Supramolecular Polymers

Abstract

Sophisticated polymeric materials with ‘responsive’ properties are beginning to reach the market. The use of reversible, noncovalent interactions is a recurring design principle for responsive materials. Now, recently developed hydrogen-bonding units allow this design principle to be taken to its extreme. Supramolecular polymers, where hydrogen bonds are the only force keeping the monomers together, form materials whose (mechanical) properties respond strongly to a change in temperature or solvent. In this review, we describe the developments that have led to hydrogen-bonded supramolecular polymers and discuss the use of these materials in advanced applications.

Full text

In the past 60 years, polymers have revolutionized
our lives. Their low price, high processability, and
exceptional mechanical properties have led to the use
of polymers in ever more sophisticated applications.
Presently, a strong scientific interest in ‘smart’
polymers – polymers that respond to changes in
temperature, solvent, or the presence of ‘signal’
chemicals – is beginning to expand the potential field
of applications for such materials even further.
Key to the responsiveness of smart materials is the
reversibility of the noncovalent interactions that lead to the
change in properties. In nature, responsive structural
polymers abound and their capacity to respond is often
brought about by reversible assembly/disassembly. Scaffolds,
such as those that constitute the cellular skeleton, are
formed only where and when they are required, and they
are disassembled into small building blocks when their task
has been fulfilled. As opposed to a purely macromolecular
approach, such a modular, supramolecular strategy allows
a fast and efficient response to changing needs in the
cellular cycle.
The field of supramolecular chemistry has been developed
over the past 25 years by synthetic chemists, inspired by the
ubiquity of reversible, yet highly specific, intermolecular
processes in nature. Supramolecular polymers form the most
recent branch in the tree of ‘chemistry beyond the covalent
bond’, as supramolecular chemistry is sometimes called.
Despite their short history, supramolecular polymers are
already beginning to find commercial use in applications
that take advantage of the reversibility and responsiveness
of noncovalent interactions.
by Anton W. Bosman*, Rint P. Sijbesma
†
, and E. W. Meijer
†
Supramolecular
polymers at work
*SupraPolix Research Center,
Horsten 1, 5612 AX Eindhoven, The Netherlands
†Laboratory of Macromolecular and Organic
Chemistry, Eindhoven University of Technology,
PO Box 513, 5300 MB, Eindhoven, The Netherlands
E-mail: E.W.Meijer@tue.nl
April 200434
ISSN:1369 7021 © Elsevier Ltd 2004
Sophisticated polymeric materials with ‘responsive’
properties are beginning to reach the market. The use
of reversible, noncovalent interactions is a recurring
design principle for responsive materials. Now,
recently developed hydrogen-bonding units allow this
design principle to be taken to its extreme.
Supramolecular polymers, where hydrogen bonds are
the only force keeping the monomers together, form
materials whose (mechanical) properties respond
strongly to a change in temperature or solvent.
In this review, we describe the developments that
have led to hydrogen-bonded supramolecular
polymers and discuss the use of these materials in
advanced applications.
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REVIEW FEATURE
Introduction
Before macromolecules were generally accepted, the majority
of scientists were convinced that polymer properties were
the result of the colloidal aggregation of small molecules or
particles. It was only after the pioneering work of Staudinger
1
that it became evident that polymeric properties in both
solution and the solid state are the result of the
macromolecular nature of the molecules. A large number of
repeating units are covalently linked into a long chain and the
entanglements of the macromolecular chains are responsible
for many typical polymer properties.
The impressive recent progress in supramolecular
chemistry has paved the way for the design of polymers and
polymeric materials that lack macromolecular structure.
Instead, highly directional secondary interactions are used to
assemble the many repeating units into a polymer array.
Polymers based on this concept hold promise as a distinctive
class of novel materials because they combine many of the
attractive features of conventional polymers with the
reversibility originating from the secondary interactions.
Consequently, the architectural and dynamic parameters that
determine polymer properties, such as the degree of
polymerization, lifetime of the chain, and its conformation,
can be reversibly adjusted, resulting in unique materials that
respond to external stimuli. These aspects of supramolecular
polymers have led to a recent surge in attention for this
promising class of compounds and have stimulated us to
bring together materials science and supramolecular
chemistry. Fig. 1 shows the required directionality in
supramolecular interactions compared with the historical and
current macromolecular view on polymers.
What are supramolecular polymers?
Loosely defined, supramolecular polymers are those in which
the monomers are held together by noncovalent
interactions
2
. In all condensed molecular materials, whether
they are liquid, glassy, or (liquid) crystalline, noncovalent
interactions with little specificity or directionality are
present. However, when highly directional forces dominate
the interaction between neighboring molecules, long chains
or networks of concatenated molecules can be formed,
resulting in many of the (mechanical) properties that have
made polymeric materials so successful. Long chains, which
lead to polymer-like behavior, are only formed when the
interactions between the monomeric units are strong enough.
The presence of linear chains, which persist when a material
is heated or dissolved, is the hallmark of a successful design
of strong and directionally interacting functionalities. In a
fundamental research context, where the goal is to
understand the relation between molecular structure and
macroscopic properties, strength and directionality are of
prime importance
3
. While the directionality and strength of
the interactions between monomers are also important when
developing applications for supramolecular polymers, there
are additional requirements that have to be met for the
successful application of these materials. The most
challenging of these requirements have been synthetic
availability, cost, and stability of appropriate functionalities.
Quadruple hydrogen bonds aka UPy
Hydrogen (H) bonds hold a prominent place in
supramolecular chemistry because of their directionality and
versatility, although they are not among the strongest
noncovalent interactions. Cooperativity holds the answer to
this problem and, consequently, several systems have been
designed that combine multiple H-bonds in a row. Indeed,
this increases the strength of the interaction and, moreover,
enhances its specificity. Very stable complexes can be
obtained when quadruple H-bonding units are employed.
Therefore, it was not until the development of the quadruple
H-bond unit by Meijer and Sijbesma
4,5
that H-bonding
systems were developed with sufficiently high association
constants to allow the formation of supramolecular polymers
with significant degrees of polymerization. These self-
Fig. 1 Schematic representation of the evolution in time of macromolecular science from
colloids, via the original work of Staudinger
1
(who showed, for the first time, that high
molecular weight linear polymers do exist), to supramolecular polymers that can be seen
as a combination of both earlier concepts: a high molecular weight linear polymer held
together by secondary interactions. (Reproduced with permission of SupraPolix BV.)
April 2004
35
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complementary quadruple H-bonding units based on
2-ureido-4[1H]-pyrimidinones (Fig. 2) dimerize in toluene
with an association constant of
K
dim
= 6·10
8
M
-
1
and a
lifetime of 1.7 s. Application of these H-bonding units as
associating end-groups in di- or multifunctional molecules
has resulted in the formation of supramolecular polymers
with high degrees of polymerization (DP). The development
of the ureidopyrimidinone (UPy) functionality, a synthetic
H-bonding unit with a very high association constant, has
helped enormously in opening the way for the exploration of
all aspects of supramolecular polymers.
Supramolecular polymers with UPy
The difunctional UPy compound shown in Fig. 2 can easily be
made in a one-step procedure from commercially available
compounds (hexyldiisocyanate and methylisocytosine). The
high association constant of UPy combined with its
difunctional nature results in the formation of a stable and
long polymer chain in solution as well as in the bulk.
Dissolving a small amount of this low molecular weight
compound in chloroform gives solutions with high viscosities.
The bulk viscoelastic properties of low molecular weight,
bifunctional UPy compounds have been studied using
dynamic mechanical thermal analysis (DMTA), rheology, and
dielectric relaxation spectroscopy
6
. One of the salient
features of the materials, with great relevance for
applications, is the extremely high activation energy for
viscous flow of 105 kJ/mol. This results in a strongly
temperature dependent melt viscosity, which increases
processability of these materials at temperatures only
moderately higher than the melting point or T
g
. The high
activation energy can be attributed to the contribution of
three mechanisms to stress relaxation in sheared melts of
supramolecular polymers. Firstly, a mechanism shared with
covalent polymers, in which they escape from entanglements
by reptation. In addition to that, supramolecular polymers
have enhanced relaxation at higher temperatures because the
chains become shorter. Finally, in a mechanism unique to
reversible polymers, the supramolecular chains lose strain by
breaking, followed by recombination of free chain ends
without strain. Breaking rates increase with temperature, and
contribute to the temperature-dependent behavior of
supramolecular polymers.
Supramolecular materials
The quadruple H-bonded unit has been further employed in
the chain extension of telechelic polysiloxanes, polyethers,
polyesters, poly(ethylene/butylenes), and polycarbonates
7
. In
these compounds, the material properties have been shown
to improve dramatically upon functionalization, resulting in
materials that combine many of the mechanical properties of
conventional macromolecules with the low melt viscosity of
low molecular weight organic compounds (Fig. 3).
REVIEW FEATURE
April 200436
Fig. 3 Illustration of the phase changes in a rubbery material based on Kraton®, (poly(ethylene-co-butylene), modified with UPy, exemplifying the diversity in phase behavior because of
the supramolecular interactions. (Reproduced with permission of SupraPolix BV.)
Fig. 2 Schematic representation of a linear supramolecular polymer. Enlargement shows a
representation of the X-ray structure of the H-bonded dimer of the UPy group that holds
the supramolecular polymers together. (Reproduced with permission of SupraPolix BV.)
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REVIEW FEATURE
The reversibility of supramolecular polymers adds new
aspects to many of the principles that are known from
condensation polymerization. For example, a mixture of
different supramolecular monomers will yield copolymers,
but it is extremely simple to adjust the copolymer
composition by adding an additional monomer. Moreover, the
use of monomers with a functionality of three or more will
give rise to network formation. However, in contrast to
condensation networks, the ‘self-healing’ supramolecular
network can reassemble to form the thermodynamically most
favorable state, thus forming denser networks (Fig. 4).
Applications of supramolecular
polymers
The strong secondary interactions of the quadruple
H-bonding unit, combined with the ease of synthesis, is
probably the main reason that, following the first publication
in 1997 by Sijbesma
et al.
5
, there have been numerous patent
applications filed using supramolecular architectures in fields
ranging from adhesives
8
, printing
9-13
, cosmetics
14
,
15
, personal
care
16
, to coatings
17
. The added value that supramolecular
polymers are foreseen to give to these everyday applications
is based on their improved processing in the melt or solution
while maintaining excellent material properties in the solid
state, the lower temperatures needed to obtain tractable
materials, the ease of synthesis, the compatibility with
existing polymeric systems, and the intrinsic reversibility of
supramolecular systems that makes the materials easily
removable. In this section, several examples of these
applications will be discussed that take advantage of the
possibilities originating from the unlocking of the processing
properties from the material properties by using
supramolecular interactions. Moreover, this unambiguously
shows that supramolecular polymers are not restricted to the
laboratory anymore.
IInnkk--jjeett iinnkkss
An application in which the dramatic differences in phase
behavior of supramolecular polymers in a relatively narrow
temperature range can be used is ink-jet printing. In this
application, images are created on a substrate (i.e. paper) by
the ejection of ink droplets through a small orifice. The ink,
therefore, has to be low viscosity in order to be ejected in
small droplets. On the other hand, the ink needs to be highly
viscous, almost a solid, once the droplet hits the paper –
otherwise, the ink will smear out through capillary action of
the paper, resulting in blurry pictures.
Xerox has filed two patents in which supramolecular
polymers are used as binders in ink compositions. One
application relates to hot-melt inks
9
, consisting of a colorant
and a binder. These inks are solid at temperatures below
50ºC and a liquid with a viscosity of around 20 cps at 160ºC.
The binder is a multifunctional low molecular weight
compound that has been functionalized with two to five
UPy groups, resulting in polyether compounds that form
supramolecular networks. Mixing these materials at elevated
temperatures with other ingredients like (UV) ultraviolet-
stabilizers, antioxidants, and colorants results in inks that can
be used in hot-melt ink printers.
In the other patent application filed by Xerox
10
, aqueous
based inks are formulated with supramolecular polymers. The
material properties of the ink (i.e. viscosity) are, in this case,
tuned with the heat and polarity of the solvent medium,
since these parameters determine the amount of H-bonding
between the different UPy groups. The ink consists of an
aqueous solvent, colorant, and supramolecular polymeric
additive. Because of the high polarity of the solvent, H-
bonding is strongly reduced in the ink solution, resulting in a
rather low viscosity. In contrast, the viscosity of the ink
rapidly increases during jetting as the solvent evaporates
from the droplet, which, together with a temperature drop in
the ink, results in a virtual increase of molecular weight of
the polymer additive because of the formation of H-bonds.
Consequently, the ink is prevented from spreading on the
paper and a clear image with good permanence
April 2004
37
Fig. 4 Different architectures possible for supramolecular polymers. Top left: linear
supramolecular polymer. Top right: linear supramolecular block-copolymer. Bottom left:
supramolecular network via branching. Bottom right: supramolecular network via
grafting. (Reproduced with permission of SupraPolix BV.)
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characteristics is produced. The supramolecular compounds
used in this application are comparable to the ones for hot-
melt ink, the only difference is the formulation, which
contains around 10% solids (binder, pigment or dye, and
surfactant) in the aqueous solvent. Interestingly, Agfa-
Gevaert in Belgium has filed a patent related to ink-jet inks
in which the dyes themselves have been modified with UPy
groups, resulting in an improved lightfastness of the ink
11
.
PPrriinnttiinngg ppllaatteess
Another way to use supramolecular polymers is shown in a
patent application filed by Kodak Polychrome Graphics
12
,
which takes advantage of the fact that the solubility of
supramolecular polymer coatings increases after heating
(thermal solubilization). This concept is applied in printing
plates that are used in lithographic printing processes.
Lithographic printing is based on the immiscibility of
hydrophobic ink and water; the printing plate consists of
hydrophilic surfaces that are wetted by the water and
hydrophobic surfaces that are wetted by the ink. In this way,
hydrophobic patterns on the printing plate can be transferred
to a substrate (i.e. paper) to create an image. Typically,
patterns are created by using a radiation-sensitive coating,
which becomes soluble upon exposure to radiation and can
be removed in a subsequent developing process to reveal the
hydrophobic surface underneath that will absorb the ink
(positive-working printing plate). If the top layer is made
insoluble by exposure to radiation, and the unexposed areas
are removed in developing, a negative-working printing plate
is obtained.
In these patent applications, a thermally imageable,
positive-working printing plate is disclosed that makes use of
a supramolecular polymer as a thermally sensitive coating.
The polymers used are obtained by reacting polyfunctional
resins (phenolic, acrylic, or polyester) with isocyanate
functional UPy. Specific examples of resins are phenol/cresol
novolaks and 4-hydroxy-styrene/styrene copolymers. The
resulting coating consists of these supramolecular cross-
linked polymers and (IR) infrared-dyes that transform laser
light into heat. Because the resins contain several reversible
cross-links to reinforce the coating, it is possible to disrupt
these cross-links thermally with short (~100 µs) IR laser
illumination and solubilize the exposed regions in the
following developing step. This procedure eliminates two
processing steps normally used in preparing printing plates
(preheating and post-development baking), and superior
press-life is obtained when compared with other digitally
imaged compositions.
PPoollyymmeerriizzaattiioonn--iinndduucceedd pphhaassee sseeppaarraattiioonn
The dynamic flexibility of supramolecular polymers is being
exploited by Keizer and coworkers in polymerization-induced
phase separation (PIPS) with H-bonded supramolecular
polymers
18
. In PIPS, a polymer is dissolved in a reactive
monomer, which is subsequently polymerized to cause phase
separation, resulting in two polymeric phases with certain
morphology (Fig. 5). PIPS is currently used to produce
multiphase composite materials like high-impact polystyrene,
avoiding the use of solvent and consequently resulting in fast
and clean production of polymeric materials.
The rate of phase separation in PIPS is generally limited by
the mobility of the dissolved polymer. Supramolecular
polymers, however, may dissociate when dissolved in a
REVIEW FEATURE
April 200438
SupraPolix
Initial work in the field of supramolecular polymer
chemistry, performed by the group of Bert Meijer and
Rint Sijbesma at the University of Technology in
Eindhoven, The Netherlands, has led to the development
of new materials combining the sophisticated use of self-
organization with strong noncovalent interactions. The
key to their success is the use of quadruple H-bonding
units that combine easy preparation with strong
secondary interactions. Great interest from industry in
the concept of reversible polymers was one of the main
motivations to start SupraPolix. SupraPolix develops
materials containing supramolecular polymers, thereby
offering a new concept in the world of plastics by
separating the processability demands from the material
demands: incorporation of even a small amount of a
UPy group in existing plastics dramatically simplifies the
processing. As a consequence, production can be
cheaper and faster, while the consumer will benefit from
innovative products that are easier to use. The reduced
energy consumption during processing and the improved
recycling of these materials makes the technology
environmentally benign. Implementation of the technology
will allow producers to comply with the environmental
constraints of the near future without any concession to
the quality of the material.
For further information: www.suprapolix.com
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REVIEW FEATURE
reactive monomer, resulting in strongly enhanced diffusion.
Hence, macroscopic phase separation of supramolecular
polymers can be reached within a very short reaction time.
Keizer
et al.
18
report the PIPS of solutions of H-bonded
supramolecular polymers in acrylates, within the short
reaction times (0.3 s) used in UV-curing. The cured films are
colorless, transparent, flexible, and show macrophase
separation in secondary electron microscopy. Moreover, their
mechanical behavior is comparable to high molecular weight
polymers. Interestingly, DSM has filed a patent that describes
the use of supramolecular polymers in coatings for glass
fibers
17
, a process which needs very fast reaction times
indeed. In addition, this strategy could be used for either
stratification or patterning via a mold in thin films.
Outlook and the modular approach
Ten years ago, the first supramolecular polymers were seen
as scientific curiosities. Now this field of research is
generating several technologically important applications.
Progress in supramolecular chemistry has made it possible to
assemble small molecules into polymer arrays, and the
created structures possess many of the well-known
properties of ‘traditional’ macromolecules. Because of the
reversibility in the bonding, these supramolecular polymers
are under thermodynamic equilibrium and their properties
can be adjusted by external stimuli.
H-bonded systems are now being shown to be of
technological relevance instead of just a scientific curiosity.
A large variety of applications are feasible, especially since
the chosen approach can also be used for the modification of
telechelic oligomers or to modify existing polymers. A highly
interesting outlook for supramolecular polymers is the option
of the modular approach. By having a ‘box’ of different and
functional UPy-based building blocks, it should be possible to
create new functions by just mixing the building blocks in the
appropriate amounts. These materials could be formed by
self-assembly and changed by adding another component.
Therefore, the possibility of tuning the properties by changing
the relative ratio of UPy-monomer in the copolymer feed
seems very attractive, while hybrids between blocks of
macromolecules and supramolecular polymers would be easy
to prepare. Novel thermoplastic elastomers, superglues, hot-
melts, and tunable polymeric materials are within reach and
are currently receiving a great deal of attention from several
industrial research labs.
MT
April 2004
39
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Fig. 5 Schematic representation of PIPS using H-bonded supramolecular polymers: the
situation before polymerization of the acrylate-monomer is depicted on the left and the
situation after the polymerization of the acrylate monomer, which has induced phase
separation, is depicted on the right.
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