous
and size exclusion chromatography equipped with a light success. As expected, SEC traces showed increased retention
scattering detector (SEC-MALS). An increase in SEC retention time (with the expected increase in absolute molecular weight)
time was observed as more and more AT pendants were and DLS displayed a decrease in Rh when the amount of
added to the chain (see ESI,‡ Fig. S3), indicating a reduction diamine introduced was increased (see ESI,‡ Fig. S4 and
in hydrodynamic volume due to folding of the chain. The Table S1).
absolute molecular weight of the chain, as measured using
We then examined thiol–ene chemistry23 as a second
the MALS detector, increased to the expected amount as approach to induce the folding process. Although this reaction
more pendants were added confirming that the change in has been used to similar ends before,24 previous examples involved
retention time is indeed due to a reduction in hydrodynamic polymers that featured a vinyl pendant, necessitating the synthesis
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In summary, we have described the synthesis of a novel
electroactive polyolefin that exhibits interesting spectroscopic
and electrochromic properties and shown that sequentially
activated intra-chain interactions can be used to fold this
polymer from an expanded coil conformation to a globular
conformation, as is evident through SEC-MALS and DLS char-
acterization. We are continuing to develop these methods in
our lab towards scaling up the amount of folded structures we
make, as well as expanding this technique to a range of other
functional polymers.
We graciously acknowledge NSF for funding via the Nano-
scale Science & Engineering Center for High-rate Nanomanu-
facturing (grant NSF EEC 0832785), as well as start-up funding
from the University of New Hampshire. We also thank Prof.
C. Barner-Kowallik, Prof. T. S. Emrick, and Prof. E.W. Meijer for
helpful and enlightening discussions.
Fig. 2 SEC traces displaying the decrease in hydrodynamic volume with each
sequential folding step. Rh numbers listed are from DLS measurements.
See ESI‡ for full DLS traces.
Notes and references
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Rh (see ESI,‡ Fig. S5).
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covalent interaction separately, we applied each sequentially
in an attempt to fold the chain in a stepwise fashion. We first
reacted half of the anhydride units in the parent copolymer
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