A Simply Synthesized, Tough Polyarylene with Transient Mechanochromic Response

Article abstract of DOI:10.1002/anie.201709142

A simple and high-yielding route to tough polyarylenes of the type poly(meta,meta,para-phenylene) (PmmpP) is developed. PmmpP is tough even in its as-synthesized state which has an intermediate molar mass of M_w≈60 kg mol^?1 and exhibits outstanding mechanical properties at further optimized molecular weight of M_w=96 kg mol^?1, E=0.9 GPa, ?=300 %. Statistical copolymers with para,para-spiropyran (SP) are mechanochromic, and the toughness allows mechanochromism to be investigated. Strained samples instantaneously lose color upon force release. DFT calculations show this phenomenon to be caused by the PmmpP matrix that allows build-up of sufficiently large forces to be transduced to SP, and the relatively unstable corresponding merocyanine (MC) form arising from the aromatic co-monomer. MC units covalently incorporated into PmmpP show a drastically reduced half life time of 3.1 s compared to 4.5 h obtained for SP derivatives with common 6-nitro substitution.

Full text of DOI:10.1002/anie.201709142

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Accepted Article
Title: A Simply Synthesized, Tough Polyarylene with Transient
Mechanochromic Response
Authors: Fabian Kempe, Oliver Brügner, Hannah Buchheit, Sarah
Noemi Momm, Felix Riehle, Sophie Hameury, Michael Walter,
and Michael Sommer
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709142
Angew. Chem. 10.1002/ange.201709142
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10.1002/anie.201709142
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A Simply Synthesized, Tough Polyarylene with Transient
Mechanochromic Response
Fabian Kempe,[a,b] Oliver Brügner,[a] Hannah Buchheit,[c] Sarah N. Momm,[c] Felix Riehle,[c] Sophie
Hameury,[c,d] Michael Walter[a] and Michael Sommer*[a,b]
Abstract: A simple, straightforward and high-yielding route to tough
polyarylenes of type poly(meta,meta,para-phenylene) (PmmpP) is
developed. PmmpP is already tough in its as-synthesized state at
intermediate molar mass of Mw ~60 kg/mol and exhibits outstanding
mechanical properties for further optimized molecular weight
(Mw = 96 kg/mol, E = 0.9 GPa, ε = 300 %). Statistical copolymers with
para,para-spiropyran (SP) are mechanochromic, whereby the tough
behavior allows to investigate mechanochromism. Strained samples
instantaneously lose color upon force release. DFT calculations show
this phenomenon to be caused by i) the tough PmmpP matrix that
allows build-up of sufficiently large forces to be transduced to SP, and
ii) the relatively unstable corresponding merocyanine (MC) form
arising from the aromatic comonomer. MC units covalently
incorporated into PmmpP show a drastically reduced half life time of
3.1 s compared to 4.5 h obtained for SP derivatives with common
6-nitro substitution.
polyarylenes are inherently more chemically stable than
polycarbonates due to the exclusive presence of aryl-aryl bonds
in the backbone. Subsequently, Schlüter et al. investigated
various kinked polyarylenes, with some of them exhibiting
toughness at high molecular weight after fractionation.[3–7]
Our interest in tough polyarylenes stems from their potentially
ideal use as stable, tough yet amorphous matrices for covalent
incorporation of mechanochromic dyes such as spiropyrans
(SPs).[8–10] SPs isomerize to their colored merocyanine (MC) form
under a number of external stimuli, including force, and can
therefore be used as mechanical force sensors.[11–15] Compared
to matrix polymers used so far in combination with covalently
linked SPs, tough polyarylenes have several distinct advantages.
Their toughness allows drawing samples and thus to transduce
mechanical force to SP. Also, they are typically amorphous,
hence mechanochromism can be investigated within an isotropic
matrix not possible with semicrystalline polymers. Here we show
that the nature and high strength of polyarylenes is of striking
additional advantage in that high forces can be transduced to SP
comonomers.
Polyarylenes have intrigued material scientists since the
invention of transition metal catalyzed cross coupling
reactions.[1,2] In 2007, Schlüter et al. introduced a high molecular
weight (MW) Poly(meta,para-phenylene) PmpP by careful
selection of catalyst, solvent mixture, concentration and
temperature.[3] In their pioneering work a crude polymer of
Mw = 83 kg/mol was synthesized and the MW further increased to
Mw = 255 kg/mol by fractionation. After fractionation, this material
showed a toughness close to that of aromatic polycarbonates
(E = 1 GPa, ε = 122 %). This was a remarkable feat both from a
synthetic as well as a material science point of view, because
We found existing protocols for kinked polyarylene synthesis
to be cumbersome, expensive and inefficient due to significant
loss of polymer during fractionation.[3,4,7] Moreover, existing
polyarylenes show limited solubility and high glass transition
temperatures (Tg) of up to 160 °C, which required processing at
too high temperatures for thermolabile SP copolymers. The
design of new, tough polyarylenes satisfying all mentioned criteria
required straightforward synthesis of monomers starting from
inexpensive compounds. Therefore, we designed a new kinked
polyarylene, Poly(meta,meta,para-phenylene) PmmpP. Para-
bromination of 2,2’-biphenol 1 results in the solid monomer 2,
which can be conveniently purified by recrystallization. Suzuki
polycondensation (SPC) with 3 gives the kinked polyarylene
PmmpP with a “double meta-” linkage in its backbone (Scheme
1). High weight average molecular weights Mw ~ 56-96 kg/mol
were obtained with almost all molecular weights showing a yield
point. Details regarding retrosynthetic analysis, molecular and
mechanical characterization are given in the Supporting
Information.
[a]
M.Sc. F. Kempe, M.Sc. O. Brügner, PD Dr. M. Walter,
Prof. Dr. M. Sommer,
Freiburg Center for Interactive Materials and Bioinspired
Technologies (FIT)
Georges-Köhler-Allee 105, 79110 Freiburg, Germany
E-mail: michael.sommer@chemie.tu-chemnitz.de
[b]
[c]
Present address: M.Sc. F. Kempe, Prof. Dr. M. Sommer
Professur Polymerchemie
TU Chemnitz
Str. der Nationen 62, 09111 Chemnitz, Germany
B.Sc. H. Buchheit, B.Sc. S. N. Momm, B.Sc. F. Riehle,
Dr. S. Hameury
Institute for Macromolecular Chemistry
Albert-Ludwigs-Universität Freiburg
Stefan-Meier-Str., 79104 Freiburg, Germany
SPs have been covalently incorporated into various polymer
architectures.[11,14,16,17] Notably, almost all of these systems use
6-nitro-SP derivatives. The nitro group lowers the energy of the
corresponding MC form and hence facilitates SP → MC
isomerization.[18] The previously reported main-chain alternating
para,para-SP copolymers did not show toughness and hence the
mechanochromic behavior of the para-linked SP without the nitro-
substituent could not be investigated using uniaxial positive
strain.[8–10]
[d]
Present address: Dr. S. Hameury
Laboratoire Hétérochimie Fondamentale et Appliquée
(UMR CNRS 5069)
Université de Tolouse, UPS
118, route de Narbonne, Bât. 2R1, 31062 Toulouse, France
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201709142
Angewandte Chemie International Edition
COMMUNICATION
mechanochromism regardless, the high strength of PmmpP of up
to 50 MPa at yield is probably key (see Supporting Info). Secondly,
once MC forms under the conditions of the stress-strain
experiment, it is only stable under the applied force due to the
discussed substitution pattern, but immediately isomerizes back
to SP when the force is released. This behavior of a rather
unstable MC form leads to the observed transient
mechanochromic response.
To further corroborate this assumption and get more insight
into the uncommon mechanochromic behavior of the SP
copolymer, density functional theory (DFT) calculations were
performed. Bond breaking under the action of an external force
퐹
mostly is a stochastic process as it is initiated by thermal
ext
fluctuations.[19–22] Only in case of extremely rapid pulling, thermal
effects are overcome and the force corresponding to the maximal
derivative of the potential gets visible. The force needed to break
a bond thus depends on the loading rate 훼 = 푑퐹ext/푑푡 . We
modeled two SP derivatives, one having the SP substitution
pattern as in PmmpP and “para,para- pulling direction”, and one
with the commonly used 6-nitro group (SP-NO2) and ”para, ortho-
pulling direction”.[23,24]
The structures are depicted in Figure 2. Calculations were
performed within the projector augmented wave method as
implemented in the GPAW package.[25,26] The exchange-
correlation energy was approximated as devised by Perdew,
Burke, and Ernzerhof.[27]
Scheme 1. Top: Retrosynthetic analysis of PmmpP compared to previous
work.[3] Bromination of 2,2’-biphenol leads to a “double meta-” linkage in the
backbone. Bottom:Synthesis of PmmpP. * = non-commercial Pd2dba3.
The mechanochromic response of PmmpP with 2 mol-%
copolymerized p,p-SPBr2 is shown in Figure 1 (Mw = 134 kg/mol,
Ð = 5.5). For the first time, copolymers with p,p-SPBr2 were
mechanochromic under positive strain. Yet more remarkable is
the observation that samples immediately lost their color upon
force release. Figure 1 shows that repeated switching was
possible for 25 times. This is a marked difference compared to
SP-based mechanochromic systems reported so far showing
persistent colors.[10,13,17]
Figure 1. Tensile experiment of PmmpP containing 2 mol-% p,p-SPBr2.
a) Structures of SP (top) and MC (bottom) in the PmmpP matrix. b) Pictures of
repeated strained and released samples. Pictures were adjusted for brightness
and contrast. c) Intensity of green color in the mid-section of the specimen
during repeated force build-up and release. The time scale corresponds to the
change in force as shown in Fig. S5.
We hypothesized the reason for the observed transient
mechanochromic behavior of PmmpP with instantaneous loss of
color upon force release is two-fold. Firstly, covalent incorporation
of SP through para-linkages compared to commonly used ortho-
linked SP mechanophores leads to a rather inefficient molecular
lever and transduction of mechanical force to the weak C-O bond.
The replacement of the commonly used nitro-group in 6-position
by the phenyl ring of comonomer 3 further adds up to this effect,
as the electronic effect of the phenyl ring destabilizes the MC form
compared to the nitro group.[18] In order to be able to observe
In order to obtain the force needed for the transition from SP to
MC, we applied a two-dimensional Constrained Geometries
Simulate External Force (COGEF) method, where the energy
barriers in dependence of 퐹 are determined (see supporting
ext
info for details).[22,28]
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We found two energy barriers with an instable intermediate state
along the transition, which belong to the C-O cleavage (increasing
of bond length b) and the cis-trans isomerization by rotation of the
dihedral angle 훽 (see Fig. 2a,b). Figure 2c depicts the rate
determining total energy barriers in forward 훥퐺푓,퐭퐨퐭퐚퐥(퐹ext) and
backward 훥퐺푏,퐭퐨퐭퐚퐥(퐹ext) direction which are relative to the
energies of the SP and MC forms, respectively. For small forces
the barrier for 훽 rotation is decisive, while for large forces it is C-O
bond breaking. While this behavior is similar in both SP
derivatives, the transition happens at much lower external force
for SP-NO2 compared to SP in PmmpP. Fig. 2c also shows that
the forward barrier for nitro is smaller than for polyarylene in a
large force range. The transition rates 푘 can be obtained from the
barriers using the Eyring equation[20,29]
backbone that are tough without prior purification and at
intermediate molecular weight. Our rational monomer design
employs inexpensive and readily available 2,2’ biphenol as a
푘퐵푇
∆퐺
푘 =
exp (−
),
(1)
ℎ
푘퐵푇
where 푘퐵 is the Boltzmann and ℎ the Planck constant. From
these the probabilities for the presence of each isomer at fixed
force and temperature 푇 can be determined through solving the
corresponding coupled rate equations for the probabilities
푃 (퐹ext) with 푆 = SP, IN, MC (see supporting info for details).
푆
Coloration happens as soon as the C-O bond breaks. Therefore
the
probability
for
the
colored
form
is
푃 = 1 − 푃SP = 푃 + 푃MC. The average force for coloration 퐹 is
푐
IN
then obtained as
∞
푑푃 (퐹
)
푐
ext
퐹 = ∫
퐹
ext
푑퐹
ext
(2)
푑퐹
ext
0
By assuming a constant loading rate 훼 , integral (2) can be
evaluated and leads to 퐹(훼) as shown in Figure 2d. The rupture
forces for SP in PmmpP are larger compared to SP-NO2 within a
wide range of loading rates except for very large values of
α ≥ 105 nN/s. This is a result of the higher forward energy barrier
for SP in PmmpP for low external forces that is decisive for small
loading rates (Fig. 2d). Only for very high loading rates external
forces 퐹 ≥ 0.7 nN are reached and SP in PmmpP isomerizes
ext
faster than SP-NO2. The sharp increase of the rupture force for
SP-NO2 and the deeper understanding of the force-dependent
energy barriers obtained from the method of calculation are
subject of future studies.
Here, we estimate the loading rate to be less than
α = 1 nN/s such that the rupture force for SP in PmmpP is larger
by more than a factor of two. This explains why SP in PmmpP
isomerizes at larger mechanical stress than the commonly used
SP-NO2. Figure 2c also shows that the backward barrier of
SP-NO2 is substantially higher than that of SP in PmmpP. As the
SP isomer is energetically favored, discoloration happens
spontaneously with a theoretical half time of 4.5 h for SP-NO2, but
only with 3.1 s for SP in PmmpP, which is in excellent agreement
with the fast de-coloration observed in the experiment.
Figure 2. a), b) Chemical structures of SP derivatives modelled with arrows
indicating pulling directions. The definitions of the pulling distance d, the bond
distance b and the dihedral angle β between olefinic protons are indicated.
c) Gibbs energy barriers in dependence of the external force corresponding to
the polyarylene and nitro-substituted spiropyran. d) Force needed for coloration
F in dependence of the loading rate α for the two SP derivatives.
Next to these unique electronic substituent effects, the high
strength and elongation at break enabling transient
mechanochromic response, the SP in PmmpP additionally
benefits from the strong absorption of UV light of PmmpP (see
supporting info), which may reduce photo-bleaching as a major
drawback of SP-based materials considerably.[30–32]
starting material, from which solid monomers can be made in high
purity by simple recrystallization in multigram scale. The
availability of highly pure and solid monomers allowed for high
molecular weight polycondensates to be made, which further
improved mechanical properties to outstanding values.
Copolymers of PmmpP with spiropyran are mechanochromic
under positive uniaxial strain, but lose color instantaneously after
In conclusion, we designed a simple and high-yielding synthetic
route to polyarylenes of type PmmpP with meta,meta,para
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Acknowledgements
F. K., O. B., M. W. and M. S. acknowledge funding from the DFG
(SO 1213/ 7-1, SO 1213/ 8-1, MW 1687/ 9). O. B. and M. W. are
grateful for computational resources from FZ-Julich (HFR08) and
̈
from the bwForCluster NEMO in Freiburg. F. K. acknowledges
Mats Moskopp for help with video analysis.
Keywords: Suzuki Polycondensation • Polyarylene •
Mechanochromism • Mechanical properties • Spiropyran
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A simple and high-yielding synthesis to
Author(s), Corresponding Author(s)*
high
molecular
weight,
tough
is
Page No. – Page No.
poly(meta,meta,para-phenylene)
reported. Statistical copolymers with
para,para-spiropyran showed
Title
a
transient mechanochromic response,
the origin of which is explained in
detail.
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