Metal-Free Curing of 3D Printable Silicone Elastomers via Thermally Triggered 2-Oxazoline Cross-Linkers
Paul Strasser, Christina Walliser, Edip Ajvazi, Felix Bauer, Oliver Brüggemann, Sebastian Lämmermann, Zoltán Major, Alžbeta Minarčíková, Monika Majerčíková, Matej Mičušík, Angela Kleinová, Zuzana Kroneková, Juraj Kronek, Ian Teasdale
Abstract
Silicone elastomers are commonly cured by hydrosilylation or condensation reactions, both of which require metal-based catalysts. Due to environmental, toxicological, and cost concerns, there is considerable interest in developing metal-free alternatives. Herein, we present a novel solution via an atom-efficient, catalyst-free ring-opening of poly(2-isopropenyl-2-oxazoline) (PiPOx) as a curing agent. PiPOx are macromolecules bearing pendant reactive 2-oxazoline groups capable of undergoing a ring-opening reaction in the presence of carboxylic acids to give covalent amide-ester bonds. Polydimethylsiloxane (PDMS) chains with COOH moieties at the chain ends could be effectively cured with PiPOx as a latent curing agent. The reaction does not proceed at room temperature but cures in less than 5 min at elevated temperatures (>80 °C) in the absence of catalysts or solvents and without the evolution of volatiles. The PDMS diacids are easily accessible via a simple thiolene addition to divinyl siloxanes in a single step, thus extending the utility of this approach to all divinyl siloxanes, which are widely commercially available in a broad range of chain lengths. The cured elastomers contain up to 98 wt % of the PDMS constituent (including the end-groups), hence mirroring the chemical structure of traditional metal-cured commercial PDMS elastomers. Solvent-free, thermally triggered curing in a practical temperature range facilitates processing using standard extrusion-based processing and additive manufacturing techniques.