Independent Control of Elastomer Properties through Stereocontrolled Synthesis

Article abstract of DOI:10.1002/anie.201606750

In most synthetic elastomers, changing the physical properties by monomer choice also results in a change to the crystallinity of the material, which manifests through alteration of its mechanical performance. Using organocatalyzed stereospecific additions of thiols to activated alkynes, high-molar-mass elastomers were isolated via step-growth polymerization. The resulting controllable double-bond stereochemistry defines the crystallinity and the concomitant mechanical properties as well as enabling the synthesis of materials that retain their excellent mechanical properties through changing monomer composition. Using this approach to elastomer synthesis, further end group modification and toughening through vulcanization strategies are also possible. The organocatalytic control of stereochemistry opens the realm to a new and easily scalable class of elastomers that will have unique chemical handles for functionalization and post synthetic processing.

Full text of DOI:10.1002/anie.201606750

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201606750
German Edition: DOI: 10.1002/ange.201606750
Step-Growth Polymerization
Independent Control of Elastomer Properties through
Stereocontrolled Synthesis
Craig A. Bell+, Jiayi Yu+, Ian A. Barker, Vinh X. Truong, Zhen Cao, Andrey V. Dobrinyin,
Matthew L. Becker,* and Andrew P. Dove*
Abstract: In most synthetic elastomers, changing the physical
properties by monomer choice also results in a change to the
crystallinity of the material, which manifests through alteration
of its mechanical performance. Using organocatalyzed stereo-
specific additions of thiols to activated alkynes, high-molar-
mass elastomers were isolated via step-growth polymerization.
The resulting controllable double-bond stereochemistry
defines the crystallinity and the concomitant mechanical
properties as well as enabling the synthesis of materials that
retain their excellent mechanical properties through changing
monomer composition. Using this approach to elastomer
synthesis, further end group modification and toughening
through vulcanization strategies are also possible. The organo-
catalytic control of stereochemistry opens the realm to a new
and easily scalable class of elastomers that will have unique
chemical handles for functionalization and post synthetic
processing.
Elastomeric materials are applied widely to demanding
applications on account of their inherent reversible deforma-
tion behavior. Many synthetic elastomer materials are tri- or
multi-block copolymers that are based on the concept of an
amorphous-crystalline or hard–soft phase-separated system in
which organization of the hard and soft domains endows the
strong but elastic properties upon the materials.[1] While these
materials have found widespread application, changes to the
monomers or stoichiometry designed to elicit a change in
physical properties also alter the chain packing and hence the
mechanical properties of the materials. Interestingly, natural
rubber and gutta percha are homopolymers of poly(cis-
isoprene) and poly(trans-isoprene) respectively. While these
materials differ by only the double bonds of the backbone, the
superior elastomeric properties of natural rubber[2] are
attributed to the enhanced chain packing afforded by its
stereochemical orientation.[3] While the design principles to
control crystallinity and the associated mechanical properties
in these materials are clear, the inability to incorporate a wide
range of functional groups in a controlled manner or ration-
ally define the chain end functionality limits the applications
of both these materials and their synthetic analogues.[4]
Combination of the control over degree of crystallinity and
mechanical properties through backbone double-bond ste-
reochemistry, as exemplified by natural rubber/gutta percha,
with the tunability of materials properties of synthetic
materials via monomer selection and end group modification,
presents a new method by which to undertake the rational
design of elastomers and presents an entire new design space
for functional materials.
A critical deficiency of synthetic methods in the ability to
control double-bond stereochemistry using monomers with
diverse functional groups limits the development of novel
materials that mimic the method of mechanical property
control exemplified in natural rubber and gutta percha
elastomers. The application of click chemistry, reactions that
are so categorized because of their modular nature, single
reaction trajectory, equimolarity, high yields, simple purifica-
tion, chemoselectivity, and fast timescales,[5] could potentially
address the synthetic pitfalls and afford access to high-molar-
mass polymers with defined stereochemistry. Indeed, we and
others have recently demonstrated that the organobase-
catalyzed addition of thiols to activated alkynes could be
stopped after a single addition and furthermore, by judicious
choice of catalyst or solvent polarity, the stereochemistry of
the resultant alkene could be controlled.[6] Application of this
method to step-growth polymerization has allowed us to
access high molar mass polymers with control over double
bond stereochemistry, monomer composition, and chain-end
N
ature has evolved the ability to create large and complex
molecules in which the precise control over the spatial
arrangement of the atoms is critical to their performance. The
three-dimensional control over the arrangement of bonds is as
important to the function and behavior of molecules as any
other factor and is critical to the structure–function relation-
ships that govern the role of a range of molecules. While the
effect of stereochemistry on functionality is probably best
known in the examples of small molecule drugs such as
thalidomide (one enantiomer is effective against morning
sickness, the other is teratogenic) or naproxen (one enantio-
mer is used to treat arthritis pain, the other causes liver
poisoning with no analgesic effect), it is less well-studied with
respect to materials design.
[*] Dr. C. A. Bell,[+] Dr. I. A. Barker, Dr. V. X. Truong, Prof. A. P. Dove
Department of Chemistry, The University of Warwick
Coventry, CV4 7AL (UK)
E-mail: a.p.dove@warwick.ac.uk
J. Yu,[+] Z. Cao, Prof. A. V. Dobrinyin, Prof. M. L. Becker
Department of Polymer Science, The University of Akron
Akron, OH 44325 (USA)
E-mail: becker@uakron.edu
[+] These authors contributed equally to this work.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606750.
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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functionality to create elastomeric materials with independ-
ently tuneable mechanical and physical properties.
Again, at lower %cis bond contents, no significant peaks were
observed in the SAXS analysis (Figure 1B). Differential
scanning calorimetry (DSC) revealed, along with glass
transitions of the materials at about ꢀ28C, that the 80% cis
material displays melting transitions in both the first and
second scan data (Figure 1C). A reduction in the %cis
content to 70% resulted in the observation of a melting peak
in only the first scan which had a decreased peak area that
corresponds to decreased crystallinity within the sample.
Additional decreases in the %cis content further decreased
the crystallinity of the sample. These data are confirmed by
the diffraction data and indicate that the ordering of the
polymer chains into microcrystalline domains is enhanced by
high cis contents, in line with that of natural and synthetic
polyisoprene and its analogues. Interestingly, the WAXD and
SAXS spectra of the 80% cis material after stretching
revealed a significant change in the crystallinity of the
materials consistent with strain induced crystallization (Sup-
porting Information, Figure S5).
The semicrystalline nature of the networks manifests itself
in a significant improvement of the network mechanical
properties with increasing fraction of the cis isomers (Fig-
ure 2A). Investigation of the elastomer mechanical proper-
ties was undertaken to quantify deformation over a wide
range of strain rates (2.34 ꢀ 10ꢀ6 to 9.38 ꢀ 10ꢀ2 sꢀ1). The data
sets collapse into a straight line which indicates that for the
studied time interval the network mechanical properties are
time (deformation rate) independent. To obtain the equilib-
rium Youngꢁs modulus at small deformations, the data set was
To access suitable materials, a dialkyne monomer (C3A
)
synthesized by simple Fischer esterification of propiolic acid
and propane diol was combined with hexane dithiol (C6S) in
chloroform (CHCl3) in the presence of 1 mol% of 1,8-
diazabicycloundecane (DBU) to promote cis double-bond
formation (Scheme 1). After only 1 h, precipitation of the
polymer into diethyl ether revealed the formation of a mate-
rial with high molar mass (Supporting Information, Figure S1
and S2; Tables S3 and S4) with a high cis content (80%).
Interestingly, changing the polarity of the reaction solvent by
blending CHCl3 with N,N-dimethylformaldehyde (DMF) and
using 1 mol% of trimethylamine (NEt3) enabled the synthesis
of materials with altered cis/trans ratios (Supporting Infor-
mation, Figure S3). Further analysis of the materials by size-
exclusion chromatography using viscometry detection (Sup-
porting Information, Figure S4) revealed a values of about
0.6, which is highly characteristic of a linear, unbranched, or
non-cross-linked material in a good solvent.[7]
Scheme 1. Synthesis of thiol–yne elastomer materials from dialkyne
˙
˙
fitted by the linear function strue(t)/e = E0e(t)/e. From the
resultant values of the Youngꢁs modulus (Table 1, Figure 2B),
the networks can be concluded to function as composite
materials for which modulus depends on the network degree
of crystallinity fc. Using a mixture rule, the Youngꢁs modulus
of the materials as a function of their degree of crystallinity is
expressed as E0 = (1ꢀfc)Ea + fcEc, where Ea is Youngꢁs
modulus of the amorphous phase and Ec is the Youngꢁs
modulus of the crystalline phase. The modulus of the
amorphous phase, Ea, is estimated by averaging the modulus
of samples with 32% and 53% cis isomer content, resulting in
Ea = 2.94 MPa. In turn, the modulus of the crystalline phase,
Ec, from fitting the data is equal to 238.4 MPa. This value
shows significant improvement over the mechanical proper-
ties of existing natural or synthetic
and dithiol precursors.
Analysis of the materials by wide-angle X-ray diffraction
(WAXD) analysis revealed that while the 80% cis material
displayed sharp 2q peaks at 21 and 238, these peaks were
significantly diminished in the 70% cis polymers. Materials at
lower %cis double bonds display broad peaks that are
consistent with amorphous materials that exhibit no strong
crystalline domains (Figure 1A). Complimentary analysis by
small-angle X-ray scattering (SAXS) confirms these observa-
tions with microcrystalline domains of ca. 18 and 22 nm
(calculated from the q value at maximum intensity) being
observed for the 80 and 70% cis materials, respectively.
elastomer analogues.
Further analysis of these data
using a value of the shear modulus,
G = E0/3, of the network in the
amorphous phase enables the esti-
mation of the maximum elongation
ratio. The molecular mass of
strands supporting stress (M) is
estimated from the network shear
modulus as G ꢁ 1RT/M!M ꢁ
1RT/G, where
1
is density
(assumed to be 1.0 gcmꢀ3 for all
materials in this study), R is the gas
constant, and T is the absolute
temperature. Furthermore, the
Figure 1. A) Wide-angle X-ray diffraction, B) small-angle X-ray scattering, and C) differential scanning
calorimetry (solid=first scan; dashed=second scan) for C3A–C6S materials.
2
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degree of control over network elastic properties by varying
the cis isomer content, is clearly illustrated from samples with
52% and 32% cis isomer content, respectively (Figures 3B
and C). In these examples, increasing the fraction of cis
isomers leads to a decrease of the elongation at break from
2970 ꢂ 137% to 2252 ꢂ 115%. This insignificant decrease in
network elongation is compensated by almost a five-fold
increase from 2.82 ꢂ 0.4 MPa to 16.6 ꢂ 3.8 MPa in ultimate
tensile strength at break (Table 1). The integral characteristic
of ultimate tensile strength and elongation at break is
materialꢁs tensile toughness. For these materials, it varies
between 35 ꢂ 3 MJmꢀ3 and 330 ꢂ 29 MJmꢀ3, which indicates
that a large amount of energy can be stored in the system
either by achieving a high ultimate tensile strength or
elongation at break (see data for 70% and 80% cis isomer
content). The values obtained are noted to differ from the
measured elongation at break, ebreak (Table 1) as a conse-
quence of elongation induced crystallization of the samples
and sample necking at large deformations which is not
captured in the emax estimated from small network deforma-
tions. Nonetheless, this is consistent with what is expected for
amorphous entangled networks with shear modulus values on
the order of 1–5 MPa.[8] Finally, a series of experiments were
conducted to investigate the effect of molar mass of precursor
polymers on the mechanical properties of resultant networks.
The preparation of a series of samples with Mw = 44, 62 and
107 kDa and subsequent analysis revealed a decrease in
ultimate tensile strength (UTS) and a slight increase in the
Youngꢁs modulus E with decreasing molar mass (Supporting
Information, Table S3). These changes are likely a result of
more facile and rapid crystallization of the lower molar mass
materials.[9]
Figure 2. A) Dependence of the normalized network true stress
˙
˙
strue(t)/e as a function of time e(t)/e for networks with different
fraction of the cis-isomers obtained from the constant strain rate
experiments (2.34ꢁ10ꢀ6 to 9.38ꢁ10ꢀ2 sꢀ1); B) Dependence of the
network Young’s modulus on the fraction of cis isomers. Inset: the
linear dependence of E0ꢀEa on the degree of crystallinity f .
c
contour length of the chain can be determined as L0 = l0M/M0,
where M0 is monomer molecular mass (330.10 gmolꢀ1) and l0
is the length of monomer in cis/trans configuration (Support-
The synthetic nature of this system also affords the ability
to control material properties via monomer design and end-
group modification. This method results in controllable and
definable end groups that can be specifically modified and
will be critical to dispersing and grafting fillers within the
elastomer network. Specifically, the use of a slight excess of
dialkyne monomer results in alkyne chain ends that can be
further modified by a simple nucleophilic addition or click
reaction. To this end, we have demonstrated that 2,2,2-
trifluoroethanethiol can be added selectively to the chain
ends of C3A–C6S (Figure 4) by addition of the thiol to the
material either directly after the polymerization reaction or in
a subsequent post-polymerization step. The two signals are
consistent with cis and trans double-bond formation upon
addition. This has been confirmed
ing Information, Figure S12, Table S2). The maximum exten-
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
sion ratio, lmax
ꢁ
l01RT=ðbKGM0Þ, where bK is the polymer
Kuhn length (here, bK = 0.8 nm is used) enables the calcu-
lation of emax, the maximum theoretical extension at break as
emax = (lmaxꢀ1) ꢀ 100%.
The remarkable mechanical properties of our materials
can further be assessed in the nonlinear deformation regime
(Figure 3, Table 1). Under this regime, the materials with
80% cis isomer content are strong and elastic materials with
an ultimate tensile strength (UTS) of 54.3 ꢂ 6.5 MPa and an
elongation at break (ebreak) of 1495 ꢂ 66% (Figure 3A). The
by 1H and 19F NMR spectroscopy of
Table 1: Mechanical properties of thiol–yne elastomers (C3A–C6S).
small molecule model compounds
[a]
(data not shown). Beyond chain-
%cis
Crystallinity
[%]
E0
[MPa]
emax
[%]
ebreak
[%]
Tensile
toughness[b]
[MJmꢀ3
UTS
[MPa]
end modification, simple variation
of the monomer structure of the
dialkyne or dithiol enables not only
access to varied mechanical proper-
ties but also materials with different
functional end groups. For example,
materials with longer alkyl chains
between ester units (introduced
through either dialkyne or dithiol
]
80
79
70
53
32
22.3
22.9
14.1
0
59.8ꢂ0.2
56.5ꢂ6.4
26.2ꢂ0.3
3.1ꢂ0.1
2.7ꢂ0.1
–
–
–
346
378
1495ꢂ66
1554ꢂ71
1874ꢂ94
2252ꢂ115
2970ꢂ137
289ꢂ45
330ꢂ29
292ꢂ58
104ꢂ22
35ꢂ3
54.3ꢂ6.5
56.7ꢂ5.2
43.5ꢂ6.5
16.6ꢂ3.8
2.8ꢂ0.4
0
[a] emax estimated from the value of the Young’s modulus at small deformations. [b] Network toughness
is calculated at a strain rate of 2.34ꢁ10ꢀ2 sꢀ1
.
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Figure 3. A)–C) Exemplary stress vs. strain curves for A) 80% cis C3A–C6S; B) 53% cis C3A–C6S; C) 32% cis C3A–C6S in the non-linear region. Data for
three samples are shown to illustrate the reproducibility. Expansions inset for clarity.
the double bond within the material also remains accessible
for further reaction. Specifically, vulcanization of the materi-
als by radical curing with 1 wt% dicumyl peroxide at 1608C in
bulk yields a significant change to the materials properties
such that, consistent with cross-linking reactions occurring,
the material becomes more elastic in nature (Figure 5). Most
notably, the modified material can now be subjected to
repeated load–unload cycles up to 500% extension before
being allowed to relax. While the non-vulcanized sample does
not significantly recover, the vulcanized sample recovers to
150% of its original length, which demonstrates the superior
Figure 4. 19F NMR spectrum of C3A–C6S thiol–yne step-growth polymer
following end capping with 2,2,2-trifluoroethanethiol (376 MHz,
CDCl3 +0.01% v/v CF3COOH).
elastic properties endowed as a result of the vulcanization
process.
monomers) typically resulted in materials with increased
strength (Supporting Information, Table S4). C3A–C3S materi-
als were also prepared and characterized and confirmed that
the manipulation of cis/trans ratio retained the effect on
mechanical properties for a different monomer composition
(Supporting Information, Table S4). Distinct changes in
physical properties were easily introduced through the
incorporation of functional dithiols including 2,2’-(ethylene-
dioxy)diethanethiol and 1,4-dithio-d-threitol. Even at only
Figure 5. Exemplary stress vs. strain curves for 80% cis C3A-C6S before
(red solid line) and after (blue dashed line) vulcanization with 1%wt
dicumyl peroxide.
a 10% incorporation of these comonomers, the polar
component of the surface energy is significantly reduced,
while the mechanical properties were largely retained (Sup-
porting Information, Tables S4 and S5). These effects will be
significant when dispersing polar filler materials.
The novel approach to materials design described herein
outlines a series of principles that transfer the unique
mechanical properties of natural rubber and gutta percha to
a fully synthetic system. The organocatalytic step-growth
polymerization affords independent control over mass, mass
distribution, stereochemistry, and the resulting mechanical
and physical properties, without the use of metals or exotic
additives. Furthermore, the synthetic precursors allow access
to starting materials and functional species and defined end
groups not commonly found in natural or even synthetic
elastomers. The benefits of being able to control monomer
composition, stoichiometry, end-group modification with
a minimal resulting effect on mechanical performance
combined with the ability to toughen via commercially
relevant vulcanization processes yields an exciting design
space for functional elastomers.
Analysis of the hydrolytic and thermal degradability of
the materials have also been investigated. Storage at ambient
temperature leads to no observable discoloration or change in
mechanical properties. Submersion of the 80% cis C3A–C6S
material in both phosphate-buffered saline solution (pH 7.4)
and 1m KOHaq solution at 278C only revealed less than 0.3%
addition of mass and no degradation after 14 days, which
indicates that the materials have an excellent stability to
hydrolysis. Furthermore, using thermogravimetric analysis
(TGA), an onset of degradation was observed at 3558C which
shows that the materials are also thermally stable and suitable
for melt processing, thermal molding and vulcanization-
based toughening. These materials are shown to be stable to
ambient oxidation and maintained their mechanical proper-
ties for more than a year. Finally, we have demonstrated that
4
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This work was supported by financial support from BBSRC
(BB/I002286/1) for funding a postdoctoral fellowship to
V.X.T.; EPSRC (EP/I014454/1) for funding a postdoctoral
fellowship to I.A.B. The Royal Society are acknowledged for
funding a Newton International Fellowship to C.A.B. and an
Industrial Fellowship to A.P.D. and the Leverhulme Trust are
thanked for financial support for this project (RPG-2015-
120). The authors acknowledge support from the Ohio
Department of Developments Innovation Platform Program.
Dr Stefan Naumann (University of Warwick) is thanked for
assistance with translation of German literature.
Keywords: click chemistry · elastomers · organocatalysis ·
step-growth polymerization · stereochemistry
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Received: July 12, 2016
Revised: August 19, 2016
Published online: && &&, &&&&
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Communications
Step-Growth Polymerization
Stereochemistry-controlled stretching:
Step-growth polymerization using orga-
nocatalytic nucleophilic thiol–yne addi-
tion chemistry is demonstrated to pro-
duce elastomeric materials in which the
stereochemistry of the double bond con-
trols the mechanical properties of the
materials independently of the mono-
mers.
C. A. Bell, J. Yu, I. A. Barker, V. X. Truong,
Z. Cao, A. V. Dobrinyin, M. L. Becker,*
A. P. Dove*
&&&& — &&&&
Independent Control of Elastomer
Properties through Stereocontrolled
Synthesis
6
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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