Metal-Free Curing of 3D Printable Silicone Elastomers via Thermally Triggered 2-Oxazoline Cross-Linkers

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.