Spontaneous Single-Crystal-to-Single-Crystal Evolution of Two Cross-Laminated Polymers

Article abstract of DOI:10.1002/anie.201812094

Two cases of spontaneous evolution of monomers to linear polymers having novel cross-laminated topology are reported. We synthesized two peptide monomers N₃-Gly-Gly-NH-CH₂-CCH and N₃-Gly-Gly-Gly-CH₂-CCH and solved their crystal structures by single-crystal X-ray diffraction. They adopt H-bonded crisscrossed layered packing in their crystals such that: (a) the monomers are aligned head-to-tail in 1D-chain-like arrays and parallel arrangement of such arrays forms a layer; (b) the proximally placed azide and alkyne motifs are in an orientation apt for their regiospecific cycloaddition; (c) each monomer having x peptide bonds is H-bonded with 2x monomers disposed in intersecting arrangement, which pre-organize 1D-chain-like arrays in adjacent layers in perpendicular orientation. These crystals underwent spontaneous single-crystal-to-single-crystal (SCSC) polymerization via azide–alkyne cycloaddition reaction to form triazolyl-polyglycines, at room temperature. The crisscrossed arrangement of monomers in adjacent layers ensured the formation of cross-laminated polymers.

Full text of DOI:10.1002/anie.201812094

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Accepted Article
Title: Spontaneous Single-Crystal-to-Single-Crystal Evolution of Two
Cross-laminated Polymers
Authors: Kana M Sureshan and Vignesh Athiyarath
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812094
Angew. Chem. 10.1002/ange.201812094
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Spontaneous Single-Crystal-to-Single-Crystal Evolution of Two
Cross-laminated Polymers
Vignesh Athiyarath, and Kana M. Sureshan*[a]
Triazole
being
isosteric
with
amide
group,[10]
pseudopolypeptides having some of the amide linkage replaced
by triazole motif are interesting. Solution-phase synthesis of
such large triazolyl-polymers is practically impossible due to the
poor solubility, difficult purification, incomplete reaction,
necessity of metal catalysts, etc. We have exploited
Topochemical Azide-Alkyne Cycloaddition (TAAC) reaction for
the catalyst-free, solvent-free polymerization of various
monomers to give their corresponding triazolyl-polymers[11] or
oligomers.[12] While the solution phase synthesis gives random
coil polymer, topochemical polymerization gives perfectly
aligned polymer having precise supramolecular structure (and
hence properties) predetermined by the molecular arrangement
of the monomers in the crystal.[13] Recent and elegant examples
of this approach are the synthesis of 2D polymers through
topochemical photocycloaddition reactions.[14] We decided to
synthesise pseudopoylglycines wherein every third or fourth
amide bonds of polyglycine (Figure 1a) is replaced with triazole
(Figure 1b,c), via TAAC polymerization of two monomers
dipeptide 1 or tripeptide 2 (Figure 2a,c), having complimentary
reactive motifs viz. azide and alkyne at their termini.
Abstract: We report two cases of spontaneous evolution of
monomers to linear polymers having novel cross-laminated topology.
We synthesized two peptide monomers N3-Gly-Gly-NH-CH2-CCH
and N3-Gly-Gly-Gly-CH2-CCH and solved their crystal structures by
Single-Crystal X-ray Diffraction. They adopt H-bonded criss-crossed
layered packing in their crystals such that: (a) the monomers are
aligned head-to-tail in 1D-chain-like arrays and parallel arrangement
of such arrays forms a layer; (b) the proximally placed azide and
alkyne motifs are in an orientation apt for their regiospecific
cycloaddition; (c) each monomer having x peptide bond is H-bonded
with 2x monomers disposed in intersecting arrangement, which pre-
organize 1D-chain-like arrays in adjacent layers in perpendicular
orientation. These crystals underwent spontaneous single-crystal-to-
single-crystal (SCSC) polymerization via azide-alkyne cycloaddition
reaction to form triazolyl-polyglycines, at room temperature. The
criss-crossed arrangement of monomers in adjacent layers ensured
the formation of cross-laminated polymers.
Polypeptides and polypeptide-mimics have received much
attention due to their application in variety of fields ranging from
medicine to materials.[1] Among polypeptides, polyglycine, the
homopolymer of the simplest and achiral amino acid, glycine
received special attention due to their unique structure, packing
and properties.[2] Though polyglycines are structurally simple
polymer, they adopt different packing or assembly in solid[3] and
solution states.[4] In crystal state, they form interesting
polyglycine-I[2c,2e] or unusual polyglycine-II[2a,2b] structures. While
polyglycine-I crystallises either as parallel chain networks or
antiparallel rippled sheet depending on the size of polymer
(Figure 1d), polyglycine-II chains adopt helical conformation with
each strand hydrogen bonded to six-neighboring strands (Figure
1e). Such a unique packing (supramolecular structure) gives
polyglycine-II attractive mechanical properties.[2d] Polyglycine is
used for developing modified electrodes for electrochemical
sensing of various chemicals,[5] solid pH sensor[6] and improving
the properties of thermoplastics.[7] In addition, polyglycine
domains in natural proteins such as silk are shown to play
important role in the mechanical properties of such proteins.[8]
Hence there is tremendous interest in synthetic polyglycine
mimics for materials science applications.[9] Here we report
spontaneous single-crystal-to-single-crystal polymerization, at
room temperature, of two small glycine-based peptides having
complimentary reactive motifs at their termini, to two
pseudopolyglycines possessing cross-laminated arrangement in
their crystals. Due to this exceptional packing, each n-mer chain
makes H-bonding with 4n/6n neighboring chains.
Figure 1. Chemical structure and packing of polyglycine and
pseudopolyglycines. (a) Chemical structure of polyglycine. (b) Chemical
structure of pseudopolyglycine P1. c) Chemical structure of pseudopolyglycine
P2. d) Antiparallel β-sheet arrangement of polyglycine-I chains. e) Hexagonal
packing of helical polyglycine-II chains.
We synthesized the monomers, dipeptide 1 and tripeptide 2
(Figure 2a,c, Supporting Information Section S1) and solved
their crystal structures by single crystal X-ray analysis (Table
S1). Both the peptides adopt similar molecular packing in their
crystals. In both dipeptide 1 and tripeptide 2, the molecules
arrange linearly in a head-to-tail fashion such that azide and
alkyne of adjacent molecules are juxtaposed at proximity,[15]
[a]
V. Athiyarath, Prof. Dr. K. M. Sureshan
School of Chemistry
Indian Institute of Science Education and Research
Thiruvananthapuram, Kerala-695 551 (India)
E-mail: kms@iisertvm.ac.in
Homepage: http://kms514.wix.com/kmsgroup
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Figure 2. Single crystal XRD analyses of monomers. (a) Chemical structure of dipeptide 1. (b) ORTEP diagram of dipeptide 1 (ellipsoids are drawn with 20 %
probability level). (c) Chemical structure of tripeptide 2. (d) ORTEP diagram of tripeptide 2 (ellipsoids are drawn with 20 % probability level). (e) Arrangement of
dipeptide 1 molecules in bc plane showing the proximity of azide and alkyne. (f) Arrangement of tripeptide 2 molecules in ab plane showing the proximity of azide
and alkyne. (g) Criss-crossed arrangement of dipeptide 1 molecules along a-direction. Each molecule (shown in red) is hydrogen bonded to four neighboring
molecules (shown in blue). (h) Criss-crossed arrangement of tripeptide 2 molecules along c-direction. Each molecule (shown in red) is hydrogen bonded to six
neighboring molecules (shown in blue). (i) View of dipeptide 1 molecules along a-axis. (j) View of tripeptide 2 molecules along c-axis. Four layers are shown. The
chains in adjacent layers are encoded in blue and red colors. The azide and alkyne are colored in magenta and green respectively.
(Figure 2e,f), in an orientation suitable for their topochemical
cycloaddition reaction to give triazol-1,4-diyl-polyglycines. Such
non-bonded chains align parallely with respect to each other in a
plane (‘bc’ plane in dipeptide 1 and ‘ab’ plane in tripeptide 2).
Along the direction normal to this plane, the NH…O hydrogen
bonds connect molecules arranged in criss-crossed orientations
(molecules in alternate layers are in same orientation). Each
amide group makes two H-bonds; one with a molecule in the
immediate top layer and the other with a molecule in the
immediate bottom layer. Thus each molecule of dipeptide 1
makes hydrogen-bonding, along ‘a’ axis, with four neighboring
molecules oriented at a tilt angle of 71° in adjacent layers; two in
one direction and two in opposite direction (Figure 2g,i).
adjacent layers are criss-crossed at a tilt angle of 74° (Figure
2h,j).
We observed that both the peptides 1 and 2 undergo
spontaneous azide-alkyne cycloaddition reaction in their crystals
and powders to form triazole-linked oligomers/polymers at room
temperature. Using various NMR techniques, we have
established that in both the peptides, the azide-alkyne
cycloaddition reaction is regiospecific yielding 1,4-disubstituted
triazole as anticipated based on their crystal structures
(Supporting Information Section S3). Detailed kinetics and
various time-dependent analyses (NMR, FT-IR, DSC and PXRD)
revealed that these reactions are spontaneous TAAC reactions
(Supporting Information Sections S4.1 and S4.2). Though the
initially formed oligomers were soluble in DMSO, higher
oligomers formed (after 12 days for dipeptide 1 and 9 days for
Similarly, each molecule of tripeptide
2 makes hydrogen-
bonding with six (twice the number of amide bonds) neighboring
molecules along ‘c’ axis and such H-bonded molecules in
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Figure 3. Single crystal XRD analyses of polymers P1 and P2. (a) Chemical structure of polymer P1. (b) ORTEP diagram of polymer P1 (ellipsoids are drawn with
20 % probability level). (c) Chemical structure of polymer P2. (d) ORTEP diagram of polymer P2 (ellipsoids are drawn with 20 % probability level). (e)
Arrangement of polymer P1 chains in bc plane. (f) Arrangement of polymer P2 chains in ab plane. (g) An n-mer chain of polymer P1 (shown in red) hydrogen
bonded to 4n chains (shown in blue). A trimer fragment H-bonded to twelve chains is shown. (h) An n-mer chain of polymer P2 (shown in red) hydrogen bonded to
6n chains (shown in blue). A trimer fragment H-bonded to eighteen chains is shown. (i) View of polymer P1 molecules along a-axis. (j) View of polymer P2
molecules along c-axis. Four layers are shown. The polymer chains in adjacent layers are shown in blue and red colors. Triazole motif is colored in orange.
tripeptide 2) were only partially soluble. Hence the, solution state
1H NMR in DMSO-d6 do not reflect the real reaction conversion.
However, comparison of CP-MAS 13C NMR spectra of
monomers 1 and 2 with corresponding polymers P1 and P2
revealed that the polymerizations are nearly quantitative
(approximately 95 % for P1 and 93 % for P2, Supporting
Information Section S4.3). Topochemical polymerization
reactions that proceed to conversions of 75 % and above are
known to yield polymers.[16] Solvent-free MALDI-TOF mass
spectra of polymers P1 and P2 showed polymers with mass
corresponding to 17-mers and 13-mers (3.3 kDa for both
polymers) respectively (Supporting Information Section S4.4).
The average mass of these polymers could not be determined
due to their poor solubilities.
chains were arranged along ‘bc’ and ‘ab’ planes in polymers P1
and P2 respectively (Figure 3e,f). The alternate layers form
cross-laminated arrangement in the crystal (Figure 3g-j). The
angles of crossing of both the polymer chains in adjacent layers
have increased by 12-13° compared to respective monomers.
Also, the unit cell parameters have undergone changes after the
polymerization. Compared to monomers (1 and 2), the unit cell
parameters ‘a’ and ‘c’ shrank by 4.9 % and 17.3 % for polymer
P1; 15.2 % and 2.7 % for polymer P2, whereas the parameter ‘b’
elongated by 21 % for polymer P1; 16.9 % for polymer P2. This
resulted in an overall reduction in the unit cell volume (4.8 % for
polymer P1 and 4.7 % for polymer P2) of the polymer crystals
compared to the respective monomers. These changes are
expected in view of the molecular arrangement in the monomer
crystals and the replacement of non-covalent linkages with
covalent linkage between monomers as a result of the TAAC
reaction. Such drastic changes in unit cell parameters as a result
of topochemical polymerizations are rare.[11b,c,13g,17d,e,18] The
hydrogen bonding pattern present in monomers 1 and 2 are not
only conserved in their polymers too but also the interactions
became stronger (as evident from the reduction in the H-bonding
distances). As monomers 1 and 2 make contacts with four and
six neighboring molecules respectively, each n-mer chain of
their polymers P1 and P2 make strong H-bonding interactions
Time-dependent PXRD studies suggested that the
crystallinity is preserved during the reaction (Figure S10,14). We
were curious to know whether these reactions are single-crystal-
to-single-crystal polymerization reactions.[17] We have solved the
crystal structures of both the crystals aged at room temperature
for 20 days by single crystal X-ray diffraction (Supporting
Information Section S5). We found that the polymerization
reaction proceeded with the conservation of the space group
(Pca21 for dipeptide 1 and its polymer P1; Cc for tripeptide 2 and
its polymer P2). As anticipated, 1,4-triazolyl linked polymer
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with 4n and 6n chains respectively (Figure 3g,h). Hence, in
these cross-laminated layered packing, the number of polymer
chains a single chain can interact (H-bond) with depends on the
size (or degree of polymerization) of the chain itself. It is to be
noted that in the case of polyglycine-I and polyglycine-II each
chain can only interact with two and six chains of similar kinds
respectively.
± 139 MPa). The difference in Young’s moduli of polymers P1
and P2 may be due to the sensitivity of nanoindentation
technique to the surface roughness.
Thermogravimetric analyses were performed to verify the
enhancement of thermal stabilities of P1 and P2 due to their
cross-laminated topologies. We dissolved the completely
polymerized polymers P1 and P2 in 7 % TFA-HFIP (solubility 3.3
mg/mL and 3.1 mg/mL for P1 and P2 respectively) and
evaporated the solvents under reduced pressure to get
randomly arranged polymers. MALDI-TOF spectra of both
randomly aligned polymers of P1 and P2 were similar to those of
corresponding crystalline polymers (Supporting Information
Figures S43, S44). This confirms that random polymers have
same molecular composition as their corresponding crystalline
polymers. TGA analyses revealed that these randomly oriented
polymers decompose in the temperature range of 120 °C-300 °C
whereas the crystalline polymers P1 and P2 decompose at
temperatures around 400 °C (Figure 5a,b). This implies that
random polymers are less stable than the cross-laminated
polymers formed in the TAAC reaction. The additional stabilities
of TAAC polymers clearly stem from their packing (Figure 3i,j).
Material strength and solubility are inversely related and both
Lamellar packing, (Figure 4a,b) wherein straight or folded
polymer chains unidirectionally stack perpendicular to their
length-axis is a very common topology for 1D polymers.[19]
Crystallizing purely organic 1D polymer into a weave or cross-
laminated topology (Figure 4f) is challenging. Yaghi et al.[20] and
Mayor et al.[21] elegantly exploited the temporary metal-co-
ordination to pre-organize the monomers such that the 1D-
polymer formed after the covalent linkage, adopt interwoven
topologies (Figure 4c-e). In this context, two fully organic
polymers in novel cross-laminated topology reported here is very
interesting. Such an ordered arrangement of polymers is only
possible by topochemical synthesis. Also, it is not possible to
synthesize the polymers of 1 and 2 by conventional solution-
phase synthesis as the cycloaddition reactions of monomers 1
and 2 in solution gave cyclic oligomers (Supporting Information
Section S6).
depend on the crystal packing (molecular interactions).[23]
A
comparison of the solubilities of the products formed in the
TAAC reaction with that of corresponding randomly aligned
Figure 4. Illustrations showing various ordered packing possible for linear 1D-
polymer chains. (a,b) lamellar packing. (c-e) various interwoven packing. (f)
cross-laminated packing.
Well oriented arrangement is known to impart enhanced
mechanical strength and stiffness in aligned polymers.[22] In
order to verify the enhancement in mechanical strength of
polymers due to the cross-laminated packing, nanoindentations
were done on the crystals of monomers (1 and 2) and polymers
(P1 and P2). The Young’s moduli of polymers (P1 and P2) were
found to be almost twice that of the corresponding monomers
(Supporting Information Section S7). The Young’s modulus of
dipeptide 1 crystal was found to be 10.18 ± 0.55 GPa (hardness
661.44 ± 52 MPa) and that of polymer P1 was 20.81 ± 0.75 GPa
(hardness 1574.4 ± 280 MPa). Similarly the Young’s modulus of
tripeptide 2 was 7.14 ± 0.84 GPa (hardness 394.54 ± 35 MPa)
and that of polymer P2 was 12.49 ± 0.71 GPa (hardness 918.93
Figure 5. Comparison of thermal stability of crystalline and random polymers
of P1 and P2 (a) TGA profiles of randomly oriented polymer P1 and crystalline
polymer P1. (b) TGA profiles of randomly oriented polymer P2 and crystalline
polymer P2.
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In conclusion, we have reported spontaneous evolution of
two polymerizable monomers into their respective 1D-polymers
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peptides terminally modified with complimentary reacting motifs
(CRMs), viz. azide and alkyne, crystallize via head-to-tail
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bonds is H-bonded with ‘2x’ monomers arranged in criss-
crossed manner in adjacent layers. The crystals undergo
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polypeptides in SCSC manner. As the monomers adopted the
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polymers adopted hitherto unknown cross-laminated packing,
wherein each polymer chain having ‘x’ amino acids is H-bonded
to ‘2x’ polymer chains. As mechanical property of the polymer
depends upon the nature of packing of polymer chains, these
novel polyglycine-mimics with an interesting packing might find
applications akin to polyglycines. This is the first report of
spontaneous SCSC polymerization of peptide derivatives.
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Acknowledgements
We thank Dr. Sujit K. Ghosh, IISER Pune, for supporting with
single crystal X-ray diffraction measurements and Prof. Prita
Pant, IIT Bombay, for supporting with the nanoindentation
studies. KMS thanks Department of Science and Technology,
India for a SwarnaJayanti Fellowship.
Conflict of interest
The authors declare no competing financial interest.
Keywords: single-crystal-to-single-crystal (SCSC) •
cycloaddition • polymerization • pseudopolyglycines • cross-
laminated
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Entry for the Table of Contents
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Vignesh Athiyarath, and Kana M.
Sureshan*
Spontaneous Single-Crystal-to-
Single-Crystal Evolution of Two
Cross-laminated Polymers
Spontaneous evolution of polymers with novel topology. Two glycine-derived
peptides having azide and alkyne as complementary reacting motifs, arranged in
criss-crossed layered packing in their crystals and spontaneously evolved, at room
temperature, to corresponding polymers in a single-crystal-to-single-crystal fashion
via Topochemical Azide-Alkyne Cycloaddition reaction. The criss-crossed
arrangement of the monomers ensued novel cross-laminated topology for the
resulting polymers, wherein each polymer chain having ‘n’ monomer units is H-
bonded with 4n or 6n chains.
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