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. Author manuscript; available in PMC: 2010 Oct 20.
Published in final edited form as: Acc Chem Res. 2009 Oct 20;42(10):1489–1500. doi: 10.1021/ar800274u

Supramolecular Interactions in Chemomechanical Polymers

HANS-JÖRG SCHNEIDER 1, ROBERT M STRONGIN 1
PMCID: PMC2765657  NIHMSID: NIHMS130154  PMID: 19839651

Conspectus

Molecular recognition is the basis for the operation of most biological functions; outside of nature, it has also been developed to a high degree of sophistication within the framework of supramolecular chemistry. More recently, selective noncovalent interactions—which constitute molecular recognition—are being used in intelligent new materials that transform chemical signals into actions, such as the release of drugs. The presence of supramolecular binding sites allows chemomechanical polymers to operate as sensors and actuators within a single unit without the need for any additional devices such as transducers or power supplies.

A polymer can be designed so that a particular chemical substance, most often in aqueous surroundings, will trigger either a large expansion or a large contraction, depending on the mechanism. The translation of binding energy into mechanical motion can, with a suitable arrangement of the materials in tubes or on flexible films, be harnessed for unidirectional drives, flow control, the liberation of drugs, or the uptake of toxic compounds, among other applications. Miniaturization of the polymer particles allows one to enhance both the sensitivity and speed of the response, which is of particular importance in sensing.

The basis for the selective response to external effector compounds, such as metal ions, amino acids, peptides, or nucleotides, is their noncovalent interaction with complementary functions covalently bound to the polymer network. With suitable polymers, selectivity between structural isomers—and even between enantiomers—as triggers can be achieved. As with supramolecular complexes in solution, the underlying interactions in polymers comprise a variety of noncovalent binding mechanisms, which are not easy to distinguish and quantify—and more so with polymers, which are not monodisperse. In this Account, we present systematic comparisons of different polymers and effector classes that allow, for the first time, the characterization of these contributions in chemomechanical polymers: they comprise ion pairing, metal coordination, stacking, cation–π, dispersive, and hydrophobic forces. In contrast, hydrogen bonding has a major role primarily in the hydrogel network structure.

The fully reversible polymer volume changes are essentially determined by water uptake or release. In gels derived from boronic acid, glucose can serve as a cross-linking effector in promoting contractions via strong, reversible covalent bond formation in a highly distinctive manner. Cooperativity between two different effector compounds is more frequently seen with such polymers than in solution: it leads to logical AND gates by different motions of the particles, with a direct communication link to the outside world. For example, with a polymer that bears several recognition sites, triggering peptides induce motion only if Zn2+ or Cu2+ ions are simultaneously present.

The molecular recognition mechanisms that cause volume changes in polymers share similarities with extensively studied supramolecular systems in solution—but there are also remarkable differences. In this Account, we bring the knowledge learned from solution studies to bear on our systematic analysis of polymeric systems in an effort to promote the effective harnessing of the forces involved in chemomechanical polymers and the smart materials that can be created with them.

1. Introduction

Chemists, including the present authors, have studied supramolecular complexes and the underlying mechanisms of molecular recognition in much detail, relying on sophisticated instrumentation and data analyses. Nature achieves highly efficient control of biological systems without a battery of electronic devices. It is thus not surprising that scientists seek to systematically produce similar control systems which do not need electronics for their operation. Chemomechanical polymers are evolving as simple devices which can perform simultaneously as self-contained sensors and actuators within tiny particles.

Intelligent materials have a built-in capacity to respond to external stimuli with specific responses.1 They are of interest for a manifold of possible applications, particularly as actuators and sensors.2 Nanoparticles may be used as carriers for drugs or also gene delivery.3 The release can be triggered by a temperature change, by magnetic fields, or by light.4 Nanopores of mesostructured silica nanoparticles can perform as nanovalves; 5 they can be triggered by pH changes, light, redox reactions, competitive binding of another compound, or even by enzymes.6 Polymers exhibiting shape changes triggered electrochemically mainly via applied voltage can perform as artificial muscles,7 or, for example, be used in microfluidics.8 They rely, however, on a suitable power supply. Molecular imprinting is used to implement cavities in hydrogels which selectively recognize compounds of the same or similar structures to those used in the imprinting process by shape and by non-covalent interactions.9 Temperature, pH changes as well as light or redox input can control, for example, the helix transitions of peptides.10

Side-chain functionalization of polymers11 leads to materials with promising applications ranging from liquid crystals to drug releasing or electro-optical systems.12 Non-covalent interactions between the side groups play a major role in controlling the structure of the resulting self-assembling materials, but also allow the selective binding of external compounds, often by hydrogen bonding,13 but also by metal coordination. Gelation processes and corresponding sol-gel transitions can also be used, e.g., for drug release, for instance by dissolution of the drug-containing gel which contains a drug after adding an effector.14 Hydrogels which undergo physico-chemical changes upon external stimulation are increasingly used for many applications.15

Chemomechanical polymers which exhibit reversible volume changes triggered by external compounds were until now usually stimulated by pH gradients.2b,16,17 They may function in drug release,18 microfluidic devices, blood circulation control etc. Only recently was it elucidated how suitable binding sites in such polymers, mostly hydrogels, may lead to selective molecular recognition of, for example, specific metal ions, aminoacids, peptides, nucleotides, and carbohydrates. Interactions with antibodies19 or with DNA20 can also trigger macroscopic signals in such smart materials. Accordingly, one can design drug release devices which are triggered by the levels of such effector compounds in the body.2b,21 The selective uptake of toxic compounds could also operate in this manner. The binding energy translated into mechanical motion can be used in actuators with linear movements.

The present review focuses on the recognition mechanisms of chemically triggered size changes of polymers, in particular on the similarities and remarkable differences in comparison to molecular recognition in solution. However, even the interactions in solution of host-guest complexes typically involve several binding mechanisms, which are not easy to distinguish and quantify.22 The interactions within polymers are even more complex, and due to their mostly statistical nature, structural characterizations pose many more challenges. Although for these reasons conclusions regarding binding mechanisms in chemomechanical polymers are particularly daunting, it will be seen that, in principle, all non-covalent interactions studied to date involving supramolecular complexes in solution can exist and can be used in these smart materials, with sometimes surprising differences.

2. Principles: Expansion and Contraction / Water Uptake and Release /Phase transitions ?

Figure 1 illustrates the fundamental events stimulated by the reaction of guest molecules with the acceptor groups A within a chemomechanical polymer, such as P1 to P5 (Scheme 1). Contraction is the result of either non-covalent or covalent crosslinking between receptor groups and the guest or effector molecules, with concomitant release of water. Expansion results from uptake of the guest molecules, which require simultaneous uptake of solvation water. That water uptake or release is the major contributor to the observed volume changes has been shown by the comparison of water content before and after exposition of the polymers to external effector molecules. The water-swollen hydrogels already contain up to 98 % water before the addition of an effector: all reported effector-induced changes refer to this starting value. Figure 2 shows that indeed most of the observed volume change is due to additional water uptake, with only minor contributions from the effector molecules themselves.23 The importance of solvation changes in host-guest complexation has been stressed for many years; in chemomechanical polymers such changes can be quantified by gravimetry,23 and also by FT-IR measurements.24 In crosslinked chitosan hydrogels the amount of bound water is related to pH.25

FIGURE 1.

FIGURE 1

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Contraction by guest molecule uptake, crosslinking and water release (upper part); expansion by guest molecule and solvation water uptake (lower part).

SCHEME 1.

SCHEME 1

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Structural elements in the chemomechanical polymers. P1: polymethyl(methyl)acrylic derivative (contains to minor degree also other units, see ref. 23); P2: chitosan; P3: chitosan-anthryl derivative ; P4: polyethylenimin; P5: polyallylamin.

FIGURE 2.

FIGURE 2

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Weight increase (scaled per mg) compared to expansion, V (in % volume).23 With permission of Wiley/VCH.

Scheme 1 lists the structural elements in the chemomechanical polymers discussed in the context of this review, together with the most relevant binding contributions. The structures are simplified, and crosslinking bridges are not shown. Observed expansions typically increase with larger effector molecules (Scheme 2),23 not only due to the space demand but also due to the need of more solvation water.

SCHEME 2.

SCHEME 2

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Size effects with aromatic effectors (expansion in one dimension, polymer P1, pH effect (–65%) subtracted.23

Size changes in chemomechanical polymers are usually attributed to phase transitions,15,26 possibly including a multitude of states.27 Phase transitions are characterized by abrupt changes in the physical properties of the gels as function of temperature, pH, solvent composition etc. However, in cases where the volume changes of polymers such as P1 to P5 triggered by specific effectors compounds were measured as a function of their concentration, no such discontinuity was observed. Rather, the profile shown for instance in Figure 3 indicates a steady expansion as function of the effector concentration, resembling a normal saturation isotherm. Although the influence of particle size and discontinuous cooperativity effects (see below) preclude an exact fitting of the isotherms23 approximate binding constants K can be inferred, which, for example, for AMP (Figure 3) amounts to K = 20 M−1. This latter value is not far in magnitude from the K values reported in homogenous solutions for the interaction of AMP and ethylendiamine-type hosts.28 Similar saturation-like profiles were observed for the contraction of the allylamine polymer P5,29 or for reactions of the chitosan hydrogel P2.30

FIGURE 3.

FIGURE 3

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Expansion (length) as function of AMP concentration; polymer P1, Lower trace (○), in absence of buffer; upper trace (●), in presence of 0.02 M NaH2PO4 buffer;23 with permission of Wiley/VCH.

The profiles in Figure 3 indicate that expansion starts only after some concentration of the effector builds up. In contrast, the spectroscopically measured absorption starts, as expected, immediately after the polymer particles are immersed into the effector solution. The explanation is that the surface of the particles first need to be loaded to some extent before the effector starts to move inside the network.

3. Interactions

3.1 Electrostatic and pH Effects, Ion pairing and Cross linking

Gel size changes induced by pH control are the oldest applications of chemomechanical polymers, and have been discussed in detail.15,17 The basis of the expansions is the electrostaticrepulsion between cationic or anionic groups of the polymer as consequence of either lowering or enhancing the pH value. In most cases the polymers bear either pH-sensitive amino- or carboxylate groups; if both are present one observes a symmetric pH-profile. The break points obviously reflect the pK values of the participating ionogenic groups. It is, however, not primarily the electrostatic repulsion which leads to expansions, but essentially the uptake of water which is needed to solvate not only the charged polymer backbone, but also the simultaneously imported counterions which neutralize the backbone charge. The change of ionic strength concomitant with the pH change is the reason why the pH profiles are smooth as in Figure 4a only in presence of excess salts such as buffers, and become quite different if neutral salt concentrations are lowered (Figure 4b).

FIGURE 4.

FIGURE 4

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a) Size changes of hydrogel P1 as function of pH; in 0.05M phosphate buffer (circles), and in 0.5M NaCl (triangles).; b) pH profiles at different salt concentrations; in 0.5 M (●-a), 0.05 M(Δ-b), and 0.025 M(∇-c) NaCl solution, and in water with very dilute HCl or NaOH (▲-d),23 with permission of Wiley/VCH.

Counterions exert another important effect by the formation of ion pairs with the charged backbone. The ensuing non-covalent crosslinking counteracts the expansion described above, and leads, in the case of ionic chemomechanical polymers, to a strong dependence of the gel volume not only on the concentration of the counterion (visible also in Figure 4), but also on the nature of added salts.

Polyallylamine P5 shows characteristic pH and ion effects,31 and distinct contraction in the presence of small anions32 and some organic anions.33. If chloride inside the polyallylamine gel is replaced by added acetate the improved ion pairing with the carboxylate, which can use both oxygen atoms for contact to the backbone, leads to a 17% contraction (in one dimension), increasing to up to a 69 % contraction with phosphate and with α,ω-dicarboxylates, due to stronger salt bridges.29 The polyethyleneimine gel P4 exhibits related pH profiles,34 and again small contractions upon added acetate, and larger ones promoted by phosphate and aliphatic dicarboxylates.35 The expansions observed with the chitosan gel P3 are large upon adding free acids in comparison to the gel at pH 7, as P3 is essentially unprotonated. The corresponding anions then lead to contraction by crosslinking (Table 1).35 The exceptions are those expected by the effect of ion pairing: phosphate is monoprotonated at low pH, leading to crosslinking which counteracts the expansion promoted by the acid. Oxalic acid acts as an anion bearing two charges, due to its low pK value; the succinate anion leads to particularly small contraction due to the longer chain between the charges which allows more freedom within the network.

Table 1.

Crosslinking by ion pairing. Expansion EF [%] of the chitosan gel P3 by free acid XH and by anion X− (EF in one direction; with EF = 0 % for the gel at pH 7).43

Acid XH XH X− Acid XH X−
HCl 135 0 (COOH)2 1 (X2−)
H3PO4 28 (XH2−) 0 CH2(COOH)2 12
CH3COOH 131 5 (CH2)2(COOH)2 105
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EF in one direction; with EF = 0 % for the gel at pH 7

3.2 Metal Ion Chelation

Guidelines for designing metal chelate binding sites in chemomechanical polymers emerge from coordination chemistry. For alkali or alkali earth ions, crown ether -type oxygen ligands are suitable, for heavier metal ions ethylendiamine-type hosts such as in P1 are most relevant. A hydrogel containing benzo-18-crown-6-acrylamide exhibits expansion in the presence of Ba++, and does not respond to K+ or other ions which are known to bind less efficiently. The expansion and a lower critical solution temperature (LCST) of the hydrogel was ascribed to the repulsion among the charged Ba++ complex groups and the osmotic pressure within the hydrogel.36

Figure 5 shows that heavy metal ions trigger either large expansion (up to 390 vol% with the largest ion Pb2+), or smaller contractions of gel P1.37 Noticeably, the profiles show, in contrast to other effectors (see e.g. Figure 3), a maximum which, for the given gel particle size, occurs around 0.02 M. Metal content measurements established that the correspondingly calculated amount of ions were present in the gel particles. The decrease after reaching the maximum expansion, at which approximately one metal ion is occupying one ethylendiamine unit, is obviously due to weaker bound additional ions, which occupy other centers in the polymer. As expected, the affinity of these metal ions is so strong that all of the ethylendiamine binding sites are occupied. In contrast other effectors such as nucleotides use only part of the available binding sites, as a function of their concentration in the solution. Obviously, the metal ions are not making use of ethylendiamine units in opposing polymer strands after binding to one en unit, as this would lead to contraction and a smaller binding capacity than measured. This, and the strong affinity of e.g. Cu or Zn ions to polymers such as P1, is the basis of the promising cooperativity effects discussed in section 3.6. Again, the uptake of solvation water is the major contributor to the expansion, as exemplified in Figure 2 with the effect of Cu(OAc)2.

FIGURE 5.

FIGURE 5

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Size changes (length increase or decrease) of the polymer film P1 in the presence of various metal ions. 37

3.3 Hydrogen bonding

Hydrogen bonding can only play a minor role in effector binding to hydrogels, due to the strong competition with excess water. This is borne out by the fact that with all the chemomechanical polymers we have investigated (P1 to P5), electroneutral compounds even possessing strong donor/acceptor functions such as ureas, never induced detectable size changes. However, the polymer network itself and the water inside the gels is strongly stabilized by bridges between H-donor and acceptor groups,38 particularly in side-chain modified synthetic polymers.12 Size changes in hydrogels are essentially due to hydrogen bonding with water, which at higher temperature breaks down, with resulting water loss and contraction.39 Amphililic hydrogelators usually contain amide or hydroxy groups which allows stabilization of fibers, etc. in the gelation process.14 Hydrogen bonding plays a major role particularly for organic gelators;14 and in hydrophobic gels or gel cavities, where water minimally competes, also in sol-gel transitions,40 and in the transition between swollen and collapsed gel phases.41

3.4 Stacking and cation -π interactions

The well known attraction between aromatic units can be used in chemomechanical polymers containing suitable binding units, such as with the chitosan derivative P3 and different amino acids (Scheme 3).42 The largest expansion is seen with Trp and Phe. Smaller but still noticeable effects of aliphatic residues are due to hydrophobic interactions (section 3.5).

SCHEME 3.

SCHEME 3

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Volume expansions [%] on chitosan-anthrhyl polymer P3 with different amino acid esters; pH and salt effects deducted.42

The importance of cation-π effects is most clearly seen in the expansions triggered by aromatic effectors with the polymer P1 (Scheme 4), where saturated rings contribute nothing.23 A dramatic manifestation of the cation-π effect is evident in the effect of aromatic effectors on chitosan gels P3.30 Tartaric acid or its O-t-butyl derivative is completely inactive; but aromatic substituents as in the O-benzoyl derivative lead to sizeable volume changes of the gel, which moreover depend on the chosen enantiomer. MAS-NMR-spectra of the complexed gel (Figure 6) exhibit large upfield shifts of the axial glucose protons of the chitosan chain of approximately 2 ppm. This clearly indicates a position of these protons in the shielding cone of the effector phenyl substituent; the corresponding conformation allows strong interactions of the effector aryl unit with the protonated aminogroup of chitosan. Noticeably, the L-enantiomer leads to much smaller signal changes, in line with its much smaller activity, which allowed for the first direct translation of chiral recognition into mechanical motions.

SCHEME 4.

SCHEME 4

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Expansion with polymer P1 (in % volume) at different pH values; values at pH 11 corrected for the effect of pH alone (about 70%).23

FIGURE 6.

FIGURE 6

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MAS-NMR-spectrum of the complex between chitosan and O-dibenzoyl-tartaric acid (DBTA) enantiomers as effectors, and underlying structure.30

3.5 Dispersion forces, lipophilic binding

Van der Waals- and CH-π interactions can also play a role in the interaction of aromatic residues with glucose axial protons in structures such as those shown in Figure 6. However, in view of the inactivity observed with neutral effectors43 such contributions are much weaker than e.g. the cation-π effect. Studies with a polyallylamine-derived hydrogel P5 show, however, that dispersive interactions between effector molecules can play a major role in chemomechanical activity.29 (Scheme 5). The presence of aromatic units leads to remarkably enhanced contractions even with monoacids, as seen with benzoic and naphthoic acids. With all these effectors a plateau of the volume change is reached only with a large excess of effector over the available binding sites within a gel piece. These observations are in line with interactions between the effector molecules, which in the case of benzoic and naphthoic acids may be due also to stacking (Figure 7). The large contraction seen with m-nitrobenzoate however points strongly to a major contribution from dispersive effects, as found in independent studies with porphyrin complexes.44 This is corroborated by the strikingly diminished gel size changes with o-nitrobenzoate: as in the independent measurements with complexes in solution steric hindrance of coplanarity with the phenyl residue leads to diminished strength of complexation ( Figure 7).

SCHEME 5.

SCHEME 5

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Typical contractions triggered by carboxylates (%, in one direction) with the polyallylamine-derived hydrogel P5.29

FIGURE 7.

FIGURE 7

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Non-covalent crosslinking in the hydrogel P5 , with (a) dicarboxylic acids; (b) p-nitrobenzoic acid; (c) o-nitrobenzoic acid ; (d) stacking of naphthyl groups.29

Lipophilic or hydrophobic interactions, which even in solution complexes are difficult to separate from dispersive forces, play a decisive role e.g in steroid-imprinted polymers,17 and in the distinction of amino acid side chains in metal-mediated complexation with chemomechanical polymers (see section 3.6). The long alkyl substituents introduced in the polymer P1 provide for a significant lipophilic or hydrophobic interaction. Besides the amino acid selectivity discussed in section 3.6 this is clearly seen in the abrupt change from expansion to contraction of the gel P1 in the variation of tetraalkylammonium effectors (Figure 8). Here the normal expansion, which increases with the size of the ammonium effector, changes to contraction with the tetrahexyl compound, due to a collapse of network parts by stronger association with the polymer alkyl chains.

FIGURE 8.

FIGURE 8

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Size changes (vol-%) of the polymer gel P1 by tetraalkylammonium hydroxides.23

3.6 Cooperativity and Logical Gate Functions

Cooperativity between two effector molecules in the sense of a logical AND gate is known from solution chemistry, mostly in the form of a signal dependence not only on the presence of an analyte molecule, but also on the pH. With chemomechanical polymers cooperativity turns out to be a more common phenomenon. Even simple gels such as the polyethyleneimine polymer P4 show cooperativity (Scheme 6) between e.g. naphthoic acid as component A and amino acids as effector B. If both A and B react, the resulting contraction is enhanced by 20 to 25 %.35 Figure 9 illustrates how (a) the replacement of chloride by naphthoate can lead to cation-π interactions and COO− +NH3 salt bridges, and how (b) the binding of the second effector aminoacid is enhanced by additional COO− +NH3 salt bridges, with measurable concomitant water content changes.

SCHEME 6.

SCHEME 6

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Cooperativity between naphthoic acid as component A and aminoacids as effector B, (% contraction in one direction) in gel P4; if A and B react simultaneously contraction is enhanced by 25 or 21 %.35

FIGURE 9.

FIGURE 9

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Model for cooperative contraction mechanism in gel P4, see text; the relevant non-covalent interactions are indicated as dashed lines.35

A more dramatic, and for many applications very useful cooperativity in the sense of yes/no responses of the polymer, is possible with metal complexes operating in the ethylendiamine-containing polymer P1.45 Here, amino acids or peptides promote volume changes only in presence of e.g. Cu++ or Zn++ ions, depending moreover on the nature of underlying amino acids. Scheme 7 shows how the presence of the metal ions, which themselves lead to only moderate size changes (see section 3.2), allows, by occupation of free coordination sites, the action of added amino acids or peptides, which otherwise are completely inactive. As mentioned above, the interaction with the lipophilic alkyl chain L implemented in polymer P1 leads to discrimination between amino acids according to the lipophilicity of their side chains.

SCHEME 7.

SCHEME 7

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Ternary complexes with cooperativity between metal ions and aminoacids or peptides; examples for expansion triggered by peptides (in one dimension); net : effect of Cu2+ alone deducted.45

3.7 Sugar recognition in chemomechanical gels

To date glucose is the most extensively studied sugar in fully synthetic chemomechanical polymers. This is driven by the need for minimal- or non-invasive glucose monitoring and insulin delivery systems.46 In polyacrylamide hydrogels, relatively unspecific interactions with glucose lead to swelling in response to glucose.47 Poly(N-isopropylacrylamide)-based hydrogels exhibit volume phase transitions upon the addition of sugars arising from changes in structured water and hydrophobic hydration around the isopropyl moieties, rather than via direct interaction between the sugars and the polymer.48

Enhanced properties are attained via glucose-imprinted gels.15,49 Molecularly imprinted polymers (MIP's) function mainly based on shape selectivity, and are therefore outside the scope of this Account. However, there are examples of chemomechanical MIPs for glucose that incorporate enhanced supramolecular interactions. One of these, inspired by natural lectins, employs amino acid-mimicking functional monomers. It has affinities for glucose in the range of 1.7 mM, comparable to concanavalin A.50 In another example of a glucose-selective chemomechanical MIP, ion pairing interactions were used in imprinting gels with glucose phosphate. This material exhibited binding selective for glucose over fructose.51

Configurationally biomimetic imprinted polymers (CBIPs) exhibit enhanced interactions between monomers and the template/analyte. For instance, hydroxyethyl methacrylate monomer was used in a recently reported CBIP to enhance hydrogen bonding interactions with glucose. The binding capacity for glucose in glucose-imprinted copolymers from hydroxyethyl methacrylate and methacrylic acid was found to be 5.5 times higher than for galactose.52

Fast covalent interactions

Reversible covalent binding to cis-diols by boronic acids has inspired the design of several chemomechanical gels responding to glucose.2b These materials can be subdivided into two main classes: (i) optical signal transduction components and (ii) those for potential use in automated insulin delivery. These potential applications have been reviewed previously.2b

To address the challenge53 of attaining materials functional in biological fluids we reported the first boronic acid-based chemomechanical polymer composed entirely of synthetic materials that exhibited selective size changes in response to glucose concentration in human plasma.54 Unlike analogous hydrogels,2b flexible supramolecular binding sites (Scheme 1, structure P1, and with arylboronic acid moieties) were appended to a pre-existing polymer (PMMA). The gel exhibited reversibility and insignificant interference from other common blood sugars.

Complexes a, c, and e in Scheme 8 predominate under neutral, nonaqueous solution conditions.55aHowever, when cyclic esters such as c and e form upon glucose addition, the boron atom becomes more Lewis acidic. In aqueous media, the equilibrium favors structures such as d and f over c and e, respectively. In a hydrogel, charge formation leads to a Donnan potential and a greater free energy of mixing with water. Thus, monodentate binding as shown in structure d promotes water uptake and swelling.

Scheme 8.

Scheme 8

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Glucose-boronate complexes. The preferential binding of the alpha glucofuranose was demonstrated in solution (ref. 64,55a) and later observed in hydrogels (ref. 58).

In biological fluids, however, high ionic strength renders Donnan potential effects insignificant. This was addressed in hydrogels designed to promote crosslinked structures such as f via the formation of a supramolecular complex in which glycol moieties localize sodium cations to stabilize the boronate dianion structures56. This promotes gel shrinkage.

In our chemomechanical polymer glucose-induced shrinkage was tuned by varying the size of the modifiers or crosslinkers.47 Boronic acid-based gels can exhibit either swelling57 or shrinkage58 at high ionic strength conditions by optimizing the concentration of appended boronic acid groups and properties including gel hydrophobicity.50-59

Ammonium cations also stabilize boronate dianion structures and promote cross-linking60 and resistance to pH fluctuations.61 The incorporation of tertiary amines in a boronic acid hydrogel has also afforded enhanced glucose-induced cross-linking .62 Interestingly, in related solution studies, boron-nitrogen interactions are most significant in relatively hydrophobic environments.63 Importantly, selectivity is automatically addressed via the cross-linking mechanism, as it is well-known that glucose is the only major physiologically-relevant monosaccharide that forms a bis-boronate structure readily under neutral conditions.

4. Conclusions

Chemomechanical polymers hold a vastly unexplored promise for a large variety of applications. It is hoped that the present review will trigger more investigations, which will address the implementation of more sophisticated molecular recognition sites, and the practical use of such smart materials. This will be aided by a better understanding of the fundamental processes involved in chemically induced size changes of these polymers. Systematic analyses with a variety of polymers and effector compounds, and advanced methods for the inherently challenging structural characterizations, will greatly contribute to this new field of supramolecular chemistry.

Biographies

Professor Hans-Jörg Schneider studied chemistry in Tübingen, München and Berlin. He obtained his diploma in 1963, and 1967 his Ph.D. degree in Tübingen (with Prof. M. Hanack). 1967-1969 he was Postgraduate Research Fellow at the Univ. of California, San Diego (with Prof. R.C. Fahey); 1969-1971 he worked as Research Assistant of Prof. W. Hückel, Univ. Tübingen. 1972 he became Professor for Organic Chemistry at the Universität des Saarlandes. The early research interest of Professor Schneider was conformational analysis, strain-reactivity relations, and NMR-spectroscopy. Later his group became involved in studies on mechanisms of molecular recognition, new enzyme and receptor analogs, DNA interactions, and artificial esterases or nucleases. More recently his major activity involved molecular recognition in chemomechanical polymers.

Professor Robert Strongin was born in Brooklyn, New York in 1955. He received a B.A. with Honors in Chemistry form Temple University and a Ph.D. in Organic Chemistry from the University of Pennsylvania under the guidance of Professor Amos B. Smith, III. He was Philip and Foymae Kelso West Distinguished Professor in Chemistry at Louisiana State University before moving to the Department of Chemistry at Portland State University. Professor Strongin's research interests include the design, synthesis and evaluation of functional chemical sensing agents and new redox and chromophore materials.

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