Low-molecular-weight gels show great potential for application in fields ranging from the petrochemical industry to healthcare and tissue engineering. These supramolecular gels are often metastable materials, which implies that their properties are, at least partially, kinetically controlled. Here we show how the mechanical properties and structure of these materials can be controlled directly by catalytic action. We show how in situ catalysis of the formation of gelator molecules can be used to accelerate the formation of supramolecular hydrogels, which drastically enhances their resulting mechanical properties. Using acid or nucleophilic aniline catalysis, it is possible to make supramolecular hydrogels with tunable gel-strength in a matter of minutes, under ambient conditions, starting from simple soluble building blocks. By changing the rate of formation of the gelator molecules using a catalyst, the overall rate of gelation and the resulting gel morphology are affected, which provides access to metastable gel states with improved mechanical strength and appearance despite an identical gelator composition.

Lipase-catalyzed ring-opening polymerizations of lactones with various ring sizes (6-to 13-and the 16-membered ring) employing Novozym 435 demonstrate fascinating differences in their polymerization rates. These differences cannot be related to variations in physical properties such as the dipole moment of the lactones only. For example, 10-decanolactone, 11-undecanolactone, and 12-dodecanolactone show dipole moments of around 1.9 D, but the initial rate constant of their Novozym 435-catalyzed polymerization was found to be 0.10, 0.38, and 4.91 h -1 , respectively. The Michaelis-Menten constants K M and V max were measured for all lactones, and this revealed that the K M was more or less independent of the ring size, suggesting similar affinities of the lipase for all lactones, while no obvious trend could be discerned for V max . However, conformational strain and transannular interactions present in medium ring lactones (ring size 8-12) affect the reactivity of these lactones in lipase catalysis in a similar way as was previously described for the alkaline hydrolysis of lactones, rationalizing the low reactivity of the 10-12-membered rings and the high reactivity of the 8-membered ring. To our surprise, there is a large discrepancy in lipase-catalyzed ring-opening polymerizations compared to the alkaline hydrolysis reactions with respect to the relative reactivity of lactones possessing a cisoid conformation of the ester bond: while the ratio in rates between δ-valerolactone and 12-dodecanolactone in a alkaline hydrolysis is 9400, the ratio in rates using Novozym 435-catalyzed polymerization is only 3. In fact, in the latter case lactones possessing a cisoid conformation are less reactive than large ring lactones possessing a transoid conformation. This mechanistic study also furnished polyesters with an increasing number of methylenes in their repeat unit. The polyesters can be readily prepared in reasonable molecular weights (>10 000 g/mol). Preliminary thermal characterization of the polyesters shows, as expected, that the melting temperature and the melting enthalpy increase with an increasing number of methylenes in the repeat unit.

In situ multicomponent hydrogelator formation enables straightforward chemical functionalisation of supramolecular hydrogels.

The use of polymeric crosslinkers is an attractive method to modify the mechanical properties of supramolecular materials, but their effects on the self-assembly of the underlying supramolecular polymer networks are poorly understood. Modulation of the gelation pathway of a reaction-coupled low molecular weight hydrogelator is demonstrated using (bio)polymeric crosslinkers of disparate physicochemical identities, providing a handle for control over materials properties.

Abstract2‐Oxo‐12‐crown‐4‐ether (OC) was procured in a novel, two‐step procedure in a 37% overall yield. This interesting hydrophilic lactone was effectively polymerized with Novozym 435 as the catalyst: within 10 min, the monomer conversion was greater than 95%. Poly(2‐oxo‐12‐crown‐4‐ether) [poly(OC)] was obtained as a viscous oil with a glass‐transition temperature of approximately −40 °C, and it was soluble in water. Subsequently, OC was copolymerized with ω‐pentadecanolactone (PDL). A kinetic evaluation of both monomers showed that for OC, the Michaelis–Menten constant (KM) and the maximal rate of polymerization (Vmax) were 2.7 mol/L and 0.24 mol/L min, respectively, whereas for PDL, KM and Vmax were 0.5 mol/L and 0.09 mol/L min, respectively. Although OC polymerized five times faster than PDL, 1H NMR analysis of the copolymers revealed a random copolymer structure. Differential scanning calorimetry traces of the copolymers showed that they were semicrystalline and that the melting temperature and melting enthalpy of the copolymers linearly decreased with an increasing amount of OC. The melting temperature of the copolymers could be adequately predicted by the Baur equation, and this suggested that poly (OC) was rejected from the poly(ω‐pentadecanolactone) [poly(PDL)] crystals. Solid‐state NMR studies confirmed that the crystalline phase exclusively consisted of poly (PDL), whereas the amorphous phase was a mixture of OC and PDL units. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2166–2176, 2006