Crystallization-driven constitutional changes of dynamic polymers in response to neat/solution conditions

Article abstract of DOI:10.1039/b713413d

Dynamic polymers (dynamers) based on reversible imine interactions were generated and found to respond to changes in neat/solution environment, thus displaying adaptive behavior through modification of their constitution in order to maximize the stability

Full text of DOI:10.1039/b713413d

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Crystallization-driven constitutional changes of dynamic polymers in
response to neat/solution conditions{
Cheuk-Fai Chow,a Shunsuke Fujiiab and Jean-Marie Lehn*a
Received (in Cambridge, UK) 31st August 2007, Accepted 24th September 2007
First published as an Advance Article on the web 4th October 2007
DOI: 10.1039/b713413d
Am1 or Am2 in 1 : 1 molar ratios in the presence of anhydrous
Na2SO4 in chloroform10 (Fig. 1). 1H-NMR spectroscopy con-
firmed the formation of the molecular polymers by the appearance
of the corresponding imine proton signals (Fig. 2a–d). Polymer P1,
which is the condensation material between Ald1 and Am2, was an
opaque solid giving an isotropic phase at 72.5 uC. P2, the material
Dynamic polymers (dynamers) based on reversible imine
interactions were generated and found to respond to changes
in neat/solution environment, thus displaying adaptive behavior
through modification of their constitution in order to maximize
the stability of their mesoscopic state as a function of
conditions.
Dynamic covalent chemistry (DCC),1,2 the molecular wing of
constitutional dynamic chemistry,3 involves the generation of sets
of molecular constituents from components linked through
reversible interactions. The combination of polymer chemistry
with DCC defines an area of constitutional dynamic polymer
chemistry4–6 and offers possibilities to develop adaptive materi-
als.3,7 Constitutional dynamic polymers, dynamers, are polymeric
entities based on monomeric components connected through
either reversible covalent bonds4,6 or labile non-covalent interac-
tion.4,8 By virtue of the lability of these connections, they are
constitutional dynamic materials capable of constitutional varia-
tion through exchange and reshuffling of their components.
Consequently, dynamers may be expected to respond to external
factors by shifting the equilibria of the system to the most stable
state, allowing them to express or fine-tune a given property under
different environmental conditions.5c,f
Amplification of a given dynamer of a dynamic combinatorial
library (CDL) may be achieved by stabilizing its microscopic
structures.5c,f,6c On the other hand, advantage may be taken of the
mesoscopic features of dynamers. Thus, amplification of a given
dynamer of a CDL under the pressure of a self-organization
process,9 such as the formation of an anisotropic phase, is of
special interest. In principle, it can influence the library composi-
tion, favouring those species that form the most stable phase, such
as solid state crystalline domains.
Here we describe our studies of constitutional dynamic polymers
that are able to exchange and reshuffle their components through
imine reversible bonding. As a consequence, these dynamers can
adapt to different environmental conditions that stabilize/destabi-
lize the mesoscopic states of the CDL members by generating
different constitutional expressions.
The polymers P1–P4, which are connected through imine
reversible bonds, have been obtained by polycondensation of the
dialdehyde monomers Ald1 or Ald2 with the diamine monomers
aISIS, Universite´ Louis Pasteur, 8 alle´e Gaspard Monge, F-
67083 Strasbourg cedex, France. E-mail: lehn@isis.u-strasbg.fr
bR&D Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura,
Chiba, 299-0265, Japan
{ Electronic supplementary information (ESI) available: Experimental
details and 1H-NMR spectra of dialdehydes Ald1 and Ald2 and polymers
P1–P4. See DOI: 10.1039/b713413d
Fig. 1 (Top) Structure of the diamines Am1 and Am2, of the dialdehydes
Ald1 and Ald2 and of the polymers obtained by polycondensation: P1
(Ald1 + Am2), P2 (Ald2 + Am1), P3 (Ald1 + Am1) and P4 (Ald2 + Am2).
The domains P1d, P2d, P3d and P4d are shown in red. ‘‘d’’ stands for
domain of polymer. (Bottom) Physical aspect of the polymers (a) P1:
opaque solid; (b) P2: stretchy film; (c) P3: viscous oil; (d) P4: brittle film.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4363–4365 | 4363

View Article Online
ratio for 24 h with 1% pentadecafluorooctanoic acid (catalyst). Its
1H-NMR spectrum showed four characteristic imine proton
signals, which were assigned to (1) the connection between Ald1
and Am2, i.e. P1d domain (at 8.42 ppm); (2) the connection
between Ald2 and Am1, i.e. P2d domain (at 8.36 ppm); most
importantly, (3) a new connection between Ald1 and Am1, i.e. P3d
domain (at 8.23 ppm); and (4) another new connection between
Ald2 and Am2, i.e. P4d domain (at 8.57 ppm). Blsol may thus be
considered as a random copolymer resulting from recombination
of all four components; it displays the four different domains in
ratio P1d : P2d : P3d : P4d determined to be about 3 : 3 : 2 : 2
(Fig. 2e).
Most importantly, these dynamic features resulted in an
adaptive behavior of the dynamer system. Indeed, blending of
the parent dynamers, P1d and P2d, in neat conditions at 80 uC for
1
24 h gave Blneat. H-NMR analyses allowed identification of the
different constitutional changes of the blend Blneat with respect to
those of the blend Blsol. The spectrum of Blneat showed only two
characteristic imine proton signals, which were assigned to (1) a
new connection between Ald1 and Am1, i.e. P3d domain (at
8.23 ppm); and (2) another new connection between Ald2 and
Am2, i.e. P4d domain (at 8.57 ppm). Thus, under neat conditions,
the dynamic system reshuffled and exchanged its components in a
different way from the solution conditions. The percentage
composition of P1d : P2d : P3d : P4d in the blend Blneat was
0 : 0 : 5 : 5, revealing the presence of only the P3d and P4d
domains, while the P1 and P2d domains were repressed in the
constitution of the system under neat conditions (Fig. 2f). In
contrast, the formation of all four domains was obtained for Blsol.
Furthermore, subjecting Blsol or Blneat to successive sol–neat
cycles by dissolution–solvent removal (evaporation) operations
showed reversible switching between the B1sol and the Blneat
constitutional states without apparent fatigue (Fig. 3).
Fig. 2 Part of the 1H-NMR spectra of polymers: (a) P1; (b) P2; (c) P3;
(d) P4, (e) blend Blsol and (f) blend Blneat in CDCl3 (ca. 5 mM) showing the
C–H proton signals of the imine or aromatic groups. The symbols identify
the characteristic proton signals of the reference polymers in the spectra of
B1sol and B1neat. The 1H-NMR spectra were taken within 10 min after
dissolving blend Blneat in CDCl3.
The constitutional evolution of the dynamic polymer systems
can be explained by the formation of the crystalline copolymer P4
in the CDL (Scheme 1). Under the neat conditions at 80 uC, P1,
from Ald2 and Am1, was obtained as a stretchy film, becoming an
isotropic phase at 68.4 uC. P3, the condensation material between
Ald1 and Am1, was a viscous oil with an isotropic phase at less
than 40 uC. P4, the material between Ald2 and Am2, gave a brittle
polymer film presenting the highest isotropic transition tempera-
ture at 159.6 uC because of the presence of strong mesogen. (Fig. 1)
Integration of the very small remaining 1H-NMR signal
for terminal aldehyde protons gave a molecular weight of
34 000 g mol21 (y50 repeating units) for P3. On the other hand,
this signal was not observable for P1, P2 and P4. The ratio
between the signals of their new imine proton of the a-CH2 of
siloxane moiety for P1, P2 and P4 and the a-CH2 of the
oligo(ethylene oxa) moiety was exactly 2 : 4, indicating that
polymerization had proceeded beyond the NMR detection limit.
The molecular weights of these polymers were roughly estimated
as ¢50 000 g mol21 (using an 1H-NMR detection limit of
1 mol%). The dynamic behavior of the present molecular
polymers, resulting from the reversibility of the imine bonds, was
demonstrated by the occurrence of ligand exchange and
recombination, as shown by 1H-NMR studies of polymer blends.
Polymer blend Blsol was prepared by just mixing solutions of the
homopolymers P1 and P2 in CDCl3 (ca. 5 mM) in equal molar
Fig. 3 Constitutional dynamic interconversion between the polymer
blends Blsol in solution and Blneat in neat conditions. The percentages of
P3d and P4d domains are calculated from the integration of the imine CH
1H-NMR signals of the blends in CDCl3 (ca. 10 mM). At 100%, the
composition is that of Blneat (P3 + P4). The 1H-NMR spectra were taken
within 10 min after dissolving blend Blneat in CDCl3.
4364 | Chem. Commun., 2007, 4363–4365
This journal is ß The Royal Society of Chemistry 2007

View Article Online
(3) Formation of Blsol from (P1 + P2) or from Blneat amounts to
a randomization giving a copolymeric material containing all four
monomeric components Am1, Am2, Ald1 and Ald2.
(4) Formation of Blneat from the Blsol random copolymer in the
neat state upon solvent removal amounts to a derandomization
giving a mixture of only the polymers P3d and P4d. This implies a
process of self-selection under the pressure of the formation of an
organized phase, the crystalline copolymer P4.
(5) The system described here represents a constitutional
dynamic material displaying adaptive behavior by constitutional
dynamic interconversion between two constitutional states in
response to a change in physical stimuli (‘‘solution’’ or ‘‘neat’’
conditions).
The present results demonstrate the generation of dynamic
imine polymers displaying constitutional dynamics by reorganiza-
tion and exchange of the monomers in the polymer chains through
bond recombination between their dialdehyde and diamine
components. More importantly, these dynamers can adapt to
different environmental conditions that stabilize/destabilize the
mesoscopic states of the CDL members. Last but not least, the
dynamic selection processes can be reversibly switched over several
interconversion cycles without significant fatigue. In more general
terms, dynamers belong to a class of materials that may respond to
chemical effectors or physical stimuli by constitutional variation,
i.e. adaptive materials.
Scheme 1 Schematic representation of adaptive behavior of the dynamic
polymer system in response to a change in physical state: solution and neat
acting as stimuli.
Notes and references
1 (a) J.-M. Lehn, Chem.–Eur. J., 1999, 5, 2455–2463; (b) J.-M. Lehn and
A. V. Eliseev, Science, 2001, 291, 2331–2332.
2 (a) S. J. Rowan, S. J. Cantrill, G. R. L. Cousins, J. K. M. Sanders and
J. F. Stoddart, Angew. Chem., Int. Ed., 2002, 41, 898–952; (b)
P. T. Corbett, J. Leclaire, L. Vial, K. R. West, J.-L. Wietor, J. K. M.
Sanders and S. Otto, Chem. Rev., 2006, 106, 3652–3711.
3 (a) J.-M. Lehn, Polym. Int., 2002, 51, 825–839J; (b) J.-M. Lehn, Science,
2002, 295, 2400–2403; (c) J.-M. Lehn, Chem. Soc. Rev., 2007, 36,
151–160.
Fig. 4 Photographs taken with
a polarized light microscope in
transmission mode (640) at 80 uC: (a) isotropic phase of homopolymer
P1 at 80 uC, (b) isotropic phase of homopolymer P2 at 80 uC and (c)
anisotropic phase of polymer blend Blneat (P3 + P4) at 80 uC.
4 J.-M. Lehn, Prog. Polym. Sci., 2005, 30, 814–831.
5 (a) T. Ono, S. Fujii, T. Nobori and J.-M. Lehn, Chem. Commun., 2007,
46–48; (b) N. Sreenivasachary, D. T. Hickman, D. Sarazin and
J.-M. Lehn, Chem.–Eur. J., 2006, 8581–8588; (c) N. Giuseppone,
G. Fuks and J.-M. Lehn, Chem.–Eur. J., 2006, 1723–1735; (d) T. Ono,
T. Nobori and J.-M. Lehn, Chem. Commun., 2005, 1522–1524; (e)
W. G. Skene and J.-M. Lehn, Proc. Natl. Acad. Sci. U. S. A., 2004, 101,
8270–8275; (f) N. Giuseppone and J.-M. Lehn, J. Am. Chem. Soc., 2004,
126, 11448–11449.
6 (a) H. Otsuka, K. Aotani, Y. Higaki and A. Takahara, J. Am. Chem.
Soc., 2003, 125, 4064–4065; (b) X. Chen, M. A. Dam, K. Ono, A. Mal,
H. Shen, S. R. Nutt, K. Sheran and F. Wudl, Science, 2002, 295,
1698–1702; (c) K. Oh, K.-S. Jeong and J. S. Moore, Nature, 2001, 414,
889–893.
7 J.-M. Lehn, in Supramolecular science: where it is and where it is going,
ed. R. Ungaro and E. Dalcanale, Kluwer, Dordrecht, 1999, pp. 287–
304.
8 (a) L. Brunsweld, B. J. B. Folder, E. W. Meijer and R. P. Sisjbesma,
Chem. Rev., 2001, 101, 4071–4197; (b) J.-M. Lehn, Polym. Int., 2002, 51,
825–839; (c) Supramolecular Polymers, ed. A. Ciferri, Taylor &
Francis, Boca Raton, 2nd edn, 2005; (d) J.-M. Lehn in ref. 3a ch. 1,
p. 3.
P2, P3 and the mixed polymers existed as isotropic liquids, while
P4 was the only crystalline member. This strong driving force
shifted the equilibria of the CDL towards P4 and, as consequence,
generated also agonistically11 the copolymer P3. As shown in
Fig. 4, both neat parent dynamers, P1 and P2, existed as isotropic
liquids at 80 uC, while an anisotropic phase grew up after stacking
them together in the same conditions. On the other hand, when the
polymer blend Blneat was dissolved in an organic solvent, the
dissolution of the crystalline phase allowed the dynamic system to
re-adapt, reshuffle and re-exchange the monomers so as to
generate the four imine domains of P1, P2, P3 and P4.
The processes described here present several aspects of adaptive
behavior in a system of covalent dynamic polymers in response to
a change in physical state (Scheme 1).
(1) The parent dynamers P1 and P2 generated two offspring
dynamer blends, a random copolymer Blsol when mixed in
solution, and a mixture Blneat of the two polymers P3 and P4 in
neat conditions.
9 (a) J. R. Nitschke and J.-M. Lehn, Proc. Natl. Acad. Sci. U. S. A., 2003,
100, 11970–11974; (b) N. Screenivasachary and J.-M. Lehn, Proc. Natl.
Acad. Sci. U. S. A., 2005, 102, 5938–5943.
10 Synthetic procedures and compounds will be described in detail in a full
paper. All new compounds have been characterized by the usual
spectroscopic and analytical techniques.
(2) This adaptive behavior is retained in the generation of the
offspring systems B1sol and Blneat, which can be reversibly switched
over several dissolution–evaporation cycles.
11 N. Giuseppone and J.-M. Lehn, Chem.–Eur. J., 2006, 12, 1715–1722.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4363–4365 | 4365