Damage and Repair in Informational Poly(N-substituted urethane)s

Article abstract of DOI:10.1002/anie.202008864

The degradation and repair of uniform sequence-defined poly(N-substituted urethane)s was studied. Polymers containing an ω-OH end-group and only ethyl carbamate main-chain repeat units rapidly degrade in NaOH solution through an ω→α depolymerization mechanism with no apparent sign of random chain cleavage. The degradation mechanism is not notably affected by the nature of the side-chain N-substituents and took place for all studied sequences. On the other hand, depolymerization is significantly influenced by the molecular structure of the main-chain repeat units. For instance, hexyl carbamate main-chain motifs block unzipping and can therefore be used to control the degradation of specific sequence sections. Interestingly, the partially degraded polymers can also be repaired; for example by using a combination of N,N′-disuccinimidyl carbonate with a secondary amine building-block. Overall, these findings open up interesting new avenues for chain-healing and sequence editing.

Full text of DOI:10.1002/anie.202008864

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Accepted Article
Title: Damages and Repair in Informational Poly(N-substituted
urethane)s
Authors: Tathagata Mondal, Laurence Charles, and Jean-Francois
Lutz
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202008864
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10.1002/anie.202008864
Angewandte Chemie International Edition
COMMUNICATION
Damages and Repair in Informational Poly(N-substituted urethane)s
Tathagata Mondal,[a] Laurence Charles[b] and Jean-François Lutz*[a]
[a]
Dr. T. Mondal, Dr. J.-F. Lutz
Precision Macromolecular Chemistry
Université de Strasbourg, CNRS, Institut Charles Sadron UPR22
23 rue du Loess, 67034 Strasbourg Cedex 2, France
E-mail: jflutz@unistra.fr
[b]
Prof. Dr. L. Charles
Institute of Radical Chemistry
Aix Marseille Université, CNRS, UMR 7273
13397, Marseille Cedex 20, France
Supporting information for this article is given via a link at the end of the document.
Abstract: The degradation and repair of uniform sequence-defined
poly(N-substituted urethanes) was studied. Polymers containing an
ω-OH end-group and only ethyl carbamate main-chain repeat units
rapidly degrade in NaOH solution via an ω→α depolymerization
mechanism with no apparent sign of random chain cleavage. The
degradation mechanism is not notably affected by the nature of the
side-chain N-substituents and took place for all studied sequences.
On the other hand, depolymerization is significantly influenced by the
molecular structure of the main-chain repeat units. For instance, hexyl
carbamate main-chain motifs block unzipping and can therefore be
used to control the degradation of specific sequence sections.
Interestingly, the partially-degraded polymers can also be repaired;
for example using a combination of N,N’-disuccinimidyl carbonate with
a secondary amine building-block. Overall, these findings open up
interesting new avenues for chain-healing and sequence editing.
and proceeds via an unzipping mechanism. This interesting
mechanism has been explored for biomedical applications, for
instance for triggered drug release.[9] Very recently, Anslyn and
coworkers[10] have reported that the depolymerization of
sequence-defined polyurethanes can be kinetically-controlled to
enable sequencing.[11] In this case, chain unzipping was triggered
by a terminal OH-group and required basic conditions and
elevated temperatures (70°C, microwave) to proceed. In the
present work, we investigated the depolymerization in basic
medium of sequence-coded poly(N-substituted urethane)s, a
promising new family of digital polymers.[7e] As described in the
following paragraphs, depolymerization occurs at room
temperature and can be finely tuned by macromolecular design.
The poly(N-substituted urethane)s studied in this work were
all synthesized using a recently-reported orthogonal solid-phase
synthesis protocol.[7e] Figure 1 shows their general structure.
These informational polymers are directional and contains
therefore two distinct chain ends denoted as α and ω. The former
one is a caproic acid residue, whereas the latter one is either an
OH or a methyl group. The coded sequences were constructed
using a set of nine different informational monomers A-I. Figure
S1 shows the molecular structure of the polymers P1-P13 studied
in this work. The molecular uniformity of the polymers was
assessed by high-resolution electrospray mass spectrometry
(ESI-HRMS) and size exclusion chromatography (SEC), while
their coded sequences were characterized by tandem mass
spectrometry (MS/MS) (Table S1 and Figures S2-S14).[12]
Informational polymers are macromolecules that store information
in a controlled comonomer sequence.[1] A typical example of that
is DNA, which is a biological storage medium. In recent years, it
has also been reported that data (e.g. in a binary format) can be
stored in synthetic macromolecules, for example in polyamides,
polyesters and polyurethanes, just to name a few.[2] This new
class of functional polymers has opened up interesting
applications in areas such as data storage, cryptography, anti-
counterfeiting, traceability and plastic recycling.[3] Yet, there are
still pronounced differences between natural and non-natural
informational polymers. Man-made polymers mainly enable
passive information storage, while the information sequences of
DNA can be copied, mutated, damaged, modified (i.e. through
epigenetics) and repaired.[4] There are currently only a couple of
examples of synthetic information sequences that can be
degraded or transformed;[2c, 5] although such behaviors could be
useful for the aforementioned applications as well as for more
distant goals such as the development of artificial Life.[6]
(A) R = Me, x = 1 (F) R = Bz, x = 2
(B) R = Et, x = 1
(C) R = Pr, x = 1
(D) R = Bu, x = 1
(E) R = Bz, x = 1
(G) R = Bz, x = 3
(H) R = Bz, x = 4
(I) R = Bz, x = 5
α
Sequence
ω
Y = OH or CH3
Figure 1. General structure of the informational poly(N-substituted urethane)s.
Here, we report the controllable degradation and possible
repair of informational poly(N-substituted urethane)s. Sequence-
defined polyurethanes can be synthesized via different solid-
Since the depolymerization of sequence-defined poly(N-
substituted urethane)s was never investigated, the degradation
mechanism and kinetics were first analyzed in details. For
polymers having an ω-hydroxyl end-group and ethyl carbamate
repeat units (x = 1 in Figure 1), complete depolymerization occurs
at room temperature in the presence of sodium hydroxide. For
example, Figure S2 shows the ESI-HRMS spectrum and SEC
chromatogram of P1 having the sequence α-DCEC-OH after 16
7]
phase chemistry approaches.[2d, Different types of functional
sequences can be prepared, including digitally-encoded ones.[2d,
7e] It is also well-known that some linear or dendritic polyurethanes
undergo controllable depolymerization (sometimes referred to as
self-immolative behavior).[8] In these macromolecules, chain-
degradation is triggered by a nucleophile (e.g. a primary amine)
1
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10.1002/anie.202008864
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COMMUNICATION
8
6
4
2
0
hours in basic medium. Both analytical methods evidence the
disappearance of P1 and the formation of three low molecular
weight species of mass 129.1, 142.1 and 177.1 Da, which
correspond to the oxazolidinone rings resulting from the
cyclization of repeat units C, D and E, respectively. As proposed
in Scheme S1, these compounds are likely formed through a
backbiting reaction leading to complete chain depolymerization.
To validate this mechanism, P2 having the same primary structure
as P1 but no terminal OH group (i.e. α-DCEC-CH3) was studied
in the same basic conditions (Figure S3). This experiment
indicated no sign of degradation, thus suggesting that
depolymerization solely occurs through an ω→α unzipping
mechanism, i.e. without contribution from other mechanisms such
as random chain cleavage.[13] Furthermore, this behavior is fully
orthogonal as shown in Figure 2, which compares the ESI-HRMS
spectra of a mixture of P1 and P2 before and after basic treatment.
The selective depolymerization of P1 is observed in these
conditions.
a-DCE-OH/P1
a-DC-OH/P1
a-D-OH/P1
0
500
1000
1500
2000
2500
3000
Time [min]
Figure 3. Evolution of SEC peak areas ratios as a function of time for the
degradation of P1 in NaOH solution in MeOH/H2O 2:1 v/v. Peak areas were
estimated by deconvoluting the chromatograms of Figure S15b using the Origin
software. For a given time, the ratios α-DCE-OH/P1, α-DC-OH/P1 and α-D-
OH/P1 are calculated from the corresponding chromatogram.
[P2+NH4]+
a.
726.5
Possible sequence effects were also investigated. Polymers
P3 and P4 having ω-hydroxyl end-group and ethyl carbamate
repeat units but a different sequence and a different terminal
monomer unit from P1 were examined (Figures S4-S5). P3 and
P4 have a similar terminal unit but a different penultimate unit.
Depolymerization proceeded in all cases and appeared to be
sequence-independent and side-group-independent (for both
penultimate and terminal positions). Yet, minor deviations in
depolymerization kinetics cannot be ruled out but could not be
evidenced in the performed experiments.
α-
α-
-OH
P1
P2
[P1+NH4]+
+
728.4
-CH3
[P2+H]+
709.5
m/z
600
650
700
750
Furthermore, the influence of the number of the methylene
groups in the repeat units was investigated. A series of polymers
P4-P8 having all the same sequence but different terminal
monomer units (R = Bz, x ranging from 1 to 5, Figure 1) was
studied (Figures S5-S9). After 16h of basic treatment in
methanol/water, the polymer having an ethyl terminal (P4) unit
was entirely degraded. The polymer having a propyl-based (P5)
terminal units was significantly degraded but a residual peak of
the initial polymer was still observed. For polymers having butyl-
(P6) and pentyl-based (P7) terminal units, the pristine polymer
was still observed as a dominant species, although traces of
degradation were detected. The polymer having a hexyl-based
terminal unit (P8) obviously did not degrade in the studied
conditions and only exhibited marginal oxazolidinone peaks.
Behaviors comparable to those described for P5-P8 were
obtained with polymers with alternative monomer sequences P9-
P12. Altogether, these results imply that the depolymerization
mechanism proceeds well when 5- and 6-membered rings are
formed but is slowed down or hindered by repeat units leading to
the formation of larger rings. To confirm this hypothesis, a mixture
of polymers P4-P8 was treated in basic medium for 16h. ESI-
HRMS analysis (Figure S17) indicated complete disappearance
of P4, significant disappearance of P5, partial disappearance of
P6 and P7, whereas P8 remained the most abundant species.
Although these results should be interpreted with caution due to
potential suppression effects during the ionization process, this
experiment confirms that main-chain alkyls strongly influence the
depolymerization mechanism.
b.
[P2+NH4]+
726.5
144.1
130.1
*
*
*
120
600
140
160
180
700
m/z
650
750
Figure 2. Positive mode ESI-HRMS spectra of a 1:1 w/w mixture of polymers
P1 and P2: (a) before basic treatment and (b) after 16h at RT in NaOH solution
in MeOH/H2O 2:1 v/v. The inset of panel b shows a zoom of the 120-180 m/z
region of the obtained spectrum. As compared to the main spectrum, the
intensity scale of the inset has been increased by a factor of about 18. The
asterisks indicate background noise peaks.
Figures 3 and S15 show the kinetics of depolymerization of
polymer P1, as monitored by SEC. In Figure 3, the evolution of
the ratio of the peak areas of the partially-degraded products (α-
DCE-OH, α-DC-OH and α-D-OH) over the peak area of the
remaining initial polymer P1 are displayed as a function of time.
These results suggest that α-DCE-OH is first predominantly
formed, followed by the gradual accumulation of α-DC-OH and α-
D-OH. Comparable data were although recorded by ESI-MS
(Figure S16), even though peak intensities should be interpreted
with caution in mass spectrometry. Altogether, these results
suggest a gradual unzipping degradation, as already reported for
regular sequence-defined polyurethanes.[10]
2
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10.1002/anie.202008864
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COMMUNICATION
2+
a.
[P13+H+NH ]
4
646.9
[P13+2NH4]2+
P13
655.4
[P13+NH4]+
1292.8
α-
-OH
(i)
m/z
600
800
1000
1200
[P13'+NH4]+
784.5
b.
d.
c.
178.1
P13’
*
α-
-OH
*
*
*
*
*
600
800
1000
1200
m/z
100
120
140
160
180
m/z
(ii)
P13 P13" P13'
[P13"+NH4]+
994.7
e.
P13”
α'-
-CH3
V [mL]
25
26
27
28
29
600
800
1000
1200
m/z
Figure 4. Partial degradation and repair of sequence-coded poly(N-substituted urethane) P13. Positive mode ESI-HRMS spectra of (a) the original polymer P13
before basic treatment, (b, c) the degraded polymer P13’ after NaOH treatment, and (d) the repaired polymer P13” after molecular healing. Panel c shows a zoom
of the 100-200 m/z range for P13’. As compared to the spectrum shown in panel b, the intensity scale of the inset in panel c has been increased by a factor of about
11. The asterisks indicate background noise peaks. Experimental conditions: (i) NaOH, MeOH/H2O, RT (ii) DSC, pyridine, ACN, 60°C, microwave then di-n-propyl-
amine, pyridine, DMF, RT. (e) SEC chromatograms recorded in THF for P13, P13’ and P13”. The high molecular weight shoulders are most likely due to the
formation of polyesters, as discussed in previous publications.[7e, 14]
The fact that chain depolymerization depends on the number
of methylene groups in a repeat unit is appealing because it
suggests that the content of an informational sequence can be
tuned by monomer design. Indeed, in informational polymers,
molecular bits are usually set by using side-chain motifs of
different molar mass but can also be obtained using main-chain
motifs of different length or composition.[15] For instance, in the
present case, informational units containing hexyl spacers, such
(i.e. α-COOH and ω-OH) and can therefore be repaired. Hence,
the same sequence (complete or partial) or a different one can be
reconstructed on the partially-degraded polymer. Here, P13’ was
partially-repaired by reacting it first with N,N’-disuccinimidyl
carbonate (DSC) and then with di-n-propyl-amine. This
modification leads to modification of both chain-ends, as shown
in Figure 4c. Yet, it should be noted that the amidification of the
α-chain-end can be avoided if needed; for instance by modifying
the terminal acid group during or after cleavage. Here, the
targeted modification was successfully performed, as evidenced
by the ESI-MS spectrum (Figure 4c and Figure S19) and SEC
chromatogram (Figure 4e) of the repaired polymer P13”. Indeed,
the transformation P13’→P13” led to the expected mass increase
of 210 Da in ESI-MS and to an apparent Mp increase of 168 Da in
SEC.
In summary, OH-terminated poly(N-substituted urethane)s
degrade via an ω→α unzipping mechanism in basic conditions at
room temperature. If the polymer chain is only composed of ethyl
carbamate repeat units, full decomposition can be attained. Yet,
if some repeat units of the chain contain longer alkyl spacers,
degradation can be slowed down or even suppressed. For
instance, polymers containing a hexyl carbamate terminal unit do
not degrade in basic conditions; at least not notably during the
investigated periods of times. Furthermore, if the hexyl carbamate
unit is placed inside an otherwise ethyl carbamate-based
as monomer I, can be used as
a stopper to interrupt
depolymerization at a precise chain-location. As a proof-of-
concept, polymer P13 with sequence α-DDBIAACE-OH was
investigated (Figure 4a). It contains a single I unit placed in
between two blocks that are composed of ethyl carbamate repeat
units. In NaOH medium, this polymer depolymerizes only until the
stopping point, as evidenced by the ESI-MS spectrum (Figure 4b
and Figure S18) and SEC chromatogram (Figure 4e) of the
resulting polymer P13’. In ESI-MS, the transformation P13→P13’
leads to the expected mass decrease of 508 Da, whereas in SEC
an apparent diminution of 466 Da is measured between peak
maxima Mp. Partial depolymerization is also supported by the
appearance of the oxazolidinone rings, expelled from the
decomposed AACE sequence, in the ESI-MS spectrum of P13’
(Figure 4d). These results confirm that preset sequence sections
can be selectively degraded using appropriate molecular design.
Furthermore, the degraded chain still exhibits reactive chain ends
3
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[10] S. D. Dahlhauser, P. R. Escamilla, A. N. VandeWalle, J. T. York, R. M.
Rapagnani, J. S. Shei, S. A. Glass, J. N. Coronado, S. R. Moor, D. P.
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sequence, partial depolymerization proceeds from the ω-terminus
to the stopping hexyl carbamate unit. Therefore, specific
information sections can be selectively erased from an
informational sequence. This property is particularly interesting
for anti-counterfeiting applications,[3b] because molecular
barcodes could be purposely altered when exposed to specific
conditions. Perhaps even more importantly, the partial
degradation mechanism found in this work opens up possibilities
of repairing informational polymers. For instance, it was
demonstrated herein that intentionally-damaged sequences can
be re-engineered using simple modification conditions. This
proof-of-concept of molecular healing paves the way for the
development of more complex deconstruction/reconstruction
mechanisms, including chain rewriting and editing.
[15] J.-F. Lutz, Macromolecules 2015, 48, 4759-4767.
Acknowledgements
This work was supported by CNRS, the University of Strasbourg
and the LabEx CSC. The SEC results shown in the publication
were obtained with the help of the polymer characterization
service of the Institut Charles Sadron. L.C. acknowledges support
from Spectropole, the Analytical Facility of Aix-Marseille
University, by allowing a special access to the instruments
purchased with European Funding (FEDER OBJ2142-3341).
Keywords: Sequence-controlled polymers • Digital polymers •
Polymer degradation • Molecular healing • Sequencing
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4
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COMMUNICATION
Entry for the Table of Contents
Insert graphic for Table of Contents here.
α-
-OH
Controlled
depolymerization
α-
-OH
Repair
α'-
-CH3
Insert text for Table of Contents here. In nature, the monomer
sequences of informational biopolymers undergo damages,
mutations and repair. In the present work, we show that
comparable modifications can be obtained in non-natural
sequence-defined poly(N-substituted urethane)s. Polymers
having ethyl carbamate main-chain repeat units and an ω-OH
end-group depolymerize in basic conditions. Yet, through careful
monomer design, this behavior can be controlled and repair can
be performed.
Institute and/or researcher Twitter usernames: @CharlesSadron
5
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