Reactions at the Interface
A molecular “screw clamp”: Interfacial reactions and polymerizations
130 years after Schotten and Baumann, reactions at the liquid-liquid interface still hold a great potential, especially as convenient and versatile route for the preparation of (nano)materials, which are not accessible by any other technique.
In our department we are investigating polymerization (chain-growth and step-growth) at the interface between hydrophilic and hydrophobic liquids, mainly in (mini)emulsions. At the droplet interface, monomers provided from either phase meet and react to generate a polymer, which is insoluble in both phases and thus forms a shell surrounding a liquid core. The liquid core can be water or any other hydrophilic organic solvent, dispersed in an inert organic continuous phase. Typically, polyaddition reactions, as the generation of polyurethanes from di- or polyols and diisocyanates, or more recently bio-orthogonal polyadditions such as alkyne-azide “click” reactions or polycondensations, were performed in this manner. In addition to polyurethane capsules obtained through a polyaddition reaction, also poly(cyanacrylate) nanocapsules can be generated by interfacial anionic polymerization or a polycondensation of alkoxysilanes by aqueous hydrolysis can be performed. By these reactions, linear, branched or crosslinked polymers can be obtained, which form the shell of the respective nanocarrier. Within this field of research we are combining classical polymerization strategies with novel demands: biocompatibility, drug delivery, sustainability, and self-healing properties.
The molecular “screw clamp” is generated at the interface forcing both reaction partners to react. This allows us to perform reactions that cannot be performed in homogeneous solution. For example, alkyne-azide click reactions normally need copper catalysts, which are not necessary in heterogeneous conditions, as the interface, i.e. the close proximity of the two components, promotes the reaction. This technique allows us to include, i.e. encapsulate, bioactive or sensitive molecules, as enzymes, vitamins, DNA, or proteins, in the interior of the nanocapsules which would not survive metal catalysis or the harsh reaction conditions of other polymerization techniques as the abovementioned polyaddition with isocyanates. Due to the presence of the polymeric shell the “payload” is protected against the organic solvent from the outside, which can be easily exchanged to water resulting in aqueous dispersions which are highly promising for the delivery of drugs or other biomedical applications as enzyme sensing. For drug delivery, biodegradable or switchable materials can be used, such as polysaccharides, providing biocompatibility of the nanocarrier and degradation in vivo.
Moreover, the huge droplet interface in a miniemulsion provides an excellent environment for interfacial catalysis. Enzymatic synthesis of small molecules can as well be performed as metathesis reactions, generating or crosslinking polymers. A recent review summarizes the field in detail.