We use the phosphorus-chemistry platform to design water-soluble, functional, and biodegradable polyphosphates and –phosphonates with controlled interactions to their environment. This allowed the design of thermoresponsive assemblies (Wolf et al. J. Am. Chem Soc. 2017) and the development of nanocarriers with controlled blood interactions (Simon et al. Angew. Chem. 2018).
My research group has reported the first water-soluble polyphosphonates by a “living” anionic polymerization in 2014 (Steinbach et al. ACS Macro Lett. 2014). A series of polymers and copolymers were developed that undergo a precise degradation (e.g. in seawater or by enzymes), depending on their structure and comonomer composition (Wolf et al. Macromolecules 2015, Polym. Chem. 2016). A new monomer family was established that allows further adjustment of their hydrophilicity and the design of temperature-responsive degradable polymers (Wolf et al. Eur. Polym J. 2017). These tools allowed us to assemble temperature-responsive coacervates or polymersomes that release a cargo upon heating or cooling.
Redox-responsive Metallocene-Containing Polyethers
Metallocene-containing polymers, particularly materials based on ferrocene (fc), are unusual polymers due to their unique physical and chemical properties. Ferrocene is thermally very stable and it can be oxidized reversibly to the ferrocenium ion, rendering it a highly interesting organometallic group for materials for various applications.
Ferrocene can be introduced into polymeric structures via several approaches: (i) either fc-containing monomers are polymerized to generate structures with fc in the main chain or as pendant groups, or (ii) fc-groups are attached to a multivalent, prefabricated polymer as side chains subsequent to polymerization. Our group uses both strategies to imply fc units within interesting and novel polymer architectures. A general synthetic protocol to generate hydroxyl terminated poly(ferrocenyl¬silane)s (PFS) was recently established and used as macroinitiators to prepare amphiphilic, water-soluble PFS-b-PEG. In the context of fast access to water-soluble fc-containing polymers, we introduced ferrocene glycidyl ether (fcGE) as a novel epoxide monomer carrying a fc side chain. Ferrocene glycidyl ether was uti¬lized for living anionic ring-opening polymerization. It was homopolymerized as well as copolymerized with ethylene oxide (EO) leading to water-soluble fc-containing copolymers (with fcGE contents up to 10 mol%), which show a lower critical solution temperature (LCST) in water. Also polyvalent copolymers by copolymerization of fcGE with allyl glycidyl ether were recently presented which were synthesized in a bulk polymerization. For the first time, a bulk polymerization was monitored via an in situ microstructure analysis via quantitative 13C NMR spectroscopy. The materials possess an adjustable number of redox-active ferrocenes and reactive double bonds, which can be addressed by further transformations.
This strategy was recently extended to bivalent vinyl ferrocenyl glycidyl ether – a novel monomer for either radical or anionic polymerization. After anionic copolymerization with ethylene oxide and postmodification of the pendant double bonds, the first fc-containing triple-responsive polyethers were obtained and used for the generation of smart surfaces, that change their hydrophilicity by external stimuli, such as temperature, pH, or redox potential.