Synthetic polymer networks have attracted considerable attention owing to their exceptional mechanical properties including high resilience and toughness. Such materials are typically based on multi-arm poly(ethylene glycol) (PEG) which is a commercially available compound. However, PEG networks suffer from restricted access to higher molecular weight which limits the network dimensions. In addition, the crystalline nature of PEG does not allow for a comprehensive understanding of the mechanical behaviour in bulk network elastomers. To overcome this challenge, Walther and co-workers introduced a new class of high molecular weight star polymer building blocks for the construction of model network elastomers and hydrogels with tuneable mechanical properties. To achieve this, triethylene glycol methyl ether acrylate was successfully polymerized via light-inducted atom transfer radical polymerization and Cu(0)-wire reversible deactivation radical polymerization, yielding well-defined polymers with narrow molecular weight distributions and high end-group fidelity. Upon synthesis, functional motifs were introduced within the polymer through either post-polymerization modification of the bromine end-groups or the use of a fluorescent star initiator. In particular, the introduction of norbornene end-groups allowed for the subsequent crosslinking of the materials in presence of a photo-radical initiator. This allowed access to thermally reversible model network hydrogels based on dynamic supramolecular bonds. Overall, this work enables the simultaneous study of the mechanical behaviour of bulk network elastomers and swollen hydrogens with the same network topology. As the authors elude in their conclusions, by elegantly exploiting precision polymer chemistry, our understanding of architecture control can be enhanced leading to the rational design of functional mechanical network materials.
Tips/comments directly from the authors:
- Water-soluble star polymers with a low Tg and quantitative end-group introduction allow the simultaneous investigation of identical model networks as hydrogels and bulk elastomers.
- The monomer triethylene glycol methyl ether acrylate (mTEGA) yields low-Tg, water-soluble polymers. A distinct advantage over other oligo(ethylene glycol) acrylates is the absence of potential diacrylate impurities compromising polymerization control.
- Polymerization of mTEGA by photo-induced and Cu0-catalyzed Cu-RDRP from commercial and functional 4-arm initiators yields narrowly dispersed star polymers up to high molecular weights. In order to achieve optimal control with minimal side reactions, a balance in the initiator-to-CuBr2 ratio is necessary.
- Cu0-mediated Cu-RDRP is suitable for scale up, and the polymers can be isolated by precipitation into 85/15 diethyl ether/n-pentane followed by salt removal through neutral alumina.
- Following end-group transformation with primary amines, both excess amines and bromide salts must be removed. The former is removed through precipitation, the latter by taking the polymer up into a diethyl ether/THF mixture and removing insoluble components.
- Constructing hydrogels by photo-crosslinking 4-arm p(mTEGA)-norbornene with a bifunctional thiol is fast (<1 min) with the photo-radical initiator LAP and slower (>30 min) with Irgacure-2959.
- Supramolecular hydrogels constructed from 4-arm p(mTEGA)-terpyridine with divalent metal ions are highly dependent on the metal. ZnII yields hydrogels which are dynamic at room temperature, and increasingly so upon heating making them suitable for thermal 3D-printing.
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Bottom-up design of model network elastomers and hydrogels from precise star polymers, Polym. Chem., 2019, 10, 3740-3750, DOI: 10.1039/c9py00731h
About the web writer
Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.