Surface-accessible detection units in self-immolative polymers enable translation of selective molecular detection events into amplified responses in macroscopic, solid-state plastics

Article abstract of DOI:10.1021/jacs.5b02799

This Communication describes a strategy for incorporating detection units onto each repeating unit of self-immolative CD_r polymers. This strategy enables macroscopic plastics to respond quickly to specific applied molecular signals that react with the plastic at the solid-liquid interface between the plastic and surrounding fluid. The response is a signal-induced depolymerization reaction that is continuous and complete from the site of the reacted detection unit to the end of the polymer. Thus, this strategy retains the ability of CD_r polymers to provide amplified responses via depolymerization while simultaneously enhancing the rate of response of CD_r-based macroscopic plastics to specific applied signals. Depolymerizable poly(benzyl ethers) were used to demonstrate the strategy and now are capable of depolymerizing in the context of rigid, solid-state polymeric materials.

Full text of DOI:10.1021/jacs.5b02799

Communication
pubs.acs.org/JACS
Surface-Accessible Detection Units in Self-Immolative Polymers
Enable Translation of Selective Molecular Detection Events into
Amplified Responses in Macroscopic, Solid-State Plastics
Kimy Yeung, Hyungwoo Kim, Hemakesh Mohapatra, and Scott T. Phillips*
Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States
*
S Supporting Information
4
CD polymer that makes up the material. This approach
r
ABSTRACT: This Communication describes a strategy
for incorporating detection units onto each repeating unit
retains the ability to provide amplified responses via continuous
and complete depolymerization, while also increasing the rate
of the detection event.
of self-immolative CD polymers. This strategy enables
r
macroscopic plastics to respond quickly to specific applied
molecular signals that react with the plastic at the solid−
liquid interface between the plastic and surrounding fluid.
The response is a signal-induced depolymerization
reaction that is continuous and complete from the site of
the reacted detection unit to the end of the polymer. Thus,
We demonstrate this concept using a new type of
depolymerizable poly(benzyl ether) (Figure 1b). First gen-
eration CD poly(benzyl ethers) are not capable of depolyme-
r
8
rizing in the solid state, but poly(benzyl ethers) of the type
depicted in Figure 1b deploymerize quickly in the solid state,
which is a rare capability among CD polymers (i.e., only
r
this strategy retains the ability of CD polymers to provide
9
10
r
poly(phthalaldehyde) and poly(4,5-dichlorophthalaldehyde)
amplified responses via depolymerization while simulta-
depolymerize on the macroscale in the context of solid-state
plastics). These new poly(benzyl ethers) are more stable than
poly(phthalaldehydes); therefore, they likely will become the
preferred reagents in future studies for creating advanced
stimuli-responsive materials with the capability of selective and
amplified responses.
neously enhancing the rate of response of CD -based
r
macroscopic plastics to specific applied signals. Depoly-
merizable poly(benzyl ethers) were used to demonstrate
the strategy and now are capable of depolymerizing in the
context of rigid, solid-state polymeric materials.
We prepared this new class of poly(benzyl ethers) using a
short synthesis (3 steps, ∼30% overall yield) to access quinone
methide monomers that contain pendant detection units
acroscopic stimuli-responsive materials could benefit
M
substantially from the ability to self-amplify a macro-
scopic response to a specific molecular-level input. An amplified
macroscopic response would allow a material to change its
global properties in response to a local signal, increase the rate
of change in the material, and decrease the quantity of signal
(
Figure 2). In theory, a variety of detection units can be
2c
incorporated into these monomers, including proof-of-
concept units that are responsive to Pd(0) (4-allyl, R =
1
allyl) or fluoride (4-TBS, R = TBS). Polymerization of the
1
monomers under anionic conditions (Figure 2) followed by
end-capping provides access to polymers with detection units
on each repeating unit (e.g., 5 and 8), as well as control
polymers either with detection units only on their termini (6
and 9) or without detection units (7 and 10).
1
necessary to induce the response. One emerging strategy for
realizing these benefits is based on self-immolative CDr
polymers that are defined as having the capability of continuous
and complete depolymerization when a molecular signal cleaves
a detection unit (or end-cap) from the terminus of the polymer
through a reaction-based detection event. The reaction-based
As designed, polymers 5 and 6 depolymerize completely
when they are dissolved in THF and exposed to excess fluoride
(tetrabutylammonium fluoride; TBAF) at 23 °C (Figure S2). In
contrast, control polymer 7 does not depolymerize, since it
does not contain a TBS detection unit. LCMS analysis of
aliquots of the depolymerization reaction of 5 with fluoride
reveals deprotected and deprotonated quinone methide (i.e., 4-
detection event provides selectivity for the response, while the2
continuous depolymerization reaction provides amplification.
Depolymerization of this type, however, suffers from slow rates
of reaction between the applied molecular signal and the
detection units that are displayed at the interface between a
solid, macroscopic material and a surrounding liquid that
−
2i,3
O , which we confirmed by LCMS (Figure S4) and via
contains the signal.
In most cases, the detection unit
−
comparison with 4-O prepared by independent synthesis;
constitutes <1% of the number of atoms in a polymeric
material; therefore, few detection units statistically are
accessible at the solid−liquid interface. Thus, the detection
11
Scheme S10) as well as TBS-protected monomer 4-TBS
where R = TBS) (Figure S4). The presence of 4-TBS
(
1
confirms that depolymerization occurs via a head-to-tail
amplification reaction, rather than via cleavage of every TBS
group in the polymer. Similar results in terms of selective
event has become the rate-limiting step in responses of CD -
r
based macroscopic materials.
To ameliorate this rate-limiting step, we have now developed
a strategy for substantially enhancing the number of detection
units that are displayed at a solid−liquid interface by
incorporating a detection unit onto each repeating unit of the
Received: March 17, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b02799
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 2. Preparation of monomers and polymers used in this study.
Reagents and conditions: (a) formaldehyde, HCl (80%); (b)
electrophile, base (to append R ) (∼50%); (c) Ag O (75%−80%);
1
2
(
d) MeOH, P -tBu; (e) electrophile, base (to append R ) (31−89%).
2
2
The detection units are highlighted in blue.
that the disks were rigid plastics rather than elastic materials.
This rigidity is an important property when comparing the
accessibility of detection units between disks made from
12
different polymers. In addition, disks made from 5 and 8 did
not swell when submerged for 5 h in acetonitrile (the solvent
used for the solid state depolymerization experiments) (Figure
S6), which demonstrates that the disks do not substantially
absorb the surrounding solvent that contains the molecular
signal, and therefore the depolymerization experiments likely
represent reactions at the solid−liquid interface. Moreover,
spin-cast films of polymers 6−10 have nearly equal contact
angles (e.g., 22° ± 1° for 7) when wet with acetonitrile, while a
film of polymer 5 is slightly higher (i.e., 35° ± 2°) (Figure S8).
These results demonstrate that surfaces of solid-state objects
made from each polymer will wet essentially equally with the
fluid that contains the molecular signal; therefore, differences in
wettability will not affect the response rates of the plastics to
the applied molecular signals. Finally, scanning electron
microscope (SEM) images of disks comprising polymers 5−7
show comparable surface morphology (Figure S7), further
supporting the expectation that solid−liquid interactions will be
similar across disks made from different polymers.
In the solid-state experiments, disks 5−7 were submerged in
a nonstirring solution of acetonitrile containing 0.23 M TBAF
at 23 °C and photographs were acquired over time (Figure 3).
The disk made from polymer 5 responded in minutes by
Figure 1. Strategy for creating stimuli-responsive rigid plastics that
display amplified responses to specific signals at the solid−liquid
interface. (a) Representation of the general design of the polymers in
this study. (b) Specific backbone structure of the polymers used in this
study. (c) Depiction of the challenge of placing detection units at the
solid−liquid interface between a rigid plastic and its surroundings.
13
producing a deep purple color (from 11) and reducing in size
Figure 3a). In less than 5 h, the entire disk was converted into
(
soluble products. In contrast to the rapid reaction of the disk
made from 5, the disk made from polymer 6 showed only
minimal production of a yellow/blue color within the first 2 h
of exposure to fluoride (presumably due to the few surface-
accessible detection units), after which no additional reaction
occurred (Figure 3b). In fact, after 7 days of exposure to
fluoride, the disk appeared equal in size to its starting point.
Likewise, the disk made from polymer 7 showed no change
over 7 days of exposure to fluoride (Figure 3c).
depolymerization were observed when polymers 8, 9, and 10
were exposed to Pd(0) in THF (Figure S3), thus confirming
that the detection units can be changed on each repeating unit
to alter the signal to which the polymer responds.
Depolymerization experiments in the solid state revealed
substantial differences in the accessibility of detection units at
the solid−liquid interface. Solid disks of polymers 5−10 were
prepared by solvent casting each polymer into a silicon mold.
Each disk was approximately 3.5 mm in diameter × 1.2 mm
thick, weighing 10.2 mg ± 1.2 mg. Young’s modulus values for
two representative disks (made from 5 and 8) ranged from 1.20
These solid-state changes in morphology are corroborated by
X-ray photoelectron spectroscopy analysis of solids made from
polymers 5 and 6. As anticipated, solids made from 5 contain
86% more silicon in the top 5 nm of the material than solids
14
±
0.06 GPa (for 5) to 1.67 ± 0.11 GPa (for 8), establishing
made from polymer 6.
B
DOI: 10.1021/jacs.5b02799
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 3. Photographs showing selective depolymerization at the solid−liquid interface using solvent-cast disks of polymers 5−7. All photographs
show a top-down view of a glass crystallization dish that contains a plastic disk immersed in a solution of acetonitrile that contains 0.23 M TBAF.
The disk in (a) comprises polymer 5, in (b) polymer 6, and in (c) polymer 7. Similar results are available in Figure S5 for disks made using polymers
8−10 that respond to Pd(0).
Similar selectivity in the solid-state depolymerization results
Aizenberg, J. Chem. Soc. Rev. 2013, 42, 7072−7085. (c) Esser-Kahn, A.
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were obtained for disks made from polymers 8−10 that
respond to Pd(0) (Figure S5), showing that inclusion of
detection units on each repeating unit of the poly(benzyl
ethers) indeed accelerates the rate of molecular detection
events at the solid−liquid interfaces of macroscopic poly(benzyl
ether)-based materials.
The combination of these capabilitiesi.e., tunable specific-
ity, signal amplification, stimuli responses in rigid plastics, and
rapid responses at solid−liquid interfacesallows us to
envisage applications of such materials. Example applications
may range from plastics that easily alter their size, shape,
structure, surface properties, or function to plastics that report
their exposure to a specific analyte (via the purple color of 11),
to plastics that form the basis for smart capsules.
7
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014, 3, 298−304. (b) Phillips, S. T.; Robbins, J. S.; DiLauro, A. M.;
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ASSOCIATED CONTENT
Supporting Information
■
Am. Chem. Soc. 2010, 132, 10266−10268. Recent CD polymers that
r
*
S
are not covered in the reviews: (j) Fan, B.; Trant, J. F.; Wong, A. D.;
Gillies, E. R. J. Am. Chem. Soc. 2014, 136, 10116−10123. (k) DiLauro,
A. M.; Phillips, S. T. Polym. Chem. 2015, DOI: 10.1039/C5PY00190K.
(3) (a) DiLauro, A.; Robbins, J. S.; Phillips, S. T. Macromolecules
Procedures, figures, NMR spectra, and GPC chromatograms.
This material is available free of charge via the Internet at
http://pubs.acs.org.
2
013, 46, 2963−2968. (b) DiLauro, A. M.; Abbaspourrad, A.; Weitz,
D. A.; Phillips, S. T. Macromolecules 2013, 46, 3309−3313.
c) DiLauro, A. M.; Zhang, H.; Baker, M. S.; Wong, F.; Sen, A.;
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AUTHOR INFORMATION
Corresponding Author
sphillips@psu.edu
■
(
*
5
6
7
(
4) Gillies, Almutairi, and Cheng used similar approaches for
Notes
degradable polymers that respond to applied signals, where each
detection event cleaves a polymer into two pieces. Thus, n − 1
detection events are required to cleave all bonds between repeating
units, where n refers to the number of repeating units in the polymer.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Defense Threat Reduction
Agency (HDTRA1-13-1-0039) and the Penn State MRSEC
■
(5) (a) Soleimani, A.; Borecki, A.; Gillies, E. R. Polym. Chem. 2014, 5,
7
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(
062−7071. (b) Mejia, J. S.; Gillies, E. R. Polym. Chem. 2013, 4,
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(
DMR-0820404) (surface-accessible detection units), and by
the U.S. Army Research Office (W911NF-14-1-0232) (solid-
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6) (a) de Gracia Lux, C.; Joshi-Barr, S.; Nguyen, T.; Mahmoud, E.;
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11) An independent synthesis was used to confirm the structure of
1 (see the Supporting Information).
12) Under the same experimental conditions, Young’s modulus
(
9
2
(
(
1
(
value was obtained for poly(styrene) (2.87 ± 0.09 GPa) to provide
internal comparison for our measurements of 5 and 8. Literature
values for poly(styrene) range from 3 to 3.6 GPa. Poly(propylene) has
Young’s modulus values that are similar to those for polymers 5 and 8
(
i.e., 1.5−2 GPa).
(
13) Quinone methide 11 is bright purple when deprotonated (λ
max
5
−1
−1
=
574 nm; ε = 1.4 × 10 M cm in MeCN containing 5 mM DBU).
These data were obtained using 11 that was prepared via an
independent synthesis (see the Supporting Information) (Figure S1)
and thus serves as a colorimetric reporter for depolymerization.
(
14) Solids made from 5 contain 7% ± 1% silicon in the top 5 nm of
the material, whereas solids from 6 contain 1% silicon.
D
DOI: 10.1021/jacs.5b02799
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX