Even 60 years after its discovery, the living anionic polymerization is still the method of choice, when it comes to well-defined polymers of high molar mass, end group functionality, block copolymers. The anionic polymerization is also very important in the industry to prepare block copolymers from vinyl monomers or epoxides, for example. The high control of the anionic polymerization allows also controlling comonomer sequences, either by sequential polymerization or a competing copolymerization of several monomers.
The “Functional Polymers” group develops comonomer systems, which undergo a selective sequenced copolymerization. By monomer design, we control the polymerization kinetics in order to prepare block copolymers (Gleede et al. J. Am. Chem. Soc. 2018) or (multi) gradient copolymers (Rieger et al. Angew. Chem. 2018 & Macromol. Rapid Comm. 2016).
In contrast to solid-supported synthetic strategies, our approach allows us to prepare sophisticated macromolecules on a large scale with high structural precision. The monomer design controls the reactivity.
A synthetic platform to realize such sequenced copolymers was established in the “Functional Polymers” Group in recent years: the living anionic polymerization of aziridines. Aziridines were missing in the monomer family for anionic polymerization and access to well-defined polyamine structures or copolymers with styrenes or epoxides are challenging or even impossible to prepare. Aziridine, or ethylene imine, is produced industrially and polymerized only by uncontrolled cationic polymerization. It has a plethora of applications, ranging from chelator, wastewater treatment to gene transfection. The combination of such building blocks with other industrially relevant materials would be desirable to prepare novel materials with unprecedented properties. However, controlled synthesis of linear or branched poly(ethylene imine) (PEI) from aziridine had been missing. The only way for preparing linear PEI is the detour via oxazoline chemistry and subsequent hydrolysis. We established a novel monomer family for the living anionic polymerization, namely activated aziridines. By activation of the aziridine-ring with sulfonamides to allow nucleophilic ring opening, a variety of novel monomers and polymers becomes available. More importantly, base-initiated polymerization proceeds via a “living” mechanism, allowing the formation of polymer architectures or combination with other monomer types. We were able to introduce several chemical functions into the polyaziridines, either in the activating group or as a pendant chain. Removal of the activating group, if desired, is possible by several methods to prepare polyamines which were not accessible so far.
The activating group can do more: by adjustment of the electron-withdrawing effect, the sulfonamide finely tunes the monomer reactivity and controls the synthetic primary structure. This allowed us to prepare sequence-controlled copolymers by a competing anionic polymerization of up to 5 different monomers in a one-pot and one-shot reaction (Macromol. Rapid Comm. 2016). This will allow us to prepare functional materials mimicking the primary structure of proteins and allow folding into hierarchical assemblies. In addition, chirality is currently installed into such synthetic primary structures to further control the chirality of the folding. The combination of aziridine chemistry with epoxides allowed us to prepare amphiphilic multi-block copolymers.
Compartmentalization is another handle to control the synthetic primary structure: By confining a copolymerization to nanodroplets in an emulsion, an ideal (random) copolymerization is forced into gradient copolymers. Polymerization began only inside the droplets. As that compound was gradually consumed, the change in concentration pulled ever-greater amounts of the other ingredient into the chains, creating a gradient effect.
Besides aziridines, also biodegradable polymers are interesting candidates to prepare gradient (amphiphilic) copolymers. We recently were able to copolymerize lactide with cyclic phosphonates and by choice of the catalyst, the gradient structure and comonomer incorporation was controlled (collaboration with University of Bath UK)).